תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
The Asymmetric Thermal Emission of the Protoplanetary Disk Surrounding HD 142527 Seen by Subaru/COMICS
Mid-infrared (MIR) images of the Herbig Ae star HD 142527 were obtainedat 18.8 and 24.5 μm with the Subaru/COMICS. Bright extended arclikeemission (outer disk) is recognized at r=0.85" together with a strongcentral source (inner disk) and a gap around r=0.6" in both images. Thethermal emission on the eastern side is much brighter than that on thewestern side in the MIR. We estimate the dust size to be a few micronsfrom the observed color of the extended emission and the distance fromthe star. The dust temperature T and the optical depth τ of theMIR-emitting dust are also derived from the two images as T=82+/-1 K,τ=0.052+/-0.001 for the eastern side and T=85+/-3 K,τ=0.018+/-0.001 for the western side. The observed asymmetry in thebrightness can be attributed to the difference in the optical depth ofthe MIR-emitting dust. To account for the present observations, wepropose an inclined disk model, in which the outer disk is inclinedalong the east-west direction with the eastern side being on the farside while the inner rim of the outer disk on the eastern side isdirectly exposed to us. The proposed model can successfully account forthe MIR observations as well as the near-infrared images of thescattering light, in which the asymmetry is seen in the opposite senseand in which the forward scattering light (near side-western side) isbrighter.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
| Infrared Spectroscopy of Symbiotic Stars. IV. V2116 Ophiuchi/GX 1+4, The Neutron Star Symbiotic
We have computed, using 17 infrared radial velocities, the first set oforbital elements for the M giant in the symbiotic binary V2116 Ophiuchi.The giant's companion is a neutron star, the bright X-ray source GX 1+4.We rule out the previously proposed period of 304 days and instead findan orbital period of 1161 days, by far the longest of any known X-raybinary. The orbit has a modest eccentricity of 0.10, with an orbitalcircularization time of <~5×106 yr. The large massfunction of the orbit significantly restricts the mass of the M giant.Adopting a neutron star mass of 1.35 Msolar, the maximum massof the M giant is 1.22 Msolar, making it the less massivestar. Spectrum synthesis analysis of several infrared spectral regionsresults in slightly subsolar abundances for most metals. Carbon andnitrogen are in the expected ratio resulting from the red-giant firstdredge-up phase. The lack of 17O suggests that the M gianthas a mass less than 1.3 Msolar, consistent with our maximummass. The surface gravity and maximum mass of the M giant result in aradius of 103 Rsolar, much smaller than its estimated Rochelobe radius. Thus, the mass loss of the red giant is via a stellar wind.These properties argue that the M giant is near the tip of thefirst-ascent giant branch. Although the M-giant companion to the neutronstar has a mass similar to the late-type star in low-mass X-raybinaries, its near-solar abundances and apparent runaway velocity arenot fully consistent with the properties of this class of stars. Thus,in many ways this symbiotic X-ray binary system is unique, and variousscenarios for its possible evolution are discussed.
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| 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.
| Study of molecular layers in the atmosphere of the supergiant star μ Cep by interferometry in the K band
Infrared interferometry of supergiant and Mira stars has recently beenreinterpreted as revealing the presence of deep molecular layers.Empirical models for a photosphere surrounded by a simple molecularlayer or envelope have led to a consistent interpretation of previouslyinconsistent data. The stellar photospheres are found to be smaller thanpreviously understood, and the molecular layer is much higher and denserthan predicted by hydrostatic equilibrium. However, the analysis wasbased on spatial observations with medium-band optical filters, whichmixed the visibilities of different spatial structures. This paperreports spatial interferometry with narrow spectral bands, isolatingnear-continuum and strong molecular features, obtained for thesupergiant μ Cep. The measurements confirm strong variation ofapparent diameter across the K-band. A layer model shows that a stellarphotosphere of angular diameter 14.11±0.60 mas is surrounded by amolecular layer of diameter 18.56±0.26 mas, with an opticalthickness varying from nearly zero at 2.15 μm to >1 at 2.39 μm.Although μ Cep and α Ori have a similar spectral type,interferometry shows that they differ in their radiative properties.Comparison with previous broad-band measurements shows the importance ofnarrow spectral bands. The molecular layer or envelope appears to be acommon feature of cool supergiants.
| 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.
