G. Surcis

Observing the onset of outflow collimation in a massive protostar

Young stars grow up and narrow their focus Stars are thought to grow by gathering spirals of material from a disk. If this is the case, to balance angular momentum, gas should flow out rapidly along the disks rotation axis. Carrasco-Gonzalez et al. now seem to have glimpsed the “before” and “after” stages of the onset of such an outflow, over the course of just 18 years (see the Perspective by Hoare). Radio monitoring of the massive protostar W75N(B)-VLA2 reveals a transition from a spherical wind to a collimated one, giving critical insight into what happens as a massive star forms. Science, this issue p. 114; see also p. 44 A stellar outflow changed from spherical to collimated while being monitored with radio telescopes. [Also see Perspective by Hoare] The current paradigm of star formation through accretion disks, and magnetohydrodynamically driven gas ejections, predicts the development of collimated outflows, rather than expansion without any preferential direction. We present radio continuum observations of the massive protostar W75N(B)-VLA 2, showing that it is a thermal, collimated ionized wind and that it has evolved in 18 years from a compact source into an elongated one. This is consistent with the evolution of the associated expanding water-vapor maser shell, which changed from a nearly circular morphology, tracing an almost isotropic outflow, to an elliptical one outlining collimated motions. We model this behavior in terms of an episodic, short-lived, originally isotropic ionized wind whose morphology evolves as it moves within a toroidal density stratification.

Very Long Baseline Interferometry with the SKA

Adding VLBI capability to the SKA arrays will greatly broaden the science of the SKA, and is feasible within the current specifications. SKA-VLBI can be initially implemented by providing phased-array outputs for SKA1-MID and SKA1-SUR and using these extremely sensitive stations with other radio telescopes, and in SKA2 by realising a distributed configuration providing baselines up to thousands of km, merging it with existing VLBI networks. The motivation for and the possible realization of SKA-VLBI is described in this paper.

Velocity and magnetic fields within 1000 AU of a massive YSO

Aims: We study the velocity and magnetic field morphology in the vicinity ( 600 AU) of the molecular envelope, where the magnetic field orientation shows a smooth change with the maser cloudlets position (0.2° AU-1). Overall, the velocity field vectors accommodate the local magnetic field direction well, but still show an average misalignment of 30°. We interpret this finding as the contribution of a turbulent velocity field of about 3.5 km s-1, which would be responsible for breaking up the alignment between the velocity and magnetic field vectors. We do resolve different gas flows that develop both along the outflow axis and across the disk plane and that have an average speed of 7 km s-1. In the direction of the outflow axis, we establish a collimation of the gas flow at a distance of about 1000 AU from the disk plane. In the disk region, gas appears to stream outward along the disk plane for radii greater than 500-600 AU and inward for shorter radii.

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions - II. First statistical results

Context. Magnetic fields have only recently been included in theoretical simulations of high-mass star formation. The simulations show that magnetic fields play an important role in the formation and dynamics of molecular outflows. Masers, in particular 6.7-GHz CH3OH masers, are the best probes of the magnetic field morphologies around massive young stellar objects on the smallest scales of 10-100 AU. Aims. Providing new observational measurements of the morphology of magnetic fields around massive young stellar objects by using 6.7-GHz CH3OH maser emission is very important for setting constraints on the numerical simulations of massive star formation. Methods. This paper focuses on 4 massive young stellar objects, IRAS 06058+2138-NIRS 1, IRAS 22272+6358A, S255-IR, and S231, which complement our previous 2012 sample (the first EVN group). From all these sources, molecular outflows have been detected in the past. Seven of the European VLBI Network antennas were used to measure the linear polarization and Zeeman-splitting of the 6.7-GHz CH3OH masers in the star-forming regions in this second EVN group. Results. We detected a total of 128 CH3OH masing cloudlets. Fractional linear polarization (0.8%-11.3%) was detected towards 18% of the CH3OH masers in our sample. The linear polarization vectors are well ordered in all the massive young stellar objects. We measured significant Zeeman-splitting in IRAS 06058+2138-NIRS 1 (ΔVZ = 3.8 ± 0.6 m s-1) and S255-IR (ΔVZ = 3.2 ± 0.7 m s-1). Conclusions. By considering the 20 massive young stellar objects towards which the morphology of magnetic fields was determined by observing 6.7-GHz CH3OH masers in both hemispheres, we find no evident correlation between the linear distributions of CH3OH masers and the outflows or the linear polarization vectors. On the other hand, we present first statistical evidence that the magnetic field (on scales 10-100 AU) is primarily oriented along the large-scale outflow direction. Moreover, we empirically find that the linear polarization fraction of unsaturated CH 3OH masers is Pl < 4.5%.

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

Context. 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. Aims. 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. Methods. 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). Results. 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 similar to 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.

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions III. The flux-limited sample ⋆

Theoretical simulations and observations at different angular resolutions have shown that magnetic fields have a central role in massive star formation. Like in low-mass star formation, the magnetic field in massive young stellar objects can either be oriented along the outflow axis or randomly. Measuring the magnetic field at milliarcsecond resolution (10-100 au) around a substantial number of massive young stellar objects permits determining with a high statistical significance whether the direction of the magnetic field is correlated with the orientation of the outflow axis or not. In late 2012, we started a large VLBI campaign with the European VLBI Network to measure the linearly and circularly polarized emission of 6.7 GHz methanol masers around a sample of massive star-forming regions. This paper focuses on the first seven observed sources, G24.78+0.08, G25.65+1.05, G29.86-0.04, G35.03+0.35, G37.43+1.51, G174.20-0.08, and G213.70-12.6. For all these sources, molecular outflows have been detected in the past. We detected a total of 176 methanol masing cloudlets toward the seven massive star-forming regions, 19% of which show linearly polarized emission. The methanol masers around the massive young stellar object MM1 in G174.20-0.08 show neither linearly nor circularly polarized emission. The linear polarization vectors are well ordered in all the other massive young stellar objects. We measured significant Zeeman splitting toward both A1 and A2 in G24.78+0.08, and toward G29.86-0.04 and G213.70-12.6. By considering all the 19 massive young stellar objects reported in the literature for which both the orientation of the magnetic field at milliarcsecond resolution and the orientation of outflow axes are known, we find evidence that the magnetic field (on scales 10-100 au) is preferentially oriented along the outflow axes.

