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Young Stellar Groups around Herbig Ae/Be Stars: A Low-Mass YSO Census
We present near-IR and mid-IR observations of eight embedded youngstellar groups around Herbig Ae/Be stars (HAEBEs) using archived SpitzerIRAC data and 2MASS data. These young stellar groups are nearby (<=1kpc) and still embedded within their molecular clouds. In order toidentify the young stellar objects in our sample, we use the color-colordiagram of J-[3.6] versus Ks-[4.5]. The Spitzer images of oursample show that the groups around HAEBEs, spectral types earlier thanB8, are usually associated with bright infrared nebulosity. Within this,there are normally 10-50 young stars distributed close to the HAEBEs(<1 pc). Not only are there young stars around the HAEBEs, there arealso young stellar populations throughout the whole cloud, some of whichare distributed and some of which are clumped. The groups around theHAEBEs are substructures of the large young population within themolecular cloud. The sizes of groups are also comparable with thosesubstructures seen in massive clusters. Young stars in groups aroundHAEBEs have generally larger SED slopes compared to those outside, whichsuggests that the young stars in groups are probably younger than thedistributed systems. This might imply that there is usually a higher andmore continuous star-forming rate in groups, that the formation ofgroups initiates later, or that low-mass stars in groups form slowerthan those outside. Finally, there is no obvious trend between the SEDslopes and the distance to the HAEBEs for those young stars within thegroups. This suggests that the clustering of young stars dominates overthe effect of massive stars on the low-mass young stars at the scale ofour study.
| Relation between the Luminosity of Young Stellar Objects and Their Circumstellar Environment
We present a new model-independent method of comparison of NIRvisibility data of YSOs. The method is based on scaling the measuredbaseline with the YSO's distance and luminosity, which removes thedependence of visibility on these two variables. We use this method tocompare all available NIR visibility data and demonstrate that itdistinguishes YSOs of luminosity L*<~103Lsolar (low L) from YSOs of L*>~103Lsolar (high L). This confirms earlier suggestions, based onfits of image models to the visibility data, for the difference betweenthe NIR sizes of these two luminosity groups. When plotted against the``scaled'' baseline, the visibility creates the following data clusters:low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. Wemodel the shape and size of clusters with different image models andfind that low-L Herbig stars are best explained by the uniformbrightness ring and the halo model, T Tauri stars with the halo model,and high-L Herbig stars with the accretion disk model. However, theplausibility of each model is not well established. Therefore, we try tobuild a descriptive model of the circumstellar environment consistentwith various observed properties of YSOs. We argue that low-L YSOs haveoptically thick disks with the optically thin inner dust sublimationcavity and an optically thin dusty outflow above the inner disk regions.High-L YSOs have optically thick accretion disks with high accretionrates enabling gas to dominate the NIR emission over dust. Althoughobservations would favor such a description of YSOs, the required dustdistribution is not supported by our current understanding of dustdynamics.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| 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.
| [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation
We present high spectral-resolution optical spectra of 49 Herbig Ae/Bestars in a search for the [O i] 6300 Å line. The vast majority ofthe stars in our sample show narrow ({FWHM} < 100 km s-1)emission lines, centered on the stellar radial velocity. In only threesources is the feature much broader ( 400 km s-1), andstrongly blueshifted (-200 km s-1) compared to the stellarradial velocity. Some stars in our sample show double-peaked lineprofiles, with peak-to-peak separations of 10 km s-1. Thepresence and strength of the [O i] line emission appears to becorrelated with the far-infrared energy distribution of each source:stars with a strong excess at 60 μm have in general stronger [O i]emission than stars with weaker 60 μm excesses. We interpret thesenarrow [O i] 6300 Å line profiles as arising in the surface layersof the protoplanetary disks surrounding Herbig Ae/Be stars. A simplemodel for [O i] 6300 Å line emission due to the photodissociationof OH molecules shows that our results are in quantitative agreementwith that expected from the emission of a flared disk if the fractionalOH abundance is 5 × 10-7.
| Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars
We present Hα spectropolarimetry observations of a sample of 10bright T Tauri stars, supplemented with new Herbig Ae/Be star data. Achange in the linear polarization across Hα is detected in most ofthe T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret interms of a compact source of line photons that is scattered off arotating accretion disc. We find consistency between the position angle(PA) of the polarization and those of imaged disc PAs from infrared andmillimetre imaging and interferometry studies, probing much largerscales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find thepolarization PA to be perpendicular to the imaged disc, which isexpected for single scattering. On the other hand, the polarization PAaligns with the outer disc PA for the T Tauri stars DR Tau and SU Aurand FU Ori, conforming to the case of multiple scattering. Thisdifference can be explained if the inner discs of Herbig Ae stars areoptically thin, whilst those around our T Tauri stars and FU Ori areoptically thick. Furthermore, we develop a novel technique that combinesknown inclination angles and our recent Monte Carlo models to constrainthe inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, weconsider the connection of the inner disc structure with the orientationof the magnetic field in the foreground interstellar medium: for FU Oriand DR Tau, we infer an alignment of the stellar axis and the largermagnetic field direction.
| The Near-Infrared Size-Luminosity Relations for Herbig Ae/Be Disks
We report the results of a sensitive K-band survey of Herbig Ae/Be disksizes using the 85 m baseline Keck Interferometer. Targets were chosento span the maximum range of stellar properties to probe the disk sizedependence on luminosity and effective temperature. For most targets,the measured near-infrared sizes (ranging from 0.2 to 4 AU) support asimple disk model possessing a central optically thin (dust-free)cavity, ringed by hot dust emitting at the expected sublimationtemperatures (Ts~1000-1500 K). Furthermore, we find a tightcorrelation of disk size with source luminosity R~L1/2 for Aeand late Be systems (valid over more than two decades in luminosity),confirming earlier suggestions based on lower quality data.Interestingly, the inferred dust-free inner cavities of the highestluminosity sources (Herbig B0-B3 stars) are undersized compared topredictions of the ``optically thin cavity'' model, likely because ofoptically thick gas within the inner AU.
| PAHs in circumstellar disks around Herbig Ae/Be stars
We investigate the presence and properties of PAHs on the surface ofcircumstellar disks around Herbig Ae/Be stars by comparing thepredictions of disk models with observations. We present results of aradiation transfer code for disks heated by the central star, inhydrostatic equilibrium in the vertical direction (flared disks). Thedust is a mixture of large grains in thermal equilibrium, transientlyheated small grains and PAHs. Special attention is given to theinfluence of the stellar, disk and PAH properties on the strength of thePAH emission lines and their spatial distribution. The models predict aninfrared SED showing PAH features at 3.3, 6.2, 7.7, and 11.3 μmclearly visible above the continuum, and with some of them very strong.The PAH emission, spatially extended, comes mostly from the outer diskregion (R100 AU) while the continuum emission at similarwavelengths, mostly due to warm large grains, is confined to theinnermost disk regions (R few AU). We compare the model results toinfrared observations from ISO and ground-based telescopes of somethirty Herbig Ae/Be stars. Most of the observed PAH features in objectswith spectral type later than B9 are well described by our disk modelsand we discuss in some detail the PAH characteristics one can derivefrom the existing data. Objects with strong radiation field (generallyearlier than about B9) have the 3.3 μm feature (often the only oneobserved) much weaker than predicted, and we discuss possibleexplanations (dissipation of the disk, photoevaporation or modificationof the PAH properties).Appendix is only available in electronic form athttp://www.edpsciences.org
| A study of high velocity molecular outflows with an up-to-date sample
A statistical study of the properties of molecular outflows is performedbased on an up-to-date sample. 391 outflows were identified in publishedarticles or preprints before February 28, 2003. The parameters ofposition, morphology, mass, energy, outflow dynamics and central sourceluminosity are presented for each outflow source. Outflow lobe polarityis known for all the sources, and 84% are found to be bipolar. Thesources are divided into low mass and high mass groups according toeither the available bolometric luminosity of the central source or theoutflow mass. The pace of discovery of outflows over the past sevenyears has increased much more rapidly than in previous periods. Surveysfor outflows are still continuing. The number of high-mass outflowsdetected (139) has considerably increased, showing that they arecommonly associated with massive as well as low mass stars. Energeticmass ejection may be a common aspect of the formation of high mass aswell as low mass stars. Outflow masses are correlated strongly withbolometric luminosity of the center sources, which was obtained for thefirst time. There are also correlations between the central