Contents
Images
Upload your image
DSS Images Other Images
Related articles
Chromospheric Activity in G and K Main-Sequence Stars, and What It Tells Us about Stellar Dynamos
For main-sequence G and K stars we study again the empirical relationsbetween the periods of the activity cycles, Pcyc, and therotational periods, prot. We use the high-quality dataselected by Brandenburg, Saar, and Turpin. As found by those authors``the Pcyc increase proportional to the prot,along two distinctly different sequences,'' the active ``A'' sequence,and the inactive ``'I'' sequence with cooler and more slowly rotatingstars. It is found here that along each sequence the number of rotationperiods per activity cycle is nearly the same, but the numbers aredifferent for the different sequences, indicating that probablydifferent kinds of dynamos are working for the stars on the differentsequences. The transition from one sequence to the other occurs at arotation period of 21 days. The rotation periods then increase abruptlyby about a factor of 2 for the cooler stars. We suggest that thisindicates abruptly increased deep mixing. Along the I sequence theoverall dependence of the Ca II emission line fluxes, F(Ca II), onrotation and Teff is consistent with F(CaII)~T4effp-4/3rot. For theA-sequence stars the dependence of F(Ca II) on rotation seems to bestronger than for the I-sequence stars.
| The Activity and Variability of the Sun and Sun-like Stars. I. Synoptic Ca II H and K Observations
Synoptic measurements of activity in Sun-like stars have been performedcontinuously since 1966, and the largest set comes from the Mount WilsonHK project, in the form of the well-known S index. We have beenmonitoring the activity and variability of the Sun and a large sample ofSun-like stars, in terms of S and absolute flux, since 1994 with theSolar-Stellar Spectrograph (SSS) at Lowell Observatory. Directlyinspired by the similar long-term program at Mount Wilson Observatory,the SSS incorporates both an HK spectrograph and an echelle for visibleand far-red observations. This is the first of three papers presentingthe results of some 20,000 observations of the Sun and Sun-like starswith the SSS. In this paper we describe our program, review thecalibration of solar and stellar fluxes to S and the chromosphericemission fraction R'HK, compare our derived stellar activitymeasures to those from other programs, and discuss the broadcharacteristics of the activity and variability in our target set, withparticular attention to good solar analogs and noncycling stars. Insubsequent papers we will discuss the echelle data and present detailedexaminations of stars of particular interest.
| A non-LTE study of neutral and singly-ionized calcium in late-type stars
Aims.Non-local thermodynamical equilibrium (NLTE) line formation forneutral and singly-ionized calcium is considered through a range ofspectral types when the Ca abundance varies from the solar value down to[Ca/H] = -5. We evaluate the influence of departures from LTE on Caabundance determinations and inspect the possibility of using Ca I / CaII line-strength ratios as indicators of surface gravity for extremelymetal-poor stars. Methods: .A comprehensive model atom for Ca Iand Ca II is presented. Accurate radiative and electron collisionalatomic data are incorporated. The role of inelastic collisions withhydrogen atoms in the statistical equilibrium of Ca I/II is estimatedempirically from inspection of their different influences on the Ca Iand Ca II lines in selected stars with well determined stellarparameters and high-quality observed spectra. Results: .Thedependence of NLTE effects on the atmospheric parameters is discussed.Departures from LTE significantly affect the profiles of Ca I lines overthe whole range of stellar parameters being considered. However, at[Ca/H] ≥ -2, NLTE abundance correction of individual lines have alow absolute value due to the different influence of NLTE effects online wings and the line core. At lower Ca abundances, NLTE leads tosystematically depleted total absorption in the line and positiveabundance corrections, exceeding +0.5 dex for Ca I λ 4226 at[Ca/H] = -4.9. In contrast, the NLTE effects strengthen the Ca II linesand lead to negative abundance corrections. NLTE corrections are small,≤0.02 dex, for the Ca II resonance lines, and they grow in absolutevalue with decreasing Ca abundance for the IR lines of multiplet 3d-4p,exceeding 0.4 dex in the metal-poor models with [Fe/H] ≤ -3. As atest and first application of the Ca I/II model atom, Ca abundances aredetermined on the basis of plane-parallel LTE model atmospheres for theSun, Procyon (F IV-V), and seven metal-poor stars, using high S/N andhigh-resolution spectra at visual and near-IR wavelengths. Lines of Ca Iand Ca II give consistent abundances for all objects (except Procyon)when collisions with hydrogen atoms are taken into account. The derivedabsolute solar Ca abundance (from Ca I and Ca II lines) is log\varepsilon_Ca,ȯ = 6.38 ± 0.06. For Procyon, the mean Caabundance from Ca I lines is markedly subsolar, [Ca/H] = -0.14 ±0.03. All metal-poor stars within our sample show an overabundance ofcalcium relative to iron with [Ca/Fe] abundance ratios of 0.26 to 0.46that are typical of the halo population. The W(Ca I 4226) / W(Ca II8498) equivalent width ratio is predicted to be sensitive to surfacegravity for extremely metal-poor stars, while this is not the case forthe ratio involving the Ca II resonance line(s).
