Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
The Tennessee State University Automatic Spectroscopic Telescope: Data Processing and Velocity Variation of Cool Giants
This paper discusses data reduction for an echelle spectrograph we havedeveloped for an automatic telescope at Tennessee State University andare using to monitor radial velocities and line profiles of cool giantand supergiant stars. Although our approach to data reduction is ratherconventional, we discuss flat-fielding and extraction of velocities inways that should be of general interest, establish a transformation tothe IAU radial velocity system (+0.35 +/- 0.09 km s-1), anddetermine the external precision for measured velocities (0.10-0.11 kms-1). Also, we present results of the first 2-3 years ofmonitoring radial velocities in about 120 cool giants and compare thoseresults with the level of variability found with photometry. These newdata confirm the widely held understanding that K and M giants are allradial velocity variables at the level of 0.1 km s-1.
| Very Large Excesses of 18O in Hydrogen-deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers
We have found that at least seven hydrogen-deficient carbon (HdC) and RCoronae Borealis (RCB) stars, have 16O/18O ratiosclose to and in some cases less than unity, values that are orders ofmagnitude lower than measured in other stars (the solar value is 500).Greatly enhanced 18O is evident in every HdC and RCB we havemeasured that is cool enough to have detectable CO bands. The three HdCstars measured have 16O/18O<1, lower valuesthan any of the RCB stars. These discoveries are important clues indetermining the evolutionary pathways of HdC and RCB stars, for whichtwo models have been proposed: the double degenerate (white dwarf [WD]merger) and the final helium-shell flash (FF). No overproduction of18O is expected in the FF scenario. We have quantitativelyexplored the idea that HdC and RCB stars originate in the mergers of CO-and He-WDs. The merger process is estimated to take only a few days,with accretion rates of 150 Msolar yr-1 producingtemperatures at the base of the accreted envelope of(1.2-1.9)×108 K. Analysis of a simplified one-zonecalculation shows that nucleosynthesis in the dynamically accretingmaterial may provide a suitable environment for a significant productionof 18O, leading to very low values of16O/18O, similar to those observed. We also findqualitative agreement with observed values of 12C/13C and with the CNO elemental ratios. H-admixture during theaccretion process from the small H-rich C/O WD envelope may play animportant role in producing the observed abundances. Overall, ouranalysis shows that WD mergers may very well be the progenitors ofO18-rich RCB and HdC stars, and that more detailedsimulations and modeling are justified.
| Low-resolution spectroscopy of high Galactic latitude objects: A search for CH stars
Properties of CH stars like iron deficiency and enrichment of carbon andheavy elements can provide valuable inputs to our understanding ofnucleosynthesis. In particular, these parameters provide strongobservational constraints for theoretical studies of nucleosynthesis ofheavy elements at low-metallicity. Accurate identification andspectroscopic characterization of CH stars are therefore very essential.We have undertaken a programme with a prime objective to search forthese objects in a mixed sample of carbon stars taken from HamburggESO survey. The spectra of the objects were obtained using OMR at VBO,Kavalur and HFOSC at HCT, IAO, Hanle, during 2005 and 2006. Here, wereport a detection of twenty-one CH stars from a sample of sixtyobjects based on low-resolution spectral analysis. Estimated effectivetemperatures, ^{12}Cg^{13}C isotopic ratios, and their location in thetwo colour J-H vs H-K plot support their identification with the classof CH stars. Detection of these potential CH star candidates and theirspectral description is the main theme of this paper.
| A Cool R Coronae Borealis Star Z UMi
High resolution spectra of a R CrB type star Z UMi are analyzed. Theatmospheric parameters of Z UMi are estimated -- Teff=5250±250 K and log; g=0.5±0.3. This places Z UMi amongthe coolest R CrB stars. The hydrogen deficiency of Z UMi is confirmed.The abundances of other elements resemble those found for the minoritygroup of R CrB stars. We note very low iron abundance, [Fe/H] = --1.85,and an excess of lithium, [Li/Fe]=+1.9.
| Search for C2- in Diffuse Clouds
A search has been carried out for the C2- ion indiffuse clouds toward HD 23180, HD 24912, HD 24398, HD 46711, and HD50064 using the HIDES spectrometer on the Okayama 188-cm telescope. Anupper limit of 8.3 × 1010, 1.8 × 1012cm-2 was obtained for the C2- columndensity. The upper limit value (3.8 × 1011cm-2) toward HD 23180 is two orders of magnitude smaller thanthat of the C2 radical. Possible production mechanisms forC2- are discussed.
