Multi-frequency VLBI observations of faint gigahertz peaked spectrum sources

We present the data and analysis of VLBI observations at 1.6, 5 and 15 GHz of a sample of faint Gigahertz Peaked Spectrum (GPS) sources selected from the Westerbork Northern Sky Survey (WENSS). The 5 GHz observations involved a global array of 16 stations and yielded data on the total sample of 47 sources. A subsample of 26 GPS sources with peak frequencies > 5 GHz and/or peak flux densities > 125 mJy was observed with the VLBA at 15 GHz. A second subsample of 29 sources, with peak frequencies <5 GHz, was observed at 1.6 GHz using a 14 station global VLBI array. In this way, 44 of the 47 sources (94%) in the sample were observed above and at or below their spectral peak. Spectral decomposition allowed us to identify 3, 11, 7, and 2 objects as compact symmetric objects, compact doubles, core-jet and complex sources respectively. However, many of the sources classified as compact double or core-jet sources show only two components making their classification rather tentative. This may explain why the strong morphological dichotomy of GPS quasars and galaxies found for radio-bright GPS sources, is not as clear in this faint sample.Comment: Latex, 18 pages, 8 figures; MNRAS, accepted. The paper, with higher quality figures, may also be obtained from http://www.ast.cam.ac.uk/~snellen . Minor comments of referee incorporate

Molecular outflows and 1000 AU structure of low mass YSO envelopes

We present the results of an observational study into the molecular outflows and small scale ( ~ 1000 AU) envelope structure of a sample of nine low mass young stellar objects (YSOs) in Taurus. The characteristics of the outflows are derived from ^(12)CO J = 3 - 2 mapping with the James Clerk Maxwell Telescope, while the envelopes are imaged in the HCO^+ 1-0, ^(13)CO 1- 0 and C^(18)O 1-0 emission lines with the Owens Valley Millimeter Array. Using dust envelope continuum fluxes at 1 mm as the basis for an evolutionary ordering, a picture emerges in which the mass, extent, and collimation of outflows decreases over time as the envelopes become less massive, the opening angle of the outflow cavity increases, and mass accretion through the disk slows down. On 1000 AU scales the HCO+ and ^(13)CO/C^(18)O emission in the envelope is closely related to the outflow cavity, often outlining the cavity walls. In addition, the envelopes are clumpy, and two sources appear surrounded by an incomplete ring or torus, 1500-3000 AU in radius. The role of the outflow in shaping the small scale molecular emission may be passive (creating a low-opacity pathway for heating radiation) rather than, or in addition to, active (compressing and shock-heating the material)

The spatial distribution of excited H_2 in T Tau: a molecular outflow in a young binary system

Strong extended emission from molecular hydrogen in the v = 1 → 0 S(l) transition is mapped around T Tau. In addition, the v = 2 → 1 S(l) line is detected close to the star. The ratio of the two transitions is consistent with an excitation process in which both fluorescence by stellar ultraviolet radiation and collisions in a warm, dense medium play a role. The morphology is interpreted as emission from a molecular outflow which appears to wiggle as a result of the fact that T Tau is a binary system seen almost pole-on. It is shown that an outflow with a small opening angle can reproduce the observed extended emission. From comparison with previous studies it is argued that the molecular outflow originates from T Tau S, the infrared component. The presented model constrains the orientation and geometry of the system

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

Magnetic fields play an important role during star formation. Direct magnetic field strength observations have proven specifically challenging in the extremely dynamic protostellar phase. 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 protostars. 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 unsuccessful, 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 rotation. We show that the large range in values of the Land\'{e} g-factors of the hyperfine components of each maser line lead to conclusions which differ substantially from the current interpretation based on a single effective g-factor. These conclusions are more consistent with other observations and confirm the presence of dynamically important magnetic fields around protostars. Additionally, our calculations show that (non-linear) Zeeman effects must be taken into account to further enhance the accuracy of cosmological electron-to-proton mass ratio determinations using methanol.Comment: 23 pages, 3 figures, excluding Supplementary information. Author manuscript version before editorial/copyediting by Nature Astronomy. Journal version available via http://rdcu.be/FPeB . Supplementary material available via https://static-content.springer.com/esm/art%3A10.1038%2Fs41550-017-0341-8/MediaObjects/41550_2017_341_MOESM1_ESM.pd

Subarcsecond Imaging at 267 GHz of a Young Binary System: Detection of a Dust Disk of Radius Less than 70 AU around T Tauri N

