Be star outbursts: transport of angular momentum by waves (original) (raw)
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Stochastic gravito-inertial modes discovered by CoRoT in the hot Be star HD 51452
Astronomy & Astrophysics, 2012
Context. Be stars are rapidly rotating stars with a circumstellar decretion disk. They usually undergo pressure and/or gravity pulsation modes excited by the κ-mechanism, i.e. an effect of the opacity of iron-peak elements in the envelope of the star. In the Milky Way, p-modes are observed in stars that are hotter than or equal to the B3 spectral type, while g-modes are observed at the B2 spectral type and cooler. Aims. We observed a B0IVe star, HD 51452, with the high-precision, high-cadence photometric CoRoT satellite and high-resolution, ground-based HARPS and SOPHIE spectrographs to study its pulsations in great detail. We also used the lower resolution spectra available in the BeSS database. Methods. We analyzed the CoRoT and spectroscopic data with several methods: Clean-NG, FreqFind, and a sliding window method. We also analyzed spectral quantities, such as the violet over red (V/R) emission variations, to obtain information about the variation in the circumstellar environment. We calculated a stellar structure model with the ESTER code to test the various interpretation of the results. Results. We detect 189 frequencies of variations in the CoRoT light curve in the range between 0 and 4.5 c d −1. The main frequencies are also recovered in the spectroscopic data. In particular we find that HD 51452 undergoes gravito-inertial modes that are not in the domain of those excited by the κ-mechanism. We propose that these are stochastic modes excited in the convective zones and that at least some of them are a multiplet of r-modes (i.e. subinertial modes mainly driven by the Coriolis acceleration). Stochastically excited gravito-inertial modes had never been observed in any star, and theory predicted that their very low amplitudes would be undetectable even with CoRoT. We suggest that the amplitudes are enhanced in HD 51452 because of the very rapid stellar rotation. In addition, we find that the amplitude variations of these modes are related to the occurrence of minor outbursts. Conclusions. Thanks to CoRoT data, we have detected a new kind of pulsations in HD 51452, which are stochastically excited gravito-inertial modes, probably due to its very rapid rotation. These modes are probably also present in other rapidly rotating hot Be stars.
Astronomy & Astrophysics, 2012
Context. HD 181231 and HD 175869 are two late rapidly rotating Be stars, which have been observed using high-precision photometry with the CoRoT satellite during about five consecutive months and 27 consecutive days, respectively. An analysis of their light curves, by Neiner and collaborators and Gutiérrez-Soto and collaborators respectively, showed that several independent pulsation g-modes are present in these stars. Fundamental parameters have also been determined by these authors using spectroscopy. Aims. We aim to model these results to infer seismic properties of HD 181231 and HD 175869, and constrain internal transport processes of rapidly rotating massive stars. Methods. We used an adiabatic (NRO) and a non-adiabatic (Tohoku) oscillation code that accounts for the combined action of Coriolis and centrifugal accelerations on stellar pulsations as needed for rapid rotator modelling. We coupled these codes with a 2D (ROTORC) stellar structure model to take the rotational deformation of the star into account. The action of transport processes was parametrised with the mixing parameter α ov , which represents the "non-standard" extension of the convective core, and determined by matching observed pulsation frequencies assuming a single star evolution scenario. In a second step, we used (Geneva) evolution models to evaluate the contribution of the secular rotational transport and mixing processes in the radiative envelope. A Monte Carlo analysis of spectropolarimetric data was also performed to examine the role of a potential fossil magnetic field. Finally, based on state-of-the-art modelling of penetrative convection and internal waves, we unravelled their respective contribution to the needed "non-standard" mixing. Results. We find that extra mixing of α ov = 0.3−0.35H p is needed in HD 181231 and HD 175869 to match the observed frequencies with those of prograde sectoral g-modes. We also detect the possible presence of r-modes. We investigated the respective contributions of several transport processes to this mixing: the hydrodynamical processes in particular the meridional circulation and shear-induced turbulence caused by the radiative envelope differential rotation, the possible magnetic field, the penetrative convection at the top of the convective core, and the transport by internal waves. Conclusions. We showed that the extension of the convective core needed to match observations and models may be explained by mixing induced by the penetrative movements at the bottom of the radiative envelope and by the secular hydrodynamical transport processes induced by the rotation in the envelope. We showed how asteroseismology opens a new door to probe transport processes in stellar interiors.
