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Heavy Element Abundances in Late-B and Early-A Stars. I. Co-Added IUE Spectra of HgMn Stars
Very heavy elements (Pt, Au, Hg, Tl, and Bi) are found to be enhanced inthe atmospheres of the chemically peculiar stars of the upper mainsequence by up to a million times the solar system levels. Suchenhancements are believed to result from atmospheric dynamics (i.e.,diffusion) rather than scenarios that dredge up nuclear-processedmaterial to the surface or transfer processed material between binarycompanions. However, the theoretical framework needs to be furtherconstrained by observations beyond the realm of the spectral types forwhich such abundance enhancements are observed at optical wavelengths.The International Ultraviolet Explorer (IUE) satellite collected spectraof bright stars for which chemical peculiarities have been derived fromground-based data. For several elements the abundance enhancements haveonly been recently measured using Hubble Space Telescope data and havetherefore not yet been exploited in the IUE data. We have initiated aprogram to analyze IUE high-dispersion spectra to more fullycharacterize the pattern of very heavy element enhancement for manymercury-manganese (HgMn) stars and to potentially extend the spectralclass (effective temperature) boundaries over which these abundanceanomalies are known to exist. The abundances of very heavy elements inchemically normal B and A-type stars provide a base level that may becompared with the solar system abundances. These early spectral typestars may therefore reveal clues for galactic chemical evolution studiessince they were formed at a later epoch than the Sun in the history ofthe Galaxy. This first paper presents the motivation for the analyses tofollow, outlines our spectral co-addition technique for IUE spectra, anddiscusses the choice of model atmospheres and the synthetic spectrumprocedures, while initiating the study by highlighting the abundance ofgold in several HgMn stars.
| A spectroscopic atlas of the HgMn star HD 175640 (B9 V) λλ 3040-10 000 Å
We present a high resolution spectral atlas of the HgMn star HD 175640covering the 3040-10 000 Å region. UVES spectra observed with 90000-110 000 resolving power and signal to noise ratio ranging from 200to 400 are compared with a synthetic spectrum computed with the SYNTHEcode (Kurucz \cite{K93b}). The model atmosphere is an ATLAS12 model(Kurucz \cite{K97}) with parameters Teff = 12 000 K, log g =3.95, ξ = 0 km s-1. The stellar individual abundances inATLAS12 were derived from an iterative procedure. The starting atomicline lists downloaded from the Kurucz website have been improved andextended by examining different sources in the literature and bycomparing the computed profiles with the observed spectrum. The highquality of the data allowed us to study the isotopic and hyperfinestructure for several lines of Mn II, Ga II, Ba II, Pt II, Hg I, and HgII. Numerous weak emission lines from Cr II and Ti II have beenidentified in the red part of the spectrum, starting at ≈λ5847 Å. Two emission lines of C I (mult. 10, mult. 9) have beenobserved for the first time. All Cr II and Ti II emission linesoriginate from the high excitation states (χlow 89000 cm-1 for Cr II and χlow 62 000cm-1 for Ti II) with large transition probabilities (log gf>-1.00). The synthetic spectrum superimposed on the observed spectrumas well as the adopted improved atomic line lists are available at theCDS and http://wwwuser.oat.ts.astro.it/castelli/stars.html. An extendeddiscussion on each identified ion and related atomic data is availableboth on the quoted website and in an electronic Appendix to the paper.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO program No. 67.D-0579).Appendices is only available in electronic form athttp://www.edpsciences.orgAtlas is available 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/425/263
| On the elemental abundance and isotopic mixture of mercury in HgMn stars
Optical region spectra of 31 HgMn stars have been studied for theabundance and isotope mixture of mercury. In the course of theinvestigation the lines Hg I lambda 4358 and Hg Ii lambda lambda3984,6149 have been studied, with abundances established for all threelines in several HgMn stars. The mercury isotope mixture has beendetermined from high resolution spectra of the lambda 3984 line.Possible signs of an ionization anomaly have been detected by thecomparison of the abundance derived from the Hg I line and the Hg Iilines in seven of the observed HgMn stars. A possible correlation of themercury abundance with Teff has been detected. Possible signsof a weak anticorrelation of the manganese and mercury abundance in HgMnstars have been found, which could be interpreted as a sign ofinhomogeneous surface distribution of these elements. For a number ofthe HgMn stars in this study the mercury abundance and isotope mixtureare reported for the first time.
