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Vega is a rapidly rotating star
Vega, the second brightest star in the northern hemisphere, serves as aprimary spectral type standard. Although its spectrum is dominated bybroad hydrogen lines, the narrower lines of the heavy elements suggestedslow to moderate rotation, giving confidence that the ground-basedcalibration of its visible spectrum could be safely extrapolated intothe ultraviolet and near-infrared (through atmosphere models), where italso serves as the primary photometric calibrator. But there have beenproblems: the star is too bright compared to its peers and it hasunusually shaped absorption line profiles, leading some to suggest thatit is a distorted, rapidly rotating star seen pole-on. Here we reportoptical interferometric observations that show that Vega has theasymmetric brightness distribution of the bright, slightly offset polaraxis of a star rotating at 93 per cent of its breakup speed. In additionto explaining the unusual brightness and line shape peculiarities, thisresult leads to the prediction of an excess of near-infrared emissioncompared to the visible, in agreement with observations. The largetemperature differences predicted across its surface call into questioncomposition determinations, adding uncertainty to Vega's age and openingthe possibility that its debris disk could be substantially older thanpreviously thought.
| High-precision photometry with the WIRE satellite .
Around 200 bright stars (V<6) have been monitored with the two-inchstar tracker on the WIRE satellite since observations started in 1999.Here we present new results for the solar-like star Procyon A, the twodelta Scuti stars Altair and epsilon Cephei, and the triple systemlambda Scorpii which consist of two B-type stars - one of which we findto be an eclipsing binary.
| First Results from the CHARA Array. VII. Long-Baseline Interferometric Measurements of Vega Consistent with a Pole-On, Rapidly Rotating Star
We have obtained high-precision interferometric measurements of Vegawith the CHARA Array and FLUOR beam combiner in the K' bandat projected baselines between 103 and 273 m. The measured visibilityamplitudes beyond the first lobe are significantly weaker than expectedfor a slowly rotating star characterized by a single effectivetemperature and surface gravity. Our measurements, when compared tosynthetic visibilities and synthetic spectrophotometry from a Roche-vonZeipel gravity-darkened model atmosphere, provide strong evidence forthe model of Vega as a rapidly rotating star viewed very nearly pole-on.Our best-fitting model indicates that Vega is rotating at ~91% of itsangular break-up rate with an equatorial velocity of 275 kms-1. Together with the measured vsini, this velocity yieldsan inclination for the rotation axis of 5deg. For this modelthe pole-to-equator effective temperature difference is ~2250 K, a valuemuch larger than previously derived from spectral line analyses. A polareffective temperature of 10,150 K is derived from a fit to ultravioletand optical spectrophotometry. The synthetic and observed spectralenergy distributions are in reasonable agreement longward of 140 nm,where they agree to 5% or better. Shortward of 140 nm, the model is upto 10 times brighter than observed. The model has a luminosity of ~37Lsolar, a value 35% lower than Vega's apparent luminositybased on its bolometric flux and parallax, assuming a slowly rotatingstar. Our model predicts the spectral energy distribution of Vega asviewed from its equatorial plane, and it may be employed in radiativemodels for the surrounding debris disk.
| Determination of Stellar Ellipticities in Future Microlensing Surveys
We propose a method that can determine the ellipticities of source starsof microlensing events produced by binary lenses. The method is based onthe fact that the products of the caustic-crossing timescale, Δt,and the cosine of the caustic incidence angle of the source trajectory,κ, of the individual caustic crossings are different for eventsinvolving an elliptical source, while the products are the same forevents associated with a circular source. The productΔt⊥=Δtcosκ corresponds to thecaustic-crossing timescale when the incidence angle of the sourcetrajectory is κ=0. For the unique determination of the sourceellipticity, resolutions of at least three caustic crossings arerequired. Although this requirement is difficult to achieve under thecurrent observational setup based on alert/follow-up mode, it will bepossible with the advent of future lensing experiments that will surveywide fields continuously at high cadence. For typical Galactic bulgeevents, the difference in Δt⊥ between causticcrossings is of the order of minutes, depending on the sourceorientations and ellipticities. Considering the monitoring frequency ofthe future lensing surveys of ~6 times hr-1 and the improvedphotometry, especially of the proposed space-based survey, we predictthat ellipticity determinations by the proposed method will be possiblefor a significant fraction of multiple caustic-crossing binary lensevents involving source stars having nonnegligible ellipticities.
