This week on HST

HST Programs: April 5, 2010 - April 11, 2010

Selected highlights

GO 11599: Distances of Planetary Nebulae from SNAPshots of Resolved Companions

HST image of the Eskimo planetary nebula, NGC 2392

Planetary nebulae are the spectacular results of the penultiumate evoutionary phase of intermediate-mass stars. Towards the conclusion of the second, or asymptotic, giant branch, ~1 to ~7 solar mass stars have achieved radii exceeding 100 RSun and their extensive envelopes are subject to long-period, opacity-driven pulsations and substantial mass loss. AGB evolution terminates with the ejection of the stellar envelope, revealing the bare core, with an initial surface temperature approaching 100,000K. The core cools to become a white dwarf but, during the initial phase, its luminosity is sufficient to excite gases in the expanding envelope, producing a planetary nebula. These objects are extremely spectacular, but also, in Galactic terms, rare. As a result, we only know reliable distances to a handful of such systems. The present program aims to tackle this issue through observations of planetary nebulae with known or suspected lower-mass companions. Once confirmed as binaries, distances can be estimated using the companions, which still reside on the main sequence.

GO 11600: Star formation, extinction and metallicity at 0.7

ACS images of a section of the GOODS fields

The Great Observatories Origins Deep Survey, originated as a Cycle 12 HST Treasury program, designed to probe galaxy formation and evolution at redshifts from z~1 to z~6. GOODS covers two ~150 sq. arcminute fields, one centred on the Hubble Deep Field in Ursa Major and the Chandra Deep Field-South in Fornax. Initially, the program combined deep optical/far-red imaging (F435W, F606W, F775W and F850LP filters) using ACS on HST with deep IRAC (3.6 to 8 micron) and MIPS (25 micron) imaging with Spitzer. These two fields have become among the most studied celestial regions. In addition to deep HST data at optical and near-infrared wavelengths (both fields have been covered by NICMOS), the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton, and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Gemini, Surbaru and the ESO VLT. Part of the GOODS South field was covered by the WFC3 Early Release Science observations (see ), and both fields are also covered partially by one of the three Multi-Cycle Treasury programs allocated time in Cycle 18-20. The present program is using the G141 grism on the WFC3 IR camera to conduct a spectroscopic survey of part of the GOODS-North field. The data will enable the identification of H-alpha+[N II] emission from galaxies at redshifts 0.7 < z < 1.5, and thereby set constraints on star formation at those redshifts.

GO 11673: Dynamics in the atmosphere of the evaporating planet HD189733b

Artist's conception of atmospheric ablation on a hot jupiter

HD 189733 is an early K-type dwarf lying at a distance of ~19.3 parsecs in the constellation of Vulpecula. It has an M dwarf companion, HD 189733B, at a separation of ~220 AU, and also harbours a planetary system. Like ~3% of sun-like stars, it has a `hot Jupiter', HD 1897733b, orbiting orbiting the parent star at a distance of 0.031 AU in a period of 2.219 days. At these distances, the planetary atmosphere is heated to temperatures exceeding 1000K, and the expectation is that it will be subject to extensive evaporation. This is a transiting system, so the planetary mass is well determined as 1.13+/- 0.03 M, and both HST and SPitzer have been employed in probing the planetary atmosphere, either by searching for enhanced absorption during the primary eclipse (HD 189733b transits HD 189733), or by searching for reduced (mid-infrared) emission as HD 189733 eclipses the hot Jupiter. The present proposal aims to build on previous HST observations using the ACS/SBC prism. Those observations were sufficient to identify signatures of planetary evaporation, but of too low resolution to quantify the process. The present proposal will use COS to obtain higher resolution data, and examine the dynamics of the evaporation process.

GO 11677: Is 47 Tuc Young? Measuring its White Dwarf Cooling Age and Completing a Hubble Legacy

Hubble image of the globular cluster, 47 Tucanae

Globular clusters are members of the Galactic halo population, which formed during 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. Hubble has conducted a significant number of observing programs targeting these systems, with the majority designed to obtain moderately deep, multicolour imaging data of a range of clusters. Those programs probw evolved stars, on the red giant and horizotal branch, and generally extend no more than a few magnitudes below the main-sequence turnoff. A few clusters, however, have been studied in detail - specifically, the two nearest clusters, NGC 6397, an extremely metal-poor cluster, and M4, a moderately metal-rich systems; Omega Centauri, one of the most massive clusters, perhaps even the remnant core of a dwarf galaxy; and 47 Tucanae, one of the higher metallicity systems, lying in the foreground of the Small Magellanic Cloud. Deep imaging of NGC 6397 and M4 has succeeded in clear detecion of the white dwarf cooling sequence in those clusters, and those data have been used to derive age estimates. The present observation aims to obtain similar data for 47 Tucanae, permitting an estimate of the relative age of these three, disparate clusters.

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Past weeks:

• Cycle 14 observations (from March 13 2006 to June 30 2006)
• Cycle 15 observations (from July 1 2006)
• Cycle 16 observations (from July 1 2007)
• Cycle 17 observations (from July 13 2009)

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page by Neill Reid, updated 19/2/2010