Massive Star Formation Process Research Articles

The ALMA-QUARKS Survey. I. Survey Description and Data Reduction

This paper presents an overview of the QUARKS survey, which stands for “Querying Underlying mechanisms of massive star formation with ALMA-Resolved gas Kinematics and Structures.” The QUARKS survey is observing 139 massive clumps covered by 156 pointings at Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 (λ ∼ 1.3 mm). In conjunction with data obtained from the ALMA-ATOMS survey at Band 3 (λ ∼ 3 mm), QUARKS aims to carry out an unbiased statistical investigation of massive star formation process within protoclusters down to a scale of 1000 au. This overview paper describes the observations and data reduction of the QUARKS survey, and gives a first look at an exemplar source, the mini-starburst Sgr B2(M). The wide-bandwidth (7.5 GHz) and high-angular-resolution (∼0.″3) observations of the QUARKS survey allow for the resolution of much more compact cores than those could be done by the ATOMS survey, and to detect previously unrevealed fainter filamentary structures. The spectral windows cover transitions of species including CO, SO, N2D+, SiO, H30 α, H2CO, CH3CN, and many other complex organic molecules, tracing gas components with different temperatures and spatial extents. QUARKS aims to deepen our understanding of several scientific topics of massive star formation, such as the mass transport within protoclusters by (hub-)filamentary structures, the existence of massive starless cores, the physical and chemical properties of dense cores within protoclusters, and the feedback from already formed high-mass young protostars.

Spectroscopic variability of massive pre-main-sequence stars in M17

Context. It is a challenge to study the formation process of massive stars: their formation time is short, there are only few of them, they are often deeply embedded, and they lie at relatively large distances. Our strategy is to study the outcome of the star formation process and to search for signatures that remain of the formation. We have access to a unique sample of (massive) pre-main-sequence (PMS) stars in the giant H II region M17. These PMS stars can be placed on PMS tracks in the Hertzsprung–Russell diagram (HRD) as we can detect their photospheric spectrum, and they exhibit spectral features indicative of the presence of a circumstellar disk. These stars are most likely in the final stage of formation. Aims. The aim is to use spectroscopic variability as a diagnostic tool to learn about the physical nature of these massive PMS stars. More specifically, we wish to determine the variability properties of the hot gaseous disks to understand the physical origin of the emission lines, identify dominant physical processes in these disks, and to find out about the presence of an accretion flow and/or jet. Methods. We obtained multiple-epoch (four to five epochs) VLT/X-shooter spectra of six young stars in M17 covering about a decade. Four of these stars are intermediate to massive PMS stars with gaseous disks. Using stacked spectra, we updated the spectral classification and searched for the presence of circumstellar features. With the temporal variance method (TVS), we determined the extent and amplitude of the spectral line variations in velocity space. The double-peaked emission lines in the PMS stars with gaseous disks were used to determine peak-to-peak velocities, V/R ratios, and the radial velocity of the systems. Simultaneous photometric variations were studied using VLT acquisition images. Results. From detailed line identification in the PMS stars with gaseous disks, we identify many (double-peaked) disk features, including a new detection of CO bandhead and CI emission. In three of these stars, we detect significant spectral variability, mainly in lines originating in the circumstellar disk, in a velocity range up to 320 km s−1, which exceeds the rotational velocity of the central sources. The shortest variability timescale is about one day. We also detect long-term (months, years) variability. The ratio of the blue and red peaks in two PMS stars shows a correlation with the peak-to-peak velocity, which might be explained by a spiral-arm structure in the disk. Conclusions. The variable PMS stars lie at similar positions in the HRD, but show significant differences in disk lines and variability. The extent and timescale of the variability differs for each star and line (sets), showing the complexity of the region where the lines are formed. We find indications for an accretion flow, slow disk winds, and/or disk structures in the hot gaseous inner disks. We find no evidence for close companions or strong accretion bursts as the cause of the variability in these PMS stars.