The young binary system T Tauri was observed with the Owens Valley Millimeter Array in the 267 GHz continuum and HCO^+ J = 3-2 emission at 0".8 resolution, with the single-baseline interferometer of the James Clerk Maxwell Telescope-Caltech Submillimeter Observatory in the 357 GHz continuum and with the W. M. Keck Telescope at λ = 4 μm. The 267 GHz emission is unresolved, with a flux of 397±35 mJy, located close to the position of the optical star T Tau N. An upper limit of 100 mJy is obtained toward the infrared companion T Tau S. The 357 GHz continuum emission is unresolved, with a flux of 1.35±0.68 Jy. HCO^+ J = 3-2 was detected from a 2" diameter core surrounding T Tau N and S. Both stars are detected at 4 μm, but there is no evidence of the radio source T Tau R. We propose a model in which T Tau S is intrinsically similar to T Tau N but is obscured by the outer parts of T Tau N's disk. A fit to the spectral energy distribution (SED) between 21 cm and 1.22 μm is constructed on this basis. Adopting an r^(−1) surface density distribution and an exponentially truncated edge, disk masses of 0.04±0.01 and 6×10^(−5) to 3×10^(−3) M_☉ are inferred for T Tau N and T Tau S, respectively. A 0.005-0.03 M_☉ circumbinary envelope is also required to fit the millimeter to mid-infrared SED

OH masers in the Milky Way and Local Group galaxies in the SKA era

The intense line emission of OH masers is a perfect tracer of regions where new stars are born aswell as of evolved stars, shedding large amounts of processed matter into the interstellar medium. From SKA deep surveys at 18 cm, where the maser lines from the ground-state of the OH molecule arise, we predict the discovery of more than 20000 sources of stellar and interstellar origin throughout the Galaxy. The study of this maser emission has many applications, including the determination of magnetic field strengths from polarisation measurements, studies of stellar kinematics using the precisely determined radial velocities, and distance determinations from VLBI astrometry. A new opportunity to study shocked gas in different galactic environments is expected to arise with the detection of lower luminosity masers. For the first time, larger numbers of OH masers will be detected in Local Group galaxies. New insights are expected in structure formation in galaxies by comparing maser populations in galaxies of different metallicity, as both their properties as well as their numbers depend on it. With the full capabilities of SKA, further maser transitions such as from excited OH and from methanol will be accessible, providing new tools to study the evolution of star-forming regions in particular.Comment: Contribution to the conference on "Advancing Astrophysics with the Square Kilometre Array" for the SKA science book, Giardini-Naxos, Sicily, June 2014; in Proceedings of Science, 14 page

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions. IV. Magnetic field strength limits and structure for seven additional sources

Magnetohydrodynamical simulations show that the magnetic field can drive molecular outflows during the formation of massive protostars. The best probe to observationally measure both the morphology and the strength of this magnetic field at scales of 10-100 au is maser polarization. We measure the direction of magnetic fields at milliarcsecond resolution around a sample of massive star-forming regions to determine whether there is a relation between the orientation of the magnetic field and of the outflows. In addition, by estimating the magnetic field strength via the Zeeman splitting measurements, the role of magnetic field in the dynamics of the massive star-forming region is investigated. We selected a flux-limited sample of 31 massive star-forming regions to perform a statistical analysis of the magnetic field properties with respect to the molecular outflows characteristics. We report the linearly and circularly polarized emission of 6.7 GHz CH3OH masers towards seven massive star-forming regions of the total sample with the European VLBI Network. The sources are: G23.44-0.18, G25.83-0.18, G25.71-0.04, G28.31-0.39, G28.83-0.25, G29.96-0.02, and G43.80-0.13. We identified a total of 219 CH3OH maser features, 47 and 2 of which showed linearly and circularly polarized emission, respectively. We measured well-ordered linear polarization vectors around all the massive young stellar objects and Zeeman splitting towards G25.71-0.04 and G28.83-0.25. Thanks to recent theoretical results, we were able to provide lower limits to the magnetic field strength from our Zeeman splitting measurements. We further confirm (based on ∼80% of the total flux-limited sample) that the magnetic field on scales of 10-100 au is preferentially oriented along the outflow axes. The estimated magnetic field strength of |B||| > 61 mG and >21 mG towards G25.71-0.04 and G28.83-0.25, respectively, indicates that it dominates the dynamics of the gas in both regions