Contents
Images
Upload your image
DSS Images Other Images
Related articles
Anharmonic and standing dynamo waves: theory and observation of stellar magnetic activity
The familiar decadal cycle of solar activity is one expression ofinterannual variability of surface magnetism observed in stars on ornear the lower main sequence. From studies of time-series of CaII H andK emission fluxes that go back more than 35 yr and have been accumulatedfor such stars at the Mount Wilson Observatory by the HK Project, wedefine a quantitative measure, called anharmonicity, of the cycliccomponent of interannual magnetic variability. Anharmonicity provides aconnection between observed variations in magnetic activity and thetwo-dimensional description of a Parker dynamo model. We explore theparameter space of the Parker dynamo model and find an excellentcounterpart in the records of several of the lowest-mass (late K-type toearly M-type) active stars in the HK Project sample to the solutionscontaining highly anharmonic, standing dynamo waves. We interpretanharmonicity apparent in the records as resulting from non-propagatingor standing dynamo waves, which operate in a regime that issubstantially supercriticial. There, for the majority of a cycle, orpulse of decadal-to-interdecadal variability, the large-scale magneticfields are generated and maintained by winding of field by differentialrotation rather than by the joint action of differential rotation andhelical convection. Among the less active stars (the Sun is consideredsuch a star in the HK Project sample) we find a correspondence betweenanharmonicity and Parker dynamo model solutions that include simpleharmonic, migratory and/or intermediate-type dynamo wave patterns over abroad range of dynamo parameters.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| α-, r-, and s-process element trends in the Galactic thin and thick disks
From a detailed elemental abundance analysis of 102 F and G dwarf starswe present abundance trends in the Galactic thin and thick disks for 14elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba, and Eu).Stellar parameters and elemental abundances (except for Y, Ba and Eu)for 66 of the 102 stars were presented in our previous studies (Bensbyet al. [CITE], A&A, 410, 527, [CITE], A&A, 415, 155). The 36stars that are new in this study extend and confirm our previous resultsand allow us to draw further conclusions regarding abundance trends. Thes-process elements Y and Ba, and the r-element Eu have also beenconsidered here for the whole sample for the first time. With this newlarger sample we now have the following results: 1) smooth and distinctabundance trends that for the thin and thick disks are clearlyseparated; 2) the α-element trends for the thick disk show typicalsignatures from the enrichment of SN Ia; 3) the thick disk stellarsample is in the mean older than the thin disk stellar sample; 4) thethick disk abundance trends are invariant with galactocentric radii(R_m); 5) the thick disk abundance trends appear to be invariant withvertical distance (Z_max) from the Galactic plane. Adding furtherevidence from the literaure we argue that a merger/interacting scenariowith a companion galaxy to produce a kinematical heating of the stars(that make up today's thick disk) in a pre-existing old thin disk is themost likely formation scenario for the Galactic thick disk. The 102stars have -1 ≲ [Fe/H] ≲ +0.4 and are all in the solarneighbourhood. Based on their kinematics they have been divided into athin disk sample and a thick disk sample consisting of 60 and 38 stars,respectively. The remaining 4 stars have kinematics that make themkinematically intermediate to the two disks. Their chemical abundancesalso place them in between the two disks. Which of the two diskpopulations these 4 stars belong to, or if they form a distinctpopulation of their own, can at the moment not be settled. The 66 starsfrom our previous studies were observed with the FEROS spectrograph onthe ESO 1.5-m telescope and the CES spectrograph on the ESO 3.6-mtelescope. Of the 36 new stars presented here 30 were observed with theSOFIN spectrograph on the Nordic Optical Telescope on La Palma, 3 withthe UVES spectrograph on VLT/UT2, and 3 with the FEROS spectrograph onthe ESO 1.5-m telescope. All spectra have high signal-to-noise ratios(typically S/N≳ 250) and high resolution (R 80 000, 45 000,and 110 000 for the SOFIN, FEROS, and UVES spectra, respectively).Based on observations collected at the Nordic Optical Telescope on LaPalma, Spain, and at the European Southern Observatories on La Silla andParanal, Chile, Proposals # 65.L-0019(B), 67.B-0108(B), 69.B-0277. FullTables [see full text], [see full text] and [see full text] are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/185
| Time-spectra of chromospheric activity of old solar-type stars: detection of rotational signals from double wavelet analysis
