Open clusters and the galactic disk

It is textbook knowledge that open clusters are conspicuous members of the thin disk of our Galaxy, but their role as contributors to the stellar population of the disk was regarded as minor. Starting from a homogenous stellar sky survey, the ASCC-2.5, we revisited the population of open clusters in the solar neighbourhood from scratch. In the course of this enterprise we detected 130 formerly unknown open clusters, constructed volume- and magnitude-limited samples of clusters, re-determined distances, motions, sizes, ages, luminosities and masses of 650 open clusters. We derived the present-day luminosity and mass functions of open clusters (not the stellar mass function in open clusters), the cluster initial mass function CIMF and the formation rate of open clusters. We find that open clusters contributed around 40 percent to the stellar content of the disk during the history of our Galaxy. Hence, open clusters are important building blocks of the Galactic disk (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper is the second in a series devoted to the use of young, optically revealed open clusters as probes of the star-formation process. The first paper in the series [Phelps & Janes, ApJS (1993) (in press)] presented the dataset used for the study, and discussed some of the results from the survey of 23 open clusters in the Cassipoeia region of the Perseus spiral arm of our Galaxy. This paper presents an analysis of the stellar content of a subset of the clusters, including determinations of the luminosity and mass functions of 8 of the clusters to ∼1M ○. . The slope of the average mass function Γ is found to be −1.40±0.13 over the mass range 1.4<M/M ○. <7.9. Significant variations from this average value are found for two of the clusters, NGC 581 and NGC 663

I present a literature search through 31 July 1997 of white dwarfs (WDs) in open and globular clusters. There are 36 single WDs and 5 WDs in binaries known among 13 open clusters, and 340 single WDs and 11 WDs in binaries known among 11 globular clusters. From these data I have calculated WD mass fractions for four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one globular cluster (NGC 6121). I develop a simple model of cluster evolution that incorporates stellar evolution but not dynamical evolution to interpret the WD mass fractions. I augment the results of my simple model with N-body simulations incorporating stellar evolution (Terlevich 1987; de la Feunte Marcos 1996; Vesperini & Heggie 1997). I find that even though these clusters undergo moderate to strong kinematical evolution the WD mass fraction is relatively insensitive to kinematical evolution. By comparing the cluster mass functions to that of the Galactic disk, and incorporating plausibility arguments for the mass function of the Galactic halo, I estimate the WD mass fraction in these two populations. I assume the Galactic disk is ~10 Gyrs old (Winget et al. 1987; Liebert, Dahn, & Monet 1988; Oswalt et al. 1996) and that the Galactic halo is ~12 Gyrs old (Reid 1997b; Gratton et al. 1997; Chaboyer et al. 1998), although the WD mass fraction is insensitive to age in this range. I find that the Galactic halo should contain 8 to 9% (alpha = -2.35) or perhaps as much as 15 to 17% (alpha = -2.0) of its stellar mass in the form of WDs. The Galactic disk WD mass fraction should be 6 to 7% (alpha = -2.35), consistent with the empirical estimates of 3 to 7% (Liebert, Dahn, & Monet 1988; Oswalt et al. 1996). (abridged)

Context. Unresolved binaries have a strong influence on the observed parameters of stellar clusters (SCs). Aims. We quantify this influence and compute the resulting mass underestimates and stellar mass function (MF). Methods. N-body simulations of realistic SCs were used to investigate the evolution of the binary population in a SC and its tidal tails. Together with an empirically gauged stellar mass-luminosity relation, the results were then used to determine how the presence of binaries changes the photometric mass and MF of the SC and its tails as deduced from observations. Results. Tail 1 (T1), which is the tidal tail caused by gas expulsion, contains a larger fraction of binaries than both the SC and Tail 2 (T2), which forms after gas expulsion. Additionally, T1 has a larger velocity dispersion. Using the luminosity of an unresolved binary, an observer would underestimate its mass. This bias sensitively depends on the companion masses due to the structure of the stellar mass-luminosity relation. Combining the effect of all binaries in the simulation, the total photometric mass of the SC is underestimated by 15%. Dark objects (black holes and neutron stars) increase the difference between the real and observed mass of the SC further. For both the SC and the tails, the observed power-law index of the MF between a stellar mass of 0.3 and 0.7 M⊙ is smaller by up to 0.2 than the real one, the real initial mass function (IMF) being steeper by this amount. This difference is larger for stars with a larger velocity dispersion or binary fraction. Conclusions. Since the stars formed in SCs are the progenitors of the Galactic field stars, this work suggests that the binary fractions of different populations of stars in the Galactic disc will differ as a function of the velocity dispersion. However, the direction of this correlation is currently unclear, and a complete population synthesis will be needed to investigate this effect. Variations in the binary fractions of different clusters can lead to perceived variations of the deduced stellar MFs.

