SPB Stars Research Articles

Maia variables and other anomalies among pulsating stars

High-precision photometry from TESS has revealed over 500 stars, located between the δ Scuti and β Cephei instability strips, which pulsate with high frequencies. Models do not predict high pulsation frequencies in these stars. These anomalous variables may be identified with the historical “Maia” variables. From the projected rotational velocities, it is shown that the rotation rates of Maia variables are no different from main sequence or SPB stars in the same effective temperature range. Some Maia stars pulsate at frequencies typical of roAp stars. It is shown that Maia stars should be considered an extension of δ Scuti variables to effective temperatures as high as 18,000 K, rather than as a separate class. The TESS data show a continuous sequence of low-frequency pulsating stars linking the γ Doradus and SPB variables, which is not predicted by the models. There are, in fact, no well-defined instability strips at all among upper main sequence stars, which means that arbitrary choices of effective temperature and frequency ranges need to be made in order to assign a particular variability class. It seems that a mixture of driving mechanisms is present in which convection may play a very important role.

Pulsating B stars in the Scorpius–Centaurus Association with TESS

ABSTRACT 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 binaries, including a very short-period heartbeat star (HD 132094), a previously unknown eclipsing binary (π Lup), and an eclipsing binary with high-amplitude tidally driven pulsations (HR 5846). The results clearly demonstrate the power of TESS for studying variability in stellar associations.

Seismic modelling of a very young SPB star – KIC 8264293

ABSTRACT KIC 8264293 is a fast-rotating B-type pulsator observed by Kepler satellite. Its photometric variability is mainly due to pulsations in high-order g modes. Besides, we detected a weak Hα emission. Thus, the second source of variability is the fluctuation in a disc around the star. The pulsational spectrum of KIC 8264293 reveals a frequency grouping and period spacing pattern. Here, we present the thorough seismic analysis of the star based on these features. Taking into account the position of the star in the HR diagram and fitting the 14 frequencies that form the period spacing, we constrain the internal structure of the star. We conclude that the star barely left the zero-age main sequence and the best seismic model has $M = 3.54\, \mathrm{M}_{\odot }$, $V_\mathrm{rot}=248\, \mathrm{km\, s}^{-1}$, and Z = 0.0112. We found the upper limit on the mixing at the edge of the convective core, with the overshooting parameter up to fov = 0.03. On the other hand, we were not able to constrain the envelope mixing for the star. To excite the modes in the observed frequency range, we had to modify the opacity data. Our best seismic model with an opacity increase by 100 per cent at the ‘nickel’ bump log T = 5.46 explains the whole instability. KIC 8264293 is the unique, very young star pulsating in high-order g modes with the Be feature. However, it is not obvious whether the source of this circumstellar matter is the ejection of mass from the underlying star or whether the star has retained its protostellar disc.

Pulsation among TESS A and B stars and the Maia variables

ABSTRACT Classification of over 50 000 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 pulsations 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 that 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.

Magnetic characterization and variability study of the magnetic SPB staroLupi

Thanks to large dedicated surveys, large-scale magnetic fields have been detected for about 10% of early-type stars. We aim to precisely characterize the large-scale magnetic field of the magnetic component of the wide binaryoLupi, by using high-resolution ESPaDOnS and HARPSpol spectropolarimetry to analyze the variability of the measured longitudinal magnetic field. In addition, we have investigated the periodic variability using space-based photometry collected with the BRITE-Constellation by means of iterative prewhitening. The rotational variability of the longitudinal magnetic field indicates a rotation periodProt = 2.95333(2) d and that the large-scale magnetic field is dipolar, but with a significant quadrupolar contribution. Strong differences in the strength of the measured magnetic field occur for various chemical elements as well as rotational modulation for Fe and Si absorption lines, suggesting a inhomogeneous surface distribution of chemical elements. Estimates of the geometry of the large-scale magnetic field indicatei = 27 ± 10°,β= 74−9+7°, and a polar field strength of at least 5.25 kG. The BRITE photometry reveals the rotation frequency and several of its harmonics, as well as two gravity mode pulsation frequencies. The high-amplitudeg-mode pulsation atf = 1.1057 d−1dominates the line-profile variability of the majority of the spectroscopic absorption lines. We do not find direct observational evidence of the secondary in the spectroscopy. Therefore, we attribute the pulsations and the large-scale magnetic field to the B5IV primary of theoLupi system, but we discuss the implications should the secondary contribute to or cause the observed variability.

