Young High-mass Stars Research Articles

A kinematic analysis of the giant molecular complex W3: Possible evidence for cloud–cloud collisions that triggered OB star clusters in W3 Main and W3(OH)

Abstract W3 is one of the most outstanding regions of high-mass star formation in the outer solar circle, and includes two active star-forming clouds: W3 Main and W3(OH). Based on a new analysis of the ${^{12}\text{CO}(J = 2-1)}$ data obtained at $38^{\prime \prime }$ resolution, we have found three clouds that have molecular masses from 2000 to $8000\, {M_\odot }$ at velocities $-50\:\rm{km\: s^{-1}}$, $-43\:\rm{km\:s^{-1}}$, and $-39\:\rm{km\:s^{-1}}$. The $-43\:\rm{km\:s^{-1}}$ cloud is the most massive one, overlapping with the $-39\:\rm{km\:s^{-1}}$ cloud and the $-50\:\rm{km\:s^{-1}}$ cloud toward W3 Main and W3(OH), respectively. In W3 Main and W3(OH), we have found typical signatures of a cloud–cloud collision, i.e., the complementary distribution with/without a displacement between the two clouds and/or a V-shape in the position–velocity diagram. We frame a hypothesis that a cloud–cloud collision triggered the high-mass star formation in each region. The collision in W3 Main involves the $-39\:\rm{km\:s^{-1}}$ cloud and the $-43\:\rm{km\:s^{-1}}$ cloud. The collision likely produced a cavity in the $-43\:\rm{km\:s^{-1}}$ cloud that has a size similar to the $-39\:\rm{km\:s^{-1}}$ cloud and triggered the formation of young high-mass stars in IC 1795 $2\:$Myr ago. We suggest that the $-39\:\rm{km\:s^{-1}}$ cloud is still triggering the high-mass objects younger than $1\:$Myr currently embedded in W3 Main. On the other hand, another collision between the $-50\:\rm{km\:s^{-1}}$ cloud and the $-43\:\rm{km\:s^{-1}}$ cloud likely formed the heavily embedded objects in W3(OH) within $\sim\! 0.5\:$Myr ago. The present results favour an idea that cloud–cloud collisions are common phenomena not only in the inner solar circle but also in the outer solar circle, where the number of reported cloud–cloud collisions is yet limited (Fukui et al. 2021, PASJ, 73, S1).

Gas kinematics around filamentary structures in the Orion B cloud

Context. Understanding the initial properties of star-forming material and how they affect the star formation process is key. From an observational point of view, the feedback from young high-mass stars on future star formation properties is still poorly constrained. Aims. In the framework of the IRAM 30m ORION-B large program, we obtained observations of the translucent (2 ≤ AV < 6 mag) and moderately dense gas (6 ≤ AV < 15 mag), which we used to analyze the kinematics over a field of 5 deg2 around the filamentary structures. Methods. We used the Regularized Optimization for Hyper-Spectral Analysis (ROHSA) algorithm to decompose and de-noise the C18O(1–0) and 13CO(1–0) signals by taking the spatial coherence of the emission into account. We produced gas column density and mean velocity maps to estimate the relative orientation of their spatial gradients. Results. We identified three cloud velocity layers at different systemic velocities and extracted the filaments in each velocity layer. The filaments are preferentially located in regions of low centroid velocity gradients. By comparing the relative orientation between the column density and velocity gradients of each layer from the ORION-B observations and synthetic observations from 3D kinematic toy models, we distinguish two types of behavior in the dynamics around filaments: (i) radial flows perpendicular to the filament axis that can be either inflows (increasing the filament mass) or outflows and (ii) longitudinal flows along the filament axis. The former case is seen in the Orion B data, while the latter is not identified. We have also identified asymmetrical flow patterns, usually associated with filaments located at the edge of an H II region. Conclusions. This is the first observational study to highlight feedback from H II regions on filament formation and, thus, on star formation in the Orion B cloud. This simple statistical method can be used for any molecular cloud to obtain coherent information on the kinematics.

