ABSTRACT
We investigated, through numerical models, the flaring variability that may arise from the rotation of maser clouds of approximately spheroidal geometry, ranging from strongly oblate to ...strongly prolate examples. Inversion solutions were obtained for each of these examples over a range of saturation levels from unsaturated to highly saturated. Formal solutions were computed for rotating clouds with many randomly chosen rotation axes, and corresponding averaged maser light curves plotted with statistical information. The dependence of results on the level of saturation and on the degree of deformation from the spherical case was investigated in terms of a variability index and duty cycle. It may be possible to distinguish observationally between flares from oblate and prolate objects. Maser flares from rotation are limited to long time-scales (at least a few years) and modest values of the variability index (≲100), and can be aperiodic or quasi-periodic. Rotation is therefore not a good model for H2O variability on time-scales of weeks to months, or of truly periodic flares.
ABSTRACT
We briefly discuss the history of pumping schemes for the common 22-GHz maser transition in o-H2O, and note that radiative mechanisms have been considered in the past, as well as the better ...known collisional pumping mechanisms associated with shock waves. Substantial IR irradiation is indeed destructive to inversion at 22-GHz under a wide range of physical conditions. We identify a small number of transitions, including 22 GHz, that can be pumped by both collisional and radiative schemes, and, through radiative transfer modelling over a grid of values in kinetic temperature, number density, and dust temperature, identify loci in density-kinetic temperature space where these schemes lead to strong inversions. The radiative pumping scheme generally operates at higher dust temperature, and lower kinetic temperature, than the collisional scheme. We identify a small network of transitions that form a radiative pumping scheme for the 22-GHz maser, involving radiative transitions of wavelength approximately 6 $\mu$m. This network is capable of supplying more than 50 per cent of the 22-GHz inversion under typical radiative pumping conditions, and it is consistent with the need for high dust and low kinetic temperatures determined from the modelling. We identify a probable case of radiative pumping in a massive star-forming region from the observation of a positive correlation between the flux densities of 22-GHz H2O masers and 6.7-GHz methanol masers. We discuss possibilities for finding radiatively pumped H2O maser lines in other source types.
As part of our effort to search for circumstellar disks around high-mass stellar objects, we observed the well-known core G31.41 +0.31 with ALMA at 1.4 mm with an angular resolution of ~0.′′22 (~1700 ...au). The dust continuum emission has been resolved into two cores namely Main and NE. The Main core, which has the stronger emission and is the more chemically rich, has a diameter of ~5300 au, and is associated with two free-free continuum sources. The Main core looks featureless and homogeneous in dust continuum emission and does not present any hint of fragmentation. Each transition of CH3CN and CH3OCHO, both ground and vibrationally excited, as well as those of CH3CN isotopologues, shows a clear velocity gradient along the NE–SW direction, with velocity linearly increasing with distance from the center, consistent with solid-body rotation. However, when comparing the velocity field of transitions with different upper level energies, the rotation velocity increases with increasing energy of the transition, which suggests that the rotation speeds up toward the center. Spectral lines towardtoward the dust continuum peak show an inverse P-Cygni profile that supports the existence of infall in the core. The infall velocity increases with the energy of the transition suggesting that the infall is accelerating toward the center of the core, consistent with gravitational collapse. Despite the monolithic appearance of the Main core, the presence of red-shifted absorption, the existence of two embedded free-free sources at the center, and the rotational spin-up are consistent with an unstable core undergoing fragmentation with infall and differential rotation due to conservation of angular momentum. Therefore, the most likely explanation for the monolithic morphology is that the large opacity of the dust emission prevents the detection of any inhomogeneity in the core.
ABSTRACT
A 3D maser model has been used to perform an inverse problem on the light curves from three high-amplitude maser flares, selected on the basis of contemporaneous infrared observations. Plots ...derived from the model recover the size of the maser cloud, and two parameters linked to saturation, from three observational properties of the light curve. Recovered sizes are consistent with independent interferometric measurements. Maser objects transition between weak and moderate saturation during a flare.
