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The Anisoplanatic Point-Spread Function in Adaptive Optics
The effects of anisoplanatism on the adaptive optics point-spreadfunction are investigated. A model is derived that combines observationsof the guide star with an analytic formulation of anisoplanatism inorder to generate predictions for the adaptive optics point-spreadfunction at arbitrary locations within the field of view. The analyticformulation captures the dependencies of anisoplanatism on aperturediameter, observing wavelength, angular offset, zenith angle, andturbulence profile. The predictions of this model are compared tonarrowband 2.12 and 1.65 μm images of a 21" binary (mv =7.3, 7.6) acquired with the Palomar adaptive optics system on the 5 mHale Telescope. Contemporaneous measurements of the turbulence profilemade with a DIMM/MASS (differential image motion monitor/multiaperturescintillation sensor) unit are used together with images of the primaryto predict the point-spread function of the binary companion. Predictedcompanion Strehl ratios are shown to match measurements to within a fewpercent, whereas predictions based on the isoplanatic angleapproximation are highly discrepant. The predicted companionpoint-spread functions are shown to agree with observations to 10%.These predictions are used to measure the differential photometrybetween binary members to an accuracy of 1 part in 103, andthe differential astrometry to an accuracy of 1 mas. Errors in thedifferential astrometry are shown to be dominated by differentialatmospheric tilt jitter. These results are compared to other techniquesthat have been employed for photometry, astrometry, and high-contrastimaging.
| The Remarkable Far-Ultraviolet Spectrum of FK Comae Berenices: King of Spin
A Far Ultraviolet Spectroscopic Explorer (FUSE) pointing on theultrafast rotating yellow giant FK Comae Berenices (HD 117555; vsini~163km s-1) recorded emission profiles of C III λ977(T~8×104 K) and O VI λ1031(T~3×105 K) that are exceptionally broad andasymmetric, but nearly identical in shape, aside from a bluewardabsorption component in the latter (identified as interstellar O I,rather than, say, a C III outflow feature). The FWHMs exceed 500 kms-1, twice the broadest far-UV line shape of any normallate-type star observed to date, but similar to the Hα profiles ofFK Com, and following the trend of other fast spinning early G giantsthat often display ``superrotational'' broadening of their UV ``hot''lines. Although the red-asymmetric O VI λ1031 profile issuggestive of an outflow at ~3×105 K, the weaker memberof the doublet, λ1037, does not display the differentialabsorption pattern expected from a warm wind. Furthermore, at times thechromospheric Mg II λ2796 + λ2803 composite profile, froma collection of International Ultraviolet Explorer (IUE) echellegramsobtained two decades earlier, is nearly identical in shape tored-asymmetric O VI λ1031. A contemporaneous optical Doppler mapplaces the photospheric dark spots mainly in the polar regions of theapproaching hemisphere. The dominantly redward biased profiles of C IIIand O VI could be explained if the associated emission zones wereleading the starspots in phase and partially rooted in lower latitudes.
| Lithium abundances and rotational behavior for bright giant stars
Aims.We study the links possibly existing between the lithium content ofbright giant stars and their rotational velocity. Methods: .Weperformed a spectral analysis of 145 bright giant stars (luminosityclass II) spanning the spectral range from F3 to K5. All these starshave homogeneous rotational velocity measurements available in theliterature. Results: .For all the stars of the sample, we provideconsistent lithium abundances (A_Li), effective temperatures (T_eff),projected rotational velocity (v sin i), mean metallicity ([Fe/H]),stellar mass, and an indication of the stellar multiplicity. The gradualdecrease in lithium abundance with T_eff is confirmed for bright giantstars, and it points to a dilution factor that is at least assignificant as in giant stars. From the F to K spectral types, the A_Lispans at least three orders of magnitude, reflecting the effects ofstellar mass and evolution on dilution. Conclusions: .We find thatthe behavior of A_Li as a function of v sin i in bright giant starspresents the same trend as is observed in giants and subgiants: starswith high A_Li are moderate or fast rotators, while stars with low A_Lishow a wide range of v sin i values.
| Shapes of Spectral Line Bisectors for Cool Stars
The shape of the line bisector for the prototype spectral line Fe Iλ6253 was measured for an array of 54 stars on the cool half ofthe HR diagram. These bisectors are given in tables along with theirerrors. The classic C shape is shown by only a rather restricted rangein effective temperature and luminosity. The detailed change in bisectorshape with effective temperature and luminosity is documented moreprecisely than in previous work. The most blueward point on the bisectorchanges its height systematically with luminosity and can be used as aluminosity or gravity discriminant. The wide range of bisector shapescontains significant information about the velocity fields in theatmospheres of these stars, but extracting that information may requireextensive modeling.
| 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.
| On the lithium abundance in F-, G- supergiants and its possible correlation with rotation.
