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Register a new userNew Compact Star Cluster Candidates in the Galactic Plane
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D. Froebrich
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The sample of known star clusters, the fundamental building blocks of galaxies, in the Milky Way is still extremely incomplete for objects beyond a distance of 1-2kpc. Many of the more distant and young clusters are compact and hidden behind large amounts of extinction. We thus utilised the deep high resolution near infrared surveys UGPS and VVV to uncover so far unknown compact clusters and to analyse their properties. Images of all objects in the area covered by these two surveys, and which are listed as Galaxy in SIMBAD have been inspected and 125 so far unknown stellar clusters and candidate clusters have been identified. Based on the frequent associations with star formation indicators (nebulosities, IRAS sources, Hii regions, masers) we find that the typical cluster in our sample is young, at distances between 1-10kpc and has a typical apparent radius of 25arcsec. We suggest more systematic searches e.g. at all positions of 2MASS extended sources to increase the completeness of the known cluster sample beyond distances of 2kpc.
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We report on the discovery of a new Milky Way companion stellar system located at (RA, Dec) = (22h10m43.15s, +14:56:58.8). The discovery was made using the eighth data release of SDSS after applying an automated method to search for overdensities in the Baryon Oscillation Spectroscopic Survey footprint. Follow-up observations were performed using CFHT-MegaCam, which reveal that this system is comprised of an old stellar population, located at a distance of 31.9+1.0-1.6 kpc, with a half-light radius of r_h = 7.24+1.94-1.29 pc and a concentration parameter of c = 1.55. A systematic isochrone fit to its color-magnitude diagram resulted in log(age) = 10.07+0.05-0.03 and [Fe/H] = -1.58+0.08-0.13 . These quantities are typical of globular clusters in the MW halo. The newly found object is of low stellar mass, whose observed excess relative to the background is caused by 96 +/- 3 stars. The direct integration of its background decontaminated luminosity function leads to an absolute magnitude of MV = -1.21 +/- 0.66. The resulting surface brightness is uV = 25.9 mag/arcsec2 . Its position in the M_V vs. r_h diagram lies close to AM4 and Koposov 1, which are identified as star clusters. The object is most likely a very faint star cluster - one of the faintest and lowest mass systems yet identified.
We present a catalog of extended objects in the vicinity of M81 based a set of 24 Hubble Space Telescope Advanced Camera for Surveys (ACS) Wide Field Camera (WFC) F814W (I-band) images. We have found 233 good globular cluster candidates; 92 candidate HII regions, OB associations, or diffuse open clusters; 489 probable background galaxies; and 1719 unclassified objects. We have color data from ground-based g- and r-band MMT Megacam images for 79 galaxies, 125 globular cluster candidates, 7 HII regions, and 184 unclassified objects. The color-color diagram of globular cluster candidates shows that most fall into the range 0.25 < g-r < 1.25 and 0.5 < r-I < 1.25, similar to the color range of Milky Way globular clusters. Unclassified objects are often blue, suggesting that many of them are likely to be HII regions and open clusters, although a few galaxies and globular clusters may be among them.
We present comprehensive characterization of the Galactic open cluster M 36. Some two hundred member candidates, with an estimated contamination rate of $sim$8%, have been identified on the basis of proper motion and parallax measured by the $Gaia$ DR2. The cluster has a proper motion grouping around ($mu_{alpha} cosdelta = -$0.15 $pm$ 0.01 mas yr$^{-1}$, and $mu_{delta} = -$3.35 $pm$ 0.02 mas yr$^{-1}$), distinctly separated from the field population. Most member candidates have parallax values 0.7$-$0.9 mas, with a median value of 0.82 $pm$ 0.07 mas (distance $sim$1.20 $pm$ 0.13 kpc). The angular diameter of 27$$ $pm$ $0farcm4$ determined from the radial density profile then corresponds to a linear extent of 9.42 $pm$ 0.14 pc. With an estimated age of $sim$15 Myr, M 36 is free of nebulosity. To the south-west of the cluster, we discover a highly obscured ($A_{V}$ up to $sim$23 mag), compact ($sim$ $1farcm9 times 1farcm2$) dense cloud, within which three young stellar objects in their infancy (ages $lesssim$ 0.2 Myr) are identified. The molecular gas, 3.6 pc in extent, contains a total mass of (2$-$3)$times$10$^{2}$ M$_{odot}$, and has a uniform velocity continuity across the cloud, with a velocity range of $-$20 to $-$22 km s$^{-1}$, consistent with the radial velocities of known star members. In addition, the cloud has a derived kinematic distance marginally in agreement with that of the star cluster. If physical association between M 36 and the young stellar population can be unambiguously established, this manifests a convincing example of prolonged star formation activity spanning up to tens of Myrs in molecular clouds.
We report a search and analysis of obscured cluster candidates in the VISTA Variables in the Via Lactea eXtended (VVVX) ESO Public Survey area encompassing the region between 229.4 < l < 295.2 and -4.3 < b < 4.4 of the southern Galactic disk. We discover and propose 88 new clusters. We improve the completeness of the embedded cluster population in this region, adding small size (linear diameters of 0.2-1.4 pc) and relatively far objects (heliocentric distance between 2 and 4 kpc) to existing catalogues. Nine candidates are proposed to be older open cluster candidates. Three of them (VVVX CL204, 207, 208) have sufficient numbers of well-resolved stellar members to allow us to determine some basic cluster parameters. We confirm their nature as older, low-mass open clusters. Photometric analysis of 15 known clusters shows that they have ages above 20Myr, and masses below 2000Msun: in general, their proper motions follow the motion of the disk. We outline some groups of clusters, most probably formed within the same dust complex. Broadly, our candidates follow the network of filamentary structure in the remaining dust. Thus, in this part of the southern disk of the Galaxy, we have found recent star formation, producing small size and young clusters, in addition to the well known, massive young clusters, including NGC3603, Westerlund2 and the Carina Nebula Complex.
Recent Galactic plane surveys of dust continuum emission at long wavelengths have identified a population of dense, massive clumps with no evidence for on-going star formation. These massive starless clump candidates are excellent sites to search for the initial phases of massive star formation before the feedback from massive star formation effects the clump. In this study, we search for the spectroscopic signature of inflowing gas toward starless clumps, some of which are massive enough to form a massive star. We observed 101 starless clump candidates identified in the Bolocam Galactic Plane Survey (BGPS) in HCO+ J = 1-0 using the 12m Arizona Radio Observatory telescope. We find a small blue excess of E = (Nblue - Nred)/Ntotal = 0.03 for the complete survey. We identified 6 clumps that are good candidates for inflow motion and used a radiative transfer model to calculate mass inflow rates that range from 500 - 2000 M /Myr. If the observed line profiles are indeed due to large-scale inflow motions, then these clumps will typically double their mass on a free fall time. Our survey finds that massive BGPS starless clump candidates with inflow signatures in HCO+ J = 1-0 are rare throughout our Galaxy.
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