This week on HST

HST Programs: October 8 - October 15, 2012

Selected highlights

GO 12202: Wide-Field Hubble Observations of NGC 1023: Testing the Origin of Low-Mass X-ray Binaries in a Lenticular Galaxy

The lenticular galaxy, NGC 1023

Low-mass X-ray binaries are generally believed to be binary systems where a compact, high-mass component, either a black hole or a neutron star, is accreting mass from a lower-mass component that is overflowing its Roche lobe. These systems are extremely luminous at short wavelengths, and are readily detected at X-ray wavelengths in nearby galaxies. The present program targets the lenticular galaxy, NGC 1023, the brightest member of the eponymous small group lying at a distance of ~10 Mpc. The Chandra satellite will be used to obtain a deep (~200 ksec) exposure that is expected to reveal ~70 field LMXBs, together with several hundred star clusters. Deep ACS imaging will be used to not only search for optical counterparts to the ray sources, but also to characterise the spatial distribution of globular clusters and open clusters.

GO 12612: Probing Weak Intergalactic Absorption with Flaring Blazar Spectra

Probing the intergalactic medium via absorption lines imprinted on QSO spexctra

One of the key issues facing modern cosmology is the "missing baryon" problem. In brief, a census of all the constituents in the local universe accounts for less than half of the baryonic mass expected based on measurements of the fractional abundanmce of deuterium and observations of the cosmic microwave background. It is generally believed that the missing material lurks in the form of extremely hot gas in the intergalactic medium. The most effective means of probing that medium, and testing this hypothesis, is to search for the appropriate absorption lines in the spectrum of a background source. QSOs are particularly effective cosmic searchlights, since they have strong continuum flux levels at the ultraviolet wavelengths where most of the important absorption lines fall. Following SM4, and the installation of the Cosmic Origins Spectrograph (COS), HST is now well equipped to tackle this issue. COS is incredibly sensitive, but the detection limits for individual species depend on photon statistics, and hence on the brightness of the backgropund source. The present program aims to capitalise on the intrinsic variability of QSOs by targetting particularly variable sources, blazars, durign otuburst. The resulting high S/N spectra will provide strong constraints on weak spectral signatures, such as broad (high velocity disperson) features due to Lyman Alpha or O VI. The present observations target 3C 66.

GO 12972: In Search of the Coldest Atmospheres: Identifying Companions to the Latest WISE Brown Dwarfs

The stellar menagerie: Sun to Jupiter, via brown dwarfs

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which do not accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion. As a result, these objects, which have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and they fade and cool on relatively short astronomical (albeit, long anthropological) timescales. Following their discovery over a decade ago, considerable observational and theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes. At their formation, most brown dwarfs have temperatures of ~3,000 to 3,500K, comparable with early-type M dwarfs, but they rapidly cool, with the rate of cooling increasing with decreasing mass. As temperatures drop below ~2,000K, dust condenses within the atmosphere, molecular bands of titanium oxide and vanadium oxide disappear from the spectrum to be replaced by metal hydrides, and the objects are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At present, the coolest T dwarfs known have temperatures of ~650 to 700K. At lower temperatures, other species, notably ammonia, are expected to become prominent, and a number of efforts have been undertaken recently to find examples of these "Y" dwarfs. The search is complicated by the fact that such objects are extremely faint instrinsically, so only the nearest will be detectable.Wide-field surveys have been undertaken at infrared wavelengths with both ground-based telescopes (eg UKIDDS) and satellite observatories. Most recently, the Wide-field Infrared Survey Exoplorer, WISE satellite mission, completed an all-sky survey and succeeded in identifying several tens of late-T and Y dwarfs. As a complement tot he wide-field approach, one can "look under the lamp-post": both stars and brown dwarfs are often found as binary or multiple systems, so one can take a sample of low-mass obejcts known to be within the Solar Neighbourhood, and look for even lower luminosity companions. That technique served in the past to identify van Biesbroeck 10, the first ultracool dwarf; GD 165B, the first L dwarf; and Gl 229B, the first T dwarf. The present program is applying the latter technique to the results of the WISE survey: thirteen brown dwarfs with spectral types T8 or later are being targeted for observation with WFC3-IR (J and H bands), with the aim of detecting even lower luminosity (and lower mass) companions.

GO 12870: The mass and temperature distribution of accreting white dwarfs

An accreting white dwarf starn in a close binary system

Supernovae are the most spectacular form of stellar obituary. Since B2FH, the physical processes underlying their eruptive deaths have been known to play a key role in populating the ISM with metals beyond the iron peak. More recently, these celestial explosions have acquired even greater significance through the use of Type Ia supernovae as distance indicators in mapping the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae (runaway fusion on the surface of a white dwarf in a binary system for Type Ia, or detonation of the core in Type II), some significant uncertainties remain concerning the physical details of the disruption, and, potentially, the overall uniformity of these events. Consequently, there is potential for systematic bias in the distance estimates. The present program aims to set constraints on the various mechanisms associated with white dwarf stars by investigating the rotational properties of over 40 degenerate companions in catclysimic variable systems. All of these systems are in the process of accreting material from the companion star, as the latter voerflows its Roche lobes. The program aims to obtain ultraviolet spectra with the Cosmic origins Spectrograph, probing both the spin rates and the orbital parameters. Over the next decade, these data may lead to the determinaton of reliable masses for both stars once accurate parallax measurements become available from Gaia.