Gyrochronology and the Rotational Evolution of Cool Stars
Dr. Sydney Barnes (Lowell Observatory)
I will show how observed patterns in the rotational evolution of cool stars permit a relatively precise derivation of the age of a star, provided that its rotation period is measured, and the star is not pathological in certain ways. This method is called GYROCHRONOLOGY. I will discuss its viability, limitations, the related age errors for field stars, and prospects for further improvements. I will also compare its characteristics to those of other age indicators. If time permits, I will say something about the possible meanings of the related mathematical expressions.
The ACS LCID Project: Variable Stars as Tracers of Population Gradients
Mr. Edouard Bernard (Instituto de Astrofisica de Canarias)
The LCID project (Local Cosmology from Isolated Dwarfs) is an international collaboration aiming at deriving detailed star formation histories for a sample of isolated, low metallicity Local Goup dwarf galaxies based on very deep, multi-epoch HST/ACS photometry. We present results concerning the search for short-period variable stars in four galaxies, namely IC1613, LGS3, Cetus and Tucana. These will be discussed in the context of the star formation histories obtained from our very deep CMDs. In particular, we show how the pulsational properties of the RR~Lyrae stars, which represent the vast majority of the observed variables, can trace subtle differences in the age and metallicity of the old population. For example, in the dwarf spheroidal galaxy Tucana we find that the fainter RR Lyrae stars, having a shorter period, are more centrally concentrated than the more luminous, longer period RR Lyrae variables. Through comparison with the predictions of theoretical models of stellar evolution and stellar pulsation, we interpret the fainter RR Lyrae stars as a more metal-rich subsample. In addition, we show that they must be older than about 10 Gyr. Therefore, a metallicity gradient must have appeared very early on in the history of this galaxy. This is the first time that the properties of RR Lyrae stars have been used to infer subtle differences in the properties of the parent stellar population, likely the oldest one in a galaxy, which would be elusive otherwise.
The Bologna Open Cluster Chemical Evolution project: a large, homogeneous sample of Galactic Open Clusters
Dr. Angela Bragaglia (INAF-Osservatorio Astronomico di Bologna)
I will present results of our study of a large sample of open clusters, the Bologna Open Cluster Chemical Evolution (BOCCE) project. We derive in a homogeneous way: a) the clusters distance, age and reddening, using comparison of observed and synthetic color-magnitude diagrams, and b) the detailed chemical composition, using high-resolution spectroscopic data. The information we derive for these (mostly old) open clusters can be used to probe the properties, the formation, and the evolution of the Galactic disk. In particular, the precise determination on a homogenoeus scale of chemical abundance, distance and age will permit to describe the distribution of metallicity of the disk (a simple negative radial gradient, or a gradient in the inner parts followed by a flattening in the outer regions ?) and of its possible evolution with time, a key ingredient of chemical evolution models. We have analyzed photometric data for more than 20 clusters (about one half of our sample) and are trying to do the same for spectroscopy. In our sample we have some of the most interesting clusters known, like the old, metal-rich, and rather peculiar NGC 6791, the very old Be 17, and the very distant Be 20 and Be 29, located in the outer disk. To study the special case of NGC 6791, with its definitely over-solar metallicity, ad hoc stellar evolution models are being computed, that present interesting and challenging features compared to stellar models of less extreme metallicity. This work is in collaboration with: Monica Tosi, Michele Cignoni, Eugenio Carretta (Bologna); Raffaele Gratton (Padova); Jason Kalirai (Lick); Gianni Marconi (ESO); Gloria Andreuzzi, Luca Di Fabrizio (TNG); Scilla Degl'Innocenti, Pier Giorgio Prada Moroni (Pisa); Paolo Ventura, Marco Castellani (Roma)
Brown Dwarfs as Galactic Chronometers
Prof. Adam Burgasser (MIT)
Brown dwarfs are very low-mass stellar objects that lack sufficient mass to sustain core hydrogen fusion. Without the stable main sequence phase of their more massive counterparts, these objects steadily cool and dim over time. The cooling of brown dwarfs, along with their ubiquity in nearly all Galactic environments, make them potentially useful chronometers in a variety of studies. In this talk, I will review several ways in which brown dwarfs have been exploited as chronometers, from individual stellar systems to young open clusters to entire populations in the vicinity of the Sun. I will review current progress toward improving the accuracy of age determinations through precise characterization of brown dwarf physical properties. Finally, I will discuss empirical tests of evolutionary models on which age determinations for brown dwarfs hinge.
The promise of GAIA and how it will influence stellar ages
Dr. Carla Cacciari (INAF - Osservatorio Astronomico Bologna)
The GAIA space project, planned for launch in 2011, is one of the ESA Cornerstone missions, and will provide astrometric, photometric and spectroscopic data of very high quality for about one billion stars brighter than V=20. This will allow to reach an unprecedented level of information and knowledge on several of the most fundamental astrophysical issues, such as mapping of the Milky Way, stellar physics (classification and parameterization), Galactic kinematics and dynamics, study of the resolved stellar populations in the Local Group, distance scale and age of the Universe, dark matter (potential tracers), reference frame (quasars, astrometry), planet detection, fundamental physics, Solar physics, Solar system science. I will present a description of the instrument and its main characteristics, and discuss a few specific science cases where GAIA data promise to contribute a fundamental improvement within the scope of this Symposium.
MAD/VLT Near-Infrared Photometry of the Galactic Globular Cluster NGC3201
Dr. Annalisa Calamida (ESO)
We present deep and accurate Near-Infrared photometry collected with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) on VLT for the Galactic globular cluster NGC3201. The unprecedented quality of the images (Full Width at Half Maximun better than 0.1 arcsec in the Ks and in the J bands) allowed us to detected main-sequence (MS) stars with mass values M~0.3Mo (K_s ~ 21). We adopt the hook of the lower MS in the Ks, J-Ks color-magnitude diagramm to estimate the absolute age of NGC3201. The difference in color between this feature and the Turn-Off point is strongly correlated to the cluster age. This method is marginally affected by distance and reddening uncertainties. Furthermore, the hook location does not depend on the cluster age and it is a robust theoretical prediction.
