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Properties of Galactic FS CMa Type Objects, a New Group of Dust-Forming Systems
The recently defined Galactic FS CMa group comprises ˜ 40 notvery luminous objects that significantly expand the luminosity range ofhot dust-forming stars. These objects exhibit extremely strongemission-line spectra and IR excesses, which are indicative of compactdust envelopes. They are most likely binary systems that currentlyundergo or have recently undergone a phase of a rapid mass exchange,associated with dust formation. Complex structure of the circumstellarenvironments significantly veils the underlying stars and requiresmultitechnique investigation, in which interferometry should play animportant role.
| On the interplay between flaring and shadowing in disks around Herbig Ae/Be stars
Context: Based on their spectral energy distribution, Herbig stars havebeen categorized into two observational groups, reflecting their overalldisk structure: group I members have disks with a high degree of flaringas opposed to their group II counterparts. Literature results show thatthe structure of the disk is a strong function of the disk mass in?m-sized dust grains. Aims: We investigate the 5-35 ?mSpitzer IRS spectra of a sample of 13 group I sources and 20 group IIsources. We focus on the continuum emission to study the underlying diskgeometry. Methods: We have determined the [30/13.5] and [13.5/7]continuum flux ratios. The 7-?m flux excess with respect to thestellar photosphere is measured, as a marker for the strength of thenear-IR emission produced by the hot inner disk. We have compared ourdata to the spectra produced by self-consistent passive-disk models, forwhich the same quantities were derived. Results: We confirm theresults by Meijer et al. (2008, A&A, 492, 451) that the differencesin continuum emission between group I and II sources can largely beexplained by a difference in amount of small dust grains. However, wereport a strong correlation between the [30/13.5] and [13.5/7] fluxratios for Meeus group II sources. Moreover, the [30/13.5] flux ratiodecreases with increasing 7-?m excess for all targets in the sample.To explain these correlations with the models, we need to introduce anartificial scaling factor for the inner disk height. In roughly 50% ofthe Herbig Ae/Be stars in our sample, the inner disk must be inflated bya factor 2 to 3 beyond what hydrostatic calculations predict. Conclusions: The total disk mass in small dust grains determines thedegree of flaring. We conclude, however, that for any given disk mass insmall dust grains, the shadowing of the outer (tens of AU) disk isdetermined by the scale height of the inner disk (~1 AU). The inner diskpartially obscures the outer disk, reducing the disk surfacetemperature. Here, for the first time, we prove these effectsobservationally.
| Toward Understanding the B[e] Phenomenon. III. Properties of the Optical Counterpart of IRAS 00470+6429
FS CMa type stars are a group of Galactic objects with the B[e]phenomenon. They exhibit strong emission-line spectra and infraredexcesses, which are most likely due to recently formed circumstellardust. The group content and identification criteria were described inthe first two papers of the series. In this paper we report ourspectroscopic and photometric observations of the optical counterpart ofIRAS 00470+6429 obtained in 2003-2008. The optical spectrum isdominated by emission lines, most of which have P Cyg typeprofiles. We detected significant brightness variations, which mayinclude a regular component, and variable spectral line profiles in bothshape and position. The presence of a weak Li I 6708 Å linein the spectrum suggests that the object is most likely a binary systemwith a B2-B3 spectral-type primary companion of a luminosity log L/Lsun = 3.9 ± 0.3 and a late-type secondary companion.We estimate a distance toward the object to be 2.0 ± 0.3 kpc fromthe Sun.Partially based on data obtained at the 6 m BTA telescope of the RussianAcademy of Sciences, 3 m IRTF, 3 m Shane telescope of the LickObservatory, 2.7 m Harlan J. Smith and 2.1 m Otto Struve telescopes ofthe McDonald Observatory, 2.1 m telescope of the San Pedro MartirObservatory, 1.5 m telescope of the Loiano Observatory, and 0.8 mtelescope of the Dark Sky Observatory.
| Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks
Context: Recently, evidence for the presence of weak magnetic fields inHerbig Ae/Be stars has been found in several studies. Aims: Weseek to expand the sample of intermediate-mass pre-main sequence starswith circular polarization data to measure their magnetic fields, and todetermine whether magnetic field properties in these stars arecorrelated with mass-accretion rate, disk inclination, companions,silicates, PAHs, or show a correlation with age and X-ray emission asexpected for the decay of a remnant dynamo. Methods:Spectropolarimetric observations of 21 Herbig Ae/Be stars and six debrisdisk stars have been obtained at the European Southern Observatory withFORS 1 mounted on the 8 m Kueyen telescope of the VLT. With the GRISM600B in the wavelength range 3250-6215 Å we were able to cover allhydrogen Balmer lines from Hβ to the Balmer jump. In allobservations a slit width of 0.4 arcsec was used to obtain a spectralresolving power of R ≈ 2000. Results: Among the 21 HerbigAe/Be stars studied, new detections of a magnetic field were achieved insix stars. For three Herbig Ae/Be stars, we confirm previous magneticfield detections. The largest longitudinal magnetic field, =-454±42 G, was detected in the Herbig Ae/Be star HD 101412 usinghydrogen lines. No field detection at a significance level of 3σwas achieved in stars with debris disks. Our study does not indicate anycorrelation of the strength of the longitudinal magnetic field with diskorientation, disk geometry, or the presence of a companion. We also donot see any simple dependence on the mass-accretion rate. However, it islikely that the range of observed field values qualitatively supportsthe expectations from magnetospheric accretion models giving support fordipole-like field geometries. Both the magnetic field strength and theX-ray emission show hints of a decline with age in the range of ~2-14Myr probed by our sample, supporting a dynamo mechanism that decays withage. However, our study of rotation does not show any obvious trend ofthe strength of the longitudinal magnetic field with rotation period.Furthermore, the stars seem to obey the universal power-law relationbetween magnetic flux and X-ray luminosity established for the Sun andmain-sequence active dwarf stars.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes 077.C-0521(A) and 081.C-0410(A)).
| The Alignment of the Polarization of Herbig Ae/Be Stars with the Interstellar Magnetic Field
We present a study of the correlation between the direction of thesymmetry axis of the circumstellar material around intermediate massyoung stellar objects and that of the interstellar magnetic field. Weuse CCD polarimetric data on 100 Herbig Ae/Be stars. A large numberof them show intrinsic polarization, which indicates that theircircumstellar envelopes are not spherical. The interstellar magneticfield direction is estimated from the polarization of field stars. Thereis an alignment between the position angle of the Herbig Ae/Be starpolarization and that of the field stars for the most polarized objects.This may be an evidence that the ambient interstellar magnetic fieldplays a role in shaping the circumstellar material around young stars ofintermediate mass and/or in defining their angular momentum axis.Based on observations made at the Observatório do Pico dos Dias,Brazil, operated by the Laboratório Nacional deAstrofísica.
| The characteristics of the IR emission features in the spectra of Herbig Ae stars: evidence for chemical evolution
Context: Infrared (IR) spectra provide a prime tool to study thecharacteristics of polycyclic aromatic hydrocarbon (PAH) molecules inregions of star formation. Herbig Ae/Be stars are a class of youngpre-main sequence stellar objects of intermediate mass. They are knownto have varying amounts of natal cloud material still present in theirdirect vicinity. Aims: We characterise the IR emission bands, due tofluorescence by PAH molecules, in the spectra of Herbig Ae/Be stars andlink observed variations to spatial aspects of the mid-IR emission. Methods: We analysed two PAH dominated spectra from a sample of 15Herbig Ae/Be stars observed with the Spitzer Space Telescope. Results:We derived profiles of the major PAH bands by subtracting appropriatecontinua. The shape and the measured band characteristics showpronounced variations between the two Spitzer spectra investigated.Those variations parallel those found between three infrared spaceobservatory (ISO) spectra of other, well-studied, Herbig Ae/Be stars.The derived profiles are compared to those from a broad sample ofsources, including reflection nebulae, planetary nebulae, H II regions,young stellar objects, evolved stars and galaxies. The Spitzer and ISOspectra exhibit characteristics commonly interpreted respectively asinterstellar matter-like (ISM), non-ISM-like, or a combination of thetwo. Conclusions: We argue that the PAH emission detected from thesources exhibiting a combination of ISM-like and non-ISM-likecharacteristics indicates the presence of two dissimilar, spatiallyseparated, PAH families. As the shape of the individual PAH bandprofiles reflects the composition of the PAH molecules involved, thisdemonstrates that PAHs in subsequent, evolutionary linked stages of starformation are different from those in the general ISM, implying activechemistry. None of the detected PAH emission can be associated with the(unresolved) disk and is thus associated with the circumstellar (natal)cloud. This implies that chemical changes may already occur in the(collapsing?) natal cloud and not necessarily in the disk.
