Statistics – Applications
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21360602c&link_type=abstract
American Astronomical Society, AAS Meeting #213, #606.02
Statistics
Applications
Scientific paper
We have used HST/ACS to produce precise proper motions (better than 0.3mas/yr in both coordinates) for >81,000 stars along a single sight-line towards the Milky Way Bulge, many of which are on the main sequence and thus allow dissection of proper motions by photometric distance. Full details on the astrometry and some example applications are provided in Clarkson et al. (2008 ApJ, hereafter C08; also 2009 Nature in final prep, see also Sahu et al. 2006); here we discuss one application to the structure of the inner Milky Way.
The partition of the Milky Way Bulge into kinematic populations is currently controversial; abundances suggest rapid formation in a classical merger-built Bulge (e.g. Ballero et al. 2007), while its integrated light and bulk kinematics suggest a bar-like structure (e.g. Dehnen & Binney 1998; also called a Boxy Bulge or Peanut Bulge in the literature), expected to form from disk instabilities. Cut by distance towards the bulge, our motions suggest a spheroid component participating in solid body-type rotation interior to cutoff radius Rc 0.5-0.7 kpc, flattening briefly interior to 2.5kpc before finally
transitioning to disk-like motion. Taken with radial velocity cuts across the Bulge (Rich et al. 2007; Howard et al. 2008; also poster 416.07), this nesting occurs in both distance D and Galactic longitude l. The Bulge thus likely contains a long bar of major axis 2.5kpc plus a spheroid of radius 0.5-0.7kpc. Our interpretation is consistent with both star counts and N-body simulations in the literature.
Controversy has recently arisen as to whether the commonly-used red clump objects and K,M giants really are tracers of the majority population; the dwarfs might belong to a more metal-rich population than the giants (e.g. Cohen et al. 2008 vs Sumi et al. 2004). Our motions show no systematic differences between the populations.
Anderson Jeffrey
Bond Howard E.
Brown Tim
Casertano Sefano
Clarkson Will
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