| New Debris Disks around Nearby Main-Sequence Stars: Impact on the Direct Detection of Planets
Using the MIPS instrument on Spitzer, we have searched for infraredexcesses around a sample of 82 stars, mostly F, G, and K main-sequencefield stars, along with a small number of nearby M stars. These starswere selected for their suitability for future observations by a varietyof planet-finding techniques. These observations provide information onthe asteroidal and cometary material orbiting these stars, data that canbe correlated with any planets that may eventually be found. We havefound significant excess 70 μm emission toward 12 stars. Combinedwith an earlier study, we find an overall 70 μm excess detection rateof 13%+/-3% for mature cool stars. Unlike the trend for planets to befound preferentially toward stars with high metallicity, the incidenceof debris disks is uncorrelated with metallicity. By newly identifyingfour of these stars as having weak 24 μm excesses (fluxes ~10% abovethe stellar photosphere), we confirm a trend found in earlier studieswherein a weak 24 μm excess is associated with a strong 70 μmexcess. Interestingly, we find no evidence for debris disks around 23stars cooler than K1, a result that is bolstered by a lack of excessaround any of the 38 K1-M6 stars in two companion surveys. Onemotivation for this study is the fact that strong zodiacal emission canmake it hard or impossible to detect planets directly with futureobservatories such as the Terrestrial Planet Finder (TPF). Theobservations reported here exclude a few stars with very high levels ofemission, >1000 times the emission of our zodiacal cloud, from directplanet searches. For the remainder of the sample, we set relatively highlimits on dust emission from asteroid belt counterparts.
| Metallicity and absolute magnitude calibrations for UBV photometry
Calibrations are presented here for metallicity ([Fe/H]) in terms of theultraviolet excess, [δ(U - B) at B - V = 0.6, hereafterδ0.6], and also for the absolute visual magnitude(MV) and its difference with respect to the Hyades(ΔMHV) in terms of δ0.6 and(B - V), making use of high-resolution spectroscopic abundances from theliterature and Hipparcos parallaxes. The relation[Fe/H]-δ0.6 has been derived for dwarf plus turn-offstars, and also for dwarf, turn-off, plus subgiant stars classifiedusing the MV-(B - V)0 plane of Fig. 11, which iscalibrated with isochrones from Bergbusch & VandenBerg (and alsoVandenBerg & Clem). The [Fe/H]-δ0.6 relations inour equations (5) and (6) agree well with those of Carney, as can beseen from Fig. 5(a). Within the uncertainties, the zero-points,+0.13(+/-0.05) of equation (5) and +0.13(+/-0.04) of equation (6), arein good agreement with the photometric ones of Cameron and of Carney,and close to the spectroscopic ones of Cayrel et al. and of Boesgaard& Friel for the Hyades open cluster. Good quantitative agreementbetween our estimated [Fe/H] abundances with those from uvby-βphotometry and spectroscopic [Fe/H]spec values demonstratesthat our equation (6) can be used in deriving quality photometric metalabundances for field stars and clusters using UBV data from variousphotometric surveys.For dwarf and turn-off stars, a new hybrid MV calibration ispresented, based on Hipparcos parallaxes withσπ/π <= 0.1 and with a dispersion of +/-0.24in MV. This hybrid MV calibration containsδ0.6 and (B - V) terms, plus higher order cross-termsof these, and is valid for the ranges of +0.37 <= (B - V)0<= +0.88,- 0.10 <= δ0.6 <= +0.29 and 3.44<= MV <= 7.23. For dwarf and turn-off stars, therelation for ΔMHV is revised and updated interms of (B - V) and δ0.6, for the ranges of -0.10<= δ0.6 <= +0.29, and +0.49 <= (B -V)0 <= +0.89, again making use of Hipparcos parallaxeswith σπ/π <= 0.1. These parallaxes formetal-poor dwarf and turn-off stars in our sample reveal that thedifference of ΔMHV(B - V) relative to Hyadesat (B - V) = +0.70 should be 1.37mag, instead of the 1.58mag given byLaird et al. In general, Hipparcos parallaxes are larger thanground-based ones, causing a divergence of ourΔMHV(B - V,δ0.6) relation(the solid line in Fig. 15b), from the one of Laird et al. (the dashedline) for the range +0.10 <= δ0.6 <= +0.29 ourabsolute magnitudes are fainter, as has been confirmed for localsubdwarfs by Reid. Our final calibrations forΔMHV(B - V, δ0.6),equations (16) and (17), are third-order polynomials inδ0.6, pass through the origin, and provide photometricdistances in reasonable agreement with those obtained directly fromHipparcos parallaxes (Fig. 18).
