Monday, September 04, 2006

D. Pourbaix: Binaries in Large-Scale Surveys (S 240 invited talk, Wed, Aug. 23)

This talk gave a thorough overview on ground- and space-based surveys relevant to binary stars.

Starting with unresolved binaries, Wielen (1996) suggested two ways of unveiling unresolved binaries:
  • Variability-induced movers (VIM), for which the position of the photocenter moves back and forth along a line, and
  • Colour-induced displacement binaries (CID), for which one takes images through different filters and sees the displacement. This was illustrated by a plot for the five SDSS filters. The five positions of the photocenter will follow the path of the binaries (a line), ordered by wavelength (as opposed to random displacements for a single star). Thus, one can detect binaries with separations below the resolving power of the instrument.

This was followed by a list of eclipsing binary surveys:
  • Robotic Optical Transient Serach Experiment
  • Optical Gravitaional Lensing Experiment
  • All Sky Automated Survey
  • MACHO
  • Discovery Channel Survey
and others. All surveys are essentially automated.

Then followed a discussion of the usefulness of general surveys with respect to binary stars. From 2MASS, one can get positions and IR magnitudes to confirm binarity, and it serves as a source of candidates.

From SDSS, binaries can be detected in several ways:
  • Spectrum analysis (Balmer lines from hot stars and TiO from a cool companion, or narrow H emission lines superposed on the WD spectrum)
  • Outliers in the color-color diagram
  • Spectroscopic binaries (but there are only 10000 objects with two or more spectra, the maximum number of spectra is 13)
  • CID binaries
Results:
  • Spectrum analysis: 747 detached close binaries (Silvestri et al. 2005)
  • Color outliers: 863 WD+MD pairs (Smolcic et al. 2004)
  • SBs: 675 candidates (19 orbits, Pourbaix et al. 2005)
  • CID binaries: 542 candidates (Pourbaix et al. 2004)

Other useful surveys are
  • High proper motion stars (bulge): A group at CfA and others have obtained more than 300 spectroscopic orbits.
  • Open clusters: Mermilliod & Paunzen (2003)
  • By-products of Planet Quest (late-type stars): Nidever et al.

Potentially useful surveys are
  • Guide star catalog II (proper motions, BVI magnitudes, positions)
  • Palomar Quest
  • CFHT Legacy Survey
  • UKIRT (looking for WD+BD and subgiant+BD binaries)
  • Radial Velocity Experiment (RAVE)
  • Pan-STARRS

Space missions:
  • IUE has observed about 150 spectroscopic orbits (Stickland et al.).
  • FUSE took over in 1999.
  • Hipparcos (double and multiple star annexes, DMSA), the so-called "Hipparcos Poor Astronomer Sky Survey" (What can be done with existing Hipparcos data?). The percentage of binaries among variable stars with a large brightness variation increases with V-I. This is a CIM effect. There is correlation between the position of the photocenter and the brightness. HIP 88848 served as an example for "changing a mess into science" (Fekel et al. 2005, third body). Torres et al. (2006) discusses the eclipsing binary V1061 Cyg. A poster by Halbwachs & Pourbaix was also mentioned. Makarov et al. discuss the identification of binaries comparing Tycho-2 proper motions with Hipparcos ones.
  • Gaia will enable the detection of binaries and multiple stars by astrometry, spectroscopy and photometry (variability, outliers). There will be enough data to investigate the binary frequency over the HR diagram, across stellar populations, etc.

Conclusions:
  • Do not put all your eggs in the same pocket (do not survey only eclipsing binaries)
  • Someone's garbage can be science for others (e.g. stars in the SDSS)
  • Public archives are gold mines for immediate scientific results (enough data exist to give one paper for each of us) and training sets for future science
  • Do not push garbage recycling too much, beyond the specs (cf. the Hipparcos-planets controversy)

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