The birth and death of star clusters in the Milky Way

The article discusses research in astrophysics as of March 2013, with a focus on the author's theory that the mass of parent clouds influences the formation, internal structure, and evolution of star clusters. Topics include the Pleiades open star cluster in the Milky Way galaxy, the formation of stellar groups within clouds of gas and dust, and the difference between T and OB Associations in clusters. Additional information is presented on the duration of open clusters, cloud contractions, and the Orion nebula cluster.

We present a catalog of high-precision proper motions in the Orion Nebula Cluster (ONC), based on Treasury Program observations with the Hubble Space Telescope’s (HST) ACS/WFC camera. Our catalog contains 2454 objects in the magnitude range of 14.2 < m F775W < 24.7, thus probing the stellar masses of the ONC from ∼0.4 M ☉ down to ∼0.02 M ☉ over an area of ∼550 arcmin2. We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at and mas yr−1) might be biased by the overlapping young stellar populations of Orion A. We identified four new ONC candidate runaways based on HST and the Gaia DR 2 data, all with masses less than ∼1 M ☉. The total census of known candidate runaway sources is 10—one of the largest samples ever found in any Milky Way open star cluster. Surprisingly, none of them have tangential velocities exceeding 20 km s−1. If most of them indeed originated in the ONC, it may compel the re-examination of dynamical processes in very young star clusters. It appears that the mass function of the ONC is not significantly affected by the lost runaways.

Context. We use present-day observations to investigate how the content of massive OB stars affects the long-term evolution of young open clusters and their tidal streams, and how such an effect influences the constraint of initial conditions. Aims. OB stars are typically found in binaries. They have a strong wind mass loss during the first few million years and many become black holes. These affect the dynamical evolution of an open star cluster and impact its dissolution in a given Galactic potential. We investigate the correlation between the mass of OB stars and the observational properties of open clusters. Hyades-like star clusters are well represented in the solar neighborhood and thus allow comparisons with observational data. Methods. We perform a large number of star-by-star numerical N-body simulations of Hyades-like star clusters using the high-performance N-body code PETAR combined with GALPY. Results. We find that OB stars and black holes have a major effect on star cluster evolution. Star clusters with the same initial conditions but a different initial content of OB stars follow very different evolutionary paths. Thus, the initial total mass and radius of an observed star cluster cannot be unambiguously determined unless the initial content of OB stars is known. We show that the stellar counts in the corresponding tidal tails, which can be identified in the Gaia data, help to resolve this issues. We thus emphasize the importance of exploring not only star clusters but also their corresponding tidal tails. These findings are relevant for studies of the formation of massive stars.

The mismatch between the mass function of the Milky Way’s embedded clusters (ECs) and that of open clusters (OCs) raises the question of whether each OC originates from a single EC. In this work, we explore a scenario in which OCs form as a result of post-gas expulsion coalescence of ECs within the same parental molecular cloud. We model this process using N-body simulations of ECs undergoing expansion due to gas expulsion. Our initial conditions are based on the observed spatial, kinematic, and mass distributions of ECs in three representative massive star-forming regions. Initially, ECs are isolated. After further expansion, interactions between ECs begin, mutually influencing their evolution. We examine this process as a function of gas expulsion time-scales, spatial separations between ECs, and their relative velocities. Our results demonstrate that, within a reasonable range of these parameters, the coalescence of ECs is robust and largely insensitive to initial conditions. The mass of ECs plays a critical role in the coalescence process. More massive ECs form stable gravitational cores, which greatly facilitate coalescence and help the resulting cluster resist expansion and Galactic tidal forces. Additionally, the number of ECs also enhances coalescence. The current mass distribution of clumps in the Milky Way suggests that directly forming massive ECs is challenging. However, the coalescence of multiple low-mass ECs can account for the observed parameter space of OCs in the Milky Way.

The Orion Nebula cluster (ONC) appears to be unusual on two grounds: the observed constellation of the OB stars of the entire ONC and its Trapezium at its centre implies a time-scale problem given the age of the Trapezium, and an initial mass function (IMF) problem for the whole OB star population in the ONC. Given the estimated crossing time of the Trapezium, it ought to have totally dynamically decayed by now. Furthermore, by combining the lower limit of the ONC mass with a standard IMF it emerges that the ONC should have formed at least about 40 stars heavier than 5 M⊙ while only 10 are observed. Using the N-body experiments we (i) confirm the expected instability of the Trapezium and (ii) show that beginning with a compact OB-star configuration of about 40 stars both the number of observed OB stars after 1 Myr within 1 pc radius and a compact trapezium configuration can be reproduced. These two empirical constraints thus support our estimate of 40 initial OB stars in the cluster. Interestingly, a more-evolved version of the ONC resembles the Upper Scorpius OB association. The N-body experiments are performed with the new C-code catena by integrating the equations of motion using the chain-multiple-regularization method. In addition, we present a new numerical formulation of the IMF.

