This week on HST

HST Programs: February 19 - February 25, 2007

ACS Solar Blind Channel comes back on line

Some selected highlights

GO 10610: Astrometric masses of extrasolar planets and brown dwarfs

Artist's impression of the planets circling the M dwarf, Gl 876

The overwhelming majority of extrasolar planetary systems have been identified through radial velocity monitoring, and the detection of the reflex motion of the parent star as it orbits the common center of mass of the system. Just as radial velocities measure the stellar "wobble" introduced along the line of sight, so high precision astrometry can be used to measure motion in the plane of the sky. Combining these data gives the full three-dimensional motions of the system, and a direct measure of the mass of the planetary companion. The Fine Guidance Sensors on HST are the only system currently capable of making observations at the required sub-milliarcscond accuracy, and has already been used for astrometry of four systems, including the M dwarf Gl 876. The current GO program pursues observations of six planetary hosts; the final set of FGS observations of HD 47536 is scheduled over the next week.

GO 10810: The Gas Dissipation Timescale: Constraining Models of Planet Formation

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 and 10864. After only ~10 million years, however, most of 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 ). Recent models suggest that, while these debris disks have a much lower gaseous content than classic T Tauri disks, they should retain some gas content, which can be crucial in influencing planet formation. The aim of the present program is to use the prism on the UV-sensitive ACS Solar Blind Channel SBC) to search for molecular hydrogen absorption at 1600 Angstroms. The program targets 11 systems with ages between 10 and 50 Myrs, including two members of the TW Hydrae association.

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, 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, conducted in early January, focused on Saturn, which is at opposition. In February, the New Horizons spacecraft flys 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 10989: Astrometric masses of extrasolar planets and brown dwarfs

Artist's impression of one of the planets circling Epsion eridani (from Nova Celestia )

The overwhelming majority of extrasolar planetary systems have been identified through radial velocity monitoring, and the detection of the reflex motion of the parent star as it orbits the common center of mass of the system. Just as radial velocities measure the stellar "wobble" introduced along the line of sight, so high precision astrometry can be used to measure motion in the plane of the sky. Combining these data gives the full three-dimensional motions of the system, and a direct measure of the mass of the planetary companion. The Fine Guidance Sensors on HST are the only system currently capable of making observations at the required sub-milliarcscond accuracy, and has already been used for astrometry of several systems, including the M dwarf Gl 876 and, most recently, Epsilon Eridani. The current GO program pursues observations of six planetary hosts, and FGS observations of HD 38529, which has at least two planetary companions, are scheduled over the next week.