A population of accreted small magellanic cloud stars in the large magellanic cloud

Abstract

We present an analysis of the stellar kinematics of the Large Magellanic Cloud (LMC) based on 5900 new and existing velocities of massive red supergiants, oxygen-rich and carbon-rich asymptotic giant branch (AGB) stars, and other giants. After correcting the line-of-sight velocities for theLMC's space motion and accounting for asymmetric drift in the AGB population, we derive a rotation curve that is consistent with all of the tracers used, as well as that of published H I data. The amplitude of the rotation curve is v 0 = 87 5km s-1 beyond a radius R 0 = 2.4 0.1kpc and has a position angle of the kinematic line of nodes of θ = 142°±5°. By examining the outliers from our fits, we identify a population of 376 stars, or ≳5% of our sample, that have line-of-sight velocities that apparently oppose the sense of rotation of theLMC disk. We find that these kinematically distinct stars are either counter-rotating in a plane closely aligned with theLMC disk, or rotating in the same sense as theLMC disk, but in a plane that is inclined by 54° ±2° to theLMC. Their kinematics clearly link them to two known H I arms, which have previously been interpreted as being pulled out from theLMC. We measure metallicities from the Ca triplet lines of 1000LMC field stars and 30 stars in the kinematically distinct population. For theLMC field, we find a median [Fe/H] = -0.56v0.02 with dispersion of 0.5 dex, while for the kinematically distinct stars the median [Fe/H] is -1.25 0.13 with a dispersion of 0.7 dex. The metallicity differences provide strong evidence that the kinematically distinct population originated in the Small Magellanic Cloud. This interpretation has the consequence that the H I arms kinematically associated with the stars are likely falling into theLMC, instead of being pulled out. © 2011. The American Astronomical Society. All rights reserved..