scholarly journals Chromospheres, Coronae, and Mass Loss in Stars Hotter than the Sun

1980 ◽  
Vol 5 ◽  
pp. 525-531 ◽  
Author(s):  
Theodore P. Snow
Keyword(s):  
Mass Loss ◽  
Main Sequence ◽  
Emission Lines ◽  
Be Stars ◽  
Main Sequence Stars ◽  
B Stars ◽  
X Ray ◽  
Ob Stars ◽  
F Stars ◽  
Simple Dependence

AbstractReviews of the mass-loss characteristics of OB stars have been published recently, and the present review therefore emphasizes the A and F stars and very recent results on O and B stars. For the F stars, chromospheric indicators are present in the form of emission lines, seen in visible and ultraviolet wavelengths. Winds are present in A supergiants, but not in main sequence stars, although at least a few of the latter are X-ray sources, indicating the possible existence of coronae. Most OB supergiants are X-ray sources as well, indicating, along with the presence of super-ionization, that these stars have coronae. On the main sequence, the O stars and some B stars (including Be stars in many cases) have mass loss with highly-ionized species in the wind. The winds in the O and B stars are commonly variable. The mass-loss rates do not show a simple dependence on luminosity, contrary to the predictions for radiatively-driven winds.

scholarly journals Infrared spectroscopy of Be stars

1994 ◽  
Vol 162 ◽  
pp. 412-413
Author(s):  
R.M. Torres ◽  
A. Damineli-Neto ◽  
J.A. de Freitas Pacheco
Keyword(s):  
Mass Loss ◽  
Near Infrared ◽  
Uv Spectra ◽  
Emission Lines ◽  
Be Stars ◽  
B Stars ◽  
Astrophysical Objects ◽  
Optical Region ◽  
Support Excitation ◽  
Loss Rates

FeII emission lines are present in a variety of astrophysical objects and, in particular, in Be stars, where in some situations they can also be seen in absorption. Selvelli & Araujo (1984) studied a sample of classical Be stars that have FeII emission lines in the optical region. The analysis of IUE spectra of those stars revealed that, for the majority of the objects, neither absorption nor emission FeII features were present in the UV. The conclusion was that their data could not support excitation of FeII by continuum fluorescence. On the other hand, FeIII of circumstellar origin is often seen in absorption in the UV spectra of Be stars (Snow & Stalio 1987 and references therein). This could be an indication that the optical FeII emission lines are originated from recombination and cascade. However, Selvelli & Araujo (1984) argued that, since the multiplet UV 191 of FeII does not appear in emission, that mechanism is probably not relevant. In the present work we report new spectroscopic observations in the near infrared of a sample of 60 Be stars, including the prominent FeII 999.7 nm emission line. This line is also present in the spectra of superluminous B stars for which mass loss rates have recently been estimated (Lopes, Damineli-Neto & Freitas Pacheco 1992). We derived mass loss rates from the infrared line luminosities, in agreement with those derived by other methods. We also found a new evidence of the Be envelope flattening through the FeII/Paδ line ratio.

scholarly journals Observations of stellar winds in early type stars

1981 ◽  
Vol 59 ◽  
pp. 1-18
Author(s):  
Peter S. Conti
Keyword(s):  
Resonance Line ◽  
Optical Spectrum ◽  
Main Sequence ◽  
Emission Lines ◽  
Stellar Winds ◽  
Be Stars ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars ◽  
Line P

I have been asked to review the “observations” of winds in “early-type” stars. This normally means stars of spectral type OB and those of the Wolf-Rayet (WR) class. In this paper I will concentrate on the massive population I stars of these types, and primarily the O and WR classes on which most of the recent work has been done. The early B type supergiants share many of the wind properties of the O stars but the later supergiant types, Be stars, and main sequence stars may not. Stellar winds are a ubiquitous phenomenon among these early type stars (Snow and Morton 1976). We see evidence of their winds in the resonance line P Cygni profiles in the UV region, in the emission lines of Hα and λ4686 He II in the optical spectrum, and in the free-free emission from the ionized plasma as observed in the IR and radio regions of the spectrum.

scholarly journals Pulsation and mass loss across the H-R diagram: From OB stars to Cepheids to red supergiants

