Young Compact Clusters Research Articles

X-ray emission from Westerlund 2 detected by SRG/ART-XC and Chandra: search for radiation of TeV leptons

ABSTRACT We present the results of current observations of the young compact cluster of massive stars Westerlund 2 (Wd2) with the Mikhail Pavlinsky ART-XC telescope aboard the Spectrum-Roentgen-Gamma (SRG) observatory which we analysed together with the archival Chandra data. In general, Wd2 was detected over the whole electromagnetic spectrum including high-energy gamma rays, which revealed a cosmic ray acceleration in this object to the energies up to tens of TeV. The detection of Wd2 with ART-XC allowed us to perform a joint spectral analysis together with the high resolution Chandra observations of the diffuse emission from a few selected regions in the vicinity of the Wd2 core in the 0.4–20 keV range. To fit the Wd2 X-ray spectrum above a few keV one needs either a non-thermal power-law emission component, or a hot plasma with temperatures ∼ 5 keV. Our magnetohydrodynamic modelling of the plasma flows in Wd2 shows substantially lower electron temperatures in the system and thus the presence of the non-thermal component is certainly preferable. A kinetic model of the particle acceleration demonstrated that the non-thermal component may originate from the synchrotron radiation of multi-TeV electrons and positrons produced in Wd2 in accordance with the TeV photons detection from the source.

Massive star cluster origin for the galactic cosmic ray population at very-high energies

ABSTRACT We demonstrate that supernova remnant (SNR) shocks embedded within massive star clusters can reproduce both the cosmic-ray proton and all-particle spectra measured in the vicinity of the Earth up to hundreds of peta-electronvolts (PeV). We model two classes of massive star clusters. The first population are ‘loose clusters’ that do not power a collective wind termination shock. SNR shocks then expand in a low-density and weakly magnetized medium, and this population mainly contributes up to the ‘knee’ of the CR spectrum around 1 PeV. The second population are young compact clusters, which are powerful and compact enough to sustain a collective wind outflow. SNR shocks then expand from the cluster into the strongly magnetized wind and accelerate nuclei up to ultra-high energies. This population, representing only about 15 per cent of all Galactic massive star clusters, nevertheless dominates the spectrum between ∼1 and 100 PeV. While these two components alone can reproduce the shape of the CR spectrum up to hundreds of PeV, adding a light subankle extragalactic component motivated by composition and anisotropy measurements, allows to reproduce the spectrum up to the highest energies. Fitting parameters are systematically linked to physical variables whose values are in line with theoretical expectations.

Inside the core of a young massive star cluster: 3D MHD simulations

ABSTRACT Young massive star clusters inhabit regions of star formation and play an essential role in the galactic evolution. They are sources of both thermal and non-thermal radiation, and are effective cosmic ray accelerators. We present the 3D magnetohydrodynamic modelling of the plasma flows in a young compact cluster at the evolutionary stage comprising multiple interacting supersonic winds of massive OB and WR stars. The modelling allows studying the partitioning of the mechanical energy injected by the winds between the bulk motions, thermal heating, and magnetic fields. Cluster-scale magnetic fields reaching the magnitudes of ∼300 $\mu$G show the filamentary structures spreading throughout the cluster core. The filaments with the high magnetic fields are produced by the Axford–Cranfill-type effect in the downstream of the wind termination shocks, which is amplified by a compression of the fields with the hot plasma thermal pressure in the central part of the cluster core. The hot (∼a few keV) plasma is heated at the termination shocks of the stellar winds and compressed in the colliding post-shock flows. We also discuss a possible role of the thermal conduction effects on the plasma flow, and analyse temperature maps in the cluster core and the diffuse thermal X-ray emission spectra. The presence of high cluster-scale magnetic fields supports the possibility of high-energy cosmic ray acceleration in clusters at the given evolutionary stage.

Can superbubbles accelerate ultrahigh energy protons?

