scholarly journals Strange Among the Strange: The B-emission Star γ Cassiopeiæ

2002 ◽  
Vol 187 ◽  
pp. 221-237 ◽  
Author(s):  
P. Harmanec
Keyword(s):  
Unsolved Problem ◽  
Central Star ◽  
Time Variability ◽  
Be Stars ◽  
Iras Source ◽  
B Stars ◽  
X Ray ◽  
Spectroscopic Binary ◽  
Present Understanding

Abstractγ Cas (HD 5394) is an important representative of Be stars, hot B stars exhibiting Balmer and other emission lines in their spectra at certain epochs. Its emission spectrum was observed as early as in 1863 and has undergone spectacular changes since then. γ Cas is also extremely interesting because it qualifies for several phenomenologically defined groups: it is a light and colour variable, long-term radial-velocity variable, X-ray source, IRAS source, rapid line-profile variable, a single-line spectroscopic binary and a central star of a reflection nebula.A critical compilation of the wealth of observational data on the star, with emphasis on its time variability on several time scales, as well as an estimate of the most probable values of all basic physical properties of the object is presented. It is pointed out that in spite of a large quantity of data, systematic and well-calibrated observations are still rather rare. The present understanding of the object is put into perspective of the more general – and as yet unsolved – problem of the formation of Be envelopes.

scholarly journals Ubiquitous C IV 1550 Variability in B and Be Stars

1987 ◽  
Vol 92 ◽  
pp. 286-288 ◽  
Author(s):  
G. Sonneborn ◽  
M.P. Garhart ◽  
C.A. Grady
Keyword(s):  
Time Scale ◽  
High Dispersion ◽  
Be Stars ◽  
Line Profiles ◽  
Standard Star ◽  
B Stars ◽  
Short And Long Term ◽  
Resonance Transitions ◽  
Long Term Behavior

Studies of line profile variability of the ultraviolet 1550 Angstrom resonance transitions of C IV in Be stars (Sonneborn et al. 1986; Grady, et al. 1986a,b) have prompted an investigation into the short- and long-term behavior of the C IV lines in other types of B stars. We present examples of two well-studied Be stars, Omega Orionis and 66 Ophiuchi, and two non-Be stars, Beta Cephei and the standard star Zeta Cassiopeiae. Zeta Cas is also known to be a 53 Per variable (see Cox 1983). The IUE SWP high-dispersion spectra of Beta Cep and Zeta Cas have been obtained from the IUE archives. It has been known for some years that the C IV line profiles in Beta Cep vary in a time scale of several days (Fishel and Sparks, 1980). However, it came as a surprise to discover C IV variability in Zeta Cas. Available data allow us to set an upper limit of several months for the time scale of Zeta Cas C IV variability. The principal difference between the C IV variability in Be and non-Be stars appears to be the magnitude and velocity range of the effect.

scholarly journals The Chandra survey of Carina OB stars

2010 ◽  
Vol 6 (S272) ◽  
pp. 608-609
Author(s):  
Marc Gagné ◽  
Garrett Fehon ◽  
Michael R. Savoy ◽  
David H. Cohen ◽  
Leisa K. Townsley ◽  
...  
Keyword(s):  
Time Variability ◽  
B Stars ◽  
X Ray ◽  
Ob Stars ◽  
High Shock

AbstractWe have combined 22 deep Chandra ACIS-I pointings to map over one square degree of the Carina complex. Our x-ray survey detects 69 of 70 known O-type stars and 61 of 130 known early B stars. The majority of single O stars display soft X-ray spectra and have a mean log LX/Lbol ≈ −7.5 suggesting shocks embedded in the O-star winds. Over OB stars show unusually high X-ray luminosities, high shock temperatures or time variability, not predicted for embedded wind shocks.

scholarly journals Long-term Variability in Be-Star disks

2000 ◽  
Vol 175 ◽  
pp. 422-434 ◽  
Author(s):  
J.H. Telting
Keyword(s):  
Recent Work ◽  
Be Stars ◽  
X Ray ◽  
Be Star ◽  
Disk Growth ◽  
X Ray Binaries ◽  
Disk Loss