Studies of the mass function of open clusters of different ages allow us to study the efficiency with which brown dwarfs are evaporated from clusters to populate the field. Surveys in relatively old clusters (age ≳100 Myr ) do not suffer from problems found in young clusters, such as intra-cluster extinction and large uncertainties in brown dwarf models. In this paper, we present the results of a photometric survey to study the mass function of the old open cluster Praesepe (age of ~590 Myr and distance of ~190 pc), down to the substellar regime. We have performed optical ( riz and Y -band) photometric survey of Praesepe with the Large Binocular Telescope Camera, for a spatial coverage of 0.61 deg 2 from ~90 M J down to a 5σ detection limit at 40 M J .

A widely supported formation scenario for the Galactic disc is that it formed inside-out from material accumulated via accretion events. The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is the best example of such an accretion, and its ongoing disruption has resulted in that its stars are being deposited in the Milky Way halo and outer disc. It is therefore appealing to search for possible signatures of the Sgr dSph contribution to the build-up of the Galactic disc. Interestingly, models of the Sgr dSph stream clearly indicate that the trailing tail passes through the outer Galactic disc, at the same Galactocentric distance as some anticentre old open star clusters. We investigate in this Letter the possibility that the two outermost old open clusters, Berkeley 29 and Saurer 1, could have formed inside the Sgr dSph and then left behind in the outer Galactic disc as a result of tidal interaction with the Milky Way. The actual location of these two star clusters, inside the Sgr dSph trailing tail, is compatible with this scenario, and their chemical and kinematical properties, together with our present understanding of the age–metallicity relationship in the Sgr dSph, lend further support to this possible association. Hence, we find it likely that the old open star clusters, Berkeley 29 and Saurer 1, have extragalactic origins.

We have compiled two new open cluster catalogs. In the first one, there are 119 objects with ages, distances, and metallicities available, while in the second one, 144 objects have both absolute proper motion and radial velocity data, of which 45 clusters also have metallicity data available. Taking advantage of the large number of objects included in our sample, we present an iron radial gradient of about -0.063 ± 0.008 dex kpc-1 from the first sample, which is quite consistent with the most recent determination of the oxygen gradient from nebulae and young stars, about -0.07 dex kpc-1. By dividing clusters into age groups, we show that the iron gradient was steeper in the past, which is consistent with the recent result from Galactic planetary nebulae data, and also consistent with inside-out galactic disk formation scenarios. Based on the cluster sample, we also discuss the metallicity distribution, cluster kinematics, and space distribution. A disk age-metallicity relation could be implied by those properties, although we cannot give conclusive result from the age- metallicity diagram based on the current sample. More observations are needed for metal-poor clusters. From the second catalog, we have calculated the velocity components in cylindrical coordinates with respect to the Galactic standard of rest for 144 open clusters. The velocity dispersions of the older clusters are larger than those of young clusters, but they are all much smaller than that of the Galactic thick disk stars.

The currently accepted paradigm for star formation assumes that field stars are born in clusters. These are not formed in isolation but in stellar complexes born out of giant molecular clouds. In the Galactic disk, molecular clouds have distinctive orbits, which, as they disappear after star formation is complete, may seed the Galactic disk with families of young clusters. These families gradually disperse to become individual clusters and, eventually, field populations. We investigate the existence of dynamical families of open clusters in the solar neighborhood using both age- and volume-limited samples from WEBDA in the framework of scan statistics. Our analysis indicates that a significant number of known young clusters organize in groups when age, spatial distribution, and kinematics are taken into account simultaneously. We find compelling statistical evidence for the presence of at least five dynamical families of young open clusters in the Milky Way disk associated to the underlying spiral structure. The young cluster population seems to be dominated by families of 10-20 objects; they are short-lived and the likely progenitors of classical moving groups, and stellar streams. Available observational data suggests that 50%-80% of newly formed open clusters dissolve within 20 Myr of formation to become field population. The overall age distribution of open clusters shows a steep decline, -->dN/dτ τβ, with -->β = − 3.6 ± 0.5 for clusters younger than 100 Myr, although it could be dependent on the local conditions as it ranges from -->β = − 1.0 ± 0.2 in the direction of Puppis to -->β = − 2.8 ± 0.5 in Norma. Due to the high rate of destruction among young clusters, any cluster-related coherent substructure must be younger than about 30 Myr unless it is the result of dynamically induced corotation resonances within the Galactic disk or minor mergers. The characteristic timescale for stars to become part of the field stellar populations is 10-20 Myr.

