Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Hα spectropolarimetry of the B[e] supergiant GG Carinae
Aims. We study the geometry of the circumstellar environment of the B[e]supergiant star GG Car. Methods: We present observations acquiredusing the IAGPOL imaging polarimeter in combination with theEucalyptus-IFU spectrograph to obtain spectropolarimetric measurementsof GG Car across Hα at two epochs. Polarization effects along theemission line are analysed using the Q-U diagram. In particular, thepolarization position angle (PA) obtained using the line effect is ableto constrain the symmetry axis of the disk/envelope. Results: Byanalysing the fluxes, GG Car shows an increase in its double-peakedHα line emission relative to the continuum within the interval ofour measurements (~43 days). The depolarization line effect aroundHα is evident in the Q-U diagram for both epochs, confirming thatlight from the system is intrinsically polarized. A rotation of the PAalong Hα is also observed, indicating a counter-clockwise rotatingdisk. The intrinsic PA calculated using the line effect (~85°) isconsistent between our two epochs, suggesting a clearly defined symmetryaxis of the disk.Based on observations obtained at the Observatório do Pico dosDias, LNA/MCT, Itajubá, Brazil.
| Revealing the sub-AU asymmetries of the inner dust rim in the disk around the Herbig Ae star R Coronae Austrinae
Context: Unveiling the structure of the disks around intermediate-masspre-main-sequence stars (Herbig Ae/Be stars) is essential for ourunderstanding of the star and planet formation process. In particular,models predict that in the innermost AU around the star, the dust diskforms a “puffed-up” inner rim, which should result in astrongly asymmetric brightness distribution for disks seen underintermediate inclination. Aims: Our aim is to constrain the sub-AUgeometry of the inner disk around the Herbig Ae star R CrA and searchfor the predicted asymmetries. Methods: Using the VLTI/AMBERlong-baseline interferometer, we obtained 24 near-infrared (H- andK-band) spectro-interferometric observations on R CrA. Observing withthree telescopes in a linear array configuration, each data set samplesthree equally spaced points in the visibility function, providing directinformation about the radial intensity profile. In addition, theobservations cover a wide position angle range (~97°), also probingthe position angle dependence of the source brightness distribution. Results: In the derived visibility function, we detect the signaturesof an extended (Gaussian FWHM ~ 25 mas) and a compact component(Gaussian FWHM ~ 5.8 mas), with the compact component contributing abouttwo-thirds of the total flux (both in H- and K-band). The brightnessdistribution is highly asymmetric, as indicated by the strong closurephases (up to ~40°) and the detected position angle dependence ofthe visibilities and closure phases. To interpret these asymmetries, weemploy various geometric as well as physical models, including a binarymodel, a skewed ring model, and a puffed-up inner rim model with avertical or curved rim shape. For the binary and vertical rim model, noacceptable fits could be obtained. On the other hand, the skewed ringmodel and the curved puffed-up inner rim model allow us tosimultaneously reproduce the measured visibilities and closure phases.From these models we derive the location of the dust sublimation radius(~0.4 AU), the disk inclination angle (~35°), and a north-south diskorientation (PA ~ 180-190°). Our curved puffed-up rim model canreproduce reasonably well the interferometric observables and the SEDand suggests a luminosity of ~29 Lȯ and the presence ofrelatively large (⪆1.2 μm) Silicate dust grains. Our study alsoreveals discrepancies between the measured interferometric observablesand the puffed-up inner rim models, providing important constraints forfuture refinements of these theoretical models. Perpendicular to thedisk, two bow shock-like structures appear in the associated reflectionnebula NGC 6729, suggesting that the detected sub-AU size disk is thedriving engine of a large-scale outflow. Conclusions: Detecting,for the first time, strong non-localized asymmetries in the innerregions of a Herbig Ae disk, our study supports the existence of apuffed-up inner rim in YSO disks.Based on observations made with ESO telescopes at the La Silla ParanalObservatory under programme IDs 079.D-0370(A), 081.C-0272(A,B,C), and081.C-0321(A).
| A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| 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.
