Star-forming Region W75N Research Articles

An SiO Toroid and Wide-angle Outflow Associated with the Massive Protostar W75N(B)-VLA2

We have carried out Atacama Large Millimeter/submillimeter Array observations of the massive star-forming region W75N(B), which contains the massive protostars VLA1, VLA2, and VLA3. Particularly, VLA2 is an enigmatic protostar associated with a wind-driven H2O maser shell, which has evolved from an almost isotropic outflow to a collimated one in just 20 yr. The shell expansion seemed to be halted by an obstacle located to the northeast of VLA2. Here we present our findings from observing the 1.3 mm continuum and H2CO and SiO emission lines. Within a region of ∼30″ (∼39,000 au) diameter, we have detected 40 compact millimeter continuum sources, three of them coinciding with VLA1, VLA2, and VLA3. While the H2CO emission is mainly distributed in a fragmented structure around the three massive protostars, but without any of the main H2CO clumps spatially coinciding with them, the SiO is highly concentrated on VLA2, indicating the presence of very strong shocks generated near this protostar. The SiO emission is clearly resolved into an elongated structure (∼0.″6 × 0.″3; ∼780 au×390 au) perpendicular to the major axis of the wind-driven maser shell. The structure and kinematics of the SiO emission are consistent with a toroid and a wide-angle outflow surrounding a central mass of ∼10 M ⊙, thus supporting previous theoretical predictions regarding the evolution of the outflow. Additionally, we have identified the expected location and estimated the gas density of the obstacle that is hindering the expansion of the maser shell.

Submillimeter Observations of Magnetic Fields in Massive Star-forming Region W75N

This paper presents the results of full polarization observations of the massive star-forming region W75N, conducted with ∼3″ spatial resolutions at 345 GHz using the Submillimeter Array. The magnetic field structures in the dense cores of the region are derived using the linearly polarized continuum emission. The overall magnetic field strength and orientation are found to agree with those from the previous observations. The plane-of-sky component of the magnetic field in the region was calculated to be ∼0.8 ± 0.1 mG using the angular dispersion function method. Further analyses involving the polarization-intensity gradient-local gravity method and H13CO+ (4–3) line data indicated that the cloud is undergoing global gravitational collapse and the magnetic field is shaped by gravity and outflows in the dense core regions.

Monitoring of the polarized H2O maser emission around the massive protostars W75N(B)-VLA 1 and W75N(B)-VLA 2

Context. Several radio sources have been detected in the high-mass star-forming region W75N(B), with the massive young stellar objects VLA 1 and VLA 2 shown to be of particular interest among them. These objects are thought to be at different evolutionary stages: VLA 1 is in the early stage of photoionization and driving a thermal radio jet, while VLA 2 is a thermal, collimated ionized wind surrounded by a dusty disk or envelope. In both sources, 22 GHz H2O masers have been detected in the past. Those around VLA 1 show a persistent linear distribution along the thermal radio jet, while those around VLA 2 have traced the evolution from a non-collimated to a collimated outflow over a period of ∼20 yr. The magnetic field inferred from the H2O masers has shown an orientation rotation following the direction of the major-axis of the shell around VLA 2, whereas it is immutable around VLA 1. Aims. By monitoring the polarized emission of the 22 GHz H2O masers around both VLA 1 and VLA 2 over a period of six years, we aim to determine whether the H2O maser distributions show any variation over time and whether the magnetic field behaves accordingly. Methods. The European VLBI Network was used in full polarization and phase-reference mode in order to determine the absolute positions of the 22 GHz H2O masers with a beam size of ∼1 mas and to determine the orientation and the strength of the magnetic field. We observed four epochs separated by two years from 2014 to 2020. Results. We detected polarized emission from the H2O masers around both VLA 1 and VLA 2 in all the epochs. By comparing the H2O masers detected in the four epochs, we find that the masers around VLA 1 are tracing a nondissociative shock originating from the expansion of the thermal radio jet, while the masers around VLA 2 are tracing an asymmetric expansion of the gas that is halted in the northeast where the gas likely encounters a very dense medium. We also found that the magnetic field inferred from the H2O masers in each epoch can be considered as a portion of a quasi-static magnetic field estimated in that location rather than in that time. This allowed us to study the morphology of the magnetic field around both VLA 1 and VLA 2 locally across a larger area by considering the vectors estimated in all the epochs as a whole. We find that the magnetic field in VLA 1 is located along the jet axis, bending toward the north and south at the northeasterly and southwesterly ends of the jet, respectively, reconnecting with the large-scale magnetic field. The magnetic field in VLA 2 is perpendicular to the expansion directions until it encounters the denser matter in the northeast, where the magnetic field is parallel to the expansion direction and agrees with the large-scale magnetic field. We also measured the magnetic field strength along the line of sight in three of the four epochs, with resulting values of −764 mG < B||VLA 1 < − 676 mG and −355 mG < B||VLA 2 < −2426 mG.

