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|Spectral observations of stars associated with nebulae. I. PP52, PP57, PP63, and PP89|
Spectral observations of stars associated with nebulae, PP52, PP57,PP63, and PP89, are reported. The star PP52 is of spectral type A and isassociated with a reflecting nebula. PP57 is a nonstationary star. Aweak object (1079-0122733) immediately adjacent to PP57 is highlyvariable. It is shown that the nebulae PP63 and PP89 are cometary.
|Radio and X-ray variability of young stellar objects in the Coronet cluster|
The Coronet cluster in the nearby R CrA dark cloud offers the rareopportunity to study at least four "class I" protostellar sources aswell as one candidate "class 0" source, a Herbig Ae star, and acandidate brown dwarf within a few square arcminutes - most of themdetected at radio- and X-ray wavelengths. These sources were observedwith the Very Large Array (VLA) at λ = 3.5 cm on nine occasionsin 1998, spread over nearly four months. The source IRS 5, shown earlierto emit circularly polarized radio emission, was observed to undergo aflux increase accompanied by changes in its polarization properties.Comparison with VLA measurements taken in January 1997 allows analysisof longer-term variability. In addition to this radio monitoring, weanalyze archival Chandra and XMM-Newton X-ray data of these sources.Three class I protostars are bright enough for X-ray spectroscopy, andwe perform a variability analysis for these sources, covering a total of154 ks spread over more than two and a half years. Also in X-rays, IRS 5shows the most pronounced variability, whilst the other two class Iprotostars IRS 1 and IRS 2 have more stable emission. X-ray data is alsoanalyzed for the recently identified candidate class 0 source IRS 7E,which shows strong variability as well as for the Herbig Ae star R CrAfor which we find extremely hot X-ray-emitting plasma. For IRS 1, 2 and5, the hydrogen column densities derived from the X-ray spectra are atabout half the values derived with near-infrared techniques, a situationsimilar to what has been observed towards some other young stellarobjects.
|The pre-stellar and protostellar population of R Coronae Australis|
We present 450- and 850-μm maps of R Coronae Australis. We comparethe maps with previous surveys of the region, and shed new light on thepreviously unknown nature of the protostellar sources at the centre ofthe cloud. We clarify the nature of two millimetre sources previouslydiscovered in lower-resolution data. We identify one new Class 0protostar, which we label SMM 1B, and we measure the envelope masses ofa number of more evolved protostars. We identify two new pre-stellarcores, which we call SMM 1A and SMM 6.
|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 (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
|Searching for the Molecules of Life in Space|
|Solid Carbon Dioxide in Regions of Low-Mass Star Formation|
We present high-resolution (R~1500-2000) spectra of the 4.27 μmasymmetric stretching feature of solid CO2 in eight lines ofsight observed with the Short Wavelength Spectrometer of the InfraredSpace Observatory. Two of the sources are field stars located behind theTaurus molecular cloud; the others are young stellar objects (YSOs) ofpredominantly low-to-intermediate mass. We find a significantsource-to-source variation in the solid CO2/H2Oabundance ratio in our sample: two lines of sight, Elias 18 and RAFGL989, have CO2 abundances of ~34%-37%, considerably higherthan in other lines of sight studied to date. In agreement with aprevious study of Elias 16, we confirm a substantial (~20%) abundance ofsolid CO2 relative to H2O in the quiescentintracloud medium. We compare the CO2 profiles withlaboratory spectra of interstellar ice analogs from the LeidenObservatory Laboratory database. Results show that the 4.27 μmprofiles toward field stars and embedded low-mass objects are remarkablysimilar to each other and seem to originate mostly in coldH2O-rich ice. In two higher mass YSOs (RAFGL 989 and S255IRS1), the profiles are clearly different, and at least the lattersource shows signs of thermal processing.
