Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
Analysis of main-sequence A-type stars showing radial velocity variability.
We obtained high-resolution spectroscopic data for 33 bright (V < 8)A-type HIPPARCOS programme stars at the ``Observatoire deHaute-Provence'' (OHP) in December 2004. All our targets show someindication of radial velocity variability (Grenier et al, 1999), arelocated in or near the lower part of the Cepheid instability strip, andare poorly studied objects. In this contribution, we present the newdata of the most interesting programme stars and we derive a physicalinterpretation for their variable nature.
| VLT-ISAAC 3-5 μm spectroscopy of embedded young low-mass stars. III. Intermediate-mass sources in Vela
Aims.We study in this paper the ice composition in the envelope aroundintermediate-mass class I Young Stellar Objects (YSOs).Methods.Weperformed a spectroscopic survey toward five intermediate-mass class IYSOs located in the Southern Vela molecular cloud in the L (2.85-4.0μm) and M (4.55-4.8 μm) bands at resolving powers λ/Δλ=600-800 up to 10 000, using the Infrared Spectrometer and ArrayCamera mounted on the Very Large Telescope-ANTU. Lower mass companionobjects were observed simultaneously in both bands.Results.Solid H{2}Oat 3 μm is detected in all sources, including the companion objects.CO ice at 4.67 μm is detected in a few main targets and one companionobject. One object (LLN 19) shows little CO ice butstrong gas-phase CO ro-vibrational lines in absorption. The CO iceprofiles are different from source to source. The amount of water iceand CO ice trapped in a water-rich mantle may correlate with the fluxratio at 12 and 25 μm. The abundance of H2O-rich CO likely correlateswith that of water ice. A weak feature at 3.54 μm attributed to solidCH3OH and a broad feature near 4.62 μm are observed towardLLN 17, but not toward the other sources. The derivedabundances of solid CH{3}OH and OCN- are 10 ± 2% and 1 ± 0.2% of the H{2}O ice abundance respectively. The H{2}Ooptical depths do not show an increase with envelope mass, nor do theyshow lower values for the companion objects compared with the mainprotostar. The line-of-sight CO ice abundance does not correlate withthe source bolometric luminosity.Conclusions.Comparison of the solid COprofile toward LLN 17, which shows an extremely broadCO ice feature, and that of its lower mass companion at a few thousandAU, which exhibits a narrow profile, together with the detection ofOCN- toward LLN 17 provide directevidences for local thermal processing of the ice.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| The chemical composition of δ Scuti*
We present chemical abundances in the photosphere of δ Scuti(δ Sct) - a prototype of the class of pulsating variables -determined from the analysis of a spectrum obtained by using the 2-mtelescope at the Peak Terskol Observatory and a high-resolutionspectrometer with R= 52000, a signal-to-noise ratio 250 and fromInternational Ultraviolet Explorer spectra. The abundance pattern ofδ Sct consists of 49 chemical elements. The abundances of 19elements have not been investigated previously. The abundances of Pr andNd obtained from the lines of the second and third spectra are equal.The abundances of heavy elements indicate the overabundances withrespect to the solar system values up to 1 dex. The abundance pattern ofδ Sct is similar to the abundance patterns of Am-Fm or δ Deltype stars.A splitting of the cores of all clean lines is observed for the spectraof δ Sct and HD 57749. This can signify evidence of non-radialpulsations in these stars.
| Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?
We present some new ideas about the possibility of life developingaround subgiant and red giant stars. Our study concerns the temporalevolution of the habitable zone. The distance between the star and thehabitable zone, as well as its width, increases with time as aconsequence of stellar evolution. The habitable zone moves outward afterthe star leaves the main sequence, sweeping a wider range of distancesfrom the star until the star reaches the tip of the asymptotic giantbranch. Currently there is no clear evidence as to when life actuallyformed on the Earth, but recent isotopic data suggest life existed atleast as early as 7×108 yr after the Earth was formed.Thus, if life could form and evolve over time intervals from5×108 to 109 yr, then there could behabitable planets with life around red giant stars. For a 1Msolar star at the first stages of its post-main-sequenceevolution, the temporal transit of the habitable zone is estimated to beseveral times 109 yr at 2 AU and around 108 yr at9 AU. Under these circumstances life could develop at distances in therange 2-9 AU in the environment of subgiant or giant stars, and in thefar distant future in the environment of our own solar system. After astar completes its first ascent along the red giant branch and the Heflash takes place, there is an additional stable period of quiescent Hecore burning during which there is another opportunity for life todevelop. For a 1 Msolar star there is an additional109 yr with a stable habitable zone in the region from 7 to22 AU. Space astronomy missions, such as proposed for the TerrestrialPlanet Finder (TPF) and Darwin, that focus on searches for signatures oflife on extrasolar planets, should also consider the environments ofsubgiants and red giant stars as potentially interesting sites forunderstanding the development of life. We performed a preliminaryevaluation of the difficulty of interferometric observations of planetsaround red giant stars compared to a main-sequence star environment. Weshow that pathfinder missions for TPF and Darwin, such as Eclipse andFKSI, have sufficient angular resolution and sensitivity to search forhabitable planets around some of the closest evolved stars of thesubgiant and red giant class.
