Young massive clusters are dense aggregates of young stars that form the fundamental building blocks of galaxies. Several examples exist in the Milky Way Galaxy and the Local Group, but they are particularly abundant in starburst and interacting galaxies. The few young massive clusters that are close enough to resolve are of prime interest for studying the stellar mass function and the ecological interplay between stellar evolution and stellar dynamics. The distant unresolved clusters may be effectively used to study the star-cluster mass function, and they provide excellent constraints on the formation mechanisms of young cluster populations. Young massive clusters are expected to be the nurseries for many unusual objects, including a wide range of exotic stars and binaries. So far only a few such objects have been found in young massive clusters, although their older cousins, the globular clusters, are unusually rich in stellar exotica. In this review we focus on star clusters younger than $\sim100$ Myr, more than a few current crossing times old, and more massive than $\sim10^4$ \Msun, irrespective of cluster size or environment. We describe the global properties of the currently known young massive star clusters in the Local Group and beyond, and discuss the state of the art in observations and dynamical modeling of these systems. In order to make this review readable by observers, theorists, and computational astrophysicists, we also review the cross-disciplinary terminology.

/pdf/young-massive-star-clusters-1mlfusxprp.pdf

Young Massive Star Clusters

Whether the stellar initial mass function (IMF) is universal or is instead sensitive to environmental conditions is of critical importance: The IMF influences most observable properties of stellar populations and thus galaxies, and detecting variations in the IMF could provide deep insights into the star formation process. This review critically examines reports of IMF variations, with a view toward whether other explanations are sufficient given the evidence. Studies of the field, young clusters and associations, and old globular clusters suggest that the vast majority were drawn from a universal system IMF: a power law of Salpeter index (Γ = 1.35) above a few solar masses, and a log normal or shallower power law (Γ ∼ 0–0.25) for lower mass stars. The shape and universality of the substellar IMF is still under investigation. Observations of resolved stellar populations and the integrated properties of most galaxies are also consistent with a universal IMF, suggesting no gross variations over much of cosm...

/pdf/a-universal-stellar-initial-mass-function-a-critical-look-at-dyi5eyxuh9.pdf

A Universal Stellar Initial Mass Function? A Critical Look at Variations

We present a high spatial resolution multiwavelength survey of 44 young binary star systems in Taurus-Auriga with separations of 10-1000 AU. These observations, which were obtained using the Hubble Space Telescope and the NASA Infrared Telescope Facility, quadruple the number of close (less than 100 AU) binary stars with spatially resolved measurements from 0.3 to 2.2 μm and are the first 3.6 μm measurements for the majority of the companion stars in the sample. Masses and ages are estimated for the components observed at optical wavelengths. The relative ages of binary star components are more similar than the relative ages of randomly paired single stars within the same star-forming region. This is the first statistically significant evidence for coeval formation. Only one of the companion masses is substellar, from which we conclude that the apparent overabundance of T Tauri star companions relative to main-sequence star companions is not due to a wealth of substellar secondaries that would have been missed in main-sequence surveys. The circumstellar environments of binary star systems are studied in this work through three diagnostics: the infrared color K-L, the ultraviolet excess ΔU, and Hα emission. Several conclusions are drawn. First, the mass accretion rates for primary stars are similar to single stars, which suggests that companions as close as 10 AU have little effect on the mass accretion rate. Second, although most classical T Tauri star binaries retain both a circumprimary and a circumsecondary disk, there are several systems with only a circumprimary disk. Systems with only a circumsecondary disk are rare. This suggests that circumprimary disks survive longer than circumsecondary disks. Third, primary stars accrete at a higher rate, on average, than secondary stars. This is most likely because of their larger stellar mass, since the mass accretion rates for both single and binary T Tauri stars exhibit a moderate mass dependence. Fourth, approximately 10% of T Tauri binary star components have very red near-infrared colors (K-L > 1.4) and unusually high mass accretion rates. This phenomenon does not appear to be restricted to binary systems, however, since a comparable fraction of single T Tauri stars exhibit the same properties. These high accretion rate stars are probably not at an earlier stage of evolution, as has been proposed. Their semblance of younger protostars at optical and infrared wavelengths is most likely because of their similar high levels of accretion, which are above the norm for T Tauri stars, and not because of similar ages. The stellar and circumstellar properties are also used to trace indirectly the evolution of circumbinary material. In contrast to single T Tauri stars, which have disk dissipation timescales comparable to their ages, the disk dissipation timescales for binary T Tauri stars are ~10 times less than their ages. Replenishment of the inner circumstellar disks may be necessary to explain the continuing disk accretion in these systems. The longer disk lifetimes of circumprimary disks, despite their higher depletion rates, suggest that circumprimary disks are being preferentially replenished, possibly from a circumbinary reservoir with low angular momentum relative to the binary. Further support for circumbinary reservoirs comes from the observed correlated presence of circumprimary and circumsecondary disks for binaries with separations of less than ~200 AU. The presence of disks appears uncorrelated for wider binaries. Additionally, binaries with separations of less than ~100 AU exhibit a higher fraction of high mass ratio (ms/mp) pairs than wider binaries. These separation-dependent properties can be explained if the components are being replenished from a common circumbinary reservoir with low angular momentum. The components of the closest pairs are expected to be more equally replenished than the widest pairs, which consequently sustains both disks and drives their mass ratios toward unity. Overall, the results of this study corroborate previous work that suggests that fragmentation is the dominant binary star formation mechanism in Taurus-Auriga; disk instabilities and capture seem unlikely.

