<i>K</i>s- and<i>L</i>p-band polarimetry on stellar and bow-shock sources in the Galactic center

Witzel, G.; Schödel, R.; Eckart, A.

doi:10.1051/0004-6361/201220338

Cited by 20 publications

(43 citation statements)

References 48 publications

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“…It also matches the results of Buchholz et al (2011) well in terms of polarization magnitude, but this is not a strong constraint, as dust inside the shock decreases the outgoing polarization ( § 4.3). However, our simulated position angles produce a centrosymmetric pattern even in cases of dust emission, whereas Buchholz et al (2011Buchholz et al ( , 2013 found a consistent polarization orientation of Φ ∼ −75 • across the bow shock (parallel to the bow head, in contrast with the results of Rauch et al 2013). Thus, while the spherical dust grains we consider could produce the behaviour of the polarization magnitudes observed in IRS 1W, the position angle behaviour suggests elongated grains are more likely, as discussed in Buchholz et al (2011).…”

Section: Resolved Bow Shockmentioning

confidence: 49%

Polarization simulations of stellar wind bow shock nebulae – II. The case of dust scattering

Shrestha

Neilson

Hoffman

et al. 2020

Monthly Notices of the Royal Astronomical Society

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We study the polarization produced by scattering from dust in a bow shock-shaped region of enhanced density surrounding a stellar source, using the Monte Carlo radiative transfer code SLIP. Bow shocks are structures formed by the interaction of the winds of fast-moving stars with the interstellar medium. Our previous study focused on the polarization produced in these structures by electron scattering; we showed that polarization is highly dependent on inclination angle and that multiple scattering changes the shape and degree of polarization. In contrast to electron scattering, dust scattering is wavelength-dependent, which changes the polarization behaviour. Here we explore different dust particle sizes and compositions and generate polarized spectral energy distributions for each case. We find that the polarization SED behaviour depends on the dust composition and grain size. Including dust emission leads to polarization changes with temperature at higher optical depth in ways that are sensitive to the orientation of the bow shock. In various scenarios and under certain assumptions, our simulations can constrain the optical depth and dust properties of resolved and unresolved bow shock-shaped scattering regions.Constraints on optical depth can provide estimates of local ISM density for observed bow shocks. We also study the impact of dust grains filling the region between the star and bow shock. We see that as the density of dust between the star and bow shock increases, the resulting polarization is suppressed for all the optical depth regimes.

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Section: Resolved Bow Shockmentioning

confidence: 49%

Polarization simulations of stellar wind bow shock nebulae – II. The case of dust scattering

Shrestha

Neilson

Hoffman

et al. 2020

Monthly Notices of the Royal Astronomical Society

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“…The alignment mechanism implies a compression of the magnetic field of the Northern Arm in the environs of the bow shocks and the increment of the local field density. These changes produce a rapid magnetic alignment of the grains (Buchholz et al 2013). From the best-fit bow-shock stand-off distances, we have established that their central sources are massive objects (WR stars) with supersonic stellar wind velocities and strong mass-loss rates.…”

Section: Discussionmentioning

confidence: 92%

“…Polarization and emission mechanisms Buchholz et al (2013) described that the bow-shock sources, particularly IRS 21 and IRS 1W, exhibit strong polarization (2) and (3)). The blue axis is centered on the position of the star and the scale is in multiples of R 0 .…”

