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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.
| The Effective Temperature Scale of FGK Stars. II. Teff:Color:[Fe/H] Calibrations
We present up-to-date metallicity-dependent temperature versus colorcalibrations for main-sequence and giant stars based on temperaturesderived with the infrared flux method (IRFM). Seventeen colors in thephotometric systems UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO,Hipparcos-Tycho, and Two Micron All Sky Survey (2MASS) have beencalibrated. The spectral types covered by the calibrations range from F0to K5 (7000K>~Teff>~4000K) with some relationsextending below 4000 K or up to 8000 K. Most of the calibrations arevalid in the metallicity range -3.5>~[Fe/H]>~0.4, although some ofthem extend to as low as [Fe/H]~-4.0. All fits to the data have beenperformed with more than 100 stars; standard deviations range from 30 to120 K. Fits were carefully performed and corrected to eliminate thesmall systematic errors introduced by the calibration formulae. Tablesof colors as a function of Teff and [Fe/H] are provided. Thiswork is largely based on the study by A. Alonso and collaborators; thus,our relations do not significantly differ from theirs except for thevery metal-poor hot stars. From the calibrations, the temperatures of 44dwarf and giant stars with direct temperatures available are obtained.The comparison with direct temperatures confirms our finding in Paper Ithat the zero point of the IRFM temperature scale is in agreement, tothe 10 K level, with the absolute temperature scale (that based onstellar angular diameters) within the ranges of atmospheric parameterscovered by those 44 stars. The colors of the Sun are derived from thepresent IRFM Teff scale and they compare well with those offive solar analogs. It is shown that if the IRFM Teff scaleaccurately reproduces the temperatures of very metal-poor stars,systematic errors of the order of 200 K, introduced by the assumption of(V-K) being completely metallicity independent when studying verymetal-poor dwarf stars, are no longer acceptable. Comparisons with otherTeff scales, both empirical and theoretical, are also shownto be in reasonable agreement with our results, although it seems thatboth Kurucz and MARCS synthetic colors fail to predict the detailedmetallicity dependence, given that for [Fe/H]=-2.0, differences as highas approximately +/-200 K are found.
| Broad-band photometric colors and effective temperature calibrations for late-type giants. I. Z = 0.02
We present new synthetic broad-band photometric colors for late-typegiants based on synthetic spectra calculated with the PHOENIX modelatmosphere code. The grid covers effective temperatures T_eff=3000dots5000 K, gravities log g=-0.5dots{+3.5}, and metallicities[M/H]=+0.5dots{-4.0}. We show that individual broad-band photometriccolors are strongly affected by model parameters such as molecularopacities, gravity, microturbulent velocity, and stellar mass. Ourexploratory 3D modeling of a prototypical late-type giant shows thatconvection has a noticeable effect on the photometric colors too, as italters significantly both the vertical and horizontal thermal structuresin the outer atmosphere. The differences between colors calculated withfull 3D hydrodynamical and 1D model atmospheres are significant (e.g.,Δ(V-K)0.2 mag), translating into offsets in effectivetemperature of up to 70 K. For a sample of 74 late-type giants in theSolar neighborhood, with interferometric effective temperatures andbroad-band photometry available in the literature, we compare observedcolors with a new PHOENIX grid of synthetic photometric colors, as wellas with photometric colors calculated with the MARCS and ATLAS modelatmosphere codes. We find good agreement of the new synthetic colorswith observations and published T_eff-color and color-color relations,especially in the T_eff-(V-K), T_eff-(J-K) and (J-K)-(V-K) planes.Deviations from the observed trends in the T_eff-color planes aregenerally within ±100 K for T_eff=3500 to 4800 K. Syntheticcolors calculated with different stellar atmosphere models agree to±100 K, within a large range of effective temperatures andgravities. The comparison of the observed and synthetic spectra oflate-type giants shows that discrepancies result from the differencesboth in the strengths of various spectral lines/bands (especially thoseof molecular bands, such as TiO, H2O, CO) and the continuum level.Finally, we derive several new T_eff-log g-color relations for late-typegiants at solar-metallicity (valid for T_eff=3500 to 4800 K), based bothon the observed effective temperatures and colors of the nearby giants,and synthetic colors produced with PHOENIX, MARCS and ATLAS modelatmospheres.
