Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
The eccentricity-mass distribution of exoplanets: signatures of different formation mechanisms?
We examine the distributions of eccentricity and host star metallicityof exoplanets as a function of their mass. Planets with M sin i ⪆ 4MJ have an eccentricity distribution consistent with that ofbinary stars, while planets with M sin i ⪉ 4 MJ are lesseccentric than binary stars and more massive planets. In addition, hoststar metallicities decrease with planet mass. The statisticalsignificance of both of these trends is only marginal with the presentsample of exoplanets. To account for these trends, we hypothesize thatthere are two populations of gaseous planets: the low-mass populationforms by gas accretion onto a rock-ice core in a circumstellar disk andis more abundant at high metallicities, and the high-mass populationforms directly by fragmentation of a pre-stellar cloud. Planets of thefirst population form in initially circular orbits and grow theireccentricities later, and may have a mass upper limit from the totalmass of the disk that can be accreted by the core. The second populationmay have a mass lower limit resulting from opacity-limitedfragmentation. This would roughly divide the two populations in mass,although they would likely overlap over some mass range. If most objectsin the second population form before the pre-stellar cloud becomeshighly opaque, they would have to be initially located in orbits largerthan ~30 AU, and would need to migrate to the much smaller orbits inwhich they are observed. The higher mean orbital eccentricity of thesecond population might be caused by the larger required intervals ofradial migration, and the brown dwarf desert might be due to theinability of high-mass brown dwarfs to migrate inwards sufficiently inradius.
| Tertiary companions to close spectroscopic binaries
We have surveyed a sample of 165 solar-type spectroscopic binaries (SB)with periods from 1 to 30 days for higher-order multiplicity. Asubsample of 62 targets were observed with the NACO adaptive opticssystem and 13 new physical tertiary companions were detected. Anadditional 12 new wide companions (5 still tentative) were found usingthe 2MASS all-sky survey. The binaries belong to 161 stellar systems; ofthese 64 are triple, 11 quadruple and 7 quintuple. After correction forincompleteness, the fraction of SBs with additional companions is foundto be 63% ± 5%. We find that this fraction is a strong functionof the SB period P, reaching 96% for P<3d and dropping to34% for P>12^d. Period distributions of SBs with and withouttertiaries are significantly different, but their mass ratiodistributions are identical. The statistical data on the multiplicity ofclose SBs presented in this paper indicates that the periods and massratios of SBs were established very early, but the periods of SB systemswith triples were further shortened by angular momentum exchange withcompanions.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| Stellar activity cycles: observing the dynamo?
The enormous complexity of the atmospheric structure observed on the Sunmakes it very difficult to compare the Sun with ``solar-type stars''.Clearly, we need to identify parameters that can be observed on the Sunas well as on other stars which can be interpreted unambiguously. Themost widely accepted dynamo signature is the presence of an activitycycle, well documented for the Sun and for main-sequence stars due tothe Mount Wilson Ca II H&K project. Only recently have we detectedspatial information, differential rotation and possibly meridional flowson other stars and thereby adding another constraint for itsinterpretation within a dynamo theory. Again, the picture is notcomplete yet, despite that there is just a single main ingredient thatacts as the driving mechanism for activity in all atmospheric layers andthe convective envelope of a solar-type star: the dynamo-relatedmagnetic field. I stress the importance of mapping stellar surfaces asfingerprints of the underlying dynamo action over long periods of time.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Spots, activity cycles, and differential rotation on cool stars
The first results are reported from a search for activity cycles instars similar to the sun based on modelling their spotting with analgorithm developed at the Crimean Astrophysical Observatory. Of themore than thirty program stars, 10 manifested a cyclical variation intheir central latitudes and total starspot area. The observed cycleshave durations of 4-15 years, i.e., analogous to the 11 year Schwabesunspot cycle. Most of the stars have a rough analog of the solarbutterfly pattern, with a reduction in the average latitude of the spotsas their area increases. A flip-flop effect during the epoch of themaximum average latitude is noted in a number of these objects (e.g.,the analog LQ Hya of the young sun or the RS CVn-type variable V711Tau), as well as a reduction in the photometric rotation period of astar as the spots drift toward the equator, an analog of thedifferential rotation effect in the sun. Unlike in the sun, the observedspot formation cycles do not correlate uniquely with other indicators ofactivity chromospheric emission in the CaII HK lines (Be Cet, EKDra, Dx Leo), H line emission (LQ Hya, VY Ari, EV Lac), or cyclicalflare activity (EV Lac). In V833 Tau, BY Dra, EK Dra, and VY Ari shortSchwabe cycles coexist with long cycles that are analogous to theGleissberg solar cycle, in which the spotted area can approach half theentire area of the star.
