Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Accretion rates in Herbig Ae stars
Aims.Accretion rates from disks around pre-main sequence stars are ofimportance for our understanding of planetary formation and diskevolution. We provide in this paper estimates of the mass accretionrates in the disks around a large sample of Herbig Ae stars.Methods: .We obtained medium resolution 2 μm spectra and used theresults to compute values of dot M_acc from the measured luminosity ofthe Brγ emission line, using a well established correlationbetween L(Brγ) and the accretion luminosity L_acc. Results:.We find that 80% of the stars, all of which have evidence of anassociated circumstellar disk, are accreting matter, with rates 3×10-9 dot M_acc 10-6 M_ȯ/yr; for 7objects, 6 of which are located on the ZAMS in the HR diagram, we do notdetect any line emission. Few HAe stars (25%) have dotM_acc>10-7 M_ȯ/yr. Conclusions: .In most HAestars the accretion rate is sufficiently low that the gas in the innerdisk, inside the dust evaporation radius, is optically thin and does notprevent the formation of a puffed-up rim, where dust is directly exposedto the stellar radiation. When compared to the dot M_acc values foundfor lower-mass stars in the star forming regions Taurus and Ophiuchus,HAe stars have on average higher accretion rates than solar-mass stars;however, there is a lack of very strong accretors among them, probablydue to the fact that they are on average older.
| VLT/NACO adaptive optics imaging of the Herbig Ae star HD 100453
We report the detection of a companion candidate to the Herbig Ae starHD 100453, using the NACO instrument at the VLT UT4. The separationbetween the companion candidate and HD 100453 is 1.6 arcsec, and theirbrightness contrast is 5.1 mag at Ks and 4.2 mag atBrγ. We discuss the possible nature of the newly detected starbased on near-infrared photometry. The photometric data are consistentwith a 0.3 M_ȯ companion to HD 100453, corresponding to a spectraltype of M3-M5. This detection could have important implications for thestructure of the disk around HD 100453.
| 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.
| ISO spectroscopy of disks around Herbig Ae/Be stars
We have investigated the infrared spectra of all 46 Herbig Ae/Be starsfor which spectroscopic data are available in the ISO data archive. Ourquantitative analysis of these spectra focuses on the emission bands at3.3, 6.2, ``7.7'', 8.6 and 11.2 micron, linked to polycyclic aromatichydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5micron, the amorphous 10 micron silicate band and the crystallinesilicate band at 11.3 micron. We have detected PAH emission in 57% ofthe Herbig stars in our sample. Although for most of these sources thePAH spectra are similar, there are clear examples of differences in thePAH spectra within our sample which can be explained by differences inPAH size, chemistry and/or ionization. Amorphous silicate emission wasdetected in the spectra of 52% of the sample stars, amorphous silicateabsorption in 13%. We have detected crystalline silicate emission in 11stars (24% of our sample), of which four (9%) also display strong PAHemission. We have classified the sample sources according to thestrength of their mid-IR energy distribution. The systems with strongermid-infared (20-100 μm) excesses relative to their near-infrared (1-5μm) excess display significantly more PAH emission than those withweaker mid-infrared excesses. There are no pronounced differences in thebehaviour of the silicate feature between the two groups. This providesstrong observational support for the disk models by \citet{dullemond01},in which systems with a flaring disk geometry display a strongmid-infrared excess, whereas those with disks that are strongly shadowedby the puffed-up inner rim of the disk only display modest amounts ofmid-infrared emission. Since the silicates are expected to be producedmainly in the warm inner disk regions, no large differences in silicatebehaviour are expected between the two groups. In contrast to this, thePAH emission is expected to be produced mainly in the part of the diskatmosphere that is directly exposed to radiation from the central star.In this model, self-shadowed disks should display weaker PAH emissionthan flared disks, consistent with our observations.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) and with the participation of ISASand NASA.Tables 1, 3-6 and Appendix are only available in electronic form athttp://www.edpsciences.org
| 3-14 Micron Spectroscopy of Comets C/2002 O4 (Hönig), C/2002 V1 (NEAT), C/2002 X5 (Kudo-Fujikawa), C/2002 Y1 (Juels-Holvorcem), and 69P/Taylor and the Relationships among Grain Temperature, Silicate Band Strength, and Structure among Comet Families
