Radio emission from the outer parts of stellar atmospheres (mantles) at centimeter wavelengths and the winds of early-type stars

1984 ◽  
Vol 276 ◽  
pp. 583 ◽  
Author(s):  
A. B. Underhill
Keyword(s):  
Radio Emission ◽  
Stellar Atmospheres ◽  
Early Type ◽  
Early Type Stars

scholarly journals The auroral radio emission of the magnetic B-type star ρ OphC

2020 ◽  
Vol 499 (1) ◽  
pp. L72-L76
Author(s):  
P Leto ◽  
C Trigilio ◽  
C S Buemi ◽  
F Leone ◽  
I Pillitteri ◽  
...  
Keyword(s):  
Radio Emission ◽  
Indirect Evidence ◽  
Magnetic Star ◽  
Early Type ◽  
Radiation Beam ◽  
Polar Caps ◽  
Thermal Radio ◽  
Thermal Radio Emission ◽  
Early Type Stars ◽  
Rotational Phase

ABSTRACT The non-thermal radio emission of main-sequence early-type stars is a signature of stellar magnetism. We present multiwavelength (1.6–16.7 GHz) ATCA measurements of the early-type magnetic star ρ OphC, which is a flat-spectrum non-thermal radio source. The ρ OphC radio emission is partially circularly polarized with a steep spectral dependence: the fraction of polarized emission is about $60{{\ \rm per\ cent}}$ at the lowest frequency sub-band (1.6 GHz) while is undetected at 16.7 GHz. This is clear evidence of coherent Auroral Radio Emission (ARE) from the ρ OphC magnetosphere. Interestingly, the detection of the ρ OphC’s ARE is not related to a peculiar rotational phase. This is a consequence of the stellar geometry, which makes the strongly anisotropic radiation beam of the amplified radiation always pointed towards Earth. The circular polarization sign evidences mainly amplification of the ordinary mode of the electromagnetic wave, consistent with a maser amplification occurring within dense regions. This is indirect evidence of the plasma evaporation from the polar caps, a phenomenon responsible for the thermal X-ray aurorae. ρ OphC is not the first early-type magnetic star showing the O-mode dominated ARE but is the first star with the ARE always on view.

scholarly journals Boundary Condition with Mass Loss: The Radiatively-Driven Wind Model

1977 ◽  
Vol 4 (2) ◽  
pp. 175-175
Author(s):  
Dimitri Mihalas
Keyword(s):  
San Francisco ◽  
Stellar Atmospheres ◽  
Current Status ◽  
Early Type ◽  
Wind Model ◽  
Wind Structure ◽  
Sonic Flow ◽  
Early Type Stars ◽  
Coronal Region ◽  
Very High

A brief summary of the current status of radiatively driven wind models for early-type stars is given. A critique of these models is made both on theoretical and observational grounds, and it is concluded that a pure radiatively driven wind is probably not a realistic approximation for 0-star winds. It is argued that probably the wind structure must have an initial high-temperature (“coronal”) region through which the trans-sonic flow takes place, followed by radiative accelerations to very high terminal velocities. Full details of the discussion can be found in Stellar Atmospheres, 2nd Edition, by D. Mihalas, to be published by W. H. Freeman and Company, San Francisco, in Fall 1977.

scholarly journals Observations of velocity fields in WN and Of stars

1979 ◽  
Vol 83 ◽  
pp. 475-478
Author(s):  
Virpi S. Niemelä
Keyword(s):  
Velocity Field ◽  
Large Scale ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Emission Lines ◽  
Temperature Structure ◽  
Early Type ◽  
Velocity Gradients ◽  
Early Type Stars

Systematic wavelength shifts of series of spectral line centers observed in many early type stars, generally interpreted as due to large scale motions, can give us information about the velocity gradients in stellar atmospheres. However, it should be borne in mind that the velocity gradients inferred from the observed displacements of spectral lines may not correspond to a unique alternative (e.g. see Karp 1978). Also, and especially when we are dealing with stars which have emission lines in their spectra, the structure of the velocity field depends on the assumed temperature structure of the atmosphere, i.e. in which atmospheric region do the lines originate.

scholarly journals Polarized Continuum Radiation from Stellar Atmospheres

2014 ◽  
Vol 10 (S305) ◽  
pp. 395-400 ◽  
Author(s):  
J. Patrick Harrington
Keyword(s):  
Rayleigh Scattering ◽  
Accurate Method ◽  
Stellar Atmospheres ◽  
Early Type ◽  
Late Type ◽  
Hydrogen Atoms ◽  
Free Electrons ◽  
Continuum Radiation ◽  
Early Type Stars ◽  
Limb Darkening