| Molecular hydrogen in the circumstellar environments of Herbig Ae/Be stars probed by FUSE
Context: Molecular hydrogen (H2) gas is the most abundant molecule inthe circumstellar (CS) environments of young stars. It is thus a keyelement in our understanding of the evolution of pre-main sequence starsand their environments towards the main sequence. Aims: At the presenttime, little is known about the gas as compared to the dust in theenvironments of young stars. We thus observed molecular hydrogen arounda sample of pre-main sequence stars in order to better characterizetheir gaseous CS environments. Methods: The FUSE (Far UltravioletSpectroscopic Explorer) spectral range offers access to electronictransitions of H2. We analyzed the FUSE spectra of a sample of HerbigAe/Be stars (HAeBes) covering a broad spectral range (from F4 to B2),including the main-sequence A5 star β Pictoris. To better diagnosethe origin of the detected molecular gas and its excitation conditions,we used a model of a photodissociation region. Results: Our analysisdemonstrates that the excitation of H2 is clearly different around mostof the HAeBes compared to the interstellar medium. Moreover, thecharacteristics of H2 around Herbig Ae and Be stars give evidence fordifferent excitation mechanisms. For the most massive stars of oursample (B8 to B2 type), the excitation diagrams are reproduced well by amodel of photodissociation regions (PDR). Our results favor aninterpretation in terms of large CS envelopes, remnants of the molecularclouds in which the stars were formed. On the other hand, the group ofAe stars (later than B9 type) known to possess disks is moreinhomogeneous. In most cases, when CS H2 is detected, the lines of sightdo not pass through the disks. The excitation conditions of H2 around Aestars cannot be reproduced by PDR models and correspond to warm and/orhot excited media very close to the stars. In addition, no clearcorrelation has been found between the ages of the stars and the amountof circumstellar H2. Our results suggest structural differences betweenHerbig Ae and Be star environments. Herbig Be stars do evolve fasterthan Ae stars, and consequently, most Herbig Be stars are younger thanAe ones at the time we observe them. It is thus more likely to findremnants of their parent cloud around them.
| Toward Understanding the B[e] Phenomenon. I. Definition of the Galactic FS CMa Stars
The B[e] phenomenon is defined as the simultaneous presence oflow-excitation forbidden line emission and strong infrared excess in thespectra of early-type stars. It was discovered in our Galaxy 30 yearsago in the course of the early exploration of the infrared sky andinitially identified in 65 Galactic objects, of which nearly halfremained unclassified. The phenomenon is associated with objects atdifferent evolutionary stages, ranging from the pre-main-sequence to theplanetary nebula stage. We review the studies of both the original 65and subsequently identified Galactic stars with the B[e] phenomenon. Anew classification is proposed for stars with the B[e] phenomenon basedon the time of dust formation in their environments. Properties of theunclassified Galactic B[e] stars are analyzed. We propose that theseobjects are binary systems that are currently undergoing or haverecently undergone a phase of rapid mass exchange, associated with astrong mass loss and dust formation. A new name, FS CMa stars, andclassification criteria are proposed for the unclassified B[e] stars.