Asteroseismology and mass loss in be stars: study with corot
2010
describes the basics of the Fourier analysis and the rudiments of the time series analysis. In this Chapter, we introduce the pasper code for the frequency analysis of photometric light curves. At the early begin of this Ph.D. thesis, the CoRoT satellite was still on ground getting ready for the launch. At this time, we were performing and improving our pasper code and we needed some variable stars to check the performance of our methods. In this context, we perform a search for short-period B and Be star variables in the low metallicity environment of the Magellanic Clouds. This study constitutes the Part I of this Ph.D. thesis. This Part has a double goal: i) to test the frequency analysis codes; and ii) to detect observationally β Cephei and SPB-like B-type pulsators in low metallicity environments, actually not predicted by the pulsational theory and models. This Part is organized in four Chapters: Chapter 4 depicts the scientific context for the search of B-type pulsators in the Magellanic Clouds. An overview of the MACHO survey and the description of the B and Be studied samples are given in Chapter 5. Finally, Chapter 6 and Chapter 7 describe the results and discussion for the SMC and LMC studies, respectively. Part II is devoted to the study of Be stars with the CoRoT space mission. This Part is also organized in four Chapters: Chapter 8 depicts a complete review on the CoRoT mission, describing the spacecraft, the different observing programmes and the mission design. Chapter 9 presents the CoRoT Be Team, a collaboration for the study of Be stars using the CoRoT data. We describe some general remarks about the instrumental effects present in the CoRoT light curves and information on the frequency analysis of the CoRoT data. In Chapter 10 we describe the results on the analysis of three Be stars from the CoRoT exoplanet field. Finally, in Chapter 11 we present the results on the frequency analysis of the late Be star HD 50 209, observed in the seismology field of the CoRoT satellite. The analysis of this Be star has revealed up to sixty frequencies, grouped in six different and separated sets, attributed to g-mode pulsations. Finally, in Chapter 12 we resume the main conclusions of the whole project, including prospects and future work to be done. An addendum with all the published results derived from this project has been added in Chapter 13. P. D. Diago with = 1 or = 2 is sometimes called a dipole mode, respectively quadrupole mode. 1.2.3 Historical background The early history of studies on stellar pulsation was concisely described in the introduction of the famous textbook The Pulsation Theory of Variable Stars (Rosseland 1949). It is interesting to see that the theory of non-radial pulsation developed by Kelvin (1863) preceded the theory of radial pulsation developed by Ritter (1879). However, the Cepheids have been the chief concern of pulsation theory, which was founded by Eddington as summarised in his book The Internal Constitution of the Stars (Eddington 1926). In spite of the remarkable progress in the development of the theory of radial pulsation, the theoretical study of nonradial pulsation remained largely within academic circles until recently. But the work of Pekeris and Cowling should be mentioned. Pekeris (1938) obtained the exact analytic solution for adiabatic non-radial oscillations in the homogeneous compressible model. Cowling (1941) extended the study for the polytrope model. For a description of these and other studies, readers can refer to the comprehensive article by Ledoux and Walraven (1958). Ledoux developed the study of non-radial pulsations in 1951 (see Ledoux 1951). He suggested that non-radial oscillations could explain the double periodicity and the large temporal variations in the broadening of spectral lines observed in β Canis Majoris (a prototype of β Cephei type variable stars). Osaki (1971) examined Ledoux's theory by calculating line profiles for a star undergoing non-radial oscillations and compared the result with observations available at that time. He also suggested (Osaki 1974) a possible mechanism for the origin and maintenance of β Cephei pulsation based on non-radial oscillation. The discovery of the five-minute Solar oscillations by Leighton et al. (1962) was also epoch-making. A number of interesting theories had been proposed to explain this phenomenon (see Stein and Leibacher 1974). Some fifteen years later, Deubner (1975) succeeded in resolving observed oscillations into discrete modes 12 Asteroseismology in the so-called diagnostic diagram. A comparison between his observation and theoretical eigenfrequencies of non-radial modes calculated by Ando and Osaki (1975) has established that the solar five-minute oscillations are global non-radial p-modes of the Sun with high spherical harmonic degree (= 200 − 1 000). Furthermore, low-degree (= 0 − 5) and intermediate-degree (= 1 − 200) have also been detected. Since the late 1960s and early 1970s, pulsations and oscillation-related phenomena have been observed in many stars that were regarded as non-pulsating stars before. They include white dwarfs, hot subdwarfs, Ap stars, γ Dor stars, and early type O and B stars. It is now believed that non-radial oscillations are responsible for variability observed in these stars in most cases. Along with these observational developments, much progress has been made in the theoretical side of non-radial oscillations theory. Since the middle of the 1970s, full equations of linear adiabatic and non-adiabatic non-radial oscillations have been solved numerically for realistic stellar models with the help of computers. The introduction of the so-called propagation diagram, the phase diagram and the concept of "wave trapping" (Scuflaire 1974; Unno 1975; Osaki 1975; Shibahashi and Osaki 1976) have greatly improved the understanding on non-radial oscillation in stars. A number of theoretical problems remain to be studied. Among them are non-linear problems, including the mixing of matter due to finite oscillations, oscillations in the presence of a strong magnetic field or rotation, and energy and momentum transport by waves. Both theory and observation are still in progress, so further developments can be expected in the near future. 1.2.4 The effect of rotation From Eqs. 1.1 and 1.4 we can derive that for modes with m = 0 the exponentials in the two equations combine to give a time dependence that goes as e −i (2πνt−mϕ). This phase factor in the time dependence means that the m = 0 modes are travelling waves. Conventionally, we assign positive values of m to modes that propagate in the same direction as the stellar rotation (pro-grade modes), and 14 http://kepler.arc.nasa.gov/.