We introduce a novel technique, called the double wavelet analysis(DWA), for the determination of stellar rotation periods from timeserial data. This first paper aims narrowly at the discussion,introduction and application of the DWA technique to records of surfacemagnetism in solar-type (relatively old) lower main sequence stars thatare obtained by the Mount Wilson Observatory (MWO) HK Project. Thetechnique takes a series of careful steps that seek to optimize waveletparameters and normalization schemes, ultimately allowing fine-tuned,arguably more accurate, estimates of rotation-modulated signals (with,e.g., periods of days to months) in records that contain longerperiodicities such as stellar magnetic activity cycles (with, e.g.,period of years). The apparent rotation periods estimated from the DWAtechnique are generally consistent with results from both ``first-pass''(i.e., ordinary) global wavelet spectrum and earlier classicalperiodogram analyses. But there are surprises as well. For example, therotation period of the ancient subdwarf Goombridge 1830 (HD 103095),previously identified as ~31 days, suggests under the DWAtechnique a significantly slower period of 60 days. DWA spectra alsogenerally reveal a shift in the cycle period toward high frequencies(hence shorter periods) compared to the first-pass wavelet spectrum. Forsolar-type stars analyzed here, the character of the DWA spectrum andslope of the first-pass global wavelet spectrum produce a classificationscheme that allows a star's record to be placed into one of threecategories.
| 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
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| Chemical enrichment and star formation in the Milky Way disk. III. Chemodynamical constraints
In this paper, we investigate some chemokinematical properties of theMilky Way disk, by using a sample composed by 424 late-type dwarfs. Weshow that the velocity dispersion of a stellar group correlates with theage of this group, according to a law proportional to t0.26,where t is the age of the stellar group. The temporal evolution of thevertex deviation is considered in detail. It is shown that the vertexdeviation does not seem to depend strongly on the age of the stellargroup. Previous studies in the literature seem to not have found it dueto the use of statistical ages for stellar groups, rather thanindividual ages. The possibility to use the orbital parameters of a starto derive information about its birthplace is investigated, and we showthat the mean galactocentric radius is likely to be the most reliablestellar birthplace indicator. However, this information cannot bepresently used to derive radial evolutionary constraints, due to anintrinsic bias present in all samples constructed from nearby stars. Anextensive discussion of the secular and stochastic heating mechanismscommonly invoked to explain the age-velocity dispersion relation ispresented. We suggest that the age-velocity dispersion relation couldreflect the gradual decrease in the turbulent velocity dispersion fromwhich disk stars form, a suggestion originally made by Tinsley &Larson (\cite{tinsley}, ApJ, 221, 554) and supported by several morerecent disk evolution calculations. A test to distinguish between thetwo types of models using high-redshift galaxies is proposed.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/517
| Oxygen trends in the Galactic thin and thick disks
We present oxygen abundances for 72 F and G dwarf stars in the solarneighbourhood. Using the kinematics of the stars we divide them into twosub-samples with space velocities that are typical for the thick andthin disks, respectively. The metallicities of the stars range from[Fe/H] ≈ -0.9 to +0.4 and we use the derived oxygen abundances of thestars to: (1) perform a differential study of the oxygen trends in thethin and the thick disk; (2) to follow the trend of oxygen in the thindisk to the highest metallicities. We analyze the forbidden oxygen linesat 6300 Å and 6363 Å as well as the (NLTE afflicted) tripletlines around 7774 Å. For the forbidden line at 6300 Å wehave spectra of very high S/N (>400) and resolution (R ≳ 215000). This has enabled a very accurate modeling of the oxygen line andthe blending Ni lines. The high internal accuracy in our determinationof the oxygen abundances from this line is reflected in the very tighttrends we find for oxygen relative to iron. From these abundances we areable to draw the following major conclusions: (i) That the [O/Fe] trendat super-solar [Fe/H] continues downward which is in concordance withmodels of Galactic chemical evolution. This is not seen in previousstudies as it has not been possible to take the blending Ni lines in theforbidden oxygen line at 6300 Å properly into account; (ii) Thatthe oxygen trends in the thin and the thick disks are distinctlydifferent. This confirms and extends previous studies of the otherα-elements; (iii) That oxygen does not follow Mg at super-solarmetallicities; (iv) We also provide an empirical NLTE correction for theinfrared O I triplet that could be used for dwarf star spectra with aS/N such that only the triplet lines can be analyzed well, e.g. stars atlarge distances; (v) Finally, we find that Gratton et al. (1999)overestimate the NLTE corrections for the permitted oxygen triplet linesat 7774 Å for the parameter space that our stars span.Based on observations collected at the European Southern Observatory, LaSilla and Paranal, Chile, Proposals #65.L-0019, 67.B-0108, and69.B-0277.The full Table 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/415/155
| 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.
