This week on HST

HST Programs: May 11 - May 17, 2015

Selected highlights

GO 13652: The frequency and chemical composition of rocky planetary debris around young white dwarfs: Plugging the last gaps

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years.In subsequent years, a significant number of DA white dwarfs have been found to exhibit narrow metallic absorption lines in their spectra. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk. The Cosmic Origins Spectrograph (COS) is an ideal instrument for probing the abundance of trace elements in white dwarfs atmospheres: more than 70 systems have been observed, with detection rates running at around 50%. The presentSNAPSHOT program aims to cap pervious investigations by targeting white dwarfs with cooling ages of 5-25 Myrs and 100-300 Myrs: models suggest that planetary collisions should be frequent at the younger ages, when the parent stars have just completed extensive mass loss on the AGB.

GO 13731: The Real Impact of Extragalactic Jets on Their Environments: Measuring the Advance Speed of Hotspots with HST

A WFPC2 image of the jet in M87

Highly collimated jets are common features of active galactic nuceli and quasars, generally believed to be generated by accretion onto supermassive black holes at the centres of these galaxies. The most famous example is the central jet in the giant elliptical M87, the second brightest galaxy in the Virgo cluster. M87 is a strong radio source, originally detected as Virgo A in 1947. The jet itself was first seen as long ago as 1918, when the lick astronomer Heber Curtis commented on a strange linear feature, a "curious straight ray", in the central regions of the galaxy. Hubble has been observing M87 for more than 20 years, and the high spatial resolution offered by those observations has allowed astronomers to measure the relative motions of features in the jet, The present observations build on that past history for both M87A and the radio galaxy Pictor A. The Advanced Camera for Surveys will be used to obtain moderately deep images of both galaxies, probing the motions of the extreme regions of the jet, the terminal hot spots where the relativistic material in the jet impacts the intergalactic medium.

GO 13767: Bright Galaxies at Hubble's Detection Frontier: The redshift z~9-10 BoRG pure-parallel survey

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). The highest redshift objects found in the UDF have redshifts approaching z~7. Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. Wide Field Camera 3, installed in Servicing Mission 4, is well suited to these observations, and a number of programs are in place in Cycle 17 that address these issues. Indeed, WFC3 is employed in pure parallel mode by several programs. These take advantage of other science programs, usually with COS, that involve 2-5 orbit pointings on sources at high galactic latitude. The WFC3 pointing is unplanned, since it depends on the orientation adopted for the prime observations, but 2-5 orbits of IR imaging can reach galaxies at redshifts exceeding z=7 (potentially even z~8) in high latitude fields. The present program builds on similar programs in Cycles 17 and 19, and aims to detect the brightest galaxies at z~9, within 600 Myrs of the Big Bang.

GO 13841: Accurate masses and distances of the binary Cepheids S Mus and SU Cyg

The radial velocity curve (left) and pulsational velocity curve (middle) of the primary component of the binary Cepheid, V1334 Cyg (Gallene et al, A&A, 552, A21)

SU Cygni and S Muscae are short-period (P = 3.8 days, 9.66 days) classical Cepheid variables lying at a distance of a few hundred parsecs from the Sun. Both are known binary systems, with SU Cyg harbouring a ~4 solar mass, B6 companion and S Muscae matched with a 3-5 solar mass, B3-B5 companion. At optical wavelengths, the combined flux is dominated by the cooler Cepheid in these systems, but hot companion is detectable at ultraviolet wavelengths. The present program aims to use STIS to obtain ultraviolet spectra that will enable measurement of the radial velocity curve for the hot companions in these systems. Those observations will be combined with existing ground-based measurements of the Cepheid radial velocities, therefore determing velocity curves for both stars. Combining those data with high-precision astrometric measurements from ground-based interferometry will allow a direct measurement of mass and distance for both systems.