Not Available
| Chandra Observations of Coronal Emission from the Early G Supergiants α and β Aquarii
We report Chandra detections of coronal X-rays from the early Gsupergiants α Aquarii (HD 209750: G2 Ib) and β Aquarii (HD204867: G0 Ib). Previous ROSAT observations of these archetypical``hybrid chromosphere'' stars were inconclusive, in the case of αAqr owing to a 38' mispointing, and for β Aqr because of a smallpositional discrepancy of the apparent source. The Chandra HighResolution Camera (HRC-I), with its superior spatial resolution andsensitivity, has obtained a positive detection of α Aqr andrecovered faint emission at the location of β Aqr, now wellseparated from the stronger source to the southeast that dominated theearlier ROSAT image. The coronal LX/LC IV luminosity ratiosof both supergiants are extremely depressed relative to early Gmain-sequence stars, continuing the ``X-ray deficiency syndrome''originally identified in late F/early G luminosity class III giants ofthe Hertzsprung gap.
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| A Hot Wind from the Classical T Tauri Stars: TW Hydrae and T Tauri
Spectroscopy of the infrared He I (10830 Å) line with NIRSPEC onKeck and CSHELL at the IRTF, and of the ultraviolet C III (977 Å)and O VI (1032 Å) emission with FUSE, reveals that the classical TTauri star TW Hydrae exhibits P Cygni profiles, line asymmetries, andabsorption indicative of a continuous, fast (~400 km s-1),hot (~300,000 K) accelerating outflow with a mass-loss rate~10-11 to 10-12 Msolar yr-1or larger. Spectra of T Tau N appear consistent with such a wind. Thesource of the emission and outflow seems restricted to the starsthemselves. Although the mass accretion rate is an order of magnitudeless for TW Hya than for T Tau, the outflow reaches higher velocities atchromospheric temperatures in TW Hya. Winds from young stellar objectsmay be substantially hotter and faster than previously thought.
| A Far-Ultraviolet Spectroscopic Survey of Luminous Cool Stars
The Far Ultraviolet Spectroscopic Explorer (FUSE) ultraviolet spectra ofeight giant and supergiant stars reveal that high-temperature(3×105 K) atmospheres are common in luminous cool starsand extend across the color-magnitude diagram from α Car (F0 II)to the cool giant α Tau (K5 III). Emission present in thesespectra includes chromospheric H Lyβ, Fe II, C I, and transitionregion lines of C III, O VI, Si III, and Si IV. Emission lines of FeXVIII and Fe XIX signaling temperatures of ~107 K and coronalmaterial are found in the most active stars, β Cet and 31 Com. Ashort-term flux variation, perhaps a flare, was detected in β Cetduring our observation. Stellar surface fluxes of the emission of C IIIand O VI are correlated and decrease rapidly toward the cooler stars,reminiscent of the decay of magnetically heated atmospheres. Profiles ofthe C III λ977 lines suggest that mass outflow is underway atT~80,000 K and the winds are warm. Indications of outflow at highertemperatures (3×105 K) are revealed by O VI asymmetriesand the line widths themselves. High-temperature species are absent inthe M supergiant α Ori. Narrow fluorescent lines of Fe II appearin the spectra of many giants and supergiants, apparently pumped by HLyα, and formed in extended atmospheres. Instrumentalcharacteristics that affect cool star spectra are discussed.