| Barium even-to-odd isotope abundance ratios in thick disk and thin disk stars
We present the Ba even-to-odd isotope abundance ratios in 25 cool dwarfstars with the metallicity [Fe/H] ranging between 0.25 and -1.35. Ourmethod takes advantage of the hyperfine structure (HFS) affecting the BaII resonance line of the odd isotopes. The fractional abundance of theodd isotopes of Ba is derived from a requirement that Ba abundances fromthe resonance line λ 4554 and subordinate lines λ 5853 andλ 6496 must be equal. The results are based on NLTE lineformation and analysis of high resolution (R 60 000) highsignal-to-noise ({S/N} ≥ 200) observed spectra. We find that thefraction of the odd isotopes of Ba grows toward the lower Ba abundance(or metallicity) and the mean value in the thick disk stars equals 33± 4%. This indicates the higher contribution of the r-process tobarium in the thick disk stars compared to the solar system matter. Theobtained fraction increases with the [Eu/Ba] abundance ratio growth inagreement with expectations. A significant fraction of the even isotopesof Ba found in old Galactic stars (the thick disk stars), 67%, is incontrast to the prediction of the "classical" model of the s-process andfavors the value predicted by the "stellar" models of Arlandini et al.(1999) and Travaglio et al. (1999).
| Spectrometric composition of nearby K dwarfs
We have obtained relatively high resolution spectra of Northernhemisphere K dwarfs. This is the first spectrometric project dedicatedonly to K dwarfs. Earlier studies have concentrated on more massive Fand G dwarfs. However, these stars have already undergone evolutionaryeffects, unlike K dwarfs, which offer more accurate information aboutthe evolution of the Solar neighbourhood. We have determined the LTEabundances of 14 elements for 42 stars with initial metallicity rangecovered by -1.52 < [Fe/H] < 0.48. We confirm the discrepancy inthe abundances derived from neutral and ionized lines. The solution tothis problem cannot just be the modification of initial physicalparameters, but requires fundamental changes in the modeling of Kdwarfs.
| The activity of late-type stars: the Sun among stars with cyclic activity
The coronal and chromospheric emission of several hundred late-typestars whose activity was recently detected are analyzed. This confirmsthe previous conclusion for stars of HK project that there exist threegroups of objects: active red M dwarfs, G-K stars with cyclic activity,and stars exhibiting high but irregular activity. The X-ray fluxes,EUV-spectra, and X-ray cycles can be used to study the main property ofstellar coronas—the gradual increase in the number ofhigh-temperature (T ≥ 10 MK) regions in the transition from the Sunto cyclically active K dwarfs and more rapidly rotating F and G starswith irregular activity. The level of X-ray emission is closely relatedto the spottedness of the stellar surface. The correlation between thechromospheric and coronal emission is weak when the cycles arewell-defined, but becomes strong when the activity is less regular.Unexpectedly, stars whose chromospheric activity is even lower than thatof the Sun are fairly numerous. Common and particular features of solaractivity among the activity of other cyclically active stars arediscussed. Our analysis suggests a new view of the problem of heatingstellar coronas: the coronas of stars with pronounced cycles areprobably heated by quasistationary processes in loops, while prolongednonstationary coronal events are responsible for heating the coronas ofF and G stars with high but irregular activity.
