내용
사진
사진 업로드
DSS Images Other Images
관련 글
Modelling the components of binaries in the Hyades: the dependence of the mixing-length parameter on stellar mass
We present our findings based on a detailed analysis of the binaries ofthe Hyades, in which the masses of the components are well known. We fitthe models of the components of a binary system to observations so as togive the observed total V and B-V of that system and the observed slopeof the main sequence in the corresponding parts. According to ourfindings, there is a very definite relationship between themixing-length parameter and the stellar mass. The fitting formula forthis relationship can be given as α= 9.19(M/Msolar-0.74)0.053- 6.65, which is valid for stellar masses greaterthan 0.77Msolar. While no strict information is gathered forthe chemical composition of the cluster, as a result of degeneracy inthe colour-magnitude diagram, by adopting Z= 0.033 and using models forthe components of 70 Tau and θ2 Tau we find thehydrogen abundance to be X= 0.676 and the age to be 670 Myr. If weassume that Z= 0.024, then X= 0.718 and the age is 720 Myr. Our findingsconcerning the mixing-length parameter are valid for both sets of thesolution. For both components of the active binary system V818 Tau, thedifferences between radii of the models with Z= 0.024 and the observedradii are only about 4 per cent. More generally, the effectivetemperatures of the models of low-mass stars in the binary systemsstudied are in good agreement with those determined by spectroscopicmethods.
| Spectral separation of two pulsating non-single stars.
theta 2 Tau and RS Cha are 2 binary systems also showingdelta Scuti-type pulsations. theta 2 Tau is a Hyades"single-lined" spectroscopic binary with an orbital period of 140.7days. Its secondary component, though of similar temperature, is lessevolved and fainter than the primary and is therefore difficult todetect spectroscopically. RS Cha is a double-lined pre-main-sequenceeclipsing spectroscopic binary orbiting with a short 1.7 days period.Since both systems address several interesting issues related to stellarevolution, metallicism, binarity and pulsation, we explore in thepresent contribution the possibility to study their spectra using theFourier disentangling technique developed by\citet{1995A&AS..114..393H} for extraction of reliable and accuratecomponent properties.
| The Hyades Binary θ2 Tauri: Confronting Evolutionary Models with Optical Interferometry
We determine the masses and magnitude difference of the components ofthe Hyades spectroscopic binary θ2 Tauri. We find thatboth components appear to be less massive and/or brighter than predictedfrom some recent evolutionary models. The rapid rotation and unknownrotational inclination of both components introduce uncertainty in theirluminosities and colors, but not enough to reconcile both of them withthe evolutionary models. We measured the visual orbit with the Mark IIIoptical interferometer and the Navy Prototype Optical Interferometer andcombined it with the Hipparcos proper-motion-based parallax to find atotal system mass ΣM of 4.03+/-0.20 Msolar.We also combined our visual orbit with three recent spectroscopic orbitsto find three spectroscopically based estimates of ΣM andcompared these to the ΣM from the visual orbit and parallax.We chose the spectroscopic orbit that agreed best and used its massratio to estimate individual masses MA,B of 2.15+/-0.12and 1.87+/-0.11 Msolar. From the interferometry, wedetermine Δm=1.13+/-0.05 mag across the 450-850 nm band. Theparallax then implies absolute V magnitudes MA,B of0.48+/-0.05 and 1.61+/-0.06 mag. If the components are rotating nearbreakup velocity and seen nearly pole-on, the true luminosities may beas faint as 1.03 and 2.13 mag; even in that case, however, the secondaryis too blue by ~0.07 mag in B-V.
