27 research outputs found

    High resolution magnetic field measurements in high-mass star-forming regions

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    Three different scenarios have been proposed to explain the formation of high-mass stars. In one of these scenarios, core accretion, massive stars form through gravitational collapse, which involves disk-assisted accretion to overcome radiation pressure. This scenario is similar to the favored picture of low-mass star-formation, in which magnetic fields are thought to play an important role by removing excess angular momentum, thereby allowing accretion to continue onto the star. However, the role of magnetic fields during the protostellar phase of high-mass star-formation is still a debated topic. In particular, it is still unclear how magnetic fields influence the formation and dynamics of disks and outflows. Most current information on magnetic fields close to high-mass protostars comes from polarized maser emissions, which allow us to investigate the magnetic field on small scales (10s - 1000s AU) by using interferometers, such as EVN, MERLIN, and VLBA. The aim of my Ph.D. was to investigate the magnetic fields in 7 massive star-forming regions by observing the polarized emission of methanol and water masers at milliarcsecond resolution

    Astronomical Masers: Polarization Properties Of 22-ghz Water And 6.7-ghz Methanol Masers.

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    By observing the astronomical masers in the Milky Way we can determine for instance high-accurate distances of the hosting Galactic sources (e.g., Galactic star-forming regions) and the kinematic of the gas where the masers arise (e.g., the kinematic of Keplerian accretion disks and outflows in massive star-forming regions). In addition, the bright and narrow spectral line emissions of water and methanol masers are ideal for measuring the Zeeman splitting as well as for determining the orientation of the magnetic field in 3-dimensions around massive young stellar objects (YSOs). Therefore, water and methanol maser species can help us to answer several crucial questions about massive star-formation. For instance, one of the most debated question is whether magnetic fields are important in the formation of high-mass stars (M>8 MsunM>8~\rm{M_{sun}}). The main difficulty in answering this question is related to the fast evolution of the high-mass stars that makes the massive YSOs rare. Furthermore, they are typically found at fairly large distance. Hence, it is very difficult to measure the magnetic fields at distances <100<100 Astronomical Units from the central protostar by using dust polarized emissions. But fortunately, the direct measurement of magnetic fields at small scale (10-100~Astronomical Units) around massive YSOs is possible by observing the polarized emission of masers.\\ \indent In my oral contribution, besides showing the polarization properties of 22-GHz water and 6.7-GHz methanol masers, I will show our most interesting results about the determination of the orientation and of the strength of magnetic fields around massive YSOs. We have also started a systematic study for determining if there exists a real alignment between magnetic fields and the large scale outflows that are launched from the central protostar, which is important to constrain future simulations. Furthermore, we are involved in laboratory and modelling efforts to calibrate the magnitude of the Zeeman effect for methanol masers

    Polarization properties of methanol masers

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    (Abridged) Astronomical masers have been effective tools to study magnetic fields for many years. In particular, methanol can be used to probe different parts of protostars such as accretion discs and outflows, since it produces one of the strongest and the most commonly observed masers in massive star-forming regions. We investigate the polarization properties of selected methanol maser transitions in light of newly calculated methanol Land\'e g-factors and considering hyperfine components. We compare our results with previous observations and we evaluate the effect of preferred hyperfine pumping and non-Zeeman effects. We run simulations using the radiative transfer code CHAMP. We find a dependence of linear and circular polarization fractions on the hyperfine transitions. Preferred hyperfine pumping can explain some high levels of linear and circular polarization and some of the peculiar features seen in the S-shape of observed V-profiles. Methanol masers are not significantly affected by non-Zeeman effects. Our models show that for methanol maser emission, both the linear and circular polarization percentages depend on which hyperfine transition is masing and the degree to which it is being pumped. Since non-Zeeman effects become more relevant at high values of brightness temperatures, it is important to obtain good estimates of these quantities and on maser beaming angles. Better constraints on the brightness temperature will help in understand about the extent to which non-Zeeman effects contribute to the observed polarization percentages. In order to detect separate hyperfine components, an intrinsic thermal line width significantly smaller than the hyperfine separation is required.Comment: Accepted for publication in Astronomy & Astrophysic

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

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    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

    Measuring Magnetic Fields from Water Masers Associated with the Synchrotron Protostellar Jet in W3(H2O)

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    The Turner-Welch 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 water 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. We report here full polarimetric VLBA observations of water masers. The linearly polarized emission from water masers provides clues on the orientation of the local magnetic field, while the measurement of the Zeeman splitting from circular polarization provides its strength. By combining the information on the measured orientation and strength of the magnetic field with the knowledge of the maser velocities, we infer that the magnetic field evolves 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 water 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 re-configured to be parallel to the shock front as a consequence of gas compression

    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

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    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

    Maser Polarization

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    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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