Contents
Images
Upload your image
DSS Images Other Images
Related articles
G 112-29 (=NLTT 18149): A Very Wide Companion to GJ 282 AB with a Common Proper Motion, Common Parallax, Common Radial Velocity, and Common Age
We have made a search for common proper motion (CPM) companions to thewide binaries in the solar vicinity. We found that the binary GJ 282ABhas a very distant CPM companion (NLTT 18149) at a separation s =1fdg09. Improved spectral types and radial velocities are obtained, andages determined for the three components. The Hipparcos trigonometricparallaxes and the new radial velocities and ages turn out to be verysimilar for the three stars, and provide strong evidence that they forma physical system. At a projected separation of 55,733 AU from GJ 282AB,NLTT 18149 ranks among the widest physical companions known.
| Mass-radius relation of low and very low-mass stars revisited with the VLTI
We measured the radii of 7 low and very low-mass stars using longbaseline interferometry with the VLTI interferometer and its VINCI andAMBER near-infrared recombiners. We use these new data, together withliterature measurements, to examine the luminosity-radius andmass-radius relations for K and M dwarfs. The precision of the newinterferometric radii now competes with what can be obtained fordouble-lined eclipsing binaries. Interferometry provides access to muchless active stars, as well as to stars with much better measureddistances and luminosities, and therefore complements the informationobtained from eclipsing systems. The radii of magnetically quiet late-Kto M dwarfs match the predictions of stellar evolution models very well,providing direct confirmation that magnetic activity explains thediscrepancy that was recently found for magnetically active eclipsingsystems. The radii of the early K dwarfs are reproduced well for amixing length parameter that approaches the solar value, asqualitatively expected.Based on data collected with the VLTI/VINCI and VLTI/AMBER instrumentsat ESO Paranal Observatory, programmes ID 60.A-9220, 080.D-0653 and082.D-0196.
| Keck Interferometer Nuller Data Reduction and On-Sky Performance
We describe the Keck Interferometer nuller theory of operation, datareduction, and on-sky performance, particularly as it applies to thenuller exozodiacal dust key science program that was carried out between2008 February and 2009 January. We review the nuller implementation,including the detailed phasor processing involved in implementing thenull-peak mode used for science data and the sequencing used for scienceobserving. We then describe the Level 1 reduction to convert theinstrument telemetry streams to raw null leakages, and the Level 2reduction to provide calibrated null leakages. The Level 1 reductionuses conservative, primarily linear processing, implemented consistentlyfor science and calibrator stars. The Level 2 processing is moreflexible, and uses diameters for the calibrator stars measuredcontemporaneously with the interferometer's K-band cophasing system inorder to provide the requisite accuracy. Using the key science data setof 462 total scans, we assess the instrument performance for sensitivityand systematic error. At 2.0 Jy we achieve a photometrically-limitednull leakage uncertainty of 0.25% rms per 10 minutes of integration timein our broadband channel. From analysis of the Level 2 reductions, weestimate a systematic noise floor for bright stars of rms null leakageuncertainty per observing cluster in the broadband channel. A similaranalysis is performed for the narrowband channels. We also provideadditional information needed for science reduction, including detailson the instrument beam pattern and the basic astrophysical response ofthe system, and references to the data reduction and modeling tools.
| High-Resolution Spectroscopy of T Tauri Stars
Not Available
| Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars
We present interferometric angular sizes for 12 stars with knownplanetary companions, for comparison with 28 additional main-sequencestars not known to host planets. For all objects we estimate bolometricfluxes and reddenings through spectral-energy distribution (SED) fits,and in conjunction with the angular sizes, measurements of effectivetemperature. The angular sizes of these stars are sufficiently smallthat the fundamental resolution limits of our primary instrument, thePalomar Testbed Interferometer, are investigated at thesub-milliarcsecond level and empirically established based upon knownperformance limits. We demonstrate that the effective temperature scaleas a function of dereddened (V – K)0 color isstatistically identical for stars with and without planets. A usefulbyproduct of this investigation is a direct calibration of the TEFF scale for solarlike stars, as a function of both spectraltype and (V – K)0 color, with an precision of\overline{\Delta T}_{\it {(V-K)}_0} = 138\,K over the range (V –K)0 = 0.0-4.0 and \overline{\Delta T}_{SpType} = 105\,K forthe range F6V-G5V. Additionally, in an Appendix we provide SED fits forthe 166 stars with known planets which have sufficient photometryavailable in the literature for such fits; this derived "XO-Rad"database includes homogeneous estimates of bolometric flux, reddening,and angular size.
