scholarly journals Rapid multicolour variations of selected southern Be stars

1994 ◽  
Vol 162 ◽  
pp. 305-306
Author(s):  
S. Štefl ◽  
P. Harmanec ◽  
D. Baade
Keyword(s):  
Variable Stars ◽  
Be Stars ◽  
Rapid Variability ◽  
Preliminary Results ◽  
Observational Effort

Many previous surveys of the rapid variability of Be stars (e.g., Balona et al. 1987, Cuypers et al. 1989, Balona et al 1992) were limited to the Strömgren b-band only. We have undertaken a new observational effort with the specific aim of studying colour variations. A search for new rapidly variable stars should provide us with statistical material needed to generalize properties of the phenomenon with respect to the stellar parameters. Some preliminary results of these items are presented here.

scholarly journals A spectroscopic study of Be-like variable stars in the Small Magellanic Cloud

2010 ◽  
Vol 6 (S272) ◽  
pp. 308-309
Author(s):  
Beatriz Sabogal ◽  
Alejandro García-Varela ◽  
Ronald E. Mennickent
Keyword(s):  
Spectroscopic Study ◽  
Variable Stars ◽  
Magellanic Cloud ◽  
Be Stars ◽  
Small Magellanic Cloud ◽  
Preliminary Results

AbstractPhotometric searches for Be stars in environments with different metallicities have led to the discovery of many Be-like star variables. The knowledge of these types of variables is still fragmentary. This work presents the preliminary results of analyzing FLAMES+GIRAFFE spectra of a sample of these Be-like stars that we have found in the Small Magellanic Cloud (SMC).

scholarly journals Main-sequence broadening, Be stars, and stellar rotation inhand χ Persei

1994 ◽  
Vol 162 ◽  
pp. 151-152
Author(s):  
J. Denoyelle ◽  
C. Aerts ◽  
C. Waelkens
Keyword(s):  
Main Sequence ◽  
Large Fraction ◽  
Variable Stars ◽  
Theoretical Studies ◽  
Be Stars ◽  
Stellar Rotation ◽  
B Stars ◽  
Fundamental Parameters ◽  
Magnitude Diagram ◽  
Double Cluster

The double cluster h andxPersei is one of the richest clusters containing early-B stars, and therefore is important for observational and theoretical studies on the fundamental parameters of massive stars. The colour-magnitude diagram of the double cluster shows an important scatter (see Figure 1). It has long been known thathandxPersei are extremely rich in Be stars (Slettebak 1968). Our previous contention (Waelkens et al. 1990) that the large-amplitude variable stars we discovered are also Be stars, could be confirmed for a few objects. Rotation velocities for stars inhandxPersei are usually high, which is not surprising in view of the large fraction of Be stars.

scholarly journals Spectro-interferometric observations of a sample of Be stars

2019 ◽  
Vol 621 ◽  
pp. A123 ◽  
Author(s):  
Y. R. Cochetti ◽  
C. Arcos ◽  
S. Kanaan ◽  
A. Meilland ◽  
L. S. Cidale ◽  
...  
Keyword(s):  
Critical Velocity ◽  
Velocity Ratio ◽  
Density Wave ◽  
Wave Structure ◽  
Maximum Size ◽  
Rotation Velocity ◽  
Variable Stars ◽  
High Mass ◽  
Be Stars ◽  
Infrared Excess

