A New Method to Determine the Local Standard of Rest Velocity Using Sagittarius Tidal Debris

Details A New Method to Determine the Local Standard of Rest Velocity Using Sagittarius Tidal Debris A New Method to Determine the Local Standard of Rest Velocity Using Sagittarius Tidal Debris

Aug 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008noao.prop..448c&link_type=abstract

NOAO Proposal ID #2008B-0448

Computer Science

Scientific paper

We still do not know the absolute value of the Sun's rotational velocity about the Milky Way to the 25% level (~55 km s^-1). This means we can not even accurately place our own galaxy on the Tully- Fisher relation. Previous methods to determine Θ_LSR, the velocity of the Local Standard of Rest, give results that are degenerate with the distance to the Galactic center, R_0. However, the fortuitous location and orientation of the Sagittarius (Sgr) dSph trailing tidal tail in the Southern Milky Way hemisphere means that it provides a fixed velocity zero-point reference that allows measurement of Θ_LSR directly and in a way virtually independent of R_0; this is because the (mu)_l component of proper motion for these Sgr stars is almost entirely due to solar reflex motion. We have derived proper motions of deep (V ≳ 19.5), magnitude- limited surveys of stars in the direction of the Sgr tails using photographic plates typically spanning a 90-year baseline; these produce mean proper-motions with an accuracy of better than 0.5 mas yr^-1 for Sgr debris in the sampled fields. However, to take advantage of these quality proper motions to make the new measurement of Θ_LSR we have to identify pure samples of Sgr stars in our proper motion survey. Fortunately, Sgr stars readily identify themselves by their kinematics in these survey directions. We propose to obtain Hectospec multifiber radial velocities of stars in our proper motion survey optimized to those with color-magnitude combinations expected for Sgr debris to identify pure Sgr samples that will allow us to derive Θ_LSR to better than 10 km s^-1. Of course, by breaking the degeneracy between Θ_LSR and R_0 we will also define the latter parameter, which is still unknown at the 20% level. This proposal was granted time by NOAO in 2006 but was not able to be scheduled by the MMT that year.

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