We present new measurements of the velocity dispersions of eleven Local Group globular clusters using spatially integrated spectra, to expand our sample of clusters with precise integrated-light velocity dispersions to 29, over 4 different host galaxies. This sample allows us to further our investigation of the stellar mass function among clusters, with a particular emphasis on a search for the driver of the apparent bimodal nature of the inferred stellar initial mass function. We confirm our previous result that clusters fall into two classes. If, as we argue, this behavior reflects a variation in the stellar initial mass function, the cause of that variation is not clear. The variations do not correlate with formation epoch as quantified by age, metallicity quantified by $[ {\rm Fe/H}] $, host galaxy, or internal structure as quantified by velocity dispersion, physical size, relaxation time, or luminosity. The stellar mass-to-light ratios, $\Upsilon_*$, of the high and low $\Upsilon_*$ cluster populations are well-matched to those found in recent studies of early and late type galaxies, respectively.

Context. Open clusters are groups of coeval stars sharing properties such as distance and metallicity, and they are key to understanding stellar evolution. Aims. Our main goal is to study the evolution of open clusters with a special focus on the universality of the luminosity function. Methods. We applied an upgraded version of the convergent point technique on about 50 open clusters. The selection of cluster members was based purely on the exquisite astrometry of the Gaia DR3 and HIPPARCOS catalogues in the five-dimensional or full six-dimensional space. Results. We present updated lists of bona fide members of ∼50 open clusters within 500 pc and younger than 1 Gyr, exploiting the full depth of the third Gaia data release complemented by HIPPARCOS at the bright end, excluding regions in the Galactic plane. Our catalogues also are complemented by optical and infrared photometry from the major large-scale public surveys. All the data will be made available on a dedicated webpage with interactive plots and a direct link to Aladin and Vizier hosted at the Centre de Données de Strasbourg. We derived luminosity functions for all bound clusters and compared them in three age groups of ∼50 Myr, ∼150 Myr, and ∼600 Myr, discussing similarities and differences to constrain their dynamical evolution. Conclusions. Luminosity functions of clusters at 50 Myr are more likely similar to each other and show a greater degree of similarity than older clusters. We explain this observation with the universal luminosity function within the volume of our sample (500 pc). Luminosity functions of clusters with ages similar to the Pleiades or Hyades are more diverse, perhaps due to internal dynamical evolution, but more work is needed to provide additional evidence.

This work analyzes the present-day mass function (PDMF) of 93 star clusters utilizing Gaia Data Release 3 data, with membership determined by the StarGo machine-learning algorithm. The impact of unresolved binary systems on mass estimation is rigorously assessed, adopting three mass ratio profiles for correction. The PDMF is characterized by the power-law index, α, derived through a robust maximum likelihood method that avoids biases associated with data binning. The value of α for stars between the completeness limited mass of Gaia (with a mean 0.3 M ⊙ for our cluster samples) and 2 M ⊙ exhibits stability for clusters younger than 200 Myr, decreasing for older clusters, particularly when considering stars within the half-mass radius. The PDMF of these star clusters is consistent with a dynamically evolved Kroupa initial mass function via the loss of low-mass stars. Cluster morphology shows a correlation with α, as α values exhibit a decreasing trend from filamentary to tidal-tail clusters, mirroring the sequence of increasing cluster age. The dependence of α on the total cluster mass is weak, with a subtle increase for higher-mass clusters, especially outside the half-mass radius. We do not observe a correlation between α and the mean metallicity of the clusters. Younger clusters have lower metallicity compared to their older counterparts, which indicates that the older clusters might have migrated to the solar neighborhood from the inner disk. A comparison with numerical models incorporating a black hole population suggests the need for observations of distant, older, massive open clusters to determine whether or not they contain black holes.

view Abstract Citations (23) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Supernovae, Globular Clusters, and Galaxy Mergers van den Bergh, Sidney Abstract A number of lines of evidence show that collisions, and subsequent mergers, of gas-rich disk galaxies can trigger bursts of star and cluster formation. Observations of NGC 4038/4039 (the Antennae) appear to show that the young clusters formed during this merger have an exponential (open cluster) luminosity function, rather than a Gaussian (globular cluster) luminosity function. This result militates against the suggestion that the high specific globular cluster frequency in elliptical galaxies is due to globular cluster formation during mergers. The observation that [α/Fe] is larger in ellipticals than it is in spirals shows that elliptical galaxies must have formed on a shorter timescale than spirals. This would appear to rule out evolutionary models in which the majority of ellipticals formed from mergers between mostly stellar disk systems. Scenarios in which the protogalactic halo was locally homogeneous, but globally heterogeneous and clumpy, may account for the puzzling observation that Galactic globular clusters exhibit both age differences of a few gigayears and high [α/Fe] values. Publication: The Astrophysical Journal Pub Date: September 1995 DOI: 10.1086/176115 Bibcode: 1995ApJ...450...27V Keywords: GALAXIES: ABUNDANCES; GALAXIES: EVOLUTION; GALAXIES: FORMATION; GALAXIES: INTERACTIONS; GALAXIES: STAR CLUSTERS; STARS: SUPERNOVAE: GENERAL full text sources ADS | data products SIMBAD (11) NED (3)

