Phases Of Massive Star Formation Research Articles

Importance of source structure on complex organics emission

Context. The hot molecular core phase of massive star formation shows emission from complex organic molecules. However, these species are only detected toward a fraction of high-mass protostars. In particular, there is a spread of ~2 orders of magnitude in methanol emission intensity from high-mass protostars. Aims. The goal of this work is to answer the question of whether high-mass disks can explain the lack of methanol emission from some massive protostellar systems. Methods. We considered an envelope-only and an envelope-plus-disk model and used the code RADMC-3D to calculate the methanol emission. High and low millimeter (mm) opacity dust (representing large and small dust distributions) were considered for both models separately, and the methanol abundance was parameterized. Viscous heating was included due to the high accretion rates of these objects in the disk. Results. In contrast with low-mass protostars, the presence of a disk does not significantly affect the temperature structure and methanol emission. The shadowing effect of the disk is not as important for high-mass objects, and the disk midplane is hot because of viscous heating, which is effective due to the high accretion rates. The methanol emission is lower for models with high mm opacity dust because the dust attenuation blocks the emission in the envelope and hides it in the disk through continuum oversubtraction, but the disk needs to be large for this to become effective. A minimum disk size of ~2000–2500 au is needed (at L = 104 L⊙) with high mm opacity dust for drop of a factor of about one order of magnitude in the methanol emission compared with the envelope-only models with low mm opacity dust. Consistent with observations of infrared absorption lines toward high-mass protostars, we find a vertical temperature inversion, that is, higher temperatures in the disk midplane than the disk surface, at radii ≲50 au for models with L = 104 L⊙ and high mm opacity dust as long as the envelope mass is ≳550 M⊙ (Ṁ = 3.6 × 10−3 M⊙ yr−1). Conclusions. The large observed scatter in methanol emission from massive protostars can be mostly explained toward lower-luminosity objects (~103 L⊙) with the envelope-plus-disk models including low and high mm opacity dust. The methanol emission variation toward sources with high luminosities (≳104 L⊙) cannot be explained by models with or without a disk with a relatively high gas-phase abundance of methanol. However, the luminosity-to-mass ratios of these objects suggest that they might be associated with hypercompact or ultracompact HII regions. Therefore, the low methanol emission toward the high-luminosity sources can be explained by them hosting an HII region in which methanol is absent.

PHANGS–JWST First Results: Duration of the Early Phase of Massive Star Formation in NGC 628

The earliest stages of star formation, when young stars are still deeply embedded in their natal clouds, represent a critical phase in the matter cycle between gas clouds and young stellar regions. Until now, the high-resolution infrared observations required for characterizing this heavily obscured phase (during which massive stars have formed, but optical emission is not detected) could only be obtained for a handful of the most nearby galaxies. One of the main hurdles has been the limited angular resolution of the Spitzer Space Telescope. With the revolutionary capabilities of the James Webb Space Telescope (JWST), it is now possible to investigate the matter cycle during the earliest phases of star formation as a function of the galactic environment. In this Letter, we demonstrate this by measuring the duration of the embedded phase of star formation and the implied time over which molecular clouds remain inert in the galaxy NGC 628 at a distance of 9.8 Mpc, demonstrating that the cosmic volume where this measurement can be made has increased by a factor of >100 compared to Spitzer. We show that young massive stars remain embedded for Myr ( Myr of which being heavily obscured), representing ∼20% of the total cloud lifetime. These values are in broad agreement with previous measurements in five nearby (D < 3.5 Mpc) galaxies and constitute a proof of concept for the systematic characterization of the early phase of star formation across the nearby galaxy population with the PHANGS–JWST survey.

Spinning Nanoparticles Impacted by C-shock: Implications for Radio-millimeter Emission from Star-forming Regions

We investigate the impact of anomalous microwave emission (AME) on the radio-millimeter spectral energy distribution for three typical interstellar medium (ISM) conditions surrounding star-forming regions—cold neutral medium, warm neutral medium, and photodissociation regions—by comparing the emissivities of three major contributors: free–free, thermal dust emission, and AME. In particular, for spinning nanoparticles (i.e., potential carriers of AME), we consider a known grain destruction mechanism due to a centrifugal force from spin-up processes caused by collisions between dust grains and supersonic neutral streams in a magnetized shock (C-shock). We demonstrate that, if the ISM in a magnetic field is impacted by a C-shock developed by a supernova explosion in the early phase of massive star formation (≲10 Myr), AME can be significantly or almost entirely suppressed relative to free–free and thermal dust continuum emission if the grain tensile strength is small enough. This study may shed light on explaining the rare observations of AME from extragalactic star-forming regions preferentially observed from massive star clusters and suggest a scenario of “the rise and fall of AME” in accordance with the temporal evolution of star-forming regions.

