내용
사진
사진 업로드
DSS Images Other Images
관련 글
Spitzer Observations of the Orion OB1 Association: Second-Generation Dust Disks at 5-10 Myr
We report new Spitzer observations of intermediate-mass stars in tworegions of the Orion OB1 association located in the subassociations OB1a(~10 Myr) and OB1b (~5 Myr). In a representative sample of stars earlierthan F5 of both stellar groups, we find a population of stars surroundedof debris disks, without excess in the IRAC bands and without emissionlines in their optical spectra, but with a varying degree of 24 μmexcess. Comparing our samples with 24 μm observations ofintermediate-mass stars in other stellar groups, spanning a range ofages from 2.5 to 150 Myr, we find that debris disks are more frequentand have larger 24 μm excess at 10 Myr (OB1a). This trend agrees withpredictions of models of evolution of solids in the outer regions ofdisks (>30 AU), where large icy objects (~1000 km) begin to form at~10 Myr; the presence of these objects in the disk initiates acollisional cascade, producing enough dust particles to explain therelatively large 24 μm excess observed in OB1a. The dust luminosityobserved in the stellar groups older than 10 Myr declines roughly aspredicted by collisional cascade models. Combining Spitzer observations,optical spectra, and 2MASS data, we found a new Herbig Ae/Be star (HD290543) and a star (HD 36444) with a large 24 μm excess, both inOB1b. This last object could be explained as a intermediate stagebetween HAeBe and true debris systems or as a massive debris diskproduced by a collision between two large objects (>1000 km).
| 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.
| 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.
| CO emission from discs around isolated HAeBe and Vega-excess stars
We describe results from a survey for J = 3-2 12CO emissionfrom visible stars classified as having an infrared excess. The line isclearly detected in 21 objects, and significant molecular gas(>=10-3 Jupiter masses) is found to be common in targetswith infrared excesses >=0.01 (>=56 per cent of objects), but rarefor those with smaller excesses (~10 per cent of objects).A simple geometrical argument based on the infrared excess implies thatdisc opening angles are typically >=12° for objects with detectedCO; within this angle, the disc is optically thick to stellar radiationand shields the CO from photodissociation. Two or three CO discs have anunusually low infrared excess (<=0.01), implying the shielding discis physically very thin (<=1°).Around 50 per cent of the detected line profiles are double-peaked,while many of the rest have significantly broadened lines, attributed todiscs in Keplerian rotation. Simple model fits to the line profilesindicate outer radii in the range 30-300 au, larger than found throughfitting continuum SEDs, but similar to the sizes of debris discs aroundmain-sequence stars. As many as five have outer radii smaller than theSolar System (50 au), with a further four showing evidence of gas in thedisc at radii smaller than 20 au. The outer disc radius is independentof the stellar spectral type (from K through to B9), but there isevidence of a correlation between radius and total dust mass. Also themean disc size appears to decrease with time: discs around stars of age3-7 Myr have a mean radius ~210 au, whereas discs of age 7-20 Myr are afactor of three smaller. This shows that a significant mass of gas (atleast 2 M⊕) exists beyond the region of planetformation for up to ~7 Myr, and may remain for a further ~10Myr withinthis region.The only bona fide debris disc with detected CO is HD9672; this shows adouble-peaked CO profile and is the most compact gas disc observed, witha modelled outer radius of 17 au. In the case of HD141569, detailedmodelling of the line profile indicates gas may lie in two rings, withradii of 90 and 250 au, similar to the dust structure seen in scatteredlight and the mid-infrared. In both AB Aur and HD163296 we also findthat the sizes of the molecular disc and the dust scattering disc aresimilar; this suggests that the molecular gas and small dust grains areclosely co-located.
| Unusual Color Variability of Eruptive Stars
We substantiate the conclusion that the unusual color variability foundpreviously in some eruptive stars is typical of a broad class ofnonstationary objects, manifests itself over a wide temperature range(from B0 to K 3), and can be regarded as a new type of stellarvariability.