Context. Close investigations of unstudied open-cluster candidates may improve the statistics of objects undergoing the dissolution phase.Aims. We plan to settle the nature and derive astrophysical (fundamental, structural, and stellar mass content) parameters for 34 unstudied open cluster candidates from the near-infrared Teutsch list.Methods. The analysis employs 2MASS photometry, field-star decontamination (to enhance the intrinsic colour–magnitude diagram morphology), and colour–magnitude filters (for high contrast in stellar radial density profiles).Results. We find 8 clusters younger than ~30 Myr, 21 with ages within 100–900 Myr, 3 older than 1 Gyr, and possibly 1 as old as ~7 Gyr. Part of the sample is affected by reddening as high as A V ~ 9, and about half is located more than ~ 3 kpc away from the Sun, with a few reaching = 6–9 kpc. The sample contains essentially low-luminosity clusters in the optical, with ≈ -3± 3. These properties are consistent with their near-infrared origin. Cluster size increases both with Galactocentric distance and height over the plane, which is consistent with the low level of tidal stress (and field contamination) associated with these regions. The average mass density falls off with cluster radius as ρ ~ R -3 , which in clusters younger than ~20 Myr and more massive than ~103 has been interpreted as diffusion-related cluster expansion.Conclusions. Besides the derivation of astrophysical parameters for a sample of unstudied open clusters, in this paper we identify a set of clusters older than several 102 Myr, with 4 of them having survived a few Gyr. Surveys of open cluster candidates should be further explored to fill in the gap between the detected and predicted number of clusters. An improved statistic, especially on the population of clusters in highly evolved phases, can be used to investigate cluster formation rates and constrain the dissolution-time scale in the Galaxy.

We present the Milky Way Star Clusters (MWSC) survey based on 470 million stars extracted from the all-sky catalogues PPMXL and 2mass. The target list comprises 3784 entries and includes all open and globular clusters, as well as all cluster candidates, known at present. By means of a combined photometric/astrometric pipeline process, we determine individual membership probabilities for cluster stars and provide fundamental cluster parameters such as cluster centre coordinates, mean proper motions, radial velocity, distance, reddening, age and tidal radius. This sample includes ~ 700 clusters with fundamental parameters based on optical data and offers a good opportunity for a comparison of the near-infrared (NIR) and optical cluster distance scales. We find that distances and reddening values obtained from the colour–magnitude diagrams (CMDs) in the NIR agree well with earlier distance estimates based on optical CMDs. These newly obtained homogeneous distances, together with the other cluster parameters, allow us to study the properties of the open cluster population in the Galactic disk.

We investigate the Fe, C, N, O, Mg, Al, Si, K, Ca, Ce, and Nd abundances of 2283 red giant stars in 31 globular clusters from high-resolution spectra observed in both the Northern and Southern hemisphere by the SDSS-IV APOGEE-2 survey. This unprecedented homogeneous data set, largest to date, allows us to discuss the intrinsic Fe spread, the shape, and statistics of Al-Mg and N-C anti-correlations as a function of cluster mass, luminosity, age, and metallicity for all 31 clusters. We find that the Fe spread does not depend on these parameters within our uncertainties including cluster metallicity, contradicting earlier observations. We do not confirm the metallicity variations previously observed in M22 and NGC 1851. Some clusters show a bimodal Al distribution, while others exhibit a continuous distribution as has been previously reported in the literature. We confirm more than two populations in ω Cen and NGC 6752, and find new ones in M79. We discuss the scatter of Al by implementing a correction to the standard chemical evolution of Al in the Milky Way. After correction, its dependence on cluster mass is increased suggesting that the extent of Al enrichment as a function of mass was suppressed before the correction. We observe a turnover in the Mg-Al anticorrelation at very low Mg in ω Cen, similar to the pattern previously reported in M15 and M92. ω Cen may also have a weak K-Mg anticorrelation, and if confirmed, it would be only the third cluster known to show such a pattern.