| The field Am and ρ Puppis-like stars: Lithium and heavier elements
With observations at the Canada-France-Hawaii telescope, photosphericabundances of two evolved field Am stars, τ UMaand HR 178, have been determined for Li, Al, Si, S,Fe, Ni, and Eu by model abundance analysis. Thanks to these newdetermined abundances and the HIPPARCOS mission, making it possible tobetter estimate the evolutionary stage of field stars, one can considerthe influence of evolution on the abundances of Am stars. No abundancetrend for Al, Si, S, and Fe is found during Main Sequence evolution, upto its very last phases. The abundance of Li is, generally, cosmic.Large Li deficiencies may be observed in the very last phases (the``hook'' region of the H-R diagram) and, too, on the red side of the Amphenomenon region where the ρ Puppis-like stars lie.Based on observations collected at the Canada-France-Hawaii telescope(Hawaii).
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| Lithium and rotation in F and G dwarfs and subgiants
Lithium abundances have been determined in 127 F and G Pop I stars basedon new measurements of the equivalent width of the lambda 6707 ÅLi I line from their high resolution CCD spectra. Distances and absolutemagnitudes of these stars have been obtained from the HipparcosCatalogue and their masses and ages derived, enabling us to investigatethe behaviour of lithium as a function of these parameters. Based ontheir location on the HR diagram superposed on theoretical evolutionarytracks, the sample of the stars has been chosen to ensure that they havemore or less completed their Li depletion on the main sequence. A largespread in the Li abundances is found at any given effective temperatureespecially in the already spun down late F and early G stars. Thisspread persists even if the ``Li-dip'' stars that have evolved from themain sequence temperature interval 6500-6800 K are excluded. Stars inthe mass range up to 2 M/Msun when divided into threemetallicity groups show a linear correlation between Li abundance andmass, albeit with a large dispersion around it which is not fullyaccounted for by age either. The large depletions and the observedspread in Li are in contrast to the predictions of the standard stellarmodel calculations and suggest that they are aided by non-standardprocesses depending upon variables besides mass, age and metallicity.The present study was undertaken to examine, in particular, the effectsof rotation on the depletion of Li. No one-to-one correlation is foundbetween the Li abundance and the present projected rotational velocity.Instead the observed abundances seem to be dictated by the rotationalhistory of the star. However, it is noted that even this interpretationis subject to the inherent limitation in the measurement of the observedLi EQW for large rotational velocities.Table 1 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/409/251
| A 3-5 μ m VLT spectroscopic survey of embedded young low mass stars I. Structure of the CO ice
Medium resolution (lambda /Delta lambda = 5000-10 000) VLT-ISAACM-band spectra are presented of 39 young stellar objects in nearbylow-mass star forming clouds showing the 4.67 μm stretching vibrationmode of solid CO. By taking advantage of the unprecedentedly largesample, high S/N ratio and high spectral resolution, similarities in theice profiles from source to source are identified. It is found thatexcellent fits to all the spectra can be obtained using aphenomenological decomposition of the CO stretching vibration profile at4.67 μm into 3 components, centered on 2143.7 cm-1, 2139.9cm-1 and 2136.5 cm-1 with fixed widths of 3.0, 3.5and 10.6 cm-1, respectively. All observed interstellar COprofiles can thus be uniquely described by a model depending on only 3linear fit parameters, indicating that a maximum of 3 specific molecularenvironments of solid CO exist under astrophysical conditions. A simplephysical model of the CO ice is presented, which shows that the 2139.9cm-1 component is indistinguishable from pure CO ice. It isconcluded, that in the majority of the observed lines of sight, 60-90%of the CO is in a nearly pure form. In the same model the 2143.7cm-1 component can possibly be explained by the longitudinaloptical (LO) component of the vibrational transition in pure crystallineCO ice which appears when the background source is linearly polarised.The model therefore predicts the polarisation fraction at 4.67 μm,which can be confirmed by imaging polarimetry. The 2152 cm-1feature characteristic of CO on or in an unprocessed water matrix is notdetected toward any source and stringent upper limits are given. Whenthis is taken into account, the 2136.5 cm-1 component is notconsistent with the available