AbstractBe stars can switch between non-disk, gradual disk growth, disk-loss events, and shell-line phases. Many aspects of this Be phenomenon are still not understood. In this paper I review recent work on variability in Be-star disks, divided in four different topics: disk growth (Section 1), long-term V/R variations and global disk oscillations (Section 2), spectacular variations (Section 3), and, concisely, the disk variability in Be/X-ray binaries (Section 4).

scholarly journals Detection of X-ray flares from AX J1714.1−3912, the unidentified source near RX J1713.7−3946

2018 ◽  
Vol 612 ◽  
pp. A32 ◽  
Author(s):  
Marco Miceli ◽  
Aya Bamba
Keyword(s):  
Supernova Remnants ◽  
Time Variability ◽  
X Rays ◽  
Energy Bands ◽  
X Ray ◽  
Nonthermal Emission ◽  
Photon Index ◽  
Unidentified Source

Context. Molecular clouds are predicted to emit nonthermal X-rays when they are close to particle-accelerating supernova remnants (SNRs), and the hard X-ray source AX J1714.1−3912, near the SNR RX J1713.7−3946, has long been considered a candidate for diffuse nonthermal emission associated with cosmic rays diffusing from the remnant to a closeby molecular cloud. Aim. We aim at ascertaining the nature of this source by analyzing two dedicated X-ray observations performed with Suzaku and Chandra. Methods. We extracted images from the data in various energy bands, spectra, and light curves and studied the long-term evolution of the X-ray emission on the basis of the ~4.5 yr time separation between the two observations. Results. We found that there is no diffuse emission associated with AX J1714.1−3912, which is instead the point-like source CXOU J171343.9−391205. We discovered rapid time variability (timescale ~ 103 s), together with a high intrinsic absorption and a hard nonthermal spectrum (power law with photon index Γ ~ 1.4). We also found that the X-ray flux of the source drops down by 1–2 orders of magnitude on a timescale of a few years. Conclusions. Our results suggest a possible association between AX J1714.1−3912 and a previously unknown supergiant fast X-ray transient, although further follow-up observations are necessary to prove this association definitively.

scholarly journals Polaris: Mass and Multiplicity

2006 ◽  
Vol 2 (S240) ◽  
pp. 102-104
Author(s):  
Nancy Remage Evans ◽  
Gail Schaefer ◽  
Howard E. Bond ◽  
Edmund Nelan ◽  
Giuseppe Bono ◽  
...  
Keyword(s):  
High Resolution ◽  
Orbital Period ◽  
Hubble Space Telescope ◽  
X Rays ◽  
Full Extent ◽  
Classical Cepheid ◽  
X Ray ◽  
Spectroscopic Binary ◽  
Advanced Camera For Surveys

AbstractPolaris, the nearest and brightest classical Cepheid, is a member of at least a triple system. It has a wide (18″) physical companion, the F-type dwarf Polaris B. Polaris itself is a single-lined spectroscopic binary with an orbital period of ∼30 years (Kamper 1996). By combining Hipparcos measurements of the instantaneous proper motion with long-term measurements and the Kamper radial-velocity orbit, Wielen et al (2000) have predicted the astrometric orbit of the close companion. Using the Hubble Space Telescope and the Advanced Camera for Surveys' High-Resolution Channel with an ultraviolet (F220W) filter, we have now directly detected the close companion. Based on the Wielen et al orbit, the Hipparcos parallax, and our measurement of the separation (0″.176 ± 0″.002), we find a preliminary mass of 5.0 ± 1.5 M⊙ for the Cepheid and 1.38 ± 0.61 M⊙ for the close companion. These values will be refined by additional HST observations scheduled for the next 3 years.We have also obtained a Chandra ACIS-I image of the Polaris field. Two distant companions C and D are not X-rays sources and hence are not young enough to be physical companions of the Cepheid. There is one additional stellar X-ray source in the field, located 253″ from Polaris A, which is a possible companion. Further investigation of such a distant companion is valuable to confirm the full extent of the system.

