This week on HST

HST Programs: January 8 - January 14, 2007

Some selected highlights

GO 10849: Imaging Scattered Light from Debris Disks Discovered by the Spitzer Space Telescope around 21 Sun-like Stars

HST ACS image of the face-on debris disk around the nearby G dwarf, HD 107146

While much debate has raged in recent months over exactly how to define a planet, there is very little debate in the astronomical community about where planets form: they form in circumstellar disks. During the earliest stages of their existence, the disks are dusty, gas-rich and high opacity; for example, see NICMOS images of T Tauri stars and IRAS sources and current HST proposals 10540, 10810 and 10864. After only ~10 million years, however, the gas dissipates, leaving a young planetary system with a rich content of dust, rocks, planetoids and planets. This period corresponds to the high bombardment phase in earth's history, when the Moon was formed. To the outside observer, the dusty disk has low surface brightness, and is much less prominent than the gaseous disk. HST can image these disks via scattered light at near-infrared and, in a few cases, optical wavelengths - probably the most spectacular example is Beta Pic (see the recent HST ACS images ) - but the most effective means of detection is to search for excess thermal radiation at mid-IR wavelengths with Spitzer. The present proposal is a follow-on to a Spitzer Legacy program that surveyed nearby G dwarfs. The aim is to use the NICMOS coronagraph to resolve the structure of the disks suspected to be present in these 21 nearby solar-type stars.

GO 10862: Comprehensive Auroral Imaging of Jupiter and Saturn during the International Heliophysical Year

Hubble ultraviolet image of auroral activity near Jupiter's north magnetic pole

2007 has been designated the International Heliophysical Year, and HST will be playing a key part in the associated scientific activities by participating in a detailed investigation of auroral activity in jupiter and Saturn. Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. They are a common phenomenon on Earth, sometimes visible at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune. Jovian auroral activity is also affected by the Galilean satellites, which generate electric currents that can produce bright auroral spots (see figure), and, in some cases, have their own auroral storms. . HST will use the ACS Solar Blind Channel to monitor activity on the two largest gas giants. The initial campaign, starting in early January, focuses on Saturn, which is at opposition. In February, the New Horizons spacecraft will fly by Jupiter, using the strong gravitational field to propel it on its way to Pluto. During the fly-by, New Horizons will carry out a number of experiments ( see this link ) Finally, Jupiter will be surveyed while it is at opposition in June 2007.

GO 10889: The Nature of the Halos and Thick Disks of Spiral Galaxies

Ground-based imaging of the edge-on spiral, NGC 891

The stars in the Milky Way are generally grouped into stellar populations, building blocks that provide insight into the process of galaxy assembly. The traditional populations are the near-spherical, metal-poor Halo, representing the first significant burst of star formation; the Disk, whose constituents have higher metallicities, a flattened density distribution (which defines the Galactic Plane) and significant angular momentum, suggesting a formation history that includes collapse and dissipation; and the central Bulge, which, with a spheroidal distribution and broad metallicity range, may be something of an amalgam of disk and halo. The original models for the Disk envisaged a relatively simple population, with a continuous star formation history and an exponential density distribution, both radially and perpendicular to the Plane. However, in the mid-1980s, starcount analyses revealed more complexity in the vertical density distribution, with evidence for two components with scaleheights ~300 and 1000-1300 pc. Subsequent investigations of the more extended component, dubbed the thick disk, suggest that it probably formed as a result of a merger with a massive satellite early in the Milky Way's history (8-10 Gyrs ago). Ground-based observations suggest that some other spiral galaxies possess a similar component. Clearly, the frequency of such systems and their age distribution offer clues to the merging history of the average spiral galaxy. The present program will use ACS, WFPC2 and NICMOS to image seven edge-on spirals at several locations perpendicular to the Plane, with the aim of resolving the underlying stellar populations and tracing the metallicity distribution and overall morphology of the extended disk components.

GO 10918: Reducing Systematic Errors on the Hubble Constant: Metallicity Calibration of the Cepheid PL Relation

M101, the spiral galaxy targetted in this program

Cepheids are the original distance indicator, and remain the primary calibrator for the extragalactic distance scale. Most investigations tie the zeropoint for the latter scale to the Large Magellanic Cloud, which has a large population of Cepheid variables that provide a well-populated period-luminosity-colour (PLC) relation. However, the Cepheid period-Luminosity relation has long been suspected of being dependent on metallicity. The LMC Cepheids have a lower average metallicity than both the Galactic Cepheids that define the local calibration and the Cepheids in more distant galaxies used to derive H0 - setting the stage for potential systematic errors in the distance scale. The present program aims to address this issue through HST ACS imaging of Cepheids in several fields in the relatively nearby spiral galaxy, M101. The M101 disk has a metallicity gradient, and the targeted fields have been chosen to sample a range of metallicities. Comparison of the perid-luminosity relations from the separate fields will allow direct measurement of any systematic metallicity-based variations.