Proper-Motion Study of the Magellanic Clouds Using SPM Material

Abstract

-Absolute proper motions are determined for stars and galaxies to V=17.5 over a 450 square-degree area that encloses both Magellanic Clouds. The proper motions are based on photographic and CCD observations of the Yale/San Juan Southern Proper Motion program, which span over a baseline of 40 years. Multiple, local relative proper motion measures are combined in an overlap solution using photometrically selected Galactic Disk stars to define a global relative system that is then transformed to absolute using external galaxies and Hipparcos stars to tie into the ICRS. The resulting catalog of 1.4 million objects is used to derive the mean absolute proper motions of the Large Magellanic Cloud and the Small Magellanic Cloud; (µ α cos δ, µ δ ) LM C = (1.89, +0.39) ± (0.27, 0.27) mas yr −1 and (µ α cos δ, µ δ ) SM C = (0.98, −1.01) ± (0.30, 0.29) mas yr −1 . These mean motions are based on best-measured samples of 3822 LMC stars and 964 SMC stars. A dominant portion (0.25 mas yr −1 ) of the formal errors is due to the estimated uncertainty in the inertial system of the Hipparcos Catalog stars used to anchor the bright end of our proper motion measures. A more precise determination can be made for the proper motion of the SMC relative to the LMC; (µ α cos δ , µ δ ) SM C−LM C = (−0.91, −1.49) ± (0.16, 0.15) mas yr −1 . This differential value is combined with measurements of the proper motion of the LMC taken from the literature to produce new absolute proper-motion determinations for the SMC, as well as an estimate of the total velocity difference of the two clouds to within ±54 kms −1 . The absolute proper motion results are consistent with the Clouds' orbits being marginally bound to the Milky Way, albeit on an elongated orbit. The inferred relative velocity between the Clouds places them near their binding energy limit and, thus, no definitive conclusion can be made as to whether or not the Clouds are bound to one another.

The VMC survey

“…Centre of mass proper motion measurements from the literature for both the SMC and 47 Tuc are indicated by empty squares whose sizes correspond to their statistical uncertainties. For the SMC the red rectangles (from smallest to largest) give the values from Kallivayalil et al (2013), Costa et al (2011), Vieira et al (2010, respectively, and for 47 Tuc the blue rectangle gives the value from Anderson & King (2003).…”

“…Oxygen-rich AGB stars analysed by Vieira et al (2010) from observations included in the Southern Proper Motion program (Wesselink 1974) result in proper motion values of (μ α cos(δ), μ δ ) = (+0.98 ± 0.30, −1.10 ± 0.29) mas yr −1 (Fig. 8).…”

SMC west halo: a slice of the galaxy that is being tidally stripped?

“…Both scenarios are limited to morphology and kinematics and have their advantages and drawbacks, but the deadlock of the discussion might be solved with proper motions of the clouds (e.g. Kallivayalil et al 2013, using a space-based telescope; Vieira et al 2010, using a ground-based telescope).…”

The VMC survey

“…On the contrary, large statistical samples from groundbased observations have large uncertainties in the measured motions and are, at present, restricted to the outer regions of the Magellanic Clouds (e.g. Vieira et al 2010;Costa et al 2011) or are limited to a specific type of stars, i.e. oxygen-rich (O-rich) asymptotic giant branch (AGB) stars (Vieira et al 2010).…”

“…Vieira et al 2010;Costa et al 2011) or are limited to a specific type of stars, i.e. oxygen-rich (O-rich) asymptotic giant branch (AGB) stars (Vieira et al 2010).…”