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| 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.
| Butterfly Diagram for Starspots on LQ Hya
Information on the latitude distribution of starspots and changes inthis distribution from year to year is very important for ourunderstanding of the nature of stellar activity and for developingdynamo theory. The concept of butterfly diagrams is introduced forhighly spotted stars of late spectral types, by analogy to the Maunderdiagrams for the Sun. Our approach is based on the zonal spottednessmodels constructed by Alekseev and Gershberg. A detailed analysis isgiven for the single active star LQ Hya, and a comparison is made tosimilar analyses for several stars with two well-separated spotbelts—EK Dra, VY Ari, V775 Her, and V833 Tau. The lower boundaryof the butterfly diagram drifts toward the equator during theactivity-rise phase, i.e., during years when the relative spotted areaincreases. This effect is clearly expressed for LQ Hya and other starswhose orientation enables observation of both hemispheres and virtuallyvanishes for V833 Tau, which is viewed nearly pole-on. The upperboundary of the diagram is virtually unchanged for all the consideredspotted stars except V775 Her, for which it moves toward the pole. Thedrift rate of the lower boundary is -1 to -2 deg/yr, a factor of two tothree smaller in magnitude than the corresponding solar value. Ouranalysis provides an independent confirmation of the occurrence ofhigh-latitude spots on stars that are younger than the Sun and whoseactivity is high but less regular than the solar activity; it alsoenables the identification of the starting times of stellar cycles.
| Completeness of USNO-B for High Proper Motion Stars
I test the completeness of USNO-B detections of high proper motion(μ>180 mas yr-1) stars and the accuracy of itsmeasurements by comparing them to the revised New Luyten Two-Tenthscatalog of Salim & Gould. For 14.5~20 mas yr-1) may actuallyhave still larger errors than tabulated.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe 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/403/247
| Nearby young stars
We present the results of an extensive all-sky survey of nearby stars ofspectral type F8 or later in a systematic search of young (zero-age mainsequence) objects. Our sample has been derived by cross-correlating theROSAT All-Sky Survey and the TYCHO catalogue, yielding a total of 754candidates distributed more or less randomly over the sky. Follow-upspectroscopy of these candidate objects has been performed on 748 ofthem. We have discovered a tight kinematic group of ten stars withextremely high lithium equivalent widths that are presumably youngerthan the Pleiades, but again distributed rather uniformly over the sky.Furthermore, about 43 per cent of our candidates have detectable levelsof lithium, thus indicating that these are relatively young objects withages not significantly above the Pleiades age.Based on observations collected at the European Southern Observatory,Chile (ESO No. 62.I-0650, 66.D-0159(A), 67.D-0236(A)).
| 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.
| 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.
| A search for the infrared spectroscopic signature of hot Jupiter planets
We present the results of an attempt to detect the hottest `hot Jupiter'planets directly in the thermal infrared. A simple method based uponhigh signal-to-noise ratio spectroscopy of the central star at lowspectral resolution is described. In the 2-4 μm region the contrastratio between planet and star is expected to be relatively low and theplanetary spectrum should appear as a faint signal on top of the stellarspectrum, distinguished by edges of H2O absorption. No wateredges were found to 3σ limits of one part in a few hundred in eachcase. These upper limits are compared with the irradiated planetaryatmosphere models of Barman, Hauschildt & Allard to derive upperlimits on the size of the hot Jupiters, which are expected to besomewhat larger than Jupiter. If reasonably strong H2Oabsorption occurs in these objects then typical upper limits of R < 3RJup are derived, the precision being limited by thestability of telluric transmission. Only a modest improvement inprecision is needed (e.g. with space-based instruments) to reach therange of greatest interest (1 < R < 2 RJup).
