Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Polarimetry of Li-rich giants
Context: .Protoplanetary nebulae typically present non-sphericalenvelopes. The origin of such geometry is still controversial. There areindications that it may be carried over from an earlier phase of stellarevolution, such as the AGB phase. But how early in the star's evolutiondoes the non-spherical envelope appear? Aims.Li-rich giants show dustycircumstellar envelopes that can help answer that question. We study asample of fourteen Li-rich giants using optical polarimetry in order todetect non-spherical envelopes around them. Methods.We used the IAGPOLimaging polarimeter to obtain optical linear polarization measurementsin {V} band. Foreground polarization was estimated using the field starsin each CCD frame.Results.After foreground polarization was removed,seven objects presented low intrinsic polarization (0.19-0.34)% and two(V859 Aql and GCSS 557) showedhigh intrinsic polarization values (0.87-1.16)%. This intrinsicpolarization suggests that Li-rich giants present a non-sphericaldistribution of circumstellar dust. The intrinsic polarization level isprobably related to the viewing angle of the envelope, with higherlevels indicating objects viewed closer to edge-on. The correlation ofthe observed polarization with optical color excess gives additionalsupport to the circumstellar origin of the intrinsic polarization inLi-rich giants. The intrinsic polarization correlates even better withthe IRAS 25 μ m far infrared emission. Analysis of spectral energydistributions for the sample show dust temperatures for the envelopesthat tend to be between 190 and 260 K. We suggest that dust scatteringis indeed responsible for the optical intrinsic polarization in Li-richgiants.Conclusions.Our findings indicate that non-spherical envelopesmay appear as early as the red giant phase of stellar evolution.
| Polarization Measurements of Post-Asymptotic Giant Branch Candidates and Related Stars
We have obtained UBVRI polarization measurements of 26 post-asymptoticgiant branch (post-AGB) candidates and related stars. The extremelymetal-poor post-AGB star HR 4049 has been observed several times. Inmost cases we find the objects to be intrinsically polarized. Thepolarization measurements presented in this paper indicate asymmetriccircumstellar dust shells and disks around these stars. For some objectsthe steep percent polarization λ-dependence and large degree ofpolarization suggest that scattering by circumstellar dust grains may beresponsible for the observed polarizations in the blue.
| Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features
A study of all full-scan spectra of optically thin oxygen-richcircumstellar dust shells in the database produced by the ShortWavelength Spectrometer on ISO reveals that the strength of severalinfrared spectral features correlates with the strength of the 13 μmdust feature. These correlated features include dust features at 19.8and 28.1 μm and the bands produced by warm carbon dioxide molecules(the strongest of which are at 13.9, 15.0, and 16.2 μm). The databasedoes not provide any evidence for a correlation of the 13 μm featurewith a dust feature at 32 μm, and it is more likely that a weakemission feature at 16.8 μm arises from carbon dioxide gas ratherthan dust. The correlated dust features at 13, 20, and 28 μm tend tobe stronger with respect to the total dust emission in semiregular andirregular variables associated with the asymptotic giant branch than inMira variables or supergiants. This family of dust features also tendsto be stronger in systems with lower infrared excesses and thus lowermass-loss rates. We hypothesize that the dust features arise fromcrystalline forms of alumina (13 μm) and silicates (20 and 28 μm).Based on observations with the ISO, a European Space Agency (ESA)project with instruments funded by ESA member states (especially thePrincipal Investigator countries: France, Germany, the Netherlands, andthe United Kingdom) and with the participation of the Institute of Spaceand Astronautical Science (ISAS) and the National Aeronautics and SpaceAdministration (NASA).
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| On the Variability of K5-M Stars
I investigate the Hipparcos Satellite photometry of K5-M stars to seethe pattern of activity of these stars. A few stars for which furtherstudy is desirable are identified.
| Detailed analysis of a sample of Li-rich giants
A detailed analysis has been carried out for a sample of 16 red giantsshowing a strong Li I 670.8 nm line. Ten of them were detected in asurvey by Castilho et al. (1998), and the other 6 stars are Li-richgiants selected from the literature. Element abundances in the sampleLi-rich giants are similar to those in normal red giants, differing onlyby their high Li abundance and infrared excess. This suggests thatLi-rich giants may correspond to a phase of stellar evolution of normalred giants, when Li is produced and transported to the atmosphere.
| Hipparcos: The Stars
Not Available
| Circumstellar shells of the mass-losing asymptotic giant branch stars: limits for the dust-driven winds.