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars
Aims.Parameters for 55 nearby metal-poor stars are determined usinghigh-resolution spectroscopy. Together with similar data taken from arecent analysis, they are used to show trends of their Galacticevolution with stellar [Fe/H] or [Mg/H] abundances. The separation ofabundance ratios between disk and halo stars is used as a basiccriterion for population membership. Methods.After carefulselection of a clean subsample free of suspected or known binaries andpeculiar stars, abundances of Mg, Na and Al are based on NLTE kineticequilibrium calculations applied to spectrum synthesis methods. Results.The relation between [Na/Mg] and [Fe/H] is a continuousenrichment through all three Galactic populations spanning a range ofvalues between a metal-poor plateau at [ Na/Mg] = -0.7 and solar values.[Al/Mg] displays a step-like difference between stars of the Galactichalo with overline[Al/Mg] -0.45 and the two disk populations withoverline[Al/Mg] +0.10. [Al/Mg] ratios, together with the [Mg/Fe]ratios, asymmetric drift velocities V, and stellar evolutionary ages,make possible the individual discrimination between stars of the thickdisk and the halo. At present, this evidence is limited by the smallnumber of stars, and by the theoretical and empirical uncertainties ofstellar age determinations, but it achieves a high significance. Conclusions.While the stellar sample is not complete with respect tospace volume, the resulting abundances indicate the necessity to revisecurrent models of chemical evolution to allow for an adequate productionof Al in early stellar generations.
| On the Predicted and Observed Color Boundaries of the RR Lyrae Instability Strip as a Function of Metallicity
The purpose of this paper is to predict the temperature at thefundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL)variables from the pulsation equation that relates temperature toperiod, luminosity, and mass. Modern data for the correlations betweenperiod, luminosity, and metallicity at the FBE for field and cluster RRLstars are used for the temperature calculation. The predictedtemperatures are changed to B-V colors using an adopted colortransformation. The predicted temperatures at the FBE become hotter as[Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicitydecreases to -2.5. The temperature range over this interval ofmetallicity is ΔlogTe=0.04, or 640 K at 6900 K. Thepredicted color variation is at the level of 0.03 mag in B-V over mostof this range. The predictions are compared with the observed RRL colorsat the FBE for both the field and cluster variables, showing generalagreement at the level of 0.02 mag in (B-V)0, which, however,is the uncertainty of the reddening corrections. The focus of theproblem is then reversed by fitting a better envelope to the observedFBE relation between color and metallicity for metallicities smallerthan -1.8, which, when inserted in the pulsation equation, gives anonlinear calibration of the absolute magnitude of the average evolvedlevel of the horizontal branch (HB) ofMV=1.109+0.600([Fe/H])+0.140([Fe/H])2, where thezero point has been set by the observed RR Lyrae stars in the LMC at0=19.064 by Clementini et al. for [Fe/H]=-1.5, andusing an adopted LMC modulus of (m-M)0=18.54 from Tammann etal. that is independent of the LMC Cepheids. This equation givesMV=0.52 at [Fe/H]=-1.5.However, the calibration fails for the extreme second-parameter clustersNGC 6388 and NGC 6441. Proof that the MV absolute magnitudesfor their RR Lyrae variables are ~0.4 mag brighter than the calibrationequation predicts is from the unusually long periods for givenamplitudes at their high metallicities of [Fe/H]~-0.5. Allsecond-parameter clusters are believed to violate the equation, but lessseverely than these two extreme examples. An additional complication inusing RRL stars as distance indicators at the 0.1 mag level is shown bythe difference of ΔlogP=0.029+/-0.007 in the position of theenvelope locus at the shortest periods for the observedperiod-metallicity correlation between the field and cluster variables.The field variables have shorter periods than cluster variables at theenvelope. This requires the cluster RRL stars to be 0.09 mag brighterthan the field variables at the same temperature and mass, or to have atemperature difference of ΔlogTe=0.008 at fixedluminosity and mass. The field and cluster variables also differ in thenear absence of cluster RR Lyrae stars in the -1.7>[Fe/H]>-2.0metallicity interval, whereas the field variables show no such gap. Atest is proposed for different origins for the field and the clustervariables by comparing the morphology of the HBs in the local dwarfspheroidal galaxies with that in the Galactic globular clusters in theinner halo and by relating the differences with the relevantsecond-parameter indicators.
