New studies of a variety of Southern pulsating B stars (original) (raw)
Pulsational properties of ten new slowly pulsating B stars
Astronomy & Astrophysics, 2019
Context. Slowly pulsating B (SPB) stars are upper main-sequence multi-periodic pulsators that show non-radial g-mode oscillations driven by the κ mechanism acting on the iron bump. These multi-periodic pulsators have great asteroseismic potential and can be employed for the calibration of stellar structure and evolution models of massive stars. Aims. We collected a sample of ten hitherto unidentified SPB stars with the aim of describing their pulsational properties and identifying pulsational modes. Methods. Photometric time series data from various surveys were collected and analyzed using diverse frequency search algorithms. We calculated astrophysical parameters and investigated the location of our sample stars in the log Teff vs. log L/L⊙ diagram. Current pulsational models were calculated and used for the identification of pulsational modes in our sample stars. An extensive grid of stellar models along with their g-mode eigenfrequencies was calculated and subsequently cross-mat...
Colour and radial velocity variations in pulsating subluminous B stars
Astrophysics and Space Science, 2004
The utility of pulsations for the investigation of the structure and evolution of subdwarf B stars is considerable. However, the small number of detected modes generally limits the potential for a traditional seismological analysis such as that carried out for the Sun. Therefore, it is crucial to acquire additional primary data to characterise the stellar oscillations more completely. We review recent studies of radial velocity amplitudes and introduce new multi-colour photometry of smallamplitude sdBV stars. We also discuss a set of models for radial velocity and colour variations and demonstrate how these may be used to infer the spherical and azimuthal degrees of observed pulsations.
New Opacities and the Beta Cephei Stars
International Astronomical Union Colloquium
The recent revision of metal opacities has opened up the possibility that the long-sought mechanism for driving pulsations in these stars has at last, been found. This hypothesis makes a testable prediction — that no β Cep variables should exist among metal-poor B-type stars. We report on the results of an intensive CCD monitoring campaign to test this prediction. The question of the pulsation mode of β Cep stars and the consequence of a revision of the absolute magnitudes to accommodate the fundamental radial mode among these stars is also discussed. Pulsational instability due to driving by ionization of metals has many other repercussions for the incidence of pulsation among the B-type stars. A classification scheme for other intrinsically variable B-type stars is suggested. It is shown that if, as generally supposed, pulsation is common among B-type stars then at least two different mechanisms must be in operation.
Monthly Notices of The Royal Astronomical Society, 2006
We report a multisite photometric campaign for the β Cephei star 12 Lacertae. 750 h of high-quality differential photoelectric Strömgren, Johnson and Geneva time-series photometry were obtained with nine telescopes during 190 nights. Our frequency analysis results in the detection of 23 sinusoidal signals in the light curves. Ten of those correspond to independent pulsation modes, and the remainder are combination frequencies. We find some slow aperiodic variability such as that seemingly present in several β Cephei stars. We perform mode identification from our colour photometry, derive the spherical degree ℓ for the five strongest modes unambiguously and provide constraints on ℓ for the weaker modes. We find a mixture of modes of 0 ≤ℓ≤ 4. In particular, we prove that the previously suspected rotationally split triplet within the modes of 12 Lac consists of modes of different ℓ; their equal frequency splitting must thus be accidental.One of the periodic signals we detected in the light curves is argued to be a linearly stable mode excited to visible amplitude by non-linear mode coupling via a 2:1 resonance. We also find a low-frequency signal in the light variations whose physical nature is unclear; it could be a parent or daughter mode resonantly coupled. The remaining combination frequencies are consistent with simple light-curve distortions.The range of excited pulsation frequencies of 12 Lac may be sufficiently large that it cannot be reproduced by standard models. We suspect that the star has a larger metal abundance in the pulsational driving zone, a hypothesis also capable of explaining the presence of β Cephei stars in the Large Magellanic Cloud.
