ABSTRACT
Differential Interferometry allows to obtain the differential visibility and phase, in addition to the spectrum. The differential phase contains important information about the structure and ...motion of stellar photosphere such as stellar spots and non-radial pulsations, and particularly the rotation. Thus, this interferometric observable strongly helps to constrain the stellar fundamental parameters of fast rotators. The spectroastrometry mainly uses the photocentre displacements, which is a first approximation of the differential phase, and is applicable only for unresolved or marginally objects. We study here the sensitivity of relevant stellar parameters to the simulated photocentres using the scirocco code: a semi-analytical algorithm dedicated to fast rotators, applied to two theoretical modelling stars based on Achernar and Regulus, in order to classify the importance of these parameters and their impact on the modelling. We compare our simulations with published VLTI/AMBER data. This work sets the limits of application of photocentre displacements to fast rotators, and under which conditions we can use the photocentres and/or the differential phase, through a pre-established physical criterion. To validate our theoretical study, we apply our method of analysis on observed data of the edge-on fast rotator Regulus. For unresolved targets, with a visibility V ∼ 1, the photocentre can constrain the main stellar fundamental parameters of fast rotators, whereas from marginally resolved objects (0.8 ≤ V < 1), mainly the rotation axis position angle ($\rm PA_{\rm rot}$) can be directly deduced from the vectorial photocentre displacement, which is very important for young cluster studies.
We report here the first observations of a rapidly rotating Be star, α Eridani, using Earth-rotation synthesis on the Very Large Telescope (VLT) Interferometer. Our measures correspond to a $2a/2b = ...1.56\pm0.05$ apparent oblate star, $2a$ and $2b$ being the equivalent uniform disc angular diameters in the equatorial and polar direction. Considering the presence of a circumstellar envelope (CSE) we argue that our measurement corresponds to a truly distorted star since α Eridani exhibited negligible Hα emission during the interferometric observations. In this framework we conclude that the commonly adopted Roche approximation (uniform rotation and centrally condensed mass) should not apply to α Eridani. This result opens new perspectives to basic astrophysical problems, such as rotationally enhanced mass loss and internal angular momentum distribution. In addition to its intimate relation with magnetism and pulsation, rapid rotation thus provides a key to the Be phenomenon: one of the outstanding non-resolved problems in stellar physics.
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Optical interferometry is entering a new age with several ground- based
long-baseline observatories now making observations of unprecedented spatial
resolution. Based on a great leap forward in the ...quality and quantity of
interferometric data, the astrophysical applications are not limited anymore
to classical subjects, such as determination of fundamental properties of
stars; namely, their effective temperatures, radii, luminosities and masses,
but the present rapid development in this field allowed to move to a
situation where optical interferometry is a general tool in studies of many
astrophysical phenomena. Particularly, the advent of long-baseline
interferometers making use of very large pupils has opened the way to faint
objects science and first results on extragalactic objects have made it a
reality. The first decade of XXI century is also remarkable for aperture
synthesis in the visual and near-infrared wavelength regimes, which provided
image reconstructions from stellar surfaces to Active Galactic Nuclei. Here I
review the numerous astrophysical results obtained up to date, except for
binary and multiple stars milliarcsecond astrometry, which should be a
subject of an independent detailed review, taking into account its importance
and expected results at microarcsecond precision level. To the results
obtained with currently available interferometers, I associate the adopted
instrumental settings in order to provide a guide for potential users
concerning the appropriate instruments which can be used to obtain the
desired astrophysical information.
