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|A Moving Cluster Distance to the Exoplanet 2M1207b in the TW Hydrae Association|
A candidate extrasolar planet companion to the young brown dwarf 2MASSWJ1207334-393254 (hereafter 2M1207) was recently discovered by Chauvin etal. They find that the temperature and luminosity of 2M1207b areconsistent with a young, ~5MJ planet. The 2M1207 system ispurported to be a member of the TW Hya association (TWA) and situated~70 pc away. Using a revised space motion vector for TWA and improvedproper motion for 2M1207, I use the moving cluster method to estimatethe distance to the 2M1207 system and other TWA members. The deriveddistance for 2M1207 (53+/-6 pc) forces the brown dwarf and planet to behalf as luminous as previously thought. The inferred masses for 2M1207Aand 2M1207b decrease to ~21 and ~3-4MJ, respectively, withthe mass of 2M1207b well below the observed tip of the planetary massfunction and the theoretical deuterium-burning limit. After removingprobable Lower Centaurus Crux (LCC) members from the TWA sample, as wellas the probable nonmember TWA 22, the remaining TWA membership is foundto have distances of 49+/-3 (s.e.m.)+/-12 (1 σ) pc and an internalone-dimensional velocity dispersion of 0.8+0.3-0.2km s-1. There is weak evidence that the TWA is expanding, andthe data are consistent with a lower limit on the expansion age of 10Myr (95% confidence).
|A Survey for Low-Mass Stars and Brown Dwarfs in the η Chamaeleontis and ɛ Chamaeleontis Young Associations|
I present the results of a search for new low-mass stars and browndwarfs in the η Cha and ɛ Cha young associations. Within radiiof 1.5d and 0.5d surrounding η Cha and ɛ Cha, respectively, Ihave constructed color-magnitude diagrams from Deep Near Infrared Survey(DENIS) and Two Micron All Sky Survey (2MASS) photometry and haveobtained spectra of the candidate low-mass members therein. The fivecandidates in η Cha are classified as four field M dwarfs and onecarbon star. No new members are found in this survey, which is completefor M/Msolar=0.015-0.15, according to the evolutionary modelsof Chabrier and Baraffe. Thus, an extended population of low-massmembers is not present in η Cha out to 4 times the radius of theknown membership. Meanwhile, the three candidate members of ɛ Chaare classified as young stars, and thus likely members of theassociation, based on Li absorption and gravity-sensitive absorptionlines. These new sources have spectral types of M2.25, M3.75, and M5.75,corresponding to masses of 0.45, 0.25, and 0.09 Msolar by themodels of Chabrier and Baraffe. For one of these stars, intense Hαemission, forbidden line emission, and strong K-band excess emissionsuggest the presence of accretion, an outflow, and a disk, respectively.This young star is also much fainter than expected for an associationmember at its spectral type, which could indicate that it is seen inscattered light. No brown dwarfs are detected in ɛ Cha down to thecompleteness limit of 0.015 Msolar. The absence of browndwarfs in these associations is statistically consistent with the massfunctions measured in star-forming regions, which exhibit only ~2 and ~1brown dwarfs for stellar samples at the sizes of the η Cha andɛ Cha associations.Based on observations performed at Las Campanas Observatory. Thispublication makes use of data products from the Two Micron All SkySurvey and the Deep Near-Infrared Survey of the Southern Sky.
|New Perspectives on the X-Ray Emission of HD 104237 and Other Nearby Herbig Ae/Be Stars from XMM-Newton and Chandra|
The origin of the X-ray emission from Herbig Ae/Be stars is not yetknown. These intermediate-mass pre-main-sequence stars lie on radiativetracks and are not expected to emit X-rays via solar-like magneticprocesses, nor are their winds powerful enough to produce X-rays byradiative wind shocks as in more massive O-type stars. The emissioncould originate in unseen low-mass companions, or it may be intrinsic tothe Herbig stars themselves if they still have primordial magneticfields or can sustain magnetic activity via a nonsolar dynamo. Wepresent new X-ray observations of the nearby Herbig Ae star HD 104237(=DX Cha) with XMM-Newton, whose objective is to clarify the origin ofthe emission. Several X-ray emission lines are clearly visible in theCCD spectra, including the high-temperature Fe Kα complex. Theemission can be accurately modeled as a multitemperature thermal plasmawith cool (kT<1 keV) and hot (kT>~3 keV) components. The presenceof a hot component is compelling evidence that the X-rays originate inmagnetically confined plasma, either in the Herbig star itself or in thecorona of an as yet unseen late-type companion. The X-ray temperaturesand luminosity (logLX=30.5 ergs s-1) are withinthe range expected for a T Tauri companion, but high-resolution Chandraand Hubble Space Telescope images constrain the separation of a putativecompanion to less than 1". We place these new results into broadercontext by comparing the X-ray and bolometric luminosities of a sampleof nearby Herbig stars with those of T Tauri stars and classicalmain-sequence Be stars. We also test the predictions of a model thatattributes the X-ray emission of Herbig stars to magnetic activity thatis sustained by a shear-powered dynamo.
