SCYON Abstract

Received on October 5 2009

Why Simple Stellar Population models do not fit the colours of Galactic open clusters

Authors	A.E. Piskunov (^1,2,3), N.V. Kharchenko (^1,3,4), E. Schilbach (³), S. Röser (³), R.-D. Scholz (¹), and H. Zinnecker (¹)
Affiliation	(¹) Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D--14482 Potsdam, Germany (²) Institute of Astronomy of the Russian Acad. Sci., 48 Pyatnitskaya Str., 109017 Moscow, Russia (³) Astronomisches Rechen-Institut, Mönchhofstrasse 12-14, Zentrum für Astronomie der Universität Heidelberg, D--69120 Heidelberg, Germany (⁴) Main Astronomical Observatory, 27 Academica Zabolotnogo Str., 03680 Kiev, Ukraine
Accepted by	Astronomy & Astrophysics
Contact	rdscholz@aip.de
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Abstract

For Galactic open clusters fundamental parameters like age or reddening are usually determined independently of their integrated colours. For extragalactic clusters, on the other hand, they are essentially derived via a comparison of their integrated colours with predictions of simple stellar population (SSP) models.
We have found a disagreement between the observed integrated colours of 650 local Galactic clusters and theoretical colours of present-day SSP models and seek an explanation for this discrepancy.
We check the hypothesis that the systematic offset between observed and theoretical colours, which is (B-V) ≈ 0.3 and (J-K_s) ≈ 0.8, is due to neglecting the discrete nature of the underlying mass function. Using Monte Carlo simulations we construct artificial clusters of coeval stars drawn from a mass distribution according to the Salpeter IMF and compare them with corresponding ``continuous-IMF'' SSP models.
If the discreteness of the IMF is taken into account, the model fits the observations perfectly and is able to explain naturally a number of red "outliers" observed in the empirical colour-age relation. We find that the systematic offset between the continuous- and discrete-IMF colours reaches its maximum of about 0.5 in (B-V) for a cluster mass M_c = 10²m(sun) at ages log t ≈ 7, and diminishes substantially but not completely to about one hundredth of a magnitude at log t > 7.9 at cluster masses M_c > 10⁵m(sun). At younger ages, it is still present even in massive clusters, and for M_c ≤ 10⁴m(sun) it is larger than 0.1 mag in (B-V). Only for very massive clusters (M_c > 10⁶m(sun)) with ages log t < 7.5 the offset is small (of the order of 0.04 mag) and falls below the typical observational error of colours of extragalactic clusters.