Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
The Formation and Evolution of Planetary Systems: Description of the Spitzer Legacy Science Database
We present the science database produced by the Formation and Evolutionof Planetary Systems (FEPS) Spitzer Legacy program. Data reduction andvalidation procedures for the IRAC, MIPS, and IRS instruments aredescribed in detail. We also derive stellar properties for the FEPSsample from available broadband photometry and spectral types, andpresent an algorithm to normalize Kurucz synthetic spectra to opticaland near-infrared photometry. The final FEPS data products include IRACand MIPS photometry for each star in the FEPS sample and calibrated IRSspectra.
| Spitzer MIPS Observations of Stars in the β Pictoris Moving Group
We present Multiband Imaging Photometer for Spitzer (MIPS) observationsat 24 and 70 μm for 30 stars, and at 160 μm for a subset of 12stars, in the nearby (~30 pc), young (~12 Myr) β Pictoris movinggroup (BPMG). In several cases, the new MIPS measurements resolve sourceconfusion and background contamination issues in the IRAS data for thissample. We find that 7 members have 24 μm excesses, implying a debrisdisk fraction of 23%, and that at least 11 have 70 μm excesses (diskfraction of >=37%). Five disks are detected at 160 μm (out of abiased sample of 12 stars observed), with a range of 160/70 flux ratios.The disk fraction at 24 and 70 μm, and the size of the excessesmeasured at each wavelength, are both consistent with an ``inside-out''infrared excess decrease with time, wherein the shorter wavelengthexcesses disappear before longer wavelength excesses, and consistentwith the overall decrease of infrared excess frequency with stellar age,as seen in Spitzer studies of other young stellar groups. Assuming thatthe infrared excesses are entirely due to circumstellar disks, wecharacterize the disk properties using simple models and fractionalinfrared luminosities. Optically thick disks, seen in the younger TW Hyaand η Cha associations, are entirely absent in the BPMG. Additionalflux density measurements at 24 and 70 μm are reported for nineTucana-Horologium association member stars. Since this is <20% of theassociation membership, limited analysis on the complete disk fractionof this association is possible.
| Mg II h+k emission lines as stellar activity indicators of main sequence F-K stars
Context: The largest dataset of stellar activity measurements availableat present is the one obtained at the Mount Wilson Observatory, wherehigh-precision Ca II H+K fluxes have been measured from 1966 for about2200 stars. Since the Mg II h and k lines at λ2800 Å areformed in a similar way to the Ca II H+K emission lines, they are alsogood indicators of chromospheric structure. The InternationalUltraviolet Explorer (IUE) provides a large database of UV spectra inthe band 1150-3350 Å from 1978 to 1995, which can also be used tostudy stellar activity. Aims: The main purpose of this study is to usethe IUE spectra in the analysis of magnetic activity of main sequenceF-K stars. Combining IUE observations of Mg II and optical spectroscopyof Ca II, the registry of activity of stars can be extended in time. Methods: We retrieved all the high-resolution spectra of F, G, and Kmain sequence stars observed by IUE (i.e. 1623 spectra of 259 F to Kdwarf stars). We obtained the continuum surface flux near the Mg II h+klines near λ2800 Å and the Mg II line-core surface fluxfrom the IUE spectra. Results: We obtained a relation between the meancontinuum flux near the Mg II lines with the colour B-V of the star. Fora set of 117 nearly simultaneous observations of Mg II and Ca II fluxesof 21 F5 to K3 main sequence stars, we obtained a colour dependentrelation between the Mount Wilson Ca II S-index and the Mg II emissionline-core flux. As an application of this calibration, we computed theMount Wilson index for all the dF to dK stars which have high resolutionIUE spectra. For some of the most frequently observed main sequencestars, we analysed the Mount Wilson index S from the IUE spectra,together with the ones derived from visible spectra. We confirm thecyclic chromospheric activity of ɛ Eri (HD 22049) and βHydri (HD 2151), and we find a magnetic cycle in α Cen B (HD128621)Table 3 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/483/903
| On the kinematic evolution of young local associations and the Scorpius-Centaurus complex
Context: Over the last decade, several groups of young (mainly low-mass)stars have been discovered in the solar neighbourhood (closer than ~100pc), thanks to cross-correlation between X-ray, optical spectroscopy andkinematic data. These young local associations - including an importantfraction whose members are Hipparcos stars - offer insights into thestar formation process in low-density environments, shed light on thesubstellar domain, and could have played an important role in the recenthistory of the local interstellar medium. Aims: To study the kinematicevolution of young local associations and their relation to other youngstellar groups and structures in the local interstellar medium, thuscasting new light on recent star formation processes in the solarneighbourhood. Methods: We compiled the data published in theliterature for young local associations. Using a realistic Galacticpotential we integrated the orbits for these associations and theSco-Cen complex back in time. Results: Combining these data with thespatial structure of the Local Bubble and the spiral structure of theGalaxy, we propose a recent history of star formation in the solarneighbourhood. We suggest that both the Sco-Cen complex and young localassociations originated as a result of the impact of the inner spiralarm shock wave against a giant molecular cloud. The core of the giantmolecular cloud formed the Sco-Cen complex, and some small cloudlets ina halo around the giant molecular cloud formed young local associationsseveral million years later. We also propose a supernova in young localassociations a few million years ago as the most likely candidate tohave reheated the Local Bubble to its present temperature.
