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Ýlgili Makaleler
Discovery of magnetic fields in the βCephei star ξ1 CMa and in several slowly pulsating B stars*
We present the results of a magnetic survey of a sample of eightβCephei stars and 26 slowly pulsating B (SPBs) stars with the FOcalReducer low dispersion Spectrograph at the Very Large Telescope. A weakmean longitudinal magnetic field of the order of a few hundred Gauss isdetected in the βCephei star ξ1CMa and in 13 SPBstars. The star ξ1CMa becomes the third magnetic staramong the βCephei stars. Before our study, the star ζCas wasthe only known magnetic SPB star. All magnetic SPB stars for which wegathered several magnetic field measurements show a field that varies intime. We do not find a relation between the evolution of the magneticfield with stellar age in our small sample. Our observations imply thatβCephei and SPB stars can no longer be considered as classes ofnon-magnetic pulsators, but the effect of the fields on the oscillationproperties remains to be studied.
| Photometric studies of three multiperiodic β Cephei stars: β CMa, 15 CMa and KZ Mus
We have carried out single and multisite photometry of the three βCephei stars β and 15 CMa as well as KZ Mus. For the two stars inCMa, we obtained 270h of measurement in the Strömgren uvy andJohnson V filters, while 150h of time-resolved Strömgren uvyphotometry was acquired for KZ Mus. All three stars are multiperiodicvariables, with three (β CMa) and four (15 CMa, KZ Mus) independentpulsation modes. Two of the mode frequencies of 15 CMa are newdiscoveries and one of the known modes showed amplitude variations overthe last 33yr. Taken together, this fully explains the diverse behaviourof the star reported in the literature.Mode identification by means of the amplitude ratios in the differentpassbands suggests one radial mode for each star. In addition, βCMa has a dominant l = 2 mode while its third mode is non-radial withunknown l. The non-radial modes of 15 CMa, which are l <= 3, form analmost equally split triplet that, if physical, would imply that we seethe star under an inclination angle larger than 55°. The strongestnon-radial mode of KZ Mus is l = 2, followed by the radial mode and adipole mode. Its weakest known mode is non-radial with unknown l,confirming previous mode identifications for the pulsations of the star.The phased light curve for the strongest mode of 15 CMa has a descendingbranch steeper than the rising branch. A stillstand phenomenon duringthe rise to maximum light is indicated. Given the low photometricamplitude of this non-radial mode this is at first sight surprising, butit can be explained by the aspect angle of the mode.
| Variations in D/H and D/O from New Far Ultraviolet Spectroscopic Explorer Observations
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to determine the interstellar abundances of D I, N I, O I, Fe II,and H2 along the sight lines to WD 1034+001, BD +39 3226, andTD1 32709. Our main focus is on determining the D/H, N/H, O/H, and D/Oratios along these sight lines, with logN(H)>20.0, that probe gaswell outside of the Local Bubble. Hubble Space Telescope (HST) andInternational Ultraviolet Explorer (IUE) archival data are used todetermine the H I column densities along the WD 1034+001 and TD1 32709sight lines, respectively. For BD +39 3226, a previously published N(HI) is used. We find(D/H)×105=2.14+0.53-0.45,1.17+0.31-0.25, and1.86+0.53-0.43 and(D/O)×102=6.31+1.79-1.38,5.62+1.61-1.31, and7.59+2.17-1.76 for the WD 1034+001, BD +39 3226,and TD1 32709 sight lines, respectively (all 1 σ). The scatter inthese three D/H ratios exemplifies the scatter that has been found byother authors for sight lines with column densities in the range19.2
| Abundance analysis of prime B-type targets for asteroseismology. I. Nitrogen excess in slowly-rotating β Cephei stars
Seismic modelling of the β Cephei stars promises major advances inour understanding of the physics of early B-type stars on (or close to)the main sequence. However, a precise knowledge of their physicalparameters and metallicity is a prerequisite for correct modeidentification and inferences regarding their internal structure. Herewe present the results of a detailed NLTE abundance study of nine primetargets for theoretical modelling: γ Peg,δ Cet, ν Eri,β CMa, ξ1 CMa,V836 Cen, V2052 Oph,β Cep and DD (12) Lac (hereafter 12Lac). The following chemical elements are considered: He, C,N, O, Mg, Al, Si, S and Fe. Our curve-of-growth abundance analysis isbased on a large number of time-resolved, high-resolution opticalspectra covering in most cases the entire oscillation cycle of thestars. Nitrogen is found to be enhanced by up to 0.6 dex in four stars,three of which have severe constraints on their equatorial rotationalvelocity, Ω R, from seismic or line-profile variation studies:β Cep (Ω R ~ 26 km s-1),V2052 Oph (Ω R ~ 56 km s-1),δ Cet (Ω R < 28 km s-1) andξ1 CMa (Ω R sin i 10 kms-1). The existence of core-processed material at the surfaceof such largely unevolved, slowly-rotating objects is not predicted bycurrent evolutionary models including rotation. We draw attention to thefact that three stars in this subsample have a detected magnetic fieldand briefly discuss recent theoretical work pointing to the occurrenceof diffusion effects in β Cephei stars possibly capable of alteringthe nitrogen surface abundance. On the other hand, the abundances of allthe other chemical elements considered are, within the errors,indistinguishable from the values found for OB dwarfs in the solarneighbourhood. Despite the mild nitrogen excess observed in someobjects, we thus find no evidence for a significantly higherphotospheric metal content in the studied β Cephei stars comparedto non-pulsating B-type stars of similar characteristics.
| Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| Observations of Hα, iron, and oxygen lines in B, Be, and shell stars
We carried out a spectroscopic survey of several B, Be, and shell starsin optical and near-infrared regions. Line profiles of the Hα lineand of selected Fe II and O I lines are presented.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| The epoch of the constellations on the Farnese Atlas and their origin in Hipparchus's lost catalogue
Not Available
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| XMM-Newton observations of β Centauri (B1 III): The temperature structure in the hot plasma and the photosphere-wind connection
We present XMM-Newton observations of the B giant β Centauri (B1III). The spectra are rich of spectral lines from a wide range ofionization stages which indicate temperatures in the range ~0.1-0.6 keV.Simultaneous fits to the rgs, epic-mos, and epic-pn spectra yield threeplasma temperatures (0.1, 0.2, and 0.6 keV), emission measures, andelemental abundances which are quite close to solar values. Thesetemperatures are confirmed by DEM modeling. According to the derivedmodels the intrinsic source X-ray luminosity in the energy range 0.3-10keV is Lx = 10 × 1030 erg s-1 ata distance of 161 pc. An analysis of the X-ray light curve suggests thatthe photospheric variability does not have much of an effect on theproperties of the X-ray luminosity. The sensitivity of the He-likeforbidden and intercombination lines to a strong ultraviolet stellarradiation field is used to constrain the radial distances at which thelines of Ne ix, O vii, and N vi originate.
| Metallicity of mono- and multiperiodic β Cephei stars
Analyzing IUE ultraviolet spectra of β Cep pulsating stars wenoticed that multiperiodic variables have a larger mean metal abundancein the photosphere, [ m/H] , than monoperiodic ones. We applystatistical tests to verify this dichotomy. We obtain that, with a largeprobability, the multiperiodic β Cep stars have greater values of [m/H] . This result is consistent with the linear non-adiabatic theory ofpulsation of early B-type stars.