| The CIDA Variability Survey of Orion OB1. I. The Low-Mass Population of Ori OB1a and 1b
We present results of a large-scale, multiepoch optical survey of theOrion OB1 association, carried out with the QUEST camera at theVenezuela National Astronomical Observatory. We identify for the firsttime the widely spread low-mass, young population in the Ori OB1a andOB1b subassociations. Candidate members were picked up by theirvariability in the V band and position in color-magnitude diagrams. Weobtained spectra to confirm membership. In a region spanning ~68deg2, we found 197 new young stars; of these, 56 are locatedin the Ori OB1a subassociation and 141 in Ori OB1b. The spatialdistribution of the low-mass young stars is spatially coincident withthat of the high-mass members but suggests a much sharper edge to theassociation. Comparison with the spatial extent of molecular gas andextinction maps indicates that the subassociation Ori OB1b isconcentrated within a ringlike structure of radius ~2°(~15 pc at 440pc), centered roughly on the star ɛ Ori in the Orion belt. The ringis apparent in 13CO and corresponds to a region with anextinction AV>=1. The stars exhibiting strong Hαemission, an indicator of active accretion, are found along this ring,whereas the center is populated with weak Hα-emitting stars. Incontrast, Ori OB1a is located in a region devoid of gas and dust. Weidentify a grouping of stars within a ~3 deg2 area located inOri OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Bestar V346 Ori is also associated with this grouping, which could be anolder analog of σ Ori. Using several sets of evolutionary tracks,we find an age of 7-10 Myr for Ori OB1a and of ~4-6 Myr for Ori OB1b,consistent with previous estimates from OB stars. Indicators such as theequivalent width of Hα and near-IR excesses show that the numberof accreting low-mass stars decreases sharply between Ori OB1b and OriOB1a. These results indicate that although a substantial fraction ofaccreting disks remain at ages ~5 Myr, inner disks are essentiallydissipated by 10 Myr.Based on observations obtained at the Llano del Hato NationalAstronomical Observatory of Venezuela, operated by Centro deInvestigaciones de Astronomía (CIDA) for the Ministerio deCiencia y Tecnología, and at the Fred Lawrence WhippleObservatory (FLWO) of the Smithsonian Institution.Based on observations obtained at the 3.5 m WIYN Telescope. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory (NOAO).
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| Pre-Main-Sequence A-type stars
Young A-type stars in the pre-main-sequence (PMS) evolutionary phase areparticularly interesting objects since they cover the mass range(1.5-4 Mȯ) which is most sensitive to the internalconditions inherited from the protostellar phase. In particular, theyundergo a process of thermal relaxation from which they emerge as fullyradiative objects contracting towards the Main Sequence. A-type starsalso show intense surface activity (including winds, accretion,pulsations) whose origin is still not completely understood, andinfrared excesses related to the presence of circumstellar disks andenvelopes. Disks display significant evolution in the dust properties,likely signalling the occurrence of protoplanetary growth. Finally,A-type stars are generally found in multiple systems and smallaggregates with lower mass companions.
| Optical Properties of the Circumstellar Dust around Stars with Aperiodic Fadings
Multicolor observations of 21 rapid, irregular variable stars withAlgol-like fadings and of R Coronae Borealis are used to derive theoptical extinction coefficients of the circumstellar dust associatedwith these objects, θ(λ). We used more than 3600 uniformmulti-color brightness measurements obtained in a rigorous U BV R systemat the Terskol High-altitude Observing Station. The mean extinctioncoefficient θ for the circumstellar dust for this sample of starscoincides almost exactly with the interstellar extinction coefficient,θis, but is somewhat higher in the ultraviolet. We suggest anexplanation for this difference.