Stellar Chronology with White Dwarfs in Wide Binaries
Dr. Silvia CatalÀn (Institut de CiÕncies de l'Espai (CSIC-IEEC))
White dwarfs are the evolutionary end product of stars with low and intermediate masses. The evolution of white dwarfs can be understood as a cooling process, which is relatively well known at the moment. For this reason, wide binaries containing white dwarfs are a powerful scenario to constrain stellar ages. When a wide binary is comprised by a white dwarf and a low-mass star as a companion, the total age of the system can be obtained through stellar isochrones or X-ray luminosity, depending on the evolutionary state of the companion. On the contrary, when this is not possible, the white dwarf itself can be used to constrain the lower limit of the age of the companion through its cooling time and the use of the initial-final mass relationship of white dwarfs. In this work, I will present results using both methods to constrain the ages of wide binaries.
The Ages of Evolved Low-Mass Stars: The Horizontal Branch, and Beyond
Prof. MÀrcio Catelan (Pontificia Universidad CatÑlica de Chile)
The "second-parameter problem" of horizontal branch (HB) morphology is characterized by the presence of globular clusters with very similar metallicities (the "first parameter") and yet widely different HB morphologies. Age has long been a prominent second-parameter candidate, but other candidates, such as helium abundance and mass loss, have also been proposed in the literature. Defining the relative weights of each of the several different factors that are (at least in principle) able to control HB morphology, in addition to metallicity, constitutes the main goal of modern second-parameter studies. In this talk an overview will be presented of how several different factors may affect the advanced evolutionary stages of low-mass stars, including the HB and beyond, and possible empirical tests that may help distinguish amongst them. An updated view of some key second parameter pairs will also be provided.
Models of Low-Metallicity Stars
Prof. Brian Chaboyer (Dartmouth College)
Models of low-metallicity stars are widely used to determine the ages of old stellar systems. I will present an overview of current models of low-metallicity stars with particular attention to the largest uncertainties in current models (1) the treatment of convection; (2) the importance of microscopic diffusion and (3) mass loss. Recent progress in improving the basic physics (such as the 14N + p reaction rate) used to construct models of low-metallicity stars will also be highlighted.
The Sun as a Fundamental Calibrator of Stellar Evolution
Prof. Joergen Christensen-Dalsgaard (University of Aarhus)
The Sun plays a central role in our understanding of stellar structure and evolution, owing to its proximity and our resulting ability to carry out very detailed observations. Furthermore, age determinations of meteorites provide a measure of the age of the solar system which can be compared with the solar age as inferred from helioseismology, including analyses based on the modes that are observed in distant stars. Thus we can directly test the asteroseismic techniques in a case where the answer is known. As discussed by Yveline Lebreton in the preceding talk the resulting inferred age depends on other uncertainties in the modeling of the stellar interior. I discuss these issues, with a view towards the application of the analysis for other stars, and drawing heavily on recent detailed investigations by Houdek and Gough on seismic inferences for solar-like stars.
The Ages of Galactic Globulars in the Context of Self-Enrichment
Dr. Thibaut Decressin (Argenlander Institut for Astronomy)
Stars in globular clusters exhibit a peculiar chemical pattern with strong abundance variations in light elements along with constant abundance in heavy elements. These abundance anomalies can be created in the H-burning core of a first generation of fast rotating massive stars and due to rotational induced mixing these elements are convoyed to the stellar surface. If the rotation of the stars is fast enough this matter is ejected at a low velocity through the equator and then it pollutes the ISM from which a second generation of chemically anomalous stars can be formed. This process of self-pollution has several dynamical consequences for the formation and evolution of two distinct populations in globular clusters. In particular the observed ratio of anomalous and standard stars in clusters needed both gas expulsion by supernovae and long term evolution to be matched. Besides such process induces a strong He-enrichment for the second generation low-mass stars. Here the consequences of this enhancement of the initial He-content is discussed with an emphasis on stellar lifetime and on the age of globular clusters.
Lithium Depletion and Age: Models and Observations
Prof. Constantine Deliyannis (Indiana University)
Lithium is a key diagnostic of big bang nucleosynthesis, Galactic chemical evolution, and physical processes occurring inside stars. I will focus on solar-type stars and discuss how the observed surface Li abundance may be used as a diagnostic of the star's age. With few exceptions, stars deplete their surface Li abundances as they age. However, the Li depletion depends also on other parameters, including effective temperature, metallicity, and rotational history. Data from stellar samples of known age, mostly open clusters, will be used to map out empirically the relative importance of these parameters. Insights from models will also be discussed, and the issue of the initial stellar Li abundance will be addressed. Finally, I will shift to the bottom of the main sequence and brown dwarfs and discuss ages in the context of the lithium depletion boundary.