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.

W 49 N MCN-a: A disk-accreting massive protostar embedded in an early-phase hot molecular core

Abstract We present ALMA archival data for 219–235 GHz continuum and line observations toward the hot molecular core (HMC) W 49 N MCN-a (UCHII region J1) at a resolution of ∼${0.^{\prime \prime }3}$. The dust continuum emission, showing an elongated structure of ${1.^{\prime \prime }40} \times {0.^{\prime \prime }95}$ (position angle = 43.°5) perpendicular to the outflow seen in SiO and SO, represents a rotating flattened envelope, or torus, with a radius of 7800 au inclined at 47.°5 or larger. The emissions from CH3CN, 13CS, HNCO, HC3N, SO2, DCN, H2CO, OCS, CH3OH, and C18O exhibit a consistent velocity gradient as a result of rotation. The magnitude of each velocity gradient is different, reflecting that each line samples a specific radial region. This allows us to derive a rotation curve as Vrot ∝ R0.44 ± 0.11 for 2400 au ≲ R ≲ 14000 au, giving the dynamical mass as $M_{\rm dyn} = 57.0^{+24.5}_{-17.1}\, (R\, [{\rm au}]/3000)^{1.88}\, M_{\odot }$. The envelope mass independently estimated from the dust emission is 910 M⊙ (for Tdust = 180 K) for R ≤ 7800 au and 32 M⊙ (for Tdust = 300 K) for R ≤ 1700 au. The dynamical mass formula agrees well with these mass estimates within an uncertainty of a factor of three in the latter. The envelope is self-gravitating and is unstable to form spiral arms and fragments, allowing rapid accretion to the inner radii with a rate of the order of 10−2 M⊙ yr−1, although inward motion was not detected. The envelope may become a non-self-gravitating Keplerian disk at R ≲ (300–1000) au. The formula is also consistent with the total mass ∼104 M⊙ of the entire HMC 0.15 pc (31000 au) in radius. Multiple transitions of CH3CN, HNCO, and CH3OH provide rotation temperatures of 278$^{+39}_{-30}$, 297$^{+52}_{-39}$, and 154$^{+73}_{-37}$ K, respectively, for R ≲ 1700 au, suggesting that the central source of MCN-a has an intrinsic bolometric luminosity of ∼106 L⊙. These results have revealed the structure and kinematics of MCN-a at its intermediate radii. With no broad-line H30α emission detected, MCN-a may be in the earliest phase of massive star formation.

Discs and outflows in the early phases of massive star formation: Influence of magnetic fields and ambipolar diffusion

