Study of a sample of faint Be stars in the exofield of CoRoT

Context. The class of Be stars are the epitome of rapid rotators in the main sequence. These stars are privileged candidates for studying the incidence of rotation on the stellar internal structure and on non-radial pulsations. Pulsations are considered possible mechanisms to trigger mass-ejection phenomena required to build up the circumstellar disks of Be stars. Aims. Time series analyses of the light curves of 15 faint Be stars observed with the CoRoT satellite were performed to obtain the distribution of non-radial pulsation (NRP) frequencies in their power spectra at epochs with and without light outbursts and to discriminate pulsations from rotation-related photometric variations. Methods. Standard Fourier techniques were employed to analyze the CoRoT light curves. Fundamental parameters corrected for rapid-rotation effects were used to study the power spectrum as a function of the stellar location in the instability domains of the Hertzsprung–Russell (H-R) diagram. Results. Frequencies are concentrated in separate groups as predicted for g-modes in rapid B-type rotators, except for the two stars that are outside the H-R instability domain. In five objects the variations in the power spectrum are correlated with the time-dependent outbursts characteristics. Time-frequency analysis showed that during the outbursts the amplitudes of stable main frequencies within 0.03 c d−1 intervals strongly change, while transients and/or frequencies of low amplitude appear separated or not separated from the stellar frequencies. The frequency patterns and activities depend on evolution phases: (i) the average separations between groups of frequencies are larger in the zero-age main sequence (ZAMS) than in the terminal age main sequence (TAMS) and are the largest in the middle of the MS phase; (ii) a poor frequency spectrum with f ≲ 1 cd−1 of low amplitude characterizes the stars beyond the TAMS; and (iii) outbursts are seen in stars hotter than B4 spectral type and in the second half of the MS. Conclusions. The two main frequency groups are separated by δf = (1.24 ± 0.28) × frot in agreement with models of prograde sectoral g-modes (m = −1, −2) of intermediate-mass rapid rotators. The changes of amplitudes of individual frequencies and the presence of transients correlated with the outburst events deserve further studies of physical conditions in the subatmospheric layers to establish the relationship between pulsations and sporadic mass-ejection events.

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The Evolution of Rapidly Rotating B Stars (Review Paper)

International Astronomical Union Colloquium ◽

10.1017/s0252921100116665 ◽

1987 ◽

Vol 92 ◽

pp. 486-499

Author(s):

Robert Connon Smith

Keyword(s):

Main Sequence ◽

Review Paper ◽

Rotation Axis ◽

Evolutionary Stage ◽

Radial Pulsation ◽

Be Stars ◽

Rotating Stars ◽

B Stars ◽

Sequence Band ◽

Main Effects

AbstractBe stars are located in or near the main-sequence band for non-rotating stars. Although this stage of evolution is relatively well understood, there are two main effects that make it impossible to say whether all Be stars are in the same stage of evolution and, if so, what that stage is. One effect is the spread in observed magnitude and colour as a result of rotation. Correction for rotation is not possible because of the unknown inclination of any particular star's rotation axis to the line of sight and because it is not clear what the internal angular momentum distribution is or how it changes as a result of evolution. The other effect is that there are uncertainties in the theoretical evolutionary tracks because the amount of convective overshooting is unclear. Other mixing mechanisms that might in principle also confuse the tracks seem to be small near the main sequence. If Be stars are related to the β Cephei and 53 Persei stars in the same part of the HR diagram, then the non-radial pulsation properties of Be stars may give a clue to their evolutionary state. The existence of a circumstellar disc or ring, however, tells us very little about the evolutionary stage of the underlying star. A useful way forward may be to try to understand individual stars in as much detail as possible.

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Photometric and H-Alpha Variability in Some Be Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100116124 ◽

1987 ◽

Vol 92 ◽

pp. 101-103

Author(s):

S. Catalano ◽

G. Umana

Keyword(s):

Magnetic Fields ◽

Time Scales ◽

Time Scale ◽

Small Amplitude ◽

Rotation Period ◽

Light Curves ◽

Radial Pulsation ◽

Be Stars ◽

Relative Movement ◽

Short Term

Short-term variations, typically with small amplitude (Δm < 0.1 mag.), time-scales of hours or near one day and in many cases correlated with the rotation period have been found to be commonplace among Be stars (see Harmanec and Pavlovski 1983 for review and papers). Radial and non-radial pulsation modes have been proposed to explain this variability. However, the light curves are often double-peaked, at first suggesting the stars may have large organized dipole magnetic fields and spots not unlike those in the Ap or Bp stars (Harmanec 1983). These rotationally modulated variations are not stable, and seem to vary in both amplitude and period in the sense that intervals with well defined light curves alternate with intervals when variations are absent. This phenomenon is very reminiscent of formation and apparent relative movement in longitude of spots (groups) in some RS CVn binaries (Catalano 1983, Rodono 1986), but on a much more rapid time scale.

