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Register a new userOld Star Clusters in the FSR catalogue
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Authors
D. Froebrich
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We investigate the old star clusters in the sample of cluster candidates from Froebrich, Scholz & Raftery 2007 -- the FSR list. Based on photometry from the 2-Micron All Sky Survey we generated decontaminated colour-magnitude and colour-colour diagrams to select a sample of 269 old stellar clusters. This sample contains 63 known globular clusters, 174 known open clusters and 32 so far unclassified objects. Isochrone fitting has been used to homogeneously calculate the age, distance and reddening to all clusters. The mean age of the open clusters in our sample is 1Gyr. The positions of these clusters in the Galactic Plane show that 80% of open clusters older than 1Gyr have a Galactocentric distance of more than 7kpc. The scale height for the old open clusters above the Plane is 375pc, more than three times as large as the 115pc which we obtain for the younger open clusters in our sample. We find that the mean optical extinction towards the open clusters in the disk of the Galaxy is 0.70mag/kpc. The FSR sample has a strong selection bias towards objects with an apparent core radius of 30 to 50 and there is an unexplained paucity of old open clusters in the Galactic Longitude range of 120deg < l < 180deg.
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We used resolved star counts from Hubble Space Telescope images to determine the center of gravity and the projected density profiles of 6 old globular clusters in the Large Magellanic Cloud (LMC), namely NGC 1466, NGC 1841, NGC 1898, NGC 2210, NGC 2257 and Hodge 11. For each system, the LMC field contribution was properly taken into account by making use, when needed, of parallel HST observations. The derived values of the center of gravity may differ by several arcseconds (corresponding to more than 1 pc at the distance of the LMC) from previous determinations. The cluster density profiles are all well fit by King models, with structural parameters that may differ from the literature ones by even factors of two. Similarly to what observed for Galactic globular clusters, the ratio between the effective and the core radii has been found to anti-correlate with the cluster dynamical age.
Discerning the nature of open cluster candidates is essential for both individual and statistical analyses of cluster properties. Here we establish the nature of thirteen cluster candidates from the FSR cluster list using photometry from the 2MASS and deeper, higher resolution UKIDSS-GPS and VISTA-VVV surveys. These clusters were selected because they were flagged in our previous studies as expected to contain a large proportion of pre-main sequence members or are at unusually small/large Galactocentric distances. We employ a decontamination procedure of JHK photometry to identify cluster members. Cluster properties are homogeneously determined and we conduct a cross comparative study of our results with the literature (where available). Seven of the here studied clusters were confirmed to contain PMS stars, one of which is a newly confirmed cluster. Our study of FSR1716 is the deepest to date and is in notable disagreement with previous studies, finding that it has a distance of about 7.3kpc and age of 10-12Gyr. As such, we argue that this cluster is a potential globular cluster candidate.
We are conducting a large program to classify newly discovered Milky Way star cluster candidates from the list of Froebrich, Scholz & Raftery (2007). Here we present deep NIR follow-up observations from ESO/NTT of 14 star cluster candidates. We show that the combined analysis of star density maps and colour-colour/magnitude diagrams derived from deep near-infrared imaging is a viable tool to reliably classify new stellar clusters. This allowed us to identify two young clusters with massive stars, three intermediate age open clusters, and two globular cluster candidates among our targets. The remaining seven objects are unlikely to be stellar clusters. Among them is the object FSR1767 which has previously been identified as a globular cluster using 2MASS data by Bonatto et al. (2007). Our new analysis shows that FSR1767 is not a star cluster. We also summarise the currently available follow-up analysis of the FSR candidates and conclude that this catalogue may contain a large number of new stellar clusters, probably dominated by old open clusters.
We analyze our accurate kinematical data for the old clusters in the inner regions of M31. These velocities are based on high S/N Hectospec data (Caldwell et al 2010). The data are well suited for analysis of M31s inner regions because we took particular care to correct for contamination by unresolved field stars from the disk and bulge in the fibers. The metal poor clusters show kinematics which are compatible with a pressure-supported spheroid. The kinematics of metal-rich clusters, however, argue for a disk population. In particular the innermost region (inside 2 kpc) shows the kinematics of the x2 family of bar periodic orbits, arguing for the existence of an inner Lindblad resonance in M31.
We present ESO/VLT FORS2 low resolution spectroscopy of red giant branch stars in three massive, intermediate age ($sim 1.7-2.3$ Gyr) star clusters in the Large Magellanic Cloud. We measure CH and CN index bands at 4300A, and 3883A, as well as [C/Fe] and [N/Fe] abundance ratios for 24, 21 and 12 member stars of NGC 1978, NGC 1651, NGC 1783, respectively. We find a significant intrinsic spread in CN in NGC 1978 and NGC 1651, a signal of multiple stellar populations (MPs) within the clusters. On the contrary, we report a null CN spread in NGC 1783 within our measurement precision. For NGC 1978, we separated the two populations in the CN distribution and we translated the CN spread into an internal N variation $Delta$[N/Fe]$=0.63pm0.49$ dex. For NGC 1651 and NGC 1783, we put upper limits on the N abundance variations of $Delta$[N/Fe]$leq 0.2, 0.4$ dex, respectively. The spectroscopic analysis confirms previous results from HST photometry, where NGC 1978 was found to host MPs in the form of N spreads, while slightly younger clusters (e.g. NGC 1783, $<$ 2 Gyr old) were not, within the limits of the uncertainties. It also confirms that intermediate age massive clusters show lower N abundance variations with respect to the ancient globular clusters, although this is in part due to the effect of the first dredge up at these stellar masses, as recently reported in the literature. We stress the importance of future studies to estimate the initial N abundance variations, free of stellar evolutionary mixing processes, by observing unevolved stars in young clusters.
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