Context. Be stars are rapid rotators surrounded by a gaseous disk envelope whose origin is still under debate. This envelope is responsible for observed emission lines and large infrared excess. Aims. To progress in the understanding of the physical processes involved in the disk formation, we estimate the disk parameters for a sample of Be stars and search for correlations between these parameters and stellar properties. Methods. We performed spectro-interferometric observations of 26 Be stars in the region of the Brγ line to study the kinematical properties of their disks through the Doppler effect. Observations were performed at the Paranal observatory with the VLTI/AMBER interferometer. This instrument provides high spectral (R ≃ 12 000) and high spatial (θmin = 4 mas) resolutions. Results. We modeled 18 Be stars with emission in the Brγ line. The disk kinematic is described by a quasi-Keplerian rotation law, with the exception of HD 28497 that presents a one-arm density-wave structure. Using a combined sample, we derived a mean value for the velocity ratio V̅/V̅c = 0.75 (where Vc is the critical velocity), and found that rotation axes are probably randomly distributed in the sky. Disk sizes in the line component model are in the range of 2–13 stellar radii and do not correlate with the effective temperature or spectral type. However, we found that the maximum size of a stable disk correlates with the rotation velocity at the inner part of the disk and the stellar mass. Conclusions. We found that, on average, the Be stars of our combined sample do not rotate at their critical velocity. However, the centrifugal force and mass of the star defines an upper limit size for a stable disk configuration. For a given rotation, high-mass Be stars tend to have more compact disks than their low-mass counterparts. It would be interesting to follow up the evolution of the disk size in variable stars to better understand the formation and dissipation processes of their circumstellar disks.

scholarly journals Short Term Photometric and Spectroscopic Variability of a Sample of Be Stars

1987 ◽  
Vol 92 ◽  
pp. 74-77
Author(s):  
M. Alvarez ◽  
R. Michel
Keyword(s):  
Be Stars ◽  
Short Term ◽  
Preliminary Results ◽  
Cooperative Program ◽  
Short Term Variability

Previous studies of variable Be stars show that the short-term variability is a common, but not well understood phenomena. From a list of ‘candidates’ given by Alvarez (1980) and Schuster & Alvarez (1983) under a cooperative program between France and México, we choose some interesting Be objects to study their behaviour both in spectroscopy and photometry. We are reporting some preliminary results of our observations.

scholarly journals The Carina Dwarf Galaxy Variable Star Population

2002 ◽  
Vol 185 ◽  
pp. 134-135
Author(s):  
V. Ripepi ◽  
M. Dall’Ora ◽  
L. Pulone ◽  
M. Castellani ◽  
C. Corsi ◽  
...  
Keyword(s):  
Variable Star ◽  
Dwarf Galaxy ◽  
Variable Stars ◽  
Photometric Data ◽  
Wide Field ◽  
Preliminary Results

AbstractWe present some preliminary results based on new observations of the variable stars belonging to the Carina Dwarf Galaxy (DG). Photometric data were collected with the two wide field imagers available at ESO ([email protected].) and CTIO (4m prime focus).

scholarly journals Multicolour studies of β Cephei stars in the LMC

2010 ◽  
Vol 6 (S272) ◽  
pp. 286-287
Author(s):  
Chris A. Engelbrecht ◽  
Fabio A. M. Frescura ◽  
Sashin L. Moonsamy
Keyword(s):  
Variable Stars ◽  
Instability Strip ◽  
Model Calculations ◽  
Preliminary Results

AbstractPreliminary results of a four-week multi-colour photometric campaign on previously identified β Cephei stars as well as newly-discovered variable stars in two respective LMC fields are presented. Besides the two targeted β Cephei stars, at least six further presumed B variables are detected. The strongest identified periods appear to lie on the longward end of the galactic β Cep instability strip, as predicted by model calculations.

scholarly journals Simultaneous Spectroscopy and Polarimetry of Be Stars

1987 ◽  
Vol 92 ◽  
pp. 84-86
Author(s):  
D. R. Gies ◽  
David McDavid
Keyword(s):  
Mass Loss ◽  
Emission Line ◽  
Absorption Line ◽  
Line Profile ◽  
Circumstellar Envelope ◽  
Be Stars ◽  
Line Profiles ◽  
Initial Attempt ◽  
Rapid Variability ◽  
Hα Emission