| Kinematics of the Scorpius-Centaurus OB association
A fine structure related to the kinematic peculiarities of threecomponents of the Scorpius-Centaurus association (LCC, UCL, and US) hasbeen revealed in the UV-velocity distribution of Gould Belt stars. Wehave been able to identify the most likely members of these groups byapplying the method of analyzing the two-dimensional probability densityfunction of stellar UV velocities that we developed. A kinematicanalysis of the identified structural components has shown that, ingeneral, the center-of-mass motion of the LCC, UCL, and US groupsfollows the motion characteristic of the Gould Belt, notably itsexpansion. The entire Scorpius-Centaurus complex is shown to possess aproper expansion with an angular velocity parameter of 46 ± 8 kms‑1 kpc‑1 for the kinematic centerwith l 0 = ‑40° and R 0 = 110 pc found.Based on this velocity, we have estimated the characteristic expansiontime of the complex to be 21 ± 4 Myr. The proper rotationvelocity of the Scorpius-Centaurus complex is lower in magnitude, isdetermined less reliably, and depends markedly on the data quality.
| The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
We provide an overview of the Spitzer Legacy Program, Formation andEvolution of Planetary Systems, that was proposed in 2000, begun in2001, and executed aboard the Spitzer Space Telescope between 2003 and2006. This program exploits the sensitivity of Spitzer to carry outmid-infrared spectrophotometric observations of solar-type stars. With asample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace theevolution of circumstellar gas and dust from primordial planet-buildingstages in young circumstellar disks through to older collisionallygenerated debris disks. When completed, our program will help define thetimescales over which terrestrial and gas giant planets are built,constrain the frequency of planetesimal collisions as a function oftime, and establish the diversity of mature planetary architectures. Inaddition to the observational program, we have coordinated a concomitanttheoretical effort aimed at understanding the dynamics of circumstellardust with and without the effects of embedded planets, dust spectralenergy distributions, and atomic and molecular gas line emission.Together with the observations, these efforts will provide anastronomical context for understanding whether our solar system-and itshabitable planet-is a common or a rare circumstance. Additionalinformation about the FEPS project can be found on the team Web site.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Accretion Disks around Young Stars: Lifetimes, Disk Locking, and Variability
We report the findings of a comprehensive study of disk accretion andrelated phenomena in four of the nearest young stellar associationsspanning 6-30 million years in age, an epoch that may coincide with thelate stages of planet formation. We have obtained ~650 multiepochhigh-resolution optical spectra of 100 low-mass stars that are likelymembers of the η Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), βPictoris (~12 Myr), and Tucanae-Horologium (~30 Myr) groups. Our datawere collected over 12 nights between 2004 December and 2005 July on theMagellan Clay 6.5 m telescope. Based on Hα line profiles, alongwith a variety of other emission lines, we find clear evidence ofongoing accretion in 3 out of 11 η Cha stars and 2 out of 32 TWHydrae members. None of the 57 β Pic or Tuc-Hor members showsmeasurable signs of accretion. Together, these results imply significantevolution of the disk-accretion process within the first several Myr ofa low-mass star's life. While a few disks can continue to accrete for upto ~10 Myr, our findings suggest that disks accreting for beyond thattimescale are rather rare. This result provides an indirect constrainton the timescale for gas dissipation in inner disks and, in turn, ongas-giant planet formation. All accretors in our sample are slowrotators, whereas nonaccretors cover a large range in rotationalvelocities. This may hint at rotational braking by disks at ages up to~8 Myr. Our multiepoch spectra confirm that emission-line variability iscommon even in somewhat older T Tauri stars, among which accretors tendto show particularly strong variations. Thus, our results indicate thataccretion and wind activity undergo significant and sustained variationsthroughout the lifetime of accretion disks.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| 51 Eridani and GJ 3305: A 10-15 Myr old Binary Star System at 30 Parsecs