Context. Circumstellar discs around massive stars could mediate the accretion onto the star from the infalling envelope, and could minimize the effects of radiation pressure. Despite such a crucial ...role, only a few convincing candidates have been provided for discs around deeply embedded O-type (proto)stars. Aims. In order to establish whether disc-mediated accretion is the formation mechanism for the most massive stars, we have searched for circumstellar, rotating discs around a limited sample of six luminous (>105L⊙) young stellar objects. These objects were selected on the basis of their IR and radio properties in order to maximize the likelihood of association with disc+jet systems. Methods. We used ALMA with ~0.̋2 resolution to observe a large number of molecular lines typical of hot molecular cores. In this paper we limit our analysis to two disc tracers (methyl cyanide, CH3CN, and its isotopologue, 13CH3CN), and an outflow tracer (silicon monoxide, SiO). Results. We reveal many cores, although their number depends dramatically on the target. We focus on the cores that present prominent molecular line emission. In six of these a velocity gradient is seen across the core,three of which show evidence of Keplerian-like rotation. The SiO data reveal clear but poorly collimated bipolar outflow signatures towards two objects only. This can be explained if real jets are rare (perhaps short-lived) in very massive objects and/or if stellar multiplicity significantly affects the outflow structure.For all cores with velocity gradients, the velocity field is analysed through position–velocity plots to establish whether the gas is undergoing rotation with νrot ∝ R− α, as expected for Keplerian-like discs. Conclusions. Our results suggest that in three objects we are observing rotation in circumstellar discs, with three more tentative cases, and one core where no evidence for rotation is found. In all cases but one, we find that the gas mass is less than the mass of any embedded O-type star, consistent with the (putative) discs undergoing Keplerian-like rotation. With the caveat of low number statistics, we conclude that the disc detection rate could be sensitive to the evolutionary stage of the young stellar object. In young, deeply embedded sources, the evidence for discs could be weak because of confusion with the surrounding envelope, while in the most evolved sources the molecular component of the disc could have already been dispersed. Only in those objects that are at an intermediate stage of the evolution would the molecular disc be sufficiently prominent and relatively less embedded to be detectable by mm/submm observations.
Context.
To better understand the formation of high-mass stars, it is fundamental to investigate how matter accretes onto young massive stars, how it is ejected, and how all this differs from the ...low-mass case. The massive protocluster G31.41+0.31 is the ideal target to study all these processes because observations at millimeter and centimeter wavelengths have resolved the emission of the Main core into at least four massive dust continuum sources, named A, B, C, and D, within 1″ or 0.018 pc, and have identified signatures of infall and several outflows associated with the core.
Aims.
We study the interplay between infall and outflow in G31.41+0.31, at a spatial resolution of a few 100 au, by investigating their properties and their possible impact on the core.
Methods.
We carried out molecular line observations of typical high-density tracers, such as CH
3
CN or H
2
CO, and shock and outflow tracers, such as SiO, with ALMA at 1.4 mm that achieved an angular resolution of ~0.′′09 (~340 au).
Results.
The observations reveal inverse P Cygni profiles in CH
3
CN and H
2
CO toward the four sources embedded in the Main core, suggesting that all of them are undergoing collapse. The infall rates estimated from the redshifted absorption are on the order of ~10
−2
M
⊙
yr
−1
. The individual infall rates imply that the accretion timescale of the Main core is an order of magnitude smaller than its rotation timescale. This confirms that rotating toroids such as the G31 Main core are non-equilibrium, transient collapsing structures that need to be constantly replenished with fresh material from a large-scale reservoir. For sources B, C, and D the infall could be accelerating inside the sources, while for source A the presence of a second emission component complicates the interpretation. The SiO observations have revealed the presence of at least six outflows in the G31.41+0.31 star-forming region, and suggest that each of the four sources embedded in the Main core drives a molecular outflow. The outflow rates are on the order of ~10
−5
–10
−4
M
⊙
yr
−1
, depending on the SiO abundance. The feedback of the outflows appears sufficient to sustain the turbulence in the core and to eventually disrupt the core or prevent its further collapse. The mass accretion rates onto the individual sources, estimated from the highest values of the outflow mass loss rates, are on the order of 10
−4
M
⊙
yr
−1
. The difference of about two orders of magnitude between the accretion rates and the infall rates suggests that the central stars and the jets and outflows might not be efficient in removing disk material, which could lead to gravitational instabilities, fragmentation of the disk, and episodic accretion events.