| WZ Cas - variability on multiple time-scales
We present results on our long-term velocity monitoring of the C-richsemi-regular variable WZ Cas with a Coravel instrument. Our findings arecompared with light changes of the star. We suggest that the two mainperiods, 366 and 186 d, are due to radial pulsation. Furthermore, wefind an interesting variation of the width and depth of thecross-correlation profile over a time-scale of at least 1000 d. Severalpossible explanations for this behaviour are discussed by comparison oftime-scales and expected variability amplitudes. The influence ofmovements of large convective cells on the line profiles seems to be themost likely explanation of some of the observed phenomena.
| First results from the ESO VLTI calibrators program
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
| Aldebaran's angular diameter: How well do we know it?
The bright, well-known K5 giant Aldebaran,α Tau, is probably the star with the largestnumber of direct angular diameter determinations, achieved over a longtime by several authors using various techniques. In spite of thiswealth of data, or perhaps as a direct result of it, there is not a verygood agreement on a single angular diameter value. This is particularlyunsettling if one considers that Aldebaran is also used as a primarycalibrator for some angular resolution methods, notably for optical andinfrared long baseline interferometry. Directly connected to Aldebaran'sangular diameter and its uncertainties is its effective temperature,which also has been used for several empirical calibrations. Among theproposed explanations for the elusiveness of an accurate determinationof the angular diameter of Aldebaran are the possibility of temporalvariations as well as a possible dependence of the angular diameter onthe wavelength. We present here a few, very accurate new determinationsobtained by means of lunar occultations and long baselineinterferometry. We derive an average value of 19.96±0.03milliarcsec for the uniform disk diameter. The correspondinglimb-darkened value is 20.58±0.03 milliarcsec, or 44.2±0.9Rȯ. We discuss this result, in connection with previousdeterminations and with possible problems that may affect suchmeasurements.Based on observations collected at TIRGO (Gornergrat, Switzerland). TIRGO is operated by CNR - CAISMI Arcetri, Italy.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165}
| Infrared Irradiance Calibration
Infrared astronomical measurements are calibrated against referencesources, usually primary standard stars that are, in turn, calibratedeither by direct or indirect means. A direct calibration compares thestar with a certified source, typically a blackbody. Indirect methodsextrapolate a direct measurement of the flux at one wavelength to theflux at another. Historically, α Lyr (Vega) has been used as theprimary standard as it is bright, easily accessible from the northernhemisphere, and is well calibrated in the visual. Until recently, thedirect absolute infrared calibrations of α Lyr and those derivedfrom the absolute solar flux scaled to the observed spectral energydistributions of solar type stars increasingly diverged with wavelengthfrom those obtained using a model atmosphere to extrapolate the absolutevisual flux of Vega into the infrared. The exception is the directcalibration by the 1996/97 Midcourse Space Experiment of the absolutefluxes for a number of the commonly used infrared standard stars,including Vega.
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| Seasonal variation of Titan's stratospheric ethylene (C2H4) observed
All previous observations of seasonal change on Titan have been ofphysical phenomena such as clouds and haze. We present here the firstobservational evidence of chemical change in Titan's atmosphere. Imagestaken during 1999-2002 (late southern spring on Titan) with the W.M.Keck I 10-meter telescope at 8-13 μm show a significant accumulationof ethylene (C2H4) in the south polar stratosphereas well as north-south stratospheric temperature variation (colder atpoles). Our observations restrict this newly discovered south polarethylene accumulation to latitudes south of 60° S. The only otherobservations of the spatial distribution of C2H4were those of Voyager I, which found a significant north polaraccumulation in early northern spring. We see no build-up in the north,although the highest northern latitudes are obstructed from view in thecurrent season. Our observations constrain any unobserved north polaraccumulation of C2H4 to north of 50° Nlatitude. Comparison of the Voyager I results with our new results showseasonal chemical change has occurred in Titan's atmosphere.
