目录
图像
上传图像
DSS Images Other Images
相关文章
The nature of stars with the B[e] phenomenon through interferometric eyes
Stars that present the B[e] phenomenon are known to form a heterogeneousgroup. This group is composed by objects in different evolutionarystages, like high- and low-mass evolved stars, intermediate-masspre-main sequence stars and symbiotic objects. However, for more than50% of the confirmed B[e] stars the evolutionary stage is still unknown,so that they are gathered in the group of the unclassified B[e] stars.High-angular resolution interferometry is certainly an important tool toanswer questions related to the circumstellar geometry of these objects.In this work, we present the results related to seven objects, based onobservations from VLTI/MIDI, VLTI/AMBER and CHARA/VEGA.
| The galactic unclassified B[e] star HD 50138. I. A possible new shell phase
Context: The observed spectral variation of HD 50138 has led differentauthors to classify it in a very wide range of spectral types andluminosity classes (from B5 to A0 and III to Ia) and at differentevolutionary stages as either HAeBe star or classical Be. Aims:Based on new high-resolution optical spectroscopic data from 1999 and2007 associated to a photometric analysis, the aim of this work is toprovide a deep spectroscopic description and a new set of parameters forthis unclassified southern B[e] star and its interstellar extinction. Methods: From our high-resolution optical spectroscopic dataseparated by 8 years, we perform a detailed spectral description,presenting the variations seen and discussing their possible origin. Wederive the interstellar extinction to HD 50138 by taking the influencesof the circumstellar matter in the form of dust and an ionized disk intoaccount. Based on photometric data from the literature and the newHipparcos distance, we obtain a revised set of parameters for HD50138. Results: Because of the spectral changes, we tentativelysuggest that a new shell phase could have taken place prior to ourobservations in 2007. We find a color excess value of E(B-V) = 0.08 mag,and from the photometric analysis, we suggest that HD 50138 is a B6-7III-V star. A discussion of the different evolutionary scenarios is alsoprovided. Based on observations: (i) with the 1.52-m and 2.2-m telescopes at theEuropean Southern Observatory (La Silla, Chile), under agreement withthe Observatório Nacional-MCT (Brazil); and (ii) with theTelescope Bernard Lyot, Observatory of Pic du Midi (France). Table 1 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/cgi-bin/qcat?J/A+A/508/309 It is with greatsadness that we have to report that, during the final stages of thispaper, we had a deep loss when Francisco X. de Araújo passedaway.
| Mid-Infrared Size Survey of Young Stellar Objects: Description of Keck Segment-Tilting Experiment and Basic Results
The mid-infrared properties of pre-planetary disks are sensitive to thetemperature and flaring profiles of disks for the regions where planetformation is expected to occur. In order to constrain theories of planetformation, we have carried out a mid-infrared (λ = 10.7 μm)size survey of young stellar objects using the segmented Keck telescopein a novel configuration. We introduced a customized pattern of tilts toindividual mirror segments to allow efficient sparse-apertureinterferometry, allowing full aperture synthesis imaging with highercalibration precision than traditional imaging. In contrast to previoussurveys on smaller telescopes and with poorer calibration precision, wefind that most objects in our sample are partially resolved. Here, wepresent the main observational results of our survey of five embeddedmassive protostars, 25 Herbig Ae/Be stars, 3 T Tauri stars, 1 FU Orisystem, and five emission-line objects of uncertain classification. Theobserved mid-infrared sizes do not obey the size-luminosity relationfound at near-infrared wavelengths and a companion paper will providefurther modeling analysis of this sample. In addition, we report imagingresults for a few of the most resolved objects, including complexemission around embedded massive protostars, the photoevaporatingcircumbinary disk around MWC 361A, and the subarcsecond binaries T Tau,FU Ori, and MWC 1080.