| The Evolutionary State of Stars in the NGC 1333S Star Formation Region
We present 2 μm near-IR spectroscopic observations of a sample of 33objects in the NGC 1333S active star-forming cluster centered on thepre-main-sequence star SSV 13. We have previously studied this regionphotometrically in the optical and near-IR, and with the addition ofthese near-IR spectra, we further probe the pre-main-sequence clustermembership and evolutionary state. From the atomic and molecularabsorption features observed, together with the earlier photometry, wederive spectral types, effective temperatures, masses, and ages of thestars and conclude that almost all (90%) the stars observed in thissample are pre-main-sequence objects. This result significantly refinesthe evolutionary information obtained from photometric evidence alone.Comparison with theoretical evolutionary tracks and isochrones suggeststhat our survey has sampled sources with masses in the range 0.2-2Msolar and stellar ages between 7×104 and1×108 yr with a preponderance of sources around3×106 yr. This implies the presence of low- tointermediate-mass T Tauri stars of evolutionary designation Class I toClass III. We conclude that star formation seems to have occurred inlikely several bursts rather than occurring coevally. Star formation insuch a region as NGC 1333S is likely significantly affected by the largenumber of active molecular outflows in the region, which could provide amechanism for cloud turbulence and the onset of subsequent starformation.
| Observational constraints for lithium depletion before the RGB
Precise Li abundances are determined for 54 giant stars mostly evolvingacross the Hertzsprung gap. We combine these data with rotationalvelocity and with information related to the deepening of the convectivezone of the stars to analyse their link to Li dilution in the referredspectral region. A sudden decline in Li abundance paralleling the onealready established in rotation is quite clear. Following similarresults for other stellar luminosity classes and spectral regions, thereis no linear relation between Li abundance and rotation, in spite of thefact that most of the fast rotators present high Li content. The effectsof convection in driving the Li dilution is also quite clear. Stars withhigh Li content are mostly those with an undeveloped convective zone,whereas stars with a developed convective zone present clear sign of Lidilution.Based on observations collected at ESO, La Silla, Chile, and at theObservatoire de Haute Provence, France, operated by the Centre Nationalde la Recherche Scientifique (CNRS).
