This week on HST

HST Programs: January 16 - January 22, 2017

Selected highlights

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 14114: A Wide-Field WFC3 Imaging Survey in the COSMOS Field

The full COSMOS field

Hubble has made significant contributions in many science areas, but galaxy formation, assembly and evolution is a topic that has been transformed by the series of deep fields obtained over the past 20 years. The largest area survey to date was conducted in Cycles 12 & 13,when the Advanced Camera for Surveys was used to obtain single filter (F814) images of 2 square degrees in 640 orbits. The resultant dataset, the COSMOS survey, has been surveyed subsequently from both ground and space, with data spanning all wavelengths from X-ray through optical and infrared to the sub-millimetre and millimetre regimes. The HST data are important in providing data with exquisite angular resolution, as well as depth, providing important morphological information on galaxies in the field. Multi-wavelength data at the same resolution can provide important information on star formation history and evolution, and the spatial distribution of dust. Additional HST observations have been obtained for subsets of the field (eg the CANDELs field), but the large areal span has made full coverage overly expensive. That is, until a new observing technique was devised for HST that enables observations of multiple distinct fields in a single orbit. In standard the observing scenario, HST moves to a field, then acquries a guidestar to ensure acurate pointing and stability during the observation; each guide star acquisition takes 4-5 minutes, and HST is generally restricted to 2 pointings at most. In the new mode, HST makes the initial guide-star acquisition, but then offsets to new fields. The telescope drifts without a guide star, but this can be dealt with for near-infrared wavelength imaging; the HST detectors use multiple non-destructive reads, and the telescope drifts by a very limited amount in the time between reads. The net result is that it is possible to cover the COSMOS fields with H-band observations in 57 orbits, adding high angular resolution near-infrared data to compleemnt the ACS I-band data.

GO 14648: A New Threshold of Precision, 30 micro-arcsecond Parallaxes and Beyond

HST WFPC2 image of NGC 4639, one of the Cepheid-rich spiral galaxies used to calibrate SNe Ia

The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. Previous HST programs have focused on identifying and measuring light curves for cepheids in external galaxies (eg GO 10802 , GO 11570 ) or quantifying the effects of variations in intrinsic stellar parameters, such as metallicity (eg GO 10918 , GO 11297 ). The present SNAP program is part of a suite of HST programs focusing on the Galactic Cepheids that form the foundation for the whole distance ladder. These programs employ a revived version of an old technique to determine both accurate astrometry, hence trigonometric parallaxes and reliable distances, and accurate photometry, hence flux emasurements. The technique is drift-scanning - tracking HST during the observation so that stars form trails on the detector. This mode of observations was available in the early years of HST's operations, and has been revived primarily as a means of obtaining high signal-to-noise grism spectroscolpic data of stars hosting transiting exoplanets. However, the same technique can be used in imaging mode, and the extended trails allow not only multiple measurements of position differences for stars in the field but also extremely high signal-to-noise photometry. The latter is crucial in obtaining direct photometry of tghe local calibrations on the same HST system, the same system that is being used for photometry of Cepheids in the external galaxies that serve as the basis for the distance scale. Observations have been obtained for more than 20 such stars. The present program aims to refine the distance estimates by obtaining four additional epochs for 9 core Cepheids (Z Sct, DD Cas, VX Per, SZ Cyg, SS CMa, XY Car, S Vul, X Pup and WZ Sgr). These data will improve the precision of the final parallaxes by identifying and eliminating binaries among ther eference stars, providing a longer baselien for proper motion determination, and providing direct overlap with Gaia observations.

GO 14752: Variability in the Escape of Water from Mars

HST images of Mars at opposition

Mars lies at an average distance of 228 million kilometres, or 1.52 AU, from the Sun, and has an orbital period of 687 days. With a diameter less than half that of Earth's, a corresponding mass 0.1 MEarth and no substantial magnetic field, Mars' atmosphere has been greatly depleted since its formation. Nonetheless, traces still remain, although the residual atmosphere is highly rarified. The overall composition is 96% carbon dioxide, with approximately 3.5% contributed argon and nitrogen and traces of oxygen and water. The MAVEN spacecraft, launched in 2013, has been in orbit round the planet since September 2014, and has been conducting in situ monitoring of the escaping gases. Hubble can also contribute, providing more global information. The present program aims to use ultraviolet imaging with the Solar Blind Camera on the Advanced Camera for Surveys to search for extended lyman-alpha emission that would indicate the presence of hydrogen escaping from the outer atmosphere.