Following the suggestion of Zuckerman and coworkers, we consider theevidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historicallyclassified as unrelated main-sequence stars in the solar neighborhood,are instead a wide physical binary system and members of the youngβ Pic moving group (BPMG). The BPMG is the nearest (d<~50 pc) ofseveral groups of young stars with ages around 10 Myr that arekinematically convergent with the Oph-Sco-Cen association (OSCA), thenearest OB star association. Combining South African AstronomicalObservatory optical photometry, Hobby-Eberly Telescope high-resolutionspectroscopy, Chandra X-Ray Observatory data, and Second US NavalObservatory CCD Astrograph Catalog kinematics, we confirm with highconfidence that the system is indeed extremely young. GJ 3305 itselfexhibits very strong magnetic activity but has rapidly depleted most ofits lithium. The 51 Eri/GJ 3305 system is the westernmost known memberof the OSCA, lying 110 pc from the main subgroups. The system is similarto the BPMG wide binary HD 172555/CD -64 1208 and the HD 104237 quintet,suggesting that dynamically fragile multiple systems can survive theturbulent environments of their natal giant molecular cloud complexes,while still having high dispersion velocities imparted. Nearby youngsystems such as these are excellent targets for evolved circumstellardisk and planetary studies, having stellar ages comparable to that ofthe late phases of planet formation.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Evolution of Cold Circumstellar Dust around Solar-type Stars
We present submillimeter (Caltech Submillimeter Observatory 350 μm)and millimeter (Swedish-ESO Submillimetre Telescope [SEST] 1.2 mm, OwensValley Radio Observatory [OVRO] 3 mm) photometry for 127 solar-typestars from the Formation and Evolution of Planetary Systems SpitzerLegacy program that have masses between ~0.5 and 2.0 Msolarand ages from ~3 Myr to 3 Gyr. Continuum emission was detected towardfour stars with a signal-to-noise ratio>=3: the classical T Tauristars RX J1842.9-3532, RX J1852.3-3700, and PDS 66 with SEST, and thedebris-disk system HD 107146 with OVRO. RX J1842.9-3532 and RXJ1852.3-3700 are located in projection near the R CrA molecular cloud,with estimated ages of ~10 Myr (Neuhäuser et al.), whereas PDS 66is a probable member of the ~20 Myr old Lower Centaurus-Crux subgroup ofthe Scorpius-Centaurus OB association (Mamajek et al.). The continuumemission toward these three sources is unresolved at the 24" SESTresolution and likely originates from circumstellar accretion disks,each with estimated dust masses of ~5×10-5Msolar. Analysis of the visibility data toward HD 107146(age~80-200 Myr) indicates that the 3 mm continuum emission is centeredon the star within the astrometric uncertainties and resolved with aGaussian-fit FWHM size of (6.5"+/-1.4")×(4.2"+/-1.3"), or185AU×120 AU. The results from our continuum survey are combinedwith published observations to quantify the evolution of dust mass withtime by comparing the mass distributions for samples with differentstellar ages. The frequency distribution of circumstellar dust massesaround solar-type stars in the Taurus molecular cloud (age~2 Myr) isdistinguished from that around 3-10 Myr and 10-30 Myr old stars at asignificance level of ~1.5 and ~3 σ, respectively. These resultssuggest a decrease in the mass of dust contained in small dust grainsand/or changes in the grain properties by stellar ages of 10-30 Myr,consistent with previous conclusions. Further observations are needed todetermine if the evolution in the amount of cold dust occurs on evenshorter timescales.