AbstractContinuum scattering by free electrons can be significant in early type stars, while in late type stars Rayleigh scattering by hydrogen atoms or molecules may be important. Computer programs used to construct models of stellar atmospheres generally treat the scattering of the continuum radiation as isotropic and unpolarized, but this scattering has a dipole angular dependence and will produce polarization. We review an accurate method for evaluating the polarization and limb darkening of the radiation from model stellar atmospheres. We use this method to obtain results for: (i) Late type stars, based on the MARCS code models (Gustafsson et al. 2008), and (ii) Early type stars, based on the NLTE code TLUSTY (Lanz and Hubeny 2003). These results are tabulated at http://www.astro.umd.edu/~jph/Stellar_Polarization.html While the net polarization vanishes for an unresolved spherical star, this symmetry is broken by rapid rotation or by the masking of part of the star by a binary companion or during the transit of an exoplanet. We give some numerical results for these last cases.

scholarly journals Surface Magnetic Fields in Early-Type Stars

2000 ◽  
Vol 175 ◽  
pp. 334-336 ◽  
Author(s):  
V. S. Airapetian
Keyword(s):  
Magnetic Fields ◽  
Magnetic Activity ◽  
Stellar Atmospheres ◽  
Early Type ◽  
Inertial Oscillations ◽  
Early Type Stars ◽  
Stressed Surface

AbstractRecent observations imply magnetic activity in atmospheres of early-type stars. We explore the possibility that stressed surface magnetic fields can be driven by inertial oscillations, such as r-modes which are vorticity waves. We show that vorticIAL MOTIOns are able to supply helicity to drive magnetic activity in stellar atmospheres.

scholarly journals Fifty Years of Research on Stellar Atmospheres

2001 ◽  
Vol 18 (3) ◽  
pp. 311-316 ◽  
Author(s):  
Dimitri Mihalas
Keyword(s):  
Adult Life ◽  
Stellar Atmospheres ◽  
The Sun ◽  
Early Type ◽  
The Third ◽  
Early Type Stars ◽  
To Come ◽  
Almost All

AbstractI would like to begin by saying what a pleasure it is for me to be here. For my entire adult life I have wanted to come to Australia. Actually, I have been invited to visit here twice before, but each time I was thwarted by circumstances beyond my control. But this time I was determined to (a) prove that the third time is indeed the charm, and (b) pay homage to Walter Stibbs, who in my mind is the epitome of a scholar and a gentleman. I have known Walter as colleague, teacher, and friend, not to mention as an inspiration, both professional and personal. So I am here today to try to give some sense of progress in the study of stellar atmospheres, a field that Walter has graced with his virtuosic touch. I will follow an unabashedly personal path, describing the development as I experienced it. I will focus almost entirely on early-type stars, where we may reasonably expect the atmospheric layers to be homogeneous, and in radiative equilibrium. Only at the end will I mention our nearest stellar neighbor, the Sun, which, because we can study it in so much detail, offers counterexamples to almost all of the the theory that works so well for early-type stars. I offer apologies in advance to anyone this approach may offend.

scholarly journals VLBI Observations of WR Stars

1991 ◽  
Vol 143 ◽  
pp. 87-94
Author(s):  
M. Felli ◽  
M. Massi
Keyword(s):  
Radio Emission ◽  
Brightness Temperature ◽  
Small Region ◽  
Early Type ◽  
Radio Observations ◽  
Thermal Component ◽  
Vlbi Observations ◽  
Early Type Stars

We present the first VLBI radio observations of two WR stars (WR140 and WR146) and of two early type stars (HD167971 and θ1 ORI A). The derived size and brightness temperature suggest that in all cases the radio emission originates from two distinct components: a steady weak emission due to the ionized wind of the primary and a smaller diameter brighter non-thermal component, probably associated to a small region within the ionized envelope of the star.

Some results of the Barbier-Chalonge-Divan system of stellar classification

1966 ◽  
Vol 24 ◽  
pp. 77-90 ◽  
Author(s):  
D. Chalonge
Keyword(s):  
Early Type ◽  
Parameter System ◽  
Early Type Stars ◽  
Two Parameter

Several years ago a three-parameter system of stellar classification has been proposed (1, 2), for the early-type stars (O-G): it was an improvement on the two-parameter system described by Barbier and Chalonge (3).

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