| 11. |
- Goddi, C., et al.
(författare)
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Measuring magnetic fields from water masers in the synchrotron protostellar jet in W3(H 2 O)
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 597
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Tidskriftsartikel (refereegranskat)abstract
- Context. Magnetic fields are invoked to launch, drive, and shape jets in both low- and high-mass protostars, but observational data on the spatial scales required to assess their role in the protostellar mass-loss process is still scarce. Aims. The Turner-Welch (TW) object in the W3(OH) high-mass star-forming complex drives a synchrotron jet, which is quite exceptional for a high-mass protostar, and is associated with a strongly polarized H2O maser source, W3(H2O), making it an optimal target to investigate the role of magnetic fields on the innermost scales of protostellar disk-jet systems. Methods. We report full polarimetric VLBA observations of H2O masers towards W3(H2O). Their linearly polarized emission provides clues on the orientation of the local magnetic field (on the plane of the sky), while the measurement of the Zeeman splitting provides its strength (along the line-of-sight). The linear scales probed by H2O masers are tens to hundreds of AU (at the W3(H2O) distance, ~2 kpc), inaccessible to other star-formation tracers. Results. We identified a total of 148 individual maser features and we measured their physical properties. Out of 148, we measured linear polarization in 34 features, with a fractional percentage varying in the range 0.9-42%, making W3(H2O) the highest-polarized H2O maser source observed with VLBI known in the Galaxy. The H2O masers trace a bipolar, biconical outflow at the center of the synchrotron jet. Although on scales of a few thousand AU the magnetic field inferred from the masers is on average orientated along the flow axis, on smaller scales (10s to 100s of AU), we have revealed a misalignment between the magnetic field and the velocity vectors, which arises from the compression of the field component along the shock front. We also detected circularly polarized emission toward ten maser features, with a fractional percentage varying in the range 0.2-1.6%. In the gas shocked by the synchrotron jet, we estimate a total field strength in the range ~100-300 mG (at densities of 109 cm-3). We conclude that fields of this order of magnitude are expected if the observed polarized water masers emerge behind magnetically supported shocks which, propagating in the W3(H2O) hot core (with an initial density of order of 107 cm-3), compress and enhance the field component perpendicular to the shock velocity (with an initial field strength of a few mG). We constrain the magnetic field strength in the pre-shock circumstellar gas (which is dominated by the component parallel to the flow motion) to at least 10-20 mG (at densities of 107 cm-3), consistent with previous estimates from a synchrotron jet model and dust polarization measurements. Conclusions. In W3(H2O), the magnetic field would evolve from having a dominant component parallel to the outflow velocity in the pre-shock gas, with field strengths of the order of a few tens of mG, to being mainly dominated by the perpendicular component of order of a few hundred of mG in the post-shock gas where the H2O masers are excited. The general implication is that in the undisturbed (i.e., not-shocked) circumstellar gas, the flow velocities would follow closely the magnetic field lines, while in the shocked gas the magnetic field would be reconfigured to be parallel to the shock front.
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| 12. |
- Keimpema, A., et al.
(författare)
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The SFXC software correlator for very long baseline interferometry: algorithms and implementation
- 2015
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Ingår i: Experimental Astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 39:2, s. 259-279
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Tidskriftsartikel (refereegranskat)abstract
- In this paper a description is given of the SFXC software correlator, developed and maintained at the Joint Institute for VLBI in Europe (JIVE). The software is designed to run on generic Linux-based computing clusters. The correlation algorithm is explained in detail, as are some of the novel modes that software correlation has enabled, such as wide-field VLBI imaging through the use of multiple phase centres and pulsar gating and binning. This is followed by an overview of the software architecture. Finally, the performance of the correlator as a function of number of CPU cores, telescopes and spectral channels is shown.
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| 13. |
- Lankhaar, Boy, 1991, et al.
(författare)
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Characterization of methanol as a magnetic field tracer in star-forming regions
- 2018
-
Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 2, s. 145-150
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic fields play an important role during star formation1. Direct magnetic field strength observations have proven particularly challenging in the extremely dynamic protostellar phase2,3,4. 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 methanol9,10,11,12.
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| 14. |
- Lankhaar, Boy, 1991, et al.