scholarly journals Three discoveries of γ Cas analogues from dedicated XMM–Newton observations of Be stars

2020 ◽  
Vol 493 (2) ◽  
pp. 2511-2517 ◽  
Author(s):  
Yaël Nazé ◽  
Christian Motch ◽  
Gregor Rauw ◽  
Shami Kumar ◽  
Jan Robrade ◽  
...  
Keyword(s):  
Spectral Characterization ◽  
Spectral Information ◽  
X Rays ◽  
Be Stars ◽  
Spectral Variability ◽  
Short Term ◽  
X Ray ◽  
Long Term Variations

ABSTRACT In the last years, a peculiarity of some Be stars – their association with unusually hard and intense X-ray emission – was shown to extend beyond a mere few cases. In this paper, we continue our search for new cases by performing a limited survey of 18 Be stars using XMM–Newton. The targets were selected either on the basis of a previous X-ray detection (exosat,rosat, XMM-slew survey) without spectral information available or because of the presence of a peculiar spectral variability. Only two targets remain undetected in the new observations and three other stars display only faint and soft X-rays. Short-term and/or long-term variations were found in one third of the sample. The spectral characterization of the X-ray brightest 13 stars of the sample led to the discovery of three new γ Cas (HD 44458, HD 45995, and V558 Lyr), bringing the total to 25 known cases, and another γ Cas candidate (HD 120678), bringing the total to two.

scholarly journals A Search for White Dwarf Companions of Be Stars

1992 ◽  
Vol 151 ◽  
pp. 347-350
Author(s):  
A.J.M. Piters ◽  
E.J.A. Meurs ◽  
J. Coté ◽  
M.H. Van Kerkwijk ◽  
J. Van Paradijs ◽  
...  
Keyword(s):  
White Dwarf ◽  
Close Binary ◽  
Be Stars ◽  
Infrared Photometry ◽  
B Stars ◽  
X Ray ◽  
Bolometric Luminosity ◽  
Sky Survey ◽  
Multi Wavelength ◽  
Binary Evolution

Preliminary results of an extended multi-wavelength study of bright B- and Be-type stars are presented. This project aims at finding evidence for the existence of white-dwarf companions to Be stars which are predicted as the result of close binary evolution. Comparison of ROSAT all-sky survey data with simultaneous measurements of Hα profiles and infrared photometry suggests that there is no difference in the X-ray behaviour of Be stars with respect to that of normal B stars. The ROSAT X-ray luminosities of B and Be stars range from 10−8 to 10−5 times the bolometric luminosity.

scholarly journals ROSAT Observations of B and Be Stars

1994 ◽  
Vol 162 ◽  
pp. 189-199
Author(s):  
Joseph P. Cassinelli ◽  
David H. Cohen
Keyword(s):  
Large Fraction ◽  
Emission Measure ◽  
X Rays ◽  
Be Stars ◽  
Hard Component ◽  
B Stars ◽  
X Ray ◽  
Wind Speeds ◽  
Sky Survey ◽  
Wind Theory

We present results from a survey of X-ray emission properties of near main-sequence B stars, including several Be and β Cephei stars. The main conclusions of our survey are: 1) The X-rays are soft, probably because the shock velocity jumps are small since the terminal wind speeds are small. 2) A major fraction of the wind emission measure is hot, assuming wind theory estimates for the density distribution. A large fraction of the wind is not expected to be hot in current wind shock models. 3) A hard component is found to be present in τ Sco; possible causes are discussed. 4) For the Be stars, the X-rays emission is from a normal B-star wind that is coming from the poles as in the WCD model of Be stars. 5) None of the stars, including the β Cep stars, show noticeable variability in their X-rays. For the normal B stars we conclude from the lack of variability that the shocks are in the form of fragments in the wind instead of spherical shells. 6) Our observations suggest that all B stars are X-ray sources and that there is a basal amount of X-ray luminosity of about 10-8.5Lboi. The hot component in τ Sco and the high X-ray luminosity of B stars detected in the all-sky survey suggests that there is a source of X-ray emission in addition to wind shocks in some B stars.

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 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.

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