| On the relationship between the mercury-manganese stars and the metallic-lined stars
An HR diagram indicating the positions of the HgMn and the Am starsanalyzed by the senior author and his collaborators shows that thecoolest HgMn stars and the hottest Am stars are found on the samestellar evolutionary tracks and hence the former must evolve into thelater. The explanation of the dividing line between these two types ofnonmagnetic chemically peculiar stars where the Hg abundances suddenlychange their degree of overabundance is a major test of the theorieswhich try to explain the anomalous abundances of such stars. Some otherimportant relationships are found which can also serve as tests oftheories which purport to explain the properties of these stars.
| On the behavior of the Cii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars
With the aim of investigating the possible particular behavior of carbonin a sample of chemically peculiar stars of the main sequence withoutturning to modeling, we performed spectroscopic observations of threeimportant and usually prominent single ionized carbon lines: 4267.261,6578.052 and 6582.882 Å. In addition, we observed a large numberof standard stars in order to define a kind of normality strip, usefulfor comparing the observed trend for the peculiar stars. We paidparticular attention to the problem of the determination of fundamentalatmospheric parameters, especially for the chemically peculiar stars forwhich the abundance anomalies change the flux distribution in such a waythat the classical photometric methods to infer effective temperaturesand gravities parameter cannot be applied. Regarding CP stars, we founda normal carbon abundance in Hg-Mn, Si (with some exceptions) and Hestrong stars. He weak stars are normal too, but with a large spread outof the data around the mean value. A more complicated behavior has beennoted in the group of SrCrEu stars: four out of seven show a strongoverabundance, being the others normal.
| The Variability of the Hg II λ3984 Line of the Mercury-Manganese Star α Andromedae
The variability of the Hg II λ3984 line in the primary of thebinary star α And was discovered through the examination ofhigh-dispersion spectra with signal-to-noise ratios greater than 500.This first definitively identified spectrum variation in anymercury-manganese star is not due to the orbital motion of thecompanion. Rather, the variation is produced by the combination of the2.38236 day period of rotation of the primary that we determined and anonuniform surface distribution of mercury that is concentrated in itsequatorial region. If the surface mercury distribution exhibitslong-term stability, then it is likely that a weak magnetic fieldoperates in its atmosphere, but if changes are observed in the lineprofile over a period of a few years, then these would constitute directevidence for diffusion.
| Light-induced drift for Hg isotopes in chemically peculiar stars
In the present paper the abundance anomalies of mercury and its isotopesin the atmospheres of HgMn stars have been studied. Observations haveshown strongly anomalous isotopic composition of Hg, Pt, Tl and He inthe atmospheres of such CP stars. Generation of elemental abundanceanomalies in quiescent atmospheres of CP stars can generally beexplained by the mechanism of diffusive segregation of elements due tooppositely directed gravitational and radiative forces. It has beenshown that the formation of the observed isotopic anomalies can besuccessfully explained by a diffusion mechanism called the light-induceddrift (LID). The observed ratios of isotopes also enable to estimate theevolutionary stages of CP stars.
| The laboratory analysis of Bi II and its application to the Bi-rich HgMn star HR 7775
The bismuth spectrum emitted from a hollow cathode discharge has beenrecorded with a Fourier Transform Spectrometer (FTS). Accuratewavelengths have been determined for 104 Bi Ii lines, and several newenergy levels have been found, while the accuracy of previously known BiIi level energies have been improved. The hyperfine structure of allobserved Bi Ii lines has been analyzed, yielding hyperfine constants Aand B for 56 Bi Ii levels. With the aid of the laboratory measurementsthe optical region spectrum of the HgMn star HR 7775 has been studiedfor all observable Bi Ii lines. The wavelengths and hfs constantsestablished from the laboratory work have been combined with theoreticalgf values to identify spectral lines and make an abundance estimation ofbismuth. It has been established that bismuth is present in HR 7775 atan enhancement level of approximately 5 orders of magnitude relative tothe meteoritic abundance, consistent with previous observations in theultraviolet region of this star. Astrophysical gf values are presentedfor a number of Bi Ii lines.