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Inflection point in the power spectrum of stellar brightness variations

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936018 ◽

2020 ◽

Vol 633 ◽

pp. A32 ◽

Cited By ~ 10

Author(s):

A. I. Shapiro ◽

E. M. Amazo-Gómez ◽

N. A. Krivova ◽

S. K. Solanki

Keyword(s):

Power Spectrum ◽

Inflection Point ◽

Rotation Period ◽

Power Spectra ◽

Light Curves ◽

Stellar Rotation ◽

Space Telescopes ◽

Rotation Periods ◽

Active Stars ◽

Total Irradiance

Context. Considerable effort has gone into using light curves observed by such space telescopes as CoRoT, Kepler, and TESS for determining stellar rotation periods. While rotation periods of active stars can be reliably determined, the light curves of many older and less active stars, such as stars that are similar to the Sun, are quite irregular. This hampers the determination of their rotation periods. Aims. We aim to examine the factors causing these irregularities in stellar brightness variations and to develop a method for determining rotation periods for low-activity stars with irregular light curves. Methods. We extended the Spectral And Total Irradiance Reconstruction approach for modeling solar brightness variations to Sun-like stars. We calculated the power spectra of stellar brightness variations for various combinations of parameters that define the surface configuration and evolution of stellar magnetic features. Results. The short lifetime of spots in comparison to the stellar rotation period, as well as the interplay between spot and facular contributions to brightness variations of stars with near solar activity, cause irregularities in their light curves. The power spectra of such stars often lack a peak associated with the rotation period. Nevertheless, the rotation period can still be determined by measuring the period where the concavity of the power spectrum plotted in the log–log scale changes its sign, that is, by identifying the position of the inflection point. Conclusions. The inflection point of the (log–log) power spectrum is found to be a new diagnostic for stellar rotation periods which is shown to work even in cases where the power spectrum shows no peak at the rotation rate.

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Fully Nonstationary Spatially Variable Ground Motion Simulations Based on a Time-Varying Power Spectrum Model

Mathematical Problems in Engineering ◽

10.1155/2014/293182 ◽

2014 ◽

Vol 2014 ◽

pp. 1-10

Author(s):

Huiguo Chen ◽

Yingmin Li ◽

Junru Ren

Keyword(s):

Power Spectrum ◽

Ground Motion ◽

Response Spectrum ◽

Power Spectra ◽

Response Spectra ◽

Simulation Method ◽

Time Varying ◽

Time Frequency ◽

Response Characteristics ◽

Spectrum Model

By analyzing the evolutionary spectrum method for multivariate nonstationary stochastic processes, a simulation method for fully nonstationary spatially variable ground motion is proposed based on the Kameda time-varying power spectrum model. This method can properly simulate nonstationary spatially variable ground motion based on a target response spectrum. Two numerical examples, in which the Kameda time-varying power spectra are calculated for different conditions, are presented to demonstrate the capabilities of the proposed method. In the first example, the nonstationary spatially variable ground motion that satisfies the time-frequency characteristics and response characteristics of the original ground motion is simulated by identifying the parameters of the given time-varying power spectrum. In the second example, the ground motion that satisfies the design response spectra is simulated by defining the parameters of the time-varying power spectrum directly. The results demonstrate that the method can effectively simulate nonstationary spatially variable ground motion, which implies that the proposed method can be used in engineering applications.

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Non-Radial Pulsation In Be Stars

Highlights of Astronomy ◽

10.1017/s1539299600006535 ◽

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.

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An empirical pipeline for determining the viscosity parameter for Be star disks

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314006541 ◽

2014 ◽

Vol 9 (S307) ◽

pp. 133-134

Author(s):

Leandro R. Rímulo ◽

Alex C. Carciofi ◽

Thomas Rivinius ◽

Xavier Haubois

Keyword(s):

Radiative Transfer ◽

Temporal Evolution ◽

Variable Mass ◽

Circumstellar Disks ◽

Light Curves ◽

Viscosity Parameter ◽

Ongoing Effort ◽

Be Star ◽

Mass Ejection

AbstractBe star phenomenology is strongly associated with their viscous circumstellar disks. Recently, models became available for the temporal evolution of these disks when subject to variable mass ejection rates. In this contribution we will discuss how these dynamical disk models, modeled with the radiative transfer code HDUST, can be used for constraining fundamental disk parameters, such as the α viscosity parameter, and we will report on an ongoing effort to model light curves of a large number of stars.