| Spectropolarimetric Observations of Herbig Ae/Be Stars. II. Comparison of Spectropolarimetric Surveys: Haebe, Be and Other Emission-Line Stars
The polarization of light across individual spectral lines containsinformation about the circumstellar environment on very small spatialscales. We have obtained a large number of high-precision,high-resolution spectropolarimetric observations of Herbig Ae/Be,Classical Be and other emission-line stars collected on 117 nights ofobservations with the Hi-Resolution Visible spectropolarimeter at aresolution of R = 13, 000 on the 3.67 m Advanced Electro-Optical Systemtelescope. We also have many observations from the ESPaDOnSspectropolarimeter at a resolution of R = 68, 000 on the 3.6 mCanada-France-Hawaii Telescope. In roughly ~2/3 of the so-called "windy"or "disky" Herbig Ae/Be stars, the detected Hα linearpolarization varies from our typical detection threshold near 0.1% toover 2%. In all but one HAe/Be star, the detected polarization effect isnot coincident with the Hα emission peak but isdetected in and around the obvious absorptive part of the line profile.The qu-loops are dominated by the polarization in this absorptiveregion. In several stars, the polarization varies in time mostly in theabsorptive component and is not necessarily tied to correspondingvariations in intensity. This is a new result not seen at lowerresolution. In the Be and emission-line stars, ten out of a sample of 30show a typical broad depolarization effect but four of these ten showweaker effects only visible at high resolution. Another five of 30 showsmaller amplitude, more complex signatures. Six stars of alternateclassification showed large amplitude (1%-3%) absorptive polarizationeffects. These detections are largely inconsistent with the traditionaldisk-scattering and depolarization models.
| A GLIMPSE into the Nature of Galactic Mid-IR Excesses
We investigate the nature of the mid-IR excess for 31 intermediate-massstars that exhibit an 8 μm excess in either the Galactic LegacyInfrared Mid-Plane Survey Extraordinaire or the Mid-Course SpaceExperiment using high-resolution optical spectra to identify starssurrounded by warm circumstellar dust. From these data we determineprojected stellar rotational velocities and estimate stellar effectivetemperatures for the sample. We estimate stellar ages from thesetemperatures, parallactic distances, and evolutionary models. Using MIPS[24] measurements and stellar parameters we determine the nature of theinfrared excess for 19 GLIMPSE stars. We find that 15 stars exhibitHα emission and four exhibit Hα absorption. Assuming thatthe mid-IR excesses arise in circumstellar disks, we use the Hαfluxes to model and estimate the relative contributions of dust andfree-free emission. Six stars exhibit Hα fluxes that implyfree-free emission can plausibly explain the infrared excess at [24].These stars are candidate classical Be stars. Nine stars exhibitHα emission, but their Hα fluxes are insufficient to explainthe infrared excesses at [24], suggesting the presence of acircumstellar dust component. After the removal of the free-freecomponent in these sources, we determine probable disk dust temperaturesof Tdisk~=300-800 K and fractional infrared luminosities ofLIR/L*~=10-3. These nine stars may bepre-main-sequence stars with transitional disks undergoing diskclearing. Three of the four sources showing Hα absorption exhibitcircumstellar disk temperatures ~=300-400 K,LIR/L*~=10-3, IR colors K-[24]<3.3,and are warm debris disk candidates. One of the four Hα absorptionsources has K-[24]>3.3 implying an optically thick outer disk and isa transition disk candidate.
