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Accretion rates in Herbig Ae stars
Aims.Accretion rates from disks around pre-main sequence stars are ofimportance for our understanding of planetary formation and diskevolution. We provide in this paper estimates of the mass accretionrates in the disks around a large sample of Herbig Ae stars.Methods: .We obtained medium resolution 2 μm spectra and used theresults to compute values of dot M_acc from the measured luminosity ofthe Brγ emission line, using a well established correlationbetween L(Brγ) and the accretion luminosity L_acc. Results:.We find that 80% of the stars, all of which have evidence of anassociated circumstellar disk, are accreting matter, with rates 3×10-9 dot M_acc 10-6 M_ȯ/yr; for 7objects, 6 of which are located on the ZAMS in the HR diagram, we do notdetect any line emission. Few HAe stars (25%) have dotM_acc>10-7 M_ȯ/yr. Conclusions: .In most HAestars the accretion rate is sufficiently low that the gas in the innerdisk, inside the dust evaporation radius, is optically thin and does notprevent the formation of a puffed-up rim, where dust is directly exposedto the stellar radiation. When compared to the dot M_acc values foundfor lower-mass stars in the star forming regions Taurus and Ophiuchus,HAe stars have on average higher accretion rates than solar-mass stars;however, there is a lack of very strong accretors among them, probablydue to the fact that they are on average older.
| On the origin of the X-ray emission from Herbig Ae/Be stars
Context: .Herbig Ae/Be stars are fully radiative and not expected tosupport dynamo action analogous to their convective lower-masscounterparts, the T Tauri stars. Alternative X-ray productionmechanisms, related to stellar winds or star-disk magnetospheres havebeen proposed, but their X-ray emission has remained a mystery. Aims: .A study of Herbig Ae/Be stars' global X-ray properties (such asdetection rate, luminosity, temperature, variability), helps toconstrain the emission mechanism by comparison to other types of stars,e.g. similar-age but lower-mass T Tauri stars, similar-mass but moreevolved main-sequence A- and B-type stars, and with respect to modelpredictions. Methods: .We performed a systematic search forChandra archival observations of Herbig Ae/Be stars. The superiorspatial resolution of this satellite with respect to previous X-rayinstrumentation has allowed us to also examine the possible role oflate-type companions in generating the observed X-rays. Results:.In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-rayemitting faint companions or other unrelated X-ray bright objects within10''. The detection fraction of Herbig Ae/Be stars is 76%, but decreasesto 35% if all emission is attributed to further known and unresolvedcompanions. The spectral analysis confirms the high X-ray temperatures( 20 MK) and large range of fractional X-ray luminosities(log{L_x/L_*}) of this class derived from earlier studies of individualobjects. Conclusions: .Radiative winds are ruled out as anemission mechanism on the basis of the high temperatures. The X-rayproperties of Herbig Ae/Be stars are not vastly different from those oftheir late-type companion stars (if such are known), nor from otheryoung late-type stars used for comparison. Therefore, either a similarkind of process takes place in both classes of objects, or there must beas yet undiscovered companion stars.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| Discovery of Extremely Embedded X-Ray Sources in the R Coronae Australis Star-forming Core
With the XMM-Newton and Chandra observatories, we detected two extremelyembedded X-ray sources in the R Corona Australis (R CrA) star-formingcore, near IRS 7. These sources, designated as XE andXW, have X-ray absorption columns of ~3×1023cm-2 equivalent to AV~180 mag. They are associatedwith the VLA centimeter radio sources 10E and 10W, respectively;XW is the counterpart of the near-infrared source IRS 7,whereas XE has no K-band counterpart above 19.4 mag. Thisindicates that XE is younger than typical Class I protostars,probably a Class 0 protostar, or in an intermediate phase between Class0 and Class I. The X-ray luminosity of XE varied between29
| X-Ray Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| Far Ultraviolet Spectroscopy of HD 76534
We present the first Far Ultraviolet Spectroscopic Explorer (FUSE)spectrum of HD 76534, a Herbig Be star. Here we focus on the analysis ofthe H2 absorption lines which allow to quantify the gaseouscontent along the line of sight. This analysis evidences large amountsof cold and warm H2 toward the star. We demonstrate that theH2 is bound to HD 76534 and we argue that it is likelydistributed in a circumstellar envelope. In addition, the present studygives clues for the existence of common properties for the circumstellarH2 of Herbig Ae/Be stars.
