This week on HST

HST Programs: October 15 - October 21, 2007

Some selected highlights

GO 11084: Probing the Least Luminous Galaxies in the Local Universe

Ground-based image of Andromeda V, one of the brighter(!) dwarf companions

Large spiral galaxies tend to be accompanied by a retinue of lesser companions. In the case of the Milky Way, the two most prominent satellites are the Large and Small Magellanic Clouds, irregular galaxies with considerable gas content and extensive ongoing star formation. But there are at least 12 other systems, the majority of which are dwarf spheroidals with masses less than ~108 Solar masses, or less than one-thousandth that of the Milky Way, and no on-going star formation. The nearby Andromeda galaxy has its own swarm of acolytes, with the most recent additions (Andromeda XI, XII and XIII) discovered in 2006. The latter galaxies populate the extremely low luminosity tail of the galaxy luminosity function, and the aim of this proposal is to obtain a better understanding of the stellar population (or even populations) in these low-mass systems. Besides the three Andromeda dwarfs, the program will target the Milky Way satellites Canes Venatici I, Canes Venatici II, Hercules, and Leo IV, together with Leo T, an isolated Local Group member. The WFPC2 camera will be used to obtain deep imaging in F606W (V-band ) and F814W (I-band).

GO 11169: Collisons in the Kuiper Belt

Visions of the Kuiper Belt

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most were relatively modest in size, with diameters of a few hundred km and photometric properties that suggested an icy composition, similar to Pluto and its main satellite, Charon. Over the last three years, however, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; indeed, one object, Eris (2003 UB13), is slightly larger than Pluto (2320 km) and 25% more massive. We know the mass for Eris because it has a much lower mass companion, Dysnomia, which orbits Eris with a period of 16 days (see this recent press release ). Pluto, itself, has three companions: Charon, which is about 1/7th the mass of Pluto, and the much smaller bodies, Hydra and Nix, discovered from HST observations in early 2005. Observations of other Kuiper Belt Objects (KBOs), mainly using HST, reveal that a significant fraction are binary. This may indicate that the Kuiper Belt is a dangerous place to live, with frequent collisions between KBOs, leading to fragmentation and satellite formation. The present program aims to probe this issue through multi-wavelength observations of a wide range of KBOs.

GO 11211: An Astrometric Calibration of Population II Distance Indicators

Measuring trigonometric parallax

Trigonometric parallax measurement remains the fundamental method of determining distances to astronomical objects. The best ground-based parallax measurements can achieve accuracies of ~1 milliarcsecond, comparable with the typical accuracies achieved by the ESA Hipparcos astrometric satellite. This level of accuracy allows us to obtain reliable distances and luminosities for main sequence stars, subgiants, red giants and even a number of metal poor subdwarfs. However, with an effective distance limit of 100-150 parsecs, the sampling volume includes less than a handful of rarer, shorter-lived celestial objects. In particular, there are no RR Lyraes or Cepheids, two of the principal calibrators in the extragalactic distance scale. There is only one instrument currently available that can achieve astrometry of higher accuracy - the Fine Guidance Sensors (FGS) on HST. The present team used the FGS to measure a parallax of 3.82+/10.2 milliarseconds for RR Lyrae, the nearest star of its type. this corresponds toa distance of 262 parsecs. The present program aims to improve the calibration by extending observations to four more relatively nearby RR Lyraes (XZ Cyg, UV Oct, RZ Cep and SU Dra) and two Pop II Cepheids (Kappa Pav and VY Pyx).

GO 11213: Distances to Eclipsing M Dwarf Binaries

Artist's impression of a cool binary system

Eclipsing binaries are stellar systems where the orbital plane lies in the line of sight, leading to the components undergoing mutual eclipses. These systems are extremely powerful probes of stellar properties, since (given the appropriate radial velocity measurements) they permit direct measurement of both stellar masses and radii. Accurate distances The present program aims to use the Fine Guidance Sensors on HST to determine sub-milliarcsecond trigonometric parallaxes for five M-dwarf binaries: YY Gem, GU Boo, CM Dra, NSVS0103 and TRES-HER0-R