Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
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.
| 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
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| A new look at the position of the 1604 Supernova (V843 Ophiuchi)
The position of the supernova of 1604 (V843 Oph) is re-assessed, withrelevant discussion pertaining to the present-day remnant, 3C 358.
| 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.
| Differential rotation and star-spot evolution on HK Aqr in 2001 and 2002
We present images of the rapidly rotating (Prot= 0.431 d)M1.5Ve dwarf HK Aqr, from data obtained during 2001 and 2002. Star-spotsare found distributed at a range of latitudes during both observingseasons, but unlike other solar-type rapid rotators of higher mass, HKAqr does not show a strong polar cap at either epoch. In addition to thesurface brightness images, we make use of the four-night time-base overwhich the 2002 observations were made, in order to derive an estimate ofthe latitudinal dependent rotation on HK Aqr. We find that theequator-lap-pole time lies in the range -1449 to +448 d. Although verysmall, we are therefore unable to determine whether the degree ofdifferential rotation is in the solar or anti-solar sense.
| 400 years of Uranometria.
Not Available
| Observations of [S IV] 10.5 μm and [Ne II] 12.8 μm in Two Halo Planetary Nebulae: Implications for Chemical Self-Enrichment
We have detected the [S IV] 10.5 μm and [Ne II] 12.8 μmfine-structure lines in the halo population planetary nebula (PN) DdDm 1and set upper limits on their intensities in the halo PN H4-1. We alsopresent new measurements of optical lines from various ions of S, Ne, O,and H for DdDm 1, based on a high-dispersion spectrum covering thespectral range 3800 Å to 1 μm. These nebulae have similar O/Habundances, (O/H)~1×10-4, but S/H and Ne/H are abouthalf an order of magnitude lower in H4-1 than in DdDm 1; thus H4-1appears to belong to a more metal-poor population. This supportsprevious suggestions that PNe arising from metal-poor progenitor starscan have elevated oxygen abundances due to internal nucleosynthesis andconvective dredge-up. It is generally accepted that high abundances ofcarbon in many PNe result from self-enrichment. To the extent thatoxygen can also be affected, the use of nebular O/H values to infer theoverall metallicity of a parent stellar population (for example, inexternal galaxies) may be suspect, particularly for low metallicities.
| Interstellar Silicon Abundance
We present 34 measurements of silicon gas phase column densities in theinterstellar medium. We have used spectra containing the SiII 1808 Angline which were obtained with the Goddard High Resolution Spectrograph(GHRS) aboard the Hubble Space Telescope (HST). Extinction curveparameters are determined for analyzed lines of sight and relationbetween Si/H ratio and extinction parameters is discussed. We find theabundance of gas phase silicon in diffuse clouds to be lower than thesolar value by a factor of four.
| A 3-5 μ m VLT spectroscopic survey of embedded young low mass stars I. Structure of the CO ice
Medium resolution (lambda /Delta lambda = 5000-10 000) VLT-ISAACM-band spectra are presented of 39 young stellar objects in nearbylow-mass star forming clouds showing the 4.67 μm stretching vibrationmode of solid CO. By taking advantage of the unprecedentedly largesample, high S/N ratio and high spectral resolution, similarities in theice profiles from source to source are identified. It is found thatexcellent fits to all the spectra can be obtained using aphenomenological decomposition of the CO stretching vibration profile at4.67 μm into 3 components, centered on 2143.7 cm-1, 2139.9cm-1 and 2136.5 cm-1 with fixed widths of 3.0, 3.5and 10.6 cm-1, respectively. All observed interstellar COprofiles can thus be uniquely described by a model depending on only 3linear fit parameters, indicating that a maximum of 3 specific molecularenvironments of solid CO exist under astrophysical conditions. A simplephysical model of the CO ice is presented, which shows that the 2139.9cm-1 component is indistinguishable from pure CO ice. It isconcluded, that in the majority of the observed lines of sight, 60-90%of the CO is in a nearly pure form. In the same model the 2143.7cm-1 component can possibly be explained by the longitudinaloptical (LO) component of the vibrational transition in pure crystallineCO ice which appears when the background source is linearly polarised.The model therefore predicts the polarisation fraction at 4.67 μm,which can be confirmed by imaging polarimetry. The 2152 cm-1feature characteristic of CO on or in an unprocessed water matrix is notdetected toward any source and stringent upper limits are given. Whenthis is taken into account, the 2136.5 cm-1 component is notconsistent with the available water-rich laboratory mixtures and wesuggest that the carrier is not yet fully understood. A shallowabsorption band centered between 2165 cm-1 and 2180cm-1 is detected towards 30 sources. For low-mass stars, thisband is correlated with the CO component at 2136.5 cm-1,suggesting the presence of a carrier different from XCN at 2175cm-1. Furthermore the absorption band from solid13CO at 2092 cm-1 is detected towards IRS 51 inthe rho Ophiuchi cloud complex and an isotopic ratio of12CO/13CO=68+/-10 is derived. It is shown that allthe observed solid 12CO profiles, along with the solid13CO profile, are consistent with grains with an irregularlyshaped CO ice mantle simulated by a Continuous Distribution ofEllipsoids (CDE), but inconsistent with the commonly used models ofspherical grains in the Rayleigh limit.Based on observations obtained at the European Southern Observatory,Paranal, Chile, within the observing programs 164.I-0605 and 69.C-0441.ISO is an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, The Netherlands and theUK) and with the participation of ISAS and NASA.Table \ref{SourceList} and Appendices A and B are only available inelectronic 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.