Pulsating B and Be stars in the Small Magellanic Cloud
Astronomy & Astrophysics, 2008
Context. Stellar pulsations in main-sequence B-type stars are driven by the κ-mechanism due to the Fe-group opacity bump. The current models do not predict the presence of instability strips in the B spectral domain at very low metallicities. As the metallicity of the SMC is lower than Z = 0.005, it constitutes a very suitable object to test these predictions. Aims. The main objective is to investigate the existence of B-type pulsators at low metallicities, searching for short-term periodic variability in absorption-line B and Be stars in the SMC. The analysis has been performed in a sample of 313 B and Be stars with fundamental astrophysical parameters accurately determined from high-resolution spectroscopy. Methods. Photometric light curves of the MACHO project have been analyzed using standard Fourier techniques and linear and nonlinear least squares fitting methods. The position of the pulsating stars in the HR diagram has been used to ascertain their nature and to map the instability regions in the SMC. Results. We have detected 9 absorption-line B stars showing short-period variability, two among them being multiperiodic. One star is most likely a β Cephei variable and the remaining 8 are SPB stars. The SPB instability strip in the SMC is shifted towards higher temperatures than the Galaxy. In the Be star sample, 32 stars are short-period variables, 20 among them multiperiodic. 4.9% of B stars and 25.3% of Be stars are pulsating stars. Conclusions. β Cephei and SPB stars do exist at the SMC metallicity. The fractions of SPB stars and pulsating Be stars in the SMC are lower than in the Galaxy. The fraction of pulsating Be stars in the SMC is much higher than the fraction of pulsating absorption-line B stars, as in the Galaxy.
2004
We undertook a multisite photometric campaign for the β Cephei star ν Eridani. More than 600 h of differential photoelectric uvyV photometry were obtained with 11 telescopes during 148 clear nights. The frequency analysis of our measurements shows that the variability of ν Eri can be decomposed into 23 sinusoidal components, eight of which correspond to independent pulsation frequencies between 5 and 8 cd −1. Some of these are arranged in multiplets, which suggests rotational m-mode splitting of non-radial pulsation modes as the cause. If so, the rotation period of the star must be between 30 and 60 d. One of the signals in the light curves of ν Eri has a very low frequency of 0.432 cd −1. It can be a high-order combination frequency or, more likely, an independent pulsation mode. In the latter case, ν Eri would be both a β Cephei star and a slowly pulsating B (SPB) star. The photometric amplitudes of the individual pulsation modes of ν Eri appear to have increased by about 20 per cent over the last 40 years. So have the amplitudes of the dominant combination frequencies of the star. Among the latter, we could only identify sum frequencies with certainty, not difference frequencies, which suggests that neither light-curve distortion in its simplest form nor resonant mode coupling is their single cause. One of our comparison stars, µ Eridani, turned out to be variable with a dominant timescale of 1.62 d. We believe either that it is an SPB star just leaving its instability strip or that its variations are of rotational origin.
Progress on the Front of Pulsating Subdwarf B Stars
Astrophysics and Space Science, 2000
We briefly review the recent advances that have been made on the front of pulsating subdwarf B (sdB) stars. The first family of sdB pulsators, the EC 14026 stars, was discovered a few years ago and consists of short-period (∼100−200 s) p-mode variables. The second type of pulsating sdB's consists of the PG 1716+426 stars, a group of variables showing long-period (∼1 h) g-mode pulsations. The existence of the latter was first reported less than a year ago. While the two types of sdB pulsators differ markedly in their observational characteristics, we recently found a unifying property in the sense that the observed modes in these objects are excited through the same driving process, a classic kappa mechanism associated with the radiative levitation of iron in the stellar envelope.
More on pulsating B-type stars in the Magellanic Clouds
Communications in Asteroseismology, 2009
We present here the results of our research for B-type pulsators in low metallicity environments, searching for short-term periodic variability in a large sample of B and Be stars in the Magellanic Clouds (MC), for which the fundamental astrophysical parameters were accurately determined. A significant number of beta Cephei and SPB-like pulsators at low-metallicity have been detected, conflicting with the current theoretical models of pulsation. In addition, we have placed these pulsating stars in the HR diagram mapping the observational instability regions for the MC metallicities. The large sample of B and Be stars analysed allows 5Aus to make a reliable statistics of the pulsating B-type stars in the MC. Finally, we have made a comparison between pulsational theory and observations in low metallicity environments.