Context. Massive star formation remains one of the most challenging problems in astrophysics, as illustrated by the fundamental issues of the radiative pressure barrier and the initial fragmentation. The wide variety of physical processes involved, in particular the protostellar radiative feedback, increase the complexity of massive star formation in comparison with its low-mass counterpart. Aims. We aim to study the details of mass accretion and ejection in the vicinity of massive star forming cores using high-resolution (5 au) three-dimensional numerical simulations. We investigated the mechanisms at the origin of outflows (radiative force versus magnetic acceleration). We characterised the properties of the disc forming around massive protostars depending on the physics included: hydrodynamics, magnetic fields, and ambipolar diffusion. Methods. We used state-of-the-art three-dimensional adaptive-mesh-refinement models of massive dense core collapse, which integrate the equations of (resistive) grey radiation magnetohydrodynamics, and include sink particle evolution. For the first time, we include both protostellar radiative feedback via pre-main-sequence evolutionary tracks and magnetic ambipolar diffusion. To determine the role of magnetic fields and ambipolar diffusion play in the formation of outflows and discs, we studied three different cases: a purely hydrodynamical run, a magnetised simulation under the ideal approximation (perfect coupling), and a calculation with ambipolar diffusion (resistive case). In the most micro-physically complex model (resistive MHD), we also investigated the effect the initial amplitude of both magnetic field and solid body rotation have on the final properties of the massive protostellar system. We used simple criteria to identify the outflow and disc material and follow their evolution as the central star accretes mass up to 20 M⊙ in most of our models. The radiative, magnetic, and hydrodynamical properties of the outflows and discs are quantitatively measured and cross-compared between models. Results. Massive stars form in all our models, together with outflows and discs. The outflow is completely different when magnetic fields are introduced, so magneto-centrifugal processes are the main driver of the outflow up to stellar masses of 20 M⊙. Then, the disc properties heavily depend on the physics included. In particular, the disc formed in the ideal and resistive runs show opposite properties in terms of plasma beta; that is, the ratio of thermal-to-magnetic pressures and of magnetic field topology. While the disc in the ideal case is dominated by the magnetic pressure and the toroidal magnetic fields, the one formed in the resistive runs is dominated by the thermal pressure and essentially has a vertical magnetic field in the inner regions (R &lt; 100−200 au). Conclusions. We find that magnetic processes dominate the early evolution of massive protostellar systems (M⋆ &lt; 20 M⊙) and shapes the accretion and ejection as well as the disc formation. Ambipolar diffusion is mainly at work at disc scales and regulates its properties. We predict magnetic field’s topology within the disc and outflows, as well as disc masses and radii to be compared with observations. Lastly, our finding for the outflow and disc properties are reminiscent of the low-mass star formation framework, suggesting that accretion and ejection in young massive and low-mass protostars are regulated by the same physical processes in the early stages.

SiO Outflows as Tracers of Massive Star Formation in Infrared Dark Clouds

Abstract To study the early phases of massive star formation, we present ALMA observations of SiO(5−4) emission and VLA observations of 6 cm continuum emission toward 32 Infrared Dark Cloud clumps, spatially resolved down to ≲0.05 pc. Out of the 32 clumps, we detect SiO emission in 20 clumps, and in 11 of them the SiO emission is relatively strong and likely tracing protostellar outflows. Some SiO outflows are collimated, while others are less ordered. For the six strongest SiO outflows, we estimate basic outflow properties. In our entire sample, where there is SiO emission, we find 1.3 mm continuum and infrared emission nearby, but not vice versa. We build the spectral energy distributions (SEDs) of cores with 1.3 mm continuum emission and fit them with radiative transfer models. The low luminosities and stellar masses returned by SED fitting suggest these are early-stage protostars. We see a slight trend of increasing SiO line luminosity with bolometric luminosity, which suggests more powerful shocks in the vicinity of more massive YSOs. We do not see a clear relation between the SiO luminosity and the evolutionary stage indicated by L/M. We conclude that, as a protostar approaches a bolometric luminosity of ∼102 L ⊙, the shocks in the outflow are generally strong enough to form SiO emission. The VLA 6 cm observations toward the 15 clumps with the strongest SiO emission detect emission in four clumps, which is likely from shock-ionized jets associated with the more massive ones of these protostellar cores.