We report 3-13 μm spectroscopy of four comets observed between 2002August and 2003 February: C/2002 O4 (Hönig) on 2002 August 1,C/2002 V1 (NEAT) on 2003 January 9 and 10, C/2002 X5 (Kudo-Fujikawa) on2003 January 9 and 10, and C/2002 Y1 (Juels-Holvorcem) on 2003 February20. In addition, we include data obtained much earlier on 69P/Taylor(1998 February 9) but not previously published. For comets Taylor,Hönig, NEAT, and Kudo-Fujikawa, the silicate emission band wasdetected, being approximately 23%, 12%, 15%, and 10%, respectively,above the continuum. The data for comet Juels-Holvorcem were ofinsufficient quality to detect the presence of a silicate band ofcomparable strength to the other three objects, and we place an upperlimit of 24% on this feature. The silicate features in both NEAT andKudo-Fujikawa contained structure indicating the presence of crystallinematerial. The shape of the silicate feature at a projected distance of1900 km from the nucleus of Kudo-Fujukawa was nearly identical to thatcentered on the nucleus, indicating that the grain size population hadnot been measurably modified by the time it had reached that distance.Combining these data with those of other comets, we confirm thecorrelation between silicate band strength and grain temperature ofGehrz & Ney and Williams and coworkers for dynamically new andlong-period comets, but the majority of Jupiter family objects maydeviate from this relation. Despite the weakness of the silicate band inKudo-Fujikawa, its structure resembles the bands seen in dynamically newand long-period objects with substantially stronger features. Thelimited data available on Jupiter family objects suggest that they mayhave silicate bands that are slightly different from the former objects.Finally, when compared to the silicate emission bands observed inpre-main-sequence stars, the dynamically new and long-period comets mostclosely resemble the more evolved stellar systems, while the limiteddata (in quantity and quality) on Jupiter family objects seem to suggestthat these have spectra more like the less evolved stars. Higher qualitydata on a larger number of Jupiter family objects are needed to confirm(or reject) this trend.
| Correlation between grain growth and disk geometry in Herbig Ae/Be systems
We have calculated the (sub-)mm spectral indices of 26 Herbig Ae/Bestars, for which we can determine the infrared spectral energydistribution (SED). We find a clear correlation between the strength ofthe ratio of the near- to mid-infrared excess of these sources, and theslope of the (sub-)mm energy distribution. Based on earliermulti-dimensional modeling of disks around Herbig Ae stars, we interpretthis as a correlation between the geometry of the disk (flared orself-shadowed) and the size of the grains: self-shadowed disks have, onaverage, larger grains than their flared counterparts. These datasuggest that the geometry of a young stellar disk evolves from flared toself-shadowed.
| Submillimetre observations and modelling of Vega-type stars
We present new submillimetre observations of Vega-excess stars, andconsistent modelling for all known Vega-excess stars with submillimetredata. Our analysis uses dust grain models with realistic opticalproperties, with the aim of determining physical parameters of theunresolved discs from just their spectral energy distributions (SEDs).For the resolved targets, we find that different objects require verydifferent dust grain properties in order to fit the image data and SEDsimultaneously. Fomalhaut and Vega require solid dust grains, while HR4796 and HD 141569 can only be fitted using porous grains. The olderstars tend to have grains which are less porous than the younger stars,which may indicate that collisions in the discs have reprocessed theinitially fluffy grains into a more solid form. ɛ Eri appears to bedeficient in small dust grains compared with our best-fitting model.This may show that it is important to include all the factors that causethe size distribution to depart from a simple power law for grains closeto the radiation pressure blow-out limit. Alternatively, thisdiscrepancy may be due to some external influence on the disc (e.g. aplanet).When the model is applied to the unresolved targets, an estimate of thedisc size can be made. However, the large diversity in dust compositionfor the resolved discs means that we cannot make a reliable assumptionas to the composition of the grains in an unresolved disc, and there iscorresponding uncertainty in the disc size. In addition, the poor fitfor ɛ Eri shows that the model cannot always account for the SEDeven if the disc size is known. These two factors mean that it may notbe possible to determine the size of a disc without actually resolvingit.
