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Science Case

The Hipparcos Survey

The Hipparcos Catalog (ESA 1997)1 provides a valuable compendium of astrometric data for a well-chosen set of stars. About half the stars in the Hipparcos Input Catalog (Turon et al. 1993)2 were selected to be complete to well-defined magnitude limits, and the catalog includes excellent statistical samples of the most important stellar types present in the extended solar neighborhood. The remaining stars were selected corresponding to planned observing programs. Despite technical difficulties, the Hipparcos mission realized an extraordinary transformation in stellar astronomy by providing accurate parallax and/or proper motion data for over 100,000 stars. These data have proved useful for many science programs from the distances to subdwarfs (Reid 1997)3 to Barium (Mashonkina et al. 1999)4 and S-type stars to the distance scale of RR Lyrae stars (Groenewegen & Salaris 1999)5.

The Hipparcos Catalog contains some 73,000 stars north of declination -20 degrees. The bulk of the stars (71%) are brighter than V=9. Counting just the stars with parallax errors less than 10% of the measured parallax, the catalog contains more than 10,000 stars north of declination -20 degrees and brighter than V=9. The stars are distributed in color from early (B-V < 0) to late (B-V > 1.6), with most of the stars being within the color range 0.3 < B-V < 1.3. All luminosity classes are represented. The distribution in apparent V magnitude and color of stars with well-measured parallaxes is shown in the figure below.

 

 

Distribution of Hipparcos stars with precise parallaxes with B-V color and apparent V magnitude. The sample totals about 104 stars brighter than V=9.

 

 

 

 

 

Hipparcos stars with accurate kinematic data are also well-characterized photometrically. The stars have been observed with uvby photometry in the ongoing survey by Nordstrom et al. (1999)6 ,who obtained not only photometric data, but also precision radial velocity observations through the Geneva group using CORAVEL. These observations allow the determination of ages and binary status for all stars in the sample.

Spectral Libraries

Comparable spectroscopic catalogs would prove equally valuable. The Observatoire de Haute-Provence, for example, hosts an archive of R=42,000 ELODIE (see Moultaka et al. 2004) 7. The ELODIE archive comprises some 18,000 spectra of about 2500 distinct objects with S/N ratios above 10 (about 3000 spectra have S/N ratios above 100), representing some 8 years of investigator programs. The archive has been utilized for numerous research studies, as discussed by Moultaka et al.

A library of digital stellar spectra at high spectral resolving power (R=60,000 to resolve thermal line profiles in solar-type stars) with a statistically significant sample of stars of all spectral types would provide the means to address many important and longstanding problems in stellar astronomy. A uniform database of spectra will allow the systematic determination of stellar properties, including temperature, radius, surface gravity, detailed composition, and projected rotational velocity across the HR diagram. The spectra will have sufficient resolution to allow studies of some line profiles, and to recognize double-line spectroscopic binaries.

Science with FHiRE and FHiREBot

The large and uniform database of spectra planned for FHiRE and FHiREBot will allow statistical approaches to the study of many important astrophysical problems. In the area of abundances, detailed composition data, analyzed in a uniform way, can be used to correlate with kinematic data to study galactic structure and to strengthen the identification of moving groups and streams. A uniform set of detailed compositions would provide us with a statistically valid picture of the chemical enrichment history of the Milky Way, not just for iron but for 20 or 30 atomic species. The spectral region to be included in the FHiRE observations, 390-680 nm, includes numerous atomic and molecular lines that are of use in various regions of the HR diagram, from O and B stars to K and M giants and dwarfs. With uniform spectroscopy, stars in all parts of the HR diagram can be compared and contrasted, and the systematic errors that affect the determination of composition in different types of stars can be better understood.

The detailed survey of Edvardsson et al. (1993)8 pioneered the comprehensive study of the chemical evolution of the disk of the Milky Way with an analysis of 189 nearby field F and G dwarf stars with ages ranging from one to ten Gyr. While they achieved a spectral resolving power of 80,000 for the brighter stars, stars fainter than V=6.5 were observed with resolution similar to that proposed here. Their observations were limited to a few spectral regions, allowing them to determine abundances for only 13 species from oxygen through neodymium. Among the many important results of this study is the observation of a considerable range of metallicity an any given age of formation in the Galactic disk.

The FHiRE spectral survey will be able to extend the work of Edvardsson et al. in several ways.

  • With broad spectral coverage from 390 to 680 nm, many more species can be studied, with the addition of carbon and its isotope 13C, many additional transition metals, and many additional heavy metals.
  • With much larger sample sizes, we will be able to investigate changes in the detailed chemical compositions with time and in the dispersion in element/element ratios with time. Larger statistical samples will allow us to identify and study sub-populations of the disk, and to explore the origins of elements with greater depth than previously possible. The heavy metals will receive particular attention, since multiple sources seem to contribute to the current distributions of heavy elements.
  • With statistical samples of stars of well-determined age we can connect changes in relative element abundances to specific progenitor masses beginning to contribute yields as the Galaxy ages.
  • With the relatively fainter limiting magnitude of the survey, we will be able to include substantially younger stars to probe the more recent chemical evolution of the disk, and identify stellar element producers among even lower mass progenitors.

Studies of the lithium abundance in stars will particularly benefit from spectroscopy of large samples. Present work to understand the diagnostic value of lithium, and its correlation with stellar rotation, is limited by the relatively small samples of stars that can be analyzed through the normal approach of limited observing runs. Indeed, studies of the angular momentum evolution of stars will be greatly aided by accurate vsini measurements of large samples of stars with ages known from analysis of photometric data and parallaxes.

Studies of interstellar features, including atomic features and diffuse interstellar bands, in a large sample of stars at high spectral resolution and S/N ratio will allow us to obtain a much more detailed model of the local interstellar medium than has heretofore been possible.

In addition to the survey science, the FHiRE spectrographs will enable targeted observations addressing particular science questions, particularly those exploiting the time domain for stellar variability.

 


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Last updated: 15 November 2005
Comments: astdept
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