water-rich laboratory mixtures and wesuggest that the carrier is not yet fully understood. A shallowabsorption band centered between 2165 cm-1 and 2180cm-1 is detected towards 30 sources. For low-mass stars, thisband is correlated with the CO component at 2136.5 cm-1,suggesting the presence of a carrier different from XCN at 2175cm-1. Furthermore the absorption band from solid13CO at 2092 cm-1 is detected towards IRS 51 inthe rho Ophiuchi cloud complex and an isotopic ratio of12CO/13CO=68+/-10 is derived. It is shown that allthe observed solid 12CO profiles, along with the solid13CO profile, are consistent with grains with an irregularlyshaped CO ice mantle simulated by a Continuous Distribution ofEllipsoids (CDE), but inconsistent with the commonly used models ofspherical grains in the Rayleigh limit.Based on observations obtained at the European Southern Observatory,Paranal, Chile, within the observing programs 164.I-0605 and 69.C-0441.ISO is an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, The Netherlands and theUK) and with the participation of ISAS and NASA.Table \ref{SourceList} and Appendices A and B are only available inelectronic form at http://www.edpsciences.org
| 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.
| The latitude and epoch for the formation of the southern Greek constellations
Not Available
| 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.
| Mode characterisation in delta Scuti stars. I. rho Pup, GN And, V1208 Aql and AV Cet
We present new spectroscopic observations of four bright delta Scutipulsators. Line indices of the Balmer lines can be used to establishmode identifications using the ratio R between such line indexamplitudes and photometric amplitudes. For the well known radialpulsator rho Pup an absolute value of the amplituderatio of the radial mode was found to R=0.43, consistent with earliervalues of R ~ 0.5 reported for other delta Scuti stars. In GNAnd we recover the recently reported newly excited mode, whilethe former dominant mode was not recovered. We confirm thatV1208 Aql is a non-radial pulsator, containing twomodes of different degree. In V1208 Aql we report a previously unseenmode behavior: the phases of both modes are reversed between the coreand wings of the Hβ line, suggesting the presence of moreundetected modes. For AV Cet we find that thedominant mode is a previously undetected one. We suggest the undertakingof new photometric observations of rho Pup, GN And, V1208 Aql and AV Cetto determine the frequencies and amplitudes of the pulsations. Based inpart on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile.
| delta Scuti stars in Praesepe. II. The STACC 1998 campaign - The spectroscopy
We present the results of a large international spectroscopic campaignon the delta Scuti star BN Cnc. Combiningobservations from five observatories taken over more than two weeks, wecalculate line indices of the Hα line. A line index is theintegrated line flux in a software filter divided by the continuum flux.We demonstrate that this can be used in combination with simultaneousphotometry to classify the oscillation modes. We recover all thefrequencies also found from photometry and assign likely modeidentifications, which differ slightly from previously published values,but are found to be consistent with simple models. The difference inidentification is found to have very little effect on the derivedluminosity and temperature.
| UBVRIJH photometry of two new luminous δ Scuti stars and the discovery of δ Scuti pulsation in the most evolved Ap star known
Time-series photometry of the Hipparcos variable stars HD 199434 and21190 is reported. Both stars are pulsators of the δ Scuti type.Reclassifications of the MK types of the stars, based on newspectrograms, are given. HD 21190 is found to be F2III SrEuSi:, makingit the most evolved Ap star known. Its Strömgren photometricindices support the peculiar spectral type. It is also one of the mostevolved δ Scuti stars known. Its combined Ap-δ Scuti naturemakes it an important test of models of pulsation in peculiar starsrecently developed by Turcotte et al., although it is more extreme thanany model they examined. Physical parameters of both stars are estimatedfrom Strömgren and Hβ photometry, and Hipparcos absolutemagnitudes. We attempt mode identifications based on amplitude ratiosand phase differences from our photometry. The dominant pulsation of HD21190 may be an overtone radial mode. The model fits for HD 199434 areeven less satisfactory, but favour an l=2 mode. Given the good qualityand wavelength coverage of our data, the poor results from theapplication of the photometric theory of mode identification may callinto question the use of that technique.