| Stellar and Circumstellar Properties of Class I Protostars
We present a study of the stellar and circumstellar properties of ClassI sources using low-resolution (R ~ 1000) near-infrared (near-IR) K- andL-band spectroscopy. We measure prominent spectral lines and features ineight objects and use fits to standard star spectra to determinespectral types, visual extinctions, K-band excesses, and water iceoptical depths. Four of the seven systems studied are close binarypairs; only one of these systems, Haro 6-10, was angularly resolvable.For certain stars, some properties found in our analysis differsubstantially from published values; we analyze the origin of thesedifferences. We determine extinction to each source using threedifferent methods and compare and discuss the resulting values. Onehypothesis that we were testing, that extinction dominates over theK-band excess in obscuration of the stellar photospheric absorptionlines, appears not to be true. Accretion luminosities and mass accretionrates calculated for our targets are highly uncertain, in part theresult of our inexact knowledge of extinction. For the six targets wewere able to place on a Hertzsprung-Russell diagram, our age estimates,<2 Myr, are somewhat younger than those from comparable studies. Ourresults underscore the value of low-resolution spectroscopy in the studyof protostars and their environments; however, the optimal approach tothe study of Class I sources likely involves a combination of high- andlow-resolution near-IR, mid-IR, and millimeter wavelength observations.Accurate and precise measurements of extinction in Class I protostarswill be key to improving our understanding of these objects.
| An evolved donor star in the long-period cataclysmic variable HS 0218+3229
Context: We present time-resolved spectroscopy and photometry of HS0218+3229, a new long-period cataclysmic variable discovered within theHamburg Quasar Survey. It is one of the few systems that allow adynamical measurement of the masses of the stellar components. Aims: We combine the analysis of time-resolved optical spectroscopyand R-band photometry with the aim of measuring the mass of the whitedwarf and the donor star and the orbital inclination. Methods:Cross-correlation of the spectra with K-type dwarf templates is used toderive the radial velocity curve of the donor star. An optimalsubtraction of the broadened templates is performed to measure therotational broadening and constrain the spectral type of the donor.Finally, an ellipsoidal model is fitted to the R-band light curve toobtain constraints upon the orbital inclination of the binary system. Results: The orbital period of HS 0218+3229 is found to be0.297229661 ± 0.000000001 d (7.13351186 ± 0.00000002 h),and the amplitude of the donor's radial velocity curve is K2= 162.4 ± 1.4 km~s-1. Modelling the ellipsoidal lightcurves gives an orbital inclination in the range i = 59° ±3°. A rotational broadening between 82.4 ± 1.2km~s-1 and 89.4 ± 1.3 km~s-1 is found whenassuming zero and continuum limb darkening, respectively. The secondarystar has most likely a spectral type K5 and contributes ~80-85% to theR-band light. Our analysis yields a mass ratio of 0.52 < q < 0.65,a white dwarf mass of 0.44 < M1 (M_ȯ) < 0.65, anda donor star mass of 0.23 < M2 (M_ȯ) < 0.44. Conclusions: We find that the donor star in HS 0218+3229 issignificantly undermassive for its spectral type. It is therefore verylikely that it has undergone nuclear evolution prior to the onset ofmass transfer.