| Resolving the Effects of Rotation in Altair with Long-Baseline Interferometry
We report the successful fitting of a Roche model, with a surfacetemperature gradient following the von Zeipel gravity darkening law, toobservations of Altair made with the Navy Prototype OpticalInterferometer. We confirm the claim by Ohishi et al. that Altairdisplays an asymmetric intensity distribution due to rotation, the firstsuch detection in an isolated star. Instrumental effects due to the highvisible flux of this first magnitude star appear to be the limitingfactor in the accuracy of this fit, which nevertheless indicates thatAltair is rotating at 0.90+/-0.02 of its breakup (angular) velocity. Ourresults are consistent with the apparent oblateness found by van Belleet al. and show that the true oblateness is significantly larger owingto an inclination of the rotational axis of ~64° to the line ofsight. Of particular interest, we conclude that instead of beingsubstantially evolved as indicated by its classification, A7 IV-V,Altair is only barely off the zero-age main sequence and represents agood example of the difficulties rotation can introduce in theinterpretation of this part of the HR diagram.
| The Hyades Binary θ2 Tauri: Confronting Evolutionary Models with Optical Interferometry
We determine the masses and magnitude difference of the components ofthe Hyades spectroscopic binary θ2 Tauri. We find thatboth components appear to be less massive and/or brighter than predictedfrom some recent evolutionary models. The rapid rotation and unknownrotational inclination of both components introduce uncertainty in theirluminosities and colors, but not enough to reconcile both of them withthe evolutionary models. We measured the visual orbit with the Mark IIIoptical interferometer and the Navy Prototype Optical Interferometer andcombined it with the Hipparcos proper-motion-based parallax to find atotal system mass ΣM of 4.03+/-0.20 Msolar.We also combined our visual orbit with three recent spectroscopic orbitsto find three spectroscopically based estimates of ΣM andcompared these to the ΣM from the visual orbit and parallax.We chose the spectroscopic orbit that agreed best and used its massratio to estimate individual masses MA,B of 2.15+/-0.12and 1.87+/-0.11 Msolar. From the interferometry, wedetermine Δm=1.13+/-0.05 mag across the 450-850 nm band. Theparallax then implies absolute V magnitudes MA,B of0.48+/-0.05 and 1.61+/-0.06 mag. If the components are rotating nearbreakup velocity and seen nearly pole-on, the true luminosities may beas faint as 1.03 and 2.13 mag; even in that case, however, the secondaryis too blue by ~0.07 mag in B-V.
| A comprehensive asteroseismic modelling of the high-amplitude δ Scuti star RV Arietis
We present a comprehensive asteroseismic study of the double-modehigh-amplitude δ Scuti star HD 187642(RV Ari). The modelling includes some of the mostrecent techniques: 1) effects of rotation on both equilibrium models andadiabatic oscillation spectrum, 2) non-adiabatic study of radial andnon-radial modes, 3) relationship between the fundamental radial modeand the first overtone in the framework of Petersen diagrams. Theanalysis reveals that two of the observed frequencies are very probablyidentified as the fundamental and first overtone radial modes. Analysisof the colour index variations, together with theoretical non-adiabaticcalculations, points to models in the range of [7065, 7245] K ineffective temperature and of [1190, 1270] Myr in stellar age. Thesevalues were found to be compatible with those obtained using the threeother asteroseismic techniques.
| Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| The polar wind of the fast rotating Be star Achernar. VINCI/VLTI interferometric observations of an elongated polar envelope
Context: .Be stars show evidence of mass loss and circumstellarenvelopes (CSE) from UV resonance lines, near-IR excesses, and thepresence of episodic hydrogen emission lines. The geometry of theseenvelopes is still uncertain, although it is often assumed that they areformed by a disk around the stellar equator and a hot polar wind.Aims: .We probe the close environment of the fast rotating Be starAchernar at angular scales of a few milliarcseconds (mas) in theinfrared, in order to constrain the geometry of a possible polar CSE. Methods: .We obtained long-baseline interferometric observations ofAchernar with the VINCI/VLTI beam combiner in the H and K bands, usingvarious telescope configurations and baseline lengths with a wideazimuthal coverage. Results: .The observed visibility measurementsalong the polar direction are significantly lower than the visibilityfunction of the photosphere of the star alone, in particular at lowspatial frequencies. This points to the presence of an asymmetricdiffuse CSE elongated along the polar direction of the star. To ourdata, we fit a simple model consisting of two components: a 2Delliptical Gaussian superimposed on a uniform ellipse representing thedistorted photosphere of the fast rotating star. Conclusions: .Weclearly detected a CSE elongated along the polar axis of the star, aswell as rotational flattening of the stellar photosphere. For theuniform-ellipse photosphere we derive a major axis of θ_eq = 2.13± 0.05 mas and a minor axis of θ_pol = 1.51 ± 0.02mas. The relative near-IR flux measured for the CSE compared to thestellar photosphere is f = 4.7 ± 0.3%. Its angular dimensions areloosely constrained by the available data at ρ_eq = 2.7 ± 1.3mas and ρ_pol = 17.6 ± 4.9 mas. This CSE could be linked tofree-free emission from the radiative pressure driven wind originatingfrom the hot polar caps of the star.
| Les etoiles, deformees par leur rotation.
Not Available
| An Improved Infrared Passband System for Ground-based Photometry: Realization
We describe new simulations and field trials of the new infraredpassband system developed and discussed by Young, Milone, & Stagg,who discussed and illustrated the state of infrared photometry andsuggested ways in which it could be improved. In particular, theypresented a new set of passbands that minimize the dependence of thephotometry on the water vapor bands of the atmospheric windows, whichdefined the edges of many previous infrared passbands, especially whenused at sites and under conditions for which they were not designed. Inthis paper, we present numerical simulations for three atmosphericmodels, demonstrate a measure of the signal-to-noise ratio in the newpassbands for these models, and present observational data obtained at arelatively low-elevation site. The latter demonstrate the utility ofthis system for most astronomical sites where photometry can beperformed, and permit the transformation of observations to this system.Publications of the Rothney Astrophysical Observatory, No. 74.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| Predicting accurate stellar angular diameters by the near-infrared surface brightness technique
I report on the capabilities of the near-infrared (near-IR) surfacebrightness technique to predict reliable stellar angular diameters asaccurate as <~2 per cent using standard broad-band Johnson photometryin the colour range -0.1 <= (V-K)O<= 3.7 includingstars of A, F, G, K spectral type. This empirical approach is fast toapply and leads to estimated photometric diameters in very goodagreement with recent high-precision interferometric diametermeasurements available for non-variable dwarfs and giants, as well asfor Cepheid variables. Then I compare semi-empirical diameters predictedby model-dependent photometric and spectrophotometric (SP) methods withnear-IR surface brightness diameters adopted as empirical referencecalibrators. The overall agreement between all these methods is withinapproximately +/-5 per cent, confirming previous works. However, on thesame scale of accuracy, there is also evidence for systematic shiftspresumably as a result of an incorrect representation of the stellareffective temperature in the model-dependent results. I also comparemeasurements of spectroscopic radii with near-IR surface brightnessradii of Cepheids with known distances. Spectroscopic radii are found tobe affected by a scatter as significant as >~9 per cent, which is atleast three times greater than the formal error currently claimed by thespectroscopic technique. In contrast, pulsation radii predicted by theperiod-radius (PR) relation according to the Cepheid period result aresignificantly less dispersed, indicating a quite small scatter as aresult of the finite width of the Cepheid instability strip, as expectedfrom pulsation theory. The resulting low level of noise stronglyconfirms our previous claims that the pulsation parallaxes are the mostaccurate empirical distances presently available for Galactic andextragalactic Cepheids.