| BS Indi: An enigmatic object in the Tucana association
BS Ind (=HD 202947) is a young K0V star in the Tucana association.Photometric observations with the Hipparcos satellite show aneclipse-like light-curve with primary and secondary eclipse. Theeclipsing binary has a period of 0.435338 days and a circular orbit. Ourspectroscopic observations however show that the K0V primary is asingle-line spectroscopic binary with a period of 3.3 years. The minimummass of the invisible component is about 0.9 M_ȯ which means thatthe mass of the companion is about the same as that of the primary. Thefirst inspection of our FEROS spectra with a resolution of 48 000, aswell as a CES spectrum with a resolution of 220 000 shows no obviouscompanion. However, when the FEROS spectra are cross-correlated with anM-star, a secondary becomes visible as a broad peak in thecross-correlation function. The width and the position of this broadpeak is variable on a short time. When phased to a period of 0.435338days, the radial velocity variations of the broad peak show thecharacteristic sine-wave of a spectroscopic binary in a circular orbit.The best interpretation of this data is that the broad peak in thecross-correlation function is caused by an eclipsing binary consistingof two late-K, or early-M stars with an orbital period of 0.435338 days.This is the eclipsing system. These two stars then orbit the K0V-primarywith a period of 3.3 years. The assumption that BS Ind is a triplesystem consisting of a K0V star and two late-K, or early-M stars alsoexplains the unusual brightness of the object and the near infraredexcess. Thus, BS Ind is unique, as it contains by far theshortest-period young binary star, and these stars are eclipsing.Based on observations obtained at the European SouthernObservatory at La Silla, Chile in program62.I-0418, 63.I-0096, 64.I-0294, 65.I-0012,67.C-0155, 68.C-0292, 69.C-0207, 70.C-0163,072.A-9012, 073.C-0355 and 67.C-0194.
| The K-band intensity profile of R Leonis probed by VLTI/VINCI
We present near-infrared K-band interferometric measurements of the Mirastar R Leonis obtained in April 2001 and January 2002 with the VLTI, thecommissioning instrument VINCI, and the two test siderostats. Theseepochs correspond to near-maximum stellar variability phases ~0.08 and~1.02 (one cycle later), respectively. The April 2001 data cover a rangeof spatial frequencies (31 35 cycles/arcsecond) within the first lobe ofthe visibility function. These measurements indicate a center-to-limbintensity variation (CLV) that is clearly different from a uniform disk(UD) intensity profile. We show that these measured visibility valuesare consistent with predictions from recent self-excited dynamic Miramodel atmospheres that include molecular shells close tocontinuum-forming layers. We derive high-precision Rosseland diametersof 28.5 ± 0.4 mas and 26.2 ± 0.8 mas for the April 2001and January 2002 data, respectively. Together with literature estimatesof the distance and the bolometric flux, these values correspond tolinear radii of 350+50-40 R_ȯ and320+50-40 R_ȯ, and to effective temperaturesof 2930 ± 270 K and 3080 ± 310 K, respectively.Based on public commissioning data released from the ESO VLTI(www.eso.org/projects/vlti/instru/vinci/ vinci_data_sets.html)
| 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
| The G9.62+0.19-F hot molecular core. The infrared view on very young massive stars
We present the results of an extensive infrared study of the massivestar-forming region G9.62+0.19. The data cover information from broad-and narrow-band filters in the wavelength range from 1 to 19 μm andare obtained with ESO's near- and thermal infrared camera ISAAC at theVLT and with the mid-infrared cameras TIMMI2 (La Silla, ESO) andSpectroCam-10 (Mt. Palomar). The high sensitivity and resolutionprovided by these facilities revealed intriguing new details of thisstar-forming region, especially of the embedded hot molecular core (HMC)-- component F. We analyse the newly found infrared sub-structure offour objects in this HMC region. While one of these objects (F2) isprobably a foreground field star, the nature of the brightest object inthe near-infrared there (F1) remains somewhat enigmatic. Our newastrometry proves that this object is not coincident with the peak ofthe molecular line emission of the HMC, but displaced by 1.7arcsecs which translates to nearly 10,000 AU on a linear scale. On thebasis of the available data we estimate this object to be an additionalembedded object with a dense dust shell. Very near the HMC location wefind L' band emission which strongly rises in flux towards longerwavelengths. We presume that this emission (F4) arises from the envelopeof the HMC which is known to be associated with a molecular outflowroughly aligned along the line of sight. Thus, the clearing effect ofthis outflow causes strong deviations from spherical symmetry whichmight allow infrared emission from the HMC to escape through the outflowcavities. This presents the first direct detection of an HMC at awavelength as short as 3.8 μm. At 11.7 μm and 18.75 μm, the HMCcounterpart F4 ultimately proves to be the most luminous IR sourcewithin the G9.62+0.19-F region. In addition, within the entireG9.62+0.19 complex our narrow-band data and the K band imagingpolarimetry reveal well-defined regions of enhanced Brγ andH2 emission as well as a sector where a large contributioncomes from scattered light. Combining our results with high-resolutionradio data we make predictions about the extinction within thisstar-forming region which clarifies why some of the associatedultracompact H II regions are not visible in the near-infrared. Ourinvestigations show the complexity of massive star formation in its fullgrandeur, but they also demonstrate that the related problems can betackled by observations using the new generation of infrared cameras.Based on observations made with the ESO VLT at the Paranal Observatoryunder programme IDs 63.I-0329 and 67.C-0264 and with ESO'S TIMMI2 on LaSilla under programme ID 71.C-0438.All appendices are only available in electronic form athttp://www.edpsciences.org