Astronomy and Astrophysics, 2007
In order to better model massive B-type stars, we need to understand the physical processes taking place in slowly pulsating B (SPB) stars, chemically peculiar Bp stars, and non-pulsating normal B stars co-existing in the same part of the H-R diagram. Methods. We carry out a comparative study between samples of confirmed and well-studied SPB stars and a sample of well-studied Bp stars with known periods and magnetic field strengths. We determine their evolutionary state using accurate HIPPARCOS parallaxes and Geneva photometry. We discuss the occurrence and strengths of magnetic fields as well as the occurrence of stellar pulsation among both groups. Further, we make a comparison of Geneva photometric variability for both kinds of stars.
New Links Between Pulsation and Stellar History
Astrophysics and Space Science Proceedings, 2012
Cepheids, particularly about their formation and history. Three approaches are discussed, using space (Hubble and Chandra) and ground-based studies (radial velocities). First, we are conducting a survey of Cepheids with the Hubble Space Telescope Wide Field Camera 3 (WFC3) to identify possible resolved companions (for example Eta Aql) and thus provide constraints on star formation. Followup Xray observations (Chandra and XMM-Newton) can confirm whether possible low mass companions are young enough to be physical companions of Cepheids. In a related study of intermediate mass stars, Chandra X-ray observations of late B stars in Tr 16 have been used to determine the fraction which have X-ray active low mass companions. Finally, the Tennessee State Automatic Spectroscopic Telescope AST and the Moscow University group have obtained velocities of a number of Cepheids. As an example, the orbit of V350 Sgr has been redetermined, providing a new level of accuracy to the orbital velocity amplitude, which is needed for mass determination.
Interpretation of the BRITE oscillation spectra of the early B-type stars : ν Eri and α
2017
ν Eridani is a well known multiperiodic β Cephei pulsator which exhibits also the SPB (Slowly Pulsating B-type stars) type modes. Recent frequency analysis of the BRITE photometry of α Lupi showed that the star is also a hybrid β Cep/SPB pulsator, in which both high and low frequencies were detected. We construct complex seismic models in order to account for the observed frequency range, the values of the frequencies themselves and the non-adiabatic parameter f for the dominant mode. Our studies suggest that significant modifications of the opacity profile at the temperature range log T ∈ (5.0 − 5.5) are necessary to fulfill all these requirements.
Kepler observations of the variability in B-type stars
Monthly Notices of the Royal Astronomical Society, 2011
The analysis of the light curves of 48 B-type stars observed by Kepler is presented. Among these are 15 pulsating stars, all of which show low frequencies, characteristic of slowly pulsating B (SPB) stars. Seven of these stars also show a few weak, isolated high frequencies and they could be considered as SPB/β Cephei (β Cep) hybrids. In all cases, the frequency spectra are quite different from what is seen from ground-based observations. We suggest that this is because most of the low frequencies are modes of high degree which are predicted to be unstable in models of mid-B stars. We find that there are non-pulsating stars within the β Cep and SPB instability strips. Apart from the pulsating stars, we can identify stars with frequency groupings similar to what is seen in Be stars but which are not Be stars. The origin of the
Pulsating B stars in the Scorpius–Centaurus Association with TESS
Monthly Notices of the Royal Astronomical Society
We study 119 B stars located in the Scorpius–Centaurus Association using data from NASA’s TESS Mission. We see pulsations in 81 stars (68 per cent) across the full range of effective temperatures. In particular, we confirm previous reports of low-frequency pulsations in stars whose temperatures fall between the instability strips of SPB stars (slowly pulsating B stars) and δ Scuti stars. By taking the stellar densities into account, we conclude that these cannot be p modes and confirm previous suggestions that these are probably rapidly rotating SPB stars. We also confirm that they follow two period–luminosity relations that are consistent with prograde sectoral g modes that are dipole (l = m = 1) and quadrupole (l = m = 2), respectively. One of the stars (ξ2 Cen) is a hybrid pulsator that shows regular spacings in both g and p modes. We confirm that α Cru has low-amplitude p-mode pulsations, making it one of the brightest β Cephei stars in the sky. We also find several interesting ...