| Spectroscopic investigation of stars on the lower main sequence
Aims. The aim of this paper is to provide fundamental parameters andabundances with a high accuracy for a large sample of cool main sequencestars. This study is part of wider project, in which the metallicitydistribution of the local thin disc is investigated from a completesample of G and K dwarfs within 25 pc. Methods: The stars were observedat high resolution and a high signal-to-noise ratio with the ELODIEechelle spectrograph. The V sin i were obtained with a calibration ofthe cross-correlation function. Effective temperatures were estimated bythe line depth ratio method. Surface gravities (log g) were determinedby two methods: parallaxes and ionization balance of iron. The Mg and Naabundances were derived using a non-LTE approximation. Abundances ofother elements were obtained by measuring equivalent widths. Results:Rotational velocities, atmospheric parameters (T_eff, log g, [Fe/H],V_t), and Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Co, Ni, and Znabundances are provided for 131 stars. Among them, more than 30 starsare active stars with a fraction of BY Dra and RS CVn type stars forwhich spectral peculiarities were investigated. We find the meanabundances of the majority of elements in active and nonactive stars tobe similar, except for Li, and possibly for Zn and Co. The lithium isreliably detected in 54% of active stars but only in 20% of nonactivestars. No correlation is found between Li abundances and rotationalvelocities. A possible anticorrelation of log A(Li) with the index ofchromospheric activity GrandS is observed. Conclusions: Active andnonactive cool dwarfs show similar dependencies of most elemental ratiosvs. [Fe/H]. This allows us to use such abundance ratios to study thechemical and dynamical evolution of the Galaxy. Among active stars, noclear correlation has been found between different indicators ofactivity for our sample stars.Based on spectra collected with the ELODIE spectrograph at the 1.93-mtelescope of the Observatoire de Haute Provence (France). Tables A.1-A3are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/489/923
| A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192
We report the detection of an extrasolar planet of mass ratioq~2×10-4 in microlensing event MOA-2007-BLG-192. Thebest-fit microlensing model shows both the microlensing parallax andfinite source effects, and these can be combined to obtain the lensmasses of M=0.060+0.028-0.021 Msolarfor the primary and m=3.3+4.9-1.6M⊕ for the planet. However, the observational coverageof the planetary deviation is sparse and incomplete, and the radius ofthe source was estimated without the benefit of a source star colormeasurement. As a result, the 2 σ limits on the mass ratio andfinite source measurements are weak. Nevertheless, the microlensingparallax signal clearly favors a substellar mass planetary host, and themeasurement of finite source effects in the light curve supports thisconclusion. Adaptive optics images taken with the Very Large Telescope(VLT) NACO instrument are consistent with a lens star that is either abrown dwarf or a star at the bottom of the main sequence. Follow-up VLTand/or Hubble Space Telescope (HST) observations will either confirmthat the primary is a brown dwarf or detect the low-mass lens star andenable a precise determination of its mass. In either case, the lensstar, MOA-2007-BLG-192L, is the lowest mass primary known to have acompanion with a planetary mass ratio, and the planet,MOA-2007-BLG-192Lb, is probably the lowest mass exoplanet found to date,aside from the lowest mass pulsar planet.
| High-dispersion absorption-line spectroscopy of AE Aqr
High-dispersion time-resolved spectroscopy of the unique magneticcataclysmic variable AE Aqr is presented. A radial velocity analysis ofthe absorption lines yields K2 = 168.7 +/- 1kms-1.Substantial deviations of the radial velocity curve from a sinusoid areinterpreted in terms of intensity variations over the secondary star'ssurface. A complex rotational velocity curve as a function of orbitalphase is detected which has a modulation frequency of twice the orbitalfrequency, leading to an estimate of the binary inclination angle thatis close to 70°. The minimum and maximum rotational velocities areused to indirectly derive a mass ratio of q = 0.6 and a radial velocitysemi-amplitude of the white dwarf of K1 = 101 +/-3kms-1. We present an atmospheric temperature indicator,based on the absorption-line ratio of FeI and CrI lines, whose variationindicates that the secondary star varies from K0 to K4 as a function oforbital phase. The ephemeris of the system has been revised, using morethan 1000 radial velocity measurements, published over nearly fivedecades. From the derived radial velocity semi-amplitudes and theestimated inclination angle, we calculate that the masses of the starsare M1 = 0.63 +/- 0.05Msolar M2 = 0.37+/- 0.04Msolar, and their separation is a = 2.33 +/-0.02Rsolar. Our analysis indicates the presence of alate-type star whose radius is larger, by a factor of nearly 2, than theradius of a normal main-sequence star of the same mass. Finally, wediscuss the possibility that the measured variations in the rotationalvelocity, temperature and spectral type of the secondary star asfunctions of orbital phase may, like the radial velocity variations, beattributable to regions of enhanced absorption on the star's surface.