|The ɛ Chamaeleontis Young Stellar Group and the Characterization of Sparse Stellar Clusters|
We present the outcomes of a Chandra X-Ray Observatory snapshot studyof five nearby Herbig Ae/Be (HAeBe) stars that are kinematically linkedwith the Oph-Sco-Cen association (OSCA). Optical photometric andspectroscopic follow-up was conducted for the HD 104237 field. Theprincipal result is the discovery of a compact group ofpre-main-sequence (PMS) stars associated with HD 104237 and itscodistant, comoving B9 neighbor ɛ Chamaeleontis AB. We name thegroup after the most massive member. The group has five confirmedstellar systems ranging from spectral type B9 to M5, including aremarkably high degree of multiplicity for HD 104237 itself. The HD104237 system is at least a quintet, with four low-mass PMS companionsin nonhierarchical orbits within a projected separation of 1500 AU ofthe HAeBe primary. Two of the low-mass members of the group are activelyaccreting classical T Tauri stars. The Chandra observations alsoincrease the census of companions for two of the other four HAeBe stars,HD 141569 and HD 150193, and identify several additional new members ofthe OSCA. We discuss this work in light of several theoretical issues:the origin of X-rays from HAeBe stars; the uneventful dynamical historyof the high-multiplicity HD 104237 system; and the origin of the ɛCha group and other OSCA outlying groups in the context of turbulentgiant molecular clouds. Together with the similar η Cha cluster, wepaint a portrait of sparse stellar clusters dominated byintermediate-mass stars 5-10 Myr after their formation.
|Observations of Star-Forming Regions with the Midcourse Space Experiment|
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
|Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr|
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
|The Dispersal of Young Stars and the Greater Sco-Cen Association|
We review topics related to the dispersal of young stars from theirbirth-sites, and focus in particular on the entourage of young starsrelated to the ongoing star-formation event in the Sco-Cen OBassociation. We conduct a follow-up kinematic study to that presented inMamajek, Lawson, & Feigelson (2000; ApJ 544, 356) amongst nearby,isolated, young stars. In addition to the eta Cha and TW Hya groups, wefind several more intriguing Sco-Cen outlier candidates: most notablyβ Pic, PZ Tel, HD 199143, and HD 100546. We discuss the connectionbetween Sco-Cen and the southern ``150 pc Conspiracy'' molecular clouds,and in particular, Corona Australis. The kinematic evidence suggeststhat many of the nearby, isolated ~10 Myr-old stars were born nearSco-Cen during the UCL and LCC starbursts 10-15 Myr ago. We hypothesizethat these stars inherited 5-10 km/s velocities moving away fromSco-Cen, either through molecular cloud turbulence, or through formationin molecular clouds associated with the expanding Sco-Cen superbubbles(e.g. Loop I).
|CCD Speckle Observations of Binary Stars from the Southern Hemisphere. III. Differential Photometry|
Two hundred seventy-two magnitude difference measures of 135 double starsystems are presented. The results are derived from speckle observationsusing the Bessel V and R passbands and a fast readout CCD camera.Observations were taken at two 60 cm telescopes, namely the Helen SawyerHogg Telescope, formerly at Las Campanas, Chile, and the Lowell-TololoTelescope at the Cerro Tololo Inter-American Observatory, Chile. Thedata analysis method is presented and, in comparing the results to thoseof Hipparcos as well as to recent results using adaptive optics, we findvery good agreement. Overall, the measurement precision appears to bedependent on seeing and other factors but is generally in the range of0.10-0.15 mag for single observations under favorable observingconditions. In four cases, multiple observations in both V and R allowedfor the derivation of component V-R colors with uncertainties of 0.11mag or less. Spectral types are assigned and preliminary effectivetemperatures are estimated in these cases.
|Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics|
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
|The η Chamaeleontis Cluster: Origin in the Sco-Cen OB Association|
A young, nearby compact aggregate of X-ray-emitting pre-main-sequencestars was recently discovered in the vicinity of η Chamaeleontis. Inthis paper, we further investigate this cluster: its membership, itsenvirons, and its origins. ROSAT High Resolution Imager X-ray data forthe cluster's T Tauri stars show high levels of magnetic activity andvariability. The cluster has an anomalous X-ray luminosity functioncompared to other young clusters, deficient in stars with low butdetectable X-ray luminosities. This suggests that many low-mass membershave escaped the surveyed core region. Photographic photometry from theUSNO-A2.0 catalog indicates that additional, X-ray-quiet members existin the cluster core region. The components of the eclipsing binary RSCha, previously modeled in the literature as post-main sequence (MS)with discordant ages, are shown to be consistent with being coevalpre-MS stars. We compute the Galactic motion of the cluster fromHipparcos data and compare it to other young stars and associations inthe fourth Galactic quadrant. The kinematic study shows that the ηCha cluster, as well as members of the TW Hya association and a newgroup near ɛ Cha, probably originated near the giant molecularcloud complex that formed the two oldest subgroups of the Sco-Cen OBassociation roughly 10-15 Myr ago. Their dispersal is consistent withthe velocity dispersions seen in giant molecular clouds. A large H Ifilament and dust lane located near η Cha has been identified aspart of a superbubble formed by Sco-Cen OB winds and supernova remnants.The passage of the superbubble may have terminated star formation in theη Cha cluster and dispersed its natal molecular gas.
|Two-colour photometry for 9473 components of close Hipparcos double and multiple stars|
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
|Photometric confirmation of the δ Scuti pulsational variability in the pre-main-sequence Herbig Ae star HD104237|
Photometric observations of the Herbig Ae star HD104237 confirm that itis a δ Scuti star. We find two periods, the highest amplitude onehaving a value of 43min. This is only the fourth known pre-main-sequenceδ Scuti star, and the only one with such short periods. Theseimply high-overtone modes which have not yet been investigatedtheoretically. Two of our comparison stars, HD104036 and 105234, arenewly discovered δ Scuti stars.
|The evolution status of HD 104237, epsilon Cha and HD 100546.|
|The Scorpius OB2 Complex|
The Sco OB2 complex is a member of the Local Association. Theassociation contains pre-main-sequence stars in addition to objects some3 x 10^7 yr old. If it is assumed that stars in wide binaries arecoeval, then the He-weak variables in the supercluster arepre-main-sequence, elevated above that sequence by an amount similar tothat of the 3 x 10^7 yr old, normal stars but contracting toward thesequence, not evolving from it. The apparent great depth of theassociation is probably a result of some foreground superclustermembers, superposed on Sco OB2, and a rift in the absorption cloudbetween Scorpius and Ophiuchus. The adjoining Chamaeleon complex alsoappears to be a member of the supercluster.
|CCD Speckle Observations of Binary Stars From the Southern Hemisphere|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2117H&db_key=AST
|ASCA Observations of HD 104237 (A4e) and the Origin of X-Ray Emission from Herbig AE Stars|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...471..987S&db_key=AST
|The ROSAT all-sky survey catalogue of optically bright OB-type stars.|
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
|Molecular gas near HD 104237 and ɛ Chamaeleontis.|
We present the results of a search for molecular gas near the candidateHerbig Ae/Be star HD104237 and its common proper motion companionɛ Cha. The molecular gas found near these stars is clumpy, ofvery low mass, and is far from virial equilibrium. Our results can beinterpreted as providing evidence in favour of the pre-main sequencenature of HD104237 and, by implication, of ɛ Cha. We suggestthat these stars may have recently cleared away the molecular cloud inwhich they have formed. The HD104237/ɛ Cha system may be anexample of isolated intermediate mass star formation within the complexof Chamaeleon dark clouds.
|Pre-Main-Sequence Stars in the Pleiades Supercluster|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.1749E&db_key=AST
|Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.|
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
|The distribution of interstellar dust in the solar neighborhood|
We surveyed the IRAS data base at the positions of the 1808 O6-B9.5stars in The Bright Star Catalog for extended objects with excessemission at 60 microns, indicating the presence of interstellar dust atthe location of the star. Within 400 pc the filling factor of theinterstellar medium, for dust clouds with a density greater than 0.5/cucm is 14.6 + or - 2.4%. Above a density of 1.0/cu cm, the densitydistribution function appears to follow a power law index - 1.25. Whenthe dust clouds are mapped onto the galactic plane, the sun appears tobe located in a low-density region of the interstellar medium of widthabout 60 pc extending at least 500 pc in the direction of longitudes 80deg - 260 deg, a feature we call the 'local trough'.