The Solar Abundance Mixture: Effects on the Atmospheric Structure and Dynamics
Prof. Pierre Demarque (Yale University)
The ages of stars depend on their chemical abundances. Stellar abundances are usually expressed relative to the solar abundance mixture, which is at the moment a source of controversy. In order to help resolve this controversy, we test the internal consistency of the derivation of the most recent solar mixture calibration due to Asplund et al (2005). This paper presents a comparison between two three-dimensional radiative hydrodynamical (3D RHD) simulations of the solar outer layers using the Grevesse & Sauval (1998; GS98) and the Asplund, Grevesse & Sauval (2005; AGS05) mixtures, respectively. The AGS05 mixture is a revision of the solar abundance mixture using a model atmosphere based on the stratification and dynamics of a solar 3D RHD simulation performed with the GS98 mixture, using the code of Stein & Nordlund (1989, 1998). Our two simulations were performed with the code of Robinson et al. (2003) including physically realistic treatment of radiative opacities and equation of state. A comparison between our GS98 and AGS05 3D simulations shows differences both in the mean vertical temperature gradient and in the turbulent velocity field in the line forming region of the solar atmosphere. The effect of chemical composition on the structure of the superadiabatic layer (SAL) is illustrated. Turbulent velocities, critical for absorption line profile calculations, are larger in the AGS05 simulation by about 10%. Since opacities for the AGS05 mixture are lower than those for the GS98 mixture, the T-$\tau$ relations derived from the two simulations are different. At 5000K, $\tau$ obtained from the GS98 simulation is about 30% larger than that obtained from the AGS05 simulation. These results show the importance of preserving self consistency in the chemical abundances between the model atmosphere and the 3D HRD simulation. When deriving the solar abundances, one must iterate the chemical composition in the model atmosphere, and hence in the 3D simulation on which the atmosphere is based. This research was supported in part by NASA/ATP grant NAG5-13299 (PD and FR) and NSF grant ATM 0348837 to SB.
Globular Cluster Ages from Main Sequence Fitting and Detached, Eclipsing Binaries: The Case of 47 Tuc
Dr. Aaron Dotter (University of Victoria, Dept. of Physics and Astronomy)
Age constraints are most often placed on globular clusters by comparing their CMDs with theoretical isochrones. The recent discoveries of detached, eclipsing binaries in globular clusters by the Cluster AgeS Experiment (CASE) will yield new insights into the ages of these systems and, at the same time, provide much needed tests for stellar evolution models. We will describe efforts to model the properties of the detached, eclipsing binary V69 in 47 Tuc and compare age constraints derived from stellar evolution models of V69A & B with ages obtained from fitting isochrones to the CMD of 47 Tuc. We will explore whether or not, under reasonable assumptions of distance, reddening, and metallicity, it is possible to simultaneously constrain the age and He content of 47 Tuc.
Confronting Substellar Evolutionary Models with Stellar Ages
Mr. Trent Dupuy (Univ. of Hawaii)
Because brown dwarfs cool and dim over time, tests of substellar models require objects with independently determined ages. Most such brown dwarfs are companions to nearby stars, so stellar ages are at the heart of this effort. However, substellar models are only fully constrained if both the mass and age are known. We have used the Keck adaptive optics system to monitor the orbit of HD 130948BC, a brown dwarf binary that is a companion to the young solar analog HD 130948A. The total dynamical mass of 0.109+-0.002 Msun is the most precise mass measurement (2%) for any brown dwarf binary to date and shows that both B and C are substellar for any plausible mass ratio. The ensemble of available age indicators from the primary star suggests an age comparable to the Hyades, with the most precise age being 0.79 Gyr based on gyrochronology. Therefore, HD 130948BC is now unique among field L and T dwarfs, with a well-determined mass, luminosity, and age. Our results indicate that substellar evolutionary models may underpredict the luminosity of brown dwarfs by as much as a factor of 2 to 3. The implications of such a systematic error in evolutionary models would be far-reaching, for example, affecting determinations of the initial mass function and predictions of the radii of extrasolar gas-giant planets. This result is largely based on the reliability of stellar age estimates, and the case study of HD 130948A highlights the difficulties in determining the age of an arbitrary field star, even with the most up-to-date chromospheric activity and gyrochronology relations. In order to better assess the potential systematic errors present in substellar models, a more refined age estimate for HD 130948A is critically needed.
The Age of the Galaxy’s Thick Disk
Dr. Sofia Feltzing (Lund Observatory)
Thick disks appear ubiquitous in disk galaxies. In most of the cases studied so far these thick disks appear to be old stellar populations. The determination of the age of the thick disk in the Milky Way is of great interest in terms of its implications for galaxy formation and evolution. Is our thick disk a homogenous entity? Did all of the thick disk stars form during a short time? The answers to these questions leads back to the more fundamental question: is our thick disk a unique stellar population or is it the result of several merger events? In the Milky Way we are able to study the ages of individual stars. This both makes the task easier and more complicated. Both random and systematic errors in the age determinations make the interpretations less robust. I will review the various methods that have been used to infer the age of the Milky Way thick disk as a whole as well as for the individual stars in the thick disk. The age of the thick disk will then be discussed in the context of galaxy formation, e.g. can we know the ages of the Milky Way thick disk stars well enough to make any firm statements about its formation.
Using ages and kinematic traceback: an scenario for the origin of young local associations
Dr. David FernÀndez (Institut de CiÕncies del Cosmos - Universitat de Barcelona)
Over the last decade, several groups of young (mainly low-mass) stars have been discovered in the solar neighbourhood (closer than ~100 pc), thanks to cross-correlation between X-ray, optical spectroscopy and kinematic data. These young local associations - including an important fraction whose members are Hipparcos stars - offer insights into the star formation process in low-density environments, shed light on the substellar domain, and could have played an important role in the recent history of the local interstellar medium. Ages estimates for these associations have been derived in the literature by several ways (HR diagram, spectra, Li and H_alpha widths, expansion motion, etc.). In this work we have studied the kinematic evolution of young local associations and their relation to other young stellar groups and structures in the local interstellar medium, thus casting new light on recent star formation processes in the solar neighbourhood. We compiled the data published in the literature for young local associations, including the astrometric data from the new Hipparcos reduction. Using a realistic Galactic potential we integrated the orbits for these associations and the Sco-Cen complex back in time. Combining these data with the spatial structure of the Local Bubble and the spiral structure of the Galaxy, we propose a recent history of star formation in the solar neighbourhood. We suggest that both the Sco-Cen complex and young local associations originated as a result of the impact of the inner spiral arm shock wave against a giant molecular cloud. The core of the giant molecular cloud formed the Sco-Cen complex, and some small cloudlets in a halo around the giant molecular cloud formed young local associations several million years later. We also propose a supernova in young local associations a few million years ago as the most likely candidate to have reheated the Local Bubble to its present temperature.