http://iopscience.iop.org/article/10.1086/321542/pdf

Observational Constraints on the Formation and Evolution of Binary Stars

The star formation rate (SFR) of the Milky Way remains poorly known, with often-quoted values ranging from 1 to 10 Myr −1 . This situation persists despite the potential for the Milky Way to serve as the ultimate SFR calibrator for external galaxies. We show that various estimates for the Galactic SFR are consistent with one another once they have been normalized to the same initial mass function (IMF) and massive star models, converging to 1.9 ± 0.4 Myr −1 . However, standard SFR diagnostics are vulnerable to systematics founded in the use of indirect observational tracers sensitive only to high-mass stars. We find that absolute SFRs measured using resolved low/intermediate-mass stellar populations in Galactic Hii regions are systematically higher by factors of ∼2-3 compared with calibrations for SFRs measured from mid-IR and radio emission. We discuss some potential explanations for this discrepancy and conclude that it could be allayed if (1) the power-law slope of the IMF for intermediate-mass (1.5 M� <m< 5M� ) stars were steeper than the Salpeter slope or (2) a correction factor was applied to the extragalactic 24 μm SFR calibrations to account for the duration of star formation in individual mid-IR-bright Hii regions relative to the lifetimes of O stars. Finally, we present some approaches for testing whether a Galactic SFR of ∼2 Myr −1 is consistent with what we would measure if we could view the Milky Way as external observers. Using luminous radio supernova remnants and X-ray point sources, we find that the Milky Way deviates from expectations at the 1σ -3σ level, hinting that perhaps the Galactic SFR is overestimated or extragalactic SFRs need to be revised upward.

/pdf/toward-a-unification-of-star-formation-rate-determinations-49v1sdozmf.pdf

Toward a unification of star formation rate determinations in the milky way and other galaxies

As members of the instrument team for the Advanced CCD Imaging Spectrometer (ACIS) on NASA's Chandra X-ray Observatory and as Chandra General Observers, we have developed a wide variety of data analysis methods that we believe are useful to the Chandra community, and have constructed a significant body of publicly available software (the ACIS Extract package) addressing important ACIS data and science analysis tasks. This paper seeks to describe these data analysis methods for two purposes: to document the data analysis work performed in our own science projects and to help other ACIS observers judge whether these methods may be useful in their own projects (regardless of what tools and procedures they choose to implement those methods). The ACIS data analysis recommendations we offer here address much of the workflow in a typical ACIS project, including data preparation, point source detection via both wavelet decomposition and image reconstruction, masking point sources, identification of diffuse structures, event extraction for both point and diffuse sources, merging extractions from multiple observations, nonparametric broadband photometry, analysis of low-count spectra, and automation of these tasks. Many of the innovations presented here arise from several, often interwoven, complications that are found in many Chandra projects: large numbers of point sources (hundreds to several thousand), faint point sources, misaligned multiple observations of an astronomical field, point source crowding, and scientifically relevant diffuse emission.