Section: Bow-shock Emissionmentioning

confidence: 99%

Properties of bow-shock sources at the Galactic center

Sánchez-Bermúdez

Schödel

Alberdi

et al. 2014

A&A

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Context. There exists an enigmatic population of massive stars around the Galactic center (GC) that were formed some Myr ago. A fraction of these stars has been found to orbit the supermassive black hole, Sgr A*, in a projected clockwise disk-like structure, which suggests that they were formed in a formerly existing dense disk around Sgr A*. Aims. We focus on a subgroup of objects, the extended, near-infrared (NIR) bright sources IRS 1W, IRS 5, IRS 10W, and IRS 21, that have been suggested to be young, massive stars that form bow shocks through their interaction with the interstellar medium (ISM). Their nature has impeded accurate determinations of their orbital parameters. We aim at establishing their nature and kinematics to test whether they form part of the clockwise disk. Methods. We performed NIR multiwavelength imaging with NACO/VLT using direct adaptive optics (AO) and AO-assisted sparse aperture masking (SAM). We introduce a new method for self-calibration of the SAM point spread function in dense stellar fields. The emission mechanism, morphology, and kinematics of the targets were examined via 3D models, combined with existing models of the gas flow in the central parsec.Results. We confirm previous findings that IRS 21, IRS 1W, and IRS 5 are bow-shocks created by the interaction between mass-losing stars and the interstellar gas. The nature of IRS 10W remains unclear. Our modeling shows that the bow-shock emission is caused by thermal emission, while the scattering of stellar light does not play a significant role. IRS 1W shows a morphology that is consistent with a bow shock produced by an anisotropic stellar wind or by locally inhomogeneous ISM density. Our best-fit models provide estimates of the local proper motion of the ISM in the Northern Arm that agree with previously published models that were based on radio interferometry and NIR spectroscopy. Assuming that all of the sources are gravitationally tied to Sagittarius A*, their orbital planes were obtained via a Monte Carlo simulation. Conclusions. Our sources appear to be Wolf-Rayet stars associated to the last starburst at the GC. Our orbital analysis suggests that they are not part of any of the previously suggested coherent stellar structures, in particular the clockwise disk. We thus add more evidence to recent findings that a large proportion of the massive stars show apparently random orbital orientations, suggesting either that not all of them were formed in the clockwise disk, or that their orbits were randomized rapidly after formation in the disk.

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“…We tested and calibrated the linear polarization measurement with GRAVITY on sources with known polarization properties (taken from Ott et al 1999;Buchholz et al 2013), namely GCIRS21 (P ≈ 14%; θ ≈ 15 • ), IRS16SW (P ≈ 3.1%; θ ≈ 20 • ), GCIRS33E (P ≈ 5.7%; θ ≈ 35 • ) and IRS1W (P ≈ 1.8%; θ ≈ −37 • ). Here the polarization angle is defined in the range [−90 • , 90 • ] with the angle increasing east of north.…”

Section: A4 Linear Polarization Analysismentioning

confidence: 99%

Detection of orbital motions near the last stable circular orbit of the massive black hole SgrA*

Abuter¹,

Amorim²,

Bauböck³

et al. 2018

A&A

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314

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We report the detection of continuous positional and polarization changes of the compact source SgrA* in high states (“flares”) of its variable near-infrared emission with the near-infrared GRAVITY-Very Large Telescope Interferometer (VLTI) beam-combining instrument. In three prominent bright flares, the position centroids exhibit clockwise looped motion on the sky, on scales of typically 150 μas over a few tens of minutes, corresponding to about 30% the speed of light. At the same time, the flares exhibit continuous rotation of the polarization angle, with about the same 45(±15) min period as that of the centroid motions. Modelling with relativistic ray tracing shows that these findings are all consistent with a near face-on, circular orbit of a compact polarized “hot spot” of infrared synchrotron emission at approximately six to ten times the gravitational radius of a black hole of 4 million solar masses. This corresponds to the region just outside the innermost, stable, prograde circular orbit (ISCO) of a Schwarzschild–Kerr black hole, or near the retrograde ISCO of a highly spun-up Kerr hole. The polarization signature is consistent with orbital motion in a strong poloidal magnetic field.

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Ks- andLp-band polarimetry on stellar and bow-shock sources in the Galactic center

Cited by 20 publications

References 48 publications

Polarization simulations of stellar wind bow shock nebulae – II. The case of dust scattering

Polarization simulations of stellar wind bow shock nebulae – II. The case of dust scattering

Properties of bow-shock sources at the Galactic center

Detection of orbital motions near the last stable circular orbit of the massive black hole SgrA*

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