The diverse chemistry of protoplanetary disks as revealed by JWST.

Early results from the James Webb Space Telescope-Mid-InfraRed Instrument (JWST-MIRI) guaranteed time programs on protostars (JOYS) and disks (MINDS) are presented. Thanks to the increased sensitivity, spectral and spatial resolution of the MIRI spectrometer, the chemical inventory of the planet-forming zones in disks can be investigated with unprecedented detail across stellar mass range and age. Here, data are presented for five disks, four around low-mass stars and one around a very young high-mass star. The mid-infrared spectra show some similarities but also significant diversity: some sources are rich in CO2, others in H2O or C2H2. In one disk around a very low-mass star, booming C2H2 emission provides evidence for a "soot" line at which carbon grains are eroded and sublimated, leading to a rich hydrocarbon chemistry in which even di-acetylene (C4H2) and benzene (C6H6) are detected. Together the data point to an active inner disk gas-phase chemistry that is closely linked to the physical structure (temperature, snowlines, presence of cavities and dust traps) of the entire disk and which may result in varying CO2/H2O abundances and high C/O ratios >1 in some cases. Ultimately, this diversity in disk chemistry will also be reflected in the diversity of the chemical composition of exoplanets.

SOFIA/GREAT observations of OD and OH rotational lines towards high-mass star forming regions

Context. Only recently, OD, the deuterated isotopolog of hydroxyl, OH, has become accessible in the interstellar medium; spectral lines from both species have been observed in the supra-Terahertz and far infrared regime. Studying variations of the OD/OH abundance amongst different types of sources can deliver key information on the formation of water, H2O. Aims. With observations of rotational lines of OD and OH towards 13 Galactic high-mass star forming regions, we aim to constrain the OD abundance and infer the deuterium fractionation of OH in their molecular envelopes. For the best studied source in our sample, G34.26+0.15, we were able to perform detailed radiative transfer modelling to investigate the OD abundance profile in its inner envelope. Methods. We used the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe the 2Π3∕2 J = 5∕2−3∕2 ground-state transition of OD at 1.3 THz (215 μm) and the rotationally excited OH line at 1.84 THz (163 μm). We also used published high-spectral-resolution SOFIA data of the OH ground-state transition at 2.51 THz (119.3 μm). Results. Absorption from the 2Π3∕2 OD J = 5∕2−3∕2 ground-state transition is prevalent in the dense clumps surrounding active sites of high-mass star formation. Our modelling suggests that part of the absorption arises from the denser inner parts, while the bulk of it as seen with SOFIA originates in the outer, cold layers of the envelope for which our constraints on the molecular abundance suggest a strong enhancement in deuterium fractionation. We find a weak negative correlation between the OD abundance and the bolometric luminosity to mass ratio, an evolutionary indicator, suggesting a slow decrease of OD abundance with time. A comparison with HDO shows a similarly high deuterium fractionation for the two species in the cold envelopes, which is of the order of 0.48% for the best studied source, G34.26+0.15. Conclusions. Our results are consistent with chemical models that favour rapid exchange reactions to form OD in the dense cold gas. Constraints on the OD/OH ratio in the inner envelope could further elucidate the water and oxygen chemistry near young high-mass stars.