AFGL 5180 and AFGL 6366S: sites of hub–filament systems at the opposite edges of a filamentary cloud

ABSTRACT We present a multiscale and multiwavelength study to unveil massive star formation (MSF) processes around sites AFGL 5180 and AFGL 6366S, both hosting a Class ii 6.7 GHz methanol maser emission. The radio continuum map at 8.46 GHz reveals a small cluster of radio sources towards AFGL 5180. Signatures of the early stages of MSF in our target sites are spatially seen at the opposite edges of a filamentary cloud (length ∼5 pc), which is observed in the submillimetre dust continuum maps. Using the near-infrared photometric data, the spatial distribution of young stellar objects is found towards the entire filament, primarily clustered at its edges. The getsf utility on the Herschel far-infrared images reveals a hub–filament system (HFS) towards each target site. The analysis of the molecular line data, which benefits from large area coverage (∼1° × 1°), detects two cloud components with a connection in both position and velocity space. This supports the scenario of a cloud–cloud collision (CCC) that occurred ∼1 Myr ago. The filamentary cloud, connecting AFGL 5180 and AFGL 6366S, seems spatially close to an H ii region Sh 2−247 excited by a massive O9.5 star. Based on the knowledge of various pressures exerted by the massive star on its surroundings, the impact of its energetic feedback on the filamentary cloud is found to be insignificant. Overall, our observational outcomes favour the possibility of the CCC scenario driving MSF and the formation of HFSs towards the target sites.

Maser science with the next generation Very Large Array (ngVLA)

AbstractImaging the bright maser emission produced by the various molecular species from 1.6 to 116 GHz provides a way to probe the kinematics of dense molecular gas at high angular resolution. Unimpeded by the high dust optical depths that affect shorter wavelength (sub)mm observations, the high brightness temperature of these emission lines have become an essential tool for understanding the process of massive star formation. Operating from 1.2–116 GHz, the next generation Very Large Array (ngVLA) of 263 antennas will provide the capabilities needed to fully exploit these powerful tracers, including the ability to resolve accretion and outflow motions down to scales as fine as ∼1-10 au in deeply embedded Galactic star-forming regions, and at sub-pc scales in nearby galaxies. I will summarize the proposed specifications of the ngVLA, describe the current status of the project, and offer examples of future experiments designed to image the vicinity of massive protostars in continuum, thermal lines, and maser lines simultaneously.

A potential new phase of massive star formation

Context. Due to the sparsity and rapid evolution of high-mass stars, a detailed picture of the evolutionary sequence of massive protostellar objects still remains to be drawn. Some of the early phases of their formation are so short that only a select number of objects throughout the Milky Way currently find themselves spending time in those phases. Aims. Star-forming regions going through the shortest stages of massive star formation present different observational characteristics than most regions. By studying the dust continuum and line emission of such unusual clouds, one might be able to set strong constraints on the evolution of massive protostellar objects. Methods. We present a detailed analysis of the G345.88-1.10 hub filament system, which is a newly discovered star-forming cloud that hosts an unusually bright bipolar infrared nebulosity at its centre. We used archival continuum observations from Berschel, WISE, Spitzer, 2MASS, and SUMSS in order to fully characterise the morphology and spectral energy distribution of the region. We further made use of APEX 12CO(2–1), 13CO(2–1), C18O(2–1), and H30α observations to investigate the presence of outflows and map the kinematics of the cloud. Finally, we performed RADMC-3D radiative transfer calculations to constrain the physical origin of the central nebulosity. Results. At a distance of 2.26-0.21+0.30 kpc, G345.88-1.10 exhibits a network of parsec-long converging filaments. At the junction of these filaments lie four infrared-quiet fragments. The fragment H1 is the densest one (with M = 210 M⊙, Reff = 0.14 pc) and sits right at the centre of a wide (opening angle of ~90 ± 15°) bipolar nebulosity where the column density reaches local minima. The 12CO(2–1) observations of the region show that these infrared-bright cavities are spatially associated with a powerful molecular outflow that is centred on the H1 fragment. Negligible radio continuum and no H30α emission is detected towards the cavities, seemingly excluding the idea that ionising radiation drives the evolution of the cavities. Radiative transfer calculations of an embedded source surrounded by a disc and/or a dense core are unable to reproduce the observed combination of a low-luminosity (≲500 L⊙) central source and a surrounding high-luminosity (~4000 L⊙) mid-infrared-bright bipolar cavity. This suggests that radiative heating from a central protostar cannot be responsible for the illumination of the outflow cavities. Conclusions. This is, to our knowledge, the first reported object of this type. The rarity of objects such as G345.88-1.10 is likely related to a very short phase in the massive star and/or cluster formation process that has been unidentified thus far. We discuss whether mechanical energy deposition by one episode or successive episodes of powerful mass accretion in a collapsing hub might explain the observations. While promising in some aspects, a fully coherent scenario that explains the presence of a luminous bipolar cavity centred on an infrared-dark fragment remains elusive at this point.