(författare)
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Quantum-Chemical calculations revealing the effects of magnetic fields on methanol masers
- 2017
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 13:S336, s. 23-26
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Tidskriftsartikel (refereegranskat)abstract
- Maser observations of both linearly and circularly polarized emission have provided unique information on the magnetic field in the densest parts of star forming regions, where non-maser magnetic field tracers are scarce. While linear polarization observations provide morphological constraints, magnetic field strengths are determined by measuring the Zeeman splitting in circularly polarized emission. Methanol is of special interest as it is one of the most abundant maser species and its different transitions probe unique areas around the protostar. However, its precise Zeeman-parameters are unknown. Experimental efforts to determine these Zeeman-parameters have failed. Here we present quantum-chemical calculations of the Zeeman-parameters of methanol, along with calculations of the hyperfine structure that are necessary to interpret the Zeeman effect in methanol. We use this model in re-analyzing methanol maser polarization observations. We discuss different mechanisms for hyperfine-state preference in the pumping of torsion-rotation transitions involved in the maser-action.
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| 15. |
- van Langevelde, H. J., et al.
(författare)
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The Synergy between VLBI and Gaia astrometry
- 2018
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Ingår i: Proceedings of Science. - 1824-8039. ; 344
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Konferensbidrag (refereegranskat)abstract
- With the publication of Gaia DR2, 1.3 billion stars now have public parallax and proper motion measurements. In this contribution, we compare the results for sources that have both optical and radio measurements, focusing on circumstellar masers. For these large, variable and bright AGB stars, the VLBI astrometry results can be more robust. Moreover, there are a number of applications where VLBI astrometry provides unique data for studying stellar populations and Galactic structure. The BeSSel project not only provides parallax and proper motions at much larger distances than Gaia can reach, but it also uniquely samples the spiral arms of the Galaxy. The evolved stars in the BAaDE sample can potentially constrain the dynamics and stellar content of the inner bulge and bar of the Milky Way, not reachable in the optical.
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| 16. |
- Surcis, G., et al.
(författare)
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EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions: III. the flux-limited sample
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 578
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Tidskriftsartikel (refereegranskat)abstract
- Context. 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. Aims. 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. Methods. 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 CH3OH 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. Results. We detected a total of 176 CH3OH masing cloudlets toward the seven massive star-forming regions, 19% of which show linearly polarized emission. The CH3OH 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. Conclusions. 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.
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| 17. |
- Surcis, G., et al.
(författare)
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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
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 623
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Tidskriftsartikel (refereegranskat)abstract
- Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.
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| 18. |
- Surcis, G., et al.
(författare)
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Magnetic field measurements around massive young stellar objects with the EVN
- 2018
-
Ingår i: Proceedings of Science. - 1824-8039. ; 344
-
Konferensbidrag (refereegranskat)abstract
- Although there has been significant progresses in the last years, the formation process of high-mass stars (M>8 Msun) is still unclear. This is mainly due to the fast evolution and large distances of the massive young stellar objects (YSOs) that make difficult to observe, with fine details, a sufficient number of massive YSOs at each evolutionary stages. However, in the last 10 years some pieces of information regarding the gas motion and the magnetic field close to the YSOs have been gathered by observing and analyzing the maser emission of mainly water and methanol molecules. In particular, we have performed full polarization observations of 6.7 GHz methanol masers and/or 22 GHz water maser with the European VLBI Network (EVN) towards a large number of sources in order to provide measurements of magnetic fields orientation and strength at milliarcsecond resolution around massive protostars. These measurements can be made at this high angular resolution only by observing and analyzing the polarized emission of masers. From the linearly and circularly polarized emissions we can determine the orientation and the strength of the magnetic field, respectively. Here, we present the updated statistics of the possible alignment between the magnetic field and the bipolar outflows ejected from the massive protostars. In addition, and for the first time, we will provide lower limits of the magnetic field strength measured from the Zeeman-splitting of the methanol maser by using the very recently determined Landé g-factors for the methanol maser transitions. Furthermore, the up-to-date results of the monitoring project of the 22 GHz water masers detected towards the massive star-forming region W75N(B) will be also presented.
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| 19. |
- Surcis, G., et al.
(författare)
-
Maser Polarization
- 2017
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 13:S336, s. 27-32
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Tidskriftsartikel (refereegranskat)abstract
- Through the observations and the analysis of maser polarization it is possible to measure the magnetic field in several astrophysical environments (e.g., star-forming regions, evolved stars). In particular from the linearly and circularly polarized emissions we can determine the orientation and the strength of the magnetic field, respectively. In these proceedings the implications, on observed data, of the new estimation of the Landé g-factors for the CH3OH maser are presented. Furthermore, some example of the most recent results achieved in observing the polarized maser emission from several maser species will also be reported.
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