| Spectroscopic monitoring of the Herbig Ae star HD 104237. I. Multiperiodic stellar oscillations
We present the results of multisite observations spanning two years ofthe pre-main-sequence Herbig Ae star HD 104237. The star was observed inhigh resolution spectroscopy (R ≥ 35 000) for a total of 157.3 h ofeffective exposure time, over 42 nights, corresponding to a data base of1888 individual echelle spectra. We confirm that the HD 104237 systemincludes a spectroscopic binary, whose primary component HD 104237 A ispulsating. The resulting high quality radial velocity curve allows us todetect for the first time by spectroscopic means multiperiodicoscillations in a pre-main sequence star. Five different modes aredetected with very high confidence, with frequencies ranging between28.5 and 35.6 d-1, typical of δ Scuti pulsations; anadditional 3 frequencies have been extracted from the data, but with alower level of confidence. The pattern of frequencies indicates that atleast some of the detected modes are non-radial. The precise orbitdetermination and the measurement of the double line spectroscopicbinary observed around periastron enabled us to determine a mass ratioof 1.29 ± 0.02 between the primary and the secondary; based onthe primary mass of 2.2 ± 0.1 Mȯ we conclude thatthe spectroscopic secondary HD 104237 b should have a mass of 1.7± 0.1 Mȯ and lie outside the pre-main sequenceinstability strip towards later spectral types. A search for pulsationsin the radial velocity curve of the much weaker secondary component wasnot conclusive at this stage. The location of the primary in the HRdiagram and its position with respect to recent pre-main sequenceevolutionary tracks and isochrones implies a location of the secondaryindicative of spectral type K3.Based on observations collected at the 1.9 m SAAO Radcliffe telescopeand the 74 inch MSSSO telescope.Table \ref{log} is only available in electronic form athttp://www.edpsciences.org
| 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.
| Oscillations in the PMS delta Scuti star V346 Ori
We report the discovery of periodicities in the light curve of thePre-Main Sequence (PMS) Herbig Ae star V346 Ori. We interpret variationsin the light curve of the star as the superposition of at least twoperiodic signals that result from delta Scuti-type stellaroscillations. The computation of linear non-adiabatic radial pulsationmodels for PMS stars reproduces these periods for a 1.55 Msunstar with Teff = 7410 K and log L/Lsun = 0.74pulsating in the fundamental and in the second overtone. If ourassumption of radial oscillations is indeed correct, the star's locationon the HR diagram places it at a distance significantly smaller thanthat of the Orion star forming region, hence raising questions regardingthe evolutionary status of V346 Ori. Observations with better timesampling and spanning a longer period of time are needed if the latterquestion is to be addressed fully.Tables 1 and 2 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/399/271
| On the simultaneous optical and near-infrared variability of pre-main sequence stars
For a complete understanding of the physical processes causing thephotometric variability of pre-main sequence systems, simultaneousoptical and near-IR observations are required to disentangle theemission from the stars and that from their associated circumstellardisks. Data of this sort are extremely rare and little systematic workhas been reported to date. The work presented in this paper is asystematic attempt in this direction. It presents an analysis of thesimultaneous optical and near-IR photometric variability of 18 HerbigAe/Be and T Tauri stars which were observed in October 98 by the EXPORTcollaboration. The time difference between the UBVRI and JHKmeasurements is less than 1 hour in ~50% of the data and the largestdifference is around 2 hours in only ~10% of the data. Twelve starsappear to show a correlation between the optical and near-IR variabilitytrends, which suggests a common physical origin such as spots and/orvariable extinction. The optical and near-IR variability is uncorrelatedin the rest of the objects, which suggests it originates in distinctlydifferent regions. In general, the optical variability qualitativelyfollows the predictions of starspots or variable extinction. As far asthe near-IR is concerned, the simultaneity of the observationsdemonstrates that for most objects the flux is largely produced by theircircumstellar disks and, consequently, in many cases the near-IRfluctuations must be attributed to structural variations of such disksproducing variations of their thermal emission and/or scattered light.The observed near-IR changes of up to around 1 mag on timescales of 1-2days provide interesting challenges for understanding the mechanismsgenerating such remarkable variabilities, an issue insufficientlyinvestigated until now but one which deserves further theoretical andmodeling efforts.
| Comparative morphology of ALIVARS and RCB type stars
Some conclusions were drawn from the analysis of the following specificfeatures of the rapid Algol-Like Irregular VARiable Stars (ALIVARS): 1)general mode of variability for ALIVARS and R CrB, 2) long periods ofnearly constant brightness, 3) specific "color - V-magnitude"dependence,4) Anticorrelation between V brightness and degree ofpolarization, 5) identity of the optical properties of the RCB andALIVARS circumstellar dust, 6) cyclic light variations at normalbrighness, 7) connection between cyclic light variations and the onsetof drastic light fadings. We found that ALIVARS have some morphologicalfeatures more in common with evolved RCB-type stars rather than withyoung Herbig Ae/Be stars. Moreover, the results of the analysis of theALIVARS evolution characteristic - a) space distribution of individualALIVARS and star formation regions, b) loci on the H-R diagram, c)equivalent widths of hydrogen absorption lines, d) relative hydrogendeficiency of two ALIVARS investigated (70% of the solar abundance for V351 Ori and 30% for RZ Psc), e) mean group parameter Vsini of ALIVARS,f) luminosity of ALIVARS �alpha emission envelopes- unambiguously point that ALIVARS are not so young as Herbig Ae/Bestars and they have left the main sequence. Thus, we have encounteredthe fact of stellar mimicry: ALIVARS tend to look out as young stars,having at that time features of rather evolved stars. Relying on suchfindings, we conclude the same ALIVARS should not be further regarded asyoung stars of the HAEBES group.
