Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Studies of the Diffuse Interstellar Bands. IV. The Nearly Perfect Correlation Between ??6196.0 and 6613.6
In a sample of 114 diffuse cloud sightlines spanning a wide range ofinterstellar environments, we find the equivalent widths of the diffuseinterstellar bands (DIBs) ?6196.0 and ?6613.6 to beextremely well correlated, with a correlation coefficient of 0.986. Amaximum likelihood functional relationship analysis shows that theobservations are consistent with a perfect correlation if theobservational errors, which are dominated by continuum placement andother systematics such as interfering lines, have been underestimated bya factor of 2. The quality of this correlation far exceeds otherpreviously studied correlations, such as that between the ?5780.5DIB and either the color excess or the atomic hydrogen column density.The unusually tight correlation between these two DIBs would seem tosuggest that they might represent the first pair of DIBs known to be dueto the same molecular carrier. However, further theoretical work will berequired to determine whether the different linewidths and band shapesof these two DIBs can be consistent with a common carrier. If the twoDIBs do not in fact share the same molecular carrier, their two carriersmust be chemically very closely related.
| Molecular Hydrogen in the Far Ultraviolet Spectroscopic Explorer Translucent Lines of Sight: The Full Sample
We report total abundances and related parameters for the full sample ofthe Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogenin 38 translucent lines of sight. New results are presented for the"second half" of the survey involving 15 lines of sight to supplementdata for the first 23 lines of sight already published. We assess thecorrelations between molecular hydrogen and various extinctionparameters in the full sample, which covers a broader range ofconditions than the initial sample. In particular, we are now able toconfirm that many, but not all, lines of sight with shallow far-UVextinction curves and large values of the total-to-selective extinctionratio, RV = AV /E(B -V)—characteristic of larger than average dust grains—areassociated with particularly low hydrogen molecular fractions (f_H_2).In the lines of sight with large RV , there is in fact a widerange in molecular fractions, despite the expectation that the largergrains should lead to less H2 formation. However, we seespecific evidence that the molecular fractions in this sub-sample areinversely related to the estimated strength of the UV radiation fieldand thus the latter factor is more important in this regime. We haveprovided an update to previous values of the gas-to-dust ratio,N(Htot)/E(B - V), based on direct measurements ofN(H2) and N(H I). Although our value is nearly identical tothat found with Copernicus data, it extends the relationship by a factorof 2 in reddening. Finally, as the new lines of sight generally showlow-to-moderate molecular fractions, we still find little evidence forsingle monolithic "translucent clouds" with f_H_2 \sim 1.
| A CO J = 1-0 survey of common optical/uv absorption sightlines
Context: Comparison of optical/uv absorption line data withhigh-resolution profiles of mm-wave CO emission provides complementaryinformation on the absorbing gas, as toward ? Oph. Over the pastthirty years a wealth of observations of CO and other molecules inoptical/uv absorption in diffuse clouds has accumulated for which nocomparable CO emission line data exist. Aims: To acquire mm-waveJ=1-0 CO emission line profiles toward a substantial sample ofcommonly-studied optical/uv absorption line targets and to compare withthe properties of the absorbing gas, especially the predicted emissionline strengths. Methods: Using the ARO 12 m telescope, weobserved mm-wavelength J=1-0 CO emission with spectral resolution R ?3× 106 and spatial resolution 1' toward a sample of 110lines of sight previously studied in optical/uv absorption lines of CO,H2, CH, etc. Results: Interstellar CO emission was detected along65 of the 110 lines of sight surveyed and there is a generalsuperabundance of CO emission given the distribution of galacticlatitudes in the survey sample. Much of the emission is optically thickor very intense and must emanate from dark clouds or warm dense gas nearHII regions. Conclusions: Judging from the statisticalsuperabundance of CO emission, seen also in the total line of sightreddening, the OB star optical/uv absorption line targets must bephysically associated with the large quantities of neutral gas whose COemission was detected, in which case they are probably influencing theabsorbing gas by heating and/or photoionizing it. This explains whyCO/H2 and 12CO/13CO ratios differ somewhat betweenuv and mm-wave absorption line studies. Because the lines of sight havebeen preselected to have AV ? 1 mag, relatively little ofthe associated material actually occults the targets, making itdifficult for CO emission line observations to isolate the foregroundgas contribution.Based on observations obtained with the ARO Kitt Peak 12 mtelescope.
