SKY-MAP.ORG
 Főoldal Alapinformációk To Survive in the Universe News@Sky Asztrofotók Kollekció Fórum Blog New! GYIK Sajtó Bejelentkezés

# HD 44627

Tartalom

### Képek

Kép feltöltése

DSS Images   Other Images

### Kapcsolódó cikkek

 An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMTWe present the results of a survey of 45 young (<~250 Myr), close(<~50 pc) stars with the Simultaneous Differential Imager (SDI)implemented at the VLT and the MMT for the direct detection ofextrasolar planets. As part of the survey, we observed 54 objects,consisting of 45 close, young stars; two more distant (<150 pc),extremely young (<=10 Myr) stars; three stars with known radialvelocity planets; and four older, very nearby (<=20 pc) solaranalogs. Our SDI devices use a double Wollaston prism and a quad filterto take images simultaneously at three wavelengths surrounding the 1.62μm methane absorption bandhead found in the spectrum of cool browndwarfs and gas giant planets. By differencing adaptive optics-correctedimages in these filters, speckle noise from the primary star issignificantly attenuated, resulting in photon (andflat-field)-noise-limited data. In our VLT data, we achieved H-bandcontrasts>~10 mag (5 σ) at a separation of 0.5" from theprimary star on 45% of our targets and H-band contrasts>~9 mag at aseparation of 0.5" on 80% of our targets. With these contrasts, we canimage (5 σ detection) a 7 MJ planet 15 AU from a 70 MyrK1 star at 15 pc or a 7.8 MJ planet at 2 AU from a 12 Myr Mstar at 10 pc. We detected no candidates with S/N>2 σ whichbehaved consistently like a real object. From our survey null result, wecan rule out (with 93% confidence) a model planet population whereN(a)~constant out to a distance of 45 AU.Steward Observatory, University of Arizona, Tucson, AZ 85721. An Improbable Solution to the Underluminosity of 2M1207B: A Hot Protoplanet Collision AfterglowWe introduce an alternative hypothesis to explain the very lowluminosity of the cool (L-type) companion to the ~25 MJup, ~8Myr old brown dwarf 2M1207A. Recently, Mohanty et al. found thateffective temperature estimates for 2M1207B (1600+/-100 K) are grosslyinconsistent with its lying on the same isochrone as the primary, beinga factor of ~10 underluminous at all bands between I (0.8 μm) andL' (3.6 μm). Mohanty et al. explain this discrepancy bysuggesting that 2M1207B is an 8 MJup object surrounded by anedge-on disk comprised of large dust grains producing 2.5 mag ofachromatic extinction. We offer an alternative explanation: the apparentflux reflects the actual source luminosity. Given the temperature, weinfer a small radius (~49,000 km), and for a range of plausibledensities, we estimate a mass < MJup. We suggest that2M1207B is a hot protoplanet collision afterglow and show that theradiative timescale for such an object is >~1% the age of the system.If our hypothesis is correct, the surface gravity of 2M1207B should bean order of magnitude lower than that predicted by Mohanty et al. Rotation and Activity of Pre-Main-Sequence StarsWe present a study of rotation (vsini) and chromospheric activity(Hα equivalent width) based on an extensive set of high-resolutionoptical spectra obtained with the MIKE instrument on the 6.5 m MagellanClay telescope. Our targets are 74 F-M dwarfs in four young stellarassociations, spanning ages from 6 to 30 Myr. By comparing Hα EWsin our sample to results in the literature, we see a clear evolutionarysequence: Chromospheric activity declines steadily from the T Tauriphase to the main sequence. Using activity as an age indicator, we finda plausible age range for the Tuc-Hor association of 10-40 Myr. Between5 and 30 Myr, we do not see evidence for rotational braking in the totalsample, and thus angular momentum is conserved, in contrast to youngerstars. This difference indicates a change in the rotational regulationat ~5-10 Myr, possibly because disk braking cannot operate longer thantypical disk lifetimes, allowing the objects to spin up. Therotation-activity relation is flat in our sample; in contrast tomain-sequence stars, there is no linear correlation for slow rotators.We argue that this is because young stars generate their magnetic fieldsin a fundamentally different way from main-sequence stars, and not justthe result of a saturated solar-type dynamo. By comparing our rotationalvelocities with published rotation periods for a subset of stars, wedetermine ages of 13+7-6 and9+8-2 Myr for the η Cha and TWA associations,respectively, consistent with previous estimates. Thus we conclude thatstellar radii from evolutionary models by Baraffe et al. (1998) are inagreement with the observed radii to