| 1. |
- Surcis, G., et al.
(författare)
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Rapidly increasing collimation and magnetic field changes of a protostellar H2O maser outflow
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 565
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Green, J.A., et al.
(författare)
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Maser astrometry with VLBI and the SKA
- 2014
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Ingår i: Proceedings of Science. - 1824-8039. ; 9-13-June-2014
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Konferensbidrag (refereegranskat)abstract
- We discuss the unique opportunities for maser astrometry with the inclusion of the Square Kilometre Array (SKA) in Very Long Baseline Interferometry (VLBI) networks. The first phase of the SKA will enable observations of hydroxyl and methanol masers, positioning the latter to an accuracy of 5 microarcseconds, and the second phase may allow water maser observations. These observations will provide trigonometric distances with errors as small as 1%. The unrivalled sensitivity of the SKA will enable large-scale surveys and, through joint operations, will turn any VLBI network into a fast astrometry device. Both evolved stars and high mass star formation regions will be accessible throughout the (Southern) Milky Way, completing our understanding of the content, dynamics and history of our Galaxy. Maser velocities and proper motions will be measurable in the Local Group of galaxies and beyond, providing new insights into their kinematics and evolution.
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| 3. |
- Surcis, G., et al.
(författare)
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EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 541
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Tidskriftsartikel (refereegranskat)abstract
- Context. The role of magnetic fields in the formation of high-mass stars is still under debate, and recent measurements of their orientation and strength by using polarized maser emissions are contributing new insights. Masers polarization, in particular of the 6.7-GHz methanol masers, are one of the best probes of the magnetic field morphologies around massive protostars.Aims. Determining the magnetic field morphology around an increasing number of massive protostars at milliarcsecond resolution by observing 6.7-GHz methanol masers is crucial to better understand the role of magnetic fields in massive star formation.Methods. The First EVN Group consists of 4 massive star-forming complexes: W51, W48, IRAS 18556+0138, and W3(OH). These contain well-studied H II regions from some of which molecular bipolar outflows were also detected (W51-e2, G35.20-0.74N). Nine of the European VLBI Network (EVN) antennas were used to measure the linear polarization and Zeeman-splitting of the 6.7-GHz methanol masers in the star-forming regions of the First EVN Group.Results. We detected a total of 154 CH3OH masers, one third of these towards W3(OH). Fractional linear polarization (1.2-11.5%) was detected towards 55 CH3OH masers. The linear polarization vectors are well-ordered in all the massive star-forming regions. We measured significant Zeeman-splitting in 3 massive star-forming regions (W51, W48, and W3(OH)) revealing a range of separations -3.5 ms(-1)
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| 4. |
- Surcis, G., et al.
(författare)
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EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions: II. First statistical results
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556:Aug., s. article no. A73-
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 5. |
- Surcis, G., et al.
(författare)
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Magnetic field measurements at milliarcsecond resolution around massive young stellar objects
- 2014
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Ingår i: Proceedings of Science. - 1824-8039.
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Konferensbidrag (refereegranskat)abstract
- Magnetic fields have only recently been included in theoretical simulations of high-mass star formation. The simulations show that magnetic fields can play a crucial role not only in the formation and dynamics of molecular outflows, but also in the evolution of circumstellar disks. Therefore, new measurements of magnetic fields at milliarcsecond resolution close to massive young stellar objects (YSOs) are fundamental for providing new input for numerical simulations and for understanding the formation process of massive stars. The polarized emission of 6.7 GHz CH3OH masers allows us to investigate the magnetic field close to the massive YSO where the outflows and disks are formed. Recently, we have detected with the EVN CH3OH maser polarized emission towards 10 massive YSOs. From a first statistical analysis we have found evidence that magnetic fields are primarily oriented along the molecular outflows. To improve our statistics we are carrying on a large observational EVN campaign for a total of 19 sources, the preliminary results of the first seven sources are presented in this contribution. Furthermore, we also describe our efforts to estimate the Landé g-factors of the CH3OH maser transition to determine the magnetic field strength from our Zeeman-splitting measurements. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
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| 6. |
- Torstensson, Karl, 1977, et al.
(författare)
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Methanol masers and millimetre lines: a common origin in protostellar envelopes
- 2012
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 8:5287, s. 146-150
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Konferensbidrag (refereegranskat)abstract
- To understand the origin of the CH3OH maser emission, we map the distribution and excitation of the thermal CH3OH emission in a sample of 14 relatively nearby (
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| 7. |
- Amiri, N., et al.
(författare)
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VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology
- 2012
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 8:5287, s. 54-58
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Konferensbidrag (refereegranskat)abstract
- We report on Very Long Baseline Array SiO maser observations of the OH/IR star OH 44.8 - 2.3. The observations show that the maser features form a ring located at a distance of 5.4 AU around the central star. The masers show high fractional linear polarization up to 100%. The polarization vectors are consistent with a dipole field morphology. Additionally, we report a tentative detection of circular polarization of 7% for the brightest maser feature. This indicates a magnetic field of 1.5 ± 0.3 G. The SiO masers and the 1612 MHz OH masers suggest a mildly preferred outflow direction in the circumstellar environment of this star. The observed polarization is consistent with magnetic field structures along the preferred outflow direction. This could indicate the possible role of the magnetic fields in shaping the circumstellar environment of this object.
