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: http://www.stsci.edu/institute/conference/iau258/posters
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| The Age-Metallicity-Velocity Relation in the Nearby Disk |
| Mr. Borja Anguiano (Astrophysikalisches Institut Potsdam) |
| Stellar ages are crucial to place the observed chemical and kinematics properties of the stars in an evolutionary context. We are making a new study of the local stellar Age-Metallicity-Velocity relation (AMVR) in the Galactic disk using a carefully selected large sample of subgiants stars, drawn from RAdial Velocity Experiment (RAVE) and Geneva-Copenhagen Survey (GCS). We are making spectrophotometric observations to confirm the subgiant status and echelle spectroscopy to get detailed element abundances. Our goal is to obtain accurate stellar parameters in order to make a reliable measurement of the AMVR in the nearby disk. |
| The Role of Ages in the Study of Formation and Evolution of Moving Groups |
| Ms. Teresa Antoja (DAM, Universitat de Barcelona) |
| Traditionally, moving groups were thought to be a result of the dispersion of stellar clusters, and thus, stars with a common origin and evolution. Recent studies have suggested that they could be caused by resonances of the Galactic spiral arms and/or bar. Stellar ages are the cornerstone to distinguish between these scenarios. Here we apply multi-scale techniques -wavelet denoising- to characterize and compare observed and simulated moving groups in the kinematic-age-metallicity space. In the observational domain, we confirm an extended age distribution for all kinematic branches, in which classic moving groups are arranged. Our analysis shows the existence of certain clumps in the age distribution of the Hyades-Pleiades branch with a periodicity of about 500-600 Myr. Besides, the birth of test particles is simulated, according to certain SFR in time and location, and their evolution is obtained from orbit integration using a model for the Galactic potential. The preliminary results show that several branches with an extended age distribution can be well reproduced near the 4:1 inner Lindblad resonance of the galactic spiral structure. Both, realistic ages for the test particles and more accurate and unbiased stellar ages on real stars are mandatory to go one step further in the physical interpretation of the formation of moving groups in terms of local or large-scale dynamics. |
| Probing the Fossil Record of Disk Galaxy Evolution in M33 |
| Dr. Mike Barker (Institute for Astronomy) |
| We present deep color-magnitude diagrams (CMDs) of two fields in M33’s outskirts imaged with the Advanced Camera for surveys on the Hubble Space Telescope. These Fields lie at 4.5 and 6 exponential scale lengths along the northeast major axis and straddle the break in disk surface brightness at 5 scale lengths observed in extensive ground surveys. The CMDs reach main sequence turn-offs of about 8 Gyr, and will provide important constraints on the epoch of disk galaxy assembly and the origin of disk truncations. Here we present preliminary constraints on the ages and metallicities present in these fields and investigate the spatial distribution of stars with different ages. We also report the discovery of a new M33 cluster lying at the outer edge of the inner field whose CMD suggests an age of a few Gyr. |
| Sizing Up the Stars: Main Sequence Stellar Diameters with the CHARA Array |
| Tabetha Boyajian (Georgia State University) |
| Possessing the longest optical interferometric baselines in the world, the CHARA Array is uniquely suited to measure the diameters of stars, which are generally unresolved by other optical interferometers. We present the current status of the survey to measure accurate angular diameters of nearby, main sequence, A, F, and G-type stars with the CHARA Array. This data then provides us with a direct determination of the stellar radius and effective temperature, critical to testing stellar atmosphere and evolution models, as well as the practical application of accurately plotting the H-R diagram. The accuracy and target sample range are aimed at refining, as well as expanding, the existing diameter measurements, which are used in the calibration of numerous less direct methods based on photometric parameters to predict fundamental properties of these types of stars. |
| Second Generation Stars in Globular Clusters: Light Elements Abundances and Relations with Global Cluster Parameters |
| Dr. Angela Bragaglia (INAF-Osservatorio Astronomico di Bologna) |
| We present the first results of our large survey of Na and O abundances in 19 halo/disk/bulge globular clusters of different metallicities, HB morphologies, masses, densities, concentrations, etc. We have analyzed medium-high resolution FLAMES-GIRAFFE spectra of about 2000 red giant stars and high resolution FLAMES-UVES spectra of more than 200 RGB stars in these 19 GCs. The Na-O anticorrelation is present in all of them, but with different extensions. We link the anticorrelation to the existence of two generations of stars in GCs and discuss the relationships between the extension of the "chemical anomalies" and the global cluster parameters. We also present implications for the formation scenarios and the early evolution of these stellar aggregates. |
| Simulations of Global-Scale Dynamo Action and Magnetism in Rapidly Rotating Suns |
| Mr. Benjamin Brown (University of Colorado, Boulder) |
| Rotation, convection and magnetism couple in stellar interiors to generate global-scale magnetic fields and cycles of stellar activity in stars like our sun. We have conducted global-scale fully 3-D MHD simulations of convection in more rapidly rotating solar-type stars with the ASH code, exploring the effects of more rapid rotation on stellar convection and the resulting magnetic dynamo action. We find that these rapidly rotating suns can build surprisingly strong and organized global-scale magnetic fields. As a great surprise, these stars can undergo cycles of magnetism, with global polarity reversals, even in the absence of tachoclines of shear and penetration. That boundary layer, at the base of the solar convection zone, has been thought crucial to global-scale dynamo action in the sun but may play a lesser role when stars are young. This may have important implications for the dynamos in these young, rapidly rotating suns and for fully convective stars. |
| Yonsei Evolutionary Population Synthesis (YEPS) model |
| Mr. Chul Chung (Department of Astronomy Yonsei University) |
| We present a new version of the Yonsei Evolutionary Population Synthesis (YEPS) model for simple stellar population based on the most up-to-date Yonsei-Yale stellar evolutionary tracks. We have modeled the integrated spectro-photometric quantities such as the absorption-line features and magnitudes in various passbands. Great care has been taken to incorporate the systematic variation of horizontal-branch morphology with respect to the 1st (Metallicity & abundance ratio), 2nd (Age) and 3rd (helium contents) parameters. Comparison of the model to observations reveals that the HB effect is a key element in understanding otherwise inexplicable phenomena found in globular cluster (GC) systems in the Galaxy and external galaxies. Wide application of the YEPS model will be discussed with a particular interest in interpreting (a) GC color bimodality found in bright galaxies, (b) HB effects in the local universe, and (c) GCs with Super-He-Rich subpopulation. |
| Star Formation Histories of SMC Fields Observed with HST/ACS |
| Dr. Michele Cignoni (Department of Astronomy, Bologna University, Italy) |
| Within the framework of a large coordinated HST effort to study the star formation processes and history of several fields in the SMC, we present preliminary results on the history of NGC602 and NGC346, a couple of very young clusters. In particular, we explore the characteristics of the many candidate pre main sequence (PMS) stars. We combine a new set of PMS stellar tracks for the metallicity Z=0.004 with a stellar population synthesis model that takes into account all evolutionary phases. The best star formation is searched by comparing observed and synthetic CMDs. |
| Pulsation in Young Brown Dwarfs: Constraints on Very Low Mass Cluster Members |
| Ms. Ann Marie Cody (California Institute of Technology) |
| Brown dwarfs and very low mass stars constitute a crucial link between the intertwined processes of star formation and planet formation. To date, however, observational methods to uncover their formation mechanism or determine important properties such as mass and age have been lacking. Pulsation powered by deuterium burning in brown dwarfs and very low mass stars is a newly suggested phenomenon that offers unprecedented opportunities to probe the interiors and evolutionary status of these objects. We have initiated a photometric campaign to search for this putative class of pulsators among low-mass members of several young star clusters. Our monitoring technique is targeted to identify low-amplitude periodic variables with periods on the order of several hours, and initial results indicate several candidate pulsators near our detection limit. The identification of brown dwarfs on the narrow pulsation instability strip should ultimately provide observational diagnostics for mass and age via the techiniques of seismology. |