Determining the Old Age of Star HE 1523-0901 from Seven Different Abundance Ratios Involving Radioactive and Stable R-Process Elements, and the Role of Lead in Nucleo-Chronometry
Dr. Anna Frebel (McDonald Observatory, Univ. of Texas)
In their atmospheres, old metal-poor Galactic stars retain detailed information about the chemical composition of the interstellar medium at the time of their birth. About 5% of metal-poor stars with [Fe/H]<-2.5 display in their spectrum a strong enhancement of neutron-capture elements associated with the rapid (r-) nucleosynthesis process that is responsible for the production of the heaviest elements in the Universe. The so-called r-process stars are formed from material enriched in these hehaviest elements that were created during an r-process nucleosynthesis event in one previous-generation supernova. The neutron-capture abundances in the few stars known (~15) with this extremely rare chemical signature all follow the scaled solar r-process pattern for the elements above Ba. This suggests that the main r-process is universal -- a surprising empirical finding and a solid result that makes feasible age measurements of r-process stars. Among the heaviest elements are the long-lived radioactive isotopes 232Th (half-life 14 Gyr) and 238U (4.5 Gyr). While Th is often detectable in these stars, U poses a real challenge because only one, extremely weak line is available in the optical spectrum. Only three stars have measured U abundances, of which HE 1523-0901 has the most confidently determined value. From comparing the stable Eu, Os, and Ir abundances with measurements of Th and U, stellar ages can be derived. Based on seven such chronometer abundance ratios, the age of HE 1523-0901 was found to be ~13 Gyr. In this context, measuring the stellar Pb abundance together with Th and U is essential. Pb is the beta- plus alpha-decay end-product of all decay chains in the mass region between Pb and the onset of dominant spontaneous fission above Th and U. Hence, in addition to Th/U also Th,U/Pb should be used to obtain a consistent picture for actinide chronometry. From recent r-process calculations within the classical "waiting-point" model, for a 13 Gyr old star we predict the respective abundance ratios of log(Th/U)=~0.85, log(Th/Pb)=~-1.32 and log(U/Pb)=~-2.16. These predictions can be compared with the measured abundance ratios in HE 1523-0901 of log(Th/U)=0.86, log(Th/Pb)>-1.0 and log(U/Pb)>-1.9. With this good level of agreement, HE 1523-0901 is already a vital probe for observational ``near-field'' cosmology by providing an independent lower limit for the age of the Universe.
The Star Formation History of the Magellanic Clouds
Dr. Carme Gallart (Instituto de AstrofÌsica de Canarias)
Does the onset of star formation in a galaxy depend on galaxy mass, type or environment? How is the global star formation history of a galaxy influenced by galaxy interactions? What is the nature of the stellar population gradients observed in galaxies? Precise, detailed answers of these questions can be obtained from colour-magnitude diagrams reaching the oldest main-sequence turnoffs with good photometric accuracy, and covering representative portions of the galaxy of interest, and via comparison with state-of-the-art stellar evolution models. Such data can be obtained and is partially available for the Magellanic Clouds. I will summarize the main results published so far on the early evolution and star formation history of both Magellanic Clouds, together with new, unpublished results covering large, representative portions of each galaxy. In order to shed some light on the above questions, some comparison will be performed with the star formation history of other galaxies, in particular with results of the HST LCID project on isolated Local Group dwarf galaxies.
The Ages of AGB Stars
Dr. Leo Girardi (Osservatorio Astronomico di Padova)
We review the state of the art of TP-AGB models including several critical effects such as third dredge up events, hot bottom burning, changes in molecular opacities driven by the evolving surface chemical composition, changes in mass loss rates between M- and C-type stars, the transition from higher modes to fundamental-mode pulsation, reprocessing of radiation by circumstellar dust, etc. These models challenge the old idea that the AGB tip monotonically decreases with age (and hence that it would be a good age indicator). Moreover, the models indicate that the ages of Carbon stars dependent very much on metallicity. Many of the theoretical predictions are supported (and in some cases calibrated) by Magellanic Cloud data, whereas the model behaviour at both low and high metallicities is significantly more uncertain.
The Sun in Time: Spin-down and angular momentum loss of the Sun and solar-type Stars: Determining Reliable Ages and X-UV Irradiances for Stars & hosted planets
Dr. Edward Guinan (Villanova University)
Multi-wavelength studies of solar analogs (G0-5 V stars) with ages from ~50 Myr to 9 Gyr are being carried out as part of the "Sun in Time" program over the last ~20 yrs. These studies infer that the young Sun was rotating over ten times faster than today. As a consequence, the early Sun and young solar-type stars, had vigorous magnetic dynamos and correspondingly strong coronal X-ray and chromospheric UV emissions up to a thousand time stronger then seen for the present Sun. Also rotational modulated light variations indicate the presence of large starspot regions on the younger stars. The chief science goals are (1) to study the solar magnetic dynamo (with rotation as the only variable) and (2) to determine the spin-down and angular momentum loss of the Sun and solar-type stars as a function of age, and (3) to determine the radiative and magnetic properties of the young Sun (and other solar-type stars) with the purpose of constructing spectral irradiances and plasma densities that can be used to study the evolution of planetary atmospheres of hosted planets. As part of this program we have developed tight relations among rotation, age & magnetic-activity indicators (Lx, UV emissions, starspots...). The strong XUV radiation and particle fluxes inferred for young suns have major implications and impacts on the photochemistry and photo-ionization of paleo-planetary atmospheres (in particular for the terrestrial planets) and also play important roles in the possible atmospheric erosion as well as the study of the suitability of the planets for life. Some the new results of this program will be discussed that focus on ages of stars and the evolution over time of their dynamo related X-UV emissions. This research is supported by grants from NASA (XMM, Chandra, HST, FUSE programs) as well as from NSF/RUI Grant 05-07536. We are very thankful for this support.