| Mg Isotope Ratios in Giant Stars of the Globular Clusters M13 and M71
We present Mg isotope ratios in four red giants of the globular clusterM13 and one red giant of the globular cluster M71 based onhigh-resolution, high signal-to-noise ratio spectra obtained with HDS onthe Subaru Telescope. We confirm earlier results by Shetrone that forM13 the ratio varies from(25Mg+26Mg)/24Mg~=1 in stars with thehighest Al abundance to(25Mg+26Mg)/24Mg~=0.2 in stars with thelowest Al abundance. However, we separate the contributions of all threeisotopes and find a considerable spread in the ratio24Mg:25Mg:26Mg, with values rangingfrom 48:13:39 to 78:11:11. As in NGC 6752, we find a positivecorrelation between 26Mg and Al, an anticorrelation between24Mg and Al, and no correlation between 25Mg andAl. In M71, our one star has a Mg isotope ratio of 70:13:17. For bothclusters, even the lowest ratios of 25Mg/24Mg and26Mg/24Mg exceed those observed in field stars atthe same metallicity, a result also found in NGC 6752. The contributionof 25Mg to the total Mg abundance is constant within a givencluster and between clusters with25Mg/(24Mg+25Mg+26Mg)~=0.13.For M13 and NGC 6752, the ranges of the Mg isotope ratios are similarand both clusters show the same correlations between Al and Mg isotopes,suggesting that the same process is responsible for the abundancevariations in these clusters. While existing models fail to reproduceall the observed abundances, we continue to favor the scenario in whichtwo generations of asymptotic giant branch (AGB) stars produce theobserved abundances. A first generation of metal-poor AGB stars pollutesthe entire cluster and is responsible for the large ratios of25Mg/24Mg and 26Mg/24Mgobserved in cluster stars with compositions identical to field stars atthe same metallicity. Differing degrees of pollution by a secondgeneration of AGB stars of the same metallicity as the cluster providesthe star-to-star scatter in Mg isotope ratios.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Ages of White Dwarf-Red Subdwarf Systems
We provide the first age estimates for two recently discovered whitedwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These unusualsystems provide a new opportunity for linking the reliable age estimatesfor the white dwarfs to the (measurable) metallicities of the redsubdwarfs. We have obtained precise photometry in theVJRKCIKCJH bands and spectroscopycovering from 6000 to 9000 Å for the two new systems, as well asfor a comparison white dwarf-main-sequence red dwarf system, GJ 283 AB.Using model grids available in the literature, we estimate the coolingages, as well as temperatures, surface gravities, masses, progenitormasses, and total lifetimes of the white dwarfs. The results indicatethat the two new systems are probably ancient thick-disk objects withages of at least 6-9 Gyr. We also conduct searches of red dwarf andwhite dwarf compendia from SDSS data, Lépine Shara Proper Motion(LSPM) catalog, and Chanamé & Gould for additional commonproper motion white dwarf-red subdwarf systems. Only seven new candidatesystems are found, which indicates the rarity of these systems.
| 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.
| UV Ceti - Sterne.
Not Available
| Galactic structure from the Calar Alto Deep Imaging Survey (CADIS)
We used 1627 faint (15.5≤ R≤ 23) stars in five fields of theCalar Alto Deep Imaging Survey (CADIS) to estimate the structureparameters of the Galaxy. The results were derived by applying twocomplementary methods: first by fitting the density distributionfunction to the measured density of stars perpendicular to the Galacticplane, and second by modelling the observed colors and apparentmagnitudes of the stars in the field, using Monte Carlo simulations. Thebest-fitting model of the Galaxy is then determined by minimising theC-statistic, a modified χ^2. Our model includes a double exponentialfor the stellar disk with scaleheights h1 and h2and a power law halo with exponent α. 24 480 different parametercombinations have been simulated. Both methods yield consistent results:the best fitting parameter combination is α=3.0 (or α=2.5,if we allow for a flattening of the halo with an axial ratio of(c/a)=0.6), h_1=300 pc, h_2=900 pc, and the contribution of thick diskstars to the disk stars in the solar neighbourhood is found to bebetween 4 and 10%.
