Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
A Comparative Study on Lithium Abundances in Solar-Type Stars With and Without Planets
We have investigated the abundance anomalies of lithium for stars withplanets in the temperature range of 5600-5900 K reported by Israelianand coworkers, as compared to 20 normal stars in the same temperatureand metallicity ranges. Our result indicates a higher probability oflithium depletion for stars with planets in the main-sequence stage. Itseems that stellar photospheric abundances of lithium in stars withplanets may be somewhat affected by the presence of planets. Twopossible mechanisms are considered to account for the lower Liabundances of stars with planets. One is related to the rotation-inducedmixing due to the conservation of angular momentum by the protoplanetarydisk, and the other is a shear instability triggered by planetmigration. These results provide new information on stellar evolutionand the lithium evolution of the Galaxy.
| 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.
| The dependence of the Rossby number and XUV-Lyα emission flux with age for solar-like G-type stars
Stellar parameters of 11 G-type stars with ages ranging from 0.1 to 8.5Gyr, from the Sun in Time programme, were used to compute the Rossbynumber, ℜ, for each star. The Rossby number for each star wascalculated from the rotation period and the convective overturn timederived from spectral type (B-V). It was found to vary essentially ast0.5, where t is the stellar age in Gyr. The Rossby number isused as an index of X-ray-ultraviolet (XUV) (1-1200 Å) andLyα activity, defined as the ratio of the total emission flux inthese spectral regions to the total bolometric emission. Expressions forthe ratio of the stellar surface XUV and Lyα emission fluxrelative to present mean solar surface flux values are given in terms ofℜ. It is shown that the observed activity in these stars varies asℜ-β, where β takes values of 2.5 and 1.5 forXUV and Lyα, respectively. Expressions for deriving the Rossbynumber from B-V and age are also given. Thus, one can use the stellarB-V and effective temperature variation with age to calculate the XUVand Lyα emission flux relative to present solar values. As anexample, the evolution of the solar XUV and Lyα with age from 0.1to 8.5 Gyr is given. The variation of the stellar ultraviolet flux withage can be used in photochemical models to study the evolution ofplanetary atmospheres orbiting such stars.
| 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.
| 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.
| Single-Visit Photometric and Obscurational Completeness
We report a method that uses ``completeness'' to estimate the number ofextrasolar planets discovered by an observing program with adirect-imaging instrument. We develop a completeness function forEarth-like planets on ``habitable'' orbits for an instrument with acentral field obscuration, uniform sensitivity in an annular detectionzone, and limiting sensitivity that is expressed as a ``deltamagnitude'' with respect to the star, determined by systematic effects(given adequate exposure time). We demonstrate our method of estimationby applying it to our understanding of the coronagraphic version of theTerrestrial Planet Finder (TPF-C) mission as of 2004 October. Weestablish an initial relationship between the size, quality, andstability of the instrument's optics and its ability to meet missionscience requirements. We provide options for increasing the fidelity andversatility of the models on which our method is based, and we discusshow the method could be extended to model the TPF-C mission as a wholeto verify that its design can meet the science requirements.
| 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.
| Low-mass companions to Hyades stars
It is now well established that a large fraction of the low-mass starsare binaries or higher order multiples. Similarly a sizable fractionhave giant planets. In contrast to these, the situation for brown dwarfcompanions is complicated: While close systems seem to be extremelyrare, wide systems are possibly more common. In this paper, we presentnew results on a survey for low-mass companions in the Hyades. Aftermeasuring precisely the radial velocity of 98 Hyades dwarf stars for 5years, we have selected all stars that show low-amplitude long-periodtrends. With AO-observations of these 14 stars we found companioncandidates around nine of them, where one star has two companions. Thetwo companions of HIP 16548 have masses between 0.07 to 0.08Mȯ, and are thus either brown dwarfs or very low massstars. In the case of HAN 172 we found a companion with a mass between0.08 to 0.10 Mȯ, which is again between a star and abrown dwarf. The other seven stars all have stellar companions. In twoadditional cases, the RV-variations are presumably caused by stellaractivity, and in another case the companion could be a short-periodbinary. The images of the remaining two stars are slightly elongated,which might imply that even these are binaries. Because at least 12 ofthe 14 stars showing low-amplitude RV trends turn out to have companionswith a mass ≥ 70 M_Jupiter, or are just active, we finally estimatethe number of companions with masses between 10 M_Jupiter and 70M_Jupiter within 8 AU of the host stars in the Hyades as ≤2%.
