Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Fundamental properties of pre-main sequence stars in young, southern star forming regions: metallicities
Aims. The primary motivation for this project is to search formetal-rich star forming regions, in which, stars of super-solarmetallicity will be created, as hopefully, will be extra-solar planetsorbiting them! The two aims of this project are: 1) to show that oursample stars are young, lithium rich, magnetically active andnon-accreting kinematic members of their respective regions. 2) Tomeasure the metallicity for such members. Methods. The feroséchelle spectrograph together with eso's 2.2 m telescope, wasused to obtain high resolution (R = 32 000) spectra for each of ourweak-lined T-Tauri target stars. The wavelength range of the spectra is≃4000{-}8000 Å. Results. We find (pre-main sequence)model-dependent isochronal ages of the Lupus, Chamaeleon and CrA targetsto be 9.1 ± 2.1 Myr, 4.5 ± 1.6 Myr and 9.0 ± 3.9Myr respectively. The majority of the stars have Li i 6707.8 Åequivalent widths similar to, or above those of, their similar massPleiades counterparts, confirming their youthfulness. Most stars arekinematic members, either single or binary, of their regions. We find amean radial velocity for objects in the Lupus cloud to be RV=+2.6± 1.8 km s-1, for the Chamaeleon i & ii clouds,RV=+12.8 ± 3.6 km s-1 whereas for the CrA cloud, wefind RV=-1.1 ± 0.5 km s-1. All stars are coronally andchromospherically active, exhibiting X-ray and Hα emission levelsmarginally less, approximately equal or superior to that of their olderIC 2602/2391 and/or Pleiades counterparts. All bar three of the targetsshow little or no signature of accretion from a circumstellarenvironment, according to their positions in a J-K/H-K'diagram. For the higher quality spectra, we have performed an iron-linemetallicity analysis for five (5) stars in Chamaeleon, four (4) stars inLupus and three (3) stars in the CrA star forming regions. These resultsshow that all three regions are slightly metal-poor, with marginallysub-solar metallicities, with <[Fe/H]> = -0.11 ± 0.14,-0.10 ± 0.04 and -0.04 ± 0.05 respectively. Conclusions. Asample of stars in several nearby, young star-forming regions has beenestablished, the majority of which is young, lithium rich, magneticallyactive and are non-accreting kinematic members of their respectiveclouds. Within the errors, each region is essentially of solarmetallicity.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| 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
| Multiplicity of X-Ray-selected T Tauri Stars in Chamaeleon
We report on a multiplicity survey of a sample of X-ray-selected youngstars in the Chamaeleon association. We used speckle interferometry anddirect imaging to find companions in the separation range 0.13"-6".After correction for chance alignment with background stars, we find amultiplicity (number of binaries or multiples divided by number ofsystems) of 14.0%+/-4.3% and a companion-star frequency (number ofcompanions divided by number of systems) of 14.7%+/-5.1%. Compared withsolar-type main-sequence stars, the companion-star frequency is lower bya factor of 0.61+/-0.27. This is remarkably different from the highmultiplicity found in the Taurus-Auriga star-forming region and for TTauri stars in Chamaeleon known before ROSAT. We find only a fewbinaries with projected separations of more than 70 AU, also in contrastto the results for stars known before ROSAT. This indicates that theX-ray-selected stars belong to a different population than the starsknown before ROSAT, a hypothesis further supported by their Hipparcosdistances and proper motions. Based on observations obtained at theEuropean Southern Observatory, La Silla, proposal 56.E-0197.
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Proper motions of faint ROSAT WTT stars in the Chamaeleon region
We present proper motions of 59 stars of the ROSAT All-Sky Survey (RASS)located in direction of the Chamaeleon star forming region (SFR) in themagnitude range B=5.1 - 17 mag. Proper motions of the fainter stars werenewly derived utilizing survey Schmidt plates from the GSC II platearchive and from a set of special plates taken with the ESO Schmidttelescope. The vector point diagram (VPD) indicates that the certifiedWTT stars cluster away from the region occupied by the brighterpre-main-sequence stars (PMS) in Cha I. The distance to this newassociation is estimated at ~ 100 pc, sensibly smaller than the 150 pcgenerally assumed for the SFR. This yields an upper limit of 2 km s(-1)for the velocity dispersion of this new kinematic group. The de-reddenedCM diagram of the group members suggest the WTT stars are still PMSobjects, but older (3-30 Myr) and less massive than previousdeterminations. These revised age estimates, the newly derived grouppeculiar velocity, and current distance estimates to the Cha I/II/IIIcomplex would favour in-situ formation against that predicted by highvelocity cloud models. Finally, based on a redetermination of thepeculiar motions of stars and gas, we speculate that the whole SFRoriginated from the local Orion spur as a result of more classicalmechanisms like interactions with the spiral arms. Based on observationscollected at the European Southern Observatory (Chile) and on data fromthe Hipparcos astrometry satellite. Table~1 is available only inelectronic form at the CDS via anonymous ftp at 130.79.128.5.