| A search for strong, ordered magnetic fields in Herbig Ae/Be stars
The origin of magnetic fields in intermediate- and high-mass stars isfundamentally a mystery. Clues towards solving this basic astrophysicalproblem can likely be found at the pre-main-sequence (PMS) evolutionarystage. With this work, we perform the largest and most sensitive searchfor magnetic fields in PMS Herbig Ae/Be (HAeBe) stars. We seek todetermine whether strong, ordered magnetic fields, similar to those ofmain-sequence Ap/Bp stars, can be detected in these objects, and if so,to determine the intensities, geometrical characteristics, andstatistical incidence of such fields. 68 observations of 50 HAeBe starshave been obtained in circularly polarized light using the FORS1spectropolarimeter at the ESO VLT. An analysis of both Balmer andmetallic lines reveals the possible presence of weak longitudinalmagnetic fields in photospheric lines of two HAeBe stars, HD 101412 andBF Ori. Results for two additional stars, CPD-53 295 and HD 36112, aresuggestive of the presence of magnetic fields, but no firm conclusionscan be drawn based on the available data. The intensity of thelongitudinal fields detected in HD 101412 and BF Ori suggest that theycorrespond to globally ordered magnetic fields with surface intensitiesof order 1 kG. On the other hand, no magnetic field is detected in 4other HAeBe stars in our sample in which magnetic fields had previouslybeen confirmed. Monte Carlo simulations of the longitudinal fieldmeasurements of the undetected stars allow us to place an upper limit ofabout 300 G on the general presence of aligned magnetic dipole magneticfields, and of about 500 G on perpendicular dipole fields. Taking intoaccount the results of our survey and other published results, we findthat the observed bulk incidence of magnetic HAeBe stars in our sampleis 8-12 per cent, in good agreement with that of magnetic main-sequencestars of similar masses. We also find that the rms longitudinal fieldintensity of magnetically detected HAeBe stars is about 200 G, similarto that of Ap stars and consistent with magnetic flux conservationduring stellar evolution. These results are all in agreement with thehypothesis that the magnetic fields of main-sequence Ap/Bp stars arefossils, which already exist within the stars at the PMS stage. Finally,we explore the ability of our new magnetic data to constrainmagnetospheric accretion in Herbig Ae/Be stars, showing that ourmagnetic data are not consistent with the general occurrence in HAeBestars of magnetospheric accretion as described by the theories ofKönigl and Shu et al..Based on observations from the ESO telescopes at the La Silla ParanalObservatory under programme ID 072.C-0447, DDT-272.C-5063, 074.C-0442.E-mail: wade-g@rmc.ca
| Evolution of Emission-Line Activity in Intermediate-Mass Young Stars
We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars.Together with the multiepoch spectroscopic and photometric data compiledfor a large sample of these stars and ages estimated for individualstars by using pre-main-sequence evolutionary tracks, we have studiedthe evolution of emission-line activity in them. We find that, onaverage, the Hα emission line strength decreases with increasingstellar age in Herbig Ae/Be stars, indicating that the accretionactivity gradually declines during the pre-main-sequence phase. Thiswould hint at a relatively long-lived (a few Myr) process beingresponsible for the cessation of accretion in Herbig Ae/Be stars. Wealso find that the accretion activity in these stars drops substantiallyby ~3 Myr. This is comparable to the timescale in which mostintermediate-mass stars are thought to lose their inner disks,suggesting that inner disks in intermediate-mass stars are dissipatedrapidly after the accretion activity has fallen below a certain level.We further find a relatively tight correlation between strength of theemission line and near-infrared excess due to inner disks in HerbigAe/Be stars, indicating that the disks around Herbig Ae/Be stars cannotbe entirely passive. We suggest that this correlation can be understoodwithin the framework of the puffed-up inner rim disk models if theradiation from the accretion shock is also responsible for the diskheating.