| Continuous and burst-like accretion on to substellar companions in Mira winds
We present numerical hydrodynamical modelling of the effects of a giantplanet or brown-dwarf companion orbiting within the extended atmosphereand wind formation zone of an approximately solar-mass Mira variablestar. The large-scale, time-dependent accretion flows within theradially oscillating and outflowing circumstellar gas around Miras arerelated to Bondi-Hoyle-Lyttleton flows, but have not, to our knowledge,been previously modelled. The new models presented in this paperillustrate the changes in accretion and wake dynamics as the companionmass is varied over a range from 10 to 50 Jupiter masses(MJ), and generalize the results of the single model wepresented in an earlier paper.The character of the accretion on to the companion changes greatly asthe companion mass is increased. At the lowest companion massesconsidered here, a low continuous rate of mass accretion is punctuatedby large, nearly periodic bursts of accretion. When the companion massis large, the mass accretion has both a continuous part and a rapidlyvarying, nearly stochastic part. Surprisingly, the angular momentum ofthe accreted gas shows an opposite trend with mass, varying nearlyperiodically at large companion masses and stochastically at low masses.These trends can be understood as the result of the interplay betweenthe shocks and radial oscillations in the circumstellar gas and the wakeflow behind the companion. Boundary conditions also affect the characterof the accretion. The equation of state, however, is found to havelittle effect, at least for gamma-law gases, with gamma in the rangefrom 1 to 5/3.Models with accretion bursts may produce observable opticalbrightenings, and may affect SiO maser emission, as we suggested inprevious papers. Interruptions of continuous accretion, or shadowingeffects, could give rise to bursts of dimming in the optical. Suchdimming effects are likely to be correlated with bursts and opticalflashes, helping to explain some rather mysterious Hipparcosobservations.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| Initial Results from the Palomar Adaptive Optics Survey of Young Solar-Type Stars: A Brown Dwarf and Three Stellar Companions
We present first results from the Palomar Adaptive Optics Survey ofYoung Stars conducted at the Hale 5 m telescope. Through direct imagingwe have discovered a brown dwarf and two low-mass stellar companions tothe young solar-type stars HD 49197, HD 129333 (EK Dra), and V522 Perand confirmed a previously suspected companion to RX J0329.1+0118(Sterzik et al.), at respective separations of 0.95" (43 AU), 0.74" (25AU), 2.09" (400 AU), and 3.78" (380 AU). Physical association of eachbinary system is established through common proper motion and/orlow-resolution infrared spectroscopy. Based on the companion spectraltypes, we estimate their masses at 0.06, 0.20, 0.13, and 0.20Msolar, respectively. From analysis of our imaging datacombined with archival radial velocity data, we find that the spatiallyresolved companion to HD 129333 is potentially identical to thepreviously identified spectroscopic companion to this star (Duquennoy& Mayor). However, a discrepancy with the absolute magnitudesuggests that the two companions could also be distinct, with theresolved one being the outermost component of a triple system. The browndwarf HD 49197B is a new member of a growing list of directly imagedsubstellar companions at 10-1000 AU separations from main-sequencestars, indicating that such brown dwarfs may be more common thaninitially speculated.
| Probing the Sun's Hidden Magnetic Past: The Two Billion Year-Old Solar Proxy 15 Sge and its Brown Dwarf Companion
Recently, 15 Sge was discovered to host a dim, cool L6 ± 1.5brown dwarf companion with a mass of 58-71 Mj and adistance of 14-21 AU from the host star (see Liu et al 2002;Bocccaletti et al. 2003). 15 Sge (HR 7276; G1V; V = +5.80 mag; π(Hipp) = 56.6 mas) has been on the Villanova Sun-in-Time program forover 15 years. As part of the Sun-in-Time program, this young solarproxy has been observed at X-ray, EUV, FUV-NUV, and Optical wavelengthsto study its coronal, chromospheric, and starspot activity. Its rotationperiod of 13.9 ± 0.4 days is well determined from photometry(light variations of starspots) and from Mt. Wilson Ca II H+K studies.From our Age-Rotation-Activity relations, its age is 1.9 ± 0.3Gyrs. This value agrees well with an age of 2.0 ± 0.5 Gyrestimated from recent stellar evolution codes. 15 Sge is extremelycritical because of its age. It serves as an important proxy for our Sunat a time in the early solar system when primitive life had justestablished a foothold on Earth and when Mars may have been warm, wet,and suitable for life. From the available data, we computed the XUVspectral irradiances. These can be used to model the radiation effectsof the younger Sun's stronger X-ray ( 10x present) and FUV (2-3xpresent) radiation on paleoplanetary atmospheres. The large XUV solarfluxes may have had major effects on the younger planets' ionospheresand resulting mass loss. The stronger FUV fluxes may have beeninfluential in photochemical reactions that could affect life. We willalso discuss the effects of star's strong XUV emission on its browndwarf companion, and the importance of the refined age determination of15 Sge. Using Age-Rotation-Activity relations pins down the age of itsbrown dwarf companion, and allows comparisons to current brown dwarfmodels. This research is supported in part by grants from NASA and NSFfor which we are very grateful.
