Young Massive Stellar Clusters Research Articles

Massive stars in the giant molecular cloud G23.3−0.3 and W41

Young massive stars and stellar clusters continuously form in the Galactic disk, generating new HII regions within their natal giant molecular clouds and subsequently enriching the interstellar medium via their winds and supernovae. Massive stars are among the brightest infrared stars in such regions; their identification permits the characterization of the star formation history of the associated cloud as well as constraining the location of stellar aggregates and hence their occurrence as a function of global environment. We present a stellar spectroscopic survey in the direction of the giant molecular cloud G23.3-0.3. This complex is located at a distance of ~ 4-5 kpc, and consists of several HII regions and supernova remnants. We discovered 11 OfK+ stars, one candidate Luminous Blue Variable, several OB stars, and candidate red supergiants. Stars with K-band extinction from ~1.3 - 1.9 mag appear to be associated with the GMC G23.3-0.3; O and B-types satisfying this criterion have spectro-photometric distances consistent with that of the giant molecular cloud. Combining near-IR spectroscopic and photometric data allowed us to characterize the multiple sites of star formation within it. The O-type stars have masses from 25 - 45 Msun, and ages of 5-8 Myr. Two new red supergiants were detected with interstellar extinction typical of the cloud; along with the two RSGs within the cluster GLIMPSE9, they trace an older burst with an age of 20--30 Myr. Massive stars were also detected in the core of three supernova remnants - W41, G22.7-0.2, and G22.7583-0.4917. A large population of massive stars appears associated with the GMC G23.3-0.3, with the properties inferred for them indicative of an extended history of stars formation.

ON THE IMPACT OF RADIATION PRESSURE ON THE DYNAMICS AND INNER STRUCTURE OF DUSTY WIND-DRIVEN SHELLS

Massive young stellar clusters are strong sources of radiation and mechanical energy. Their powerful winds and radiation pressure sweep-up interstellar gas into thin expanding shells which trap the ionizing radiation produced by the central clusters affecting the dynamics and the distribution of their ionized gas. Here we continue our comparison of the star cluster winds and radiation pressure effects on the dynamics of shells around young massive clusters. We calculate the impact that radiation pressure has on the distribution of matter and thermal pressure within such shells as well as on the density weighted ionization parameter $U_w$ and put our results on the diagnostic diagram which allows one to discriminate between the wind-dominated and radiation-dominated regimes. We found that model predicted values of the ionization parameter agree well with typical values found in local starburst galaxies. Radiation pressure may affect the inner structure and the dynamics of wind-driven shells only at the earliest stages of evolution or if a major fraction of the star cluster mechanical luminosity is dissipated or radiated away within the star cluster volume and thus the star cluster mechanical energy output is significantly smaller than star cluster synthetic models predict. However, even in these cases radiation dominates over the wind dynamical pressure only if the exciting cluster is embedded into a high density ambient medium.

HOW TO FIND YOUNG MASSIVE CLUSTER PROGENITORS

We propose that bound, young massive stellar clusters form from dense clouds that have escape speeds greater than the sound speed in photo-ionized gas. In these clumps, radiative feedback in the form of gas ionization is bottled up, enabling star formation to proceed to sufficiently high efficiency so that the resulting star cluster remains bound even after gas removal. We estimate the observable properties of the massive proto-clusters (MPCs) for existing Galactic plane surveys and suggest how they may be sought in recent and upcoming extragalactic observations. These surveys will potentially provide a significant sample of MPC candidates that will allow us to better understand extreme star-formation and massive cluster formation in the Local Universe.

Discovery of extended VHEγ-ray emission from the vicinity of the young massive stellar cluster Westerlund 1