Aims. We built Galactic open star cluster mass functions (CMFs) for different age sub-samples and spatial locations in the wider solar neighbourhood. Here, we present a simple cluster formation and evolution model to reproduce the main features of the CMFs. Methods. We used an unbiased working sample of 2227 clusters of the Milky Way Star Cluster (MWSC) catalogue, which occupy the heliocentric cylinders with magnitude-dependent completeness radii of 1–5 kpc. The MWSC survey provides an extended set of open star cluster parameters, including tidal radii, distances, and ages. From an analytic three-component Galaxy model, we derived tidal masses of clusters with a typical accuracy of about 70%. Our simple model includes a two-section cluster initial mass function, constant cluster formation rate, supervirial phase after a sudden expulsion of the remaining gas, and cluster mass loss due to stellar evolution and the clusters’ gradual destruction in the Galactic tidal field. The dynamical evolution model is based on previous N-body simulations. Results. The obtained tidal masses have been added to the MWSC catalogue. A general CMF (GCMF), built for all cluster ages around the Sun, has a bell-like shape and extends over four decades in mass. The high-mass slope found for tidal mass log mt/M⊙ ≥ 2.3 is equal to 1.14 ± 0.07. The CMFs for different age groups show the same high-mass slopes, while the low-mass slope is nearly flat for the youngest sub-sample (clusters younger than 20 Myr) and about −0.7 for the others. The inner and outer sub-samples covering Galactocentric radii R = 4.2–8.1 kpc and 8.9–13.5 kpc, respectively, are consistent with the GCMF, once the exponential decline of the Galactic disc density is taken into account. The model suggests star formation with low efficiency of 15–20%, where only 10% of stars remain bound in a cluster after gas expulsion and subsequent violent relaxation. The cluster formation rate required to reproduce the observed distributions in age and mass is about 0.4 M⊙ pc−2 Gyr−1. Conclusions. The obtained high-mass slope of the GCMF for the wide neighbourhood of the Sun is similar to slopes determined earlier in nearby galaxies for more luminous clusters with log m/M⊙ > 3.8. The MWSC catalogue supports models with a low star-formation efficiency, where 90% of stars are lost quickly after gas expulsion. The obtained cluster formation rate corresponds to open clusters’ contribution to the stellar content of the thin disc at the level of 30%.

External photoevaporation of protoplanetary discs, by massive O stars in stellar clusters, is thought to be a significant process in the evolution of a disc. It has been shown to result in significant mass loss and disc truncation, ultimately reducing the lifetime of the discs, and possibly affecting potential planet populations. It is a well-studied process in the Orion Nebula Cluster (ONC) where the cometary morphology of proplyds is spatially resolvable due to its proximity to Earth. However, we need to study external photoevaporation in additional stellar clusters to better understand its prevalence and significance more globally. Unfortunately, more massive stellar clusters where the majority of stars form are much farther away than the ONC. In these more distant clusters the proplyds are spatially unresolvable with current facilities, hence the cometary morphology is not a useful identification of external photoevaporation. Therefore, in order to identify and interpret external photoevaporation, the only observations we have are of spatially unresolved emission lines. To resolve this issue we have used the cloudy code to develop an approximate general model of the emission lines emanating from the hot ionized wind of a proplyd. We have used the model to determine which line ratios are most sensitive to the distance from an OB star, and found that the most sensitive line ratios vary by multiple orders of magnitude over an far-ultraviolet field of between 10$^3$ G$_0$ to 10$^6$ G$_0$. By identifying spatial gradients of line ratios in stellar clusters, we can identify regions of ongoing external photoevaporation.

