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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.
| Single-Visit Photometric and Obscurational Completeness
We report a method that uses ``completeness'' to estimate the number ofextrasolar planets discovered by an observing program with adirect-imaging instrument. We develop a completeness function forEarth-like planets on ``habitable'' orbits for an instrument with acentral field obscuration, uniform sensitivity in an annular detectionzone, and limiting sensitivity that is expressed as a ``deltamagnitude'' with respect to the star, determined by systematic effects(given adequate exposure time). We demonstrate our method of estimationby applying it to our understanding of the coronagraphic version of theTerrestrial Planet Finder (TPF-C) mission as of 2004 October. Weestablish an initial relationship between the size, quality, andstability of the instrument's optics and its ability to meet missionscience requirements. We provide options for increasing the fidelity andversatility of the models on which our method is based, and we discusshow the method could be extended to model the TPF-C mission as a wholeto verify that its design can meet the science requirements.
| Rotation and Lithium Surface Abundances, Revisited
For giants in the Hertzsprung gap, the relations betweenTeff, vsini, and lithium surface abundances arereinvestigated and compared with the relations found for Hyadesmain-sequence F stars. For the Hyades main-sequence F stars, the vsinidecrease steeply around Teff~6450 K. At the same temperaturethe lithium surface abundances show a narrow, deep dip. For most giantsthere is also a steep decrease of vsini for Teff around 6450K. At this temperature the lithium surface abundances of the giants alsodecrease steeply and remain low for Teff<6400 K. Thechanges in rotation and Li surface abundances occur over a temperatureinterval of less than 300 K, which for a 2 Msolar giantcorresponds to an age interval of about 106 yr. Thesimultaneous steep decreases of rotation velocities and Li surfaceabundances indicate that for the giants these changes are due to thesame cause, which we suggest to be deep mixing. It then seems ratherlikely that for the Hyades main-sequence F5 V stars the decrease ofrotation and Li surface abundance is also caused by deep mixing. Wesuggest that in both cases the changes are related to the merging of thehydrogen and helium convection zones.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Rotation in Globular Cluster stars. Turn-off and subgiant stars in NGC 104, NGC 6397 and NGC 6752
We present a derivation of upper limits to the rotation of Main Sequencestars in three globular clusters using spectra obtained during timeallocated to the ESO Large Programs 165-L0263 and 167.D-0173, with UVESat VLT2 (Kueyen). The stars analyzed in this work do not show anyevidence of high values of rotational velocities as far as the outerlayers are concerned, in particular robust estimates for the upperlimits of the values of the mean projected rotational velocities areplaced, about /line{vrot sin i} = 3.5+/- 0.2 kms-1 and about /line{vrot sin i}=4.7+/-0.2 kms-1 for, respectively, the program turn-off and subgiantstars. On the basis of statistical considerations, assuming thatinclination of the rotational axis i is randomly oriented, and that allstars within the same group rotate at the same rate, we obtain estimatesfor the values of the true rotational velocities. These values are upperlimits to the true rotational velocities if there is some star-to-starscatter in rotational or macro-turbulent velocities. The mean values ofthese upper limits for vrot for the stars of the samespectral type averaged over the three clusters are found to berespectively, /line{v}rot<=3.5+/-0.4 km s-1 and/line{v}rot<=2.6+/-1.1 km s-1. Thus, theexplanation for the large rotational velocities found for the HorizontalBranch stars must be looked for either in the rotation of the core notdetectable in the outer layers (Sills & Pinsennault \cite{sill}) orin the acquisition of angular momentum during their evolution.Based on observations collected at the European Southern Observatory,Chile.