| From B[e] to A[e]. On the peculiar variations of the SMC supergiant LHA 115-S 23 (AzV 172)
Context: Optical observations from 1989 of the Small Magellanic Cloud(SMC) B[e] supergiant star LHA 115-S 23 (in short: S 23) revealed thepresence of photospheric Hei absorption lines, classifying S 23 as a B8supergiant. In our high-resolution optical spectra from 2000, however,we could not identify any Hei line. Instead, the spectral appearance ofS 23 is more consistent with the classification as an A1 supergiant,maintaining the so-called B[e] phenomenon. Aims: The observedchanges in spectral behaviour of S 23 lead to different spectralclassifications at different observing epochs. The aim of this researchis, therefore, to find and discuss possible scenarios that might cause adisappearance of the photospheric Hei absorption lines within a periodof only 11 years. Methods: From our high-resolution opticalspectra, we perform a detailed investigation of the different spectralappearances of S 23 based on modern and revised classification schemes.In particular, we derive the contributions caused by the interstellar aswell as the circumstellar extinction self-consistently. The latter isdue to a partly optically thick wind. We further determine the projectedrotational velocities of S 23 in the two epochs of spectroscopicobservations. Results: Based on its spectral appearance in 2000,we classify S 23 as A1 Ib star with an effective temperature of about9000 K. This classification is supported by the additional analysis ofthe photometric UBV data. An interstellar extinction value of E(B-V)? 0.03 is derived. This is considerably lower than the previouslypublished value, which means that, if the circumstellar extinction dueto the stellar wind is neglected, the interstellar extinction, and hencethe luminosity of the star, are overestimated. We further derive arotation velocity of ? sin i ? 150 km s-1, whichmeans that S 23 is rotating with about 75% of its critical speed. Theobject S 23 is thus the fourth B[e] supergiant with confirmed highprojected rotational velocity. The most striking result is the apparentcooling of S 23 by more than 1500 K with a simultaneous increase of itsrotation speed by about 35% within only 11 years. Since such a behaviouris excluded by stellar evolution theories, we discuss possible scenariosfor the observed peculiar variations in S 23.
| Spectroscopic analysis of two peculiar emission-line stars: RJHA 49 and SS73 21
Aims:We investigate the spectra and the evolutionary stages of twopeculiar emission-line stars: RJHA 49 and SS73 21. Methods: Weused low and high resolution optical data. Line identifications andmeasurements were performed for several features in their spectra. Results: For each object, we derived the extinction and the excitationtemperature from a set of [Fe ii] lines and the electron density from [Nii] lines. For RJHA 49, no detailed spectroscopic study has been done sofar. Our low resolution spectrum have confirmed the main characteristicsfound in previous works on SS73 21. On the other hand, from our highresolution data, we find that the Hα line presents a double peak,in contrast with the suggestion in the literature that it should reveala P-Cygni profile. Surprisingly, we found a few He i transitionsresembling P-Cygni profiles (e.g. He i λ 5876), directlysuggesting that mass loss is active in SS73 21. We also discuss thenature of these two objects based on data obtained. Although theevolutionary status of SS73 21 seems well established from previousstudies (a proto-planetary nebula), the situation for RJHA 49 is notvery clear mainly due to its unknown distance. However, from thestrength of [N ii] λ 5754 relative to [O i] λ 6300, thepossibility of RJHA 49 being a LBV object is reduced, and aB[e]-supergiant or a proto-planetary nebula status is more plausible.Based on observations made with the 1.52 m telescope at the EuropeanSouthern Observatory (La Silla, Chile) under the agreement with theObservatório Nacional, Brazil. Spectra and fits files are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/477/877
| Extended shells around B[e] stars. Implications for B[e] star evolution
Aims.The position of B[e] stars in the upper left part of theHertzsprung-Russell diagram creates a quandary. Are these stars youngstars evolving onto the main sequence or old stars that are evolving offof it? Spectral characteristics suggest that B[e] stars can be placedinto five subclasses and are not a homogeneous set. Suchsub-classification is believed to coincide with varying origins anddifferent evolutions. However, the evolutionary connection of B[e] stars- and notably sgB[e] - to other stars is unclear, particularly toevolved massive stars. We attempt to provide insight into theevolutionary past of B[e] stars. Methods: We performed an Hαnarrow-band CCD imaging survey of B[e] stars, in the northernhemisphere. Prior to the current work, no emission-line survey of B[e]stars had yet been made, while only two B[e] stars appeared to have ashell nebula as seen in the Digital Sky Survey. Of nebulae around B[e]stars, only the ring nebula around MWC 137 has been previously observedextensively. Results: In this presentation we report the findings fromour narrow-band optical imaging survey of the environments of 25 B[e]stars. Of the objects surveyed, 7 show bipolar or uni- polar structuresup to 15 arcmin across; 5 show faint, large, or filamentary shells; and2 are compact planetary nebula-type systems. The most spectacular systemobserved is a large bipolar structure associated with MWC 314. Conclusions: The possible links between B[e] stars and other evolvedstars, implied by our observations, are investigated.