| The presence of Nd and Pr in HgMn stars
Optical region spectra for a number of upper main sequence chemicallypeculiar (CP) stars have been observed to study singly and doublyionized praseodymium and neodymium lines. In order to improve existingatomic data of these elements, laboratory measurements have been carriedout with the Lund VUV Fourier Transform Spectrometer (FTS). From thesemeasurements wavelengths and hyperfine structure (hfs) have been studiedfor selected Pr Ii, Pr Iii and Nd Iii lines of astrophysical interest.Radiative lifetimes for some excited states of Pr Ii have beendetermined with the aid of laser spectroscopy at the Lund Laser Center(LLC) and have been combined with branching fractions measured in thelaboratory to calculate gf values for some of the stronger optical linesof Pr Ii. With the aid of the derived gf values and laboratorymeasurements of the hfs, a praseodymium abundance was derived fromselected Pr Ii lines in the spectrum of the Am star 32 Aqr. Thisabundance was used to derive astrophysical gf values for selected Pr Iiilines in 32 Aqr, and these gf values were used to get a praseodymiumabundance for the HgMn star HR 7775. The praseodymium abundance in HR7775 was then utilized to derive astrophysical gf values for allobservable Pr Iii lines in this star. The neodymium abundance, derivedfrom unblended lines of Nd Ii in HR 7775, has been utilized to establishastrophysical gf values for observed Nd Iii lines in the optical regionof this star. Selected Pr Iii and Nd Iii lines have been identified andstudied in a number of HgMn stars and three hot Am stars. Thepraseodymium and neodymium abundance change rapidly from an approximate1-1.2 dex enhancement for the hot Am stars to 1.5-3 dex enhancement forthe cool HgMn stars, indicating a well-defined boundary between the hotAm and HgMn stars in the vicinity of 10 500 K. The enhancement ofpraseodymium and neodymium in Am and HgMn stars may be explained bydiffusive processes active in the stellar atmosphere, while the observeddiscontinuity might be explained by a thin hydrogen convection zonethought to be present for the Am stars, but absent in the HgMn stars.The absence of a convection zone would cause the diffused elements togather higher in the atmosphere of HgMn stars compared to Am stars, andexplain the observed increase in abundance.
| An Abundance Study in the Hg-Mn Star 46 Aquilae (HD 186122) with the SUBARU/HDS
A detailed abundance analysis has been carried out for the Hg-Mn star 46Aql using high-resolution spectra in the visual region (5100--6400Å) obtained with the High Dispersion Spectrograph of the SubaruTelescope. Our attention has been mainly focused on those elements whichhave not been analyzed previously. He, C, N, and O are underabundant,while Na shows the solar abundance. We have confirmed previouslyreported underabundances of Mg, Al, and Si, and have found anoverabundances of P (+1.5 dex) and Ti (+1.0 dex) and a significantunderabundance of S (-1.8 dex). Fe is overabundant by +0.8 +/- 0.1 dexand a microturbulent velocity of 0.3 km s-1 was found from ananalysis of the FeII lines. Among a few hundred unidentified absorptionlines, we identified lines of AsII and XeII and a very large (+4.0 dex)overabundance of Xe is obtained. We have confirmed emission lines ofMnII and TiII near 6100 Å, while those of CrII cannot have beenseen.
| An elemental abundance analysis of the mercury manganese star HD 29647
The sharp-lined mercury manganese (HgMn) star HD 29647, which is locatedbehind the outer envelope of the Taurus Molecular Cloud 1, is ofinterest for both its stellar properties and its utility as a probe ofinterstellar gas and dust along its line of sight. In this paper wereview the properties of the star, summarize its line identificationsand present an abundance analysis based on spectrograms obtained at theMcDonald Observatory and the Observatoire de Haute-Provence. This starhas elemental abundances similar to those of other HgMn stars exceptthat its He/H ratio is closer to solar, possibly indicating a young age,and that it, like HR 7775, is overabundant in selected elementsincluding the rare earths. The stellar radial velocity, unfortunately,closely matches that of the foreground cloud.
| A Study of Rare Earth Elements in the Atmospheres of Chemically Peculiar Stars. Pr III and Nd III Lines
We determine the abundances of Pr and Nd in the atmospheres of magneticand non-magnetic chemically peculiar stars from the lines of rare earthelements in the first and second ionization states. The computations forthe magnetic stars take into account the influence of the magnetic fieldon line formation. We studied the influence of errors in thestellar-atmosphere parameters and the atomic parameters of the spectrallines on the accuracy of abundance determinations. Within the derivedaccuracy, ionization equilibrium is satisfied in the atmospheres ofnon-pulsating magnetic and non-magnetic stars (so that abundancesderived separately from lines of first and second ions agree). For allthe pulsating magnetic (roAp) stars studied, the abundances derived fromlines of second ions are 1.0 to 1.7 dex higher than those derived fromfirst ions. The violation of ionization equilibrium in the atmospheresof pulsating stars is probably due to, first, considerable enrichment ofPr and Nd in the uppermost atmospheric layers, and second, a higherlocation for the layer of enhanced elemental abundance in roAp starsthan in non-pulsating stars. Two objects from the list of non-pulsatingmagnetic stars, HD 62140 and HD 115708, exhibit anomalies of their Prand Nd lines characteristic of roAp stars. The differences in the rareearth anomalies for the pulsating and non-pulsating peculiar stars canbe used as a selection criterion for candidate roAp stars.