| Kinematics of T Tauri stars in Chamaeleon
We study the kinematics of T Tauri stars (TTS) located in the cores ofthe Chamaeleon clouds as well as far off these clouds. Our samplecomprises 2 early type stars known to be related to Cha i, 6 classical(CTTS) and 6 weak-line T Tauri stars (WTTS) known before the ROSATmission, and 8 bona-fide pre-main sequence (PMS) stars as well as 23presumably older stars discovered with ROSAT (Alcalaet al. 1995; Covinoet al. 1997). Altogether we present proper motions for 45 stars, takenfrom the Hipparcos, ACT and STARNET catalogues. For 12 stars of oursample parallaxes measured by Hipparcos are available, and we use themto derive constraints on the distance distribution of the other stars inour sample. Our analysis of the proper motions allows us to divide thesample into several subgroups. We analyse the motions of the stars inconnection with different star formation scenarios and find themconsistent with both the high velocity cloud (HVC) impact model (Lepine& Duvert 1994) and the cloudlet model (Feigelson 1996), whereas thedata seem to be inconsistent with any kind of a dynamical ejectionmodel.
| HIPPARCOS results for ROSAT-discovered young stars
Out of ~ 500 Lithium-rich ROSAT counterparts, which were presumed to below-mass pre-main sequence stars, 21 stars have been observed byHIPPARCOS. We study their parallaxes, proper motions, and photometricdata. For 7 out of 10 Taurus and Lupus stars in our sample, propermotions and parallaxes are not inconsistent with membership to theseassociations, while most of the stars in Chamaeleon and Scorpius appearto be young foreground stars. Combined with ground based photometry andspectroscopy, HIPPARCOS parallaxes allow us to place 15 stars on an H-Rdiagram. All these 15 stars lie above the Zero-Age-Main-Sequence andthus are indeed pre-main sequence stars with ages from 1 to 15 Myr. Onlytwo of the stars are located on the Hayashi-tracks, whereas the other 13are post-TTauri stars located on radiative tracks. Although the sampleis admittedly small, containing only 3% of the total sample ofLithium-rich ROSAT counterparts, it does not confirm recent predictionsby other authors: We find no stars in the age range from 20 to 100 Myr.The foreground pre-main sequence stars may have been ejected toward us,or they belong to the Gould Belt system, a plane filled with youngstars.
| A study of the Chamaeleon star-forming region from the ROSAT all-sky survey. III. High resolution spectroscopic study
We present the results of a high-resolution spectroscopic study on some70 stars, discovered recently on the basis of the ROSAT all-sky surveyspread over a wide area in the Chamaeleon star forming region andclassified as new weak-line T Tauri stars. We refine the previousspectral type classification, based on low-resolution spectra, andcharacterize each star in the sample according to the Hα lineprofile. We use the strength of Li I 6708, compared to Pleiades stars ofthe same spectral type, as a youth discriminator in order to recognizebona-fide pre-main sequence stars. According to the adopted ``lithiumcriterion'', more than 50% of the stars in our sample are confirmed tobe truly young, PMS stars (most having age less than 5x 10(6) yr), whilethe remaining part seems mostly composed by active, young, foregroundmain-sequence stars (possibly Pleiades-like), which contaminate theoriginal sample. We confirm the existence of some very young stars farfrom the main Chamaeleon clouds, while we do not find clear evidence forthe presence of post-T Tauri stars in our sample. We find that 5 starsin the sample are spectroscopic binaries and 1 is a spectroscopic triplesystem. We derive radial and rotational velocities for all the stars insample and analyse their distributions for different spectral typeintervals. The radial velocity distribution shows a clear peak at about15 km s(-1) , which coincides with the radial velocity of stars and gasin the Cha I cloud. However, the velocity dispersion of the weak-line TTauri stars appears much broader and, possibly, a second peak is presentaround 16-18 km s(-1) . A clear segregation in radial velocity isobserved between the strong-lithium and the weak-lithium stars, with theformer showing radial velocities which, in most cases, fall in theinterval 12
| A study of the Chamaeleon star-forming region from the ROSAT All-Sky Survey. II. The pre-main sequence population.
