Young Massive Stellar Clusters Research Articles

A deep spectromorphological study of the γ-ray emission surrounding the young massive stellar cluster Westerlund 1

Context. Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible for a significant fraction of cosmic rays (CRs) that are accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy, is a prime candidate for studying this hypothesis. While the very-high-energy γ-ray source HESS J1646−458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified. Aims. We aim to identify the physical processes responsible for the γ-ray emission around Westerlund 1 and thus to understand the role of massive stellar clusters in the acceleration of Galactic CRs better. Methods. Using 164 h of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the γ-ray emission of HESS J1646−458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs. Results. We detected large-scale (∼2° diameter) γ-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with γ-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and it is uniform across the entire source region. We did not find a clear correlation of the γ-ray emission with gas clouds as identified through H I and CO observations. Conclusions. We conclude that, of the known objects within the region, only Westerlund 1 can explain the majority of the γ-ray emission. Several CR acceleration sites and mechanisms are conceivable and discussed in detail. While it seems clear that Westerlund 1 acts as a powerful particle accelerator, no firm conclusions on the contribution of massive stellar clusters to the flux of Galactic CRs in general can be drawn at this point.

Chandra large-scale mapping of the Galactic Centre: probing high-energy structures around the central molecular zone

ABSTRACT Recent observations have revealed interstellar features that apparently connect energetic activity in the central region of our Galaxy to its halo. The nature of these features, however, remains largely uncertain. We present a Chandra mapping of the central 2° × 4° field of the Galaxy, revealing a complex of X-ray-emitting threads plus plume-like structures emerging from the Galactic Centre (GC). This mapping shows that the northern plume or fountain is offset from a well-known radio lobe (or the GCL), which however may represent a foreground H ii region, and that the southern plume is well wrapped by a corresponding radio lobe recently discovered by MeerKAT. In particular, we find that a distinct X-ray thread, G0.17−0.41, is embedded well within a non-thermal radio filament, which is locally inflated. This thread with a width of ∼1.6 arcsec (FWHM) is ∼2.6 arcmin or 6 pc long at the distance of the GC and has a spectrum that can be characterized by a power law or an optically-thin thermal plasma with temperature ≳ 3 keV. The X-ray-emitting material is likely confined within a strand of magnetic field with its strength ≳ 1 mG, not unusual in such radio filaments. These morphological and spectral properties of the radio/X-ray association suggest that magnetic field re-connection is the energy source. Such re-connection events are probably common when flux tubes of antiparallel magnetic fields collide and/or become twisted in and around the diffuse X-ray plumes, representing blowout superbubbles driven by young massive stellar clusters in the GC. The understanding of the process, theoretically predicted in analog to solar flares, can have strong implications for the study of interstellar hot plasma heating, cosmic ray acceleration and turbulence.

Stellar-mass black holes in young massive and open stellar clusters – V. comparisons with LIGO-Virgo merger rate densities

ABSTRACT I study the contribution of young massive star clusters (YMCs) and open star clusters (OCs) to the present day, intrinsic merger rate density of dynamically assembled binary black holes (BBHs). The BBH merger event rate is estimated based on a set of state-of-the-art evolutionary models of star clusters, as presented in Banerjee (2021). The merger-event rates are obtained by constructing a cluster population of the Universe, out of the models, taking into account mass distribution of clusters and cosmic star formation and enrichment histories, as per observations. The model BBH merger rate density ranges from a pessimistic to a reference value of 0.5−37.9 yr−1 Gpc−3, for a LIGO-Virgo-like detector horizon. The reference rate well accommodates the BBH merger rate densities estimated from GWTC-1 and GWTC-2 merger-event catalogues. The computed models also yield differential BBH merger rate densities that agree reasonably with those from GWTC-1 and, as well, with the much more constrained ones from GWTC-2. These results suggest that dynamical interactions in YMCs and OCs can, in principle, alone explain the BBH merger rate density and its dependence on the merging-binary properties, as inferred from to-date gravitational-wave (GW) events. The cosmic merger rate density evolution also agrees with GWTC-2. The models predict a rate of ≈5 yr−1 Gpc−3 for eccentric LIGO-Virgo mergers from YMCs and OCs. The improving constraints on BBH merger rate density with mounting GW events will help constraining scenarios of star cluster formation across cosmic time and as well the relative contributions of the various compact binary merger channels.