| A search for evolved dust in Herbig Ae stars
We present observations of six isolated, pre-main-sequence, intermediatemass stars selected for shallow spectra at submillimeter wavelengths at1.3, 2.6, 7.0, and 36 millimeters from the IRAM PdBI and the VLA. Weanalyze the new observations of these stars (HD 34282, HD 35187, HD142666, HD 143006, HD 150193, HD 163296) together with similarobservations of three additional stars from the literature (CQ Tau, UXOri, TW Hya), in the context of self-consistent irradiated disk models.Our aim is to constrain the wavelength dependence of the dust opacityand the total dust mass in the disks. The shallow wavelength dependenceof the opacity is confirmed and for a few stars extended tosignificantly longer wavelengths. For any plausible dust properties,this requires grain growth from interstellar sizes to maximum sizes ofat least a few millimeters, and very likely to several centimeters ormore. For four of the stars (HD 34282, HD 163296, CQ Tau, TW Hya), themillimeter emission has been spatially resolved, and the large diskradii (>100 AU) rule out that high optical depths play a role. Themass of dust that has been processed into large grains is substantial,and in some cases implies a disk mass comparable to the mass of thecentral star.
| 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.
| 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.
| The MIR Dust Features of Vega-like Stars: A Comparison With Cometary Spectra
Vega-like stars, which are main-sequence (or approaching the mainsequence), have substantial debris disks around them. We observed 8-13micron spectra towards several Vega-like stars. We present models thatfit the mid-infrared spectra of the Vega-like stars using the SAMEsilicate minerals used to fit Comet Hale-Bopp (Wooden et al. 1999). Wefind that the Hale-Bopp mixture, which contains Mg-rich pyroxene grains- a possible signature of pristine material, fits our Vega-like 8-13micron spectra extremely well. Models of HD 144432 indicate thatpyroxene's Mg-richness might be a sensitive parameter when fitting 8-13micron spectra.
| Circumstellar disks around Herbig Ae/Be stars: Polarization, outflows and binary orbits
The geometrical relationship between the distribution of circumstellarmatter, observed optical linear polarization, outflows and binaryorbital plane in Herbig Ae/Be stars is investigated. Optical linearpolarization measurements carried out for a number of Herbig Ae/Be starsthat are either known to be in binary systems and/or have bipolar jetsare presented in this paper. Available information on the positionangles of polarization, outflows and binary companions for Herbig Ae/Bestars is compiled and analysed for any possible correlations. In ~85% ofthe sources the outflow position angle is within 30deg ofbeing parallel or perpendicular to the polarization position angle. In~81% of the sources the binary position angle is within 30degof being parallel or perpendicular to the polarization position angle.Out of 15 sources with bipolar outflows, 10 sources have the binaryposition angle within 30deg of being perpendicular to theoutflow position angle. These results favour those binary formationmechanisms in which the binary components and the disks aroundindividual stars or circumbinary disks are coplanar.
| Discovery of Reflection Nebulosity around Five Vega-like Stars
Coronagraphic optical observations of six Vega-like stars revealreflection nebulosities, five of which were previously unknown. Thenebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, andHD 49662 resemble that of the Pleiades, indicating an interstellarorigin for dust grains. The reflection nebulosity around HD 123160 has adouble-arm morphology, but no disklike feature is seen as close as 2.5"from the star in K-band adaptive optics data. We demonstrate that auniform density dust cloud surrounding HD 23362, HD 23680, and HD 123160can account for the observed 12-100 μm spectral energy distributions.For HD 4881, HD 26676, and HD 49662, an additional emission source, suchas from a circumstellar disk or nonequilibrium grain heating, isrequired to fit the 12-25 μm data. These results indicate that insome cases, particularly for Vega-like stars located beyond the LocalBubble (>100 pc), the dust responsible for excess thermal emissionmay originate from the interstellar medium rather than from a planetarydebris system.