2013 ◽  
Vol 9 (S301) ◽  
pp. 205-212
Author(s):  
Hilding R. Neilson
Keyword(s):  
Mass Loss ◽  
Stellar Evolution ◽  
Main Sequence ◽  
Massive Stars ◽  
Main Sequence Stars ◽  
Ob Stars ◽  
Classical Cepheids ◽  
Growing Body ◽  
Red Supergiants ◽  
Enhance Mass

AbstractBoth pulsation and mass loss are commonly observed in stars and are important ingredients for understanding stellar evolution and structure, especially for massive stars. There is a growing body of evidence that pulsation can also drive and enhance mass loss in massive stars and that pulsation-driven mass loss is important for stellar evolution. In this review, I will discuss recent advances in understanding pulsation-driven mass loss in massive main-sequence stars, classical Cepheids and red supergiants and present some challenges remaining.

scholarly journals Non-Radial Pulsation In Be Stars

1986 ◽  
Vol 7 ◽  
pp. 265-272
Author(s):  
John R. Percy
Keyword(s):  
Mass Transfer ◽  
Critical Velocity ◽  
Main Sequence ◽  
Radial Pulsation ◽  
Be Stars ◽  
Hydrogen Line ◽  
Main Sequence Stars ◽  
Rapid Rotation ◽  
B Stars ◽  
Effective Gravity

Be stars are B stars in which emission has been observed in at least one hydrogen line on at least one occasion. Some Be stars are pre-main-sequence stars, mass-transfer binaries, or supergiant stars with extended atmospheres. The majority, however, are classical Be stars: single stars on or near the main sequence. An important characteristic of these stars seems to be their rapid rotation – close to but not at the “critical” velocity at which the effective gravity vanishes at the equator.

scholarly journals Radio Observations of Massive OB Stars

1991 ◽  
Vol 143 ◽  
pp. 315-315 ◽  
Author(s):  
Ian D. Howarth ◽  
Alexander Brown
Keyword(s):  
Mass Loss ◽  
Main Sequence ◽  
Thermal Emission ◽  
Stellar Winds ◽  
X Ray ◽  
Ob Stars ◽  
Hydrogen Recombination ◽  
Flux Limit ◽  
Model Independent ◽  
Loss Rates

The mass-loss rates of O stars and B supergiants are of interest because of their influence on the evolution of these massive stars (among other matters). In principle, the ‘safest’ (i.e. most model-independent) method of determining M is to measure the free-free emission from stellar winds at radio wavelengths. This method is complicated, however, by the existence of poorly understood non-thermal emission in some stars, and by the possibility of hydrogen recombination in the winds of B supergiants.We are in the process of carrying out a VLA survey of OB stars, initially at 3.5cm, to a flux limit of ~0.1mJy. Because all our targets should have thermal emission at detectable levels (based on mass-loss rates from Howarth & Prinja 1989 and terminal velocities from Prinja, Barlow & Howarth 1990), the survey is yielding an unbiassed estimate of the frequency of non-thermal emission. The improved sensitivity of our survey over earlier work defines the log M – log L relationship much more precisely than was previously possible, over a large range in luminosities; and allows us to make definitive statements on recombination in B supergiant winds. Our sample includes the first radio detections of an OC star, of a massive X-ray binary, and of thermal emission from a main-sequence star.

scholarly journals Hot stars observed by XMM-Newton

2018 ◽  
Vol 619 ◽  
pp. A148 ◽  
Author(s):  
Yaël Nazé ◽  
Christian Motch
Keyword(s):  
Spectral Analysis ◽  
Main Sequence ◽  
Be Stars ◽  
Hot Stars ◽  
X Ray ◽  
Ob Stars ◽  
Wide Range

We perform a survey of Oe and Be stars in the X-ray range. To this aim, we cross-correlated XMM-Newton and Chandra catalogs of X-ray sources with a list of Be stars, finding 84 matches in total. Of these, 51 objects had enough counts for a spectral analysis. This paper provides the derived X-ray properties (X-ray luminosities, and whenever possible, hardness ratios, plasma temperatures, and variability assessment) of this largest ever sample of Oe and Be stars. The targets display a wide range in luminosity and hardness. In particular, the significant presence of very bright and hard sources is atypical for X-ray surveys of OB stars. Several types of sources are identified. A subset of stars display the typical characteristics of O-stars, magnetic OB stars, or pre-main-sequence (PMS) objects: their Be nature does not seem to play an important role. However, another subset comprises γ Cas analogs, which are responsible for the luminous and hard detections. Our sample contains seven known γ Cas analogs, but we also identify eight new γ Cas analogs and one γ Cas candidate. This nearly doubles the sample of such stars.