Results obtained in very-high-energy (VHE; E > 100 GeV) \gamma-ray observations performed with the H.E.S.S. telescope array are used to investigate particle acceleration processes in the vicinity of the young massive stellar cluster Westerlund 1 (Wd 1). Imaging of Cherenkov light from \gamma-ray induced particle cascades in the Earth's atmosphere is used to search for VHE \gamma\ rays from the region around Wd 1. Possible catalogued counterparts are searched for and discussed in terms of morphology and energetics of the H.E.S.S. source. The detection of the degree-scale extended VHE \gamma-ray source HESS J1646-458 is reported based on 45 hours of H.E.S.S. observations performed between 2004 and 2008. The VHE \gamma-ray source is centred on the nominal position of Wd 1 and detected with a total statistical significance of ~20\sigma. The emission region clearly extends beyond the H.E.S.S. point-spread function (PSF). The differential energy spectrum follows a power law in energy with an index of \Gamma=2.19 \pm 0.08_{stat} \pm 0.20_{sys} and a flux normalisation at 1 TeV of \Phi_0 = (9.0 \pm 1.4_{stat} \pm 1.8_{sys}) x 10^{-12} TeV^{-1} cm^{-2} s^{-1}. The integral flux above 0.2 TeV amounts to (5.2 \pm 0.9) x 10^{-11} cm^{-2} s^{-1}. Four objects coincident with HESS J1646-458 are discussed in the search of a counterpart, namely the magnetar CXOU J164710.2-455216, the X-ray binary 4U 1642-45, the pulsar PSR J1648-4611 and the massive stellar cluster Wd 1. In a single-source scenario, Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent population would be accelerated within the stellar cluster. Beside this, there is evidence for a multi-source origin, where a scenario involving PSR J1648-4611 could be viable to explain parts of the VHE \gamma-ray emission of HESS J1646-458.

On the association of ULXs with young superclusters: M82 X-1 and a new candidate in NGC 7479

ABSTRACT We investigate the spatial coincidence of ultraluminous X-ray sources (ULXs) with young massive stellar clusters. In particular, we perform astrometry on Chandra and Hubble Space Telescope (HST) data of two ULXs that are possibly associated with such clusters. To date, M82 X-1 is the only ULX claimed to be coincident with a young massive stellar cluster. We remeasure the position of this source with a high accuracy and find that the position of the X-ray source is 0.65 arcsec away from the stellar cluster, corresponding to an offset significance of 3σ. We also report the discovery of a new candidate, based on observations of NGC 7479. One of the ULXs observed in three X-ray observations is found to be spatially coincident (within 1σ of the position error) with a young supercluster observed in the HST images. In the brightest state, the absorbed luminosity of the ULX is a few times 1040 erg s−1, and in the faintest state below the detection limit of ∼4 times 1039 erg s−1. The luminosity in the brightest state requires an accreting black hole mass of at least 100 M⊙ assuming isotropic emission. However, it is possible that the source is contaminated by X-ray emission from the nearby supernova SN2009jf. In this case, the luminosity of the ULX is in a range where it is strongly debated whether it is a super-Eddington stellar mass black hole or an intermediate-mass black hole. The colours of the host cluster indicate a young stellar population, with an age between 10 and 100 Myr. The total stellar mass of the cluster is ∼5 × 105 M⊙.

MASSIVE STARS IN THE Cl 1813-178 CLUSTER: AN EPISODE OF MASSIVE STAR FORMATION IN THE W33 COMPLEX

Young massive (M >10^4 Msun) stellar clusters are a good laboratory to study the evolution of massive stars. Only a dozen of such clusters are known in the Galaxy. Here we report about a new young massive stellar cluster in the Milky Way. Near-infrared medium-resolution spectroscopy with UIST on the UKIRT telescope and NIRSPEC on the Keck telescope, and X-ray observations with the Chandra and XMM satellites, of the Cl 1813-178 cluster confirm a large number of massive stars. We detected 1 red supergiant, 2 Wolf-Rayet stars, 1 candidate luminous blue variable, 2 OIf, and 19 OB stars. Among the latter, twelve are likely supergiants, four giants, and the faintest three dwarf stars. We detected post-main sequence stars with masses between 25 and 100 Msun. A population with age of 4-4.5 Myr and a mass of ~10000 Msun can reproduce such a mixture of massive evolved stars. This massive stellar cluster is the first detection of a cluster in the W33 complex. Six supernova remnants and several other candidate clusters are found in the direction of the same complex.

