The Curious Case of Palomar 13: The Influence of the Orbital Phase on the Appearance of Galactic Satellites

TL;DR

This study uses N-body simulations to show that Palomar 13's peculiar properties are likely due to its orbital phase near apogalacticon, affecting its appearance and internal dynamics, with implications for other galactic satellites.

Contribution

It demonstrates that the orbital phase, especially near apogalacticon, can significantly influence the observed properties of globular clusters and satellites, providing an alternative explanation to tidal shocking.

Findings

Palomar 13 is likely near apogalacticon, appearing supervirial.

Orbital phase affects the observed mass-to-light ratio and surface density.

Discrepancies in proper motion suggest observational uncertainties.

Abstract

We investigate the dynamical status of the low-mass globular cluster Palomar 13 by means of N-body computations to test whether its unusually high mass-to-light ratio of about 40 and its peculiarly shallow surface density profile can be caused by tidal shocking. Alternatively, we test - by varying the assumed proper motion - if the orbital phase of Palomar 13 within its orbit about the Milky Way can influence its appearance and thus may be the origin of these peculiarities, as has been suggested by Kuepper et al. (2010). We find that, of these two scenarios, only the latter can explain the observed mass-to-light ratio and surface density profile. We note, however, that the particular orbit that best reproduces those observed parameters has a proper motion inconsistent with the available literature value. We discuss this discrepancy and suggest that it may be caused by an underestimation of the observational uncertainties in the proper motion determination. We demonstrate that Palomar 13 is most likely near apogalacticon, which makes the cluster appear supervirial and blown-up due to orbital compression of its tidal debris. Since the satellites of the Milky Way are on average closer to apo- than perigalacticon, their internal dynamics may be influenced by the same effect, and we advocate that this needs to be taken into account when interpreting their kinematical data. Moreover, we briefly discuss the influence of a possible binary population on such measurements.