| Stellar encounters with the solar system
We continue our search, based on Hipparcos data, for stars which haveencountered or will encounter the solar system(García-Sánchez et al. \cite{Garcia}). Hipparcos parallaxand proper motion data are combined with ground-based radial velocitymeasurements to obtain the trajectories of stars relative to the solarsystem. We have integrated all trajectories using three different modelsof the galactic potential: a local potential model, a global potentialmodel, and a perturbative potential model. The agreement between themodels is generally very good. The time period over which our search forclose passages is valid is about +/-10 Myr. Based on the Hipparcos data,we find a frequency of stellar encounters within one parsec of the Sunof 2.3 +/- 0.2 per Myr. However, we also find that the Hipparcos data isobservationally incomplete. By comparing the Hipparcos observations withthe stellar luminosity function for star systems within 50 pc of theSun, we estimate that only about one-fifth of the stars or star systemswere detected by Hipparcos. Correcting for this incompleteness, weobtain a value of 11.7 +/- 1.3 stellar encounters per Myr within one pcof the Sun. We examine the ability of two future missions, FAME andGAIA, to extend the search for past and future stellar encounters withthe Sun.
| The Naini Tal - Cape survey for pulsations in chemically peculiar A-type stars. I. Methods and preliminary results
A new survey for pulsating, chemically peculiar A- and F-type stars inthe northern hemisphere has been initiated using the 1-m telescope atUttar Pradesh State Observatory in Naini Tal. The survey is primarilystructured to reveal high-overtone pulsations in chemically peculiar A-Fstars, but it is also revealing low-overtone delta Scuti-type pulsationsin stars with peculiar colours. This paper describes the scope andmethods of the survey. Preliminary observations of 28 candidate starsare presented and the discovery of three new pulsators, HD 12098, HD13038 and HD 13079, is announced. Null results for the remaining 25stars are also discussed.
| delta Scuti and related stars: Analysis of the R00 Catalogue
We present a comprehensive analysis of the properties of the pulsatingdelta Scuti and related variables based mainly on the content of therecently published catalogue by Rodríguez et al.(\cite{retal00a}, hereafter R00). In particular, the primaryobservational properties such as visual amplitude, period and visualmagnitude and the contributions from the Hipparcos, OGLE and MACHOlong-term monitoring projects are examined. The membership of thesevariables in open clusters and multiple systems is also analyzed, withspecial attention given to the delta Scuti pulsators situated ineclipsing binary systems. The location of the delta Scuti variables inthe H-R diagram is discussed on the basis of HIPPARCOS parallaxes anduvbybeta photometry. New borders of the classical instability arepresented. In particular, the properties of the delta Scuti pulsatorswith nonsolar surface abundances (SX Phe, lambda Boo, rho Pup, delta Deland classical Am stars subgroups) are examined. The Hipparcos parallaxesshow that the available photometric uvbybeta absolute magnitudecalibrations by Crawford can be applied correctly to delta Scutivariables rotating faster than v sin i ~ 100 km s{-1} withnormal spectra. It is shown that systematic deviations exist for thephotometrically determined absolute magnitudes, which correlate with vsin i and delta m1. The photometric calibrations are found tofit the lambda Boo stars, but should not be used for the group ofevolved metallic-line A stars. The related gamma Dor variables and thepre-main-sequence delta Scuti variables are also discussed. Finally, thevariables catalogued with periods longer than 0fd 25 are examined on astar-by-star basis in order to assign them to the proper delta Scuti, RRLyrae or gamma Dor class. A search for massive, long-period delta Scutistars similar to the triple-mode variable AC And is also carried out.
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| On the Variability of A6 to F9 Supergiants
Hipparcos Satellite photometry of A6 to F9 supergiants reveals theiractivity. A few are especially quiescient. A0-A5II stars are examinedto help connect this study with that of earlier supergiants.
