This week on HST

HST Programs: May 2 - May 8, 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 14181: A Snapshot WFC3 IR Survey of Spitzer/Hershel-Identified Protostars in Nearby Molecular Clouds

An image of the Orion Nebula superimposed on the 13CO map of Orion A (from this link ).

Giant molecular cloud complexes serve as nurseries for star formation. Deeply embedded in dust and gas, young stars are generally extremely difficult to detect at optical wavelengths. Consequently, these complexes have been subject to extensive scrutiny at near- and mid-infrared wavelengths, initially through ground-based observing campaigns and more recently by the Spitzer and Herschel space missions. Those observations have resulted in the identification of numerous embedded sources, young stellar objects (YSOs) that are still accreting from the surrounding molecular gas .he present proposal aims to follow up on those discoveries by obtaining WFC3-IR SNAPs of candidate protostars in several molecular cloud complexes. These observations will provide an excellent complement to Spitzer and Herschel since, while HST cannot offer either the same areal coverage or sensitivity at mid-infrared wavelegths, the imaging has a resolution close to 0.1 arcsecond, an order of magnitude higher than the Spitzer images. The observations are therefore capable of detecting very faint companions, with luminosities consistent with sub-stellar masses, as well as identifying jets and outflows associated with the star formation process. The present program is using the F160W filter to obtain H-band images and determine the true nature of these objects.

GO 14268: Project AMIGA: Mapping the Circumgalactic Medium of Andromeda

The extent of Andromeda's gaseous halo, as sampled by COS

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore depends on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. Extensive observations of galaxies at modest redshift (0.15 < z < 0.35) have shown that material extends to radii of hundreds of kpc, with a total mass in metals that is at least comparable with the mass in the central galaxy. Andromeda, the nearest large spiral to the Milky Way,provides an unparalleled opportunity to probe the detailed structure of the gaseous halo. The present program will target background QSOs along 18 sightlines at radial distances between 25 and 330 kpc. from Andromeda's nucleus. Combined with archival data for 7 other background targets, these data will be sensitive to a wide range of key species OI 1302, CII 1334, SiII 1190, 1193, 1523, SiIII 1206, SiIV 1393,1402, CIV 1548, 1550), probing the composition and ionisation of the halo.

GO 14500: Two Birds One Stone: Simultaneous Atmospheric Pre-Screening of Two Temperate Earth-Sized Exoplanets During Their Double Transit

An artist's impression of the view from the third planet in the Trappist 1 system

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Transiting systems provide particularly powerful insight into exoplanet characteristics since they provide a direct measurement of size (at least relative to the star) as well as an opportunity to probe the atmospheric composition and temperature structure via the transmission and emissive spectra. Over the following decade the number of known systems grew slowly through extensive ground-based campaigns, but the launch of Kepler in March 2009 marked a turning point. In its four year survey, Kepler has identified more than 1030 confirmed transiting exoplanets, with a further ~4700 candidates and the potential for more discoveries through increasingly detailed analysis of the archival data. Ground-based surveys can still play a major role, however, as illustrated by the recent discovery by the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) team of at least three terrestrial planets orbiting the nearby ultracool dwarf, 2MASS 2306-0502, also known as TRAPPIST 1. This team is one of several that are using relatively small telescopes to monitor nearby M dwarfs for planetary transits. Lower mass and cooler than solar-type stars, M dwarfs constitute approximately 70% of the stars in the Galaxy, and are therefore likely to host most of the planets near the Sun. Since they have substantially smaller diameters than solar-type stars, terrestial planets have a larger covering factor and are therefore easier to detect. Moreover, the cooler stellar temperatures mean that the Habitable Zone lies closer to the parent star, and the orbits are correspondingly shorter. In the case of 2MASS 2306-0502, an M8 dwarf, the three known planets have periods of 1.5, 2.4 and more than 4.5 days, with surface temperatures that have been compared with Venus, Earth and Mars. The present program will use the G141 grism on WFC3 to obtain near-infrared spectra as the two innermost planets transit the parent star.