Not Available
| The circumstellar molecular envelope of HD 101584
CO radio line observations reveal a molecular gas envelope around thepeculiar star HD101584 with characteristics very similar to those ofwellknown young post-AGB objects. We estimate that there is at least 0.1M_sun of molecular gas, very likely remnant gas from a formerAGB-envelope. This gas has been efficiently accelerated to very highvelocities (>50 {km s(-1) , and a significant fraction to >100 {kms(-1) ). There is evidence for an expanding disk-like structure seenclose to edge-on, and a high-velocity bipolar outflow. In the latter theexpansion velocity increases linearly with distance from the star,suggesting either a brief period of ejection or a fast wind interactingwith a slower wind. A significant fraction of the high-velocity gas hasreached a welldefined terminal velocity. Momentum well above theavailable radiation momentum has been transferred to the gas. There arealso indications of high-density, low-velocity molecular gas, possiblyin a disk close to the star. The (12) CO/(13) CO-ratio in the envelopeis uncertain, but probably quite low ( ~ 10). The systemic heliocentricvelocity is 50+/-2 {km s(-1) .
| SiO masers in OH/IR stars, proto-planetary and planetary nebulae
We present a search for SiO masers towards a sample of 126 objectsincluding OH/IR stars, proto-planetary and planetary nebulae. Allobjects are classified as oxygen-rich, and most of them are associatedwith OH or H_2O masers. SiO masers were found only in variable objectslike the OH/IR stars and a few objects classified as proto-planetarynebulae, but with variable central stars that may be part of binarysystems. In one object, OH 15.7+0.8, which appears to be varyingirregularly and most likely recently left the AGB, an SiO maser wastentatively detected. Thus, we conclude that variability and SiO maseremission are closely linked, and that SiO masers disappear very soonafter a star has reached the end of the AGB, when pulsation and massloss cease. Tables 1 and 2 are available at the CDS via anonymous ftp130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html and Fig. 1can be found in the on-line version of A\&AS viahttp://www.ed-phys.fr
| Li-rich giants: A survey based on IRAS colours
In a previous work we studied the IRAS colours of known Li-rich redgiants and showed that they have flux ratiosF12/F25 and F25/F60 in welldefined ranges. By using this result as a selection criterion, weprepared a list of 280 IRAS Point Source candidates to be Li-rich giantstars. Up to the present we have obtained spectra for 57% of our targetlist. We identified five stars showing a strong LiI 670.079 nm line andsix ones with a Li line of medium strength. Most of the candidates showfeatures typical of normal giants having circumstellar dust, asindicated by their IRAS colours. Observations collected at theLaboratorio Nacional de Astrofisica - LNA, Brazil; Observatoire de HauteProvence - OHP, France; European Southern Observatory - ESO, Chile.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| On a Rapid Lithium Enrichment and Depletion of K Giant Stars
A model scenario has recently been introduced by de la Reza andcolleagues to explain the presence of very strong Li lines in thespectra of some low-mass K giant stars. In this scenario all ordinary,Li-poor, K giants become Li rich during a short time (~105 yr) whencompared to the red giant phase of 5 x 107 yr. In this "Li period," alarge number of the stars are associated with an expanding thincircumstellar shell supposedly triggered by an abrupt internal mixingmechanism resulting in a surface new 7Li enrichment. This Letterpresents nearly 40 Li-rich K giants known up to now. The distribution ofthese Li-rich giants, along with 41 other observed K giants that haveshells but are not Li rich, in a color-color IRAS diagram confirms thisscenario, which indicates, also as a new result, that a rapid Lidepletion takes place on a timescale of between ~103 and 105 yr. Thismodel explains the problem of the presence of K giants with far-infraredexcesses presented by Zuckerman and colleagues. Other present and futuretests of this scenario are briefly discussed.
| Lithium Enrichment--Mass-Loss Connection in K Giant Stars
Based on observed far-infrared properties of K giant stars, we propose ascenario linking the high Li abundances of some of these stars to theevolution of circumstellar shells. In this model, every K giant withmasses between 1.0 and 2.5 Mȯ become Li rich during the red giantbranch stage, and the internal mechanism responsible for the Lienrichment will initiate a prompt mass-loss event. The evolutionarypaths of the detached shells are compatible with observations for lowexpansion velocities of the order of 2 km s-1 and mass loss of (2--5) x10-8 Mȯ yr-1. This modest mass loss is, however, 2 orders ofmagnitude larger than those of normal, Li-poor K giants. A "Li time" ofthe order of 80,000 yr or somewhat larger is possible. This Li phase is,nevertheless, not related to the 12C/13C ratio which appears to evolve,for these low-mass stars, over a much longer time. This model requires arapid internal process of Li enrichment and depletion. New argumentsappearing in the literature concerning this internal process arediscussed. Reference is also made to the importance of these stars tothe Galactic Li evolution.
| The far-infrared colors of carbon stars revisited.