within +/-15%. Rotational period of GQ LupiAims. We wanted to determine the rotation parameters of GQ Lup A,thereby constraining the evolutionary history of the GQ Lup system. Methods: We have undertaken a photometric monitoring campaign on GQ LupA consisting of two epochs spaced one year apart. We also searched thephotometric archives to enlarge the data set. Results: We were able todetermine the photometric period (8.45±0.2 days) in both epochsin several photometric bands. This periodicity could also be found insome of the archival data. The combined false-alarm probability is0.015. The variation is most likely caused by hot spots on the surfaceof GQ Lup A. This, combined with high-resolution spectra (v sin i)allows calculation of GQ Lup A's inclination (i=27±5°).Radial velocity data also contains this period but is inconclusive.Nevertheless, the RV data supports the interpretation that hot spotscause the photometric variation. We use the known K-band variability,amplitude, and phase of GQ Lup A together with a new image of GQ LupA+b, taken quasi-simultaneously with our monitoring of the star, toconfirm the magnitude and, hence, luminosity of the companion.Based on observations obtained on Cerro Paranal, Chile, in ESO program075.C-0710(C) and on La Silla, Chile, in ESO programs074.C-0034(A),075.C-0710(E), 075.C-0710(F), 075.C-0202(A), 076.C-0010(A)as well as with ANDICAM of the SMARTS consortium. Photometry data ofepochs I and II are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/468/1039 Unraveling the Origins of Nearby Young StarsA systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago. The Planetary Mass Companion 2MASS 1207-3932B: Temperature, Mass, and Evidence for an Edge-on DiskWe present J-band imaging and H+K-band low-resolution spectroscopy of2MASS 1207-3932AB, obtained with VLT NACO. For the putative planetarymass secondary, we find J=20.0+/-0.2 mag. The HK spectra of bothcomponents imply low gravity and a dusty atmosphere for the secondary.Comparisons to synthetic spectra yield Teff,A~2550+/-150 Kand Teff,B~1600+/-100 K, consistent with their late-M andmid- to late-L types. For these Teff and an age of 5-10 Myr,evolutionary models imply MA~24+/-6 MJup andMB~8+/-2 MJup. Independent comparisons of thesemodels to the observed colors, spanning ~I to L', also yield the samemasses and temperatures. Our primary mass agrees with other recentanalyses; however, our secondary mass, while still in the planetaryregime, is 2-3 times larger than claimed previously. This discrepancycan be traced to the luminosities: while the absolute photometry andMbol of the primary agree with theoretical predictions, thesecondary is ~2.5+/-0.5 mag fainter than expected in all bands from I toL' and in Mbol. This accounts for the much lower secondarymass (and temperature) derived earlier. We argue that this effect ishighly unlikely to result from a variety of model-related problems andis instead real. This conclusion is bolstered by the absence of anyluminosity problems in either the primary or AB Pic B, which we alsoanalyze. We therefore suggest gray extinction in 2M1207B, due toocclusion by an edge-on circumsecondary disk. This is consistent withthe observed properties of edge-on disks around T Tauri stars and withthe known presence of a high-inclination evolved disk around theprimary. Finally, the system's implied mass ratio of ~0.3 suggests abinary-like formation scenario. First High-Contrast Science with an Integral Field Spectrograph: The Substellar Companion to GQ LupiWe present commissioning data from the OSIRIS integral fieldspectrograph (IFS) on the Keck II 10 m telescope that demonstrate theutility of adaptive optics IFS spectroscopy in studying faint close-insubstellar companions in the halos of bright stars. Our R~2000 J- andH-band spectra of the substellar companion to the 1-10 Myr old GQ Lupcomplement existing K-band spectra and photometry and improve on theoriginal estimate of its spectral type. We find that GQ Lup B issomewhat hotter (M6-L0) than reported in the discovery paper byNeuhäuser and collaborators (M9-L4), mainly due to the surfacegravity sensitivity of the K-band spectral classification indices usedby the discoverers. Spectroscopic features characteristic of low surfacegravity objects, such as lack of alkali absorption and a triangularH-band continuum, are indeed prominent in our spectrum of GQ Lup B. Thepeculiar shape of the H-band continuum and the difference between thetwo spectral type estimates is well explained in the context of thediminishing strength of H2 collision-induced absorption withdecreasing surface gravity, as recently proposed for young ultracooldwarfs