HST astrometry in the Orion Nebula Cluster: census of low-mass runaways

We present a catalog of high-precision proper motions in the Orion Nebula Cluster (ONC), based on Treasury Program observations with the Hubble Space Telescope's (HST) ACS/WFC camera. Our catalog contains 2,454 objects in the magnitude range of $14.2<m_{\rm F775W}<24.7$, thus probing the stellar masses of the ONC from $\sim$0.4 $M_\odot$ down to $\sim$0.02 $M_\odot$ over an area of $\sim$550 arcmin$^2$. We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on the stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at $\sigma_{v,x}=0.94$ and $\sigma_{v,y}=1.25$ mas yr$^{-1}$) might be biased by the overlapping young stellar populations of Orion A. We identified 4 new ONC candidate runaways based on HST and the Gaia DR2 data, all with masses less than $\sim$1 $M_\odot$. The total census of known candidate runaway sources is 10 -- one of the largest samples ever found in any Milky Way open star cluster. Surprisingly, none of them has the tangential velocity exceeding 20 km s$^{-1}$. If most of them indeed originated in the ONC, it may compel re-examination of dynamical processes in very young star clusters. It appears that the mass function of the ONC is not significantly affected by the lost runaways.Comment: 16 pages, 10 figures, 5 tables. Accepted for publication in A

The Southern Proper Motion Program III. A Near-Complete Catalog to V=17.5

We present the third installment of the Yale/San Juan Southern Proper Motion Catalog, SPM3. Absolute proper motions, positions, and photographic B,V photometry are given for roughly 10.7 million objects, primarily stars, down to a magnitude of V=17.5. The Catalog covers an irregular area of 3700 square degrees, between the declinations of -20 and -45 degrees, excluding the Galactic plane. The proper-motion precision, for well-measured stars, is estimated to be 4.0 mas/yr. Unlike previous releases of the SPM Catalog, the proper motions are on the International Celestial Reference System by way of Hipparcos Catalog stars, and have an estimated systematic uncertainty of 0.4 mas/yr. The SPM3 Catalog is available via electronic transfer,(http://www.astro.yale.edu/astrom/) As an example of the potential of the SPM3 proper motions, we examine the Galactocentric velocities of a group of metal-poor, main-sequence A stars. The majority of these exhibit thick-disk kinematics, lending support to their interpretation as thick-disk blue stragglers, as opposed to being an accreted component.Comment: 23 pages, 10 figures, accepted for publication in Astronomical Journa

Ground-based astrometry with wide field imagers. V. Application to near-infrared detectors: HAWK-I@VLT/ESO

High-precision astrometry requires accurate point-spread function modeling and accurate geometric-distortion corrections. This paper demonstrates that it is possible to achieve both requirements with data collected at the high acuity wide-field K-band imager (HAWK-I), a wide-field imager installed at the Nasmyth focus of UT4/VLT ESO 8m telescope. Our final astrometric precision reaches ~3 mas per coordinate for a well-exposed star in a single image with a systematic error less than 0.1 mas. We constructed calibrated astro-photometric catalogs and atlases of seven fields: the Baade's Window, NGC 6656, NGC 6121, NGC 6822, NGC 6388, NGC 104, and the James Webb Space Telescope calibration field in the Large Magellanic Cloud. We make these catalogs and images electronically available to the community. Furthermore, as a demonstration of the efficacy of our approach, we combined archival material taken with the optical wide-field imager at the MPI/ESO 2.2m with HAWK-I observations. We showed that we are able to achieve an excellent separation between cluster members and field objects for NGC 6656 and NGC 6121 with a time base-line of about 8 years. Using both HST and HAWK-I data, we also study the radial distribution of the SGB populations in NGC 6656 and conclude that the radial trend is flat within our uncertainty. We also provide membership probabilities for most of the stars in NGC 6656 and NGC 6121 catalogs and estimate membership for the published variable stars in these two fields.Comment: 36 pages (included appendix), 13 tables, 35 figures (26 in low resolution), accepted for publication in Astronomy and Astrophysics. Online materials will be soon available on CDS. Meanwhile, online materials can be requested directly to the first autho