| A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| The 78th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1706 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| On the binarity of Herbig Ae/Be stars
We present high-resolution spectro-astrometry of a sample of 28 HerbigAe/Be and three F-type pre-main-sequence stars. The spectro-astrometry,which is essentially the study of unresolved features in long-slitspectra, is shown from both empirical and simulated data to be capableof detecting binary companions that are fainter by up to 6mag atseparations larger than ~0.1arcsec. The nine targets that werepreviously known to be binary are all detected. In addition, we reportthe discovery of six new binaries and present five further possiblebinaries. The resulting binary fraction is 68 +/- 11 per cent. Thisoverall binary fraction is the largest reported for any observed sampleof Herbig Ae/Be stars, presumably because of the exquisite sensitivityof spectro-astrometry for detecting binary systems. The data hint thatthe binary frequency of the Herbig Be stars is larger than that of theHerbig Ae stars. The Appendix presents model simulations to assess thecapabilities of spectro-astrometry and reinforces the empiricalfindings. Most spectro-astrometric signatures in this sample of HerbigAe/Be stars can be explained by the presence of a binary system. Twoobjects, HD 87643 and Z CMa, display evidence for asymmetric outflows.Finally, the position angles of the binary systems have been comparedwith available orientations of the circumprimary disc and these appearto be coplanar. The alignment between the circumprimary discs and thebinary systems strongly suggests that the formation of binaries withintermediate-mass primaries is due to fragmentation as the alternative,stellar capture, does not naturally predict aligned discs. The alignmentextends to the most massive B-type stars in our sample. This leads us toconclude that formation mechanisms that do result in massive stars, butpredict random angles between the binaries and the circumprimary discs,such as stellar collisions, are also ruled out for the same reason.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Properties of galactic B[e] supergiants. IV. Hen 3-298 and Hen 3-303
We present the results of optical and near-IR spectroscopic and near-IRphotometric observations of the emission-line stars Hen 3-298 and Hen3-303. Strong emission in the H? line is found in both objects.The presence of Fe ii and [O i] emission lines in the spectrum of Hen3-298 indicates that it is a B[e] star. The double-peaked CO lineprofiles, found in the infrared spectrum of Hen 3-298, along with theoptical line profiles suggest that the star is surrounded by a rotatingcircumstellar disc. Both objects also show infrared excesses, similar tothose of B[e] stars. The radial velocities of the absorption andemission lines as well as a high reddening level suggest that theobjects are located in the Norma spiral arm at a distance of 3-4.5 kpc.We estimated a luminosity of log L/L? ˜ 5.1 and aspectral type of no earlier than B3 for Hen 3-298. Hen 3-303 seems to bea less luminous B-type object (log L/L? ˜ 4.3),located in the same spiral arm.
| Circular Polarization of Starlight
Of the 7500 stars cited in the Catalog of starlight polarization, thosewhich satisfy the condition P obs % and A V 0m.5 are selected. It ispresumed that the selected stars (n=216) have circularly polarizedlight.
| Investigation of 131 Herbig Ae/Be Candidate Stars
We present a new catalog of 108 Herbig Ae/Be candidate stars identifiedin the Pico dos Dias Survey, together with 19 previously knowncandidates and four objects selected from the IRAS Faint Source Catalog.These 131 stars were observed with low- and/or medium-resolutionspectroscopy, and we complement these data with high-resolution spectraof 39 stars. The objects present a great variety of Hα lineprofiles and were separated according to them. Our study suggests thatmost of the time a Herbig Ae/Be star will present a double peak Hαline profile. Correlations among different physical parameters, such asspectral type and vsini with Hα line profiles were searched. Wefound no correlation among Hα line profiles and spectral type orvsini except for stars with P Cygni profiles, where there is acorrelation with vsini. We also use preliminary spectral energydistributions to seek for any influence of the circumstellar medium inthe Hα line profiles. The presence of [O I] and [S II] forbiddenlines is used together with the Hα line profiles and thesepreliminary spectral energy distributions to discuss the circumstellarenvironment of the Herbig Ae/Be candidates. The distribution of thedetected [O I] and [S II] forbidden lines among different spectral typespoints to a significantly higher occurrence of these lines among Bstars, whereas the distribution among different Hα profile typesindicates that forbidden lines are evenly distributed among eachHα line-profile type. Combining the distance estimates of theHerbig candidates with the knowledge of the interstellar mediumdistribution, we have found that 84 candidates can be associated withsome of the more conspicuous SFRs, being in the right direction and at acompatible distance. As a further means of checking the properties ofthe HAeBe candidates, as well as their present evolutionary status, thederived luminosities and effective temperatures of the stars withpossible association to the star-forming regions and/or Hipparcosdistances were plotted together with a set of pre-main-sequenceevolutionary tracks on an HR diagram. A set of 14 stars were found outof their expected positions in the HR diagram.Based on observations made at the Obsevatório do Pico dosDias/LNA (Brazil), ESO (Chile), and the Lick Observatory.