Recovering the Ages of Stars of a Complex Stellar Population System: IAC-pop/MINNIAC
Dr. Sebastian L. Hidalgo (Instituto de Astrofisica de Canarias)
We present IAC-pop/MINNIAC, a code to recover the age of stars of a complex stellar population system, like a galaxy. It uses a genetic algorithm to minimize a $\chi^2$ merit function comparing the star distributions in the observed color-magnitude diagram (CMD) and the CMD of a synthetic stellar population. A parameterization of the CMDs is used, which is the main input of the code. The computation of the synthetic CMD can be done using the code IAC-star. Since this is a quite time consuming step, a method is presented requiring computation of only a single synthetic CMD. IAC-pop/MINNIAC has been run through several consistency test and has been proved with deep ACS@HST photometry from the LCID project. We present the first results for the star formation histories of six galaxies using IAC-pop/MINNIAC code.
Observations of Low Mass Pre Main Sequence and Zero Age Main Sequence Stars
Prof. Lynne Hillenbrand (California Institute of Technology)
This talk will summarize the age constraints available for young stars, including the Hertzsprung Russell diagram, spectroscopic surface gravity indicators, and lithium depletion in the pre-main sequence, then rotation (period or vsini) and activity (coronal and chromospheric) diagnostics along with lithium depletion near and beyond the zero age main sequence. Other speakers will present more detail. Recent results on the Orion Nebula Cluster population will be highlighted, time permitting.
The angular momentum evolution of low-mass stars
Dr. Jonathan Irwin (Harvard-Smithsonian Center for Astrophysics)
The previous two talks in this meeting have examined the use of rotational evolution as a tool for determining ages of low-mass stars. In this talk, we attempt to examine the physical processes giving rise to the observed mass- and age-dependence used for this "gyrochronology", by looking at the rotational evolution on the pre-main sequence. Large samples of rotation period and v sin i measurements are now available in open clusters over the age range 1-650 Myr spanning from solar masses down to near the hydrogen burning limit. I shall demonstrate that much of the observed evolution can be explained surprisingly well by extremely simple models, based on standard stellar evolution tracks and incorporating two sources of angular momentum loss: solar-type winds for stars on the main sequence, and some form of (presumably) accretion-disc-related losses on the pre-main sequence. Nonetheless, while this formalism works well for solar-type stars, there are a number of unanswered questions for lower-mass stars, particularly below 0.4 solar masses where the stars are fully-convective throughout their lifetimes.
The Ages of Open Clusters
Ms. Elizabeth Jeffery (University of Texas at Austin)
Open clusters have long been objects of interest in astronomy. As a good approximation of essentially pure stellar populations, they have proved very useful for studies in a wide range of astrophysically interesting questions, including stellar evolution and atmospheres, the chemical and dynamical evolution of our Galaxy, and the structure of our Galaxy. Of fundamental importance to our understanding of open clusters is accurate determinations of cluster ages. Currently there are two main techniques for independently determining the ages of stellar populations: main sequence evolution theory (via cluster isochrones) and white dwarf cooling theory. We will provide an overview of these two methods, the current level of agreement between them, as well as a look to the current state of increasing precision in the determination of each. Particularly we will discuss a new Bayesian statistical technique that has been developed by our group and its applications in improving and determining white dwarf ages of open clusters.
Measuring Age Spreads in Star Forming Regions and Young Clusters
Dr. Robin Jeffries (Keele University)
In principle, the ages of individual stars can be estimated from their position in the Hertzsprung-Russell (or color-magnitude) diagram. Such an exercise typically reveals significant age spreads in star forming regions of order 10 Myr. Whether these spreads are real is still debatable. Binarity, accretion, variability and patchy extinction conspire to indicate age variations where none may exist. I will describe two independent techniques - estimating radii from rotation data and measuring lithium depletion dispersions - that largely circumvent these problems to yield model dependent age spread determinations. These techniques are applied in some illustrative cases and their limitations discussed.
White Dwarfs as Astrophysical Probes
Dr. Jason Kalirai (UC Santa Cruz)
White dwarfs represent the eventual end products of 98% of all stars. As such, their luminosity and mass distributions can be used to understand the properties of their progenitor populations (e.g., the age and initial mass function). In this talk, I will summarize several ways in which studies of local white dwarfs have aided in our understanding of general astrophysics. For example, photometric studies of white dwarfs in the Galactic disk and halo have firmly established the ages of the first structures that formed in these components. Spectroscopic follow up of these stellar cinders over a wide range in age has led to unprecedented constraints on the initial-to-final mass relation (i.e., what mass main-sequence star maps to white dwarf mass), and therefore are a powerful input to chemical evolution models of galaxies including enrichment in the interstellar medium over the age of the Universe. Finally, the study of nearby white dwarfs in old stellar populations that are metal-rich has led to direct empirical evidence that stellar mass loss is much more efficient in high metallicity environments. This result is critical in interpreting the UV upturn in elliptical galaxies, the dearth of planets around white dwarfs, and the different rates (and properties) of type Ia SNe in elliptical vs spiral galaxies.