| Non-radial pulsations in the γ Doradus star HD 195068
We present high resolution spectroscopic observations of the γDoradus star HD 195068. About 230 spectra werecollected over 2 years. Time series analysis performed on radialvelocity data shows a main peak at 1.61 d-1 , a frequency notyet detected in photometry. The Hipparcos photometric 1.25d-1 frequency is easily recovered as is 1.30 d-1while the third photometric frequency, 0.97 d-1 , is onlymarginally present. The good quality of our data, which includes 196spectra collected over seven consecutive nights, shows that both the1.61 d-1 and intermediate 1.27 d-1 (mixture of1.25 and 1.30 d-1 ) frequencies are present in the lineprofile variations. Using the Fourier-Doppler Imaging (FDI) method, thevariability associated with 1.61 d-1 can be successfullymodeled by a non-radial pulsation mode ℓ=5± 1, |m|=4±1. For the intermediate frequency 1.27 d-1 we deduceℓ=4± 1, |m|=3± 1. Evidence that the star is notpulsating in the radial mode (ℓ=0) rules out a previousclassification as an RR Lyrae type star. We investigate the timevariability of FDI power spectra concluding that the observed temporalvariability of modes can be explained by a beating phenomenon betweenclosely spaced frequencies of two non-radial modes. The distribution ofthe oscillation power within the line profile indicates that there is asignificant tangential velocity component of oscillations characteristicof high radial order gravity modes which are predicted to be observed inγ Doradus type stars.
| A Catalog of Temperatures and Red Cousins Photometry for the Hyades
Using Hyades photometry published by Mendoza and other authors,Pinsonneault et al. have recently concluded that Cousins V-I photometrypublished by Taylor & Joner is not on the Cousins system. Extensivetests of the Taylor-Joner photometry and other pertinent results aretherefore performed in this paper. It is found that in part, thePinsonneault et al. conclusion rests on (1) a systematic error inMendoza's (R-I)J photometry and (2) a small error in anapproximate Johnson-to-Cousins transformation published by Bessell. Forthe Taylor-Joner values of (V-R)C, it is found that there arepossible (though not definite) differences of several mmag with otherresults. However, the Taylor-Joner values of (R-I)C data aresupported at the 1 mmag level. Using the (R-I)C data andother published results, an (R-I)C catalog is assembled for146 Hyades stars with spectral types earlier than about K5. For singlestars with multiple contributing data, the rms errors of the catalogentries are less than 4.4 mmag. Temperatures on the Di Benedettoangular-diameter scale are also given in the catalog and are used tohelp update published analyses of high-dispersion values of [Fe/H] forthe Hyades. The best current mean Hyades value of [Fe/H] is found to be+0.103+/-0.008 dex and is essentially unchanged from its previous value.In addition to these numerical results, recommendations are made aboutimproving attitudes and practices that are pertinent to issues likethose raised by Pinsonneault et al.
| Detection of 75+ pulsation frequencies in the δ Scuti star FG Virginis
Extensive photometric multisite campaigns of the δ Scuti variableFG Vir are presented. For the years 2003 and 2004, 926 h of photometryat the millimag precision level were obtained. The combinations withearlier campaigns lead to excellent frequency resolution and highsignal/noise. A multifrequency analysis yields 79 frequencies. Thisrepresents a new record for this type of star. The modes discoveredearlier were confirmed. Pulsation occurs over a wide frequency band from5.7 to 44.3 c/d with amplitudes of 0.2 mmag or larger. Within this wideband the frequencies are not distributed at random, but tend to clusterin groups. A similar feature is seen in the power spectrum of theresiduals after 79 frequencies are prewhitened. This indicates that manyadditional modes are excited. The interpretation is supported by ahistogram of the photometric amplitudes, which shows an increase ofmodes with small amplitudes. The old question of the "missing modes" maybe answered now: the large number of detected frequencies as well as thelarge number of additional frequencies suggested by the power spectrumof the residuals confirms the theoretical prediction of a large numberof excited modes. FG Vir shows a number of frequency combinations of thedominant mode at 12.7162 c/d (m = 0) with other modes of relatively highphotometric amplitudes. The amplitudes of the frequency sums are higherthan those of the differences. A second mode (20.2878 c/d) also showscombinations. This mode of azimuthal order m = -1 is coupled with twoother modes of m = +1.
