This week on HST

HST Programs: August 22 - August 28, 2016

Selected highlights

GO 14076: An HST legacy ultraviolet spectroscopic survey of the 13pc white dwarf sample

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 present program is using COS to refine the statistics by targeting a volume-limited sample of 37 white dwarfs within 13 parsecs of the Sun. This sample is sufficient to provide an estimate of the overall occurence of accreting systems.

GO 14212: A Snapshot Imaging Survey of Spitzer-selected Young Stellar Objects in Nearby Star Formation Regions

HST image of the face-on debris disk in the G2 dwarf, HD 107146

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The structure of those disks is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems. HST currently provides almost the only means of achieving the high-contrast required for the detection of scattered light from these disks in the presence of the bright parent stars. While many such systems have been observed, only a relatively small number of disks have been imaged successfully at visual or near-infrared wavelengths. The present SNAPSHOT program aims to expand the sample by targetting sources within nearby star-forming regions that have past Spitzer observations that indicate a significant infra-red excess, suggesting the presence of a circumstellar disk.The Advanced Camera for Surveys will be used to obtain images in the V and I-bands (F606W and F814W filter), providing high resolution data that can be analysed for direct evidence of disks or companions.

GO 14227: The CANDELS Lyman-alpha Emission At Reionization (CLEAR) Experiment

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

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. CANDELS, one of three Multi-Cycle Treasury Program executed in cycles 18 through 20, is one of the more recent additions to this genre.Building on past investment of both space- and ground-based observational resources, it covers five five fields including both the Great Observatory Origins Deep Survey (GOODS), centred on the northern Hubble Deep Field (HDF) in Ursa Major and the Chandra Deep Field-South in Fornax. In addition to deep HST data at optical and near-infrared wavelengths, the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton; at mid-infrared wavelengths with Spitzer; and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Surbaru and the ESO VLT. This represents an accumulation of almost 1,000 orbits of HST time, and comparable scale allocations on Chandra, Spitzer and ground-based facilities. CANDELS added new optical and near-infrared observations with WFC3 and ACS (see this link for more details). Those data have been processed and analysed by both the CANDELS team and by other groups within the community. The present program builds on this foundation by adding 16 pointings within the CANDELS fields with the WFC3 G102 grism. The goal is to probe reionisation by measuring the strength of Lyman-alpha absorption in galaxies at redshifts between z=6.5 and z=8.2. The expectation is that the ovall absorption strength should decrease with decreasing redshift as the intergalactic medium is ionised, and the proportion of neutral gas decreases.

GO 14332: Precision Measurement of the Mass of the Cepheid V350 Sgr

The radial velocity curve for V350 Sgr (from ground-based observations)

V350 Sgr ia a classical Cepheid variable lying at a distance of a few hundred parsecs from the Sun. Crucially, the variable is in a binary system with a hot (~B-type) companion orbiting with a period of ~4 years. Ground-based observations suggest that the Cepheid has a mass of ~5.8 solar masses and the companions a mass of ~2.5 MSun, but the uncertainties are significant. At optical wavelengths, the combined flux is dominated by the cooler Cepheid in these systems, but hot companion is detectable at ultraviolet wavelengths. The present program aims to use STIS to obtain ultraviolet spectra that will enable measurement of the radial velocity curve for the hot companions in these system. Those observations will be combined with existing ground-based measurements of the Cepheid radial velocities, therefore determing velocity curves for both stars. Combining those data with high-precision astrometric measurements from ground-based interferometry will allow a direct measurement of mass and distance for these systems.