| A revised catalogue of delta Sct stars
An extensive and up-dated list of delta Sct stars is presented here.More than 500 papers, published during the last few years, have beenrevised and 341 new variables have been added to our last list, sixyears ago. This catalogue is intended to be a comprehensive review onthe observational characteristics of all the delta Sct stars known untilnow, including stars contained in earlier catalogues together with othernew discovered variables, covering information published until January2000. In summary, 636 variables, 1149 references and 182 individualnotes are presented in this new list. Tables 1 and 2 will be accessibleonly in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Metal abundances of field A and Am stars
Detailed abundances of 9 field A and Am stars are derived for Mg, Ca,Sc, Cr, Fe, and Ni using high resolution spectroscopy. Most of oursample Am stars show the characteristic deficiencies of Ca and Sc on onehand, and the overabundances of the iron peak elements on the other,which is consistent with the predictions of the diffusion model for MainSequence or slightly evolved stars. There seems to be a correlationbetween abundances of Ca and Sc and the effective temperature for starsof similar age: the abundance of Ca increases with increasing masswhereas that of Sc decreases. Two of our Am stars have a differentabundance pattern: Ca is overabundant in the hot magnetic Am star o Pegand its abundance is nearly normal in the metallic giant star rho Pup.The other objects of our sample have globally a solar composition except28 And, which shows significant deficiencies for most the studiedelements. Based on observations collected at the Observatoire deHaute-Provence (France), and on data from the ESA\protect\linebreakHIPPARCOS astrometry satellite.
| Ca II activity and rotation in F-K evolved stars
Ca II H and K high resolution observations for 60 evolved stars in thefield and in 5 open clusters are presented. From these spectrachromospheric fluxes are derived, and a homogeneous sample of more than100 giants is built adding data from the literature. In addition, formost stars, rotational velocities were derived from CORAVELobservations. By comparing chromospheric emission in the cluster starswe confirm the results of Pasquini & Brocato (1992): chromosphericactivity depends on the stellar effective temperature, and mass, whenintermediate mass stars (M ~ 4 Msun) are considered. TheHyades and the Praesepe clump giants show the same level of activity, asexpected from stars with similar masses and effective temperatures. Adifference of up to 0.4 dex in the chromospheric fluxes among the Hyadesgiants is recorded and this sets a clear limit to the intrinsic spreadof stellar activity in evolved giants. These differences in otherwisevery similar stars are likely due to stellar cycles and/or differencesin the stellar initial angular momentum. Among the field stars none ofthe giants with (V-R)o < 0.4 and Ia supergiants observedshows a signature of Ca II activity; this can be due either to the realabsence of a chromosphere, but also to other causes which preclude theappearance of Ca II reversal. By analyzing the whole sample we find thatchromospheric activity scales linearly with stellar rotational velocityand a high power of stellar effective temperature: F'k ~Teff7.7 (Vsini)0.9. This result can beinterpreted as the effect of two chromospheric components of differentnature: one mechanical and one magnetic. Alternatively, by using theHipparcos parallaxes and evolutionary tracks, we divide the sampleaccording to the stellar masses, and we follow the objects along anevolutionary track. For each range of masses activity can simply beexpressed as a function of only one parameter: either theTeff or the angular rotation Omega , with laws F'k~ Omega alpha , because angular velocity decreases witheffective temperature along an evolutionary track. By using theevolutionary tracks and the observed Vsini we investigate the evolutionof the angular momentum for evolved stars in the range 1-5Msun. For the 1.6-3 solar mass stars the data are consistentwith the IOmega =const law while lower and higher masses follow a lawsimilar to IOmega 2=const, where I is the computed stellarmomentum of inertia. We find it intriguing that Vsini remains almostconstant for 1Msun stars along their evolution; if a similarbehavior is shared by Pop II stars, this could explain the relativelyhigh degree of activity observed in Pop II giants. Finally, through theuse of models, we have verified the consistency of the F'k ~Omega alpha and the IOmega beta = Const lawsderived, finding an excellent agreement. This representation, albeitcrude (the models do not consider, for instance, mass losses) representsthe evolution of Ca II activity and of the angular momentum in asatisfactory way in most of the portion of HR diagram analyzed.Different predictions could be tested with observations in selectedclusters. Based on observations collected at ESO, La Silla. Tables 1-3are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| UV observations of B to F-type stars.
