Context. When a given observational quantity depends on several stellar physical parameters, it is generally very difficult to obtain observational constraints for each of them individually. ...Therefore, we studied under which conditions constraints for some individual parameters can be achieved for fast rotators, knowing that their geometry is modified by the rapid rotation which causes a non-uniform surface brightness distribution. Aims. We aim to study the sensitivity of interferometric observables on the position angle of the rotation axis (PA) of a rapidly rotating star, and whether other physical parameters can influence the determination of PA, and also the influence of the surface differential rotation on the determination of the β exponent in the gravity darkening law that enters the interpretation of interferometric observations, using α Cep as a test star. Methods. We used differential phases obtained from observations carried out in the Hα absorption line of α Cep with the VEGA/CHARA interferometer at high spectral resolution, R = 30 000 to study the kinematics in the atmosphere of the star. Results. We studied the influence of the gravity darkening effect (GDE) on the determination of the PA of the rotation axis of α Cep and determined its value, PA = −157-10°+17°. We conclude that the GDE has a weak influence on the dispersed phases. We showed that the surface differential rotation can have a rather strong influence on the determination of the gravity darkening exponent. A new method of determining the inclination angle of the stellar rotational axis is suggested. We conclude that differential phases obtained with spectro-interferometry carried out on the Hα line can in principle lead to an estimate of the stellar inclination angle i. However, to determine both i and the differential rotation parameter α, lines free from the Stark effect and that have collision-dominated source functions are to be preferred.
Aims. The rapidly rotating Be star ϕ Persei was spun up by mass and angular momentum transfer from a now stripped-down, hot subdwarf companion. Here we present the first high angular resolution ...images of ϕ Persei made possible by new capabilities in long-baseline interferometry at near-IR and visible wavelengths. We analyzed these images to search for the companion, to determine the binary orbit, stellar masses, and fluxes, and to examine the geometrical and kinematical properties of the outflowing disk surrounding the Be star. Methods. We observed ϕ Persei with the MIRC and VEGA instruments of the CHARA Array. MIRC was operated in six-telescope mode, whereas VEGA was configured in four-telescope mode with a change of quadruplets of telescopes during two nights to improve the (u,v) plane coverage. Additional MIRC-only observations were performed to track the orbital motion of the companion, and these were fit together with new and existing radial velocity measurements of both stars to derive the complete orbital elements and distance. We also used the MIRC data to reconstruct an image of the Be disk in the near-IR H-band. VEGA visible broadband and spectro-interferometric Hα observations were fit with analytical models for the Be star and disk, and image reconstruction was performed on the spectrally resolved Hα emission line data. Results. The hot subdwarf companion is clearly detected in the near-IR data at each epoch of observation with a flux contribution of 1.5% in the H band, and restricted fits indicate that its flux contribution rises to 3.3% in the visible. A new binary orbital solution is determined by combining the astrometric and radial velocity measurements. The derived stellar masses are 9.6 ± 0.3 M⊙ and 1.2 ± 0.2 M⊙ for the Be primary and subdwarf secondary, respectively. The inferred distance (186 ± 3 pc), kinematical properties, and evolutionary state are consistent with membership of ϕ Persei in the α Per cluster. From the cluster age we deduce significant constraints on the initial masses and evolutionary mass transfer processes that transformed the ϕ Persei binary system. The interferometric data place strong constraints on the Be disk elongation, orientation, and kinematics, and the disk angular momentum vector is coaligned with and has the same sense of rotation as the orbital angular momentum vector. The VEGA visible continuum data indicate an elongated shape for the Be star itself, due to the combined effects of rapid rotation, partial obscuration of the photosphere by the circumstellar disk, and flux from the bright inner disk.
Context. As is the case of several other Be stars, Achernar is surrounded by an envelope, recently detected by near-IR interferometry. Aims. We search for the signature of circumstellar emission at ...distances of a few stellar radii from Achernar, in the thermal IR domain. Methods. We obtained interferometric observations on three VLTI baselines in the N band (8-13 μm), using the MIDI instrument. Results. From the measured visibilities, we derive the angular extension and flux contribution of the N band circumstellar emission in the polar direction of Achernar. The interferometrically resolved polar envelope contributes 13.4 ± 2.5% of the photospheric flux in the N band, with a full width at half maximum of 9.9 ± 2.3 mas (≈$6~R_\star$). This flux contribution is in good agreement with the photometric IR excess of 10-20% measured by fitting the spectral energy distribution. Due to our limited azimuth coverage, we can only establish an upper limit of 5-10% for the equatorial envelope. We compare the observed properties of the envelope with an existing model of this star computed with the SIMECA code. Conclusions. The observed extended emission in the thermal IR along the polar direction of Achernar is well reproduced by the existing SIMECA model. Already detected at 2.2 μm, this polar envelope is most probably an observational signature of the fast wind ejected by the hot polar caps of the star.
Interferometry concepts Millour, F.