| X-Rays from Hybrid Stars
The late-type giants and supergiants of the ``hybrid chromosphere''class display signatures of cool (T<~2×104 K) windstogether with hot emission lines from species like C IV(T~105 K). A survey of such stars by Reimers et al. usingROSAT reported numerous X-ray detections (T~106 K),strengthening the (then heretical) idea that hot coronae and cool windscan coexist in luminous giants. However, several of the candidatesources were offset from the predicted stellar coordinates, calling intoquestion the identifications. In an effort to secure better knowledge ofthe X-ray luminosities of the hybrids, the ROSAT fields from the Reimerset al. survey were reexamined, exploiting the USNO-A2.0 astrometriccatalog to register the pointings to a few arcseconds accuracy. On thebasis of positional mismatches, at least two of the previously reporteddetections of key hybrid stars-γ Dra (K5 III) and β Aqr (G0Ib)-must be rejected. The new X-ray upper limits for these stars,combined with the remaining candidate detections (and nondetections)from the original survey, place the hybrids into the same ``X-raydeficient'' category as the ``noncoronal'' red giants like Arcturus(α Boo: K1.5 III) and Aldebaran (α Tau: K5 III). A few ofthe hybrid X-ray sources are exceptional, however. The archetype αTrA (K2 II-III), in particular, is securely detected in terms ofpositional coincidence, but its anomalous, contradictory coronalproperties suggest that an unseen companion-a young hyperactive Gdwarf-might dominate the X-ray emission.
| Magnetic field measurements on four yellow supergiants. I
Multiyear high precision measurements of the longitudinal component ofthe magnetic field (Be) of four supergiants are reported: Aqr (G0 Ib),Aqr (G2 Ib), Gem (G8 Ib), and Peg (K2 Ib). The best measurementaccuracy, =0.8 G, was achieved for Peg. A Monte Carlo method was used totest the reliability of the derived measurement errors. The differencesbetween the observational errors and the calculated Monte Carlo errorswere 3.2%. For Aqr and Aqr no statistically significant value of themagnetic field was recorded when averaged over a night. For eGem thefollowing overnight average values of the magnetic field were recordedon five nights: 11.1±2.7 G, 9.8±2.5 G, -10.5±3.0 G,38.1±7.4 G, and 5.3±1.5 G. For Peg the magnetic fieldrecorded over two nights was -5.3±0.9 G and - 2.7±0.8 G.
| 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
| 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.
| HST/STIS high resolution echelle spectra of α Centauri A (G2 V)
We describe and analyze HST/STIS observations of the G2 V star αCentauri A (α Cen A, HD128620), a star similar to the Sun. Thehigh resolution echelle spectra obtained with the E140H and E230Hgratings cover the complete spectral range 1133-3150 Å with aresolution of 2.6 km s-1, an absolute flux calibrationaccurate to ± 5%, and an absolute wavelength accuracy of 0.6-1.3km s-1. We present here a study of the E140H spectrumcovering the 1140-1670 Åspectral range, which includes 671emission lines representing 37 different ions and the molecules CO andH_2. For α Cen A and the quiet and activeSun, we intercompare the redshifts, nonthermal linewidths, and parameters of two Gaussian representations of transitionregion lines (e.g., Si IV, C IV), infer the electron density from the OIV intersystem lines, and compare their differential emission measuredistributions. One purpose of this study is to compare theα Cen A and solar UV spectra to determine howthe atmosphere and heating processes in α Cen Adiffer from the Sun as a result of the smalldifferences in gravity, age, and chemical composition of the two stars.A second purpose is to provide an excellent high resolution UV spectrumof a solar-like star that can serve as a proxy for theSun observed as a point source when comparing otherstars to the Sun.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withproposal GO-07263.Table 4 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/331
| A Far Ultraviolet Spectroscopic Explorer Survey of Coronal Forbidden Lines in Late-Type Stars
We present a survey of coronal forbidden lines detected in FarUltraviolet Spectroscopic Explorer (FUSE) spectra of nearby stars. Twostrong coronal features, Fe XVIII λ974 and Fe XIX λ1118,are observed in 10 of the 26 stars in our sample. Various other coronalforbidden lines, observed in solar flares, also were sought but notdetected. The Fe XVIII feature, formed at logT=6.8 K, appears to be freeof blends, whereas the Fe XIX line can be corrupted by a C I multiplet.FUSE observations of these forbidden iron lines at spectral resolutionλ/Δλ~15,000 provides the opportunity to studydynamics of hot coronal plasmas. We find that the velocity centroid ofthe Fe XVIII feature deviates little from the stellar rest frame,confirming that the hot coronal plasma is confined. The observed linewidths generally are consistent with thermal broadening at the hightemperatures of formation and show little indication of additionalturbulent broadening. The fastest rotating stars, 31 Com, α AurAb, and AB Dor, show evidence for excess broadening beyond the thermalcomponent and the photospheric vsini. The anomalously large widths inthese fast-rotating targets may be evidence for enhanced rotationalbroadening, consistent with emission from coronal regions extending anadditional ΔR~0.4-1.3R* above the stellar photosphere,or represent the turbulent broadening caused by flows along magneticloop structures. For the stars in which Fe XVIII is detected, there isan excellent correlation between the observed Röntgensatellit(ROSAT) 0.2-2.0 keV soft X-ray flux and the coronal forbidden line flux.As a result, Fe XVIII is a powerful new diagnostic of coronal thermalconditions and dynamics that can be utilized to study high-temperatureplasma processes in late-type stars. In particular, FUSE provides theopportunity to obtain observations of important transition region linesin the far-UV, as well as simultaneous measurements of soft X-raycoronal emission, using the Fe XVIII coronal forbidden line.