ATLASGAL-selected massive clumps in the inner Galaxy

Context. Deuteration has been used as a tracer of the evolutionary phases of low- and high-mass star formation. The APEX Telescope Large Area Survey (ATLASGAL) provides an important repository for a detailed statistical study of massive star-forming clumps in the inner Galactic disc at different evolutionary phases. Aims. We study the amount of deuteration using NH2D in a representative sample of high-mass clumps discovered by the ATLASGAL survey covering various evolutionary phases of massive star formation. The deuterium fraction of NH3 is derived from the NH2D 111−101 ortho transition at ~86 GHz and NH2D 111−101 para line at ~110 GHz. This is refined for the first time by measuring the NH2D excitation temperature directly with the NH2D 212–202 para transition at ~74 GHz. Any variation of NH3 deuteration and ortho-to-para ratio with the evolutionary sequence is analysed. Methods. Unbiased spectral line surveys at 3 mm were conducted towards ATLASGAL clumps between 85 and 93 GHz with the Mopra telescope and from 84 to 115 GHz using the IRAM 30m telescope. A subsample was followed up in the NH2D transition at 74 GHz with the IRAM 30m telescope. We determined the deuterium fractionation from the column density ratio of NH2D and NH3 and measured the NH2D excitation temperature for the first time from the simultaneous modelling of the 74 and 110 GHz line using MCWeeds. We searched for trends in NH3 deuteration with the evolutionary sequence of massive star formation. We derived the column density ratio from the 86 and 110 GHz transitions as an estimate of the NH2D ortho-to-para ratio. Results. We find a large range of the NH2D to NH3 column density ratio up to 1.6 ± 0.7 indicating a high degree of NH3 deuteration in a subsample of the clumps. Our analysis yields a clear difference between NH3 and NH2D rotational temperatures for a fraction. We therefore advocate observation of the NH2D transitions at 74 and 110 GHz simultaneously to determine the NH2D temperature directly. We determine a median ortho-to-para column density ratio of 3.7 ± 1.2. Conclusions. The high detection rate of NH2D confirms a high deuteration previously found in massive star-forming clumps. Using the excitation temperature of NH2D instead of NH3 is needed to avoid an overestimation of deuteration. We measure a higher detection rate of NH2D in sources at early evolutionary stages. The deuterium fractionation shows no correlation with evolutionary tracers such as the NH3 (1,1) line width, or rotational temperature.

On the duration of the embedded phase of star formation

ABSTRACT Feedback from massive stars plays a key role in molecular cloud evolution. After the onset of star formation, the young stellar population is exposed by photoionization, winds, supernovae, and radiation pressure from massive stars. Recent observations of nearby galaxies have provided the evolutionary timeline between molecular clouds and exposed young stars, but the duration of the embedded phase of massive star formation is still ill-constrained. We measure how long massive stellar populations remain embedded within their natal cloud, by applying a statistical method to six nearby galaxies at $20{-}100~\mbox{${\rm ~pc}$}$ resolution, using CO, Spitzer 24$\rm \, \mu m$, and H α emission as tracers of molecular clouds, embedded star formation, and exposed star formation, respectively. We find that the embedded phase (with CO and 24$\rm \, \mu m$ emission) lasts for 2−7 Myr and constitutes $17{-}47{{\ \rm per\ cent}}$ of the cloud lifetime. During approximately the first half of this phase, the region is invisible in H α, making it heavily obscured. For the second half of this phase, the region also emits in H α and is partially exposed. Once the cloud has been dispersed by feedback, 24$\rm \, \mu m$ emission no longer traces ongoing star formation, but remains detectable for another 2−9 Myr through the emission from ambient CO-dark gas, tracing star formation that recently ended. The short duration of massive star formation suggests that pre-supernova feedback (photoionization and winds) is important in disrupting molecular clouds. The measured time-scales do not show significant correlations with environmental properties (e.g. metallicity). Future JWST observations will enable these measurements routinely across the nearby galaxy population.

Probing the early phases of high-mass star formation with 6.7 GHz methanol masers

ABSTRACT Methanol masers at 6.7 GHz are the brightest of class II methanol masers and have been found exclusively towards massive star-forming regions. These masers can thus be used as a unique tool to probe the early phases of massive star formation. We present here a study of the spectral energy distributions of 320 6.7 GHz methanol masers chosen from the Methanol Multibeam catalogue, which fall in the Hi-GAL range (|l| ≤ 60°, |b| ≤ 1°). The spectral energy distributions are constructed from 870 to 70 µm using data from the ATLASGAL and Hi-GAL surveys. The emission from cold dust is modelled by a single grey body component fit. We estimate the clump properties such as mass, far-infrared luminosity, and column density using the best-fitting parameters of the SED fits. Considering the Kauffman criteria for massive star formation, we find that all but a few maser hosts have the potential to harbour at least one high-mass star. The physical properties of the methanol maser hosts are also discussed. The evolutionary stages of 6.7 GHz maser sources, explored using the mass luminosity diagram, suggest that they are predominantly associated with high-mass stars with the majority being in the accretion phase. However, we observe a small number of sources that could possibly be related to intermediate- or low-mass stars.