| Lithium and Lithium Depletion in Halo Stars on Extreme Orbits
We have determined Li abundances in 55 dwarfs and subgiants that aremetal-poor (-3.6<[Fe/H]<-0.7) and have extreme orbital kinematics.Our purpose is to examine the Li abundance in the Li plateau stars andits decrease in low-temperature, low-mass stars. For the stars in oursample we have determined chemical profiles given in 2002 by Stephens& Boesgaard. The Li observations are primarily from the echellespectrograph on the 10 m Keck I telescope, with HIRES covering 4700-6800Å with a spectral resolution of ~48,000. The spectra have highsignal-to-noise ratios, from 70 to 700 pixel-1, with a medianof 140. The Li I resonance doublet was detected in 42 of the 55 stars.Temperatures were found spectroscopically by Stephens & Boesgaard.Abundances or upper limits were determined for all stars, with typicalerrors of 0.06 dex. Corrections for the deviations from nonlocalthermodynamical equilibrium for Li in the stellar atmospheres have beenmade, which range from -0.04 to +0.11 dex. Our 14 dwarf and turnoffstars on the Li plateau with temperatures greater than 5700 K and[Fe/H]<-1.5 give A(Li)=logN(Li)/N(H)+12.00 of 2.215+/-0.110,consistent with earlier results. We find a dependence of the Liabundance on metallicity as measured by [Fe/H] and the Fe-peak elementsCr and Ni, with a slope of ~0.18. We have examined the possible trendsof A(Li) with the chemical abundances of other elements and find similardependences of A(Li) with the α-elements Mg, Ca, and Ti. Theseslopes are slightly steeper at ~0.20, resulting from an excess in[α/Fe] with decreasing [Fe/H]. For the n-capture, rare-earthelement Ba, we find a relation between A(Li) and [Ba/H] that has ashallower slope of ~0.13 over a range of 2.6 dex in [Ba/H], the Liabundance spans only a factor of 2. We have also examined the possibletrends of A(Li) with the characteristics of the orbits of our halostars. We find no trends in A(Li) with kinematic or dynamic properties.For the stars with temperatures below the Li plateau, there are severalinteresting results. The group of metal-poor stars possess, on average,more Li at a given temperature than metal-rich stars. When we divide thecool stars into smaller subsets with similar metallicities, we findtrends of A(Li) with temperature for the different metallicity groups.The decrease in A(Li) sets in at hotter temperatures for the highermetallicity stars than for the lower metallicity stars. The increased Lidepletion in cooler stars could be a result of the increased action ofconvection, since cooler stars have deeper convection zones. This wouldalso make it easier for additional mixing mechanisms, such as thoseinduced by rotation, to have a greater effect in cooler stars. Since themodel depth of the convection zone is almost independent of metallicityat a given effective temperature, the apparent metallicity dependence ofthe Li depletion in our data may be pointing to subtle but poorlyunderstood mixing effects in low-mass halo dwarfs. Predictions for Lidepletion from standard and nonstandard models seem to underestimate thedegree of depletion inferred from the observations of the cool stars.
| CCD Photometry of M92
We present Johnson B and V photometry for the galactic globular clusterM92 (NGC 6341). Photometric results for a total of 30,000 starsare obtained and are plotted on a V versus (B-V) diagram. We fittheoretical isochrones to this diagram in order to get an estimate forthe age of M92. The age which we find is 16×109years with the following values for the metallicity and He-abundance:[Fe/H] = -2.03, Y = 0.235. The distance modulus to this cluster turnsout to be m - M = 14.6 in accordance with that obtained by Stetson &Harris (1988). We also perform stellar counts in order to produce aluminosity function which is successfully fitted by the same theoreticalmodels fitted to the colour-magnitude diagram.
| Predicting the Length of Magnetic Cycles in Late-Type Stars
In this paper we present a modification of a local approximation of theso-called interface dynamo in an attempt to reproduce the length of themagnetic cycles for a sample of late-type stars. The sample consists of25 stars, observed during the Mount Wilson and Las Campanas long-termmonitoring campaigns, for which well-defined cycles have been detected.We have focused our efforts on reproducing general trends observed,namely, the dependence of the cycle length, Pcyc, on thestellar rotation period, Prot, rather than attempting toinfer from the dynamo model individual cycle lengths for each star. Inspite of the simplicity of the model, the results are promising. Thetrend of increasing cycle length with increasing rotation period isreproduced with a minimum of assumptions.