LISA sources from young massive and open stellar clusters

I study the potential role of young massive (YMCs) and open star clusters (OCs) in assembling stellar-mass binary black holes (BBHs) which would be detectable as persistent gravitational-wave (GW) sources by the forthcoming LISA mission. The energetic dynamical interactions inside star clusters make them factories of assembling BBHs and other types of double-compact binaries that undergo general-relativistic (GR) inspiral and merger. The initial phase of such inspirals would, typically, sweep through the LISA GW band. Here, such LISA sources are studied from a set of evolutionary models of star clusters with masses ranging over $10^4M_\odot-10^5M_\odot$ that represent YMCs and intermediate-aged OCs in metal-rich and metal-poor environments of the Local Universe. These models are evolved with long-term, direct, relativistic many-body computations incorporating state-of-the-art stellar-evolutionary and remnant-formation models. Based on models of Local Universe constructed with such model clusters, it is shown that YMCs and intermediate-aged OCs would yield several 10s to 100s of LISA BBH sources at the current cosmic epoch with GW frequency within $10^{-3}{\rm~Hz} - 10^{-1}{\rm~Hz}$ and signal-to-noise-ratio (S/N) $>5$, assuming a mission lifetime of 5 or 10 years. Such LISA BBHs would have a bimodal distribution in total mass, be generally eccentric ($\lesssim0.7$), and typically have similar component masses although mass-asymmetric systems are possible. Intrinsically, there would be 1000s of present-day, LISA-detectable BBHs from YMCs and OCs. That way, YMCs and OCs would provide a significant and the dominant contribution to the stellar-mass BBH population detectable by LISA. A small fraction, $<5$%, of these BBHs would undergo GR inspiral to make it to LIGO-Virgo GW frequency band and merge, within the mission timespan; $<15$% would do so within twice the timespan.

Stellar-mass black holes in young massive and open stellar clusters – IV. Updated stellar-evolutionary and black hole spin models and comparisons with the LIGO-Virgo O1/O2 merger-event data

ABSTRACT I present a set of long-term, direct, relativistic many-body computations of model dense stellar clusters with up-to-date stellar-evolutionary, supernova (SN), and remnant natal-kick models, including pair instability and pulsation pair instability supernova (PSN and PPSN), using an updated version of ${\rm{\small NBODY7}}$ N-body simulation program. The N-body model also includes stellar evolution-based natal spins of black holes (BHs) and treatments of binary black hole (BBH) mergers based on numerical relativity. These, for the first time in a direct N-body simulation, allow for second-generation BBH mergers. The set of 65 evolutionary models have initial masses $10^4{\!-\!}10^5\, \mathrm{M}_{\odot }$, sizes 1–3 pc, metallicity 0.0001–0.02, with the massive stars in primordial binaries and they represent young massive clusters (YMC) and moderately massive open clusters (OC). Such models produce dynamically paired BBH mergers that agree well with the observed masses, mass ratios, effective spin parameters, and final spins of the LVC O1/O2 merger events, provided BHs are born with low or no spin but spin-up after undergoing a BBH merger or matter accretion on to it. In particular, the distinctly higher mass, effective spin parameter, and final spin of GW170729 merger event is naturally reproduced, as also the mass asymmetry of the O3 event GW190412. The computed models produce intermediate-mass, $\sim 100\, \mathrm{M}_{\odot }$ BBH mergers with primary mass within the ‘PSN gap’ and also yield mergers involving remnants in the ‘mass gap’. They also suggest that YMCs and OCs produce persistent, Local-Universe GW sources detectable by LISA. Such clusters are also capable of producing eccentric LIGO-Virgo mergers.