| Seismic constraints on open clusters
The aim of this theoretical and modelling paper is to derive knowledgeon the global and structural parameters of low-mass stars usingasteroseismology and taking advantage of the stellar collective behaviorwithin open clusters. We build stellar models and compute the seismicsignal expected from main sequence objects in the 0.8-1.6Mȯ range. We first evaluate apparent magnitudes andoscillations-induced luminosity fluctuations expected in the Hyades, thePleiades and the α Persei clusters. The closest cluster presents afeasible challenge to observational asteroseismology in the present andnear future. The remainder of the work therefore focuses on the Hyades.We combine seismological and classical computations to address threequestions: what can be inferred about 1) mass; 2) composition; and 3)extension of outer convection zones of solar analogs in the Hyades. Thefirst issue relies on the strong sensitivity of the large separation tomass. We show that seismic constraints provide masses to a precisionlevel (0.05 Mȯ) that is competitive with the actualmass estimations from binary systems. Then large separations (Δν) and second differences (δ2 ν) are used torespectively constrain metal and helium fractions in the Hyades. Whenplotted for several masses, the relation of effective temperature(Teff) vs. large separation (Δ ν) is found to bestrongly dependent on the metal content. Besides this the seconddifference main modulation is related to the second ionization ofhelium. An accuracy in the helium mass fraction of 0.02 to 0.01 can beachieved provided mass and age are accurately known, which is the casefor a few Hyades binary systems. The second difference modulations arealso partly due to the discontinuity in stellar stratification at theconvective envelope/radiative core transition. They permit directinsight in the stellar structure. We compute acoustic radii of theconvective bases for different values of the mixing length theoryparameter αMLT in convection modelling, i.e. differentconvective efficiency in the superadiabatic layers. For a giveneffective temperature we show that the acoustic radius changes withconvection efficiency. This suggests that seismology can provideconstraints on the extension of outer convection and also more generallyon the direct approaches of convection and dynamical phenomena beingcurrently developed.
| Detection of double stars with the use of Kyiv database of lunar occultations
On the basis of 40-year series of observations of occultations of starsby the Moon from the Kyiv database, we found 83 stars which are likelyto be double ones. Additional components close to basic stars arerevealed for four known wide pair.
| XMM-Newton EPIC observations of stellar clusters and star forming regions
We report on observations of open clusters (OCs) and star formingregions (SFRs) obtained with the EPIC camera as part of the MissionScientist Guaranteed Time on XMM-Newton. These observations provide apowerful tool to investigate the evolution of coronal activity inlate-type convective stars and its dependence on magnetic fieldgeneration by dynamo processes. We discuss the motivations for thisprogram and present some results for the SFRs sigma Orionis (2-5Myr) and Taurus-Auriga (1-10 Myr) as well as for the OCs IC 2602(30 Myr), alpha Persei (50 Myr), Praesepe (600 Myr) andthe Hyades (600 Myr). We discuss imaging and spectral data providedby the EPIC MOS and PN detectors focussing on the determination of thecluster X-ray luminosity function and of the temperature structure,chemical abundances and time variability of cluster stars.Based on observations collected with the ESA mission XMM-Newton as partof the Mission Scientist (R. Pallavicini) Guaranteed Time
| Binarity as a tool for determining the physical properties and evolutionary aspects of A-stars
Double-lined-eclipsing binaries are the essential systems for themeasurement of stellar masses and radii. About 50-60 systems havecomponents (mostly A-stars) for which these values are known with anuncertainty less than 1-2%. Therefore, these systems are very suitableto improve our understanding of stellar structure and evolution. In thispaper, special attention is given to the assessment of the role ofinternal rotation of the early-type stars in selected double-linedbinaries (i.e., EK Cep, PV Cas, and θ2 Tau): it isshown that adoption of rapidly rotating cores for such stars permits themodels to be in very good agreement with the observational resultsincluding the apsidal advance rates.
