Biases on initial mass function determinations. II. Real multiple systems and chance superpositions

J. Maíz Apellániz


When calculating stellar initial mass functions (IMFs) for young clusters, one has
to take into account that most massive stars are born in multiple systems and that
most IMFs are derived from data that cannot resolve such systems. It is also
common to measure IMFs for clusters that are located at distances where multiple
chance superpositions between members are expected to happen. In this article I
model the consequences of both of those phenomena, real multiple systems and
chance superpositions, on the observed color-magnitude diagrams and the IMFs
derived from them. Using numerical experiments I quantify their influence on the
IMF slope for massive stars and on the generation of systems with apparent masses
above the stellar upper mass limit. The results in this paper can be used to
correct for the biases induced by real and chance-alignment multiple systems when
the effects are small and to identify when they are so large that most information
about the IMF in the observed color-magnitude diagram is lost. Real multiple
systems affect the observed or apparent massive-star MF slope little but can
create a significant population of apparently ultramassive stars. Chance
superpositions produce only small biases when the number of superimposed stars is
low but, once a certain number threshold is reached, they can affect both the
observed slope and the apparent stellar upper mass limit. In the second part of
the paper, I apply the experiments to two well known massive young clusters in the
Local Group, NGC 3603 and R136. In both cases I show that the observed
population of stars with masses above 120 solar masses can be explained by the
effects of unresolved objects, mostly real multiple systems for NGC 3603 and a
combination of real and chance-alignment multiple systems for R136. Therefore,
the case for the reality of a stellar upper mass limit at solar or near-solar
metallicities is strengthened, with a possible value even lower than 150 solar
masses. An IMF slope somewhat flatter than Salpeter or Kroupa with gamma
between -1.6 and -2.0 is derived for the central region of NGC 3603, with a
significant contribution to the uncertainty arising from the imprecise knowledge
of the distance to the cluster. The IMF at the very center of R136 cannot be
measured with the currently available data but the situation could change with new
Hubble Space Telescope (HST) observations.

Reference: Accepted for publication in ApJ
Status: Manuscript has been accepted


Comments: arXiv version has low-quality figures due to their size limitations. See for a full-quality version.