| Understanding the Time Scale of Star Formation by Characterizing the Age and Stellar Content of B59 & the Pipe Nebula |
| Dr. Kevin Covey (Harvard-Smithsonian Center for Astrophysics) |
| The Pipe Nebula has recently been recognized as a valuable new laboratory for studying molecular cloud properties during the earliest stages of star formation. The Pipe contains only one small cluster (B59) of ~20 young stars; until recently, therefore, the Pipe has escaped detailed scrutiny despite its proximity (~130 pc) and size (~10^4 M_sun). New studies of the Pipe, however, have revealed valuable clues to the time scale of the star formation process, and thus the extent to which stellar populations can be considered co-evol. In particular, detailed studies of dense cores in the Pipe (Rathborne et al. 2008, Muench et al. 2007, Lada et al. 2008) find that thermal pressure provides the dominant source of support against collapse, and that non-thermal support is constrained to sub-sonic flows. With sizes less than 0.1 parsec, the sound crossing times and minimum ages of these cores are of order 1 million years. These cores are also found to be stable, suggesting that they are evolving quasi-staticly over several of crossing times, counter to the more rapid evolution predicted by theoretical models whereby turbulence provides the dominant means of support against collapse. Understanding the nature of this seeming conflict between observations and theory is essential to ensuring a proper description of time scale of star formation, and thus the limit to which stellar populations can be considered co-evol. To shed light on the true age of the dense core population in the Pipe, we have made spectroscopic observations of the stellar content of B59, the most evolved core in the Pipe. Measuring stellar temperatures from these spectra, we have created an observational HR diagram for B59; comparing this HR diagram to theoretical pre-main sequence evolutionary tracks, we have derived the mean stellar age of B59, providing a first estimate of the ages of dense cores in the Pipe, and constrained the characteristic time scale of the star formation process in the Pipe Nebula. |
| WIYN Open Cluster Study: Effects of Metallicity on Initial Li and Li Depletion Rates in Young Open Clusters |
| Mr. Jeff Cummings (Indiana University) |
| Lithium abundance is a promising age indicator for G and K dwarfs, but the effects of metallicity on initial Li and Li depletion rates must first be considered. These effects are illustrated in comparisons of Li abundances derived for stars in three young (30-40 Myr) open clusters that span a range of metallicities: M36 ([Fe/H]~-0.25), NGC 2547 ([Fe/H]~-0.02), and IC 4665 ([Fe/H]~+0.12). Young clusters provide important information about the effects of metallicity on initial Li abundances, because these clusters exhibit Li-Teff plateaus in F dwarfs with little scatter, which thus provide direct measurements of each cluster's initial Li. These measurements of initial Li increase with increasing [Fe/H], consistent with the expectation that the Galaxy produces Li. Even at this young age the effects of metallicity on Li depletion rate is apparent in the K and M dwarfs, with the more metal-rich NGC 2547 exhiting significantly more depletion than the more metal-poor M36. The G and K dwarfs do show scatter in Li, which we discuss. |
| Watching Stars Grow |
| Dr. Guido De Marchi (ESA) |
| Although high-mass stars govern the energy budget of the Universe, it is low-mass stars that make up the bulk of its mass and that are most likely to harbour habitable planets. At present, not enough is known on the timescale of low-mass star formation and on the duration of the pre-main sequence (PMS) phase. So far, determinations of the mass accretion rate onto T-Tauri stars have been obtained for about a hundred objects in nearby star forming regions, all with solar metalliticy, and no data exist for low metallicity environments. To alleviate this problem, we have developed a novel method that allows us to determine PMS mass accretion rates using narrow-band (Halpha) and broad-band (V,I) photometry and that can efficiently be applied to large areas of extra-galactic star-forming regions. We will present the first results of our study of both Magellanic Clouds, revealing that at any age the mass accretion rate is systematically higher there than in the Milky Way, by as much as two orders of magnitude for ages above 10 Myr. These preliminary results provide tantalising evidence that the radiation pressure on low-metallicity disc material in the Magellanic Clouds might not be as efficient at dissolving circum-stellar discs as it is in the Milky Way. This could have tremendous implications for the formation of planetary systems and would suggest a much higher number of planets around stars in the outer regions of the Galaxy, where metallicity is lower. |
| Inverting Color-Magnitude Diagrams to Access Precise Star Cluster Parameters: A New White Dwarf Age for the Hyades |
| Mr. Steven DeGennaro (University of Texas at Austin) |
| We demonstrate a Bayesian technique to invert color-magnitude diagrams (CMDs) of main sequence and white dwarf stars in open clusters to reveal the underlying cluster properties of age, distance, metallicity, and line-of-sight absorption, as well as individual stellar masses and cluster membership probabilities. In contrast to most traditional analyses of CMDs, our technique, which uses main-sequence stellar evolution models, a mapping between initial masses and white dwarf masses, white dwarf cooling models, and white dwarf atmospheres, is objective and repeatable by any other group. As a critical test of our Bayesian modeling technique, we apply it to Hyades $UBV$ photometry, with membership priors based on proper motions and radial velocities, where available. Under the assumtion of a particular set of WD cooling and atmosphere models, we estimate the age of the Hyades based on cooling white dwarfs to be 648 $\pm$ 45 Myr, consistent with the best prior analysis of the cluster main-sequence turn-off age. Since the faintest white dwarfs have most likely evaporated from the Hyades, prior work provided only a lower limit to the cluster's white dwarf age. Our result demonstrates that we are able to derive reasonable white dwarf ages from the brighter white dwarfs alone, eliminating the requirement of observing the coolest (i.e., faintest) white dwarfs. |
| Age Determination and Characterization of Single-lined Eclipsing Binary Stars |
| Mr. Jose Fernandez (SAO) |
| We derived ages, masses and radii for both components in five short-period single-lined eclipsing binary stars, which consist of a primary F-star and an M-dwarf secondary (M_B < 0.4 M_sun). The radial velocity orbital solution combined with a high S/N light curve gave sufficient information to obtain precise values for the density of the primary star and the surface gravity of the secondary. The scale of each system was set estimating the age and mass of the primary star using stellar evolution models. The solutions were compared to results obtained under the assumption of synchronization between the rotation of the primary star and the orbital motion of the secondary. Two systems showed evidence of being synchronized. On the remaining three, the rotational velocity of the primary star was significantly lower than the expected for a tidally locked binary. These results combined with previous works suggest that single-lined eclipsing binaries with primaries more massive than the sun (i.e. with radiative envelopes) must be treated carefully when trying to obtain fundamental properties like age, mass and radius of both members via synchronization assumptions only. The observed low rotational velocities may indicate an ongoing synchronization process or a misalignment between the rotation and orbital axes in the system. |
| On the Evolutionary Status of Be Stars |
| Dr. Yves FrÈmat (Royal observatory of Belgium) |
| Be stars are fast rotating main sequence emission line stars. They are probably the best laboratories to study the variation and redistribution of the angular momentum in main sequence stars and to study empirically the effects of the fast rotation on stellar evolution. Large samples of B and Be stars in our Galaxy and in the Magellanic Clouds have been analyzed by our team. In the present contribution, we present an overview of the work done and on the results we have obtained on the Be phenomenon and on its connection with the stellar evolution at high angular velocity rates and as a function of different metallicity regimes. |
| Time-evolution of High-Energy Emissions of GKM Stars |
| Ane GarcÈs (Institut de CiÕncies de l'Espai (CSIC-IEEC)) |