| Triggered Star Formation on the Border of the Orion-Eridanus Superbubble
A census of classical T Tauri stars and Herbig Ae/Be stars has beenperformed around the Orion-Eridanus Superbubble that is ionized andcreated by the Ori OB1 association. This sample is used to study thespatial distribution of newborn stars, hence the recent star formationsequence, in the region that includes two giant molecular clouds (OrionsA and B) and additional smaller clouds (NGC 2149, GN 05.51.4, VdB 64,the Crossbones, the Northern Filament, LDN 1551, LDN 1558, andLDN 1563). Most of the molecular clouds are located on the borderof the Superbubble, and associated with Hα filaments and starformation activity, except the Northern Filament which is probablylocated outside the Superbubble. This suggests that while star formationprogresses from the oldest Ori OB1a subgroup to 1b, 1c, and 1d, theSuperbubble compresses and initiates starbirth in clouds such asNGC 2149, GN 05.51.4, VdB 64, and the Crossbones, whichare located more than 100 pc away from the center of the Superbubble,and even in clouds some 200 pc away, i.e., in LDN 1551,LDN 1558, and LDN 1563. A superbubble appears to havepotentially a long-range influence in triggering next-generation starformation in an OB association.
| Circumstellar disc geometry constrained by infrared line fluxes
Lenorzer et al. introduce ratios of hydrogen infrared recombinationlines as a diagnostic tool to constrain the spatial distribution andphysical condition of circumstellar material around hot massive stars.They demonstrate that the observed line flux ratios Hu14/Brα andHu14/Pfγ from different types of objects associated withcircumstellar material, such as Be stars, B[e] stars, and Luminous BlueVariable (LBV) stars are well separated in a diagnostic diagram. In thispaper, we investigate this diagnostic tool using a non-LTE disc codedeveloped by Sigut & Jones focusing on Be discs. We find goodagreement between the empirical and predicted locations of Be stars inthe Hu14/Brα versus Hu14/Pfγ diagram and show that indeedthis diagnostic tool can be used to constrain basic properties of thediscs of these stars.
| 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 Tale of Two Herbig Ae Stars, MWC 275 and AB Aurigae: Comprehensive Models for Spectral Energy Distribution and Interferometry
We present comprehensive models for the Herbig Ae stars MWC 275 and ABAur that aim to explain their spectral energy distribution (from UV tomillimeter) and long-baseline interferometry (from near-infrared tomillimeter) simultaneously. Data from the literature, combined with newmid-infrared (MIR) interferometry from the Keck Segment TiltingExperiment, are modeled using an axisymmetric Monte Carlo radiativetransfer code. Models in which most of the near-infrared (NIR) emissionarises from a dust rim fail to fit the NIR spectral energy distribution(SED) and sub-milliarcsecond NIR CHARA interferometry. Following recentwork, we include an additional gas emission component with similar sizescale to the dust rim, inside the sublimation radius, to fit the NIR SEDand long-baseline NIR interferometry on MWC 275 and AB Aur. In theabsence of shielding of starlight by gas, we show that the gas-dusttransition region in these YSOs will have to contain highly refractorydust, sublimating at ~1850 K. Despite having nearly identical structurein the thermal NIR, the outer disks of MWC 275 and AB Aur differsubstantially. In contrast to the AB Aur disk, MWC 275 lacks smallgrains in the disk atmosphere capable of producing significant 10-20μm emission beyond ~7 AU, forcing the outer regions into the``shadow'' of the inner disk.
| Searching for molecular hydrogen mid-infrared emission in the circumstellar environments of Herbig Be stars
Context: Molecular hydrogen (H2) is the most abundant molecule in thecircumstellar (CS) environments of young stars, and is a key element ingiant planet formation. The measurement of the H2 content provides themost direct probe of the total amount of CS gas, especially in the innerwarm planet-forming regions of the disks. Aims: Most Herbig Bestars (HBes) are distant from the Sun and their nature and evolution arestill debated. We therefore conducted mid-infrared observations of H2 asa tracer of warm gas around HBes known to have gas-rich CS environments.Methods: We report a search for the H2 S(1) emission line at17.0348 μm in the CS environments of 5 HBes with the high resolutionspectroscopic mode of VISIR (ESO VLT Imager and Spectrometer for themid-InfraRed). Results: No source shows evidence for H2 emissionat 17.0348 μm. Stringent 3σ upper limits on the integrated linefluxes are derived. Depending on the adopted temperature, limits oncolumn densities and masses of warm gas are also estimated. Thesenon-detections constrain the amount of warm (>150 K) gas in theimmediate CS environments of our target stars to be less than ˜1-10~ M_Jup.
