Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Mid-infrared imaging of the circumstellar dust around three Herbig Ae stars: HD 135344, CQ Tau, and HD 163296
Aims.Planet formation has been known for many years to be tied to thespatial distribution of gas and dust in disks around young stars. Toconstrain planet formation models, imaging observations ofprotoplanetary disks are required. Methods: . Given this, we haveundertaken a mid-infrared imaging survey of Herbig Ae stars, which arepre-main sequence stars of intermediate mass still surrounded by a largeamount of circumstellar material. The observations were made at awavelength of 20.5 μm with the CAMIRAS camera mounted at theCassegrain focus of the Canada France Hawaii Telescope. Results: .We report the observations of three stars, HD 135344, CQ Tau, and HD163296. The circumstellar material around the three objects is spatiallyresolved. The extensions feature a disk-like shape. The images providedirect information on two key parameters of the disk: its inclinationand its outer radius. The outer radius is found to be quite differentfrom the one deduced from disk models, which is only constrained byfitting the Spectral Energy Distribution of the object. Other parametersof the disk, such as flaring and dust mass have been deduced fromfitting both the observed extension and the spectral energy distributionwith sophisticated disk models. Conclusions: .Our results showhow important imaging data are to tighten constraints on the disk modelparameters.
| 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.
| Deep Imaging Surveys of Star-forming Clouds. IV. The Meek and the Mighty: Outflows from Young Stars in Chamaeleon I
We present a survey of shocks and outflows in the Chamaeleon Istar-forming complex using Hα, [S II], and SDSS i' images obtainedfrom the ground, an i' image obtained with the Hubble Space Telescope,and 4.5 μm images obtained with the Infrared Array Camera (IRAC) onthe Spitzer Space Telescope. We find new Herbig-Haro (HH) objects andextensions to the previously cataloged shocks that trace parts of atleast 20 distinct outflows from young stars. Some HH objects mark thepresence of giant outflows, the largest of which is powered by Cha-MMS1and associated with HH 49/50 near the Ced 110 region. Other large flowsare powered by Cha-MMS2 in the Ced 112 region and the IRN in the Ced 111region. Although some shocks exhibit infrared emission in the IRACbands, most notably HH 49/50 (the ``tornado''), most outflows in the ChaI clouds are not detected in the Spitzer IRAC bands. This result isconsistent with the general lack of extensive 2.12 μm H2emission from Cha I.
| 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.
| Blue Luminescence and the Presence of Small Polycyclic Aromatic Hydrocarbons in the Interstellar Medium
Blue luminescence (BL) was first discovered in a proto-planetary nebula,the Red Rectangle (RR), surrounding the post-AGB star HD 44179. BL hasbeen attributed to fluorescence by small, 3-4 ringed neutral polycyclicaromatic hydrocarbon (PAH) molecules and was thought to be unique to theRR environment, where such small molecules are actively being producedand shielded from the harsh interstellar radiation by a densecircumstellar disk. In this paper we present the BL spectrum detected inseveral ordinary reflection nebulae illuminated by stars havingtemperatures between 10,000 and 23,000 K. All these nebulae are known toalso exhibit the infrared emission features called aromatic emissionfeatures (AEFs) attributed to large PAHs. We present the spatialdistribution of the BL in these nebulae. In the case of Ced 112, the BLis spatially correlated with mid-IR emission structures attributed toAEFs. These observations provide evidence for grain processing andpossibly for in situ formation of small grains and large molecules fromlarger aggregates. Most importantly, the detection of BL in theseordinary reflection nebulae suggests that the BL carrier is anubiquitous component of the interstellar medium and is not restricted tothe particular environment of the RR.
| 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.
| 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| A Near-Infrared Multiplicity Survey of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds
We present new near-IR observations of 76 Class I/flat-spectrum objectsin the nearby (d<~320 pc) Perseus, Taurus, Chamaeleon I and II, ρOphiuchi, and Serpens dark clouds. These observations are part of alarger systematic infrared multiplicity survey of self-embedded objectsin the nearest dark clouds. When combined with the results of ourpreviously published near-infrared multiplicity survey, we find arestricted companion star fraction of 14/79 (18%+/-4%) of the sourcessurveyed to be binary or higher order multiple systems over a separationrange of ~300-2000 AU with a magnitude difference ΔK<=4 andwith no correction for background contamination or completeness. This isconsistent with the fraction of binary/multiple systems found amongolder pre-main-sequence T Tauri stars in each of the Taurus, ρ Oph,and Chamaeleon star-forming regions over a similar separation range, aswell as the combined companion star fraction for these regions. However,the companion star fraction for solar-type, and lower mass M dwarf,main-sequence stars in the solar neighborhood in this separation range(11%+/-3%) is approximately one-half that of our sample. Together withmultiplicity statistics derived for previously published samples ofClass 0 and Class I sources, our study suggests that a significantnumber of binary/multiple objects may remain to be discovered at smallerseparations among our Class I/flat-spectrum sample and/or most of theevolution of binary/multiple systems occurs during the Class 0 phase ofearly stellar evolution.