|ICCD speckle observations of binary stars. VIII - Measurements during 1989-1991 from the Cerro Tololo 4 M telescope|
One-thousand eighty-eight observations of 947 binary star systems,observed by means of speckle interferometry with the 4 m telescope onCerro Tololo, are presented. These measurements, made during the period1989-1991, comprise the second installment of results stemming from theexpansion of our speckle program to the southern hemisphere.
|The Pleiades supercluster in FK 5|
The age distribution of the early-type supercluster members whichdominate FK 5 is studied by means of the proper motions derived byFricke et al. (1988). The main values of the total space velocity of themembers and their dependence on radial distance from the sundemonstrates the characteristic dependence of isoperiodic galacticorbits. Models incorporating convective overshoot yield a wide range ofages, and dual epochs of star formation are identified for someindividual clusters. The results of the analysis include three widepairs of premain-sequence stars, three late-type supergiants, and oneevolved close visual binary. Recent speckle observations can be utilizedto determine the orbit of the latter binary and to determine thestructure of a potential equal-component binary comprising twomain-sequence stars. Orbital and stellar parameters are listed for theclusters which make up the Pleiades supercluster including periods,cluster parallaxes, and individual masses.
|An optical search for Beta Pictoris-like disks around nearby stars|
A coronagraphic survey of more than one hundred stars has been carriedout in a search at optical wavelengths for circumstellar materialsimilar to that found in the Beta Pictoris disk. The survey stars wereprimarily dwarfs in the spectral range A to K and most were closer than100 pc. No evidence of circumstellar material was found around any ofthe stars, suggesting that Beta Pictoris is an abnormal object,surrounded by an unusually large amount of optically scatteringmaterial.
|IUE observations of the bright Herbig AE star HD 104237|
IUE observations are noted to indicate that HD 104237 exhibits the sameUV, visible, and IR spectral properties as the Herbig Ae stars AB Aurand HR 5999. It is accordingly concluded that HD 104237 is a 2.1solar-mass Herbig AE star. With respect to HR 5999, the chromosphericand transition region emission-line flux of HD 104237 is sufficientlylarge to permit the detection of these lines. A study of the relation ofdifferent levels of chromospheric activities and evolutionary stage ofintermediate, premain-sequence stars is projected to be able tocontribute to the understanding of the premain-sequence evolution of theintermediate-mass stars, during the final contraction phase.
|ICCD speckle observations of binary stars. V - Measurements during 1988-1989 from the Kitt Peak and the Cerro Tololo 4 M telescopes|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990AJ.....99..965M&db_key=AST
|The correlation between rotation and IR colour excess for B-types dwarfs|
The present derivation of 12-micron IR color excess for B-type dwarfs inthe IV and V luminosity classes, on the basis of IRAS observations,notes a large IR color excess for most of these stars. An examination ofthe relation between IR color excess and v sin i shows a clear tendencyin B0-B4 type stars with low v sin i values to exhibit no excess, whilethe fraction of stars with large IR excesses increases toward high v sini values. It is suggested that rotation facilitates the presence of acircumstellar shell, although other mechanisms determine whether or nota star is a Be star. Once a shell has come into existence, there is nocorrelation between v sin i and IR color excess.
|Comparison of formaldehyde and hydroxyl parameters in the Epsilon Chamaeleonis I dark cloud with the distribution of optical extinction and young stars|
Star counts have been performed using a blue ESO/SRC sky survey plate inorder to study the dust distribution in the Epsilon Cha I dark cloud.The extinction maps obtained indicate a lower limit of 8 m for themaximum extinction at the center of the cloud; a comparison between theextinction and the H2CO and OH line intensities shows good correlationbetween the molecular gas and the dust components. The connectionbetween the young stellar and protostellar objects is also studied bycomparing their distributions with the H2CO distribution.
|Erratum - Discordances Between SAO and HD Numbers for Bright Stars|
|Spectral classification from the ultraviolet line features of S2/68 spectra. V - Supplement series|
The paper presents the ultraviolet classification of more than 450 B-Aand F spectra obtained with the sky survey telescope mounted on the TD1satellite. Concerning abnormal objects, attention is given to the Bestars, the binary stars, the helium weak stars, and the lambda Booobjects. With regard to Be stars, it is concluded that the 1930 Awavelength feature is linked to the existence of either shell structuresor Fe II emission lines in the visual spectral range. In addition, it issuggested that stellar spectra at wavelengths of 1410, 1610-20 and 2400A are equivalent to the He-weak group in the classical range.
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