sourceluminosity and the parameters of mechanical luminosity and the thrust orforce necessary to drive the outflow. The results show that flow mass,momentum and energy depend on the nature of the central source. Despitetheir similarity, there are differences between the high mass and lowmass outflows. Low mass outflows are more collimated than high massoutflows. On average, the mass of high mass sources can be more than twoorders of magnitude larger than those of low mass outflows. The relationbetween flow mass and dynamical time appears to differ for the two typesof outflows. Low mass sources make up 90% of outflows associated with HHobjects while high mass outflows make up 61% of the sources associatedwith H_2O masers. Sources with characteristics of collapse or infallcomprise 12% of the entire outflow sample. The spatial distribution ofthe outflow sources in the Galaxy is presented and the local occurrencerate is compared with the stellar birth rate.Tables 1a and 1b are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/503
| Spectral and Photoelectric Studies of the Herbig Ae/Be Star HD 259431
Spectral and photoelectric (ubvy, H, H) observations of the Herbig Ae/Bestar HD 259431 are reported. It is found that as its brightness fades,this star becomes bluer in the Paschen continuum and the intensity andequivalent width of the hydrogen emission lines increase. The spectralobservations reveal significant variations in the intensity of the Mg II4481 Å photospheric absorption line. A rise and fall in theluminosity by 0m.04 within a period of 5-7 minutes was recorded. Radicalvariations in the H lineshape ("double" "P Cyg") and flare activity arenot only observed in this star, but also in a number of HAEBE stars. Itis suggested that flare activity may initiate a change in the velocitygradient at the base of the wind and, thereby, induce "double P Cyg" or"P Cyg single" transitions. The nonradial pulsations of this star arealso discussed.
| ISO spectroscopy of disks around Herbig Ae/Be stars
We have investigated the infrared spectra of all 46 Herbig Ae/Be starsfor which spectroscopic data are available in the ISO data archive. Ourquantitative analysis of these spectra focuses on the emission bands at3.3, 6.2, ``7.7'', 8.6 and 11.2 micron, linked to polycyclic aromatichydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5micron, the amorphous 10 micron silicate band and the crystallinesilicate band at 11.3 micron. We have detected PAH emission in 57% ofthe Herbig stars in our sample. Although for most of these sources thePAH spectra are similar, there are clear examples of differences in thePAH spectra within our sample which can be explained by differences inPAH size, chemistry and/or ionization. Amorphous silicate emission wasdetected in the spectra of 52% of the sample stars, amorphous silicateabsorption in 13%. We have detected crystalline silicate emission in 11stars (24% of our sample), of which four (9%) also display strong PAHemission. We have classified the sample sources according to thestrength of their mid-IR energy distribution. The systems with strongermid-infared (20-100 μm) excesses relative to their near-infrared (1-5μm) excess display significantly more PAH emission than those withweaker mid-infrared excesses. There are no pronounced differences in thebehaviour of the silicate feature between the two groups. This providesstrong observational support for the disk models by \citet{dullemond01},in which systems with a flaring disk geometry display a strongmid-infrared excess, whereas those with disks that are strongly shadowedby the puffed-up inner rim of the disk only display modest amounts ofmid-infrared emission. Since the silicates are expected to be producedmainly in the warm inner disk regions, no large differences in silicatebehaviour are expected between the two groups. In contrast to this, thePAH emission is expected to be produced mainly in the part of the diskatmosphere that is directly exposed to radiation from the central star.In this model, self-shadowed disks should display weaker PAH emissionthan flared disks, consistent with our observations.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.Tables 1, 3-6 and Appendix are only available in electronic form athttp://www.edpsciences.org
| Spectral Analysis and Classification of Herbig Ae/Be Stars
We present an analysis of the optical spectra of 75 early-typeemission-line stars, many of which have been classified previously asHerbig Ae/Be (HAeBe) stars. Accurate spectral types were derived for 58members of the sample; high continuum veiling, contamination bynonphotospheric absorption features, or a composite binary spectrumprevented accurate spectral typing for the rest. Approximately half ofour sample exhibited [O I] λ6300 forbidden-line emission down toour detection limit of 0.1 Å equivalent width; a third of thesample exhibited Fe II emission (multiplet 42). A subset of 11 of theHAeBe sample showed abnormally strong Fe II absorption; 75% of thissubset are confirmed UX Ori objects. Combining our spectral typingresults with photometry from the literature, we confirm previousfindings of high values of total-to-selective extinction(RV~5) in our larger sample, suggesting significant graingrowth in the environments of HAeBe stars. With this high value ofRV, the vast majority of HAeBe stars appear younger than withthe standard RV=3.1 extinction law and are more consistentwith being pre-main-sequence objects.