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Spectroscopic Hα and Hγ survey of field Be stars: 2004–2009

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311010611 ◽

2010 ◽

Vol 6 (S272) ◽

pp. 290-291

Author(s):

Erika D. Grundstrom ◽

Douglas R. Gies ◽

Christina Aragona ◽

Tabetha S. Boyajian ◽

E. Victor Garcia ◽

...

Keyword(s):

Main Sequence ◽

Massive Stars ◽

Line Emission ◽

Circumstellar Disks ◽

Circumstellar Disk ◽

Sequence Evolution ◽

Be Stars ◽

B Stars ◽

Significant Variability ◽

The Universe

AbstractMassive O- and B-type stars are “cosmic engines” in the Universe and can be the dominant source of luminosity in a galaxy. The class of Be stars are rapidly rotating B-type stars that lose mass in an equatorial, circumstellar disk (Porter & Rivinius 2003) and cause Balmer and other line emission. Currently, we are unsure as to why these stars rotate so quickly but three scenarios are possible: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence evolution of B stars. In order to investigate these scenarios for this population of massive stars, we have been spectroscopically observing a set of 115 field Be stars with the Kitt Peak Coudè Feed telescope in both the Hα and Hγ wavelength regimes since 2004. This time baseline allows for examination of variability properties of the circumstellar disks as well as determine candidates for closer examination for binarity. We find that 90% of the observed stars show some variability with 8% showing significant variability over the 5-year baseline. Such values may be compared with the significant variability seen in some clusters such as NGC 3766 (McSwain 2008). Also, while ~20% of the sample consists of known binaries, we find that another 15–30% of the sample shows indications of binarity.

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Short-term variability and mass loss in Be stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731187 ◽

2018 ◽

Vol 610 ◽

pp. A70 ◽

Cited By ~ 11

Author(s):

D. Baade ◽

A. Pigulski ◽

Th. Rivinius ◽

A. C. Carciofi ◽

D. Panoglou ◽

...

Keyword(s):

Mass Loss ◽

Circumstellar Matter ◽

Radial Pulsation ◽

Be Stars ◽

Solar Mass ◽

Solar Mass Ejection Imager ◽

Feeder Mechanism ◽

The Difference ◽

Mass Ejection

Context. Be stars are important reference laboratories for the investigation of viscous Keplerian discs. In some cases, the disc feeder mechanism involves a combination of non-radial pulsation (NRP) modes. Aims. We seek to understand whether high-cadence photometry can shed further light on the role of NRP modes in facilitating rotation-supported mass loss. Methods. The BRITE-Constellation of nanosatellites obtained mmag photometry of 28 Cygni for 11 months in 2014–2016. We added observations with the Solar Mass Ejection Imager (SMEI) in 2003–2010 and 118 Hα line profiles, half of which were from 2016. Results. For decades, 28 Cyg has exhibited four large-amplitude frequencies: two closely spaced frequencies of spectroscopically confirmed g modes near 1.5 c/d, one slightly lower exophotospheric (Štefl) frequency, and at 0.05 c/d the difference (Δ) frequency between the two g modes. This top-level framework is indistinguishable from η Cen (Paper I), which is also very similar in spectral type, rotation rate, and viewing angle. The circumstellar (Štefl) frequency alone does not seem to be affected by the Δ frequency. The amplitude of the Δ frequency undergoes large variations; around maximum the amount of near-circumstellar matter is increased and the amplitude of the Štefl frequency grows by a factor of a few. During such brightenings dozens of transient spikes appear in the frequency spectrum; these spikes are concentrated into three groups. Only 11 frequencies were common to all years of BRITE observations. Conclusions. Be stars seem to be controlled by several coupled clocks, most of which are not very regular on timescales of weeks to months but function for decades. The combination of g modes to the slow Δ variability and/or the atmospheric response to it appears significantly non-linear. As in η Cen, the Δ variability seems to be mainly responsible for the modulation of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of Δ frequencies may reach the longest known timescales of the Be phenomenon.