| Metallicities of the β Cephei stars from low-resolution ultraviolet spectra
We derive basic stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddening for all βCephei stars observed during the IUE satellite mission, including thosebelonging to three open clusters. The parameters are derived by means ofan algorithmic procedure of fitting theoretical flux distributions tothe low-resolution IUE spectra and ground-based spectrophotometricobservations. Since the metallicity has a special importance forpulsating B-type stars, we focus our attention in particular on thisparameter.Tables 1, 2, 4 and 5 are only available in electronic form athttp://www.edpsciences.org
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Atomic and Molecular Emission Lines from the Red Rectangle
HD 44179 is the binary, post-asymptotic giant branch, central star ofthe Red Rectangle Nebula. Echelle spectra of the star have been obtainedover a wavelength range from about 3800 to 10000 Å at a resolvingpower R=38,000. A maximum S/N of 850 was achieved near 6800 Å.Fifty-seven identified atomic or ionic emission lines of 12 elements aredetected in the star's spectrum, along with 76 emission lines of CH,CH+, or CN. Three other CN lines are also present inabsorption. Fewer than 30 of these 136 lines apparently have beenpreviously reported, and the newly detected species include N II, Mg I,S II, K I, Fe I, Fe II, Rb I, Ba II, CH, and CN. On the basis of theirshapes and widths, the line profiles of the various species can beclassified into three groups: narrow, broad, or double-peaked. Theemission apparently originates in an unusual, compact H II region and inassociated neutral gas, both concentrated primarily within the small,dusty torus that optically obscures the central star. The detection ofthe Ba II emission from the gas may support the hypothesis of Waelkenset al. that HD 44179 is destined to become a barium star.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Two New Low Galactic D/H Measurements from the Far Ultraviolet Spectroscopic Explorer
We analyze interstellar absorption observed toward two subdwarf O stars,JL 9 and LS 1274, using spectra taken by the Far UltravioletSpectroscopic Explorer (FUSE). Column densities are measured for manyatomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, FeII, and H2), but our main focus is on measuring the D/Hratios for these extended lines of sight, as D/H is an importantdiagnostic for both cosmology and Galactic chemical evolution. We findD/H=(1.00+/-0.37)×10-5 toward JL 9 andD/H=(0.76+/-0.36)×10-5 toward LS 1274 (2 σuncertainties). With distances of 590+/-160 and 580+/-100 pc,respectively, these two lines of sight are currently among the longestGalactic lines of sight with measured D/H. With the addition of thesemeasurements, we see a significant tendency for longer Galactic lines ofsight to yield low D/H values, consistent with previous inferences aboutthe deuterium abundance from D/O and D/N measurements. Short lines ofsight with H I column densities of logN(HI)<19.2 suggest that thegas-phase D/H value within the Local Bubble is(D/H)LBg=(1.56+/-0.04)×10-5. However, thefour longest Galactic lines of sight with measured D/H, which haved>500 pc and logN(HI)>20.5, suggest a significantly lower valuefor the true local disk gas-phase D/H value,(D/H)LDg=(0.85+/-0.09)×10-5. Oneinterpretation of these results is that D is preferentially depletedonto dust grains relative to H and that longer lines of sight thatextend beyond the Local Bubble sample more depleted material. In thisscenario, the higher Local Bubble D/H ratio is actually a betterestimate than (D/H)LDg for the true local disk D/H,(D/H)LD. However, if (D/H)LDg is different from(D/H)LBg simply because of variable astration and incompleteinterstellar medium mixing, then (D/H)LD=(D/H)LDg.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE), which is operated for NASA by JohnsHopkins University under NASA contract NAS5-32985.
| β Cep stars from a spectroscopic point of view
In this review we present the current status of line-profile-variationstudies of β Cep stars. Such studies have been performed for 26bright members of this class of pulsating stars in the past 25 years. Wedescribe all these currently available data and summarize theinterpretations based on them in terms of the excited pulsation modes.We emphasize that line-profile variations offer a much more detailedpicture of the pulsational behaviour of pulsating stars compared toground-based photometric data. The latter, however, remain necessary tounravel the often complex frequency pattern and to achieve unambiguousmode identification for multiperiodic β Cep stars and also toderive the pulsational properties of the faint members of the class. Wehighlight the statistical properties of the sample of 26 stars for whichaccurate spectroscopic studies are available and point out some futureprospects.
| Far Ultraviolet Spectroscopic Explorer Survey of the Local Interstellar Medium within 200 Parsecs