| A near-infrared stellar spectral library: I. H-band spectra.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
| The Distances to Open Clusters as Derived from Main-Sequence Fitting. II. Construction of Empirically Calibrated Isochrones
We continue our series of papers on open cluster distances by comparingmulticolor photometry of single stars in the Hyades with theoreticalisochrones constructed with various color-temperature relations. Afterverifying that the isochrone effective temperatures agree well withspectroscopically determined values, we argue that mismatches betweenthe photometry and the theoretical colors likely arise from systematicerrors in the color-temperature relations. We then describe a method forempirically correcting the isochrones to match the photometry anddiscuss the dependence of the isochrone luminosity on metallicity.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| Gas in the Terrestrial Planet Region of Disks: CO Fundamental Emission from T Tauri Stars
We report the results of a high-resolution spectroscopic survey for COfundamental emission from T Tauri stars. CO fundamental emission isfrequently detected, with the likely origin of the emission in thecircumstellar disk. An initial assessment of the line profiles indicatesthat the emission region includes the equivalent of the terrestrialplanet region of our solar system, a result that suggests the utility ofCO fundamental emission as a probe of disks at planet formationdistances. Since fundamental emission is detected frequently from bothclose binary and apparently single stars, it appears that both lowcolumn density regions, such as disk gaps, and temperature inversionregions in disk atmospheres can produce significant emission. Theestimated excitation temperature of the emitting gas is unexpectedlywarm for the disk radii that they appear to probe. Thus, the surfacegaseous component of inner disks may be significantly warmer than thesurface dust component. We also detect CO emission from a transitional TTauri star. Because fundamental emission from CO and its isotopes issensitive to a wide range of gas masses, including masses<
| Empirically Constrained Color-Temperature Relations. I. BV(RI)C
This investigation presents a set of transformations to Johnson B-V,Cousins V-R, and Cousins V-I, as well as bolometric corrections to V,for [Fe/H]=-3, -2, -1, -0.5, 0.0, and +0.3 and, in each case, values oflogg from -0.5 to 5.0 for 3000 K<=Teff<=5500 K and from2.0 to 5.0 for 6000 K<=Teff<=40,000 K. Thesetransformations employ the predictions from Kurucz model atmospheres athigh temperatures (Teff>=8000 K) and from MARCS modelatmospheres at intermediate temperatures (from 7000 K down to atemperature in the range 4000 K<=Teff<=5500 K,depending on [Fe/H], where adjustments to satisfy observationalconstraints become necessary). Thus, theoretical color-Teffrelations are used exclusively down to a minimum temperature that iscooler than the temperatures of turnoff stars in open and globular starclusters. To better represent the color transformations obeyed by coolstars (down to 3000 K), corrections to the synthetic transformationshave been determined from a careful consideration of observations for afew globular clusters (M92, M68, and 47 Tucanae), the color-magnitudediagrams (CMDs) of several open clusters (M67, the Pleiades, the Hyades,and NGC 6791), the CMDs and mass-luminosity diagram for solarneighborhood stars having good distance measurements from Hipparcos,empirical (B-V)-Teff and (V-K)-Teff relations, andcolor-color diagrams for field giants. The semiempirical colortransformations that have been produced as a result of our analysis arealso compared with several others that have been published in recentyears: some of the deficiencies of the latter are revealed.
| δ Scuti stars and their related objects
δ Scuti stars are a group of stars located on or a little abovethe main sequence of H-R diagram with spectral type from A3 to F5. Theyare low amplitude single or multi period pulsators with period shorterthan 0.3 d. Within the same area there are several groups of variablesor special stars correlated with them, e.g., Dwarf Cepheids, γ Dorvariables, Blue Stragglers, Am stars, Ap stars, ROAp variables, λBoo variables and δ Del variables. In this paper a general reviewin this field, including the number of new variables discovered after1995, is presented. The most reliable period variation rates for all thehigh amplitude variables and several low amplitude variables are listed.Statistic shows the higher the rotation rate v sin i is, the lower thelight variation amplitude is. Thus within young open clusters highamplitude variables cannot be found. The amplitudes-periods distributionhave 3 peaks with the highest of 1.0 mag in V at 0.17 d in period. Forδ Scuti variables in stellar systems the shorter the averageperiod is, the lower the metallicity and the older the age of thestellar system are.