| Projected Rotation Velocities of Several Algol-like Irregular Variable Stars with H-alpha Emission (ALIVARS)
We present results of projected rotation velocities determination ofseveral algol-like irregular variable stars with Ha-emission (ALIVARS).The analysis of overall observational data on rotation velocity of theALIVARS has shown, that: 1. Mean value of rotation velocity of theALIVARS AA is less than that of stars luminosity class V.This fact can be considered as the indicating on rather advancedevolution status of the ALIVARS, which in larger measure have lost itsinitial angular momentum than dwarfs. In any case the relative smallgroup values of v sin i of the ALIVARS explicitly contradict theconjecture of youth of these stars. 2. The presence among the ALIVARS ofstars with small velocities v sin i (V346 Ori, RR Tau) points out thatthey are visible in a position, close to a position ``on pole''. Howeverthese stars are typical antiflare stars and its brightness is undergoneby algol-like fadings. Then the reason of algol-like brightnessvariation seems to be irrelevant with respect to the presence ofgas-dust equatorial discs, the last being considered last time as thespecific indicator of young stars. 3. It is now firmly established thatclassic HAEBES really have the status of Pre-Main Sequence stars anddemonstrate all inherent evidences of young objects. The ALIVARS aredepleted of several such evidences and otherwise demonstrate theproperties of moderately evolved stars being in an advanced stage ofevolution. Hence, the ALIVARS exhibit property of ``stellar mimicry''being not dissimilar in aspect to HAEBES. On the basis of this findingsthe ALIVARS should be classified as an independent group of variablestars.
| 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
| EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of theEXPORT sample obtained at the 2.5 m Nordic Optical Telescope. Thedatabase consists of multi-epoch photo-polarimetry of 68pre-main-sequence and main-sequence stars. An investigation of thepolarization variability indicates that 22 objects are variable at the3sigma level in our data. All these objects are pre-main sequence stars,consisting of both T Tauri and Herbig Ae/Be objects while the mainsequence, Vega type and post-T Tauri type objects are not variable. Thepolarization properties of the variable sources are mostly indicative ofthe UXOR-type behaviour; the objects show highest polarization when thebrightness is at minimum. We add seven new objects to the class of UXORvariables (BH Cep, VX Cas, DK Tau, HK Ori, LkHα 234, KK Oph and RYOri). The main reason for their discovery is the fact that our data-setis the largest in its kind, indicating that many more young UXOR-typepre-main sequence stars remain to be discovered. The set of Vega-likesystems has been investigated for the presence of intrinsicpolarization. As they lack variability, this was done using indirectmethods, and apart from the known case of BD+31o643, thefollowing stars were found to be strong candidates to exhibitpolarization due to the presence of circumstellar disks: 51 Oph,BD+31o643C, HD 58647 and HD 233517. Table A1 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/379/564
| EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars
In this paper we present the first comprehensive results extracted fromthe spectroscopic campaigns carried out by the EXPORT (EXoPlanetaryObservational Research Team) consortium. During 1998-1999, EXPORTcarried out an intensive observational effort in the framework of theorigin and evolution of protoplanetary systems in order to obtain clueson the evolutionary path from the early stages of the pre-main sequenceto stars with planets already formed. The spectral types of 70 stars,and the projected rotational velocities, v sin i, of 45 stars, mainlyVega-type and pre-main sequence, have been determined from intermediate-and high-resolution spectroscopy, respectively. The first part of thework is of fundamental importance in order to accurately place the starsin the HR diagram and determine the evolutionary sequences; the secondpart provides information on the kinematics and dynamics of the starsand the evolution of their angular momentum. The advantage of using thesame observational configuration and methodology for all the stars isthe homogeneity of the set of parameters obtained. Results from previouswork are revised, leading in some cases to completely new determinationsof spectral types and projected rotational velocities; for some stars noprevious studies were available. Tables 1 and 2 are only, and Table 6also, available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/116 Based onobservations made with the Isaac Newton and the William Herscheltelescopes operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofísica de Canarias.
