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 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 Radial velocities of early-type stars

1970 ◽  
Vol 38 ◽  
pp. 290-290
Author(s):  
J. P. Kaufmann
Keyword(s):  
Velocity Gradient ◽  
Milky Way ◽  
Radial Velocities ◽  
Circular Motion ◽  
The Sun ◽  
Spectral Classification ◽  
Early Type ◽  
Objective Prism ◽  
Minimum Distances ◽  
Early Type Stars

With the Fehrenbach objective prism radial velocities of about 700 stars of type B0 to A0 were determined in two fields of the Southern Milky Way (lII = 295° bII = −0.6°; lII = 320° bII = −2.5°), with a mean error of ±20 km s−1. An additional photographic UBV-photometry with plates of the ADH-telescope at Boyden Observatory was accomplished. Minimum distances for the stars resulted from absorption-corrected magnitudes and a MK-spectral classification. About 200 stars lay at distances greater than 1.5 kpc from the sun. The largest distances determined were 5 kpc. From the radial velocities and distances circular velocities were derived and plotted against galactocentric distances R. Even within the possible error limits a positive velocity gradient showed up in the range 8 kpc < R < 9.5 kpc, which French authors had already found for the region 10.5 kpc < R < 12.5 kpc. If there do not exist significant deviations from circular motion for these stars, a conformity with Schmidt's 1965 model cannot be obtained.

scholarly journals NRP Modes in a Be Star Zeta Oph

1995 ◽  
Vol 155 ◽  
pp. 301-302
Author(s):  
E. Kambe ◽  
R. Hirata ◽  
M. Kato ◽  
H. Ando ◽  
E. J. Kennelly ◽  
...  
Keyword(s):  
Line Profile ◽  
Early Type ◽  
Period Analysis ◽  
Be Star ◽  
Early Type Stars ◽  
Almost All

AbstractTwo large sinusoidal variations with periods of 3.337 hrs and 2.018 hrs and other smaller variations have been detected from the period analysis of Hel Λ 6678 spectra obtained by our multi-site campaign for ζ Oph in May, 1993. The resultant periodgram is considerably improved (aliasing free) from those in previous publications. The periodicity may be consistent with previous studies except ambiguities of aliasing. It is surprising that almost all detected periods have a common superperiodicity of about 10.05 hrs. We emphasise the importance of multi-site campaign for the study of line-profile variations (lpv) in early-type stars.

scholarly journals Line Profile Variations of Rotating Pulsating Stars

1993 ◽  
Vol 139 ◽  
pp. 182-182
Author(s):  
C. Aerts ◽  
M. De Pauw
Keyword(s):  
Line Profile ◽  
Be Stars ◽  
Line Profiles ◽  
Rotating Stars ◽  
Early Type ◽  
Pulsating Stars ◽  
Doppler Image ◽  
Early Type Stars ◽  
Almost All ◽  
High Degree

Line-profile variations (LPVs) are often seen in early-type stars. They were first detected in the β Cephei stars, where they are due to radial as well as to nonradial pulsations (NRP). It is still a matter of debate whether NRP is also the cause of LPVs in broad-lined B- and Be stars.The study of LPVs of nonradially pulsating stars is often considered to be especially interesting in the case of rapid rotators, because in such stars the line profiles may offer a Doppler image of the stellar surface. Almost all investigators conclude that only sectorial modes with high -values can properly account for the observations of LPVs in broad-lined stars; on the other hand, high-degree modes are almost never seen in slowly rotating stars.We point out here that the expression for the velocity field which is usually adopted, is not valid for rapid rotators.

scholarly journals Review of ground-based observations of spectra relevant to the ultraviolet

1970 ◽  
Vol 36 ◽  
pp. 187-198
Author(s):  
M. W. Feast
Keyword(s):  
Mass Loss ◽  
Extreme Ultraviolet ◽  
The Sun ◽  
Temperature Minimum ◽  
Early Type ◽  
Ultraviolet Spectra ◽  
Early Type Stars

Up to the present the chief results of investigations of stellar line spectra in the extreme ultraviolet have been limited to two fairly distinct fields. In the case of the Sun the data have referred mainly to the chromosphere and also to the region of the temperature minimum at the top of the photosphere and to the corona. We may hope that observations will eventually be extended to the chromospheric ultraviolet spectra of other stars. In the case of the early type stars the most spectacular results so far obtained have concerned the evidence for mass loss by several of these stars. I have therefore chosen to spend most of the available time discussing some of the ways of studying from the ground, first the chromospheres of stars other than the Sun and secondly mass loss from stars.

Sign in / Sign up

Export Citation Format

Share Document