This week on HST

HST Programs: August 31 - September 6, 2015

Selected highlights

GO 13693: The Ultraviolet Spectrum of Ceres

Images from the Dawn spacecraft of the dwarf planet, Ceres, including the (in)famous bright spots

The asteroid Ceres has undergone more changes in classification than most astronomical objects. Discovered by Giuseppe Piazzi on New Year's day 1801, Ceres was first identified as a planet lying between Mars and Jupiter (consistent with the Titius-Bode sequence). It soon became clear that Ceres was much smaller than any known planet (a diameter of only 950 km), and William Herschel suggested that it should be called an "asteroid" (star-like body), since it was unresolved in telescopes of the time. Within 6 years. Pallas, Juno and Vesta had been added to the roster, and the descriptor "minor planet" was applied. Most recently, Ceres has changed status again, becoming, with Pluto, a "dwarf planet", the only such body within the asteroid belt. Nomenclature apart, observations of Ceres provide important insight into the chemical composition of the bodies within the main asteroid belt. Both Ceres and Vesta were targets the NASA Discovery Mission, Dawn, launched on 27 September 2007. Dawn rendezvoused with Vesta in July 2011 and remained in orbit through September 2012 before transitioning to Ceres. Dawn arrived at Ceres earlier this year, and immediately raised the level of attention by identifying a number of bright spots within one of the larger craters. Most hypotheses suggest that these are highly reflective icy regions. Dawn lacks ultraviolet instrumentation, so the present HST program aims to complement the in-situ observations by using the Space Telescope Imaging Spectrograph to obtain blue and UV spectra.

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 13834: The Proper Motion Field along the Magellanic Bridge: a New Probe of the LMC-SMC interaction

The Large Magellanic Cloud (upper left) with the Small Magellanic Cloud (right) and the (foreground) Galactic globular cluster47 Tucanae

The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are the most massive satellites of the Milky Way galaxy. The orbital motions of these systems can be used to probe the mass distribution of Milky Way, and backtracking the orbits can shed light on how the three systems have interacted, In particular, the well known Magellanic Stream, stretching between the two Clouds, is thought to be a product either of interactions between the Clouds, or of ram-stripping of gas from the LMC on its last passage through the Plane of the Milky Way. Understanding the full scope of the interactions demands knowledge of the tangential motions of these systems - that is, proper motion measurements. Given the distances of the Clouds (~50 kpc.), the actual motions amount to only a few milliarcseconds, but the high spatial resolution and high stability of HST imaging makes such measurements possible. Past observing programs (eg GO 11730) have concentrated on the LMC, using the now-defunct ACS High Resolution Camera (ACS/HRC), the Planetary Camera on WFPC2 and the UVIS camera on WFC3 to target known QSOs lying behind the Clouds; the QSOs serve as fixed reference points for absolute astrometry of the numerous foreground LMC/SMC stars. A recent Cycle 21 program focused on the SMC, targeting 30 newly identified background QSos for WFC3 observations over a two-year period. The present program expands observations to several fields along the Magellanic bridge, a complex of gas and stars that conencts the two clouds, and will test the hypothsesis that the clouds are undergoing their first interaction with teh Milky Way.

GO 14040: Unique Hubble Investigation of a Newly Discovered Main Belt Comet

Asteroid 596-Scheila, the prototype main-belt coment, imaged by Peter Lake in December, 2010

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. The objects include the asteroids Scheila, which underwent an outburst in late 2010, 1979 OW7/1996 N2 and the Pan-STARRs discoveries 2006 VW139, imaged during an outburst; and Main Belt Comet 2013-P5. The initial hypothesis was that these bursts of activity might represent the effect of collisions, but the presence of multiple episodes in some objects suggests that an intrinsic origin is possible. The asteroid P/2014 S4 was identified (by Pan-STARRs observations) as a possible MBC in September 2014, and initial follow-up HST observations showed significant variation in the structure of the "coma" over an interval of only 2 weeks. The present HST program builds on those results by using high-resolution visual imaging with Wide-Feld Camera 3 to monitor the progression of the outburst and probe the underlying physical nature of the event.