| Analysis of the variability of the luminous emission line star MWC 314
Context: We investigated the surroundings of MWC 314 in the framework ofthe study of hot emission line star environments using the SAC method.This star is either a B[e] supergiant or a luminous blue variable andappears to be extremely luminous and massive. Aims: We determinethe structure and physical conditions of the emitting region and studythe possible variations. Methods: We measured the absorption andemission line radial velocities and the emission line fluxes onhigh-resolution spectra obtained with Aurelie at the 1.52 m OHPtelescope in July 1998, with Elodie at the 1.93 m OHP telescope atvarious epochs, and with echelle spectrographs of the Asiago and Loianoobservatories (Italy) in 2006. We used the statistical approach of theself-absorption curve method (SAC) to derive physical parameters of theline-emitting region. Results: We detected drastic variations ofthe photospheric absorption line radial velocities with time, while theemission line velocities appear to be stable. The Cr II, Ti II, and FeII emission lines have a complex structure. They are double-peaked, andeach of these two 60 km s-1 separated components, is composedof a narrow and a broad component, while the [Fe II] line components arenarrower. The fit of the various components of the Fe II lines to a SACcurve indicates that their intensities are affected by some selfabsorption. We obtained a Boltzmann-type population law whose meanexcitation temperature is 6500-1000+1500 K for thenarrow component lower and upper levels. We obtained a higherBoltzmann-type population law of 10 500-2000+3000K for the forbidden transition upper levels. Conclusions: Fromthe absorption lines we confirm the binarity for MWC 314. Theperiodicity has nevertheless to be improved with a higher samplingfrequency. Our results from the emission lines are consistent with lineformation in a rotating disk around a star. The typical minimum radiusof the line emitting region obtained from the SAC study is 3.5 ×1013 cm (2.0 × 1013 cm < R < 6.3× 1013 cm). We argue, in the framework of a verysimplified geometrical model, that the [Fe II] lines are emitted fartherout than the permitted Cr II, Ti II, and Fe II lines, in a disk inclined25 ± 5 degrees to the plane of sky. If the rotation of the diskis Keplerian, the Fe II lines are emitted in a zone defined by 4 ×1012 cm < R < 7 × 1013 cm, while for arotation with conservation of angular momentum, they are emitted from 4× 1012 cm < R < 2 × 1013 cm.based on observations obtained at the Loiano and Asiago Observatoriesand with AURELIE spectra and ELODIE archival spectra of the HauteProvence Observatory.
| Spectropolarimetry of the massive post-red supergiants IRC +10420 and HD 179821
We present medium resolution spectropolarimetry and long-termphoto-polarimetry of two massive post-red supergiants, IRC +10420 and HD179821. The data provide new information on their circumstellar materialas well as their evolution. In IRC +10420, the polarization of theHα line is different to that of the continuum, which indicatesthat the electron-scattering region is not spherically symmetric. Theobserved long-term changes in the polarimetry can be associated with anaxisymmetric structure, along the short axis of the extended reflectionnebulosity. Long-term photometry reveals that the star increased intemperature until the mid-nineties, after which the photospheric flux inthe optical levelled off. As the photometric changes are mostly probedin the red, they do not trace high stellar temperatures sensitively. So,it is not obvious whether the star has halted its increase intemperature or not. For HD 179821, we find no polarization effectsacross any absorption or emission lines, but observe very largepolarization changes of the order of 5 per cent over 15 yr. During thesame period, the optical photometry displayed modest variability at the0.2 mag level. This is unexpected, because large polarization changesare generally accompanied by strong photometric changes. Severalexplanations for this puzzling fact are discussed. Most of which,involving asymmetries in the circumstellar material, seem to fail asthere is no evidence for the presence of hot, dusty material close tothe star. A caveat is that the sparsely available near-infraredphotometry could have missed periods of strong polarization activity.The variations may be explained by the presence of a non-radiallypulsating photosphere. Changes in the photometry hint at an increase intemperature corresponding to a change through two spectral subclassesover the past 10 yr.
