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Register a new userThe HST large programme on omega Centauri -- IV. catalogue of two external fields
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M. Scalco
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In the fourth paper of this series, we present -- and publicly release -- the state-of-the-art catalogue and atlases for the two remaining parallel fields observed with the Hubble Space Telescope for the large programme on omega Centauri. These two fields are located at ~12 from the centre of the globular cluster (in the West and South-West directions) and were imaged in filters from the ultraviolet to the infrared. Both fields were observed at two epochs separated by about 2 years that were used to derive proper motions and to compute membership probabilities.
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In this second installment of the series, we look at the internal kinematics of the multiple stellar populations of the globular cluster $omega$ Centauri in one of the parallel Hubble Space Telescope (HST) fields, located at about 3.5 half-light radii from the center of the cluster. Thanks to the over 15-year-long baseline and the exquisite astrometric precision of the HST cameras, well-measured stars in our proper-motion catalog have errors as low as $sim 10 mu$as 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 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 to 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 $eta_{rm 1G}=-0.007pm0.026$ for the former, and $eta_{rm 2G}=0.074pm0.029$ for the latter, where $eta$ is defined so that the velocity dispersion $sigma_mu$ scales with stellar mass as $sigma_mu propto m^{-eta}$. The kinematical differences reported here can help constrain the formation mechanisms for the multiple stellar populations in $omega$ Centauri and other globular clusters. We make our astro-photometric catalog publicly available.
In this paper we report a new estimate of the absolute proper motion (PM) of the globular cluster NGC 5139 ($omega$ Cen) as part of the HST large program GO-14118+14662. We analyzed a field 17 arcmin South-West of the center of $omega$ Cen and computed PMs with an epoch span of $sim$15.1 years. We employed 45 background galaxies to link our relative PMs to an absolute reference-frame system. The absolute PM of the cluster in our field is: $(mu_alpha cosdelta , mu_delta) = (-3.341 pm 0.028 , -6.557 pm 0.043)$ mas yr$^{-1}$. Upon correction for the effects of viewing perspective and the known cluster rotation, this implies that for the cluster center of mass $(mu_alpha cosdelta , mu_delta) = (-3.238 pm 0.028, -6.716 pm 0.043)$ mas yr$^{-1}$. This measurement is direct and independent, has the highest random and systematic accuracy to date, and will provide an external verification for the upcoming Gaia Data Release 2. It also differs from most reported PMs for $omega$ Cen in the literature by more than 5$sigma$, but consistency checks compared to other recent catalogs yield excellent agreement. We computed the corresponding Galactocentric velocity, calculated the implied orbit of $omega$ Cen in two different Galactic potentials, and compared these orbits to the orbits implied by one of the PM measurements available in the literature. We find a larger (by about 500 pc) perigalactic distance for $omega$ Cen with our new PM measurement, suggesting a larger survival expectancy for the cluster in the Galaxy.
We have constructed the most-comprehensive catalog of photometry and proper motions ever assembled for a globular cluster (GC). The core of $omega$Cen has been imaged over 650 times through WFC3s UVIS and IR channels for the purpose of detector calibration. There exist from 4 to over 60 exposures through each of 26 filters, stretching continuously from F225W in the UV to F160W in the infrared. Furthermore, the 11-year baseline between these data and a 2002 ACS survey has allowed us to more than double the proper-motion accuracy and triple the number of well-measured stars compared to our previous groundbreaking effort. This totally unprecedented complete spectral coverage for over 470,000 stars within the clusters core, from the tip of the red-giant branch down to the white dwarfs, provides the best astro-photometric observational data base yet to understand the multiple-population phenomenon in any GC. In this first paper of the series we describe in detail the data-reduction processes and deliver the astro-photometric catalog to the astronomical community.
We take advantage of the exquisite quality of the Hubble Space Telescope astro-photometric catalog of the core of wCen presented in the first paper of this series to derive a high-resolution, high-precision, high-accuracy differential-reddening map of the field. The map has a spatial resolution of 2x2 square arcsecs over a total field of view of about 4.3x4.3. The differential reddening itself is estimated via an iterative procedure using five distinct color-magnitude diagrams, which provided consistent results to within the 0.1% level. Assuming an average reddening value E(B-V)=0.12, the differential-reddening within the clusters core can vary by up to +/- 10%, with a typical a standard deviation of about 4%. Our differential-reddening map is made available to the astronomical community in the form of a multi-extension FITS file. This differential-reddening map is essential for a detailed understanding of the multiple stellar populations of wCen, as presented in the next paper in this series. Moreover, it provides unique insight into the level of small spatial-scale extinction variations in the Galactic foreground.
We present results of a search for optical counterparts of X-ray sources in and toward the globular cluster Omega Centauri (NGC 5139) using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The ACS data consist of a mosaic of Wide Field Channel (WFC) images obtained using F625W, F435W, and F658N filters; with 9 pointings we cover the central ~10x10 of the cluster and encompass 109 known Chandra sources. We find promising optical counterparts for 59 of the sources, ~40 of which are likely to be associated with the cluster. These include 27 candidate cataclysmic variables (CVs), 24 of which are reported here for the first time. Fourteen of the CV candidates are very faint, with absolute magnitudes in the range M_625 = 10.4 - 12.6, making them comparable in brightness to field CVs near the period minimum discovered in the SDSS (Gansicke et al. 2009). Additional optical counterparts include three BY Dra candidates, a possible blue straggler, and a previously-reported quiescent low-mass X-ray binary (Haggard et al. 2004). We also identify three foreground stars and 11 probable active galactic nuclei. Finally, we report the discovery of a group of seven stars whose X-ray properties are suggestive of magnetically active binaries, and whose optical counterparts lie on or very near the metal-rich anomalous giant and subgiant branches in {omega} Cen. If the apparent association between these seven stars and the RGB/SGB-a stars is real, then the frequency of X-ray sources in this metal-rich population is enhanced by a factor of at least five relative to the other giant and subgiant populations in the cluster. If these stars are not members of the metal-rich population, then they bring to 20 the total number of red stragglers (also known as sub-subgiants) that have been identified in {omega} Cen, the largest number yet known in any globular cluster.
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