| SIMBA observations of the R Corona Australis molecular cloud
We have mapped the R Corona Australis molecular cloud at 1.2 mm withSIMBA on SEST and detected 25 distinct dust emission peaks. While 7 ofthem coincide with positions of previously known young stars, 18 areseemingly not associated with any known stellar object. We discuss thenature of individual sources and conclude that there are at least foursmall concentrations of young objects located along the filamentaryshaped cloud. A comparison with C18O data hints at thedepletion of molecules in some of the cores. Our new results yield someconflicting arguments about whether star formation proceeds fromnorth-west to south-east in the R Cr A cloud.Based on observations collected at the European Southern Observatory, LaSilla, Chile.Appendices A and B are only available in electronic form athttp://www.edpsciences.org
| VLT/NACO adaptive optics imaging of the TY CrA system. A fourth stellar component candidate detected
We report the detection of a possible subsolar mass companion to thetriple young system TY CrA using the NACO instrument at the VLT UT4during its commissioning. Assuming for TY CrA a distance similar to thatof the close binary system HD 176386, the photometric spectral type ofthis fourth stellar component candidate is consistent with an ~ M 4star. We discuss the dynamical stability of this possible quadruplesystem as well as the possible location of dusty particles inside oroutside the system.
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud
We present the ROSAT All-Sky Survey data in a 126 deg2 areain and around the CrA star forming region. With low-resolutionspectroscopy of unidentified ROSAT sources we could find 19 new pre-mainsequence stars, two of which are classical T Tauri stars, the othersbeing weak-lined. The spectral types of these new T Tauri stars rangefrom F7 to M6. The two new classical T Tauri stars are located towardstwo small cloud-lets outside of the main CrA cloud. They appear to be ~10 Myrs old, by comparing their location in the H-R diagram withisochrones for an assumed distance of 130 pc, the distance of the mainCrA dark cloud. The new off-cloud weak-line T Tauri stars may haveformed in similar cloudlets, which have dispersed recently.High-resolution spectra of our new T Tauri stars show that they havesignificantly more lithium absorption than zero-age main-sequence starsof the same spectral type, so that they are indeed young. From thosespectra we also obtained rotational and radial velocities. For somestars we found the proper motion in published catalogs. The directionand velocity of the 3D space motion - south relative to the galaticplane - of the CrA T Tauri stars is consistent with the dark cloud beingformed originally by a high-velocity cloud impact onto the galacticplane, which triggered the star formation in CrA. We also present VRIJHKphotometry for most of the new T Tauri stars to derive theirluminosities, ages, and masses. Partly based on observations collectedat the 1.52 m and 3.5 m telescopes of the European Southern Observatory,Chile, in programs 55.E-0549, 57.E-0646, and 63.L-0023, and onobservations collected at the 0.9 m, 1.5 m, and 4.0 m CTIO telescope.
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Mid infrared emission of nearby Herbig Ae/Be stars
We present mid IR spectro-photometric imaging of a sample of eightnearby (D <= 240 pc) Herbig Ae/Be stars. The spectra are dominatedby photospheric emission (HR6000), featureless infrared excess emission(T Cha), broad silicate emission feature (HR5999) and the infraredemission bands (HD 97048, HD 97300, TY CrA, HD 176386). The spectrum ofHD179218 shows both silicate emission and infrared emission bands (IEB).All stars of our sample where the spectrum is entirely dominated by IEBhave an extended emission on scales of a few thousand AU ( ~ 10''). Weverify the derived source extension found with ISOCAM by multi-aperturephotometry with ISOPHT and compare our ISOCAM spectral photometry withISOSWS spectra. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, the Netherlands and the United Kingdom) with theparticipation of ISAS and NASA.