Conclusions.
Infall and outflows are simultaneously present in all four sources embedded in the high-mass star-forming core G31.41+0.31. This indicates that these sources are still actively accreting and have not reached their final mass yet, which confirms the youth of this massive core.
ABSTRACT
We simulate maser flares by varying either the pump rate or the background level of radiation in a 3D model of a maser cloud. We investigate the effect of different cloud shapes, saturation ...levels, and viewpoints. Results are considered for clouds with both uniform and internally variable unsaturated inversion. Pumping and background variations are represented by several different driving functions, some of which are light curves drawn from observations. We summarize the pumping variability results in terms of three observable parameters: the maximum flux density achieved, a variability index, and duty cycle. We demonstrate typical ranges of the flux density that may result from viewing an aspherical object from random viewpoints. The best object for a flare is a prolate cloud, viewed close to its long axis and driven from unsaturated conditions to at least modest saturation. Results for variation of the background level are qualitatively different from the variable pumping results in that they tend to produce short intervals of low flux density under conditions of moderate saturation and sufficient variability to be consistent with strong flaring. Variable background models typically have a significantly higher duty cycle than those with variable pumping.
Context. The formation process of high-mass stars (with masses >8 M⊙) is still poorly understood, and represents a challenge from both the theoretical and observational points of view. The advent of ...the Atacama Large Millimeter Array (ALMA) is expected to provide observational evidence to better constrain the theoretical scenarios. Aims. The present study aims at characterizing the high-mass star forming region G35.20−0.74 N, which is found associated with at least one massive outflow and contains multiple dense cores, one of them recently found associated with a Keplerian rotating disk. Methods. We used the radio-interferometer ALMA to observe the G35.20−0.74 N region in the submillimeter continuum and line emission at 350 GHz. The observed frequency range covers tracers of dense gas (e.g., H13CO+, C17O), molecular outflows (e.g., SiO), and hot cores (e.g., CH3CN, CH3OH). These observations were complemented with infrared and centimeter data. Results. The ALMA 870 μm continuum emission map reveals an elongated dust structure (~0.15 pc long and ~0.013 pc wide; full width at half maximum) perpendicular to the large-scale molecular outflow detected in the region, and fragmented into a number of cores with masses ~1–10 M⊙ and sizes ~1600 AU (spatial resolution ~960 AU). The cores appear regularly spaced with a separation of ~0.023 pc. The emission of dense gas tracers such as H13CO+ or C17O is extended and coincident with the dust elongated structure. The three strongest dust cores show emission of complex organic molecules characteristic of hot cores, with temperatures around 200 K, and relative abundances 0.2–2 × 10-8 for CH3CN and 0.6–5 × 10-6 for CH3OH. The two cores with highest mass (cores A and B) show coherent velocity fields, with gradients almost aligned with the dust elongated structure. Those velocity gradients are consistent with Keplerian disks rotating about central masses of 4–18 M⊙. Perpendicular to the velocity gradients we have identified a large-scale precessing jet/outflow associated with core B, and hints of an east-west jet/outflow associated with core A. Conclusions. The elongated dust structure in G35.20−0.74 N is fragmented into a number of dense cores that may form high-mass stars. Based on the velocity field of the dense gas, the orientation of the magnetic field, and the regularly spaced fragmentation, we interpret this elongated structure as the densest part of a 1D filament fragmenting and forming high-mass stars.
We report on the measurement of the trigonometric parallaxes of 1612 MHz hydroxyl masers around two asymptotic giant branch stars, WX Psc and OH 138.0+7.2, using the NRAO Very Long Baseline Array ...with in-beam phase referencing calibration. We obtain a 3 upper limit of ≤5.3 mas on the parallax of WX Psc, corresponding to a lower limit distance estimate of 190 pc. The obtained parallax of OH 138.0+7.2 is 0.52 0.09 mas ( 18%), corresponding to a distance of , making this the first hydroxyl maser parallax below one milliarcsecond. We also introduce a new method of error analysis for detecting systematic errors in the astrometry. Finally, we compare our trigonometric distances to published phase-lag distances toward these stars and find a good agreement between the two methods.
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