| Pulsation of the δ Scuti star θ2 Tau: new multisite photometry and modelling of instability
The results of a multisite photometric campaign of θ2Tau are reported. This binary system consists of an evolved and amain-sequence A star, both situated inside the instability strip. The12th Delta Scuti Network campaign included 152 h of high-precisionphotometry obtained at four observatories. This leads to the derivationof 11 frequencies of pulsation in the 10.8 to 14.6 cycle d-1range. These frequencies confirm the results from previous Earth-based(1982-1986) as well as satellite (2000) photometry, although amplitudevariability on a time-scale of several years is present. We show that atleast two high frequencies (26.18 and 26.73 cycle d-1) arealso present in θ2 Tau. Arguments are given that thesehigh frequencies originate in the main-sequence companion and are notcombination frequencies, fi+fj, from the primary.Models for both the primary and the secondary components were checkedfor the instability against radial and non-radial oscillations. All hotmodels of the primary with Teff > 8000 K are stable in theobserved frequency range. The best fit between the theoretical andobserved frequency ranges is achieved for models with Teff~7800 K (or slightly higher), in agreement with photometric calibrations.The instability range spans two or three radial orders in the rangep4 to p6 for radial modes. Post-main-sequencemodels (with or without overshooting) are preferable for the primary,but main-sequence models with overshooting cannot be excluded. For theless-luminous secondary component the instability range is wider andspans 5 to 7 radial orders from p2 to p8. Theobserved frequencies lie around radial modes p5 top6. The main uncertainties of these results are caused by asimple treatment of the convective flux in the hydrogen ionization zone.
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| Multiperiodicity in the light variations of the beta Cephei star beta Crucis
High-resolution spectroscopic observations have led recently to thediscovery that the beta Cephei star beta Crucis (Mimosa) ismultiperiodic with at least three non-radial pulsation modes. Dataobtained by the star tracker of the WIRE satellite have now allowed usto confirm this multiperiodicity in the light variations. A total of 5million observations covering 17 days was analysed and the three mainperiods we find in this work are in perfect agreement with the resultsderived from the line profile variations. The photometric amplitudes aresmall (3, 2.7 and 0.6 millimag for the dominant modes), but this is notsurprising in view of the mode identifications derived earlier from theline profiles. Additional periods of low-amplitude modes (between0.2-0.3 mmag) are also derived, including one suggested earlier by theradial velocity data.
| On the Period-Luminosity-Colour-Metallicity relation and the pulsational characteristics of lambda Bootis type stars
Generally, chemical peculiarity found for stars on the upper mainsequence excludes delta Scuti type pulsation (e.g. Ap and Am stars), butfor the group of lambda Bootis stars it is just the opposite. This makesthem very interesting for asteroseismological investigations. The groupof lambda Bootis type stars comprises late B- to early F-type,Population I objects which are basically metal weak, in particular theFe group elements, but with the clear exception of C, N, O and S. Thepresent work is a continuation of the studies by Paunzen et al.(\cite{Pau97}, \cite{Pau98}), who presented first results on thepulsational characteristics of the lambda Bootis stars. Since then, wehave observed 22 additional objects; we found eight new pulsators andconfirmed another one. Furthermore, new spectroscopic data (Paunzen\cite{Pau01}) allowed us to sort out misidentified candidates and to addtrue members to the group. From 67 members of this group, only two arenot photometrically investigated yet which makes our analysis highlyrepresentative. We have compared our results on the pulsationalbehaviour of the lambda Bootis stars with those of a sample of deltaScuti type objects. We find that at least 70% of all lambda Bootis typestars inside the classical instability strip pulsate, and they do sowith high overtone modes (Q < 0.020 d). Only a few stars, if any,pulsate in the fundamental mode. Our photometric results are inexcellent agreement with the spectroscopic work on high-degree nonradialpulsations by Bohlender et al. (\cite{Boh99}). Compared to the deltaScuti stars, the cool and hot borders of the instability strip of thelambda Bootis stars are shifted by about 25 mmag, towards smaller(b-y)_0. Using published abundances and the metallicity sensitiveindices of the Geneva 7-colour and Strömgren uvbybeta systems, wehave derived [Z] values which describe the surface abundance of theheavier elements for the group members. We find that thePeriod-Luminosity-Colour relation for the group of lambda Bootis starsis within the errors identical with that of the normal delta Scutistars. No clear evidence for a statistically significant metallicityterm was detected. Based on observations from the Austrian AutomaticPhotoelectric Telescope (Fairborn Observatory), SAAO and Siding SpringObservatory.