| The SCUBA Legacy Catalogues: Submillimeter-Continuum Objects Detected by SCUBA
We present the SCUBA Legacy Catalogues, two comprehensive sets ofcontinuum maps (and catalogs) using data at 850 and 450 μm of thevarious astronomical objects obtained with the Submillimetre Common UserBolometer Array (SCUBA). The Fundamental Map Data Set contains data onlywhere superior atmospheric opacity calibration data were available. TheExtended Map Data Set contains data regardless of the quality of theopacity calibration. Each data set contains1.2deg×1.2deg maps at locations where dataexisted in the JCMT archive, imaged using the matrix inversion method.The Fundamental Data Set is composed of 1423 maps at 850 μm and 1357maps at 450 μm. The Extended Data Set is composed of 1547 maps at 850μm. Neither data set includes high sensitivity, single-chop SCUBAmaps of ``cosmological fields'' nor solar system objects. Each data setwas used to determine a respective object catalogue, consisting ofobjects identified within the respective 850 μm maps using anautomated identification algorithm. The Fundamental and Extended MapObject Catalogues contain 5061 and 6118 objects, respectively. Objectsare named based on their respective J2000.0 position of peak 850 μmintensity. The catalogues provide for each object the respective maximum850 μm intensity, estimates of total 850 μm flux and size, andtentative identifications from the SIMBAD Database. Where possible, thecatalogues also provide for each object its maximum 450 μm intensityand total 450 μm flux and flux ratios.
| 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.
| The Role of Evolutionary Age and Metallicity in the Formation of Classical Be Circumstellar Disks. II. Assessing the Evolutionary Nature of Candidate Disk Systems
We present the first detailed imaging polarization observations of sixSMC and six LMC clusters, known to have large populations of B-typestars that exhibit excess Hα emission from 2-CD photometricstudies, to constrain the evolutionary status of these stars and hencebetter establish links between the onset of disk formation in classicalBe stars and cluster age and/or metallicity. We parameterize and removethe interstellar polarization (ISP) associated with each line of sight,thereby isolating the presence of any intrinsic polarization. We use thewavelength dependence of this intrinsic polarization to discriminatepure gas disk systems, i.e., classical Be stars, from compositegas-plus-dust disk systems, i.e., Herbig Ae/Be or B[e] stars. Ourintrinsic polarization results, along with available near-IR colorinformation, support the suggestion of Wisniewski et al. that classicalBe stars are present in clusters of age 5-8 Myr and contradictassertions that the Be phenomenon only develops in the second half of aB star's main-sequence lifetime, i.e., no earlier than 10 Myr. Theprevalence of polarimetric Balmer jump signatures decreases withmetallicity; we speculate that either it is more difficult to form largedisk systems in low-metallicity environments or that average disktemperatures are higher in low-metallicity environments. Thepolarimetric signatures of ~25% of our sample appear unlikely to arisefrom true classical Be star disk systems, suggesting one should proceedwith caution when attempting to determine the role of evolutionary ageand/or metallicity in the Be phenomenon purely via 2-CD results.
