Open cluster stability and the effects of binary stars (original) (raw)
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Star cluster stability and the effects of binary stars: Galactic open clusters
arXiv (Cornell University), 2008
The diagnostic age versus mass-to-light ratio diagram is often used in attempts to constrain the shape of the stellar initial mass function (IMF), and the potential longevity of extragalactic young to intermediate-age massive star clusters. Here, we explore its potential for Galactic open clusters. On the basis of a small, homogenised cluster sample we provide useful constraints on the presence of significant binary fractions. Using the massive young Galactic cluster Westerlund 1 as a key example, we caution that stochasticity in the IMF introduces significant additional uncertainties. We conclude that for an open cluster to survive for any significant length of time, and in the absence of substantial external perturbations, it is a necessary but not a sufficient condition to be located close to or (in the presence of a significant binary population) somewhat below the predicted photometric evolutionary sequences for 'normal' simple stellar populations (although such a location may be dominated by a remaining 'bound' cluster core and thus not adequately reflect the overall cluster dynamics).
Critical tests of stellar evolution in open clusters
Astronomy and Astrophysics, 2002
Based on new, accurate photometry, radial velocities, and proper motions for the intermediate-age open cluster NGC 3680, we identify individual single and binary cluster members and field stars in the colour-magnitude diagram (CMD). This basic step turns out to be crucial for a proper understanding of the cluster CMD: ∼60% of the stars are found to be field stars, and over 50% of the cluster stars are binaries. No bona f ide cluster star is found more than 1. m 5 below the turnoff, and cluster stars below 1.4 M are only found in binary systems. The total present mass of NGC 3680 is ∼100M , excluding any as yet unseen stellar remnants, and its half-mass radius is 3.3 (1.2 pc). Comparison with plausible IMFs indicates that only ∼3% of the original stars and < ∼ 10% of the mass now survive, ∼30% of the initial mass being in the form of massive stars that have now completed their evolution, and ∼60% in low-mass stars which may now be located in a distant cluster halo or perhaps have been lost entirely.
The evolution of the luminosity function of young open clusters
Chinese Astronomy and Astrophysics, 1998
The luminosity function of a star cluster evolves markedly during the pre-main sequence phase. With an assumed initial mass function and premain sequence tracks, we calculate a set of monochromatic luminosity functions which, when compared with observations, can be used to infer the age and star formation history (coeval versus intermittent) of a star cluster. Applied to the Trapezium cluster, our model suggests an age close to a million years, whereas in IC 348 the age estimate yields 4-6 million years and continual bursts of star formation seem to have occurred in this cluster. CCD imaging observations in the I-band are presented for NGC 663, for which an age of lo-30 million years is inferred. The initial mass function for NGC 663 in the range 2-7.1 M. has a slope of-0.77f 0.20, much shallower than that for the solar neighborhood field stars. We interpret this being due to mass segregation in the cluster. We interpret this as being due to mass segregation in the cluster.
2003
The ages of 203 open clusters from the list of Dambis (1999) are computed in terms of the evolutionary tracks of Pols et al. (1998) with and without the allowance for convective overshooting. The vertical scaleheight of the cluster layer at Galactocentric distances R_0-1 kpc < R_g< R_0+ 1 kpc varies nonmonotonically with age, exhibiting a wavelike pattern similar to the one earlier found for the Cepheid population (Joeveer 1974), with a period of P_Z = 74 +/- 2 Myr and P_Z = 92 +/- 2 Myr if cluster ages are computed in terms of evolutionary models without and with overshooting, respectively. The period of vertical oscillations can be reconciled with the known local mass density only if cluster ages are computed with no or just mild overshooting: the overshooting-based ages imply a local mass density of rho = 0.075 +/- 0.003 M_Sun/pc^3, which is incompatibe with the recent Hipparcos-based estimate of rho = 0.102 M_Sun/pc^3 (Holmberg and Flynn 2000). Our results imply a maximum local dark-mass density of rho_DM <= 0.027 M_Sun/pc^3. At the time of their formation open clusters have, on the average, excess vertical kinetic energy and as a population are not in virial equilibrium; moreover, their initial vertical coordinates (at the time of birth) are strongly and positively correlated with initial vertical velocities (r = 0.81 +/-0.08), thus favoring a scenario where star formation in the disk is triggered by some massive objects falling onto the Galactic plane.