The first interferometric survey of massive YSOs in the K-band

Context. Circumstellar discs are essential for the formation of high mass stars, while multiplicity, and in particular binarity, appears to be an inevitable outcome, as the vast majority of massive stars (>8 M⊙) are found in binaries (up to 100%). Our understanding of the innermost regions of accretion discs around massive stars and the binarity of high-mass young stars is sparse because of the high spatial resolution and sensitivity required to trace these rare and distant objects. Aims. We aim to spatially resolve and constrain the sizes of the dust and ionised gas emission from the innermost regions of a sample of massive young stellar objects (MYSOs) for the first time, and to provide high-mass binary statistics for young stars at 2–300 au scales using direct interferometric measurements. Methods. We observed six MYSOs using long-baseline near-infrared K-band interferometry on the VLTI (GRAVITY, AMBER) in order to resolve and characterise the 2.2 μm hot dust emission originating from the inner rim of circumstellar discs around MYSOs, and the associated Brγ emission from ionised gas. We fitted simple geometrical models to the interferometric observables, and determined the inner radius of the dust emission. We placed MYSOs with K-band measurements in a size–luminosity diagram for the first time, and compared our findings to their low- and intermediate-mass counterparts (T Tauris and Herbig AeBes). We also compared the observed K-band sizes (i.e. inner rim radius) to the sublimation radius predicted by three different disc scenarios: a classical thick flattened structure with oblique heating in action, and direct heating from the protostar via an optically thin cavity with and without backwarming effects. Lastly, we applied binary geometries to trace close binarity among MYSOs. Results. The characteristic size of the 2.2 μm continuum emission towards this sample of MYSOs shows a large scatter at the given luminosity range. When the inner sizes of MYSOs are compared to those of lower mass Herbig AeBe and T Tauri stars, they appear to follow a universal trend in that the sizes scale with the square-root of the stellar luminosity. The Brγ emission originates from a similar or somewhat smaller and co-planar area compared to the 2.2 μm continuum emission. We discuss this new finding with respect to a disc-wind or jet origin. Finally, we report an MYSO binary fraction of 17–25% at milli-arcsecond separations (2–300 au). Conclusions. The size–luminosity diagram indicates that the inner regions of discs around young stars scale with luminosity independently of the stellar mass. The observed fraction of MYSO binaries in K-band is almost ‘flat’ for a wide range of separations (2–10 000 au). At the targeted scales (2–300 au), the MYSO binary fraction is lower than what was previously reported for the more evolved main sequence massive stars, which, if further confirmed, could implicate predictions from massive binary formation theories. Lastly, with this study, we can finally spatially resolve the crucial star–disc interface in a sample of MYSOs, showing that au-scale discs are prominent in high-mass star formation and are similar to their low-mass equivalents, while the ionised gas can be linked to disc wind and disc accretion models similar to Herbig AeBes.

Dust polarized emission observations of NGC 6334

Context.Molecular filaments and hubs have received special attention recently thanks to new studies showing their key role in star formation. While the (column) density and velocity structures of both filaments and hubs have been carefully studied, their magnetic field (B-field) properties have yet to be characterized. Consequently, the role of B-fields in the formation and evolution of hub-filament systems is not well constrained.Aims.We aim to understand the role of the B-field and its interplay with turbulence and gravity in the dynamical evolution of the NGC 6334 filament network that harbours cluster-forming hubs and high-mass star formation.Methods.We present new observations of the dust polarized emission at 850μm toward the 2 pc × 10 pc map of NGC 6334 at a spatial resolution of 0.09 pc obtained with theJames Clerk MaxwellTelescope (JCMT) as part of the B-field In STar-forming Region Observations (BISTRO) survey. We study the distribution and dispersion of the polarized intensity (PI), the polarization fraction (PF), and the plane-of-the-sky B-field angle (χB_POS) toward the whole region, along the 10 pc-long ridge and along the sub-filaments connected to the ridge and the hubs. We derived the power spectra of the intensity andχBPOSalong the ridge crest and compared them with the results obtained from simulated filaments.Results.The observations span ~3 orders of magnitude in StokesIandPIand ~2 orders of magnitude inPF(from ~0.2 to ~ 20%). A large scatter inPIandPFis observed for a given value ofI. Our analyses show a complex B-field structure when observed over the whole region (~ 10 pc); however, at smaller scales (~1 pc),χBPOSvaries coherently along the crests of the filament network. The observed power spectrum ofχBPOScan be well represented with a power law function with a slope of − 1.33 ± 0.23, which is ~20% shallower than that ofI. We find that this result is compatible with the properties of simulated filaments and may indicate the physical processes at play in the formation and evolution of star-forming filaments. Along the sub-filaments,χBPOSrotates frombeing mostly perpendicular or randomly oriented with respect to the crests to mostly parallel as the sub-filaments merge with the ridge and hubs. This variation of the B-field structure along the sub-filaments may be tracing local velocity flows of infalling matter in the ridge and hubs. Our analysis also suggests a variation in the energy balance along the crests of these sub-filaments, from magnetically critical or supercritical at their far ends to magnetically subcritical near the ridge and hubs. We also detect an increase inPFtoward the high-column density (NH2≳ 1023 cm−2) star cluster-forming hubs. These latter largePFvalues may be explained by the increase in grain alignment efficiency due to stellar radiation from the newborn stars, combined with an ordered B-field structure.Conclusions.These observational results reveal for the first time the characteristics of the small-scale (down to ~ 0.1 pc) B-field structure of a 10 pc-long hub-filament system. Our analyses show variations in the polarization properties along the sub-filaments that may be tracing the evolution of their physical properties during their interaction with the ridge and hubs. We also detect an impact of feedback from young high-mass stars on the local B-field structure and the polarization properties, which could put constraints on possible models for dust grain alignment and provide important hints as to the interplay between the star formation activity and interstellar B-fields.