Unraveling the Observational Signatures of Cloud–Cloud Collision and Hub-filament Systems in W31

To understand the formation process of massive stars, we present a multiscale and multiwavelength study of the W31 complex hosting two extended H ii regions (i.e., G10.30-0.15 (hereafter, W31-N) and G10.15-0.34 (hereafter, W31-S)) powered by a cluster of O-type stars. Several Class i protostars and a total of 49 ATLASGAL 870 μm dust clumps (at d = 3.55 kpc) are found toward the H ii regions where some of the clumps are associated with the molecular outflow activity. These results confirm the existence of a single physical system hosting the early phases of star formation. The Herschel 250 μm continuum map shows the presence of a hub-filament system (HFS) toward both W31-N and W31-S. The central hubs harbor H ii regions and they are depicted with extended structures (with T d ∼ 25–32 K) in the Herschel temperature map. In the direction of W31-S, an analysis of the NANTEN2 12CO(J = 1−0) and SEDIGISM 13CO(J = 2−1) line data supports the presence of two cloud components around 8 and 16 km s−1, and their connection in velocity space. A spatial complementary distribution between the two cloud components is also investigated toward W31-S, where the signposts of star formation, including massive O-type stars, are concentrated. These findings favor the applicability of cloud–cloud collision (CCC) around ∼2 Myr ago in W31-S. Overall, our observational findings support the theoretical scenario of CCC in W31, which explains the formation of massive stars and the existence of HFSs.

The origin of close massive binaries in the M17 star-forming region

Context. Spectroscopic multiplicity surveys of O stars in young clusters and OB associations have revealed that a large portion (∼70%) of these massive stars (Mi > 15 M⊙) belong to close and short-period binaries (with a physical separation of less than a few astronomical units). Follow-up VLT(I) high-angular-resolution observations led to the detection of wider companions (up to d ∼ 500 au), increasing the average companion fraction to > 2. Despite the recent and significant progress, the formation mechanisms leading to such close massive multiple systems remain to be elucidated. As a result, young massive close binaries (or higher-order multiple systems) are unique laboratories for determining the pairing mechanism of high-mass stars. Aims. We present the first VLTI/GRAVITY observations of six young O stars in the M17 star-forming region (≲1 Myr) and two additional foreground stars. VLTI/GRAVITY provides the K-band high-angular-resolution observations needed to explore the close environment of young O-type stars, and, as such, offers an excellent opportunity to characterise the multiplicity properties of the immediate outcome of the massive star formation process. Methods. From the interferometric model fitting of visibility amplitudes and closure phases, we search for companions and measure their positions and flux ratios. Combining the resulting magnitude difference with atmosphere models and evolutionary tracks, we further constrain the masses of the individual components. Results. All six high-mass stars are in multiple systems, leading to a multiplicity fraction of 100% and yielding a 68% confidence interval of 94–100%. We detect a total of nine companions with separations of up to 120 au. Including previously identified spectroscopic companions, the companion fraction of the young O stars in our sample reaches 2.3 ± 0.6. The derived masses span a wide range, from 2.5 to 50 M⊙, with a great tendency towards high-mass companions. However, we do not find a significant correlation between the mass of the companions and their separation. Conclusions. While based on a modest sample, our results clearly indicate that the origin of the high degree of multiplicity is rooted in the star formation mechanism of the sample stars. No clear evidence for one of the competing concepts of massive star formation (core accretion or competitive accretion) could be found. However, given that we find all of the companions within ∼120 au, our results are compatible with migration as a scenario for the formation of close massive binaries.

Effect of Dust Rotational Disruption by Radiative Torques on Radiation Pressure Feedback from Massive Protostars

Abstract Radiation pressure on dust is thought to play a crucial role in the formation process of massive stars by acting against gravitational collapse onto the central protostar. However, dust properties in dense regions irradiated by the intense radiation of massive protostars are poorly constrained. Previous studies usually assume the standard interstellar dust model to constrain the maximum mass of massive stars formed by accretion, which appears to contradict dust evolution theory. In this paper, using the fact that stellar radiation exerts on dust simultaneous radiation pressure and radiative torques, we study the effects of grain rotational disruption by radiative torques (RATs) on radiation pressure and explore its implications for massive star formation. For this paper, we focus on the protostellar envelope and adopt a spherical geometry. We find that originally large grains of micron sizes presumably formed in very dense regions can be rapidly disrupted into small grains by RATs due to infrared radiation from the hot dust shell near the sublimation front induced by direct stellar radiation. Owing to the modification of the size distribution by rotational disruption, the radiation pressure opacity can be decreased by a factor of ∼3 from the value expected from the original dust model. However, to form massive stars via spherical accretion, the dust-to-gas mass ratio needs to be reduced by a factor of ∼5 as previously found.

