Accretion and outflow structures within 1000 AU from high-mass protostars with ALMA longest baselines

Understanding the formation of massive stars is one of the unsolved problems in modern astronomy. The main difficulty is that the intense radiation from the high-luminosity stars and the thermal pressure from the resulting ionized gas (both insignificant for low-mass stars) may be able to reverse the accretion flow and prevent the star from accreting fresh material. Such feedback effects can naturally be mitigated if accretion proceeds through discs, which is the established mechanism to form sun-like stars. However, recent 3D MHD simulations have shown that accretion on 1000 au scales is through filaments rather than a large disc. This theoretical prediction has never been confirmed via observations owing to the poor linear resolution of previous studies (>1000 au). Here we present the first observational evidence that mass assembly in young high-mass stars forming in protoclusters is predominantly asymmetric and disordered. In particular, we observed the innermost regions around three deeply embedded high-mass protostars with very high spatial resolution (~100 au). We identified multiple massive (several solar masses), warm (50-150 Kelvin) filamentary streamers pointing onto the central sources, which we interpret as multi-directional accretion channels. These structures inhibit the formation of a large, steady disc. Nevertheless, the identification of fast collimated outflows in the three observed systems indicates that (non-steady) compact discs may be present (we measure upper limits on their radii of <80 for one object and <350 astronomical units for the remaining two objects). Our finding contrasts with the simplified classic paradigm of an ordered (and stable) disc/jet system and provides an experimental confirmation of a multi-directional and unsteady accretion model for massive star formation supported by recent 3D (magneto)hydrodynamic simulations.Comment: Submitted to Nature on Dec 19 2017, transferred to Nature Astronomy after review on February 8 2018, rejected after a recommendation for acceptance by one reviewer, and a more critical report by a second reviewer. To be submitted to ApJ. Comments from colleagues (even critical ones) are welcom

High resolution imaging of massive young stellar objects and a sample of molecular outflow sources

This thesis contains a study of millimetre wavelength observations of massive young stellar objects (MYSOs) both via interferometric and single dish observations. First, the high angular resolution observations ( up to ∼0.1”) from a variety of interferometers of the MYSO, S140 IRS1, are presented. This source is one of only two prototypes that have ionised equatorial emission from a radiatively driven disc wind. The observations confirm that IRS1 has a dusty disc at a position angle compatible with that of the disc wind emission, and confirms the disc wind nature for the first time. Secondly, the observations of S140 IRS1 are modelled using a 2D axisymmetric radiative transfer code. Extensive models producing synthetic data at millimetre wavelengths were developed. These models show that on the largest scales, typically accessible with single dish observations or compact interferometric configurations, the spectral energy distribution is relatively unchanged by the addition of a compact dust disc. However, a disc is required to match the interferometric visibilities at the smaller scales. The position angle of the disc is well constrained via a newly developed 2D visibility fitting method. The models however, are degenerate and there are a range of realistic best fitting discs. The third section presents the single dish observations of the core material traced by C18O around 99 MYSOs and compact HII regions from the RMS survey. A method to calculate the core masses and velocity extent is reported. The method is accurate and robust, and can be applied to any molecular line emission. An updated distance limited sample contains 87 sources and is complete to 103 L⊙. It is a representative sample of MYSOs and HII regions. All of the cores harbour at least one massive protostar. Finally, methodologies to establish outflow parameters via 12CO (3-2) and 13CO (3-2) data are investigated. Multiple techniques are trialed for a well studied test source, IRAS 20126+4104, and a repeatable outflow analysis pathway is described. In more complex regions using the 12CO emission to identify outflows and determine the mass is more difficult and an alternative method is suggested. Moreover, the dynamical timescale of the outflows and the dynamical parameters are estimated in a spatial sense rather than using a simple average. Such analysis will aid in categorising different outflows from the full sample