| CN column densities and excitation temperatures
We analyse abundances and rotational temperatures of the interstellar CNmolecule. We have calculated the column densities and excitationtemperatures of the molecule along 73 lines of sight basing on ouroriginal measurements of the B 2?+ -X2?+ (0,0) vibrational band recorded in highsignal-to-noise ratio spectra and also for 88 directions based onmeasurements already available in literature. We compare the columndensities obtained from different bands of CN molecule available toground-based instruments. The obtained excitation temperatures in theanalysed directions show always an excess over the cosmic microwavebackground radiation (CMBR) temperature.
| On the origin of field O-type stars
Aims. We aim to identify the origins of field O-stars in the nearest 2to 3 kpc around the Sun using the best presently available kinematicdata on O-stars and on young open clusters. We investigate the questionof whether the present-day data are consistent with the assumption thatO-stars have formed in groups (clusters, associations), or in isolation. Methods: We apply the epicycle theory to back-trace the orbits ofO-type stars and of candidate parent open clusters. Results: From the370 O-stars in the “Galactic O star catalog v 2.0” (GOSV2)we have investigated 93 stars classified as field, and found the originfor 73 of them in 48 open clusters younger than 30 Myr. Only for 32stars or about 9% of all O-stars from this catalogue is the question oftheir origin in groups not solved; some of them may have originated inisolation or may have disintegrated the group in which they formed.Fifty percent of the young open clusters (age < 30 Myr) in the“Catalogue of Open Cluster Data” (COCD) have O-stars asmembers, or have ejected at least one O-star in the first 10 Myr oftheir life, or both. During this period the average mass loss from openclusters by ejecting O-stars is found to be 3 to 5 M_ȯ per Myr. Weprove that ζ Pup had its origin in the open cluster Trumpler 10which it left about 2.5 Myr ago, and that its present-day distance is300 pc (compared to 440 pc before). The revised distance implies asignificant revision of the stellar parameters (a radius of 14 R_ȯ,a mass of 22.5 M_ȯ, and a luminosity of log L/L_ȯ of 5.74)i.e., ζ Pup is closer, less massive, and less luminous thanpreviously thought. Our findings provide independent estimates of thepresent-day distances and absolute magnitudes of field O-stars.
| The relation between CH and CN molecules and carriers of 5780 and 5797 diffuse interstellar bands
Optical absorption bands of the interstellar CN (near 3875 Å) andCH molecules (the violet and blue ones near 4300 and 3886 Å,respectively) were applied to determine the column densities of thesetwo radicals in a statistically meaningful sample of 84 reddened OBstars. Equivalent widths of the major 5780 and 5797 diffuse bands (DIBs)were measured along the lines of sight toward the same stars in spectraacquired using four echelle spectrographs situated in both the northernand southern hemispheres. The mutual relation between abundances of CHand CN molecules shows a large scatter; and especially the CN moleculeabundance varies strongly from cloud to cloud. The carriers of the major5780 and 5797 DIBs seem to be spatially correlated with column densitiesof CH rather than of the CN molecule. This is most likely true in thecase of a narrower feature: the 5797 DIB correlates with CH columndensity better than 5780 does. The correlations do suggest that the DIBcarriers are likely hydrocarbons. They apparently occupy molecularclouds since the H{2} abundance is closely related to that ofmethylidyne (CH), as has already been demonstrated.
| Automated classification of ELODIE stellar spectral library using probabilistic artificial neural networks
A Probabilistic Neural Network model has been used for automatedclassification of ELODIE stellar spectral library consisting of about2000 spectra into 158 known spectro-luminosity classes. The full spectrawith 561 flux bins and a PCA reduced set of 57, 26 and 16 componentshave been used for the training and test sessions. The results show aspectral type classification accuracy of 3.2 sub-spectral type andluminosity class accuracy of 2.7 for the full spectra and an accuracy of3.1 and 2.6 respectively with the PCA set. This technique will be usefulfor future upcoming large databases and their rapid classification.