by Kirkpatrick and collaborators. Using our updated spectroscopicclassification of GQ Lup B and a reevaluation of the age andheliocentric distance of the primary, we perform a comparative analysisof the available substellar evolutionary models to estimate the mass ofthe companion. We find that the mass of GQ Lup B is 0.010-0.040Msolar. Hence, it is unlikely to be a wide-orbit counterpartto the known radial velocity extrasolar planets, whose masses are<~0.015 Msolar. Instead, GQ Lup A/B is probably a memberof a growing family of very low mass ratio widely separated binariesdiscovered through high-contrast imaging. Binarity of the Nearby 30 Myr Old Solar Analog HIP 16853 in the Tucana-Horologium StreamA robust unconstrained orbital solution is obtained for the G2 V starHIP 16853 = HD 22705 at 42 pc, which is a probable member of the 28-30Myr old Tucana-Horologium stream of post-T Tauri stars. The solutionyields an apparent semimajor axis of 5.1+/-0.7 mas, a period of 201+/-2days, and an inclination of 80deg+/-7deg. Assuminga mass of 1 Msolar for the primary, the close companion isonly 0.4 Msolar, which implies a spectral type M0.5 at thisage. The expected maximum separation (a) between the companions is 18mas, which makes this system amenable for high-resolution observations.The wide companion HIP 16853 B at 14" is investigated as a possibletertiary component but rejected on account of the near-infraredphotometric data inconsistent with the well-defined H-R diagram of theTucana-Horologium group. Are isolated planetary-mass objects really isolated?. A brown dwarf-exoplanet system candidate in the σ Orionis clusterContext: .Free-floating planetary-mass objects have masses below thedeuterium burning mass limit at about 13 Jupiter masses, and have mostlybeen found in very young open clusters. Their origin and relationship tostars and brown dwarfs are still a mystery. Aims: .The recentdetection by direct imaging of three giant planets at wide separation(50-250 AU) from their primaries has raised the question about the true"isolation" of planetary-mass objects in clusters. Our goal was to testthe possibility that some free-floating planetary-mass objects could infact be part of wide planetary systems. Methods: .We searched inthe literature for stellar and brown-dwarf candidates members of theσ Orionis cluster (~3 Ma, ~360 pc) at small angular separationsfrom published candidate planetary-mass objects. We found one candidateplanetary system composed of an X-ray source, SE 70, and aplanetary-mass object, S Ori 68, separated by only 4.6 arcsec. In orderto assess the cluster membership of the X-ray source, we obtainedmid-resolution optical spectroscopy using ISIS on the William HerschelTelescope. We also compiled additional data on the target from availableastronomical catalogues. Results: .We have found that SE 70follows the spectrophotometric sequence of the cluster and displaysspectroscopic features of youth, such as lithium in absorption andchromospheric Hα emission. The radial velocity is consistent withcluster membership. Hence, SE 70 is very probably a member of theσ Orionis cluster. The projected physical separation between SE 70and S Ori 68 is 1700~± 300 AU at the distance of the cluster. Ifa common proper motion is confirmed in the near future, the system wouldbe composed of an M5-6 brown dwarf with an estimated mass of ~45 M_Jupand an L5 ± 2 giant planet with an estimated mass of ~5 M_Jup. Itwould be the widest and one of the lowest-mass planetary systems knownso far. HST NICMOS Imaging of the Planetary-mass Companion to the Young Brown Dwarf 2MASSW J1207334-393254Multiband (0.9-1.6 μm) images of the TW Hydrae association (TWA)brown dwarf 2MASSW J1207334-393254 (also known as 2M 1207) and itscandidate planetary-mass companion (2M 1207b) were obtained on 2004August 28 and 2005 April 26 with HST NICMOS. The images from these twoepochs unequivocally confirm the two objects as a common proper motionpair (16.0 σ confidence). A new measurement of the proper motionof 2M 1207 implies a distance to the system of 59+/-7 pc and a projectedseparation of 46+/-5 AU. The NICMOS and previously published VLTphotometry of 2M 1207b, extending overall from 0.9 to 3.8 μm, arefully consistent with an object of a few Jupiter masses at the canonicalage of a TWA member (~8 Myr) based on evolutionary models of young giantplanets. These observations provide information on the physical natureof 2M 1207b and unambiguously establish that the first direct image of aplanetary-mass companion in orbit around a self-luminous body, otherthan our Sun, has been secured. HD 203030B: An Unusually Cool Young Substellar Companion