| Six intermediate-mass stars with far-infrared excess: a search for evolutionary connections
We present the results of high-resolution spectroscopic, low-resolutionspectrophotometric and spectropolarimetric and broad-band multicolourobservations of four B-type stars (HD 4881, 5839, 224648 and 179218) andtwo A-type stars (HD 32509 and 184761) with strong far-infrared (IR)excesses. The excess in HD 184761, which is located at a distance of 65pc from the Sun, was recognized for the first time. Double-peakedHα emission line profiles are found in HD 4881 and HD 5839, whileHD 184761, HD 224648 and HD 32509 display no emission in Hα. Theremarkable variations observed in the Hα profile of HD 179218 arealso observed in some classical Be and Herbig Ae/Be stars. An intrinsiccomponent of polarization is clearly present in HD 179218, only aninterstellar component is detected in HD 4881 and HD 224648, and HD184761 was found to be unpolarized. Improved effective temperatures forall six objects were derived. Parallaxes measured by the Hipparcossatellite were used to determine positions of the stars in the HRdiagram. HD 4881 and HD 5839 are an order of magnitude more luminousthan main-sequence stars of similar temperatures and are most likelynewly discovered classical Be stars. Study of the high-resolution IRASmaps and modelling of the spectral energy distributions of HD 4881, HD5839 and HD 224648 suggest that the observed large IR excesses arecaused by radiation from circumstellar dust rather than free--freeradiation or infrared cirrus, so they may be higher mass counterparts ofbeta Pictoris stars. HD 32509, HD 224648 and HD 184761, which have verysmall near-IR excesses, are probably young main-sequence stars. HD179218, which exhibits the largest near- and far-IR excess in thesample, is an isolated pre-main-sequence Herbig Be star.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Deuterium fractionation and the degree of ionization in the R Coronae Australis molecular cloud core
The fractionation of D and (13) C in HCO(+) was investigated in the RCoronae Australis molecular cloud core. The distributions of H(13) CO(+)and DCO(+) were found to be morphologically similar but their columndensity maxima were found to lie in different locations. The H(13) CO(+)/HC(18) O(+) abundance ratio was found to vary little from 10 within themapped region, in excellent agreement with the (13) CO/C(18) O abundanceratios derived earlier towards the cloud by Harjunpää &Mattila (1996). This corroborates the close relationship between HCO(+)and CO predicted by the chemistry models. The DCO(+) /HCO(+) abundanceratio ranges from 0.006 to 0.04, being lowest towards two locations nearthe embedded infrared source IRS 7 where the kinetic temperature, asderived from methyl acetylene (CH_3CCH) observations, is somewhatelevated. The variation of the degree of deuterium fractionation withinthe core is due to an increase in the kinetic temperature near thecluster of newly born stars. This temperature rise results in twoeffects: Firstly, the reaction H_2D(+) -> H_3(+) becomes faster; andsecondly, an intensified desorption from grain surfaces increases theabundance of neutral atoms and molecules in the gas phase leading to thedestruction of H_3(+) and H_2D(+) ions. Both processes decrease theDCO(+) /HCO(+) abundance ratio. Far from the active region the derivedabundances of neutral species indicate the presence of depletion ontograin surfaces. The observations suggest furthermore that the fractionalelectron abundance, chi (e(-)) , is lowest in the dense clump near IRS7. This region also exhibits a low degree of gas phase depletion. Infact, increased fractional abundances of neutral species such as atomicoxygen and CO lead to a decrease in the [H_3(+]/[) HCO(+]) abundanceratio which is directly proportional to chi (e(-)) .
| The Pre-Main-Sequence Eclipsing Binary TY Coronae Australis: Precise Stellar Dimensions and Tests of Evolutionary Models
We analyze new photometric data for the Herbig Be eclipsing binary TYCrA, which securely reveal the secondary eclipse, ~0.03 mag deep in y.From the light-curve solution and our previous spectroscopic data,absolute dimensions of the primary and secondary stars are derived. Themasses are found to be M_1 = 3.16 +/- 0.02 M_ȯ and M_2 = 1.64 +/-0.01 M_ȯ, the radii are R_1 = 1.80 +/- 0.10 R_ȯ and R_2 = 2.08+/- 0.14 R_ȯ, the luminosities are L_1 = 67 +/- 12 L_ȯ and L_2 =2.4 +/- 0.8 L_ȯ, and the effective temperatures are T_1 = 12,000 +/-500 K and T_2 = 4900 +/- 400 K. Here the uncertainties representhigh-confidence limits, not standard deviations. The secondary star is apre-main-sequence star located at the base of the Hayashi tracks. Assuch, it is the least evolved star with a dynamically measured mass.Given higher effective temperatures for the primary (e.g., 12,500 K),the solar-composition 1.64 M_ȯ evolutionary tracks of Swenson etal., Claret, and D'Antona & Mazzitelli are all consistent with theproperties of the TY CrA secondary and suggest an age of order 3 Myr.The radius and projected rotational velocity of the secondary star areconsistent with synchronous rotation. The primary star is located nearthe zero-age main sequence, which, for solar compositions, is consistentwith an age of 3 Myr. However, the primary star is not well representedby any of the 3.16 M_ȯ evolutionary models, which predict somewhathigher effective temperatures than observed.