| The brown dwarf population in the Chamaeleon I cloud
We present the results of a multiband survey for brown dwarfs in theChamaeleon I dark cloud with the Wide Field Imager(WFI) camera at the ESO/MPG 2.2-m telescope on La Silla (Chile). Thesurvey has revealed a substantial population of brown dwarfs in thissouthern star-forming region. Candidates were selected from R, I andHα imaging observations. We also observed in two medium-bandfilters, M 855 and M 915, for spectral type determination. The formerfilter covers a wavelength range containing spectral featurescharacteristic of M-dwarfs, while the latter lies in a relativelyfeatureless wavelength region for these late-type objects. A correlationwas found between spectral type and (M 855-M 915) colour index for mid-to late M-type objects and early L-type dwarfs. With this method, weidentify most of our object candidates as being of spectral type M 5 orlater. Our results show that there is no strong drop in the number ofobjects for the latest spectral types, hence brown dwarfs may be asabundant as low-mass stars in this region. Also, both kind of objectshave a similar spatial distribution. We derive an index α = 0.6± 0.1 of the mass function in this region of dispersed starformation, in good agreement with the values obtained in other starforming regions and young clusters. Some of the brown dwarfs have strongHα emission, suggesting mass accretion. For objects with publishedinfrared photometry, we find that strong Hα emission is related toa mid-infrared excess, indicative of the existence of a circumstellardisk.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Isophot-S Spectral Atlas of Young Stellar Objects
We present a mid-infrared spectral atlas containing 146 observations of122 young stellar objects. The 2.5--11.6 μ m low-resolution spectrawere obtained with the ISOPHOT-S instrument on-board of the l InfraredSpace Observatory. The atlas will be a valuable database for studyingthe emission/absorption profile of the amorphous silicate band at 10μ m, looking for the signatures of silicate grain processing,characterizing the PAH bands and ice features and selecting candidatevariable sources at mid-infrared wavelengths.
| The short period multiplicity among T Tauri stars
We present the results of high-resolution spectroscopic observationscarried out over three years aimed at estimating the short-period(P_orb<100 days) binary frequency of a sample of T Tauri stars inOph-Sco, Cha, Lup, CrA star forming regions (SFRs), already observedwith high angular resolution techniques by Ghez et al.(\cite{Ghezetal93}) and by Ghez et al. (\cite{Ghezetal97}) to detectwider components. When combining all four SFRs, the short-period binaryfrequency is indistinguishable from that found by Duquennoy & Mayor(\cite{DuqMay91}) for the solar-type field stars which is alsoconsistent with the previous result obtained by Mathieu(\cite{Mathieu92}, \cite{Mathieu94}). When Oph-Sco is analyzedseparately, it seems that there is an excess of short-period binaries ofa factor 2-2.5. On the contrary, short-period binary systems seem to beabsent in the sample containing stars in Cha/Lup/CrA. Such a trend wasequally found by Mathieu (\cite{Mathieu92}) in Taurus. An excess ofspectroscopic systems among the components of visual multiple systems isalso observed.Based on observations collected with the Swiss Euler Telescope and 1.5-mESO, proposal 63.I-0112.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| ISOCAM-CVF spectroscopy of the circumstellar environment of young stellar objects
We present the results of a mid-infrared (5-16.5 mu m) imagingspectroscopy survey of Young Stellar Objects (YSOs) and theirsurrounding environment in four low-mass star formation regions: RCrA,rho Ophiuchi, Serpens and Chamaeleon I. This survey was performed usingISOCAM and its Circular Variable Filters (CVF) and observed 42 YSOcandidates: we were able to obtain complete 5-16.5 mu m spectra for 40of these with a spectral resolving power of lambda /Delta lambda =~ 40.A number of spectral features were measured, most notably the 9.7 mu msilicate feature, the bending modes of both water and CO2ices at 6.0 and 15.2 mu m respectively and the well-known unidentifiedfeature at 6.8 mu m. The strength of the unidentified feature wasobserved to correlate very well with that of the water ice bending modeand far less strongly with the CO2 ice bending mode. Thissuggests, in a manner consistent with previous observations, that thecarrier of the unidentified feature is a strongly polar ice. Absorptionprofiles of the bending mode of CO2 ice are observed to showa significant long wavelength wing, which suggests that a significantfraction of the CO2 ice observed exists in a polar(H2O-rich) phase. The sources observed in RCrA, rho Oph andSerpens show similar spectral characteristics, whilst the sourcesobserved in Cha I are somewhat anomalous, predominantly showing silicateemission and little or no absorption due to volatile ices. However thisis consistent with previous studies of this region of the Cha I cloud,which contains an unusual cluster of YSOs. From comparisons of thestrengths of the water ice and silicate bands we detect an apparentunder-abundance of water ice towards the sources in rho Oph, relativeto both RCrA and Serpens. This may be indicative of differences inchemical composition between the different clouds, or may be due toevaporation. Finally the CO2:H2O ice ratiosobserved towards the sources in rho Oph show significantly greaterscatter than in the other regions, possibly due to varying localconditions around the YSOs in rho Oph.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with the participation of ISASand NASA.