| The Bismuth Abundance in the HGMN Stars χ Lupi and HR 7775 and Improved Atomic Data for Selected Transitions of BI I, BI II, and BI III
High-resolution spectra of the chemically peculiar HgMn stars χ Lupiand HR 7775, obtained with the Hubble Space Telescope/Goddard HighResolution Spectrograph, are investigated for their abundance of bismuthby comparison with LTE synthetic spectrum modeling. HR 7775, previouslyknown from International Ultraviolet Explorer spectra to display stronglines of Bi II, is determined to have bismuth present at an enhancementlevel of nearly 5 orders of magnitude from the lines Bi IIλλ1436, 1902 and Bi III λ1423. The bismuthenhancement for χ Lupi is found to be near a level of 1.5 dex, andan ionization anomaly between Bi+ and Bi++ isapparent. HR 7775 abundance enhancements of the heavy elements platinum,[Pt/H]=4.7 dex, and gold, [Au/H]=3.8 dex, have also been determined. Newlaboratory measurements for wavelengths and hyperfine structure patternsof Bi I/Bi II/Bi III lines are presented, as well as the results ofcalculations for hyperfine structure constants and oscillator strengthsfor selected lines of Bi II and Bi III. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555.
| The long-period companions of multiple stars tend to have moderate eccentricities
We examined the statistics of an angle gamma between the radius vectorof a visual companion of a multiple star and the vector of its apparentrelative motion in the system. Its distribution f(gamma ) is related tothe orbital eccentricity distribution in the investigated sample. Wefound that for the wide physical subsystems of the 174 objects from theMultiple Star Catalogue f(gamma ) is bell-shaped. The Monte-Carlosimulations have shown that our f(gamma ) corresponds to the populationof the moderate-eccentricity orbits and is not compatible with thelinear distribution f(e)=2e which follows from stellar dynamics andseems to hold for wide binaries. This points to the absence of highlyelongated orbits among the outer subsystems of multiple stars. Theconstraint of dynamical stability of triple systems is not sufficient toexplain the ``rounded-off'' outer orbits; instead, we speculate that itcan result from the angular momentum exchange in multiple systems duringtheir early evolution.
| New results of magnetic field diagnosis in HgMn stars and normal late B-type stars
We suggested in a previous paper that three HgMn stars, HD 175640, HD178065, and HD 186122, may be suspected to possess a magnetic field thatcould be larger than 2 kG. We report here new observations of thesethree stars, three more HgMn stars, and four normal late B-type stars.The search was carried out by measuring the equivalent width of the FeII lambda 6147.7 Å line relative to the equivalent width of the FeII lambda 6149.2 Å line. The observed relative differences betweenthe equivalent widths of these Fe II lines are compared with thosederived from synthetic spectra computed by neglecting magnetic fieldeffects. To investigate the effect of oscillator strength uncertaintieson the results, we computed equivalent widths by using both Fe II loggf-values taken from Kurucz & Bell (\cite{KB95}) and Fe II loggf-values taken from Raassen & Uylings (2000). The comparison of thecomputed and observed equivalent widths based on the Kurucz & Bell(\cite{KB95}) atomic data leads us to conclude that all the stars of oursample, except HD 175640, are very likely to possess a magnetic field.On the other hand, the comparison of the computed and observedequivalent widths based on the Raassen & Uylings (2000) loggf-values suggests the possible presence of magnetic fields only inthree stars, the HgMn star HD 16717 and the two normal B-type stars HD179761 and HD 186568. The latter two are those in the sample with thelargest vsin i (15 km s-1 and 18 km s-1,respectively), so that the results for them are the most uncertain ones.