| Magnetic activity in the Sun, and low-mass stars in general, manifests itself in the form of high-energy and particle emissions. Besides an associated strong level of variability over various timescales (rotational modulation, flares, cycles), the overall activity decreases very rapidly with time. This is related to the rotational spin down and the subsequent loss of efficiency of energy generation mechanisms. For the young Sun (and young solar type stars) we found that the emissions were stronger than today by factors of 1000-100, 60-20, and 20-10 in X rays, far ultraviolet and ultraviolet, respectively. It has also been found that the particle emissions are much stronger at young ages. Such environment of intense energy and particle emissions should greatly influence the atmospheres of planets orbiting those stars. Here we present the results of our efforts in extending the magnetic evolution investigation to stars with masses below that of the Sun. A major component of our study is the determination of stellar ages, since these are crucial to define the rate of time decay of the magnetic emissions. For this we use a combination of various techniques, including gyro-chronology, cluster membership, kinematics, wide binaries,..., which are appropriate for different ages. |
| The VISTA Survey of the Magellanic Clouds: Unveiling Their Star Formation History |
| Dr. Leo Girardi (Osservatorio Astronomico di Padova) |
| The Magellanic Clouds (MC) have always been a rich laboratory for studies of stellar evolution, stellar populations, and the calibration of primary standard candles, thanks to the simultaneous presence of a wide variety of interesting objects such as red clump giants, Cepheis, RR Lyrae, LPVs, C stars, PNe, the TRGB, dust-enshrouded giants, etc. These objects are however irremediably mixed by a complex (and little known) history of star formation, and partially hidden by the presence of variable and patchy extinction across the MCs, which hinder the accurate calibration of their properties - absolute luminosities, lifetimes, periods, etc. - as a function of age and metallicity. The VISTA Survey of the Magellanic System (VMC) is an ESO Public Survey which will provide, in the next 5 years, critical near-infrared data to unveil the space-resolved star formation history (SFH) all over the MC system, hopefully opening the possibility a more accurate calibration of stellar models and primary standard candles. We describe the preparatory work for the derivation of the SFH from VMC data. SFH-recovery performed over simulated VMC images allow us to access the random and systematic errors in the derivation of the SFH. We present thee errors as a function of age, for the entire range of stellar densities met in the surveyed area. We investigate the sensitivity of our results to errors in the assumed reddening and distance. |
| Testing Stellar Evolution Models at Young Ages using Two New Pre-Main Sequence Eclipsing Binary Stars |
| Dr. Leslie Hebb (University of St Andrews) |
| Here we present the analysis of two new pre-main sequence eclipsing binary systems discovered in the Orion Star Forming Region, JW 380 and ASAS J052821+0338.5 and discuss how the two young stellar systems compare to theoretical stellar evolution models used to predict ages of young stars. ASAS J052821+0338.5 has two nearly equal mass components, with M~1.35 Msun. The stellar radii are 1.83 and 1.73 Rsun for the primary and secondary components, and the temperatures are found to be 4700 and 5100K. The system is found in the Orion OB1a subassociation (10-12 Myr). We find the system is well matched in the mass-radius and radius-temperature plane to the Baraffe et al. 1998 stellar evolution models with low mixing length values (1.0 H_P). JW 380 is a very young, 1-2 Myr, eclipsing binary with masses of 0.26 and 0.15 Msun. The radii of the two low mass stars are both > 1 Msun. We measure new temperatures for the system components and compare the object to a range of theoretical models to place constraints on the models at the lowest masses and youngest ages. |
| Two New K-type Eclipsing Binaries from the ASAS Database. |
| Mr. Krzysztof Helminiak (Nicolaus Copernicus Astronomical Center) |
| We present observations and results of modeling of two eclipsing binaries with K-type components found in the All Sky Automated Survey database: ASAS_DEB_001 and ASAS_DEB_003. Radial velocities obtained from the 10-m Keck I telescope with HIRES combined with photometry from the 1.0-m Elizabeth telescope (SAAO) allowed us to determine absolute physical parameters of both systems. Obtained masses and radii are (all in solar units): M1 = 0.846 +/- 0.003, M2 = 0.840 +/- 0.003, R1 = 0.994 +/- 0.011 and R2 = 0.979 +/- 0.010 for ASAS_DEB_001 and M1 = 0.714 +/- 0.006, M2 = 0.694 +/- 0.006, R1 = 0.705 +/- 0.007 and R2 = 0.701 +/- 0.003 for ASAS_DEB_003. |
| Stellar Age Estimation from ~3 Myr to ~3 Gyr |
| Prof. Lynne Hillenbrand (California Institute of Technology) |
| In contrast to mass, radius, and angular momentum which are -- for a limited set of stars at present -- derivable through fundamental observables such as orbital motion, eclipses, and period measurements, stellar ages have no firm basis or anchor other than in the case of the Sun for which radiometric dating of primitive solar system materials is possible. Yet, stellar ages are critical to establish for investigations such as the time scales for formation and long term evolution of planetary systems. We present recent progress on quantitative estimation of stellar ages using tools such as theoretical evolutionary tracks, rotation, rotation-driven activity diagnostics, and lithium depletion. Our focus is on roughly solar-mass and solar-metallicity stars younger than the Sun. We attempt to characterize the systematic and random error sources and then derive ``best” ages along with the age dispersion arising among the age estimation methods. We also present an application of our techniques to the evolution of debris disks. |
| Are the Spin Axes of Stars Randomly Aligned within Young Clusters? |
| Mr. Richard Jackson (Keele University UK) |
| If one can assume that the spin axes of stars in a young cluster are randomly orientated then it becomes possible, with measurements of period and projected equatorial velocity, to obtain accurate estimates of distance, mean age and age dispersion of the cluster (see Jeffries 2007a,b). A new technique is described whereby the measured inclination of a set of stars determined from their projected equatorial velocity, period and angular diameter can be analysed to determine whether their spin axes are randomly distributed in space or if there is a preferred direction of rotation such that the spin axes of a group of stars lie within a conical section of the celestial sphere. The method is used to assess the degree of alignment of stars in the Pleiades and Alpha Per clusters. Both clusters shows no significant alignment based on distances derived from main sequence fitting. If the lower distance to Pleiades given by Hipparcos measurements is used then this would suggest significant alignment of stars within the cluster. |
| Calibrating Lithium Depletion Ages in Young Clusters: The Case of IC4665 |
| Dr. Robin Jeffries (Keele University) |
| The "lithium depletion boundary" (LDB) technique is a precise and probably very accurate method for obtaining the absolute ages of young clusters, being based on the relatively simple physics of young, fully convective low-mass ($<0.5\,M_{\odot}$) stars. However, it requires high resolution spectroscopy of very faint objects to accomplish and is limited to nearby clusters. Brighter, higher-mass (0.5--1\,$M_{\odot}$) stars also undergo time-dependent Li-depletion, but here the physics is complicated by the presence of a developing radiative core. Observing Li-depletion among the higher mass stars of clusters with LDB ages allows us to probe this physics and empirically calibrate the time dependence of Li-depletion in stars which potentially could be observed in much more distant clusters. This poster discusses the case of IC4665 a cluster with a published LDB age of $27\pm 5$\,Myr. |
| Globular Clusters Ages from Detached Eclipsing Binaries |
| Dr. Janusz Kaluzny (Nicolaus Copernicus Astronomical Center) |
| Detached eclipsing binaries can be used for accurate and direct determination of ages and distances of globular clusters. We will present determination of absolute parameters for two binaries discovered in M4 and Omega Cen clusters by the Cluster AgeS Experiment (CASE). The M4 binary has an orbital period of 48 days and is composed of two upper-main sequence stars. The binary in Omega Cen has and orbital period of 76 days. It is composed of a subgiant and an upper-main sequence companion. Age constraints imposed by these binaries on their host clusters will be discussed. |
| The Ages of Young Stellar Populations in M31 |
| Mr. Yongbeom Kang (Johns Hopkins University) |