Spectral population synthesis including massive binaries

We have constructed a new code to produce synthetic spectra of stellar populations that includes massive binaries. We have tested this code against the broadband colours of unresolved young massive stellar clusters in nearby galaxies, the equivalent widths of the Red and Blue Wolf-Rayet bumps in star-forming SDSS galaxies and the UV and optical spectra of the star forming regions Tol-A and B in NGC5398. In each case we find a good agreement between our models and observations. We find that in general binary populations are bluer and have fewer red supergiants, and thus significantly less flux in the I-band and at longer wavelengths, than single star populations. Also we find that Wolf-Rayet stars occur over a wider range of ages up to 10^7 years in a stellar population including binaries, increasing the UV flux and Wolf-Rayet spectral features at later times. In addition we find that nebula emission contributes significantly to these observed properties and must be considered when comparing stellar models with observations of unresolved stellar populations. We conclude that incorporation of massive stellar binaries can improve the agreement between observations and synthetic spectral synthesis codes, particularly for systems with young stellar populations.

TheChandraACIS Survey of M33 (ChASeM33): Investigating the Hot Ionized Medium in NGC 604

NGC 604 is the largest H II region in M33, second only within the Local Group to 30 Dor, and is important as a laboratory for understanding how massive young stellar clusters interact with the surrounding interstellar medium. Here we present deep (300 ks) X-ray imagery of NGC 604 obtained as part of the Chandra ACIS Survey of M33 (ChASeM33), which shows highly structured X-ray emission covering ~70% of the full Hα extent of NGC 604. The main bubbles and cavities in NGC 604 are filled with hot (kT = 0.5 keV) X-ray-emitting gas, and X-ray spectra extracted from these regions indicate that the gas is thermal. For the western part of NGC 604 we derive an X-ray gas mass of ~4300 M☉ and an unabsorbed (0.35-2.5 keV) X-ray luminosity of LX = 9.3 × 1035 erg s−1. These values are consistent with a stellar mass-loss bubble entirely powered by ~200 OB stars. This result is remarkable because the standard bubble model tends to underpredict the luminosity of X-ray-bright bubbles and usually requires additional heating from SNRs. Given a cluster age of ~3 Myr, it is likely that the massive stars have not yet evolved into SNe. We detect two discrete spots of enhanced and harder X-ray emission, which we consider to be fingerprints from a reverse shock produced by a supersonic wind after it collided with the shell wall. In the eastern part of NGC 604 the X-ray gas mass amounts to ~1750 M☉. However, mass loss from young stars cannot account for the unabsorbed X-ray luminosity of LX = 4.8 × 1035 erg s−1. Off-center SNRs could produce the additional luminosity. The bubbles in the east seem to be much older and were most likely formed and powered by young stars and SNe in the past. A similar dichotomy between east and west is seen in the optical, implying that a massive wall of neutral and ionized gas shields the dynamically quiescent east from the actively star-forming west.

Discovery of a Young Massive Stellar Cluster near HESS J1813-178

We present the serendipitous discovery of a young stellar cluster in the Galactic disk at l = 12°. Using Keck/NIRSPEC, we obtained high- and low-resolution spectroscopy of several stars in the cluster, and we identified one red supergiant and two blue supergiants. The radial velocity of the red supergiant provides a kinematic cluster distance of 4.7 ± 0.4 kpc, implying luminosities of the stars consistent with their spectral types. Together with the known Wolf-Rayet star located 2.4' from the cluster center, the presence of the red supergiant and the blue supergiants suggests a cluster age of 6-8 Myr and an initial mass of 2000 M☉. Several stars in the cluster are coincident with X-ray sources, including the blue supergiants and the Wolf-Rayet star. This is indicative of a high binary fraction and is reminiscent of the massive young cluster Westerlund 1. The cluster is coincident with two supernova remnants, SNR G12.72–0.0 and G12.82–0.02, and the highly magnetized pulsar associated with the TeV γ-ray source HESS J1813–178. The mixture of spectral types suggests that the progenitors of these objects had initial masses of 20-30 M☉.

Spherically Symmetric Accretion onto a Black Hole at the Center of a Young Stellar Cluster

We present a self-consistent, bimodal stationary solution for spherically symmetric flows driven by young massive stellar clusters with a central supermassive black hole. We demonstrate that the hydrodynamic regime of the flow depends on the location of the cluster in the 3D (star cluster mechanical luminosity - BH mass - star cluster radius) parameter space. We show that a threshold mechanical luminosity (L_crit) separates clusters which evolve in the BH dominated regime frome those whose internal structure is strongly affected by the radiative cooling. In the first case(below the threshold energy) gravity of the BH separates the flow into two distinct zones: the inner accretion zone and the outer zone where the star cluster wind is formed. In the second case (above the critical luminosity), catastrophic cooling sets in inside the cluster. In this case the injected plasma becomes thermally unstable that inhibits a complete stationary solution. We compared the calculated accretion rates and the BH luminosities with those predicted by the classic Bondi accretion theory and found that Bondi's theory is in good agreement with our results in the case of low mass clusters. However, it substantially underestimates the accretion rates and BH luminosities if the star cluster mechanical luminosity, L_sc, approaches the threshold value (L_sc > 0.1 L_crit).