| 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
| Rotation and lithium in single giant stars
In the present work, we study the link between rotation and lithiumabundance in giant stars of luminosity class III, on the basis of alarge sample of 309 single stars of spectral type F, G and K. We havefound a trend for a link between the discontinuity in rotation at thespectral type G0III and the behavior of lithium abundances around thesame spectral type. The present work also shows that giant starspresenting the highest lithium contents, typically stars earlier thanG0III, are those with the highest rotation rates, pointing for adependence of lithium content on rotation, as observed for otherluminosity classes. Giant stars later than G0III present, as a rule, thelowest rotation rates and lithium contents. A large spread of about fivemagnitudes in lithium abundance is observed for the slow rotators.Finally, single giant stars with masses 1.5 < M/Msun<=2.5 show a clearest trend for a correlation between rotational velocityand lithium abundance. Based on observations collected at theObservatoire de Haute -- Provence (France) and at the European SouthernObservatory, La Silla (Chile). Table 2 is only available electronicallywith the On-Line publication athttp://link.springer.de/link/service/00230/
| Evolution of X-ray activity of 1-3 Msun late-type stars in early post-main-sequence phases
We have investigated the variation of coronal X-ray emission duringearly post-main-sequence phases for a sample of 120 late-type starswithin 100 pc, and with estimated masses in the range 1-3Msun, based on Hipparcos parallaxes and recent evolutionarymodels. These stars were observed with the ROSAT/PSPC, and the dataprocessed with the Palermo-CfA pipeline, including detection andevaluation of X-ray fluxes (or upper limits) by means of a wavelettransform algorithm. We have studied the evolutionary history of X-rayluminosity and surface flux for stars in selected mass ranges, includingstars with inactive A-type progenitors on the main sequence and lowermass solar-type stars. Our stellar sample suggests a trend of increasingX-ray emission level with age for stars with masses M > 1.5Msun, and a decline for lower-mass stars. A similar behaviorholds for the average coronal temperature, which follows a power-lawcorrelation with the X-ray luminosity, independently of their mass andevolutionary state. We have also studied the relationship between X-rayluminosity and surface rotation rate for stars in the same mass ranges,and how this relationships departs from the Lx ~vrot2 law followed by main-sequence stars. Ourresults are interpreted in terms of a magnetic dynamo whose efficiencydepends on the stellar evolutionary state through the mass-dependentchanges of the stellar internal structure, including the properties ofenvelope convection and the internal rotation profile.
| 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
| The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Evolution of X-ray activity and rotation on G-K giants
The recent availability of stellar parallaxes provided by the Hipparcosstar catalogue (ESA 1997) enables an accurate determination of thepositions of single field giants in a theoretical H-R diagram and areliable estimate of their masses. The present study combines these newastrometric data with previously published X-ray fluxes and rotationalvelocities. The results confirm the existence of a sharp decrease ofX-ray emission at spectral type K1 for 2.5 M_sun < M < 5 M_sungiants. The study shows that the rotational velocity of these starsreaches a minimum at the same location in the H-R diagram. However, notight relationship between X-ray luminosities and projected equatorialvelocities was found among the sample stars. I suggest that theseresults could reflect the importance of differential rotation indetermining the level of coronal emission among >= 2.5Msun G and K giants. The restoration of rigid rotation at thebottom of the red giant branch could prevent the maintenance of largescale magnetic fields, thus explaining the sharp decrease of coronalX-ray emission at spectral type K1.
| X-ray emission from A0-F6 spectral type stars
We use the ROSAT public data archive to study the X-ray emission of asample of supposedly single A0-F6 spectral type stars from the BrightStar Catalogue. We detected X-ray emission from 19 A- and 33 F-typestars. However, our results are not sufficient to associate withcertainty the X-ray emission to the A-type stars themselves, since theusual argument that it may originate from a binary companion cannot beexcluded. A spectral analysis was conducted for 14 sources (3 A and 11F), finding that for 12 of them a two temperature thermal plasma modelis needed to reproduce the observed spectra. The two temperatures arecentered at 0.13 and 0.54 keV, respectively. The values found for thehigher temperature are lower than that ones of X-ray selected singlelate-type stars. The X-ray luminosities are in the range LX ~10(28}-10({30)) erg s(-1) , with a distribution similar to that ofactive late-type stars. No correlation is found between LXand B-V color, V sin i and Lbol, while a positive correlationis found between the X-ray luminosity and the hardness ratio.