| We present a comprehensive study of star-forming regions in the nearest large spiral galaxy M31. We use GALEX far-UV and near-UV imaging to detect the radiation from young massive stars and trace the recent star formation across the galaxy. The far-UV and near-UV flux measurements, combined with ground-based data for estimating the reddening by interstellar dust, are used to derive ages and masses of the star-forming associations. The GALEX imaging, combining deep sensitivity and entire coverage of the galaxy, provides a complete picture of the recent star formation in M31 and its variation with environment throughout the galaxy. |
| Stellar Clusters in LMC |
| Vera Kozhurina-Platais (STScI) |
| Eight stellar clusters in Large Magellanic Clouds selected within the metallicity range of -1<[Fe/H]<0.1 and the age range of 0.5 - 3 Gyr, were observed with HST/ACS Wide Field Chanel through F435W, F55W and F814W filters. Newly derived Color-Magnitudes Diagrams from high precision ePSF photometry reach up to 5 magnitudes bellow of MS turn-off. Comparison of observed CMDs with the stellar evolutionary isochrones from different stellar models (BaSti, Padova and Dartmouth) allowed us to chose the best fit and obtained the accurate age, metallicity [Fe/H] and [alpha/Fe] estimates for each cluster. In some clusters we notice the presence of a double MS turn off, which has also been observed recently in some of the older Galactic globular clusters. |
| The Coevality of Young Binary Systems |
| Mr. Adam Kraus (Caltech) |
| Binary systems are commonly assumed to form simultaneously, making them excellent tests of stellar models. However, several recent results for young eclipsing binary systems have cast doubt on this long-held assumption. We will summarize the apparent ages and coevality of binary systems in several young associations, based on their positions in the HR diagram. We then will analyze the degree of coevality as a function of binary separation, comparing it to the typical coevality for random pairs of association members. Finally, we will use a sample of high-order multiples (such as the archetypical quadruple GG Tau) to test the calibration of stellar evolutionary models across a range of masses. |
| Metallicity Distribution of Stellar Clusters in the Large Magellanic Cloud |
| Dr. Arunas Kucinskas (Institute of Theoretical Physics and Astronomy) |
| We present preliminary results of a program aimed at studying chemical evolution of stellar clusters in the Large and Small Magellanic Clouds (MCs). Our ultimate goal is to derive photometric metallicities for a large sample of clusters in the LMC and SMC based on the data obtained during a large-scale near-infrared SIRIUS photometric survey of the MCs. In this contribution we present photometric metallicities for ~100 intermediate age and old clusters in the LMC derived from the infrared properties of their red giant branches. We make a detailed assessment of the new metallicity estimates looking into various sources of uncertainties that may affect the accuracy of photometric metallicities (in particular, the effect of age on the RGB slope – metallicity calibrations, which is typically overlooked in this type of studies). Using these data we derive a spatial metallicity distribution in the LMC as traced by intermediate-age and old stellar clusters and find that there is significant variation in metallicities accross the LMC face. We compare our results with those derived previously by other authors and discuss possible possible evolutionary scenarios that may have governed and/or contributed to the formation of the MC clusters to produce their observed metallicity distribution. |
| The Impacts of Super-Helium-Rich Population on the Age Datings of Old Stellar Systems |
| Prof. Young-Wook Lee (Yonsei University) |
| There is a growing body of evidence for the presence of super-helium-rich subpopulations in a significant fraction of globular clusters. Here we present the effects of this on the age datings of old stellar systems from HR diagrams, integrated colors & spectra. |
| Age Distribution of Galactic Globular Clusters using HST Snapshot Photometry |
| Mr. Youngdae Lee (Chungnam National University) |
| We present relative ages for a sample of Galactic globular clusters (GCs) using their color-magnitude diagrams (CMDs) observed with the HST/WFPC2 camera in the F439W and F555W bands. The ages are obtained by differential comparison of the CMDs of GCs using △(B-V) method, the color difference between main-sequence turnoff and the lower red-giant branch. All metal-poor GCs with [Fe/H] < -1.7 show old (~12 Gyr) ages and are coeval. Intermediate-metallicity clusters (-1.7 < [Fe/H] < -0.8) are on average 2 Gyr younger than the most metal-poor counterparts, with a large age dispersion and a total age range of ~4 Gyr. We also discuss the correlation of relative ages with physical parameters of GCs. |
| Determining Age on Two Cooling Sequences: Sub-Stellar Brown Dwarfs and Evolved White Dwarfs |
| Dr. Sandy Leggett (Gemini Observatory) |
| We present new discoveries of populations of very low mass, sub-stellar, brown dwarfs, and of high-velocity, cool and evolved, white dwarfs. These discoveries were made possible by the UKIRT Infrared Deep Sky Survey (UKIDSS), and in particular the Large Area Survey (LAS) component of UKIDSS. By pairing the LAS with the Sloan Digital Sky Survey (SDSS), very cool (600 - 1000 K) brown dwarfs can be identified as objects with extremely red SDSS colors, and blue LAS colors due to strong H2O, CH4 and possibly NH3 absorption bands in the near-infrared. The cool (4000 - 6500 K) white dwarfs were also identified by pairing the SDSS with the LAS; these are high proper motion objects with neutral to red SDSS colors and blue LAS colors due to pressure-induced H2 absorption. Both types of object cool with time, and their age is a crucial parameter for full characterisation. In the case of a brown dwarf, if effective temperature and gravity can be determined by fitting synthetic spectra, then both mass and age can be tightly constrained. If a white dwarf is cooler than 4500 K, its cooling age, post-main sequence, is >~ 8 Gyr, and hence it is necessarily an old member of the Galaxy. We use synthetic spectra to show that the LAS contains ~2 Gyr-old brown dwarfs with very low temperatures and masses at distances ~10 pc, as well as ~10 Gyr-old white dwarfs at ~50 pc. |
| Precise Ages for Field Brown Dwarfs |
| Dr. Michael Liu (University of Hawaii) |
| Hundreds of field brown dwarfs have been found from wide-field sky surveys, but the vast majority of them have unknown ages. Since brown dwarfs steadily cool as they grow older, they obey a mass-luminosity-age relation. Thus, ages are crucial for interpreting the luminosities and spectra of individual objects and for understanding the formation history of the population. We discuss age determinations for the very rare subset of brown dwarfs that are "benchmarks", namely objects where at least two of the three key physical properties of luminosity, mass, and age are known. For brown dwarf companions to main-sequence stars ("age benchmarks"), ages and metallicities can be assigned based on the properties of the primary stars and masses determined using substellar evolutionary models. For brown dwarfs with dynamical mass determinations ("mass benchmarks"), ages can be inferred using the observed luminosities and evolutionary models, with age uncertainties of ~10% or smaller. Analysis of these benchmarks provide very precise determinations of effective temperatures and surface gravities and thereby serve as calibration points for theoretical ultracool atmospheres and for the properties of the field brown dwarf population as a whole. |
| Ages of Central Stars of Planetary Nebulae |
| Dr. Walter Maciel (University of Sao Paulo) |
| Planetary nebulae (PN) are the offspring of intermediate mass stars with main sequences masses between 0.8 and 8 solar masses approximately. As a consequence, their properties reflect different physical conditions depending on the masses - and therefore ages - of their central stars (CSPN), which makes these objects extremely important in the study of the chemical evolution of the Galaxy. As an example, some recent theoretical models predict a time flattening of the observed radial abundance gradients in the galactic disk, while other models predict just the opposite behaviour. This can be analyzed on the basis of abundance determinations in planetary nebulae or in open cluster stars. In both cases, the results depend critically on the ages of the considered stars. The determination of ages of CSPN is a complex problem, and there is presently no single method that can be generally applied. Our group has pioneered in the treatment of this problem, and we have developed several methods to estimate the ages of the PN progenitor stars, based both on the observed nebular properties and in some properties of the stars themselves. We have obtained a large sample of well observed nebulae, located in the solar neighbourhood, in the galactic bulge and anticenter, and in the Magellanic Clouds, so that we can apply our methods to objects in different environments with different ages and metallicities. In principle, the traditional methods to derive the ages of galactic stars can be applied to CSPN, such as the use of theoretical isochrones. However, the physical properties of these objects are not as well known as in the case of normal stars, so that the derived isochrones are generally uncertain, leading to the need of alternative methods. In this work, we will discuss several of the methods developed so far by our group, such as (i) the determination of ages from the central star masses obtained directly of from the observed nitrogen abundance; (ii) the use of some type of age-metallicity relation, and (iii) the use of theoretical isochrones recently obtained for AGB stars. We discuss these and other possibilities and apply their results to some problems regarding the chemical evolution of the Galaxy. (FAPESP/CNPq) |