| Visual Star Colours from Instrumental Photometry
In order to display graphically the visual colours of stars and otherastronomical objects, photometric broadband R, V, B colours are used toproxy for the r, g, b colours of the three visual sensors of the eye.From photometric Johnson B-V and V-R colour indices, R, V, and Bmagnitudes (V = 0) are calculated, and from these the respectivebrightnesses (r, v = 1 = g, and b) are calculated. After suitablenormalization these are then placed in a ternary diagram having r, g,and b as the vertices. All B-V and V-R are adjusted so that the Sunfalls in the same place as a blackbody at 5800 K. The resulting ternaryplot shows all of its objects (stars, planets) in their visual coloursat their relative positions in the ternary diagram. The star coloursdisplayed on a computer monitor screen or as a print with a colourprinter are more vivid than the usual visual impressions of isolatedstars, undoubtedly because of properties of the dark-adapted eye, butdouble-star pairs with contrasting colours correspond nicely totelescopic visual impressions.
| Observations and Modeling of the 2-25 μm Emission from High-Mass Protostellar Object Candidates
This is a report on detailed modeling of young high-mass protostellarcandidates during their most embedded and obscured phases. We performednarrowband mid-infrared imaging of three candidate high-massprotostellar objects in G11.94-0.62, G29.96-0.02, and G45.07+0.13 atGemini Observatory using the Thermal-Region Camera and Spectrograph(T-ReCS). The sources were imaged through up to 11 narrowband filters,sampling their SEDs over the entire 2-25 μm infrared range. For thefirst time, we have fit the observed SEDs of massive protostars withmodels that take into account departures from spherical symmetry in theinfalling envelopes. In this way, we have been able to derive from themodels the detailed physical parameters for these earliest stages ofmassive stellar life. Our detailed modeling suggests that massive starformation can proceed in a way very similar to the formation of low-massstars.
| Crystalline Silicate Emission in the Protostellar Binary Serpens SVS 20
We present spatially resolved mid-infrared spectroscopy of the ClassI/flat-spectrum protostellar binary system SVS 20 in the Serpens cloudcore. The spectra were obtained with the mid-infrared instrument T-ReCSon Gemini South. SVS 20-South, the more luminous of the two sources,exhibits a mid-infrared emission spectrum peaking near 11.3 μm, whileSVS 20-North exhibits a shallow amorphous silicate absorption spectrumwith a peak optical depth of τ~0.3. After removal of theline-of-sight extinction by the molecular common envelope, the``protostar-only'' spectra are found to be dominated by strong amorphousolivine emission peaking near 10 μm. We also find evidence foremission from crystalline forsterite and enstatite associated with bothSVS 20-S and SVS 20-N. The presence of crystalline silicate in such ayoung binary system indicates that the grain processing found in moreevolved Herbig Ae/Be and T Tauri pre-main-sequence stars likely beginsat a relatively young evolutionary stage, while mass accretion is stillongoing.
| First Results from the CHARA Array. I. An Interferometric and Spectroscopic Study of the Fast Rotator α Leonis (Regulus)
We report on K-band interferometric observations of the bright, rapidlyrotating star Regulus (type B7 V) made with the CHARA Array on MountWilson, California. Through a combination of interferometric andspectroscopic measurements, we have determined for Regulus theequatorial and polar diameters and temperatures, the rotational velocityand period, the inclination and position angle of the spin axis, and thegravity darkening coefficient. These first results from the CHARA Arrayprovide the first interferometric measurement of gravity darkening in arapidly rotating star and represent the first detection of gravitydarkening in a star that is not a member of an eclipsing binary system.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Diameters of Mira Stars Measured Simultaneously in the J, H, and K' Near-Infrared Bands
We present the first spatially resolved observations of a sample of 23Mira stars simultaneously measured in the near-infrared J, H, and K'bands. The technique used was optical long-baseline interferometry, andwe present for each star visibility amplitude measurements as a functionof wavelength. We also present characteristic sizes at each spectralband, obtained by fitting the measured visibilities to a simple uniformdisk model. This approach reveals the general relation J diameter < Hdiameter < K' diameter.
| Altair: The Brightest δ Scuti Star
We present an analysis of observations of the bright star Altair(α Aql) obtained using the star camera on the Wide Field InfraredExplorer (WIRE) satellite. Although Altair lies within the δ Scutiinstability strip, previous observations have not revealed the presenceof oscillations. However, the WIRE observations show Altair to be alow-amplitude (Δm<1 parts per thousand [ppt]) δ Scutistar with at least seven modes present.
| A new look at the position of the 1604 Supernova (V843 Ophiuchi)
The position of the supernova of 1604 (V843 Oph) is re-assessed, withrelevant discussion pertaining to the present-day remnant, 3C 358.