| Sodium abundances in nearby disk stars
We present sodium abundances for a sample of nearby stars. All resultshave been derived from NLTE statistical equilibrium calculations. Theinfluence of collisional interactions with electrons and hydrogen atomsis evaluated by comparison of the solar spectrum with very precise fitsto the Na I line cores. The NLTE effects are more pronounced inmetal-poor stars since the statistical equilibrium is dominated bycollisions of which at least the electronic component is substantiallyreduced. The resulting influence on the determination of sodiumabundances is in a direction opposite to that found previously for Mgand Al. The NLTE corrections are about -0.1 in thick-disk stars with[Fe/H] -0.6. Our [Na/Fe] abundance ratios are about solar forthick- and thin-disk stars. The increase in [Na/Fe] as a function of[Fe/H] for metal-rich stars found by Edvardsson et al. (\cite{EAG93}) isconfirmed. Our results suggest that sodium yields increase with themetallicity, and quite large amounts of sodium may be produced by AGBstars. We find that [Na/Fe] ratios, together with either [Mg/Fe] ratio,kinematic data or stellar evolutionary ages, make possible theindividual discrimination between thin- and thick-disk membership.Based on observations collected at the Germany-Spanish AstronomicalCenter, Calar Alto, Spain.Tables \ref{table2} and \ref{table3} are only available in electronicform at http://www.edpsciences.org
| On the determination of oxygen abundances in chromospherically active stars
We discuss oxygen abundances derived from [O I] λ6300s and the OI triplet in stars spanning a wide range in chromospheric activitylevel, and show that these two indicators yield increasingly discrepantresults with higher chromospheric/coronal activity measures. While theforbidden and permitted lines give fairly consistent results forsolar-type disk dwarfs, spuriously high O I triplet abundances areobserved in young Hyades and Pleiades stars, as well as in individualcomponents of RS CVn binaries (up to 1.8 dex). The distinct behaviour ofthe [O I]-based abundances which consistently remain near-solar suggeststhat this phenomenon mostly results from large departures from LTEaffecting the O I triplet at high activity level that are currentlyunaccounted for, but also possibly from a failure to adequately modelthe atmospheres of K-type stars. These results suggest that some cautionshould be exercised when interpreting oxygen abundances in activebinaries or young open cluster stars.Based on observations collected at the European Southern Observatory,Chile (Proposals 64.L-0249 and 071.D-0260).Table \ref{tab_data} is only available in electronic form athttp://www.edpsciences.org
| 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
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| 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.
