This week on HST

HST Programs: March 16 - March 22, 2009

Selected highlights

GO 11343: Identifying the host galaxies for optically dark gamm-ray bursts

Artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts. The long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are coalescing binary systems (probably pairs of neutron stars or black holes). In both cases, the high energies observed are believed to be due to beaming of the radiation. The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature. Since then, more than 50 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. The present program focuses on the subset of GRBs which have no optical counterparts. These are likely to be highly obscured, and may originate in different environments. The proposers use Chandra to obtain precise positions of the GRB, and deep follow-up HST observations to search for the host galaxy.

GO 11944: Binaries at the Extremes of the H-R Diagram

The MV-mass relation for low-mass stars (from T. Henry)

The mass-luminosity relation remains one of the key underpinnings of stellar astrophysics, notably in probing the grey area that separates hydrogen-burning stars from cooling-powered brown dwarfs. The calibration of thsi relation rests on observations of binary systems, primarily eclipsing binaries at masses above 1 MSun, and primarily astrometric binaries at sub-solar masses. In the latter case, reliable mass determinations require orbital measurements of extremely high accuracy, which, in turn, requires high precision astrometry over at least one orbital period. The Fine Guidance Sensors on HST have proven invaluable in this regard, since they permit both the detection of closely-separated binary systems, and sub-milliarcsecond accuracy astrometry of resolved binary systems with sub-arcsecond separations; in other words, HST allows measurement of nearby, low-mass binaries with periods short enough to allow completion of the observations in significantly less than an astronomer's lifetime. The current program is using the FGS to search for new binary systems among the more extreme constituents of the HR diagram: specifically, massive, luminous O stars; nearby lower main-sequence K and M dwarfs and subdwarfs; and cool white dwarfs. Suitable binary systems detected through these initial observations will be targeted for detailed monitoring in subsequent observational programs

GO 11983: An imaging survey of protoplanetary disks and brown dwarfs in the Chamaeleon I region

Multi-colour image of the Chamaeleon I region

Chamaeleon I is a star-forming region that lies within a molecular cloud complex at a distance of ~150 parsecs from the Sun. The young cluster has been the subject of extensive ground-based observations, which have succeeded in identifying over 200 members, spread over an area of ~0.5 square degrees, with masses ranging from 2-3 solar masses to below the hydrogen burning limit. The cluster is much less massive than the Orion Nebula Cluster (ONC), and appears to break into two sub-units. Matched against theoretical isochrones, the colour-magnitude data suggest that the cluster has an age comparable to the ONC, with estimates of 3-4 Myrs for the southern sub-unit and 5-6 Myrs for the northern. This cluster therefore presents an interesting opportunity to investigate the extent to which environment plays a role in determining the properties of stars and brown dwarfs. The present program aims to focus on the lower mass members, using WFPC2 to obtain high resolution images that can probe the structure of protoplanetary disks and search for jets and low-mass binary companions.