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
| Early star formation in the Galaxy from beryllium and oxygen abundances
We investigate the evolution of the star formation rate in the earlyGalaxy using beryllium and oxygen abundances in metal poor stars.Specifically, we show that stars belonging to two previously identifiedkinematical classes (the so-called "accretion" and "dissipative"populations) are neatly separated in the [O/Fe] vs. log (Be/H) diagram.The dissipative population follows the predictions of our model ofGalactic evolution for the thick disk component, suggesting that theformation of this stellar population occurred on a timescalesignificantly longer (by a factor 5-10) than the accretioncomponent. The latter shows a large scatter in the [O/Fe] vs. log (Be/H)diagram, probably resulting from the inhomogeneous enrichment in oxygenand iron of the protogalactic gas. Despite the limitation of the sample,the data suggest that the combined use of products of spallationreactions (like beryllium) and elemental ratios of stellarnucleosynthesis products (like [O/Fe]) can constrain theoretical modelsfor the formation and early evolution of our Galaxy.
| The Planet-Metallicity Correlation
We have recently carried out spectral synthesis modeling to determineTeff, logg, vsini, and [Fe/H] for 1040 FGK-type stars on theKeck, Lick, and Anglo-Australian Telescope planet search programs. Thisis the first time that a single, uniform spectroscopic analysis has beenmade for every star on a large Doppler planet search survey. We identifya subset of 850 stars that have Doppler observations sufficient todetect uniformly all planets with radial velocity semiamplitudes K>30m s-1 and orbital periods shorter than 4 yr. From this subsetof stars, we determine that fewer than 3% of stars with-0.5<[Fe/H]<0.0 have Doppler-detected planets. Above solarmetallicity, there is a smooth and rapid rise in the fraction of starswith planets. At [Fe/H]>+0.3 dex, 25% of observed stars have detectedgas giant planets. A power-law fit to these data relates the formationprobability for gas giant planets to the square of the number of metalatoms. High stellar metallicity also appears to be correlated with thepresence of multiple-planet systems and with the total detected planetmass. This data set was examined to better understand the origin of highmetallicity in stars with planets. None of the expected fossilsignatures of accretion are observed in stars with planets relative tothe general sample: (1) metallicity does not appear to increase as themass of the convective envelopes decreases, (2) subgiants with planetsdo not show dilution of metallicity, (3) no abundance variations for Na,Si, Ti, or Ni are found as a function of condensation temperature, and(4) no correlations between metallicity and orbital period oreccentricity could be identified. We conclude that stars with extrasolarplanets do not have an accretion signature that distinguishes them fromother stars; more likely, they are simply born in higher metallicitymolecular clouds.Based on observations obtained at Lick and Keck Observatories, operatedby the University of California, and the Anglo-Australian Observatories.
| Kinematics of nearby subdwarf stars
We present an analysis of the space motions of 742 subdwarf stars basedon the sample of Carney et al. (1994, AJ, 107, 22 40). Hipparcosparallaxes, TYC2+HIP proper motions and Tycho 2 proper motions werecombined with radial velocities and metallicities from CLLA. Thekinematical behavior is discussed in particular in relation to theirmetallicities. The majority of these sample stars have metal abundancesof [Fe/H] >-1 and represent the thick disk population. The halocomponent, with [Fe/H] <-1.6, is characterized by a low mean rotationvelocity and a radially elongated velocity ellipsoid. In theintermediate metallicity range (-1.6≤ [Fe/H] ≤-1), we find asignificant number of subdwarfs with disklike kinematics. We interpretthis population of stars as a metal-weak thick disk (MWTD) population.
| Lithium Abundances of F-, G-, and K-Type Stars: Profile-Fitting Analysis of the Li I 6708 Doublet
An extensive profile-fitting analysis was performed for the Li(+Fe)6707-6708Å feature of nearby 160 F-K dwarfs/subgiants (including27 planet-host stars) in the Galactic disk ( 7000 K ≳Teff ≳ 5000 K, -1 ≲ [Fe/H] ≲ +0.4), in orderto establish the photospheric lithium abundances of these stars. Thenon-LTE effect (though quantitatively insignificant) was taken intoaccount based on our statistical equilibrium calculations, which werecarried out on an adequate grid of models. Our results confirmed most ofthe interesting observational characteristics revealed by recentlypublished studies, such as the bimodal distribution of the Li abundancesfor stars at Teff ≳ 6000 K, the satisfactory agreementof the upper envelope of the A(Li) vs. [Fe/H] distribution with thetheoretical models, the existence of a positive correlation betweenA(Li) and the stellar mass, and the tendency of lower lithium abundancesof planet-host stars (as compared to stars without planets) at thenarrow ``transition'' region of 5900 K ≳ Teff ≳5800 K. The solar Li abundance derived from this analysis is 0.92 (H =12.00), which is by 0.24dex lower than the widely referenced standardvalue of 1.16.