| The ROSAT all-sky survey catalogue of optically bright late-type giants and supergiants
We present X-ray data for all late-type (A, F, G, K, M) giants andsupergiants (luminosity classes I to III-IV) listed in the Bright StarCatalogue that have been detected in the ROSAT all-sky survey.Altogether, our catalogue contains 450 entries of X-ray emitting evolvedlate-type stars, which corresponds to an average detection rate of about11.7 percent. The selection of the sample stars, the data analysis, thecriteria for an accepted match between star and X-ray source, and thedetermination of X-ray fluxes are described. Catalogue only available atCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Angular Momentum of Main Sequence Stars and Its Relation to Stellar Activity
Rotational velocities are reported for intermediate-mass main sequencestars it the field. The measurements are based on new, high S/N CCDspectra from the Coudé Feed Telescope of the Kitt Peak NationalObservatory. We analyze these rotation rates for a dependence on bothmass and age. We compare the average rotation speeds of the field starswith mean velocities for young stars in Orion, the Alpha Persei cluster,the Pleiades, and the Hyades. The average rotation speeds of stars moremassive than $\sim1.6$ \msun\experience little or no change during theevolutionary lifetimes of these stars on the zero age main sequence orwithin the main sequence band. Less massive stars in the range betwee n1.6\msun\ and 1.3\msun\ also show little decline in mean rotation ratewhile they are on the main sequence, and at most a factor of 2 decreasein velocity as they evolve off the main sequence. The {\it e}-foldingtime for the loss of angular momentum b y the latter group of stars isat least 1--2 billion years. This inferred characteristic time scale forspindown is far longer than the established rotational braking time forsolar-type stars with masses below $\sim1.3$ \msun. We conclude from acomparison of the trends in rotation with trends in chromospheric andcoronal activity that the overall decline in mean rotation speed alongthe main sequence, from $\sim2$ \msun\ down to $\sim1.3$ \msun, isimposed during the pre-main sequence phase of evolution, and that thispattern changes little thereafter while the star resides on the mainsequence. The magnetic activity implicated in the rotational spindown ofthe Sun and of similar stars during their main sequence lifetimes mus ttherefore play only a minor role in determining the rotation rates ofthe intermediate mass stars, either because a solar-like dynamo is weakor absent, or else the geometry of the magnetic field is appreciablyless effective in removing angular momentu m from these stars. (SECTION:Stars)
| Correlating Coronal Temperature and Gravitational Potential: A Test of the Nonthermal Boundary Hypothesis
It has been suggested that a nonthermal boundary condition can accountfor the existence of a coronal temperature inversion in stars. Thenonthermal hypothesis makes falsifiable predictions of the maximumcoronal temperature obtainable. This maximum temperature corresponds tothe depth of the potential well at the boundary, which is effectivelyR*, and so varies as M*/R*. Here, we compare this prediction withone-temperature fits to single stars in Einstein data. We find that thecoronal temperatures in dwarfs are consistent with a nonthermal boundarycondition, but the nonthermal hypothesis cannot readily account for thecoronal temperatures of class III giants. However, interpreting theemission from the non-compliant giants in alternate terms suggests thata dwarf companion may be the source of the X-rays. More generally,gravitational potential is found to be a useful quantity to orderingcoronal observations: (1) the "dividing line" near K4, beyond whichgiant star coronae are not observed, corresponds to a rather precipitousfactor of 10 drop in the strength of the stellar gravitationalpotential; and (2) M dwarfs and A stars of class III--V, whose coronaehave been somewhat of a puzzle, all have relatively deep gravitationalpotentials.
| A microwave survey of southern early-type stars
A multi-epoch survey with the Parkes telescope of a completedistance-limited sample of 57 stars earlier than F6 has detectedpossible 8.4-GHz emission from 16 stars. Single-epoch partial synthesisobservations with the Australia Telescope Compact Array (ATCA) at 4.8GHz on 27 stars from the same sample (including the possible Parkesdetections) found no emission at the stellar positions above a fluxdensity limit of 1.2-1.9 mJy, but the maps show that the Parkesdetections are not merely the results of confusion of sources within theParkes beam. Three early F stars with UV and/or X-ray emission wereobserved simultaneously at 4.8 and 8.4 GHz in 12-h syntheses with the6-element ATCA. Two of these stars were from the above sample and thethird was the supergiant Alpha Carinae. We detected only alphaCar withflux densities of 300+/-65 and 140+/-65 muJy at 4.8 and 8.6 GHz(S~nu^-1.3+/-1.3). We discuss the legitimacy of the Parkes 3-6sigmadetections and show that, although none has been detected by synthesisobservations, there is no compelling reason for rejecting them on theinternal evidence. The power emitted by the supergiant alphaCar issimilar to that of the 16 possible Parkes detections, although itsactivity index is orders of magnitude lower. We show that this emissioncannot be thermal bremsstrahlung from the 10^7.2-K corona of the starbut is probably synchrotron emission from a magnetically maintainedcorona.