Monitoring of the polarized H2O maser emission around the massive protostars W75N(B)-VLA 1 and W75N(B)-VLA 2

AbstractSeveral radio sources have been detected in the high-mass star-forming region W75N(B), among them the massive young stellar objects VLA 1 and VLA 2 are of great interest. These are thought to be in different evolutionary stages. In particular, VLA 1 is at the early stage of the photoionization and it is driving a thermal radio jet, while VLA 2 is a thermal, collimated ionized wind surrounded by a dusty disk or envelope. In both sources 22 GHz H2O masers have been detected in the past. Those around VLA 1 show a persistent linear distribution along the thermal radio jet and those around VLA 2 have instead traced the evolution from a non-collimated to a collimated outflow over a period of ∼20 years. The magnetic field inferred from the H2O masers showed a rotation of its orientation according to the direction of the major-axis of the shell around VLA 2, while it is immutable around VLA 1.We further monitored the polarized emission of the 22 GHz H2O masers around both VLA 1 and VLA 2 over a period of six years with the European VLBI Network for a total of four epochs separated by two years from 2014 to 2020. We here present the results of our monitoring project by focusing on the evolution of the maser distribution and of the magnetic field around the two massive young stellar objects.

Characterizing the radio continuum nature of sources in the massive star-forming region W75N (B)

ABSTRACT The massive star-forming region W75N (B) is thought to host a cluster of massive protostars (VLA 1, VLA 2, and VLA 3) undergoing different evolutionary stages. In this work, we present radio continuum data with the highest sensitivity and angular resolution obtained to date in this region, using the VLA-A and covering a wide range of frequencies (4–48 GHz), which allowed us to study the morphology and the nature of the emission of the different radio continuum sources. We also performed complementary studies with multi-epoch Very Large Array (VLA) data and Atacama Large Millimeter Array (ALMA) archive data at 1.3 mm wavelength. We find that VLA 1 is driving a thermal radio jet at scales of ≈0.1 arcsec (≈130 au), but also shows signs of an incipient hypercompact H ii region at scales of ≲1 arcsec (≲1300 au). VLA 3 is also driving a thermal radio jet at scales of a few tenths of arcsec (few hundred of au). We conclude that this jet is shock exciting the radio continuum sources Bc and VLA 4 (obscured Herbig–Haro objects), which show proper motions moving outward from VLA 3 at velocities of ≈112–118 km s−1. We have also detected three new weak radio continuum sources, two of them associated with millimetre continuum cores observed with ALMA, suggesting that these two sources are also embedded young stellar objects in this massive star-forming region.

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.