| Time series photometric spot modelling V. Phase coherence of spots on UZ Librae
We present spot models for nine years of continuous VI_C photometry ofUZ Lib from 1993-2001. The relatively stable double-humped light curveshape suggests extreme phase coherence. From the spot-modellinganalysis, we found that the major spots or spot groups are alwayslocated on the hemisphere facing the secondary star and exactly in theopposite hemisphere anti-facing the secondary. Several single-humpedlight curves and our suggested binary scenario rule out a pureellipsoidal variability as the cause of the double-humped light curveshape. We try to explain this preferred spot pattern with amagnetic-field structure that connects the two components, as suggestedearlier for RS CVn stars in general. A possible 4.8 years spot cycle isfound from the long-term brightness variations but needs confirmation.We rediscuss the basic astrophysical data of UZ Lib. The Hipparcosparallax is likely wrong, a possible reason could be that UZ Lib ise.g., a triple system.
| Starspot cycles from long-term photometry
We continue investigating the photometric cycle lengths of some activestars. Using datasets now up to 34 years long we confirm previouslyderived activity cycles for the shorter period systems of our sample (LQHya, V833 Tau, EI Eri, V711 Tau, HU Vir and IL Hya), but find differentcycle lengths for the two long-period giants HK Lac and IM Peg. We addUZ Lib to our previous sample. The connection between the rotationalperiod and cycle length seems evident for the shortest derived cyclelengths. A similar connection between the rotational period and thelonger cycle lengths is not as clear, taking into account recent resultson the time variability of the longer term solar cycles. While the timebase of the observations puts an upper limit to the detectable lengthsof the longer cycles, a general dependence of the cycle period on therotation rate is maintained. The length of the shortest cycle that wefound for LQ Hya agrees with the cycle period derived from dynamomodelling of this star.
| Measuring starspots on magnetically active stars with the VLTI
We present feasibility studies to directly image stellar surfacefeatures, which are caused by magnetic activity, with the Very LargeTelescope Interferometer (VLTI). We concentrate on late typemagnetically active stars, for which the distribution of starspots onthe surface has been inferred from photometric and spectroscopic imaginganalysis. The study of the surface spot evolution during consecutiverotation cycles will allow first direct measurements (apart from theSun) of differential rotation which is the central ingredient ofmagnetic dynamo processes. The VLTI will provide baselines of up to 200m, and two scientific instruments for interferometric studies at near-and mid-infrared wavelengths. Imaging capabilities will be made possibleby closure-phase techniques. We conclude that a realistically modeledcool surface spot can be detected on stars with angular diametersexceeding ~ 2 mas using the VLTI with the first generation instrumentAMBER. The spot parameters can then be derived with reasonable accuracy.We discuss that the lack of knowledge of magnetically active stars ofthe required angular size, especially in the southern hemisphere, is acurrent limitation for VLTI observations of these surface features.
| Multiwavelength optical observations of chromospherically active binary systems. IV. The X-ray/EUV selected binary BK Psc (2RE J0039+103)
We present high resolution echelle spectra taken during four observingruns from 1999 to 2001 of the recently X-ray/EUV selectedchromospherically active binary BK Psc (2RE J0039+103). Our observationsconfirm the single-lined spectroscopic binary (SB1) nature of thissystem and allow us to obtain, for the first time, the orbital solutionof the system as in the case of a SB2 system. We have determined preciseradial velocities of both components: for the primary by using the crosscorrelation technique, and for the secondary by using its chromosphericemission lines. We have obtained a circular orbit with an orbital periodof 2.1663 days, very close to its photometric period of 2.24 days(indicating synchronous rotation). The spectral type (K5V) we determinedfor our spectra and the mass ratio (1.8) and minimum masses (Msin3i) resulting from the orbital solution are compatible withthe observed K5V primary and an unseen M3V secondary. Using thisspectral classification, the projected rotational velocity (vsin i, of17.1 km s-1) obtained from the width of the cross-correlationfunction and the data provided by HIPPARCOS, we have derived otherfundamental stellar parameters. The kinematics and the non-detection ofthe Li I line indicate that it is an old star. The analysis of theoptical chromospheric activity indicators from the Ca II H & K to CaII IRT lines, by using the spectral subtraction technique, indicatesthat both components of the binary system show high levels ofchromospheric activity. Hα emission above the continuum from bothcomponents is a persistent feature of this system during the period 1999to 2001 of our observations as well as in previous observations. TheHα and Hβ emission seems to arise from prominence-likematerial, and the Ca II IRT emission from plage-like regions. Based onobservations made with the 2.2 m telescope of the German-SpanishAstronomical Centre, Calar Alto (Almería, Spain), operated by theMax-Planck-Institute for Astronomy, Heidelberg, jointly with the SpanishNational Commission for Astronomy, with the Nordic Optical Telescope(NOT), operated on the island of La Palma jointly by Denmark, Finland,Iceland, Norway and Sweden, in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias, and withthe Isaac Newton Telescope (INT) operated on the island of La Palma bythe Isaac Newton Group in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias.