| Four-colour photometry of eclipsing binaries. XXXIX. Light curves of the pre-main sequence triple system TY Coronae Australis
Complete uvby light curves of the detached Herbig Be eclipsing binary TYCoronae Australis are presented. A total of 1789 photometricmeasurements in each of the four colours were obtained in 1989 and in1992-1994. A detailed analysis of the y light curve obtained in1992-1993 is published separately \cite[(Casey et al. 1997)]{cas97}. Thereflection nebula around the system contributes about 30% of the lightin all four passbands. Here we present and discuss thenon-eclipse-related photometric variability of the system. We suggestthat these variations are the result of variable obscuration, possiblylinked to dust shells physically associated to the system. Based onobservations collected with the Danish 50~cm Strömgren AutomaticTelescope, SAT, at the European Southern Observatory (ESO), La Silla,Chile.
| Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars
Not Available
| A study of the candidates for Herbig Ae/Be stars HD 35929 and HD 203024.
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| Comparison of Star Formation in Five Nearby Molecular Clouds
We have calculated bolometric temperature (Tbol) and bolometricluminosity (Lbol) for 383 young stellar objects (YSOs) in five molecularclouds within 200 pc in Corona Australis (CrA), Ophiuchus (Oph), Taurus(Tau), Chamaeleon (Cha), and Lupus (Lup). We used Tbol, Lbol, andbolometric luminosity-temperature (BLT) diagrams to characterize andcompare the overall-star-formation activity of the clouds on aself-consistent basis. The main results are the following: (1) the YSOpopulations in the clouds can be differentiated by the fraction of theirlow-Tbol sources, which increases systematically from Lup and Cha to Tauand to Oph and CrA. This trend is interpreted as increasing currentstar-forming activity in the same order; (2) the clouds with higher coldsource fractions also seem to have higher bright source fractions; (3)In the BLT diagram, the CrA and Oph sources are more uniformlydistributed while the Cha and Lup sources are aggregated near thezero-age mainsequence (ZAMS). Tau sources appear to be an intermediatecase. Taurus also seems to contain more cold (Tbol < 1000 K) andlow-luminosity (Lbol < 1 Lȯ) sources than the other complexes;(4) the YSOs show a characteristic distribution in the median BLTdiagram. This distribution is qualitatively consistent with the earlyYSO evolution from a protostar to a pre--main-sequence star and providesa unique observational test to star-formation models; (5) for Luppre--main-sequence stars, the ratio of their Tbol to Teff increasesduring their approach to the ZAMS. This increase can be explained by thedisk and envelope dissipation during the pre--main-sequence evolution;(6) the most active star-forming clouds (Oph and CrA) also have densermolecular cores as measured by C18O J = 1--0 line emission, suggestingthat the star formation occurs in the densest parts of the molecularclouds; and (7) we find an anti-correlation between Tbol and C18Oemission for the class 0 and I Tau sources (Tbol < 650 K). This showsthat Tbol measures the intrinsic redness of YSOs, rather than theirdisk-envelope orientation. The disk orientation may have a moreimportant effect on Tbol of the pre--main-sequence stars.
| Deep Infrared Imaging of the R Coronae Australis Cloud Core
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2029W&db_key=AST
| MSC - a catalogue of physical multiple stars
The MSC catalogue contains data on 612 physical multiple stars ofmultiplicity 3 to 7 which are hierarchical with few exceptions. Orbitalperiods, angular separations and mass ratios are estimated for eachsub-system. Orbital elements are given when available. The catalogue canbe accessed through CDS (Strasbourg). Half of the systems are within 100pc from the Sun. The comparison of the periods of close and widesub-systems reveals that there is no preferred period ratio and allpossible combinations of periods are found. The distribution of thelogarithms of short periods is bimodal, probably due to observationalselection. In 82\% of triple stars the close sub-system is related tothe primary of a wide pair. However, the analysis of mass ratiodistribution gives some support to the idea that component masses areindependently selected from the Salpeter mass function. Orbits of wideand close sub-systems are not always coplanar, although thecorresponding orbital angular momentum vectors do show a weak tendencyof alignment. Some observational programs based on the MSC aresuggested. Tables 2 and 3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| HIPPARCOS data on Herbig Ae/Be stars: an evolutionary scenario.
