History of NStars Database/Project

The Nearby Stars Database Project at NAU

I. Introduction and History - the 1997 TPF / Exozodi Dust Workshop

In October, 1997, the Exozodiacal Dust Workshop was held at NASA-Ames (summarized in http://astrobiology.arc.nasa.gov/workshops/zodiac/, proceedings edited by Backman et al. 1998). This workshop was driven by a need to understand the effects of exozodiacal dust clouds on the proposed Terrestrial Planet Finder (TPF)'s ability to detect Earth-sized life-bearing planets around nearby stars, so that TPF's design could be optimized. The workshop participants summarized what was known about exozodiacal clouds and how existing, planned, or possible new missions and projects could pave the way for TPF.

II. The NStars Project

A main recommendation of the Exozodiacal Dust Workshop was development of a master database regarding stars in the solar neighborhood. The ultimate goal in establishing the NStars Database is to produce a carefully selected list of target stars for TPF. The baseline design for TPF would be sensitive to Earth-like planets around Sun-like stars to a distance of about 15 pc, therefore the distance horizon for NStars is set conservatively at 25 pc. Along the road to TPF, an important secondary goal of the NStars project is to support study of nearby stars and circumstellar matter by NASA Origins missions such as SIRTF, SOFIA and SIM, plus ground-based interferometers such as Keck and LBT, leading to understanding evolution of the Sun, solar system, Earth, and life on Earth in context of our nearest stellar and planetary neighbors.

The NStars Project was established in April 1998 not only to produce the Database but also to foster new research on nearby stars. The first term of the project's NASA funding support ran through December 1999, the second term supposedly through December 2001.

The National Science Foundation announced a funding opportunity specifically to support research on nearby stars using ground-based facilities. Substantial motivation for this initiative came from NSF staff on the original NStars project's science working group. In turn, NStars personnel were directly involved in discussions in October '98 at NSF headquarters to define the NSF initiative. The NStars database is meant as the final destination for the data gathered by the NSF-funded research. NSF expects the NStars project in turn to specify formats and quality criteria for incoming data, and also to help coordinate the target lists and observations of the funded teams.

III. The NStars Database and TPF Targeting

The total number of stars within 25 pc (scaled from the better-known sample within 5 pc) is expected to be at least 7500, but only ~2600 objects are presently known to that distance. Thus the catalog is now only about 30% complete. In addition, substellar objects are being found rapidly enough that their total space density may be comparable to stars. Planets and brown dwarfs will be included in the NStars database.

In contrast to the European general stellar database SIMBAD and the nearby stars database CNS which are simply compendia of all published data on their catalog objects, NStars will include only quality-filtered data. Unlike the earlier general and "passive" stellar databases, the NStars project is focused on: (1) evaluation of stellar parameters related to astrobiology concerns; (2) determining which stellar characteristics need further definition and measurement, and promoting research by the astronomical community in those directions; and (3) critical examination of completeness of the sample, with active research aimed at adding to the nearby stars list.

Also unlike SIMBAD and CNS, the NStars project will do in-house analyses to determine the most likely values for higher-order parameters such as metallicity and age. Those stellar characteristics in particular require modeling and judicious weighing of disparate input information for their definition, but their astrobiological and TPF-target implications are straightforward. For example, stars with radial-velocity planet detections, and, significantly, our Sun, are all above the local median metallicity, suggesting that metal content of protoplanetary material has a strong effect on efficiency of planet formation. If this correlation remains true for larger and better-determined samples, then we will gain improved understanding of the planet-formation process, particulary for metal-rich planets resembling Earth, and also have a good criterion for first cut at a TPF target star list.

Ages of individual field (non-cluster) stars are also difficult to determine, requiring combination of a number of semi-correlated indices such as chromospheric emission intensity and rotation speed. Most astrobiological thought inclines toward the notion that only stars older than a certain age (in the few-Gyr range) can have planets where life has progressed enough to produce non-equilibrium atmospheric compositions detectable by TPF. Thus, TPF should be aimed first at the older stars in a given volume, but the ages of almost all nearby stars are presently unknown, waiting to be determined by use of NStars database information.

Completing the sample of nearby stars down to low-luminosity M dwarfs has a special connection to defining the TPF target list. Common wisdom has it that M stars are not expected to have Earth-like planets because such planets would need to be so near their primaries that tidal friction would halt or slow planets' rotation, rendering them sterile. But, some research indicate that a substantial atmosphere can redistribute heat around an otherwise too-slowly rotating planet. Also, an M0 star has fully half the Sun's mass and would heat a planet to terrestrial temperatures at the position of Mercury's orbit where tidal braking around a 0.5 Msun star might not be severe. Thus, blanket exclusion from TPF focus of the 80% of nearby stars that fall into the arbitrarly- defined spectral class M might be a mistake. NStars studies of M dwarf metallicity, age, variability, etc. could lead to substantial broadening of the TPF target list.