| Search for Nanosecond Optical Pulses from Nearby Solar-Type Stars
With ``Earth 2000'' technology we could generate a directed laser pulsethat outshines the broadband visible light of the Sun by 4 orders ofmagnitude. This is a conservative lower bound for the technicalcapability of a communicating civilization; optical interstellarcommunication is thus technically plausible. We have built a pair ofsystems to detect nanosecond pulsed optical signals from a target listthat includes some 13,000 Sun-like stars, and we have made some 16,000observations totaling nearly 2400 hr during five years of operation. Abeam splitter-fed pair of hybrid avalanche photodetectors at the 1.5 mWyeth Telescope at the Harvard/Smithsonian Oak Ridge Observatory(Agassiz Station) triggers on a coincident pulse pair, initiatingmeasurement of pulse width and intensity at subnanosecond resolution. Anidentical system at the 0.9 m Cassegrain at Princeton's Fitz-RandolphObservatory performs synchronized observations with 0.1 μs eventtiming, permitting unambiguous identification of even a solitary pulse.Among the 11,600 artifact-free observations at Harvard, the distributionof 274 observed events shows no pattern of repetition, and is consistentwith a model with uniform event rate, independent of target. With onepossible exception (HIP 107395), no valid event has been seensimultaneously at the two observatories. We describe the search andcandidate events and set limits on the prevalence of civilizationstransmitting intense optical pulses.
| High-Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer
We present first results of an experiment to combine data from Keckaperture masking and the Infrared-Optical Telescope Array to image thecircumstellar environments of evolved stars with ~20 mas resolution. Theunique combination of excellent Fourier coverage at short baselines andhigh-quality long-baseline fringe data allows us to determine thelocation and clumpiness of the innermost hot dust in the envelopes andto measure the diameters of the underlying stars themselves. We findevidence for large-scale inhomogeneities in some dust shells and alsosignificant deviations from uniform brightness for the photospheres ofthe most evolved M stars. Deviations from spherically symmetric massloss in the red supergiant NML Cyg could be related to recent evidencefor dynamically important magnetic fields and/or stellar rotation. Wepoint out that dust shell asymmetries, like those observed here, canqualitatively explain the difficulty recent workers have had insimultaneously fitting the broadband spectral energy distributions andhigh-resolution spatial information, without invoking unusual dustproperties or multiple distinct shells (from hypothetical``superwinds''). This paper is the first to combine opticalinterferometry data from multiple facilities for imaging, and we discussthe challenges and potential for the future of this method, givencurrent calibration and software limitations.
| Long Secondary Periods in Pulsating Asymptotic Giant Branch Stars: An Investigation of their Origin
Approximately 25%-30% of pulsating asymptotic giant branch (AGB) starsshow long secondary periods (LSPs) of typical length ~400-1500 days,roughly 10 times longer than the primary period of pulsation. Here weseek an explanation for the LSPs. We describe the spectral variationsover a 4 yr interval for three of these stars. The radial velocity isfound to vary during the LSP with full amplitude of ~5 kms-1, a result similar to that found by Hinkle and coworkersfor six other variables of this type. Variations in the Hα and NaD line profiles throughout the LSP suggest that chromospheric activityand mass loss vary with the period of the LSP. Possible explanations forthe photometric and radial velocity variations include eccentric motionof an orbiting companion of mass ~0.1 Msolar, radial andnonradial pulsation, rotation of an ellipsoidal-shaped red giant,episodic dust ejection, and star spot cycles. We discuss each of thesemodels and show that they all have problems. The most likely explanationis that the LSPs result from a low degree g+ mode confined tothe outer radiative layers of the red giant, combined with large-scalestar spot activity to give the observed chromosphere and theirregularity of the light curve. We suggest that these stars may be theprecursors of asymmetric planetary nebulae.