| On the Rotation of Post-T Tauri Stars in Associations
Nearby associations or moving groups of post-T Tauri stars with agesbetween ~10 and 30 Myr are excellent objects for the study of theinitial spin-up phase during the pre-main-sequence evolution. Anempirical approach is adopted here for the first time with these starsto infer their rotations, properties, and relations to X-ray emission.Three nearby associations with distances less than 100 pc areconsidered: the TW Hydrae association (TWA) with an age of 8 Myr, theβ Pictoris moving group (BPMG) with an age of 12 Myr, and acombination of Tucana and Horologium associations (Tuc/HorA; 30 Myr).Two low- and high-rotation modes are considered for each association,with stellar masses of0.1Msolar<=M<1.5Msolar and1.5Msolar<=M<=2.6Msolar, respectively.Because no observed rotational periods are known for these stars, we usea mathematical tool to infer representative equatorial rotationvelocities v0(eq) from the observed distribution of projectedrotational velocities (vsini). This is done for each mode and for eachassociation. A spin-up is found for the high-rotation mode, whereas inthe low-rotation mode the v0(eq) do not increasesignificantly. This insufficient increase of v0(eq) isprobably the cause of a decrease of the total mean specific angularmomentum for the low-mass stars between 8 and 30 Myr. However, for thehigh-mass stars, where a sufficient spin-up is present, the specificangular momentum is practically conserved in this same time interval. Atwo-dimensional (mass and vsini) K-S statistical test yields resultscompatible with a spin-up scenario. By supposing that the distributionof the masses of these three associations follows a universal massfunction, we estimate the number of members of these associations thatremain to be detected. The analysis of rotational and stellar massesusing the luminosity X-ray indicators LX andLX/Lb present similar properties, as does thedependence on stellar mass and rotation, at least for the youngerassociations TWA and BPMG, to those obtained for T Tauri stars in theOrion Nebula Cluster (1 Myr). A strong desaturation effect appears at~30 Myr, the age of Tuc/HorA, measured essentially by the early-G andlate-F type stars. This effect seems to be provoked by the minimumconfiguration of the stellar convection layers, attained for the firsttime for the higher mass stars at ~30 Myr. The desaturation appears tobe independent of rotation at this stage.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| A Submillimeter Search of Nearby Young Stars for Cold Dust: Discovery of Debris Disks around Two Low-Mass Stars
We present results from a James Clerk Maxwell Telescope/SCUBA 850 μmsearch for cold dust around eight nearby young stars belonging to theβ Pic (t~12 Myr) and the Local Association (t~50 Myr) movinggroups. Unlike most past submillimeter studies, our sample was chosensolely on the basis of stellar age. Our observations achieve about anorder of magnitude greater sensitivity in dust mass compared to previouswork in this age range. We detected two of the three M dwarfs in oursample at 850 μm, GJ 182 and GJ 803 (M*~0.5Msolar), with inferred dust masses of only ~0.01-0.03M⊕. GJ 182 may also possess a 25 μm excess, which isindicative of warm dust in the inner few AU of its disk. For GJ 803 (AUMic; HD 197481), submillimeter mapping finds that the 850 μm emissionis unresolved. A nondetection of the CO 3-2 line indicates the system isgas-poor, and the spectral energy distribution suggests the presence ofa large inner disk hole (~17AU=1.7" in radius for blackbody grains).These are possible indications that planets at large separations canform around M dwarfs within ~10 Myr. In a companion paper, we confirmthe existence of a dust disk around GJ 803 using optical coronagraphicimaging. Given its youthfulness, proximity, and detectability, the GJ803 disk will be a valuable system for studying disk, and perhapsplanet, formation in great detail. Overall, submillimeter measurementsof debris disks point to a drop in dust mass by a factor of~103 within the first ~10 Myr, with the subsequent decline inthe masses of submillimeter-detected disks consistent witht-0.5-t-1.
| 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
| HD 199143 and HD 358623: Two recently identified members of the β Pictoris moving group
HD 199143 and HD 358623 (BD-17°6128) are two sets of binary starswhich are physically associated and 48 pc from Earth. We presentheliocentric radial velocities and high lithium abundances whichestablish these stars as members of the 12 Myr-old β PictorisMoving Group. We also present mid-IR photometric measurements which showno firm evidence for warm dust around all four stars.