A Photoionized Accretion Disk around a Young High-mass Star

Abstract We present high spatial resolution (52 au) observations of the high-mass young stellar object (HMYSO) G345.4938+01.4677 made using the Atacama Large Millimeter/submillimeter Array. This O-type HMYSO is located at 2.38 kpc and is associated with a luminosity of 1.5 × 105 L ☉. We detect circumstellar emission from the H38β hydrogen recombination line showing a compact structure rotating perpendicularly to the previously detected radio jet. We interpret this emission as tracing a photoionized accretion disk around the HMYSO. While this disk-like structure seems currently too small to sustain continued accretion, the data present direct observational evidence of how disks can effectively survive the photoionization feedback from young high-mass stars. We also report the detection of a low-mass young stellar object in the vicinity of the HMYSO and suggest that it forms a high-mass and low-mass star binary system.

The most evolved sources in the Hi-GAL survey

Far-infrared and submillimetre surveys as the Herschel Galactic Plane Infrared Survey (Hi-GAL) represent an irreplaceable knowledge base about early phases of star formation, permitting statistical analysis based on thousands of Galaxy-wide distributed sources. Those with a regular spectral energy distribution in the Herschel wavelength range 70-500 μm span a variety of evolutionary stages, from quiescent to star forming clumps and, within the latter class, from mid-infrared dark clumps to sources appearing very bright also at shorter wavelengths (e.g. Spitzer 24 μm). A fraction of these clumps hosts the formation of high mass stars, which are expected to reach the zero-age main sequence and to develop a HII region in their surroundings while they are still embedded in their parental large-scale dusty envelope. This paper aims at selecting and studying in detail a robust sample of Hi-GAL clumps supposed to be candidate to host a HII region in their interior. They are expected to be the most evolved sources in the Hi-GAL catalogue. The Galactic locations and the physical properties (temperature, mass, bolometric luminosity and temperature, and surface density) of these sources are discussed here. The large number (1199) of selected sources constitutes an important starting point for planning further interferometric programs, aimed at resolving possible cores hosting a young high-mass star.

Anomalous peculiar motions of high-mass young stars in the Scutum spiral arm

We present trigonometric parallax and proper motion measurements toward 22 GHz water and 6.7 GHz methanol masers in 16 high-mass star-forming regions. These sources are all located in the Scutum spiral arm of the Milky Way. The observations were conducted as part of the Bar and Spiral Structure Legacy (BeSSeL) survey. A combination of 14 sources from a forthcoming study and 14 sources from the literature, we now have a sample of 44 sources in the Scutum spiral arm, covering a Galactic longitude range from 0° to 33°. A group of 16 sources shows large peculiar motions of which 13 are oriented toward the inner Galaxy. A likely explanation for these high peculiar motions is the combined gravitational potential of the spiral arm and the Galactic bar.