Opticka interferometrija ulazi u novo doba sa nekoliko zemaljskih
opservatorija koje, koristeci dugacke interferometrijske baze, vrse
posmatranja sa prostornom rezolucijom bez presedana. Zasnovane na velikom
skoku unapred u kvalitetu i kvantitetu interferometrijskih podataka,
astrofizicke primene nisu vise ogranicene samo na klasicne teme, kao sto su
odredjivanje fundamentalnih parametara zvezda; naime njihove efektivne
temperature, precnici, sjaj i mase, nego je sadasnji brzi razvoj na tom polju
dozvolio da se dodje u situaciju gde opticka interferometrija postaje opsti
alat za proucavanje mnogih astrofizickih problema. Narocito je ulazak u
upotrebu interferometara sa dugackim bazama, koji koriste teleskope vrlo
velikih apertura, otvorio put za proucavanje slabih objekata, tako da su prvi
ekstragalakticki rezultati postali realnost. Prva dekada XXI veka je takodje
obelezena sintezom apertura u vidljivom i bliskom infracrvenom domenu
talasnih duzina, omogucavajuci rekonstrukciju slike od povrsine zvezda do
aktivnih galaktickih jezgara. Ovde dajem pregled brojnih do danas ostvarenih
astrofizickih rezultata, izuzimajuci astrometriju dvojnih i visestrukih
zvezda sa preciznoscu od mililucne sekunde, koja bi trebalo da bude tema
nezavisnog detaljnog pregleda, uzevsi u obzir njenu vaznost i ocekivane
rezultate na nivou mikro-lucne sekunde. Rezultatima koji su dobijeni trenutno
dostupnim interferometrima, pridruzujem koriscene instrumentalne
konfiguracije da bih naveo potencijalne korisnike na odgovarajuce instrumente
koji mogu biti upotrebljeni da bi se dobila zeljena astrofizicka informacija.
PR Projekat Ministarstva nauke Republike Srbije, br. 176004: Stellar physics
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As previously demonstrated on Achernar, one can derive the angular radius, rotational velocity, axis tilt, and orientation of a fast-rotating star from the differential phases obtained by spectrally ...resolved long baseline interferometry using earth-rotation synthesis. We applied this method on a small sample of stars for different spectral types and classes, in order to generalize the technique to other rotating stars across the H-R diagram and determine their fundamental parameters. Beyond the theoretical diffraction limit of an interferometer (ratio of the wavelength to the baseline), spatial super resolution is well suited to systematically estimating the angular diameters of rotating stars and their fundamental parameters with a few sets of baselines and the Earth-rotation synthesis provided a high enough spectral resolution.
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Context. As previously demonstrated on Achernar, one can derive the angular radius, rotational velocity, axis tilt, and orientation of a fast-rotating star from the differential phases obtained by ...spectrally resolved long baseline interferometry using earth-rotation synthesis. Aims. We applied this method on a small sample of stars for different spectral types and classes, in order to generalize the technique to other rotating stars across the H-R diagram and determine their fundamental parameters. Methods. We used differential phase data from the AMBER/VLTI instrument obtained prior to refurbishing its spectrometer in 2010. With the exception of Fomalhaut, which has been observed in the medium-resolution mode of AMBER (λ/δλ ≈ 1500), our three other targets, Achernar, Altair, and δ Aquilae offered high-resolution (λ/δλ ≈ 12 000) spectro-interferometric data around the Brγ absorption line in K band. These data were used to constrain the input parameters of an analytical, still realistic model to interpret the observations with a systematic approach for the error budget analysis in order to robustly conclude on the physics of our 4 targets. We applied the super resolution provided by differential phases φdiff to measure the size (equatorial radius Req and angular diameter ⌀eq), the equatorial rotation velocity (Veq), the inclination angle (i), and the rotation axis position angle (PArot) of 4 fast-rotating stars: Achernar, Altair, δ Aquilae, and Fomalhaut. The stellar parameters of the targets were constrained using a semi-analytical algorithm dedicated to fast rotators SCIROCCO. Results. The derived parameters for each star were Req = 11.2 ± 0.5 R⊙, Veqsini = 290 ± 17 km s-1, PArot = 35.4° ± 1.4°, for Achernar; Req = 2.0 ± 0.2 R⊙, Veqsini = 226 ± 34 km s-1, PArot = −65.5° ± 5.5°, for Altair; Req = 2.2 ± 0.3 R⊙, Veqsini = 74 ± 35 km s-1, PArot = −101.2° ± 14°, for δ Aquilae; and Req = 1.8 ± 0.2 R⊙, Veqsini = 93 ± 16 km s-1, PArot = 65.6° ± 5°, for Fomalhaut. They were found to be compatible with previously published values from differential phase and visibility measurements, while we were able to determine, for the first time, the inclination angle i of Fomalhaut (i = 90° ± 9°) and δ Aquilae (i = 81° ± 13°), and the rotation-axis position angle PArot of δ Aquilae. Conclusions. Beyond the theoretical diffraction limit of an interferometer (ratio of the wavelength to the baseline), spatial super resolution is well suited to systematically estimating the angular diameters of rotating stars and their fundamental parameters with a few sets of baselines and the Earth-rotation synthesis provided a high enough spectral resolution.