| GasDust Shells around Some Early-Type Stars with an IR Excess (of Emission)
The results of an investigation of IR (IRAS) observations of 58OBAF stars of different luminosity classes, whichare mainly members of various associations, are presented. The colorindices of these stars are determined and two-color diagrams areconstructed. The emission excesses at 12 and 25 mm (E 12 and E 25) arealso compared with the absorption A1640 of UV radiation. It is concludedthat 24 stars (of the 58 investigated) are disk systems of the Vegatype, to which Vega = N 53 also belongs. Eight known stars of the Vegatype are also given in the figures for comparison. The remaining 34stars may have gasdust shells and/or shelldisks. The IRemission excesses of the 34 investigated stars and 11 comparison stars(eight of them are Be-Ae stars) are evidently due both to thermalemission from grains and to the emission from freefreetransitions of electrons in the gasdust shells of these stars.
| Mid-Infrared Imaging of Candidate Vega-like Systems
We have conducted deep mid-infrared imaging of a relatively nearbysample of candidate Vega-like stars using the OSCIR instrument on theCerro Tololo Inter-American Observatory 4 m and Keck II 10 m telescopes.Our discovery of a spatially resolved disk around HR 4796A has alreadybeen reported in 1998 by Jayawardhana et al. Here we present imagingobservations of the other members of the sample, including the discoverythat only the primary in the HD 35187 binary system appears to harbor asubstantial circumstellar disk and the possible detection of extendeddisk emission around 49 Ceti. We derive global properties of the dustdisks, place constraints on their sizes, and discuss several interestingcases in detail. Although our targets are believed to be main-sequencestars, we note that several have large infrared excesses compared withprototype Vega-like systems and may therefore be somewhat younger. Thedisk size constraints we derive, in many cases, imply emission fromrelatively large (>~10 μm) particles at mid-infrared wavelengths.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| ISOPHOT Observations of Dust Disks around Main Sequence (Vega-Like) Stars
The photometer (ISOPHOT) on the Infrared Space Observatory (ISO) hasproved to be invaluable for investigating the dust around main sequencestars (both prototypes and candidate Vega-like stars). The longwavelength camera (at 60 and 90 μm) has been used to map the areaaround the stars to establish whether the dust disk is extended.Low-resolution spectra between 5.8 and 11.6 μm show whether the dustis composed of silicate grains, and whether molecular features arepresent. The four prototype Vega-like stars (Vega, β Pic,Fomalhaut, ɛ Eri) are studied, as well as eight other stars,which are main sequence stars with cool dust associated with them. Wefind that the spectra of β Pic, 49 Cet, HD98800, and HD135344 showexcess emission from the cool dust around the star, HD144432 andHD139614 show silicate dust emission, HD169142 and HD34700 show emissionfeatures from carbon-rich molecules (possibly PAHs, polycyclic aromatichydrocarbon molecules), and HD142666 shows emission features from bothcarbon-rich molecules and silicate dust. Up to 11.6 μm, the emissionfrom Vega, Fomalhaut, and ɛ Eri is dominated by the stellarphotosphere. At 60 and 90 μm, the extended dust emission is mapped,and the disk resolved in eight cases. The dust mass in the disks isfound to range from around 10-9 to 10-4Msolar. Since several of the stars are younger than the Sun,and the disks have sufficient material of the type found in the SolarSystem, these disks could be in the early stages of planet formation.
