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 EUV Radiation from B Stars: The Broad Implications for Stellar and Interstellar Astronomy

1996 ◽  
Vol 152 ◽  
pp. 367-374
Author(s):  
Joseph P. Cassinelli
Keyword(s):  
Variable Star ◽  
Extreme Ultraviolet ◽  
Early Type ◽  
Large Excess ◽  
Continuum Radiation ◽  
B Stars ◽  
Lyman Continuum ◽  
High Ionization ◽  
Early Type Stars ◽  
Warm Ionized Medium

Observations made with the Extreme Ultraviolet Explorer (EUVE) of the two bright stars ϵ CMa (B2 II) and β CMa (Bl II-III) are discussed. The photospheres show excess EUV radiation. The wind of ϵ CMa exhibits the Bowen Fluorescence mechanism, along with high ionization stages that help explain the nature of the wind shocks. The pulsation and beat phenomena exhibited by the variable star β CMa suggest that deposition of residual pulsation energy might heat and modify the structure of the atmospheres of early-type stars near the β Cephei strip. The possibility that many other B stars show a large excess Lyman continuum radiation is considered as a possible source of the ionization of the warm ionized medium (WIM) in the galactic ISM.

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 Radio and X-Ray Emissions from Chemically Peculiar B- and A-Type Stars: Observations and a Model

1993 ◽  
Vol 138 ◽  
pp. 507-516 ◽  
Author(s):  
Jeffrey L. Linsky
Keyword(s):  
Energy Source ◽  
Stellar Wind ◽  
Mechanical Energy ◽  
Early Type ◽  
Late Type ◽  
X Ray ◽  
Nonthermal Radio ◽  
Chemically Peculiar ◽  
Early Type Stars

AbstractConventional wisdom holds that early-type and late-type stars have very different outer atmospheres, because the early-type stars lack deep convective zones. I argue that the magnetic chemically peculiar (CP) stars hotter than about spectral type A2 display many of the activity phenomena seen in the most active late-type stars. In particular, many CP stars are luminous nonthermal radio and coronal x-ray sources like the RS CVn systems. A wind-fed magnetosphere model has been proposed to explain both the nonthermal radio and the x-ray emission. In this model the stellar wind plays the role of a mechanical energy source analogous to the role played by convection in the active late-type stars.

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 Nonlinear Limb Darkening for Early-Type Stars

1970 ◽  
Vol 75 ◽  
pp. 175 ◽  
Author(s):  
D. A. Klinglesmith ◽  
S. Sobieski
Keyword(s):  
Early Type ◽  
Early Type Stars ◽  
Limb Darkening

scholarly journals Flares in A-type stars?

2015 ◽  
Vol 11 (S320) ◽  
pp. 150-152 ◽  
Author(s):  
May G. Pedersen ◽  
Victoria Antoci ◽  
Heidi Korhonen
Keyword(s):  
High Resolution ◽  
Magnetic Reconnection ◽  
Light Curves ◽  
Spectral Classification ◽  
Early Type ◽  
Late Type ◽  
Optical Telescope ◽  
Preliminary Results ◽  
Stellar Flares ◽  
Early Type Stars

AbstractStellar flares are known to originate from magnetic reconnection in the atmospheres of late–type stars or through radiatively driven wind instabilities in early–type stars. Situated right between these two groups, the A–type stars are not expected to support either of the two mechanisms. However, recent studies report flare features in the Kepler light curves of 32 A–type stars, contradicting theory. We investigate the stars reported in literature, setting strong constraints on the detection criteria. Although significantly fewer, we conclude that flare-like features are present. To determine the origin we obtained high-resolution spectra from the Nordic Optical Telescope (NOT) for the ten brightest, flaring A-type stars for 3-4 epochs. Here we present the preliminary results of these spectroscopic observations, with respect to spectral classification and binarity.

scholarly journals X-ray properties of early-type stars

1994 ◽  
Vol 162 ◽  
pp. 200-201
Author(s):  
Thomas W. Berghöfer ◽  
Jürgen H. M. M. Schmitt
Keyword(s):  
Survey Data ◽  
Early Type ◽  
Late Type ◽  
X Ray ◽  
Sky Survey ◽  
Low Mass ◽  
Early Type Stars ◽  
Einstein Observatory ◽  
Hr Diagram

Extensive stellar surveys with the Einstein Observatory (Chlebowski et al., 1989) and with ROSAT have clearly confirmed the presence of stellar X-ray emission over nearly the whole range of the HR diagram. In the ROSAT all-sky survey data approximately 20000 stellar X-ray sources were detected (Schmitt et al., 1992). Most of these stellar X-ray emitters are low mass late-type stars, the origin of their X-ray emission is thought to be coronal.

scholarly journals Observed Periodic Phenomena

2000 ◽  
Vol 175 ◽  
pp. 178-191 ◽  
Author(s):  
D. Baade
Keyword(s):  
Circumstellar Disks ◽  
Be Stars ◽  
Early Type ◽  
Late Type ◽  
Intrinsic Variability ◽  
Wide Range ◽  
Early Type Stars ◽  
Nonradial Pulsation

AbstractThe wide range of periodic and cyclic variabilities commonly observed in Be stars and their circumstellar disks is reviewed. Many of them are related to the basic nonradial pulsation, the effects of which may range from the photosphere to the disk. Through pulsation-triggered outbursts, some stars may owe their Be characteristics largely to their intrinsic variability. However, because late-type Be stars do not normally show detectable short-periodic variability, this is probably not true of all Be stars. Comparisons are made with other variable early-type stars.

Sign in / Sign up

Export Citation Format

Share Document