| Variability of the Photospheric Radiation of Active K-M Dwarfs and Their X-ray Luminosities
Variability of the photospheric radiation of 40 (dKe-dMe) dwarfs in thesolar neighborhood due to variations in the spottedness of theirsurfaces is analyzed based on the behavior of their mean annualbrightnesses over long time intervals. The amplitudes and characteristictime scales of the variations of the mean annual brightness are taken tobe indicators of photospheric activity and were used to infer the levelsof photospheric activity in the stars studied. The influence of axialrotation on the development of cyclic activity in young red dwarfs andF-M main-sequence stars is analyzed. The durations and amplitudes of thephotospheric variability of rapidly rotating (dK0e-dK5e) stars testifiesto a higher level of photospheric activity among red dwarfs andsolar-type stars. The X-ray luminosities of these stars grow with theamplitude of the variations of the mean annual brightness. However, thisis not typical of rapidly rotating M dwarfs, for which the X-rayemission varies by more than two orders of magnitude, although theirdegrees of spottedness are all virtually the same. A linear relationshipbetween the X-ray and bolometric luminosities is observed for young(dKe-dMe) stars, with their ratios log(L x/L bol) being about -3. Theseproperties can be used to determine whether a red dwarf is a young staror is already on the main sequence.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe 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/386/492, and from theauthors on CD-Rom.
| Planet Host Stars: Mass, Age and Kinematics
We determine the mass, age and kinematics of 51 extra-solar planet hoststars. The results are then used to search for signs of connection ofthe data with metallicity and to investigate the population nature. Wefind that the increase in mean metallicity with stellar mass is similarto that in normal field stars, so it seems unsuitable to use thisrelation as a constraint on the theory of planet formation. The age andkinematic distributions seem to favour the metallicity of extra-solarplanet host stars being initial. Although the kinematic data of thesestars indicate their origin from two populations -- the thin and thethick disks, kinematics may not help in the maintenance of the planetaround the host. Stars with planets, brown dwarfs or stellar companionsare sorted into three groups and re-investigated separately for theirformation mechanism. The main results indicate that stars withM2 < 25 MJ have [Fe/H] > -0.1 and a wideperiod range, but there are no other differences. Thus, there does notseem to be any physically distinguishable characteristics among thethree star groups.
| The Smallest Mass Ratio Young Star Spectroscopic Binaries
Using high-resolution near-infrared spectroscopy with the KeckTelescope, we have detected the radial velocity signatures of the coolsecondary components in four optically identified pre-main-sequence,single-lined spectroscopic binaries. All are weak-lined T Tauri starswith well-defined center-of-mass velocities. The mass ratio for oneyoung binary, NTTS 160905-1859, isM2/M1=0.18+/-0.01, the smallest yet measureddynamically for a pre-main-sequence spectroscopic binary. These newresults demonstrate the power of infrared spectroscopy for the dynamicalidentification of cool secondaries. Visible-light spectroscopy, to date,has not revealed any pre-main-sequence secondary stars with masses<0.5 Msolar, while two of the young systems reported hereare in that range. We compare our targets with a compilation of thepublished young, double-lined spectroscopic binaries and discuss ourunique contribution to this sample.
| Convective Intensification of Magnetic Flux Tubes in Stellar Photospheres
The convective collapse of thin magnetic flux tubes in the photospheresof Sun-like stars is investigated using realistic models of thesuperadiabatic upper convection zone layers of these stars. Thestrengths of convectively stable flux tubes are computed as a functionof surface gravity and effective temperature. We find that while starswith Teff>=5500 K and logg>=4.0 show flux tubes highlyevacuated of gas, and hence strong field strengths due to convectivecollapse, cooler stars exhibit flux tubes with lower field strengths.Observations reveal the existence of field strengths close to thermalequipartition limits even in cooler stars, implying highly evacuatedtubes, for which we suggest possible reasons.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| The X-ray Emission and the Magnetic Fields of Late-Type Stars
We present the results of X-ray luminosities of some active late-typestars, based on data primarily from the ROSAT position sensitiveproportional counter (PSPC). According to the observations, we dividethe stars into four groups: single, wide binary, binary and RS CVn. Weinvestigated the correlation between the X-ray emission and the hardnessratio, coronal temperature, magnetic field strength, magnetic fluxdensity. Our results suggest that the magnetic field plays a veryimportant role in stellar X-ray emission.