| 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 3 μM Survey of the Chamaeleon I Dark Cloud
We describe an L-band photometric survey of ~ 0.5 deg2 of theChamaeleon I dark cloud. The survey has a completeness limit ofL<11.0. Our survey detects 124 sources, including all knownpre-main-sequence stars with L<=11. The fraction of sources withnear-IR excess emission is 58%+/-4% for K=9-11. Cha I sources have bluerH-K and K-L colors than pre-main-sequence stars in Taurus-Auriga. Thesesources also have a strong correlation between EW(Hα) and K-L.Stars with K-L<=0.6 have weak Hα emission; stars withK-L>=0.6 have strong Hα emission. Because many Cha I sourcesare heavily reddened, this division between weak emission T Tauri starsand classical T Tauri stars occurs at a redder K-L than inTaurus-Auriga. Based on observations obtained with the SPIREX/Abu systemin Antarctica. SPIREX/Abu was operated for the 1999 observing seasonunder agreement between the National Optical Astronomy Observatories(NOAO) and the Center for Astrophysical Research in Antarctica (CARA).
| A Near-Infrared Imaging Survey of the Chamaeleon I Dark Cloud
We describe a near-infrared imaging survey covering ~1 deg2of the Chamaeleon I dark cloud. The survey is complete for K<15.0,H<16.0, and J<16.5, roughly 2 magnitudes more sensitive thanprevious large-scale surveys. We use the large number of backgroundstars detected to derive an accurate near-infrared extinction law forthe cloud and select new candidate members with near-infrared colorexcesses. We list ~100 candidates of the cloud with K>=12.0, based ontheir positions in the J-H, H-K color-color diagram. These new starshave low luminosities (K~12-16, H-K>~0.5-1.5) and may have massesclose to or even below the hydrogen-burning limit.
| 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.
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
| ISOCAM observations of the Chamaeleon I dark cloud
We present the results of an ISOCAM survey of the Chamaeleon I darkcloud conducted in two broad-band filters at 6.7 and 14.3 mu m. In anarea of 0.59 sq.deg. we have detected a total of 282 mid-IR sources with103 sources observed in both filters. Combining the ISOCAM observationswith the I, J, and K_s data obtained with DENIS, we have found 108pre-main-sequence (PMS) stars in the region, of which 34 were previouslyunidentified. Several of these newly discovered young stellar objectsare relatively faint suggesting a population in Cha I of very low massobjects that probably includes brown dwarfs in their early contractionphases. Finally, most of the PMS stars show the spectral index computedbetween 2.2 and 14.3 mu m typical of Class II sources. The luminosityfunction (LF) derived for our detected PMS stars is discussed. ISO is anESA project with instruments funded by ESA Member States (especially thePI countries: France, Germany, the Netherlands and the United Kingdom)and with the participation of ISAS and NASA. Tables 1 and 2 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Tables 3 and 4 are onlyavailable with the on-line publication athttp://link.springer.de/link/service/journals/00230/
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| A Deep Near-Infrared Survey of the Chamaeleon I Dark Cloud Core
We have carried out a deep near-infrared imaging survey to search forlow-mass young stellar objects (YSOs) in the densest star-forming coreof the Chamaeleon I dark cloud. Our observations cover an area of 30arcmin2, including an early B9 star (HD 97300) and an outflowsource (HM 23). The 10 σ limiting magnitudes are 18.1, 17.0, and16.2 mag at J, H, and K, respectively, which is sensitive enough toprovide a census of the embedded stellar population down to substellarobjects in the cloud. Source classification is performed based on thenear-infrared (NIR) color-color diagram. Many of the YSO candidates withNIR excesses are more than 7 mag fainter than typical T Tauri stars inthe same cloud. Some of them are even fainter than the known browndwarfs in the Pleiades. The luminosities of newly identified YSOcandidates and the recent evolutionary models for very low mass objectssuggest that they appear to be substellar, if their typical age isassumed to be similar to that of classic T Tauri stars or, namely, 1 Myrwith an upper limit of 10 Myr. Therefore it is highly likely that youngbrown dwarfs form in this molecular cloud core. The J-band luminosityfunction of the YSO candidates does not appear to turn over down to thecompleteness limit. In the Chamaeleon I dark cloud core, stars form in aclustered mode characterized by both a high star formation efficiencyand high stellar density such as in the ρ Oph core.
