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Register a new userInstabilities and pulsations in models of the B-type supergiant $kappa$ Cassiopeiae (HD 2905)
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For the B-type supergiant $kappa$ Cassiopeiae (HD 2905) variabilities with periods between several hours and a few days have been observed both photometrically and spectroscopically. A recent study of this star by Simon-D{i}az et al. (2018) has revealed variability with a dominant period of 2.7 days. To understand this variability, we present a linear non-adiabatic stability analysis with respect to radial perturbations for models of $kappa$ Cassiopeiae. Instabilities associated with the fundamental mode and the first overtone are identified for models with masses between 27 M$_{odot}$ and 44 M$_{odot}$. For selected models, the instabilities are followed into the non-linear regime by numerical simulations. As a result, finite amplitude pulsations with periods between 3 and 1.8 days are found. The model with a mass of 34.5 M$_{odot}$ exhibits a pulsation period of 2.7 days consistent with the observations. In the non-linear regime, the instabilities may cause a substantial inflation of the envelope.
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{We aim to detect and interpret photometric and spectroscopic variability of the bright CoRoT B-type supergiant target HD,46769 ($V=5.79$). We also attempt to detect a magnetic field in the target.} {We analyse a 23-day oversampled CoRoT light curve after detrending, as well as spectroscopic follow-up data, by using standard Fourier analysis and Phase Dispersion Minimization methods. We determine the fundamental parameters of the star, as well as its abundances from the most prominent spectral lines. We perform a Monte Carlo analysis of spectropolarimetric data to obtain an upper limit of the polar magnetic field, assumping a dipole field.} {In the CoRoT data, we detect a dominant period of 4.84,d with an amplitude of 87,ppm, and some of its (sub-)multiples. Given the shape of the phase-folded light curve and the absence of binary motion, we interpret the dominant variability in terms of rotational modulation, with a rotation period of 9.69,d. Subtraction of the rotational modulation signal does not reveal any sign of pulsations. Our results are consistent with the absence of variability in the Hipparcos light curve. The spectroscopy leads to a projected rotational velocity of 72$pm 2$,km,s$^{-1}$ and does not reveal periodic variability nor the need to invoke macroturbulent line broadening. No signature of a magnetic field is detected in our data. A field stronger than $sim 500$,G at the poles can be excluded, unless the possible non-detected field were more complex than dipolar.} {The absence of pulsations and of macroturbulence of this evolved B-type supergiant is placed into context of instability computations and of observed variability of evolved B-type stars.}
Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the stars evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind. To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind. We modeled the H, He I, Si II and Si III lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations. The Halpha line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7-2 on different timescales. We also observe changes in the ionization rate of Si II and determine a multiperiodic oscillation in the He I absorption lines, with periods ranging from a few hours to 22.5 days. We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of alpha Cyg variables.
We review the main results obtained from our seismic studies of B-type main sequence pulsators, based on the ground-based, MOST, Kepler and BRITE observations. Important constraints on stellar opacities, convective overshooting and rotation are derived. In each studied case, a significant modification of the opacity profile at the depths corresponding to the temperature range $log{T}in (5.0-5.5)$ is indispensable to explain all pulsational properties. In particular, a huge amount of opacity (at least 200%) at the depth of the temperature $log T = 5.46$ (the nickel opacity) has to be added in early B-type stellar models to account for low frequencies which correspond to high-order g modes. The values of the overshooting parameter, $alpha_{rm ov}$, from our seismic studies is below 0.3. In the case of a few stars, the deeper interiors have to rotate faster to get the g-mode instability in the whole observed range.
Constraining substellar evolutionary models (SSEMs) is particularly difficult due to a degeneracy between the mass, age, and luminosity of a brown dwarf. In cases where a brown dwarf is found as a directly imaged companion to a star, as in HD 4747 and HD 19467, the mass, age, and luminosity of the brown dwarf are determined independently, making them ideal objects to use to benchmark SSEMs. Using the Center for High Angular Resolution Astronomy Array, we measured the angular diameters and calculated the radii of the host stars HD 4747 A and HD 19467 A. After fitting their parameters to the Dartmouth Stellar Evolution Database, MESA Isochrones and Stellar Tracks, and Yonsei-Yale isochronal models, we adopt age estimates of $10.74^{+6.75}_{-6.87}$ Gyr for HD 4747 A and $10.06^{+1.16}_{-0.82}$ Gyr for HD 19467 A. Assuming the brown dwarf companions HD 4747 B and HD 19467 B have the same ages as their host stars, we show that many of the SSEMs under-predict bolometric luminosities by $sim$ 0.75 dex for HD 4747 B and $sim 0.5$ dex for HD 19467 B. The discrepancies in luminosity correspond to over-predictions of the masses by $sim$ 12% for HD 4747 B and $sim$ 30% for HD 19467 B. We also show that SSEMs that take into account the effect of clouds reduce the under-prediction of luminosity to $sim 0.6$ dex and the over-prediction of mass to $sim 8%$ for HD 4747 B, an L/T transition object that is cool enough to begin forming clouds. One possible explanation for the remaining discrepancies is missing physics in the models, such as the inclusion of metallicity effects.
We report the discovery of a square axisymmetric circumstellar nebula around the emission-line star HD 93795 in archival Spitzer Space Telescope 24 micron data. We classify HD 93795 as an B9 Ia star using optical spectra obtained with the Southern African Large Telescope (SALT). A spectral analysis carried out with the stellar atmosphere code FASTWIND indicates that HD 93795 only recently left the main sequence and is evolving redward for the first time. We discuss possible scenarios for the origin of the nebula and suggest that HD 93795 was originally a binary system and that the nebula was formed because of merger of the binary components. We also discuss a discrepancy between distance estimates for HD 93795 based on the Gaia data and the possible membership of this star of the Car OB1 association, and conclude that HD 93795 could be at the same distance as Car OB1.
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