| Dependence of coronal X-ray emission on spot-induced brightness variations in cool main sequence stars
The maximum amplitude (Amax) of spot-induced brightnessvariations from long-term V-band photometry and the ratioLX/Lbol between X-ray and bolometric luminositiesare suitable indicators of the level of magnetic activity in thephotosphere and in the corona of late-type stars, respectively. By usingthese activity indicators we investigate the dependence of coronal X-rayemission on the level of photospheric starspot activity in a homogeneoussample of low mass main sequence field and cluster stars of differentages (IC 2602, IC 4665,IC 2391, alpha Persei,Pleiades and Hyades). First, theactivity-rotation connection at the photospheric level is re-analysed,as well as its dependence on spectral type and age. The upper envelopeof Amax increases monotonically with decreasing rotationalperiod (P) and Rossby number (R0) showing a break around 1.1d that separates two rotation regimes where the starspot activity showsdifferent behaviours. The Amax-P andAmax-R0 relations are fitted with linear,exponential and power laws to look for the function which bestrepresents the trend of the data. The highest values of Amaxare found among K-type stars and at the ages of alphaPersei and Pleiades. We also analyse theactivity-rotation connection at the coronal level as well as itsdependence on spectral type. The level of X-ray emission increases withincreasing rotation rate up to a saturation level. The rotational periodat which saturation occurs is colour-dependent and increases withadvancing spectral type. Also the LX/Lbol-P andLX/Lbol-R0 relations are fitted withlinear, exponential and power laws to look for the best fittingfunction. Among the fastest rotating stars (P<=0.3 d) there isevidence of super-saturation. Also the highest values ofLXLbol are found among K-type stars. Finally, thephotospheric-coronal activity connection is investigated by using forthe first time the largest ever sample of light curve amplitudes asindicators of the magnetic filling factor. The activity parametersLX/Lbol and Amax are found to becorrelated with each other, thus confirming the dependence of coronalactivity on photospheric magnetic fields. More precisely, theLX/Lbol-Amax distribution shows thepresence of an upper envelope, which is constant at theLX/Lbol =~ -3.0 saturation level, and of a lowerenvelope. The best fit to the lower envelope is given by a power lawwith steepness decreasing from F-G to M spectral types. However, it isconsidered a tentative result, since the fit reduced chi-squares arelarge. Such spectral-type dependence may be related to a colourdependence of Amax on the total starspot filling factor, aswell as to the coronal emission being possibly more sensitive tostarspot activity variations in F- and G-type than in M-type stars. TheLX/Lbol-Amax mean values for eachcluster in our sample decrease monotonically with increasing age,showing that the levels of photospheric and coronal activity evolve intime according to a single power law till the Sun's age.Tables of the photometric and X-ray data sets are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/671
| Near-IR coronagraphic imaging of the companion to HR 7672
This article presents coronagraphic images of the low-mass companion tothe star HR 7672 observed at the Palomar 200 inch telescope and firstdetected at Gemini and Keck in the K band by \citet{liu02}. We obtainedadditional photometry in J(1.2 mu m), H(1.6 mu m) and Ks(2.2mu m) bands to cover the full near-IR domain and hence to furtherconstrain the nature of the companion. A mass estimate of 58-71MJ is derived from evolutionary models of very low-massobjects.Based on data collected at the Palomar 200-inch.
| Magnetic activity of six young solar analogues II. Surface Differential Rotation from long-term photometry
The present paper is the second of a series dedicated to the study ofthe magnetic activity in a selected sample of young solar analogues. Thesample includes five single G0-G5V stars with ages between =~ 130 Myrand 700 Myr: EK Dra, pi 1 UMa,HN Peg, k1 Cet and BECet. In this study we also include the Pleiades-age ( =~ 130Myr) K0V star DX Leo. Our analysis is based on highprecision photometric observations carried out as part of The Sun inTime project, aimed at a multiwavelength study of stars with solar-likeglobal properties, but with different ages and thus at different stagesof evolution. In the first paper of this series we presented thephotometric observations and determined the existence of starspot cyclesand their correlation with the global stellar properties. In the presentpaper we investigate the surface differential rotation (SDR). Theperiodogram analysis of the photometric data time series has allowed usto determine the rotational periods and to derive the following results:i) all the selected stars show variations of the rotational period. Suchvariations are definitely periodic and in phase with the starspot cyclefor BE Cet and DX Leo. They are likely periodic and in phase also for pi1 UMa, EK Dra and HN Peg, but still need confirmation. Byanalogy with the solar butterfly diagram, the rotational periodvariations are interpretable in terms of surface differential rotation,that is, they are attributable to the existence of active latitude beltsmigrating during the activity cycle on a differentially rotating star;ii) BE Cet, pi 1 UMa andEK Dra show a solar-like pattern of SDR, that is therotational period steadily decreases along the activity cycle, jumpingback to higher values at the beginning of the next cycle; on thecontrary, DX Leo, k1 Cet and HN Peg show an antisolarpattern; iii) the amplitude of the rotational period variations shows apower law dependence on the rotational period similar to that found inprevious studies. Contrary to theoretical predictions, the cycle lengthis not correlated to the Dynamo number, it is indeed positivelycorrelated to the SDR amplitude. More precisely, stars tend toconcentrate along three different branches with the cycle lengthincreasing with increasing Delta Omega /Omega . Moreover, we found thatthe SDR amplitude changes from cycle to cycle, which is reminiscent of awave of excess rotation propagating in latitude; iiii) the apparentlydifferent solar and antisolar behaviours are probably due to differentinclinations of the stellar rotation axis under which the star is seen.The long-term photometry of the young single star LQ Hya, although notincluded in the initial project, is also used in the present analysis toenlarge the investigated sample. We determined for LQ Hya threedifferent starspot cycles and an antisolar pattern of SDR.
