15 research outputs found
In Search of Possible Associations between Planetary Nebulae and Open Clusters
We consider the possibility of cluster membership for 13 planetary nebulae
that are located in close proximity to open clusters lying in their lines of
sight. The short lifetimes and low sample size of intermediate-mass planetary
nebulae with respect to nearby open clusters conspire to reduce the probability
of observing a true association. Not surprisingly, line of sight coincidences
almost certainly exist for 7 of the 13 cases considered. Additional studies are
advocated, however, for 6 planetary nebula/open cluster coincidences in which a
physical association is not excluded by the available evidence, namely M
1-80/Berkeley 57, NGC 2438/NGC 2437, NGC 2452/NGC 2453, VBRC 2 & NGC 2899/IC
2488, and HeFa 1/NGC 6067. A number of additional potential associations
between planetary nebulae and open clusters is tabulated for reference
purposes. It is noteworthy that the strongest cases involve planetary nebulae
lying in cluster coronae, a feature also found for short-period cluster
Cepheids, which are themselves potential progenitors of planetary nebulae.Comment: Accepted for publication in PASP (December 2007
Exploring the consequences of pairing algorithms for binary stars
Knowledge of the binary population in stellar groupings provides important
information about the outcome of the star forming process in different
environments (see, e.g., Blaauw 1991, and references therein). Binarity is also
a key ingredient in stellar population studies, and is a prerequisite to
calibrate the binary evolution channels. In this paper we present an overview
of several commonly used methods to pair individual stars into binary systems,
which we refer to as pairing functions. These pairing functions are frequently
used by observers and computational astronomers, either for their mathematical
convenience, or because they roughly describe the expected outcome of the star
forming process. We discuss the consequences of each pairing function for the
interpretation of observations and numerical simulations. The binary fraction
and mass ratio distribution generally depend strongly on the selection of the
range in primary spectral type in a sample. The mass ratio distribution and
binary fraction derived from a binarity survey among a mass-limited sample of
targets is thus not representative for the population as a whole. Neither
theory nor observations indicate that random pairing of binary components from
the mass distribution, the simplest pairing function, is realistic. It is more
likely that companion stars are formed in a disk around a star, or that a
pre-binary core fragments into two binary components. The results of our
analysis are important for (i) the interpretation of the observed mass ratio
distribution and binary fraction for a sample of stars, (ii) a range of
possible initial condition algorithms for star cluster simulations, and (iii)
how to discriminate between the different star formation scenarios.Comment: 43 pages, 18 figures, accepted for publication in A&