This week on HST

HST Programs: March 21 - March 27, 2016

Selected highlights

GO 14157: Testing Model Atmospheres with the Coldest Known Brown Dwarf

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 MJup, 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 completed an all-sky survey in 2011 (and is currently being employed in a search for Near-Earth Objects, NEOWISE). WISE has identified several Y dwarfs, including several with temperatures lower than 350K. The most interesting source was discovered by Kevin Luhman (Penn State), WISE0855-071, a brown dwarf lying only 2.2 parsecs from the Sun with a surface temperature around 250 K. The object is extremelty faint and is detectable only by virtue of its proximity. Previously, Hubble obtained J-band imaging with the F110W filter on WFC3-IR; the present program aims add detections in the far red (F850LP) with ACS and near-infrared (F105W and F127M) with WFC3-IR.

GO 14163: Honing Type Ia Supernovae as Distance Indicators, Exploiting Environmental Bias for H0 and w.

Supernova in M101

Supernovae have long attracted the attention of both amateur and professional astronomers as a means of studying the violent eruption and death of massive stars and degenerates. However, in the last decade they have also acquired considerable importance as distance indicators, tracing the expansion of the universe to redshifts well beyond the reach of more conventional yardsticks, such as cepheids, and providing a key underpinning for the hypothesised existcen of dark energy. Understanding the supernovae themselves, and, in particular, their progenitors, is key to accurately interpreting their luminosities and distances. Recent observations have suggested that there may be a correlation between the brightnesses of Type Ia supernovae and some characteristics of their local environment; specifically, supernovae found in close proximity to star formation appear to be sub-luminous. The present SNAP program uses the UVIS camera on WFC3 to image the locations of supernovae that have been used to map the Hubble flow, and will use the UV-to-optical flux ratio to characterise the local environment, potentially offering a means of tightening the distribution in the Hubble diagram.

GO 14199: Refsdal Redux: Precise Measurements of the Reappearance of the First Supernova with Multiple Resolved Image

Finding chart for the multiply imaged supernova, SN Refsdal, discovered in November 2014 in cluster MACJ1149

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters, providing information on the mass distribution within the lensing cluster. Over the past three cycles, Hubble has been undertaking deep imaging observations 6 galaxy clusters as the Frontier Fields Director's Time program (GO 13495/13496). Those observations have provided a basis for several synergistic programs. In particular, the observations enabled a search for supernovae at high redshifts, z> 1.5, aiming to set further constraints on dark energy and probing the frequency of supernovae as a function of redshift, the delay time and hence the likely progenitors. In 2014, observations of the fourth cluster, MACSJ1149.5+2223, resulted in the detection of a particularly unusual object - multiple lensed images of a supernova in a redshift z=1.49 galaxy that is itself multiply lensed. Each of those images results from light following a different path due to the gravitational potential of the foreground cluster and galaxies. Dubbed Supernova Refsdahl, after the gravitational lensing pioneer, the original detections were followed over the course of their fading. But, more spectacularly, models of the cluster potential and the consequent light paths led to a prediction that the supernova should appear in one of the other lensed images of the parent galaxy in late 2015. The present program set out to check those predictions by re-imaging the cluster. Update: the supernova was not present in observations obtained on November 14, 2015, but has been detected in the December 11 observations, thus representing the first time that a supernova has been "predicted" successfully.Further observations are being obtained to monitor the light curve.

GO 14474: Comet P/2010 V1 fragmentation event

Comet P/2010 V1 Ikeya-Murakami shortly after its disovery in 2010

Comet P/2010 V1 is a member of the family of short period comets that live in the inner Solar System. Discovered near perihelion in 2010 by the Japanese astronomers Kaoru Ikeya and Shigeki Murakami, the comet has an orbital period of 5.49 years, an orbital ecentricity of 0.49 and comes within 1.58 AU of the Sun at perihelion. Shortly after its discovery, the comet was observed to undergo significant brightening, suggesting a major burst of activity. The comet was recovered in late December 2015, and observations revealed that the system had fragmented, with 4 components identified from deep ground-based observations by early January. The present program aims to take advantage of Hubble's unparalleled resolution at optical wavelengths and probe the number and size distribution of the fragments, with the aim of setting constraints on the likely mechanism driving this disruptive event.