| On the determination of the ages of FGK stars in the solar neighborhood |
| Miss Raquel M. Martinez-Arnaiz (Universidad Complutense de Madrid (UCM)) |
| We present the most recent results of our ongoing long-term high resolution spectroscopic study of nearby (d < 25 pc) FGK stars, which aim is to characterize the local properties of the Galaxy, in particular, the star-formation history. Up to now we have obtained lithium abundances, activity levels, rotation rates (vsini), and radial velocities of 253 nearby cool stars, although our complete sample amounts to 716 stars inside a 25 pc radius sphere, which represents a statistically valid sample to use as a probe to contrast the ages obtained using different methods. In this contribution we contrast the ages obtained using lithium abundances with that given by the membership of a star in a moving group. We also analyze the dependence of the activity-rotation relations with age. |
| Ages of Delta Scuti Variables |
| Dr. Delbert McNamara (Brigham Young University) |
| The ages of Delta Scuti Variables have been determined using stellar evolutionary models. The long period Delta Scuti stars found in the large and small Magelanic clouds are found to be less than 1 billion years old, and surprisingly have solar metallicities rather than being metal poor. The pulsating blue stragglers found in globular clusters originate from a very small mass range (approximately .1) and have ages typically of the order of 1 billion years. The shortest period pulsating variables are found to be the youngest stars. |
| Resolved Stellar Populations of Local Dwarf Irregular KKH 98 from HST and Keck Laser Guide Star Adaptive Optics |
| Dr. Jason Melbourne (California Institute of Technology) |
| Photometry of resolved stellar populations have long been used to determine ages, metallicities, and dust reddening of star clusters within the Milky Way. More recently these same techniques have been applied to composite stellar populations in nearby galaxies. For instance, the ACS Nearby Galaxy Survey Treasury (ANGST) has obtained HST optical imaging of a volume limited ( < 4 Mpc) galaxy sample to constrain star formation histories. While the bulk of stars imaged by ANGST are red giants, the bolometrically most luminous tend to populate the asymptotic giant branch (AGB). For intermediate age populations (i.e. 0.5 - 2 Gyr) AGB stars can dominate the near-IR flux. This makes the AGB especially important for the study of unresolved stellar populations in intermediate and high redshift galaxies. Unfortunately, the dusty stellar envelopes of AGB stars limit our understanding of the AGB at optical wavelengths. We have obtained Keck Laser Guide Star Adaptive Optics K-band imaging of ANGST dwarf irregular KKH98 as the first in a series of efforts to obtain high spatial resolution near-IR imaging of the ANGST galaxies. We demonstrate that near-IR photometry is less sensitive to dust reddening, giving tightened AGB sequences in color-magnitude space, and allowing us to better constrain models of the AGB. These efforts will inform the study of intermediate and high redshift galaxies which require adequate models of the AGB across a range of metallicities. They will also pave the way for future studies of resolved stellar populations out to Virgo and beyond which will be possible with the James Webb Space Telescope and adaptive optics on the next generation of extremely large telescopes. |
| Ages Measured from Lithium Depletion in Nearby Young Stellar Associations |
| Miss Erin Mentuch (University of Toronto) |
| We present lithium depletion ages for five nearby (<100 pc), young (10-150 Myrs) groups: TW Hydrae Association, Eta Chamaeleontis Cluster, Beta Pictoris Moving Group, Tucanae-Horologium Association and AB Doradus Moving Group. We have determined surface gravities, effective temperatures and lithium abundances for over 900 spectra through least squares fitting to model-atmosphere spectra. For each group, we compare the dependence of lithium abundance on temperature with isochrones from pre-main-sequence evolutionary tracks to obtain model dependent ages. The precise ages depend on our choice of both atmospheric and evolutionary models. As a result, while our ages are consistent with estimates based on Hertzsprung-Russell isochrone fitting and dynamical expansion, they are not yet more accurate. Our observations do show that with improved models, much stronger constraints should be feasible. |
| Evidence for a Non-Uniform Initial Mass Function and Implications for Steallar Populations |
| Dr. Gerhardt Meurer (Johns Hopkins U.) |
| Many of the results in modern astrophysics rest on the notion that the Initial Mass Function (IMF) is universal and fully populated to an upper mass limit of ~ 100 Msun. I will show Halpha and UV observations of a sample of HI selected galaxies that challenge this result. The extinction corrected Halpha/FUV flux ratio from these two tracers of star formation shows strong correlations with the surface-brightness in Halpha and the R band: Low Surface Brightness galaxies have lower ratios compared to High Surface Brightness galaxies as well as compared to expectations from equilibrium models of constant star formation rate using commonly favored IMF parameters. The most plausible explanation for the correlations is the systematic variation of the upper mass limit and/or the slope which define the upper end of the IMF, although we can not rule out systematic variations in the escape fraction of ionizing photons. The implications of this result are major. Most relevant to this symposium is that the ages estimated for young stellar populations under the assumption of a fully populated IMF may be overestimated. |
| "The Age of M32: Resolving the stellar populations of the nearest elliptical with ACS/HRC" |
| Antonela Monachesi (Kapteyn Astronomical Institute) |
| We present a (F435W - F555W, F555W) color-magnitude diagram for a region 90" from the center of M32 which we have obtained from deep (F435W,F555W) photometry based on Hubble Space Telescope ACS/HRC images. Only the superior resolution and blue sensitivity of ACS/HRC make the observation of the M32 main sequence turnoff (MSTO) - which is necessary for an accurate determination of its age and star formation history - possible. The M32 color magnitude diagram goes as deep as F555W = 28. It displays a wide color distribution of red giant branch stars, a strong red clump - which is expected for metal rich populations - and asymptotic giant branch stars rising to M_v ~ -2.0. There is not a noticeable presence of blue horizontal branch stars, suggesting that a well-developed old, metal poor population is unlikely to be present. The detection of a blue component of stars ("blue plume") may be indicating the presence of a very young stellar population in the galaxy. A comparison of the color magnitude diagram with theoretical isochrones gives us a rough estimation of the MSTO age, showing that the youngest solar-metallicity stellar population of M32 may have an age between 2 and 4 Gyr old. |
| The LCID Project: a Comparative Analysis of the Star Formation History of Isolated Local Group Galaxies |
| Dr. Matteo Monelli (Instituto de AstrofÌsica de Canarias) |
| The star formation history (SFH) of resolved dwarf galaxies in the Local Group is a fundamental instrument to understand when galaxies formed and how they evolved. In particular, isolated dwarfs are ideal laboratories since their evolution has not been complicated by the vicinity of giant galaxies. Therefore, it is possible to dig into the details of the oldest populations of these galaxies in order to study their formation epochs and chemical properties. In this context, we will discuss the main results of the LCID project (Local Cosmology from Isolated Dwarfs), aimed at deriving detailed star formation histories for a sample of six Local Group isolated dwarf galaxies. To accomplish this goal we have collected, using the ACS and WFPC2 on board the HST, color-magnitude diagrams reaching their oldest main sequence turnoff (V~28) with good photometric accuracy for a sample of six galaxies: IC1613 and LeoA (dIrr), LGS3 and Phoenix (dIrr/dSph), Cetus and Tucana (dSph). We will present accurate star formation histories derived through comparison with synthetic color-magnitude diagrams, with particular emphasis on a comparative analysis of the oldest populations in these galaxies. The most insteresting result, in this respect, is a clear delay in the onset of the star formation occurred in LeoA, and possibly in IC1613. |