|Polarization models of young stellar objects - II. Linear and circular polarimetry of R Coronae Australis|
Near-infrared linear imaging polarimetry of the young stellar objects RCrA and T CrA in the J, H and Kn bands, and circular imaging polarimetryin the H band, is presented. The data are modelled with the Clark andMcCall scattering model. The R CrA and T CrA system is shown to be aparticularly complex scattering environment. In the case of R CrA thereis evidence that the wavelength dependence of polarization changesacross the nebula. MRN dust grain models do not explain this behaviour.Depolarization by line emission is considered as an alternativeexplanation. The dust grain properties could also be changing across thenebula. Although surrounded by reflection nebulosity, there is a regionof particularly low polarization surrounding R CrA that is best modelledby the canonical bipolar outflow being truncated by an evacuatedspherical cavity surrounding the star. The symmetry axis of the nebulaappears inclined by 50° to the plane of the sky. The H-band circularpolarimetry of R CrA clearly shows a quadrupolar structure of positiveand negative degrees of circular polarization that reach peak magnitudesof ~5per cent within our limited map. It is shown that spherical MRNgrains are incapable of producing this circular polarization given theobserved linear polarization of the R CrA system. Instead, scatteringfrom aligned non-spherical grains is proposed as the operatingmechanism. T CrA is a more archetypical bipolar reflection nebula, andthis object is modelled as a canonical parabolic reflection nebula thatlies in the plane of the sky. The wavelength independence of linearpolarization in the T CrA reflection nebula suggests that the scatteringparticles are Rayleigh sized. This is modelled with the MRN interstellargrain size distribution.
|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.
|Herbig Ae/Be stars with the IRAS low-resolution spectra.|
|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
|Transitional YSOs: candidates from flat-spectrum IRAS sources|
We are searching for Young Stellar Objects (YSOs) near the boundarybetween protostars and pre-main-sequence objects, what we termTransitional YSOs. We have identified a sample of 125 objects ascandidate transitional YSOs on the basis of IRAS colors and the opticalappearance on POSS plates. We have obtained optical and near-IR imagingof 82 objects accessible from the Northern Hemisphere and optical imagesof 62 sources accessible from the South. We also created deconvolved 60mu m IRAS images of all sources. We have classified the objects on thebasis of their morphology in the optical and near-IR images. We findthat the majority of our objects are associated with star-formingregions, confirming our expectation that the bulk of these objects areYSOs. Of the 125 objects, 28 have a variety of characteristics verysimilar to other transitional YSOs, while another 22 show some of thesecharacteristics. Furthermore we have found seven objects to be goodcandidates for members of the Herbig Ae/Be stellar group, of which threeare newly identified as such. We have placed a set of images for each ofthe objects in the archives of the Centre de Données astronomiquede Strasbourg (CDS). Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile.
|ISO-LWS observations of Herbig Ae/Be stars. I. Fine structure lines|
We present the results of the first spectrophotometric survey of asample of eleven Herbig Ae/Be stars (HAEBE) obtained with the LongWavelength Spectrometer (LWS) on board the Infrared Space Observatory(ISO). The [OI] 63mu m and the [CII] 158mu m lines are observed in allthe investigated sources, while the [OI] 145mu m transition, due to itsrelative faintness, sometimes remains undetected. By comparing lineintensity ratios with model predictions, photodissociation, due to theUV photons from the central star, results the dominating excitationmechanism although contributions of C-shocks to the [OI] emission cannotbe ruled out. A clear example for the presence of a photodissociationregion (PDR) illuminated by an HAEBE is shown by LWS spectroscopicmapping of NGC 7129. Some diagnostic probes of the radiation field anddensity are provided for the objects in our sample: these substantiallyagree with the known characteristics of both the star and itscircumstellar environment, although the observed ratio [OI]63/[OI]145tends to be smaller than predicted by PDR models. The most likelyexplanation for this behaviour is self-absorption at 63mu m by coldatomic oxygen. Fine structure lines of the ionised species [OIII], [NII]were detected whenever the star had a spectral type of B0 or earlier; inparticular, around the star CoD-42(deg) 11721, besides a compact HIIregion, evidence is given for an extended low electron density ionisedregion. Finally, molecular line emission is associated with starspowering a CO outflow, and clumpy PDR models, better than C-shockmodels, predict for them relative cooling (CO vs OI and CO vs OH)similar to the observed ones. Based on observations with ISO, an ESAproject with instruments funded by ESA Member States and with theparticipation of ISAS and NASA}
|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.