Evidence is now accumulating that many Be stars display photospheric line profile variations on timescales of days or less that are probably caused by nonradial pulsations (Baade 1984; Penrod 1986). In some circumstances these pulsations can promote mass loss into the circumstellar envelope, and consequently the conditions in the inner part of the envelope may vary on similar timescales. Changes in the envelope could produce variations in the polarization and emission line profiles, and observers have reported rapid variability in both. We describe here an initial attempt to search for simultaneous variations in continuum polarization, Hα emission, and the He I λ6678 photospheric absorption line in order to investigate correlated changes on short timescales.

scholarly journals Multimode Detections from Surveys

2000 ◽  
Vol 175 ◽  
pp. 192-204
Author(s):  
C. Aerts
Keyword(s):  
High Accuracy ◽  
The Other ◽  
Time Base ◽  
Be Stars ◽  
Preliminary Results ◽  
Other Hand ◽  
Multiple Frequencies

AbstractIn this study we present an overview of the search for multiple frequencies in variable Be stars from surveys. We review the recent ground-based photometric surveys and conclude from them that multiperiodicity is not detected. The photometric experiment on board Hipparcos, on the other hand, does allow us to derive multiple periods for a few Be stars. We present the results of our frequency searches in the Hipparcos data for 50 λ Eri stars.Further we review the recent spectroscopic campaigns dedicated to some stars. It seems that all of these campaigns point to multiperiodic phenomena, although for most of them the time base is insufficient to derive the periods with high accuracy. Moreover, the spectroscopic periods are often not the same as those found from the photometry. This clearly demonstrates the necessity of long-term simultaneous photometric and spectroscopic campaigns. We present the preliminary results of such a campaign for the λ Eri star HD 105382.We close with reflections on the cause of the variability observed in the λ Eri stars and on the best viewpoint for further research in this area.

scholarly journals Short-term Photometric Variability in Be Stars in the Far-Ultraviolet - A Preliminary Report

1987 ◽  
Vol 92 ◽  
pp. 282-285
Author(s):  
R. S. Polidan ◽  
G. J. Peters
Keyword(s):  
Preliminary Report ◽  
Be Stars ◽  
Ultraviolet A ◽  
Short Term ◽  
Photometric Variability ◽  
Preliminary Results ◽  
Far Ultraviolet

AbstractIn this paper we report preliminary results on the first search for short term photometric variations in the far-UV in Be stars. Results for mu Cen and 28 Cyg indicate no significant variations.

scholarly journals Long-Term Optical Monitoring of Blazars

Galaxies ◽  
2019 ◽  
Vol 7 (2) ◽  
pp. 58
Author(s):  
Kosmas Gazeas
Keyword(s):  
Variable Stars ◽  
X Rays ◽  
Rapid Variability ◽  
New Era ◽  
Sky Surveys ◽  
Long Term Study ◽  
Detailed Classification ◽  
Wide Range ◽  
The Time Domain

Systematic monitoring of specific targets in the optical regime was historically applied on a very narrow sample of known variable stars. The discovery of blazars in the 20th century brought to the foreground the need for new global sky surveys, covering the entire sky and fainter sources. Full-sky surveys are conducted more easily from space observatories, while radio telescopes perform follow up observations from the ground. Blazars are detected in a wide range of energies, while they exhibit strong variability in various wavelengths from γ-rays and X-rays to the optical and radio domain. This results in a detailed classification, according to their emission properties in each region. The rapid variability in optical domain makes blazars interesting targets for optical sky surveys, offering a new opportunity to study their variability in the time domain. Digital sky surveys in optical and near-IR found a fertile ground with the aid of sensitive sensors. Only a few dedicated programs are focusing on blazar variability, a trend which evolved rapidly in the last decade. Modern techniques, in combination with dedicated sky survey programs lead towards a new era of long-term monitoring of blazars, aiming towards the search or variability on various time scales. In this work, an overview of blazar optical surveys and monitoring projects is given, addressing the major points of each one, and highlighting the constraints that the long-term study of blazars will bring through future international campaigns.

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