Rest-frame UV properties of luminous strong gravitationally lensed Lyα emitters from the BELLS GALLERY Survey

ABSTRACT We present deep rest-frame UV spectroscopic observations using the Gran Telescopio Canarias of six gravitationally lensed Lyα emitters (LAEs) at 2.36 &amp;lt; z &amp;lt; 2.82 selected from the BELLS GALLERY survey. By taking the magnifications into account, we show that LAEs can be as luminous as LLyα ≃ 30 × 1042 erg s−1 and MUV ≃ −23 (AB) without invoking an AGN component, in contrast with previous findings. We measure Lyα rest-frame equivalent widths, $EW_{0}\,\rm (Ly\alpha)$, ranging from 16 to 50 Å and Lyα escape fractions, $f_{\rm esc}\, \rm (Ly\alpha)$, from 10 per cent to 40 per cent. Large $EW_{0}\, \rm (Ly\alpha)$ and $f_{\rm esc}\, \rm (Ly\alpha)$ are found predominantly in LAEs showing weak low-ionization ISM absorption (EW0 ≲ 1 Å) and narrow Lyα profiles (≲300 km s−1 FWHM) with their peak close (≲80 km s−1) to their systemic redshifts, suggestive of less scatter from low H i column densities that favours the escape of Lyα photons. We infer stellar metallicities of Z/Z⊙ ≃ 0.2 in almost all LAEs by comparing the P-Cygni profiles of the wind lines N v1240 Å and C iv1549 Å with those from stellar synthesis models. We also find a trend between MUV and the velocity offset of ISM absorption lines, such as the most luminous LAEs experience stronger outflows. The most luminous LAEs show star formation rates up to ≃180 M⊙ yr−1, yet they appear relatively blue (βUV ≃ −1.8 to −2.0) showing evidence of little dust attenuation [E(B − V) = 0.10–0.14]. These luminous LAEs may be particular cases of young starburst galaxies that have had no time to form large amounts of dust. If so, they are ideal laboratories to study the early phase of massive star formation, stellar and dust mass growth, and chemical enrichment histories of starburst galaxies at high-z.

Simultaneous 22 GHz Water and 44 GHz Methanol Maser Survey of Ultracompact H ii Regions

We have carried out a multi-epoch simultaneous survey of 22 GHz H$_2$O and 44 GHz Class I CH$_3$OH masers toward 103 ultracompact HII regions (UCHIIs) between 2010 and 2017. We detected H$_2$O and CH$_3$OH maser emission in 74 (72%) and 55 (53%) UCHIIs, respectively. Among them, 3 H$_2$O and 27 CH$_3$OH maser sources are new detections. These high detection rates suggest that the occurrence periods of both maser species are significantly overlapped with the UCHII phase of massive star formation. The CH$_3$OH maser lines always have small (< 10 km s$^{-1}$) relative velocities with respect to the natal molecular cores, while H$_2$O maser lines often show larger relative velocities. Twenty four H$_2$O maser-detected sources have maser lines at relative velocities > 30 km s$^{-1}$, and thirteen of them show extremely high-velocity (> 50 km s$^{-1}$) components. The appearance and disappearance of H$_2$O maser lines are quite frequent over six-month or one-year time intervals. In contrast, CH$_3$OH maser lines usually do not exhibit significant variations in the line intensity, velocity, or shape for the same periods. The isotropic luminosities of both masers appear to correlate with the bolometric luminosities of the central stars. This correlation becomes much stronger in the case that data points in the low- and intermediate-mass regimes are added. The maser luminosities also tend to increase with the radio continuum luminosities of UCHIIs and the submillimeter continuum luminosities of the associated dense cores.

Searching for Inflow toward Massive Starless Clump Candidates Identified in the Bolocam Galactic Plane Survey

Abstract Recent Galactic plane surveys of dust continuum emission at long wavelengths have identified a population of dense, massive clumps with no evidence for ongoing star formation. These massive starless clump candidates are excellent sites to search for the initial phases of massive star formation before the feedback from massive star formation affects the clump. In this study, we search for the spectroscopic signature of inflowing gas toward starless clumps, some of which are massive enough to form a massive star. We observed 101 starless clump candidates identified in the Bolocam Galactic Plane Survey (BGPS) in J = 1−0 using the 12 m Arizona Radio Observatory telescope. We find a small blue excess of for the complete survey. We identified six clumps that are good candidates for inflow motion and used a radiative transfer model to calculate mass inflow rates that range from 500 to 2000 Myr−1. If the observed line profiles are indeed due to large-scale inflow motions, then these clumps will typically double their mass on a freefall time. Our survey finds that massive BGPS starless clump candidates with inflow signatures in J = 1−0 are rare throughout our Galaxy.

