Although fundamental for astrophysics, the processes that produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. This review offers a basic description of the collapse of a massive molecular core and a critical discussion of the three competing concepts of massive star formation: ▪  monolithic collapse in isolated cores ▪  competitive accretion in a protocluster environment ▪  stellar collisions and mergers in very dense systems We also review the observed outflows, multiplicity, and clustering properties of massive stars, the upper initial mass function and the upper mass limit. We conclude that high-mass star formation is not merely a scaled-up version of low-mass star formation with higher accretion rates, but partly a mechanism of its own, primarily owing to the role of stellar mass ...

Toward Understanding Massive Star Formation

Although fundamental for astrophysics, the processes that produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. This review offers a basic description of the collapse of a massive molecular core and a critical discussion of the three competing concepts of massive star formation: 
- monolithic collapse in isolated cores 
- competitive accretion in a protocluster environment 
- stellar collisions and mergers in very dense systems 
We also review the observed outflows, multiplicity, and clustering properties of massive stars, the upper initial mass function and the upper mass limit. We conclude that high-mass star formation is not merely a scaled-up version of low-mass star formation with higher accretion rates, but partly a mechanism of its own, primarily owing to the role of stellar mass and radiation pressure in controlling the dynamics.

We study the stellar and wind properties of massive stars in the central cluster of the Galaxy. We use non-LTE atmosphere models including winds and line-blanketing to fit their H and K band spectra obtained with the 3D spectrograph SINFONI on the VLT. We derive the main stellar (Teff, L, abundances, ionizing flux) and wind (mass loss rate, terminal velocity) properties. They are found to be similar to other galactic massive stars. We show that a direct evolutionary link between Ofpe/WN9, WN8 and WN/C stars exists. Using individual SEDs for each massive star, we construct the total spectral energy distribution of the cluster and use it to compute photoionization models. We show that the nebular properties of the central HII region are well reproduced. We conclude that, contrary to previous claims, standard stellar evolution and atmosphere models are well suited to explain the properties of the central cluster. Our results indicate that massive stars in the central cluster do not have a peculiar evolution as could be expected from their proximity to the supermassive black hole SgrA*.

/pdf/stellar-and-wind-properties-of-massive-stars-in-the-central-3kvsxeq4r3.pdf

Stellar and wind properties of massive stars in the central parsec of the Galaxy

We present a study of carbon in N159/N160, an HII region complex in the low metallicity Large Magellanic Cloud. We have mapped this region, which comprises four distinct molecular clouds spanning a wide range of star-formation activity, in four transitions: 13CO (1-0) CO (2-1) and (4-3), and [CI] (1-0). Combining these data with existing [CII] observations provides a complete picture of the predominant forms of carbon in the gas phase of the ISM. The new CO (2-1) data show that the complex is immersed in an envelope of extended, low level emission, undetected by previous (1-0) mapping efforts. The CO (2-1)/(1-0) ratio in this envelope is >~3, a value consistent with optically thin CO emission. The envelope is also relatively bright in [CI] and [CII], and calculations show that it is mostly photodissociated: it appears to be translucent (Av<1). Neutral carbon emission in the complex unexpectedly peaks at the quiescent southern cloud (N159S). In the northern portion of the map (the N160 nebula), the HII regions prominent in [CII] correspond to holes in the [CI] distribution. Overall we find that, while the [CII]/CO ratio is enhanced with respect to similar complexes in the Milky Way, the [CI]/CO ratio appears to be similar or reduced.

Carbon in the N159/N160 Complex of the Large Magellanic Cloud

We present the ALMA observations of CO isotopes and 1.3 mm continuum emission toward the N159E-Papillon Nebula in the Large Magellanic Cloud (LMC). The spatial resolution is 0"25-0"28 (0.06-0.07 pc), which is a factor of 3 higher than the previous ALMA observations in this region. The high resolution allowed us to resolve highly filamentary CO distributions with typical widths of $\sim$0.1 pc (full width half maximum) and line masses of a few 100 $M_{\odot}$ pc$^{-1}$. The filaments (more than ten in number) show an outstanding hub-filament structure emanating from the nebular center toward the north. We identified for the first time two massive protostellar outflows of $\sim$10$^4$ yr dynamical age along one of the most massive filaments. The observations also revealed several pillar-like CO features around the Nebula. The H II region and the pillars have a complementary spatial distribution and the column density of the pillars is an order of magnitude higher than that of the pillars in the Eagle nebula (M16) in the Galaxy, suggesting an early stage of pillar formation with an age younger than $\sim$10$^5$ yr. We suggest that a cloud-cloud collision triggered the formation of the filaments and protostar within the last $\sim$2 Myr. It is possible that the collision is more recent, as part of the kpc-scale H I flows come from the tidal interaction resulting from the close encounter between the LMC and SMC $\sim$200 Myr ago as suggested for R136 by Fukui et al.