| EXPORT: Near-IR observations of Vega-type and pre-main sequence stars
We present near-IR JHK photometric data of a sample of 58 main-sequence,mainly Vega-type, and pre-main sequence stars. The data were takenduring four observing runs in the period May 1998 to January 1999 andform part of a coordinated effort with simultaneous optical spectroscopyand photo-polarimetry. The near-IR colors of the MS stars correspond inmost cases to photospheric colors, although noticeable reddening ispresent towards a few objects, and these stars show no brightnessvariability within the observational errors. On the other hand, the PMSstars show near-IR excesses and variability consistent with previousdata. Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/110
| Do Herbig-Ae stars pulsate ?
Not Available
| Spectral Features of RZ Psc, a Cool Star with Algol-like Brightness Minima
We have determined the physical (T eff, logg, ζ) and kinematic (V esini,V r ) parameters and abundances for 14 chemical elements in theatmosphere of the “antiflare” variable RZ Psc, usingmedium-resolution spectra obtained with the Coudé spectrographsof the 6-m telescope of the Special Astrophysical Observatory and theCrimean Astrophysical Observatory 2.6-m Shain telescope. The chemicalcomposition of the star is characterized by a slight metal deficiency;however, the iron and calcium abundances are consistent with the solarvalues within the errors. We also detected a peculiar dip (depression)of the continuum level near the Hα line. Assuming that thisdepression and the photospheric Hα line have independent origins,we calculated the hydrogen abundance X in the atmosphere of RZ Psc. Theresulting value X=0.70 (of the solar value) implies a relativedeficiency of hydrogen. Together with the spatial location of the star,these properties provide evidence that RZ Psc is an evolved star, mostlikely belonging to population II.
| 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.
| Pulsation in two Herbig Ae stars: HD 35929 and V351 Ori
New photometric observations of seven intermediate mass pre-mainsequence delta Scuti candidates are presented. The periods and pulsationmodes are derived for two of these stars, namely HD 35929 and V351 Ori.The comparison between observations and nonlinear pulsational modelsallows us to provide some initial constraints on their mass andevolutionary state. As an illustration we discuss the use of periods toidentify the mode of pulsation in these two stars and to have anindependent estimate of their distances. Based on observations carriedout at the European Southern Observatory, La Silla, Chile underproposals number 62-I-0533, 63-I-0053
| On the evolutionary status of the variable AE star V351 Ori.
Not Available
| Photoelectric Photometry of Herbig Ae/be and Related Stars in the Vilnius Photometric System
A catalog of photoelectric photometry of 62 Herbig Ae/Be and relatedstars in the Vilnius system is presented. It contains stars down to V =12 mag located mainly in the northern hemisphere and observed in theperiod of 1994--1996.
| The Incidence of lambda Boötis Stars via an Extension of the MK Spectral Classification System to Very Young A-Type Stars
In this paper we introduce an extension to the MK system of spectralclassification that allows the precise classification ofpre-main-sequence (PMS) A-type stars, including most Herbig Ae stars.This classification scheme characterizes the star by a standard MK type,the presence and strength of emission and/or shell lines, and thestrength of the Balmer decrement. It can be used to summarize temporalspectral changes in PMS A-type stars, and to search for peculiar types.We have used this scheme to classify 38 Herbig Ae stars as well as 22PMS stars in the young open clusters NGC 2264, NGC 7160, and IC 348. Wehave also used this extended system to search for lambda Boötisstars among PMS A-type stars. We have found one definite lambdaBoötis star among the Ae stars, and one marginal lambda Boötisstar in NGC 2264, yielding statistics not significantly different fromthose of the lambda Boötis stars in the field. This, in addition toother considerations from previous studies, leads us to conclude thatthe lambda Boötis mechanism is operable from very early ages (PMS)to well into the main-sequence life (a few times 10^8 yr) of only 2%-3%of A-type stars.
| 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
| 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.
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