| Photometric activity of the unique X-ray transient CI Camelopardalis (XTE J0421+560)
We present an analysis of the photometric observations of the peculiarX-ray binary and X-ray transient CI Cam (XTE J0421+560), mostly coveringthe interval following its 1998 outburst. We show that the mostprominent variations are observed in the I band, with the amplitudedecreasing toward the V (and U) passband. We find that CI Cam displayscomplicated shifts in the colour diagrams and show that the variationsof the continuum play a significant role in the colour changes. We alsoresolve the signatures of the variations of the very strong Hαemission with respect to the combination of the continuum and otherlines in the colours. On the basis of the shifts in the colour diagramsand of the spectral energy distribution determined from the photometrywe propose that the division of the dominant contributions of thesuperimposed spectral components (free free emission (Clark, J.S.,Miroshnichenko, A.S., Larionov, V.M., et al., 2000, A&A, 356, 50)and the (pseudo)photospheric emission) occurs near λ = 550 nm. Wemake use of this division for an explanation of the decoupling of thevariations of U B and B V from those in V R, R I and V I. We observedtwo maxima of brightness (the second one only in R and I) separated by1350 d in which the (pseudo)photospheric emission, f f/f b and Hαemission appear to be involved in a complicated way. We find someindications that the variations of the source of the optical light canbe related to those of the X-ray source in quiescence, particularly inan event which we interpret as a density enhancement in the matterexchange. We explain the huge changes of the absorption of the X-rayspectrum, NH, not reflected in the colour variations in theoptical region, in terms of the variations of NH confined tothe region hotter than the temperature of the condensation of the dust,maybe related to the filling of the disk embedding the compact object.We argue that the dominant part of the X-ray emission comes from theclose vicinity of the compact object, not from the donor. Verylow-amplitude (˜0.02 mag) intranight optical variations can bepresent in four nights of our observations in the post-outburst period;they have the form of bumps on the time-scale of about an hour, butwithout any coherent component. We also point out the similarities inthe situation of CI Cam and the microquasar LS 5039/RX J1826 1450, asregards the variations of brightness coming from the vicinity of thecompact object.
| 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.
| Galactic distributions and statistics of the HD stars in the michigan spectral catalogue.
Not Available
| Relation between the Luminosity of Young Stellar Objects and Their Circumstellar Environment
We present a new model-independent method of comparison of NIRvisibility data of YSOs. The method is based on scaling the measuredbaseline with the YSO's distance and luminosity, which removes thedependence of visibility on these two variables. We use this method tocompare all available NIR visibility data and demonstrate that itdistinguishes YSOs of luminosity L*<~103Lsolar (low L) from YSOs of L*>~103Lsolar (high L). This confirms earlier suggestions, based onfits of image models to the visibility data, for the difference betweenthe NIR sizes of these two luminosity groups. When plotted against the``scaled'' baseline, the visibility creates the following data clusters:low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. Wemodel the shape and size of clusters with different image models andfind that low-L Herbig stars are best explained by the uniformbrightness ring and the halo model, T Tauri stars with the halo model,and high-L Herbig stars with the accretion disk model. However, theplausibility of each model is not well established. Therefore, we try tobuild a descriptive model of the circumstellar environment consistentwith various observed properties of YSOs. We argue that low-L YSOs haveoptically thick disks with the optically thin inner dust sublimationcavity and an optically thin dusty outflow above the inner disk regions.High-L YSOs have optically thick accretion disks with high accretionrates enabling gas to dominate the NIR emission over dust. Althoughobservations would favor such a description of YSOs, the required dustdistribution is not supported by our current understanding of dustdynamics.
| A polarimetric study of the B[e] star HD 45677
We present new medium-resolution spectropolarimetric observations of HD45677 in the B and R bands. A change in polarization is detected acrossHα, Hβ and Hγ confirming that the ionized region aroundthe star is aspherical. (Q, U) points associated with these emissionlines occur away from the continuum, defining a polarization vectorwhich points in the same direction for each of the lines at an averageintrinsic polarization angle of 164° +/- 3°.These data were combined with past photometric and polarimetric datafrom the literature to investigate any variability. We find that HD45677 is both photometrically and polarimetrically variable and thatthese changes are linked. We suggest that these variations may be causedby an aspherical blowout, and by deriving a least-squares fit to theB-band polarimetric data in the Q-U space, we show that the blowoutoccurs at an intrinsic polarization angle of 175° +/- 1°, alongthe same angle as the proposed geometry of the ionized region.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The present status of four luminous variables in M 33