Morphological and Spectral Study of 4FGL J1115.1–6118 in the Region of the Young Massive Stellar Cluster NGC 3603

Abstract We report a detailed study of an unidentified gamma-ray source located in the region of the compact stellar cluster NGC 3603. This is a star-forming region (SFR) powered by a massive cluster of OB stars. A dedicated analysis of about 10 yr of data from 10 GeV to 1 TeV, provided by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, yields the detection of a pointlike source at a significance of 9σ. The source photon spectrum can be described by a power-law model with a best-fit spectral index of 2.35 ± 0.03. In addition, the analysis of a deep Chandra image in the 0.5–7 keV band reliably rules out an extragalactic origin for the gamma-rays. We also conclude that the broadband spectral energy distribution of the point source can be explained well with both leptonic and hadronic models. No firm evidence of association with any other classes of known gamma-ray emitters is found; therefore, we speculate that 4FGL J1115.1−6118 is a gamma-ray-emitting SFR.

High-Energy Particles and Radiation in Star-Forming Regions

Non-thermal particles and high-energy radiation can play a role in the dynamical processes in star-forming regions and provide an important piece of the multiwavelength observational picture of their structure and components. Powerful stellar winds and supernovae in compact clusters of massive stars and OB associations are known to be favourable sites of high-energy particle acceleration and sources of non-thermal radiation and neutrinos. Namely, young massive stellar clusters are likely sources of the PeV (petaelectronvolt) regime cosmic rays (CRs). They can also be responsible for the cosmic ray composition, e.g., 22Ne/20Ne anomalous isotopic ratio in CRs. Efficient particle acceleration can be accompanied by super-adiabatic amplification of the fluctuating magnetic fields in the systems converting a part of kinetic power of the winds and supernovae into the magnetic energy through the CR-driven instabilities. The escape and CR propagation in the vicinity of the sources are affected by the non-linear CR feedback. These effects are expected to be important in starburst galaxies, which produce high-energy neutrinos and gamma-rays. We give a brief review of the theoretical models and observational data on high-energy particle acceleration and their radiation in star-forming regions with young stellar population.

The Massive Star-forming Regions Omnibus X-ray Catalog, Third Installment

Abstract We offer to the star formation community the third installment of the Massive Star-forming Regions (MSFRs) Omnibus X-ray Catalog (MOXC3), a compilation of X-ray point sources detected in 50 archival Chandra Advanced CCD Imaging Spectrometer observations of 14 Galactic MSFRs and surrounding fields. The MOXC3 MSFRs are NGC 2264, NGC 6193, RCW 108-IR, Aur OB1, DR15, NGC 6231, Berkeley 87, NGC 6357, AFGL 4029, h Per (NGC 869), NGC 281, Onsala 2S, G305, and RCW 49 (Wd 2); they have distances of 0.7–4.2 kpc. Most exhibit clumped or clustered young stellar populations; several contain at least two distinct massive young stellar clusters. The total MOXC3 catalog includes 27,923 X-ray point sources. We take great care to identify even the faintest X-ray point sources across these fields. This allows us to remove this point source light, revealing diffuse X-ray structures that pervade and surround MSFRs, often generated by hot plasmas from massive star feedback. As we found in MOXC1 and MOXC2, diffuse X-ray emission is traceable in all MOXC3 MSFRs; here we perform spectral fitting to investigate the origins of selected diffuse regions. Once again, MOXC3 shows the value of high spatial resolution X-ray studies of MSFRs enabled by Chandra.