Establishing the evolutionary timescales of the massive star formation process through chemistry

Context.Understanding the details of the formation process of massive (i.e.M≳ 8–10M⊙) stars is a long-standing problem in astrophysics. They form and evolve very quickly, and almost their entire formation process takes place deeply embedded in their parental clumps. Together with the fact that these objects are rare and at a relatively large distance, this makes observing them very challenging.Aims.We present a method for deriving accurate timescales of the evolutionary phases of the high-mass star formation process.Methods.We modelled a representative number of massive clumps of the ATLASGAL-TOP100 sample that cover all the evolutionary stages. The models describe an isothermal collapse and the subsequent warm-up phase, for which we followed the chemical evolution. The timescale of each phase was derived by comparing the results of the models with the properties of the sources of the ATLASGAL-TOP100 sample, taking into account the mass and luminosity of the clumps, and the column densities of methyl acetylene (CH3CCH), acetonitrile (CH3CN), formaldehyde (H2CO), and methanol (CH3OH).Results.We find that the molecular tracers we chose are affected by the thermal evolution of the clumps, showing steep ice evaporation gradients from 103to 105AU during the warm-up phase. We succeed in reproducing the observed column densities of CH3CCH and CH3CN, but H2CO and CH3OH agree less with the observed values. The total (massive) star formation time is found to be ~5.2 × 105yr, which is defined by the timescales of the individual evolutionary phases of the ATLASGAL-TOP100 sample: ~5 × 104yr for 70-μm weak, ~1.2 × 105yr for mid-IR weak, ~2.4 × 105yr for mid-IR bright, and ~1.1 × 105yr for HII-region phases.Conclusions.With an appropriate selection of molecular tracers that can act as chemical clocks, our model allows obtaining robust estimates of the duration of the individual phases of the high-mass star formation process. It also has the advantage of being capable of including additional tracers aimed at increasing the accuracy of the estimated timescales.

The ionized heart of a molecular disk

Context. Hyper-compact (HC) or ultra-compact H II regions are the first manifestations of the radiation feedback from a newly born massive star. Therefore, their study is fundamental to understanding the process of massive (≥8 M⊙) star formation. Aims. We employed Atacama Large Millimeter/submillimeter Array (ALMA) 1.4 mm Cycle 6 observations to investigate at high angular resolution (≈0.′′050, corresponding to 330 au) the HC H II region inside molecular core A1 of the high-mass star-forming cluster G24.78+0.08. Methods. We used the H30α emission and different molecular lines of CH3CN and 13CH3CN to study the kinematics of the ionized and molecular gas, respectively. Results. At the center of the HC H II region, at radii ≲500 au, we observe two mutually perpendicular velocity gradients, which are directed along the axes at PA = 39° and PA = 133°, respectively. The velocity gradient directed along the axis at PA = 39° has an amplitude of 22 km s−1 mpc−1, which is much larger than the other’s, 3 km s−1 mpc−1. We interpret these velocity gradients as rotation around, and expansion along, the axis at PA = 39°. We propose a scenario where the H30α line traces the ionized heart of a disk-jet system that drives the formation of the massive star (≈20 M⊙) responsible for the HC H II region. Such a scenario is also supported by the position-velocity plots of the CH3CN and 13CH3CN lines along the axis at PA = 133°, which are consistent with Keplerian rotation around a 20 M⊙ star. Conclusions. Toward the HC H II region in G24.78+0.08, the coexistence of mass infall (at radii of ~5000 au), an outer molecular disk (from ≲4000 au to ≳500 au), and an inner ionized disk (≲500 au) indicates that the massive ionizing star is still actively accreting from its parental molecular core. To our knowledge, this is the first example of a molecular disk around a high-mass forming star that, while becoming internally ionized after the onset of the H II region, continues to accrete mass onto the ionizing star.

