This week on HST

HST Programs: November 25 - December 1, 2013

Selected highlights

GO 13046: RAISIN: Tracers of cosmic expansion with SN IA in the IR

The first supernova discoevered by the Pan-STARRs survey

Supernovae are the most spectacular form of stellar obituary. In recent years, these celestial explosions have acquired even more 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 uncertainties remain as to the uniformity of the events. Moreover, as the sample of known supernova has grown, so has the range of photometric systems and the methods used to fit the light curve and account for the ever-present uncertainites inroduced by dust absorption. Consequently, the potential remains for systematic bias in distance estimates due both to intrinsic differences and to measurement errors. The persent program aims to minimise these systematics by compiling standard sequences of observations, primarily in the Y, J, and H filters, of supernovae at redshifts between z~0.3 and 0.5. Focusing on those wavelengths minises the effects, and hence the uncertainties, due to dust absorption. The supernovae themselves are drawn from the Pan-STARRS survey, with the WFC3-IR camera on HST employed to obtain the photometry.

GO 13329: Discovering the nature of the star-planet interaction at WASP-12b

Artist's conception of atmospheric ablation on a hot jupiter

Among the most surprising discoveries of the latter part of the twentieth century was the finding that exoplantery systems were not simple copies of the Solar System architecture. Specifically, Jovian-mass gas giants were a relatively common occurrence at sub-Mercurian separations from sun-like stars. Characterised as "hot jupiters", the hottest jupiter is the ~1.4 MJup planet in a 1.1 day period orbit around a fairly anonymous 11th magnitude G dwarf. This is a transiting system, identified by the WASP team and designated as WASP-12b, so the orbital inclination is known, and the absolute semi-major axis is only 0.023 AU, less than one-tenth the radius of Mercury's orbit. Consequently, WASP-12b experiences substantial heating, and is estimated to have a temperature of ~2500K (comparable with an M7/M8 dwarf). At these temperatures, the expectation is that the planetary atmosphere will be subject to extensive evaporation, potentially leading to a substantial gas tail. In Cycle 17, COS was used to observe this system, and the results appeared to indicate that the transit ingress occurs earlier at near-UV wavelengths than at optical wavelengths;that is, the planet seems to have a larger cross-section in the near-UV. The same phenomenon was not observed at transit egress. A possible explanation for this effect is that the planet has a substantial magnetic field, which concentrates dense shocked material in the supersonic stellar wind upstream of the field. If so, this could provide a mechanism for not only probing the origins of magnetic fields in exoplanets, but also constraining the interior structure of the planet. The data from the Cycle 17 program, however, are sparse. The present program aims to confirm the results through observations of additional transits.

GO 13482: The Evolving Gas Content of Galaxy Halos: A Complete Census of MgII Absorption Line Host Galaxies at 0.7 < z < 2.5

How background QSOs are used to probe the gas content of galactic halos

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution of individal stellar systems therefore depends on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. Halo gas is expected to exist over a range of temperatures, ranging from extremely hot gas (>106K) participating in accretion and outflows, to much cooler gas forming reservoirs within the halo. The most effective means of detecting that gas is through spectroscopy of background soruces, usually quasars, where the aim is to detect the absorption signature imprinted by the foreground material on the intrisic QSO spectrum. Hot gas is best detected at far UV wavelengths, but cooler gas has less energetic transitions and therefore tends to exhibit absorption at longer wavelengths. The present program aims to search for cool gas in galactic halos by looking for evidence of singly ionised magnesium, Mg II, absorption in the spectra of nine moderate-redshift QSOs. Since the goal is to probe redshifts between z=0.7 and z=2.5, the near-UV lines are shifted to the near-infrared, and consequently the observatons are being made using the G141 prism on the WFC3-IR camera.

GO 13510: Joint Chandra and HST Monitoring of the Crab Nebula

The Crab Nebula

Messier 1, the Crab Nebula, provides astronomy with one of its iconic images. The remnant of a bright supernova observed in 1054 by Arabian and Chinese astronomers, the Crab was first recorded in 1731 by the English astronomer, John Bevis, thirt-seven years before Messier compiled his catalogue of non-comets. The energy source for the gaseous emission is the neutron star that lies in the centre of nebulosity, and was one of the first pulsars to be identified. The Crab is also a source of high energy emission, including radiation at X-ray and gamma ray wavelengths. Overall, this system plays a crucial role in aiding our understanding of post-supernova evolutionary processes. However, there are still some notable undertainties in the detailed processes within even this system. In particular, in September of 2010 the Crab surprised the astronomial community by producing a powerful flare at gamma-ray wavelengths that persisted for 4 days (see GO 12381 ).A second flare of similar magnitude occurred in May, 2011. Observations taken during the 2010 flare by HST and Chandra provided some insight into the effects of the flare, but analysis was hampered by the absence of a comparison set of pre-outburst images of comparable resolution and depth. The present program aims to address that issue through coordinated monitoring of the Crab at X-ray and optical wavelengths. Tne Advanced Camera for Surveys on HST was used to take images in the F550M filter at 6 epochs in 2012, with the observations timed to be within 10 days of X-ray images taken by Chandra using the AXAF CCD Imaging Spectrometer; the present program will obtain a further series of observations through 2013 and 2014. These data will establish a reference set should a further flare occur.