KIC 3240411 – the hottest known SPB star with the asymptotic g–mode period spacing

We report the discovery of the hottest hybrid B--type pulsator, KIC\,3240411, that exhibits the period spacing in the low--frequency range. This pattern is associated with asymptotic properties of high-order gravity (g) modes. Our seismic modelling made simultaneously with the mode identification shows that dipole axisymmetric modes best fit the observations. Evolutionary models are computed with MESA code and pulsational models with the linear non-adiabatic code employing the traditional approximation to include the effects of rotation. The problem of mode excitation is discussed. We confirm that significant modification is indispensable to explain an instability of both pressure and gravity modes in the observed frequency ranges of KIC\,3240411.

Triple system HD 201433 with a SPB star component seen by BRITE - Constellation: Pulsation, differential rotation, and angular momentum transfer

Stellar rotation affects the transport of chemical elements and angular momentum and is therefore a key process during stellar evolution, which is still not fully understood. This is especially true for massive stars, which are important for the chemical enrichment of the universe. It is therefore important to constrain their physical parameters and internal angular momentum distribution to calibrate stellar structure and evolution models. Stellar internal rotation can be probed through asteroseismic studies of rotationally split oscillations but such results are still quite rare, especially for stars more massive than the Sun. The SPB star HD201433 is known to be part of a single-lined spectroscopic triple system, with two low-mass companions orbiting with periods of about 3.3 and 154 d. Our results are based on photometric observations made by BRITE - Constellation and the SMEI on board the Coriolis satellite, high-resolution spectroscopy, and more than 96 years of radial velocity measurements. We identify a sequence of 9 rotationally split dipole modes in the photometric time series and establish that HD201433 is in principle a solid-body rotator with a very slow rotation period of 297+/-76 d. Tidal interaction with the inner companion has, however, significantly accelerated the spin of the surface layers by a factor of approximately one hundred. The angular momentum transfer onto the surface of HD201433 is also reflected by the statistically significant decrease of the orbital period of about 0.9 s during the last 96 years. Combining the asteroseismic inferences with the spectroscopic measurements and the orbital analysis of the inner binary system, we conclude that tidal interactions between the central SPB star and its inner companion have almost circularised the orbit but not yet aligned all spins of the system and have just begun to synchronise rotation.

Short-term variability and mass loss in Be stars

Context: [abbreviated] Aims: Kepler data of three known Be stars are re-visited to establish their pulsational nature and assess the properties of additional, non-pulsational variations. The three program stars turned out to be one inactive Be star, one active, continuously outbursting Be star, and one Be star transiting from a non-outbursting into an outbursting phase, thus forming an excellent sample to distill properties of Be stars in the various phases of their life-cycle. Methods: [abbreviated] Results: The short-term photometric variability of Be stars must be disentangled into a stellar and a circumstellar part. The stellar part is on the whole not different from what is seen in non-Be stars. However, some of the observed phenomena might be to be due to resonant mode coupling, a mechanism not typically considered for B-type stars. Short-term circumstellar variability comes in the form of either a group of relatively well-defined, short-lived frequencies during outbursts, which are called \v{S}tefl frequencies, and broad bumps in the power spectra, indicating aperiodic variability on a time scale similar to typical low-order $g$-mode pulsation frequencies, rather than true periodicity. Conclusions: From a stellar pulsation perspective, Be stars are rapidly rotating SPB stars, that is they pulsate in low order $g$-modes, even if the rapid rotation can project the observed frequencies into the traditional high-order $p$-mode regime above about 4 c/d. However, when a circumstellar disk is present, Be star power spectra are complicated by both cyclic, or periodic, and aperiodic circumstellar phenomena, possibly even dominating the power spectrum.

Stars with discrepant v sin i as derived from the Ca II λ3933 Å and Mg II λ4481 Å lines. V. HD 182255 and HD 214923—SPB stars in binary systems

The axial rotation of a star plays an important role in its evolution, the physical conditions in its atmosphere and the appearance of its spectrum.We analyzed the CCD spectra of two stars for which their projected rotational velocity differs remarkably when derived from Ca II λ3933 A and Mg II λ4481 A lines. We estimated the projected rotational velocity of HD182255 to be 15.5 kms−1, although in various spectra of this star the line widths correspond to values as high as 28.5 km s−1. We found the HeI λ4471.498 A line to be shifted to longer wavelengths by 0.046 A, thus indicating a presence of the 3He I isotope in the atmosphere of this star with the 3He : 4He ratio from 0.2 to 0.6.We also found an absorption feature at the position of the forbidden line He I λ4470.02A. We found the lines ofMg II and CII originating from higher excited levels to be missing in the spectra of HD 182255. For HD 214923 we determined the projected rotational velocity v sin i = 165km s−1 from the profiles of the metallic lines and Ca II λ3933A, whereas for helium lines v sin i ≈ 130km s−1 is more appropriate. Radial velocity analysis results in three long periods of ≈ 105, 34, and 15 days, and a short period of ≈ 22 hours, close to the pulsational one mentioned earlier in the literature.

