This week on HST

HST Programs: February 17 - February 23, 2014

Selected highlights

GO 13282: A Search for Surviving Companions of Type Ia Supernovae in the Large Magellanic Cloud

Combined HST and Chnadra imaging of the supernova remnant SNR 0509-67.5 in the LMC

Type Ia supernovae are generally believed to be produced by the explosive deflagration of white dwarf star that exceeds the Chandrasekhar due to accretion from a binary companion, either a hydrogen-burning main-sequence/red giant star or another degenerate. In double degenerate systems, the white dwarfs merge and explode; in the single degenerate mode, the non-degenerate companion is likely to survive, albeit in a stripped, ablated form. Besides providing crucial information on stellar evolution and how stars enrich the interstellar medium, Type Ia supernovae have acquired global importance in recent years through their use as distance indicators. Indeed, these objects played a crucial role in identifying dark energy and the accelerating universe. In that context, it is important to obtain a better udnerstanding of the underlying mechanism and the distribution of intrinsic properties of these exploding stars.Relatively nearby supernovae that can be probed in detail are therefore crucial to the large mapping of the cosmic flow. The present program aims to probe whether the double degenerate or single degenerate mode of formation is more important by using HST imaging to search for survivors in a sample of nine supernova remnants in the LMC. If the double degenerate mechanism is the dominant mechanism, then few survivors of the progenitor explosion will be found.

GO 13330: Mapping the AGN Broad Line Region by Reverberation

Simulations of the appearance and velocity structure within an AGN disk (see

Active galaxies (AGNs) are generally luminous systems, characterised by the presence of strong nuclear emission lines of numerous species including H, He I, He II, and Fe, Ca, O, C and S over a range of ionisations. These features originate from gas clouds in the nuclear regions, with the energy supplied through accretion onto a central massive black hole. The high-temperature, rapidly-rotating gas clouds nearest the central engine are responsible for producing broad emission lines (hence, the "Broad Line Region"). The structure of the BLR can be discerned using a technique known as reverberation mapping: variations in the accretion rate lead to fluctuations in luminosity; those variations lead, in turn, to variations in the photoionisation of the BLR, and corresponding changes in spectral line strengths and velocities; monitoring those changes, and correlating them with the photometric variability of the central source, measures the light travel time from nucleus to BLR gas, and hence maps the size of the BLR. The present prorgam will use the Cosmic Origins Spectrograph to undertake systematic monitoring of the nuclear regions of the Seyfert I galaxy, NGC 5548. The observatons are spread over the next 6 months, with one orbit per day for 179 days.

GO 13406: Hydrogen, Deuterium and Nitrogen in the Beta Pictoris disk

The extended disk around Beta Pictoris

It is now well established that planet formation occurs within circumstellar disks.The past decade has seen the identification of many examples of such phenomena around young stars, notably through infrared observations by the Spitzer space telescope and ground-based data from millimetre-wave interferometers.. The general indications are that the planet-formation process occurs rapidly, with the disks largely dissipating within 10-15 million years of formation. Key questions that remain, however, include the rate of gas depletion within the disks, the extent to gas survives within the debris disks and the abundance distribution of any residual gaseous materials. Most examples of circumstellar disks are found in stars in young star-forming associations, but there are a number among isolated, young stars in the Solar neighbourhood. The most extensively studied system is beta Pictoris, an A star at a distance of ~ 19 parsecs and an estimated age of ~10-20 million years. The disk was originally discovered in the early 1980s and has since been observed extensively from both the ground and space, including coronagraphic observations with STIS and direct imaging with WFPC2, ACS and WFC3 on HST. The present program aims to build on those past observations by using the Cosmic Origins Spectrograph to probe the gas content of the system, specifically searching for ulrtaviolet signatuers of neutral hydrogen, deuterium and nitrogen.

GO 13428: Characterizing the Ultra-cold Brown Dwarf WD 0806-661B

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. One approach that has long-standing success is to look for companions to stars already known to lie near the Sun; the first ultracool M dwarfs (VB 8, VB 10), the first L dwarf (GD 165B) and the first T dwarf (Gl 229B) were all discovered through this method of fortunate association. The present program targets a ~Y2 dwarf that, like GD 165B and Gl 229B, happens to be a companion of a white dwarf in the Solar neighbourhood. The obejct was found seredipitously from multi-epoch Spitzer data. WFC3 on HST will be used to obtain photometry at near-infrared wavelengths. be used