Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Observational Properties of Rotationally Excited Molecular Hydrogen in Translucent Lines of Sight
The Far Ultraviolet Spectroscopic Explorer (FUSE) has allowed precisedeterminations of the column densities of molecular hydrogen(H2) in Galactic lines of sight with a wide range ofpathlengths and extinction properties. However, survey studies of linesof sight with greater extinction have been mostly restricted to thelow-J states (lower total angular momentum) in which most molecularhydrogen is observed. This paper presents a survey of column densitiesfor the molecular hydrogen in states of greater rotational excitation (J>= 2) in Galactic lines of sight with log N(H2) >~ 20.This study is comprehensive through the highest excited state detectablein each line of sight. J = 5 is observed in every line of sight, and wedetect J = 7 in four lines of sight, J = 8 in one line of sight, andvibrationally excited H2 in two lines of sight. We comparedthe apparent b-values and velocity offsets of the higher-J statesrelative to the dominant low-J states and we found no evidence of anytrends that might provide insight into the formation of higher-JH2, although these results are the most affected by thelimits of the FUSE resolution. We also derive excitation temperaturesbased on the column densities of the different states. We confirm thatat least two distinct temperatures are necessary to adequately describethese lines of sight, and that more temperatures are probably necessary.Total H2 column density is known to be correlated with othermolecules; we explore if correlations vary as a function of J forseveral molecules, most importantly CH and CH+. Finally, webriefly discuss interpretations of selected lines of sight by comparingthem to models computed using the Meudon PDR code.
| A Detailed Far-Ultraviolet Spectral Atlas of Main-Sequence B Stars
We have constructed a detailed spectral atlas covering the wavelengthregion 930-1225 Å for 10 sharp-lined B0-B9 stars near the mainsequence. Most of the spectra we assembled are from the archives of theFar Ultraviolet Spectroscopic Explorer satellite, but for nine stars,wavelength coverage above 1188 Å was taken from high-resolutionInternational Ultraviolet Explorer or echelle Hubble SpaceTelescope/Space Telescope Imaging Spectrograph spectra. To represent thetenth star at type B0.2 V, we used the Copernicus atlas of ? Sco. Wemade extensive line identifications in the region 949-1225 Å ofall atomic features having published oscillator strengths at types B0,B2, and B8. These are provided as a supplementary dataproduct—hence the term detailed atlas. Our list of found featurestotals 2288, 1612, and 2469 lines, respectively. We were able toidentify 92%, 98%, and 98% of these features with known atomictransitions with varying degrees of certainty in these spectra. Theremaining lines do not have published oscillator strengths. Photosphericlines account for 94%, 87%, and 91%, respectively, of all ouridentifications, with the remainder being due to interstellar (usuallymolecular H2) lines. We also discuss the numbers of lineswith respect to the distributions of various ions for these three moststudied spectral subtypes. A table is also given of 162 least blendedlines that can be used as possible diagnostics of physical conditions inB star atmospheres.
| The Extension of the Transition Temperature Plasma into the Lower Galactic Halo
Column densities for H I, Al III, Si IV, C IV, and O VI toward 109 starsand 30 extragalactic objects have been assembled to study the extensionsof these species away from the Galactic plane into the Galactic halo. HI and Al III mostly trace the warm neutral and warm ionized medium,respectively, while Si IV, C IV, and O VI trace a combination of warmphotoionized and collisionally ionized plasmas. The much larger objectsample compared to previous studies allows us to consider and correctfor the effects of the sample bias that has affected earlier but smallersurveys of the gas distributions. We find that Si IV and C IV havesimilar exponential scale heights of 3.2(+1.0, -0.6) and 3.6(+1.0,-0.8) kpc. The scale height of O VI is marginally smaller with h =2.6 ± 0.6 kpc. The transition temperature gas is ~3 times moreextended than the warm ionized medium traced by Al III with h =0.90(+0.62, -0.33) kpc and ~12 times more extended than the warmneutral medium traced by H I with h = 0.24 ± 0.06 kpc. There is afactor of 2 decrease in the dispersion of the log of the column densityratios for transition temperature gas for lines of sight in the Galacticdisk compared to extragalactic lines of sight through the entire halo.The observations are compared to the predictions of the various modelsfor the production of the transition temperature gas in the halo. Theappendix presents a revision to the electron scale height of Gaensler etal.'s 2008 study based on electron dispersion measures.