Weak magnetic field, solid-envelope rotation, and wave-induced N-enrichment in the SPB starζCassiopeiae

Aims. The main-sequence B-type star $\zeta$ Cassiopeiae is known as a N-rich star with a magnetic field discovered with the Musicos spectropolarimeter. We model the magnetic field of the star by means of 82 new spectropolarimetric observations of higher precision to investigate the field strength, topology, and effect. Methods. We gathered data with the Narval spectropolarimeter installed at T\'elescope Bernard Lyot (TBL, Pic du Midi, France) and applied the least-squares deconvolution technique to measure the circular polarisation of the light emitted from $\zeta$ Cas. We used a dipole oblique rotator model to determine the field configuration by fitting the longitudinal field measurements and by synthesizing the measured Stokes V profiles. We also made use of the Zeeman-Doppler Imaging technique to map the stellar surface and to deduce the difference in rotation rate between the pole and equator. Results. $\zeta$ Cas exhibits a polar field strength $B_{\rm pol}$ of 100-150 G, which is the weakest polar field observed so far in a massive main-sequence star. Surface differential rotation is ruled out by our observations and the field of $\zeta$ Cas is strong enough to enforce rigid internal rotation in the radiative zone according to theory. Thus, the star rotates as a solid body in the envelope. Conclusions. We therefore exclude rotationally-induced mixing as the cause of the surface N-enrichment. We discuss that the transport of chemicals from the core to the surface by internal gravity waves is the most plausible explanation for the nitrogen overabundance at the surface of $\zeta$ Cas.

Seismic modelling of the rotating, slowly pulsating B-type star HD 21071

Interpretation of the oscillation spectrum of the slowly pulsating B-type star HD21071 is presented. We show that non-rotating models cannot account for the two highest amplitude frequencies and taking into account the effects of rotation is necessary. Rotating seismic models are constructed using various chemical compositions, opacity data, core overshooting parameters and rotational velocities. There are prospects for seismic modelling of SPB stars, even if no asymptotic pattern is observed in their oscillation spectra, provided an unambiguous mode identification is doable and the effects of rotation are properly included.

Wider pulsation instability regions forβCephei and SPB stars calculated using new Los Alamos opacities

Aims. Our goal is to test the newly developed OPLIB opacity tables from Los Alamos National Laboratory and check their influence on the pulsation properties of B-type stars.Methods. We calculated models using MESA and Dziembowski codes for stellar evolution and linear, nonadiabatic pulsations, respectively. We derived the instability domains of β Cephei and SPB-types for different opacity tables OPLIB, OP, and OPAL.Results. The new OPLIB opacities have the highest Rosseland mean opacity coefficient near the so-called Z -bump. Therefore, the OPLIB instability domains are wider than in the case of OP and OPAL data.

Oscillation modes in the rapidly rotating slowly pulsating B-type star μ Eridani

We present results of a search for identification of modes responsible for the six most significant frequency peaks detected in the rapidly rotating SPB star $\mu$ Eridani. All published and some unpublished photometric data are used in our new analysis. The mode identification is carried out with the method developed by Daszy\'nska-Daszkiewicz et al. employing the phases and amplitudes from multi-band photometric data and relying on the traditional approximation for the treatment of oscillations in rotating stars. Models consistent with the observed mean parameters are considered. For the five frequency peaks, the candidates for the identifications are searched amongst unstable modes. In the case of the third frequency, which is an exact multiple of the orbital frequency, this condition is relaxed. The systematic search is continued up to a harmonic degree $\ell =6$. Determination of the angular numbers, $(\ell,m)$, is done simultaneously with the rotation rate, $V_{\rm rot}$, and the inclination angle, $i$, constrained by the spectroscopic data on the projected rotational velocity, $V_{\rm rot}\sin i$, which is assumed constant. All the peaks may be accounted for with g-modes of high radial orders and the degrees $\ell\le 6$. There are differences in some identifications between the models. For the two lowest--amplitude peaks the identifications are not unique. Nonetheless, the equatorial velocity is constrained to a narrow range of (135, 140) km/s. Our work presents the first application of the photometric method of mode identification in the framework of the traditional approximation and we believe that it opens a new promising direction in studies of SPB stars.