Trigonometric Parallaxes of High-mass Star-forming Regions: Our View of the Milky Way

Abstract We compile and analyze approximately 200 trigonometric parallaxes and proper motions of molecular masers associated with very young high-mass stars. Most of the measurements come from the BeSSeL Survey using the VLBA and the Japanese VERA project. These measurements strongly suggest that the Milky Way is a four-arm spiral, with some extra arm segments and spurs. Fitting log-periodic spirals to the locations of the masers, allowing for “kinks” in the spirals and using well-established arm tangencies in the fourth Galactic quadrant, allows us to significantly expand our view of the structure of the Milky Way. We present an updated model for its spiral structure and incorporate it into our previously published parallax-based distance-estimation program for sources associated with spiral arms. Modeling the three-dimensional space motions yields estimates of the distance to the Galactic center, , the circular rotation speed at the Sun's position, km s−1, and the nature of the rotation curve. Our data strongly constrain the full circular velocity of the Sun, km s−1, and its angular velocity, km s−1 kpc–1. Transforming the measured space motions to a Galactocentric frame which rotates with the Galaxy, we find non-circular velocity components typically ≲10 km s−1. However, near the Galactic bar and in a portion of the Perseus arm we find significantly larger non-circular motions. Young high-mass stars within 7 kpc of the Galactic center have a scale height of only 19 pc, and thus are well suited to define the Galactic plane. We find that the orientation of the plane is consistent with the IAU-defined plane to within ±0.°1, and that the Sun is offset toward the north Galactic pole by pc. Accounting for this offset places the central supermassive black hole, Sgr A*, in the midplane of the Galaxy. The measured motions perpendicular to the plane of the Galaxy limit precession of the plane to ≲4 km s−1 at the radius of the Sun. Using our improved Galactic parameters, we predict the Hulse–Taylor binary pulsar to be at a distance of 6.54 ± 0.24 kpc, assuming its orbital decay from gravitational radiation follows general relativity.

On the episodic excursions of massive protostars in the Hertzsprung–Russell diagram

Massive protostars grow and evolve under the effect of rapid accretion of circumstellar gas and dust, falling at high rates ($\ge 10^{-4}$-$10^{-3}\, \rm M_{\odot}\, \rm yr^{-1}$). This mass infall has been shown, both numerically and observationally, to be episodically interspersed by accretion of dense gaseous clumps migrating through the circumstellar disc to the protostellar surface, causing sudden accretion and luminous bursts. Using numerical gravito-radiation-hydrodynamics and stellar evolution calculations, we demonstrate that, in addition to the known bloating of massive protostars, variable episodic accretion further influences their evolutionary tracks of massive young stellar objects (MYSOs). For each accretion-driven flare, they experience rapid excursions toward more luminous, but colder regions of the Hertzsprung-Russell diagram. During these excursions, which can occur up to the end of the pre-main-sequence evolution, the photosphere of massive protostars can episodically release much less energetic photons and MYSOs surreptitiously adopt the same spectral type as evolved massive (supergiants) stars. Each of these evolutionary loop brings the young high-mass stars close to the forbidden Hayashi region and might make their surrounding H II regions occasionally fainter, before they recover their quiescent, pre-burst surface properties. We interpret such cold, intermittent pre-main-sequence stellar evolutionary excursions and the dipping variability of HII regions as the signature of the presence of a fragmenting circumstellar accretion disc surrounding the MYSOs. We conjecture that this mechanism might equivalently affect young stars in the intermediate-mass regime.