| A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
A study of gas-phase element abundances reported in the literature for17 different elements sampled over 243 sight lines in the local part ofour Galaxy reveals that the depletions into solid form (dust grains) areextremely well characterized by trends that employ only three kinds ofparameters. One is an index that describes the overall level ofdepletion applicable to the gas in any particular sight line, and theother two represent linear coefficients that describe how to derive eachelement's depletion from this sight-line parameter. The information fromthis study reveals the relative proportions of different elements thatare incorporated into dust at different stages of grain growth. Anextremely simple scheme is proposed for deriving the dust contents andmetallicities of absorption-line systems that are seen in the spectra ofdistant quasars or the optical afterglows of gamma-ray bursts. Contraryto presently accepted thinking, the elements sulfur and krypton appearto show measurable changes in their depletions as the general levels ofdepletions of other elements increase, although more data are needed toascertain whether or not these findings are truly compelling. Nitrogenappears to show no such increase. The incorporation of oxygen into solidform in the densest gas regions far exceeds the amounts that can takethe form of silicates or metallic oxides; this conclusion is based ondifferential measurements of depletion and thus is unaffected byuncertainties in the solar abundance reference scale.Based in large part on published observations from (1) the NASA/ESAHubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555, (2) theFar Ultraviolet Spectroscopic Explorer (FUSE) mission operated by JohnsHopkins University, supported by NASA contract NAS5-32985, and (3) TheCopernicus satellite, supported by NASA grant NAGW-77 to PrincetonUniversity.
| Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
We carried out a comprehensive far-UV survey of 12CO andH2 column densities along diffuse molecular Galactic sightlines. This sample includes new measurements of CO from HST spectraalong 62 sight lines and new measurements of H2 from FUSEdata along 58 sight lines. In addition, high-resolution optical datawere obtained at the McDonald and European Southern Observatories,yielding new abundances for CH, CH+, and CN along 42 sightlines to aid in interpreting the CO results. These new sight lines wereselected according to detectable amounts of CO in their spectra andprovide information on both lower density (<=100 cm-3) andhigher density diffuse clouds. A plot of logN(CO) versuslogN(H2) shows that two power-law relationships are neededfor a good fit of the entire sample, with a break located atlogN(CO,cm-2)=14.1 and logN(H2)=20.4,corresponding to a change in production route for CO in higher densitygas. Similar logarithmic plots among all five diatomic molecules revealadditional examples of dual slopes in the cases of CO versus CH (breakat logN=14.1, 13.0), CH+ versus H2 (13.1, 20.3),and CH+ versus CO (13.2, 14.1). We employ both analytical andnumerical chemical schemes in order to derive details of the molecularenvironments. In the denser gas, where C2 and CN moleculesalso reside, reactions involving C+ and OH are the dominantfactor leading to CO formation via equilibrium chemistry. In thelow-density gas, where equilibrium chemistry studies have failed toreproduce the abundance of CH+, our numerical analysis showsthat nonequilibrium chemistry must be employed for correctly predictingthe abundances of both CH+ and CO.
| The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in the Disk of the Milky Way
To probe the distribution and physical characteristics of interstellargas at temperatures T~3×105 K in the disk of the MilkyWay, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) toobserve absorption lines of O VI λ1032 toward 148 early-typestars situated at distances >1 kpc. After subtracting off a mildexcess of O VI arising from the Local Bubble, combining our new resultswith earlier surveys of O VI, and eliminating stars that showconspicuous localized X-ray emission, we find an average O VI midplanedensity n0=1.3×10-8 cm-3. Thedensity decreases away from the plane of the Galaxy in a way that isconsistent with an exponential scale height of 3.2 kpc at negativelatitudes or 4.6 kpc at positive latitudes. Average volume densities ofO VI along different sight lines exhibit a dispersion of about 0.26 dex,irrespective of the distances to the target stars. This indicates that OVI does not arise in randomly situated clouds of a fixed size anddensity, but instead is distributed in regions that have a very broadrange of column densities, with the more strongly absorbing cloudshaving a lower space density. Line widths and centroid velocities aremuch larger than those expected from differential Galactic rotation, butthey are nevertheless correlated with distance and N(O VI), whichreinforces our picture of a diverse population of hot plasma regionsthat are ubiquitous over the entire Galactic disk. The velocity extremesof the O VI profiles show a loose correlation with those of very stronglines of less ionized species, supporting a picture of a turbulent,multiphase medium churned by shock-heated gas from multiple supernovaexplosions.
