This week on HST

HST Programs: September 11 - September 17, 2006

Some selected highlights

GO 10578: Eclipsing Binaries in the Local Group: Calibration of the Zero-point of the Cosmic Distance Scale and Fundamental Properties of Stars in M31

Schematic of an eclipsing binary system

Placing the cosmic distance scale on a reliable foundation remains a prime concern of cosmological astrophysics. Several HST programs are directed towards this end, sampling various rungs of the distance ladder. The current program is directed towards measurements of eclipsing binaries in nearby galaxies. In principle, these systems provide a purely geometrical means of measuring distances. We know the viewing angle for these systems: close to edge-on. Consequently, absolute stellar radii can be derived by combining the radial velocity curves with the photometric eclipse timing. Given the radius and a temperature measurement, the stellar luminosities can be determined; comparing those luminosities against the apparent brightness (making due allowance for foreground reddening) gives a direct distance estimate. The prime targets of the current program are two B-type eclipsing systems in the Andromeda spiral, M31. The ACS prism will be used to obtain spectra covering wavelengths from Lyman-alpha (1150 Angstroms) to the far red (9000 Angstroms), and those data will be used to derive temperature estimates for these stars.

GO 10847: Coronagraphic Polarimetry of HST-Resolved Debris Disks

HST ACS/HRC coronagraphic images of debris disks around two nearby stars

The last decade has seen the identification of debris disks around a number of stars in the Solar neighbourhood. The coronagraphic capabilities available on HST have been particularly useful in this regards, since the lower background present in space-based observations offers significant advantages in detecting these low surface brightness features. Those observations are complemented by mid-infrared surveys (notably by Spitzer) of denser, warmer disks in young stars. The latter observations detect thermal emission from the dust grains, permitting some constraints to be set on the chemical composition. The present program aims to use polarimetric observations of debris disks to probe the distribution of grain sizes and the density distribution of dust in those somewhat older systems. Observations of the A0 star, HD 32297, are scheduled in the coming week.

GO 10915: ACS Nearby Galaxy Survey

NGC 253, the galaxy currently targeted by the ACS NGS

Colour-magnitude diagrams derived from photometric surveys have proven invaluable in achieving an understanding of the main properties of the galactic stellar populations. Large ground-based telescopes allowed extension of this type of analysis to the principal satellites of the Milky Way and, to a limited extent, the Andromeda spiral. With the advent of HST, particularly following SM3B and the installation of the Advance Camera for Surveys, those fundamental CMD studies can be extended to higher-density star fields, fainter magnitudes and intrinsically lower luminosity stars. Until recently, those studies have concentrated on Local Group galaxies; the ambitious aim of the current program is to conduct a systematic survey of all major star-forming galaxies within ~3.5Mpc of the Milky Way, together with a number of galaxies in the M81 group at a distance of ~4 Mpc. The program includes a total of 45 galaxies, ranging from massive spiral systems to dwarf galaxies. The observations are being made using the wide-field camera on ACS, sampling selected fields in the wide-V (F606W) and I (F814W) passbands. Observations of thebarred spiral, NGC 253, are scheduled during the coming week.

GO 10929: Calibrating the Mass-Luminosity Relation at the End of the Main Sequence

The MV-mass relation for low-mass stars (from T. Henry)

The mass-luminosity relation remains one of the key underpinnings of stellar astrophysics, notably in probing the grey area that separates hydrogen-burning stars from cooling-powered brown dwarfs. The calibration of thsi relation rests on observations of binary systems, primarily eclipsing binaries at masses above 1 MSun, and primarily astrometric binaries at sub-solar masses. In the latter case, reliable mass determinations require orbital measurements of extremely high accuracy, which, in turn, requires high precision astrometry over at least one orbital period. The Fine Guidance Sensors on HST have proven invaluable in this regard, since they allow sub-milliarcsecond accuracy astrometry of binary systems with sub-arcsecond separations; in other words, HST allows measurement of nearby, low-mass binaries with periods short enough to allow completion of the observations in significantly less than an astronomer's lifetime. The current program is using the FGS to monitor six close binary systems. Observations are scheduled of G250-29 (also known as LHS 221). Lying at a distance of 10.6 parsecs, this system comprises an M2.5 primary and an ~M3.5 secondary, with a magnitude difference of ~1.6 magnitudes at visual wavelengths.

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Past weeks:

• Cycle 14 observations (from March 13 2006 to Jul 1 2006)
• Cycle 15 observations (from July 1 2006)

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page by Neill Reid, updated 2/9/2006