near the L/T TransitionWe present the discovery of a brown dwarf companion to the star HD203030: a G8 V Solar analog with an estimated age between 130 and 400Myr old G8 V. Separated by 11.9" (487 AU in projection) from its hoststar, HD 203030B has an estimated mass of0.023+0.008-0.011 Msolar. The K-bandspectral type of L7.5+/-0.5 places HD 203030B near the critical L/Ttransition in brown dwarfs, which is characterized by the rapiddisappearance of dust in substellar photospheres. From a comparativeanalysis with well-characterized field L/T transition dwarfs, we findthat, despite its young age, HD 203030B has a bolometric luminositysimilar to the >1 Gyr old field dwarfs. Adopting a radius fromcurrent models of substellar evolution, we hence obtain that theeffective temperature of HD 203030B is only1206+74-116 K, markedly lower than the ~1440 Keffective temperatures of field L/T transition dwarfs. The temperaturediscrepancy can be resolved if either (1) the ages of field brown dwarfshave been overestimated by a factor of ~1.5, leading to underestimatedradii, or (2) the lower effective temperature of HD 203030B is relatedto its young age, implying that the effective temperature at the L/Ttransition is gravity dependent. Discovery of a Young Substellar Companion in ChamaeleonDuring an imaging survey of the Chamaeleon I star-forming region withthe Advanced Camera for Surveys aboard the Hubble Space Telescope, wehave discovered a candidate substellar companion to the young low-massstar CHXR 73 (τ~2 Myr, M~0.35 Msolar). We measure aprojected separation of 1.30"+/-0.03" for the companion, CHXR 73 B,which corresponds to 210 AU at the distance of the cluster. Anear-infrared spectrum of this source obtained with the CornellMassachusetts Slit Spectrograph at the Magellan II telescope exhibitsstrong steam absorption that confirms its late-type nature (>~M9.5).In addition, the gravity-sensitive shapes of the H- and K-band continuademonstrate that CHXR 73 B is a young, pre-main-sequence object, ratherthan a field star. The probability that CHXR 73 A and B are unrelatedmembers of Chamaeleon I is ~0.001. We estimate the masses of CHXR 73 Band other known substellar companions in young clusters with a methodthat is consistent with the dynamical measurements of the eclipsingbinary brown dwarf 2M 0535-0546, which consists of a comparison of thebolometric luminosities of the companions to the values predicted by theevolutionary models of Chabrier et al. and Burrows et al. We arrive atmass estimates of 0.003-0.004, 0.024+/-0.012,0.011+0.01-0.003, and0.012+0.008-0.005 Msolar for 2M1207-3932 B, GQ Lup B, DH Tau B, and CHXR 73 B, respectively. Thus, DHTau B and CHXR 73 B appear to be the least massive companions to starsoutside the solar system that have been detected in direct images, andmay have masses that are within the range observed for extrasolarplanetary companions (M<~0.015 Msolar). However, becausethese two objects (as well as 2M 1207-3932 B) probably did not formwithin circumstellar disks around their primaries, we suggest that theyshould be viewed as brown dwarf companions rather than planets. Accretion Disks around Young Stars: Lifetimes, Disk Locking, and VariabilityWe 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. Early ComeOn+ adaptive optics observation of GQ Lupi and its substellar companionWe present an analysis of adaptive optics K-band imaging data of GQ Lupacquired in 1994 by the first generation adaptive optics system ComeOn+at the ESO 3.6 m optical telescope in La Silla. The data reveal a likelycandidate for the low-mass companion recently reported in theliterature. An a posteriori detection in the 11-year old data wouldprovide a useful astrometric data point for the very long-period (~1000yr) orbit of the GQ Lup system. However, the data is severelycontaminated by speckle noise at the given projected separation, whichdecreases the confidence of the detection. Still, from the data we canconclude that GQ Lup B is not an unrelated background source, butinstead a physical companion to GQ Lup A. We present here the reductionand analysis of the ComeOn+ images, as well as the results. We alsodiscuss the nature of the companion based on data and models availablein the scientific literature and examine claims made regarding theclassification of the object as a planet. The low-mass companion of GQ LupUsing NACO on the VLT in the imaging mode we have detected an object ata distance of only 0.7 arcsec from GQ Lup. The object turns out to beco-moving. We have taken two K-band spectra with a resolution of \lambda/ \Delta \lambda=700. In here, we analyze the spectra in detail. We showthat the