ABSTRACT We critically assess limits on the maximum energy of protons accelerated within superbubbles around massive stellar clusters, considering a number of different scenarios. In particular, we derive under which circumstances acceleration of protons above peta-electronvolt (PeV) energies can be expected. While the external forward shock of the superbubble may account for acceleration of particles up to 100 TeV, internal primary shocks such as supernova remnants expanding in the low density medium or the collective wind termination shock which forms around a young compact cluster provide more favourable channels to accelerate protons up to 1 PeV, and possibly beyond. Under reasonable conditions, clustered supernovae launching powerful shocks into the magnetized wind of a young and compact massive star cluster are found to be the most promising systems to accelerate protons above 10 PeV. On the other hand, stochastic re-acceleration in the strongly turbulent plasma is found to be much less effective than claimed in previous works, with a maximum proton energy of at most a few hundred TeV.

3D dynamics of the Orion cloud complex

We present the first study of the three-dimensional (3D) dynamics of the gas in the entire southern Orion cloud complex. We used the parallaxes and proper motions of young stellar objects (YSOs) from Gaia DR2 as a proxy for gas distance and proper motion, and the gas radial velocities from archival CO data, to compute the space motions of the different star-forming clouds in the complex, including subregions in Orion A, Orion B, and two outlying cometary clouds. From the analysis of the clouds’ orbits in space and time, we find that they were closest about 6 Myr ago and are moving radially away from roughly the same region in space. This coherent 100-pc scale radial motion supports a scenario where the entire complex is reacting to a major feedback event, which we name the Orion-BB (big blast) event. This event, which we tentatively associate with the recently discovered Orion X stellar population, shaped the distribution and kinematics of the gas we observe today, although it is unlikely to have been the sole major feedback event in the region. We argue that the dynamics of most of the YSOs carry the memory of the feedback-driven star formation history in Orion and that the majority of the young stars in this complex are a product of large-scale triggering, which can raise the star formation rate by at least an order of magnitude, as for the head of Orion A (the Integral Shape Filament). Our results imply that a feedback, compression, and triggering process lies at the genesis of the Orion Nebula Cluster and NGC 2023/2024 in Orion B, thus confirming broadly the classical feedback-driven scenario proposed in Elmegreen & Lada (1977, ApJ, 214, 725). The space motions of the well-known young compact clusters, σ Orionis and NGC 1977, are consistent with this scenario. A momentum estimate suggests that the energy of a few to several supernovae is needed to power the coherent 3D gas motion we measure in this paper.

High-energy cosmic rays from supernovae in young clusters of massive stars

While the bulk of GeV–100 TeV cosmic rays (CRs) can be accelerated at the shock of the most common supernova remnants the problem of CR acceleration to and above the PeV regime is still an open issue. We give a brief discussion of the production of 100 PeV regime cosmic rays by supernovae in young compact clusters of massive stars. This implies a spatial and temporal correlation of the SNe in the cluster over $$\sim 10^7\,\hbox {year}$$ which may affect the high-energy CR anisotropy. Here, we discuss the temporal evolution of the dipole angular anisotropy of 300 PeV CR protons accelerated within young massive star clusters.

SN 2017ein and the Possible First Identification of a Type Ic Supernova Progenitor

Abstract We have identified a progenitor candidate in archival Hubble Space Telescope (HST) images for the Type Ic supernova (SN Ic) SN 2017ein in NGC 3938, pinpointing the candidate’s location via HST Target of Opportunity imaging of the SN itself. This would be the first identification of a stellar-like object as a progenitor candidate for any SN Ic to date. We also present observations of SN 2017ein during the first ∼49 days since explosion. We find that SN 2017ein most resembles the well-studied SN Ic SN 2007gr. We infer that SN 2017ein experienced a total visual extinction of A V ≈ 1.0–1.9 mag, predominantly because of dust within the host galaxy. Although the distance is not well known, if this object is the progenitor, it was likely of high initial mass, ∼47–48 M ⊙ if a single star, or ∼60–80 M ⊙ if in a binary system. However, we also find that the progenitor candidate could be a very blue and young compact cluster, further implying a very massive (>65 M ⊙) progenitor. Furthermore, the actual progenitor might not be associated with the candidate at all and could be far less massive. From the immediate stellar environment, we find possible evidence for three different populations; if the SN progenitor was a member of the youngest population, this would be consistent with an initial mass of ∼57 M ⊙. After it has faded, the SN should be reobserved at high spatial resolution and sensitivity, to determine whether the candidate is indeed the progenitor.