| A Preliminary Investigation of the Diffuse Interstellar Line at 8621 Å
We have obtained high-resolution spectra at the Dominion AstrophysicalObservatory, the Calar Alto Observatory, and the European SouthernObservatory of hot stars in the near-infrared region to study thediffuse interstellar band (DIB) at 8621 Å. Field stars as well asselected members of the Perseus spiral arm, the ρ Ophiuchi complex,and the Cygnus OB2 association were observed and the equivalent widthsof the band measured, as well as interstellar K I absorption at 7699Å. In total we measure the equivalent width of the 8621 band in 64stars. In a series of figures we show the correlations of the DIB withreddening, polarization, K I, and strengths of other DIBs at 5780, 5797,and 6613 Å. The quality of the correlations are discussed by meansof the Spearman rank correlation test. Good correlations are found withreddening and, among the other DIBs, 8621 correlates best with 5780. Byexamining the three special regions mentioned above we confirm theweakness of the 8621 DIB in the hot stars of the ρ Oph star-formingregion. In the Perseus spiral arm region we find that the DIBs areformed largely in foreground clouds but not in the gas that appears tohave been blown away from the young massive stars of the Perseus arm. InCyg OB2 we find that the equivalent widths of the 8621 line lie abovethe linear correlation of equivalent width with E(B-V) and theirstrength does not vary as strongly with reddening. The heavily reddenedCyg OB2 star 12 is the most extreme example of the relativeinsensitivity of the 8621 equivalent width to reddening in the CygnusOB2 region.
| Is There Enhanced Depletion of Gas-Phase Nitrogen in Moderately Reddened Lines of Sight?
We report on the abundance of interstellar neutral nitrogen (N I) for 30sight lines, using data from the Far Ultraviolet Spectroscopic Explorer(FUSE) and the Hubble Space Telescope (HST). N I column densities arederived by measuring the equivalent widths of several ultravioletabsorption lines and subsequently fitting those to a curve of growth. Wefind a mean interstellar N/H of 51+/-4 ppm. This is below the mean foundby Meyer et al. of 62+4-3 ppm (adjusted for adifference in f-values). Our mean N/H is similar, however, to the(f-value adjusted) mean of 51+/-3 ppm found by Knauth et al. for alarger sample of sight lines with larger hydrogen column densitiescomparable to those in this study. We discuss the question of whether ornot nitrogen shows increased gas-phase depletion in lines of sight withcolumn densities logN(Htot)>~21, as claimed by Knauth etal. The nitrogen abundance in the line of sight toward HD 152236 isparticularly interesting. We derive very small N/H and N/O ratios forthis line of sight that may support a previous suggestion that membersof the Sco OB1 association formed from an N-deficient region.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| New Elements for 80 Eclipsing Binaries VII.
This research presents new elements for 80 eclipsing binaries found withthe help of the ASAS-3, Hipparcos and NSVS databases.
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| Some Diffuse Interstellar Bands Related to Interstellar C2 Molecules
We have investigated the correlations between the equivalent widths of21 selected diffuse interstellar bands (DIBs) and the correspondinginterstellar column densities N(C2), N(CN), and N(CH), toward53 stars with color excesses 0.11<=E(B-V)<=1.99. The observationaldata were derived primarily from echelle spectra acquired at R=38,000 aspart of our extensive, continuing survey of the bands. All but six ofthe 53 final spectra show signal-to-noise ratios >=800 at 5780Å. The principal result presented here is that seven of the 21bands prove to be examples of ``the C2 DIBs,'' a class ofweak, narrow bands whose normalized equivalent widthsWλ(X)/Wλ (λ6196) are wellcorrelated specifically with N(C2)/E(B-V) via power laws. Incontrast, the similarly normalized equivalent widths of the 14 other,well-known DIBs analyzed here are uncorrelated, or weaklyanticorrelated, with N(C2)/E(B-V), to within theobservational uncertainties. Thus, the polyatomic molecule(s) presumedto cause these seven C2 DIBs may bear a direct chemicalrelation to C2 that is not shared by the polyatomic moleculesputatively responsible for the other 14 bands. The C2 DIBsalso show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) inour particular sample of light paths, although generally with shallowerslopes in the case of N(CN) and with greater scatter in the case ofN(CH). Eleven additional C2 DIBs are also identified but arenot analyzed here. Among the 18 C2 DIBs identified, fourapparently have not been previously detected. The λ4963 band isgenerally the strongest of the 18 C2 DIBs, while theλ4734 band shows the most sensitive correlation withN(C2).Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Masses and other parameters of massive binaries
Binary stars provide us with the means to measure stellar mass. Here Ipresent several lists of known O-type stars with reliable mass estimatesthat are members of eclipsing, double-lined spectroscopic binaries. Themasses of young, unevolved stars in binaries are suitable for testingthe predictions of evolutionary codes, and there is good agreementbetween the observed and predicted masses (based upon temperature andluminosity) if the lower temperature scale from line-blanketed modelatmospheres is adopted. A final table lists masses for systems in awide variety of advanced evolutionary stages.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Atlas of Interstellar Extinction Curves of OB Stars Covering the Whole Available Wavelength Range
The paper presents a collection of 436 extinction curves covering thewhole available range of wavelengths from satellite UV to near-IR. Thedata were taken from the ANS photometric catalogue and from thecompilations of IR photometric measurements. The data curves have beenobtained with the aid of ``artificial standards": Papaj et al. (1993)and Wegner (1994, 1995). The visual magnitudes and spectralclassifications of O and B type stars with EB-V>= 0.05were taken from the SIMBAD database. The curves are given in the form ofplots and tables E{lambda - V} / EB-V versus1/λ. The observed variety of extinction laws among slightlyreddened stars is apparently due to the various physical parameters ofinterstellar clouds.