| The Wilson-Bappu effect: A tool to determine stellar distances
Wilson & Bappu (\cite{orig}) have shown the existence of aremarkable correlation between the width of the emission in the core ofthe K line of Ca II and the absolute visual magnitude of late-typestars.Here we present a new calibration of the Wilson-Bappu effect based on asample of 119 nearby stars. We use, for the first time, widthmeasurements based on high resolution and high signal to noise ratio CCDspectra and absolute visual magnitudes from the Hipparcos database.Our primary goal is to investigate the possibility of using theWilson-Bappu effect to determine accurate distances to single stars andgroups.The result of our calibration fitting of the Wilson-Bappu relationshipis MV=33.2-18.0 log W0, and the determinationseems free of systematic effects. The root mean square error of thefitting is 0.6 mag. This error is mostly accounted for by measurementerrors and intrinsic variability of W0, but in addition apossible dependence on the metallicity is found, which becomes clearlynoticeable for metallicities below [Fe/H] ~ -0.4. This detection ispossible because in our sample [Fe/H] ranges from -1.5 to 0.4.The Wilson-Bappu effect can be used confidently for all metallicitiesnot lower than ~ -0.4, including the LMC. While it does not provideaccurate distances to single stars, it is a useful tool to determineaccurate distances to clusters and aggregates, where a sufficient numberof stars can be observed.We apply the Wilson-Bappu effect to published data of the open cluster M67; the retrieved distance modulus is of 9.65 mag, in very goodagreement with the best distance estimations for this cluster, based onmain sequence fitting.Observations collected at ESO, La Silla.
| Multiplicity among solar-type stars. III. Statistical properties of the F7-K binaries with periods up to 10 years
Two CORAVEL radial velocity surveys - one among stars in the solarneighbourhood, the other in the Pleiades and in Praesepe - are merged toderive the statistical properties of main-sequence binaries withspectral types F7 to K and with periods up to 10 years. A sample of 89spectroscopic orbits was finally obtained. Among them, 52 relate to afree-of-bias selection of 405 stars (240 field stars and 165 clusterstars). The statistics corrected for selection effects yield thefollowing results: (1) No discrepancy is found between the binariesamong field stars and the binaries in open cluster. The distributions ofmass ratios, of periods, the period-eccentricity diagram and the binaryfrequencies are all within the same error intervals. (2) Thedistribution of mass ratios presents two maxima: a broad peak from q ~0.2 to q ~ 0.7, and a sharp peak for q > 0.8 (twins). Both arepresent among the early-type as well as among the late-type part of thesample, indicating a scale-free formation process. The peak for q >0.8 gradually decreases when long-period binaries are considered.Whatever their periods, the twins have eccentricities significantlylower than the other binaries, confirming a difference in the formationprocesses. Twins could be generated by in situ formation followed byaccretion from a gaseous envelope, whereas binaries with intermediatemass ratios could be formed at wide separations, but they are madecloser by migration led by interactions with a circumbinary disk. (3)The frequency of binaries with P<10 years is about 14%. (4) About0.3% of binaries are expected to appear as false positives in a planetsearch. Therefore, the frequency of planetary systems among stars ispresently 7+4-2%. The extension of thedistribution of mass ratios in the planetary range would result in avery sharp and very high peak, well separated from the binary stars withlow mass ratios. Based on photoelectric radial-velocity measurementscollected at Haute-Provence observatory and on observations made withthe ESA Hipparcos astrometry satellite.