The magnetic field at milliarcsecond resolution around IRAS 20126+4104

Context. IRAS 20126+4104 is a well studied B0.5 protostar that is surrounded by a similar to 1000 au Keplerian disk and is where a large-scale outflow originates. Both 6.7-GHz CH3OH masers and 22-GHz H2O masers have been detected toward this young stellar object. The CH3OH masers trace the Keplerian disk, while the H2O masers are associated with the surface of the conical jet. Recently, observations of dust polarized emission (350 mu m) at an angular resolution of 9 arcsec (similar to 15 000 au) have revealed an S-shaped morphology of the magnetic field around IRAS 20126+4104. Aims. The observations of polarized maser emissions at milliarcsecond resolution (similar to 20 au) can make a crucial contribution to understanding the orientation of the magnetic field close to IRAS 20126+4104. This will allow us to determine whether the magnetic field morphology changes from arcsecond resolution to milliarcsecond resolution. Methods. The European VLBI Network was used to measure the linear polarization and the Zeeman splitting of the 6.7-GHz CH3OH masers toward IRAS 20126+4104. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman splitting of the 22-GHz H2O masers toward the same region. Results. We detected 26 CH3OH masers and 5 H2O masers at high angular resolution. Linear polarization emission was observed toward three CH3OH masers and toward one H2O maser. Significant Zeeman splitting was measured in one CH3OH maser (Delta V-Z = -9.2 +/- 1.4 ms(-1)). No significant (5 sigma) magnetic field strength was measured using the H2O masers. We found that in IRAS 20126+4104 the rotational energy is less than the magnetic energy.

Planar infall of CH3OH gas around Cepheus A HW2

Aims. In order to test the nature of an (accretion) disk in the vicinity of Cepheus A HW2, we measured the three-dimensional velocity field of the CH 3 OH maser spots, which are projected within 1000 au of the HW2 object, with an accuracy on the order of 0.1 km s -1 . Methods. We made use of the European VLBI Network (EVN) to image the 6.7 GHz CH 3 OH maser emission toward Cepheus A HW2 with 4.5 milliarcsec resolution (3 au). We observed at three epochs spaced by one year between 2013 and 2015. During the last epoch, in mid-March 2015, we benefited from the newly deployed Sardinia Radio Telescope. Results. We show that the CH 3 OH velocity vectors lie on a preferential plane for the gas motion with only small deviations of 12° ± 9° away from the plane. This plane is oriented at a position angle of 134° east of north, and inclined by 26° with the line of sight, closely matching the orientation of the previously reported disk-like structure. Knowing the orientation of the equatorial plane, we can reconstruct a face-on view of the CH 3 OH gas kinematics onto the plane. CH 3 OH maser emission is detected within a radius of 900 au from HW2, and down to a radius of about 300 au, the latter coincident with the extent of the dust emission at 0.9 mm. The velocity field is dominated by an infall component of about 2 km s -1 down to a radius of 300 au, where a rotational component of 4 km s -1 becomes dominant. We discuss the nature of this velocity field and the implications for the enclosed mass. Conclusions. These findings directly support the interpretation that the high-density gas and dust emission that surrounds Cepheus A HW2 traces an accretion disk.

Measuring Magnetic Fields Near and Far with the SKA via the Zeeman Effect

The measurement of Zeeman splitting in spectral lines-both in emission and absorption-can provide direct estimates of the magnetic field strength and direction in atomic and molecular clouds, both in our own MilkyWay and in external galaxies. This method will probe the magnetic field in the warm and cold neutral components of the interstellar medium, providing a complement to the extensive SKA Faraday studies planning to probe the field in the ionized components.

Characterization of methanol as a magnetic field tracer in star-forming regions

Magnetic fields play an important role during star formation1. Direct magnetic field strength observations have proven particularly challenging in the extremely dynamic protostellar phase2–4. Because of their occurrence in the densest parts of star-forming regions, masers, through polarization observations, are the main source of magnetic field strength and morphology measurements around protostars2. Of all maser species, methanol is one of the strongest and most abundant tracers of gas around high-mass protostellar disks and in outflows. However, as experimental determination of the magnetic characteristics of methanol has remained largely unsuccessful5, a robust magnetic field strength analysis of these regions could hitherto not be performed. Here, we report a quantitative theoretical model of the magnetic properties of methanol, including the complicated hyperfine structure that results from its internal rotation6. We show that the large range in values of the Landé g factors of the hyperfine components of each maser line lead to conclusions that differ substantially from the current interpretation based on a single effective g factor. These conclusions are more consistent with other observations7,8 and confirm the presence of dynamically important magnetic fields around protostars. Additionally, our calculations show that (nonlinear) Zeeman effects must be taken into account to further enhance the accuracy of cosmological electron-to-proton mass ratio determinations using methanol9–12.Methanol maser lines are key tracers of the magnetic field strength in high-mass star-forming regions. Here the authors model the magnetic properties of methanol in detail, including the hyperfine structure arising from its internal rotation.