Pulsation among TESS A and B stars reveals unexpected results
arXiv (Cornell University), 2020
Classification of over 50000 TESS stars in sectors 1-18 has resulted in the detection of 766 pulsating main sequence B stars as well as over 5000 δ Scuti, 2300 γ Doradus and 114 roAp candidates. Whereas it has been assumed that high frequency pulsation among B-type main sequence stars are confined to the early B-type β Cephei stars, the observations indicate that high frequencies are to be found over the whole B-star range, eventually merging with δ Scuti stars. The cool B stars pulsating in high frequencies are called Maia variables. It is shown that Maia variables are not rapidly-rotating and thus cannot be β Cephei pulsators which appear to have lower temperatures due to gravity darkening. In the region where β Cephei variables are found, the proportion of pulsating stars is larger and amplitudes are higher and a considerable fraction pulsate in a single mode and low rotation rate. There is no distinct region of slowly-pulsating B stars (SPB stars). Stars pulsating solely in low frequencies are found among all B stars. At most, only one-third of B stars appear to pulsate. These results, as well as the fact that a large fraction of A and B stars show rotational modulation, indicate a need for a revision of current ideas regarding stars with radiative envelopes.
The Potential of Asteroseismology for Hot, Subdwarf B Stars: A New Class of Pulsating Stars
Astrophysical Journal, 1996
We present key sample results of a systematic survey of the pulsation properties of models of hot B subdwarfs. We use equilibrium structures taken from detailed evolutionary sequences of solar metallicity (Z = 0.02) supplemented by grids of static envelope models of various metallicities (Z = 0.02, 0.04, 0.06, 0.08, and 0.10). We consider all pulsation modes with l = 0, 1, 2, and 3 in the 80--1500 s period window, the interval currently most suitable for fast photometric detection techniques. We establish that significant driving is often present in hot B subdwarfs and is due to an opacity bump associated with heavy element ionization. We find that models with Z >= 0.04 show low radial order unstable modes; both radial and nonradial (p, f, and g) pulsations are excited. The unstable models have Teff > 30,000 K, and log g > 5.7, depending somewhat on the metallicity. We emphasize that metal enrichment needs only occur locally in the driving region. On this basis, combined with the accepted view that local enrichments and depletions of metals are common place in the envelopes of hot B subdwarfs, we predict that some of these stars should show luminosity variations resulting from pulsational instabilities.
A Driving Mechanism for the Newly Discovered Long-Period Pulsating Subdwarf B Stars
Astrophysical Journal, 2003
We present the results of a stability survey carried out for a sequence of representative models of subdwarf B stars spanning the range of effective temperature 22; 000 K T eff 38; 000 K. We show that long-period, high-order g-modes are excited in the cooler models through the same -mechanism that successfully explains the presence of short-period, low-order p-modes in the hotter EC 14026 pulsators. This is analogous to the case of the Cep/slowly pulsating B stars on the main sequence. We stress that radiative levitation is needed to boost the iron abundance in the driving region for both types of pulsating subdwarf B stars. And indeed, we find that pulsation modes cannot be excited in B subdwarf models if the metallicity is assumed to be uniform and solar. On the basis of our current models, we propose that the pulsation modes detected in longperiod pulsating subdwarf B stars have values of the degree index l ¼ 3 and/or 4, not the canonical values l ¼ 1; 2, a suggestion that is, in principle, testable through multicolor photometry or time-resolved spectroscopy. In this way, we are able to explain quite well, at least at the qualitative level, the main observed characteristics of these pulsators. On the first account, the excited high-order g-modes with l ¼ 3 and 4 in our models have periods that overlap with the range of quasi-periods observed in these stars. On the second account, if the observable modes in these pulsators have indeed such '' high '' values of l as we suggest, we find a natural explanation for the fact that their amplitudes are distinctly and systematically smaller than the amplitudes observed in EC 14026 stars. Finally, our results are also consistent with the observed fact that the long-period pulsators appear systematically cooler than the short-period EC 14026 stars. We point out, however, that our analysis suggests effective temperatures for the long-period B subdwarf pulsators that are somewhat lower than current spectroscopic estimates. The solution to this problem may come from future improvements in the models, the establishment of an effective temperature scale for subdwarf B stars that is free of systematic effects, or both.