This week on HST

HST Programs: October 22 - October 28, 2012

Selected highlights

GO 11622: A Search for Water and Methane on a Neptune-Mass Transiting Planet

Artist's impression of the exo-Neptune in orbit around Gliese 436

Gliese 436 is an early-type M dwarf (spectral type M2.5) with a mass approximately 40% that of the Sun lying at a distance of ~10 parsecs. In August 2004, the Lick/Carnegie planet search team (led by Geoff Marcy and Paul Butler) announced the discovery of a ~22 Earth-mass planet in a 2.64 day orbit around this star. Unlike most "hot jupiters", this "hot Neptune" is on an elliptical orbit, e=0.16, which, with a semi-major axis of 0.0278 AU, brings it within 3.5 million kilometres of the central star. Gl 436 is significantly cooler than the Sun, with a surface temperature close to ~3400 degrees Kelvin; even so, the "surface" temperatures on Gl 436b are expected to reach ~740 K (~370 C). In May of this year, a team led by F. Pont demonstrated that Gl 436b transits the parent star. The initial ground-based observations allowed them to derive a diameter approximately 4 times that of Earth, directly comparable with Uranus and Neptune. This provides key insight into the likely origins of Gl 436b, since combining the diameter with the measured mass gives the mean density, and, by inference, the likely composition. For Gl 436b, the indications are that the planet is a displaced "ice giant". Subsequent observations with HST (eg GO 11306) have refined the radius determination. The present program aims to probe the atmospheric structure by using drift-scanning to obtain high signal-to-noise spectra with the WFC3-IR G141 grism. The spectral regions covered (1.3 to 1.7 microns) span several strong absorption bands due to water and methane; differencing the spectra when the system is in and out of transit may lead to detection of significnt atmospheric absorption.

GO 12873: Search for Planetary Mass Companions around the Coolest Brown Dwarfs

NICMOS images of the ultracool L/T binary, 2MASS J22521073-1730134; the northern component, notably fainter at F160W, is the T dwarf.

Brown dwarfs are objects that form like stars, but lack sufficient mass to drive the central temperature above a few million degrees, and therefore never succeed in igniting core hydrogen fusion. Discovered almost 15 years ago, these objects initially have surface temperatures of ~3,500K, but cool rapidly and move through spectral types M, L and T. Following their discovery, considerable theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T and beyond. This transition marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest this transition occurs at ~1400-1200K, and that the spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds"). The overall timescales associated with this process remain unclear. Recent discoveries from both wide-field ground-based surveys (such as UKIDDS, on the UK infrared Telescope) and from space (notably by WISE, the Wide-field Infrared Survey Explorer) have resulted in the detection of significant numbers of extremely cool T dwarfs, and, indeed, a handful of dwarfs beyond T, reaching spectra type Y. Identifying even cooler systems in the field is extremely challenging, due to their extremely low luminosities. However, since around 15-20% of low-mass brown dwarfs appear to be binary (at least among spectral classes L and T), the known systems offer potential search locations for cooler, lower-mass companions. The present program is using imagign with the WFC3-IR camera in the F127M and F138M filters to search for such systems among 34 recently discovered very cool (>78) brownd warfs.

GO 12999: Are the Progenitors of SN 2002cx-like Objects Massive Stars or White Dwarfs?

The discovery image of the August 2011 SNe in M101

Supernovae are the most spectacular form of stellar obituary. Since B2FH, the physical processes underlying their eruptive deaths have been known to play a key role in populating the ISM with metals beyond the iron peak. More recently, these celestial explosions have acquired even greater significance through the use of Type Ia supernovae as distance indicators in mapping the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae (runaway fusion on the surface of a white dwarf in a binary system for Type Ia, or detonation of the core in Type II), some significant uncertainties remain concerning the physical details of the disruption, and, potentially, the overall uniformity of these events. Consequently, there is potential for systematic bias in the distance estimates. In particular, a subset of supernovae have light-curves that are superficially similar i morphoogy to type Ia, but which can decline at significantly different rates and appear to have substantially lower maximum luminosities. Classed as type Ia peculiar, SN 2002cx is the prototype of a subset of these objects. It is not clear whether these are the product of core collapse within a massive star, or partial deflagration of a white dwarf. The present program aims to address this issue by using deep, high spatial resolution HST observations with tha Advance Camera for Surveys to probe the stellar population in the immediate vicinity of four recent supernovae of this class: SN 2008ge, SN2008ha, SN 2010ae and SN2010el. Should these systems all lie close to regions with on-going star formation, then this may argue in favour of their origin in young, massive stars; location in quiescent inter-arm regions would favour the (evolevd) whtie dwarf origin.

GO 13012: Near-equinox spectro-imaging of Uranus aurorae sampling two planetary rotation

Nicmos image of aurorae on Uranus

The atmospheres of the gas giant planets in the solar system are dynamic entities that can exhibit dramatic changes over a variety of timescales. In addition to changes within the atmospheres themselves, due the formation and dissipation of storms, these systems can exhibit auroral activity. Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. Aururae are common phenomena on Earth, sometimes visible at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune.The Uranian aurorae are less intense, and were first detected by Voyager 2 during its flyby in 1986. At that time, Uranus was oriented almost pole-on to Earth, allowing observations of only one hemisphere. Now, 25 years later, Uranus has completed more than a quarter of its 84-year-period orbit, and passed through the equator-on ring plane crossing in late 2007 (see Program GO 10870 ). . As a result, we now have clear access to both the northern and southern polar regions. The peresent observing program aims to capitalise on that by targeting Uranus for observation close to its 2012 opposition, and using the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) to obtain spectra and ultraviolet imaging of the planet. The goal is to trace the evolution of active phenomena structures over a full diurnal rotation period.