Cosmic Ray Production in Supernovae

We give a brief review of the origin and acceleration of cosmic rays (CRs), emphasizing the production of CRs at different stages of supernova evolution by the first-order Fermi shock acceleration mechanism. We suggest that supernovae with trans-relativistic outflows, despite being rather rare, may accelerate CRs to energies above $10^{18}\mbox{ eV}$ over the first year of their evolution. Supernovae in young compact clusters of massive stars, and interaction powered superluminous supernovae, may accelerate CRs well above the PeV regime. We discuss the acceleration of the bulk of the galactic CRs in isolated supernova remnants and re-acceleration of escaped CRs by the multiple shocks present in superbubbles produced by associations of OB stars. The effects of magnetic field amplification by CR driven instabilities, as well as superdiffusive CR transport, are discussed for nonthermal radiation produced by nonlinear shocks of all speeds including trans-relativistic ones.

Formation of the young compact cluster GM 24 triggered by a cloud–cloud collision

Abstract High-mass star formation is an important step which controls galactic evolution. GM 24 is a heavily obscured star cluster including a single O9 star with more than ∼100 lower-mass stars within a 0.3 pc radius toward (l, b) ∼ (${350{{^{\circ}_{.}}}5}$, ${0{{^{\circ}_{.}}}96}$), close to the Galactic mini-starburst NGC 6334. We found two velocity components associated with the cluster by new observations of 12CO J =2–1 emission, whereas the cloud was previously considered to be single. We found that the distribution of the two components of 5 $\rm {km}\:$s−1 separation shows complementary distribution; the two fit well with each other if a relative displacement of 3 pc is applied along the Galactic plane. A position–velocity diagram of the GM 24 cloud is explained by a model based on numerical simulations of two colliding clouds, where an intermediate velocity component created by the collision is taken into account. We estimate the collision time scale to be ∼Myr in projection of a relative motion tilted to the line of sight by 45°. The results lend further support for cloud–cloud collision as an important mechanism of high-mass star formation in the Carina–Sagittarius Arm.

A SURVEY OF IRRADIATED PILLARS, GLOBULES, AND JETS IN THE CARINA NEBULA

We present wide-field, deep narrowband H$_2$, Br$\gamma$, H$\alpha$, [S II], [O III], and broadband I and K-band images of the Carina star formation region. The new images provide a large-scale overview of all the H$_2$ and Br$\gamma$ emission present in over a square degree centered on this signature star forming complex. By comparing these images with archival HST and Spitzer images we observe how intense UV radiation from O and B stars affects star formation in molecular clouds. We use the images to locate new candidate outflows and identify the principal shock waves and irradiated interfaces within dozens of distinct areas of star-forming activity. Shocked molecular gas in jets traces the parts of the flow that are most shielded from the intense UV radiation. Combining the H$_2$ and optical images gives a more complete view of the jets, which are sometimes only visible in H$_2$. The Carina region hosts several compact young clusters, and the gas within these clusters is affected by radiation from both the cluster stars and the massive stars nearby. The Carina Nebula is ideal for studying the physics of young H II regions and PDR's, as it contains multiple examples of walls and irradiated pillars at various stages of development. Some of the pillars have detached from their host molecular clouds to form proplyds. Fluorescent H$_2$ outlines the interfaces between the ionized and molecular gas, and after removing continuum, we detect spatial offsets between the Br$\gamma$ and H$_2$ emission along the irradiated interfaces. These spatial offsets can be used to test current models of PDRs once synthetic maps of these lines become available.