| Unveiling Mira stars behind the molecules. Confirmation of the molecular layer model with narrow band near-infrared interferometry
We have observed Mira stars with the FLUOR beamcombiner on the IOTAinterferometer in narrow bands around 2.2 μm wavelength. We findsystematically larger diameters in bands contaminated by water vapor andCO. The visibility measurements can be interpreted with a modelcomprising a photosphere surrounded by a thin spherical molecular layer.The high quality of the fits we obtain demonstrates that this simplemodel accounts for most of the star's spatial structure. For each starand each period we were able to derive the radius and temperature of thestar and of the molecular layer as well as the optical depth of thelayer in absorption and continuum bands. The typical radius of themolecular layer is 2.2 R* with a temperature ranging between1500 and 2100 K. The photospheric temperatures we find are in agreementwith spectral types of Mira stars. Our photospheric diameters are foundsmaller than in previous studies by several tens of percent. We believeprevious diameters were biased by the use of unsuited geometrical modelsto explain visibilities. The conclusions of this work are various.First, we offer a consistent view of Mira stars over a wide range ofwavelengths. Second, the parameters of the molecular layer we find areconsistent with spectroscopic studies. Third, from our diametermeasurements we deduce that all Mira stars are fundamental modepulsators and that previous studies leading to the conclusion of thefirst-overtone mode were biased by too large diameter estimates.Based on observations collected at the IOTA interferometer, WhippleObservatory, Mount Hopkins, Arizona.Table 3 is only available in electronic form athttp://www.edpsciences.org
| Interferometric observations of the supergiant stars α Orionis and α Herculis with FLUOR at IOTA
We report the observations in the K band of the red supergiant starα Orionis and of the bright giant star α Herculis with theFLUOR beamcombiner at the IOTA interferometer. The high quality of thedata allows us to estimate limb-darkening and derive precise diametersin the K band which combined with bolometric fluxes yield effectivetemperatures. In the case of Betelgeuse, data collected at high spatialfrequency although sparse are compatible with circular symmetry andthere is no clear evidence for departure from circular symmetry. We havecombined the K band data with interferometric measurements in the L bandand at 11.15 μm. The full set of data can be explained if a 2055 Klayer with optical depths τK=0.060±0.003,τL=0.026±0.002 and τ11.15 μm=2.33±0.23 is added 0.33 R* above the photosphereproviding a first consistent view of the star in this range ofwavelengths. This layer provides a consistent explanation for at leastthree otherwise puzzling observations: the wavelength variation ofapparent diameter, the dramatic difference in limb darkening between thetwo supergiant stars, and the previously noted reduced effectivetemperature of supergiants with respect to giants of the same spectraltype. Each of these may be simply understood as an artifact due to notaccounting for the presence of the upper layer in the data analysis.This consistent picture can be considered strong support for thepresence of a sphere of warm water vapor, proposed by \cite{tsuji2000}when interpreting the spectra of strong molecular lines.Based on observations collected at the IOTA interferometer, WhippleObservatory, Mount Hopkins, Arizona.
| On the analysis of band 3 of the ISO-SWS calibration sources
We analyse ISO-SWS 01 (R 1500) 12-27.5 μm (band 3) spectra ofthe 10 standard calibration stars with the highest flux using syntheticspectra generated from (MARCS) atmosphere models. The comparison betweenthe observed and synthetic spectra reveals the quality of (1) theatmospheric model construction and subsequent synthetic spectracomputation and of (2) the (OLP 10.1) calibration and data reduction ofthe spectrometer at these wavelengths.The models represent the general features of the observations, but thesynthetic spectrum computation is hampered by the lack of comprehensivemolecular and atomic line lists. We also suspect some problems with thetemperature distribution in the outer layers of the model andinaccuracies in the extrapolation of the collision-induced absorptioncoefficients of H2 pairs. We detect baseline ripples andfringes in the observed spectra, that survive the calibration anddetailed reduction process. Photometric calibration uncertainties areestimated by means of the scaling factors between the synthetic andobserved spectra.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| HD 199143 and HD 358623: Two recently identified members of the β Pictoris moving group
HD 199143 and HD 358623 (BD-17°6128) are two sets of binary starswhich are physically associated and 48 pc from Earth. We presentheliocentric radial velocities and high lithium abundances whichestablish these stars as members of the 12 Myr-old β PictorisMoving Group. We also present mid-IR photometric measurements which showno firm evidence for warm dust around all four stars.