| An infrared imaging search for low-mass companions to members of the young nearby β Pic and Tucana/Horologium associations
We present deep high dynamic range infrared images of young nearby starsin the Tucana/Horologium and β Pic associations, all 10 to 35Myrs young and at 10 to 60 pc distance. Such young nearby starsare well-suited for direct imaging searches for brown dwarf and evenplanetary companions, because young sub-stellar objects are stillself-luminous due to contraction and accretion. We performed ourobservations at the ESO 3.5m NTT with the normal infrared imagingdetector SofI and the MPE speckle camera Sharp-I. Three arc sec north ofGSC 8047-0232 in Horologium a promising brown dwarf companion candidateis detected, which needs to be confirmed by proper motion and/orspectroscopy. Several other faint companion candidates are alreadyrejected by second epoch imaging. Among 21 stars observed inTucana/Horologium, there are not more than one to five brown dwarfcompanions outside of 75 AU (1.5'' at 50 pc); most certainly only <=5% of the Tuc/HorA stars have brown dwarf companions (13 to 78 Jupitermasses) outside of 75 AU. For the first time, we can report an upperlimit for the frequency of massive planets ( 10 Mjup) atwide separations ( 100 AU) using a meaningfull and homogeneoussample: Of 11 stars observed sufficiently deep in β Pic (12 Myrs),not more than one has a massive planet outside of 100 AU, i.e.massive planets at large separations are rare (<= 9%).Based on observations obtained on La Silla, Chile, in ESO programs65.L-0144(B), 66.D-0135, 66.C-0310(A), 67.C-0209(B), 67.C-0213(A),68.C-0008(A), and 68.C-0009(A)} }
| The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun
Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSATsurveys, a sample of 100 stars most luminous in X-rays within or arounda distance of 50 pc is culled. The smallest X-ray luminosity in thesample, in units of 1029 ergs s-1, isLX=9.8 the strongest source in the solar neighborhood is IIPeg, a RS CVn star, at LX=175.8. With respect to the originof X-ray emission, the sample is divided into partly overlapping classesof pre-main-sequence, post-T Tauri, and very young ZAMS objects (typeXY), RS CVn-type binary stars (type RS), other active short-periodbinaries, including binary BY Dra-type objects (type XO), apparentlysingle or long-period binary active evolved stars (type XG), contactbinaries of WU UMa kind (type WU), apparently single or long-periodbinary variable stars of BY Dra kind (type BY), and objects of unknownnature (type X?). Chromospherically active, short-period binaries (RSand XO) make up 40% of the brightest X-ray emitters, followed by youngstars (XY) at 30% and unknown sources (X?) at 15%. The fraction ofspectroscopically single evolved X-ray emitters of spectral classes IVand III is quite large (10%). The sources identified as RS CVn-typestars (RS, 23 objects) are considerably stronger in X-ray than theXY-objects and the other active binaries (XO and WU, 20 objects). Sevenobjects have LX>100, all RS except one XY, viz., BO Mic. Onlyfive (22%) RS objects have LX<25, while only three (10%)XY stars have LX>25. Formally, the limit of LX=25could serve as a statistical criterion to differentiate RS and XY stars.However, the other short-period binaries (including eclipsing stars ofAlgol and β Lyr type) have a distribution of LX verysimilar to the XY objects. The contact binaries (WU) appear to be muchweaker in X-rays than their detached counterparts of RS type, but thesample of the former is too small (three objects) to reach a firmconclusion. Sources matched with giants (either single or in binaries)are found to be significantly harder, with only 7% of hardness ratiosbelow 0, than subgiants (66% of HR1<0) and dwarfs (59% of HR1<0).Almost all objects in the sample are binary or multiple stars; thefraction of components (FC), defined as the total number of componentsin all binary and multiple systems divided by the sum of the totalnumber of components and single stars, is at least 0.90. The FC for theXY objects reaches 0.81, and for the unknown type 0.89. About 70% of RSobjects have also visual or astrometric companions, which makes themhierarchical multiple systems. The RS objects (mostly old, evolvedstars) and the XY stars have quite different kinematics. While the RSobjects move at considerable velocities in apparently random directionswith respect to the local standard of rest, the young stars have smallerand orderly velocities and tend to comprise expanding mini-associationssuch as the β Pic and the Tucana groups. The majority of the youngX-ray active stars belong to the Pleiades stream with the meanheliocentric velocity (U,V,W)=(-9.6,-21.8,-7.7) km s-1.