| The domain of γ Doradus variables in the Hertzsprung-Russell diagram
70 new γ Doradus candidates were identified from Hipparcosphotometry, which represents more than a doubling of the presently knownnumber. Selecting the objects with good evidence for multiperiodicity,it is found that these stars, together with bona fide members of theclass, occupy a well-defined region in a colour-magnitude diagram. Thisdomain corresponds to a range of 7200-7700K on the zero-age mainsequence (ZAMS) and 6900-7500K one magnitude above it, which partlyoverlaps with the instability strip of δ Scuti stars. For thefirst time, γ Doradus stars can be discussed as a group. They canbe found over a significant fraction of the main sequence lifetimes forobjects in the relevant temperature range. An upper limit on the surfacemetallicity of γ Doradus stars is apparent, which may guide thesearch for their pulsation driving mechanism. The importance of possibleobjects exhibiting both γ Doradus and δ Scuti-typepulsations is discussed.
| Spectroscopic, photometric, and polarimetric study of the Herbig AE candidate HD 36112
We describe observations of the Herbig Ae candidate HD 36112 includinghigh-resolution spectroscopy of photospheric and circumstellar lines,multi-colour photometry and polarimetry. These observations reveal thatmany features of the observational appearance of HD 36112 are similar tothose of `classical' Herbig Ae stars. Among these are: the far IRexcess, manifestations of the high-temperature zone in the innerenvelope, distinctive types of the Hα profile variations, andvariable multi-component polarization and brightness parameters. Thefundamental characteristics of the star have been determined. Inparticular, HD 36112 proves to be an unreddened A8 v of age t ~ 5/10x10(6) years. This means, that the star is still at the PMS phase ofevolution, but close to the ZAMS. The anomalous abundances of barium andsilicon discovered in HD 36112 allow to suggest that chemical anomaliescan arise already at the early stages of stellar evolution. The totalityof observed properties of HD 36112 give evidence of complicatedstructure of the gaseous-dusty envelope around this object, includingthe variable stellar wind with an extended acceleration zone, as well aslarge and small-grained dust, located in the orthogonal planes.
| Six intermediate-mass stars with far-infrared excess: a search for evolutionary connections
We present the results of high-resolution spectroscopic, low-resolutionspectrophotometric and spectropolarimetric and broad-band multicolourobservations of four B-type stars (HD 4881, 5839, 224648 and 179218) andtwo A-type stars (HD 32509 and 184761) with strong far-infrared (IR)excesses. The excess in HD 184761, which is located at a distance of 65pc from the Sun, was recognized for the first time. Double-peakedHα emission line profiles are found in HD 4881 and HD 5839, whileHD 184761, HD 224648 and HD 32509 display no emission in Hα. Theremarkable variations observed in the Hα profile of HD 179218 arealso observed in some classical Be and Herbig Ae/Be stars. An intrinsiccomponent of polarization is clearly present in HD 179218, only aninterstellar component is detected in HD 4881 and HD 224648, and HD184761 was found to be unpolarized. Improved effective temperatures forall six objects were derived. Parallaxes measured by the Hipparcossatellite were used to determine positions of the stars in the HRdiagram. HD 4881 and HD 5839 are an order of magnitude more luminousthan main-sequence stars of similar temperatures and are most likelynewly discovered classical Be stars. Study of the high-resolution IRASmaps and modelling of the spectral energy distributions of HD 4881, HD5839 and HD 224648 suggest that the observed large IR excesses arecaused by radiation from circumstellar dust rather than free--freeradiation or infrared cirrus, so they may be higher mass counterparts ofbeta Pictoris stars. HD 32509, HD 224648 and HD 184761, which have verysmall near-IR excesses, are probably young main-sequence stars. HD179218, which exhibits the largest near- and far-IR excess in thesample, is an isolated pre-main-sequence Herbig Be star.
| 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.