We present Far Ultraviolet Spectroscopic Explorer observations of theinterstellar gas toward 30 white dwarf stars and one subdwarf (SdO)star. These sight lines probe the Local Bubble (LB) and the localinterstellar medium (LISM) near the LB. We systematically measure thecolumn densities for the following species: C II, C II*, C III, N I, NII, O I, Ar I, Si II, P II, Fe II, Fe III, and H2. Our surveydetected only diffuse H2 molecular clouds(fH2<~10-4) along six sight lines.There is no evidence from this study that H2 exists wellinside the perimeter of the LB. The kinematical temperature forH2 is less than the usual temperature observed in the localinterstellar clouds, implying different gas phases in the LISM. Therelative abundance ratios of Si II, P II, and Fe II give insight aboutthe dust content. These ratios vary but are similar to the depletionpatterns observed in warm and halo diffuse clouds in more distant sightlines in the Galaxy. The N I/O I and Ar I/O I ratios are significantlysubsolar within the LB. Outside the LB a larger scatter is observed fromsubsolar to solar. Because Ar and N are only weakly depleted into dustgrains if at all, the deficiencies of their neutral forms are likely dueto photoionization. The evidence for significant ionization of N (andhence Ar) is strengthened by the detection and measurement of N II,which is a dominant ion for this element toward many sight lines. C IIIappears to be ubiquitous in the LISM toward our sight lines, but C IIremains the dominant ionization stage of C. The limits on Fe III/Fe IIimply that Fe II is the dominant ion. These observations imply thatphotoionization is the main ionization mechanism in the LISM and do notsupport the existence of a highly ionized condition in the past. In viewof the variations observed in the different atomic and ionic ratios, thephotoionization conditions vary significantly in the LB and the LISM.The cooling rate in the LISM, lc [in ergs s-1 (H Iatom)-1], derived from the emission of the C II 157.7 μmline has a mean value of loglc=-25.59+/-0.32 dex, verysimilar to previous determinations.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are 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/411/447
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe 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/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Quantitative Stellar Spectral Classification. II. Early Type Stars
The method developed by Stock & Stock (1999) for stars of spectraltypes A to K to derive absolute magnitudes and intrinsic colors from theequivalent widths of absorption lines in stellar spectra is extended toB-type stars. Spectra of this type of stars for which the Hipparcoscatalogue gives parallaxes with an error of less than 20% were observedwith the CIDA one-meter reflector equipped with a Richardsonspectrograph with a Thompson 576×384 CCD detector. The dispersionis 1.753 Å/pixel using a 600 lines/mm grating in the first order.In order to cover the spectral range 3850 Å to 5750 Å thegrating had to be used in two different positions, with an overlap inthe region from 4800 Å to 4900 Å . A total of 116 stars wasobserved, but not all with both grating positions. A total of 12measurable absorption lines were identified in the spectra and theirequivalent widths were measured. These were related to the absolutemagnitudes derived from the Hipparcos catalogue and to the intrinsiccolors (deduced from the MK spectral types) using linear and secondorder polynomials and two or three lines as independent variables. Thebest solutions were obtained with polynomials of three lines,reproducing the absolute magnitudes with an average residual of about0.40 magnitudes and the intrinsic colors with an average residual of0.016 magnitudes.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Extrasolar Micrometeors Radiating from the Vicinity of the Local Interstellar Bubble
Over 12,000 micrometeor detections obtained mainly during 1997 November15-20 and over a similar period in 1998 with the Arecibo UHF (430 MHz)radar have been processed so far. Out of this particular sample, nearly3000 particles show measurable decelerations from which particle sizescan be estimated and extra-atmospheric velocities obtained by directnumerical integration through an appropriate MSIS model atmosphere. Fromthis subsample, 143 objects that either had distinct 1 AU heliocentrichyperbolic orbits or were on elliptical orbits of high eccentricity withsemimajor axes greater than 5 AU were found to have originallyhyperbolic orbits at distances between 50 and 100 AU. Originalextrasolar orbits, velocities, and radiants have been calculated by aCowell-method, numerical integration back 10 yr into the past, whenthese particles first encountered the solar system at greater than 50AU. The perturbation calculations included solar radiation pressure,solar wind magnetic field deflections, and the gravitational influenceof Jupiter, Saturn, Uranus, and Neptune. The original extrasolarparticle sample was parsed down in several ways to make a final list ofinterstellar particle (ISP) candidates. In Galactic coordinates relativeto an assumed LSR (α=18h,δ=30deg, v=20km s-1), most of the remaining 108 ISP radiants form anextended cluster that appears to be coming from the region of the LocalBubble or the Local Cavity. In a new LSR determined by the mean ISPvelocities, many of the particles appear to radiate from a regionstretching from the Vela supernova to the vicinity of the Gemingapulsar. The Geminga supernova is a more likely source or transport agentof the Arecibo ISPs because of its greater age and relatively smalldistance from the Sun. The likelihood of a nearby supernova as an ISPsource or at least an ISP transport mechanism is discussed is somedetail. It is suggested that the Arecibo ISPs are samples at 1 AU of theparticles detected earlier by the Pioneer 10 and 11 spacecraft between 5and 18 AU.