A METHOD FOR MEASURING VARIATIONS IN THE STELLAR INITIAL MASS FUNCTION

We present a method for investigating variations in the upper end of the stellar Initial Mass Function (IMF) by probing the production rate of ionizing photons in unresolved, compact star clusters with ages <~10 Myr and with different masses. We test this method by performing a pilot study on the young cluster population in the nearby galaxy NGC5194 (M51a), for which multi-wavelength observations from the Hubble Space Telescope are available. Our results indicate that the proposed method can probe the upper end of the IMF in galaxies located out to at least ~10 Mpc, i.e., a factor ~200 further away than possible by counting individual stars in young compact clusters. Our results for NGC5194 show no obvious dependence of the upper mass end of the IMF on the mass of the star cluster down to ~1000 M_sun, although more extensive analyses involving lower mass clusters and other galaxies are needed to confirm this conclusion.

No evidence of mass segregation in massive young clusters

Aims. We investigate the validity of the mass segregation indicators commonly used in analysing young stellar clusters. Methods. We simulate observations by constructing synthetic seeing-limited images of a 1000 massive clusters (10 4 M� ) with a standard IMF and a King-density distribution function. Results. We find that commonly used indicators are highly sensitive to sample incompleteness in observational data and that radial completeness determinations do not provide satisfactory corrections, rendering the studies of radial properties highly uncertain. On the other hand, we find that, under certain conditions, the global completeness can be estimated accurately, allowing for the correction of the global luminosity and mass functions of the cluster. Conclusions. We argue that there is currently no observational evidence of mass segregation in young compact clusters since there is no robust way to differentiate between true mass segregation and sample incompleteness effects. Caution should then be exercised when interpreting results from observations as evidence of mass segregation.

NGC 346 in The Small Magellanic Cloud. IV. Triggered Star Formation in the HiiRegion N66

Stellar feedback, expanding H II regions, wind-blown bubbles, and supernovae are thought to be important triggering mechanisms of star formation. Stellar associations, being hosts of significant numbers of early-type stars, are the loci where these mechanisms act. In this part of our photometric study of the star-forming region NGC 346/N66 in the Small Magellanic Cloud, we present evidence based on previous and recent detailed studies that it hosts at least two different events of triggered star formation, and we reveal the complexity of its recent star formation history. In our earlier studies of this region (Papers I and III) we find that besides the central part of N66, where the bright OB stellar content of the association NGC 346 is concentrated, an arclike nebular feature, north of the association, hosts recent star formation. This feature is characterized by a high concentration of emission-line stars and young stellar objects, as well as embedded sources seen as IR emission peaks that coincide with young compact clusters of low-mass pre-main-sequence stars. All these objects indicate that the northern arc of N66 encompasses the most current star formation event in the region. We present evidence that this star formation is the product of a different mechanism than that in the general area of the association, and that it is triggered by a wind-driven expanding H II region (or bubble) blown by a massive supernova progenitor, and possibly other bright stars, a few megayears ago. We propose a scenario according to which this mechanism triggered star formation away from the bar of N66, while in the bar of N66 star formation is introduced by the photoionizing OB stars of the association itself.

Investigating the borderline between a young star cluster and a small stellar association: a test case with Bochum 1

Context. Usually, a loose stellar distribution can be classified as an OB stellar group, an association, or a young open cluster. We compare data with the typical OB association Vul OB1. Aims. Here, we discuss the nature of Bochum 1, a typical example of an object affected by the above classification problem. Methods. Field-decontaminated 2MASS photometry is used to analyse colour–magnitude diagrams (CMDs) and stellar radial density profiles (RDPs) of the structures present in the region of Bochum 1. Results. The field-decontaminated CMD of Bochum 1 shows main sequence (MS) and pre-main sequence (PMS) stars. We report two new small angular-size, compact young clusters and one embedded cluster in the area of Bochum 1. Vul OB1 harbours the young open cluster NGC 6823 and the very compact embedded cluster Cr 404. The Vul OB1 association includes the H II region Sh2-86, and its stellar content is younger (≈3 Myr) than that of Bochum 1 (≈9 Myr), which shows no gas emission. Bochum 1 harbours one of the newly found compact clusters as its core. The RDP of Bochum 1 is irregular and cannot be fitted by a King-like profile, which suggests significant erosion or dispersion of stars from a primordial cluster. Similarly to Bochum 1, the decontaminated CMD of NGC 6823 presents conspicuous MS and PMS sequences. Taken separately, RDPs of MS and PMS stars follow a King-like profile. The core shows an excess density of MS stars that mimics the profile of a post-core collapse cluster. At such a young age, it can be explained by an excess of stars formed in the prominent core. Conclusions. The present study suggests that Bochum 1 is a star cluster fossil remain that might be dynamically evolving into an OB association. Bochum 1 may be a missing link connecting early star cluster dissolution with the formation of low-mass OB associations.