| The eccentric accretion disc of the black hole A0620-00
We present spectroscopic observations of the quiescent black hole binaryA0620-00 with the 6.5-m Magellan Clay telescope at Las CampanasObservatory. We measure absorption-line radial velocities of thesecondary and make the most precise determination to date (K2= 435.4 +/- 0.5kms-1). By fitting the rotational broadeningof the secondary, we refine the mass ratio to q = 0.060 +/- 0.004; theseresults, combined with the orbital period, imply a minimum mass for thecompact object of 3.10 +/- 0.04Msolar. Although quiescenceimplies little accretion activity, we find that the disc contributes 56+/- 7 per cent of the light in B and V, and is subject to significantflickering. Doppler maps of the Balmer lines reveal bright emission fromthe gas stream-disc impact point and unusual crescent-shaped features.We also find that the disc centre of symmetry does not coincide with thepredicted black hole velocity. By comparison with smoothed particlehydrodynamics (SPH) simulations, we identify this source with aneccentric disc. With high signal-to-noise ratio (S/N), we pursuemodulation tomography of Hα and find that the aforementionedbright regions are strongly modulated at the orbital period. Weinterpret this modulation in the context of disc precession, and discusscases for the accretion disc evolution.This paper includes data gathered with the 6.5-m Magellan telescopelocated at Las Campanas Observatory, Chile.E-mail: jneilsen@cfa.harvard.edu (JN); dsteeghs@cfa.harvard.edu (DS);svrtilek@cfa.harvard.edu (SDV)
| Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants
We present the parameters of 891 stars, mostly clump giants, includingatmospheric parameters, distances, absolute magnitudes, spatialvelocities, galactic orbits and ages. One part of this sample consistsof local giants, within 100 pc, with atmospheric parameters eitherestimated from our spectroscopic observations at high resolution andhigh signal-to-noise ratio, or retrieved from the literature. The otherpart of the sample includes 523 distant stars, spanning distances up to1 kpc in the direction of the North Galactic Pole, for which we haveestimated atmospheric parameters from high resolution but lowsignal-to-noise Echelle spectra. This new sample is kinematicallyunbiased, with well-defined boundaries in magnitude and colours. Werevisit the basic properties of the Galactic thin disk as traced byclump giants. We find the metallicity distribution to be different fromthat of dwarfs, with fewer metal-rich stars. We find evidence for avertical metallicity gradient of -0.31 dex kpc-1 and for atransition at ~4-5 Gyr in both the metallicity and velocities. Theage-metallicity relation (AMR), which exhibits a very low dispersion,increases smoothly from 10 to 4 Gyr, with a steeper increase for youngerstars. The age-velocity relation (AVR) is characterized by thesaturation of the V and W dispersions at 5 Gyr, and continuous heatingin U.
| Debris Disks around Sun-like Stars
We have observed nearly 200 FGK stars at 24 and 70 μm with theSpitzer Space Telescope. We identify excess infrared emission, includinga number of cases where the observed flux is more than 10 times brighterthan the predicted photospheric flux, and interpret these signatures asevidence of debris disks in those systems. We combine this sample of FGKstars with similar published results to produce a sample of more than350 main sequence AFGKM stars. The incidence of debris disks is4.2+2.0-1.1% at 24 μm for a sample of 213Sun-like (FG) stars and 16.4+2.8-2.9% at 70 μmfor 225 Sun-like (FG) stars. We find that the excess rates for A, F, G,and K stars are statistically indistinguishable, but with a suggestionof decreasing excess rate toward the later spectral types; this may bean age effect. The lack of strong trend among FGK stars of comparableages is surprising, given the factor of 50 change in stellar luminosityacross this spectral range. We also find that the incidence of debrisdisks declines very slowly beyond ages of 1 billion years.