Context: . Understanding the origin of the instabilities of LBVs isimportant for shedding light on the late evolutionary stages of massivestars and on the chemical evolution of galaxies. Aims: . Toinvestigate the physical nature of variable stars in the upper H-Rdiagram, we performed a spectrophotometric study of the Romano's star GR290 and the Hubble-Sandage variables A, B, and C in the close galaxy M33. Methods: . New spectroscopic and photometric data wereemployed in conjunction with already published data of these stars inorder to derive spectral types, energy distribution and bolometricluminosities. Results: . The yellow hypergiant Var A is still atminimum, with a ~G-type spectrum and strong Hα emission (W_eq≃ -35 Å). Var B is in a low luminosity hot state (V=17.5,B-V = -0.15) with very strong Hα emission (W_eq=-310 Å). Itsabsolute bolometric luminosity is 0.6× 106Lȯ. Var C, at V=16.4, is fainter than in the mid 1980s,but its spectrum shows the typical features of LBVs at maximum, aspectrum that is very rich in Fe ii emission lines. Its L_bol is about0.7× 106 Lȯ. The Romano's star GR 290has a rich hot emission-line spectrum and is very bright withL_bol=3× 106 Lȯ. During 2004 the starbrightened by ~half magnitude in each of the BVR filters. Conclusions: . Our observations confirm that Var A probably is anintermediate type hypergiant star surrounded by an expanding envelopewith a collisionally excited hydrogen emission, largely obscured bydusty disk and nebula. In recent years, Var B has undergone a bluewardtransition in the H-R diagram, probably at constant bolometricluminosity, while Var C is in a post-maximum phase with an ηCar-type spectrum. GR 290 is notable for its spectrum and luminosity,and it is likely to develop ample spectral variations in the nearfuture, similar to those observed in AG Car.
| Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be Stars
Using the three-telescope IOTA interferometer on Mount Hopkins, wereport results from the first near-infrared (λ=1.65 μm)closure-phase survey of young stellar objects (YSOs). These closurephases allow us to unambiguously detect departures from centrosymmetry(i.e., skew) in the emission pattern from YSO disks on the scale of ~4mas, expected from generic ``flared disk'' models. Six of 14 targetsshowed small, yet statistically significant nonzero closure phases, withlargest values from the young binary system MWC 361-A and the(pre-main-sequence?) Be star HD 45677. Our observations are quitesensitive to the vertical structure of the inner disk, and we confrontthe predictions of the ``puffed-up inner wall'' models of Dullemond,Dominik, & Natta (DDN). Our data support disk models with curvedinner rims because the expected emission appears symmetricallydistributed around the star over a wide range of inclination angles. Incontrast, our results are incompatible with the models possessingvertical inner walls because they predict extreme skewness (i.e., largeclosure phases) from the near-IR disk emission that is not seen in ourdata. In addition, we also present the discovery of mysterious H-band``halos'' (~5%-10% of light on scales 0.01"-0.50") around a few objects,a preliminary ``parametric imaging'' study for HD 45677, and the firstastrometric orbit for the young binary MWC 361-A.
| Analysis of the circumstellar environment of the B[ e] star HD 45677 (FS Canis Majoris)
Aims.We studied the circumstellar environment of the B[ e] star HD 45677through the analysis of the emission lines from ionized metals.Methods: .We used the statistical approach of the self absorption curvemethod (SAC) to derive physical parameters of the line emittingregion. Results: .The Fe II and Cr II double-peaked emission linestructure is explained by the presence of a thin absorption componentred shifted by ~3 km s-1. This absorption component can beinterpreted geometricaly as being due to infalling materialperpendicularly to the disk seen nearly pole-on, as indicated by theemission line structure. The Cr II and Fe II emission lines have acomplex structure with two (narrow and broad) components, of 45 and 180km s-1 FWHM for the permitted lines and 25 and 100 kms-1 FWHM for the forbidden ones, respectively. From our bestdata set of 1999, we obtained a Boltzmann-type population law whoseexcitation temperature is 3900-600+900 K and3150-300+350 K for the narrow component lower andupper levels respectively. We obtained an excitation temperature of3400-300+350 K for the broad component upperlevels. The forbidden lines are found to be formed in the outer regionswith higher excitation temperatures of 10 500 ± 1000 K and 8000± 1500 K for the narrow and broad components respectively in1999. Our results are consistent with line formation in a rotating disk,around a young star. In the framework of a very simplified geometricalmodel, we argue that the narrow components are principaly emitted by anoptically thin disk seen nearly pole-on, in a region whose minimumradius is estimated to be 4×1012 cm, while the broadones are formed in a disk-linked wind.