| Exploring interstellar titanium and deuterium abundances and other correlations
Aims. The origin of the observed variability of the gas-phase D/H ratioin the local interstellar medium is still debated, and in particular therole of deuterium depletion onto dust grains. Here we extend the studyof the relationship between deuterium and titanium, a refractory speciesand tracer of elemental depletion, and explore other relationships. Methods: We have acquired high resolution spectra for nine early-typestars using the VLT/UVES spectrograph, and detected the absorption linesof interstellar TiII. Using a weighted orthogonal distance regression(ODR) code and a special method to treat non symmetric errors, wecompare the TiII columns with the corresponding HI, DI and also OIcolumns. In parallel we perform the same comparisons for available FeIIdata. Results: We find a significant correlation between TiII/HI andD/H in our data set, and, when combined with published results, weconfirm and better constrain the previously established trends andextend the trends to low HI columns. We exclude uncertainties in HI andOI columns as the main contributor to the derived metals-deuteriumcorrelations by showing that the TiII/HI ratio is positively correlatedwith DI/OI. We find a similar correlation between FeII/HI and DI/OI. TheTiII gradients are similar or slightly smaller than for FeII, while onewould expect larger variations on the basis of the higher condensationtemperature of titanium. However we argue that ionisation effectsintroduce biases that affect iron and not titanium and may explain thegradient similarity. We find a less significant negative correlationbetween the TiII/DI ratio and the hydrogen column, possibly a sign ofdifferent evaporation of D and metals according to the cloud properties.More TiII absorption data along very low H column lines-of-sight wouldbe useful to improve the correlation statistics.Based on observations taken with the Ultraviolet and Visual EchelleSpectrograph (UVES) on the Very Large Telescope (VLT) Unit 2 (Kueyen) atParanal, Chile, operated by ESO.
| The Variation of Magnesium Depletion with Line-of-Sight Conditions
In this paper we report on the gas-phase abundance of singly ionizedmagnesium (Mg II) in 44 lines of sight, using data from the Hubble SpaceTelescope (HST). We measure Mg II column densities by analyzing medium-and high-resolution archival Space Telescope Imaging Spectrograph (STIS)spectra of the 1240 Å doublet of Mg II. We find that Mg IIdepletion is correlated with many line-of-sight parameters [e.g.,f(H2), EB-V, EB-V/r, AV, andAV/r] in addition to the well-known correlation with. These parameters should be more directly relatedto dust content and thus have more physical significance with regard tothe depletion of elements such as magnesium. We examine the significanceof these additional correlations as compared to the known correlationbetween Mg II depletion and . While none of thecorrelations are better predictors of Mg II depletion than, some are statistically significant even assumingfixed . We discuss the ranges over which thesecorrelations are valid, their strength at fixed ,and physical interpretations.
| New Insights on Interstellar Gas-Phase Iron
In this paper we report on the gas-phase abundance of singly ionizediron (Fe II) for 51 lines of sight, using data from FUSE. Fe II columndensities are derived by measuring the equivalent widths of several UVabsorption lines and subsequently fitting those to a curve of growth.Our derivation of Fe II column densities and abundances creates thelargest sample of iron abundances in moderately to highly reddened linesof sight explored with FUSE, lines of sight that are on average morereddened than lines of sight in previous Copernicus studies. We presentthree major results. First, we observe the well-established correlationbetween iron depletion and and also find trendsbetween iron depletion and other line-of-sight parameters [e.g.,f(H2), EB-V, and AV], and we examinethe significance of these trends. Of note, a few of our lines of sightprobe larger densities than previously explored and we do not seesignificantly enhanced depletion effects. Second, we present twodetections of an extremely weak Fe II line at 1901.773 Å in thearchival STIS spectra of two lines of sight (HD 24534 and HD 93222). Wecompare these detections to the column densities derived through FUSEspectra and comment on the line's f-value and utility for future studiesof Fe II. Finally, we present strong anecdotal evidence that the Fe IIf-values derived empirically through FUSE data are more accurate thanprevious values that have been theoretically calculated, with theprobable exception of f1112.