ALMA observations of RCW 120 Fragmentation at 0.01 pc scale

Context. Little is known about how high-mass stars form. Around 30% of the young high-mass stars in the Galaxy are observed at the edges of ionized (H II) regions. Therefore these are places of choice to study the earliest stages of high-mass star formation, especially toward the most massive condensations. High spatial resolution observations in the millimeter range might reveal how these stars form and how they assemble their mass. Aims. We want to study the fragmentation process down to the 0.01 pc scale in the most massive condensation (1700 M⊙) observed at the southwestern edge of the H ii region RCW 120 where the most massive Herschel cores (~124 M⊙ in average) could form high-mass stars. Methods. Using ALMA 3 mm continuum observations toward the densest and most massive millimetric condensation (Condensation 1) of RCW 120, we used the getimages and getsources algorithms to extract the sources detected with ALMA and obtained their physical parameters. The fragmentation of the Herschel cores is discussed through their Jeans mass to understand the properties of these future stars. Results. We extracted 18 fragments from the ALMA continuum observation at 3 mm toward eight cores detected with Herschel, whose mass and deconvolved size range from 2 M⊙ to 32 M⊙ and from 1.6 mpc to 28.8 mpc, respectively. The low degree of fragmen- tation observed regarding thermal Jeans fragmentation suggests that the observed fragmentation is inconsistent with ideal gravitational fragmentation and other ingredients such as turbulence or magnetic fields should be added to explain this inconsistency. Finally, the range of the mass of the fragments indicates that the densest condensation of RCW 120 is a favorable place for the formation of high-mass stars with the presence of a probable UCH ii region associated with the 27 M⊙ Fragment 1 of Core 2.

Forming spectroscopic massive protobinaries by disc fragmentation

The surroundings of massive protostars constitute an accretion disc which has numerically been shown to be subject to fragmentation and responsible for luminous accretion-driven outbursts. Moreover, it is suspected to produce close binary companions which will later strongly influence the star's future evolution in the Hertzsprung-Russel diagram. We present three-dimensional gravitation-radiation-hydrodynamic numerical simulations of 100 Mo pre-stellar cores. We find that accretion discs of young massive stars violently fragment without preventing the (highly variable) accretion of gaseous clumps onto the protostars. While acquiring the characteristics of a nascent low-mass companion, some disc fragments migrate onto the central massive protostar with dynamical properties showing that its final Keplerian orbit is close enough to constitute a close massive proto-binary system, having a young high-mass and a low-mass component. We conclude on the viability of the disc fragmentation channel for the formation of such short-period binaries, and that both processes -close massive binary formation and accretion bursts- may happen at the same time. FU-Orionis-type bursts, such as observed in the young high-mass star S255IR-NIRS3, may not only indicate ongoing disc fragmentation, but also be considered as a tracer for the formation of close massive binaries - progenitors of the subsequent massive spectroscopic binaries - once the high-mass component of the system will enter the main-sequence phase of its evolution. Finally, we investigate the ALMA-observability of the disc fragments.

The Structure of the Radio Recombination Line Maser Emission in the Envelope of MWC349A

AbstractThe Submillimeter Array (SMA) has been used to image the emission from radio recombination lines of hydrogen at subarcsecond angular resolution from the young high-mass star MWC349A in the H26α, H30α, and H31α transitions at 353, 232, and 211 GHz, respectively. Emission was seen over a range of 80 km s−1 in velocity and 50 mas (corresponding to 60 AU for a distance of 1200 pc). The emission at each frequency has two distinct components, one from gas in a nearly edge-on annular disk structure in Keplerian motion, and another from gas lifted off the disk at distances of up to about 25 AU from the star. The slopes of the position-velocity (PV) curves for the disk emission show a monotonic progression of the emission radius with frequency with relative radii of 0.85 ± 0.04, 1, and 1.02 ± 0.01 for the H26α, H30α, and H31α transitions, respectively. This trend is consistent with theoretical excitation models of maser emission from a region where the density decreases with radius and the lower transitions are preferentially excited at higher densities. The mass is difficult to estimate from the PV diagrams because the wind components dominate the emission at the disk edges. The mass estimate is constrained to be only in the range of 10–30 solar masses. The distribution of the wind emission among the transitions is surprisingly different, which reflects its sensitivity to excitation conditions. The wind probably extracts significant angular momentum from the system.

