M. Desmet

Post-AGB stars with hot circumstellar dust: binarity of the low-amplitude pulsators

Context. The influence of binarity on the late stages of stellar evolut ion. Aims. While the first binary post-AGB stars were serendipitously d iscovered, the distinct characteristics of their Spectral Energy Distribution (SED) allowed us to launch a more systematic search for binaries. We selected post-AGB objects which show a broad dust excess often starting already at H or K, pointing to the presence of a gravitationally bound dusty disc in the system. We started a very extensive multi-wavelength study of those systems and here we report on our radial velocity and photometric monitoring results for six stars of early F type, which are pulsators of small amplitude. Methods. To determine the radial velocity of low signal-to-noise time-series, we constructed dedicated auto-correlation masks based on high signal-to-noise spectra, used in our published chemical studies. The radial velocity variations were subjecte d to detailed analysis to differentiate between pulsational variability and variabilit y due to orbital motion. When available, the photometric monitoring data were used to complement the time series of radial velocity data and to establish the nature of the pulsation. Finally orbital minimalisation was performed to constrain the orbital elements. Results. All of the six objects are binaries, with orbital periods ran ging from 120 to 1800 days. Five systems have non-circular orbits. The mass functions range from 0.004 to 0.57 M⊙ and the companions are likely unevolved objects of (very) low initial mass. We argue that these binaries must have been subject to severe binary interaction when the primary was a cool supergiant. Although the origin of the circumstellar disc is not well understood, the disc is generally believed to be formed during this strong interaction phase. The eccentric orbits of these highly evolved objects remain poorly understood. In one object the line-of-sight is grazi ng the edge of the puffed-up inner rim of the disc. Conclusions. These results corroborate our earlier statement that evolved objects in binary stars create a Keplerian dusty circumbinary disc. With the measured orbits and mass functions we conclude that the circumbinary discs seem to have a major impact on the evolution of a significant fraction of binary systems.

Periodic Mass-loss episodes due to an oscillation mode with variable amplitude in the hot supergiant HD 50064

Aims. We aim to interpret the photometric and spectroscopic variability of the luminous blue variable supergiant HD 50064 ( V = 8.21). Methods. CoRoT space photometry and follow-up high-resolution spectroscopy with a time base of 137 d and 169 d, respectively, was gathered, analysed, and interpreted using standard time series analysis and light curve modelling methods, as well as spectral line diagnostics. Results. The space photometry reveals one period of 37 d, which undergoes a sudden amplitude change with a factor 1.6. The pulsation period is confirmed in the spectroscopy, which additionally reveals metal line radial velocity values differing by ∼30 km s ―1 depending on the spectral line and on the epoch. We estimate T eff ∼ 13 500 K, log g ∼ 1.5 from the equivalent width of Si lines. The Balmer lines reveal that the star undergoes episodes of changing mass loss on a time scale similar to the changes in the photometric and spectroscopic variability, with an average value of log M ≃ ―5 (in M ⊙ yr ―1 ). We tentatively interpret the 37 d period as the result of a strange mode oscillation.

An asteroseismic study of the β Cephei star 12 Lacertae: multisite spectroscopic observations, mode identification and seismic modelling

We present the results of a spectroscopic multisite campaign for the β Cephei star 12 (DD) Lacertae. Our study is based on more than thousand high-resolution high S/N spectra gathered with eight different telescopes in a time span of 11 months. In addition, we make use of numerous archival spectroscopic measurements. We confirm 10 independent frequencies recently discovered from photometry, as well as harmonics and combination frequencies. In particular, the slowly pulsating B-stars (SPB)-like g-mode with frequency 0.3428 d −1 reported before is detected in our spectroscopy. We identify the four main modes as (� 1, m1) = (1, 1), (� 2, m2) = (0, 0), (� 3, m3) = (1, 0) and (� 4, m4) = (2, 1) for f 1 = 5.178 964 d −1 , f 2 = 5.334 224 d −1 , f 3 = 5.066 316 d −1 and f 4 = 5.490 133 d −1 , respectively. Our seismic modelling shows that f 2 is likely the radial first overtone and that the core overshooting parameter αov is lower than 0.4 local pressure scale heights.

An asteroseismic study of the β Cephei star β Canis Majoris

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 .

CoRoT photometry and high-resolution spectroscopy of the interacting eclipsing binary AU Monocerotis

Analyses of very accurate CoRoT space photometry, past Johnson V photoelectric photometry and high-resolution echelle spectra led to the determination of improved and consistent fundamental stellar properties of both components of AU Monocerotis. We derived new, accurate ephemerides for both the orbital motion (with a period of 11 d . 113) and the long-term, overall brightness variation (with a period of 416 d .9) of this strongly interacting Be + G semi-detached binary. It is shown that this long-term variation must be due to attenuation of the total light by some variable circumbinary material. We derived the binary mass ratio M G /M B = 0.17 ± 0.03 based on the assumption that the G-type secondary fills its Roche lobe and rotates synchronously. Using this value of the mass ratio as well as the radial velocities of the G-star, we obtained a consistent light curve model and improved estimates of the stellar masses, radii, luminosities and effective temperatures. We demonstrate that the observed lines of the B-type primary may not be of photospheric origin. We also discover rapid and periodic light changes visible in the high-quality residual CoRoT light curves. AU Mon is put into perspective by a comparison with known binaries exhibiting long-term cyclic light changes.

The Spitzer spectroscopic survey of S-type stars

Context. S-type AGB stars are thought to be in the transitional phase between M-type and C-type AGB stars. Because the composition of the circumstellar environment reflects the photospheric abundances, one may expect a strong influence of the stellar C/O ratio on the molecular chemistry and the mineralogy of the circumstellar dust. Aims. In this paper, we present a large sample of 87 intrinsic galactic S-type AGB stars, observed at infrared wavelengths with the Spitzer Space Telescope, and supplemented with ground-based optical data. Methods. On the one hand, we derive the stellar parameters from the optical spectroscopy and photometry, using a grid of model atmospheres. On the other, we decompose the infrared spectra to quantify the flux-contributions from the different dust species. Finally, we compare the independently determined stellar parameters and dust properties. Results. For the stars without significant dust emission features, we detect a strict relation between the presence of SiS absorption in the Spitzer spectra and the C/O ratio of the stellar atmosphere. These absorption bands can thus be used as an additional diagnostic for the C/O ratio. For stars with significant dust emission, we define three distinct groups, based on the relative contribution of certain dust species to the infrared flux. We find a strong link between group-membership and C/O ratio. Furthermore, we show that these groups can be explained by assuming that the dust-condensation can be cut short before silicates are produced, while the remaining free atoms and molecules can then be used to form the observed magnesium sulfides or the carriers of the unidentified 13 μm and 20 μm features. Finally, we present the detection of emission features attributed to molecules and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons and magnesium sulfide grains. We show that we often detect magnesium sulfides together with molecular SiS and we propose that it is formed by a reaction of SiS molecules with Mg.