Ages From Asteroseismology
Dr. Yveline Lebreton (Paris Observatory)
Asteroseismology has been recognized for a long time as a very powerful mean to probe stellar interiors. In particular, the oscillations frequencies are closely related to stellar internal structure properties via the density and the sound speed profiles. Since these properties are in turn tightly linked with the mass and evolutionary state of stars, we can expect to determine the age and mass of a star from the comparison of its oscillation spectrum with the predictions of stellar models. Such a comparison will of course suffer both from the problems we face when modeling a particular star (for instance the uncertainties on its global parameters and chemical composition) and from our general misunderstanding of the physical processes at work in stellar interiors (for instance the various transport processes that may lead to core mixing and affect the ages predicted by models). However for stars where observations have provided very precise and numerous oscillation frequencies together with accurate global parameters and additional information (as the radius or the mass of the star if it is member of a binary system or the radius if it observable in interferometry), we can also expect to better constrain the physical description of the stellar structure and transport processes and to finally get a more reliable age estimation. After a brief survey of stellar pulsations, I will present some general seismic diagnostics that can be used to infer the age of an oscillating star as well as their limitations. I will then illustrate the ability of asteroseismology to scrutinize stellar interiors on the basis of a few exemples of recent studies of well-known pulsating stars. In the years to come, extended very precise asteroseismological observations are expected, either in photometry or in spectroscopy, from present and future ground-based (HARPS, CORALIE, ELODIE, UVES, UCLES, SIAMOIS, SONG…) or spatial observational devices (MOST, CoRoT, WIRE, Kepler, PLATO). This will considerably enlarge the sample of stars eligible to asteroseismic age determination.
Massive star clusters in the Magellanic Clouds
Dr. Dougal Mackey (University of Edinburgh)
Massive star clusters in the Magellanic Clouds have ages spanning the full range 10^6 - 10^10 years: from very newly formed objects such as 30 Doradus to clusters apparently coeval with the Galactic globulars. As such, these systems represent ideal locations for testing models of stellar evolution and calibrating the integrated properties of stellar populations. They are also central to our understanding of star cluster formation and evolution, as well as the star formation histories of the LMC and SMC. I will describe some recent developments in the study of Magellanic Cloud star clusters, including work investigating the effect of blue stragglers on a cluster's integrated light and the resulting implications for derived properties such as age and metal abundance. I will also review the recent discovery of a number of intermediate-age Magellanic Cloud clusters which do not appear to conform to the standard paradigm of star clusters as simple, single stellar populations.
The Evolution of Activity on Solar-Type Stars
Dr. Eric Mamajek (University of Rochester)
Observations of solar-type stars in age-dated clusters suggest that these stars are generally born with fast rotation rates which spin up as the star contracts to the main sequence, and spin down as the star evolves on the main sequence. Evolution of the moment of nertia of the star and magnetic breaking (respectively) appear to be dominant physical mechanisms responsible. Strong correlations are observed between stellar rotation and both coronal X-ray and chromospheric activity -- indicative of stellar magnetic dynamos. In this talk, I discuss the evolution of stellar activity with age for Sun-like stars from the (often frustrating) perspective of trying to inferring ages for field stars. Mamajek & Hillenbrand (2008) recently completed a comprehensive analysis of the relationships between the rotation of solar-mass dwarfs (F7V-K2V) and hromospheric activity (Ca H and K)and coronal activity (X-rays). The goal of the analysis was to provide a modern toolkit for observers to estimate the ages and age uncertainties of solar-type field dwarfs given three common diagnostics (R'HK, log(Lx/Lbol), and rotation), and builds upon the rich history of activity and rotation studies by Noyes, Baliunas, Hartmann, Stauffer, Soderblom, Guinan, Donahue, and Barnes, among others. Age inference from chromospheric and coronal activity for the youngest, most active stars is fraught with danger (~>1 dex uncertainties in age), however for main sequence stars with modest activity levels, ages can be estimated to accuracies of ~0.2 dex.
Refining the Relationships Between Stellar Age, Rotation, and Mass from Late-Type Dwarfs in Open Clusters
Dr. Soren Meibom (Harvard-Smithsonian Center for Astrophysics)
Stars spin down over time at a rate that is dependent on their mass. For late-type dwarfs in open clusters, the connection between stellar rotation, age, and mass was found decades ago by Skumanich and Kraft. The relations derived from these discoveries and later confirmations were limited by the inherent ambiguity of the measurements of projected stellar rotation from spectral line-broadening. Today, stellar rotation periods can be measured to a high precision from spot-modulation, and ages and masses for late-type stars in open clusters can be determined by the isochrone method. If well-defined relationships between age, mass, and rotation can be established empirically, then the ages of stars can be derived from observations of their colors (masses) and rotation periods alone. Recently, a new relationship between stellar age, rotation period, and mass was suggested based on rotation period data in open clusters (Gyrochronology). In this talk, I will present tests of the method of gyrochronology based on rotation periods for ~500 late-type radial-velocity and photometric members of two open clusters M35 and M34. Our results are based on a combination of decade-long radial-velocity surveys for cluster membership, and photometric monitoring campaigns over 5 consecutive months of the same stars in the two clusters. The combination of comprehensive cluster membership information and rotation periods ranging over two orders of magnitude for FGK dwarfs, reveal well-defined relations between stellar rotation period and stellar color (mass) in both clusters. I determine the gyro-ages, with uncertainties, for M35 and M34 and compare them to the cluster isochrone ages. I will discuss modifications of the age-rotation-mass relations based on our new data, and present the prospects for extending the calibration of the relations to older stars using observations of cluster obtained with NASA's Kepler mission.
Circumstellar Disks: Observational Constraints on the Formation and Evolution of Planetary Systems
Prof. Michael Meyer (The University of Arizona)
Observations of circumstellar disks around stars as a function of stellar properties such as mass, metallicity, multiplicity, and age, provide constraints on theories concerning the formation and evolution of planetary systems. Utilizing ground- and space-based data from the far-UV to the radio, astronomers can assess the amount, composition, and location of circumstellar gas and dust as a function of time. We will review recent results from the Spitzer Space Telescope, as well as chronologies of our solar system revealed from a variety of extinct radionuclide dating techniques. Comparing these results with those from a variety of exoplanet search techniques, theoretical models, as well as the inferred history of the solar system, help us to assess whether planetary systems like our own, and the potential for life that they represent, are common or rare in the Milky Way galaxy.
