AAS 203rd Meeting, January 2004
Session 49 Stars Looking Forward to Retirement
Poster, Tuesday, January 6, 2004, 9:20am-6:30pm, Grand Hall

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[49.03] Understanding the Effects of Starspots and Pulsations of K Giants on the SIM Astrometric Grid

D. R. Ciardi (Michelson Science Center)

The Space Interferometry Mission (SIM), scheduled for launch in 2009, will be able to perform ~1 micro-arcsec (\muas) astrometry in a narrow-angle mode (within ~1\circ) and ~4 \muas astrometry in a wide-angle model (within ~15\circ). In order to achieve such high precision astrometry, SIM requires a set of astrometrically stable stars spread quasi-uniformly across the sky - collectively known as ``the grid''. The grid will be composed of 1302 early K giant (K0 - K2 III) stars, located at \gtrsim 1000 pc. The stars must be astrometrically stable to ~4 \muas over the 5 year lifetime of the SIM mission. Here we model the effect of starspots on the astrometric position of the photocenter. We find for early-type K giants that starspots shift the apparent photocenter by \lesssim 1 \muas. Additionally, a radial velocity program with a precision of 50 m/s is being performed to remove giant stars with stellar and sub-stellar companions from the grid. Here we model the effect of pulsations on the intrinsic radial velocity stability of the giant stars. We find the early K giants are atmospherically stable to \lesssim 50 m/s, as determined empirically in earlier studies. We attribute the cause of milli-magnitude photometric variability observed in giant stars (III) to stellar pulsations and not starspots.

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.