Rapidly increasing collimation and magnetic field changes of a protostellar H2O maser outflow

W75N(B) is a massive star-forming region that contains three radio continuum sources (VLA 1, VLA 2, and VLA 3), which are thought to be three massive young stellar objects at three different evolutionary stages. VLA 1 is the most evolved and VLA 2 the least evolved source. The 22 GHz H2O masers associated with VLA 1 and VLA 2 have been mapped at several epochs over eight years. While the H2O masers in VLA 1 show a persistent linear distribution along a radio jet, those in VLA 2 are distributed around an expanding shell. Furthermore, H2O maser polarimetric measurements revealed magnetic fields aligned with the two structures. Using new polarimetric observations of H2O masers, we aim to confirm the elliptical expansion of the shell-like structure around VLA 2 and, at the same time, to determine if the magnetic fields around the two sources have changed. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman-splitting of the 22 GHz H2O masers towards the massive star-forming region W75N(B). The H2O maser distribution around VLA 1 is unchanged from that previously observed. We made an elliptical fit of the H2O masers around VLA 2. We find that the shell-like structure is still expanding along the direction parallel to the thermal radio jet of VLA 1. While the magnetic field around VLA 1 has not changed in the past 7 years, the magnetic field around VLA 2 has changed its orientation according to the new direction of the major-axis of the shell-like structure and it is now aligned with the magnetic field in VLA 1.

Flares and proper motions of ground-state OH masers in W75N

The star-forming region W75N hosts bright OH masers that are observed to be variable. We present observations taken in 2008 of the ground-state OH maser transitions with the Very Long Baseline Array (VLBA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN) and with the Nancay Radio Telescope in 2011. Several of the masers in W75N were observed to be flaring, with the brightest 1720-MHz maser in excess of 400 Jy. The 1720-MHz masers appear to be associated with the continuum source VLA 1, unlike the bright flaring 1665- and 1667-MHz masers, which are associated with VLA 2. The 1720-MHz masers are located in an outflow traced by water masers and are indicative of very dense molecular material near the H II region. The magnetic field strengths are larger in the 1720-MHz maser region than in most regions hosting only main-line OH masers. The density falls off along the outflow, and the order of appearance of different transitions of OH masers is consistent with theoretical models. The 1665- and 1667-MHz VLBA data are compared against previous epochs over a time baseline of over 7 years. The median maser motion is 3.5 km/s, with a scatter that is comparable to thermal turbulence. The general pattern of maser proper motions observed in the 1665- and 1667-MHz transitions is consistent with previous observations.

The structure of the magnetic field in the massive star-forming region W75N

A debated topic in star formation theory is the role of magnetic fields during the protostellar phase of high-mass stars. It is still unclear how magnetic fields influence the formation and dynamics of massive disks and outflows. Most current information on magnetic fields close to high-mass protostars comes from polarized maser emissions, which allows us to investigate the magnetic field on small scales by using very long-baseline interferometry. The massive star-forming region W75N contains three radio continuum sources (VLA1, VLA2, and VLA3), at three different evolutionary stages, and associated masers, while a large-scale molecular bipolar outflow is also present. Very recently, polarization observations of the 6.7 GHz methanol masers at milliarsecond resolution have been able to probe the strength and structure of the magnetic field over more than 2000 AU around VLA1. The magnetic field is parallel to the outflow, suggesting that VLA1 is its powering source. The observations of water masers at 22 GHz can give more information about the gas dynamics and the magnetic fields around VLA1 and VLA2. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman-splitting of the 22 GHz water masers in the star-forming region W75N. We detected 124 water masers, 36 around VLA1 and 88 around VLA2 of W75N, which indicate two different physical environments around the two sources, where VLA1 is in a more evolved state. The linear polarization of the masers confirms the tightly ordered magnetic field around VLA1, which is aligned with the large-scale molecular outflow, and also reveals an ordered magnetic field around VLA2, which is not parallel to the outflow. [abridged]

Evolution of the spectrum and VLBI structure of W75N during the huge OH maser flare in 2003-2007

