This week on HST

HST Programs: June 24 - June 30, 2013

Selected highlights

GO 12873: Search for Planetary Mass Companions around the Coolest Brown Dwarfs

NICMOS images of the ultracool L/T binary, 2MASS J22521073-1730134; the northern component, notably fainter at F160W, is the T dwarf.

Brown dwarfs are objects that form like stars, but lack sufficient mass to drive the central temperature above a few million degrees, and therefore never succeed in igniting core hydrogen fusion. Discovered almost 15 years ago, these objects initially have surface temperatures of ~3,500K, but cool rapidly and move through spectral types M, L and T. Following their discovery, considerable theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T and beyond. This transition marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest this transition occurs at ~1400-1200K, and that the spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds"). The overall timescales associated with this process remain unclear. Recent discoveries from both wide-field ground-based surveys (such as UKIDDS, on the UK Infrared Telescope) and from space (notably by WISE, the Wide-field Infrared Survey Explorer) have resulted in the detection of significant numbers of extremely cool T dwarfs, and, indeed, a handful of dwarfs beyond T, reaching spectra type Y. Identifying even cooler systems in the field is extremely challenging, due to their extremely low luminosities. However, since around 15-20% of low-mass brown dwarfs appear to be binary (at least among spectral classes L and T), the known systems offer potential search locations for cooler, lower-mass companions. The present program is using imagign with the WFC3-IR camera in the F127M and F138M filters to search for such systems among 34 recently discovered very cool (>78) brownd warfs.

GO 12898: Discovering the Dark Side of CDM Substructure

WFC3 image of the SDSS Einstein ring glaxy, LRG 3-757

Most current models of galaxy formation are tied to Lambda-CDM cosmology. That scheme makes predictions of the expected density profile and density stucture (specifically, the clumpiness of sub-structure) of the dark matter halos that underlie visible galaxies. Specifically, the models predict extensive sub-structure, and that prediction is clearly at odds with the number of low-mass satellite galaxies that are observed in close proximity to major galaxies like the Milky Way or Andromeda. This is the well known "missing satellite' problem, and a variety of explanations have been put forth to explain why one might have a large population of dark matter clumps that somehow failed to acquire a correspondingly substantive quantity of baryonic matter. The present program aims to probe the dark matter distribution directly through gravitational lensing. The WFC3-UVIS camera will be used to target highly magnified Einstein rings and arcs produced by foreground elliptical galaxies lensing background star-forming systems. The observations aill be made at ultraviolet wavelengths (specifically, with the F390W filter) where the background galaxy has most structure. The goal is to match the lensed image against models, and use the level of surface brightness aberrations to constrain clumpiness in the dark-matter halos of the foreground lensing galaxy.

GO 12982: Are the Milky Way's High Velocity Clouds Fuel for Star Formation or for the Galactic Corona?

A map of the high velocity cloud systems surrounding the Milky Way (B. Wakker, U. Wisconsin).

The stellar components of the Milky Way Galaxy are well known: the disk, the central bulge and the old, metal-poor stellar halo. However, the Milky Way is also surrounded by a halo of hot, gas that is itself embedded within a much more tenuous corona of even hotter, ionised gas. Within that structure lie high velocity clouds. Originally discovered in the 1930s as absorption features in stellar spectra, these clouds have velocities that differ significantly from the rotational velocity along that line of sight, and they are generally believed to be undergoing infall into the Galaxy. The origin and nature of these systems remains uncertain, with some favouring a Galactic origin, driven by star formation and feedback between disk and halo, and others supporting their origin within the warm-hot intergalactic medium. HVCs are not self luminous, so indirect methods need to be applied to examine their characteristics. The most effective is to identify stars that lie behind individual systems and, as with their discovery in the 1930s, search the stellar spectra for signature absorption lines produced by material within the cloud. Many, indeed most, of the key absorption features lie at ultraviolet wavelengths, a spectral region that has been opened up with the installation of the Cosmic Origins Spectrograph on HST. The present observations build on a Cycle 17 program that used COS to obtain spectra of distant halo stars aligned with a subset of the known HVCs within the Milky Way. The results indicate that HVCs are streams of gas in the lower halo, ruling out several models for their formation and evolution. The current observations will probe several known features for structure closer to the Disk by using COS to target stars significantly closer to the Sun.

GO 13025: Unveiling the progenitor of the most luminous supernova

Chandra X-ray image of G292.0+1.9, a ~3000-year old supernoa remnant

Supernovae are generally believed to originate through two mechanisms: accretion onto a white dwarf in a close binary system, driving the white dwarf above the Chandrasekhar limit; and the implosion of the core of very massive (> 7 solar masses) stars. The resultant supernovae have distinctive spectral characteristics: in the former case, type I supernovae, the spectra lack any hydrogen lines and the majority have strong absorption due to ionised silicon and are classified as type Ia; in the latter case, Type II, hydrogen features are present in the spectra, which show very broad emission lines. Both processes result in explosive nucleosythesis that enriches the interstellar medium, with the ejecta forming a rapidly expanding shell. In general, the luminosities of these explosive events fall within well-defined limits. However, recent years have seen the identification of several highly luminous Type II supernovae in nearby galaxies. This program aims to set limits on the likely progenitors of these unusual stars by using the UVIS channel on Wide-Field Camera 3 to obtain high-resolution images of the appropriate regions within the galaxies. Those data will be used to derive colour-magnitude diagrams for the local stellar populations, and hence constrain the likely age distribution and progenitor mass.