| 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).
| A Photometric and [S II] Survey of the Young Cluster Roslund 4
Optical B- and V-band CCD data have been obtained for the young clusterRoslund 4 in conjunction with a large-scale photometric survey of openclusters. In addition, [S II] and off-line continuum CCD emission-lineimaging data have also been obtained, revealing three regions in thecluster field where shock-excited gas is present. Two Herbig-Haro-likeemission features are found, while a more extended emission feature,along an apparent cloud boundary, may be a result of stellar windsencountering the edge of a molecular cloud. The emission-line featuresare located near IRAS sources with properties consistent withstar-forming regions. A previously published age for Roslund 4 (~10 Myr)would seemingly suggest that shocked gas features should not be presentin the cluster region, but an upper limit on the age (less than 4 Myr)derived from the new photometry is more consistent with their presence.Finally, the distance to Roslund 4 is found to be 1700-2000 pc,considerably closer than the previous published estimate.
| A 13CO and C18O Survey of the Molecular Gas Around Young Stellar Clusters within 1 Kiloparsec of the Sun
As the first step of a multiwavelength investigation into therelationship between young stellar clusters and their environment, wepresent fully sampled maps in the J=1-0 lines of 13CO andC18O and the J=2-1 line of C18O for a selectedgroup of 30 young stellar groups and clusters within 1 kpc of the Sun.This is the first systematic survey of these regions to date. Theclusters range in size from several stars to a few hundred stars. Thirtyfields ranging in size from 8'×8' to 30'×60' were mappedwith 47" resolution simultaneously in the two J=1-0 lines at the FiveCollege Radio Astronomy Observatory. Seventeen sources were mapped overfields ranging in size from 3'×3' to 13'×13' in the J=2-1line with 35" resolution at the Submillimeter Telescope Observatory. Wecompare the cloud properties derived from each of the three tracers inorder to better understand systematic uncertainties in determiningmasses and line widths. Cloud masses are determined independently usingthe 13CO and C18O transitions; these masses rangefrom 30 to 4000 Msolar. Finally, we present a simplemorphological classification scheme, which may serve as a roughindicator of cloud evolution.
| 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
| Probing the circumstellar structure of Herbig Ae/Be stars
We present Hα spectropolarimetry observations of a sample of 23Herbig Ae/Be stars. A change in the linear polarization across Hαis detected in a large fraction of the objects, which indicates that theregions around Herbig stars are flattened (disc-like) on small scales. Asecond outcome of our study is that the spectropolarimetric signaturesfor the Ae stars differ from those of the Herbig Be stars, withcharacteristics changing from depolarization across Hα in theHerbig Be stars, to line polarizations in the Ae group. The frequency ofdepolarizations detected in the Herbig Be stars (seven out of 12) isparticularly interesting as, by analogy with classical Be stars, it maybe the best evidence to date that the higher-mass Herbig stars aresurrounded by flattened structures. For the Herbig Ae stars, nine out of11 show a line polarization effect that can be understood in terms of acompact Hα emission that is itself polarized by a rotatingdisc-like circumstellar medium. The spectropolarimetric differencebetween the Herbig Be and Ae stars may be the first indication thatthere is a transition in the Hertzsprung-Russell diagram from magneticaccretion at spectral type A to disc accretion at spectral type B.Alternatively, the interior polarized line emission apparent in the Aestars may be masked in the Herbig Be stars owing to their higher levelsof Hα emission.