Context.Data from theGaiasatellite are revolutionising our understanding of the Milky Way. With every new data release, there is a need to update the census of open clusters.Aims.We aim to conduct a blind, all-sky search for open clusters using 729 million sources fromGaiaDR3 down to magnitudeG ∼ 20, creating a homogeneous catalogue of clusters including many new objects.Methods.We used the Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN) algorithm to recover clusters. We validated our clusters using a statistical density test and a Bayesian convolutional neural network for colour-magnitude diagram classification. We inferred basic astrometric parameters, ages, extinctions, and distances for the clusters in the catalogue.Results.We recovered 7167 clusters, 2387 of which are candidate new objects and 4782 of which crossmatch to objects in the literature, including 134 globular clusters. A more stringent cut of our catalogue contains 4105 highly reliable clusters, 739 of which are new. Owing to the scope of our methodology, we are able to tentatively suggest that many of the clusters we are unable to detect may not be real, including 1152 clusters from the Milky Way Star Cluster (MWSC) catalogue that should have been detectable inGaiadata. Our cluster membership lists include many new members and often include tidal tails. Our catalogue’s distribution traces the galactic warp, the spiral arm structure, and the dust distribution of the Milky Way. While much of the content of our catalogue contains bound open and globular clusters, as many as a few thousand of our clusters are more compatible with unbound moving groups, which we will classify in an upcoming work.Conclusions.We have conducted the largest search for open clusters to date, producing a single homogeneous star cluster catalogue which we make available with this paper.

The Galactic anticenter stellar structure (GASS) has been identified with excess surface densities of field stars in several large-area sky surveys and with an unusual, stringlike grouping of five globular clusters. At least two of these are diffuse, young clusters between open and globular types. Here we call attention to the fact that four younger open or transitional clusters extend the previously identified, stringlike cluster grouping, with at least one having a radial velocity consistent with the previously found GASS velocity-longitude trend. All nine clusters lie close to a plane tipped 17° to the Galactic plane. This planar orientation is used to forage for additional potential cluster members in the inner Galaxy, and a number are found along the same plane and stringlike sequence, including almost all 15 known outer, old open clusters. Tidal accretion of a dwarf satellite galaxy on a low-inclination orbit—perhaps the GASS system—appears to be a plausible explanation for the origin of the outer, old open and transitional clusters of the Milky Way. We use these clusters to explore the age-metallicity relation of the putative accreted GASS progenitor. Finally, we provide the first radial velocity of a star in the cluster BH 176 and discuss its implications.

The aim of this work is to characterize the stellar population between Earth and the Orion A molecular cloud where the well known star formation benchmark Orion Nebula Cluster (ONC) is embedded. We use the denser regions the Orion A cloud to block optical background light, effectively isolating the stellar population in front of it. We then use a multi-wavelength observational approach to characterize the cloud's foreground stellar population. We find that there is a rich stellar population in front of the Orion A cloud, from B-stars to M-stars, with a distinct 1) spatial distribution, 2) luminosity function, and 3) velocity dispersion from the reddened population inside the Orion A cloud. The spatial distribution of this population peaks strongly around NGC 1980 (iota Ori) and is, in all likelihood, the extended stellar content of this poorly studied cluster. We infer an age of ~4-5 Myr for NGC 1980 and estimate a cluster population of the order of 2000 stars, which makes it one of the most massive clusters in the entire Orion complex. What is currently taken in the literature as the ONC is then a mix of several intrinsically different populations, namely: 1) the youngest population, including the Trapezium cluster and ongoing star formation in the dense gas inside the nebula, 2) the foreground population, dominated by the NGC 1980 cluster, and 3) the poorly constrained population of foreground and background Galactic field stars. Our results support a scenario where the ONC and L1641N are not directly associated with NGC 1980, i.e., they are not the same population emerging from its parental cloud, but are instead distinct overlapping populations. This result calls for a revision of most of the observables in the benchmark ONC region (e.g., ages, age spread, cluster size, mass function, disk frequency, etc.). (abridged)

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.