shape of spectrum is not spoiled by differences in the Strehlratio in the blue and in the red part, as well as differentialrefraction. We reanalyze the spectra and derive the spectral type of thecompanion using classical methods. We find that the object has aspectral type between M9V and L4V, which corresponds to a T_eff between1600 and 2500 K. Using GAIA-dusty models, we find that the spectral typederivation is robust against different log(g)-values. The T_eff derivedfrom the models is again in the range between 1800 and 2400 K. While themodels reproduce nicely the general shape of the spectrum, the12CO lines in the spectrum have about half the depth as thosein the model. We speculate that this difference might be caused byveiling, like in other objects of similar age, and spectral class. Wealso find that the absolute brightness of the companion matches that ofother low-mass free-floating objects of similar age and spectral type. Acomparison with the objects in USco observed by Mohanty et al.(\cite{mohanty04b}) shows that the companion of GQ Lup has a lower massthan any of these, as it is of later spectral type, and younger. Thesame is as true, for the companion of AB Pic. To have a first estimateof the mass of the object we compare the derived T_eff and luminositywith those calculated from evolutionary tracks. We also point out thatfuture instruments, like NAHUAL, will finally allow us to derive themasses of such objects more precisely. Extrasolar Planets: Pictured at Last?Not Available Direct imaging of planets around young stars, the case of GQ Lup b.Not Available A companion to AB Pic at the planet/brown dwarf boundaryWe report deep imaging observations of the young, nearby star AB Pic, amember of the large Tucana-Horologium association. We have detected afaint, red source 5.5'' South of the star with JHK colors compatiblewith that of a young substellar L dwarf. Follow-up observations at twoadditional epochs confirm, with a confidence level of 4.7σ, thatthe faint red object is a companion to AB Pic rather than it being astationary background object. A low resolution K-band spectrum indicatesan early-L spectral type for the companion. Finally, evolutionary modelpredictions based on the JHK photometry of AB Pic b indicate a mass of13 to 14 MJup if its age is ~30 Myr. Is AB Pic b a massiveplanet or a minimum mass brown dwarf? On the Rotation of Post-T Tauri Stars in AssociationsNearby 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 SunUntil 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. The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfsWe 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. Our63 000 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 New Members of the TW Hydrae Association, β Pictoris Moving Group, and Tucana/Horologium AssociationWe have identified five new members of the TW Hydrae association (TWA),11 new members of the β Pic moving group, and 11 newTucana/Horologium association members. These are the three youngest(<~30 Myr) known kinematic stellar groups near the Earth. Newlyidentified β Pic group members are located mostly in the northernhemisphere, and they have a slightly different U-component of Galacticvelocity compared to that of previously known members. Tracing themotion of β Pic members backward in time for 12 Myr indicates thatthey might have formed in a small region with an initial velocitydispersion of ~8 km s-1. A couple of mid-M spectral typeβ Pic members show emission features [He Iλ5876+λ6678) and Na D λ5890+λ5896)] seenamong earlier spectral type stars in the TWA and β Pic groups. Toderive the distances of the non-Hipparcos members of these groups, wehave constructed a V-K versus MK color-magnitude diagram thatis very useful in separating young K/M stars from older main-sequencecounterparts and constraining theoretical pre-main-sequence evolutionarytracks. All newly identified K- and M-type members of the three groupsshow saturated X-ray activity(LX/Lbol~10-3). One newly identifiedTWA member, SSS 101727-5354, is estimated to be only 22 pc away fromEarth. Its extreme youth, late spectral type (~M5), and proximity toEarth make SSS 101727-5354 perhaps the best target for direct imagingdetection of cooling planets. A systematic study of X-ray variability in the ROSAT all-sky surveyWe 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 Detection of moving clusters by a method of cinematic pairs.Not Available The 74th Special Name-list of Variable StarsWe present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission. An All-Sky Catalog of Faint Extreme Ultraviolet SourcesWe present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects. Optical