| The Galactic deuterium abundance and dust depletion: insights from an expanded Ti/H sample
The primordial abundance of deuterium (D/H) yields a measure of thedensity of baryons in the Universe and is an important complement todeterminations from cosmic microwave background (CMB) experiments.Indeed, the current small samples of high-redshift D/H measurements fromquasar absorption line studies are in excellent agreement withCMB-derived values. Conversely, absorption line measurements of theGalactic D/H ratio in almost 50 stellar sightlines show a puzzlinglylarge scatter outside the local bubble which is difficult to explainsimply by astration from the primordial value. The currently favouredexplanation for these large variations is that D is differentiallydepleted relative to H in some parts of the local interstellar medium(ISM). Here, we test this scenario by studying the correlation betweenD/H and the abundance of titanium, one of the most refractory elementsreadily observed in the ISM. Previous work by Prochaska, Tripp &Howk found tentative evidence for a correlation between Ti/H and D/Hbased on seven sightlines. Here we almost triple the number of previousTi measurements and include several sightlines with very high or low D/Hthat are critical for quantifying any correlations with D/H. With ourlarger sample, we confirm a correlation between Ti/H and D/H at the 97per cent confidence level. However, the magnitude of this dependence isdifficult to reconcile with a simple model of dust depletion for tworeasons. First, contrary to what is expected from local depletion rates,the gradient of the highly refractory Ti is much shallower than thatobserved for Fe and Si. Secondly, we do not observe the establishedtight, steep correlation between [Ti/H] and the mean volume density ofhydrogen. Therefore, whilst dust remains a plausible explanation for thelocal D/H variations, the abundances of at least some of the refractoryelements do not provide unanimous support for this scenario. We alsoargue that the correlations of [Si/H], [Fe/H] and [Ti/H] with D/H areinconsistent with a simple infall model of low-metallicity gas withapproximately solar abundances as the dominant cause for D variations.Based on observations made with ESO Telescopes at the ParanalObservatories under programme ID 076.C-0503(A).E-mail: sarae@uvic.ca
| The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| Composition of the Interstellar Medium
We present an analysis of the FeII, GeII, MgI, MgII, SI, SII, SiII andZnII interstellar lines for 63 stars. Column density of MgII, SII, SiIIand ZnII is well correlated with distance. However, the column density-- distance relation should be used with care for the estimation of thedistance to OB stars. For stars with large f(H_2) this relation can leadto a large overestimation of the distance. The column densities of MgII,SiII and GeII (our largest samples of data) normalized to their totalhydrogen column densities decrease with interstellar reddening E(B-V) asexpected for elements that are incorporated into dust grains. The GeIIabundance (GeII/H) is lower in dense molecular clouds. The abundances ofall analyzed elements are generally lower than their Solar Systemvalues.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The Interstellar N/O Abundance Ratio: Evidence for Local Infall?
Sensitive measurements of the interstellar gas-phase oxygen abundancehave revealed a slight oxygen deficiency (~15%) toward stars within 500pc of the Sun as compared to more distant sight lines. Recent FUSEobservations of the interstellar gas-phase nitrogen abundance indicatelarger variations, but no trends with distance were reported due to thesignificant measurement uncertainties for many sight lines. Byconsidering only the highest quality (>=5 σ) N/O abundancemeasurements, we find an intriguing trend in the interstellar N/O ratiowith distance. Toward the seven stars within ~500 pc of the Sun, theweighted mean N/O ratio is 0.217 +/- 0.011, while for the six starsfarther away the weighted mean value (N/O = 0.142 +/- 0.008) iscuriously consistent with the current solar value (N/O =0.138+0.20-0.18). It is difficult to imagine ascenario invoking environmental (e.g., dust depletion or ionization)variations alone that explains this abundance anomaly. Is the enhancednitrogen abundance localized to the solar neighborhood or evidence of amore widespread phenomenon? If it is localized, then recent infall oflow-metallicity gas in the solar neighborhood may be the bestexplanation. Otherwise, the N/O variations may be best explained bylarge-scale differences in the interstellar mixing processes for AGBstars and Type II supernovae.Based on data obtained by the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE) mission operated by the Johns HopkinsUniversity. Financial support to US participants has been provided byNASA contract NAS5-32985.
| What Is the Total Deuterium Abundance in the Local Galactic Disk?