Evidence for the start of planet formation in a young circumstellar disk

The growth of dust grains in protoplanetary disks is a necessary first step towards planet formation. This growth has been inferred via observations of thermal dust emission towards mature protoplanetary systems (age >2 million years) with masses that are, on average, similar to Neptune3. In contrast, the majority of confirmed exoplanets are heavier than Neptune. Given that young protoplanetary disks are more massive than their mature counterparts, this suggests that planet formation starts early, but evidence for grain growth that is spatially and temporally coincident with a massive reservoir in young disks remains scarce. Here, we report observations on a lack of emission of carbon monoxide isotopologues within the inner ~15 au of a very young (age ~100,000 years) disk around the Solar-type protostar TMC1A. By using the absence of spatially resolved molecular line emission to infer the gas and dust content of the disk, we conclude that shielding by millimeter-size grains is responsible for the lack of emission. This suggests that grain growth and millimeter-size dust grains can be spatially and temporally coincident with a mass reservoir sufficient for giant planet formation. Hence, planet formation starts during the earliest, embedded phases in the life of young stars.Comment: Accepted for publication in Nature Astronomy, 3 figures, 3 extended figure

Salt-bearing disk candidates around high-mass young stellar objects

Molecular lines tracing the orbital motion of gas in a well-defined disk are valuable tools for inferring both the properties of the disk and the star it surrounds. Lines that arise only from a disk, and not also from the surrounding molecular cloud core that birthed the star or from the outflow it drives, are rare. Several such emission lines have recently been discovered in one example case, those from NaCl and KCl salt molecules. We studied a sample of 23 candidate high-mass young stellar objects (HMYSOs) in 17 high-mass star-forming regions to determine how frequently emission from these species is detected. We present five new detections of water, NaCl, KCl, PN, and SiS from the innermost regions around the objects, bringing the total number of known briny disk candidates to nine. Their kinematic structure is generally disk-like, though we are unable to determine whether they arise from a disk or outflow in the sources with new detections. We demonstrate that these species are spatially coincident in a few resolved cases and show that they are generally detected together, suggesting a common origin or excitation mechanism. We also show that several disks around HMYSOs clearly do not exhibit emission in these species. Salty disks are therefore neither particularly rare in high-mass disks, nor are they ubiquitous.Comment: accepted to Ap

ALMA High-frequency Long Baseline Campaign in 2021: Highest Angular Resolution Submillimeter Wave Images for the Carbon-rich Star R Lep

The Atacama Large Millimeter/submillimeter Array (ALMA) was used in 2021 to image the carbon-rich evolved star R Lep in Bands 8-10 (397-908 GHz) with baselines up to 16 km. The goal was to validate the calibration, using band-to-band (B2B) phase referencing with a close phase calibrator J0504-1512, 1.2 deg from R Lep in this case, and the imaging procedures required to obtain the maximum angular resolution achievable with ALMA. Images of the continuum emission and the hydrogen cyanide (HCN) maser line at 890.8 GHz, from the J=10-9 transition between the (1110) and (0400) vibrationally excited states, achieved angular resolutions of 13, 6, and 5 mas in Bands 8-10, respectively. Self-calibration (self-cal) was used to produce ideal images as to compare with the B2B phase referencing technique. The continuum emission was resolved in Bands 9 and 10, leaving too little flux for self-cal of the longest baselines, so these comparisons are made at coarser resolution. Comparisons showed that B2B phase referencing provided phase corrections sufficient to recover 92%, 83%, and 77% of the ideal image continuum flux densities. The HCN maser was sufficiently compact to obtain self-cal solutions in Band 10 for all baselines (up to 16 km). In Band 10, B2B phase referencing as compared to the ideal images recovered 61% and 70% of the flux density for the HCN maser and continuum, respectively.Comment: 37 pages, 12 figures, 9 tables, accepted by ApJ (Aug 30, 2023

ALMA High-frequency Long Baseline Campaign in 2017:Band-to-band Phase Referencing in Submillimeter Waves

In 2017, an Atacama Large Millimeter/submillimeter Array (ALMA) high-frequency long baseline campaign was organized to test image capabilities with baselines up to 16 km at submillimeter (submm) wavelengths. We investigated image qualities using ALMA receiver Bands 7, 8, 9, and 10 (285-875 GHz) by adopting band-to-band (B2B) phase referencing in which a phase calibrator is tracked at a lower frequency. For B2B phase referencing, it is expected that a closer phase calibrator to a target can be used, comparing to standard in-band phase referencing. In the first step, it is ensured that an instrumental phase offset difference between low- and high-frequency Bands can be removed using a differential gain calibration in which a phase calibrator is certainly detected while frequency switching. In the next step, comparative experiments are arranged to investigate the image quality between B2B and in-band phase referencing with phase calibrators at various separation angles. In the final step, we conducted long baseline imaging tests for a quasar at 289 GHz in Band 7 and 405 GHz in Band 8 and complex structure sources of HL Tau and VY CMa at ~670 GHz in Band 9. The B2B phase referencing was successfully applied, allowing us to achieve an angular resolution of 14x11 and 10x8 mas for HL Tau and VY CMa, respectively. There is a high probability of finding a low-frequency calibrator within 5.4 deg in B2B phase referencing, bright enough to use an 8 s scan length combined with a 7.5 GHz bandwidth.Comment: 61 pages, 17 figures, 8 table