| Radial Velocity Survey of Members and Candidate Members of the TW Hydrae Association
We report spectroscopic observations of stars belonging to the youngnearby group known as the TW Hydrae association, as well as of a numberof potential members of the association identified in kinematic andX-ray surveys. Multiple radial velocity measurements were obtained foreach object, several of which turn out to be multiple systems. Orbitalsolutions are presented for three double-lined binaries, onesingle-lined binary, and a double-lined triple system, all with shortperiods. Effective temperatures and projected rotational velocities arepresented for each visible object. None of the candidate members of theassociation in our sample are confirmed as a true member. The largefraction of close binaries among the candidate members has to do withtheir selection based on X-ray emission from ROSAT, which tends to favorthe inclusion of tidally locked systems that are active but notnecessarily young.Some of the observations reported here were obtained with the MultipleMirror Telescope, a joint facility of the Smithsonian Institution andthe University of Arizona.
| Infrared imaging and spectroscopy of companion candidates near the young stars HD 199143 and HD 358623 in Capricornius
We present JHK images of the young ( ~ 20 Myrs) nearby ( ~ 48 pc) starsHD 199143 and HD 358623 (van den Ancker et al. \cite{van00}) with highsensitivity and high dynamic range in order to search for (sub-)stellarcompanions around them. The images were obtained in JHK with the specklecamera SHARP-I in July 2001 and in H with the infrared imaging cameraSofI in December 2000, July 2001, and December 2001, all at the ESO 3.5mNTT. We present a companion candidate with a 2 arcsec offset beingalmost 2 mag fainter than HD 358623 with proper motion (over one yearbaseline) consistent with the known proper motion of the primary star HD358623 A and 6 sigma deviant from the assumption that it is a non-movingbackground object. Then, we obtained a spectrum in the H-band (with SofIat the NTT) of this companion showing that it has spectral type M2 (+/-1), consistent with its JHK colors (for negligible extinction) and withbeing a companion (i.e. at same age and distance) of HD 358623 A(K7-M0), given the magnitude difference. Also, a companion candidatewith a 1 arcsec offset being 2 mag fainter than HD 199143 is detected,but clearly resolved from the primary only with SHARP-I, so that we haveno proper motion information. Also, we could not obtain a spectrum withSofI due to the high dynamic range. The JHK colors of this candidatesand the magnitude difference between primary (F8) and companioncandidate are consistent with a spectral type M0-2. This companioncandidate was predicted by van den Ancker et al. (\cite{van00}) toexplain some unusual properties of the primary star HD 199143 A. Wedetermine the ages and masses of all four objects from theoreticaltracks and isochrones all four stars appear to be co-eval with ~ 20Myrs. Both these companions were presented previously by Jayawardhana& Brandeker (\cite{Jay01}), but only with single-epoch images in Jand K obtained in May 2001. Our limits for additional detectable butundetected companions are such that we would have detected all stellarcompanions with separations ge 0.5{' '} (24 AU at 48 pc).Based on observations obtained on La Silla, Chile, in ESO programs66.D-0135, 67.C-0209, 67.C-0213, 68.C-0009.
| No disk needed around HD 199143 B
We present new, high angular resolution images of HD 199143 in theCapricornus association, obtained with the adaptive optics systemADONIS+SHARPII at the ESO 3.6 m Telescope of La Silla Observatory. HD199143 and its neighbour star HD 358623 (separation ~ 5' away) havepreviously been imaged with adaptive optics. For each star, a companionhas been detected in the J and K bands at respective separations of1.1'' and 2.2'' (Jayawardhana & Brandeker \cite{Jayawardhana2001}).Our new photometry of HD 199143 B suggests that it is a M2 star and thatthe presence of circumstellar dust proposed by van den Ancker et al.(\cite{Van den Ancker2000}) is no longer necessary. We show that the 12mu m flux detected by IRAS previously interpreted as an IR excess, canbe explained by the presence of the late-type companion.