| 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.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| 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
| J - K DENIS photometry of a VLTI-selected sample of bright southern stars
We present a photometric survey of bright southern stars carried outusing the DENIS instrument equipped with attenuating filters. Theobservations were carried out not using the survey mode of DENIS, butwith individual target pointings. This project was stimulated by theneed to obtain near-infrared photometry of stars to be used in earlycommissioning observations of the ESO Very Large TelescopeInterferometer, and in particular to establish a network of brightcalibrator sources.We stress that near-infrared photometry is peculiarly lacking for manybright stars. These stars are saturated in 2MASS as well as in regularDENIS observations. The only other observations available for brightinfrared stars are those of the Two Micron Sky Survey dating from overthirty years ago. These were restricted to declinations above≈-30°, and thus cover only about half of the sky accessible fromthe VLTI site.We note that the final 2MASS data release includes photometry of brightstars, obtained by means of point-spread function fitting. However, thismethod only achieves about 30% accuracy, which is not sufficient formost applications.In this work, we present photometry for over 600 stars, each with atleast one and up to eight measurements, in the J and K filters. Typicalaccuracy is at the level of 0\fm05 and 0\fm04 in the J and K_s bands,respectively.Based on observations collected at the European Southern Observatory, LaSilla.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/413/1037
| 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.
| Kinematics and Luminosity Function of Dwarf Populations in Three Areas of the Calán-ESO Proper-Motion Catalog
We have completed the analysis of a sample of 112 stars in the solarneighborhood taken from the statistically complete Calán-ESOcatalog. From medium-resolution spectroscopy we classified every star,both by direct comparison with spectroscopic standards and by usingspectral indices. The latter also allowed discrimination betweenmain-sequence (MS) dwarfs and subdwarfs. Several useful spectral typeversus color relations were obtained from CCD photometry of the sample(observed magnitudes were dereddened). Distances and absolute magnitudeswere determined. From measured radial velocities and proper motions, wedetermined the kinematics [Galactocentric velocity components (U,V,W)],which allowed the classification of each star as belonging to the diskor halo population. Luminosity functions (LFs) were then obtained usingthe 1/Vmax method for the different populations. The maximumin the LF for MS dwarfs was found to be near MV=12.5+/-0.5,in accord with previous determinations. On the other hand, we found anincrease in the LF of the subdwarf at its faint end, which is in strongdisagreement with determinations by other authors. A mass density of MSdwarfs of ~0.047+/-0.021 Msolar pc-3 was derived,while the contribution of subdwarfs was found to be negligible.Based on observations obtained with the VLT (ESO), project 67.D-0224A.
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| The Wilson-Bappu effect: A tool to determine stellar distances
Wilson & Bappu (\cite{orig}) have shown the existence of aremarkable correlation between the width of the emission in the core ofthe K line of Ca II and the absolute visual magnitude of late-typestars.Here we present a new calibration of the Wilson-Bappu effect based on asample of 119 nearby stars. We use, for the first time, widthmeasurements based on high resolution and high signal to noise ratio CCDspectra and absolute visual magnitudes from the Hipparcos database.Our primary goal is to investigate the possibility of using theWilson-Bappu effect to determine accurate distances to single stars andgroups.The result of our calibration fitting of the Wilson-Bappu relationshipis MV=33.2-18.0 log W0, and the determinationseems free of systematic effects. The root mean square error of thefitting is 0.6 mag. This error is mostly accounted for by measurementerrors and intrinsic variability of W0, but in addition apossible dependence on the metallicity is found, which becomes clearlynoticeable for metallicities below [Fe/H] ~ -0.4. This detection ispossible because in our sample [Fe/H] ranges from -1.5 to 0.4.The Wilson-Bappu effect can be used confidently for all metallicitiesnot lower than ~ -0.4, including the LMC. While it does not provideaccurate distances to single stars, it is a useful tool to determineaccurate distances to clusters and aggregates, where a sufficient numberof stars can be observed.We apply the Wilson-Bappu effect to published data of the open cluster M67; the retrieved distance modulus is of 9.65 mag, in very goodagreement with the best distance estimations for this cluster, based onmain sequence fitting.Observations collected at ESO, La Silla.
