Letter to the Editor

Why simple stellar population models do not reproduce the colours of Galactic open clusters

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: rdscholz@aip.de
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, 69120 Heidelberg, Germany
⁴ Main Astronomical Observatory, 27 Academica Zabolotnogo Str., 03680 Kiev, Ukraine

Received: 9 September 2009
Accepted: 3 October 2009

Abstract

Context. For Galactic open clusters, fundamental parameters such as age or reddening are usually determined independently of their integrated colours. For extragalactic clusters, on the other hand, they are derived by comparing their integrated colours with predictions of simple stellar population (SSP) models.

Aims. We search for an explanation of the disagreement between the observed integrated colours of 650 local Galactic clusters and the theoretical colours of present-day SSP models.

Methods. We check the hypothesis that the systematic offsets between observed and theoretical colours, which are (B - V) ≈ 0.3 and (J - K_s) ≈ 0.8, are caused by neglecting the discrete nature of the underlying mass function. Using Monte Carlo simulations, we construct artificial clusters of coeval stars taken from a mass distribution defined by an Salpeter initial mass function (IMF) and compare them with corresponding “continuous-IMF” SSP models.

Results. 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 for a cluster mass M_c = 10² 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 > 10⁵ . At younger ages, it is still present even in massive clusters, and for M_c ≤ 10⁴ it is larger than 0.1 mag in . Only for very massive clusters (M_c > 10⁶ ) with ages log t < 7.5 is the offset small (of the order of 0.04 mag) and smaller than the typical observational error of colours of extragalactic clusters.