| Boron Abundances in B-Type Stars: A Test of Rotational Depletion during Main-Sequence Evolution
Boron abundances have been derived for seven main-sequence B-type starsfrom Hubble Space Telescope STIS spectra around the B III λ2066line. In two stars, boron appears to be undepleted with respect to thepresumed initial abundance. In one star, boron is detectable but isclearly depleted. In the other four stars, boron is undetectable,implying depletions of 1-2 dex. Three of these four stars are nitrogenenriched, but the fourth shows no enrichment of nitrogen. Onlyrotationally induced mixing predicts that boron depletions areunaccompanied by nitrogen enrichments. The inferred rate of borondepletion from our observations is in good agreement with thesepredictions. Other boron-depleted nitrogen-normal stars are identifiedfrom the literature. In addition, several boron-depleted nitrogen-richstars are identified, and while all fall on the boron-nitrogen trendpredicted by rotationally induced mixing, a majority have nitrogenenrichments that are not uniquely explained by rotation. The spectrahave also been used to determine iron group (Cr, Mn, Fe, and Ni)abundances. The seven B-type stars have near-solar iron groupabundances, as expected for young stars in the solar neighborhood. Wehave also analyzed the halo B-type star PG 0832+676. We find[Fe/H]=-0.88+/-0.10, and the absence of the B III line gives the upperlimit [B/H]<-2.5. These and other published abundances are used toinfer the star's evolutionary status as a post-asymptotic giant branchstar. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal GO 07400.
| The Ionization of Nearby Interstellar Gas
We present new calculations of the photoionization of interstellarmatter within ~5 pc of the Sun (which we refer to as the ``complex oflocal interstellar clouds,'' or CLIC) by directly observed radiationsources, including nearby hot stars and the diffuse emission of the softX-ray background (SXRB). In addition, we model the important, unobservedextreme-ultraviolet (EUV) emission both from the hot gas responsible forthe SXRB and from a possible evaporative boundary between the CLIC andthe hot gas. We carry out radiative transfer calculations and show thatthese radiation sources can provide the ionization and heating of thecloud required to match a variety of observations. The ionizationpredicted in our models shows good agreement with pickup ion results,interstellar absorption-line data toward ɛ CMa, and EUV opacitymeasurements of nearby white dwarf stars. Including the radiation fromthe conductive boundary improves agreement with data on the temperatureand electron density in the cloud. The presence of dust in the cloud, orat least depleted abundances, is necessary to maintain theheating/cooling balance and reach the observed temperature. Using thecolumn density observations as inputs, we derive the gas-phaseabundances of C, N, O, Mg, Si, S, and Fe. Based on these inferreddepletions, it appears that silicate and iron dust exists in the CLIC,while carbonaceous dust has been destroyed. In addition, we findevidence that the Ne abundance in the CLIC is larger than solar.
| Far Ultraviolet Spectroscopic Explorer (FUSE) observations of emitting and absorbing gas in the Local Interstellar Chimney
We present Far Ultraviolet Spectroscopic Explorer (FUSE) satellitemeasurements of the absorption and emission characteristics ofinterstellar gas associated with the Local Interstellar Chimney, whichis an extension of the rarefied Local Bubble cavity that extends outwardfrom the galactic disk towards the lower galactic halo. Far ultraviolet(FUV) diffuse background emission has been detected in the highionization line of O VI (lambda 1032 Å) for two lines-of-sight (l= 162.7deg, b = +57.0deg) and (l =156.3deg, b = +57.8deg) at emission levels of2500+/-700 photons cm-2 s-1 sr-1 (LU)and 3300+/-1100 LU respectively. These levels of O VI emission are verysimilar to those found for four other lines-of-sight sampled thus far bythe FUSE satellite, implying a fairly constant level of average O VIsurface brightness emission at high galactic latitudes of about 2700 LUwith a standard deviation of 450 LU. These emission-line data aresupplemented by FUV interstellar absorption line measurements takentowards the hot DA white dwarf star, REJ 1032+532 (l =157.5deg, b = +53.2deg), whose distance of 116 pcplaces it within the Local Bubble region. No high ionizationinterstellar O VI lambda 1032 Å absorption has been detected (N(OVI) < 13.0 cm-2), which is consistent with thenon-detections of interstellar C IV and Si IV absorption reportedtowards this star by Holberg et al. (\cite{holberg99a}). Taken together,our FUV absorption and emission data may be explained by a scenario inwhich the O VI emission and absorption lines are both formed at theconductive interface of the neutral boundary to the Local Bubble. Forthe presently sampled sight-lines we have found no correlation betweenthe OVI emission line intensity and the associated 0.25 keV soft X-raybackground flux as measured in the R1 and R2 bands by the ROSATsatellite. The OVI line intensities also show no correlation with thesoft X-ray background flux attributable to emission from the milliondegree K gas of the Local Hot Bubble as modeled by Kuntz & Snowden(\cite{kuntz00}). Any (new) model of the Local Bubble must now be ableto explain (i) the low levels of variability in both the O VIemission-line intensity and the associated soft X-ray background fluxfor galactic sight-lines >|40|o, (ii) the observedpressure of P/k ~ 10 000 cm-3 K for the local hotinterstellar gas, and (iii) the paucity of high ionization absorptionlines observed within the local ISM and the sudden increase in theirmeasured column density for distances beyond the Local Bubble neutralboundary.