The Current State of Stellar Models for Population I
Prof. Georges Meynet (Observatory of Geneva University)
We shall review the various improvements brought to the stellar models in the recent years. In particular the updates of the initial solar composition, of the opacities, of the nuclear reaction rates, will be reviewed. Also the impacts on the outputs of stellar models of various physical processes as the treatment of convection, the inclusion of extra-mixing processes, of axial rotation and mass loss by stellar winds, will be discussed. Suggestions of some future lines of research aiming at improving the stellar age determinations will be presented.
New Methods for Determining the Ages of Pre-Main-Sequence Stars.
Prof. Tim Naylor (University of Exeter)
Pre-main-sequence stars fade with age, and thus move down in a colour-magnitude diagram. In principle, this can be used to measure their ages, but in practice the ages are highly model dependent and degenerate with distance. This is unfortunate as accurate ages for pre-main-sequence stars are crucial for understanding the evolution of their planet-forming discs. We have overcome these problems for young (<15Myr-old) groups to create a self-consistent age ladder, to which other groups can be added in a straight-forward way. We use the stars which have already reached the main-sequence in these groups to derive a distance using the tau^2 technique, removing the distance degeneracy. We then fit splines through the positions of the pre-main-sequence stars in the colour-magnitude diagram to create emprical isochrones. Placing these in absolute magnitude results in a ladder, with the brightest, and hence youngest groups at the top of the diagram, and the older ones below. After placing the groups in this revised age order, we can clearly see that there is a gap in colour between the (radiative) main-sequence and (convective) pre-main-sequence in these young groups, which closes as a function of age. This "radiative-convective gap" therefore holds the promise of being a new, distance independent method of determining ages.
Disk heating in the galactic thin disk
Prof. Birgitta Nordstrom (Niels Bohr Institute, Copenhagen Univeristy)
Observations show that the velocity dispersion of stars in the galactic thin disk increases with time as a function of stellar age. The age-velocity relation for each of the three velocity components needs to be studied separately as they might be caused by different effects.
PMS evolutionary models
Dr. Francesco Palla (INAF-Osservatorio Astrofisico di Arcetri)
Unlike the hydrodynamical protostellar phase, the PMS evolution of a star can be followed by models in hydrostatic and thermal equilibrium. This approach has been extensively used in the past and has allowed researchers to build a variety of numerical codes that produce evolutionary tracks and isochrones for comparison with the observations of individual stars/systems and young stellar clusters. Some of the major uncertainties on PMS stellar models involve the poor knowledge of the initial conditions and of the physical processes (rotation, magnetic fields) that alter the simple 1D picture of the classical models. I will briefly discuss some of the latest improvements on PMS models. Then, I will address two issues, lithium depletion and stellar pulsations, that can offer an insight into the thorny issue of age dating young PMS stars and BDs.
Ages of Open Clusters
Prof. Marc Pinsonneault (Ohio State University)
Open clusters provide one of the most powerful methods for inferring ages of young and intermediate aged stars. However, there have been major systematic uncertainties in the absolute age scale, particularly for clusters where the turnoff stars have convective cores. Photometry can now be used to infer distance, abundance, and reddening to high precision, so the major error sources are systematic in nature. The advantages and disadvantages of ages based on the turnoff, lithium in low mass stars, and the main sequence/pre-main sequence boundary are reviewed in detail, and I also examine other techniques for inferring cluster ages.
The Timescales of Chemical Enrichment in the Galaxy
Dr. Antonio Pipino (Oxford University/University of Southern California)
The timescales of chemical enrichment are fundamental to understand the evolution of abundances and abundance ratios in galaxies. In particular, the timescales for the enrichment by SNe II and SNe Ia are crucial in interpreting the evolution of abundance ratios such as [alpha/Fe]. In fact, the alpha-elements are produced mainly by SNe II on timescales of the order of 3 to 30 Myr, whereas the Fe is mainly produced by SNe Ia on a larger range of timescales, going from 30 Myr to a Hubble time. This produces differences in the [alpha/Fe] ratios at high and low redshift and it is known as "time-delay" model. In this talk we review the most common progenitor models for SNe Ia and the derived rates together with the effect of the star formation history on the [alpha/Fe] versus [Fe/H] diagram in the Galaxy. From these diagrams we can derive the timescale for the formation of the inner halo (roughly 2 Gyr), the timescale for the formation of the local disk (roughly 7-8 Gyr) as well the the timescales for the formation of the whole disk. These are functions of the galactocentric distance and vary from 2-3 Gyr in the inner disk up to a Hubble time in the outer disk (inside-out formation). Finally, the timescale for the formation of the Bulge is found to be no longer than 0.3 Gyr, similar to the timescale for the formation of larger spheroids such as elliptical galaxies. Finally, we show how the time-delay model applied to galaxies of different morphological type, identified by different star formation histories, produces different abundance patterns which represent a very strong tool to understand the nature and the age of high redshift objects.
On the Use of Lithium to Derive Ages of Stars Like Our Sun
Dr. Sofia Randich (INAF-Osservatorio di Arcetri)
Along with chromospheric emission, lithium abundances are widely used to infer the ages of solar-like stars. We re-assess the validity and limits of this approach, based on new high quality Li measurements among solar-type members of 8 open clusters observed with Giraffe on the VLT. The sample clusters span a factor of 10 in age and a factor of four in metallicity; spectra of 100-150 stars per cluster have been obtained, thus providing a statistically significant sample to quantitatively define a Li vs. age relationship and its possible dependence on the metal content.