| 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.
| 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.
| 8-13 μm Spectroscopy of Young Stellar Objects: Evolution of the Silicate Feature
Silicate features arising from material around pre-main-sequence starsare useful probes of the star and planet formation process. In order toinvestigate possible connections between dust processing and diskproperties, 8-13 μm spectra of 34 young stars, exhibiting a range ofcircumstellar environments and including spectral types A-M, wereobtained using the Long Wavelength Spectrometer at the W. M. KeckObservatory. The broad 9.7 μm amorphous silicate (SiO stretching)feature that dominates this wavelength regime evolves from absorption inyoung, embedded sources, to emission in optically revealed stars, and tocomplete absence in older ``debris'' disk systems for both low- andintermediate-mass stars. This is similar to the evolutionary patternseen in Infrared Space Observatory (ISO) observations ofhigh/intermediate-mass young stellar objects (YSOs). The peak wavelengthand FWHM are centered about 9.7 and ~2.3 μm, respectively,corresponding to amorphous olivine, with a larger spread in FWHM forembedded sources and in peak wavelength for disks. In a few of ourobjects that have been previously identified as class I low-mass YSOs,the observed silicate feature is more complex, with absorption near 9.5μm and emission peaking around 10 μm. Although most of theemission spectra show broad classical features attributed to amorphoussilicates, small variations in the shape/strength may be linked to dustprocessing, including grain growth and/or silicate crystallization. Forsome of the Herbig Ae stars in the sample, the broad emission featurehas an additional bump near 11.3 μm, similar to the emission fromcrystalline forsterite seen in comets and the debris disk βPictoris. Only one of the low-mass stars, Hen 3-600A, and one Herbig Aestar, HD 179218, clearly show strong, narrow emission near 11.3 μm.We study quantitatively the evidence for evolutionary trends in the 8-13μm spectra through a variety of spectral shape diagnostics. Based onthe lack of correlation between these diagnostics and broadband infraredluminosity characteristics for silicate emission sources, we concludethat although spectral signatures of dust processing are present, theycannot be connected clearly to disk evolutionary stage (for opticallythick disks) or optical depth (for optically thin disks). Thediagnostics of silicate absorption features (other than the centralwavelength of the feature), however, are tightly correlated with opticaldepth and thus do not probe silicate grain properties.
| On the nature of pre-main sequence candidate stars in the Large Magellanic Cloud
We investigate a sample of 18 Large Magellanic CloudHerbig Ae/Be candidate stars looking at their (1) spectral types (2)brightness variability mechanism and (3) near infra-red JHK emission. Wefind that the majority of the target stars have Hα emission, areof spectral type early- to mid-B and lack strong JHK excess emission.Their Balmer decrements are found to be similar to that of Galactic Bestars in general. Their erratic brightness variability is evaluated byusing the observed optical color excess and the color gradient from thelight curves and is subsequently interpreted as being due to variabledust obscuration or variable bf-ff emission from circumstellar ionizedgas. For approximately half of the target stars in our sample the typeof variability seems to be dissimilar to the mechanism involvingbound-free and free-free emission, but could be interpreted as caused byvariable dust obscuration, as we have proposed in earlier studies. It istherefore suggested that they are pre-main sequence objects, despite thefact that they nearly all lack thermal dust emission in the nearinfra-red; mid/far infra-red observations for these objects arewarranted. One star is observed to have JHK excess emission and aninspection of its 7.5 year MACHO light curve confirms its erraticphotometric behavior. The object displays deep photometric minima with aquasi-period of 191.3 days, as generally seen in the Galactic pre-mainsequence subgroup of the UX Orionis stars.Based on observations collected at ESO, La Silla.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cane Maggiore |
| Ascensione retta: | 06h28m17.42s |
| Declinazione: | -13°03'11.1" |
| Magnitudine apparente: | 8.057 |
| Distanza: | 354.61 parsec |
| Moto proprio RA: | 2 |
| Moto proprio Dec: | 2.2 |
| B-T magnitude: | 8.114 |
| V-T magnitude: | 8.062 |
Cataloghi e designazioni:
|