| Detection of Near-Infrared CO Absorption Bands in R Coronae Borealis Stars
R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich,pulsating, post-asymptotic giant branch stars that experience massiveirregular declines in brightness caused by circumstellar dust formation.The mechanism of dust formation around RCB stars is not well understood.It has been proposed that CO molecules play an important role in coolingthe circumstellar gas so that dust may form. We report on a survey forCO in a sample of RCB stars. We obtained H- and K-band spectra includingthe first- and second-overtone CO bands for eight RCB stars, theRCB-like star DY Per, and the final-helium-flash star FG Sge. The first-and second-overtone CO bands were detected in the cooler(Teff<6000 K) RCB stars, Z Umi, ES Aql, SV Sge, and DYPer. The bands are not present in the warmer (Teff>6000 K)RCB stars, R CrB, RY Sgr, SU Tau, and XX Cam. In addition,first-overtone bands are seen in FG Sge, a final-helium-flash star thatis in an RCB-like phase at present. Effective temperatures of the eightRCB stars range from 4000 to 7250 K. The observed photospheric COabsorption bands were compared to line-blanketed model spectra of RCBstars. As predicted by the models, the CO bands are strongest in thecoolest RCB stars and not present in the warmest. No correlation wasfound between the presence or strength of the CO bands and dustformation activity in the stars.
| CH stars at high Galactic latitudes
Carbon-rich stars of Population II, such as CH stars, can provide directinformation on the role of low- to intermediate-mass stars of the haloin early Galactic evolution. Thus accurate knowledge of the CH stellarpopulation is a critical requirement for building up scenarios for earlyGalactic chemical evolution. In the present work, we report on severalCH stars identified in a sample of faint high-latitude carbon stars fromthe Hamburg survey and discuss their medium-resolution spectra coveringthe wavelength range 4000-6800 Å. Estimation of the depths of the(1,0)12C12C λ4737 and(1,0)12C13C λ4744 bands in these starsindicates an isotopic ratio 12C/13C ~ 3, apartfrom a few exceptions; these ratios are consistent with existingtheories of CH stellar evolution. The stars of the Hamburg survey, atotal of 403 objects, were reported to be carbon star candidates withstrong C2 and CN molecular bands. In the first phase ofobservation, we acquired spectra of 91 objects. Inspection of thespectra of those objects shows 51 objects with C2 molecularbands in their spectra, of which 13 stars have low flux below about 4300Å. There are 25 objects that show weak or moderate CH and CNbands, 12 objects that show weak but detectable CH bands, and threeobjects that do not show any molecular bands due to C2, CN orCH in their spectra. Objects with C2 molecular bands and withgood signals bluewards of 4300 Åthat show prominent CH bands intheir spectra are potential candidate CH stars. There were 35 suchcandidates found in the present sample of 91 objects observed so far.The set of CH stars identified could be the targets of subsequentobservation at high resolution for a detailed and comprehensive analysisto understand their role in early Galactic chemical evolution.
| An Extremely Large Excess of 18O in the Hydrogen-deficient Carbon Star HD 137613
We report the discovery of a uniquely large excess of 18O inthe hydrogen-deficient carbon (HdC) star HD 137613 based on a spectrumof the first-overtone bands of CO at 2.3-2.4 μm in which three strongabsorption bands of 12C18O are clearly present.Bands of 12C16O also are present, but no bands of13C16O or 12C17O are seen.We estimate an isotopic ratio 16O/18O <~ 1. Thesolar value of this ratio is ~500. Neither He-core burning nor He-shellflash burning can produce the isotopic ratios of oxygen and carbonobserved in HD 137613. However, a remarkable similarity exists betweenthe observed abundances and those found in the outer layers of the broadHe shell of early-asymptotic giant branch (AGB) stars, soon after theend of He-core burning. It is not known how the outer envelope down tothe He shell could be lost, but some mechanism of enhanced mass lossmust be involved. HD 137613 may be a post-early-AGB star with the outerlayers of the former He-burning shell as its photosphere. The unusualelemental abundances of the HdC stars resemble those of the R CoronaeBorealis (RCB) stars, but HdC stars do not produce clouds of dust thatproduce declines in brightness. None of the other RCB or HdC starsobserved show significant 18O.