Innovations in the analysis of chandra-acis observations

Aims. We present the deepest and highest resolution near-infrared imaging to date of cluster Trumpler 14 in Carina. Our goal is to identify and characterise the young stellar population of this massive cluster. Methods. We made use of deep and wide-field NIR images from NTT and VLT observations, that were sensitive enough to detect substellar sources at the distance to this cluster, and at high enough resolution (VLT diffraction limited) to fully resolve the core of the cluster crowded with O stars. Results. We find that Tr14 has a well-defined core-halo structure, where less than 30% of the cluster's members reside in the core. The core is well characterised by a King function with a core radius of $0\farcm17$ (0.14 pc at the adopted distance) and a constant baseline, the halo, of 125 sources/pc 2 . Despite the unusually large number of OB stars, the central number density at zero radius is ~7.3 $\times$ 10 3  pc -3 , which is loose in comparison with similar clusters. We find a normal reddening law towards the cluster and derive a global reddening of A v = 2.6 ± 0.3 mag. We find convincing evidence of a sparse foreground population (~5 sources/arcmin 2 ) reddened by about A v = 1.4 mag, which we suggest is not associated with Tr14 but is most likely an older population produced in the nearby young clusters of this complex. The colour–magnitude diagrams are compatible with ages between “zero” and ~5 Myr, although the sources from the core of the cluster appear to concentrate on the youngest isochrones, suggesting that the halo population is, on average, slightly older than the core population. Using a set of simplistic, fixed-age, mass-luminosity relations, we derive a mass of 10 4  $M_\odot$ for the cluster. From the NACO JHK s L ' data, we estimate a fraction of infrared-excess sources of 35%, although this is likely to be an underestimate given the bright completeness limits of the L'  band. Finally, we argue that the formerly identified proplyd candidates that fall inside our survey are not proplyds but remnants of the disrupted molecular cloud that surround the cluster. We also find a series of interesting objects in our field that are worthy of future attention: a candidate photoionised proplyd best seen in the L'  band, a compact nebula surrounding an early type star, and a tentative proplyd/small shock associated with a faint source.

https://www.aanda.org/articles/aa/pdf/2007/46/aa7210-07.pdf

NTT and VLT diffraction limited imaging of Trumpler 14: revealing a massive core-halo cluster

High{resolution spectroscopic monitoring of the exceptionally active classical T Tauri star (CTTS) RW Aur A was carried out in three seasons of 1996, 1998 and 1999 with simultaneous B, V photometry. The high quality spectra revealed a multicomponent structure of the spectrum, which includes: 1) a veiled photospheric spectrum of a K1{K4 star, 2) broad emission lines of neutrals and ions, 3) narrow emission lines of He i and He ii, 4) red{shifted accretion features of many lines, 5) shell lines at about the stellar velocity, 6) blue{shifted wind features and 7) forbidden lines. Periodic modulations in many spectral features were found. The photospheric absorption lines show sinusoidal variations in radial velocity with an amplitude of 6k m s 1 and a period of about 2: 77. The radial velocities of the narrow emission lines of He vary with the same period but in anti{phase to the photospheric lines. The equivalent widths of the narrow emissions vary with a phase{shift with respect to the velocity curve. The strength of the red{shifted accretion components of Na D and other lines is also modulated with the same period. The broad emission lines of metals vary mostly with the double period of about 5: d 5. One unexpected result is that no correlation was found between the veiling and the brightness, although both parameters varied in wide ranges. This is partly due to a contribution of the shell absorption to the photospheric line proles, which make them vary in width and depth thus simulating lower veiling. The spectral lines of the accreting gas show two distinct components: one is formed at low velocity at the beginning of the accretion column, and the other at high velocity near the stellar surface. The low velocity components are strong in low excitation lines of neutrals, while the high velocity components are strong in high excitation lines of ions, thus showing the gradients of temperature and density along the accretion column. Most of the observed features can be interpreted in the framework of non{axisymmetric magnetospheric accretion, but the origin of this asymmetry can be explained in dierent ways. We consider two possible models. The rst model suggests that RW Aur A is a binary with a brown dwarf secondary in a nearly circular orbit with a period of 2: d 77. The orbiting secondary generates a moving stream of enhanced accretion from one side of the disk towards the primary. The other model assumes that RW Aur A is a single star with a rotational period of 5: d 5a nd with two footpoints of channeled accretion streams within a global magnetosphere which is tilted relative to the rotational axis or otherwise non{axisymmetric. Both models can explain qualitatively and quantitatively most of the observed variations, but there are some details which are less well accounted for.