| Asteroseismology from space: The delta Scuti star theta 2 Tauri monitored by the WIRE satellite
The bright variable star theta 2 Tau was monitored with thestar camera on the Wide-Field Infrared Explorer satellite. Twelveindependent frequencies were detected down to the 0.5 mmag amplitudelevel. Their reality was investigated by searching for them using twodifferent algorithms and by some internal checks: both proceduresstrengthened our confidence in the results. All the frequencies are inthe range 10.8-14.6 cd-1. The histogram of the frequencyspacings shows that 81% are below 1.8 cd-1; rotation may thusplay a role in the mode excitation. The fundamental radial mode is notobserved, although it is expected to occur in a region where the noiselevel is very low (55 mu mag). The rms residual is about two times lowerthan that usually obtained from successful ground-based multisitecampaigns. The comparison of the results of previous campaigns with thenew ones establishes the amplitude variability of some modes.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| Optical Interferometry
The field of optical and infrared (IR) interferometry has seen rapidtechnical and scientific progress over the past few years. A number ofinstruments capable of precise visibility measurements have been built,and closure-phase imaging with multitelescope arrays has beendemonstrated. Astronomical results from these instruments includemeasurements of stellar diameters and their wavelength dependence, limbdarkening, stellar surface structure, and distances of Cepheids and ofNova Cygni 1992. Precise stellar masses have been obtained frominterferometric observations of spectroscopic binaries, andcircumstellar disks and shells have been resolved. Searches forsubstellar companions and extrasolar planets with interferometricastrometry will begin soon. Nulling interferometry will enable studiesof exozodiacal disks from the ground and the detection andcharacterization of terrestrial extrasolar planets from space. Thesedevelopments are reviewed, as well as progress in some key technologicalareas.
| Evidence for Residual Material in Accretion Disk Gaps: CO Fundamental Emission from the T Tauri Spectroscopic Binary DQ Tauri
We present the discovery of CO fundamental rovibrational emission fromthe classical T Tauri spectroscopic binary DQ Tau. The high-resolutioninfrared echelle spectra reveal emission lines from both the ν=1 andν=2 vibrational levels with line widths of roughly 70 kms-1. The average CO excitation temperature is approximately1200 K. We model the spectra as arising from gas in Keplerian rotationabout the center of mass of the binary. The disk model requires gas withan average surface density of 5×10-4 g cm-2that extends outward to 0.5+/-0.1 AU and inward to at least 0.1 AU fromthe center of mass. The radial extent for the emitting gas is close tothe predicted size of the gap in the DQ Tau accretion disk that isexpected to be dynamically cleared by the binary. We interpret theseresults, and previous modeling of DQ Tau's spectral energy distribution,as evidence for a small amount (~10-10 Msolar) ofdiffuse material residing within the optically thin disk gap. Thus,dynamical clearing has not been completely efficient in the DQ Taubinary. We suggest that the material is associated with a flow from thecircumbinary disk which feeds the ongoing accretion at the stellarsurfaces.