Star cluster evolution, dynamical age estimation and the kinematical signature of star formation
Monthly Notices of the Royal Astronomical Society, 1995
We distribute 400 stars in N bin = 200 binary systems in clusters with initial half mass radii 0.077 ≤ R 0.5 ≤ 2.53 pc and follow the subsequent evolution of the stellar systems by direct N-body integration. The stellar masses are initially paired at random from the KTG(1.3) initial stellar mass function. The initial period distribution is flat ranging from 10 3 to 10 7.5 days, but we also perform simulations with a realistic distribution of periods which rises with increasing P > 3 days and which is consistent with pre-main sequence observational constraints. For comparison we simulate the evolution of single star clusters. After an initial relaxation phase, all clusters evolve according to the same n(t) ∝ exp(−t/τ e ) curve, where n(t) is the number density of stars in the central 2 pc sphere at time t and τ e ≈ 230 Myrs. All clusters have the same lifetime τ . n(t) and τ are thus independent of (i) the inital proportion of binaries and (ii) the initial R 0.5 . Mass segregation measures the dynamical age of the cluster: the mean stellar mass inside the central region increases approximately linearly with age. The proportion of binaries in the central cluster region is a sensitive indicator of the initial cluster concentration: it declines within approximately the first 10-20 initial relaxation times and rises only slowly with age, but for initial R 0.5 < 0.8 pc, it is always significantly larger than the binary proportion outside the central region. If most stars form in binaries in embedded clusters that are dynamically equivalent to a cluster specified initially by (N bin , R 0.5 ) = (200, 0.85 pc), which is located at the edge of a 1.5 × 10 5 M ⊙ molecular cloud with a diameter of 40 pc, then we estimate that at most about 10 per cent of all pre-main sequence stars achieve near escape velocities from the molecular cloud. The large ejection velocities resulting from close encounters between binary systems imply a distribution of young stars over large areas surrounding star forming sites. This 'halo' population of a molecular cloud complex is expected to have a significantly reduced binary proportion (about 15 per cent or less) and a significantly increased proportion of stars with depleted or completely removed circumstellar disks. In this scenario, the distributed population is expected to have a similar proportion of binaries as the Galactic field (about 50 per cent). If a distributed population shows orbital parameter distributions not affected by stimulated evolution (e.g. as in Taurus-Auriga) then it probably originated in a star-formation mode in which the binaries form in relative isolation rather than in embedded clusters. The Hyades Cluster luminosity function suggests an advanced dynamical age. The Pleiades luminosity function data suggest a distance modulus m − M = 6, rather than 5.5. The total proportion of binaries in the central region of the Hyades and Pleiades Clusters are probably 0.6-0.7. Any observational luminosity function of a Galactic cluster must be corrected for unresolved binaries when studying the stellar mass function. Applying our parametrisation for open cluster evolution we estimate the birth masses of both clusters. We find no evidence for different dynamical properties of stellar systems at birth in the Hyades, Pleiades and Galactic field stellar samples. Parametrising the depletion of low mass stars in the central cluster region by the ratio, ζ(t), of the stellar luminosity function at the 'H 2 -convection peak' (M V ≈ 12) and 'H − plateau' (M V ≈ 7), we find good agreement with the Pleiades and Hyades ζ(t) values. The observed proportion of binary stars in the very young Trapezium Cluster is consistent with the early dynamical evolution of a cluster with a very high initial stellar number density.