FAUST X: Formaldehyde in the Protobinary System [BHB2007] 11: Small Scale Deuteration

Context. Deuterium in H-bearing species is enhanced during the early stages of star formation, however, only a small number of high spatial resolution deuteration studies exist towards protostellar objects, leaving the small-scale structures unrevealed and understudied. Aims. We aim to constrain the deuterium fractionation ratios in a Class 0/I protostellar object in formaldehyde (H2CO), which has abundant deuterated isotopologues in this environment. Methods. We observed the Class 0/I protobinary system [BHB2007] 11, whose emission components are embedded in circumstellar disks that have radii of 2-3 au, using ALMA within the context of the Large Program FAUST. The system is surrounded by a complex filamentary structure connecting to the larger circumbinary disk. In this work we present the first study of formaldehyde D-fractionation towards this source with detections of H2CO 3(0,3)-2(0,2), combined with HDCO 4(2,2)-3(2,1), HDCO 4(1,4)-3(1,3) and D2CO 4(0,4)-3(0,3). These observations enable multiple velocity components associated with the methanol hotspots also uncovered by FAUST data, as well as the external envelope, to be resolved. In addition, based on the kinematics seen in the observations of the H2CO emission, we propose the presence of a second large scale outflow. Results. HDCO and D2CO are only found in the central regions of the core while H2CO is found more ubiquitously. From radiative transfer modelling, the column densities ranges found for H2CO, HDCO and D2CO are (3-8)x10$^{14}$ cm$^{-2}$, (0.8-2.9)x10$^{13}$ cm$^{-2}$ and (2.6-4.3)x10$^{12}$ cm$^{-2}$, respectively, yielding an average D/H ratio of 0.01-0.04. Following the results of kinematic modelling, the second large scale feature is inconsistent with a streamer-like nature and we thus tentatively conclude that the feature is an asymmetric molecular outflow launched by a wide-angle disk wind.Comment: 17 pages, 15 figure

The ALMA Interferometric Pipeline Heuristics

We describe the calibration and imaging heuristics developed and deployed in the ALMA interferometric data processing pipeline, as of ALMA Cycle 9. The pipeline software framework is written in Python, with each data reduction stage layered on top of tasks and toolkit functions provided by the Common Astronomy Software Applications package. This framework supports a variety of tasks for observatory operations, including science data quality assurance, observing mode commissioning, and user reprocessing. It supports ALMA and VLA interferometric data along with ALMA and NRO45m single dish data, via different stages and heuristics. In addition to producing calibration tables, calibrated measurement sets, and cleaned images, the pipeline creates a WebLog which serves as the primary interface for verifying the data quality assurance by the observatory and for examining the contents of the data by the user. Following the adoption of the pipeline by ALMA Operations in 2014, the heuristics have been refined through annual development cycles, culminating in a new pipeline release aligned with the start of each ALMA Cycle of observations. Initial development focused on basic calibration and flagging heuristics (Cycles 2-3), followed by imaging heuristics (Cycles 4-5), refinement of the flagging and imaging heuristics with parallel processing (Cycles 6-7), addition of the moment difference analysis to improve continuum channel identification (2020 release), addition of a spectral renormalization stage (Cycle 8), and improvement in low SNR calibration heuristics (Cycle 9). In the two most recent Cycles, 97% of ALMA datasets were calibrated and imaged with the pipeline, ensuring long-term automated reproducibility. We conclude with a brief description of plans for future additions, including self-calibration, multi-configuration imaging, and calibration and imaging of full polarization data.Comment: accepted for publication by Publications of the Astronomical Society of the Pacific, 65 pages, 20 figures, 10 tables, 2 appendice