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Post Info TOPIC: Trumpler 20
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Title: The effects of differential reddening and stellar rotation on the appearance of multiple populations in star clusters: the case of Trumpler 20
Authors: I. Platais (1), C. Melo (2), S. N. Quinn (3), J. L. Clem (4), S. E. de Mink (5), A. Dotter (5), V. Kozhurina-Platais (5), D. W. Latham (3), A. Bellini (6) ((1) Johns Hopkins U., (2) ESO, (3) CfA, (4) Louisiana State U., (5) STScI, (6) U. di Padova)

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 colour-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.

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Blobrana


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Title: Trumpler 20 - an old and rich open cluster
Authors: I. Platais (1), C. Melo (2), J.P. Fulbright (1), V. Kozhurina-Platais (3), P. Figueira (4), S.A. Barnes (5), R.A. Mendez (6) ((1) JHU, (2) ESO, Garching, (3) STScI, (4) Observatoire de Geneve, (5) Lowell Obs, (6) Universidad de Chile)

We show that the open cluster Trumpler 20, contrary to the earlier findings, is actually an old Galactic open cluster. New CCD photometry and high-resolution spectroscopy are used to derive the main parameters of this cluster. At [Fe/H]=-0.11 for a single red giant star, the metallicity is slightly subsolar. The best fit to the colour-magnitude diagrams is achieved using a 1.3 Gyr isochrone with convective overshoot. The cluster appears to have a significant reddening at E(B-V)=0.46 (for B0 spectral type), although for red giants this high reddening yields the colour temperature exceeding the spectroscopic T_eff by about 200 K. Trumpler 20 is a very rich open cluster, containing at least 700 members brighter than M_V=+4. It may extend over the field-of-view available in our study at 20'x20'.

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