| Elemental abundance trends in the Galactic thin and thick disks as traced by nearby F and G dwarf stars
Based on spectra from F and G dwarf stars, we present elementalabundance trends in the Galactic thin and thick disks in the metallicityregime -0.8< [Fe/H] < +0.4. Our findings can besummarized as follows. 1) Both the thin and the thick disks show smoothand distinct abundance trends that, at sub-solar metallicities, areclearly separated. 2) For the alpha -elements the thick disk showssignatures of chemical enrichment from SNe type Ia. 3) The age of thethick disk sample is in the mean older than the thin disk sample. 4)Kinematically, there exist thick disk stars with super-solarmetallicities. Based on these findings, together with other constraintsfrom the literature, we discuss different formation scenarios for thethick disk. We suggest that the currently most likely formation scenariois a violent merger event or a close encounter with a companion galaxy.Based on kinematics the stellar sample was selected to contain starswith high probabilities of belonging either to the thin or to the thickGalactic disk. The total number of stars are 66 of which 21 belong tothe thick disk and 45 to the thin disk. The analysis is based onhigh-resolution spectra with high signal-to-noise (R ~ 48 000 and S/Ngtrsim 150, respectively) recorded with the FEROS spectrograph on LaSilla, Chile. Abundances have been determined for four alpha -elements(Mg, Si, Ca, and Ti), for four even-nuclei iron peak elements (Cr, Fe,Ni, and Zn), and for the light elements Na and Al, from equivalent widthmeasurements of ~ 30 000 spectral lines. An extensive investigation ofthe atomic parameters, log gf-values in particular, have been performedin order to achieve abundances that are trustworthy. Noteworthy is thatwe find for Ti good agreement between the abundances from Ti I and TiIi. Our solar Ti abundances are in concordance with the standardmeteoritic Ti abundanceBased on observations collected at the European Southern Observatory, LaSilla, Chile, Proposals #65.L-0019(B) and 67.B-0108(B).Full Tables \ref{tab:linelist} and \ref{tab:abundances} are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/527
| Post-T Tauri Stars in the Nearest OB Association
We present results of a spectroscopic survey of X-ray- andproper-motion-selected samples of late-type stars in the LowerCentaurus-Crux (LCC) and Upper Centaurus-Lupus (UCL) subgroups of thenearest OB association: Scorpius-Centaurus. The primary goals of thesurvey are to determine the star formation history of the OB subgroupsand to assess the frequency of accreting stars in a sample dominated by``post-T Tauri'' pre-main-sequence (PMS) stars. We investigate twosamples: (1) proper-motion candidates from the ACT Catalog and TychoReference Catalog (TRC) with X-ray counterparts in the ROSAT All-SkySurvey (RASS) Bright Source Catalog and (2) G- and K-type stars in theHipparcos catalog found to be candidate members by de Zeeuw et al. Weobtained optical spectra of 130 candidates with the Siding Spring 2.3 mdual-beam spectrograph. PMS stars were identified by (1) strong Liλ6707 absorption, (2) subgiant surface gravities, (3) propermotions consistent with Sco-Cen membership, and (4) H-R diagrampositions consistent with being PMS. We find 93% of the RASS-ACT/TRCstars to be probable PMS members, compared with 73% of the Hipparcoscandidates. We demonstrate that measuring the gravity-sensitive bandratio of Sr II λ4077 to Fe I λ4071 is a valuable means ofdiscriminating PMS and zero-age main-sequence (ZAMS) stars. Usingsecular parallaxes and Hipparcos, Tycho-2, and Two Micron All Sky Surveyphotometry, we construct an H-R diagram. Depending on the choice ofpublished evolutionary tracks, we find the mean ages of the PMSpopulations to range between 17 and 23 Myr for LCC and 15 and 22 Myr forUCL. Taking into account observational errors, it appears that 95% ofthe low-mass star formation in each subgroup must have occurred in lessthan 8 Myr (LCC) and 12 Myr (UCL). Using the Bertelli et al. tracks, wefind main-sequence turnoff ages for Hipparcos B-type members to be16+/-1 Myr for LCC and 17+/-1 Myr for UCL. Contrary to previousfindings, it appears that LCC is coeval with, or slightly older than,UCL. The secular parallaxes of the Sco-Cen PMS stars yield distances of85-215 pc, with 12 of the LCC members lying within 100 pc of the Sun.Only one out of 110 (0.9+2.1-0.8%; 1 σ) PMSsolar-type stars in the sample with ages of 13+/-1 (s.e.)+/-6 (1σ) Myr and masses of 1.3+/-0.2 (1 σ) Msolar showsboth enhanced Hα emission and a K-band excess indicative ofaccretion from a truncated circumstellar disk: the nearby (d~=86 pc)classical T Tauri star PDS 66.