| Lithium in the symbiotic Mira V407 Cyg
We report an identification of the lithium resonance doublet Li I 6708Å in the spectrum of V407 Cyg, a symbiotic Mira with a pulsationperiod of about 745 d. The resolution of the spectra used wasR=λ/Δλ~ 18 500 and the measured equivalent width ofthe line is ~0.34 Å. It is suggested that the lithium enrichmentis due to hot bottom burning in the intermediate-mass asymptotic giantbranch (AGB) variable, although other possible origins cannot be totallyruled out. In contrast to lithium-rich AGB stars in the MagellanicClouds, ZrO λλ5551, 6474 absorption bands were not foundin the spectrum of V407 Cyg. These are the bands used to classify theS-type stars at low-resolution. Although we identified weak ZrOλλ5718, 6412 these are not visible in the low-resolutionspectra, and we therefore classify the Mira in V407 Cyg as an M type.This, together with other published work, suggests lithium enrichmentcan precede the third dredge up of s-process enriched material ingalactic AGB stars.
| The Mid-Infrared-emitting Dust around AB Aurigae
Using the Keck I telescope, we have obtained 11.7 and 18.7 μm imagesof the circumstellar dust emission from AB Aur, a Herbig Ae star. Wefind that AB Aur is probably resolved at 18.7 μm with an angulardiameter of 1.2" at a surface brightness of 3.5 Jy arcsec-2.Most of the dust mass detected at millimeter wavelengths does notcontribute to the 18.7 μm emission, which is plausibly explained ifthe system possesses a relatively cold, massive disk. We find thatmodels with an optically thick, geometrically thin disk surrounded by anoptically thin spherical envelope fit the data somewhat better thanflared disk models.
| Angular Diameters of Stars from the Mark III Optical Interferometer
Observations of 85 stars were obtained at wavelengths between 451 and800 nm with the Mark III Stellar Interferometer on Mount Wilson, nearPasadena, California. Angular diameters were determined by fitting auniform-disk model to the visibility amplitude versus projected baselinelength. Half the angular diameters determined at 800 nm have formalerrors smaller than 1%. Limb-darkened angular diameters, effectivetemperatures, and surface brightnesses were determined for these stars,and relationships between these parameters are presented. Scatter inthese relationships is larger than would be expected from themeasurement uncertainties. We argue that this scatter is not due to anunderestimate of the angular diameter errors; whether it is due tophotometric errors or is intrinsic to the relationship is unresolved.The agreement with other observations of the same stars at the samewavelengths is good; the width of the difference distribution iscomparable to that estimated from the error bars, but the wings of thedistribution are larger than Gaussian. Comparison with infraredmeasurements is more problematic; in disagreement with models, coolerstars appear systematically smaller in the near-infrared than expected,warmer stars larger.