| Measurements of double stars 1993.67 - 1998.13
624 Micrometer Measurements of 224 pairs with a 32.5 cm Cassegrain, 719Measurements of 310 double stars with a 360 mm Newtonian are given.Tables 1 to 4 are available in electronic form only at the CDS130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html
| Spatially resolved optical spectroscopy of the Herbig Ae/Vega-like binary star HD 35187
We report on observations of the young binary system HD 35187 (SAO77144). For the first time, we have obtained spatially resolved opticalspectra of the individual stars. Analysis of their effectivetemperatures indicates that the stars have spectral types of A2 (HD35187B) and A7 (HD 35187A). Analysis of the Hγ Balmer lineindicates a luminosity class V for both stars. At the time of theseobservations, net Hα emission was present only towards HD 35187B.However, there is evidence that the photospheric Hα line in HD35187A has been `filled in' relative to its expected strength in an A7star, so this star may also be associated with some process leading toHα emission. Moreover, both stars exhibit absorption in the He ilambda5876 line well in excess of that expected for their spectraltypes. Comparison with earlier observations reveals that both theHα and He i lines are variable, so both stars are `active' in somesense. We suggest that the variable He i absorption detected towardsboth stars is a result of chromospheric activity, and is not necessarilyrelated to the circumstellar environment. We find tentative evidence fora narrow Ca K circumstellar absorption line and excess redshiftedabsorption of the Na D line profiles in the spectrum of HD 35187B, bothof which are absent in the spectrum of HD 35187A. The heliocentricradial velocity of the presumed circumstellar Ca K line (+54.5 km s^-1)is similar to that of redshifted circumstellar absorption linespreviously identified in IUE spectra of this star, and the velocityrange of the Na D absorption precisely matches that of the UVcircumstellar components. Moreover, by placing the stars on the HRdiagram (with the aid of the reliable Hipparcos distances) we findevidence that HD 35187B is dimmed by about 0.4 mag of grey circumstellarextinction. The detection of net Hα emission, circumstellarabsorption lines, and significant circumstellar extinction for HD 35187Bsuggests that it has far more mass in its circumstellar environment thanits companion, and that the observed IR excess of this system originatesfrom a disc surrounding HD 35187B alone.
| Infrared and submillimetre studies of Vega-excess stars
We report new infrared and submillimetre observations of a sample of 24candidate Vega-excess stars, and derive CO masses, dust masses, gas todust ratios and the strengths of various emission lines. Most of thesestars have dustier discs than the class archetypes (Vega, Fomalhaut,beta Pic, etc.), yet, like the archetypes, all the stars observed in COshow the gas content of their discs to be depleted compared withmolecular cloud values. We discuss how the extra dust content mightimply that these stars are less evolved than the archetypes, and useother infrared and submillimetre characteristics to support thiscontention.
| 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.
| Near infrared photometry of IRAS sources with colours like planetary nebulae. III
We present the near infrared photometry of a new sample of 225 IRASsources, many of them previously unidentified in the literature,selected because their far infrared colours are similar to those shownby known planetary nebulae. The results obtained are used to establishthe main source of near infrared emission. Combining this informationwith the far infrared IRAS data and a few additional criteria wedetermine the nature and evolutionary stage of all the sources observedso far, including those for which near infrared photometry waspreviously reported in Papers I and II. Among the unidentified IRASsources in our sample we find only a small percentage of planetarynebulae, many of them very young and dusty, showing peculiar nearinfrared colours. Most of the new objects observed in the near infraredare identified as transition objects in the previous stages of thestellar evolution. Among them, we find heavily obscured late-AGB stars,early post-AGB stars still obscured by thick circumstellar envelopeswhich are probably the true progenitors of planetary nebulae, and asignificant fraction of stars with bright optical counterparts showinglittle or no near infrared excess, which we associate with highlyevolved post-AGB stars with low mass progenitors, which may never becomeplanetary nebulae. In addition, we also find a small percentage of youngstellar objects, as well as a few Seyfert galaxies. We conclude that, inmost cases, based on near infrared data alone, it is not possible togive a confident classification of the unidentified IRAS source.However, the near infrared is shown to be a powerful tool, speciallywhen dealing with objects which are heavily obscured in the optical. Inthis case, the detection of the near infrared counterpart is the onlyway in which we can extend the study of these sources to other spectralranges and may be crucial to understand the short-lived phase whichprecedes the formation of a new planetary nebula. Based on observationscollected at the European Southern Observatory, La Silla (Chile) and atthe Spanish Observatorio del Teide, Tenerife, Spain. Table 6 is onlyavailable electronically at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| High-resolution spectroscopy of Vega-like stars - II. Age indicators, activity and circumstellar gas
