This week on HST

HST Programs: August 27 - September 2, 2012

Selected highlights

GO 12450: Understanding a new class of mid-IR transient

SN 1999bw in NGC 3198, one of the transients targeted by this proposal

Searching for transient objects, particularly supernovae, novae, cataclysmic variables and flare stars, has factored prominently in astronomy since at least the cataloguing of "guest stars" by the Chinese over 2000 years ago. For most of the 19th and 20th centuries, this field was largely the pursuit of amateur astronomers, coupling visual scans of the heavens with pesonal encyclopaedic knowledge of star patterns. However, the development of large-format, highly-sensitive digital imaging devices oevr the past decade or more has opened the subject for investment by professional observatories. Several transient surveys are currently underway, notably the Berekeley Automated Supernova Survey, using the Leischner Observatory in Lafeyette, california; the Palomar Transient Factory, using the 48-inch Oschin Schmidt of Palomar Sky Survey fame, now equipped with multiple CCDs; and Pan-STARRS, a dedicated survey telescope operating on Haleakala. Further down the road lies LSST, the Large Synoptic Survey Telescope, the highest ground-based priority from the Astro2010 Decadal Survey. In the meantime, the increasingly extensive catalogues generated by the ongoing surveys have turned up a variety of unusual transients. The present program focuses on one such example, a set of luminous spernova-like transients, detected in external galaxies, where pre-outburst observations show no evidence for a progenitor. The hypothesis is that these originate in highly obscured AGB stars. This program couples WFC3-IR J and H observations with Chandra and Spitzer measurements to probe the underlying nature of these sources.

WFC3 image of star forming regions in M83

M83 is a grand design barred spiral galaxy lying almost face-on at a distance of ~4.5 Mpc from the Milky Way. Lying in the southern constellation of Hydra, the galaxy was originally discovered by the French astronomer Nicolas Louis de LaCaille from observations at the Cape of Good Hope in 1752, before being catalogued by Messier in 1781. The galaxy lies towards the centre of one of the two sub-groups of the Centaurus A association, and has a total luminosity estimated as ~2 x 1010LSun, or comparable with that of the Milky Way. As a nearby galaxty, M83 has been well observed by both amateurs and professonal astronomers, and has been the site of six SNe over the last century (SN 1923A, 1945B, 1950B, 1957D, 1968L and 1983N). At least four of these supernovae are classed as Type II, indicating their origin as massive stars (SN45B has few observations and SN1983N was a peculiar type I) and testifying tot he extensive star formation currently under way within the galaxy. The present program aims to investigate the global star-formation characteristics through combining multi-band WFC3-UVIS observations with a deep (750 ksec) imaging observatons with the Chandra x-ray satellite. The multi-colour observations will enable fine-scale age-dating of the star forming regions, potentially enabling spatial mapping of the star-formation history. Similarly, the HST data will enable investigation of the immediate environment of the many X-ray binaries detected in the Chandra observations. Taken together, these data will provide a valuable archival reference for investigating future SNe detected within this near neighbour of the Milky Way.

GO 12544: Confirming Ultra-cold (Teff < 500K) Brown Dwarf Suspects Identified with WISE

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. Identifying such ultra-ultracool dwarfs was a goal of the WISE satellite mission, which recently completed its all-sky survey. WISE has succeeded in identifying a number of extremely interesting sources, including at least 4 objects that have been confirmed as dwarfs with temperatures lower than 350K. These are among the first examples of Y dwarfs. The current program is combining WFC3-grism imaging with warm-Spitzer photometry to verify the nature of further candidates.

GO 12586: Detecting Isolated Black Holes through Astrometric Microlensing

A rather spectacular version of black hole lensing.

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The expected astrometric signal from a black hole lens is > 1.4 millarcseconds, just measureable with HST. This program aims to capitalise on this fact by searching for lensing by black holes in the Galactic field. The observations target long-duration lensing events in the Galactic Bulge.