| Spectropolarimetry of the H? Line in Herbig Ae/Be Stars
Using the HiVIS spectropolarimeter built for the Haleakala 3.7 m AEOStelescope, we have obtained a large number of high-precisionspectropolarimetrc observations (284) of Herbig AeBe stars collectedover 53 nights totaling more than 300 hr of observing. Our sample offive HAeBe stars, AB Aurigae, MWC 480, MWC 120, MWC 158, and HD 58647,all show systematic variations in the linear polarization amplitude anddirection as a function of time and wavelength near the H? line.In all our stars, the H? line profiles show evidence of anintervening disk or outflowing wind, evidenced by strong emission withan absorptive component. The linear polarization varies by 0.2%-1.5%with the change typically centered in the absorptive part of the lineprofile. These observations are inconsistent with a simpledisk-scattering model or a depolarization model that producepolarization changes centered on the emissive core. We speculate thatpolarized absorption via optical pumping of the intervening gas may bethe cause.
| 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.
| The Complex Structure of the Mg II λλ 2795.523, 2802.698 Å Regions of 64 Be Stars
Here we consider the presence of absorption components shifted to theviolet or red side of the main spectral line (satellite or discreteabsorption components, i.e., SACs or DACs) in the regions of the Mg IIresonance lines in Be stars as well as their kinematicalcharacteristics. Namely, our objective is to check whether there existsa common physical structure for the atmospheric regions creating SACs orDACs of the Mg II resonance lines. In order to do this, a statisticalstudy of the Mg II λλ 2795.523, 2802.698 Å lines inthe spectra of 64 Be stars of all spectral subtypes and luminosityclasses was performed. We found that the atmospherical absorptionregions where the Mg II resonance lines originated may be formed ofseveral independent density layers of matter that rotate with differentvelocities. It was also attempted to separate SACs and DACs according tolow or high radial velocity. The emission lines were detected only inthe earliest and latest spectral subtypes.
| Large-scale variability in the profiles of Hα and Hβ in the spectrum of the Herbig B8e star MWC 419 and a model interpretation of it
Spectroscopic data taken with a moderate resolution spectrograph in theregion of the Hα and Hβ lines are presented for the HerbigB8e star MWC 419. The spectroscopic observations were accompanied bybroad band BVR photometric measurements. The observations reveal avariability in the line profiles that is typical of Herbig Ae/Be starswith signs of a strong stellar wind. The greatest changes are observedin the region of the absorption components of the line profiles, whichconvert the profile from a type P CygII to P CygIII, as well as in theintensities of the central emission components. A model technique isused for quantitative interpretation of this variability and it showsthat the P Cyg profile conversion of the absorption component can beexplained in terms of a stellar wind model in which its distributionover latitude varies on a time scale of a few days.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Fundamental parameters of Be stars located in the seismology fields of COROT
In preparation for the COROT space mission, we determined thefundamental parameters (spectral type, temperature, gravity, V sin i) ofthe Be stars observable by COROT in its seismology fields (64 Be stars).We applied a careful and detailed modeling of the stellar spectra,taking into account the veiling caused by the envelope, as well as thegravitational darkening and stellar flattening due to rapid rotation.Evolutionary tracks for fast rotators were used to derive stellar massesand ages. The derived parameters will be used to select Be stars assecondary targets (i.e. observed for 5 consecutive months) and short-runtargets of the COROT mission. Furthermore, we note that the main part ofour stellar sample falls in the second half of the main sequence lifetime, and that in most cases the luminosity class of Be stars isinaccurate in characterizing their evolutionary status.