| 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.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only 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/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| 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
| Polaris: Amplitude, Period Change, and Companions
Polaris has presented us with the rare phenomenon of a Cepheid with apulsation amplitude that has decreased over the last 50 yr. In thisstudy we have used this property to see whether the amplitude decreaseduring the last 15 yr has had any effect on upper atmosphere heating. Weobtained IUE high- and low-resolution spectra but found no change ineither the Mg II chromospheric emission or the flux at 1800 Åbetween 1978 and 1993 when the pulsation amplitude dropped by 50% (from2.8 to 1.6 km s-1). The energy distribution from 1700 Åthrough V, B, R(KC), and I(KC) is like that of a nonvariable supergiantof the same color rather than a full amplitude Cepheid in that it hasmore flux at 1800 Å than the full amplitude Cepheid δ Cep.Polaris also has a rapidly changing period (3.2 s yr-1), incommon with other overtone pulsators. We argue that this is a naturalconsequence of the different envelope locations that dominate pulsationgrowth rates in fundamental and overtone pulsation. In fundamental modepulsators, the deeper envelope is more important in determining growthrates than for overtone pulsators. For fundamental mode pulsators,evolutionary changes in the radius produce approximately linear changesin period. In overtone pulsators, pulsation reacts to small evolutionarychanges in a more unstable way because the modes are more sensitive tohigh envelope features such as opacity bumps, and the growth rates forthe many closely spaced overtone modes change easily. Finally, the upperlimit to the X-ray flux from an Einstein observation implies that thecompanion in the astrometric orbit is earlier than F4 V. The combinationof upper and lower limits on the companion from IUE and Einsteinrespectively catch the companion mass between 1.7 and 1.4Msolar. The X-ray limit is consistent with the more distantcompanion α UMi B being a physical companion in a hierarchaltriple system. However the X-ray limits require that the even moredistant companions α UMi C and D are too old to be physicallyassociated with Polaris.
| XMM-NEWTON/RGS First View of Coronas of Two Luminous Cool Stars
Two single cool stars, β Dra (G2Ib--IIa, HD 159181) and α TrA(K2 II--III, HD 150798) offer a contrast in the structure and evolutionof their outer atmospheres. Alpha TrA is a well-known hybrid object,from a class of cool luminous single stars originally identified basedon the presence of strong C IV emission accompanied by absorptionfeatures at low and high velocities indicating a massive stellar windand circumstellar material. The atmosphere of Beta Dra appears to besimilar to the solar coronal example with high temperature emission, butlacking a massive cool wind. X-Ray observations can help to delineatestructure and evolution of stellar coronae from the solar examplethrough the intermediate hybrid configuration. Spectra of these starsobtained with XMM/Newton under the Guest Investigator program containemission lines of many elements and ionic species. An emission measuredistribution is constructed from a line-based analysis. Differences instructure, density, and element abundance between the coronas of theseluminous stars are quantified. This research is supported in part by aNASA Grant NAG5-9977 to the Smithsonian Astrophysical Observatory.
| 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.
| Sodium enrichment of stellar atmospheres. I. Non-variable supergiants and bright giants
48 supergiants and bright giants have been observed in order toinvestigate the sodium enrichment of their atmospheres and itsconnection with stellar gravity. We present the equivalent widths of the6154 Å and 6160 Å Na I lines measured from the programspectra, the results of effective temperature determinations, the NLTEsodium abundances, and the derived relation between the sodiumoverabundance and surface gravity.
| 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).
| Absolute spectrophotometry of late-type stars.