First spectro-interferometric survey of Be stars
Astronomy & Astrophysics, 2012
Context. Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed infrared-excess and emission lines. The phenomena involved in the disk formation still remain highly debated. Aims. To progress in the understanding of the physical process or processes responsible for the mass ejections and test the hypothesis that they depend on the stellar parameters, we initiated a survey on the circumstellar environment of the brightest Be stars. Methods. To achieve this goal, we used spectro-interferometry, the only technique that combines high spectral (R=12000) and high spatial (θ min =4 mas) resolutions. Observations were carried out at the Paranal observatory with the VLTI/AMBER instrument. We concentrated our observations on the Brγ emission line to be able to study the kinematics within the circumstellar disk. Our sample is composed of eight bright classical Be stars : α Col, κ CMa, ω Car, p Car, δ Cen, µ Cen, α Ara, and o Aqr. Results. We managed to determine the disk extension in the line and the nearby continuum for most targets. We also constrained the disk kinematics, showing that it is dominated by rotation with a rotation law close to the Keplerian one. Our survey also suggests that these stars are rotating at a mean velocity of V/V c = 0.82 ± 0.08. This corresponds to a rotational rate of Ω/Ω c = 0.95 ± 0.02 Conclusions. We did not detect any correlation between the stellar parameters and the structure of the circumstellar environment. Moreover, it seems that a simple model of a geometrically thin Keplerian disk can explain most of our spectrally resolved K-band data. Nevertheless, some small departures from this model have been detected for at least two objects (i.e, κ CMa and α Col). Finally, our Be stars sample suggests that rotation is the main physical process driving the mass-ejection. Nevertheless, smaller effects from other mechanisms have to be taken into account to fully explain how the residual gravity is compensated.
First results on Be stars with CoRoT
2008
In this paper we present an overview of the analysis of some of the Be stars observed with the CoRoT satellite up to this date. Be stars are very fast-rotating B-type stars which may pulsate as β Cephei or SPB stars. CoRoT has already observed 5 bright Be stars in the seismology fields and several tens of fainter ones in the exoplanet fields with an unprecedented quality and with a time duration from 20 to 150 days. Multiple frequencies are detected in the majority of the stars. Pulsations, outbursts, beating phenomenon, rotation, amplitude variability, etc. have been found in their light curves. In order to complement this study, ground-based spectroscopic data have also been analysed for the stars located in the seismology fields.
The Astrophysical Journal, 2012
Using CHARA and VLTI near-infrared spectro-interferometry with hectometric baseline lengths (up to 330 m) and with high spectral resolution (up to λ/∆λ = 12 000), we studied the gas distribution and kinematics around two classical Be stars. The combination of high spatial and spectral resolution achieved allows us to constrain the gas velocity field on scales of a few stellar radii and to obtain, for the first time in optical interferometry, a dynamical mass estimate using the position-velocity analysis technique known from radio astronomy. For our first target star, β Canis Minoris, we model the H+K-band continuum and Brγ-line geometry with a near-critical rotating stellar photosphere and a geometrically thin equatorial disk. Testing different disk rotation laws, we find that the disk is in Keplerian rotation (v(r) ∝ r −0.5±0.1 ) and derive the disk position angle (140 ± 1.7 • ) inclination (38.5 ± 1 • ), and the mass of the central star (3.5 ± 0.2 M ⊙ ). As a second target star, we observed the prototypical Be star ζ Tauri and spatially resolved the Brγ emission as well as nine transitions from the hydrogen Pfund series (Pf 14-22). Comparing the spatial origin of the different line transitions, we find that the Brackett (Brγ), Pfund (Pf 14-17), and Balmer (Hα) lines originate from different stellocentric radii (R cont < R Pf < R Brγ ∼ R Hα ), which we can reproduce with an LTE line radiative transfer computation. Discussing different disk-formation scenarios, we conclude that our constraints are inconsistent with wind compression models predicting a strong outflowing velocity component, but support viscous decretion disk models, where the Keplerian-rotating disk is replenished with material from the near-critical rotating star.