| 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.
| Properties of galactic B[e] supergiants. V. Two-dimensional radiative transfer model of RY Sct and its dusty disc
We present results of the first two-dimensional radiative transfermodelling of the eclipsing binary RY Sct and itsdusty disc. Assuming an effective temperature Tstar = 27 000K for both components and the distance D = 1.8 kpc, we derive the totalluminosity Lstar = 4.2 × 105Lȯ. The optically thin dusty disc (τV≈ 0.04 in the equatorial plane) extends from its inner boundary atR1 = 60 AU to the distances of R2 ≈105 AU, where it blends into the interstellar medium. Thevery high energy output of the supergiants heats up the interstellardust, well beyond the outer boundary, to temperatures of 100 K. It isthe large interstellar extinction towards RY Sct(AV = 4.5 mag) that defines its spectral energy distributionin the ultraviolet, optical and near infrared. The disc has a fullopening angle {ψ} = 26° and we observe it at a viewing angleθv = 14° from its midplane (inclination i =76°). There is a strong density enhancement in the disc within anarrow ring at r = 1500 AU, that emits most of the infrared flux and isprominent in Keck telescope images (Gehrz et al. 2001, ApJ, 559, 395).The dust mass contained in the disc within 1´´ from the star(r < 1800 AU) is md = 3.2 × 10-7Mȯ, by a factor of 3 lower than in previous estimates.However, in our model there is 30 times more mass in the surroundingsof the binary system than within the dense ring. As much as 95% of thetotal dust mass Md = 9 × 10-6Mȯ and gas mass M = 0.017 Mȯ of thecircumbinary material is contained in the outer, old wind at 1800 < r< 105 AU. Presumably the dense ring has been created by afast wind that swept out and compressed the previously lost material inthe older and slower stellar wind. Based on the new Keck data, our modelpredicts that presently there is a relatively large number of small, hotdust grains in the dust formation zone, whose emission substantiallychanges the shape of the SED of RY Sct in the nearinfrared. This suggests a higher mass-loss rate or dust-to-gas massratio or lower wind velocity, or a combination of these factors.
| 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.
| Upper limits on CO 4.7 μm emission from disks around five Herbig Ae/Be stars
We present the results of medium-resolution spectroscopy of five nearbyHerbig Ae/Be stars at 4.7 μm: UX Ori, HD 34282, HD 50138, V380 Ori,HK Ori. The goal was to search for CO fundamental ro-vibrationalemission. None of the targets show CO features, either in absorption norin emission. We derive a 5σ upper limit of <10-12cm-2 to the column density of hot CO (T ≈ 1500 K) in thesources. These upper limits are considerably lower than the values ofHerbig Ae/Be stars for which warm and hot CO emission has been reported.The non-detection of CO ν=1-0 emission in these five targets suggestthat Herbig Ae/Be stars are not a homogeneous group with respect to thestructure of the gaseous disk and/or the amount of CO in the inner 50 AUof their disks.
| 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.
| 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.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Long-Term Variations in the Mid-IR Spectra of Dusty Protostellar Disks
As part of a survey of the mid-IR spectral characteristics of stars withcircumstellar dust disks (Herbig Ae/Be stars, T Tauri stars, and a fewother objects with similar characteristics) we have been monitoring afew select object for variability. In a few cases, such as the isolatedHerbig stars HD 31648 (MWC 480) and HD 163296, as well as stars whosenature remains uncertain, such as HD 45677 and HD 50138, lowerresolution photometry extend the data set back a quarter of a century.HD 163296 recently underwent a major "flare" in its 3 micron emission,suggesting the possible ejection of its next Herbig-Haro object. HD50138 continues to exhibit fluctuations on time scales of years, and isfound to possess organic emission bands at 7.7 and 8.6 microns. The TTauri star DG Tau went from exhibiting weak silicate emission to strongsilicate absorption. The absorption profile contained the 11.2 micronfeature due to crystalline olivine, the first time it has been observedin absorption in an astronomical object.This work has been supported by NASA's Origins of Solar Systems Program,the University Research Council of the University of Cincinnati, and theAerospace Corporation's Independent Research and Development Program.