| 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 Serpens OB2 Association and Its Thermal ``Chimney''
UBVRI photometry and MK spectral types have been obtained for a nearlycomplete sample of the massive star population of the Serpens OB2association. This relatively neglected association is found to containover 100 OB stars at a distance of 1.9+/-0.3 kpc and with a common ageof 5+/-1 Myr. These stars illuminate the large H II region S54, and aredirectly connected to the unusual thermal ``chimney'' detected byMüller et al. Evidence is given of continuing star formation withinthe association, and the relationship between the association stars andthe chimney is discussed.
| Observational Evidence of Supershell Blowout in GS 018-04+44: The Scutum Supershell
Emission in the H I 21 cm line has been mapped for a region of theGalaxy that includes two known supershells, GS 018-04+44 and GS034-06+65. We focus on the GS 018-04+44, hereafter referred to as theScutum Supershell, which is an elongated shell about 5 deg in diameterextending to -7 deg below the Galactic plane. The Scutum shell lies at akinematic distance of ~3300 pc, implying a shell diameter of ~290 pcwith a vertical extension of ~400 pc away from the Galactic plane. TheScutum shell contains 6.2x105 Msolar swept intothe walls. We observe that the top of the shell is missing, and asubstantial column of H I rises from the shell walls tob=-11deg, culminating in a large cloud of neutral hydrogen,3.74x104 Msolar, located ~630 pc from the plane.ROSAT data show X-ray emission that closely anticorrelates with the 21cm emission. This emission probably originates from hot gas within theScutum Supershell. After approximately correcting for the foregroundabsorption, we find that the 1.5 keV X-rays peak at the base of theshell, the 0.75 keV emission peaks in the interior and at the top of theshell, and the 0.25 keV emission extends to high latitudes above theshell. The X-ray luminosity is roughly ~5x1036 ergss-1. The Wisconsin Hα Mapper (WHAM) survey shows thepresence of Hα emission that exhibits a morphology similar to thatof the H I. Spectra indicate the presence of ionized hydrogen atvelocities similar to the H I, placing ionized material at the samekinematic distance as the neutral material. IRAS images in the 60 and100 μm wavebands reveal the presence of dust correlated with theneutral hydrogen. Infrared surface brightness indicates an excess in the100 μm emission, which could indicate a molecular hydrogen componentwith a column density of 2.4x1021 cm-2 in thedensest regions of the high-latitude cloud of neutral hydrogen. IUEultraviolet high dispersion spectra of HD 177989 (l=17.89d, b=-11.88d)and HD 175754 (l=16.40d, b=-9.92d) reveal the presence of very strongabsorption by highly ionized gas at a velocity that associates theabsorption with the ejecta of the Scutum Supershell. In the case of HD177989, the high ion column density ratios suggest an origin in aturbulent mixing layer where hot and cool gases mix in the presence ofshear flows. The Hα and X-ray emission suggest that a multitude ofenergetic phenomena exist in this region, providing the necessaryionizing radiation. Indeed, there are multiple supernova remnants, H IIregions, and hot stars, which could all contribute sizeable amounts ofenergy and ionizing radiation. The combination of these data setsindicates observational evidence of a ``blowout'' phenomena whereby hotmaterial produced within the Scutum Supershell has blown through the topof the shell and been pushed to high latitude. Based on observationsfrom the Green Bank 43m radio telescope, the ROSAT All-Sky Survey, theWHAM All-Sky Survey, IRAS, and IUE.