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| 8. |
- Amiri, N., et al.
(författare)
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VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 538
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Tidskriftsartikel (refereegranskat)abstract
- Context. SiO maser emission occurs in the extended atmosphere of evolved stars and can be studied at high angular resolution. As compact, high-brightness components they can be used as important tracers of the dynamics at distances close to the central star. The masers also serve as probes of the evolutionary path from spherically symmetric AGB stars to aspherical PNe. Very long baseline interferometry (VLBI) observations of Mira variables indicate that SiO masers are significantly linearly polarized with linear polarization fractions up to 100%. However, no information is available at high angular resolution for SiO masers in higher mass loss OH/IR stars. Theory indicates a different SiO pumping mechanism in higher mass loss evolved stars.Aims. We extend the VLBI SiO maser studies to OH/IR stars. The observations enable us to understand the SiO pumping mechanisms in higher mass-loss evolved objects and to compare them with Mira variables. Additionally, polarimetric observations of SiO masers help us to understand the magnetic field strength and morphology and to distinguish between conflicting polarization theories.Methods. The 43 GHz SiO maser observations of the OH/IR star OH 44.8-2.3 were performed with the VLBA in full polarization spectral line mode. Auxiliary EVLA observations were performed to allow for the absolute calibration of the polarization angle. The Zeeman splitting was measured by cross correlating the right and left circular polarization spectra, as well as the S-curve fitting. Additionally, we analyzed the 1612 MHz OH maser observations of OH 44.8-2.3 from the VLA archive.Results. The SiO masers of OH 44.8-2.2 form a ring located at ~5.4 AU around the star. The masers appear to be highly linearly polarized with fractional linear polarization up to 100%. The linear polarization vectors are consistent with a dipole field morphology in this star. We report a tentative detection of circular polarization of ~0.7% for the brightest maser feature. The magnetic field measured for this feature corresponds to 1.5 ± 0.3 G. Additionally, the distribution of the 1612 MHz OH maser emission could indicate an elongated morphology.Conclusions. The SiO masers in OH 44.8-2.3 exhibit a ring morphology. Even though the central AGB star of OH 44.8-2.3 is expected to be larger than typical Mira variables, the SiO masers occur at a similar distance from the stellar photosphere as Mira variables. The SiO masers and the 1612 MHz OH maser emission suggest a mildly preferred direction of the outflow in the CSE of this star. Significant linear polarization is measured for the SiO region of this star, which could develop from either collisional or radiative pumping. In either case, the observed polarization is also consistent with magnetic field structures along the preferred outflow direction. This could reflect the possible role of the magnetic field in shaping the circumstellar environment of this object. Although we cannot firmly distinguish between the different polarization theories, the derived magnetic field strength assuming standard Zeeman emission is fully consistent with other maser polarization measurements.
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| 9. |
- Richards, A. M. S., et al.
(författare)
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Sub-mm maser VLBI: How do stellar winds break free from the star's gravity?
- 2014
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Ingår i: Proceedings of Science. - 1824-8039.
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Konferensbidrag (refereegranskat)abstract
- Sub-mm VLBI with ALMA will be a superb tool for imaging masers, as well as its capabilities as the "Event Horizon Telescope". Individual masing clouds will be resolved in multiple velocity channels, providing both beamed and unbeamed sizes. The relationship between beaming and intensity can then be used to distinguish between steady or turbulent and shocked outflows. Sub-au resolution will also confirm which transitions are co-spatial or segregated, compared with maser models predicting the required small-scale variations in density, temperature, velocity structure and radiation field. This will be aided further by micro-arcsec cm-wave maser results which are already starting to appear from RadioAstron. ALMA by itself will resolve the overall structure and differing extents of the various maser species, along with thermal lines, dust and continuum. Science verification data has already provided the first-ever resolved images of sub-mm water masers and continuum around an evolved star (VY CMa), showing that the high excitation lines are surprisingly extended. This requires local heating, maybe shocks, at many tens of stellar radii. These lines straddle the dust formation zone and future higher resolution observations will also resolve dust clumps and measure their acceleration. Such studies are vital to understanding how material ejected from cool stars breaks free from the stellar gravity, and the role of dust in this process.
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| 10. |
- Surcis, G., et al.
(författare)
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High resolution magnetic field measurements in high-mass star-forming regions using masers
- 2012
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 8:5287, s. 69-73
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Konferensbidrag (refereegranskat)abstract
- The bright and narrow spectral line emission of masers is ideal for measuring the Zeeman-splitting as well as for determining the orientation of magnetic fields in 3-dimensions around massive protostars. Recently, polarization observations at milliarcsecond resolution of 6.7-GHz CH3OH masers have uniquely been able to resolve the morphology of magnetic fields close to massive protostars. The observations reveal that the magnetic fields are along outflows and/or on the surfaces of circumstellar tori. Here we present three different examples selected from a total number of 7 massive star-forming regions that were investigated at 6.7-GHz with the EVN in the last years.
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