| Measuring starspot temperature from line depth ratios. I. The method
Gray and collaborators have recently demonstrated that line-depth ratiosare a powerful tool for temperature discrimination, able to resolvedifferences <=10 K. The method has been applied to detect temperaturevariations in the 5-15 K range due to activity cycles (e.g. Gray et al.\cite{Gray96a}, \cite{Gray96b}) or to rotation modulation produced bylarge surface features, called ``star-patches", like that detected in xiBoo A by Toner & Gray (\cite{Toner88}). Cool starspots of a fewtenths of the stellar surface produce bumps in a line profile, whichmigrate through the line profile allowing Doppler-imaging in fastrotating stars. In the hypothesis that in slowly-rotating stars thepassage of dark spots produces modulation of the center line depth ofdifferent amount in lines of different sensitivity to temperature, wehave made test observations on three active binaries of the RS CVn type.Based on observations made at the Catania Astrophysical Observatory at aresolution R = 14 000, we show that line-depth ratios can be effectivelyused to determine spot temperatures of active binary systems. Using, onaverage, ten line pairs, selected in the 6100-6300 Å wavelengthrange, with the help of observations of 30 main sequence and giantstars, we have derived a calibration relation of line-depth ratios (LDR)in an absolute temperature scale, taking into account the gravity effectin the calibration relation. Single LDRs converted to temperaturethrough the calibration relations have led to clear rotationalmodulation of the average surface temperature with amplitudes of 177 K,119 K, and 127 K for VY Ari, IM Peg and HK Lac, with average estimatederrors of about 10 K. We show that the observed temperature variationamplitude allows us to define a minimum fractional spotted area coverageas a function of spot-photosphere temperature ratio. Adopting themaximum value of average temperature, determined from the LDRs, as thatof the unspotted photosphere, we computed the average spot temperaturecorresponding to the minimum spot coverage. Although not univocallyconstrained, the temperature difference (Delta T =Tph-Tsp) obtained for the three systems, Delta T =890 K for VY Ari, Delta T = 750 K for IM Peg, and Delta T = 810 K for HKLac, are in good agreement with values derived with other methods. Basedon observations collected at Catania Astrophysical Observatory, Italy.}
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http:/ /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/ A+A/394/927
| Levels of coronal and chromospheric activity in late-type stars and various types of dynamo waves
We analyze the X-ray emission and chromospheric activity of late-type F,G, and K stars studied in the framework of the HK project. More powerfulcoronas are possessed by stars displaying irregular variations of theirchromospheric emission, while stars with cyclic activity arecharacterized by comparatively modest X-ray luminosities and ratios ofthe X-ray to bolometric luminosity L X/L bol. This indicates that thenature of processes associated with magnetic-field amplification in theconvective envelope changes appreciably in the transition from small tolarge dynamo numbers, directly affecting the character of the(α-Ω) dynamo. Due to the strong dependence of both thedynamo number and the Rossby number on the speed of axial rotation,earlier correlations found between various activity parameters and theRossby number are consistent with our conclusions. Our analysis makes itpossible to draw the first firm conclusions about the place of solaractivity among analogous processes developing in active late-type stars.
| The Flux Deficits in Star Spots
The bolometric flux deficits of the photospheres of spotted stars arederived for the first time in the framework of zonal spottedness modelsfor red dwarfs computed at the Crimean Astrophysical Observatory. Theresulting flux deficits are compared to the estimated radiative lossesfrom the chromospheres and coronas measured during quasi-simultaneousobservations. A linear correlation is found between the logarithms ofthese quantities, with the Sun fitting these relations. Radiative lossesfrom the outer stellar atmospheres in quiescence and during individualsporadic flares are significantly lower than the bolometric deficits ofthe spotted photospheres of active stars. This suggests that the fluxdeficit due to spots leads to global reconstruction of the atmospheresof red dwarfs, analogous to the local atmospheric reconstruction thatoccurs during solar and stellar flares. This process may be realized viathe superposition of a large number of weak impulsive flares and otherdynamic events, which develop on these stars and heat their coronas(i.e., in this view, microflaring is favored as the principal coronalheating mechanism for these stars). A brief analysis of the long-termvariations in the chromospheric and photospheric radiation of F-K starsfrom the HK project and of the Sun suggests that such dynamicalreconstruction of the outer atmosphere by energy associated with theflux deficit of the spotted photosphere occurs at times of increasedsurface activity in all F-M stars.