NGC 6334 and NGC 6357

Aims. The formation of high-mass stars is still debated. For this reason, several projects such as Herschel-HOBYS are focussed on the study of the earliest phases of massive star formation. As a result, massive star-forming complexes such as NGC 6334 and NGC 6357 have been observed in the far-infrared to study their massive dense cores where massive stars are expected to form. However, to better characterise the environments of these cores we need to understand the previous massive star formation history. To better characterise the environment of these massive dense cores we study the previous high-mass star formation and how these stars act on their environments. Methods. This study is based on the spectral classification of the OB stars identified towards NGC 6334 and NGC 6357 with spectra taken with the AAOmega spectrograph on the Anglo-Australian Telescope (AAT). From the subsequent spectral classification of 109 stars across these regions we were able to evaluate the following: distance, age, mass, global star-forming efficiency (SFE), and star formation rate (SFR) of the regions. The physical conditions of the ionised gas for both complexes was also derived. Results. We confirm that NGC 6334 and NGC 6357 belong to the Saggitarius-Carina arm which, in this direction, extends from 1 kpc to 2.2 kpc. From the location of the stars in Hertzprung-Russell diagram we show that stars older than ∼10 Myr are broadly spread across these complexes, while younger stars are mainly located in the H ii regions and stellar clusters. Our data also suggests that some of the young stars can be considered runaway stars. We evaluate a SFE of 0.019 +0.008 −0.007 and 0.021 +0.004 −0.003 and a SFR of 1.1 × 10 3 ± 300 M Myr −1 and 1.7 × 10 3 ± 400 M Myr −1 for NGC 6334 and NGC 6357, respectively. We note that 29 OB stars have X-ray counterparts, most of them belonging to NGC 6357. This suggests that molecular clouds in NGC 6357 are more impacted by X-ray flux and stellar winds than in NGC 6334. Finally, from the analysis of nebular lines (Hα, [NII], and [SII]) from spectra from several regions of ionised gas, we confirm that the filaments in NGC 6357 are shock heated.

Quenching of expanding outflow in massive star-forming region W75N(B)-VLA 2

AbstractVLBI observation of masers is a powerful mean to understand the early evolutionary phase of massive star formation. A few different scenarios of outflow evolution in the massive protostars have been proposed, and cannot be readily examined because the precise timing of appropriate maser phenomena is difficult. In particular, it has been a matter of debate whether a well-collimated or a less-collimated outflow comes first in the very early phase of the massive protostellar evolution. Long-term, multi-epoch VLBI monitoring is probably the most important method to trace the outflow evolution. Such a monitoring of a massive star-forming region W75N(B) has been very successful. Since the first detection of the expanding water maser shell associated with the star-forming region VLA 2 of W75N(B) in 1999, the observations in 2005 and 2007 displayed that the expanding water maser shell has been evolved to well-collimated from a less collimated morphology. Observations in 2012 also confirmed such a transition. It would be a major breakthrough in our knowledge of the formation and evolution of the first stages of massive protostars. We performed multi-epoch VLBI observations in mid-2014. On the contrary to its expansion for 13 years, the maser shell at VLA 2 observed in 2014 is comparable to the size observed in 2012. The quenching of the maser shell size indicates that the previously expanding outflow has been decelerated plausibly due to the interaction with surrounding interstellar medium.

Probing Early Phases of High Mass Stars with 6.7 GHz Methanol Masers

AbstractMethanol masers at 6.7 GHz are the brightest of class II methanol masers and have been found exclusively towards massive star forming regions. These masers can thus be used as a unique tool to probe the early phases of massive star formation. We present here the SED studies of 284 methanol masers chosen from the MMB catalogue, which falls in the Hi-GAL range (|l| ≤ 60°, |b| ≤ 1°). The masers are studied using the ATLASGAL, MIPSGAL and Hi-GAL data at wavelengths ranging from 24−870 micrometers. A single grey body component fit was used to model the cold dust emission whereas the emission from the warm dust is modelled by a black body. The clump properties such as isothermal mass, FIR luminosity and MIR luminosity were obtained using the best fit parameters of the SED fits. We discuss the physical properties of the sources and explore the evolutionary stages of the sources having 6.7 GHz maser emission in the timeline of high mass star formation.