We present results of single-dish and very long baseline interferometry (VLBI) observations of a strong 1000-Jy OH maser flare in the star-forming region W75N. The flare was first seen in 2003, and persisted for at least four years. Three major spectral features were present during the whole period of observations, with N1 being the strongest initially and N2 becoming stronger later. The large velocity range of 43 km s−1 observed during the flare is not kinematic in origin, but is caused by the Zeeman splitting in the strong magnetic field generated by the central star. The maps and proper motion of the maser spots were monitored over eight epochs, between 1998 and 2006, using the European VLBI Network (EVN) supplemented by published Very Large Baseline Array (VLBA) data. Mapping of the maser spots has shown that all the new spectral features of the flare were located close to the ultracompact H ii region VLA2, while the quiescent maser features were found near VLA1. We suggest that the flare was initiated by a disturbance, probably an magnetohydrodynamic (MHD) shock launched from the massive young star, also responsible for the excitation of the ultracompact H ii region. The proper motion of the majority of the maser spots was not detected at 1665 and 1667 MHz with the upper limit of several km s−1. The only positive detection of the proper motion was made for the two brightest flare features N1 and N2. The spot N2 is moving along the filamentary spot N3, with a tangential velocity of 14 km s−1. This motion is caused by the MHD shock propagating along the filament. The measured low upper limit for the velocity of OH maser spots is consistent with models of a slowly rotating Keplerian disc rather than with outflows, jets or expanding shells.

Methanol masers probing the ordered magnetic field of W75N

The role of magnetic fields during the protostellar phase of high-mass star-formation is a debated topic. In particular, it is still unclear how magnetic fields influence the formation and dynamic of disks and outflows. Most current information on magnetic fields close to high-mass protostars comes from H2O and OH maser observations. Recently, the first 6.7 GHz methanol maser polarization observations were made, and they reveal strong and ordered magnetic fields. The morphology of the magnetic field during high-mass star-formation needs to be investigated on small scales, which can only be done using very long baseline interferometry observations. The massive star-forming regionW75N contains three radio sources and associated masers, while a large-scale molecular bipolar outflow is also present. Polarization observations of the 6.7 GHz methanol masers at high angular resolution probe the strength and structure of the magnetic field and determine its relation to the outflow. Eight of the European VLBI network antennas were used to measure the linear polarization and Zeeman-splitting of the 6.7 GHz methanol masers in the star-forming region W75N. We detected 10 methanol maser features, 4 of which were undetected in previous work. All arise near the source VLA1 of W75N. The linear polarization of the masers reveals a tightly ordered magnetic field over more than 2000 AU around VLA1 that is exactly aligned with the large-scale molecular outflow. This is consistent with the twisted magnetic field model proposed for explaining dust polarization observations. The Zeeman-splitting measured on 3 of the maser features indicates a dynamically important magnetic field in the maser region of the order of 50mG. We suggest VLA1 is the powering sources of the bipolar outflow.

Spectral and VLBI-structure monitoring of an OH maser flare in W75N

AbstractWe present VLBI and single dish observations of a strong (1000 Jy) OH maser flare at 1665 MHz in the star-forming region W75N. This new flare appeared in 2003, after a 100 Jy flare in 2000. Three major spectral features N1, N2, and N3 were present at all epochs, with N1 declining and N2 increasing in flux density during observations taken in 2003–2006. All three features have strong linear polarization approaching 90%. VLBI maps show that along with the strong features N1-3, many weaker features have appeared within 100 mas of the ultra compact HII region VLA 2. The proper motion of the flare feature N1 was measured: Δα=1.36±0.3; Δδ=-0.3±0.3 mas in 1.75 years, corresponding to a tangential velocity of about 8 km s−1 relative to the rest of the features, and is comparable to the radial velocity range in the VLA 2 disk.