| The Hydrogen-Deficient Carbon Star XX Cam
The high resolution and high S/N spectra of a nonvariablehydrogen-deficient carbon star XX Cam were analyzed and the chemicalcomposition derived. The atmospheric parameters of XX Cam areTeff=7250 K, log g= 0.8 and mean ξt=5 kms-1. The microturbulent velocity is variable with depth inthe star's atmosphere. The extreme hydrogen deficiency of XX Cam isconfirmed. No lithium is found. The abundances of other light elementsare close to the abundances in the majority group of R CrB stars. Theabundances of s-process elements are not enhanced.
| Suspected cool R Coronae Borealis stars in the Magellanic Clouds
Six stars out of a sample of ~2300 carbon stars in the Magellanic Cloudshave been identified as having strong C2 bands but CN bandsthat are very weak or absent. It is argued that five of these are likelyto be R Coronae Borealis (RCB) stars on the basis of their spectralcharacteristics and peculiar colours. Most are variables and the LargeMagellanic Cloud (LMC) members have extreme radial velocities that aremore like the planetary nebula population than the carbon stars. Thissample consists of four LMC members (only one of them previouslyrecognized as an RCB star), one Small Magellanic Cloud (SMC) member (thefirst RCB star reported in the SMC) and one foreground Galactic star.
| 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
| Carbon-rich giants in the HR diagram and their luminosity function
The luminosity function (LF) of nearly 300 Galactic carbon giants isderived. Adding BaII giants and various related objects, about 370objects are located in the RGB and AGB portions of the theoretical HRdiagram. As intermediate steps, (1) bolometric corrections arecalibrated against selected intrinsic color indices; (2) the diagram ofphotometric coefficients 1/2 vs. astrometric trueparallaxes varpi are interpreted in terms of ranges of photosphericradii for every photometric group; (3) coefficients CR andCL for bias-free evaluation of mean photospheric radii andmean luminosities are computed. The LF of Galactic carbon giantsexhibits two maxima corresponding to the HC-stars of the thick disk andto the CV-stars of the old thin disk respectively. It is discussed andcompared to those of carbon stars in the Magellanic Clouds and Galacticbulge. The HC-part is similar to the LF of the Galactic bulge,reinforcing the idea that the Bulge and the thick disk are part of thesame dynamical component. The CV-part looks similar to the LF of theLarge Magellanic Cloud (LMC), but the former is wider due to thesubstantial errors on HIPPARCOS parallaxes. The obtained meanluminosities increase with increasing radii and decreasing effectivetemperatures, along the HC-CV sequence of photometric groups, except forHC0, the earliest one. This trend illustrates the RGB- and AGB-tracks oflow- and intermediate-mass stars for a range in metallicities. From acomparison with theoretical tracks in the HR diagram, the initial massesMi range from about 0.8 to 4.0 Msun for carbongiants, with possibly larger masses for a few extreme objects. A largerange of metallicities is likely, from metal-poor HC-stars classified asCH stars on the grounds of their spectra (a spheroidal component), tonear-solar compositions of many CV-stars. Technetium-rich carbon giantsare brighter than the lower limit Mbol =~ -3.6+/- 0.4 andcentered at =~-4.7+0.6-0.9 at about =~(2935+/-200) K or CV3-CV4 in our classification. Much like the resultsof Van Eck et al. (\cite{vaneck98}) for S stars, this confirms theTDU-model of those TP-AGB stars. This is not the case of the HC-stars inthe thick disk, with >~ 3400 K and>~ -3.4. The faint HC1 and HC2-stars( =~ -1.1+0.7-1.0) arefound slightly brighter than the BaII giants ( =~-0.3+/-1.3) on average. Most RCB variables and HdC stars range fromMbol =~ -1 to -4 against -0.2 to -2.4 for those of the threepopulation II Cepheids in the sample. The former stars show the largestluminosities ( <~ -4 at the highest effectivetemperatures (6500-7500 K), close to the Mbol =~ -5 value forthe hot LMC RCB-stars (W Men and HV 5637). A full discussion of theresults is postponed to a companion paper on pulsation modes andpulsation masses of carbon-rich long period variables (LPVs; Paper IV,present issue). This research has made use of the Simbad databaseoperated at CDS, Strasbourg, France. Partially based on data from theESA HIPPARCOS astrometry satellite. Table 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/390/967
| Hydrogen-Deficient Cargon Star HM Lib
The high and medium resolution spectra of a nonvariablehydrogen-deficient carbon star (HdC) HM Lib were analyzed and thechemical composition derived. The atmospheric parameters of HM Lib areTeff = 6000+/-200 K, log g = 1.3+/-0.5 and the microturbulentvelocity xi_t = 6.8+/-1.0 km/s. The high hydrogen deficiency isconfirmed. The abundances of other elements are close to those found forthe majority of warmer and variable R CrB stars. The medium resolutionspectrum of another HdC star HD 182040 is compared to that of HM Lib. Inthe spectrum of HD 182040 the CN red system bands are weaker due to 0.3dex lower N abundance.