An infrared study of the high-mass, multistage star-forming region IRAS 12272−6240

ABSTRACT IRAS 12272−6240 is a complex star-forming region with a compact massive dense clump (DC) and several associated masers, located at a well-determined distance of d = 9.3 kpc from the Sun. For this study, we obtained sub-arcsec broad- and narrow-band near-infrared (near-IR) imaging and low-resolution spectroscopy with the Baade/Magellan telescope and its camera PANIC. Mosaics of size 2 × 2 arcmin2 in the JHKs bands and with narrow-band filters centred in the 2.12 μm H2 and 2.17 μm Br γ lines were analysed in combination with Hi-GAL/Herschel and archive IRAC/Spitzer and WISE observations. We found that the compact DC houses two Class I young stellar objects (YSOs) that probably form a 21000 -au-wide binary system. Its combined 1–1200 μm spectral energy distribution is consistent with an O9V central star with a $10^{-2} \, \mathrm{M}_\odot$ disc and a $1.3 \times 10^4 \, \mathrm{M}_\odot$ dust envelope. Its total luminosity is $8.5 \times 10^4 \, \mathrm{L}_\odot$. A series of shocked H2 emission knots are found in its close vicinity, confirming the presence of outflows. IRAS 12272−6240 is at the centre of an embedded cluster with a mean age of 1 Myr and 2.6 pc in size that contains more than 150 stars. At its nucleus, we found a more compact and considerably younger subcluster containing the YSOs. We also identified and classified the O-type central stars of two dusty radio/IR H ii regions flanking the protostars. Our results confirm that these elements form a single giant young complex where massive star formation processes started some 1 Myr ago and are still active.

The IACOB project

Context. The apparent lack of massive O-type stars near the zero-age main sequence, or ZAMS (at ages <2 Myr), is a topic that has been widely discussed in the past 40 yr. Different explanations for the elusive detection of these young massive stars have been proposed from the observational and theoretical side, but no firm conclusions have been reached yet. Aims. We reassess this empirical result here, benefiting from the high-quality spectroscopic observations of (more than 400) Galactic O-type stars gathered by the IACOB and OWN surveys. Methods. We used effective temperatures and surface gravities resulting from a homogeneous semi-automatized IACOB-GBAT/FASTWIND spectroscopic analysis to locate our sample of stars in the Kiel and spectroscopic Hertzsprung-Russell (sHR) diagrams. We evaluated the completeness of our magnitude-limited sample of stars as well as potential observational biases affecting the compiled sample using information from the Galactic O star catalog. We discuss limitations and possible systematics of our analysis method, and compare our results with other recent studies using smaller samples of Galactic O-type stars. We mainly base our discussion on the distribution of stars in the sHR diagram in order to avoid the use of still uncertain distances to most of the stars in our sample. However, we also performed a more detailed study of the young cluster Trumpler-14 as an illustrative example of how Gaia cluster distances can help to construct the associated classical HR diagram. Results. We find that the apparent lack of massive O-type stars near the ZAMS with initial evolutionary masses in the range between ≈30 and 70 M⊙ still persist even when spectroscopic results from a large non-biased sample of stars are used. We do not find any correlation between the dearth of stars close to the ZAMS and obvious observational biases, limitations of our analysis method, and/or the use of one example spectroscopic HR diagram instead of the classical HR diagram. Finally, by investigating the effect of the efficiency of mass accretion during the formation process of massive stars, we conclude that an adjustment of the mass accretion rate towards lower values than canonically assumed might reconcile the hotter boundary of the empirical distribution of optically detected O-type stars in the spectroscopic HR diagram and the theoretical birthline for stars with masses above ≈30 M⊙. Last, we also discuss how the presence of a small sample of O2-O3.5 stars found much closer to the ZAMS than the main distribution of Galactic O-type star might be explained in the context of this scenario when the effect of nonstandard star evolution (e.g. binary interaction, mergers, and/or homogeneous evolution) is taken into account.

VLT/X-shooter spectroscopy of massive young stellar objects in the 30 Doradus region of the Large Magellanic Cloud

The process of massive star (M ≥ 8 M⊙) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud. We took advantage of the spectral resolution and wavelength coverage of X-shooter (300−2500 nm), which is mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect Hα, β, [O I] 630.0 nm, Ca II, infrared triplet [Fe II] 1643.5 nm, fluorescent Fe II1687.8 nm, H22121.8 nm, Brγ, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of ten candidates as bona fide MYSOs. We compared our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We computed lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Brγluminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features, such as fluorescent Fe IIand CO bandheads, which is consistent with the Galactic rate (40%). Most of our MYSOs show outflow features.

