Massive 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.

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.

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]

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.

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.