KIC 10526294: a slowly rotating B star with rotationally split, quasi-equally spaced gravity modes

Massive stars are important for the chemical enrichment of the universe. Since internal mixing processes influence their lives, it is very important to place constraints on the corresponding physical parameters, such as core overshooting and the internal rotation profile, so as to calibrate their stellar structure and evolution models. Although asteroseismology has been shown to be able to deliver the most precise constraints so far, the number of detailed seismic studies delivering quantitative results is limited. Our goal is to extend this limited sample with an in-depth case study and provide a well-constrained set of asteroseismic parameters, contributing to the ongoing mapping efforts of the instability strips of the beta Cep and SPB stars. We derived fundamental parameters from high-resolution spectra using spectral synthesis techniques. We used custom masks to obtain optimal light curves from the original pixel level data from the Kepler satellite. We used standard time-series analysis tools to construct a set of significant pulsation modes that provide the basis for the seismic analysis carried out afterwards. We find that KIC 10526294 is a cool SPB star, one of the slowest rotators ever found. Despite this, the length of Kepler observations is sufficient to resolve narrow rotationally split multiplets for each of its 19 quasi-equally spaced dipole modes. The number of detected consecutive (in radial order) dipole modes in this series is higher than ever before. The observed amount of splitting shows an increasing trend towards longer periods, which - largely independent of the seismically calibrated stellar models - points towards a non-rigid internal rotation profile. From the average splitting we deduce a rotation period of ~188 d. From seismic modelling, we find that the star is young with a central hydrogen mass fraction X_c>0.64; it has a core overshooting alpha_ov<=0.15.

The puzzling new class of variable stars in NGC 3766: old friend pulsators?

The recent variability survey of the NGC 3766 cluster revealed a considerable number of periodic variable stars in a region of the H-R diagram where no pulsation is expected. This region lies between the instability strips of the delta Scuti and SPB stars. Moreover the periods of the new phenomenon, P~0.1-0.7 d, do not allow to associate it a priori to either of these two types of pulsations. Stars in the NGC 3766 cluster are known as fast rotators with rotational velocities typically larger than half of their critical velocity. Rotation can affect both the geometrical properties and period domain of pulsations. It also alters the apparent stellar luminosity through gravity darkening, effect seldom taken considered in theoretical studies of the rotation-pulsation interaction. We explore if both of these effects are able to deliver a consistent interpretation for the observed properties of the "new variables" in NGC 3766: explaining their presence outside the known instability strips and their variability periods. We carry out an instability analysis of SPB models within the framework of the Traditional Approximation of Rotation and study the visibility of modes according to the angle of view and rotation. We also check how gravity darkening affects the effective temperature and luminosity of stellar models for different angles of view and rotation velocities. At the red (cold) border of the instability strip, prograde sectoral modes are preferentially excited and their visibilities are maximum when seen equator-on. Furthermore low-mass SPB models seen equator-on can appear in the gap between non-rotating SPB and delta Scuti stars due to gravity darkening. In that case, periods of these most visible modes are shifted to the 0.2-0.5 d range due to the effects of the Coriolis force. We hence suggest that the new variable stars observed in NGC 3766 are actually fast rotating SPB pulsators.

Iron-group opacities in the envelopes of massive stars

Abstractβ Cephei and SPB stars are pulsating stars for which the excitation of modes by the κ mechanism, due to the iron-group opacity peak, seems puzzling. We have first investigated the origins of the differences noticed between OP and OPAL iron and nickel opacity calculations (up to a factor 2), a fact which complicates the interpretation. To accomplish this task, new well-qualified calculations (SCO-RCG, HULLAC and ATOMIC) have been performed and compared to values of these tables, and most of the differences are now well understood. Next, we have exploited a dedicated experiment on chromium, iron and nickel, conducted at the LULI 2000 facilities. We found that, in the case of iron, detailed calculations (OP, ATOMIC and HULLAC) show good agreement, contrary to all of the non-detailed calculations. However, in the case of nickel, OP calculations show large discrepancies with the experiments but also with other codes. Thus, the opacity tables need to be revised in the thermodynamical conditions corresponding to the peak of the iron group. Consequently we study the evolution of this iron peak with changes in stellar mass, age, and metallicity to determine the relevant region where these tables should be revised.