A proper motion study of the globular cluster M55

We have derived the absolute proper motion (PM) of the globular cluster M55 using a large set of CCD images collected with the du Pont telescope between 1997 and 2008. We find (PM_RA*cos(DEC), PM_DEC) = (-3.31 +/- 0.10, -9.14 +/- 0.15) mas/yr relative to background galaxies. Membership status was determined for 16 945 stars with 14<V<21 from the central part of the cluster. The PM catalogue includes 52 variables of which 43 are probable members of M55. This sample is dominated by pulsating blue straggler stars but also includes 5 eclipsing binaries, three of which are main sequence objects. The survey also identified several candidate blue, yellow and red straggler stars belonging to the cluster. We detected 15 likely members of the Sgr dSph galaxy located behind M55. The average PM for these stars was measured to be (PM_RA*cos(DEC), PM_DEC)=(-2.23 +/- 0.14, -1.83 +/- 0.24) mas/yr.Comment: 12 figures and 4 tables, accepted for publication in MNRAS Main Journal; full PM catalogue (Table 3) at http://case.camk.edu.p

The effects of differential reddening and stellar rotation on the appearance of multiple populations in star clusters: the case of Trumpler 20

We present a detailed analysis of the upper main sequence of the 1.3 Gyr old open cluster Trumpler 20. High accuracy BV photometry combined with the Very Large Telescope/FLAMES medium-resolution spectroscopy of 954 stars is essential to understanding the unusual appearance of the color-magnitude diagram (CMD), initially suggesting multiple populations in Trumpler 20. We show that differential reddening is a dominant contributor to the apparent splitting/widening of the main-sequence turnoff region. At its extreme, the excess differential reddening reaches Delta(B-V)=0.1 while the adopted minimum reddening for the cluster is E(B-V)=0.36. A unique sample of measured projected rotational velocities indicates that stellar rotation is high near the main-sequence turnoff, reaching vsin i=180 km/s. By dividing the upper main-sequence stars into equal groups of slow and fast rotators, we find that fast rotators have a marginal blueshift of delta(V-I)=-0.01, corresponding to a difference in the median vsin i of 60 km/s between these subsamples. We conclude that stellar rotation has an insignificant effect on the morphology of the upper main sequence of this intermediate-age open cluster. Trumpler 20 appears to contain a single coeval population of stars but there is evidence that the red clump is extended.Comment: 16 pages, 5 figures, accepted for publication in ApJ

The Unique Na:O Abundance Distribution in NGC 6791: The First Open(?) Cluster with Multiple Populations

Almost all globular clusters investigated exhibit a spread in their light element abundances, the most studied being a Na:O anticorrelation. In contrast, open clusters show a homogeneous composition and are still regarded as Simple Stellar Populations. The most probable reason for this difference is that globulars had an initial mass high enough to retain primordial gas and ejecta from the first stellar generation and thus formed a second generation with a distinct composition, an initial mass exceeding that of open clusters. NGC 6791 is a massive open cluster, and warrants a detailed search for chemical inhomogeneities. We collected high resolution, high S/N spectra of 21 members covering a wide range of evolutionary status and measured their Na, O and Fe content. We found [Fe/H]=+0.42$\pm 0.01$, in good agreement with previous values, and no evidence for a spread. However, the Na:O distribution is completely unprecedented. It becomes the first open cluster to show intrinsic abundance variations that cannot be explained by mixing, and thus the first discovered to host multiple populations. It is also the first star cluster to exhibit two subpopulations in the Na:O diagram with one being chemically homogeneous while the second has an intrinsic spread that follows the anticorrelation so far displayed only by globular clusters. NGC 6791 is unique in many aspects, displaying certain characteristics typical of open clusters, others more reminiscent of globulars, and yet others, in particular its Na:O behavior investigated here, that are totally unprecedented. It clearly had a complex and fascinating history.Comment: 16 pages, 4 figures, accepted for publication in the Astrophysical Journal Letter