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Interferometric observations have revealed that the rapid rotator Altair is a flattened star with a non-centrally symmetric intensity distribution. In this work we perform for the first time a ...physically consistent analysis of all interferometric data available so far, corresponding to three different interferometers operating in several spectral bands. These observations include new data (squared visibilities in the H and K bands from VLTI-VINCI) as well as previously published data (squared visibilities in the K band from PTI and squared visibilities, triple amplitudes, and closure phases in the visible between 520 nm and 850 nm from NPOI). To analyze these data we perform a $\chi^2$ minimization using an interferometry-oriented model for fast rotators, which includes Roche approximation, limb-darkening, and von Zeipel-like gravity-darkening. Thanks to the rich interferometric data set available and to this physical model, the main uniqueness problems were avoided. As a result, we show that the observations can only be explained if Altair has a gravity-darkening compatible with the expected value for hot stars, i.e., the von Zeipel effect ($T_\mathrm{eff}\propto g^{0.25}$).
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ω Ori is a B2IIIe star for which rotational modulation and non-radial pulsations (NRP) have been recently investigated from two independent observational campaigns in 1998 and 1999. Putting the data ...of these 2 campaigns together, and adding data obtained in 2001, we search for multiperiodicity in the line profile variations and evidence for outbursts. From new spectropolarimetric data obtained at the Télescope Bernard Lyot (TBL, Pic du Midi, France) in 2001 we also measure the Stokes V parameter in the polarised light. We find evidence for the presence of a weak magnetic field in ω Ori sinusoidally varying with a period of 1.29 d. The equivalent widths (EW) of the wind sensitive UV resonance lines also show a variation with the same period, which we identify as the rotational period of the star. We propose an oblique rotator model and derive $B_{\rm pol} =530\, \pm\, 230$ G to explain the observations. Moreover, we carry out an abundance analysis and find the star to be N-enriched, a property which is shared with other magnetic stars. We propose ω Ori as the first known classical Be star hosting a magnetic field.
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The past several decades have seen accelerating progress in improving binary
stars fundamental parameters determinations, as new observational techniques
have produced visual orbits of many ...spectroscopic binaries with a milli
arcsecond precision. Modern astrometry is rapidly approaching the goal of
sub-milli arcsecond precision, and although presently this precision has been
achieved only for a limited number of binary stars, in the near future this
will become a standard for very large number of objects. In this paper we
review the representative results of techniques which have already allowed
the sub-milli arcsecond precision like the optical long baseline
interferometry, as well as the precursor techniques such as speckle
interferometry, adaptive optics and aperture masking. These techniques
provide a step forward from milli to sub-milli arcsecond precision, allowing
even short period binaries to be resolved, and often resulting in orbits
allowing precisions in stellar dynamical masses better than 1%. We point out
that such unprecedented precisions should allow for a significant improvement
of our comprehension of stellar physics and other related astrophysical
topics.
nema
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