| Behaviour of the periods in double mode RR Lyrae, V26 in M15
The changes in the periods of the fundamental and first overtonepulsation of the variable V26 in the globular cluster M15 are studied.The analysis was based on published observations. The method ofinvestigation was outlined in our previous paper (Paparó et al.\cite{pap98}). The Fourier phase diagrams for both frequencies have beenconstructed. General guidelines concerning the cycle counting problemsare discussed. In both periods a sudden increase took place around JD2431500 (~1945) and the changes were the same in sign but slightlydifferent in magnitude. The period change behaviour of the fundamentalmode was noticed for the first time. Before the abrupt change thefundamental and first overtone periods were P0 = 0fd538898and P1 = 0fd402256 days and after it P0 =0fd538979 and P1 = 0fd402273 days. Although the fundamentalcomponent is very weak, there is a remarkable concordance between ourperiod after the abrupt change and the period derived by Clement &Walker (\cite{cle91}). The disagreement between the observed periodchanges and the expectation of stellar evolutionary theory stronglysuggest that period changes are dominated by short term changes governedby the physics of the star. Fast changes in oscillation frequency suchas in V26/M15 cannot take place without sudden changes in the stellarstructure. The possible physical processes are the mixing events in asmall part of the semiconvective zone (Sweigart & Renzini\cite{sr79}) or the dredge up of a small amount of helium by occasionalconvection overshooting (Cox \cite{cox98}).
| The origin of the young stellar population in the solar neighborhood -- A link to the formation of the Local Bubble?
We have analyzed the trajectories of moving stellar groups in the solarneighborhood in an attempt to estimate the number of supernovaexplosions in our local environment during the past 20 million years.Using Hipparcos stellar distances and the results of kinematicalanalyses by Asiain et al. (\cite{Asiain1999a}a) on the Pleiades movinggroups, we are able to show that subgroup B1, consisting of early type Bstars up to 10 Msun, but lacking more massive objects, haspassed through the local interstellar medium within less than 100 pc.Comparing the stellar content of B1 with the initial mass functionderived from the analysis of galactic OB associations, we estimate thenumber of supernova explosions and find that about 20 supernovae musthave occurred during the past ~ 10 - 20 million years, which issuggested to be the age of the Local Bubble; the age of the star clusteris about ~ 20 - 30 million years. For the first time, this providesstrong evidence that the Local Bubble must have been created and shapedby multi-supernova explosions and presumably been reheated more than 1million years ago, consistent with recent findings of an excess of60Fe in a deep ocean ferromanganese crust. Calculatingsimilarity solutions of an expanding superbubble for time-dependentenergy input, we show that the number of explosions is sufficient toexplain the size of the Local Bubble. The present energy input rate isabout dot ESN ~ 5 x 1036 erg/s, in good agreementwith the estimated local soft X-ray photon output rate. It seemsplausible that the origin of the Local Bubble is also linked to theformation of the Gould Belt, which originated about 30-60 Myrs ago.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe 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/386/492, and from theauthors on CD-Rom.
| The accretion/diffusion theory for lambda Bootis stars in the light of spectroscopic data
Most of the current theories suggest the lambda Bootis phenomenon tooriginate from an interaction between the stellar surface and its localenvironment. In this paper, we compare the abundance pattern of thelambda Bootis stars to that of the interstellar medium and find largerdeficiencies for Mg, Si, Mn and Zn than in the interstellar medium. Acomparison with metal poor post-AGB stars showing evidence forcircumstellar material indicates a similar physical process possiblybeing at work for some of the lambda Bootis stars, but not for all ofthem. Despite the fact that the number of spectroscopically analysedlambda Bootis stars has considerably increased in the past, a test ofpredicted effects with observations shows current abundance andtemperature data to be still controversial.
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