| Elemental abundance analyses with DAO spectrograms. XXIV. The Mercury Manganese stars upsilon Her, phi Her, and HR 7018
Elemental abundances analyses are performed for the Mercury-Manganesestars upsilon Her, phi Her, and HR 7018 consistent with previous studiesof this series using spectrograms obtained with Reticon and CCDdetectors. Comparisons of the first two analyses with those performedusing coadded photographic plates show the general consistency of thederived elemental abundances. For upsilon Her and for phi Her,abundances were newly found for O, and for Al, V, Zn, and Ce,respectively. HR 7018 is discovered to be a single-lined spectroscopicbinary. Its Sc abundance is the smallest of any class member withderived abundances and its Sr abundance the largest of any known HgMnstar. A correlation analysis of the most complete abundance sets for 20HgMn stars shows that the abundances of some elements are correlatedwith one another and some are functions of the stellar effectivetemperature. Table 6 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/367/597
| Neon abundances in normal late-B and mercury-manganese stars
We make new non-local thermodynamic equilibrium calculations to deducethe abundances of neon from visible-region echelle spectra of selectedNei lines in seven normal stars and 20HgMn stars. We find that the beststrong blend-free Ne line that can be used at the lower end of theeffective temperature Teff range is λ6402, althoughseveral other potentially useful Nei lines are found in the red regionof the spectra of these stars. The mean neon abundance in the normalstars (logA=8.10) is in excellent agreement with the standard abundanceof neon (8.08). However, in HgMn stars neon is almost universallyunderabundant, ranging from marginal deficits of 0.1-0.3dex tounderabundances of an order of magnitude or more. In many cases, thelines are so weak that only upper limits can be established. The mostextreme example found is υ Her with an underabundance of at least1.5dex. These underabundances are qualitatively expected from radiativeacceleration calculations, which show that Ne has a very small radiativeacceleration in the photosphere, and that it is expected to undergogravitational settling if the mixing processes are sufficiently weak andthere is no strong stellar wind. According to theoretical predictions,the low Ne abundances place an important constraint on the intensity ofsuch stellar winds, which must be less than10-14Msolaryr-1 if they arenon-turbulent.
| Atomic Data for Resonance Absorption Lines. II. Wavelengths Longward of the Lyman Limit for Heavy Elements
This compilation extends the 1991 listing of atomic data by Morton tothe heavier stable elements from germanium to bismuth. Technetium,thorium, and uranium are added because they can live long enough to beastrophysically detectable. The tabulation emphasizes resonance lines,i.e., lines the lower level of which is the ground state, or an excitedfine-structure state of the ground term, and is restricted towavelengths longward of the H I Lyman limit at 911.75 Å. Thispaper has attempted to review all data published by mid-1999 andincludes some later material. The tables contain the best data availableto the author on level designations, ionization potentials, vacuum andair wavelengths, lower and upper energy levels, statistical weights,transition probabilities, natural damping constants (reciprocallifetimes), oscillator strengths, and the often-used combinations ofloggf and logλf. All ion stages with classified lines areincluded. Individual components resulting from isotope shift andhyperfine structure are listed explicitly for Rb I, Cs I, Hg I and HgII, Ti II, and Pb II. The accompanying text provides references,explanations for the critical selection of data, and notes indicatingwhere new measurements or calculations are needed. This compilationshould be particularly useful in the analysis of interstellar and quasarabsorption lines and other astrophysical sites where the density ofparticles and radiation is low enough to excite only the lowest atomiclevels. The data also are relevant to the study of stellar atmospheres,particularly those with enhanced abundances of heavy elements.
| An Optical Region Elemental Abundance Analysis of the HGMN Type Star HR 7775
Optical region spectra of the chemically peculiar HgMn type star HR 7775have been analyzed for the determination of elemental abundances andspectral line identification as a complementary study to the analysis ofHubble Space Telescope Goddard High Resolution Spectrograph spectra. Thespectra were obtained with the SOFIN echelle spectrograph on the NordicOptical Telescope at nominal resolving powers of R=25,000, 80,000, and170,000 and signal-to-noise ratios typically in excess of 100:1. Theoverall nature of the elemental abundance distribution is similar toother cool, HgMn stars; somewhat enhanced abundances for the iron-groupelements, with progressively stronger enhancements for heavier elements.Of particular note are the presence of lines from the rare-earth spectraNd III and Pr III, and the heavy elements Pt, Au, Hg, Tl, and Bi. Theisotopic mixture for several lines of mercury and platinum has beeninvestigated. For mercury a possible difference is found between themixtures as derived from lines of Hg I and Hg II. Three lines of Pt IIdisplay the same mixture, which is different from that found from lowerexcitation lines at ultraviolet wavelengths. We present new laboratoryFourier Transform spectrometer measurements for isotopic and hyperfinestructure components of Hg I λ4046 and 4358 and Hg IIλ3984 Å. Based on observations obtained with the NordicOptical Telescope, operated on the island of La Palma jointly byDenmark, Finland, Iceland, Norway, and Sweden, in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
| Accuracy of radial-velocity measurements for early-type stars. II. Investigations of spectrum mismatch from high-resolution observations