Not Available
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| Identification of Fe II Emission Lines in FUSE Stellar Spectra
We identify two complexes of Fe II emission lines in far-ultravioletspectra of the stars α TrA and HD 104237. Using spectra from boththe Far Ultraviolet Spectroscopic Explorer (FUSE) and the SpaceTelescope Imaging Spectrograph (STIS) on board the Hubble SpaceTelescope (HST), we show that these emission lines, which represent themajority of previously unidentifed emission features in cool starspectra between 912 and 1180 Å, are fluorescent decays in Fe IIfollowing excitation by H Lyα. Specifically, followingphotoexcitation from the third term (4s a 4D) of Fe II,subsequent decays are observed to the two lowest terms (4s a6D and 3d7 a 4F) which are observednear 1100 and 1135 Å, respectively. Decays to higher terms, andhence longer wavelengths, also are clearly seen in the STIS spectra.Differences in the fluorescent Fe II spectra of α TrA and HD104237 are tentatively identified as resulting from differences in theintrinsic width of the density-weighted H Lyα radiation fields.The additional Fe II lines observed in α TrA result from abroadened H Lyα profile. Two features near 1060 Å appear tobe fluorescent lines of Cr II, also excited by H Lyα.
| Simultaneous Observations of Variability at All Atmospheric Levels of V824 Arae (HD 155555)
We conducted a multiwavelength campaign observing V824 Ara (HD 155555,G5 IV+K0 IV-V) continuously throughout one complete orbital cycle (~1.7days) in early May of 1996. At the core of this campaign wereobservations using the GHRS on the Hubble Space Telescope (HST). In all,about 48,000 spectra, many in rapid readout mode, were obtained with theGHRS covering the C IV, Mg II, and Fe XXI wavelength regions at 11-15separate phases. Simultaneous observations were made with the ExtremeUltraviolet Explorer (EUVE). Radio observations (3.5 and 6 cm) wereconducted at the Australian Telescope, while ground-based visualspectroscopic and photometric observations were made at EuropeanSouthern Observatory, Cerro Tololo Inter-American Observatory, theAnglo-Australian Telescope, and South African Astronomical Observatory.Additional ground-based observations were obtained before, during, andafter the campaign. Our primary intent was to obtain a three-dimensionalmodel of the atmosphere extending from the photosphere to the corona.Variability was clearly detected, including several flares observed inthe HST, EUVE, and radio data. We present results from modeling theultraviolet transition region lines using an anisotropic macroturbulencemodel. Previous studies of transition region lines in late-type activestars have used multiple Gaussians to fit the observed line profiles,adding broad components to account for the extended wings observed inseveral active systems, including V711 Tau (HR 1099). This broadcomponent has been interpreted as arising from the continuous presenceof microflaring. We demonstrate that anisotropic macroturbulence modelscan also explain the observed Mg II profiles. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS 5-26555.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. II. Basic Parameters of Program Stars and the Role of Microturbulence
Paper I of this series presented precise MK spectral types for 372 lateA-, F-, and early G-type stars with the aim of understanding the natureof luminosity classification on the MK spectral classification systemfor this range of spectral types. In this paper, a multidimensionaldownhill simplex technique is introduced to determine the basicparameters of the program stars from fits of synthetic spectra andfluxes with observed spectra and fluxes from Strömgren uvbyphotometry. This exercise yields useful calibrations of the MK spectralclassification system but, most importantly, gives insight into thephysical nature of luminosity classification on the MK spectralclassification system. In particular, we find that in this range ofspectral types, microturbulence appears to be at least as important asgravity in determining the MK luminosity type.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Sárkány |
| Rektaszcenzió: | 17h30m26.00s |
| Deklináció: | +52°18'05.0" |
| Vizuális fényesség: | 2.79 |
| Távolság: | 110.865 parszek |
| RA sajátmozgás: | 0 |
| Dec sajátmozgás: | 0 |
| B-T magnitude: | 3.98 |
| V-T magnitude: | 2.874 |
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