| 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.
| Observing roAp Stars with WET: A Primer
We give an extensive primer on roAp stars -- introducing them, puttingthem in context and explaining terminology and jargon, and giving athorough discussion of what is known and not known about them. Thisprovides a good understanding of the kind of science WET could extractfrom these stars. We also discuss the many potential pitfalls andproblems in high-precision photometry. Finally, we suggest a WETcampaign for the roAp star HR 1217.
| Magnitudes absolues des étoiles standards MK des types G à M à partir des parallaxes Hipparcos. The absolute magnitudes of the G to M type MK standards from the Hipparcos parallaxes
We analyse a sample of about 500 MK standards of cool spectral types (Gto M) for to compare the visual absolute magnitudes obtained from bothHipparcos data and Schmidt-Kaler calibrations. Our purpose is tovalidate our spectroscopic work \cite[(Ginestet et al. 1997, 1999)]{G97}on stars with composite spectra with the help of Hipparcos data.Contrary to what is claimed in other papers, the absolute magnitudedomain devoted to the giant stars does not overlap the domain of dwarfs.We find that the discrepancies between absolute magnitudes fromHipparcos data and absolute magnitudes deduced from Schmidt-Kalercalibrations increase with the relative error sigma (pi )/pi on theparallaxes. So, for sigma (pi )/pi <= 0.05 only 3% of the starspresent a discrepancy of one luminosity class, while this percentagereaches 54% for 0.25 < sigma (pi )/pi <= 0.50. Curiously, theluminosity of the giants seems to increase with the distance of thestars, whereas the supergiants of the sample appear underluminous atleast for d < 600 pc! We point out a list of 14 MK standards whoseluminosity classes may be erroneous and need a new spectralclassification, in the near infrared. The case of composite-spectrumbinaries is also discussed. Most of these are too distant for accurateparallaxes even with Hipparcos: only sixteen stars have sigma (pi )/pi<= 0.10; for these, we give new spectral classifications in agreementwith both our classifications in the near infrared of the coolcomponents and Hipparcos data. Finally, for stars having high-precisionparallaxes (sigma (pi )/pi <= 5%) there is no serious problem forSchmidt-Kaler calibrations whith respect to Hipparcos data. The datacorresponding to parallaxes of lower precisions should be used withcaution and only for statistical analyses.
| Library of Medium-Resolution Fiber Optic Echelle Spectra of F, G, K, and M Field Dwarfs to Giant Stars
We present a library of Penn State Fiber Optic Echelle (FOE)observations of a sample of field stars with spectral types F to M andluminosity classes V to I. The spectral coverage is from 3800 to 10000Å with a nominal resolving power of 12,000. These spectra includemany of the spectral lines most widely used as optical and near-infraredindicators of chromospheric activity such as the Balmer lines (Hαto Hepsilon), Ca II H & K, the Mg I b triplet, Na I D_1, D_2, He ID_3, and Ca II IRT lines. There are also a large number of photosphericlines, which can also be affected by chromospheric activity, andtemperature-sensitive photospheric features such as TiO bands. Thespectra have been compiled with the goal of providing a set of standardsobserved at medium resolution. We have extensively used such data forthe study of active chromosphere stars by applying a spectralsubtraction technique. However, the data set presented here can also beutilized in a wide variety of ways ranging from radial velocitytemplates to study of variable stars and stellar population synthesis.This library can also be used for spectral classification purposes anddetermination of atmospheric parameters (T_eff, logg, [Fe/H]). A digitalversion of all the fully reduced spectra is available via ftp and theWorld Wide Web (WWW) in FITS format.