Not Available
| Red supergiants in the LMC - III: luminous F and G stars
New BVRI observations for 40 and spectrophotometric measurements for 23F to G LMC supergiant candidates (and 3 galactic F to G supergiants) arepresented. The errors of the BVRI data are 0.01 to 0.03 mag in mostcases. The wavelength range of the spectra is 3400 to 6400 Angstroms,their resolution 10 Angstroms. The mean error of the fluxes is 0.03 mag.Spectral indices measuring the strengths of the Hβ , Hγ ,Hdelta , NaD and CaII H+K lines, the CHα_ {0} and CNbeta_ {0}bands, of the Balmer jump and the slope of the continuum redwards arediscussed as measures of effective temperature and luminosity on thebasis of galactic stars with accurate MK types and parallaxes. TheHγ line and the continuum gradient are very good temperaturecriteria, the CHα_ {0} band and especially the Balmer jump forluminosity. The luminosity classification given for F to G supergiantcandidates in the LMC in the literature is often doubtful. 5 of the 23stars observed spectrophotometrically turn out to be probably galacticforeground dwarfs on the basis both of the Balmer jump and thecomparison of their flux distributions with synthetic ones based on theKurucz model atmospheres. Surface gravities derived purely on the basisof flux distributions and such ones given by models of stellar evolutionagree with each other for dwarfs and giants only. For supergiants theformer are about 1.0 dex higher than the latter. As a consequenceeffective temperatures and metallicities given by these two methodsdeviate from each other for such stars, too. The intrinsic colours andtemperatures of galactic and LMC supergiants do not differ. Withabsolute magnitudes up to -9.6 mag the upper luminosity limit in the LMCdoes not exceed that in the Galaxy, where Ia-0 supergiants haveMV of up to -9.5 mag. The metallicities of the supergiantsshow a rather large scatter. Nevertheless the mean metallicities of 0.02+/- 0.09 dex for the Galaxy and -0.26 +/- 0.10 dex for the LMC agreewell with other observations.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Stellar Encounters with the Oort Cloud Based on HIPPARCOS Data
We have combined Hipparcos proper-motion and parallax data for nearbystars with ground-based radial velocity measurements to find stars thatmay have passed (or will pass) close enough to the Sun to perturb theOort cloud. Close stellar encounters could deflect large numbers ofcomets into the inner solar system, which would increase the impacthazard at Earth. We find that the rate of close approaches by starsystems (single or multiple stars) within a distance D (in parsecs) fromthe Sun is given by N= 3.5D^2.12 Myr^-1, less than the number predictedby a simple stellar dynamics model. However, this value is clearly alower limit because of observational incompleteness in the Hipparcosdata set. One star, Gliese 710, is estimated to have a closest approachof less than 0.4 pc 1.4 Myr in the future, and several stars come within1 pc during a +/-10 Myr interval. We have performed dynamicalsimulations that show that none of the passing stars perturb the Oortcloud sufficiently to create a substantial increase in the long-periodcomet flux at Earth's orbit.
| A catalog of rotational and radial velocities for evolved stars
Rotational and radial velocities have been measured for about 2000evolved stars of luminosity classes IV, III, II and Ib covering thespectral region F, G and K. The survey was carried out with the CORAVELspectrometer. The precision for the radial velocities is better than0.30 km s-1, whereas for the rotational velocity measurementsthe uncertainties are typically 1.0 km s-1 for subgiants andgiants and 2.0 km s-1 for class II giants and Ib supergiants.These data will add constraints to studies of the rotational behaviourof evolved stars as well as solid informations concerning the presenceof external rotational brakes, tidal interactions in evolved binarysystems and on the link between rotation, chemical abundance and stellaractivity. In this paper we present the rotational velocity v sin i andthe mean radial velocity for the stars of luminosity classes IV, III andII. Based on observations collected at the Haute--Provence Observatory,Saint--Michel, France and at the European Southern Observatory, LaSilla, Chile. Table \ref{tab5} also available in electronic form at CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| On the occurrence of a shock wave in the atmosphere of rho Puppis
New high temporal and spectral resolution observations of the Ca Ii Kabsorption profile of the delta Scuti star rho Puppis are presented. Thedata cover 1.2 pulsation cycle. Contrary to the result of Dravins et al.( te{dls}), the line profile corresponding to the phase just before themaximum acceleration (monitored twice here), does not show any emissionfeature. Thus, the Ca Ii K emission may be of transient nature.Consequently, the intensity of the shock wave should be weak andstrongly variable. This result is consistent with a previous study(Mathias et al. te{mgab}) where no shock wave signature was present inthe Hα and metallic line profiles observed. Thus, additionalobservations are required to specify the frequency of occurrence of thetransient emission. Based on observations obtained at the EuropeanSouthern Observatory (La Silla, Chile)
| The central depth of the Ca II triplet lines as a discriminant of chromospheric activity in late type stars
Not Available
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Корма |
| Прямое восхождение: | 08h07m32.60s |
| Склонение: | -24°18'15.0" |
| Видимая звёздная величина: | 2.81 |
| Расстояние: | 19.234 парсек |
| Собственное движение RA: | 0 |
| Собственное движение Dec: | 0 |
| B-T magnitude: | 3.322 |
| V-T magnitude: | 2.842 |
Каталоги и обозначения:
|
INHABITED SKY