Large Magnetic Fields and Motions of OH Masers in W75N

We report on a second epoch of VLBA observations of the 1665 and 1667 MHz OH masers in the massive star-forming region W75N. We find evidence to confirm the existence of very strong (~40 mG) magnetic fields near source VLA 2. The masers near VLA 2 are dynamically distinct and include a very bright spot apparently moving at 50 km s-1 relative to those around VLA 1. This fast-moving spot may be an example of a rare class of OH masers seen in outflows in star-forming regions. Due to the variability of these masers and the rapidity of their motions, tracking these motions will require multiple observations over a significantly shorter time baseline than obtained here. Proper motions of the masers near VLA 1 are more suggestive of streaming along magnetized shocks rather than Keplerian rotation in a disk. The motions of the easternmost cluster of masers in W75N(B) may be tracing slow expansion around an unseen exciting source.

A model of polarized OH maser emission in W75N

Multi-element Radio-linked Interferometer Network (MERLIN) observations of 18-cm lines of OH from the star-forming region W75N indicate a rotating molecular disc orthogonal to the bipolar molecular outflow, with the maser polarization tracing twisted magnetic field lines. The observations are compared with synthetic maser features generated by a polarization-dependent model of maser amplification, which uses input physical conditions drawn from a numerical simulation of a star-forming disc/outflow source. The model helps to explain the complexity of the observed magnetic field structure on the basis of maser emission originating from several different depths within the protostellar disc. Line-ratios, spectral richness and polarization of ground-state (18-cm) masers agree broadly with observations. Although the detailed agreement is crude, the model is able to generate lines that are observed, whilst excluding those that are not: for example, the main lines and 1720 MHz are generated in the ground state, but not 1612 MHz; 4765 MHz is generated, but neither of the other 4.7-GHz lines appear as masers.

A Radio Jet–H2O Maser System in W75N(B) at a 200 Au Scale: Exploring the Evolutionary Stages of Young Stellar Objects

We have observed simultaneously with the VLA in its A configuration the 1.3 cm continuum and H2O maser emission toward the star-forming region W75N(B) with 01 resolution using a powerful cross-calibration technique. Three continuum sources (VLA 1, VLA 2, and VLA 3) were detected in a region of 15. VLA 1 is elongated (043 × 012) approximately in the direction of the bipolar molecular outflow observed at scales of 2'. The frequency dependence of the flux density and size are consistent with an optically partially thick ionized thermal biconical jet. VLA 2 appears unresolved, while VLA 3 shows a bright core plus extended emission. We detected 29 H2O maser spots (spatial components) in a region of 13'' × 7'' around W75N(B). These masers are mainly distributed in two clusters, one associated with VLA 1 (11 maser spots), and the other one associated with VLA 2 (eight maser spots). One H2O maser spot is associated with VLA 3. The masers associated with VLA 1 are distributed along the major axis of the radio jet. We conclude that VLA 1 is the powering source of the extended bipolar molecular outflow and that the water masers along the radio jet axis are delineating the outflow at scales of 1''. On the other hand, the eight masers coincident with VLA 2 are distributed in a shell of 018 × 010, with a rough north-south velocity segregation that could indicate bound motions around this continuum source. From the comparison of H2O and OH maser distribution in the region with respect to the three radio sources, we consider an evolutionary scheme in which H2O masers are excited in gravitationally bound material (e.g., in circumstellar disks) in less evolved young stellar objects (YSOs), while in more evolved YSOs H2O masers preferentially trace outflows.

UKIRT observations of the mid-infrared and submillimetre thermal continuum in W75N

Maps in the mid-infrared and submillimetre thermal continuum are presented for the star-forming region W75N. Observations at 10 and 20 μm, with ∼ 5-arcsec resolution, trace warm dust heated by members of the embedded young stellar cluster. The submillimetre continuum emission traces the cool dust in the cloud core surrounding the luminosity sources. The core has been spatially resolved at 350 μm but shows no disc-like structure related to the known molecular outflow on scales larger than ∼ 0.2 pc.