Silvia Leurini

Context . Observing the earliest stages of high-mass star formation in the mid-infrared (mid-IR) is challenging because the extinction is high and the resolution limited. Millimeter vibrationally and torsionally excited transitions of several molecules have been proposed as indirect probes of local mid-IR radiation fields around young stellar objects (YSOs). Aims . We investigate the excitation and spatial distribution of torsionally excited methanol (CH ₃ OH) in 12 high-mass star-forming clumps in different evolutionary stages, assessing their potential as tracers of the mid-IR luminosity of embedded protostars. Methods . We analyzed rotational transitions of methanol within the same torsionally excited states v _t ≥1 in the Atacama Large Millimeter/submillimeter Array (ALMA) 890 µm observations and compared them with Spitzer 24 µm data by plotting line luminosities normalized by methanol abundance against mid-IR luminosities. Results . The torsionally excited methanol emission is generally compact and systematically coincident with ALMA dust continuum peaks and 24 µm emission, even in faint sources. The only clump without an IR counterpart also shows no v _t ≥1 emission, supporting radiative pumping as the dominant excitation mechanism. We found a promising power-law trend between v _t ≥1 methanol lines and 24 µm luminosities, largely independent of the methanol abundance. Conclusions . CH ₃ OH v _t ≥1 transitions appear to be promising tracers of local mid-IR radiation fields of embedded protostars, providing core-scale information even in highly extincted regions. As a pilot study, our results suggest that extending our study to a larger statistical sample is crucial for testing and improving this method.

Context . Although there has been significant progress, the physical properties and potential fragmentation of accretion disks around high-mass protostars remain poorly constrained. Aims . We characterize at high angular resolution one of the most nearby (~700 pc) high-mass accretion disk candidates, CepA HW2. Methods . Using the new long-baseline array configuration (~1700 m) of the Northern Extended Millimeter Array (NOEMA), we study CepA HW2 with a resolution of ≤0.2″ or ≤100 au at 1.3 mm in dust continuum and spectral line emission. Results . The millimeter continuum emission resolves the central disk candidate into several substructures. Conducting a Toomre Q stability analysis based on CH ₃ CN and continuum data, and a comparison to 3D radiation hydrodynamic simulations shows that the data are consistent with an almost edge-on disk whereby the observed substructures may represent fragments within the disk. The CO and SiO spectral line data confirm a second bipolar outflow (in addition to the well-known jet) emanating from the central peak position. This indicates that this central peak should host at least a binary if not even a higher-order multiple system. The usually assumed dense gas tracer CH ₃ CN also shows contributions from the outflows, which complicates further kinematic analysis of the disk. Conclusions . The high-resolution outflow-disk data of CepA reveal a multiply fragmented disk that drives several outflows. These observations reinforce the picture of high-mass star formation in which multiplicity and fragmentation can happen on the smallest spatial scales related to the inner accretion disks.

Magnetohydrodynamic (MHD) disk winds (DWs) can be responsible for removing angular momentum from the accretion disks of protostars, for driving mass accretion and disk evolution and dispersal, and, ultimately, for setting the conditions for planet formation. We want to investigate two massive young stellar objects (YSOs), and where evidence for MHD DWs has been previously obtained at scales of 10--100 au through measurements of the 22 GHz water maser velocity distribution within the Protostellar Outflows at the EarliesT Stages (POETS) survey. The different mass, 5.6±2 and ≈20 of the two YSOs allows us to investigate the dependence of MHD DWs on the YSO environment, too. We employed IRAM Northern Extended Millimeter Array and archival Atacama Large Millimeter Array observations of and respectively, to study the gas kinematics and physical conditions of the corresponding protostellar winds on scales of 100--1000 au using the same molecular tracers. In the emissions of several molecules, particularly SO, SO_2, CH_3CN, and CH_3OH, are distributed along the axis of the previously identified radio jet, and present a local standard of rest (LSR) velocity ( gradient transversal to the jet axis. Position-velocity (PV) plots of the SO lines show patterns consistent with Keplerian rotation. The SO_2 emission comes from high-velocity gas flowing close to the jet axis, while CH_3CN and CH_3OH present a larger radial extension than the S-bearing species. In the same molecules are instead distributed along the (sky-projected) major axis of the rotating disk reported previously, and their gradients consistently trace the disk rotation. The corresponding PV plots present Keplerian profiles. SO is the only molecular species whose emission extends well outside the disk. Our analysis shows that, in both YSOs, the spatial and velocity distributions of SO are consistent with a rotating wind magneto-centrifugally launched from the YSO disk. The comparison with models of molecule formation and excitation in shocks indicates that the different radial extension of the molecular species observed in the protostellar wind of as well as the lack of molecules, except SO, in the wind, can be well explained in terms of a radially extended MHD DW, rather than a compact X-wind. This study provides compelling evidence that the SO and SO_2 emissions can be used to trace MHD DWs in luminous YSOs at scales of 100--1000 au, and confirms the results derived with water masers at scales of 10--100 au.