IV. General Astrobiology Scientific Return

The NStars database will have substantial value toward astrobiology in addition to serving as the basis for an eventual TPF target list. For example, careful observation and modeling of photospheric spectral energy distributions in the near- and mid-IR will allow more sensitive searches in the thermal infrared (with SIRTF and SOFIA) for excesses from exozodical dust representing asteroid and comet populations. As an example of such research, NStars project scientist Backman is co-investigator on a SIRTF Legacy proposal to examine a large sample of solar-type stars, partly overlapping the NStars volume, with ages bracketing the period of "heavy bombardment" in our solar system. This Legacy team hopes to infer the amount of planetesimal collisions and comet activity in these systems from measurements of circumstellar dust density, seeking information on whether the hypothesized impact "frustration of life" on the early Earth was a phenomenon that occurs commonly in young planetary systems.

NStars will allow the Sun to be placed in a better-determined context of normal stars in regards characteristics such as variability, surface activity, and luminosity changes, crucial to understanding the persistence and evolution of life on Earth. NStars will also contribute to our understanding of the star formation process by substantially improving statistics on stellar characteristics such as rotation and multiplicity. NStars has the potential to become the largest, most complete, and best determined sample of typical stars in a typical spiral galaxy, providing important observational and statistical constraints on models of stellar interiors and stellar evolution.

V. NStars Education and Outreach

Because of the very strong public interest in the search for other planetary systems and signs of life elsewhere, this program presents an excellent opportunity to reach students in the local school districts. The staff of the NStars project devote 5% of their time to presenting enrichment talks in the local public schools (near NASA-Ames, Franklin & Marshall College, and Georgia State) and at other public forums.

VI. Personnel and Management

The NStars Database project scientists are Dr. Dana Backman of Franklin and Marshall College and Dr. Todd Henry of Georgia State University. They are assisted by research associate Jerry Blackwell and programmer Takeshi Okimura.

Dana Backman has worked since his Ph.D. (1985 at U. Hawaii) on: infrared characteristics of nearby stars; structure and evolution of proto- planetary systems such as beta Pictoris; angular momentum history of young solar-type stars; and connections between solar system dust and exozodiacal clouds. He has also co-authored a paper on the persistence of habitable conditions on for 0.7 Gyr Mars after the formation of the solar system. Backman has permission from Franklin & Marshall College to take leaves of absence for 3 spring semesters ('99, '00, '01) to manage the NStars project. This will allow him to spend January through August of each of those years at NASA-Ames.

Todd Henry has studied various aspects of the nearest stars since beginning his Ph.D. in 1986 at the University of Arizona. In addition to searching for low mass stellar and substellar companions to the nearest stars, he has investigated spectral characteristics and stellar activity, and calibrated the important relation between luminosity and mass. Highlights of his research include examining hundreds of stars like the Sun for those with ages and activity levels appropriate for SETI searches, providing the definitive data on nearby stars for a National Geographic map of nearby space, and discovering the 20th nearest star. He is now leading a large effort in Chile to discover up to 300 new members of the NStars sample. Henry will continue with NStars as a major component of his research, including developing a research group with graduate and undergraduate students at Georgia State University.

Jerry Blackwell has a engineering background in metal products design & manufacturing. He worked for the same national corporation (Anchor Fence Company) for 33 years in the areas of R&D, Plant Management and Sales Engineering. A lifelong interest in physics and computers turned into an exclusive pursuit to study the nearby stars. Utilizing the then new and growing power of the internet Jerry formed a worldwide astronomy organization (NBSO.ORG) to concentrate on nearby star issues. He also compiled the most complete dedicated near star (20 parsec radius) computer database available prior to the launching of the NASA NStars project. Jerry is committed to help NStars become the premier, research grade nearby star catalog and database.

The NStars programmer and web-page maintainer is Takeshi Okimura, a contractor from Symtech Inc. working at NASA-Ames while continuing to take courses for a degree at San Jose State University. Collectively, the Symtech web design team possesses all the skills needed to build effective web sites. The team's experience covers every phase of web site programming. Moreover, the Symtech team has a strong background in the system administration and web server maintenance essential to keeping web sites up and running. Symtech is expert in developing databases, and uses them to generate many of its web pages with dynamic content. This dramatically reduces the laborious process of building web pages one at a time, and permits non technical personnel to add up-to-the-minute content quickly and easily. The early NStars software, database, and web page development involved previous part-time employment of Symtech contract personnel Sarah West, Eric Vacin, Mick Storm, Rob Christensen, and Peter Mariani.