Young Massive Clusters

AbstractOver the past ten years, there has been a revolution in our understanding of massive young stellar clusters in the Galaxy. Initially, there were no known examples having masses >104, yet we now know that there are at least a half dozen such clusters in the Galaxy. In all but one case, the masses have been determined through infrared observations. Several had been identified as clusters long ago, but their massive natures were only recently determined. Presumably, we are just scratching the surface, and we might look forward to having statistically significant samples of coeval massive stars at all important stages of stellar evolution in the near future. I review the efforts that have led to this dramatic turn of events and the growing sample of young massive clusters in the Galaxy.

Gas expulsion and the destruction of massive young clusters

We examine the luminosity and dynamical mass estimates for young massive stellar clusters. For many young (<50 Myr) clusters, the luminosity and dynamical mass estimates differ by a significant amount. We explain this as being due to many young clusters being out of virial equilibrium (which is assumed in dynamical mass estimates) because the clusters are undergoing violent relaxation after expelling gas not used in star formation. We show that, if we assume that luminous mass estimates are correct (for a standard initial mass function), at least 50 per cent of young clusters for which dynamical masses are known are likely to be destroyed within a few tens of Myr of their formation. Even clusters which will retain a bound core may lose a large fraction of their stellar mass. We also show that the core radius and other structural parameters change significantly during the violent relaxation that follows gas expulsion and that they should be considered instantaneous values only, not necessarily reflecting the final state of the cluster. In particular we note that the increasing core radii observed in young LMC/SMC clusters can be well explained as an effect of rapid gas loss.

The Discovery of Diffuse X-Ray Emission in NGC 2024, One of the Nearest Massive Star-forming Regions

We analyzed deep 75 ks Chandra ACIS-I data of NGC 2024 with the aim of searching for diffuse X-ray emission in this most nearby (415 pc) of massive star-forming regions. After removing point sources, extended emission was detected in the central circular region with a radius of 0.5 pc, and it is spatially associated with this young massive stellar cluster. Its X-ray spectrum exhibits a very hard continuum (kT > 8 keV) and shows signs of having a He-like Fe Kα line with a 0.5-7 keV absorption-corrected luminosity of 2 × 1031 ergs s-1. Undetected faint point sources, estimated from the luminosity function of the detected sources, contribute less than 10% to this emission. Hence, the emission is truly diffuse in nature. Because of the proximity of NGC 2024 and the long exposure, this discovery is one of the strongest pieces of evidence in support of the existence of diffuse X-ray emission in massive star-forming regions.

X-Ray and Infrared Point Source Identification and Characteristics in the Embedded, Massive Star-Forming Region RCW 38

We report on results of a 96.7 ks Chandra observation of one of the youngest, most embedded, and most massive young stellar clusters studied in X-rays: RCW 38. We detect 460 sources in the field, of which 360 are confirmed to be associated with the RCW 38 cluster. The cluster members range in luminosity from 1030 to 1033.5 ergs s-1. Over 10% of the cluster members with over 100 counts exhibit flares, while about 15% of the cluster members with over 30 counts are variable. Of the sources identified as cluster members, 160 have near-infrared (NIR) counterparts either in the Two Micron All Sky Survey database or detected via Very Large Telescope observations. Of these, about 20% appear to have optically thick disks. An additional 353 members are identified through NIR observations, of which at least 50% possess optically thick disks. We fit over 100 X-ray sources as absorbed Raymond-Smith-type plasmas and find that the column to the cluster members varies from 1021.5 to 1023 cm-2. We compare the gas to dust absorption signatures in these stars and find NH = AV cm-2. We find that the cluster contains 31 candidate OB stars and is centered about 10'' (0.1 pc) west of the primary source of the ionization, the O5 star IRS 2. The cluster has a peak central density of about 400 X-ray sources pc-2. We estimate that the total cluster membership exceeds 2000 stars.