Compact Star Clusters as Sources of Cosmic Rays and Neutrinos

The origin of cosmic rays is one of the most important questions in modern astrophysics. Leading world observatories are engaged in their search in our Galaxy and beyond. Current observations of bright sources of hard X-rays and gamma radiation require precise models of cosmic-ray accelerators and emission generation. Recent detection of astrophysical neutrinos with the energy exceeding 1015 eV achieved by the IceCube Neutrino Observatory and registration by the LOFAR observatory of the light cosmic-ray component at 1016–1017 eV have given rise to a wide discussion of whether its sources are galactic or extragalactic. H.E.S.S. observations of high-energy gamma-ray sources in compact massive clusters Westerlund 1 and Cl*1806-20 triggered a new search for the models of effective accelerators of high-energy cosmic rays, socalled galactic PeVatrons. In this paper, the properties of compact massive young stellar clusters as possible sources of cosmic rays, neutrinos, and gamma emission are discussed. We suppose that an expanding supernova remnant in a compact massive cluster is an effective cosmic-ray accelerator up to the energies of 1015 eV and beyond. Energy spectra of particles being accelerated in the system of colliding shocks generated by a supernova remnant interaction with nearby stellar winds have a broken power-law shape with a strong upturn around several teraelectronvolts and a cutoff energy at tens of petaelectronvolts. These sources provide the maximal emission flux at the maximal energies of the spectrum. Teraelectronvolt sources detected by H.E.S.S. could be related to supernova remnants in Westerlund 1 and Cl*1806-20 and be responsible for the 10% of the IceCube neutrino events. We have also shown that PeVatrons in the systems of colliding shocks can provide up to 80% of the light cosmic-ray component at the energies of 1016–1017 eV detected at the Earth.

Pyroclastic Blowout: Dust Survival in Isolated versus Clustered Supernovae

Abstract Following the current debate on the fate of SN-condensed dust grains, here we explore by means of three-dimensional hydrodynamical simulations the interaction of dusty supernova remnants (SNRs) with the shocked winds of neighboring massive stars within young massive stellar clusters (SSCs). As a comparison, we have also explored the evolution of SNRs in the diffuse ISM with constant density. Since the hydrodynamics of SNRs is intimately related to the properties of their immediate environment, the lifecycle of dust grains in SNRs within SSCs is radically different from that in the diffuse ISM. Moreover, off-centered SNRs evolving in the steep density gradient established due to a star cluster wind experience a blowout phase: shell fragmentation due to protruding Rayleigh–Taylor instabilities and the venting of SN ejecta. Our main finding is that clustered SN explosions will cause a net increase in the amount of dust in the surroundings of young massive stellar clusters. Our analysis considers the multiple dust processing resulting from the passage of the SN reverse shock, including its reflection at the SNR’s center, the injection of shocked stellar winds within the respective remnant’s volume and the effect of secondary forward shocks produced in sequential SN explosions. With our code Cinder, on-the-fly, we have calculated the rates of thermal sputtering and dust-induced radiative cooling from an initial distribution of grain sizes and dust content. Fast-moving elongated dusty SN ejecta resemble mushroom clouds violently ascending in a stratified atmosphere after volcanic super-eruptions, where the pyroclasts carried by the clouds are wind-driven and eventually accumulate into the vast surroundings.

Stellar-mass black holes in young massive and open stellar clusters and their role in gravitational-wave generation III: dissecting black hole dynamics

Stellar-remnant black holes (BH) in dense stellar clusters comprise a natural setup to trigger general-relativistic (GR) inspiral and merger of binary black holes (BBH), detectable by the LISA and the LIGO-Virgo, through dynamical encounters inside such environments. In this work, the intricacies of such dynamical interactions are probed utilizing realistic, self-consistent, post-Newtonian, direct N-body evolutionary models of young massive and open stellar clusters. Particularly, the configurations of the compact subsystems, that drive the in-cluster GR BBH coalescences, are tracked on the fly. Such an approach reveals that the GR coalescences within the open clusters take place primarily via chaotic interactions involving triple BH systems. Although less frequently, such mergers are found to happen also in higher-order subsystems such as quadruples and in subsystems involving non-BH members; the mergers can themselves be BH – non-BH, which events would leave electromagnetic signatures. Close, fly-by encounters inside the clusters can also make BBHs and other types of double-compact binaries temporarily post-Newtonian; such binaries would potentially contribute to the GW background for the LISA and the PTA. These calculations, furthermore, suggest that open clusters are potential hosts for not only detached BH – main-sequence binaries, as recently identified in the globular cluster NGC 3201, but also a wide variety of other types of remnant – non-remnant binaries, which are assembled via dynamical interactions inside the clusters and which have the prospects of being discovered in radial-velocity surveys.