| Ages for Illustrative Field Stars Using Gyrochronology: Viability, Limitations, and Errors
We here develop an improved way of using a rotating star as a clock, setit using the Sun, and demonstrate that it keeps time well. Thistechnique, called gyrochronology, derives ages for low-massmain-sequence stars using only their rotation periods and colors. Thetechnique is developed here and used to derive ages for illustrativegroups of nearby field stars with measured rotation periods. We firstdemonstrate the reality of the interface sequence, the unifying featureof the rotational observations of cluster and field stars that makes thetechnique possible, and extend it beyond the proposal of Skumanich byspecifying the mass dependence of rotation for these stars. We delineatewhich stars it cannot currently be used on. We then calibrate the agedependence using the Sun. The errors are propagated to understand theirdependence on color and period. Representative age errors associatedwith the technique are estimated at ~15% (plus possible systematicerrors) for late F, G, K, and early M stars. Gyro ages for the MountWilson stars are shown to be in good agreement with chromospheric agesfor all but the bluest stars, and probably superior. Gyro ages are thencalculated for each of the active main-sequence field stars studied byStrassmeier and collaborators. These are shown to have a median age of365 Myr. The sample of single field stars assembled by Pizzolato andcollaborators is then assessed and shown to have gyro ages ranging fromunder 100 Myr to several Gyr, with a median age of 1.2 Gyr. Finally, wedemonstrate that the individual components of the three wide binariesξ Boo AB, 61 Cyg AB, and α Cen AB yield substantially the samegyro ages.
| Patterns of activity in stars with cycles becoming established
Late-type stars with chromospheric and coronal activities exceedingthose of the Sun and other stars with well-defined cycles areconsidered. These rotate more rapidly than stars with well establishedcycles; for single stars, this appears to be due to their younger ages.The spots on such stars cover several per cent of the total area, whichis an order of magnitude higher than for the Sun at its activitymaximum. Our wavelet analysis of the chromospheric-emission variability,which has been observed since 1965 in the framework of the HK project,indicates that the period of the axial rotation of some of these startsvaries from year to year. This is most pronounced in two“Good” stars according to the classification of Baliunas etal., HD 149661 and HD 115404, and also in a star with a more complexvariability, HD 101501. No similar effect is exhibited by the“Excellent” cyclic-activity stars. Such variations in theperiod can be observed during epochs of appreciable rotationalmodulations of the chromospheric-emission fluxes, most likely,immediately after the maximum of a long-period wave (cycle?). This seemsto provide evidence for the existence of huge activity complexes in thechromospheres of these stars, whose longitudes remain virtually constantover several years; they drift from fairly high latitudes to the equatorat speeds close to the value typical of sunspots. The observed periodvariations are most likely due to differential rotation of the same signthat is known for the Sun. Our results provide independent confirmationof similar conclusions obtained by us previously using zonal models forhighly spotted stars. Other activity features of a selected star groupand the implications of the results for the theory of stellar and solardynamos are discussed.
| Near-Infrared Spectra of the Black Hole X-Ray Binary A0620-00
We present broadband near-IR (NIR) spectra of A0620-00 obtained withSpeX on the IRTF. The spectrum is characterized by a blue continuum onwhich are superimposed broad emission lines of H I and He II and a hostof narrower absorption lines of neutral metals and molecules. Spectraltype standard star spectra scaled to the dereddened spectrum of A0620-00in K exceed the A0620-00 spectrum in J and H for all stars of spectraltype K7 V or earlier, demonstrating that the donor star, unless laterthan K7 V, cannot be the sole NIR flux source in A0620-00. In addition,the atomic absorption lines in the K3 V spectrum are too weak withrespect to those of A0620-00 even at 100% donor star contribution,restricting the spectral type of the donor star in A0620-00 to laterthan K3 V. Comparison of the A0620-00 spectrum to scaled K star spectraindicates that the CO absorption features are significantly weaker inA0620-00 than in field dwarf stars. Fits of scaled model spectra of aRoche lobe-filling donor star to the spectrum of A0620-00 show that thebest match to the CO absorption lines is obtained when the C abundanceis reduced to [C/H]=-1.5. The donor star contribution in the H wave bandis determined to be (82+/-2)%. Combined with previous published resultsfrom Froning & Robinson and Marsh et al., this gives a precise massfor the black hole in A0620-00 of M1=9.7+/-0.6Msolar.
