תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
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.
| On the nature of lithium-rich giant stars. Constraints from beryllium abundances
We have derived beryllium abundances for 7 Li-rich giant (A(Li) >1.5) stars and 10 other Li-normal giants with the aim of investigatingthe origin of the lithium in the Li-rich giants. In particular, we testthe predictions of the engulfment scenario proposed by Siess & Livio(1999, MNRAS, 308, 1133), where the engulfment of a brown dwarf or oneor more giant planets would lead to simultaneous enrichment of7Li and 9Be. We show that regardless of theirnature, none of the stars studied in this paper were found to havedetectable beryllium. Using simple dilution arguments we show thatengulfment of an external object as the sole source of Li enrichment isruled out by the Li and Be abundance data. The present results favor theidea that Li has been produced in the interior of the stars by aCameron-Fowler process and brought up to the surface by an extra mixingmechanism.
| Catalogue of [Fe/H] determinations for FGK stars: 2001 edition
The catalogue presented here is a compilation of published atmosphericparameters (Teff, log g, [Fe/H]) obtained from highresolution, high signal-to-noise spectroscopic observations. This newedition has changed compared to the five previous versions. It is nowrestricted to intermediate and low mass stars (F, G and K stars). Itcontains 6354 determinations of (Teff, log g, [Fe/H]) for3356 stars, including 909 stars in 79 stellar systems. The literature iscomplete between January 1980 and December 2000 and includes 378references. The catalogue is made up of two tables, one for field starsand one for stars in galactic associations, open and globular clustersand external galaxies. The catalogue is distributed through the CDSdatabase. Access to the catalogue with cross-identification to othersets of data is also possible with VizieR (Ochsenbein et al.\cite{och00}). The catalogue (Tables 1 and 2) is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/373/159 and VizieRhttp://vizier.u-strasbg.fr/.
| 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.
| 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.
| 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.
| High resolution spectra of Li-rich giants
We present high resolution spectra for 5 Lithium rich red giantsdiscovered in an ongoing survey based on IRAS colours. Other 6 starsshowing medium strength of the Li line are also presented here. Highresolution one order spectra for the program stars were obtained at theLaboratorio Nacional de Astrofisica (LNA) and Observatoire de HauteProvence (OHP), and the echelle spectrum of HD 146850 was obtained atthe European Southern Observatory (ESO). The 11 one order spectra andthe 40 orders of the echelle spectra were calibrated in wavelength andnormalized in relative intensity.
| 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.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| 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.
| The Li K giant stars
Not Available
| A search for Li-rich giants
Not Available
| Courbes de lumière visuelle et photographique et indice de couleur de Nova E U Scuti 1949
Not Available
| H 1175. Nova Aquilae, no. 2. 185604.
Not Available
| Nova Aquilae of 1905.
Not Available
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | נשר |
| התרוממות ימנית: | 19h01m09.50s |
| סירוב: | -04°26'20.3" |
| גודל גלוי: | 6.906 |
| מרחק: | 235.294 פארסק |
| תנועה נכונה: | -5.7 |
| תנועה נכונה: | 9.2 |
| B-T magnitude: | 9.13 |
| V-T magnitude: | 7.09 |
קטלוגים וכינוים:
|