Analyses of spectra obtained with the Far Ultraviolet SpectroscopicExplorer (FUSE) satellite, together with spectra from the Copernicus andinterstellar medium absorption profile spectrograph (IMAPS) instruments,reveal an unexplained, very wide range in the observeddeuterium/hydrogen (D/H) ratios for interstellar gas in the Galacticdisk beyond the Local Bubble. We argue that spatial variations in thedepletion of deuterium onto dust grains can explain these localvariations in the observed gas-phase D/H ratios. We present a variabledeuterium depletion model that naturally explains the constant measuredvalues of D/H inside the Local Bubble, the wide range of gas-phase D/Hratios observed in the intermediate regime [logN(HI)=19.2-20.7], and thelow gas-phase D/H ratios observed at larger hydrogen column densities.We consider empirical tests of the deuterium depletion hypothesis: (1)correlations of gas-phase D/H ratios with depletions of the refractorymetals iron and silicon, and (2) correlation with the H2rotational temperature. Both of these tests are consistent withdeuterium depletion from the gas phase in cold, not recently shockedregions of the ISM, and high gas-phase D/H ratios in gas that has beenshocked or otherwise heated recently. We argue that the mostrepresentative value for the total (gas plus dust) D/H ratio within 1kpc of the Sun is >=23.1+/-2.4(1σ) parts per million (ppm).This ratio constrains Galactic chemical evolution models to have a verysmall deuterium astration factor, the ratio of primordial to total (D/H)ratio in the local region of the Galactic disk, which we estimate to befd<=1.19+0.16-0.15(1σ) or<=1.12+/-0.14(1σ) depending on the adopted light-elementnuclear reaction rates.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| Measuring interstellar gas-phase D/H ratios in the presence of H_2
Aims.To clarify the circumstances under which it is acceptable toapproximate the interstellar gas-phase D/H ratio by D I/H I.Methods: .Observed column densities of H I, D I, H2 and HD are comparedfor six lines of sight having appreciable fractions of H2.Results: .The overall fraction of deuterium in HD varies by a factor 3-4but is found to be much smaller than the fraction of H in H2 in allcases, implying that deuterium appears as D I and N(D I)/N(H I) exceedsthe gas-phase D/H ratio in H2-bearing gas. Conclusions: .Deuteriumin H2-bearing gas contributes to the observed D I absorption and the D/Hratio should be inferred from N(D)/N(H) where N(D) = (N(D I) + N(HD)),N(H) = N(H I) + 2N(H2): failure to do so biases the resulting D/H ratioupward, typically by 5%-15% in present data. Along sightlines withmultiple kinematic components having different molecular fractions,fractionation can cause velocity differences between D I and H Iprofiles. Shifts between H2 and HD velocity centroids may arise when themolecule-bearing gas has kinematic substructure reflecting regions ofdifferent ionization balance and HD/H2 ratios.
| 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
| FUSE Determination of a Low Deuterium Abundance along an Extended Sight Line in the Galactic Disk
We present a study of the deuterium abundance along the extended sightline toward HD 90087 with the Far Ultraviolet Spectroscopic Explorer(FUSE). HD 90087 is a O9.5 III star located in the Galactic disk at adistance of ~2.7 kpc away from the Sun. Both in terms of distance andcolumn densities, HD 90087 has the longest and densest sight lineobserved in the Galactic disk for which a deuterium abundance has beenmeasured from ultraviolet absorption lines so far. Because manyinterstellar clouds are probed along this sight line, possiblevariations in the properties of individual clouds should be averagedout. This would yield a deuterium abundance that is characteristic ofthe interstellar medium on scales larger than the Local Bubble. The FUSEspectra of HD 90087 show numerous blended interstellar and stellarfeatures. We have measured interstellar column densities of neutralatoms, ions, and molecules by simultaneously fitting the interstellarabsorption lines detected in the different FUSE channels. As far aspossible, saturated lines were excluded from the fits in order tominimize possible systematic errors. IUE (International UltravioletExplorer) archival data are also used to measure neutral hydrogen. Wereport D/O=(1.7+/-0.7)×10-2 andD/H=(9.8+/-3.8)×10-6 (2 σ). Our new resultsconfirm that the gas-phase deuterium abundance in the distantinterstellar medium is significantly lower than the one measured withinthe Local Bubble. We supplement our study with a revision of the oxygenabundance toward Feige 110, a moderately distant (~200 pc) sdOB star,located ~150 pc below the Galactic plane. Excluding saturated lines fromthe fits of the FUSE spectra is critical; this led us to derive an O Icolumn density about 2 times larger than the one previously reported forFeige 110. The corresponding updated D/O ratio on this sight line isD/O=(2.6+/-1.0)×10-2 (2 σ), which is lower thanthe one measured within the Local Bubble. The data set available nowoutside the Local Bubble, which is based primarily on FUSE measurements,shows a contrast between the constancy of D/O and the variability ofD/H. As oxygen is considered to be a good proxy for hydrogen within theinterstellar medium, this discrepancy is puzzling.
