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Register a new userMassive young clusters in the disc of M31
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We have studied the properties of a sample of 67 very blue and likely young massive clusters in M31 extracted from the Bologna Revised Catalog of globular clusters, selected according to their color [(B-V) < 0.45] and/or to the strength of their Hbeta spectral index (Hbeta > 3.5 A). Their existence in M31 has been noted by several authors in the past; we show here that these Blue Luminous Compact Clusters (BLCCs) are a significant fraction (>~ 15%) of the whole globular cluster system of M31. Compared to the global properties of the M31 globular cluster system, they appear to be intrinsically fainter, morphologically less concentrated, and with a shallower Balmer jump and enhanced $Hbeta$ absorption in their spectra. Empirical comparison with integrated properties of clusters with known age as well as with theoretical SSP models consistently indicate that their typical age is less than ~2 Gyr, while they probably are not so metal-poor as deduced if considered to be old. Either selecting BLCCs by their (B-V) colors or by the strength of their Hbeta index the cluster sample turns out to be distributed onto the outskirts of M31 disc, sharing the kinematical properties of the thin, rapidly rotating disc component. If confirmed to be young and not metal-poor, these clusters indicate the occurrence of a significant recent star formation in the thin disc of M31, although they do not set constraints on the epoch of its early formation.
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In the last decade we have come to realize that the traditional classification of stellar clusters into open and globular clusters cannot be easily extended beyond the realm of the Milky Way, and that even for our Galaxy it is not fully valid. The main failure of the traditional classification is the existence of Massive Young Clusters (MYCs), which are massive like Globular Clusters (GCs) but also young like open clusters. We describe here the mass and age distributions of clusters in general with an emphasis on MYCs. We also discuss the issue of what constitutes a cluster and try to establish a general classification scheme.
We report on Gemini/GMOS observations of two newly discovered globular clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc, onto a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.
As of August 2019, among the more than 4000 confirmed exoplanets, only one has been detected in a globular cluster (GC) M4. The scarce of exoplanet detections motivates us to employ direct $N$-body simulations to investigate the dynamical stability of planets in young massive clusters (YMCs), which are potentially the progenitors of GCs. In an $N=128{rm k}$ cluster of virial radius 1.7 pc (comparable to Westerlund-1), our simulations show that most wide-orbit planets ($ageq 20$~au) will be ejected within a timescale of 10 Myr. Interestingly, more than $70%$ of planets with $a<5$~au survive in the 100 Myr simulations. Ignoring planet-planet scattering and tidal damping, the survivability at $t$ Myr as a function of initial semi-major axis $a_0$ in au in such a YMC can be described as $f_{rm surv}(a_0, t)=-0.33 log_{10}(a_0) left(1 - e^{-0.0482t} right) + 1$. Upon ejection, about $28.8%$ of free-floating planets (FFPs) have sufficient speeds to escape from the host cluster at a crossing timescale. The other FFPs will remain bound to the cluster potential, but the subsequent dynamical evolution of the stellar system can result in the delayed ejection of FFPs from the host cluster. Although a full investigation of planet population in GCs requires extending the simulations to multi-Gyr, our results suggest that wide-orbit planets and free-floating planets are unlikely to be found in GCs.
We have retrieved multicolor WFPC2/HST data from the STScI archive for 27 nearby Massive (>= 3x10^4 M_Sun) Young (< 20 Myr) star Clusters (MYCs). The data represents the most-complete-to-date sample of clearly resolved MYCs. We have analyzed their structural properties and have found that they can be classified as either Super Star Clusters (SSCs) or as Scaled OB Associations (SOBAs). SSCs have a compact core possibly surrounded by a halo while SOBAs have no core. A morphological sequence can be established from SSCs with weak halos to SSCs with strong halos to SOBAs and we propose that this is linked to the original mass distribution of the parent giant molecular clouds. Our results indicate that a significant fraction of the stars in MYCs dissipate on timescales of 10 Gyr or less due to the extended character of some of the clusters. Also, SSCs with ages < 7 Myr have smaller cores on average than those with ages > 7 Myr, confirming predictions of numerical simulations with mass loss.
Stars mostly form in groups consisting of a few dozen to several ten thousand members. For 30 years, theoretical models provide a basic concept of how such star clusters form and develop: they originate from the gas and dust of collapsing molecular clouds. The conversion from gas to stars being incomplete, the left over gas is expelled, leading to cluster expansion and stars becoming unbound. Observationally, a direct confirmation of this process has proved elusive, which is attributed to the diversity of the properties of forming clusters. Here we take into account that the true cluster masses and sizes are masked, initially by the surface density of the background and later by the still present unbound stars. Based on the recent observational finding that in a given star-forming region the star formation efficiency depends on the local density of the gas, we use an analytical approach combined with mbox{N-body simulations, to reveal} evolutionary tracks for young massive clusters covering the first 10 Myr. Just like the Hertzsprung-Russell diagram is a measure for the evolution of stars, these tracks provide equivalent information for clusters. Like stars, massive clusters form and develop faster than their lower-mass counterparts, explaining why so few massive cluster progenitors are found.
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