| The RIASS coronathon: Joint X-ray and ultraviolet observations of normal F-K stars
Between 1990 August and 1991 January the ROSAT/IUE All Sky Survey(RIASS) coordinated pointings by the International Ultraviolet Explorer(IUE) with the continuous X-ray/EUV mapping by the Roentgensatellit(ROSAT). The campaign provided an unprecedented multiwavelength view ofa wide variety of cosmic sources. We report findings for F-K stars, alarge proportion of the RIASS targets. Forty-eight of our 91'Coronathon' candidates were observed by the IUE during the campaign.For stars missed by the IUE, we supplemented the ROSAT survey fluxeswith archival UV spectra and/or follow-on observations.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| On the link between rotation and coronal activity in evolved stars.
We analyse the behaviour of coronal activity as a function of rotationfor a large sample of single and binary evolved stars for which we haveobtained CORAVEL high precision rotational velocities. This study showsthat tidal effects play a direct role in determining the X-ray activitylevel in binary evolved stars. The circularisation of the orbit is anecessary property for enhanced coronal activity in evolved binarystars.
| The lithium content and other properties of F2-G5 giants in the Hertzsprung Gap
As stars of 2-5 solar mass evolve across the Hertzsprung Gap they shouldfirst deplete their surface lithium by convective dilution and then,when convection penetrates deeper, begin to bring CN processed materialto their surfaces. To investigate this process we have observed 52giants, 25 of which have known C/N ratios, for their Li abundances.After eliminating four stars that may actually be dwarfs and includingthe two components of Capella analyzed by Pilachowski and Sowell we havecompared our (Li/Fe) ratios with models of Swenson. For stars showing vsin i greater than 50 km/s we find (Li/Fe) to be unaffected by mixingfor B - V less than 0.7 as predicted. For stars cooler than B - V = 0.7both v sin i and (Li/Fe) drop to smaller values. For the sharp linedstars (v sin i less than 50 km/s) we find a drop in Li between B - V =0.45 and 0.60 which cannot be understood in terms of dilution byconvection. Various possible causes of such an early depletion ordilution of surface Li are discussed including diffusion at the base ofthe convection zone, mass loss possibly enhanced by pulsation, andmagnetic activity as in the magnetic A and B type stars. The models ofRicher & Michaud (1993) with diffusion point toward a satisfactorysolution. A few giants with low v sin i values stand out with muchhigher than expected (Li/Fe) values despite their cool effectivetemperatures. We do not understand why those stars have not depletedtheir lithium as have most giants of similar color. The correlation of(N/C) with (Li/Fe) follows expectations in so far as almost all starswith enhanced (N/C) have depleted their Li as well.
| Silicon abundance in Population I giants
Silion to carbon abundance ratios for Population I giants weredetermined from emission lines originating in the transition layersbetween stellar chromospheres and coronae. For effective temperatureslarger than 6200 K we find a group of stars for which the silicon tocarbon abundance ratio appears to be increased. These stars arepresumably descendents from AP stars on Am stars with increased surfacesilicon to carbon abundance ratios. Around B-V approximately equal to0.45 this anomaly disappears as is to be expected due to the increaseddepth of the convection zone and therefore deeper mixing which dilutesthe surface overabundances. Unexplained is the apparent increasedsilicon to carbon abundance ratio observed for several stars cooler than5100 K. RS CVn and related stars do not show this increased abundanceratio.
| Rotation and transition layer emission in cool giants
Gray (1981, 1982) found that field giants with T(eff) less than about5500 K experience a steep decrease in rotational velocities coupled witha decrease in transition layer emission. This decrease may beattributable to fast magnetic braking or to redistribution of angularmomentum for rapidly increasing depths of the convection zones if theserotate with depth independent specific angular momentum. Additionalarguments in favor of the latter interpretation are presented. Theincrease of N/C abundances due to deep mixing occurs at the same pointas the decrease in v sin i. On the other hand, the ratios of the C IV toC II emission line fluxes decrease at this point indicating smallercontributions of MHD wave heating. The X-ray fluxes decrease at nearlythe same T(eff). Thus, no observations are found which would indicatelarger magnetic activity which could lead to fast magnetic braking.Theory predicts a rapid increase in the convection zone depth at theT(eff) where the decrease in v sin i is observed. This can explain theobserved phenomena.