| The Catania Automatic Photoelectric Telescope on Mt. Etna: a systematic study of magnetically active stars
A photometric monitoring of about 50 magnetically active stars, that arespread almost all over the H-R diagram, was initiated at the mountainstation of Catania Observatory on Mt. Etna (1750-m a.s.l.) in 1992 withan 80-cm robotic telescope (APT-80) built by AutoScope Co. (USA). Thissystematic survey is now approaching its 10th year anniversary. For mostof the stars, quite well defined solar-like spot maps have been derivedfrom UBV data obtained in different epochs. These data have allowed usto investigate some relevant characteristics of spot activity andvariability on stars, and to obtain clear evidence of long-term activitycycles, in the range from a few to about 10 years, on some of theobserved targets. Starspot maps are constructed by using advanced tools,such as massive parallel computing and are based on Maximum Entropy andTikhonov regularization criteria. Selected results are here presented.Our systematic observation program is still underway and a secondAPT80/2, equipped with a CCD camera, will pair the APT80/1 on the samesite. Its operation is foreseen for mid 2002.
| Analysis of the Hipparcos Observations of the Extrasolar Planets and the Brown Dwarf Candidates
We analyzed the Hipparcos astrometric observations of 47 stars that werediscovered to harbor giant planets and 14 stars with brown dwarfsecondary candidates. The Hipparcos measurements were used together withthe corresponding stellar radial velocity data to derive an astrometricorbit for each system. To find out the significance of the derivedastrometric orbits, we applied a ``permutation'' technique by which weanalyzed the permuted Hipparcos data to get false orbits. The sizedistribution of these false orbits indicated the range of possiblyrandom, false orbits that could be derived from the true data. Thesetests could not find any astrometric orbit of the planet candidates withsignificance higher than 99%, suggesting that most if not all orbits arenot real. Instead, we used the Hipparcos data to set upper limits on themasses of the planet candidates. The lowest derived upper limit is thatof 47 UMa-0.014 Msolar, which confirms the planetary natureof its unseen companion. For 13 other planet candidates, the upperlimits exclude the stellar nature of their companions, although browndwarf secondaries are still an option. These negate the idea that all ormost of the extrasolar planets are disguised stellar secondaries. Of the14 brown dwarf candidates, our analysis reproduced the results ofHalbwachs et al., who derived significant astrometric orbits for sixsystems that imply secondaries with stellar masses. We show that anotherstar, HD 164427, which was discovered only very recently, also has asecondary with stellar mass. Our findings support Halbwachs et al.'sconclusion about the possible existence of the ``brown dwarf desert''that separates the planets and the stellar secondaries.
| Hydroxyl 1.563 Micron Absorption from Starspots on Active Stars
We present results from a study of starspots on active stars using apair of vibrational-rotational absorption lines of the OH molecule near1.563 μm. We detect excess OH absorption due to dark, cool starspotson several active stars of the RS CVn and BY Dra classes. Our resultsfor the single-lined spectroscopic binaries II Pegasi, V1762 Cygni, andλ Andromedae augment those from a previous study that used a lesssensitive detector. In this study, we were able for the first time touse molecular absorption features to measure starspot properties ondouble-lined spectroscopic binaries. Measuring the equivalent widths ofthese OH lines in inactive giant and dwarf stars of spectral types G, K,and M, we find that the total equivalent width of the line pairincreases approximately linearly as effective temperature decreases from5000 to 3000 K. We measure starspot filling factors by fitting thespectra of active stars with linear combinations of comparison starspectra representing the spot and nonspot regions of the star.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Телец |
| Прямое восхождение: | 04h36m48.24s |
| Склонение: | +27°07'55.9" |
| Видимая звёздная величина: | 8.109 |
| Расстояние: | 17.851 парсек |
| Собственное движение RA: | 233.2 |
| Собственное движение Dec: | -148.6 |
| B-T magnitude: | 9.5 |
| V-T magnitude: | 8.224 |
Каталоги и обозначения:
|
