| 1. |
- Beltran, M. T., et al.
(författare)
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Self-similarity of the magnetic field at different scales: The case of G31.41+0.31
- 2024
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 686
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Tidskriftsartikel (refereegranskat)abstract
- Context . Dust polarization observations of the massive protocluster G31.41+0.31 carried out at similar to 1 '' (3750 au) resolution with the SMA at 870 mu m have revealed one of the clearest examples to date of an hourglass-shaped magnetic field morphology in the high-mass regime. Additionally, similar to O.'' 24 (similar to 900 au) resolution observations with ALMA at 1.3 mm have confirmed these results. The next step is to investigate whether the magnetic field maintains its hourglass-shaped morphology down to circumstellar scales. Aims . To study the magnetic field morphology toward the four (proto)stars A, B, C, and D contained in G31.41+0.31 and examine whether the self-similarity observed at core scales (1 '' and 0.'' 24 resolution) still holds at circumstellar scales, we carried out ALMA observations of the polarized dust continuum emission at 1.3 mm and 3.1 mm at an angular resolution of similar to 0.'' 068 (similar to 250 au), sufficient to resolve the envelope emission of the embedded protostars. Methods . We used ALMA to perform full polarization observations at 233 GHz (Band 6) and 97.5 GHz (Band 3) with a synthesized beam of 0.'' 072 x 0.'' 064. We carried out polarization observations at two different wavelengths to confirm that the polarization traces magnetically aligned dust grains and is not due to dust self-scattering. Results . The polarized emission and the direction of the magnetic field obtained at the two wavelengths are basically the same, except for an area between the embedded sources C and B. In such an area, the emission at 1.3 mm could be optically thick and affected by dichroic extinction. In the rest of the core, the similarity of the emission at the two wavelengths suggests that the polarized emission is due to magnetically aligned grains. The polarized emission has been successfully modeled with a poloidal field with a small toroidal component on the order of 10% of the poloidal component, with a position angle phi = -63 degrees, an inclination i = 50 degrees, and a mass-to-flux ratio lambda = 2.66. The magnetic field axis is oriented perpendicular to the NE-SW velocity gradient detected in the core. The strength of the plane-of-the-sky component of the mean magnetic field, estimated using both the Davis-Chandrasekhar-Fermi and the polarization-intensity gradient methods, is in the range similar to 10-80 mG, for a density range 1.4 x 10(7)-5 x 10(8) cm(-3). The mass-to-flux ratio is in the range lambda similar to 1.9-3.0, which suggests that the core is "supercritical". The polarization-intensity gradient method indicates that the magnetic field cannot prevent gravitational collapse inside the massive core. The collapse in the external part of the core is (slightly) sub-Alfvenic and becomes super-Alfvenic close to the center. Conclusions . Dust polarization measurements from large core scales to small circumstellar scales, in the hot molecular core G31.41+0.31 have confirmed the presence of a strong magnetic field with an hourglass-shaped morphology. This result suggests that the magnetic field could have a relevant role in regulating the star-forming process of massive stars at all scales, although it cannot prevent the collapse. However, it cannot be ruled out that the large opacity of the central region of the core may hinder the study of the magnetic field at circumstellar scales. Therefore, high-angular resolution observations at longer wavelengths, tracing optically thinner emission, are needed to confirm this self-similarity.
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| 2. |
- Fedriani, Rubén, 1991, et al.
(författare)
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The sharpest view on the high-mass star-forming region S255IR: Near infrared adaptive optics imaging of the outbursting source NIRS3
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 676
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Tidskriftsartikel (refereegranskat)abstract
- Context. Massive stars have an impact on their surroundings from early in their formation until the end of their lives. However, very little is known about their formation. Episodic accretion may play a crucial role in the process, but only a handful of observations have reported such events occurring in massive protostars. Aims. We aim to investigate the outburst event from the high-mass star-forming region S255IR where the protostar NIRS3 recently underwent an accretion outburst. We follow the evolution of this source both in photometry and morphology of its surroundings. Methods. We performed near infrared adaptive optics observations on the S255IR central region using the Large Binocular Telescope in the Ks broadband as well as the H2 and Brγ narrow-band filters with an angular resolution of 07.06, close to the diffraction limit. Results. We discovered a new near infrared knot north-east of NIRS3 that we interpret as a jet knot that was ejected during the last accretion outburst and observed in the radio regime as part of a follow-up after the outburst. We measured a mean tangential velocity for this knot of 450 ± 50 km s1. We analysed the continuum-subtracted images from H2, which traces jet-shocked emission, and Brγ, which traces scattered light from a combination of accretion activity and UV radiation from the central massive protostar. We observed a significant decrease in flux at the location of NIRS3, with K = 13.48 mag being the absolute minimum in the historic series. Conclusions. Our observations strongly suggest a scenario where the episodic accretion is followed by an episodic ejection response in the near infrared, as was seen in the earlier radio follow-up. The ~2 μm photometry from the past 30 yr suggests that NIRS3 might have undergone another outburst in the late 1980s, making it the first massive protostar with such evidence observed in the near infrared.
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| 3. |
- 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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| 4. |
- Sanna, A., et al.
(författare)
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Velocity and magnetic fields within 1000 AU of a massive YSO
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 583, s. L3-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Surcis, G., et al.
(författare)
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The magnetic field at milliarcsecond resolution around IRAS 20126+4104
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 563
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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