scholarly journals X-ray Emission from Isolated Be Stars

2000 ◽  
Vol 175 ◽  
pp. 156-169 ◽  
Author(s):  
David H. Cohen
Keyword(s):  
High Flux ◽  
Be Stars ◽  
B Stars ◽  
X Ray ◽  
Ob Stars ◽  
Spectral Diagnostics ◽  
Survey Results ◽  
New Generation

AbstractI discuss the X-ray observations of Be stars, and how their properties compare to non-emission B stars. I focus on several specific stars that show high flux levels and variability but also report on several interesting survey results. The Be X-ray properties are discussed in the context of wind-shock X-ray emission from normal OB stars as well as in the context of general mechanisms that have been proposed to explain the Be phenomenon. Finally, I conclude with a discussion of the spectral diagnostics that will be available from the new generation of X-ray telescopes.

scholarly journals Asteroseismic observations of OB stars

2010 ◽  
Vol 6 (S272) ◽  
pp. 433-444 ◽  
Author(s):  
Peter De Cat ◽  
Katrien Uytterhoeven ◽  
Juan Gutiérrez-Soto ◽  
Pieter Degroote ◽  
Sergio Simón-Díaz
Keyword(s):  
Main Sequence ◽  
Massive Stars ◽  
Space Missions ◽  
High Order ◽  
Be Stars ◽  
B Stars ◽  
Ob Stars ◽  
Pulsating Stars ◽  
Different Types

AbstractThe region of the hot end of the main-sequence is hosting pulsating stars of different types and flavours. Pulsations are not only observed for Slowly pulsating B stars (mid to late B-type stars; high order g-modes) and β Cephei stars (early B-type stars; low order p/g-modes) but are also causing variability in Be stars and OB-supergiants. In this review we give an overview of the asteroseismic observations that are currently available for these types of stars. The first asteroseismic results were solely based on ground-based observations. Recently, the arrival of space-based data gathered by space missions like most, corot and kepler has led to important discoveries for massive stars, highlighting their excellent asteroseismic potential. We show that, despite the unprecedented precision of the space-based data, there is still a clear need for ground-based follow-up observations.

scholarly journals Dynamics and variability of winds from single Wolf-Rayet stars

1999 ◽  
Vol 193 ◽  
pp. 157-167
Author(s):  
Stanley P. Owocki ◽  
Kenneth G. Gayley
Keyword(s):  
Mass Loss ◽  
Multiple Scattering ◽  
Optical Depth ◽  
Spectral Distribution ◽  
Radiation Flux ◽  
Emission Lines ◽  
List Type ◽  
Type Number ◽  
Ob Stars ◽  
Realistic Line

We review the dynamics of winds from single Wolf-Rayet stars, with emphasis on the following specific points: (a)The classical “momentum problem” (to explain the large inferred ratio of wind to radiative momentum, η Mv∞/(L/c) ≫ 1) is in principle readily solved through multiple scattering of radiation by an opacity that is sufficiently “gray” in its spectral distribution. In this case, one simply obtains η ≃ τ, where τ is the wind optical depth;(b)Lines with a Poisson spectral distribution yield an “effectively gray” cumulative opacity, with multi-line scattering occuring when the velocity separation between thick lines Δv is less than the wind terminal speed v∞. In this case, one obtains η ≃ v∞/Δv;(c)However, realistic line lists are not gray, and leakage through gaps in the line spectral distribution tends to limit the effective scattering to η ≲ 1;(d)In WR winds, ionization stratification helps spread line-bunches and so fill in gaps, allowing for more effective global trapping of radiation, and thus η > 1;(e)However, photon thermalization can reduce the local effectiveness of line-driving near the stellar core, making it difficult for radiation alone to initiate the wind;(f)The relative complexity of WR wind initiation may be associated with the extensive turbulent structure inferred from observed variabililty in WR wind emission lines;(g)Overall, the understanding of WR winds is perhaps best viewed as an “opacity problem”, i.e., identifying the enhanced opacity that can adequately block the radiation flux throughout the wind, and thus drive a WR mass loss that is much greater than from OB stars of comparable luminosity.

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