EAS publications series,
01/2014, Volume:
69-70
Journal Article
Open access
This paper serves as an introduction to the current book. It provides the basic notions of long-baseline optical/infrared interferometry prior to reading all the subsequent chapters, and is not an ...extended introduction to the field.
The signature of activity in general, and of stellar magnetic spots in particular, is present in every measurements, including interferometric ones. Indeed, stellar spots can be found on many stellar ...surfaces, their size and number varying according to their host's magnetic field and rotational velocity. To correctly determine stellar parameters, it is thus necessary to determine and extract stellar activity's signals. Interferometric observables are disturbed by activity, and this observing technique thus constitutes a good way of probing stellar surface. However, magnetic spots sometimes mimic other phenomenon, like a transiting exoplanet. In that case, the combination of several observing techniques, like photometry and interferometry, is mandatory to extract the planetary signal from the spot's one, and then characterize the exoplanet.
Stars are not smooth. Their photosphere is covered by a granulation pattern associated with the heat transport by convection. The convection-related surface structures have different size, depth, and ...temporal variations with respect to the stellar type. The related activity (in addition to other phenomena such as magnetic spots, rotation, dust, etc.) potentially causes bias in stellar parameters determination, radial velocity, chemical abundances determinations, and exoplanet transit detections. The role of long-baseline interferometric observations in this astrophysical context is crucial to characterize the stellar surface dynamics and correct the potential biases. In this Chapter, we present how the granulation pattern is expected for different kind of stellar types ranging from main sequence to extremely evolved stars of different masses and how interferometric techniques help to study their photospheric dynamics.
The galactic unclassified B[e] star HD 50138 Borges Fernandes, M.; Kraus, M.; Chesneau, O. ...
Astronomy and astrophysics (Berlin),
12/2009, Volume:
508, Issue:
1
Journal Article
Peer reviewed
Open access
Context. The observed spectral variation of HD 50138 has led different authors to classify it in a very wide range of spectral types and luminosity classes (from B5 to A0 and III to Ia) and at ...different evolutionary stages as either HAeBe star or classical Be. Aims. Based on new high-resolution optical spectroscopic data from 1999 and 2007 associated to a photometric analysis, the aim of this work is to provide a deep spectroscopic description and a new set of parameters for this unclassified southern Be star and its interstellar extinction. Methods. From our high-resolution optical spectroscopic data separated by 8 years, we perform a detailed spectral description, presenting the variations seen and discussing their possible origin. We derive the interstellar extinction to HD 50138 by taking the influences of the circumstellar matter in the form of dust and an ionized disk into account. Based on photometric data from the literature and the new Hipparcos distance, we obtain a revised set of parameters for HD 50138. Results. Because of the spectral changes, we tentatively suggest that a new shell phase could have taken place prior to our observations in 2007. We find a color excess value of $E(B-V) = 0.08$ mag, and from the photometric analysis, we suggest that HD 50138 is a B6-7 III-V star. A discussion of the different evolutionary scenarios is also provided.
Context. The observed spectral variation of HD 50138 has led different authors to classify it in a very wide range of spectral types and luminosity classes (from B5 to A0 and III to Ia) and at ...different evolutionary stages as either HAeBe star or classical Be. Aims. Based on new high-resolution optical spectroscopic data from 1999 and 2007 associated to a photometric analysis, the aim of this work is to provide a deep spectroscopic description and a new set of parameters for this unclassified southern Be star and its interstellar extinction. Methods. From our high-resolution optical spectroscopic data separated by 8 years, we perform a detailed spectral description, presenting the variations seen and discussing their possible origin. We derive the interstellar extinction to HD 50138 by taking the influences of the circumstellar matter in the form of dust and an ionized disk into account. Based on photometric data from the literature and the new Hipparcos distance, we obtain a revised set of parameters for HD 50138. Results. Because of the spectral changes, we tentatively suggest that a new shell phase could have taken place prior to our observations in 2007. We find a color excess value of E(B-V) = 0.08 mag, and from the photometric analysis, we suggest that HD 50138 is a B6-7 III-V star. A discussion of the different evolutionary scenarios is also provided.
Surface inhomogeneities are found in many stars. In active sun-like stars and cool giants temperature spots similar to that found in the Sun are recovered from the inversion of spectroscopic data. ...M-dwarfs also show highly inhomogeneous surface structures in polarized light and photometric variability that are believed to be connected with cool regions on their surface, too. Supergiants have highly inhomogeneous atmospheres because large convective cells, and therefore bright and dark regions, have sizes comparable to the size of stars themselves. Hot luminous stars can have dense clamps in their atmospheres caused by an interaction of fast stellar wind, rotation, and the magnetic fields. Finally, a special class of main-sequence B-F stars, called chemically peculiar stars, have spots of abundance origin. Spots in these stars can look bright or dark depending on wavelength of observation. Modern interferometry is capable of reaching a very high angular resolution that will make it possible to study starspots in very detail. In this review we will focus on starspots in general and abundance spots in chemically peculiar stars in particular, as well as possible application of interferometry to study them.
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