CHASING THE STELLAR ROAD RUNNERS

By measuring the accurate distances to the fastest moving stars in the sky, astronomers at Georgia State University (GSU) in Atlanta have discovered 26 new neighbors within 25 parsecs (82 light-years) of the Sun. Two other cosmic ``road runners'' are close examined to be pairs and are the first confirmed binary systems comprising a red subdwarf and a white dwarf. The results are being presented at the American Astronomical Society meeting in San Diego, California, and have been accepted for publication to the April edition of the Astronomical Journal.

Research scientist Wei-Chun Jao and the RECONS (Research Consortium on Nearby Stars) team at GSU have found that four of the systems are nearer than the 10 pc (33 light-years) horizon of the RECONS sample. All four are single, faint, red dwarf stars. The nearest is DENIS 1048-3956, which is only 13 light-years away and ranks as the 28th nearest stellar system. Noteworthily, all of these new neighbors are fainter and cooler than our Sun.

High proper motion stars are superb candidates to be nearby stars because the closest stars appear to be moving faster than more distant stars. This is much like what happens when watching a marathon --- a close-by runner appears to be running faster than a more distant one, even if they are running at the same speed. Jao is studying the complete sample of $\sim$500 stellar systems that slide across the sky more than 1 arcsecond each year, a rate at which it would take the star 1800 years to cross a piece of sky as wide as the full Moon. By measuring accurate distances to these road runners, astronomers are able to build a three-dimensional map (available on the website) of our Sun's place in the Milky Way, and to make a movie showing the motions of the nearby stars. Currently, 53\% of the 250 systems known in the RECONS sample are stellar road runners, but many more are still being discovered.

During Jao's work, two additional road runner systems, LHS 193AB and LHS 300AB (each of which is slightly further than 30 pc, or 100 light-years away), have been found to be the first confirmed binary systems containing a red subdwarf and a white dwarf. The two stars in each system have been shown to have identical proper motions, indicating that they are physically associated and traveling through the Milky Way as pairs. LHS 193AB and LHS 300AB are separated on the sky by 12.6 arcseconds and 4.3 arcseconds, respectively (see Figure 1), corresponding to distances of about 10 and 3 times the separation from our Sun to Pluto.

Subdwarfs are rare stars formed during the early history of our Milky Way Galaxy that have very low ``metallicity'', meaning that they have few elements heavier than hydrogen and helium (in general, each generation of stars has more metals than the previous generation). Subdwarfs are 1.5 to 2.0 magnitudes fainter than their main sequence counterparts, which are called dwarfs. The RECONS team's accurate trigonometric parallaxes and photometry allows them to determine the stars' positions on the Hertzsprung-Russell diagram (HR diagram), which shows the relation between the luminosities and colors of stars. As shown in Figure 2, LHS 193A and LHS 300A are clearly subdwarfs and the B components are small, dense white dwarfs --- burned out cores of stars that were once more massive than our Sun. The two new white dwarfs are located in a previously unpopulated region of the HR diagram, indicating that they are new beasts in the stellar zoo.

Another indicator that both systems are old is that each travels through the Galaxy at nearly 150 km/sec (roughly 100 miles/sec). Contrary to people, older stars like the Jupiter-sized red subdwarfs generally move faster than their younger counterparts that are still hanging around their stellar nurseries, and the two discovered systems are moving about three times faster than even middle aged stars. ``It's amazing to see two senior citizens rushing through our neighborhood'', says Jao, ``and they are carrying extremely dense baggages with them.'' The white dwarf companions are small objects the size of the Earth that contain roughly half the mass of our Sun, but they are unable to burn any elemental fuel in internal reactions, so they cool down as time passes. Consequently, the two galactic fossils discovered provide a new laboratory that can be used by astronomers to understand the cooling character of white dwarfs, and to probe the earliest moments of our Galaxy's life.

One of the ``base camps'' for taking the census of the Sun's nearest neighbors is located at GSU. The RECONS group there is conducting a long-term survey using the 0.9m (36in) telescope at the Cerro Tololo Interamerican Observatory in Chile, a U.S. facility located in the foothills of the Andes. The GSU team includes Associate Professor Todd Henry (RECONS Director), research scientists Jao and Hektor Monteiro, graduate students Thom Beaulieu, Misty Brown, Deepak Raghavan, and John Subasavage, and undergraduate students Charlie Finch and Jennifer Winters. The crucial measurements made by RECONS are the trigonometric parallaxes for the target stars, which tell astronomers their distances to 2\%. One of the keys to the team's success is that they carry out the work in the southern sky, a happy hunting ground for new nearby star discoveries. To date, they have found 23 new systems within 10 pc (33 light-years).

The GSU RECONS team has been supported by the National Optical Astronomy Observatories (NOAO), the National Science Foundation (NSF), and NASA via the Nearby Stars Project at NASA-Ames and the Space Interferometry Mission at the Jet Propulsion Laboratory.

CONTACTS:
Wei-Chun Jao
Research Scientist
Georgia State University
(404)651-1882
jao at chara.gsu.edu

Todd Henry
Associate Professor
Georgia State University
(404)463-9954
thenry at chara.gsu.edu

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