This week on HST

HST Programs: October 5 - October 11, 2015

Selected highlights

GO 13773: H-alpha LEGUS: Unveiling the Interplay Between Stars, Star Clusters, and Ionized Gas

UGC 4305 = Holmberg II - Arp 268, one of the star-forming galaxies targeted by LEGUS

Understanding the global architecture of star formation is a key step towards understanding the morphological evolution of galaxies and the characteristics of the underlying stellar populations. HST has devoted extensive resources to observations of nearby galaxies over its lifetime, including detailed surveys of a handful of systems, notably the PHAT survey of M31, with the enhanced imaging capabilities made available following SM4. Most programs, however, have focused on optical, far-red and, to a lesser extent, near-infrared wavelengths. While those observations provide high-quality colour-magnitude data that enable an exploration of relatively mature populations, they are less effective at probing active star-formation sites populated by young, high-mass stars. Those regions are most prominent at ultraviolet wavelengths. The HST LEGUS capitalises on the past heritage of HST observations by adding near-UV imaging for 50 nearby galaxies, drawn from a catalogue of 400 systems within ~11 Mpc of the Milky Way. They have been selected to provide a fair sampling of the wide variety of galactic systems within that volume. LEGUS itself is imaging star-forming regions at near-UV and blue wavelengths using the WFC3 UVIS channel with the F275W, F336W and F438W filters, supplemented by F55W and F814W (V and I) where necessary. The present program builds on that foundation by adding WFC3 narrow-band (F657N) H-alpha imaging of 32 galaxies from the sample.

GO 14149: Continuing a Snapshot Survey of the Sites of Recent, Nearby Supernovae

A recent supernova in M100

Supernovae mark the (spectacular) evolutionary endpoint for a subset of stellar systems. Standard models predict that they originate from massive stars and (probably) close binaries with a compact (WD, neutron star) component, but there are still some questions remaining over whether we fully understand the range of possible progenitors. The last decade has seen the development of a number of large-scale programs, usually using moderate-sized telescopes, that are dedicated to monitoring (relatively nearby galaxies, searching for new supernovae. This program builds on observations taken in several previous cycles, and aims to obtain follow-up multi-waveband images of nearby galaxies, focusing on the sites of recent supernovae. The program concentrates on systems within 20 Mpc of the Milky Way. The observations are taken well after maximum, with the aim of using the unparalleled angular resolution of WFC3 to identify the fading remnant, search for evidence for light echoes,characterise the local stellar population and perhaps determine the nature of the likely progenitor.

GO 14241: Cloud Atlas: Vertical Cloud Structure and Gravity in Exoplanet and Brown Dwarf Atmospheres

Ground-based imaging the the very low-mass brown dwarf binary, 2MASS1207

Recent years have seen the discovery of numerous extrasolar planets. Initially, most were discovered through radial velocity monitoring; more recently the superb photometric properties of the Kepler mission have led to the discovery of numerous transiting systems. Lagging behind in numbers, but offering the greatest prospect for probing physical conditions, is direct imaging. A handful of systems with resolved planetary companions have been discovered, most notably the multi-planet system around the nearby A-type star, HR 8799, and 2MASS1207B, the planetary-mass wide companion to the brown dwarf, 2MASS1207A, a member of the TW Hydrae association. These systems are expected to have gross properties that are similar to the Solar System gas giants, particularly atmospheric cloud structure. but they are much more massive and therefore have higher gravities. The present program aims to gain insight into their structure by comparing resolved exoplanets and higher mass, but similar temperature, brown dwarfs. Observations of isolated brown dwarfs have shown evidence for systematic variations in brightness, possibly due to dust within the atmosphere forming clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. The clouds themselves may appear and disappear over relatively short timescales, leading to photometric variations at particular wavelengths. Past programs have used both Spitzer and HST to monitor spectral variability in a number of systems. The present program will use Wide-field Camera 3 to obtain time-series near-infrared grism spectra of ten brown dwarfs and high-mass exoplanets. Variations in those spectra map the atmospheric cloud structure in each system, and those variations can be examined for correlations with the mass/gravity of the parent object.

GO 14263: Hubble imaging of a newly discovered active asteroid

The multiple nuclei of the main belt comet P/2013 R3 as imaged by Hubble in late 2013 and 2014

The term 'comet" is generally associated with low-mass, volatile-rich solar system objects that spend most of their life at very lage distances from the Sun, plunging only rarely into the inner regions where they acquire extended tails due to outgassing. Sometimes those obejcts are captured into short-period, eccentric orbits, leading to rapid depletion of the volatile content in rapidly-successive perihelion passages. However, recent years have seen growing evidence of another class of cometes exist: comets with near-circular orbits that place them between Mars and Jupiter, within the realm of the Main Belt of asteroids. One of the first candidate main belt comets, as these objects have been dubbed, is the asteroid Scheila. Discovered by the Heidelberg astronomer August Kopff in 1906, and named after an English student with whom he was acquainted, this is one of the larger known asteroids, with a diameter estimated as ~110 km. Early December 2010, Steve Larson (of Arizona's Lunar and Planetary Laboratory) noted that Scheila had sprouted a coma halo in observations taken by the Catalina Sky Survey. An examination of archival images revealed no evidence for activity throughout October and November, but a possible onset on December 3rd. The asteroid 1979 OW7/1996 N2 exhibited similar behaviour in 1996 and again in 2002; the initial outburst was ascribed to a collision, but the second event suggests that the activity is intrinsic rather than externally stimulated. More recently, the Pan-STARRS survey has contributed several objects, including the asteroid 2006 VW139, imaged during an outburst, MBC 2013-P5, which has exhibited a spectacular set of dusty tails, and MBC P/2013 R3, which has multiple "nuclei" that are gradually drifting apart at speeds of less than 1 metre/second. The present observations target the asteroid 324P/La Sagra. Discovered in August 2007 from Observatorio Astronomico de La Sagra in Granada, this small object has an orbit that takes it from 2.979 at aphelion to 1.9096 AU at perihelion, with a period of 3.82 years. In 2010 and 2011, it showed evidence for a comet-like tail. The asteroid is now approaching perihelion and has reactivated, suggesting that ice sublimation is responsible for the activity. The present program will use imaging with Hubble's Wide Field Camera 3 to map the detailed structure of the ejected materials.