Fundamental astrophysical parameters (distance, temperature, luminosity,mass, age) of a sample of 10 Herbig Ae/Be candidates and 3 non-emissionline A and B stars in star forming regions were computed combiningHipparcos parallaxes with data from literature. All genuine Herbig starsin our sample are located between the birthline and the zero-age mainsequence (ZAMS) in the Hertzsprung-Russell diagram (HRD), in accordancewith what is expected for pre-main sequence stars. The region in the HRDclose to the birthline is relatively devoid of stars when compared tothe region closer to the ZAMS, in agreement with the expectedevolutionary time scales. The Herbig Ae/Be stars not associated withstar forming regions were found to be located close to the ZAMS.Additionally we discuss a possible evolutionary scenario for thecircumstellar environment of Herbig stars.
| Discovery of Hard X-Rays from a Cluster of Protostars
We report on the detection of hard X-rays from protostar candidates(Class I sources) in the R Coronae Australis (R CrA) molecular cloud.The hard X-rays are found even in the quiescent states with extremelyhigh temperature of about 6 keV. One of the sources, probably theembedded far-infrared source R1, exhibited a powerful X-ray flare. Thesephenomena are similar to, but with a significantly higher temperaturethan, those seen in T Tauri stars. The flare spectrum is unusual,showing a broadened or double emission-line structure between 6.2 and6.8 keV. Since X-rays are unexpected from current protostellar theory,these ASCA observations should have a significant impact on ourunderstanding of star formation and the earliest phases of stellarevolution.
| Infrared excesses in A-type stars
Not Available
| The β Pictoris phenomenon among Herbig Ae/Be stars. UV and optical high dispersion spectra.
We present a survey of high dispersion UV and optical spectra of HerbigAe/Be (HAeBe) and related stars. We find accreting, circumstellar gasover the velocity range +100 to +400km/s, and absorption profilessimilar to those seen toward β Pic, in 36% of the 33 HAeBe starswith IUE data as well as in 3 non-emission B stars. We also findevidence of accretion in 7 HAeBe stars with optical data only. Lineprofile variability appears ubiquitous. As a group, the stars withaccreting gas signatures have higher vsini than the stars withoutflowing material, and tend to exhibit large amplitude (>=1^m^)optical light variations. All of the program stars with polarimetricvariations that are anti-correlated with the optical light, previouslyinterpreted as the signature of a dust disk viewed close to equator-on,also show spectral signatures of accreting gas. These data imply thataccretion activity in HAeBe stars is preferentially observed when theline of sight transits the circumstellar dust disk. Our data imply thatthe spectroscopic signatures of accreting circumstellar material seen inβ Pic are not unique to that object, but instead are consistentwith interpretation of β Pic as a comparatively young A star withits associated circumstellar disk.
| Star counts in southern dark clouds: Corona Australis and Lupus.
Star counts technique is used towards southern dark globular filamentssituated in the cloud complexes of Corona Australis and Lupus. Tablesand maps of the distribution of visual extinction are presented for eachfilament. Lower limit masses for the filaments and condensations havebeen estimated and the central coordinates of the condensations are alsogiven. R CrA is the most active star forming region among the filamentsstudied in this work whereas Lupus 1, with almost the same lower limitof mass, has only a few T Tauri stars and just one young embeddedobject. The distribution of direction of the magnetic field in thecondensations of Lupus, suggests that the condensation morphologies doesnot have any apparent relation with the magnetic field orientation.
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Corona Australis |
| Ascensió Recta: | 19h01m38.93s |
| Declinació: | -36°53'26.5" |
| Magnitud Aparent: | 7.329 |
| Distancia: | 136.054 parsecs |
| Moviment propi RA: | 2.8 |
| Moviment propi Dec: | -26.2 |
| B-T magnitude: | 7.468 |
| V-T magnitude: | 7.341 |
Catàlegs i designacions:
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