We present an extensive and up-to-date catalog of Galactic β Cephei stars. This catalog is intended to give a comprehensive overview of observational characteristics of all known β Cephei stars, covering information until 2004 June. Ninety-three stars could be confirmed to be β Cephei stars. We use data from more than 250 papers published over the last nearly 100 years, and we provide over 45 notes on individual stars. For some stars we reanalyzed published data or conducted our own analyses. Sixty-one stars were rejected from the final β Cephei list, and 77 stars are suspected to be β Cephei stars. A list of critically selected pulsation frequencies for confirmed β Cephei stars is also presented.     We analyze the β Cephei stars as a group, such as the distributions of their spectral types, projected rotational velocities, radial velocities, pulsation periods, and Galactic coordinates. We confirm that the majority of the β Cephei stars are multiperiodic pulsators. We show that, besides two exceptions, the β Cephei stars with high pulsation amplitudes are slow rotators. Those higher amplitude stars have angular rotational velocities in the same range as the high-amplitude δ Scuti stars (Prot 3 days).     We construct a theoretical HR diagram that suggests that almost all 93 β Cephei stars are main-sequence objects. We discuss the observational boundaries of β Cephei pulsation and the physical parameters of the stars. We corroborate that the excited pulsation modes are near to the radial fundamental mode in frequency and we show that the mass distribution of the stars peaks at 12 M⊙. We point out that the theoretical instability strip of the β Cephei stars is filled neither at the cool nor at the hot end and attempt to explain this observation.

https://iopscience.iop.org/article/10.1086/429408/pdf

Catalog of Galactic β Cephei Stars

Massive X-ray binaries are usually classified by the properties of the donor star in classical, supergiant and Be X-ray binaries, the main difference being the mass transfer mechanism between the two components. The massive X-ray binary 4U 2206+54 does not fit in any of these groups, and deserves a detailed study to understand how the transfer of matter and the accretion on to the compact object take place. To this end we study an IUE spectrum of the donor and obtain a wind terminal velocity (v∞ )o f∼350 km s −1 , which is abnormally slow for its spectral type. We also analyse here more than 9 years of available RXTE/ASM data. We study the long-term X-ray variability of the source and find it to be similar to that observed in the wind-fed supergiant system Vela X-1, reinforcing the idea that 4U 2206+54 is also a wind-fed system. We find a quasi-period decreasing from ∼270 to ∼130 d, noticed in previous works but never studied in detail. We discuss possible scenarios for its origin and conclude that long-term quasi-periodic variations in the mass-loss rate of the primary are probably driving such variability in the measured X-ray flux. We obtain an improved orbital period of Porb = 9.5591 ± 0.0007 d with maximum X-ray flux at MJD 51856.6 ± 0.1. Our study of the orbital X-ray variability in the context of wind accretion suggests a moderate eccentricity around 0.15 for this binary system. Moreover, the low value of v∞ solves the long-standing problem of the relatively high X-ray luminosity for the unevolved nature of the donor, BD +53 ◦ 2790, which is probably an O9.5 V star. We note that changes in v∞ and/or the mass-loss rate of the primary alone cannot explain the different patterns displayed by the orbital X-ray variability. We finally emphasize that 4U 2206+54, together with LS 5039, could be part of a new population of wind-fed HMXBs with main sequence donors, the natural progenitors of supergiant X-ray binaries.