| delta Scorpii: Visual Photometric Variability in 2000-2002
Not Available
| 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.
| Far-Ultraviolet Spectra of B Stars near the Ecliptic
Spectra of B stars in the wavelength range of 911-1100 Å have beenobtained with the Espectrógrafo Ultravioleta de RadiaciónDifusa (EURD) spectrograph on board the Spanish satellite MINISAT-01with ~5 Å spectral resolution. International Ultraviolet Explorer(IUE) spectra of the same stars have been used to normalize Kuruczmodels to the distance, reddening, and spectral type of thecorresponding star. The comparison of eight main-sequence stars studiedin detail (α Vir, ɛ Tau, λ Tau, τ Tau, αLeo, ζ Lib, θ Oph, and σ Sgr) shows agreement withKurucz models, but observed fluxes are 10%-40% higher than the models inmost cases. The difference in flux between observations and models ishigher in the wavelength range between Lyα and Lyβ. Wesuggest that Kurucz models underestimate the far-ultraviolet (FUV) fluxof main-sequence B stars between these two Lyman lines. Computation offlux distributions of line-blanketed model atmospheres including non-LTEeffects suggests that this flux underestimate could be due to departuresfrom LTE, although other causes cannot be ruled out. We found that thecommon assumption of solar metallicity for young disk stars should bemade with care, since small deviations can have a significant impact onFUV model fluxes. Two peculiar stars (ρ Leo and ɛ Aqr) and twoemission-line stars (ɛ Cap and π Aqr) were also studied. Ofthese, only ɛ Aqr has a flux in agreement with the models. The resthave strong variability in the IUE range and/or uncertain reddening,which makes the comparison with models difficult. Based on thedevelopment and utilization of the Espectrógrafo Ultravioleta deRadiación Difusa, a collaboration of the Spanish InstitutoNacional de Técnica Aeroespacial and the Center for EUVAstrophysics, University of California, Berkeley.
| EURD: The Mission and the Stellar Absolute Fluxes of B-Type Stars
We present here stellar spectra of B stars obtained with the EURDspectrograph, one of the three instruments on board MINISAT-01. EURD isa spectrograph specially designed to detect diffuse radiation in thewavelength range between 350 and 1100 Å with 5 Å spectralresolution. EURD main scientific targets are: the spectrum ofinterstellar medium, atmospheric airglow, decaying neutrinos, Moon andearly type stars.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| Doppler images from dual-site observations of southern rapidly rotating stars - I. Differential rotation on PZ Tel
We present Doppler images of the young, late-type star PZ Tel fromspectroscopic observations at two epochs. The 0.95-d period necessitateddual-site observations in order to obtain full phase coverage. Theresulting maximum entropy reconstructions based on least-squaresdeconvolved profiles derived from ~2800 photospheric absorption linesreveal the presence of starspots at all latitudes. By cross-correlationof constant-latitude strips from the images, we derive a measure ofrotation as a function of latitude. The rate of surface shear is foundto be in close agreement with the solar value. Hα absorptiontransients reveal several prominences to be present at the time ofobservation. We also examine further evidence for the young evolutionarystatus of PZ Tel through radial velocity, and lithium abundance.
| Star Patterns on the Aztec Calendar Stone.
Not Available
| He, CNO abundances and v sin i values in He-rich stars
We present projected rotational velocities distribution in He-rich starsagainst normal B type stars as well as the abundance analysis of lightelements and their comparison with results of earlier series. Theanalysis is based on high-resolution observations (R=40000) collected atESO, La Silla, Chile in the optical region and includes 11 stars,amongst them 5 new stars in comparison with an earlier sample. Thedistribution of projected rotational velocities shows a significantexcess of slow rotators, no He-rich stars having v sin i > 130 km/sat the significance level of 99.5%. Based on fully consistent LTEmodels, the helium abundance ranges from 0.1 (solar) to 0.4. Theanalysis on the new sample confirms the results of earlier paper: thelight element abundances display a diverse pattern from under-solar upto above-solar values. Carbon has the same abundance in He-rich and instandard stars for cool He-rich stars, following the predictions ofradiative diffusion in the presence of wind. The star HD149363 appearsrather as He-weak as based on the equivalent widths from R=40 000spectra.