| Optical Spectroscopy of V4334 Sgr: 1996-2000
Due to a combination of rapid expansion and changes in chemicalcomposition, the optical spectrum of V4334 Sgr changed rapidly betweendiscovery in 1996 February and extinction during 1999. As one of themost rapidly evolving stars ever observed, optical spectroscopy mustanswer a range of important questions. The development of the opticalspectrum and its implications are discussed through a review ofpublished literature. A previously unpublished sequence of spectra fromnear discovery through 1996 demonstrates the thermal evolution of V4334Sgr, while detailed analysis of an unpublished echelle spectrum obtainedin 1996 May raises questions about the model atmospheres used to date.As a result, requirements for future models of the atmosphere of V4334Sgr are analysed. The future evolution of V4334 Sgr and the dispersionof its dusty cocoon are briefly discussed.
| The effective temperatures of carbon-rich stars
We evaluate effective temperatures of 390 carbon-rich stars. Theinterstellar extinction on their lines of sights was determined andcircumstellar contributions derived. The intrinsic (dereddened) spectralenergy distributions (SEDs) are classified into 14 photometric groups(HCi, CVj and SCV with i=0,5 and j=1,7). The newscale of effective temperatures proposed here is calibrated on the 54angular diameters (measured on 52 stars) available at present from lunaroccultations and interferometry. The brightness distribution on stellardiscs and its influence on diameter evaluations are discussed. Theeffective temperatures directly deduced from those diameters correlatewith the classification into photometric groups, despite the large errorbars on diameters. The main parameter of our photometric classificationis thus effective temperature. Our photometric < k right >1/2 coefficients are shown to be angular diameters on arelative scale for a given photometric group, (more precisely for agiven effective temperature). The angular diameters are consistent withthe photometric data previously shown to be consistent with the trueparallaxes from HIPPARCOS observations (Knapik, et al. \cite{knapik98},Sect. 6). Provisional effective temperatures, as constrained by asuccessful comparison of dereddened SEDs from observations to modelatmosphere predictions, are in good agreement with the values directlycalculated from the observed angular diameters and with those deducedfrom five selected intrinsic color indices. These three approaches wereused to calibrate a reference angular diameter Phi 0 and theassociated coefficient CT_eff. The effective temperatureproposed for each star is the arithmetic mean of two estimates, one(``bolometric'') from a reference integrated flux F0, theother (``spectral'') from calibrated color indices which arerepresentative of SED shapes. Effective temperatures for about 390carbon stars are provided on this new homogeneous scale, together withvalues for some stars classified with oxygen-type SEDs with a total of438 SEDs (410 stars) studied. Apparent bolometric magnitudes are given.Objects with strong infrared excesses and optically thick circumstellardust shells are discussed separately. The new effective temperaturescale is shown to be compatible and (statistically) consistent with thesample of direct values from the observed angular diameters. Theeffective temperatures are confirmed to be higher than the mean colortemperatures (from 140 to 440 K). They are in good agreement with thepublished estimates from the infrared flux method forTeff>= 3170 K, while an increasing discrepancy is observedtoward lower temperatures. As an illustration of the efficiency of thephotometric classification and effective temperature scale, the C/Oratios and the Merrill-Sanford (M-S) band intensities are investigated.It is shown that the maximum value, mean value and dispersion of C/Oincrease along the photometric CV-sequence, i.e. with decreasingeffective temperature. The M-S bands of SiC2 are shown tohave a transition from ``none'' to ``strong'' at Teff =~(2800+/- 150right ) K. Simultaneously, with decreasing effectivetemperature, the mean C/O ratio increases from 1.04 to 1.36, thetransition in SiC2 strength occurring while 1.07<= C/O<= 1.18. This research has made use of the Simbad database operatedat CDS, Strasbourg, France. Table 10 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)}or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/178
| General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third Edition
The catalog is an updated and revised version of Stephenson's Catalogueof Galactic Cool Carbon Stars (2nd edition). It includes 6891 entries.For each star the following information is given: equatorial (2000.0)and galactic coordinates, blue, visual and infrared magnitudes, spectralclassification, references, designations in the most significantcatalogs and coordinate precision classes. The main catalog issupplemented by remarks containing information for which there was noplace in entries of the main part, as well as some occasional notesabout the peculiarities of specific stars.