/pdf/wind-accretion-in-the-massive-x-ray-binary-4u-2206-54-2j8m82peq1.pdf

Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit

Aims. We present the results of a detailed analysis of 452 ground-based, high-resolution high S/N spectroscopic measurements spread over 4.5 years for β Canis Majoris with the aim of determining the pulsational characteristics of this star, and then using them to derive seismic constraints on the stellar parameters. Methods. We determined pulsation frequencies in the Si III 4553 A line with Fourier methods. We identified the m-value of the modes by taking the photometric identifications of the degrees l into account. To this end we used the moment method together with the amplitude and phase variations across the line profile. The frequencies of the identified modes were used for a seismic interpretation of the structure of the star. Results. We confirm the presence of the three pulsation frequencies already detected in previous photometric datasets: f 1 = 3.9793 c d -1 (46.057 μHz), f 2 = 3.9995 cd -1 (46.291 μHz), and f 3 = 4.1832 cd -1 (48.417 μHz). For the two modes with the highest amplitudes, we unambiguously identify (l 1 , m 1 ) = (2,2) and (l 2 , m 2 ) = (0,0). We cannot conclude anything for the third mode identification, except that m 3 > 0. We also deduce an equatorial rotational velocity of 31 ± 5 km s -1 for the star. We show that the mode f 1 must be close to an avoided crossing. Constraints on the mass (13.5 ± 0.5 M ⊙ ), age (12.4 ± 0.7 Myr), and core overshoot (0.20 ± 0.05 Hp) of β CMa are obtained from seismic modelling using f 1 and f 2 .

/pdf/an-asteroseismic-study-of-the-b-cephei-star-b-canis-majoris-3nrjc13c12.pdf

An asteroseismic study of the β Cephei star β Canis Majoris

Aims. We have performed a set of high- and low-spectral resolution phase-resolved X-ray observations of the magnetic B star β Cep, for which theoretical models predict the presence of a confined wind emitting X-rays from stationary shocks. Given the peculiar geometry of β Cep, some of the models predict strong rotational modulation of the X-ray emission, while other models predict a much lower amplitude modulation at 90 deg phase shift from the modulation predicted from the first group of models. Our observations were designed to provide a stringent test of such models. Methods. We obtained four observations spaced in rotational phase with XMM-Newton (using both the EPIC cameras and the RGS spectrograph) and with Chandra (using the LETG spectrograph). A detailed analysis of the data was performed to derive both photometric and spectral parameters from the EPIC data, searching for rotational modulation, and to derive the location of the X-ray plasma from the line ratios in the He-like triplets of N, O, and Ne from the RGS data. The LETG data were used to constrain the presence of bulk motions in the plasma. Results. The strong rotational modulation predicted by the early, static magnetically-confined wind model for the X-ray emission is not observed in β Cep. The small modulation present goes in the opposite direction, pointing to the absence of any optically thick disk of neutral material, and showing a modulation consistent with the later, dynamic models of magnetically-confined wind models in B stars. The lack of observed bulk motion points to the plasma being confined by a magnetic field, but the low plasma temperature and lack of any flaring show that the plasma is not heated by magnetic reconnection. Therefore, the observations point to X-ray emission from shocks in a magnetically confined wind, with no evidence of an optically thick, dense disk at the magnetic equator.

/pdf/testing-magnetically-confined-wind-shock-models-for-b-cephei-4yndc51d7q.pdf

Testing magnetically confined wind shock models for β Cephei using XMM-Newton and Chandra phase-resolved X-ray observations

We present a spectroscopic study of the Cephei star Canis Majoris. Hundreds of high- resolution spectra were gathered during 4.5 years and the Si III line proles centered on 4560 A were subjected to a detailed line prole analysis. We searched for oscillation frequencies in dieren t line diagnostics, compared them with those known from photometric studies and concluded the presence of three frequencies in our dataset: f1 = 3:97933 c d 1 , f2 = 3:99959 c d 1 and f3 = 4:1832 c d 1 , which were already found earlier from photometric data. By means of the moment method and from amplitude and phase variations across the proles we could identify two of the modes as ('1; m1) = (2; 2) and ('2; m2) = (0; 0). We present our rst results of modelling based on the oscillation frequencies and place constraints on the mass, age and core overshooting of this massive evolved Cephei star.

/pdf/an-asteroseismic-study-of-the-cephei-star-canis-majoris-3vitqqlwb7.pdf

An asteroseismic study of the Cephei star Canis Majoris

We present a study of the beta Cephei instability strip based on a sample of 49 stars of this type. After deriving their effective temperatures and luminosities from their observed (B - V), (U - B) colors and parallaxes we find their positions in the HR diagram to be mostly confined to the main sequence, and their masses to lie between 7M(circle dot) and 30 M-circle dot. Their distribution on the HR diagram matches well with our previous theoretical instability strip which has an upper bound in the luminosity and rather tight boundaries in the effective temperature.

Bin Tian

Papers

Statistics of the Instability Strip of Cephei Stars