and X-Ray Characteristics of Stars Detected in the Einstein Slew SurveyWe detect X-rays for the first time from 63 cool (types AM) stars. Thesestars are part of the 229 total stellar X-ray sources identified to datein the Einstein Slew Survey (hereafter Slew). We also list new X-raydata on one A star that may have a corona, five OB stars, and reportdiscoveries of two new T Tauri stars and two new cataclysmic variables.The stellar content of the Slew high-latitude subset (currently 93%identified for |b_II_| > 20^deg^) is 26%. This agrees well with theEinstein Extended Medium Sensitivity Survey (EMSS) stellar sample, whichonly considered high latitudes. Because of the large solid angle coveredby the Slew, and its shallow limiting flux, the sample will better probethe bright end of the stellar X-ray luminosity function (L_X_ >10^30^ ergs s^-1^). Presently (based on the 221 Slew stars with knownspectral types), the sample is dominated by late-type systems (coolerthan F; 68%). These include dMe's, BY Dra, RS CVn, and FK Comae systems.Based on the limiting magnitudes of catalogs searched to date, the Slewis assessed to be complete for spectral types earlier than K. Hence, Kand M systems will be prominent in the ~50 as yet unidentified stars. Wehave embarked on an extensive program to (1) confirm the X-rayidentifications with the optical counterparts; (2) search for Ca II Hand K and Balmer line emission as activity signatures; and (3) searchfor supporting evidence of magnetic activity by measuring rotationalvelocities and relating them to X-ray luminosity level. Of 64 proposedcoronal systems observed to date, we have confirmed 44 active stars,while in the remaining 20 we have been unable to find definitiveactivity. We have confirmed an additional 19 active stars from stellardatabase searches, and reclassified nine systems as having nonstellaroptical counterparts from optical follow-up identification work andextragalactic database searches. We discuss notable new discoveries.From the sample of single active F7-MS Slew stars with measured ν sini-values, we find a strong (99.9% confidence level) linear correlationof X-ray luminosity with ν sin i and with stellar radius (R).However, L_X_ is uncorrelated with angular rotation speed at the 99%level. For the combined Slew and EMSS single star F7-MS sample, we findthe same 99.9% ν sin i-L_X_ and R-L_X_ correlations. The L_X_-νsin i relation for the combined sample appears to flatten with respectto the quadratic behavior seen for optically selected stellar samples atrotational velocities in excess of ~16 km s^-1^. For the unevolvedsubset of the Slew single star sample, we also find a correlationbetween L_X_ and Rossby number (R_0_; more than 99% confidence). Aleast-squares fit gives L_X_ ~ R_0_^-0.4^, which is similarly flatterthan the quadratic dependence seen in optical samples. Using the stellarsurface X-ray flux F_X_ versus B - V diagram, we interpret these resultsas saturation of the stellar surface by active regions at F_X_/F_bol_ ~10^-3^. A large, complete, volume-limited sample of G-type dwarfs. I. Completion of Stroemgren UVBY photometryFour-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. The Einstein Slew SurveyA catalog of 819 sources detected in the Einstein IPC Slew Survey of theX-ray sky is presented; 313 of the sources were not previously known asX-ray sources. Typical count rates are 0.1 IPC count/s, roughlyequivalent to a flux of 3 x 10 exp -12 ergs/sq cm s. The sources havepositional uncertainties of 1.2 arcmin (90 percent confidence) radius,based on a subset of 452 sources identified with previously knownpointlike X-ray sources (i.e., extent less than 3 arcmin).Identifications based on a number of existing catalogs of X-ray andoptical objects are proposed for 637 of the sources, 78 percent of thesurvey (within a 3-arcmin error radius) including 133 identifications ofnew X-ray sources. A public identification data base for the Slew Surveysources will be maintained at CfA, and contributions to this data baseare invited. Orbital variability and the white dwarf spectrum of BD + 16 516 /V471 Tau/Observations in the blue and red regions of the spectrum of the binarysystem V471 Tau show that the chromospheric activity of the K2Vsecondary star is phase dependent. H-alpha has been detected clearly inemission and is also phase dependent. The spectrum of the white dwarfprimary is isolated, and the photometric temperature T approximately35,000 K is confirmed. A radial velocity curve of the K star is given,and it is shown that the orbit is circular.

### Kapcsolódó hivatkozások

• - (nincs kapcsolódó hivatkozás) -