Connecting low- and high-mass star formation: the intermediate-mass protostar IRAS 05373+2349 VLA 2

Until recently, there have been few studies of the protostellar evolution of intermediate-mass (IM) stars, which may bridge the low- and high-mass regimes. This paper aims to investigate whether the properties of an IM protostar within the IRAS 05373+2349 embedded cluster are similar to that of low- and/or high-mass protostars. We carried out Very Large Array as well as Combined Array for Research in Millimeter Astronomy continuum and 12CO(J=1-0) observations, which uncover seven radio continuum sources (VLA 1-7). The spectral index of VLA 2, associated with the IM protostar is consistent with an ionised stellar wind or jet. The source VLA 3 is coincident with previously observed H2 emission line objects aligned in the north-south direction (P.A. -20 to -12 deg), which may be either an ionised jet emanating from VLA 2 or (shock-)ionised cavity walls in the large-scale outflow from VLA 2. The position angle between VLA 2 and 3 is slightly misaligned with the large-scale outflow we map at ~5-arcsec resolution in 12CO (P.A. ~30 deg), which in the case of a jet suggests precession. The emission from the mm core associated with VLA 2 is also detected; we estimate its mass to be 12-23 Msun, depending on the contribution from ionised gas. Furthermore, the large-scale outflow has properties intermediate between outflows from low- and high-mass young stars. Therefore, we conclude that the IM protostar within IRAS 05373+2349 is phenomenologically as well as quantitatively intermediate between the low- and high-mass domains.

CALIBRATION OF EVOLUTIONARY DIAGNOSTICS IN HIGH-MASS STAR FORMATION

ABSTRACT The evolutionary classification of massive clumps that are candidate progenitors of high-mass young stars and clusters relies on a variety of independent diagnostics based on observables from the near-infrared to the radio. A promising evolutionary indicator for massive and dense cluster-progenitor clumps is the L/M ratio between the bolometric luminosity and the mass of the clumps. With the aim of providing a quantitative calibration for this indicator, we used SEPIA/APEX to obtain CH3C2H(J = 12–11) observations, which is an excellent thermometer molecule probing densities ≥ 10 5 cm−3, toward 51 dense clumps with M ≥ 1000 M ⊙ and uniformly spanning −2 ≲ Log(L/M) [L ⊙ /M ⊙ ] ≲ 2.3. We identify three distinct ranges of L/M that can be associated to three distinct phases of star formation in massive clumps. For L/M ≤ 1 no clump is detected in CH3C2H, suggesting an inner envelope temperature below ∼30K. For 1 ≲ L/M ≲ 10 we detect 58% of the clumps with a temperature between ∼30 and ∼35 K independently from the exact value of L/M; such clumps are building up luminosity due to the formation of stars, but no star is yet able to significantly heat the inner clump regions. For L/M ≳ 10 we detect all the clumps with a gas temperature rising with Log(L/M), marking the appearance of a qualitatively different heating source within the clumps; such values are found toward clumps with UCH ii counterparts, suggesting that the quantitative difference in T versus L/M behavior above L/M ∼ 10 is due to the first appearance of ZAMS stars in the clumps.

