The HST Large Programme on ω Centauri. II. Internal Kinematics

Abstract

In this second installment of the series, we look at the internal kinematics of the multiple stellar populations of the globular cluster ωCentauri in one of the parallel Hubble Space Telescope (HST) fields, located at about 3.5 halflight radii from the center of the cluster. Thanks to the over 15 yr long baseline and the exquisite astrometric precision of the HST cameras, well-measured stars in our proper-motion catalog have errors as low as ∼10μasyr −1 , and the catalog itself extends to near the hydrogen-burning limit of the cluster. We show that second-generation (2G) stars are significantly more radially anisotropic than first-generation (1G) stars. The latter are instead consistent with an isotropic velocity distribution. In addition, 1G stars have excess systemic rotation in the plane of the sky with respect to 2G stars. We show that the six populations below the main-sequence (MS) knee identified in our first paper are associated with the five main population groups recently isolated on the upper MS in the core of cluster. Furthermore, we find both 1G and 2G stars in the field to be far from being in energy equipartition, with h = - 0.007 0.026 m . The kinematical differences reported here can help constrain the formation mechanisms for the multiple stellar populations in ωCentauri and other globular clusters. We make our astro-photometric catalog publicly available.

The proper motion of sub-populations in ω Centauri

“…Moreover, the metal-poor stars show systemic rotation, while the metal-rich stars seem to be non-rotating. These results were confirmed by van de Ven et al (2006) and Bellini et al (2018), who used different sets of data to find differences in the radial distribution and rotation of the sub-populations, as well as possible differences in their anisotropy. However, based on their radial velocity investigations, Pancino et al (2007) and van Loon et al (2007) did not find any significant difference in the rotation or velocity spreads among the sub-populations.…”

“…The authors suggested that an internal stellar proper motions investigation was required, but at that time a catalogue of sufficient quality was not available. More recently, Bellini et al (2018) and Libralato et al (2018) investigated the proper motions of small external regions of the main sequence (MS) sub-populations of the cluster using HST data and found that all the sub-populations -which do not correspond exactly to those defined by Ferraro et al (2002) -share the same median proper motions within the uncertainties. Any difference they found was very small, much smaller than the 0.8 mas yr −1 found by Ferraro et al (2002).…”

The HST Large Programme on ω Centauri. III. Absolute Proper Motion

“…In PaperII (Bellini et al 2018), we began to investigate the internal kinematics of the multiple stellar populations of the cluster.…”

Extending Gaia DR2 with HST narrow-field astrometry – II. Refining the method on WISE J163940.83−684738.6★

“…For the same reason it would also be interesting to combine them properly. Indeed, while unaccounted systematic errors in our estimated parallax could be as large as ∼5 mas, due to the problematic epochs around 2013.8 (see Sect 4.3), based on our experience we can hardly expect residual systematic errors larger than 1 mas yr −1 in the estimated proper motions derived from HST data (e.g., Bellini et al 2018 and reference therein). As we do not have the competence to analyse Spitzer data at the same level of accuracy as we have done for the HST data (not only distortion and positioning, but particularly the way to simultaneously fit HST data with data from a telescope in a significantly different, Earth-trailing, Heliocentric orbit), we list in Table 3 our HST individual measurements to allow future investigators to be able to properly combine the two space-based datasets.…”