| 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
| 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.
| Velocity Observations of Multiple-Mode Asymptotic Giant Branch Variable Stars
Numerous infrared spectroscopic observations were obtained of nineasymptotic giant branch (AGB) field M giants that have multiple periodsof light variability. Each star has a short period of several months,which is typical of low amplitude pulsation for stars on the AGB, aswell as a long period of 1-3 yr, which is significantly longer than thepredicted fundamental-mode pulsations for these stars. The location ofthese stars in the AGB period-luminosity relation is discussed. For sixof the nine giants we found radial-velocity periods that confirm thelong-period light variability. Although we considered the possibilitythat the velocity variations result from orbital motion, we concludethat the long-period velocity changes in most, if not all of the samplestars, likely result from a currently unknown type of pulsation ratherthan duplicity.
| Infrared spectral classification of normal stars.
Moderate resolution (~400) 2.38-45.2 mu m infrared spectra of starswithout dust features were obtained with the Short WavelengthSpectrometer (SWS) on board the Infrared Space Observatory (ISO). Theobservations are part of a larger program with the objective to extendand refine the current infrared classification schemes. In particular,our data provide the basis for a more detailed classification of the1.N-1.NO sources (ordinary and oxygen rich naked stars) as defined byKraemer et al. (\cite{kraemer}) in a comprehensive classification of theISO-SWS spectra. For our analysis, the continuum was determined byfitting Engelke's function (Engelke \cite{engelke}) to the SWS data. Thestellar angular diameters derived from these estimates of the continuumare in good agreement with values obtained by other methods. Analysis ofthe equivalent widths of the CO fundamental and first overtone molecularbands, the SiO fundamental and first overtone, as well as theH2O bending mode band as a function of MK class, reveals thatthere is sufficient information in the SWS spectra to distinguishbetween hot (B, A, F) and cool stars. Furthermore, it is possible todetermine the spectral type for the G, K and M giants, and subtyperanges in a sequence of K and M giants. The equivalent widths of the COand SiO bands are found to be well correlated in K and M stars, suchthat the equivalent widths of the CO fundamental, the SiO first overtoneand the SiO fundamental can be reasonably well extrapolated from thedepth of the CO first overtone. We have identified two stars,HR 365 and V Nor, whosemid-infrared spectrum does not correspond to their respective opticalclassification. HR 365 may have a late M companion,which dominates the observed infrared spectrum while VNor is a late type giant that was included because itsspectrum was classified as featureless under the IRAS LRS scheme.According to Kraemer et al. (\cite{kraemer}), V Norhas a thin dust shell, which distorts the analysis of its mid-infraredabsorption bands. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA.
| Velocity variability of semiregular and irregular variables
We compare velocities from near infrared lines with center-of-massvelocities for a sample of semiregular and irregular variables to searchfor indications for a convective blueshift. It is shown that a generalblueshift is present but that the light variability is obviously notdominated by convective cells but rather by stellar pulsation. All starsof our sample show a similar shape and amplitude in the velocityvariations. Long secondary periods are a common feature in these objectsand strongly influence the measured velocity shifts. The star V366 Aqlis found to be the first SRV showing line doubling.