| Some Characteristics of the Circumstellar Shells of Stars with Variability of the R Coronae Borealis Type
The transformation of the line and molecular spectra of R CrB fromabsorption to emission at the 1985 minimum is traced from spectroscopicobservations, and it is concluded for the first time, from a comparisonwith light curves and color indices, that the rapid variation of thecolor index U - B by -0m.6 during fading is due exclusively to thistransformation. The transformation of the spectrum can result in anincrease in the star's brightness in the UBV photometric bands by about1m.4, 0m.75, and up to 0m.75, respectively. This removes one of the mainobjections to using the concept of a homogeneous spherical shell tointerpret the R Coronae Borealis phenomenon. High-resolutionspectroscopic observations of SV Sge, a star with variability of the RCoronae Borealis type, showed the presence of an intense circumstellarcomponent in Na I D lines with a violet shift corresponding to 240km/sec, which is unrelated to visible brightness minima. It is suggestedthat a stream of matter with such a velocity through the star's vicinityis the cause of the excitation not only of the broad emission featuresobserved during brightness minima upon fading of the photosphericemission but also of the narrow ones. Assuming that a brightness minimumis associated with the temporary formation of another sphericalscreening shell, some characteristics of the permanent shell aredetermined. Its optical depth and geometrical thickness are 0.2-0.7 andat least 0.4 of its radius, respectively.
| The Rise and Fall of V4334 Sagittarii (Sakurai's Object)
CCD UBVRi photometry of the final helium flash object V4334 Sgr(Sakurai's object) carried out during 1997-1999 is presented, and thelight curve from its prediscovery rise to the dust obscuration phase isconstructed. The optical light curve can be divided into four sections,the rise to maximum, the maximum, the dust onset, and the massive dustshell phase. The color indices show a general increase with time, firstbecause of the photospheric expansion and cooling and later because ofthe dust-forming events. The energy distributions for the years1996-1999 show that an increasing part of the energy is radiated atinfrared wavelengths. In 1996 the infrared excess is likely caused byfree-free radiation in the stellar wind. Starting from 1997 or 1998 atthe latest, carbon dust grains are responsible for the more and moredramatic decrease of optical radiation and the growing infrared excess.Its photometric behavior in 1998-1999 mimics the ``red declines'' of RCrB variables; the amplitude, however, is more extreme than any fadingever observed in an R CrB star. Evidence is given that a complete dustshell has formed around V4334 Sgr. It therefore shows similarities withdust-forming classical novae, although it is evolving ~20 times moreslowly. Its luminosity increased by a factor 4 between 1996 and 1998. Acomparison of timescales of the final helium flash objects FG Sge, V605Aql, and V4334 Sgr shows that the observed photometric and spectroscopicfeatures are similar, while V4334 Sgr is the most rapidly evolvingobject to date.