| Post T Tauri and Young Main-Sequence Stars in the Solar Neighborhood: Isolated or Members of Young Stellar Kinematic Groups |
| Prof. David Montes (Universidad Complutense de Madrid, UCM) |
| Post T Tauri stars (PTTS) and young main-sequence stars (YMS) are late-type stars (K/M and F/G respectively) in the age range between 10 and 100 Myrs filling the gap between T Tauri (or Fe/Ge pre-main sequence) and zero-age main-sequence phases. This period of evolution remains ambiguous and until now different studies of young stars have failed to find the numbers of PTTS and YMS that are expected. In the last years some PTTS and YMS have been identified among the X-ray detected pre main-sequence stars in some star-forming regions. More recently, additional candidates have been identified in young associations and moving groups (Beta Pic, TW Hya, Tucana/Horologium, and the AB Dor). However, many isolated PTTS and YMS still remain undiscovered. In this contribution, we compiled the PTTS and YMS previously identified in the literature, and identified new candidates using the information provided by the high resolution spectra obtained during our surveys of late-type stars possible members to young moving groups (Montes et al. 2001, LÑpez-Santiago, et al. 2005, 2006), FGK stars in the solar neighborhood (MartÌnez-ArnÀiz et al. 2008), and RasTyc sample (cross-correlation of the ROSAT All-Sky Survey (RASS) with the TYCHO catalog, Klutsch et al. 2008; Guillout et al. 2008). To identify these stars we applied an age-oriented definition using relative age indicators (lithium abundance, chromospheric and coroanal emission and kinematics) as well as color-magnitude diagrams and pre-main sequence isochrones. |
| On the Determination of the Ages of FGK Stars in the Solar Neighborhood |
| Prof. David Montes (Universidad Complutense de Madrid, UCM) |
| We present the most recent results of our ongoing long-term high resolution spectroscopic study of nearby (d < 25 pc) FGK stars, which aim is to characterize the local properties of the Galaxy, in particular, the star-formation history. Up to now we have obtained lithium abundances, activity levels, rotation rates (vsini), and radial velocities of 253 nearby cool stars, although our complete sample amounts to 716 stars inside a 25 pc radius sphere, which represents a statistically valid sample to use a a probe to contrast the ages obtained using different methods. In this contribution we contrast the ages obtained using lithium abundances with that given by the membership of a star in a moving group. We also analyze the dependence of the activity-rotation relations with age. |
| A Maximum Likelihood Method for Fitting Colour-Magnitude Diagrams |
| Prof. Tim Naylor (University of Exeter) |
| For galactic clusters the majority of comparisons between colour-magnitude data and model isochrones are carried out "by eye". Thus although the ages of hundreds of galactic clusters have been measured from colour-magnitude diagrams, these measurements almost universally lack statistically robust uncertainties. The reason is that whilst fitting an isochrone to a set of datapoints may appear to be a chi-squared problem, the presence of binaries means that the model is a distribution, not a line in colour-magnitude space, and the data have uncertainties in two dimensions. I present a maximum likelihood solution to this problem, which can be viewed as a generalization of chi-squared to many dimensions. The technique tests goodness of fit, and gives statistically robust uncertainties. It can, therefore, test whether there are inconsistencies between ages derived from different techniques, and make clear decisions as to which isochrones fit the data best. The method has been used for main-sequence and pre-main-sequence fitting, and is currently being extended to determine ages from post-main-sequence isochrones. |
| Connecting the Evolution of YSOs and their Surrounding Disks in Serpens |
| Ms. Isa Oliveira (Leiden Observatory) |
| Based on the "Cores to Disks" (c2d) Spitzer Legacy maps, a rich population of young stellar objects (YSO) was uncovered in a region of ~0.5 sq. degrees in the Serpens Molecular Cloud. This sample contains 150 YSOs with infrared excess in a broad range of temperatures and luminosities, making Serpens an unique target region for obtaining a complete, well-defined sample of multi-wavelength observations of young stars in a possible evolutionary sequence. Follow-up complimentary observations in the optical and mid-infrared (Spitzer/IRS GO3) allow us to characterize the central stars, as well as the surrounding disks. The optical data provide estimation of the stellar parameters. The shape and slope of the mid-infrared excess provide information on the flaring geometry of the disks. The spectral features give constraints on grain growth and mineralogy, which in turn probes heating and radial mixing. The presence of PAH features traces UV radiation, whereas Halpha is a diagnostic of accretion. Assuming that the stars within a small region are nearly coeval, this provides direct constraints on the importance of environment and initial conditions on disk evolution. We are studying this rich population in order to connect the evolution of the disks with the evolution of their harboring stars, attempting to establish the mechanisms that determine the evolutionary sequence of protoplanetary disks. |
| The Ages of Massive Protostars with Outflows and X-ray Emission |
| Dr. Steven Pravdo (Caltech/JPL) |
| High sensitivity and high resolution observations with radio, IR, and X-ray observatories have enabled us to discern the properties of all classes of protostars. A few interesting targets for study are identifiable massive protostars that are the energy sources for at least a portion of the star formation regions wherein they reside. Since these protostars are often associated with compact HII regions, masers, and outflows, the ages of the protostars and the associated objects are potentially relatable. We discuss the evidence with regards to the ages of the small set of these that are identified via their IR and radio emission and are also outflow and X-ray sources. |
| A study of the age of the Orion Nebula Cluster |
| Mrs. Maddalena Maria Reggiani (STScI) |
| We present a new HR diagram of the Orion Nebula Cluster (ONC), obtained using the broad-band photometry taken for the HST Treasury Program on the ONC, an expanded set of spectral types and the most recent estimate of the cluster's distance. We compare the source distribution in the HR diagram with the theoretical isochrones derived by different groups to derive the mean cluster age, the age dispersion and to search for evidence of evolutionary trends of the age against the stellar mass or source distance from the cluster center. |
| Age Distribution of Globular Clusters in M31 and NGC 5128 from GALEX Ultraviolet Observations |
| Prof. Soo-Chang Rey (Chungnam National University) |
| We present UV photometry of Globular CLusters(GCs) in nearby galaxies (M31 and NGC 5128) which were obtained as part of the Nearby Galaxy Survey (NGS) carried out by Galaxy Evolution Explorer (GALEX) in two UV bands: FUV (1350 - 1750 angstrom) and NUV (1750 - 2750 angstrom). The UV properties of GCs have been analyzed using various combinations of UV - optical and optical - optical colors, which are considered to be excellent tracers of ages and metallicities for simple stellar populations. Comparing data with those of Galactic GCs in the UV with the aid of population models that fully take into account the detailed systematic variation of HB morphology with age and metallicity, we studied the age range of GCs in galaxies. The implication of results from UV observations is that UV – optical colors provide powerful means to distinguish GCs with different ages: young, intermediate-age, and old GCs. |
| Nucleosynthesis of Lead and Thorium in the Early Galaxy |
| Mr. Ian Roederer (University of Texas at Austin) |
| It is imperative to understand the nucleosynthesis of the heaviest products of the rapid (r-) process in order to correctly interpret the ages of individual stars derived from thorium and uranium nuclear chronometry. We examine the lead (Pb) and thorium (Th) abundances in a large sample of metal-poor stars whose neutron-capture material was produced almost exclusively in the r-process. We combine our results with other abundance measurements from the literature to form a more complete picture of nucleosynthesis of the heaviest elements produced in the r-process, including (1) revised r-process elemental production ratios based on the derived stellar ratios, (2) new assessment of stellar Th/Pb ratios, which are very high in a few stars and predict very young ages for stars otherwise expected to be very old based on their metallicity,(3) relative and absolute ages for individual halo stars derived from the radioactive decay of Th, and (4) the timescales necessary for r-process enrichment of the ISM and recycling of material from one stellar generation to the next. We discuss the consequences of each of these results in the context of radioactive age dating of a single star. |