Ages of Star Clusters from Cooling White Dwarfs
Prof. Harvey Richer (UBC)
Cooling white dwarfs provide a new and precise method of determining the ages of star clusters. The physics entering into these ages is quite different from that used in deriving ages from the turnoff - making these two techniques complementary. The white dwarf cooling method is, however, not without its own difficulties - both theoretical and observational. In this talk I discuss the technique of deriving cluster ages in this way and point out some of the difficulties in its usage. The age for NGC 6397 is derived both from cooling white dwarfs and the turnoff and the results are compared.
White Dwarf Cosmochronology: Techniques and Uncertainties
Prof. Maurizio Salaris (ARI - Liverpool John Moores University)
White dwarfs represent the endpoint of the evolution of the large majority of stars formed in the Galaxy. Their usefulness as cosmochronometers has been recognized since about 50 years ago, but only in the last two decades observations and theory have improved to a level that made possible to employ white dwarfs for determining ages of the stellar populations in the solar neighbourhood, and in the nearest star clusters. This review will be centered on the theory behind the methods for white dwarf age-dating, and the related uncertainties, with particular attention paid to the problem of the CO stratification, envelope thickness and chemical composition, and the white dwarf initial-final-mass relationship.
Relative and Absolute Ages of Globulars
Prof. Ata Sarajedini (University of Florida)
The past several years have seen an explosion of high precision color-magnitude diagrams for Galactic globular clusters. This is largely due to that fact that new observations from HST have built upon an existing archive of data to produce a rich database of high resolution imaging for globular clusters. The color-magnitude diagrams resulting from these data as well as those based on ground-based observations have been used to study, among other things, the absolute and relative ages of Galactic globulars. I will review the latest work on this topic with a focus on the results from our HST Treasury project, which has observed 65 Galactic globular clusters with the Advanced Camera for Surveys. These data extend well-down the main sequence and provide the deepest and most complete photometry for a large sample of clusters.
Eclipsing Binaries as Tests of Stellar Evolution Models: Insights and Limitations
Dr. Keivan Stassun (Vanderbilt University)
We present an overview of recent work on eclipsing binary stars, and summarize the key implications for theoretical stellar evolution models. We also highlight a few systems as exemplars of some of the important physical insights that are newly emerging. For example, there is now strong evidence that many low-mass eclipsing binaries possess sufficiently strong surface activity to significantly suppress convection, which in turn alters the mass-radius and mass-temperature relationships. There is also now evidence from a recently discovered, very young eclipsing binary that such systems can be non-coeval at the level of ~0.5 Myr. These results reveal important limitations on the use of eclipsing binaries to constrain stellar models specifically in the context of stellar ages.
CMDs: A Homogeneous Calibration
Dr. Peter Stetson (Herzberg Institute of Astrophysics)
I will present a progress report on my attempts to establish a homogeneous photometric scale in the Johnson-Cousins-Landolt system for star clusters and nearby resolved galaxies. An effort to extend this calibration chain to ACS and WFPC2 will be discussed. If there is time, I will try to overlay some isochrones and draw some highly preliminary conclusions.
Star Formation Histories of Resolved Galaxies
Prof. Monica Tosi (INAF - Osservatorio Astronomico di Bologna)
The exploitation of the power and spatial resolution of HST and new generation ground-based telescopes allows to measure with good precision the individual stars of galaxies in and beyond the Local Group. This allows very successful studies of the star formation histories of galaxies of various morphological types. I will review our current knowledge of the star formation history of galaxies within 10-20 Mpc, as derived from the colour-magnitude diagrams of their resolved stellar populations.
Stellar Ages from Asteroseismology: a Few Precise Cases
Prof. Sylvie Vauclair (LATT/OMP)
I will show how asteroseismology can give more precise ages than any other methods for solar type stars. I will present, as examples, the asteroseismic studies of a few solar type stars, which have been observed with HARPS and/or SOPHIE spectrometers. The acoustic oscillation frequencies have been obtained and compared with models, which give precise constraints on several parameters like stellar masses and ages.
Using Magnetic Activity and Galactic Dynamics to Constrain the Ages of M dwarfs
Dr. Andrew West (MIT)
I will review several methods for estimating the ages of M dwarfs but focus mainly on results that estimate a star's age based on its magnetic activity. I will present a study of the dynamics and magnetic activity of M dwarfs using the largest spectroscopic sample of low-mass stars ever assembled. The age at which strong surface magnetic activity (as traced by H-alpha) ceases in M dwarfs has been inferred to have a strong dependence on mass (spectral type, surface temperature) and explains previous results showing a large increase in the fraction of active stars at later spectral types. Using spectral observations of more than 40,000 M dwarfs from the SDSS, I will show that the fraction of active stars decreases as a function of vertical distance from the Galactic plane (a statistical proxy for age), and that the magnitude of this decrease changes significantly for different M spectral types. Adopting a simple dynamical model for thin disk vertical heating, I assign an age for the activity decline at each spectral type, and thus determine the activity lifetimes for M dwarfs. In addition, I derive a statistical age-activity relation for each spectral type using the dynamical model as well as the vertical distance from the Plane and the H-alpha luminosity of each star (the latter of which also decreases with vertical height above the Galactic plane).
Multiple Populations in Simple Stellar Populations: Theory
Prof. Sukyoung Yi (Yonsei University)
The massive Galactic globular clusters, omega Cen and NGC 2808, appear to have multiple distinct subpopulations and often considered as the remnants of the oldest galaxy building blocks. Their blue main sequence populations are at the centre of debate because they have been suggested to have an extremely high helium abundance, Y ~ 0.4. The same helium abundance is claimed to explain the presence of extreme horizontal branch stars as well. This demands a relative helium to metal enrichment of dY/dZ ~ 70; that is, more than one order of magnitude larger than the generally accepted value. Similar globular clusters appear to be present in abundance in the external galaxy M87. Candidate solutions, namely, AGB stars, massive stars, supernovae, and population III stars, have been suggested. I present a short review on the validity (or lack of validity)of these scenarios.