| Time Evolution of the Magnetic Activity Cycle Period. II. Results for an Expanded Stellar Sample
We further explore nondimensional relationships between the magneticdynamo cycle period P_cyc, the rotational period P_rot, the activitylevel (as observed in Ca II HK), and other stellar properties byexpanding the stellar sample studied in the first paper in this series.We do this by adding photometric and other cycles seen in active starsand the secondaries of CV systems and by selectively adding less certaincycles from the Mount Wilson HK survey; evolved stars, long-term HKtrends and secondary P_cyc are also considered. We confirm that moststars with age t>~0.1 Gyr occupy two roughly parallel branches,separated by a factor of ~6 in P_cyc, with the ratio of cycle androtational frequencies ω_cyc/Ω~Ro^-0.5, where Ro is theRossby number. Using the model of the first paper in this series, thisresult implies that the α effect increases with mean magneticfield (contrary to the traditional α-quenching concept) and thatα and ω_cyc decrease with t. Stars are not strictlysegregated onto one or the other branch by activity level, though thehigh-ω_cyc/Ω branch is primarily composed of inactive stars.The expanded data set suggests that for t>~1 Gyr, stars can havecycles on one or both branches, though among older stars, those withhigher (lower) mass tend to have their primary P_cyc on the lower(upper) ω_cyc/Ω branch. The Sun's ~80 yr Gleissberg cycleagrees with this scenario, suggesting that long-term activity ``trends''in many stars may be segments of long (P_cyc~50-100 yr) cycles not yetresolved by the data. Most very active stars (P_rot<3 days) appear tooccupy a new, third branch with ω_cyc/Ω~Ro^0.4. Many RS CVnvariables lie in a transition region between the two most activebranches. We compare our results with various models, discuss theirimplications for dynamo theory and evolution, and use them to predictP_cyc for three groups: stars with long-term HK trends, stars in youngopen clusters, and stars that may be in Maunder-like magnetic minima.
| CCD spectra of MK standards and a preliminary extension of the MK classification to the yellow-red region.
Not Available
| 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
| High resolution spectroscopy over lambda lambda 8500-8750 Å for GAIA. I. Mapping the MKK classification system
We present an Echelle+CCD high resolution spectroscopic atlas (0.25Ä/pix dispersion, 0.43 Ä FWHM resolution and 20 000 resolvingpower) mapping the MKK classification system over the interval lambdalambda 8500-8750 Ä. The wavelength interval is remarkably free fromtelluric lines and it is centered on the near-IR triplet of Ca II, thehead of hydrogen Paschen series and several strong metallic lines. Thespectra of 131 stars of types between O4 and M8 and luminosity classes Ithrough V are included in the atlas. Special care was put in maintainingthe highest instrumental homogeneity over the whole set of data. Thecapability to derive accurate MKK spectral types from high resolutionobservations over the interval lambda lambda 8500-8750 Ä isdiscussed. The observations have been performed as part of an evaluationstudy of possible spectroscopic performances for the astrometric missionGAIA planned by ESA. Tables~3 and 4 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/ Abstract.html}\fnmsep\thanks{ Thespectra of the stars listed in Table~2 are also available in electronicform at the CDS or via the personal HomePagehttp://ulisse.pd.astro.it/Astro/Atlases/}\fnmsep\thanks{ Figures 3--28are only available in electronic form at http://www.edpsciences.com
| Brightening of the T Tauri star RY Tauri in 1996. Photometry, polarimetry and high-resolution spectroscopy
The T Tauri star RY Tau has increased its brightnessfrom V=10fm6 to V=9fm6 in October-November 1996. By February-March 1997,the star has faded again to V=10fm8 . High-resolution échellespectra of RY Tau were obtained with the SOFIN spectrograph at theNordic Optical Telescope (La Palma, Spain) at low and high brightnesslevels of the star. No significant changes in the photospheric lines,which are sensitive to temperature and gravity, were noticed. Thespectral type of RY Tau is defined as G1-2 IV, which in combination withphotometric data implies A_V = 1fm0 -1fm3 . Polarimetric patrol of RYTau during the fading of the star showed an increase of its intrinsicpolarization from 0.5-1.0%\ at high brightness to about 2%\ at lowbrightness in the V, R and I bands. The flux radiated in Hα andthe IR Ca Ii emission lines remained about the same, in spite of the onemagnitude difference in the continuum flux. These results indicate thatvariable obscuration of the star by circumstellar dust clouds wasresponsible for the brightness change of RY Tau, and that the emissionline source is mostly outside of the obscured region. Tables~2 and 3 areonly 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
| Patterns of Variation among Sun-like Stars
We examine the patterns of variation among a sample of 35 stars thatincludes the Sun, particularly on the timescale of the 11 yr solaractivity cycle. Our investigation uses contemporaneous photometric andchromospheric HK emission time series measurements from the Lowell andMount Wilson Observatories, and comparable solar data. We find that thephotometric and HK variability of the stars in our sample can be relatedto their average level of chromospheric activity by power laws. Thephotometric variability of the Sun may be somewhat subdued for itsaverage activity level. We find that the younger, more active stars inour sample tend to become fainter as their HK emission increases,whereas the older, less active stars tend to become brighter as their HKemission increases, as the Sun does during its activity cycle.