We have completed a high-resolution optical study of 14 stars classifiedas Vega-like, having an IR excess attributable to dust emission. Surfacelithium abundances were measured for the four G- and K-type stars of thesample, to test the suggestion that these Vega-like stars areintermediate in evolutionary state between pre-main-sequence objects andestablished main-sequence stars. Abundances ranged from a very highvalue in the G5e star HD 143006, implying a very low age of only 1Myr,to below the limit of measurement for the K2V star HD 23362, which weconclude to be already well established on the main sequence. Theemission-line characteristics of all the stars in our sample werestudied to compare with those seen in the classical pre-main-sequenceHerbig Ae/Be (HAeBe) stars and T Tauri stars. Activity levels rangedfrom stars showing little or no activity, such as HD 23362, to thoseexhibiting extensive activity, such as the A9/F0Ve star HD 144432, whichshowed distinctive P Cyg profiles in its spectrum, and HD 143006, whichis young enough to be included in the T Tauri class of stars. The A2/3estar HD 35187 shows evidence of variability in its Hα and Heilambda5876 lines, with four other A-type stars in our sample alsoshowing evidence of Hei lambda5876 activity in the form of emission orabsorption. We interpret the excess absorption and/or emission in thelambda5876 line as providing direct evidence for ongoing accretionactivity on these systems. We find that the emission characteristics ofthe Hα, Nai D, Hei and Caii K lines are not significantly differentfrom those of HAeBe stars and T Tauri stars. Some of our sample havealso been previously classified as pre-main-sequence or candidatepre-main-sequence stars, which would seem to suggest that there is nodistinct boundary between Vega-like stars and the HAeBe and T Tauristars. The surface gravities of the A- and F-type stars in our sampleimply that they have already reached the main sequence, consistent withthe short time-scales to reach the main sequence predicted for stars oftheir mass and the fact that they are not located close to starformation regions. On the other hand, the ages derived for the threeemission-line G- and K-type stars in our sample imply that they haveprobably not yet reached the main sequence. It is likely that theseemission-line Vega-like stars represent the intermediate stage betweenclassical pre-main-sequence stars with `dusty' IR excesses and starsthat are well established on the main sequence. We also studied oursample for evidence of optical circumstellar gas absorption features. Ofthe 14 stars, seven show evidence for narrow absorption lines in theirspectra. Most of these appear to be of interstellar origin. One of thesestars, HD 144432, has a narrow absorption component in the absorptiontrough of its Nai D P Cyg profile, at a heliocentric velocity of-91kms^-1, which excludes an interstellar origin and therefore morelikely originates in its circumstellar environment. We also detectnarrow absorption lines in the spectrum of HD 158643 (51 Oph) arisingfrom excited-state Feii lines, which can only come from thecircumstellar environment of the star.
| Optical, infrared and millimetre-wave properties of Vega-like systems - III. Models with thermally spiking grains
Vega-like stars are main-sequence stars that exhibit excess IR emissiondue to circumstellar dust grains which are probably distributed indiscs. We have recently published an observational data base for a largesample of candidate Vega-like systems, comprising optical, near-IR andmm/submm-wave photometry, and mid-IR spectra. In a previous paper wepresented radiative transfer models of eight sources from our samplethat had low fractional excess luminosities. Here we present models of afurther eight sources, all with large fractional excess luminositiesdominated by excess emission at near-IR wavelengths. It was found thatno single distribution of dust grains at thermal equilibrium in a disccould simultaneously match the excess emission at near-IR and longerwavelengths. We attempted to model the near-IR emission as due tothermally spiking small grains, which can temporarily attain the hightemperatures required to produce excess near-IR emission. A near-IRspectrum of SAO 186777 shows the 3.3-μm UIR emission band, confirmingour earlier detection of UIR emission at longer wavelengths, andsuggesting that small carbonaceous particles are responsible for some ofthe near-IR emission. The thermally spiking models were only partiallysuccessful and many of the sources required the presence of grainsemitting in thermal equilibrium at ~ 1000-1500 K. These grains musteither be located very close to the stars (<1 au), or else be poweredby accretion luminosity. Calculations of the optical depths of the modeldiscs suggest the discs are optically thick at visual wavelengths;optically thick modelling of these sources is desirable. The discs areoptically thin at mm wavelengths, allowing us to confirm the presence oflarge grains in the discs. The stars presented in this paper may well beyounger than the prototype Vega-like stars.