Stellar-mass black holes in young massive and open stellar clusters and their role in gravitational-wave generation – II

The study of stellar-remnant black holes (BH) in dense stellar clusters is now in the spotlight, especially due to their intrinsic ability to form binary black holes (BBH) through dynamical encounters, that potentially coalesce via gravitational-wave (GW) radiation. In this work, which is a continuation of a recent study (Paper I), additional models of compact stellar clusters with initial masses $\lesssim10^5M_\odot$ and also those with small fractions of primordial binaries ($\lesssim10$%) are evolved for long term, applying the direct N-body approach, assuming state-of-the-art stellar-wind and remnant-formation prescriptions. That way, a substantially broader range of computed models than that in Paper I is achieved. As in Paper I, the general-relativistic BBH mergers continue to be mostly mediated by triples that are bound to the clusters rather than happen among the ejected BBHs. In fact, the number of such in situ BBH mergers, per cluster, tend to increase significantly with the introduction of a small population of primordial binaries. Despite the presence of massive primordial binaries, the merging BBHs, especially the in situ ones, are found to be exclusively dynamically assembled and hence would be spin-orbit misaligned. The BBHs typically traverse through both the LISA's and the LIGO's detection bands, being audible to both instruments. The "dynamical heating" of the BHs keeps the Electron-Capture-Supernova (ECS) neutron stars (NS) from effectively mass segregating and participating in exchange interactions; the dynamically-active BHs would also exchange into any NS binary within $\lesssim1$ Gyr. Such young massive and open clusters have the potential to contribute to the dynamical BBH merger detection rate to a similar extent as their more massive globular-cluster counterparts.

Can Dust Injected by SNe Explain the NIR–MIR Excess in Young Massive Stellar Clusters?

Abstract We present a physically motivated model involving the different processes affecting supernova dust grains as they are incorporated into the thermalized medium within young massive star clusters. The model is used to explain the near- to mid-infrared (NIR–MIR) excess found in such clusters and usually modeled as a blackbody with temperature ∼ ( 400 – 1000 ) K. In our approach, dust grains are efficiently produced in the clumpy ejecta of core-collapse supernovae, shattered into small pieces ( ≲ 0.05 μm) as they are incorporated into the hot and dense ISM, heated via frequent collisions with electrons and the absorption of energetic photons. Grains with small sizes can more easily acquire the high temperatures (∼1000 K) required to produce an NIR–MIR excess with respect to the emission of foreground PAHs and starlight. However, the extreme conditions inside young massive clusters make it difficult for these small grains to have a persistent manifestation at NIR–MIR wavelengths as they are destroyed by efficient thermal sputtering. Nevertheless, the chances for a persistent manifestation are increased by taking into account that small grains become increasingly transparent to their impinging ions as their sizes decrease. For an individual SN event, we find that the NIR–MIR excess lasts longer if the time required to incorporate all the grains into the thermalized medium is also longer, and, in some cases, comparable to the characteristic interval between supernova explosions. Our models can successfully explain the near-infrared excesses found in the star clusters observed in M33 assuming a low heating efficiency and mass loading. In this scenario, the presence of the NIR–MIR excess is an indication of efficient dust production in SNe and its subsequent destruction.

The varying mass distribution of molecular clouds across M83

The work of Adamo et al. (2015) showed that the mass distributions of young massive stellar clusters were truncated above a maximum-mass scale in the nearby galaxy M83 and that this truncation mass varies with galactocentric radius. Here, we present a cloud-based analysis of ALMA CO($1\to 0$) observations of M83 to search for such a truncation mass in the molecular cloud population. We identify a population of 873 molecular clouds in M83 that is largely similar to those found in the Milky Way and Local Group galaxies, though clouds in the centre of the galaxy show high surface densities and enhanced turbulence, as is common for clouds in high-density nuclear environments. Like the young massive clusters, we find a maximum-mass scale for the molecular clouds that decreases radially in the galaxy. We find the most massive young massive cluster tracks the most massive molecular cloud with the cluster mass being $10^{-2}$ times that of the most massive molecular cloud. Outside the nuclear region of M83 ($R_{g}>0.5$ kpc), there is no evidence for changing internal conditions in the population of molecular clouds, with the average internal pressures, densities, and free-fall times remaining constant for the cloud population over the galaxy. This result is consistent with the bound cluster formation efficiency depending only on the large-scale properties of the ISM, rather than the internal conditions of individual clouds.