B) Students

Five undergraduate students working directly under Backman have contributed as research assistants: Avi Mandell of Vassar College (now at Yale), Mike Connelley of Santa Clara University, plus Aaron Burgman, Candace GrandPre and Peter Nothstein of F&M College. Several undergrad students have also contributed to NStars under Henry: Kouri Coleman of Washington College, MD (now at Houston Space Flight Center), Todd Barto (also of Washington College, now at Goddard Space Flight Center), and three undergraduates at Johns Hopkins University (where Henry was located until August 2000): Sharon Jue, Lucianne Walkowicz, and Christina Williams.

VII. Progress and Accomplishments, April '98 to October '00

A. Database status

As of 10/1/00 the NStars database had the most complete and high-quality list of all stars within 25 pc available to the world scientific community. The core of the Database consists of Todd Henry's RECONS database on all stars within 10 pc plus data from other sources on stars in the 10 to 25 pc range. Visual photometry, near-IR photometry from 2MASS, and IRAS far-IR fluxes and limits are loaded or within weeks of being loaded for all the catalog stars. Optical 10x10 arcmin field images / finding charts for 1000 of the catalog stars were available as of 10/00, with another 1000 systems' images to be loaded in October and November. Michigan Catalog spectral types from the half of the sky covered by that survey have been extracted and will be loaded within a few weeks.

B. NStars Workshop

The Project staff hosted a Nearby Stars workshop at NASA-Ames in June 1999. The format involved a small number of invited speakers plus poster presentations. The invited talks addressed major topics in astrophysical research on nearby stars including: filling in the sample; searches for sub-stellar companions; precise definition of spectral types; details of atmospheric modeling; studies of microvariability, and searches for planets. The invited talks, posters, and notes from discussion sessions will be published as a NASA conference book in 2000. A preliminary version of the Database was demonstrated to the workshop participants. Capabilities to display the entire database as a scroll-through menu, to display all the data on individual stars, and to define catalog subsets based on parameter values and ranges were shown. Substantial comments from the researchers present were collected for further improvement of the Database and its user interface.

C. NStars "in-house" research

Avi Mandell of Vassar College (now at Yale) did a reading project on star ages and then calculated isochrone ages (incl. effects of estimated metallicity) for the A, F, and G stars within 20 pc using newly precise luminosities available through Hipparcos parallaxes. Mike Connelley of Santa Clara University used Hipparcos parallaxes and proper motions to find moving groups among the NStars sample and nearby stars moving parallel with more distant known clusters and stellar groups. Mandell's and Connelley's results were combined into a poster presentation at the January '00 AAS meeting in Atlanta, "Isochrone Ages and Moving Groups of Nearby Stars" (Mandell, Connelley, and Backman). Aaron Burgman of F&M College compiled published spectra and visual wavelength (Digital Sky Survey) images of nearby stars for loading into the database. Peter Nothstein of F&M College began a project, continued by Candace GrandPre, of monitoring microvariability of nearby stars using the Phoenix-10 automatic telescope in Arizona that is controlled from F&M College. Henry's students at Johns Hopkins and Georgia State were involved in aspects of identifying candidate nearby stars from proper motion surveys and quality-checking data being loaded into the NStars database.

D. Support for NASA Missions

Backman addressed the SIRTF SWG meeting in March '99 about NStars and support for definition of SIRTF observing programs. Partly as a result of NStars Backman was invited to join the SIM SWG, the TPF SWG, and two SIRTF instrument GTO teams (IRAC and MIPS). Backman gave a talk to the SOFIA star formation workshop in Santa Cruz during July '99 on possible SOFIA key projects regarding nearby stars.

VIII. Work continuing in 2000-2001

A. Complete version 1 of the Database: Enter quality-controlled photometric, image, and spectral data on catalog objects. Collect bibliographic data from published work containing only significant mention of each star. Improve and extend the Database web user interface.

B. Study methods for precise determination of star ages to support evolutionary interpretation of detected planets and planetary material, and to help choose preliminary astrobiology (e.g. TPF) targets from among the older systems.

C. Sponsor a second NStars workshop in summer 2001 to bring together investigators funded under the NSF/NASA nearby stars initiative, to review the completed v.1 NStars Database, and to evaluate the astrobiological prospects for stars within 25 pc (e.g., revisit the 'common wisdom' that M dwarfs are not likely hosts for Earth-like planets).

D. Coordinate gathering of new data by researchers receiving grant support under the joint NASA/NSF Nearby Stars initiative, and devise schemes to quality-check and combine the incoming data.

E. Study calibration and modeling of stellar photospheres in the mid- and far-IR to high precision such that limits to detection of circumstellar planetary debris / exozodiacal dust by SOFIA and SIRTF can go well below the approximately 500-zodi level possible with IRAS and ISO data.