| High-resolution spectroscopy of Vega-like stars - I. Effective temperatures, gravities and photospheric abundances
Vega-like stars are young main-sequence stars exhibiting an excessemission of infrared radiation. Modelling this excess depends not onlyon the parameters assigned to the grains, but on those assigned to thestars themselves. In an effort to update and improve the informationavailable on this class of star, we have analysed 13 stars classed asVega-like, having an infrared excess attributable to dust emission,along with two spectral standards which have also been found to showexcess emission from dust. In this, the first of two papers, we derivestellar properties (spectral type, effective temperature and log g) andphotospheric abundances. The spectral types derived revealed that one ofthe sample was a luminosity class III giant, ruling it out of theVega-like class, and two others underwent a significantreclassification. The remainder had their type confirmed. All but twoprogramme stars have been found to be emission-line stars - theiremission-line properties are discussed in Paper II. Attention hasrecently been drawn to the possible link between Vega-like stars and thephotospheric metal-depleted class of A-type stars, the lambda Bootisstars. These latter stars are hypothesized to have obtained theirunderabundances by the accretion of depleted circumstellar gas on to thephotosphere of the star. Since Vega-like stars are expected to havediscs of dust, it might be expected that accretion may cause this samephenomenon. We have analysed four A-type stars in our sample and twoA-type standards, deriving photospheric abundances for up to 10elements. No pattern of underabundance similar to lambda Bootis starswas found, although a depletion of silicon was found in two stars (up to0.86 dex below solar) and of magnesium in one star (0.56 dex lower). Thedepletion could be attributable to the accretion of those elements on tograins in the circumstellar environment of these stars.
| The β Pictoris phenomenon among Herbig Ae/Be stars. UV and optical high dispersion spectra.
We present a survey of high dispersion UV and optical spectra of HerbigAe/Be (HAeBe) and related stars. We find accreting, circumstellar gasover the velocity range +100 to +400km/s, and absorption profilessimilar to those seen toward β Pic, in 36% of the 33 HAeBe starswith IUE data as well as in 3 non-emission B stars. We also findevidence of accretion in 7 HAeBe stars with optical data only. Lineprofile variability appears ubiquitous. As a group, the stars withaccreting gas signatures have higher vsini than the stars withoutflowing material, and tend to exhibit large amplitude (>=1^m^)optical light variations. All of the program stars with polarimetricvariations that are anti-correlated with the optical light, previouslyinterpreted as the signature of a dust disk viewed close to equator-on,also show spectral signatures of accreting gas. These data imply thataccretion activity in HAeBe stars is preferentially observed when theline of sight transits the circumstellar dust disk. Our data imply thatthe spectroscopic signatures of accreting circumstellar material seen inβ Pic are not unique to that object, but instead are consistentwith interpretation of β Pic as a comparatively young A star withits associated circumstellar disk.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Taurus |
| Ascensió Recta: | 05h24m01.16s |
| Declinació: | +24°57'37.0" |
| Magnitud Aparent: | 8.515 |
| Distancia: | 150.376 parsecs |
| Moviment propi RA: | 1.2 |
| Moviment propi Dec: | -26.8 |
| B-T magnitude: | 8.807 |
| V-T magnitude: | 8.54 |
Catàlegs i designacions:
|