| Deuterium Depletion and Magnesium Enhancement in the Local Disk
The local disk deuterium is known to be depleted in comparison to thelocal bubble. We used Hubble Space Telescope (HST) spectra to obtaincolumn densities of Si, Mg and Fe. We compared normalized columndensities of these elements in the directions with high and lowdeuterium abundances.We show, that the lines of sight that are depleted in deuterium, areenhanced in magnesium. This observation implicates that astration isresponsible for both deuterium depletion and magnesium enhancement.
| 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.
| The D/H Ratio toward PG 0038+199
We determine the D/H ratio in the interstellar medium toward the DOwhite dwarf PG 0038+199 using spectra from the Far UltravioletSpectroscopic Explorer (FUSE), with ground-based support from KeckHIRES. We employ curve-of-growth, apparent optical depth, andprofile-fitting techniques to measure the column densities and limits ofmany other species (H2, Na I, C I, C II, C III, N I, N II, OI, Si II, P II, S III, Ar I, and Fe II), which allows us to determinerelated ratios such as D/O, D/N, and the H2 fraction. Ourefforts are concentrated on measuring gas-phase D/H, which is key tounderstanding Galactic chemical evolution, and comparing it topredictions from big bang nucleosynthesis. We find column densitieslogN(HI)=20.41+/-0.08, logN(DI)=15.75+/-0.08, andlogN(H2)=19.33+/-0.04, yielding a molecular hydrogen fractionof 0.14+/-0.02 (2 σ errors), with an excitation temperature of143+/-5 K. The high H I column density implies that PG 0038+199 liesoutside of the Local Bubble; we estimate its distance to be297+164-104 pc (1 σ).[DI+HD]/[HI+2H2] toward PG 0038+199 is1.91+0.52-0.42×10-5 (2 σ).There is no evidence of component structure on the scale ofΔv>8 km s-1, based on Na I, but there is marginalevidence for structure on smaller scales. The D/H value is high comparedto the majority of recent D/H measurements but consistent with thevalues for two other measurements at similar distances. D/O is inagreement with other distant measurements. The scatter in D/H valuesbeyond ~100 pc remains a challenge for Galactic chemical evolution.This paper is dedicated in memory of Ervin J. Williger, father of thefirst author, who passed away on 2003 September 13. His enthusiasticsupport and encouragement were essential to its successful completion.Based on data from the Far Ultraviolet Spectroscopic Explorer and the W.M. Keck Observatory.
| Evidence of Correlated Titanium and Deuterium Depletion in the Galactic Interstellar Medium
Current measurements indicate that the deuterium abundance in diffuseinterstellar gas varies spatially by a factor of ~4 among sight linesextending beyond the Local Bubble. One plausible explanation for thescatter is the variable depletion of D onto dust grains. To test thisscenario, we have obtained high signal-to-noise, high- resolutionprofiles of the refractory ion Ti II along seven Galactic sight lineswith D/H ranging from 0.65 to 2.1×10-5. Thesemeasurements, acquired with the recently upgraded Keck/HIRESspectrometer, indicate a correlation between Ti/H and D/H at the betterthan 95% confidence level Therefore, our observations support theinterpretation that D/H scatter is associated with differentialdepletion. We note, however, that Ti/H values taken from the literaturedo not uniformly show the correlation. Finally, we identify significantcomponent-to-component variations in the depletion levels amongindividual sight lines and discuss complications arising from thisbehavior.
| New runaway OB stars with HIPPARCOS
A Monte Carlo method for detection of runaway OB stars fromobservational data is proposed. 61 runaway OB star candidates have beendetected by an analysis of Hipparcos proper motions. The peculiartangential and total transverse velocities have been determined forthese stars. A list of the detected runaway star candidates ispresented. The evidence of a discrepancy between photometric andparallactic distances of runaway OB star candidates is presented.
| 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.
