The photometric evolution of dissolving star clusters

Abstract

Context. Evolutionary synthesis models are the primary means of constructing spectrophotometric models of stellar populations, and deriving physical parameters from observations compared with these models. One of the basic assumptions of evolutionary synthesis models has been the time-independence of the stellar mass function, apart from the successive removal of high-mass stars by stellar evolution. However, dynamical simulations of star clusters in tidal fields have demonstrated that the mass function can be changed by the preferential removal of low-mass stars from clusters. Aims. We combine the results of dynamical simulations of star clusters in tidal fields with our evolutionary synthesis code GALEV. We extend the models to consider the total cluster disruption time as additional parameter. Methods. Following up on our earlier work, which was based on simplifying assumptions, we reanalyse the mass-function evolution found in N-body simulations of star clusters in tidal fields, parametrise it as a function of age and total disruption time of the cluster, and use this parametrisation to compute GALEV models as a function of age, metallicity, and total cluster disruption time. Results. We study the impact of cluster dissolution on colours (which generally become redder) and magnitudes (which become fainter) of star clusters, their mass-to-light ratios (which can deviate by a factor of ∼2-4 from predictions of standard models without cluster dissolution), and quantify the effect of the altered integrated photometry on cluster age determination. In most cases, clusters appear to be older than they are, where the age difference can range from 20% to 200%. By comparing our model results with observed M/L ratios for old compact objects in the mass range 10 4.5 -10 8 M , we find a strong discrepancy for objects more massive than 10 7 M , such that observed M/L ratios are higher than predicted by our models. This could be caused either by differences in the underlying stellar mass function or be an indication of the presence of dark matter in these objects. Less massive objects are well described by the models.

The evolution of luminosity, colour, and the mass-to-luminosity ratio of Galactic open clusters

“…At the same time more massive models are practically insensitive to the mass loss. Note the good agreement of our data with recent calculations of Anders et al (2009) (see their Fig. 5).…”

“…On the other hand, masses of extragalactic clusters are usually determined from the mass-luminosity relation (M/L ratio), which is also based on the standard SSP models. Recent attempts to incorporate dynamical processes into SSP models were made by Anders et al (2009), who included the results of N-body calculation of Baumgardt & Makino (2003) into a standard GALEV model (i.e., with continuous IMF), and by Lamers et al (2006), Kruijssen & Lamers (2008), and Kruijssen (2009) using a simplified analytical approach.…”

“…In contrast, this extrapolation is of minor importance for high-mass stars, because the lifetime of stars with m > 8 m is much shorter than the time of a significant decline of q(m, t) from unity. For comparison, we plot the results of Anders et al (2009) in Fig. 3 as filled dots.…”

The colour–magnitude relation of globular clusters in Centaurus and Hydra

“…Harris et al 2006;Mieske et al 2006Mieske et al , 2010. A&A 567, A105 (2014) It is worth noting that colour changes as a function of GC mass can also occur as a result of dynamical evolution, via preferential loss of low-mass stars (Kruijssen & Lamers 2008;Anders et al 2009;Kruijssen 2009). This makes lower-mass GCs bluer than their colour for an unaltered stellar mass function, thus qualitatively creating a trend in the direction of the blue tilt.…”

“…ad B) Recent studies investigated the effect of this dynamical dissolussion of star clusters on their integrated colours (Kruijssen & Lamers 2008;Anders et al 2009). These studies predict star cluster colours at a determined age and metallicity as a function of the ratio t dyn between the current age and the dissolution time.…”