PRESS RELEASE, January 2001
TREASURE HUNT IN THE SUN'S NEIGHBORHOOD
Dr. Todd J. Henry
Dr. Philip A. Ianna
Dr. Rene A. Mendez
Like children scouring their neighborhood for hidden treasure, astronomers from the United States and Chile are revealing many previously unrecognized small stars near our solar system. Using the 0.9m and 1.5m telescopes at CTIO, Todd Henry of Georgia State University, Phil Ianna of the University of Virginia, and Rene Mendez of the European Southern Observatory in Santiago, Chile have discovered five new star systems, one of them a triple, lying only 7 to 15 pc away. These seven stars are the first results from CTIOPI (Cerro-Tololo Interamerican Observatory Parallax Investigation), a three-year project to determine parallaxes for stars lurking undiscovered in the solar neighborhood, carried out under the auspices of the NOAO Surveys Program.
All of the new discoveries are small red dwarfs (spectral types M2 to M5) in the southern sky, where historically research on nearby stars has been less extensive than in the north. Although they are among the nearest few hundred star systems, these five have been overlooked because they are thousands of times fainter than stars seen with the naked eye. The nearest of the new neighbors is LHS 3746, a small ember of a star only one-third the mass and diameter of the Sun, emitting less than 1% of the Sun's light. With a parallax of 0.1322 +/- 0.0074, it is only 7.6 pc away. It ranks as the 113th nearest system and is the newest member of the 10 pc RECONS sample established by Henry, which now includes 316 objects in 228 systems. Each of the new discoveries provides a fresh target for planet hunters and, eventually, for astronomers searching for signatures of biological activity on those planets.
These new solar neighbors are merely the first wave of results from a large effort that will determine distances to 250 nearby star systems. During the three-year duration of CTIOPI, which was awarded 4 nights on the 0.9m and 2 nights on the 1.5m each month, nearby stars are identified as those tracing out wiggly snake-like paths on the sky relative to more distant stars. The length of the path is determined by the star's proper motion and its wiggle by its parallax. At present, there are more than 150 systems on the program at the 0.9m (6.8' field using the central 1K of the CCD) and nearly 100 more on the 1.5m (8.2' field using the entire CCD). Due to good weather, reliable telescopes, sensitive CCDs, and superb support at CTIO, CTIOPI is on track to exceed its goal of determining parallaxes better than 3 milliarcseconds (mas) for 150 systems. These first, albeit preliminary, parallaxes have errors of 2.3 to 7.4 mas using only 16 months of data (rather than the ultimate 36 months) and typically only half of the total number of frames anticipated (about 40) for 3 mas precision. We are doubly encouraged by the success rate for these first targets --- five for five within 15 pc (let alone the goal of 25 pc) is an excellent start.
Target systems --- white, red, and brown dwarfs with R magnitudes between 10 and 20 --- have been selected from a variety of sources. Proper motion candidates are chosen from the historic Luyten and Giclas surveys, for which 15% of systems with proper motions in excess of 1"/year have no parallaxes at all. In addition, several new proper motion surveys that illustrate the international flavor of nearby star work have provided new candidates, including several from Wroblewski et al (Chile), Scholz et al (Germany, United Kingdom), Mendez et al (Chile, United States), and Ruiz et al (Chile). Photometric candidates are chosen from the extensive published work of Weis (United States) and from the historical archives of Jahreiss (Germany), who has been generous in providing targets with unpublished data. Recently, several brown dwarf candidates (many of which are not yet published) have been provided by Delfosse of the DENIS team. A half dozen calibration stars are done at each telescope, and of course, Proxima is included. There are also a few dozen targets that are being observed intensely for perturbations caused by unseen companions, a classic astrometric experiment that has not been done for southern sky targets. When combined with data from Ianna's long-term Australia program, companions of brown dwarf mass with orbital periods of a decade might be revealed.
There's something very Star Trek-like about CTIOPI,'' says Todd Henry, a professor of astronomy at Georgia State University in Atlanta. ``We're filling in the explorer's map of nearby space, discovering the stars around which planets may someday be found, and eventually, perhaps even life.'' Since discovering the twentieth nearest star in 1997, Henry and Ianna, a professor of astronomy at the University of Virginia in Charlottesville, have expanded their search to include not only the small, red M dwarfs stars that make up 70% of stars in our galaxy, but also white dwarf stars that are leftover cinders of massive stars that have blown off their outer layers, plus the not-quite-stars known as brown dwarfs. ``Right now, the sample of nearby stars seems like a few fireflies hovering near out campfire. But we are sure that the real solar neighborhood is more like an entire field full of fireflies --- we just haven't seen them all yet'' comments Ianna.
At present, there are roughly 2100 systems known closer than 25 pc, the distance limit of a sample of stars being studied in great detail by NASA's Nearby Stars (NStars) Project. CTIOPI hopes to increase that number by 10% or more. NStars is part of a long-term initiative to promote research on nearby stars from both the ground and space, including work on fundamental stellar quantities, stellar populations and kinematics, history of the galaxy, extrasolar planets, habitable zones, and the search for life elsewhere. Both NASA and the NSF have teamed up to support the effort. Goals of the effort include (1) providing a robust, web-based data set for use by researchers and the public regarding stellar objects within 25 pc of the Earth, (2) characterizing stars especially in terms of astrobiology-related properties such as age, metallicity, variability, substellar companions, and circumstellar planetary "exozodi" dust, and (3) supporting definition of research programs using upcoming NASA Origins facilities such as SIRTF (Space Infrared Telescope Facility), SOFIA (Stratospheric Observatory for Infrared Astronomy), SIM (Space Interferometry Mission), and TPF (Terrestrial Planet Finder).
It is anticipated that there will be more than 10,000 stars, brown dwarfs, and planets within 25 pc when the NStars sample is complete. These are the stars that offer the best promise for answers to questions about the census of the Sun's neighbors, how many stars have planets circling them, and whether or not any of these planets harbor life. In order to have the best sample possible to answer these questions, CTIOPI hopes to discover a lot of hidden treasure, and to make a significant contribution to the neighborhood.
This work has been supported by the National Science Foundation (observing time at CTIO) and financially by NASA via the Nearby Stars Project at NASA-Ames and the Space Interferometry Mission at the Jet Propulsion Laboratory.