HIGH-MASS STAR FORMATION TRIGGERED BY COLLISION BETWEEN CO FILAMENTS IN N159 WEST IN THE LARGE MAGELLANIC CLOUD

We have carried out 13CO(J = 2–1) observations of the active star-forming region N159 west in the Large Magellanic Cloud with ALMA. We have found that the CO distribution at a sub-parsec scale is highly elongated with a small width. These elongated clouds called "filaments" show straight or curved distributions with a typical width of 0.5–1.0 pc and a length of 5–10 pc. All the known infrared young stellar objects are located toward the filaments. We have found broad CO wings of two molecular outflows toward young high-mass stars in N159W-N and N159W-S, whose dynamical timescale is ∼104 years. This is the first discovery of protostellar outflow in external galaxies. For N159W-S, which is located toward an intersection of two filaments, we set up a hypothesis that the two filaments collided with each other ∼105 years ago and triggered the formation of the high-mass star having ∼37 . The colliding clouds show significant enhancement in linewidth in the intersection, suggesting excitation of turbulence in the shocked interface layer between them, as is consistent with the magnetohydrodynamical numerical simulations. This turbulence increases the mass accretion rate to yr−1, which is required to overcome the stellar feedback to form the high-mass star.

THE SLOW IONIZED WIND AND ROTATING DISKLIKE SYSTEM THAT ARE ASSOCIATED WITH THE HIGH-MASS YOUNG STELLAR OBJECT G345.4938+01.4677

We report the detection, made using ALMA, of the 92 GHz continuum and hydrogen recombination lines (HRLs) H40$\alpha$, H42$\alpha$, and H50$\beta$ emission toward the ionized wind associated with the high-mass young stellar object G345.4938+01.4677. This is the luminous central dominating source located in the massive and dense molecular clump associated with IRAS 16562$-$3959. The HRLs exhibit Voigt profiles, a strong signature of Stark broadening. We successfully reproduce the observed continuum and HRLs simultaneously using a simple model of a slow ionized wind in local thermodynamic equilibrium, with no need a high-velocity component. The Lorentzian line wings imply electron densities of $5\times10^7$ cm$^{-3}$ on average. In addition, we detect SO and SO$_2$ emission arising from a compact ($\sim3000$ AU) molecular core associated with the central young star. The molecular core exhibits a velocity gradient perpendicular to the jet-axis, which we interpret as evidence of rotation. The set of observations toward G345.4938+01.4677 are consistent with it being a young high-mass star associated with a slow photo-ionized wind.

Investigating the inner discs of Herbig Ae/Be stars with CO bandhead and Brγ emission★

Herbig Ae/Be stars lie in the mass range between low and high mass young stars, and therefore offer a unique opportunity to observe any changes in the formation processes that may occur across this boundary. This paper presents medium resolution VLT/X-Shooter spectra of six Herbig Ae/Be stars, drawn from a sample of 91 targets, and high resolution VLT/CRIRES spectra of five Herbig Ae/Be stars, chosen based on the presence of CO first overtone bandhead emission in their spectra. The X-Shooter survey reveals a low detection rate of CO first overtone emission (7 per cent), consisting of objects mainly of spectral type B. A positive correlation is found between the strength of the CO v=2-0 and Br {\gamma} emission lines, despite their intrinsic linewidths suggesting a separate kinematic origin. The high resolution CRIRES spectra are modelled, and are well fitted under the assumption that the emission originates from small scale Keplerian discs, interior to the dust sublimation radius, but outside the co-rotation radius of the central stars. In addition, our findings are in very good agreement for the one object where spatially resolved near-infrared interferometric studies have also been performed. These results suggest that the Herbig Ae/Be stars in question are in the process of gaining mass via disc accretion, and that modelling of high spectral resolution spectra is able to provide a reliable probe into the process of stellar accretion in young stars of intermediate to high masses.

Westerlund 1 is a Galactic Treasure Chest: The Wolf-Rayet Stars

AbstractThe Westerlund 1 Galactic cluster hosts an eclectic mix of coeval massive stars. At a modest distance of 4–5 kpc, it offers a unique opportunity to study the resolved stellar content of a young (~5 Myr) high mass (5·104M⊙) star cluster. With the aim of testing single-star evolutionary predictions, and revealing any signatures of binary evolution, we discuss on-going analyses of NTT/SOFI near-IR spectroscopy of Wolf-Rayet stars in Westerlund 1. We find that late WN stars are H-poor compared to their counterparts in the Milky Way field, and nearly all are less luminous than predicted by single-star Geneva isochrones at the age of Westerlund 1.