MWC349A and B Are Not Gravitationally Bound: New Evidence [SSA]

The age and evolutionary status of MWC349A, the unique emission line star with maser and laser radiation in hydrogen recombination lines, remain unknown because the spectrum of the star is veiled by bright emission from the ionized disk and wind. The major argument for this massive ($>$10 M_sun) star being evolved is its association with a close-by (2.4 arcsec) companion, MWC349B, whose B0III spectrum implies an age of a few Myrs. However, newly obtained high-resolution spectra of MWC349B reveal a difference $\approx$ 35 km/s in the radial velocities of the two stars, which makes their being gravitationally bound highly improbable. An estimate of the relative proper motion of the two stars seems to confirm this conclusion. This reopens the previously suggested possibility that MWC349A is a young massive star in a region of active star formation close to the Cyg OB2 association. MWC349B, which moves with a speed $\geq$ 35 km/s relative to Cyg OB2, may be a runaway star from this association.

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P. Drew, V. Strelnitski, H. Smith, et. al.
Tue, 21 Nov 17

Comments: 5 pages, 2 figures. ApJ accepted

Surface mapping of magnetic hot stars. Theories versus observations [SSA]

This review summarises results of recent magnetic and chemical abundance surface mapping studies of early-type stars. We discuss main trends uncovered by observational investigations and consider reliability of spectropolarimetric inversion techniques used to infer these results. A critical assessment of theoretical attempts to interpret empirical magnetic and chemical maps in the framework of, respectively, the fossil field and atomic diffusion theories is also presented. This confrontation of theory and observations demonstrates that 3D MHD models of fossil field relaxation are successful in matching the observed range of surface magnetic field geometries. At the same time, even the most recent time-dependent atomic diffusion calculations fail to reproduce diverse horizontal abundance distributions found in real magnetic hot stars.

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O. Kochukhov
Tue, 21 Nov 17

Comments: 8 pages, 3 figures; review presented at the conference “Stars with a stable magnetic field: from pre-main sequence to compact remnants”; to be published by Contributions of the Astronomical Observatory Skalnat\’e Pleso

Observations of Running Penumbral Waves emerging in a Sunspot [SSA]

We present results from the investigation of 5-min umbral oscillations in a single-polarity sunspot of active region NOAA 12132. The spectra of TiO, H$\alpha$, and 304 \AA{} are used for corresponding atmospheric heights from the photosphere to lower corona. Power spectrum analysis at the formation height of H$\alpha$ – 0.6 \AA{} to H$\alpha$ center resulted in the detection of 5-min oscillation signals in intensity interpreted as running waves outside the umbral center, mostly with vertical magnetic field inclination $>15\deg$. A phase-speed filter is used to extract the running wave signals with speed $v_{ph}> 4$ km s$^{-1}$, from the time series of H$\alpha$ – 0.4 \AA{} images, and found twenty-four 3-min umbral oscillatory events in a duration of one hour. Interestingly, the initial emergence of the 3-min umbral oscillatory events are noticed closer to or at umbral boundaries. These 3-min umbral oscillatory events are observed for the first time as propagating from a fraction of preceding Running Penumbral Waves (RPWs). These fractional wavefronts rapidly separates from RPWs and move towards umbral center, wherein they expand radially outwards suggesting the beginning of a new umbral oscillatory event. We found that most of these umbral oscillatory events develop further into RPWs. We speculate that the waveguides of running waves are twisted in spiral structures and hence the wavefronts are first seen at high latitudes of umbral boundaries and later at lower latitudes of the umbral center.