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| Dust Morphology and Composition in FU Orionis Systems
FU Orionis stars are a small group of pre-main-sequence stars known forlarge-amplitude optical variability. These objects also exhibitmultiwavelength phenomena suggestive of active accretion from acircumstellar disk. We present high spatial resolution mid-IR imagingand spectroscopy, submillimeter photometry, and 3-4 μm photometry offour FU Ori-class objects, RNO 1B and C, Z CMa, and Par 21, and oneobject classified as a pre-FU Ori star, V380 Ori. We resolve multiple IRsources and extended emission in the RNO 1B/C system, and we discuss indetail their association with disk activity and the source of theInfrared Astronomical Satellite far-IR and radio maser emission in thisfield. We derive dust temperatures and masses for all sources anddiscuss how dust composition and morphology is related to theevolutionary stage of these objects.
| Gravitational-darkening of Altair from interferometry
Interferometric observations have revealed that the rapid rotator Altairis a flattened star with a non-centrally symmetric intensitydistribution. In this work we perform for the first time a physicallyconsistent analysis of all interferometric data available so far,corresponding to three different interferometers operating in severalspectral bands. These observations include new data (squaredvisibilities in the H and K bands from VLTI-VINCI) as well as previouslypublished data (squared visibilities in the K band from PTI and squaredvisibilities, triple amplitudes, and closure phases in the visiblebetween 520 nm and 850 nm from NPOI). To analyze these data we perform aχ2 minimization using an interferometry-oriented modelfor fast rotators, which includes Roche approximation, limb-darkening,and von Zeipel-like gravity-darkening. Thanks to the richinterferometric data set available and to this physical model, the mainuniqueness problems were avoided. As a result, we show that theobservations can only be explained if Altair has a gravity-darkeningcompatible with the expected value for hot stars, i.e., the von Zeipeleffect (T_eff∝ g0.25).
| Detection of X-ray emission from β Pictoris with XMM-Newton: a cool corona, a boundary layer or what?
β Pictoris (HR 2020) is the most prominentprototype of stars with circumstellar disks and has generated particularinterest in the framework of young planetary systems. Given its spectraltype A5, stellar activity is not expected. Nevertheless, resonance linesof C iii and O vi typical for a chromosphere and transition region havebeen unambiguously detected with FUSE. We present results from anXMM-Newton observation of β Pic and find evidence for X-rayemission. In particular, we detected an emission of O vii at 21.6Å with the MOS detectors. These findings present a challenge forthe development of both stellar activity and disk models. We discuss andinvestigate various models to explain the observed emission includingthe presence of a cool corona and a boundary layer.
| New grids of stellar models including tidal-evolution constants up to carbon burning. II. From 0.8 to 125 Mȯ: the Small Magellanic Cloud (Z = 0.002{-0.004})
New stellar models spefically designed for the Small Magellanic Cloudare presented in this paper. In order to take into account theuncertainties in the metal content we computed two grids with differentmetallicities: Z = 0.002 and Z = 0.004. The covered mass range is from0.8 up to 125 Mȯ and the models are followed until theexhaustion of carbon in the core, for the more massive ones. We haveintroduced a recent measurement of the nuclear rate14N(p,γ)15O. A comparison among models withthe old and new rate was carried out revealed that the former areslightly hotter than the first ones. Such differences depend on the massrange. The opacities, the equation of state, the remaining nuclearreactions rates, the core overshooting parameterization and theconvective transport of energy are the same as discussed previously byus. We also give, besides the classical evolutionary models outputs, theinternal structure constants needed to investigate apsidal motion andtidal evolution in close binaries. This aspect acquires importance inthe light of recent investigations on circularization andsynchronization levels in binary systems belonging to the MagellanicClouds. The role of rotation can also be investigated through thegravity-darkening exponents which allow us to compute the brightnessdistribution of a given stellar surface.