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| 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
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| The luminous B[e] binary AS 381
We present the results of optical and near-IR spectroscopic andbroadband multicolour photometric observations of the emission-line starAS 381. Its properties were found to be similar to those of Be starswith warm dust, a group of galactic objects recently defined by Sheikinaet al. (\cite{smc}). The spectrum of AS 381 indicates the presence ofboth a hot (early B-type) and a cool (K-type) star in the system. A highinterstellar reddening (AV ~ 7 mag) suggests that it islocated at a distance of ge 3 kpc, and the companions have luminositytypes II or higher. The emission-line profiles indicate that the systemis surrounded by a flattened circumstellar envelope, which is viewedclose to pole-on. The hot companion is found to be ~ 2 mag brighter inthe V-band and more massive ( ~ 20 Msun) than the cool one (~ 7 Msun). The strong line emission and position of thecompanions in the Hertzsprung-Russell diagram indicate that the systemis experiencing mass exchange. We suggest that AS 381 is the first B[e]supergiant binary discovered in the Milky Way.
| Research on the Properties of a Group of Southern B-Type Emission-Line Stars
Not Available
| 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
| Spectroscopy of B-type emission-line stars with compact dusty envelopes: HD 85567, Hen 3-140, and Hen 3-1398
We report the results of our spectroscopic observations (3600-5900Å, R =~ 16 000 and 5750-8750 Å, R =~ 11 000) of threesouthern emission-line B-type stars associated with IRAS sources, HD85567, Hen 3-140, and Hen 3-1398, whose nature and evolutionary stateare not well-established yet. We also obtained new multicolourphotometry of HD 85567 (UBV(RI)cJHKL) and Hen 3-140 (JHKLM).A large near-IR excess is found in Hen 3-140 for the first time. Allthree objects show double-peaked Fe ii lines in emission. The Balmeremission lines are double-peaked in HD 85567 and Hen 3-140, while the Hei lines are found in absorption. Hen 3-1398 displays triple-peakedBalmer lines as well as single-peaked He i and He ii lines in emission.Fundamental parameters of the stars are estimated on the basis of bothphotometric and spectroscopic data. The whole set of their propertiesresemble that of a recently defined group of Be stars with warm dust(Sheikina et al. \cite{smc}).
| 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
| An improved classification of B[e]-type stars
We review the classification criteria for the B[e]-type stars (B typestars with forbidden emission lines in their optical spectrum) and weexpress these in terms of physical characteristics of the stars and thecircumstellar (CS) matter. We show on the basis of observations thatthese criteria can be met in different kinds of stars of different massand different evolutionary stages. We propose that the name "B[e]phen"is more appropriate than the name "B[e] stars". We propose thedefinition of five classes of stars which show the B[e]phen:B[e]gin{itemize} [(a)] B[e] supergiants or "sgB[e] stars" [(b)] pre-mainsequence B[e]-type stars or "HAeB[e] stars" [(c)] compact planetarynebulae B[e]-type stars or "B[e]ppn\ stars" [(d)] symbiotic B[e]-typestars or "SymB[e] stars" [(e)] unclassified B[e]-type stars or "unclB[e]stars" The primary and secondary classification criteria for each ofthese groups are defined. We also present lists of objects for eachgroup, except for the SymB[e] stars. It is possible that some starssatisfy the criteria for more than one of the classes sgB[e], HAeB[e],cPNB[e] and SymB[e]. In that case the evolutionary phase of the star isunclear and the star should be assigned to class unclB[e].