Interpretation of the oscillation spectrum of HD 50230 — a failure of richness

AbstractAttempts to interpret the observed oscillation spectrum of the SPB star HD 50230 are reported. We argue that a nearly equidistant period spacing found in the oscillation spectrum of the star is most likely accidental. The observed period distribution requires excitation of modes with the degree l &gt; 4. Much more may be learned from the rich oscillation spectrum of the star but most of the work is still ahead of us.

Two new SB2 binaries with main sequence B-type pulsators in theKeplerfield

Context: OB stars are important in the chemistry and evolution of the Universe, but the sample of targets well understood from an asteroseismological point of view is still too limited to provide feedback on the current evolutionary models. Our study extends this sample with two spectroscopic binary systems. AIMS. Our goal is to provide orbital solutions, fundamental parameters and abundances from disentangled high-resolution high signal-to-noise spectra, as well as to analyse and interpret the variations in the Kepler light curve of these carefully selected targets. This way we continue our efforts to map the instability strips of beta Cep and SPB stars using the combination of high-resolution ground-based spectroscopy and uninterrupted space-based photometry. Methods: We fit Keplerian orbits to radial velocities measured from selected absorption lines of high-resolution spectroscopy using synthetic composite spectra to obtain orbital solutions. We use revised masks to obtain optimal light curves from the original pixel-data from the Kepler satellite, which provided better long term stability compared to the pipeline processed light curves. We use various time-series analysis tools to explore and describe the nature of variations present in the light curve. Results: We find two eccentric double-lined spectroscopic binary systems containing a total of three main sequence B-type stars (and one F-type component) of which at least one in each system exhibits light variations. The light curve analysis (combined with spectroscopy) of the system of two B stars points towards the presence of tidally excited g modes in the primary component. We interpret the variations seen in the second system as classical g mode pulsations driven by the kappa mechanism in the B type primary, and explain the unexpected power in the p mode region as a result of nonlinear resonant mode excitation.

The triple B–star system DV Cam

DV Cam is a triple system showing the diversity of the physics of photospheres of B-type stars at a common age: an SPB star in a wide orbit around a close binary consisting of an ultra-slowly rotating helium-weak star and a much faster rotating mid-B star.

News from the opacity consortium OPAC

The international OPAC consortium (see list below) was formed three years ago. It is composed of astrophysicists, plasma physicists and experimentalists from different laboratories b . This consortium examines specific opacity calculations used in stellar physics. They contribute to solve the problems suggested by the astrophysical community in performing new calculations and new experiments with laser installation. We show here the specific example of the iron opacity peak that plays an important role in the envelope of intermediate-mass and massive stars and we present our first conclusions on iron and nickel. 1. OPEN PROBLEMS COMING FROM SEISMIC OBSERVATIONS The stellar community hopes to progress on the internal dynamics of stars thanks to the space-borne seismic missions SoHO, CoRoT and Kepler. But good identification and interpretation of acoustic frequencies are based on a proper microscopic physics. Along the last decade, some questions on energy transfer, linked to plasma opacities, have arisen and two cases have retained our attention. 1. The extraction of the solar internal sound speed and density has revealed clear discrepancies with the Standard Solar Model predictions (1-3). These discrepancies are not explained by the transport of momentum by rotation during the main sequence evolution (4). So the energy balance and (or) energy transfer stay clearly under suspicion. In parallel, the recent OPAS calculations (5) have revealed large differences in individual element spectra with previous calculations and in particular with OP (6, 7). OPAS calculations also show some compensation effects between low Z nuclei and the iron group contributions in the Rosseland mean opacity values. All these facts encourage further works and experimental verifications. 2. The second case concerns massive stars. SPB and Cephei stars are interesting pulsators that might put important constraints on rotation and magnetic fields, two key actors for massive stars. Presently, the 40 stars observed by Kepler show acoustic and gravity modes probably up to � = 20 (deformed stars) that are difficult to identify (8). The first difficulty comes from the iron group opacities, which drive their pulsations. The observed modes do not agree with the stability of modes deduced from the OP or OPAL tables (9), which suggests that those opacities are underestimated (10, 11). Up to now, our effort is dedicated to the second case. The first analysis of the OPAC consortium confirms an underestimate of these opacities through comparison with more recent opacity calculations based on old and new measurements (12-15). We shall here present our first conclusions and new orientations for the coming years.