| Spectroscopic Study on the Atmospheric Parameters of Nearby F--K Dwarfs and Subgiants
Based on a collection of high-dispersion spectra obtained at OkayamaAstrophysical Observatory, the atmospheric parameters (Teff,log g, vt, and [Fe/H]) of 160 mid-F through early-K starswere extensively determined by the spectroscopic method using theequivalent widths of Fe I and Fe II lines along with the numericaltechnique of Takeda et al. (2002, PASJ, 54, 451). The results arecomprehensively discussed and compared with the parameter values derivedby different approaches (e.g., photometric colors, theoreticalevolutionary tracks, Hipparcos parallaxes, etc.) as well as with thepublished values found in various literature. It has been confirmed thatour purely spectroscopic approach yields fairly reliable and consistentresults.
| Stars within 15 Parsecs: Abundances for a Northern Sample
We present an abundance analysis for stars within 15 pc of the Sunlocated north of -30° declination. We have limited our abundancesample to absolute magnitudes brighter than +7.5 and have eliminatedseveral A stars in the local vicinity. Our final analysis list numbers114 stars. Unlike Allende Prieto et al. in their consideration of a verysimilar sample, we have enforced strict spectroscopic criteria in thedetermination of atmospheric parameters. Nevertheless, our results arevery similar to theirs. We determine the mean metallicity of the localregion to be <[Fe/H]>=-0.07 using all stars and -0.04 when interlopersfrom the thick disk are eliminated.
| The Rise of the s-Process in the Galaxy
From newly obtained high-resolution, high signal-to-noise ratio spectrathe abundances of the elements La and Eu have been determined over thestellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarfstars. Lanthanum is predominantly made by the s-process in the solarsystem, while Eu owes most of its solar system abundance to ther-process. The changing ratio of these elements in stars over a widemetallicity range traces the changing contributions of these twoprocesses to the Galactic abundance mix. Large s-process abundances canbe the result of mass transfer from very evolved stars, so to identifythese cases we also report carbon abundances in our metal-poor stars.Results indicate that the s-process may be active as early as[Fe/H]=-2.6, although we also find that some stars as metal-rich as[Fe/H]=-1 show no strong indication of s-process enrichment. There is asignificant spread in the level of s-process enrichment even at solarmetallicity.
| Comparison of Nuclear Starburst Luminosities between Seyfert 1 and 2 Galaxies Based on Near-Infrared Spectroscopy
We report on infrared K- (2-2.5 μm) and L-band (2.8-4.1 μm) slitspectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 μm samples. Apolycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 μm inthe L band is primarily used to investigate nuclear star-formingactivity in these galaxies. The 3.3 μm PAH emission is detected in 10sources (=43%), demonstrating that detection of nuclear star formationin a significant fraction of Seyfert 1 galaxies is now feasible. For thePAH-detected nuclei, the surface brightness values of the PAH emissionare as high as those of typical starbursts, suggesting that the PAHemission probes the putative nuclear starbursts in the dusty tori aroundthe central active galactic nuclei (AGNs). The magnitudes of the nuclearstarbursts are quantitatively estimated from the observed 3.3 μm PAHemission luminosities. The estimated starburst luminosities relative tosome indicators of AGN powers in these Seyfert 1 galaxies are comparedwith 32 Seyfert 2 galaxies in the same samples that we have previouslyobserved. We find that there is no significant difference in nuclearstarburst to AGN luminosity ratios of Seyfert 1 and 2 galaxies and thatnuclear starburst luminosity positively correlates with AGN power inboth types. Our results favor a slightly modified AGN unification model,which predicts that nuclear starbursts occurring in the dusty tori ofSeyfert galaxies are physically connected to the central AGNs, ratherthan the classical unification paradigm, in which the dusty tori simplyhide the central AGNs of Seyfert 2 galaxies and reprocess AGN radiationas infrared dust emission in Seyfert galaxies. No significantdifferences in nuclear star formation properties are recognizablebetween Seyfert 1 galaxies in the CfA and 12 μm samples.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | おおぐま座 |
| Right ascension: | 11h52m56.03s |
| Declination: | +37°43'55.0" |
| Apparent magnitude: | 6.433 |
| Proper motion RA: | 0 |
| Proper motion Dec: | 0 |
| B-T magnitude: | 7.353 |
| V-T magnitude: | 6.509 |
Catalogs and designations:
|