We analyse the nature of the optical counterparts of the ROSAT all-skysurvey (RASS) X-ray sources identified with new weak-line T Tauri (WTTS)stars in the Chamaeleon star forming region (SFR). The new WTTS aredistributed throughout the whole SFR, while the classical T Tauri stars(CTTS) are found only in the cloud cores. Adopting a distance of 150pcwe derive the stellar parameters and place the new WTTS in the HRdiagram. By comparison with theoretical pre-main sequence (PMS)evolutionary tracks, we find masses in the range of0.2-2.5Msun_ and ages from a few 10^5^yr to 5x10^7^yr. Manyof the youngest WTTS are located far away from the main Chamaeleon darkclouds. By comparing the properties of the new WTTS with those of thepreviously known Chamaeleon members, we obtain the following results: i)the new WTTS are, on average, the more massive and luminous PMS stars inChamaeleon, while the Cha II population contains the lower-mass PMSstars; ii) for stellar masses between 2.5 and 0.5Msun_, thecombined mass distribution of the PMS stars is consistent with theinitial mass function (IMF) for field stars, but declines rapidly formasses between 0.5 and 0.1Msun_, where the strongestselection effects are expected; iii) a weak trend for increasing agewith increasing angular distance from the cloud cores is observed but wecannot establish an age segregation since very young WTTS are also foundfar away from the molecular clouds; iv) the age distributions of the newWTTS and the Cha I population are nearly identical, while that of theCha II population is shifted towards younger stars indicating that ChaII is probably in an earlier evolutionary phase as compared with Cha Iand the new WTTS; v) no decrease of the number density of WTTS isobserved with increasing distance to the clouds; vi) the level of X-rayemission of the new WTTS is higher than that of the previously knownChamaeleon members, and the fraction of energy released as X-rayemission, is higher in the new WTTS than in the Cha I TTS. The latter issimilar to the X-ray emission level found in open clusters. Finally, wediscuss possible mechanisms which may give rise to the observed spatialdistribution of the PMS stars in Chamaeleon.
| A study of the Chamaeleon star forming region from the ROSAT all-sky survey. I. X-ray observations and optical identifications.
We present the observations of the ROSAT all-sky survey (RASS) in thedirection of the Chamaeleon cloud complex, as well as the spectroscopicidentifications of the detected X-ray sources. The main purpose of thisidentification program was the search for low mass pre-main sequencestars. Sixteen previously known PMS stars were detected with highconfidence by ROSAT. Eight are classical T Tauri stars and eight areweak-line T Tauri stars, Seventy-seven new weak-line T Tauri stars wereidentified on the basis of the presence of strong Li λ 6707absorption, spectral type later than F0 and chromospheric emission. Wegive coordinates and count rates of the X-ray sources, and presentoptical spectra and finding charts for the sources identified opticallyas new pre-main sequence stars. Optical UBV(RI)_c_ and near-infraredJHKLM photometry for this sample of stars is also provided. In addition,6 new dKe-dMe candidates are found among the RASS sources.
| A large, complete, volume-limited sample of G-type dwarfs. I. Completion of Stroemgren UVBY photometry
Four-colour photometry of potential dwarf stars of types G0 to K2,selected from the Michigan Spectral Catalogues (Vol. 1-3), has beencarried out. The results are presented in a catalogue containing 4247uvby observations of 3900 stars, all south of δ = -26deg. Theoverall internal rms errors of one observation (transformed to thestandard system) of a program star in the interval 8.5 < V < 10.5are 0.0044, 0.0021, 0.0039, and 0.0059, respectively, in V, b-y, m_1_ ,and c_1_. The purpose of the catalogue, combined with earliercatalogues, is to allow selection of a large, complete, volume-limitedsample of G- and K-type dwarfs, investigate their metallicitydistribution, and compare it to predictions of various models ofgalactic chemical evolution. Future papers in this series will discussthese subjects.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Мува |
| Ректацензија: | 12h33m29.78s |
| Deклинација: | -75°23'11.2" |
| Apparent магнитуда: | 9.452 |
| Proper motion RA: | -95.6 |
| Proper motion Dec: | 16.4 |
| B-T magnitude: | 10.442 |
| V-T magnitude: | 9.534 |
Каталог и designations:
|
INHABITED SKY