| B Stars as a Diagnostic of Star Formation at Low and High Redshift
We have extended the evolutionary synthesis models by Leitherer et al.by including a new library of B stars generated from the IUEhigh-dispersion spectra archive. We present the library and show how thestellar spectral properties vary according to luminosity classes andspectral types. We have generated synthetic UV spectra for prototypicalyoung stellar populations varying the IMF and the star formation law.Clear signs of age effects are seen in all models. The contribution of Bstars in the UV line spectrum is clearly detected, in particular forgreater ages when O stars have evolved. With the addition of the newlibrary we are able to investigate the fraction of stellar andinterstellar contributions and the variation in the spectral shapes ofintense lines. We have used our models to date the spectrum of the localsuper-star cluster NGC 1705-1. Photospheric lines of C III λ1247,Si III λ1417, and S V λ1502 were used as diagnostics todate the burst of NGC 1705-1 at 10 Myr. Interstellar lines are clearlyseen in the NGC 1705-1 spectrum. Broadening and blueshifts of severalresonance lines are stronger in the galaxy spectrum than in our modelsand are confirmed to be intrinsic of the galaxy. Si II λ1261 andAl II λ1671 were found to be pure interstellar lines with anaverage blueshift of 78 km s-1 owing to a directed outflow ofthe interstellar medium. We have selected the star-forming galaxy1512-cB58 as a first application of the new models to high-z galaxies.This galaxy is at z=2.723, it is gravitationally lensed, and its highsignal-to-noise ratio Keck spectrum shows features typical of localstarburst galaxies, such as NGC 1705-1. Models with continuous starformation were found to be more adequate for 1512-cB58 since there arespectral features typical of a composite stellar population of O and Bstars. A model with Z=0.4 Zsolar and an IMF with α=2.8reproduces the stellar features of the 1512-cB58 spectrum.
| Interstellar Carbon Abundance
We present 10 new measurements of carbon gas phase column density in theinterstellar medium. We have used spectra made with the Goddard HighResolution Spectrograph aboard the Hubble Space Telescope containing theCII 1334.5 Ang and CII* 1335.7 Ang lines. The continuum reconstructionmethod has been used to obtain the carbon column density from theLorentzian damped lines. Extinction curve parameters are determined inselected directions and relation between C/H ratio and extinctionparameters is discussed. A correlation has been found between C/H andthe strength of the 2175 Ang bump. Unlike previous results, we noticethat C/H changes with fractional abundance of molecular hydrogen,f(H_2). The average value of C/H=3.55*10^{-4} for lines of sight withf(H_2)<1*10^{-3} is the same as solar photospheric abundance fromGrevese and Noels (1993) and may represent the real cosmic carbonabundance.
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| 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.
| Age variation of helium abundance in He-rich stars.
Not Available
| On the normal spectral energy distribution of stars: Spectral types O9-B5
The normal energy distributions for fifteen spectral subtypes from O9 toB5 for luminosity classes V, IV, and III are derived. Threephotometrically uniform catalogs served as the source of thespectrophotometric data used. Synthetic color indices for all spectraltypes are calculated using the energy distribution curves obtained.Comparison of these indices with the expected normal color indicessuggests that the energy distributions derived are reliable.
| The absolute magnitude of the early type MK standards from HIPPARCOS parallaxes
We analyse the standards of the MK system with the help of Hipparcosparallaxes, using only stars for which the error of the absolutemagnitude is <= 0.3 mag. We find that the main sequence is a wideband and that, although in general giants and dwarfs have differentabsolute magnitudes, the separation between luminosity classes V and IIIis not clear. Furthermore, there are a number of exceptions to thestrict relation between luminosity class and absolute magnitude. Weanalyse similarly the system of standards defined by Garrison & Gray(1994) separating low and high rotational velocity standards. We findsimilar effects as in the original MK system. We propose a revision ofthe MK standards, to eliminate the most deviant cases. Based on datafrom the ESA Hipparcos astrometry satellite
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| O VI + Ly beta + C II from Starburst and Poststarburst Galaxies. I. Stellar Library and Evolutionary Synthesis Profiles
Evolutionary synthesis models of a stellar population in thefar-ultraviolet are presented. The spectra include the lines O VI lambdalambda 1032, 1038, Ly beta , and C II lambda lambda 1036, 1037. They arebased on a stellar library built with observations of O and B starscollected with Copernicus and the Hopkins Ultraviolet Telescope (HUT).This library is used as input into an evolutionary synthesis code. Theline profile of O VI + Ly beta + C II is computed for different starformation histories (instantaneous burst and continuous star formation)and different assumptions about the initial mass function (IMF). Themetallicity is near the solar value. O VI lambda lambda 1032, 1038 is avery sensitive indicator of the presence or absence of O stars. O VIdevelops a P Cygni profile when formed in stellar winds of the mostmassive stars. When these stars are absent, no O VI is formed. Incontrast, Ly beta and C II are very sensitive indicators for B stars. Ifthese stars dominate, as is the case in poststarburst galaxies, Ly betaand C II are present as strong absorption features, and they are formedin the photosphere of B stars. An equivalent width of Ly beta + C IIlarger than 1 Angstroms always indicates a population younger than 1Gyr. Because of the universal strength of O VI in O stars, O VI is not agood discriminator between instantaneous versus continuous starformation for ages in the starburst phase, but the absence of O VI andthe presence of stellar Ly beta and C II is a good indicator of a shortburst duration and for the galaxy being in a poststarburst phase.Application of this technique to starburst or poststarburst galaxieswill require careful attention to interstellar absorption.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
NEWS@SKY (Science&Space News)