| Infrared spectra of young stars in Chamaeleon
We present infrared spectra covering the wavelength ranges 3.1-5.0 mu mand 5.8-11.6 mu m for 9 young stars located in the Chamaeleon I darkcloud. The spectra were taken with the spectrophotometer of ISO(ISOPHOT-S). The spectra of three stars which illuminate reflectionnebulae are dominated by the series of strong emission featuresgenerally ascribed to polycyclic aromatic hydrocarbons. Three orpossibly four stars classified as T Tauri stars show the 10 mu msilicate band in emission, whereas another object, T42, shows thesilicate band in absorption and could be a T Tau star with a disk seenedge-on. The last source, Cha IRN, is a deeply embedded young stellarobject. Its spectrum is characterized by deep absorption bands due tovarious interstellar ices, but shows a very weak silicate band only.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany,Netherlands and the United Kingdom) with the participation of ISAS andNASA
| A search for spectroscopic binaries among Herbig Ae/Be stars
We present the results of a spectroscopic survey of binaries among 42bright (m_V<11) Herbig Ae/Be stars in both hemispheres. Radialvelocity variations were found in 7 targets, 4 are new spectroscopicbinaries. The Li I 6 708 Angstroms absorption line (absent feature insimple HAeBe stars spectra) indicates the presence of a cooler companionin 6 HAeBe spectrum binaries, 4 of which are new detections. Few starsclassified as possible Herbig Ae/Be stars are not confirmed as such.While for short-period (P<100 days) spectroscopic binaries, theobserved binary frequency is 10%, the true spectroscopic binaryfrequency for Herbig Ae/Be stars may be as high as 35%. Based onobservations collected at the European Southern Observatory (ESO), LaSilla, Chile and at the Observatoire de Haute--Provence (OHP),Saint--Michel l'Observatoire, France. Table 1 only available inelectronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Extended mid-infrared emission around the outflow source and the compact H II region in AFGL 4029
We present the first sub-arsecond 8-13 μm images of the young stellarcluster AFGL 4029, obtained using the camera CAMIRAS mounted at theCanada-France-Hawaii Telescope. The two dominant components of thecluster, a highly obscured young stellar object (YSO) and a compact H iiregion excited by a B1 star, are both spatially resolved and show the11.3 mu m band in emission. The dust composition and excitation in theAFGL 4029 H ii region appear similar to those in other compact H iiregions of moderate excitation observed by ISO-SWS. In addition, weobserve that the 11.3 mu m feature/continuum ratio decreases toward themain exciting star on scales <= 8800 AU. The large extent of themid-infrared emission from the YSO is so far a unique example amongdeeply embedded objects. It suggests that there is no simple trendrelating the mid-IR size and the evolutionary status in young stars. Theemission arises in optically thin circumstellar dust, possibly in acavity carved by the bipolar outflow. An optical and near-infraredreflection nebulosity present 5({') '} away from the YSO is alsodetected in the mid infrared. It is particularly bright in the 11.3 mu mfeature, with a feature/continuum ratio ~ 4 comparable to that seen inprototypical reflection nebulae. Finally, mid-IR emission associatedwith an optical jet knot in the same zone suggests that shocks, bycreating small particles, might explain the high surface brightness inthis region. Based on observations made at the Canada-France-HawaiiTelescope, operated by the National Research Council of Canada, theCentre National de la Recherche Scientifique, and University of Hawaii
| 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.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Bukalemun |
| Sað Açýklýk: | 11h09m50.03s |
| Yükselim: | -76°36'47.7" |
| Görünürdeki Parlaklýk: | 9.042 |
| Uzaklýk: | 187.617 parsek |
| özdevim Sað Açýklýk: | -19.1 |
| özdevim Yükselim: | -0.1 |
| B-T magnitude: | 9.48 |
| V-T magnitude: | 9.079 |
Kataloglar ve belirtme:
|