Context. Filamentary accretion flows as gas-funneling mechanisms are a key aspect in high-mass star formation research. The kinematic properties along these structures are of particular interest. Aims. This paper focuses on the question of whether gas is transported to dense clumps inside high-mass star-forming regions through filamentary structures, from scales of several parsecs down to the subparsec scale. Methods. We quantified the gas flows from a scale of up to several parsecs down to the subparsec scale along filamentary structures. For this work the accretion flow mechanisms based on gas kinematic data in the three high-mass star-forming regions G75.78, IRAS21078+5211, and NGC7538 were studied with data obtained from the IRAM 30 m telescope. The analysis was carried out using the surface density derived from 1.2 mm continuum emission and velocity differences estimated from HCO ⁺ (1 − 0) and H ¹³ CO ⁺ (1 − 0) molecular line data. Results. The mass flow behavior of the gas in the vicinity of high-mass star-forming clumps shows characteristic dynamical patterns, for example an increased mass flow rate toward the clumps. We assume that the velocity differences originate from filamentary-gas infall onto the high-mass star-forming clumps; however, the inclination of the filament structures along the line of sight is unknown. Nevertheless, using the velocity differences and mass surface densities, we can estimate the mean flow rates along the filamentary structures with respect to the line of sight and toward the clumps. We quantified the flow rates toward the clumps in a range from about 10 ⁻³ M _⊙ yr ⁻¹ to 10 ⁻⁵ M _⊙ yr ⁻¹ , inferred from clump-centered polar plots. Slight variations in the flow rates along the filamentary structures may be caused by overdensities and velocity gradients along the filaments. Conclusions. While the initial studies presented here already reveal interesting results such as an increasing mass flow rate toward clumps, the properties of filamentary gas flows from large to small spatial scales, as well as potential variations over the evolutionary sequence, are subject to future studies.

Context. Almost all the physics of star formation critically depends on the number density of the molecular gas involved. However, the methods to estimate this keystone property often rely on very uncertain assumptions about the geometry of the molecular fragment, or depend on overly simplistic, uniform models, or require time-expensive observations to simultaneously constrain the gas temperature as well. An easy-to-use method to observationally derive the number density that is valid under realistic conditions is conspicuously absent, causing an evident asymmetry in how accurately the volume density is estimated, and how often dedicated tracers are used, compared to the gas temperature. Aims. To fill this gap, we propose and calibrate a versatile diagnostic tool based on methanol spectral lines that greatly simplifies the inference of molecular number density. Methanol is abundant in both cold and hot gas, and has a dense spectrum of lines, which maximises observational efficiency. It can therefore be applied to a wide variety of scales, from entire clouds to protostellar discs, and both in our Galaxy and beyond. Moreover, this tool does not need to be tailored to the specific source properties (such as distance, temperature, and mass). Methods. We construct large grids of clump models and perform radiative transfer calculations to investigate the robustness of different line ratios as density probes with different assumptions, also in the presence of density and temperature gradients. Results. We find that the line ratios of the (2_K − 1_K) band transitions around 96.7 GHz are able to fully constrain the average number density along the line of sight within a factor of two-three in the range ~5 × 10⁴−3 × 10⁷ cm⁻³. The range can be extended down to a few times 10³ cm⁻³, when also using line ratios from the (5_K − 4_K) and/or (7_K − 6_K) bands, around 241.7 GHz and 338.1 GHz, respectively. We provide the reader with practical analytic formulas and a numerical method for deriving volume density and its uncertainty from observed values of the line ratios. Conclusions. Thanks to our calibration of line ratios, we make the estimate of the number density much simpler, with an effort comparable or inferior to deriving excitation temperatures. By providing directly applicable recipes that do not require the creation of a full large velocity gradient model grid, but are equally accurate, we contribute to offsetting the disparity between these two fundamental parameters of the molecular gas. Applying our method to a sub-sample of sources from the ATLASGAL TOP100 we show that the material in the clumps is being compressed, and this compression accelerates in the latest stages.