| Interstellar C_2 absorption lines towards CH+ forming regions
Interstellar C_2 absorption line observations towards the southern OBassociations NGC 2439, Vela OB1, and Cen OB1 are used to infergaskinetic temperatures Tkin and densities nctowards lines of sight with previously determined large CH andCH+ column densities. Towards NGC 2439, the material ischaracterised by temperatures of Tkin = 75-85 K and densitiesexceeding nc > 1000 C_2m, and a fractional C_2 abundanceof about x(C_2) = 1.5 10-8. Temperatures and densitiesinferred towards two stars in Vela OB1 are Tkin = 65-85 K andnc >= 600 C_2m. C_2 fractional abundances in the cloudcomplex obscuring Vela OB1 are about half the value found in NGC 2439.For the lines of sight towards Cen OB1, values of Tkin =70-95 K and fractional abundances of x(C_2) ~ 5 10-9 arefound, and towards HD 114213, Tkin = 25 K and nc =400+/-100 C_2m. The observations demonstrate that C_2 resides in coolgas at temperature of Tkin < 100 K. The C_2 columndensities are correlated with those of CH, which is expected fromtheoretical models. This suggests that CH is efficiently formed in thecool material as well, together with C_2. This finding does not supportexpectations from recent MHD and vortex models of CH+formation, which stipulate that neutrals such as CH are abundantlyproduced in hot regions of many 100 K temperature.
| 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.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| The ``Mass Discrepancy'' for Massive Stars: Tests of Models Using Spectroscopic Binaries
Stellar evolutionary models are often used to infer a star's mass viaits luminosity, but empirical checks on the accuracy of the theoreticalmass-luminosity relation for very massive stars have been lacking. Thisis of particular concern given that modern atmosphere models yieldsystematically smaller masses for massive stars than do evolutionarymodels, with the discrepancy being a factor of 2 for Of stars. Weattempt to resolve this mass discrepancy by obtaining new,high-resolution optical data on seven early-type spectroscopic binaries:V453 Cyg, HD 191201, V382 Cyg, Y Cyg, HD 206267, DH Cep, and AH Cep. Ourstudy produces improved spectral subtypes for the components of thesesystems, which are crucial for evaluating their luminosities andlocations in the H-R diagram. Our radial velocity study utilizes ameasuring method that explicitly accounts for the effects of pairblending. We combine our new orbit solutions with existing data oninclinations and distances when available to compare the orbital masseswith evolutionary models, and we find good agreement in all cases wherethe stars are noninteracting. (The components of V382 Cyg and DH Cepfill their Roche lobes, and in both cases we find masses substantiallylower than the masses inferred from evolutionary tracks, suggesting thatsignificant material has been lost rather than transferred. We confirmthat this same trend exists for other systems drawn from theliterature.) Our own data extends to only 15 Mȯ, althoughphotometric inclination determinations for HD 191201 and HD 206267should prove possible and will provide examples of higher mass systems.We briefly discuss suitable systems from the literature and concludethat orbit solutions provide good agreement with the evolutionary modelsto 25 Mȯ. Beyond this, most known binaries either fill their Rochelobes or have other complications. We also discuss five systems forwhich our improved data and analysis failed to yield acceptable orbitsolutions: EO Aur, IU Aur, V640 Mon (Plaskett's star), LY Aur, and 29 UWCMa all remained intractable, despite improved data.
| Accurate Two-dimensional Classification of Stellar Spectra with Artificial Neural Networks
We present a solution to the long-standing problem of automaticallyclassifying stellar spectra of all temperature and luminosity classeswith the accuracy shown by expert human classifiers. We use the 15Angstroms resolution near-infrared spectral classification systemdescribed by Torres-Dodgen & Weaver in 1993. Using the spectrum withno manual intervention except wavelength registration, artificial neuralnetworks (ANNs) can classify these spectra with Morgan-Keenan types withan accuracy comparable to that obtained by human experts using 2Angstroms resolution blue spectra, which is about 0.5 types (subclasses)in temperature and about 0.25 classes in luminosity. Accuratetemperature classification requires a hierarchy of ANNs, whileluminosity classification is most successful with a single ANN. Wepropose an architecture for a fully automatic classification system.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Serpiente |
| Ascensión Recta: | 18h12m24.66s |
| Declinación: | -10°43'53.0" |
| Magnitud Aparente: | 8.462 |
| Distancia: | 10000000 parsecs |
| Movimiento Propio en Ascensión Recta: | -0.1 |
| Movimiento Propio en Declinación: | 0.8 |
| B-T magnitude: | 9.598 |
| V-T magnitude: | 8.556 |
Catálogos y designaciones:
|