| A catalogue of calibrator stars for long baseline stellar interferometry
Long baseline stellar interferometry shares with other techniques theneed for calibrator stars in order to correct for instrumental andatmospheric effects. We present a catalogue of 374 stars carefullyselected to be used for that purpose in the near infrared. Owing toseveral convergent criteria with the work of Cohen et al.(\cite{cohen99}), this catalogue is in essence a subset of theirself-consistent all-sky network of spectro-photometric calibrator stars.For every star, we provide the angular limb-darkened diameter, uniformdisc angular diameters in the J, H and K bands, the Johnson photometryand other useful parameters. Most stars are type III giants withspectral types K or M0, magnitudes V=3-7 and K=0-3. Their angularlimb-darkened diameters range from 1 to 3 mas with a median uncertaintyas low as 1.2%. The median distance from a given point on the sky to theclosest reference is 5.2degr , whereas this distance never exceeds16.4degr for any celestial location. The catalogue is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/183
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| Lick Spectral Indices for Super-Metal-rich Stars
We present Lick spectral indices for a complete sample of 139 candidatesuper-metal-rich stars of different luminosity classes (MK type from Ito V). For 91 of these stars we were able to identify, in anaccompanying paper, the fundamental atmosphere parameters. This confirmsthat at least 2/3 of the sample consists of stars with [Fe/H] in excessof +0.1 dex. Optical indices for both observations and fiducialsynthetic spectra have been calibrated to the Lick system according toWorthey et al. and include the Fe I indices of Fe5015, Fe5270, andFe5335 and the Mg I and MgH indices of Mg2 and Mg b at 5180Å. The internal accuracy of the observations is found to beσ(Fe5015)=+/-0.32 Å, σ(Fe5270)=+/-0.19 Å,σ(Fe5335)=+/-0.22 Å, σ(Mg2)=+/-0.004 mag,and σ(Mg b)=+/-0.19 Å. This is about a factor of 2 betterthan the corresponding theoretical indices from the synthetic spectra,the latter being a consequence of the intrinsic limitations in the inputphysics, as discussed by Chavez et al. By comparing models andobservations, we find no evidence for nonstandard Mg versus Fe relativeabundance, so [Mg/Fe]=0, on the average, for our sample. Both theWorthey et al. and Buzzoni et al. fitting functions are found tosuitably match the data and can therefore confidently be extended forpopulation synthesis application also to supersolar metallicity regimes.A somewhat different behavior of the two fitting sets appears, however,beyond the temperature constraints of our stellar sample. Its impact onthe theoretical output is discussed, as far as the integratedMg2 index is derived from synthesis models of stellaraggregates. A two-index plot, such as Mg2 versus Fe5270, isfound to provide a simple and powerful tool for probing distinctiveproperties of single stars and stellar aggregates as a whole. The majoradvantage, over a classical CM diagram, is that it is both reddeningfree and distance independent. Based on observations collected at theInstituto Nacional de Astrofísica, Optica y Electrónica(INAOE) ``G. Haro'' Observatory, Cananea (Mexico).
| On the Accuracy of GAIA Radial Velocities
We have obtained 782 real spectra and used them as inputs for 6700automatic cross-correlation runs to investigate the GAIA potential interms of radial velocity accuracy. We have explored the dispersions0.25, 0.5, 1 and 2 Å/pix over the 8490--8740 Å GAIA range.We have investigated late-F to early-M stars (constituting the vastmajority of GAIA targets), slowly rotating ( = 4 km/s), of solar metallicity (<[Fe/H]> = --0.07) and notbinary. The results are accurately described by the simple law: {logsigma = 0.6 (log S/N)2 -- 2.4 log S/N + 1.75 log D + 3, wheresigma is the cross-correlation standard error (in km/s) and D is thespectral dispersion (in Å/pix). The spectral dispersion has turnedout to be the dominant factor, with S/N being less important and thespectral mis-match being a weak player at the lowest S/N. Our resultsshow that mission-averaged radial velocities of faint GAIA targets (V ~15 mag) can match the ~ 0.5 km/s accuracy of tangential motions,provided the observations are performed at a dispersion not less than0.5 Å/pix.
| Detection of the 1.6 μm E 4Π-A 4Π FeH System in Sunspot and Cool Star Spectra
Lines of a previously undescribed molecular band have been identified inhigh-resolution spectra of sunspots and in the spectra of two M dwarfsand one L dwarf. This band has also been found in laboratory spectra ofa source known to produce FeH. The band has a head at 1.583 μm (6317cm-1) and is degraded to longer wavelengths. Based onobservational evidence and a published predicted band position, it issuggested that this is the 0-0 band of the E 4Π-A4Π system of FeH.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | נושא הנחש |
| התרוממות ימנית: | 16h14m20.70s |
| סירוב: | -03°41'40.0" |
| גודל גלוי: | 2.74 |
| מרחק: | 52.192 פארסק |
| תנועה נכונה: | 0 |
| תנועה נכונה: | 0 |
| B-T magnitude: | 4.808 |
| V-T magnitude: | 2.896 |
קטלוגים וכינוים:
|