Aims. W43-main is a massive molecular complex undergoing starburst activities, located at the interaction of the Scutum arm and the Galactic bar. We aim to investigate the gas dynamics, in particular, the prevailing shock signatures from cloud to clump scales. We also look to assess the impact of shocks on the formation of dense gas and early-stage cores in OB cluster formation processes. Methods. We carried out NOEMA and IRAM-30 m observations at 3 mm towards five molecular gas clumps in W43 main located within large-scale interacting gas components. We used CH₃CCH and H₂CS lines to trace the extended gas temperature and CH₃OH lines to probe the volume density of the dense gas components (≳10⁵ cm⁻³). We adopted multiple tracers that are sensitive to different gas density regimes to reflect the global gas motions. The density enhancements constrained by CH₃OH and a population of NH₂D cores are correlated (in the spatial and velocity domains) with SiO emission, which is a prominent indicator of shock processing in molecular clouds. Results. The emission of SiO (2–1) is extensive across the region (~4 pc) and it is contained within a low-velocity regime, hinting at a large-scale origin for the shocks. Position-velocity maps of multiple tracers show systematic spatio-kinematic offsets supporting the cloud-cloud collision-merging scenario. We identified an additional extended velocity component in the CCH emission, which coincides with one of the velocity components of the larger scale ¹³CO (2−1) emission, likely representing an outer, less-dense gas layer in the cloud merging process. We find that the ‘V-shaped’, asymmetric SiO wings are tightly correlated with localised gas density enhancements, which is direct evidence of dense gas formation and accumulation in shocks. The dense gas that is formed in this way may facilitate the accretion of the embedded, massive pre-stellar and protostellar cores. We resolved two categories of NH₂D cores: those exhibiting only subsonic to transonic velocity dispersions and those with an additional supersonic velocity dispersion. The centroid velocities of the latter cores are correlated with the shock front seen via SiO. The kinematics of the ~0.1 pc NH₂D cores are heavily imprinted by shock activities and may represent a population of early-stage cores forming around the shock interface.

In the last years there has been a substantial increase in the number of the reported massive and luminous star-forming regions with related explosive outflows thanks to the superb sensitivity and angular resolution provided by the new radio, infrared, and optical facilities. Here, we report one more explosive outflow related with the massive and bright star-forming region IRAS 12326−6245 using Band 6 sensitive and high-angular-resolution (∼0.″2) Atacama Large Millimeter/Submillimeter Array observations. We find over 10 molecular and collimated well-defined streamers, with Hubble–Lemaitre–like expansion motions, and pointing right to the center of a dusty and molecular shell (reported for the first time here) localized in the northern part of the UC H ii region known as G301.1A. The estimated kinematic age and energy for the explosion are ∼700 yr and 10⁴⁸ erg, respectively. Taking into account the recently reported explosive outflows together with IRAS 12326−6245, we estimate an event rate of once every 90 yr in our Galaxy, similar to the formation rate of massive stars.