The Young Massive Stellar Cluster Associated with RCW 121

We report near-infrared (NIR) broadband and narrowband photometric observations in the direction of the IRAS 17149-3916 source that reveal the presence of a young cluster of massive stars embedded in an H II region coincident with RCW 121. These observations, together with published radio data and Midcourse Space Experiment and Spitzer images, were used to determine some of the physical parameters of the region. We found 96 cluster member candidates in an area of about 1.5 × 2.0 arcmin2, 30% of them showing excess emission in the near-IR. IRS 1, the strongest source in the cluster with an estimated spectral type of O5 V-O6 V zero-age main sequence based on the color-magnitude diagram, is probably the main ionizing source of the H II region detected at radio wavelengths. Using the integrated Brγ and the 5 GHz flux densities, we derived a mean visual extinction AV = 5.49 ± mag. From the observed size of the Brγ extended emission, we calculated the emission measure E = 4.5 × 1024 cm-5 and the electron density ne = 2.6 × 103 cm-3, characteristic of compact H II regions.

The Stellar Population Associated with theIRASSource 16132-5039

We report the discovery of a young massive stellar cluster and infrared nebula in the direction of the CS molecular cloud associated with the IRAS point source 16132-5039. Analysis of mid-infrared images from the more accurate Midcourse Space Experiment catalog reveals that there are two independent components associated with the IRAS source. The integral of the spectral energy distribution for these components between 8.28 and 100 μm gives lower limits for the bolometric luminosity of the embedded objects of 8.7 × 104 and 9 × 103 L⊙, which correspond to zero-age main-sequence O8 and B0.5 stars, respectively. The number of Lyman continuum photons expected from the stars that lie along the reddening line for early-type stars is about 1.7 × 1049 s-1, enough to produce the detected flux densities at 5 GHz. The near-infrared spectrum of the nebula increases with frequency, implying that free-free emission cannot be the main source of the extended luminosity, from which we conclude that the observed emission must be mainly dust-scattered light. A comparison of the cluster described in this paper with the young stellar cluster associated with the IRAS source 16177-5018, which is located at the same distance and in the same direction, shows that the mean visual absorption of the newly discovered cluster is about 10 mag smaller and that it contains less massive stars, suggesting that it was formed from a less massive molecular cloud.

Discovery of a Young Massive Stellar Cluster Associated withIRASSource 16177-5018

We report the discovery of a young massive stellar cluster embedded in an extended HII region, invisible at optical wavelengths where the extinction is $A_V \approx 28$ magnitudes, associated with the IRAS source 16177-5018. $J, H$ and nb$K$ imaging photometry combined with the $K_S$ 2MASS data show the presence of sources with infrared excess emission at 2.2 $\mu$m, concentrated in an area of about one square parsec around a massive young stellar object identified as the IRAS source. This object has a near-mid infrared spectral index betweem 2.2 and 25 $\mu$m $\alpha({\rm IR}) = d {\rm log}(\lambda F_\lambda)/d {\rm log} \lambda $ =4.78, characteristic of compact H II regions, with bolometric luminosity, inferred from the integrated near to far-infrared flux density of $2.8 \times 10^5 L_\odot$, which corresponds to a ZAMS star of about $42 M_\odot$. From the color-magnitude diagram we were able to classify the majority of the cluster members as reddened massive stars earlier than spectral type B5.

Discovery of Nonthermal X-Ray Emission from the Embedded Massive Star-forming Region RCW 38

We report on results of a 96.7 ks Chandra observation of one of the youngest, most embedded and massive young stellar clusters studied to date in X-rays—RCW 38. We find a region of extended emission about 1.25 # pc. The emission is consistent with synchrotron emission. The power index of the emission steepens toward 1.75 the cluster core, implying that the magnetic field originates there. This is the first evidence of synchrotron emission filling a region of active star formation. The source of the necessary magnetic field is unclear. We measure the hydrogen column to the emission and find that NH increases from to cm , increasing from 21 22 2 7.6 # 10 1.61 # 10 the northwest to the southeast quadrants of the cluster, consistent with near-infrared extinction data. Subject headings: H ii regions — ISM: individual (RCW 38) — stars: formation — X-rays: diffuse background — X-rays: stars On-line material: color figure