| Masses and Radii of Nearby Young Stars |
| Dr. Gail Schaefer (Georgia State University (CHARA Array)) |
| We present initial results from a high spatial resolution interferometric survey of young stars (10-150 Myr) in nearby moving groups based on observations obtained with the CHARA Array at infrared wavelengths. The observational goals of this program are two-fold: (1) measure the angular diameters of stars larger than 0.4 milli-arcseconds in size (roughly one-fourth of the moving group members in our sample) and (2) search for companions at unprecedentedly close separations. The spatially resolved measurements will establish a grid of empirical radii of young stars at coeval ages and any identified binaries will be ideal for determining dynamical masses on a short timescale. The results of this work will directly test age and mass predictions of stellar evolutionary models, elucidate how chromospheric activity might bias these predictions, and improve our overall understanding of our nearest young neighbors. |
| The Ages and Abundance Patterns of Stars in Early-Type Galaxies |
| Dr. Ricardo Schiavon (Gemini Observatory) |
| I will present recent results of the application of state-of-the-art stellar population synthesis models to the determination of the mean ages and abundance patterns of stars in early-type galaxies from analysis of their integrated spectra. I will discuss the reliability of age determinations based on measurements of Balmer lines in the integrated spectra of stellar populations, showing that the new models constrain not only the mean ages of the stars in early type galaxies, but also their age spread. Further constraints on the duration of star formation coming from estimates of the mean abundances of Fe, Mg, C, N, and Ca, will be presented. |
| Age-Related Abundance Anomalies of Cool Open Cluster Dwarfs |
| Dr. Simon Schuler (NOAO/CTIO) |
| We present the results of our work on the chemical abundances of solar-type G and K dwarfs in Galactic open clusters and moving groups. In dwarfs cooler than about 5400 K, Fe abundances derived from singly ionized lines relative to those derived from neutral lines show a dramatic increase with decreasing stellar T_eff. Abundances derived from spectral lines of high-excitation atomic transitions, such as those of O I, show a similar increase in the cooler stars. We present new and extant abundance results of cool open cluster and moving group dwarfs, and discuss the apparent age-related diminution of the abundance anomalies. |
| Identifying the Young Low-Mass Stars within 25 pc |
| Dr. Evgenya Shkolnik (DTM/CIW) |
| The dominance of low-mass stars in the solar neighborhood suggests that most extrasolar planets, including most of the habitable ones, could orbit M dwarfs. The intrinsic faintness, proximity to Earth, and ubiquity of M dwarfs allow for much higher chances of directly detecting planets. By adding youth as a criterion, when circumstellar disks have not yet dissipated and newly-formed planets are at their hottest (and most self-luminous), we have generated a generally untapped and well-characterized resource of nearby, young M dwarfs, laying the foundation for intensive planet and disk searches. Our targets are drawn from the NStars 20-pc census constructed from the 2MASS catalogs along with the Lepine et al. (2002) proper motion catalogs. In addition to these, we include previously-uncatalogued M dwarfs that exhibit significant proper motion between the POSSI and 2MASS surveys (i.e. the Moving-M sample). Distances are available either from parallaxes or spectrophotometric relations and are limited to 25 pc from the Sun. Nearly half of the resulting M dwarfs are new discoveries not present in the Gliese catalog. We have completed an optical spectroscopic survey of the 191 most X-ray luminous M dwarfs identified using ROSAT X-ray data to select the most active objects with fractional X-ray luminosities comparable to or greater than Pleiades members. The high spectral resolution and multi-epoch observations allowed us to filter the deceivingly-young spectroscopic binaries from the sample. We estimate stellar ages using youth indicators such as gravity indices, lithium absorption and H_alpha emission. We estimate that our final sample of the 162 youngest and nearest low-mass objects is less than 300 Myr old with 25% of them being younger than 50 Myr old and 7 very young (<10 Myr), all within 25 pc. |
| AGB stars in Local Group Dwarf Irregular Galaxies - Using LPVs and the Fuel Consumption Theorem as tracers of the stellar population |
| Dr. Jan Snigula (MPE Garching) |
| From our multi-year observing campaign of a sample of Northern Dwarf Irregular Galaxies of the Local Group we extracted a sample of long period pulsating variable stars in these galaxies. Based on the long baseline of continuous observations in the R-Band and thorough completeness simulations this sample can be considered to be complete. The easiest way to draw conclusions on the star formation history in these systems is a rough comparison of the CMDs with the location of different isochrones. Since this approach can be complicated by the Age-Metallicity degeneracy, we applied a second technique. We used the prediction of the number of AGB stars based on the Fuel Consumption Theorem to estimate the age of the stellar population independent from the metallicity. To explore the fraction of AGB stars undergoing long period variability, we acquired additional data in two narrow-band filters. The photometry in these filters, compared with broad-band colors allows us to select Carbon rich AGB stars. We were able to show, that the combination of LPV counts with the Fuel Consumption Theorem can be used to provide further constraints on the presence of an intermediate-age population. For two of the galaxies we found a strong contribution from a 2-8 Gyr old population, for two others the presence of a population of such age could be ruled out. |
| Reddening-Free Color-Magnitude Diagrams for Southern Galactic Clusters Containing Cepheids |
| Ms. Laura Sturch (Carnegie Observatories) |
| We are recalibrating the Cepheid Period-Luminosity Relation using Galactic open clusters known to contain one or more classical Cepheids. UBVI data have been obtained for seven southern clusters using the duPont 2.5m telescope at Las Campanas, Chile. For each filter, at least fifteen images were taken consisting of three groups of five exposures, each of 2-8 seconds, 20-80 seconds, and 200-800 seconds. This method gives a large dynamic range over the main sequence (10-12 magnitudes deep), with consistently high signal-to-noise. Preliminary results for the clusters will be presented in the form of standard color-magnitudes and color-color plots. These clusters are: NGC 6087, NGC6649, NGC5662, NGC6664, NGC6067, M25, and Lynga 6. In addition, we will introduce a novel method for determining distances using reddening-free W-Q plots, and demonstrate it with its first application to our cluster data. |
| The Determination of the Evolutive Status of Stars Using Asteroseismology |
| Dr. Marian Doru Suran (Astronomical Institute of the Romanian Academy) |
| In this paper we try to present the modality (asteroseismological) in which we could determine the evolutive status for pulsating stars which show both pre- and post-MS properties. As exemple, we determine the evolutive status of the star V351 Ori. |
| Age Determination of T Tauri Stars by High-Resolution Near-Infrared Spectroscopy |
| Mr. Yuhei Takagi (Kobe University) |
| We present a new method to determine the age of T Tauri stars from deriving stellar surface gravity by near-infrared spectroscopy. An equivalent width depends on surface gravity, effective temperature, and metal abundance; although an indicator of the surface gravity is not established. We took high-resolution near-infrared spectra of dwarfs, T Tauri, and giants of which effective temperature are almost equal. We calculated the equivalent width ratio to eliminate the effect of veiling, and found that the Sc/Na is a good indicator of surface gravity. It is expected that the the age determination of T Tauri stars can be determined with a precision of the factor of 1.5 by our new method. |
| Analysis of the Eclipsing Binary V69 in the Globular Cluster 47 Tuc |
| Dr. Ian Thompson (Carnegie Observatories) |
| We use photometric and spectroscopic observations of the eclipsing binary V69-47Tuc to derive the masses, radii, and luminosities of the component stars. Based on measured systemic velocity, distance, and proper motion, the system is a member of the globular cluster 47 Tuc. The system has an orbital period of 29.5 d and the orbit is slightly eccentric with e = 0.055. These components of V69 are the first Population~II stars with masses and radii derived directly and with an accuracy of better than 1%. We measure an apparent distance modulus of (m-M)v = 13.38+/-0.0 to V69. We compare the absolute parameters of V69 with five sets of of stellar evolution models and estimate the age of V69 using mass-luminosity-age, mass-radius-age, and turnoff mass - age relations. The masses, radii, and luminosities of the component stars are determined well enough that the measurement of ages is dominated by systematic differences between the evolutionary models, in particular, the adopted helium abundance. We estimate for the age of V69 is 11.21+/-0.21+/-0.45 GYr for a helium abundance of Y =0.255. Comparing the measured luminosities and radii of the component stars to Dartmouth tracks we estimate a helium abundance of Y = 0.269+0.017-0.021. The determination of the distance to V69, and hence to 47 Tuc, can be further improved when infrared eclipse photometry is obtained for the variable. |