| Circumstellar disks around Herbig Ae/Be stars: Polarization, outflows and binary orbits
The geometrical relationship between the distribution of circumstellarmatter, observed optical linear polarization, outflows and binaryorbital plane in Herbig Ae/Be stars is investigated. Optical linearpolarization measurements carried out for a number of Herbig Ae/Be starsthat are either known to be in binary systems and/or have bipolar jetsare presented in this paper. Available information on the positionangles of polarization, outflows and binary companions for Herbig Ae/Bestars is compiled and analysed for any possible correlations. In ~85% ofthe sources the outflow position angle is within 30deg ofbeing parallel or perpendicular to the polarization position angle. In~81% of the sources the binary position angle is within 30degof being parallel or perpendicular to the polarization position angle.Out of 15 sources with bipolar outflows, 10 sources have the binaryposition angle within 30deg of being perpendicular to theoutflow position angle. These results favour those binary formationmechanisms in which the binary components and the disks aroundindividual stars or circumbinary disks are coplanar.
| The history of mass dispersal around Herbig Ae/Be stars
We present a systematic study of the material surroundingintermediate-mass stars. Our sample includes 34 Herbig Ae/Be (HAEBE)stars of different ages and luminosities. This is a quite completerepresentation of the whole class of HAEBE stars and consequently, ourconclusions should have a solid statistical meaning. In addition, wehave observed 2 intermediate-mass protostars and included published dataon 15 protostellar objects in order to determine the evolution of thecircumstellar material in the early stages of stellar evolution. All theHAEBE stars have been classified according with the three Types alreadydefined in Fuente et al. (\cite{fuen98}): Type I stars are immersed in adense clump and have associated bipolar outflows, their ages are ~ 0.1Myr; Type II stars are still immersed in the molecular cloud though notin a dense clump, their ages are between ~ a few 0.1 to ~ a few Myr;Type III stars have completely dispersed the surrounding material andare located in a cavity of the molecular cloud, their ages are >1Myr. Our observations are used to reconstruct the evolution of thecircumstellar material around intermediate-mass stars and investigatethe mass dispersal mechanisms at the different stages of the stellarevolution. Our results can be summarized as follows: intermediate-massstars disperse >=90% of the mass of the parent clump during theprotostellar phase. During this phase, the energetic outflows sweep outthe gas and dust forming a biconical cavity while the equatorialmaterial is infalling to feed the circumstellar disk and eventually theprotostar. In this way, the density structure of the parent clumpremains well described by a density law n~ r\beta with -2=1 Myr. Since the outflowdeclines and the stars are still too cold to generate UV photons,stellar winds are expected to be the only dispersal mechanism at work.In 1 Myr an early-type star (B0-B5) and in >=1 to 10 Myr a late-typestar (later than B6) meets the ZAMS. Now the star is hot enough toproduce UV photons and starts excavating the molecular cloud.Significant differences exist between early-type and late-type stars atthis evolutionary stage. Only early-type stars are able to create large(R>0.08 pc) cavities in the molecular cloud, producing a dramaticchange in the morphology of the region. This difference is easilyunderstood if photodissociation plays an important role in the massdispersal around these objects.