Stellar Mass Segregation in the Aged Galactic Open Star Cluster Berkeley 17
arXiv: Solar and Stellar Astrophysics, 2016
We present the analysis of the morphology of Berkeley\,17, the oldest known open cluster ($\sim10$ Gyr), using a probabilistic star counting of Pan-STARRS point sources, and confirm its core-tail shape, plus an antitail, previously detected with 2MASS data. The stellar population, as diagnosed by the color-magnitude diagram and theoretical isochrones, shows more massive than lower-mass members in the cluster core, whereas there is a paucity of massive members in both tails. This manifests mass segregation in this aged star cluster with the low-mass members being stripped away from the system. It has been claimed that Berkeley 17 is associated with an excessive number of blue stragglers. Our analysis in comparison of the cluster with nearby reference fields indicates that about half of the blue stragglers may be field contaminations, and some may be confused with the rare blue horizontal-branch stars in this cluster.
The Dynamical Evolution of Young Clusters and Galactic Implications
Eso Astrophysics Symposia, 2009
Star clusters are observed to form in a highly compact state and with low star-formation efficiencies. If the residual gas is expelled on a dynamical time the clusters disrupt thereby (i) feeding a hot kinematical stellar component into their host-galaxy's field population, and (ii) if the gas-evacuation time-scale depends on cluster mass, then a power-law embedded-cluster mass function transforms within ten to a few dozen Myr to a mass function with a turnover near 10 5 M ⊙ , thereby possibly explaining this universal empirical feature.
Monthly Notices of the Royal Astronomical Society, 2013
We present a survey of 130 Galactic and extragalactic young massive clusters (YMCs, 10 4 < M/M ⊙ < 10 8 , 10 < t/Myr < 1000) with integrated spectroscopy or resolved stellar photometry (40 presented here and 90 from the literature) and use the sample to search for evidence of ongoing star-formation within the clusters. Such episodes of secondary (or continuous) star-formation are predicted by models that attempt to explain the observed chemical and photometric anomalies observed in globular clusters as being due to the formation of a second stellar population within an existing first population. Additionally, studies that have claimed extended star-formation histories within LMC/SMC intermediate age clusters (1-2 Gyr), also imply that many young massive clusters should show ongoing star-formation. Based on visual inspection of the spectra and/or the colour-magnitude diagrams, we do not find evidence for ongoing star-formation within any of the clusters, and use this to place constraints on the above models. Models of continuous star-formation within clusters, lasting for hundreds of Myr, are ruled out at high significance (unless stellar IMF variations are invoked). Models for the (nearly instantaneous) formation of a secondary population within an existing first generation are not favoured, but are not formally discounted due to the finite sampling of age/mass-space.
The study of open clusters has a classic feel to it since the subject predates anyone alive today. Despite the age of this topic, I show via an ADS search that its relevance and importance in astronomy has grown faster in the last few decades than astronomy in general. This is surely due to both technical reasons and the interconnection of the field of stellar evolution to many branches of astronomy. In this review, I outline what we know today about open clusters and what they have taught us about a range of topics from stellar evolution to Galactic structure to stellar disk dissipation timescales. I argue that the most important astrophysics we have learned from open clusters is stellar evolution and that its most important product has been reasonably precise stellar ages. I discuss where open cluster research is likely to go in the next few years, as well as in the era of 20m telescopes, SIM, and GAIA. Age will continue to be of wide relevance in astronomy, from cosmology to planet formation timescales, and with distance errors soon no longer a problem, improved ages will be critically important to many of the most fascinating astrophysical questions.
Binary Stars in Young Clusters - a Theoretical Perspective
Proceedings of the International Astronomical Union
The preponderance of binary systems in all known stellar populations makes them exciting dynamical agents for research on topics as varied as star formation, star-cluster dynamics and the interiors of young and old stars. Today we know that the Galactic-field binary population is probably a dynamically evolved version of the Taurus-Auriga pre-main sequence population, and that the initial distributions of binarystar orbital elements are probably universal. Furthermore, N -body calculations tentatively suggest that OB stars form in energetic binaries near cluster cores, and that binaries with 'forbidden' orbital elements that are produced in stellar encounters, may turn out to be very useful windows into stellar interiors, potentially allowing tests of pre-main sequence evolution theory as well as of models of main-sequence stars.