The accuracy with which radial velocities of early-type stars can bemeasured is limited in practice by the existence of asymmetricaldifferences between object and template spectrum, constituting``spectrum mismatch''. Our studies of the magnitude of spectrum-mismatcherrors, commenced in Paper I (Verschueren et al. \cite{VDG99}) on thebasis of synthetic spectra having different attributes of effectivetemperature (Teff and {log g}, are continued here in acomplementary approach that employs observed spectra. From over 60de-archived observations we derive accurate wavelength scales for thespectra of 16 dwarfs of spectral types B8-F7, and examine the results ofcross-correlating the spectra against different (observed) templatespectra. We also test the effects of (a) truncating the spectra atdifferent levels below the continuum, (b) adding rotational broadeningto enforce a visual match of line-width between object and template, (c)applying rotational broadening to exacerbate a rotational mismatch, and(d) neglecting the presence of faint companion spectra. We alsocross-correlate pairs of spectra such that the differences between theirTeff are minimal. We conclude that it will be possible tomeasure radial velocities to an accuracy considerably better than 1 kms-1 for slowly-rotating stars in the range of spectral typesexamined, and a careful discussion of the nature and sources of therandom and systematic errors that become significant in work of thisnature enables us to specify conditions that are important for achievingsuch accuracy routinely. We find that both rotational broadening, andthe star-to-star variations in line strengths that are so prevelantamong A-type spectra, can give rise to more deleterious mismatch shifts(RV errors) than do differences in Teff alone, even for DeltaTeff as great as 300-400 K. By intercomparing the resultsgiven by wide regions of spectrum ( ~ 800 Å) with those obtainedby isolating small groups of features in very narrow windows ( ~ 30Å), we have been able to designate a window near lambda 4570Åthat should be particularly reliable for high-accuracy results,and we propose further studies at very high S/N ratio in that specificwindow to complement and extend the results of the present paper.
| Analysis of the hyperfine and isotopic structure of Ga bt II in the optical spectrum of kappa Cnc and HR 7775
Spectra of the HgMn stars kappa Cnc and HR 7775 have been analyzed,under the assumption of local thermodynamic equilibrium (LTE), withrespect to the abundance and isotopic mixture of gallium. The primarypurpose of the investigation was to level the contradictory resultsobtained using spectral lines in different spectral regions \cite{S96}.The analysis has been performed by Ga Ii lines in the optical regionincluding hyperfine and isotopic components from new Fourier TransformSpectrometer measurements \cite{KL00}. Gallium has, for both stars, beendetermined to be overabundant by two orders of magnitude compared to thesun, using oscillator strengths derived by Ryabchikova & Smirnov\cite*{RS94}. The difference in abundance obtained using spectral linesin the UV and the visual spectral region is unexplainable by accountingfor hyperfine and isotopic structure. Astrophysical log gf values havebeen determined for the Ga Ii 5p - 5d transitions at 5360, 5363 and 5421Å.
| Emission lines in the spectra of late-B type stars
We report detections of weak emission lines in the red spectral regionof sharp-lined chemically normal and peculiar (HgMn) late-B type starsfrom high spectral resolution and high signal-to-noise data. Mostemission lines originate from high-excitation states of the ions Cr IIand Mn II, with others likely to be attributed to Ti II and Fe II. Theemission is observed to extend over the entire line profile width forrotational velocities up to 18 km s-1, implying that itoriginates within the same rotational framework as the absorption linespectrum. Within the sample no obvious correlation is noted for thepresence of emission with regard to stellar effective temperature orluminosity. A dependence upon element abundance is evident from theabsence of Mn II emission for HgMn stars for which the manganeseenhancement is greater than 1.3 dex. This trend is mildly reinforced bythe chromium emission spectrum being most developed amongst stars richerin chromium. We postulate that the Cr II and Mn II emissions in the redspectral region arise from a selective excitation process involvinghydrogen Lyalpha photon energies.
| 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.
| Lithium in the Super-Beryllium-rich HG-MN Stars HR 6158 and HR 8915
The chemically peculiar stars that inhabit the upper main sequenceexhibit very extreme enhancements and deficiencies of several elements.In all likelihood, the primary cause of these anomalies is radiativelydriven diffusion, although other mechanisms may also be important-suchas mass loss and various kinds of mixing. Only by examining as manyelements as possible will a cohesive theory explaining these abundanceoddities materialize. To this end, we present high-resolution(R~62,500), high signal-to-noise ratio (S/N pixel^-1~120) observationsof the 6708 Å Li I resonance doublet in a pair of Hg-Mn stars. Norecord of a lithium detection has been found for this type of chemicallypeculiar star. The atmospheres of the two program stars (HR 6158=28Her=HD 149212; HR 8915=69 Peg=HD 220933) contain an inordinate amount ofberyllium (Be) in fact, the Be abundances in these stars are among thehighest known. While the LTE concentration of Be is several thousandtimes larger than cosmic [A(Be)_i~1.27] in both stars, lithium (Li) isdetected in neither HR 6158 nor HR 8915. The calculated 3 sigma (99%confidence level) upper limits place the maximum Li enhancements at 50times and 80 times cosmic [A(Li)_i~3.23] for HR 6158 and HR 8915,respectively, or ~100 times smaller than the Be overabundances. These Liupper limits, which argue against a ``nucleosynthesis'' origin for theBe surfeit, provide yet another means of constraining the physicalprocess (or processes) responsible for the Hg-Mn phenomenon in some Astars.
| Oxygen 6156-8 Angstroms Triplet in Chemically Peculiar Stars of the Upper Main Sequence: Do HgMn Stars Show an Oxygen Anomaly?