| The Fall and Rise of V854 Centauri: Long-Term Ultraviolet Spectroscopy of a Highly Active R Coronae Borealis Star
We examine long-term low-dispersion International Ultraviolet ExplorerSWP and LWP spectroscopy of the R Coronae Borealis (RCB) star V854 Cen,obtained across the deep (DeltaV>6 mag) 1991, 1992-1993, and 1994declines. We also report the optical light curve for the star in theinterval 1987-1998, including multicolor photometry obtained during1989-1998. The light curve includes at least eight major declines wherethe amplitude exceeds 5 mag, many of which appear to be multiple declineevents. Analysis of the UV emission-line spectra indicates most linesdecay during the deep declines on characteristic timescales comparableto that reported for optical features. Fe, Mg, and neutral C lines decayon timescales of typically 50-100 days. Other lines, notably ionized Clines, decay on longer timescales (>200 days) or appear to beunaffected by the declines. The general nature of the UV emission linesand other UV features during the declines is consistent with theE1/E2/BL line region model developed from the behavior of opticalspectral features during declines. However, the detailed line behaviorindicates large intrinsic variability between decline eventsinconsistent with the simple E1/E2/BL model. Limited temporal coverageprevents detailed examination of the geometry of the emission-lineregion or the obscuring dust. We also report the first detection of thetransition-region line C IV] lambda1550 in the spectrum of an RCB star.We fit the onset times of all declines from maximum light within the1987-1998 interval, irrespective of decline amplitude, with a 43.23 daylinear solution, thus improving the decline ephemeris. The linear termis probably the pulsation period of V854 Cen, further supporting thesuspected link between radial pulsations and mass loss in these types ofstars.
| First Observations of an R Coronae Borealis Star with the Space Telescope Imaging Spectrograph: RY Sagittarii near Maximum Light
We describe the far-UV (1140-1740 Å) spectrum of thehydrogen-deficient R Coronae Borealis (RCB) star RY Sgr, obtained nearmaximum light (pulsational phase ~0.1) by the Space Telescope ImagingSpectrograph (STIS) on Hubble Space Telescope. The far-UV spectrum showsa photospheric continuum rising steeply toward longer wavelengths andtwo prominent emission features at the shorter wavelengths: C IIlambda1335 and Cl I lambda1351 (the latter is radiatively fluoresced bythe 10 times stronger C II multiplet). We also find evidence for CO A-X4th-positive system absorption band heads and possible weak COfluorescent emissions pumped by C II lambda1335, but the inferred columndensities are low (~few times 10^16 cm^-2), consistent with formation ina warm (~5000 K) atmospheric layer. The detection of CO molecules, ifconfirmed, would be significant, because they are thought to play a keyrole in the dust ejection episodes of RCB stars through the initiationof ``molecular cooling catastrophes.''
| The spectrum of Sakurai's object in 1998
The spectra obtained just before deep minimum in 1998 are described. Thetemperature of the object is found to be T_eff=5750+-200 andlogg=0.0-0.5 corresponding to an evolution at almost constantluminosity.
| Surface compositional changes observed during last stages of stellar evolution
Not Available
| A Search for an Emission-Line Region in the Hydrogen-deficient Carbon Star HD 182040
A long-exposure short-wavelength IUE spectrum of the hydrogen-deficientcarbon (HdC) star HD 182040 does not show any detectable emission in theC II lambda1335 line. It is not certain whether this absence of emissionis intrinsic or due to the large uncertainties in the distance, absoluteluminosity, and reddening toward HD 182040. If intrinsic, this absenceof emission along with the absence of an IR excess from circumstellardust imply that little or no mass loss is occurring at present from thisstar even though it is very similar spectroscopically to the R CoronaeBorealis (RCB) stars. This result also suggests that the emission-linegas and the circumstellar dust may result from the same mass-lossmechanism in the RCB stars.
| Spectra of Cool Stars in the J Band (1.0-1.3 μm) at Medium Resolution
A spectroscopic survey of 103 cool S-, C-, and M-type stars wasundertaken with the Kitt Peak cryogenic spectrograph on the 1.3 mtelescope to make a detailed search for new molecular bands in the1.0-1.3 μm J-band region. While the spectra have high signal-to-noiseratios and good repeatability, no strong new features were found. Higherresolution spectra of a small sample of representative stars observedwith the 4 m Fourier transform spectrometer were invoked in order toidentify the features in these spectra. With few exceptions, the detailsof these spectra are well understood. Laboratory spectra were obtainedto aid in the identification of some weak features. Spectral featuresfrom dominant lines of Ti, Fe, Al, Si, Mn, Na, and K, and molecularbands due to TiO, TiS, ZrO, ZrS, VO, H_2O, and CN, have been identifiedin the spectra. Several weak unidentified bands are found.
