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Target Selection for Planet Searches
David Soderblom
Space Telescope Science Institute
Baltimore MD
Planets may, in principle, exist around any type of star, but some stars are better than others as places to look just because they are amenable to our techniques. Stars substantially more massive than the Sun tend to have broad lines, for example, precluding radial velocity searches. They also tend to exhibit intrinsic variability. Stars much less massive than the Sun are very faint, limiting the attainable signal-to-noise for even the nearest examples.
Our curiosity is naturally drawn to the solar-type stars (G dwarfs, roughly) since we live around one ourselves, and they are also - perhaps not coincidentally - a preferred sample when looking for planets beyond the solar system. This is because G dwarfs, especially old ones like the Sun, are:
- Photometrically quiet and stable compared to other stars.
- Fairly bright at optical wavelengths, so that a reasonable sample can be accessed within a modest distance.
- Rich in sharp absorption lines, which facilitates radial-velocity observations.
- Well understood, since we have a role model close at hand.
The stars that host the planets found so far are virtually all old, single, solar-type stars, for just these reasons.
A census of the nearby solar-type stars is in progress. It will soon use colors and parallaxes from Hipparcos to define a volume-limited sample of ~5,000 G dwarfs within 50 pc (i.e., down to a V magnitude of about 9). A preliminary estimate of which stars these are has been made, based on existing information. Additional observations are being made to determine which stars are in binary systems, and it is also possible to estimate ages. Stars that have stellar companions within ~10 AU are not promising targets for planet searches, nor are the very young stars. Both qualities - duplicity and age - weed out about 1/2 of the starting sample, meaning that ~1/4 of the stars are suitable targets. That is about 1,000 stars, roughly, or 1 µSagan.
In this talk I will discuss some of what is known about stars like the Sun, how that can be applied to the search for extra-solar planets, and some of the other motivations for pursuing these studies.