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تسجيل مستخدم جديدAn HST/WFPC2 survey of bright young clusters in M31. IV. Ages and mass estimates
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S. Perina
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{Aims.} We present the main results of an imaging survey of possible young massive clusters (YMC) in M31 performed with the Wide Field and Planetary Camera2 (WFPC2) on the Hubble Space Telescope (HST). We present the images and color magnitude diagrams (CMDs) of all of our targets. {Methods.} The reddening, age and, metallicity of the clusters were estimated by comparing the observed CMDs and luminosity functions with theoretical models. Stellar masses were estimated by comparison with theoretical models in the log(Age) vs. absolute integrated magnitude plane. {Results.} Nineteen of the twenty surveyed candidates were confirmed to be real star clusters. Three of the clusters were found not to be good YMC candidates from newly available integrated spectroscopy and were in fact found to be old from their CMD. Of the remaining sixteen clusters, fourteen have ages between 25 Myr and 280 Myr, two have older ages than 500 Myr (lower limits). By including ten other YMC with HST photometry from the literature we have assembled a sample of 25 clusters younger than 1 Gyr, with mass ranging from 0.6 x 10^4 M_sun to 6 x 10^4 M_sun, with an average of ~ 3 x 10^4 M_sun. {Conclusions.} The clusters considered here are confirmed to have masses significantly higher than Galactic open clusters in the same age range. Our analysis indicates that YMCs are relatively common in all the largest star-forming galaxies of the Local Group.
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{Aims.} We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We present here details of the data reduction pipeline that i
s being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case. {Methods.} The reddening, the age and the metallicity of the cluster are estimated by comparison of the observed Color Magnitude Diagram (CMD) with theoretical isochrones. {Results.} Under the most conservative assumptions the stellar mass of VdB0 is M > 2.4 x 10^4 M_sun, but our best estimates lie in the range ~ 4-9 x 10^4 M_sun. The CMD of VdB0 is best reproduced by models having solar metallicity and age = 25 Myr. Ages smaller than = 12 Myr and larger than = 60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of Red Super Giants (> 18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example. {Conclusions.} VdB0 is significantly brighter (>~ 1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius (r_h=7.4 pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31 it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~ 4 Gyr.
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of blended stellar images and required field subtraction. We thus reach 1 mag fainter than the horizontal branch (HB) even in unfavorable cases. We present the color-magnitude diagrams (CMDs) and discuss their main features also in comparison with the properties of the Galactic GCs. This analysis is augmented with CMDs previously obtained and discussed by Fusi Pecci et al. (1996) on 8 other M31 clusters. We report the following significant results: 1. The locus of the red giant branches give reliable metallicity determinations which compare generally very well with ground-based integrated spectroscopic and photometric measures, as well as giving good reddening estimates. 2. The HB morphologies show the same behavior with metallicity as the Galactic GCs, with indications that the 2nd-parameter effect can be present in some GCs of our sample. However, at [Fe/H] ~ -1.7 we observe a number of GCs with red HB morphology such that the HB type versus [Fe/H] relation is offset from the MW and resembles that of the Fornax dwarf spheroidal galaxy. One explanation for the offset is that they are younger than their MW counterparts by 1-2 Gyr. 3. The Mv(HB)-[Fe/H] relationship has been determined and the slope (~0.20) is very similar to the values derived from RR Lyrae stars in the MW and the LMC. The zero-point of this relation based on the assumed distance modulus (m-M)o(M31)=24.47+/-0.03 is consistent with (m-M)o(LMC)=18.55.
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structure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.
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