| The Origin of the β Pictoris Moving Group
The β Pictoris moving group (BPMG) as proposed by Zuckerman et al.is a group of 19 low-mass star systems containing β Pictoris andits protoplanetary disk. This nearest moving group is at a mean distanceof 36 pc from the Earth, and these authors have proposed, by means of anH-R diagram, an evolutionary age of 12+8-4 Myr forthis group. Here, by retracing the three-dimensional orbits of all themembers of the BPMG and using a realistic Galactic potential, we findthat a first maximum concentration of orbits occurs at 11.5 Myr and in aspace region having a maximum size of 24 pc, 3 times smaller than itspresent size. We consider this region to be the birthplace of the BPMG.This interesting similarity of independently obtained evolutionary andkinematical ages indicates that this group could have already beenformed as an unbound system, as observed today. The birthplace of theBPMG is located in a three-dimensional space, at ~45 pc from the regionwhere the Lower Centaurus Crux (LCC) and Upper Centaurus Lupus (UCL)subgroups of the Scorpius-Centaurus OB association were when they werebetween 4 and 6 Myr old. At that age, both subgroups were able toproduce supernovae (SNe) capable of triggering the formation of theBPMG. This interaction distance could even be smaller, up to ~26 pc, ifSNe exploded in the outer regions of the LCC or UCL near the proposedbirthplace of the BPMG. This scenario confirms, at least for the BPMG,the suggestion made by Mamajek & Feigelson and other authors thatyoung unbound nearby stellar associations with ages around ~10 Myroriginated in this OB complex. In contrast to the BPMG, for which wepropose a coeval formation, the LCC and UCL subgroups appear not to betruly coeval.
| High-Resolution Optical Observations of Extreme-Ultraviolet-selected Active Late-type Stars
We present high-resolution optical spectra of EUV-selected late-typestars. We have detected seven new spectroscopic binaries, including apair of F stars and an RS Canum Venaticorum candidate. Many of the starsin our sample show remarkable Hα emission with equivalent widthsup to 8 Å. Several sources show strong He I 5876 Å emission.We use cross-correlation techniques to estimate line-of-sight rotationalvelocities and find that they range from less than 5 to over 80 kms-1. Several of the stars show a strong Li 6707 Å line,which we use to determine their Li abundances. The EUV-to-bolometricluminosity, a measure of the coronal heating efficiency, is as high as10-3. The present sample extends the EUV-selected late-typestars to lower Rossby numbers and makes the coronal saturation boundarybetter pronounced. Our results indicate that there is a drop in the EUVemission from the corona in late M dwarfs possibly because of a changein the nature of the magnetic dynamo in fully convective stars.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| The β Pictoris Moving Group
Following the 1983 IRAS detection and subsequent imaging of itsextensive dusty circumstellar disk, β Pictoris became theprototypical and most studied example of a potential forming planetarysystem. Here we report the identification of 17 star systems, each withone or more characteristics indicative of extreme youth, that are movingthrough space together with β Pic. This diverse set of ~12 millionyr old star systems, which includes a ~35 Jupiter mass brown dwarf, anda wide assortment of dusty circumstellar disks, is the comoving,youthful group closest to Earth. Their unique combination of youth andproximity to Earth makes group members-many of which have masses similarto that of the Sun-prime candidates for imaging of warm planets anddusty circumstellar disks with ground- and space-based telescopes.
| Discovery of Close Companions to the Nearby Young Stars HD 199143 and HD 358623
Young stellar systems in the solar neighborhood provide valuablelaboratories for detailed studies of star and planet formation. Thebright F8 V star HD 199143 and the Li-rich late-type emission-line starHD 358623 are among the nearest young stars identified to date and maybe members of a young association in Capricornus. We presenthigh-resolution near-infrared images of these two sources, obtainedusing the adaptive optics system on the 3.6 m telescope at the EuropeanSouthern Observatory in La Silla, Chile. Our observations reveal thatboth are, in fact, close binary systems. The newly discovered companionat a separation of ~1" may account for the unusual characteristics of HD199143-rapid rotation, emission lines, ultraviolet variability, andexcess infrared emission-recently discussed by van den Ancker andcoworkers. HD 199143 may be a rare example of a close binary with only acircumsecondary disk. With the detection of a ~2" companion, HD 358623is now possibly one of the closest known T Tauri binaries. Both binarysystems are prime targets for follow-up spectroscopic and astrometricobservations. Based on observations collected at the European SouthernObservatory, Chile.
| EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of theEXPORT sample obtained at the 2.5 m Nordic Optical Telescope. Thedatabase consists of multi-epoch photo-polarimetry of 68pre-main-sequence and main-sequence stars. An investigation of thepolarization variability indicates that 22 objects are variable at the3sigma level in our data. All these objects are pre-main sequence stars,consisting of both T Tauri and Herbig Ae/Be objects while the mainsequence, Vega type and post-T Tauri type objects are not variable. Thepolarization properties of the variable sources are mostly indicative ofthe UXOR-type behaviour; the objects show highest polarization when thebrightness is at minimum. We add seven new objects to the class of UXORvariables (BH Cep, VX Cas, DK Tau, HK Ori, LkHα 234, KK Oph and RYOri). The main reason for their discovery is the fact that our data-setis the largest in its kind, indicating that many more young UXOR-typepre-main sequence stars remain to be discovered. The set of Vega-likesystems has been investigated for the presence of intrinsicpolarization. As they lack variability, this was done using indirectmethods, and apart from the known case of BD+31o643, thefollowing stars were found to be strong candidates to exhibitpolarization due to the presence of circumstellar disks: 51 Oph,BD+31o643C, HD 58647 and HD 233517. Table A1 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/379/564
| EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars
In this paper we present the first comprehensive results extracted fromthe spectroscopic campaigns carried out by the EXPORT (EXoPlanetaryObservational Research Team) consortium. During 1998-1999, EXPORTcarried out an intensive observational effort in the framework of theorigin and evolution of protoplanetary systems in order to obtain clueson the evolutionary path from the early stages of the pre-main sequenceto stars with planets already formed. The spectral types of 70 stars,and the projected rotational velocities, v sin i, of 45 stars, mainlyVega-type and pre-main sequence, have been determined from intermediate-and high-resolution spectroscopy, respectively. The first part of thework is of fundamental importance in order to accurately place the starsin the HR diagram and determine the evolutionary sequences; the secondpart provides information on the kinematics and dynamics of the starsand the evolution of their angular momentum. The advantage of using thesame observational configuration and methodology for all the stars isthe homogeneity of the set of parameters obtained. Results from previouswork are revised, leading in some cases to completely new determinationsof spectral types and projected rotational velocities; for some stars noprevious studies were available. Tables 1 and 2 are only, and Table 6also, available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/116 Based onobservations made with the Isaac Newton and the William Herscheltelescopes operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofísica de Canarias.
| Tucana Association
Among star clusters, only the sparse Ursa Major nucleus is closer toEarth than the recently identified Tucana association. Based on newphotometric VRI magnitudes, we construct a color-magnitude diagramcomposed of likely and possible Tucana members. The implied age of theTucana association stars, <~40 Myr, is consistent with the agedetermined from a previous analysis by Stelzer & Neuhäuser ofthe X-ray luminosities of plausible association members. Based primarilyon space motions and X-ray fluxes, we identify potential new members ofthe Tucana association not considered in earlier studies. Torres andcoworkers recently identified a group of post-T Tauri stars, theHorologium association, which is located near the Tucana association inright ascension and declination. Because the Horologium stars have thesame space motions, age, distance from Earth, volume density, and rangeof spectral types as Tucana stars, we suggest that, rather than beingcharacterized as a separate group, it would be appropriate andeconomical to subsume the Horologium association stars into the Tucanastream.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Capricorno |
| Ascensione retta: | 20h55m47.68s |
| Declinazione: | -17°06'51.1" |
| Magnitudine apparente: | 7.328 |
| Distanza: | 47.71 parsec |
| Moto proprio RA: | 62.2 |
| Moto proprio Dec: | -65.4 |
| B-T magnitude: | 7.944 |
| V-T magnitude: | 7.379 |
Cataloghi e designazioni:
|