An extensive spectrum-fitting analysis in the lambda ~ 6150 Angstromsregion was performed for forty late-B and A chemically peculiar (HgMn,Ap, Am, weak-lined) and normal stars of the upper main sequence, inorder to quantitatively establish the abundance of oxygen from the O I6156-8 triplet and to study its behavior/anomaly for each peculiaritygroup, where special attention was paid to HgMn-type stars. Magnetic Apvariables (Si or SrCrEu type) generally show a remarkably large oxygendepletion (by ~ -1.6 dex to ~ -0.4 dex relative to the solar abundance),and classical Am stars also show a clear underabundance (by ~ -0.6 dexon the average). In contrast, the oxygen abundance in HgMn stars,exhibiting only a mild deficiency relative to the Sun typically by ~0.3-0.4 dex, is not markedly different from the tendency of normalstars, also showing a subsolar abundance ([O/H] =~ -0.2). In view of therecent observational implication that the present solar oxygen abundanceis enriched by ~ 0.2-0.3 dex relative to the interstellar gas (fromwhich young stars are formed), and thus unsuitable for the comparisonstandard, we concluded that the extent of the average O-deficiency inHgMn stars is appreciably reduced down to only ~ 0.1-0.2 dex (i.e., notmuch different from the initial composition), which contrasts with theirwell-known drastically large N-depletion. Yet, as can be seen from thedelicate and tight O vs. Fe anticorrelation, this marginal deficiencyshould be real, and thus some physical process simultaneously producingO-depletion/Fe-enrichment must have actually worked in the atmosphere ofHgMn stars.
| Manganese abundances in mercury-manganese stars
We use exact curve-of-growth analysis and spectral synthesis to deducethe abundance of Mn from high signal-to-noise ratio visible-regionechelle spectra of selected Mn i and MnII lines in 24 HgMn stars. Theresults are compared with the Mn abundances derived from UV resonancelines by Smith & Dworetsky. We find excellent agreement for severalunblended Mn lines and confirm the temperature dependence of the Mnabundance found by Smith & Dworetsky. The MnII lines at lambdalambda 4206 and 4326 are much stronger than one would predict from themean Mn abundances. The lack of agreement is greatest for stars with thestrongest MnII lines. Using ad hoc multicomponent fits to the profilesof sharp-lined stars, we show that most of the discrepancies can beexplained by hyperfine structure that desaturates the lines, with fullwidths of the order of 0.06-0.09 A.
| Mercury Elemental and Isotopic Abundances in Mercury-Manganese Stars
Hg II abundances have been determined for 42 mercury-manganese (HgMn)stars by fitting synthetic spectra to observed spectra of the 3984Å Hg II line. Twenty of the stars had lines sharp enough to allowtheir Hg isotopic abundance mixes to be estimated. The Hg abundance isreported for more HgMn stars here than in any other single work. Nocorrelation was found between Hg II abundance and T_eff or the meancentral wavelength of HgMn lambda3984 stars. The mean central wavelengthof lambda3984 , an indicator of the Hg isotopic mix, is looselycorrelated with T_eff: stars with primarily heavy Hg isotopes tend to becooler, although one star, 46 Aql, has almost pure ^204Hg and T_eff inabout the middle of the temperature range for HgMn stars. We find thatthere is no evidence that any of the HgMn stars have ^196Hg or ^198Hg.For the very sharp-lined stars, the ^204Hg abundance decreases withincreasing T_eff. No correlation is seen between the mean centralwavelength and the surface gravity. No correlation was found between theprojected rotational velocity and the Hg II abundance or the centralwavelength of lambda3984, although this result may be biased by theselection of stars with low reported vsini. Hg I lambda4358 was measuredat high spectral resolution for seven HgMn stars. The isotopic shiftsare too small, and the hyperfine components are too weak to allowunambiguous isotopic abundance ratios to be found. Hg I abundancescorrelate fairly well with Hg II abundances. Some of the Hg isotopicmixtures are difficult to explain using only diffusion. HR 7245 hasapproximately equal abundances of ^199Hg, ^200Hg, ^202Hg, and ^204Hg butvery little ^201Hg, and 11 Per has Hg that is mostly ^199Hg and ^204Hg.Calculations show that hyperfine splitting of ^201Hg changes theradiative forces it feels compared with other isotopes, which may alterdiffusion of that isotope enough to explain its absence in HR 7245, butwe have found no possible explanation for the Hg isotopic mix found in11 Per. These are the first very high resolution measurements of Hg IIlambda3984 for HR 7245 and 11 Per. Although diffusion may be acting inHgMn stars, either there are one or more other mechanisms acting to helpproduce the overabundances and isotopic mixtures seen or ourunderstanding of diffusion is lacking on some important point.
