This week on HST

HST Programs: December 28, 2015 - January 3, 2016

Selected highlights

GO 13804: Important Nearby Galaxies without Accurate Distances

M104, the Sombrero galaxy, one of the systems targeted in this program

The Spitzer Infrared Nearby Galaxies Survey (SINGS) was a highly successful Spitzer Legacy program that compiled near- and mid-infrared imaging and spectroscopy for 75 galaxies within 75 Mpc of the Milky Way. SINGS' success stimulated a series of follow-up programs, including THINGS (HI gas), HERACLES (CO gas), and KINGFISH (Herschel mid- and far-IR data), that have resulted in exhaustive multi-wavelength observations that characterise the star-formation properties in many of the SINGS galaxies. However, a surprising number of these systems still lack accurate distance determinations. HST is well suited to determining distances to the closer SINGS galaxies : the high sensitivity of the Advanced Camera for Surveys and Wide-Field Camera 3 combined with the unparalleled angular resolution enables resolution of the most luminous stars; constructing the colour-magnitude provides access to a number of distance indicators, including the tip of the first red giant branch (RGB). Red giants have completed the core hydrogen-burning main-sequence stage of evolution and have moved to burning hydrogen in an inner shell. The maximum luminosity in this phase, and hence the location of the tip of the RGB, is set when the core reaches a sufficiently high temperature to ignite helium burning, the so-called helium flash. At that point, hydrogen shell-burning is extinguished, the star contracts and moves onto the horizontal branch. The present program is using HST to target several SINGS spiral galaxies within 10 Mpc, including M74, M104 (the Sombrero), NGC 1291, M51, NGC 4559, NGC 4625 and NGC 5398.

GO 14076: An HST legacy ultraviolet spectroscopic survey of the 13pc white dwarf sample
GO 14077: The frequency and chemical composition of rocky planetary debris around young white dwarfs: Plugging the last gaps

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typical targets), they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years.In subsequent years, a significant number of DA white dwarfs have been found to exhibit narrow metallic absorption lines in their spectra. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk. The Cosmic Origins Spectrograph (COS) is an ideal instrument for probing the abundance of trace elements in white dwarfs atmospheres: more than 70 systems have been observed, with detection rates running at around 50%. The two programs highlighted here are using COS to refine the statistics for such systems. GO 14076 targets a volume-limited sample of 37 white dwarfs within 13 parsecs of the Sun, sufficient to provide an estimate of the overall occurence of accreting systems. GO 14077 builds on previous investigations on younger white dwarfs, targeting white dwarfs with cooling ages of 5-25 Myrs and 100-300 Myrs: models suggest that planetary collisions (and debris) should be more frequent at the younger ages, when the parent stars have just completed extensive mass loss on the AGB.

GO 14204: Multiple stellar populations in two young Large Magellanic Cloud clusters: NGC1755 and NGC1866

HST image of the LMC cluster, NGC 1866

Globular clusters are remnants of the first substantial burst of star formation in the Milky Way. With typical masses of a few x 105 solar masses, distributed among several x 106 stars, the standard picture holds that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. Until recently, the only known exception to this rule was the cluster Omega Centauri, which is significantly more massive than most clusters and has both double main sequence and a range of metallicities among the evolved stars. Over the past 5-10 years, Omega Cen has been joined by numerous other Galactic clusters, including NGC 2808, which shows evidence for three distinct branches to the main sequence, NGC 1851, 47 Tucanae and NGC 6752 - all relatively massive clusters. In almost all cases, the complexity of these systems has only emerged through the high precision observations possible with HST. Hubble is now being turned towards clusters in one of our nearest neighbours, the Large Magellanic Cloud. Previously, observations were obtained of the massive cluster, NGC 1846. The present program aims to extend coverage to two other clusters, NGC 1866 and NGC 1755. Both clusters are much younger than the halo Milky Way globulars, with ages of ~150 Myrs, but are similar in mass. The WFC3-UVIS camera will be used to obtain deep UV (F336W) and I-band (F814W) images to search for evidence of multiple populations.

GO 14138: Absolute Measurement of the Cosmic Near-Infrared Background Using Eclipsed Galilean Satellites as Occulters

Jupiter and the Galilean satellite Ganymede

The Cosmic Infrared Background is generally conjectured to represent the diffuse, redshifted light from star formation early in the post-recombination Universe. It provides an important link between the resolved structure that we see today and the primordial fluctuations measured by the cosmic mcirowave background. Measuring the CIB, however, is not a straightforward task, since there are several other sources of infrared radiation that dominate the measured fluxes, notably stars at near-infrared wavelengths, the zodiacal light at mid-infrared wavelengths and emission from Galactic cirrus in the far infrared. The present program proposes a novel mean of isolating the near-infrared contribution from one of those components, the zodiacal light. The WFC3 IR camera will be used to observe the two of the Galilean satellites during the period when they have entered the jovian shadow, and are therefore under a solar eclipse, but are still visible from Earth. The overwhelming majority of the zodiacal light is contributed by scattered light from dust particles between us and Jupiter; the Galilean satellites obscure any contribution to the near-infrared background from sources that lie beyond Jupiter's orbit, including contributions from the CIB. If the latter contributions are significant, then one would expect to see reduced flux (ie dark spots) in the satellite locations. The present observations target Europa and Ganymede during eclipse.