| A Semi-Empirical Study of the Mass and Age Distribution of Horizontal Branch Stars in M3 (NGC 5272) |
| Mr. Aldo Valcarce (Pontificia Universidad CatÑlica de Chile) |
| Horizontal branch (HB) stars in globular clusters offer us a probe of the mass loss mechanisms taking place in red giants. For M3 (NGC 5272), in particular, different shapes for the HB mass distribution have been suggested in the literature, including Gaussian and sharply bimodal alternatives. Here, we study the mass distribution of HB stars in M3 by comparing evolutionary tracks for a suitable chemical composition and photometric observations. Our approach is thus of a semi-empirical nature, describing as it does the mass distribution that is favored from the standpoint of canonical stellar evolutionary predictions for the distribution of stars across the color-magnitude diagram. More specifically, we locate, for each individual HB star in M3, the evolutionary track whose distance from the star’s observed color and magnitude is a minimum. We carry out artificial tests that reveal that our method would be able to detect a bimodal mass distribution resembling the one previously suggested in the literature, if present. We also study the impact of different procedures for taking into account the evolutionary speed, and conclude that they have but a small effect upon the inferred mass distribution. We find that a Gaussian shape, though providing a reasonable first approximation, fails to account for the detailed shape of M3’s HB mass distribution. Indeed, the latter may have skewness and kurtosis that deviate slightly from a perfectly Gaussian solution. Alternatively, the excess of stars towards the wings of the distribution may also be accounted for in terms of a bimodal distribution in which both the low- and the high-mass modes are normal, the former being significantly wider than the latter. However, we also show that the inferred distribution of evolutionary times is inconsistent with the expected shape. This suggests that the present models underestimate the effects of evolution away from the zero-age HB, and warns against considering our inferred mass distribution as definitive. |
| Formation and Dynamical Evolution of Multiple Stellar Generations in Globular Clusters |
| Dr. Enrico Vesperini (Drexel University) |
| We present the results of hydrodynamical and N-body simulations aimed at studying the formation and subsequent stellar dynamical evolution of multiple stellar generations in globular clusters. |
| The Distance to Sigma Orionis and Implications for the Ages of the Orion OB1b Association |
| Prof. Frederick Walter (Stony Brook University) |
| There are very strong correlations between the distances to and the inferred ages and masses of very young (<10 Myr) low mass stellar and substellar mass objects. This is because the distance sets the luminosity, which, for a given temperature, determines the mass and age. In the optical, the strongest ambiguity is between the distance and age; in the near-IR where the isochrones are nearly vertical there is a three-fold ambiguity. We have run up against this problem in our large scale survey of the very low mass population of the Orion OB1a and b association. In order to break the degeneracy, we need an accurate determination of the distance to the association. Published distances for Orion OB1b (the belt of Orion) range from about 350 pc to 470 pc. We have completed a reanalysis of the upper main sequence stars in the sigma Ori cluster, and find a distance from main sequence fitting of 420 +/- 30 pc. We will summarize the implications for the age of the sigma Ori cluster and the low mass stars in Orion's belt. We are extending this analysis to a larger area of Ori OB1. We will discuss our attempts to separate the high mass members of Ori OB1a from those of Ori OB1b. We will also present preliminary results of our study of the very low mass population of this association. |
| White Dwarf Ages of Open Star Clusters, Young and Old |
| Dr. Kurtis Williams (Univ. of Texas at Austin) |
| Because of their relatively simple structures and evolution, white dwarfs are often considered useful chronometers for dating the ages of stellar systems such as the Galactic disk and globular clusters. Yet white dwarf chronology has its own inherent uncertainties, ranging from the stars' structures and composition to the physics of crystallization. Observations of white dwarfs in open star clusters provide one means of empirically calibrating white dwarf cooling rates. I present observational studies of the white dwarf cooling sequences in three open star clusters: the intermediate-age cluster Messier 35, and the older open clusters Messier 67 and NGC 188. I also discuss implications of these cluster white dwarfs on both the cluster ages and on the cooling rates of white dwarfs. |
| New Benchmarks and Metrics for the Ages of Old, Cool Stars |
| Dr. Jason Wright (Cornell University) |
| Extending the stellar age-activity relation to old, cool stars requires activity measurements of stars in old, benchmark clusters, a difficult task given their typically large distance moduli. I have begun a multi-telescope campaign to characterize a new benchmark cluster, prove its status as the closest (200 pc) old (2 Gyr) cluster, and identify its G, K, and possibly M dwarfs. Activity levels in old stars are so low that they are outside of the canonical calibration of the R'HK metric used to measure chromospheric activity. I have established a new estimate of the photospheric component of the R'HK metric for old, slightly evolved, and non-solar metallicity stars, a necessary step for a universal field-star age diagnostic. |
| Deriving Stellar Ages with Modern Isochrones |
| Prof. Sukyoung Yi (Yonsei University) |
| "Isochrones" were by definition invented to derive stellar ages. The modern observing technology produces data with unprecedented accuracy, and the stellar models are hence put to stringent tests. A new version of the Yonsei-Yale Isochrones (aka Y^2 Isochrones), one of the most widely used, is available for improved accuracy for lower main sequence and for a wide range of helium abundance. We demonstrate that the new isochrones reproduce the recent HST data on the lower main sequence of globular clusters with superb precision. The wide range of helium abundance provides a unique tool to explore the extreme helium abundances found/claimed to exist in some Galactic (omega Cen, NGC 2808, etc) and extragalactic globular clusters (e.g., those in M87). We describe the new isochrone database and demonstrate its power in deriving the ages of stellar populations. |
| Turbulent Convection in Stellar Interiors |
| Prof. Patrick Young (Arizona State University) |
| We analyze stellar convection with the aid of 3D hydrodynamic simulations, introducing the turbulent cascade into our theoretical analysis. We devise closures of the Reynolds-decomposed mean field equations by simple physical modeling of the simulations; the procedure (CABS, Convection Algorithm Based on Simulations) is terrestrially testable and allows systematic improvement. The interpretation of mixing-length theory (MLT) as generally used in astrophysics is incorrect. Direct tests show that the damping associated with the turbulent cascade is that suggested by Kolmogorov (εK ≈ (u′)3r ms/ℓCZ, where ℓCZ is the depth of the convection zone). This implies that the mixing-length parameter αML is a simple function of the depth of the convection zone, and not freely adjustable. Appropriate adjustment of the “geometric form factor” parameter can leave solar models almost unchanged, except for their velocity scale. The enthalpy flux (stellar luminosity) is directly connected to the buoyancy flux, and hence to the scale of convective velocity. MLT systematically underestimates the velocity scale, which affects estimates of coronal heating, mass loss, and wave generation. Extension of this approach to deal with rotational shear and MHD is indicated. Direct comparison with 3D radiative simulations of the solar surface layers (Asplund, et al. 2005) indicate consistency in the deeper, adiabatic regions, and suggest that the old problem of a lack of surface convection in MLT atmospheres is related to use of a local stability criterion (Schwarzschild) rather than a nonlocal one (bulk Richardson). Mixing leading to changes in stellar lifetimes for stars with convective cores and age diagnostics in low mass stars is a consequence of this treatment. |