| The frequency of planets in multiple systems
Context: The frequency of planets in binaries is an important issue inthe field of extrasolar planet studies, because of its relevance inestimating of the global planet population of our Galaxy and the cluesit can give to our understanding of planet formation and evolution.However, only preliminary estimates are available in the literature. Aims: We analyze and compare the frequency of planets in multiplesystems to the frequency of planets orbiting single stars. We also tryto highlight possible connections between the frequency of planets andthe orbital parameters of the binaries (such as the periastron and massratio.) Methods: A literature search was performed for binariesand multiple systems among the stars of the sample with uniform planetdetectability defined by Fischer & Valenti (2005, ApJ, 622, 1102),and 202 of the 850 stars of the sample turned out to be binaries,allowing a statistical comparison of the frequency of planets inbinaries and single stars and a study of the run of the planet frequencyas a function of the binary separation. Results: We found that theglobal frequency of planets in the binaries of the sample is notstatistically different from that of planets in single stars. Evenconservatively taking the probable incompleteness of binary detection inour sample into account, we estimate that the frequency of planets inbinaries can be no more than a factor of three lower than that ofplanets in single stars. There is no significant dependence of planetfrequency on the binary separation, except for a lower value offrequency for close binaries. However, this is probably not as low asrequired to explain the presence of planets in close binaries only asthe result of modifications of the binary orbit after the planetformation. Table 8 and Appendix A are only available in electronic form athttp://www.aanda.org
| On prospects for sounding activity cycles of Sun-like stars with acoustic modes
Data are now available on the Sun-like, p-mode oscillations of a growingnumber of late-type stars. With extension of these observations todedicated, long-term campaigns, it will soon become possible to probeacoustically magnetic activity, and stellar cycles, by observation ofsystematic shifts in the mode frequencies giving additional informationto the stellar dynamo theorists. Here, we use model computations of thedamping rates of stochastically excited radial p modes to makepredictions of the precision with which it will be possible to measurestellar-cycle frequency shifts of Sun-like stars along the lower mainsequence. We assume the first analyses will average shifts across themost prominent modes to reduce uncertainties. We also make somepredictions of the expected frequency shifts, based on existing stellarCaII H& K data. Our main conclusion is that the basic properties ofthe acoustic signatures of the cycles should be measurable to reasonablyhigh precision given only a few multimonth segments of data. It shouldalso be possible to make inference on the surface distribution of theactivity, through measurement of shifts of modes of different degree.Our computations also reveal an interesting feature in the predictedappearance of the acoustic mode spectra of stars cooler than about5400K: the modelled power spectral density of the modes shows twomaxima, at different frequencies. By computing average shifts of modesacross the two maxima, where the signal-to-noise ratio is highest, itshould be possible to get the first measures of the frequency dependenceof the p-mode shifts. This dependence provides information on themechanism responsible for driving these shifts.
| On the Correlation between the Magnetic Activity Levels, Metallicities, and Radii of Low-Mass Stars
The recent increase in the number of radius measurements of very lowmass stars from eclipsing binaries and interferometry of single starshas raised more questions about what could be causing the discrepancybetween the observed radii and those predicted by models. The two mainexplanations being proposed are a correlation between the radii of thestars and either their activity levels or their metallicities. Thispaper presents a study of such correlations using all the data publishedto date. The study also investigates correlations between the radiusdeviations from the models and the masses of the stars. There is noclear correlation between activity level and radius for the single starsin the sample. These single stars are slow rotators, with typicalvelocities vrotsini<3.0 km s-1. A clearcorrelation however exists in the case of the faster rotating members ofbinaries. This result is based on the X-ray emission levels of thestars. There also appears to be an increase in the deviation of theradii of single stars from the models as a function of metallicity, aspreviously indicated by Berger et al. The stars in binaries do not seemto follow the same trend. Finally, the Baraffe et al. models reproducewell the radius observations below 0.30-0.35 Msolar, wherethe stars become fully convective, although this result is preliminarysince almost all the sample stars in that mass range are slow rotatorsand metallicities have not been measured for most of them. The resultsindicate that stellar activity and metallicity play an important role indetermining the radius of very low mass stars, at least above 0.35Msolar.
| 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.