| Time Evolution of Spectral Energy Distribution Fits of Four Stars
HD 45677, HD 50138, two B type stars, HD 31468 and HD 163296, two A typestars with circumstellar disks, have displayed variability in their dataover a period of a few years. Using computer modeling codes, spectralenergy distribution fits of the 0.1-100 micron region for these objectsat different times are presented. Rough estimates of density andmaterial component distribution of the circumstellar dust are obtained.It is hoped that the comparison of these time dependent fits can help todetermine how the circumstellar dust is evolving in size and shape andhow this is causing the variability in the data of these two objects.This work has been supported by NASA's Origins of Solar Systems Program,the University Research Council of the University of Cincinnati, and theAerospace Corporation's Independent Research and Development Program.
| Spectral Classification of Stars in A Supplement to the Bright Star Catalogue
MK spectral types are given for about 584 stars in A Supplement to theBright Star Catalogue. These are compared with Hipparcos parallaxes tocheck the reliability of those classifications. The estimated errors are+/-1.2 subtypes, and 10% of the luminosity classes may be wrong.
| A new approach to IRAS observations of Be stars
The IRAS associations for 193 Be stars are identified in this paper.From the infrared colors, the IRAS low-resolution spectra (LRS) and thespectral types, some physical properties and environment of the samplesare discussed. It can be concluded that not only free-free emission orfree-bound emission from the circumstellar ionized gas can beresponsible for the large IR excesses of Be stars as suggestedpreviously, but also, for some Be stars, thermal radiation from thecircumstellar dust and/or nebula around the star can produce large IRexcess as well. It is also found that the far IR excess of Be starsincreases with wavelengths.
| Automated Classification of 2000 Bright IRAS Sources
An artificial neural network (ANN) scheme has been employed that uses asupervised back-propagation algorithm to classify 2000 bright sourcesfrom the Calgary database of Infrared Astronomical Satellite (IRAS)spectra in the region 8-23 μm. The database has been classified into17 predefined classes based on the spectral morphology. We have beenable to classify over 80% of the sources correctly in the firstinstance. The speed and robustness of the scheme will allow us toclassify the whole of the Low Resolution Spectrometer database,containing more than 50,000 sources, in the near future.
| Active phenomena in the circumstellar environmentof the Herbig Ae star HD 31648
We present the results of observations of the Herbig Ae star HD 31648including high-resolution spectroscopy near the Hα, He I λ5876 Å and DNa I lines, multi-color photometry and polarimetry.These observations have revealed many features of the observationalappearance of HD 31648 similar to those of the ``classical'' Herbig Aestars. Among them are spectral manifestations of both the mass loss andthe disk accretion onto the star and certain types of line profilevariations. The totality of the observed properties of HD 31648 providesevidence in favor of complicated structure in the stellar wind zone,exhibiting latitudinal stratification and containing loinhomogeneitiesin the form of outflowing streams and density condensations. Structuralvariations of the stellar wind have been found to correlate with changesin the accretion process in the envelope. The circumstellar environmentof HD 31648 is supposed to contain dust particles of different formsmanifesting themselves, in particular, in the variable multi-componentintrinsic polarization.
| 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.
| Diffuse interstellar bands in the spectra of Herbig Ae/Be stars and related objects
We report a detection of weak diffuse interstellar bands (DIBs) in theclose vicinity of 6 Herbig Ae/Be pre-main sequence stars. A commonfeature of these objects is the presence of a dusty shell where DIBcarriers are apparently formed/destroyed. The possible influence ofultraviolet flux on carriers of diffuse interstellar bands is discussedas well as DIB to DIB intensity ratios in the spectra of the programstars and in the general interstellar medium.Based on data collected at the 2 m telescope operated on IC AMERObservatory (Terskol, Russia).
| 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.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