The young stellar content of the giant H II regions M 8, G333.6−0.2, and NGC 6357 with VLT/KMOS

Context.The identification and characterisation of populations of young massive stars in (giant) H IIregions provides important constraints on (i) the formation process of massive stars and their early feedback on the environment, and (ii) the initial conditions for population synthesis models predicting the evolution of ensembles of stars.Aims.We identify and characterise the stellar populations of the following young giant H IIregions: M 8, G333.6−0.2, and NGC 6357.Methods.We have acquiredH- andK-band spectra of around 200 stars using theK-band Multi Object Spectrograph on the ESO Very Large Telescope. The targets for M 8 and NGC 6357 were selected from the Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX), which combines X-ray observations with near-infrared (NIR) and mid-infrared data. For G333.6−0.2, the sample selection is based on the NIR colours combined with X-ray data. We introduce an automatic spectral classification method in order to obtain temperatures and luminosities for the observed stars. We analysed the stellar populations using their photometric, astrometric, and spectroscopic properties and compared the position of the stars in the Hertzprung-Russell diagram with stellar evolution models to constrain their ages and mass ranges.Results.We confirm the presence of candidate ionising sources in the three regions and report new ones, including the first spectroscopically identified O stars in G333.6−0.2. In M 8 and NGC 6357, two populations are identified: (i) OB main-sequence stars (M > 5M⊙) and (ii) pre-main sequence stars (M ≈ 0.5 − 5 M⊙). The ages of the clusters are ∼1−3 Myr, < 3 Myr, and ∼0.5−3 Myr for M 8, G333.6−0.2, and NGC 6357, respectively. We show that MYStIX selected targets have > 90% probability of being members of the H IIregion, whereas a selection based on NIR colours leads to a membership probability of only ∼70%.

Herschel-HOBYS study of the earliest phases of high-mass star formation in NGC 6357

Aims. To constrain models of high-mass star formation it is important to identify the massive dense cores (MDCs) that are able to form high-mass star(s). This is one of the purposes of the Herschel/HOBYS key programme. Here, we carry out the census and characterise of the properties of the MDCs population of the NGC 6357 H II region. Methods. Our study is based on the Herschel/PACS and SPIRE 70−500 μm images of NGC 6357 complemented with (sub-)millimetre and mid-infrared data. We followed the procedure established by the Herschel/HOBYS consortium to extract ~0.1 pc massive dense cores using the getsources software. We estimated their physical parameters (temperatures, masses, luminosities) from spectral energy distribution (SED) fitting. Results. We obtain a complete census of 23 massive dense cores, amongst which one is found to be IR-quiet and twelve are starless, representing very early stages of the star-formation process. Focussing on the starless MDCs, we have considered their evolutionary status, and suggest that only five of them are likely to form a high-mass star. Conclusions. We find that, contrarily to the case in NGC 6334, the NGC 6357 region does not exhibit any ridge or hub features that are believed to be crucial to the massive star formation process. This study adds support for an empirical model in which massive dense cores and protostars simultaneously accrete mass from the surrounding filaments. In addition, the massive star formation in NGC 6357 seems to have stopped and the hottest stars in Pismis 24 have disrupted the filaments.

Massive pre-main-sequence stars in M17

The formation process of massive stars is still poorly understood. Massive young stellar objects (mYSOs) are deeply embedded in their parental clouds, they are rare and thus typically distant, and their reddened spectra usually preclude the determination of their photospheric parameters. M17 is one of the best studied HII regions in the sky, is relatively nearby, and hosts a young stellar population. With X-shooter on the ESO Very Large Telescope we have obtained optical to near-infrared spectra of candidate mYSOs, identified by Hanson et al. (1997), and a few OB stars in this region. The large wavelength coverage enables a detailed spectroscopic analysis of their photospheres and circumstellar disks. We confirm the pre-main sequence (PMS) nature of six of the stars and characterise the O stars. The PMS stars have radii consistent with being contracting towards the main sequence and are surrounded by a remnant accretion disk. The observed infrared excess and the (double-peaked) emission lines provide the opportunity to measure structured velocity profiles in the disks. We compare the observed properties of this unique sample of young massive stars with evolutionary tracks of massive protostars by Hosokawa & Omukai (2009), and propose that these mYSOs near the western edge of the HII region are on their way to become main-sequence stars ($\sim 6 - 20$ $M_{\odot}$) after having undergone high mass-accretion rates (${\dot{M}_{\rm acc}} \sim 10^{-4} - 10^{-3}$ $M_{\odot}$ $\rm yr^{-1}$). Their spin distribution upon arrival at the zero age main sequence (ZAMS) is consistent with that observed for young B stars, assuming conservation of angular momentum and homologous contraction.