| The Deuterium-to-Hydrogen Ratio in a Low-Metallicity Cloud Falling onto the Milky Way
Using Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble SpaceTelescope observations of the QSO PG 1259+593, we detect D I Lymanseries absorption in high-velocity cloud Complex C, a low-metallicitygas cloud falling onto the Milky Way. This is the first detection ofatomic deuterium in the local universe in a location other than thenearby regions of the Galactic disk. We construct a velocity model forthe sight line based on the numerous O I absorption lines detected inthe ultraviolet spectra. We identify eight absorption-line components,two of which are associated with the high-velocity gas in Complex C at~-128 and ~-112 km s-1. A new Westerbork Synthesis RadioTelescope (WSRT) interferometer map of the H I 21 cm emission toward PG1259+593 indicates that the sight line passes through a compactconcentration of neutral gas in Complex C. We use the WSRT data togetherwith single-dish data from the Effelsberg 100 m radio telescope toestimate the H I column density of the high-velocity gas and toconstrain the velocity extents of the H I Lyman series absorptioncomponents observed by FUSE. We find N(HI)=(9.0+/-1.0)×1019 cm-2, N(DI)=(2.0+/-0.6)×1015 cm-2, and N(OI)=(7.2+/-2.1)×1015 cm-2 for the Complex Cgas (68% confidence intervals). The corresponding light-elementabundance ratios are D/H=(2.2+/-0.7)×10-5,O/H=(8.0+/-2.5)×10-5, and D/O=0.28+/-0.12. Themetallicity of Complex C gas toward PG 1259+593 is approximately 1/6solar, as inferred from the oxygen abundance[O/H]=-0.79+/-0.120.16. While we cannot rule out avalue of D/H similar to that found for the local ISM (i.e.,D/H~1.5×10-5), we can confidently exclude values as lowas those determined recently for extended sight lines in the Galacticdisk (D/H<1×10-5). Combined with the sub-solarmetallicity estimate and the low nitrogen abundance, this conclusionlends support to the hypothesis that Complex C is located outside theMilky Way, rather than inside in material recirculated between theGalactic disk and halo. The value of D/H for Complex C is consistentwith the primordial abundance of deuterium inferred from recentWilkinson Microwave Anisotropy Probe observations of the cosmicmicrowave background and simple chemical evolution models that predictthe amount of deuterium astration as a function of metallicity.
| The Deuterium-to-Oxygen Ratio in the Interstellar Medium
Because the ionization balances for H I, O I, and D I are lockedtogether by charge exchange, the deuterium-to-oxygen ratio, D/O, is animportant tracer for the value of the D/H ratio and for potentialspatial variations in the ratio. As the D I and O I column densities areof similar orders of magnitude for a given sight line, comparisons ofthe two values will generally be less subject to systematic errors thancomparisons of D I and H I, which differ by about 5 orders of magnitude.Moreover, D/O is additionally sensitive to astration, because as starsdestroy deuterium, they should produce oxygen. We report here theresults of a survey of D/O in the interstellar medium performed with theFar Ultraviolet Spectroscopic Explorer. We also compare these resultswith those for D/N. Together with a few results from previous missions,the sample totals 24 lines of sight. The distances range from a few to~2000 pc and logN(D I) from ~13 to ~16 cm-2. The D/O ratio isconstant in the local interstellar medium out to distances of ~150 pcand N(DI)~=1×1015 cm-2, i.e., within theLocal Bubble. In this region of the interstellar space, we findD/O=(3.84+/-0.16)×10-2 (1 σ in the mean). Thehomogeneity of the local D/O measurements shows that the spatialvariations in the local D/H and O/H must be extremely few, if any. Acomparison of the Local Bubble mean value with the few D/O measurementsavailable for low-metallicity quasar sight lines shows that the D/Oratio decreases with cosmic evolution, as expected. Beyond the LocalBubble, we detected significant spatial variations in the value of D/O.This likely implies a variation in D/H, as O/H is known to not varysignificantly over the distances covered in this study. Our data setsuggests a present-epoch deuterium abundance below1×10-5, i.e., lower than the value usually assumed,around 1.5×10-5.
| Potential Variations in the Interstellar N I Abundance
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and SpaceTelescope Imaging Spectrograph observations of the weak interstellar N Iλ1160 doublet toward 17 high-density sight lines[N(Htot)>=1021 cm-2]. When combinedwith published data, our results reveal variations in the fractional N Iabundance showing a systematic deficiency at large N(Htot).At the FUSE resolution (~20 km s-1), the effects ofunresolved saturation cannot be conclusively ruled out, although O Iλ1356 shows little evidence of saturation. We investigated thepossibility that the N I variability is due to the formation ofN2 in our mostly dense regions. The 0-0 band of thec'41Σ+u-X1Σ+gtransition of N2 at 958 Å should be easily detected inour FUSE data; for 10 of the denser sight lines, N2 is notobserved at a sensitivity level of a few times 1014cm-2. The observed N I variations are suggestive of anincomplete understanding of nitrogen chemistry.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS 5-32985, and the NASA/ESA HubbleSpace Telescope, obtained from the Multimission Archive at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under the NASA contractNAS 5-26555.