| X-Ray Activity in the Open Cluster IC 4665
We present the results of a joint ROSAT High Resolution Imager (HRI) andoptical investigation of the open cluster IC 4665. The ROSAT datacontains detections for 28 stellar sources in the field, including 22cluster members and candidate members spanning the color range -0.18<= (B-V)0 <= 1.63 (~B3-M3). Upper limits are given for theremaining members (or candidate members) in the HRI field. Keck HIRESspectra have been obtained that yield radial and rotational velocitymeasures, respectively, for faint, low-mass candidate members locatedwithin the field of the ROSAT HRI observation. In addition, photometryof possible optical counterparts to previously uncataloged X-ray sourcesin the HRI field is presented. The trends in X-ray properties with B-Vcolor in IC 4665 are found to be quite similar to that for other, morenearby young clusters such as the Pleiades and alpha Persei. Inparticular a maximum in normalized X-ray luminosity of log (LX/Lbol) ~=-3 is observed, beginning in the color range of (B-V)0 = 0.7-0.8. Thisis similar to the corresponding color range among Pleiades members, inagreement with the earlier estimate by Prosser & Giampapa that theage of IC 4665 is similar to the age of the Pleiades. The correlation ofrotation and X-ray emission levels is consistent with that in otheryoung clusters. Among the high-mass stars in IC 4665, five B stars aredetected as X-ray sources. Of these, one is a spectroscopic binary whilethe remaining objects are apparently single stars. The level ofintrinsic X-ray emission observed in the rapidly rotating (v sin i >200 km s-1), single B stars is consistent with an origin due to shockheating of the ambient medium by radiatively driven, rotationallyenhanced winds. On the basis of these observations and the results forother clusters, we argue that observed levels of X-ray emission inhigh-mass stars of log (LX/Lbol) > -6.0 are likely due to intrinsicemission associated with the B star itself rather than emission from alate-type secondary. Finally, our results suggest that the initialangular momentum distributions and the subsequent evolution of rotation,LX, and (LX/Lbol) occur in practically identical fashions in clusterswith ages <~100 Myr.
| Increase of emission and shell features in the spectrum of the FUor V1057 CYG
High-resolution echelle spectra of the FUor V1057 Cyg were taken duringits minimum brightness in 1996-97. The spectra show the appearance ofstrong shell-components in the lines of low excitation, an increase ofthe splitting of the photospheric absorption lines, and the appearanceof weak emission lines. It is argued that the mysterious line splittingof the absorption lines in the spectrum of V1057 Cyg, which has beeninterpreted as originating from an accretion disk, can be explainedsimply by the presence of central emission cores in the broad absorptionlines. This emission spectrum of V1057 Cyg has increased in strengthrelative to the continuum as the star has faded. Based on observationsobtained at the 2.56m Nordic Optical Telescope, Roque de Los MuchachosObservatory, La Palma/Spain.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Herkules |
| Right ascension: | 17h43m21.60s |
| Declination: | +24°19'40.0" |
| Apparent magnitude: | 5.71 |
| Distance: | 38.27 parsecs |
| Proper motion RA: | -115.2 |
| Proper motion Dec: | 73.2 |
| B-T magnitude: | 6.544 |
| V-T magnitude: | 5.805 |
Catalogs and designations:
|
INHABITED SKY