| The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs
We present the results of a new study on the relationship betweencoronal X-ray emission and stellar rotation in late-type main-sequencestars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0,including 110 field stars and 149 members of the Pleiades, Hyades, alphaPersei, IC 2602 and IC 2391 open clusters. All the stars have beenobserved with ROSAT, and most of them have photometrically-measuredrotation periods available. Our results confirm that two emissionregimes exist, one in which the rotation period is a good predictor ofthe total X-ray luminosity, and the other in which a constant saturatedX-ray to bolometric luminosity ratio is attained; we present aquantitative estimate of the critical rotation periods below which starsof different masses (or spectral types) enter the saturated regime. Inthis work we have also empirically derived a characteristic time scale,taue , which we have used to investigate the relationshipbetween the X-ray emission level and an X-ray-based Rossby numberRe = Prot/taue: we show that ourempirical time scale taue resembles the theoreticalconvective turnover time for 0.4 <~ M/Msun <~ 1.2, butit also has the same functional dependence on B-V asLbol-1/2 in the color range 0.5 <~ B-V <~1.5. Our results imply that - for non-saturated coronae - theLx - Prot relation is equivalent to theLx/Lbol vs. Re relation. Tables 1 and 2are only available in electronic form at \ http://www.edpsciences.org
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| Self-gravitating Magnetically Supported Protostellar Disks and the Formation of Substellar Companions
Isolated low-mass stars are formed, in the standard picture, from thecollapse of dense cores condensed out of strongly magnetized molecularclouds. The dynamically collapsing inflow traps nearly half of thecritical magnetic flux needed for the core support and deposits it in asmall region surrounding the protostar. It has been argued previouslythat the deposited flux can slow down the inflow, allowing matter topile up and settle along field lines into a magnetically supported,circumstellar disk. Here we show that the disk typically contains ~10%of the stellar mass and that it could become self-gravitating underplausible conditions during the rapidly accreting, ``Class 0'' phase ofstar formation. Subsequent fragmentation of the self-gravitating,magnetically subcritical disk, driven by magnetic diffusion, couldproduce fragments of substellar masses, which collapse to form browndwarfs and possibly massive planets. This scenario predicts substellarobject formation at distances of order 100 AU from the central star,although orbital evolution is possible after formation. It may providean explanation for the small, but growing, number of brown dwarfcompanions found around nearby stars by direct imaging. The relativelylarge formation distances make the substellar companions vulnerable todynamic ejection, particularly in binary (multiple) systems and denseclusters. Those ejected may account for, at least in part, the isolatedbrown dwarfs and perhaps free-floating planetary mass objects.
| Crossing the Brown Dwarf Desert Using Adaptive Optics: A Very Close L Dwarf Companion to the Nearby Solar Analog HR 7672
We have found a very faint companion to the active solar analog HR 7672(HD 190406; GJ 779; 15 Sge). Three epochs of high-resolution imagingusing adaptive optics (AO) at the Gemini-North and Keck II Telescopesdemonstrate that HR 7672B is a common proper motion companion, with aseparation of 0.79" (14 AU) and a 2.16 μm flux ratio of 8.6 mag.Using follow-up K-band spectroscopy from Keck AO+NIRSPEC, we measure aspectral type of L45 with an uncertainty of +/-1.5 subclasses. This isthe closest ultracool companion around a main-sequence star found todate by direct imaging. We estimate that the primary has an age of 1-3Gyr. Assuming coevality, the companion is most likely substellar, with amass of 55-78 MJ based on theoretical models. The primarystar shows a long-term radial velocity trend, and we combine the radialvelocity data and AO imaging to set a firm (model-independent) lowerlimit of 48 MJ. In contrast to the paucity of brown dwarfcompanions at <~4 AU around FGK dwarfs, HR 7672B implies that browndwarf companions do exist at separations comparable to those of thegiant planets in our own solar system. Its presence is at variance withscenarios in which brown dwarfs form as ejected stellar embryos.Moreover, since HR 7672B is likely too massive to have formed in acircumstellar disk as planets are believed to, its discovery suggeststhat a diversity of physical processes act to populate the outer regionsof exoplanetary systems.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Flecha |
| Ascensión Recta: | 20h04m06.20s |
| Declinación: | +17°04'12.0" |
| Magnitud Aparente: | 5.8 |
| Distancia: | 17.668 parsecs |
| Movimiento Propio en Ascensión Recta: | -392.5 |
| Movimiento Propio en Declinación: | -407.6 |
| B-T magnitude: | 6.521 |
| V-T magnitude: | 5.857 |
Catálogos y designaciones:
|