In view of recent developments in the theory of carbon star evolution ithas become necessary to re-examine the source of the excess 100μmflux associated with carbon stars. We demonstrate that the observed farinfrared colors of carbon stars are not due to cirrus contamination andthat a geometrically thin detached shell provides insufficient far-IRflux to match the observations. We also show that more recent models areconsistent with a geometrically thick detached shell.
| The Li K giant stars
Not Available
| A search for Li-rich giants
Not Available
| Analysis of the moderately Li-rich giant HD 146850.
We report the identification of 3 Li-rich giants discovered in anongoing survey based on IRAS colours. A detailed analysis of one ofthese, HD 146850, is presented here. High-resolution spectra wereobtained for this star, using CASPEC at the 3.6m telescope of ESO. Wederived the stellar parameters (T_eff_, log g, [M/H])=4000, 1.5, -0.3. Alithium abundance of ɛ(Li)=1.6 is found, whereas [C/Fe]~0.0 and[N/Fe]~-0.1 indicating that no convective mixing has occurred. Theabundances of oxygen [O/Fe]=~+0.1 and α-elements[Mg/Fe]=[Ti/Fe]=[Ca/Fe]=~0.0 suggest that we are dealing with a diskstar, for which the radial velocity v_r_=42km/s is also moderate.
| Nebulosity around the S-Star Rz-Sagittarii
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.270L..15W&db_key=AST
| 86 GHz SiO, v=1, J=2--> 1 survey of southern IRAS point sources. II. Detection of 74 new maser sources
The detection of 74 new southern stellar SiO, v=1, J=2->1 masersassociated with IRAS point sources is reported. 57 of these were foundin an IRAS based survey of oxygen rich stellar envelopes. The detectionrate in this survey was 45%. 17 further new maser sources were detectedduring a search for strong pointing sources for the Swedish-ESOSubmillimeter Telescope (SEST). The distribution of the IRAS lowresolution spectral (LRS) classes of all the SiO masers (this paper andliterature) peaks sharply at class 15. The distribution of the LRSclasses of the maser sources from 21 to 29 is similar to thedistribution of these classes of all the point sources in the pointsource catalogue except for class 25. Only few maser sources of thisspectral class are known. This is possibly explained by a selectioneffect.
| An infrared study of the remarkable dusty M star HR 3126
HR 3126 is a unique M giant star embedded in the bipolar reflectionnebula IC 2220. In this paper we present new photometric andspectroscopic observations of HR 3126 and combine them with existingdata from the literature in an extensive reappraisal of the star'sproperties at infrared wavelengths. The spectral energy distribution isconsistent with an optical classification of M0-3 II with infraredexcess. The dust shell cannot be fitted by a single-temperatureblackbody: at least three components are required, with temperatures inthe range 35-1300 K. On the basis of spectroscopy at 1-4 and 7-25microns, combined with an assessment of various color-color diagrams, weare able to reject the hypothesis that HR 3126 is carbon-rich. Weaksilicate emission features are detected at 10 and 19 microns, and aprevious report of silicon carbide emission at 11.2 microns is notsubstantiated. It seems likely that HR 2136 is in a phase of advancedand rapid postmain-sequence evolution, possibly beginning its ascent ofthe asymptotic giant branch.
| CO and HCN observations of circumstellar envelopes. A catalogue - Mass loss rates and distributions
We have searched the literature for all observations of the (C-12)O(1-0), (C-12)O (2-1), and HCN (1-0) lines in circumstellar envelopes oflate type stars published between January 1985 and September 1992. Wereport data for 1361 observations (stellar velocity, expansion velocity,peak intensity, integrated area, noise level). This CO-HCN sample nowcontains 444 sources; 184 are identified as O-rich, 205 as C-rich, andthere are 9 S stars. About 85 percent of the sources are AGB stars.There are 32 planetary nebulae and about thirty post-AGB starscandidates. Besides results of mm-observations, we also listidentifications, coordinates, IRAS data, and chemical and spectral typesfor every source. For AGB stars, we have estimated (or compiled)bolometric fluxes and distances for 349 sources, and mass loss ratesdeduced from CO results for 324 sources, taking into account theinfluence of the CO photodissociation radius. We also list mass lossrates derived from detailed models of CO emission in the literature.