| Mercury in the HGMN Stars chi LUPI and HR 7775
Observations of mercury lines in the HgMn stars chi Lupi and HR 7775made with the Hubble Space Telescope Goddard High ResolutionSpectrograph are presented and analyzed. In chi Lupi we find that allobserved lines are consistent with the same isotopic mixture(essentially pure ^204Hg). Strong ionization anomalies are present, withUV Hg I lines being too weak and Hg III lines too strong for theabundance derived from lines of the majority ionization state, Hg II.Observations of mercury in HR 7775 show less extreme isotope andionization anomalies. We find that the ionization anomaly in the Hg Iresonance lines can be plausibly explained as a non-LTE effect, but thesame non-LTE calculations show that the Hg III ionization anomaly in chiLupi cannot be explained in this way. Radiative force calculations showthat the observed mercury abundance cannot be supported in theatmosphere by the radiative forces alone. We suggest that weak mixingbrings mercury into the line-forming region from below the photosphere,while a wind of order 10^-14 M_solar yr^-1 supports a cloud of Hg III atvery small optical depths.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Very High Resolution Ultraviolet Spectroscopy of a Chemically Peculiar Star: Results of the chi LUPI Pathfinder Project
We summarize here the results of a major eight-year investigation of theextraordinarily detailed UV spectrum of the sharp-lined, nonmagnetic,main-sequence, chemically peculiar star chi Lupi (B9.5p HgMn + A2 Vm).The UV observations are composed of 345 Å of the spectrum acquiredwith the Goddard High Resolution Spectrograph (GHRS) on board the HubbleSpace Telescope at an average resolution of 0.023 Å. The completeset of echelle spectrograms is presented as an atlas in a companionpaper. These data were supplemented by optical-wavelength spectraobtained at the Anglo-Australian Telescope. Quantitatively accurateanalysis and theoretical interpretation of these data required majorimprovements in the accuracy and completeness of available atomicdata-wavelengths, transition probabilities, hyperfine structure, andisotope shifts-for the lowest ionization states of many elements. Alarge, international group of theoretical and experimental atomicphysicists has collaborated in this investigation, and their results aresummarized or referenced in this paper. In turn, the GHRS observationsof chi Lupi have become a useful source of data for atomic spectroscopy,displaying many transitions that are difficult to observe in alaboratory setting. Measured abundances or upper limits are presentedfor 72 ions of 51 chemical elements, spanning the periodic table. Wehave confirmed and refined previously identified isotopic abundanceanomalies in mercury and platinum and have discovered similar isotopicanomalies in thallium and, tentatively, in lead. Large discrepanciesamong the LTE abundances derived, using a chemically homogeneous modelatmosphere, from two or three ionization states of the same element arefound to be common. In some cases these are due to departures from LTEin the ionization equilibria, but the largest such discrepanciesprobably result from chemical stratification within the photosphere. Wefind qualitative trends in the abundances of the elements that clearlysignify radiatively driven diffusion and gravitational settling as theprimary mechanism producing abundance anomalies. However, detailednon-LTE diffusion calculations for mercury and thallium show that thereis insufficient unsaturated radiative force within the chemicallyenriched atmosphere to sustain the observed huge overabundances of theseelements in equilibrium with gravity. Either other hydrodynamicprocesses, such as slow mass motions or unexpectedly strong stellarwinds must assist radiation pressure in supporting the enrichedmaterial, or the observed abundance patterns simply provide a snapshotin time of a nonequilibrium, time-variable phenomenon.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Bak |
| Rektaszcenzió: | 20h20m46.60s |
| Deklináció: | -14°47'06.0" |
| Vizuális fényesség: | 6.1 |
| Távolság: | 96.154 parszek |
| RA sajátmozgás: | 42.1 |
| Dec sajátmozgás: | 1.7 |
| B-T magnitude: | 6.045 |
| V-T magnitude: | 6.077 |
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