|HIPPARCOS photometry of Herbig Ae/Be stars|
The photometric behaviour of a sample of 44 Herbig Ae/Be (HAeBe)candidate stars was studied using a uniform set of optical photometryobtained by the Hipparcos mission. Astrophysical parameters (distance,temperature, luminosity, mass, age) of this sample of stars were derivedas well by combining the astrometric data provided by Hipparcos withdata from literature. Our main conclusions can be summarized as follows:(1) More than 65% of all HAeBe stars show photometric variations with anamplitude larger than 0\fm05; (2) HAeBes with a spectral type earlierthan A0 only show moderate (amplitude < 0\fm5) variations, whereasthose of later spectral type can (but not necessarily have to) showvariations of more than 2\fm5. We explain this behaviour as being due tothe fact that stars with lower masses become optically visible, andhence recognizable as Herbig Ae stars, while still contracting towardsthe zero-age main sequence (ZAMS), whereas their more massivecounterparts only become optically visible after having reached theZAMS; (3) The Herbig stars with the smallest infrared excesses do notshow large photometric variations. This can be understood by identifyingthe stars with lower infrared excesses with the more evolved objects inour sample; (4) No correlation between the level of photometricvariability and the stellar v sin i could be found. If the largephotometric variations are due to variable amounts of extinction by dustclouds in the equatorial plane of the system, the evolutionary effectsprobably disturb the expected correlation between the two. Based on datafrom the Hipparcos astrometry satellite.
|Linear and circular imaging polarimetry of the Chamaeleon infrared nebula|
We present linear and circular imaging polarimetry observations of theChamaeleon infrared nebula, a bipolar reflection nebula in theChamaeleon I dark cloud, at near-infrared (JHK_n) wavelengths. These areamongst the first imaging circular polarimetry results for astar-forming region. The detection of both high degrees of linearpolarization and a significant degree of circular polarization in theextended nebulosity allows us to comment on the scattering geometry andthe range of particle sizes present. We develop a model incorporating apolarized source which can successfully account for the observed linearand circular polarimetry and for the asymmetries in nebular brightness(the `bright rim' structures) seen in this and other objects (e.g., NGC2261/R Mon). In order to do so, the model requires a non-axisymmetricillumination of the nebula, and we discuss possible origins for thisasymmetry, including disruption of a circumstellar disc by binaryprotostars.
|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.
|Variable Radio Sources in the Corona Australis Cloud|
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....111..320S
|Meet the Milky Way.|
|The nature of the optical nebulosity surrounding the star RNO 91 in the L 43 dark cloud|
Polarization maps of the L43/RNO 91 nebulosity show that it is alarge-scale reflection nebula illuminated by the T Tauri star RNO 91.The highly structured nebulosity within 40 arcsec of the centralilluminator, which includes knots and a possible jet, is also seensolely by reflected light, and we have no evidence for the presence ofemission-line features such as Herbig-Haro objects and stellar jets. Wefind evidence for a small-scale circumstellar disc of diameter no largerthan 1200 AU and possibly as small as 160 AU, or even less, which webelieve has collimated the outflow leading to the visible nebula. Wespeculate on the cause of the lack of alignment between the optical andmolecular features.
|Imaging polarimetry of the bipolar nebula Parsamyan 22|
Par 22 shows high levels of polarization which confirm that it is abipolar reflection nebula. We suggest that circumstellar material, whichis very patchy and forms fingers which penetrate the central regions ofthe nebula, is the remnant of the circumstellar disk which is in theprocess of disintegration. We take this as an indication that the systemis in the later stages of pre-main-sequence evolution. We discuss theorigin of the sharp rim/diffuse boundary of each of the nebular lobes.
|Star formation in dark globules in the GUM Nebula|
Optical polarization maps of small nebulosities (Re5 and IRAS08211-4158) associated with IRAS sources located in dark globules arepresented and used to determine the positions of the illuminators. TheIRAS 08211 - 4158 system is found to have two illuminators, onecorresponding to the hidden IRAS source while the other is a visiblestar shrouded in nebulosity. These results are combined with earlierones from CG30/HH120 and HH46/47 to look at the overall picture oflow-mass star formation in the complex of dark globules within the GumNebula.
|Clumpy accretion onto pre-main-sequence stars|
Medium-resolution spectroscopic observations covering the range5150-9100 A are made of five premain-sequence stars in the CoronaAustralis star-forming region. They are the known variable stars R CrA,S CrA, T CrA, TY CrA, and VV CrA. During the week of observationsignificant changes, sometimes occurring between successive nights, wereobserved in the spectra of R CrA and S CrA. They are interpreted asbeing due to variable accretion onto the star. Velocities are measuredconfirming that the photosphere of R CrA in this region of the spectrumis invisible. Photospheric absorption lines are observed in S CrA, TCrA, and TY CrA. In VV CrA only Li I lambda-6708 is detected. Thevelocity variation of TY CrA is demonstrated. An infrared image of VVCrA is reproduced which shows the infrared companion 1.9 arcsec NE ofthe primary star.