| Infrared Colors and Variability of Evolved Stars from COBE DIRBE Data
For a complete 12 μm flux-limited sample of 207 IRAS sources(F12>=150 Jy, |b|>=5deg), the majority ofwhich are AGB stars (~87%), we have extracted light curves in seveninfrared bands between 1.25 and 60 μm using the database of theDiffuse Infrared Background Experiment (DIRBE) instrument on the CosmicBackground Explorer (COBE) satellite. Using previous infrared surveys,we filtered these light curves to remove data points affected by nearbycompanions and obtained time-averaged flux densities and infraredcolors, as well as estimates of their variability at each wavelength. Inthe time-averaged DIRBE color-color plots, we find clear segregation ofsemiregulars, Mira variables, carbon stars, OH/IR stars, and red giantswithout circumstellar dust (i.e., V-[12]<5) and with little or novisual variation (ΔV<0.1 mag). The DIRBE 1.25-25 μm colorsbecome progressively redder and the variability in the DIRBE databaseincreases along the oxygen-rich sequence nondusty slightly varying redgiants-->SRb/Lb-->SRa-->Mira-->OH/IR and the carbon-richSRb/Lb-->Mira sequence. This supports previous assertions that theseare evolutionary sequences involving the continued production andejection of dust. The carbon stars are redder than their oxygen-richcounterparts for the same variability type, except in theF12/F25 ratio, where they are bluer. Of the 28sources in the sample not previous noted to be variable, 18 are clearlyvariable in the DIRBE data, with amplitudes of variation of ~0.9 mag at4.9 μm and ~0.6 mag at 12 μm, consistent with them being verydusty Mira-like variables. We also present individual DIRBE light curvesof a few selected stars. The DIRBE light curves of the semiregularvariable L2 Pup are particularly remarkable. The maxima at1.25, 2.2, and 3.5 μm occur 10-20 days before those at 4.9 and 12μm, and, at 4.9 and 12 μm, another maximum is seen between the twonear-infrared maxima.
| ISO-SWS calibration and the accurate modelling of cool-star atmospheres. IV. G9 to M2 stars
A detailed spectroscopic study of 11 giants with spectral type from G9to M2 is presented. The 2.38-4.08 mu m wavelength-range of band 1 ofISO-SWS (Short-Wavelength Spectrometers on board of the Infrared SpaceObservatory) in which many different molecules - with their owndependence on each of the stellar parameters - are absorbing, enables usto estimate the effective temperature, the gravity, the microturbulence,the metallicity, the CNO-abundances, the12C/13C-ratio and the angular diameter from theISO-SWS data. Using the Hipparcos' parallax, the radius, luminosity andgravity-inferred mass are derived. The stellar parameters obtained arein good agreement with other published values, though also somediscrepancies with values deduced by other authors are noted. For a fewstars (delta Dra, xi Dra, alpha Tuc, H Sco and alpha Cet) someparameters - e.g. the CNO-abundances - are derived for the first time.By examining the correspondence between different ISO-SWS observationsof the same object and between the ISO-SWS data and the correspondingsynthetic spectrum, it is shown that the relative accuracy of ISO-SWS inband 1 (2.38-4.08 mu m) is better than 2% for these high-flux sources.The high level of correspondence between observations and theoreticalpredictions, together with a confrontation of the estimatedTeff (ISO) value with Teff values derived fromcolours - which demonstrates the consistency between V-K,BCK, Teff and thetad derived fromoptical or IR data - proves that both the used MARCS models to derivethe stellar quantities and the flux calibration of the ISO-SWS detectorshave reached a high level of reliability.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Appendices A-D are only available in electronic form athttp://www.edpsciences.org
| ISO-SWS calibration and the accurate modelling of cool-star atmospheres. II. General results
The fine calibration of the ISO-SWS detectors (Infrared SpaceObservatory - Short Wavelength Spectrometer) has proven to be a delicateproblem. We therefore present a detailed spectroscopic study in the2.38-12 mu m wavelength range of a sample of 16 A0-M2 stars used for thecalibration of ISO-SWS. By investigating the discrepancies between theISO-SWS data of these sources, the theoretical predictions of theirspectra, the high-resolution FTS-KP (Kitt Peak) spectrum of alpha Booand the solar FTS-ATMOS (Atmospheric Trace Molecule Spectroscopy)spectrum, both calibration problems and problems in computing thetheoretical models and the synthetic spectra are revealed. Theunderlying reasons for these problems are sought for and the impact onthe further calibration of ISO-SWS and on the theoretical modelling isdiscussed extensively.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Appendix is only available in electronic form athttp://www.edpsciences.org
