Target Star Selection

David Latham
Harvard University

David Soderblom
Todd Henrey
Space Telescope Science Institute

Where would YOU look for signs of extra-terrestrial intelligence (ETI)? This is the question astronomers involved in the selection of target stars for Project Phoenix have been asking.

Given so many targets to choose from, scientists on the project have chosen to concentrate on those stars from which a signal would be strongest, the nearest stars, and those that are similar to our Sun, near which is found the only known example of life. For those stars like the Sun, we've also gone two steps further. We've chosen to concentrate on the older stars (because we expect that advanced life takes a long time to evolve) and the single stars among those (because binary star systems -- two stars in orbit around one another -- cannot have planets in stable orbits that will endure the gravitational effects of the companion stars). In all, Project Phoenix will observe about 1000 stars scattered over the entire sky. The stars comprise three distinct samples, each having a different character, in order to allow various stellar environments to be examined.

The Nearest 100 Sample includes the 100 stellar systems closest to the Sun, all of which are within a distance of 7.2 parsecs (about 25 light years). The members of this sample make up stars that are hotter and larger than the Sun, like Sirius, and many that are much fainter, like the nearest star, the small red dwarf Proxima Centauri. Several stars you might be familiar with are found in this sample, including Alpha Centauri (which is in the same stellar system as Proxima), Procyon, Altair and Fomalhaut. Many of the targets are multiples, with 25 binary star systems, 7 triples, and even a quintuple.

The Best and Brightest Sample includes 140 stars within 20 parsecs (65 light years) that are very similar to our Sun in color, size and temperature. These stars will be observed for longer periods of time than those in the other samples, because of their proximity and because they have survived a rigorous selection process. This sample has been studied in detail to discover whether or not the members have other stars as close companions, and whether they are young or old. The search for companions involves measuring the velocity of the star to very high precision. If a faint companion is tugging on the target star, astronomers see a change in the velocity as the companion orbits. The age of a star can be estimated from how much activity is seen -- young stars tend to be more active. Observations of a specific spectral line of the element calcium, which is found in the upper atmospheres of stars, can reveal the activity level of the star, thereby revealing whether it is young or old. Of the 140 stars originally in this sample, only about half have been found to be both old and single.

The largest sample of stars is called the G Dwarf Sample, and includes stars nearer than about 200 light years. Our Sun is a G dwarf, and while all of the stars in this sample are much like the Sun, the criteria for membership have been relaxed from those used in the very strict Best and Brightest Sample. Nonetheless, just like in the Best and Brightest Sample, continuing observations will eliminate those that are young and those that have close stellar companions.

Because Project Phoenix is headed south to Australia for its first observing run, the current G Dwarf target list includes 650 stars in the southern sky, only half of which are old and single.

As you can see, many of the targets for Project Phoenix have been selected for characteristics that might encourage (or, at least, not discourage!) the development of life. While the bulk of the observing list is made up of stars similar to the Sun, the Nearest 100 Sample is a mixed bag of stars of all sizes, colors, temperatures and ages. It is hoped that with a combination of target types, Project Phoenix can improve its chances of detecting a signal. Regardless, we will certainly come to a better understanding of how our own Sun compares to other stars.

• Latham, D.W. and Soderblom, D.R.; SPIE Proc. 1867, 20, 1993.

• Henry, T.J., Soderblom, D.R., Baliunas, S.L., Davis, R.J., Donahue, R.A., Latham, D.W., Stefanik, R.P., Torres, G., Duquennoy, A., Mayor, M., Andersen, J., Nordstrom, B., Olsen, E.; "The Current State of Target Selection for the High Resolution Microwave Survey", in Progress in the Search for Extrasterrestrial Life, S. Shostak (ed.), Astronomical Society of the Pacific, San Francisco, in press.

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