No evidence for mass segregation in young clusters

<i>Aims. <i/>We investigate the validity of the mass segregation indicators commonly used in analysing young stellar clusters.<i>Methods. <i/>We simulate observations by constructing synthetic seeing-limited images of a 1000 massive clusters (10<sup>4<sup/> ) with a standard IMF and a King-density distribution function.<i>Results. <i/>We find that commonly used indicators are highly sensitive to sample incompleteness in observational data and that radial completeness determinations do not provide satisfactory corrections, rendering the studies of radial properties highly uncertain. On the other hand, we find that, under certain conditions, the global completeness can be estimated accurately, allowing for the correction of the global luminosity and mass functions of the cluster.<i>Conclusions. <i/>We argue that there is currently no observational evidence of mass segregation in young compact clusters since there is no robust way to differentiate between true mass segregation and sample incompleteness effects. Caution should then be exercised when interpreting results from observations as evidence of mass segregation.

On the massive stellar population of the super star cluster Westerlund 1

We present new spectroscopic and photometric observations of the young Galactic open cluster Westerlund 1 (Wd 1) that reveal a unique population of massive evolved stars. We identify ∼200 cluster members and present spectroscopic clas- sifications for ∼25% of these. We find that all stars so classified are unambiguously post-Main Sequence objects, consistent with an apparent lack of an identifiable Main Sequence in our photometric data to V ∼ 20. We are able to identify rich popu- lations of Wolf Rayet stars, OB supergiants and short lived transitional objects. Of these, the latter group consists of both hot (Luminous Blue Variable and extreme B supergiant) and cool (Yellow Hypergiant and Red Supergiant) objects - we find that half the known Galactic population of YHGs resides within Wd 1. We obtain a mean V − MV ∼ 25 mag from the cluster Yellow Hypergiants, implying a Main Sequence turnoff at or below MV = −5 (O7 V or later). Based solely on the masses inferred for the 53 spectroscopically classified stars, we determine an absolute minimum mass of ∼1.5 × 10 3 Mfor Wd 1. However, considering the complete photometrically and spectroscopically selected cluster population and adopting a Kroupa IMF we infer a likely mass for Wd 1 of ∼10 5 M� , noting that inevitable source confusion and incompleteness are likely to render this an underestimate. As such, Wd 1 is the most massive compact young cluster yet identified in the Local Group, with a mass exceeding that of Galactic Centre clusters such as the Arches and Quintuplet. Indeed, the luminosity, inferred mass and com- pact nature of Wd 1 are comparable with those of Super Star Clusters - previously identified only in external galaxies - and is consistent with expectations for a Globular Cluster progenitor.

The Progenitor of the Type II-P SN 2004dj in NGC 2403

The Type II-P supernova SN 2004dj in the nearby galaxy NGC 2403 occurred at a position coincident with object 96 in the list of luminous stars and clusters in this galaxy published by Sandage in 1984. The coincidence is established definitively through astrometric registration of our ground-based archival images of NGC 2403 with our recent images showing the SN. The archival images show that Sandage 96 is slightly resolved from the ground. Preoutburst blue spectrograms obtained by Humphreys & Aaronson reveal that Sandage 96 has a composite spectrum, dominated in the blue region by A- and B-type stars, while infrared photometry shows that Sandage 96 also contains red supergiants. These results demonstrate that Sandage 96 is a young compact cluster. We have studied the stellar population of Sandage 96, using published photometric measurements combined with a χ2 fitting code. We derive a cluster age of 13.6 Myr, a reddening of E(4405 - 5495) = 0.172, and a total stellar mass of 24,000 M☉. For this cluster age, the SN progenitor had a main-sequence mass of 15 M☉. Postoutburst photometry of Sandage 96 may establish whether the progenitor was a red or blue supergiant.