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| The Stellar and Gas Kinematics of Several Irregular Galaxies
We present long-slit spectra of three irregular galaxies from which wedetermine the stellar kinematics in two of the galaxies (NGC 1156 andNGC 4449) and ionized gas kinematics in all three (including NGC 2366).We compare this to the optical morphology and H I kinematics of thegalaxies. In the ionized gas, we see a linear velocity gradient in allthree galaxies. In NGC 1156 we also detect a weak linear velocitygradient in the stars of (5+/-1/sini) km s-1 kpc-1to a radius of 1.6 kpc. The stars and gas are rotating about the sameaxis, but this is different from the major axis of the stellar bar,which dominates the optical light of the galaxy. In NGC 4449 we do notdetect organized rotation of the stars and place an upper limit of(3/sini) km s-1 kpc-1 to a radius of 1.2 kpc. ForNGC 4449, which has signs of a past interaction with another galaxy, wedevelop a model to fit the observed kinematics of the stars and gas. Inthis model the stellar component is in a rotating disk seen nearlyface-on, while the gas is in a tilted disk with orbits whose planesprecess in the gravitational potential. This model reproduces theapparent counterrotation of the inner gas of the galaxy. The peculiarorbits of the gas are presumed as a result of acquisition of gas in thepast interaction.
| L'-Band Interferometric Observations of Evolved Stars
Ten bright Miras, six semiregular variable giants, and two semiregularvariable supergiants have been observed with the Infrared-OpticalTelescope Array interferometer in the L' band (from 3.4 to 4.1 μm).Observations were carried out in 2000 March and November with theFLUOR/TISIS instrument, using optimized single-mode waveguides foroptical recombination and a dedicated chopping system for accuratesubtraction of slow thermal background drifts. Four of the sources (theMira stars R Leo and R Cnc, α Ori, and RS Cnc) were observed inboth runs. We report on visibility measurements and derive L' broadbanduniform disk (UD) diameter best fits for all 18 stars in our sample. Wealso detect strong departures from UD models in some peculiar cases.
| The Effect of TiO Absorption on Optical and Infrared Angular Diameters of Cool Stars
We review the systematic variation between optical- andinfrared-wavelength angular diameters reported for stars in theapproximate range of spectral types K0-M6. We show that there is acorrelation between the ratio of angular diameters and the depth of TiOabsorption, in the sense that the optical diameters are larger. We arguethat this validates a recent proposal by Houdashelt et al. that TiOabsorption affects certain, but not all, optical-wavelength angulardiameters significantly. Those authors pointed out that the infraredangular diameters appear to yield better effective temperatures than dothe optical diameters, even though the latter are of higher precision.The observed angular diameter differences may arise either from limbdarkening, atmospheric extension, or a combination of these twoprocesses. Model atmosphere calculations of limb-darkening coefficientsare needed to see whether the diameter discrepancy may be resolved.These models need to contain the correct opacity sources and a realisticestimate of the atmospheric geometry and dynamics. A comparison withobservations such as those described in this paper will be useful fortesting the validity of atmosphere models.
| Speckle Observations of Binary Stars with the WIYN Telescope. II. Relative Astrometry Measures during 1998-2000
Five hundred twelve relative astrometry measures are presented for 253double stars, including 53 double stars discovered by Hipparcos. In 15cases, relative astrometry is reported for the first time for newlyconfirmed pairs. In addition, 20 high-quality nondetections ofcompanions are reported for stars suspected of being nonsingle byHipparcos. Observations were taken using a fast-readout CCD camerasystem at the WIYN 3.5 m telescope at Kitt Peak, Arizona. In comparingthese measures with ephemeris predictions for binary stars with verywell known orbits, we find that the measurement precision is better than3 mas in separation and 1° in position angle per individualobservation. Measurement precision and detection capabilities are fullydiscussed, and confirmed orbital motion is reported in four cases of theHipparcos double star discoveries. The WIYN Observatory is a jointfacility of the University of Wisconsin-Madison, Indiana University,Yale University, and the National Optical Astronomy Observatory.
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