| The Size and Age of Sakurai's Planetary Nebula and the Temperature of Its Central Star
We present high-resolution, spatially resolved spectra of the nebulasurrounding Sakurai's object. We find the expansion velocity of thenebula to be 30.9 +/- 0.7 km s^-1, and the nebula extent to be 44" indiameter. We discuss the wide range of reported distances to Sakurai'sobject and the dilemma that these present, since some evidence stronglyfavors a short distance (1.1 kpc) while other data strongly favor alonger distance (5-8 kpc). We also present spectra of Sakurai's starcentered near the lambda4216 CN band that suggests the star had cooledsignificantly between 1996 October and 1997 May, but had not cooledfurther by 1997 July.
| HIPPARCOS observations of hydrogen-deficient carbon stars
Parallax measurements for 21 hydrogen-deficient carbon stars have beenmade by the Hipparcos satellite. These stars include most of thebrighter R Coronae Borealis (RCB) variables, other coolhydrogen-deficient carbon (HdC) stars, and several higher-temperatureextreme helium (eHe) stars. Most of these stars have either negative orstatistically insignificant parallaxes, indicating that they lie beyondthe detection capability of Hipparcos. Although the distances to thegalactic hydrogen-deficient carbon stars remain unknown, at least theHipparcos observations do confirm that these objects must have highluminosity like the LMC RCB stars, for which M_bol = -4 to -5. Basedupon Hipparcos proper motions, we derive UVW velocities for the RCB andHdC stars, assuming M_bol = -3 and -5. The UW-velocity dispersion of theRCB/HdC stars is similar to that already reported for the eHe stars,further supporting that these groups of stars have predominantly bulgedistributions. However, UW Cen may be a second example of a halo RCBstar currently seen transitting the galactic plane.
| The Evolution of the Final Helium Shell Flash Star V605 Aquilae From 1917 to 1997
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.2679C
| The Spectrum Near Maximum Light of the Unusual R Coronae Borealis Variable DY Persei
DY Per was confirmed photometrically as an R CrB variable by Alksnis in1994. Spectra near maximum light show the high-speed ejection of matter(sodium at -174 km/sec in DY Per) characteristic of many of thesevariable stars. Moderate hydrogen deficiency may also be present. DY Peris remarkable in having a temperature many hundreds of degrees lowerthan most of the R CrB variables. It is unusual also in not showingclear spectroscopic evidence of high luminosit. (SECTION: Stars)
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The onset of photometric variability in red giant stars.
The onset of variability in red giant stars is studied from the datacollected by the `Long-Term Photometry of Variables' project whichoperates at ESO (La Silla) since 1982. The differential nature of theobservations combined with a multi-night reduction algorithm enables tostudy the variability of over 50 red giants with an accuracy of theorder of 2 to 3 millimagnitudes on time scales ranging from days toyears. All red giants with spectral types late G to early K (i.e.b-y~0.6-0.8) are stable at the level σ_y_<6millimag (whereσ_y_ is the standard deviation of the Stroemgren y magnitude). Redgiants with later spectral types are all variable, and it is found thatthe minimum variability level σ_y, min_ increases with increasingb-y, thus defining a minimum-variability boundary. The new data obtainedin this paper reveal that the time scale of the associated variabilityincreases with increasing b-y (and thus amplitude), and that thestability of this time scale improves concomitantly. More precisely,irregular variations on time scales of 5 to 10-days (andσ_y_~7millimag) characterize red giants with b-y~1.0, whereas thevariations (with σ_y_~100millimag) become more regular withperiods of the order of 50d for red giants with b-y~1.3. Aradial-velocity jitter (of the order of 1.5km/s r.m.s.) is associatedwith this photometric variability, and suggests that stellaroscillations may be responsible for the observed variations.
|
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: | Aquila |
| Ascensió Recta: | 19h23m10.08s |
| Declinació: | -10°42'11.6" |
| Magnitud Aparent: | 6.995 |
| Distancia: | 10000000 parsecs |
| Moviment propi RA: | 9.3 |
| Moviment propi Dec: | 0.7 |
| B-T magnitude: | 8.32 |
| V-T magnitude: | 7.105 |
Catàlegs i designacions:
|