| Determination of stellar shape in microlensing event MOA 2002-BLG-33
We report a measurement of the shape of the source star in microlensingevent MOA 2002-BLG-33. The lens for this event was a close binary whosecentre-of-mass passed almost directly in front of the source star. Atthis time, the source star was closely bounded on all sides by a causticof the lens. This allowed the oblateness of the source star to beconstrained. We found that a/b = 1.02+0.04-0.02where a and b are its semi-major and semi-minor axes respectively. Theangular resolution of this measurement is approximately 0.04μ{arcsec}. We also report HST images of the event that confirm aprevious identification of the source star as an F8-G2 turn-offmain-sequence star.
| Modelling of the fast rotating δ Scuti star Altair
We present an asteroseismic study of the fast rotating star HD187642 (Altair), recently discovered to bea δ Scuti pulsator. We have computed models taking into accountrotation for increasing rotational velocities. We investigate therelation between the fundamental radial mode and the first overtone inthe framework of Petersen diagrams. The effects of rotation on suchdiagrams, which become important at rotational velocities above 150 {km}{s}-1, as well as the domain of validity of our seismic toolsare discussed. We also investigate the radial and non-radial modes inorder to constrain models fitting the five most dominant observedoscillation modes.
| A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Aestars in the 10 μm spectral region. We perform compositional fits ofthe spectra based on properties of homogeneous as well as inhomogeneousspherical particles, and derive the mineralogy and typical grain sizesof the dust responsible for the 10 μm emission. Several trends arereported that can constrain theoretical models of dust processing inthese systems: i) none of the sources consists of fully pristine dustcomparable to that found in the interstellar medium; ii) all sourceswith a high fraction of crystalline silicates are dominated by largegrains; iii) the disks around more massive stars (M 2.5{M}ȯ, L 60 {L}ȯ) have a higherfraction of crystalline silicates than those around lower mass stars,iv) in the subset of lower mass stars (M 2.5 {M}ȯ)there is no correlation between stellar parameters and the derivedcrystallinity of the dust. The correlation between the shape andstrength of the 10 micron silicate feature reported by van Boekel et al.(2003) is reconfirmed with this larger sample. The evidence presented inthis paper is combined with that of other studies to present a likelyscenario of dust processing in Herbig Ae systems. We conclude that thepresent data favour a scenario in which the crystalline silicates areproduced in the innermost regions of the disk, close to the star, andtransported outward to the regions where they can be detected by meansof 10 micron spectroscopy. Additionally, we conclude that the finalcrystallinity of these disks is reached very soon after active accretionhas stopped.
| The sub-arcsecond dusty environment of Eta Carinae
The core of the nebula surrounding Eta Carinae has been observed withthe VLT Adaptive Optics system NACO and with the interferometerVLTI/MIDI to constrain spatially and spectrally the warm dustyenvironment and the central object. In particular, narrow-band images at3.74 μm and 4.05 μm reveal the butterfly shaped dusty environmentclose to the central star with unprecedented spatial resolution. A voidwhose radius corresponds to the expected sublimation radius has beendiscovered around the central source. Fringes have been obtained in theMid-IR which reveal a correlated flux of about 100 Jy situated 0.3arcsec south-east of the photocenter of the nebula at 8.7 μm, whichcorresponds with the location of the star as seen in other wavelengths.This correlated flux is partly attributed to the central object, andthese observations provide an upper limit for the SED of the centralsource from 2.2 μm to 13.5 μm. Moreover, we have been able tospectrally disperse the signal from the nebula itself at PA = 318degree, i.e. in the direction of the bipolar nebula (~310°) withinthe MIDI field of view of 3 arcsec. A large amount of corundum (Al2O3)is discovered, peaking at 0.6 arcsec-1.2 arcsec south-east from thestar, whereas the dust content of the Weigelt blobs is dominated bysilicates. We discuss the mechanisms of dust formation which are closelyrelated to the geometry of this Butterfly nebulae.
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