Metallic Winds in Dwarf Galaxies

Abstract We present results from models of galactic winds driven by energy injected from nuclear (at the galactic center) and non-nuclear starbursts. The total energy of the starburst is provided by very massive young stellar clusters, which can push the galactic interstellar medium and produce an important outflow. Such outflow can be a well or partially mixed wind, or a highly metallic wind. We have performed adiabatic 3D N-Body/Smooth Particle Hydrodynamics simulations of galactic winds using the gadget-2 code. The numerical models cover a wide range of parameters, varying the galaxy concentration index, gas fraction of the galactic disk, and radial distance of the starburst. We show that an off-center starburst in dwarf galaxies is the most effective mechanism to produce a significant loss of metals (material from the starburst itself). At the same time, a non-nuclear starburst produces a high efficiency of metal loss, in spite of having a moderate to low mass loss rate.

Stellar-mass black holes in young massive and open stellar clusters and their role in gravitational-wave generation

The dynamical processes involving stellar-remnant black holes (BH) in stellar clusters has always drawn attention due to the BHs' potential in a number of astrophysical phenomena, especially the dynamical formation of binary black holes (BBH), which would potentially coalesce via radiation of gravitational waves (GW). This study presents a preliminary set of evolutionary models of compact stellar clusters with initial masses ranging over $1.0\times10^4M_\odot-5.0\times10^4M_\odot$, and half-mass radius of 2 pc or 1 pc, that is typical for young massive and starburst clusters. They have metallicities between $0.05Z_\odot-Z_\odot$. Including contemporary schemes for stellar wind and remnant-formation, such model clusters are evolved, for the first time, using the state-of-the-art direct N-body evolution program NBODY7, until their dissolution or at least for 10 Gyr. That way, a self-regulatory behaviour in the effects of dynamical interactions among the BHs, especially while heating and expanding the cluster and self-depleting the BHs, is demonstrated. The BBH mergers obtained here show a prominence in triple-mediated mergers while being bound to the clusters, compared to those occurring among the BBHs that are dynamically ejected from the clusters. This is in contrast with earlier N-body computations and also with recent Monte-Carlo method based ones. A broader mass spectrum of BHs and ejection of BBHs generally of wider orbits and in lower numbers, for the cluster masses explored here, might cause this which is yet to be fully understood. Among the BBH coalescences obtained here, there are ones that resemble the detected GW151226, LVT151012, and GW150914 events and also ones which are even more massive. A preliminary estimate suggests few 10s - 100s of BBH coalescences per year, originating due to dynamics in stellar clusters, that can be detected by the LIGO at its design sensitivity.

HARD X-RAY EMISSION FROM SH 2‐104: A NuSTAR SEARCH FOR GAMMA-RAY COUNTERPARTS

ABSTRACT We present NuSTAR hard X-ray observations of Sh 2–104, a compact H ii region containing several young massive stellar clusters (YMSCs). We have detected distinct hard X-ray sources coincident with localized VERITAS TeV emission recently resolved from the giant gamma-ray complex MGRO J2019+37 in the Cygnus region. Fainter, diffuse X-rays coincident with the eastern YMSC in Sh2-104 likely result from the colliding winds of a component star. Just outside the radio shell of Sh 2–104 lies 3XMM J201744.7+365045 and a nearby nebula, NuSTAR J201744.3+364812, whose properties are most consistent with extragalactic objects. The combined XMM-Newton and NuSTAR spectrum of 3XMM J201744.7+365045 is well-fit to an absorbed power-law model with cm−2 and a photon index . Based on possible long-term flux variation and the lack of detected pulsations (≤43% modulation), this object is likely a background active galactic nucleus rather than a Galactic pulsar. The spectrum of the NuSTAR nebula shows evidence of an emission line at E = 5.6 keV, suggesting an optically obscured galaxy cluster at z = 0.19 ± 0.02 (d = 800 Mpc) and L X = 1.2 × 1044 erg s−1. Follow-up Chandra observations of Sh 2–104 will help identify the nature of the X-ray sources and their relation to MGRO J2019+37. We also show that the putative VERITAS excess south of Sh 2–104, is most likely associated with the newly discovered Fermi pulsar PSR J2017+3625 and not the H ii region.