Perspectives on star formation: the formation of high-mass stars

AbstractThe formation process of high-mass stars has puzzled the astrophysical community for decades from both a theoretical and an observational point of view. Here, we present an overview of the current theories and status of the observational research on this field, outlining the progress achieved in recent years on our knowledge of the initial phases of massive star formation, the fragmentation of cold, infrared-dark clouds, and the evidence for circumstellar accretion disks around OB stars. The role of masers in helping us to understand the mechanism leading to the formation of a high-mass star are also discussed.

AN ORDERED BIPOLAR OUTFLOW FROM A MASSIVE EARLY-STAGE CORE

ABSTRACT We present ALMA follow-up observations of two massive, early-stage core candidates, C1-N and C1-S, in IRDC G028.37+00.07, that were previously identified by their N 2 D + (3-2) emission, and show high levels of deuteration of this species. The cores are also dark at far-infrared wavelengths up to ∼ 100 μ m . We detect 12CO(2-1) from a narrow, highly collimated bipolar outflow that is being launched from near the center of the C1-S core, which is also the location of the peak 1.3 mm dust continuum emission. This protostar, C1-Sa, has associated dense gas traced by C 18 O (2-1) and DCN(3-2), from which we estimate that it has a radial velocity that is near the center of the range exhibited by the C1-S massive core. A second outflow-driving source is also detected within the projected boundary of C1-S, but it appears to be at a different radial velocity. After considering the properties of the outflows, we conclude that C1-Sa is a promising candidate for an early-stage massive protostar and as such it shows that these early phases of massive star formation can involve highly ordered outflow, and thus accretion, processes, similar to models developed to explain low-mass protostars.

PHYSICAL CONDITIONS OF THE EARLIEST PHASES OF MASSIVE STAR FORMATION: SINGLE-DISH AND INTERFEROMETRIC OBSERVATIONS OF AMMONIA AND CCS IN INFRARED DARK CLOUDS

Infrared Dark Clouds (IRDCs) harbor the earliest phases of massive star formation, and many of the compact cores in IRDCs, traced by millimeter continuum or by molecular emission in high critical density lines, host massive young stellar objects (YSOs). We used the Robert C. Byrd Green Bank Telescope (GBT) and the Karl G. Jansky Very Large Array (VLA) to map NH$_{3}$ and CCS in nine IRDCs to reveal the temperature, density, and velocity structures and explore chemical evolution in the dense ($>10^{22}$ cm$^{-2}$) gas. Ammonia is an excellent molecular tracer for these cold, dense environments. The internal structure and kinematics of the IRDCs include velocity gradients, filaments, and possibly colliding clumps that elucidate the formation process of these structures and their YSOs. We find a wide variety of substructure including filaments and globules at distinct velocities, sometimes overlapping at sites of ongoing star formation. It appears that these IRDCs are still being assembled from molecular gas clumps even as star formation has already begun, and at least three of them appear consistent with the morphology of ``hub-filament structures'' discussed in the literature. Furthermore, we find that these clumps are typically near equipartition between gravitational and kinetic energies, so these structures may survive for multiple free-fall times.Keywords: molecular data -- ISM: clouds -- (ISM:) dust, extinction -- ISM: molecules -- Stars: formation -- radio lines: ISM

The Spectral Energy Distribution of the Earliest Phases of Massive Star Formation

AbstractWe have selected cold and massive (M &gt; 100M⊙) cores as candidates for early phases of star formation from millimeter continuum surveys without associations at short wavelengths. We compared the millimeter continuum peak positions with IR and radio catalogs and excluded cores that had sources associated with the cores’ peaks. We compiled a list of 173 cores in over 117 regions that are candidates for very early phases of Massive Star Formation (MSF). Now with the Spitzer and Herschel archives, these cores can be characterized further. We are compiling this data set to construct the complete spectral energy distribution (SED) in the mid- and far-infrared with good spatial resolution and broad spectral coverage. This allow us to disentangle the complex regions and model the SED of the deeply embedded protostars/clusters. We present a status report of our efforts: a preview of the IR properties of all cores and their embedded source inferred from a grey body fit to the compiled SEDs.