Context. The fragmentation mode of high-mass molecular clumps and the accretion processes that form the most massive stars (M ≳ 8 M_⊙) are still not well understood. A growing number of case studies have found massive young stellar objects (MYSOs) to harbour disk-like structures, painting a picture that the formation of high-mass stars may proceed through disk accretion, similar to that of lower-mass stars. However, the properties of such structures have yet to be uniformly and systematically characterised. Aims. The aim of this work is to uniformly study the kinematic properties of a large sample of MYSOs and characterise the stability of possible circumstellar disks against gravitational fragmentation. Methods. We have undertaken a large observational programme (CORE) making use of interferometric observations from the Northern Extended Millimetre Array (NOEMA) for a sample of 20 luminous (L > 10⁴ L_⊙) protostellar objects in the 1.37 mm wavelength regime in both continuum and spectral line emission, reaching 0.4″ resolution (800 au at 2 kpc). Results. We present the gas kinematics of the full sample and detect dense gas emission surrounding 15 regions within the CORE sample. Using the dense gas tracer CH₃CN, we find velocity gradients across 13 cores perpendicular to the directions of bipolar molecular outflows, making them excellent disk candidates. The extent of the CH₃CN emission tracing the disk candidates varies from 1800 to 8500 au. Analysing the free-fall to rotational timescales, we find that the sources are rotationally supported. The rotation profiles of some disk candidates are well described by differential rotation while for others the profiles are poorly resolved. Fitting the velocity profiles with a Keplerian model, we find protostellar masses in the range of ~ 10–25 M_⊙. Modelling the level population of CH₃CN (12_K–11_K) K = 0–6 lines, we present temperature maps and find median temperature in the range 70–210 K with a diversity in distributions. Radial profiles of the specific angular momentum (j) for the best disk candidates span a range of 1–2 orders of magnitude, on average ~10⁻³ km s⁻¹ pc, and they follow j ∝ r^1.7, which is consistent with a poorly resolved rotating and infalling envelope-disk model. Studying the Toomre stability of the disk candidates, we find almost all (11 out of 13) disk candidates to be prone to fragmentation due to gravitational instabilities at the scales probed by our observations, as a result of their high disk to stellar mass ratio. In particular, disks with masses greater than ~ 10–20% of the mass of their host (proto)stars are Toomre unstable, and more luminous YSOs tend to have disks that are more massive compared to their host star and hence more prone to fragmentation. Conclusions. In this work, we show that most disk structures around high-mass YSOs are prone to disk fragmentation early in their formation due to their high disk to stellar mass ratio. This impacts the accretion evolution of high-mass protostars which will have significant implications for the formation of the most massive stars.

Context. We exploit the increased sensitivity of the recently installed adaptive optics SOUL at the LBT to obtain new high-spatial-resolution near-infrared images of the massive young stellar object IRAS20126+4104 and its outflow. Aims. We aim to derive the jet proper motions and kinematics, as well as to study its photometric variability by combining the novel performances of SOUL together with previous near-infrared images. Methods. We used both broad-band (K_s, K′) and narrow-band (Brγ, H2) observations from a number of near-infrared cameras (UKIRT/UFTI, SUBARU/CIAO, TNG/NICS, LBT/PISCES, and LBT/LUCI1) to derive maps of the continuum and the H₂ emission in the 2.12 µm line. Three sets of images, obtained with adaptive optics (AO) systems (CIAO, in 2003; FLAO, in 2012; SOUL, in 2020), allowed us to derive the proper motions of a large number of H₂ knots along the jet. Photometry from all images was used to study the jet variability. Results. We derived knot proper motions in the range of 1.7–20.3 mas yr⁻¹ (i.e. 13–158 km s⁻¹ at 1.64 kpc), implying an average outflow tangential velocity of ~80 km s⁻¹. The derived knot dynamical age spans a ~200–4000 yr interval. A ring-like H₂ feature near the protostar location exhibits peculiar kinematics and may represent the outcome of a wide-angle wind impinging on the outflow cavity. Both H₂ geometry and velocities agree with those inferred from proper motions of the H₂O masers, located at a smaller distance from the protostar. Although the total H₂ line emission from the knots does not exhibit time variations at a ⪞0.3 mag level, we have found a clear continuum flux variation (radiation scattered by the dust in the cavity opened by the jet) which is anti-correlated between the blue-shifted and red-shifted lobes and may be periodic (with a period of ~12–18 yr). We suggest that the continuum variability might be related to inner-disc oscillations which have also caused the jet precession. Conclusions. Our analysis shows that multi-epoch high-spatial-resolution imaging in the near-infrared is a powerful tool to unveil the physical properties of highly embedded massive protostars.