/pdf/an-alma-view-of-molecular-filaments-in-the-large-magellanic-3j0ub7en4k.pdf

An ALMA View of Molecular Filaments in the Large Magellanic Cloud. I. The Formation of High-mass Stars and Pillars in the N159E-Papillon Nebula Triggered by a Cloud─Cloud Collision

We study the Large Magellanic Cloud Papillon Nebula (N159-5), a conspicuous High Excitation Blob (HEB) lying in the star forming complex N159. Using JHK near-infrared photometry obtained at the ESO VLT with the ISAAC camera, we examine the stellar populations associated with the Papillon, tracing their history using stellar evolution models. Two populations are revealed: one composed of young, massive stars with an age ~ 3 Myr, and a second consisting of older lower mass stars of age spreading between 1 and 10 Gyr. We analyze the properties of those populations and discuss their significance in the context of N159. We also estimate that if the star at the center of the Papillon is single its initial mass is ~ 50 solar masses and it is affected by an extinction Av ~ 7 mag.

/pdf/stellar-populations-associated-with-the-lmc-papillon-nebula-54qghpkiu4.pdf

Stellar populations associated with the LMC Papillon Nebula

Accurate physical parameters of newborn massive stars are essential ingredients to shed light on their formation, which is still an unsolved problem. The rare class of compact H II regions in the Magellanic Clouds (MCs), termed ``high-excitation blobs'' (HEBs), presents a unique opportunity to acquire this information. These objects (~ 4" to 10", ~ 1 to 3 pc, in diameter) harbor the youngest massive stars of the OB association/molecular cloud complexes in the MCs accessible through high-resolution near-IR and optical techniques. We present a brief overview of the results obtained with HST mainly on two HEBs, one in the LMC (N159-5) and the other in the SMC (N81).

HST's view of the youngest massive stars in the Magellanic Clouds

We present near infrared JHK imaging of the Small Magellanic Cloud compact Hii region N81 using the ISAAC camera at the ESO Very Large Telescope (Antu). Our analysis of the stellar environment of this young massive star region reveals the presence of three new stellar populations in the surrounding field which are mainly composed of low mass stars. The main population is best fitted by evolutionary models for 2 M stars with an age of 1 Gyr. We argue that these populations are not physically associated with the Hii region N81. Instead they are the result of a number of low mass star forming events through the depth of the SMC south of its Shapley's wing. The populations can rather easily be probed due to the low interstellar extinction in that direction.

/pdf/the-stellar-environment-of-smc-n81-3ulr38gvvy.pdf

The stellar environment of SMC N81

We present near infrared JHK imaging of the Small Magellanic Cloud compact H II region N81 using the ISAAC camera at the ESO Very Large Telescope (Antu). Our analysis of the stellar environment of this young massive star region reveals the presence of three new stellar populations in the surrounding field which are mainly composed of low mass stars. The main population is best fitted by evolutionary models for about 2 solar mass stars with an age of 1 Gyr. We argue that these populations are not physically associated with the H II region N81. Instead they are the result of a number of low mass star forming events through the depth of the SMC south of its Shapley's wing. The populations can rather easily be probed due to the low interstellar extinction in that direction.

/pdf/the-stellar-environment-of-smc-n81-1w9760dqon.pdf

Accurate physical parameters of newborn massive stars are essential ingredients to shed light on their formation, which is still an unsolved problem. The rare class of compact H ii regions in the Magellanic Clouds (MCs), termed “high-excitation blobs” (HEBs), presents a unique opportunity to acquire this information. These objects (4″ to 10″, ∼1 to 3 pc, in diameter) harbor the youngest massive stars of the OB association/molecular cloud complexes in the MCs accessible through high-resolution near-IR and optical techniques. We present a brief overview of the results obtained with HST mainly on two HEBs, one in the LMC (N159-5) and the other in the SMC (N81).

HST’s View of the Youngest Massive Stars in the Magellanic Clouds

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Stellar populations associated with the LMC Papillon Nebula

HST's view of the youngest massive stars in the Magellanic Clouds

The stellar environment of SMC N81

The stellar environment of SMC N81

HST’s View of the Youngest Massive Stars in the Magellanic Clouds