| The Young Cluster IC 5146
The B0 V star BD +46°3474 lies near the front surface of a densemolecular cloud and illuminates the emission/reflection nebula IC 5146.The HAeBe variable BD +46°3471 is embedded in the same cloud, about10' (3.5 pc) away. CCD photometry in BVRI (to V=22) and in JHK (to aboutK=16.5) has been obtained for the young clusters surrounding each ofthese two bright stars. Some 100 emission-Hα stars brighter thanR=20.5 have been found in the area, most of them in IC 5146. (Amongthese are two that have spectra resembling a high-excitation Herbig-Haro[HH] object plus a stellar continuum.) A distance of 1.2 kpc followsfrom the photometry of several late-type IC 5146 cluster members; theaverage extinction from 38 stars classified spectroscopically isAV=3.0+/-0.2 mag. Although optical photometry is availablefor 700 stars in the IC 5146 field, only about half (including all theHα emitters) lie above the main sequence, while a substantialfraction of these are estimated to be foreground. A number of suchinterlopers have been identified on the basis of proper motion orabnormally low AV. The age distribution of the Hαemitters has been estimated by reference to several sets of theoreticalisochrones. There is substantial disagreement, but the median age doesappear to be near 1 Myr. The spectrum of +46°3474 is unexceptionalexcept for an unusually low vsini (10 km s-1), but+46°3471 has a complex emission plus absorption spectrum. Ourinterpretation of the structure of IC 5146 on the basis of optical andradio radial velocities follows a proposal by Roger & Irwin in 1982,namely, that +46°3474 formed near the near surface of the presentcloud and evacuated a blister cavity out of which gas and dust are nowflowing through a funnel-shaped volume in the approximate direction ofthe Sun. It is suggested that the IC 5146 cluster stars formed in adense foreground section of the molecular cloud that was dissipatedfollowing the appearance of +46°3474.
| A database of high and medium-resolution stellar spectra
We present a database of 908 spectra of 709 stars obtained with theELODIE spectrograph at the Observatoire de Haute-Provence. 52 orders ofthe echelle spectra have been carefully fitted together to providecontinuous, high-resolution spectra in the wavelength range lambdalambda = 410-680 nm. The archive provides a large coverage of the spaceof atmospheric parameters: T_eff from 3700 K to 13 600 K, log g from0.03 to 5.86 and [Fe/H] from -2.8 to +0.7. At the nominal resolution,R=42 000, the mean signal-to-noise ratio is 150 per pixel. The spectragiven at this resolution are normalized to their pseudo-continuum andare intended to serve for abundance studies, spectral classification andtests of stellar atmosphere models. A lower resolution version of thearchive, at R=10 000, is calibrated in physical flux with a broad-bandphotometric precision of 2.5% and narrow-band precision of 0.5%. It iswell suited to stellar population synthesis of galaxies and clusters,and to kinematical investigations of stellar systems. The archive isdistributed in FITS format through the HYPERCAT and CDS databases. Basedon observations made on the 193 cm telescope at the Haute-ProvenceObservatory, France. Table 1 is 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/cgi-bin/qcat?J/A+A/369/1048
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 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
| 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.
| Analysis of correlations between polarimetric and photometric characteristics of young stars. A new approach to the problem after eleven years' study
We present the results of the investigation of correlations between thepolarimetric and photometric characteristics of a sample (496 objects)of young Herbig Ae/Be (HAEBE) stars and T Tauri (TT) stars. It is shownthat, for 85% of the sample stars there is a general relation betweenthe degree of optical polarization and the infrared colour index(V-L)_obs and the colour excess E(V-L) due to the contribution of acircumstellar dust shell. Polarimetric data were also compared with thevalue of vsin i to search for a possible correlation between thepolarization and an inclination of circumstellar disks. Polarimetricdata as well as IR excesses are considered and compared for differentsubgroups of young stars namely: HAEBE and TT stars with Algol-likeminima of brightness (26 objects), Vega-type stars and post HAEBE stars(114 objects) and young solar-type stars (58 objects). For statisticalpurposes the data for young stars were compared with those collected fordifferent groups of evolved objects such as: classical Be stars (~300objects), Mira Ceti stars (39 objects), early-type supergiants from theSerkowski et al. (\cite{serk}) catalogue (120 objects) and main sequence(MS) stars within 50 pc from the Sun from the Leroy (\cite{leroy})catalogue (68 objects). The value of polarization is discussed incontext with the stages of evolution of circumstellar shells which wereestablished by comparison of spectral energy distribution in the far IR(using the IRAS data). It is shown that most young stars havestatistically larger value of polarization in comparison with the starswhich are on a stage of evolution close to MS. We are able to contendthat the changes in polarimetric behaviour of young stars are connectedwith evolution of their circumstellar shells. Appendices 1 to 5 are onlyavailable in electronic form at http://www.edpsciences.org