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Spectroscopic Hα and Hγ survey of field Be stars: 2004-2008

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310003157 ◽

2009 ◽

Vol 5 (S262) ◽

pp. 343-344

Author(s):

Erika Grundstrom ◽

Christina Aragona ◽

Tabetha Boyajian ◽

Douglas Gies ◽

Amber ◽

...

Keyword(s):

Main Sequence ◽

Line Emission ◽

Circumstellar Disks ◽

Circumstellar Disk ◽

Sequence Evolution ◽

Be Stars ◽

B Stars ◽

Significant Variability ◽

Vanderbilt University ◽

The Universe

AbstractMassive O- and B-type stars are “cosmic engines” in the Universe and can be the dominant source of luminosity in a galaxy. Be stars are rapidly rotating B-type stars that lose mass in an equatorial, circumstellar disk (Porter & Rivinius 2003) and cause Balmer and other line emission. Currently, we are unsure as to why these stars rotate so quickly but three scenarios are possible: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence evolution of B stars. In order to investigate these scenarios for this population of massive stars, we have been spectroscopically observing a set of 115 field Be stars with the Kitt Peak Coudè Feed telescope in both the Hα and Hγ wavelength regimes since 2004. This time baseline allows for examination of variability properties of the circumstellar disks as well as determine candidates for closer examination for binarity.We find that 90% of the observed stars show some variability with only 4% showing significant variability over the 4-year baseline. Such values may be compared with the significant variability seen in some clusters such as NGC 3766 (McSwain 2008). Also, while 20% of the sample consists of known binaries, we find that another 15-30% of the sample shows indications of binarity.This work has been supported in part by a grant from the Vanderbilt University Learning Sciences Institute, NASA grant # NNX08AV70G, and NSF Career grant AST-0349075.

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Mass-ejection events in Be stars triggered by coupled nonradial pulsation modes

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317001600 ◽

2016 ◽

Vol 12 (S329) ◽

pp. 384-384

Author(s):

D. Baade ◽

Th. Rivinius ◽

A. Pigulski ◽

A. Carciofi ◽

Keyword(s):

Power Spectra ◽

Unknown Origin ◽

Circumstellar Matter ◽

List Type ◽

Be Stars ◽

Aspect Angle ◽

Stellar Pulsation ◽

Mass Ejection ◽

Nonradial Pulsation ◽

High Cadence

AbstractBe stars (for an in-depth review see Rivinius, Carciofi & Martayan 2013) rotate at ⩾80% of the critical velocity and are multi-mode nonradial pulsators. Magnetic dipole fields are not detected, and binaries with periods less than 30 days are rare. The name-giving emission lines form in a Keplerian decretion disk, which is viscously re-accreted and also radiatively ablated unless replenished by outburts of unknown origin.Months-long, high-cadence space photometry with the BRITE-Constellation nanosatellites (Pablo et al. 2016) of about 10 early-type Be stars reveals the following (cf. Baade et al. 2016a, Baade et al. 2016b): ○Many Be stars exhibit 1 or 2 so-called Δ frequencies, which are differences between two nonradial-pulsation (NRP) frequencies and much lower (mostly less than 0.1 c/d) than the parent frequencies. The associated light curves are roughly sinusoidal. The amplitudes can exceed that of the sum of the parent amplitudes.○Conventional beat patterns also occur.○Amplitudes of both Δ and beat frequencies can temporarily be enhanced. Around phases of maximal amplitude the mean brightness is in- or decreased, and the scatter can be enhanced.○During high-activity phases (outbursts), broad and dense groups of numerous spikes arise in the power spectra. The two strongest groups often have a frequency ratio near 2. The phase coherence seems to be low.○Time coverage (less than half a year) is not yet sufficient to infer whether two Δ or beat frequencies can combine to cause long-lasting (years) superoutbursts (cf. Carciofi et al. 2012).From these observations it is concluded: •The variable mean brightness and the increased Δ-frequency amplitude and scatter trace the amount of near-circumstellar matter.•Increase or decrease of mean brightness is aspect-angle dependent (pole-on vs. equator-on).•Increased amounts of near-circumstellar matter are due to rotation-assisted mass ejections caused by coupled NRP modes.•Observations do not constrain the location of the coupling (atmosphere or stellar interior).•Broad frequency groups do not represent stellar pulsation modes but circumstellar variability.•Be stars later than B5 are less active and may in some cases even behave differently.

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