Fire from Ice - Massive Star Birth from Infrared Dark Clouds

AbstractI review massive star formation in our Galaxy, focussing on initial conditions in Infrared Dark Clouds (IRDCs), including the search for massive pre-stellar cores (PSCs), and modeling of later stages of massive protostars, i.e., hot molecular cores (HMCs). I highlight how developments in astrochemistry, coupled with rapidly improving theoretical/computational and observational capabilities are helping to improve our understanding of the complex process of massive star formation.

A dearth of short-period massive binaries in the young massive star forming region M 17

The formation of massive stars remains poorly understood and little is known about their birth multiplicity properties. Here, we investigate the strikingly low radial-velocity dispersion measured for a sample of 11 massive pre- and near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive star forming region M17 to obtain first constraints on the multiplicity properties of young massive stellar objects. Methods: We compute the RV dispersion of synthetic populations of massive stars for various multiplicity properties and we compare the simulated sigma_rv distributions to the observed value. We specifically investigate two scenarios: a low binary fraction and a dearth of short-period binary systems. Results: Simulated populations with low binary fractions (f_bin = 0.12_{-0.09}^{+0.16}) or with truncated period distributions (P_cutoff > 9 months) are able to reproduce the low sigma_rv observed within their 68%-confidence intervals. Parent populations with f_bin > 0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level. Both constraints are contrast with the high binary fraction and plethora of short-period systems found in few Myr-old, OB-type populations. To explain the difference, the first scenario requires a variation of the outcome of the massive star formation process. In the the second scenario, compact binaries must form later on, and the cut-off period may be related to physical length-scales representative of the bloated pre-main-sequence stellar radii or of their accretion disks. Conclusions: If the obtained constraints are representative of the overall properties of massive young stellar objects, our results may provide support to a formation process in which binaries are initially formed at larger separations, then harden or migrate to produce the typical (untruncated) power-law period distribution observed in few Myr-old OB binaries.

The multiplicity of massive stars: a 2016 view

AbstractMassive stars like company. Here, we provide a brief overview of progresses made over the last 5 years by a number of medium and large surveys. These results provide new insights on the observed and intrinsic multiplicity properties of main sequence massive stars and on the initial conditions for their future evolution. They also bring new interesting constraints on the outcome of the massive star formation process.

G11.92−0.61 MM1: a Keplerian disc around a massive young proto-O star

The formation process of massive stars is not well understood, and advancement in our understanding benefits from high resolution observations and modelling of the gas and dust surrounding individual high-mass (proto)stars. Here we report sub-arcsecond (<1550 au) resolution observations of the young massive star G11.92-0.61 MM1 with the SMA and VLA. Our 1.3 mm SMA observations reveal consistent velocity gradients in compact molecular line emission from species such as CH$_3$CN, CH$_3$OH, OCS, HNCO, H$_2$CO, DCN and CH$_3$CH$_2$CN, oriented perpendicular to the previously reported bipolar molecular outflow from MM1. Modelling of the compact gas kinematics suggests a structure undergoing rotation around the peak of the dust continuum emission. The rotational profile can be well fit by a model of a Keplerian disc, including infall, surrounding an enclosed mass of 30-60M$_{\odot}$, of which 2-3M$_{\odot}$ is attributed to the disc. From modelling the CH$_3$CN emission, we determine that two temperature components, of 150 K and 230 K, are required to adequately reproduce the spectra. Our 0.9 and 3.0cm VLA continuum data exhibit an excess above the level expected from dust emission; the full centimetre-submillimetre wavelength spectral energy distribution of MM1 is well reproduced by a model including dust emission, an unresolved hypercompact H{\i}{\i} region, and a compact ionised jet. In combination, our results suggest that MM1 is an example of a massive proto-O star forming via disc accretion, in a similar way to that of lower mass stars.