Pulsations in the late-type Be star HD 50 209 detected by CoRoT
2009
Context. The presence of pulsations in late-type Be stars is still a matter of controversy. It constitutes an important issue to establish the relationship between non-radial pulsations and the mass-loss mechanism in Be stars. Aims. To contribute to this discussion, we analyse the photometric time series of the B8IVe star HD 50 209 observed by the CoRoT mission in the seismology field. Methods. We use standard Fourier techniques and linear and non-linear least squares fitting methods to analyse the CoRoT light curve. In addition, we applied detailed modelling of high-resolution spectra to obtain the fundamental physical parameters of the star. Results. We have found four frequencies which correspond to gravity modes with azimuthal order m = 0, −1, −2, −3 with the same pulsational frequency in the co-rotating frame. We also found a rotational period with a frequency of 0.679 cd −1 (7.754 µHz). Conclusions. HD 50 209 is a pulsating Be star as expected from its position in the HR diagram, close to the SPB instability strip.
Investigating the transport of angular momentum from young stellar objects
Astronomy and Astrophysics, 2008
Aims. In this pilot study, we examine molecular jets from the embedded Class I sources, HH 26 and HH 72, to search, for the first time, for kinematic signatures of jet rotation from young embedded sources. Methods. High-resolution long-slit spectroscopy of the H 2 1-0 S(1) transition was obtained using VLT/ISAAC. The slit was placed perpendicular to the flow direction about 2 ′′ from the sources. Position-velocity (PV) diagrams are constructed and intensityweighted radial velocities transverse to the jet flow are measured. Results. Mean intensity-weighted velocities vary between v LSR ∼ -90 and -65 km s −1 for HH 26, and -60 and -10 km s −1 for HH 72; maxima occur close to the intensity peak and decrease toward the jet borders. Velocity dispersions are ∼ 45 and ∼ 80 km s −1 for HH 26 and HH 72, respectively, with gas motions as fast as -100 km s −1 present. Asymmetric PV diagrams are seen for both objects, which a simple empirical model of a cylindrical jet section shows could in principle be reproduced by jet rotation alone. Assuming magneto-centrifugal launching, the observed HH 26 flow may originate at a disk radius of 2-4 AU from the star with the toroidal component of the magnetic field dominant at the observed location, in agreement with magnetic collimation models. We estimate that the kinetic angular momentum transported by the HH 26 jet is ∼ 2 × 10 −5 M ⊙ yr −1 AU km s −1 . This value (a lower limit to the total angular momentum transported by the flow) already amounts to 70% of the angular momentum that has to be extracted from the disk for the accretion to proceed at the observed rate.
Recent Spectroscopic Evidence for Aperiodic Activity in Classical Be Stars
International Astronomical Union Colloquium, 2000
This review focuses on aperiodic line profile activity in several Be stars, ignoring attributes of pulsations probably occurring in any of the stars studied. Herein, I discuss spectroscopic evidence of heating (and cooling) of regions of Be star atmospheres, of line profile transients called dimples and high velocity absorptions. These features will be interpreted in terms of mass ejections (not necessarily permanent). Some of these events are violent and spasmodic, some are ordered and quasisteady, and some are associated with downflows. I close with new evidence for time-varying, corotating clouds suspended over a few Be stars.
Photometric variability of the Be star CoRoT-ID 102761769
2010
Context. Classical Be stars are rapid rotators of spectral type late O to early A and luminosity class V-III, wich exhibit Balmer emission lines and often a near infrared excess originating in an equatorially concentrated circumstellar envelope, both produced by sporadic mass ejection episodes. The causes of the abnormal mass loss (the so-called Be phenomenon) are as yet unknown.