This week on HST

HST Programs: October 7 - October 13, 2013

Selected highlights

GO 12903: The Evolutionary Link Between Type 2 and Type 1 Quasars

Artist's impression of the black hole and surrounding torus in an AGN

This year (2013) is the fiftieth anniversary of the recognition that QSOs (quasars, quasi-stellar objects) were extremely luminous objects lying at substantial redshifts. The central engine powering these luminous objects is now recognised as a supermassive black hole, marking the central regions of a galaxy. As such, QSOs are clearly related to (and more luminous than) active galactic nuclei (AGN). Like AGNs, QSOs have been segregated into two categories based on their spectral properties: systems with broad lines are characterised as Type 1 QSOs; systems with narrow lines are classed as Type 2. As with AGN, the underlying cause of these differences is generally believed to reside more in our perspective than on the sources themselves: heavily obscured systems, where the central accretion disk lies behind a thick veil of dust, are observed as Type 2 systems; they are expected to evolve to form Type 1 systems as the dust is ablated and destroyed. The present SNAP program aims to test this scenario by coupling mid-infrared Herschel observations, probing the dust environment, with HST near-infrared WFC3 imaging of targets drawn from two matched samples of Type 1 and Type 2 QSOs.

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 13297: The HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations and Formation

Hubble image of the metal-poor globular cluster, M15

Globular clusters are members of the Galactic halo population, representing remnants from the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. Until recently, conventional wisdom was that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. One of the most surprising disoveries in recent years is the realisation that this simple picture no longer holds. Up until about 5 years ago, the only known counter-example to convention was the cluster Omega Centauri, which is significantly more massive than most clusters and has both a complex main sequence structure and a range of metallicities among the evolved stars. High precision photometric observations with HST has demonstrated that Omega Cen is far from unique, with multiple populations evident in numerous other clusters, including NGC 2808, NGC 1851, 47 Tuc and NGC 6752. Multiple populations have also been discerned in a number of clusters in the Magellanic clouds. Sustaining multiple bursts of star formation within these systems demands that they retain gas beyond the first star forming event, which appears to set a requirement that these clusters were significantly more massive during their epoch of formation; put another way, the current globulars may represent the remnant cores of dwarf galaxy-like systems. That, in turn, implies that the stars ejected from those systems make a significant contribution to the current galactic halo. One of the most effective means of identifying and studying multi-population clusters is combining high-precision photometry over a wide wavelength range, particularly extending to UV wavelengths. Sixty-five globular clusters already have R/I (F606W, F814W) Hubble imaging and photometry thanks to the Cycle 14 program, An ACS Survey of Galactic Globular Clusters (GO 10775). The present program aims to build on those data by adding UV/blue observations using the F275W, F336W and F438W filters on the WFC3-UVIS camera. The colorus derived from these filters enable characterisation of the C, N and O abundances of the component stellar populations in these systems.

GO 13492: Hubble Spectroscopy of Sungrazing Comet ISON

HST image of Comet ISON, late April 2013

At least once every decade, the astronomical community gets excited by the discovery of a comet that offers the prospect of becoming a "comet of the century", developing a spectacular tail speading across the sky at sunset or sunrise. Unfortunately, while Comet McNaught put on a spectacular shows for the southern hemisphere in 2007, most of the predicted rivals of the Great Comets of 1680/1811/1882 have fizzled. The latest in line is Comet ISON. Discovered in late 2012 when it lay beyond jupiter's orbit, the comet is scheduled to pass within 700,000 miles of the Sun on November 28th. Despite its distance, at that time the comet already had a substahtial coma and tail, leading to hopes that it might be highly volatile rich. However, subsequent observations have shown that the brightness has increased at a slower rate than hoped for, suggesting that Comet ISON may prove to be another disappointment - at least for the public. The comet passed interior to Mars' orbit in late September, and will cross the Earth's orbit on its inward journey around November 2nd. As a sun-grazer, there is a significant chance that the comet might break up close to perihelion, potentially offering an even more spectacular show and more volatiles are exposed to solar radiation. Hubble caught observations of Comet ISON in April, before it entered the solar avoidance zone, withi 50 degrees of the Sun.The comet has become accessible again for most of October, and the present program will use STIS and COS to obtain ultraviolet spectra of the pre-perihelion gases.