|Linear polarization maps of bipolar and cometary nebulae - A polarized source interpretation|
A simple model of a polarized source is used to produce intensityisophotes and linear polarization maps simulating the scattering oflight in bipolar nebulae. The results indicate that features such aspolarization null points and bands of parallel vectors can be explainedin terms of a polarized source without recourse to aligned grains. Theparticular case of the reflection nebula NGC 2261 is considered indetail and it is shown how the model can account for the observedvariability in the polarization of this object.
|OH- and HI-observations of the CrA dark cloud complex|
This paper presents OH 1667-MHz observations covering the CrA dark cloudcomplex, as well as HI 21-cm observations in a large region around thecomplex. The OH profiles are of simple structure indicating the presenceof only one component with a mean velocity V(OH) about +5.5 km/s. ThreeOH clumps are apparent, which are coincident with regions of highphotographic extinction. The gas to dust ratio derived from theseobservations is similar to the average value obtained for other darkclouds and shows that OH and dust are well mixed in the CrA dark cloudcomplex. The total mass estimated for the molecular cloud amounts toM(T) about 7000 solar masses. The HI observations show the well-knownridge of local gas of low positive velocities. The accordance betweenthe velocities of molecular and atomic gas in the region suggests thatthe molecular complex is embedded in the local HI-ridge. It is suggestedthat they both are related to Lindblad's feature A (i.e., to Gould'sbelt).
|The nature of the GN20.18.3 nebulosity|
GN20.18.3 is a bipolar reflection nebula illuminated by a central starwhich is surrounded by an extensive circumstellar disk at position angle125 deg. The associated Herbig-Haro object is a knot-like concentrationof gas and dust and is seen in both scattered light and shock excitedintrinsic emission.
|Polarization studies of nebulae associated with young stars and outflow regions|
Consideration is given to the polarization disks observed in opticalpolarization maps of the central regions of reflection nebulaeassociated with young stars and outflows. Several models to explain theorigin of polarization disks are reviewed, including multiple scatteringmodels, the polarized source model, and the magnetized disk model. It issuggested that the properties of polarization disks can be explained bya hybrid of the polarized source and the magnetized disk models.
|The nature of the optical nebulosity near GL 989|
The small nebula near GL 989 does not show the optical polarizationfeatures of a simple reflection nebula illuminated by a nearby source,and it is suggested that the polarization arises from dichroicextinction by aligned grains. It is proposed that the nebula is thefirst visible sign of the formation of a cometary/bipolar nebula createdby collimated outflows from the IR source.
|The optical nebulosity around the T Tauri star RNO 91 in the L43 dense molecular core|
Imaging photometric and spectroscopic observations of RNO 91, a T Tauristar associated with the dense molecular core in the dark cloud L43, arepresented. A maximum entropy algorithm has been used to increase theresolution to the F, I, and H-alpha CCD images. The restored imagesreveal substantial structure in the nebular emission around the star onspatial scales of 100-1000 AU in projection. Several of the structuralfeatures consist of knots roughly aligned on a radius from the star. Themost prominent feature extends in a direction precisely coincident withthe axis of the blue lobe of the associated molecular outflow known tobe disrupting the molecular core. While similar in morphology to opticaljets observed around other T Tauri stars, the spectra of these elongatednebular features are indistinguishable from that of RNO 91, indicatingthat the extended structure is illuminated by reflected light. Thespectra show no evidence for the emission features characteristic ofpreviously observed optical jets.
|Evidence for circumstellar discs around nebulous stars in the Taurus dark clouds|
Optical polarization data for the nebulosity surrounding the HL/XZ Tauand Haro 6-5/6-5B complexes show that the stars HL Tau, LkH-alpha 358,Haro 6-5, and Haro 6-5B all possess circumstellar discs and illuminatebipolar nebulae. The position angles of the discs are compared with thedirections of the local cloud rotation axes and magnetic field.
|An optical polarization study of the Chamaeleon IR nebula|
An optical linear polarization map shows that the Chamaeleon infrarednebula is a bipolar reflection nebula illuminated by a central objectlocated at the position of a known IR source. The source is surroundedby an extensive disk of gas and dust, and the data indicate that amagnetic field is concentrated in the disk and remains detectablethroughout the lobes of the nebula.
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