| Asymptotic giant branch stars near the sun
Available red and near-infrared photometry and apparent motions of M, S,and C asymptotic giant branch (AGB) stars in the Bright Star Catalogueare tabulated and discussed. It is shown that the red and near infraredindices normally used for late-type stars are interchangeable except forcarbon stars. The M-type giants are variable with visual amplitudegreater than 0.05 mag. The reddening-free parameter m2 from Genevaphotometry is essentially a temperature parameter for M giants, whilethe reddening-free parameter d is a sensitive detector of blue stellarcompanions. The space density of AGB stars near the sun decreases by afactor of 35 in a temperature range 3800 to 3400 K. Two of the S starsnear the sun were found to have nearly equal space motions and may becomembers of the Arcturus group.
| Near-infrared photometry of a sample of IRAS point sources
This paper presents the J, H, K, L, M photometry of 516 sourcespertaining to a sample of 787 sources which has been extracted from theIRAS Point Source Catalog in order to study the late stages of stellarevolution and the concomitant phenomena of mass loss. Three differentclssifications of these sources based on broad-band photometry and IRASlow-resolution spectra are given, and the distributions of the sourcesin terms of these classifications are presented. A subsample of peculiarsources, believed to have recently undergone a helium flash, has beenisolated using the K - L, (12-micron) color diagram. Some objectsmeriting further study are also mentioned.
| A survey of circumstellar CO emission from a sample of IRAS point sources
The first results from a survey of circumstellar CO(1-0) emission arepresented. The sources were selected from the IRAS point source catalogaccording to the IRAS color criteria described in van der Veen andHabing (1988). The sources have good quality fluxes at 12, 25, and 60microns, flux densities larger than 20 Jy at 25 microns, and aresituated more than 5 deg away from the Galactic plane. The survey isundertaken to study the relationship between mass loss rates, dustproperties, and the evolution along the AGB. The sample consists of 787sources and contains both oxygen and carbon-rich stars, including Miravariables, OH/IR objects, protoplanetary nebulae, planetary nebulae, and60-micron excess sources. So far, 519 objects, situated on both thenorthern and the southern sky, have been observed; 163 sources werefound to have circumstellar CO emission, and in 58 of these CO emissionhas not previously been detected.
| CCD surface photometry of IC 2220
BVRI color-difference measurements are reported for the unusual bipolarreflection nebula IC 2220 which surrounds an early-M giant. The datawere obtained through BVRI imaging with a CCD detector. Analysis of thecolor difference between the nebula and the illuminating star HR 3126indicates that the nebular dust, having formed in the outflow from thestar, is similar to average interstellar dust. No evidence is found forthe presence of extended red emission in this nebula, although there isan indication of nonequilibrium continuum emission. The Milky Wayprovides a significant amount of external illumination to IC 2220 in Band U.
| First giant branch and asymptotic giant branch stars in nearby aggregates
The properties of the brightest red stars in several aggregates in theGalaxy are compared with theoretical models. 22 asymptotic giant branch(AGB) stars are identified, four of which are in the thermally pulsingAGB (TPAGB) phase and four of which are TPAGB carbon stars. Also, fourcases of RGB stars are identified which have accreted substantial massfrom the carbon-rich TPAGB precursor of a current white dwarf companion.There is general agreement between the observed and theoretical slopesof the RGB and early AGB branches, and quantitative differences betweenthe positioning of observed sequences can be understood in terms ofdifferences in metallicity and mass predicted by the theory.
| Observations of the nebulosities near SN 1987A
Direct images of SN 1987A taken during periods of good seeing at LaSilla Observatory are combined with spectra to map the velocity andspatial structure of the dust and gas that surround SN 1987A. The datafrom December 1989 show that the supernova is embedded in a filamentarynebula of mixed gases and dust that is morphologically and kinematicallysimilar to planetary nebulae. The narrowness of the filaments, theirshape, their radial velocities, and the lack of detectable expansion inthe plane of the sky imply that this is a physical structure, not justthe SN 1987A light curve seen reflected from a continuous sheet ofbackground material. This inner, highly structured nebula is immersed inan outer, larger nebula which may be due to reflected light from SK -69deg 202, the progenitor of SN 1987A. Alternatively, if it is due toreflected light from SN 1987A, it must lie in front of the supernova.
|
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: | Carina |
| Ascensió Recta: | 07h56m50.70s |
| Declinació: | -59°07'35.0" |
| Magnitud Aparent: | 6.25 |
| Distancia: | 296.736 parsecs |
| Moviment propi RA: | -3.5 |
| Moviment propi Dec: | 10.7 |
| B-T magnitude: | 9.659 |
| V-T magnitude: | 7.295 |
Catàlegs i designacions:
|
INHABITED SKY