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T. Priya, W. Cao, J. Su, et. al.
Tue, 21 Nov 17

Comments: 13 pages, 13 figures, Accepted for publication in The Astrophysical Journal

The Near-infrared Optimal Distances Method Applied to Galactic Classical Cepheids Tightly Constrains Mid-infrared Period–Luminosity Relations [SSA]

Classical Cepheids are well-known and widely used distance indicators. Since distance and extinction are usually degenerate, it is important to develop suitable methods to robustly anchor the distance scale. Here, we introduce a near-infrared (near-IR) optimal distance method to determine both the extinction values of and distances to a large sample of 289 Galactic classical Cepheids. The overall uncertainty in the derived distances is less than 4.9%. We compare our newly determined distances to the Cepheids in our sample with previously published distances to the same Cepheids with Hubble Space Telescope parallax measurements and distances based on the IR surface brightness method, Wesenheit functions, and the main-sequence fitting method. The systematic deviations in the distances determined here with respect to those of previous publications is less than 1-2%. We hence constructed Galactic mid-IR period-luminosity (PL) relations for classical Cepheids in the four Wide-Field Infrared Survey Explorer (WISE) bands (W1, W2, W3, and W4) and the four Spitzer Space Telescope IRAC bands ([3.6], [4.5], [5.8] and [8.0]). Based on our sample of hundreds of Cepheids, the WISE PL relations have been determined for the first time; their dispersion is approximately 0.10 mag. Using the currently most complete sample, our Spitzer PL relations represent a significant improvement in accuracy, especially in the [3.6] band which has the smallest dispersion (0.066 mag). In addition, the average mid-IR extinction curve for Cepheids has been obtained: A_W1/A_Ks=0.560, A_W2/A_Ks=0.479, A_W3/A_Ks=0.507, A_W4/A_Ks=0.406, A_[3.6]/A_Ks=0.481, A_[4.5]/A_Ks=0.469, A_[5.8]/A_Ks=0.427, and A_[8.0]/A_Ks=0.427 mag.

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S. Wang, X. Chen, R. Grijs, et. al.
Tue, 21 Nov 17

Comments: 19 pages, 5 figures, accepted for publication in The Astrophysical Journal

Putting atomic diffusion theory of magnetic ApBp stars to the test: evaluation of the predictions of time-dependent diffusion models [SSA]

A series of recent theoretical atomic diffusion studies has address the challenging problem of predicting inhomogeneous vertical and horizontal chemical element distributions in the atmospheres of magnetic ApBp stars. Here we critically assess the most sophisticated of such diffusion models – based on a time-dependent treatment of the atomic diffusion in a magnetised stellar atmosphere – by direct comparison with observations as well by testing the widely used surface mapping tools with the spectral line profiles predicted by this theory. We show that the mean abundances of Fe and Cr are grossly underestimated by the time-dependent theoretical diffusion model, with discrepancies reaching a factor of 1000 for Cr. We also demonstrate that Doppler imaging inversion codes, based either on modelling of individual metal lines or line-averaged profiles simulated according to theoretical three-dimensional abundance distribution, are able to reconstruct correct horizontal chemical spot maps despite ignoring the vertical abundance variation. These numerical experiments justify a direct comparison of the empirical two-dimensional Doppler maps with theoretical diffusion calculations. This comparison is generally unfavourable for the current diffusion theory, as very few chemical elements are observed to form overabundance rings in the horizontal field regions as predicted by the theory and there are numerous examples of element accumulations in the vicinity of radial field zones, which cannot be explained by diffusion calculations.

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O. Kochukhov and T. Ryabchikova
Tue, 21 Nov 17

Comments: 9 pages, 8 figures; accepted for publication in MNRAS

The clumpy absorber in the high-mass X-ray binary Vela X-1 [HEAP]

Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase ~0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium and neon whose strengths and presence depend on the overall level of absorption. They imply a co-existence of cool and hot gas phases in the system that we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.

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V. Grinberg, N. Hell, I. Mellah, et. al.
Tue, 21 Nov 17

Comments: 17 pages, 14 figures; A&A accepted

Les relations de Milne-Barbier-Unsöld ? [SSA]

This short review aims at clarifying upon the origins of the well-known Eddington-Barbier relationships. The later relate indeed both the emergent specific intensity and the flux to the stellar photospheric source function at certain specific optical depths. Hereafter we shall discuss about the very assumptions used for the original derivation of Barbier (1943). We also point to the fact that Milne (1921) had already formulated these two relations.

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F. Paletou
Tue, 21 Nov 17

Comments: A historical-educational note on analytical radiative transfer (text in French)