Early dust formation and a massive progenitor for SN 2011ja?

SN 2011ja was a bright (I = -18.3) Type II supernova occurring in the nearby edge on spiral galaxy NGC 4945. Flat-topped and multi-peaked H-alpha and H-beta spectral emission lines appear between 64 - 84 days post-explosion, indicating interaction with a disc-like circumstellar medium inclined 30-45 degrees from edge-on. After day 84 an increase in the H- and K-band flux along with heavy attenuation of the red wing of the emission lines are strong indications of early dust formation, likely located in the cool dense shell created between the forward shock of the SN ejecta and the reverse shock created as the ejecta plows into the existing CSM. Radiative transfer modeling reveals both ~1.5 x 10^-4 Msun of pre-existing dust located ~ 10^16.7 cm away and ~ 5 x 10^-5 Msun of newly formed dust. Spectral observations after 1.5 years reveal the possibility that the fading SN is located within a young (3-6 Myr) massive stellar cluster, which when combined with tentative 56Ni mass estimates of 0.2 Msun may indicate a massive (> 25 Msun) progenitor for SN 2011ja.

INFRARED OBSERVATIONAL MANIFESTATIONS OF YOUNG DUSTY SUPER STAR CLUSTERS

ABSTRACT The growing evidence pointing at core-collapse supernovae as large dust producers makes young massive stellar clusters ideal laboratories to study the evolution of dust immersed in a hot plasma. Here we address the stochastic injection of dust by supernovae, and follow its evolution due to thermal sputtering within the hot and dense plasma generated by young stellar clusters. Under these considerations, dust grains are heated by means of random collisions with gas particles which result in the appearance of infrared spectral signatures. We present time-dependent infrared spectral energy distributions that are to be expected from young stellar clusters. Our results are based on hydrodynamic calculations that account for the stochastic injection of dust by supernovae. These also consider gas and dust radiative cooling, stochastic dust temperature fluctuations, the exit of dust grains out of the cluster volume due to the cluster wind, and a time-dependent grain size distribution.

X-RAY PROPERTIES OF LOW-MASS PRE-MAIN SEQUENCE STARS IN THE ORION TRAPEZIUM CLUSTER

The Chandra High Energy Transmission Gratings (HETG) Orion Legacy Project (HOLP) is the first comprehensive set of observations of a very young massive stellar cluster which provides high resolution X-ray spectra of very young stars over a wide mass range (0.7 - 2.3 Msun). In this paper, we focus on the six brightest X-ray sources with T Tauri stellar counterparts which are well-characterized at optical and infra-red wavelengths. All stars show column densities which are substantially smaller than expected from optical extinction indicating that the sources are located on the near side of the cluster with respect to the observer as well as that these stars are embedded in more dusty environments. Stellar X-ray luminosities are well above $10^{31}$ erg/s, in some cases exceeding $10^{32}$ erg/s for a substantial amount of time. The stars during these observations show no flares but are persistently bright. The spectra can be well fit with two temperature plasma components of 10 MK and 40 MK, of which the latter dominates the flux by a ratio 6:1 on average. The total EMs range between 3 - 8$\times10^{54}$ cm$^{-3}$ and are comparable to active coronal sources. Limits on the forbidden to inter-combination line ratios in the He-Like K-shell lines show that we observe a predominantely optically thin plasma with electron densities below $10^{12}$ cm$^{-3}$. Observed abundances compare well with active coronal sources underlying the coronal nature of these sources. The surface flux in this sample of 0.6 to 2.3 Msun classical T Tauri stars shows that coronal activity and possibly coronal loop size increase significantly between ages 0.1 to 10 Myrs.