| 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
| Survey of bipolar outflows and methanol masers in the C(32) S (2-1) and C(34) S (2-1) lines in the Northern sky
A survey of 158 sources (bipolar outflows and methanol masers) wascarried out in the CS(2-1) and C(34) S(2-1) lines to measure and comparedensities in a large number of bipolar outflows and in Class I and ClassII methanol masers, both associated with and unrelated to bipolaroutflows. The statistical characteristics of the regions, forming ClassI methanol masers differ from those both in the centres of bipolaroutflows and in the regions forming Class II methanol masers. It ispossible that physical conditions in bipolar outflows are closer tophysical conditions of Class II methanol masers. Figures i and ii areonly available in electronic form at http://www.edpsciences.com
| A Photometric Catalog of Herbig AE/BE Stars and Discussion of the Nature and Cause of the Variations of UX Orionis Stars
UBVR photometric monitoring of Herbig Ae/Be stars and some relatedobjects has been carried out at Maidanak Observatory in Uzbekistan since1983. More than 71,000 observations of about 230 stars have beenobtained and are made available for anonymous ftp. Virtually all HerbigAe/Be stars observed are irregular variables (called ``UXors'' after UXOri), but there is a wide range of amplitudes from barely detectable tomore than 4 mag in V. Our data confirm the results of previous studies,which indicate that large-amplitude variability is confined to starswith spectral types later than B8. The distribution of variabilityranges is quite similar to what is seen in classical T Tauri stars. Acareful search has failed to reveal any evidence for periodic variationsup to 30 days, which can be interpreted as rotation periods. This is aclear distinction between the light variations of low-mass and high-masspre-main-sequence stars. The Herbig Ae/Be stars evidently do not possesseither the large, stable cool spots or persistent hot spots associatedwith strong surface magnetic fields and magnetically funneled accretionin classical T Tauri stars. A wide variety of shapes, timescales, andamplitudes exists, but the most common behavior is well illustrated bythe light curve of LkHα 234. There are two principal components:(1) irregular variations on timescales of days around a mean brightnesslevel that changes on a much longer timescale (typically years),sometimes in a quasi-cyclic fashion, and (2) occasional episodes of deepminima, occurring at irregular intervals but more frequently near thelow points of the brightness cycles. Our data suggest that many T Tauristars of K0 and earlier spectral type share the same variabilitycharacteristics as Herbig Ae/Be stars and should be regarded as UXors.Two FU Orionis stars (``FUors''), FU Ori and V1515 Cyg, also have recentlight curves that are similar, in some respects, to UXors. The mostdeveloped model to account for the variations of some large-amplitudeUXors involves variable obscuration by circumstellar dust clumpsorbiting the star in a disk viewed nearly edge-on. However, there areproblems in extending this model to the entire class, which lead us topropose an alternative mechanism, i.e., unsteady accretion. Evidencefavoring the accretion model over the obscuration model is presented. Itis suggested that the thermal instability mechanism responsible foroutbursts in interacting binary system disks, and possibly FUors, may bethe cause of the deep minima in UXors.
| 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
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| The Medicina survey of methanol masers at 6.7 GHz
A survey of Class II methanol masers at 6.7 GHz was made in the Northernhemisphere with the 32-m Medicina radio telescope. 42 objects weredetected, 20 of them are new detections at 6.7 GHz. Our results showthat the detection rate of 6.7 GHz masers toward the inner part of theGalaxy is higher than in other directions. It is confirmed that most ofthe methanol masers are associated with faint compact HII regions. The6.7 GHz methanol masers show large velocity dispersion and largevelocity offset from the velocity of parent molecular clouds. Table 2 isonly available 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
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