This week on HST

HST Programs: August 17 - August 23, 2009

SMOV still under way, but science observations being made.

Selected highlights

GO 11548: NICMOS Imaging of Protostars in the Orion A Cloud: The Role of Environment in Star Formation

An image of the orion Nebula superimposed on the 13CO map of Orion A (from this link ).

The Orion association is the largest nearby star-forming complex, providing a key laboratory for unlocking the secrets of star formation. As such, it has been subject to intense scrutiny at all wavelengths from both ground and space. Surveys at near-infrared and mid-infrared wavelenths, notably by Spitzer, have identified an extensive number of embedded sources, young stellar objects (YSOs) that are still accreting from the surrounding molecular gas. This proposal focuses on 252 sources within the Orion A molecular cloud, the complex that includes the Orion Nebula Cluster. In the original Cycle 16 incarnation of this program, NICMOS was used to survey a subset of the protostars; the program has since been transferred to the WFC3-IR camera. The observations are an excellent complement to Spitzer since, while HST cannot offer either the same areal coverage or sensitivity at mid-infrared wavelegths, the camera can provide a resolution close to 0.1 arcsecond, an order of magnitude higher than the Spitzer images.

GO 11656: A Comprehensive Survey of Neptune's Small Moons and Faint Rings

HST images of Neptune - the leftmost image is "natural" colour, and also shows 4 of neptune's moons: Proteus (the brightest), Larissa, despina and Galatea.

Over the past 17 years, HST has not only provided an unparalleled means of probing galaxy formation in the high redshift universe, but has also been used for detailed scrutiny of our immediate neighbours within the Solar System. The past cycle saw program investigating aurorae on Jupiter, and imaging Saturn during the ringplane pasage. the present program moves to the outermost gas giant (or ice giant). The UVIS camera on WFC3 is being used to probe the inner rings, arcs and moons of the planet Neptune. The main goals include determinign whether there is a size cutoff in the moon system, and searching for shepherding moons for Neptune's ring system.

GO 11704: The Ages of Globular Clusters and the Population II Distance Scale

Hubble Heritage image of the globular cluster, M15

Globular clusters are the oldest structures within the Milky Way that are directly accessible to observation. They are relatively simple systems, with relatively simple colour-magnitude diagrams (albeit with some complexities adduced from recent HST observations, see GO 11233 ). Matching those CMDs against theoretical models allows us to set constraints on the age of the oldest stars in the Galaxy, and hence on the age of the Milky Way and the epoch of galaxy formation. However, the accuracy of those age determinations rest crucially on the accuracy of the cluster distance determinations. The clusters themselves lie at distances of several kpc at best, and tens of kpc at worst; thus, direct trigonometric parallax measurements must await microacrsecond astrometric missions. The classical method of deriving distances is main sequence fitting - using nearby stars, with similar chemical abundances and accurate parallax measurements, to map out the main sequence in absolute units, and then scaling the cluster data to fit. The problem with this method is that metal-poor subdwarfs are rare, so even Hipparcos was only able to obtain accurate distances to a handful of stars. The present program aims to improve the distance measurements by using the Fine Guidance Sensors on HST to determine sub-millarcsecond trigonometric parallaxes to 9 subdwarfs, almost doubling the sample available for MS fitting.