| 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.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| The pre-main-sequence star IP Persei
We present the results of high- and low-resolution spectroscopic andbroadband multicolour photometric observations of the emission-lineA-type star IP Per. Significant variations of the Balmer line profilesand near-IR brightness are detected. Comparison with the spectra ofother stars and theoretical models allowed us to derive its fundamentalparameters as follows: T_eff =~ 8000 K, log g =~ 4.4, logLbol/Lsun =~ 1.0. They correspond to the MK typeA7 v. We also found that the metallicity of the object's atmosphere isnearly 40 per cent that of the Sun. Our result for the star's gravityimplies that it is located at the zero-age main-sequence. We concludethat IP Per is a pre-main-sequence Herbig Ae star, and belongs to thegroup of UX Ori-type stars showing irregular photometric minima. Arecent result by Kovalchuk & Pugach (\cite{kp97}), that IP Per is anevolved high-luminosity star, is not confirmed. The discrepancy in thelog g determination, which led to the difference in the luminosity,seems to be due to uncertainties in the échelle data reductionfor broad lines and a different estimate for the star's temperature.
| The helium content and age of the Hyades:. Constraints from five binary systems and Hipparcos parallaxes
We compare the accurate empirical mass-luminosity (M-L) relation basedon five Hyades binary systems to predictions of stellar modelscalculated with various input parameters (helium, metallicity and age)or physics (mixing-length ratio, model atmosphere, equation of state andmicroscopic diffusion). Models based on a helium content Y ~ 0.28inferred from the Delta Y/Delta Z enrichment law are more than 3sigmabeyond the observations, suggesting that the Hyades initial heliumabundance is lower than expected from its supersolar metallicity. Withthe photometric metallicity ([Fe/H] = 0.144+/- 0.013 dex, Grenon\cite{gre00}) we derive Y=0.255+/-0.009. Because of the (Y, [Fe/H])degeneracy in the M-L plane, the uncertainty grows to Delta Y=0.013 ifthe metallicity from spectroscopy is adopted ([Fe/H] = 0.14+/-0.05 dex,Cayrel de Strobel et al. \cite{cayG97}). We use these results to discussthe Hertzsprung-Russell (HR) diagram of the Hyades, in the(MV, B-V) plane, based on the very precise Hipparcosdynamical parallaxes. Present models fit the tight observed sequencevery well except at low temperatures. We show that the HR diagram doesnot bring further constraints on the helium abundance or metallicity ofthe cluster. In the low mass region of the HR diagram sensitive to themixing-length parameter (alpha MLT), the slope of the mainsequence (MS) suggests that alpha MLT could decrease from asolar (or even supersolar) value at higher mass to subsolar values atlow mass, which is also supported by the modeling of the vB22 M-Lrelation. We find that the discrepancy at low temperatures ((B-V)>≈ 1.2) remains, even if an improved equation of state or bettermodel atmospheres are used. Finally, we discuss the positions of thestars at turn-off in the light of their observed rotation rates and wededuce that the maxiμm age of the Hyades predicted by the presentmodels is ~650 Myr.
| Period and amplitude variations in the high-amplitude delta Scuti star AE Ursae Majoris
We present a comprehensive investigation of the variations of period andamplitude in the high-amplitude delta Scuti star AE UMa based on our newJohnson V time-series measurements and the existing data. No additionalfrequencies were detected even though all the available data sets from1974 to 2001 were analysed. The light variations of AE UMa can bewell-reproduced with the fundamental and first-overtone radial modes andtheir coupled terms. New observations and analyses support the mostrecent results of Pócs & Szeidl (\cite{pocs}). Thefundamental period was essentially constant over the past 27 years withits standard value of 0fd 086017066 (f_0= 11.625600 cd-1),while the first overtone period decreased at a rate of(1)/(P_1)(dP_1)/(dt) = -4.3x 10-8 yr-1. Theamplitude variations in the two modes of AE UMa are detected at themilli-magnitude level on a time-scale of years. It seems that theamplitudes vary in opposite phases, implying an energy conservation orsome kind of intrinsic variability cause. We deny theover-interpretation of the period change given by Hintz et al.(\cite{hintz97}) and explore its reason. Figure~\ref{Fig:lightcurve} isonly available in electronic form at http://www.edpsciences.org
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
INHABITED SKY
