NGC 6791: Space Interferometry Mission Plans For Binaries, Colors, And Parallaxes

Details NGC 6791: Space Interferometry Mission Plans For Binaries, Colors, And Parallaxes NGC 6791: Space Interferometry Mission Plans For Binaries, Colors, And Parallaxes

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21345511p&link_type=abstract

American Astronomical Society, AAS Meeting #213, #455.11; Bulletin of the American Astronomical Society, Vol. 41, p.355

Astronomy and Astrophysics

Astronomy

Scientific paper

At 8Gyr and three times solar metallicity, the open cluster NGC 6791 is uniquely old, massive, homogenous, metal-rich, and nearby, and can test stellar isochrones and provide a resolved template for ancient 'red-and-dead' elliptical galaxies. Largely neglected to date is the influence of its binary population, despite a 50% main-sequence binary fraction. To better understand the frequency of production of merger products such as extreme blue horizontal branch stars and blue stragglers, whose blue light can mimic that of young stars, we have undertaken radial-velocity surveys of its giants and red and blue horizontal branch stars, which we summarize here.
Long-period binaries are difficult to detect, however, because velocity amplitudes are generally a few km/s since eccentricities are significant. As part of our SIM Science Studies program, we will photometrically examine membership and binarity as well. SIM is capable of measuring the parallax and binary orbits of well-chosen giant stars, a significant goal here, but the technique can be applied all across the color-magnitude diagram (CMD).
We will begin empirically by constructing color-color diagrams from existing photometry in a multitude of bandpasses for the cluster. We will then attempt to model each diagram theoretically, by extending calculations of fluxes and colors for solar-metallicity and metal-rich stars across the range of temperatures from the giant branch to the main sequence. By itself this will yield color conversions from the observational CMD colors to the physical stellar parameters of temperature and metallicity, also of critical importance to age and metallicity determinations based on comparing cluster color-magnitude diagrams (CMD's) to theoretical isochrones. Applied directly to NGC 6791 photometry, it will yield constraints on the frequency and mass ratio distribution of cluster binaries.

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