/pdf/non-axisymmetric-accretion-on-the-classical-tts-rw-aur-a-1h0q581n8r.pdf

Non{axisymmetric accretion on the classical TTS RW Aur A ?

Context. Most stars in the Galaxy were formed in massive clusters. To understand nature's favorite mode of star formation and the initial stages of the life of most stars one needs to characterize the youngest and resolved massive clusters in the Milky Way. Unfortunately young massive clusters are challenging observational targets as they are rare, hence found at large distances, are still embedded in their parental molecular cloud, and are swamped by relatively bright nebulae. Aims. In this paper we propose to use deep subarcsec resolution NIR data to derive the basic parameters of the unstudied population of massive cluster Westerlund 2. Methods. We present deep JHK s images (∼0.6" seeing) and photometry of Westerlund 2. This is the most complete photometric census of the cluster's population to date. Results. We detect a total of 4701, 5724, and 5397 sources in the J, H, and K s bands respectively. By comparison with main-sequence and pre-main-sequence model tracks, we determine an average visual extinction toward the cluster of 5.8 mag, a likely distance of 2.8 kpc, and an age of 2.0 ± 0.3 Myr for the core of the cluster. Although we have the sensitivity to reach beyond the hydrogen burning limit in the cluster, we are only complete to about 1 M ⊙ due to source confusion. We find no evidence for a top-heavy MF, and the slope of the derived mass function is -1.20 ± 0.16. Based on the extrapolation of a field IMF, we roughly estimate the total mass of the cluster to be about 10 4 M ⊙ . We find compelling evidence for mass segregation in this cluster.

/pdf/near-ir-imaging-of-galactic-massive-clusters-westerlund-2-4zk5vf9gy6.pdf

Near-IR imaging of Galactic massive clusters : Westerlund 2

(Abridged) High-resolution spectroscopic monitoring of RW Aur A was carried out in 1996, 1998 and 1999 with simultaneous B, V photometry A multicomponent spectrum is revealed with a veiled photospheric spectrum, broad emissions, narrow emission lines of helium, and accretion, wind and shell features 
Periodic modulations in many spectral features were found The photospheric absorption lines show sinusoidal variations in radial velocity with an amplitude of +-6 km/s and a period of about 277 days The radial velocities of the narrow emission lines of He vary with the same period but in anti-phase to the photospheric lines The equivalent widths of the narrow emissions vary with a phase-shift with respect to the velocity curve The strength of the red-shifted accretion components of Na D and other lines is also modulated with the same period The broad emission lines of metals vary mostly with the double period of about 55 days 
One unexpected result is that no correlation was found between the veiling and the brightness, although both varied in wide ranges This is partly due to a contribution of the shell absorption to the photospheric line profiles, which make them vary in width and depth thus simulating lower veiling 
Most of the observed features can be interpreted in the framework of non-axisymmetric magnetospheric accretion We consider two possible models In the first the asymmetry is induced by orbital motion of an invisible, low mass secondary, which also influences the gasflows around the star, the second considers rotational modulation of a single star with an inclined or asymmetric magnetosphere

/pdf/non-axisymmetric-accretion-on-the-classical-tts-rw-aur-a-1xiqcmhw07.pdf

Non-axisymmetric accretion on the classical TTS RW Aur A

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.

/pdf/no-evidence-of-mass-segregation-in-massive-young-clusters-2uorgfsycy.pdf

M. T. V. T. Lago

Papers

NTT and VLT diffraction limited imaging of Trumpler 14: revealing a massive core-halo cluster

Non{axisymmetric accretion on the classical TTS RW Aur A ?

Near-IR imaging of Galactic massive clusters : Westerlund 2

Non-axisymmetric accretion on the classical TTS RW Aur A

No evidence of mass segregation in massive young clusters