view Abstract Citations (104) References (152) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Star Clusters in the Clouds of Magellan van den Bergh, Sidney Abstract Star clusters in the Magellanic Clouds differ from those in the Galaxy in a number of important respects: 1. Both old and young clusters in the Clouds have radii r_h_ that are typically 3-4 times larger than those of their Galactic counterparts. 2. Luminous clusters of all ages in the LMC and the SMC are, on average, significantly more flattened than Galactic clusters. 3. The clouds contain a class of populous star clusters, with masses that are typically an order of magnitude smaller than those of average globulars, but an order of magnitude larger than those of most Galactic open clusters. 4. Many young clusters in the Magellanic Clouds are embedded in rich unbound stellar coronae. Such halos are probably the remnants of the associations in which these clusters were born. Stellar coronae around clusters can survive longer in the Clouds than in the Galaxy because tidal forces are much lower in the LMC and SMC than they are in the Galaxy. 5. The Large Cloud is surrounded by at least seven real globular clusters with ages greater than 10 Gyr (six of which contain RR Lyrae stars), whereas the Small Cloud has only one such companion. These clusters have luminosities and mass-to-light ratios similar to those of Galactic globulars. All six of the Large Cloud globular clusters for which color-magnitude diagrams are available have blue horizontal branches. This may indicate that these objects all have very large ages. Available data do not yet entirely exclude the possibility that these globular clusters belong to a spheroidal halo. 6. The specific globular cluster frequency is found to be S = 0.4 +/- 0.15 for the LMC, and S = 0.3 +/- 0.3 for the SMC. 7. In both the Large Cloud and the Small Cloud old clusters are found to be more widely distributed than are younger clusters. Known old clusters (and nova) (exhibit no concentration to the Bar of the LMC. 8. Both the metal enrichment history and the ages of star clusters indicate that star formation in the SMC got off to a slow start. The large residual gas mass in the Small Cloud also indicates that the Small Cloud never went through an intense prolonged starburst phase. No Large Cloud clusters are presently known with ages between about 3 and 10 Gyr. The end of this hiatus may have coincided with a major burst of star formation in the LMC. 9. The fact that the two largest bursts of star and cluster formation in the LMC appear to have no counterparts in the SMC indicates that the bursts of star formation in the Large Cloud were not triggered by close encounters between the Clouds. Publication: The Astrophysical Journal Pub Date: March 1991 DOI: 10.1086/169732 Bibcode: 1991ApJ...369....1V Keywords: Globular Clusters; Magellanic Clouds; Star Clusters; Star Formation; Stellar Evolution; Color-Magnitude Diagram; Early Stars; Elliptical Galaxies; Milky Way Galaxy; Astrophysics; CLUSTERS: GLOBULAR; GALAXIES: MAGELLANIC CLOUDS; GALAXIES: STELLAR CONTENT; STARS: EVOLUTION; STARS: FORMATION full text sources ADS | data products SIMBAD (17) NED (2)

The Setteducati-Weaver catalog of new open star clusters discovered on the Palomar Sky Survey plates was compared recently with the catalogs of OB stars published by the Hamburg and Warner and Swasey Observatories. This resulted in the identification of eleven clusters which appear to contain OB stars and which may thus serve as important additional spiral arm tracers in the galactic longitude range of 75 to 135 . Published photometric studies are available for only two of these clusters. Key words: open clusters - galactic structure - OB stars

We have classified on the MK system a total of 455 stars in 12 open clusters and associations. The classification is based on wide (1.2 mm) spectra of two reciprocal dispersions (39, 128 Å mm−1) obtained with the Kitt Peak 2.1 m and 90 cm reflectors, respectively. The higher dispersion is necessary to show the subtle peculiarities found in some stars. The clusters are the Orion Nebula cluster, Orion OB1 association, Lacerta OB1 association, IC 2602, IC 4665, Pleiades, M39, M34, NGC 2516, NGC 6633, NGC 6475, and Coma; two of these were done with Morgan.

view Abstract Citations (29) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rotation Periods for Seven Stars in the Trapezium Cluster Mandel, Gregory N. ; Herbst, William Abstract Results of a periodogram analysis of seven stars in the Trapezium cluster in the Orion Nebula are presented. These stars are found to have significant peaks in their power spectra, which are intepreted as the rotation periods of the stars. Periods range from 6.2 to 14.3 d. Three of them have periods longer than any previously detected for premain-sequence objects, and the group average, 8.7 d, is more than twice as long as the average period of previously measured T Tauri stars. A real difference is found in the rotational velocities of Trapezium cluster stars and T Tauri stars in associations, perhaps because the Trapezium cluster stars are extremely young. Selection effects are hindering the discovery of longer rotation periods among T Tauri stars in associations, and selection effects are hindering the discovery of short-period stars in the Trapezium cluster. Classical and weak T Tauri stars clearly coexist in this extremely young cluster, but the classical stars appear to be confined to I magnitudes fainter than 12.5. Publication: The Astrophysical Journal Pub Date: December 1991 DOI: 10.1086/186245 Bibcode: 1991ApJ...383L..75M Keywords: Astronomical Photometry; Open Clusters; Orion Nebula; Stellar Rotation; Charge Coupled Devices; Power Spectra; Space Observations (From Earth); Spectrum Analysis; Astronomy; CLUSTERS: ASSOCIATIONS; CLUSTERS: OPEN; STARS: ROTATION full text sources ADS | data products SIMBAD (3)