Stars, Star Clusters, and Dust in NGC 3077

Images obtained with the CFHTIR camera on the Canada-France-Hawaii Telescope are used to investigate the near-infrared photometric properties of the star-forming M81 group galaxy NGC 3077. The spectral-energy distribution (SED) of the near-infrared light within 10 arcsec of the nucleus is consistent with the 2um light being dominated by hot young (log(t_{yr}) < 6.8) stars reddened by A_V = 3 - 4, with A_V < 8 mag in some regions. A population like that near the center of NGC 205 likely contributes only a modest fraction of the light near 2um. A number of candidate star clusters are detected in and around NGC 3077. The specific frequency of globular clusters falls within the range measured in nearby dEs. The candidate young clusters have photometric masses that are similar to those of compact young clusters in other active star-forming systems, and SEDs consistent with ages log(t_{yr}) < 6.6. Based on the masses and ages of the young clusters, it is estimated that the star formation rate in NGC 3077 was at least 0.25 - 0.50 solar masses per year during the past few million years.

Mass segregation in young compact clusters in the Large Magellanic Cloud — III. Implications for the initial mass function

The distribution of core radii of rich clusters in the Large Magellanic Cloud (LMC) systematically increases in both upper limit and spread with increasing cluster age. Cluster-to-cluster variations in the stellar initial mass function (IMF) have been suggested as an explanation. We discuss the implications of the observed degree of mass segregation in our sample clusters for the shape of the initial mass function. Our results are based on Hubble Space Telescope/WFPC2 observations of six rich star clusters in the LMC, selected to include three pairs of clusters of similar age, metallicity and distance from the LMC centre, and exhibiting a large spread in core radii between the clusters in each pair. All clusters show clear evidence of mass segregation: (i) their luminosity function slopes steepen with increasing cluster radius, and (ii) the brighter stars are characterized by smaller core radii. For all sample clusters, both the slope of the luminosity function in the cluster centres and the degree of mass segregation are similar to each other, within observational errors of a few tenths of power-law slope fits to the data. This implies that their initial mass functions must have been very similar, down to ∼0.8–1.0 M⊙. We therefore rule out variations in the IMF of the individual sample clusters as the main driver of the increasing spread of cluster core radii with cluster age.

Measuring Sizes of Marginally Resolved Young Globular Clusters with the Hubble Space Telescope 11Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under contract to NASA.

ABSTRACTWe present a method for deriving sizes of marginally resolved star clusters from Hubble Space Telescope/WFPC2 observations by fitting King models to observations. We describe results both on simulated images and on observations of young compact clusters in NGC 3597 and NGC 1275. From the simulations, we find that we can measure King model concentrations (c) to an accuracy of about a factor of 2 for all combinations of c and King radius (r0) of interest if the data have high signal‐to‐noise ratio (S/N) (≳500 for the integrated brightness). If the concentration is accurately measured, we can measure the King radius accurately. For lower S/N, marginally resolved King profiles suffer from a degeneracy; different values of the concentration give different r0 but have comparable reduced χ2 values. In this case, neither the core radius nor the concentration can be constrained individually, but the half‐light radius can be recovered accurately.In NGC 3597, we can differentiate between concentrations only for the very brightest clusters; these suggest a concentration of ∼2. Assuming a concentration of 2 for the rest of the objects, we find an average King radius for the clusters in NGC 3597 of 0.7 pc, while the clusters in NGC 1275 have an average radius of 1.1 pc. These are similar to the average core radii for Galactic globular clusters, 0.92 pc. We find average half‐light radii of 5.4 and 6.2 pc for the young clusters in NGC 3597 and NGC 1275, respectively, while the average half‐light radii of Galactic globular clusters is 3.4 pc. The spread in the derived radial parameters in each cluster system is comparable to that observed in the Galactic globular cluster system.