| Polarimetry of southern peculiar early-type stars
We present and discuss optical polarimetry of a large group (60 objects)of peculiar early-type stars (Herbig Ae/Be stars and candidate members,B[e] stars, extreme emission line objects etc.). Most were taken from``A new catalogue of members and candidate members of Herbig Ae/Bestellar group'' \cite[The et al. (1994)]{the2}. For 40 out of the 60objects polarization was measured for the first time. On the basis ofthe data we draw conclusions about the circumstellar shell configurationand orientation for several objects. Polarimetric variability ondifferent time-scales (minutes \ldots hours \ldots days) wasinvestigated, as well as comparison with earlier measurements toinvestigate variability over longer time-scales. No circularpolarization was detected to a 5sigma level in most of the investigatedstars. Polarimetric data obtained here as well as data available fromthe literature were analyzed from the point of investigating theevolutionary status of the objects. The relationship between thepolarimetric and photometric data was also investigated. Tables 6 to 22are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsarc.u-strasbg.fr/Abstract.html
| An ultraviolet, optical and infrared study of Herbig Ae/Be stars
We have selected a list of 45 Herbig Ae/Be-type candidates on the baseof their IRAS colors and their spectral types. We propose the presenceof a broad infrared excess as a defining criterion for these stars,rather than the detection of circumstellar nebulosity. In this way, ourselection also includes more evolved young stars, that are no longerembedded in their star-forming region. A few objects in our sample arewell-known Herbig Ae/Be stars, others are new. New optical andnear-infrared photometric observations, as well as ultraviolet ones, arepresented. The position of the objects in several color-color diagrams,as well as their de-reddened energy distributions, permit a reliableclassification. Three objects probably are binaries with a coolsecondary, 9 appear to be related to the Vega-type stars and 33 objectscan be classified as genuine Herbig Ae/Be stars. The majority of theHerbig Ae/Be stars have a dusty environment consisting of a distinct hotand cool component. These isolated Herbig Ae/Be stars suggest anevolution from embedded Herbig Ae/Be stars to beta Pictoris-likemain-sequence stars, an evolution in which planet formation may play animportant role. Based on observations obtained at the European SouthernObservatory, La Silla, Chile; and at the Swiss Telescope, La Silla,Chile; and at the Swiss Telescope, Hochalpine ForschungsstationJungfraujoch, Switzerland, and with the International UltravioletExplorer (IUE)
| 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.
| A new catalogue of members and candidate members of the Herbig Ae/Be (HAEBE) stellar group
A new up-to-date catalogue of Herbig Ae/Be (HAEBE) stars and relatedobjects is certainly needed, for both well-seasoned researchers and, inparticular, for new investigators starting to study the many interestingastrophysical properties of these very young objects. We present a briefdiscussion of the current observational characteristics that distinguishthis class from their main sequence counterparts. The HAEBE and relatedstars are listed in five tables, containing 287 objects. Table 1contains all Ae and Be stars which historically are recognized as trueHAEBE stars or potential candidate members. Table 2 gives the stars ofspectral type Fe, and emission line stars with very uncertain or unknownspectral type. In Table 3 are given all known Extreme Emission LineObjects (EELOs), of which most have not been identified to belong to anyspecific group. Table 4a and b list other Bep or B[e] stars with strongIR-excess and unknown spectral type. Table 5 contains the non-emissionline possible young objects. Furthermore, Table 6 contains 35 starsrejected from former published lists of HAEBE stars. In these tables weare including coordinates, spectral types, visual magnitudes, ranges inphotometric variability and references of several key publicationsrelated to each object. Relevant remarks, such as the presence of anebula in the vicinity of an object, are also given.
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| Сазвежђа: | Прамац |
| Ректацензија: | 09h50m28.54s |
| Deклинација: | -60°58'02.9" |
| Apparent магнитуда: | 8.569 |
| Даљина: | 1000 parsecs |
| Proper motion RA: | -8.9 |
| Proper motion Dec: | 6.3 |
| B-T magnitude: | 8.709 |
| V-T magnitude: | 8.581 |
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INHABITED SKY