| New Distant Companions to Known Nearby Stars. II. Faint Companions of Hipparcos Stars and the Frequency of Wide Binary Systems
We perform a search for faint, common proper motion companions ofHipparcos stars using the recently published Lépine-Shara ProperMotion-North catalog of stars with proper motionμ>0.15'' yr-1. Our survey uncovers a totalof 521 systems with angular separations3''<Δθ<1500'', with 15 triplesand 1 quadruple. Our new list of wide systems with Hipparcos primariesincludes 130 systems identified here for the first time, including 44 inwhich the secondary star has V>15.0. Our census is statisticallycomplete for secondaries with angular separations20''<Δθ<300'' and apparentmagnitudes V<19.0. Overall, we find that at least 9.5% of nearby(d<100 pc) Hipparcos stars have distant stellar companions withprojected orbital separations s>1000 AU. We observe that thedistribution in orbital separations is consistent with Öpik's law,f(s)ds~s-1ds, only up to a separation s~4000 AU, beyond whichit follows a more steeply decreasing power law f(s)ds~s-ldswith l=1.6+/-0.1. We also find that the luminosity function of thesecondaries is significantly different from that of the single stars'field population, showing a relative deficiency in low-luminosity(8
| Structure and Evolution of Nearby Stars with Planets. II. Physical Properties of ~1000 Cool Stars from the SPOCS Catalog
We derive detailed theoretical models for 1074 nearby stars from theSPOCS (Spectroscopic Properties of Cool Stars) Catalog. The Californiaand Carnegie Planet Search has obtained high-quality (R~=70,000-90,000,S/N~=300-500) echelle spectra of over 1000 nearby stars taken with theHamilton spectrograph at Lick Observatory, the HIRES spectrograph atKeck, and UCLES at the Anglo Australian Observatory. A uniform analysisof the high-resolution spectra has yielded precise stellar parameters(Teff, logg, vsini, [M/H], and individual elementalabundances for Fe, Ni, Si, Na, and Ti), enabling systematic erroranalyses and accurate theoretical stellar modeling. We have created alarge database of theoretical stellar evolution tracks using the YaleStellar Evolution Code (YREC) to match the observed parameters of theSPOCS stars. Our very dense grids of evolutionary tracks eliminate theneed for interpolation between stellar evolutionary tracks and allowprecise determinations of physical stellar parameters (mass, age,radius, size and mass of the convective zone, surface gravity, etc.).Combining our stellar models with the observed stellar atmosphericparameters and uncertainties, we compute the likelihood for each set ofstellar model parameters separated by uniform time steps along thestellar evolutionary tracks. The computed likelihoods are used for aBayesian analysis to derive posterior probability distribution functionsfor the physical stellar parameters of interest. We provide a catalog ofphysical parameters for 1074 stars that are based on a uniform set ofhigh-quality spectral observations, a uniform spectral reductionprocedure, and a uniform set of stellar evolutionary models. We explorethis catalog for various possible correlations between stellar andplanetary properties, which may help constrain the formation anddynamical histories of other planetary systems.
| Accurate M Dwarf Metallicities from Spectral Synthesis: A Critical Test of Model Atmospheres
We describe a method for accurately determining M dwarf metallicitieswith spectral synthesis based on abundance analyses of visual binarystars. We obtained high-resolution, high-signal-to-noise ratio spectraof each component of five visual binary pairs at McDonald Observatory.The spectral types of the components range from F7 to K3 V for theprimaries and from M0.5 to M3.5 V for the secondaries. We havedetermined the metallicities of the primaries differentially withrespect to the Sun by fitting synthetic spectra to Fe I line profiles inthe observed spectra. In the course of our analysis of the M dwarfsecondaries, we have made significant improvements to the PHOENIXcool-star model atmospheres and the spectrum analysis code MOOG. Ouranalysis yields an rms deviation of 0.11 dex in metallicity valuesbetween the binary pairs. We estimate the uncertainties in the derivedstellar parameters for the M dwarfs to be 48 K, 0.10 dex, 0.12 dex, 0.15km s-1, and 0.20 km s-1 for Teff, logg,[M/H], ξ, and η, respectively. Accurate stellar evolutionarymodels are needed to progress further in the analysis of cool-starspectra; the new model atmospheres warrant recalculation of theevolutionary models.
| Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables
Context: . Barnes-Evans type relations provide an empirical relationshipbetween the surface brightness of stars and their color. They are widelyused for measuring the distances to stars of known radii, as theRoche-lobe filling secondaries in cataclysmic variables (CVs).Aims: . The calibration of the surface brightness of field dwarfs ofnear-solar metalicity with spectral types A0 to L8 covers all secondaryspectral types detectable in CVs and related objects and will aid in themeasurement of their distances. Methods: . The calibrations arebased on the radii of field dwarfs measured by the Infrared Flux Methodand by interferometry. Published photometry is used and homogenized tothe Cousins Rc and Ic and the CIT JHK photometricsystems. The narrow band surface brightness at 7500 Å is based onour own and published spectrophotometry. Care is taken to select thedwarfs for near-solar metalicity, appropriate to CVs, and to avoiderrors caused by unrecognized binarity. Results: . Relations areprovided for the surface brightness in V, R_c, I_c, J, H, K and in anarrow band at 7500 Å as functions of V-K and of spectral type.The method is tested with selected CVs for which independent informationon their distances is available. The observed spread in the radii ofearly M-dwarfs of given mass or luminosity and its influence on thedistance measurements of CVs is discussed. Conclusions: . As longas accurate trigonometric parallaxes are not routinely available for alarge number of CVs, the surface brightness method remains a reliablemeans of determining distances to CVs in which a spectral signature ofthe secondary star can be discerned.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Long-term V-band monitoring of the bright stars of M33 at the Wise Observatory
We have conducted a long-term V-band photometric monitoring of M33 on 95nights during four observing seasons (2000-2003). A total number of 6418light curves of bright objects in the range of 14-21mag have beenobtained. All measurements are publicly available. A total of 127 newvariables were detected, of which 28 are periodic. 10 previously knownnon-periodic variables were identified as periodic, three of which areCepheids, and another previously known periodic variable was identifiedas an eclipsing binary. Our derived periods range from 2.11 to almost300d. For 50 variables we have combined our observations with those ofthe DIRECT project, obtaining light curves of up to 500 measurements,with a time-span of ~7yr. We have detected a few interesting variables,including a 99.3d periodic variable with a 0.04mag amplitude, at theposition of supernova remnant (SNR) 19.
| 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.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample
We are obtaining spectra, spectral types, and basic physical parametersfor the nearly 3600 dwarf and giant stars earlier than M0 in theHipparcos catalog within 40 pc of the Sun. Here we report on resultsfor 1676 stars in the southern hemisphere observed at Cerro TololoInter-American Observatory and Steward Observatory. These resultsinclude new, precise, homogeneous spectral types, basic physicalparameters (including the effective temperature, surface gravity, andmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. We include notes on astrophysically interesting stars inthis sample, the metallicity distribution of the solar neighborhood, anda table of solar analogs. We also demonstrate that the bimodal nature ofthe distribution of the chromospheric activity parameterlogR'HK depends strongly on the metallicity, andwe explore the nature of the ``low-metallicity'' chromosphericallyactive K-type dwarfs.
| MK Classification and Dynamical Masses for Late-Type Visual Binaries
On the basis of slit spectra obtained with the SCORPIO spectral cameraattached to the 2.6 m telescope of the V. Ambartsumian ByurakanAstrophysical Observatory (Armenia), MK classifications for 30 visualbinaries comprising mostly late K and M type stars are presented.Comparison with other determinations shows that this configurationprovides a reliable MK classification. Dynamical masses for 25 systemsare computed. Using standard mass-luminosity calibrations, individualmass sums for 11 pairs consisting of virtually single, nonvariabledwarfs are calculated, showing a good agreement with correspondingdynamical masses. The dynamical parallax of HIP 112354 is closer to thetrigonometric parallax given in the Yale General Catalogue ofTrigonometric Stellar Parallaxes (van Altena et al.) than to theHipparcos parallax.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Cetus |
| Right ascension: | 02h36m03.90s |
| Declination: | +06°53'00.8" |
| Apparent magnitude: | 5.825 |
| Proper motion RA: | 0 |
| Proper motion Dec: | 0 |
| B-T magnitude: | 7.063 |
| V-T magnitude: | 5.928 |
Catalogs and designations:
|
INHABITED SKY