| Einstein Observatory coronal temperatures of late-type stars
The results are presented of a survey of the coronal temperatures oflate-type stars using the Einstein Observatory IPC. The spectralanalysis shows that the frequently found one- and two-temperaturedescriptions are mainly influenced by the SNR of the data and thatmodels using continuous emission measure distributions can provideequally adequate and physically more meaningful and more plausibledescriptions. Intrinsic differences in differential emission measuredistributions are found for four groups of stars. M dwarfs generallyshow evidence for high-temperature gas in conjunction withlower-temperature material, while main-sequence stars of types F and Ghave the high-temperature component either absent or very weak. Very hotcoronae without the lower-temperature component appearing in dwarf starsare evident in most of the giant stars studied. RS CVn systems showevidence for extremely hot coronae, sometimes with no accompanyinglower-temperature material.
| Einstein Observatory magnitude-limited X-ray survey of late-type giant and supergiant stars
Results are presented of an extensive X-ray survey of 380 giant andsupergiant stars of spectral types from F to M, carried out with theEinstein Observatory. It was found that the observed F giants orsubgiants (slightly evolved stars with a mass M less than about 2 solarmasses) are X-ray emitters at the same level of main-sequence stars ofsimilar spectral type. The G giants show a range of emissions more than3 orders of magnitude wide; some single G giants exist with X-rayluminosities comparable to RS CVn systems, while some nearby large Ggiants have upper limits on the X-ray emission below typical solarvalues. The K giants have an observed X-ray emission level significantlylower than F and F giants. None of the 29 M giants were detected, exceptfor one spectroscopic binary.
| BVRI photometry of the Gliese Catalogue stars
Photoelectri BVRI photometry on the Cousins (Kron-Cape) system has beenobtained for many of the southern faint stars in the Gliese Catalog(1969). This extends the work of Cousins (1980) and provides a uniformset of data for the nearby stars. Several red dwarfs are noted, whichwere used to define the red end of the Cousins system.
| The evolution of chromospheric activity of cool giant and subgiant stars
IUE spectra for a large sample of cool subgiant stars are examined, andevidence is found that subgiants in the mass range 1.2-1.6 solar massesundergo a sudden decline in UV transition region emission near B - V =0.6, which corresponds to spectral type G0 IV. The decline in UVemission coincides with a sharp decrease in stellar rotation rates, andit is suggested that this decay in activity and rotation marks atransformation from acoustic heating in the early F stars to magneticdynamo-driven activity in the cooler stars, resulting in a strongrotational braking action by stellar wind. For more massive giant stars,there is a similar transformation in the nature of chromosphericactivity near B - V = 0.7, or spectral type G0 III, from acousticheating in the F-type giants to a solarlike dynamo mechanism in thecooler giants. No sign of an abrupt drop in activity near spectral typeG5 III at the location of Gray's proposed rotational boundary line isseen.
| Stellar integrated fluxes in the wavelength range 380 NM - 900 NM derived from Johnson 13-colour photometry
Petford et al. (1988) have reported measured integrated fluxes for 216stars with a wide spread of spectral type and luminosity, and mentionedthat a cubic-spline integration over the relevant Johnson 13-colormagnitudes, converted to fluxes using Johnson's calibration, is inexcellent agreement with those measurements. In this paper a list of thefluxes derived in this way, corrected for a small dependence on B-V, isgiven for all the 1215 stars in Johnson's 1975 catalog with completeentries.
| Absolute flux calibration of the H and K lines of CA II - Chromospheric radiative losses in F and G-type stars
Ca II H and K spectra of 81 (mainly Southern) F and G stars are analyzedusing two different calibration methods. It is shown that, for spectraof sufficiently high resolution, and for stars of relatively lowrotation rates, the calibrations of Linsky et al. (1979) and of Pasquiniet al. (1988) give essentially the same results. These calibrations areused to derive absolute surface fluxes in the H and K lines of Ca II for64 stars. It is shown that several late-F and early-G giants andsupergiants have Ca II H and K fluxes in excess of about 10 to the 6therg/sq cm s, much larger than those typically observed for normal giantsof later spectral types.
| Energy Distribution in the Stellar Spectra of Different Spectral Types and Luminosities - Part Five - Normal Stars
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| Yükselim: | -76°55'11.0" |
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