| Far Ultraviolet Spectroscopic Explorer Snapshot Survey of O VI Variability in the Winds of 66 OB-Type Stars
We have used the Far Ultraviolet Spectroscopic Explorer to conduct asnapshot survey of O VI variability in the winds of 66 OB-type stars inthe Galaxy and the Magellanic Clouds. These time series consist of twoor three observations separated by intervals ranging from a few days toseveral months. Although these time series provide the bare minimum ofinformation required to detect variations, this survey demonstrates thatthe O VI doublet in the winds of OB-type stars is variable on variousscales in both time and velocity. For spectral types from O3 to B1, 64%vary in time. At spectral types later than B1, no wind variability isobserved. In view of the limitations of this survey, this fractionrepresents a lower limit on the true incidence of variability in the OVI wind lines, which is very common and probably ubiquitous. Incontrast, for S IV and P V, only a small percentage of the whole sampleshows wind variations, although this may be principally due to selectioneffects. The observed variations extend over several hundreds ofkilometers per second of the wind profile and can be strong. The widthover which the wind O VI profile varies is only weakly correlated withthe terminal velocity (v&infy;), but a significantcorrelation (close to a 1:1 relationship) is derived between the maximumvelocity of the variation and v&infy;. High-velocity O VIwind absorption features (possibly related to the discrete absorptioncomponents seen in other wind lines) are also observed in 46% of thecases for spectral types from O3 to B0.5. These features are variable,but the nature of their propagation cannot be determined from thissurvey. If X-rays can produce sufficient O VI by Auger ionization of OIV and the X-rays originate from strong shocks in the wind, this studysuggests that stronger shocks occur more frequently nearv&infy;, causing an enhancement of O VI nearv&infy;.
| 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.
| Deuterium toward Two Milky Way Disk Stars: Probing Extended Sight Lines with the Far Ultraviolet Spectroscopic Explorer
We have carried out an investigation of the abundance of deuterium alongtwo extended sight lines through the interstellar medium (ISM) of theGalactic disk. The data include Far Ultraviolet Spectroscopic Explorer(FUSE) observations of HD 195965 (B1 Ib) and HD 191877 (B0 V), as wellas Space Telescope Imaging Spectrograph (STIS) observations of HD195965. The distances to HD 195965 and HD 191877, derived fromspectroscopic parallax, are 794+/-200 and 2200+/-550 pc, respectively,making these the longest Galactic disk sight lines in which deuteriumhas been investigated with FUSE. The FUSE spectra contain all of the H ILyman series transitions (and the corresponding D transitions) exceptLyα. The higher Lyman lines clearly show the presence ofdeuterium. We use a combination of curve-of-growth analyses and lineprofile fitting to determine the D I abundance toward each object. Wealso present column densities for O I and N I toward both stars, as wellas H I measured from Lyα absorption in the STIS spectrum of HD195965. Toward HD 195965 we findD/H=(0.85+0.34-0.24)×10-5 (2σ),O/H=(6.61+1.03-1.11)×10-4, andN/H=(7.94+1.69-1.34)×10-5. TowardHD 191877 we findD/H=(0.78+0.52-0.25)×10-5 (2σ) andN/H=(6.76+2.22-1.97)×10-5. The OI column density toward HD 191877 is very uncertain. Our preferred valuegives O/H=(3.09+1.98-0.98)×10-4,but we cannot rule out O/H values as low asO/H=1.86×10-4, so the O/H value for this sight lineshould be taken with caution. The D/H ratios along these sight lines arelower than the average value of (1.52+/-0.15)×10-5 (2σ in the mean) found with FUSE for the local interstellar medium(~37-179 pc from the Sun). These observations lend support to earlierdetections of variation in D/H over distances greater than a few hundredparsecs. The O/H ratio toward HD 195965 is supersolar. This star is partof an OB association, so there may be local enrichment by nearby massivestars. The D/H and O/H values measured along these sight lines supportthe expectation that the ISM is not well mixed on distances of ~1000 pc.These observations demonstrate that although D/H studies through Lymanabsorption may become impractical at d>2500 pc and logN(HI)>21,D/H studies in the distance range from 500 to 2500 pc may be very usefulfor investigating mixing and chemical evolution in the ISM.
|
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: | Cisne |
| Ascensión Recta: | 20h32m25.59s |
| Declinación: | +48°12'59.3" |
| Magnitud Aparente: | 6.969 |
| Distancia: | 523.56 parsecs |
| Movimiento Propio en Ascensión Recta: | -8.3 |
| Movimiento Propio en Declinación: | 4.8 |
| B-T magnitude: | 6.883 |
| V-T magnitude: | 6.962 |
Catálogos y designaciones:
|
