Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Blueshifted diffuse interstellar bands in the spectrum of HD 34078
In this paper, we report the very first observation of diffuseinterstellar bands (DIBs) that, in the spectrum of HD 34078 (AE Aur),are blueshifted with respect to the normal position that they have inother objects, where the rest-wavelength velocity frame is determinedusing very sharp interstellar atomic lines or molecular features. Onlyreasonably narrow DIBs seemingly show this effect, which is absent inbroader ones. The result is confirmed independently using threedifferent spectrographs attached to two different telescopes.
| The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
We present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA.
| Physical Conditions in the Interstellar Medium toward HD 185418
We have developed a complete model of the hydrogen molecule as part ofthe spectral simulation code Cloudy. Our goal is to apply this tospectra of high-redshift star-forming regions where H2absorption is seen, but where few other details are known, to understandits implication for star formation. The microphysics of H2 isintricate, and it is important to validate these numerical simulationsin better understood environments. This paper studies a well-definedline of sight through the Galactic interstellar medium (ISM) as a testof the microphysics and methods we use. We present a self-consistentcalculation of the observed absorption-line spectrum to derive thephysical conditions in the ISM toward HD 185418, a line of sight withmany observables. We deduce density, temperature, local radiation field,cosmic-ray ionization rate, and chemical composition and compare theseconclusions with conditions deduced from analytical calculations. Wefind a higher density and similar abundances, and we require acosmic-ray flux enhanced over the Galactic background value, consistentwith enhancements predicted by MHD simulations.
| B-type supergiants in the Small Magellanic Cloud: rotational velocities and implications for evolutionary models
High-resolution spectra for 24 SMC and Galactic B-type supergiants havebeen analysed to estimate the contributions of both macroturbulence androtation to the broadening of their metal lines. Two differentmethodologies are considered, viz. goodness-of-fit comparisons betweenobserved and theoretical line profiles and identifying zeros in theFourier transforms of the observed profiles. The advantages andlimitations of the two methods are briefly discussed with the lattertechniques being adopted for estimating projected rotational velocities(v sin i) but the former being used to estimate macroturbulentvelocities. The projected rotational velocity estimates range fromapproximately 20 to 60 km s-1, apart from one SMC supergiant,Sk 191, with a v sin i ≃ 90 km s-1. Apart from Sk 191,the distribution of projected rotational velocities as a function ofspectral type are similar in both our Galactic and SMC samples withlarger values being found at earlier spectral types. There is marginalevidence for the projected rotational velocities in the SMC being higherthan those in the Galactic targets but any differences are only of theorder of 5-10 km s-1, whilst evolutionary models predictdifferences in this effective temperature range of typically 20 to 70 kms-1. The combined sample is consistent with a linearvariation of projected rotational velocity with effective temperature,which would imply rotational velocities for supergiants of 70 kms-1 at an effective temperature of 28 000 K (approximately B0spectral type) decreasing to 32 km s-1 at 12 000 K (B8spectral type). For all targets, the macroturbulent broadening wouldappear to be consistent with a Gaussian distribution (although otherdistributions cannot be discounted) with an 1/e half-width varying fromapproximately 20 km s-1 at B8 to 60 km s-1 at B0spectral types.
| Neutral carbon and CO emission in the core and the halo of dark cloud Barnard 5
Aims.The physical conditions and chemical structure in the dark cloud ofBarnard 5 and its surrounding atomic halo is studied.The impact of the halo on the line emission emerging from the molecularcloud is investigated.Methods.We present observations of the [CI]^3P1→ ^3P0 transition of neutral carbon and the low-J transitionsof 12CO and 13CO. The CO maps extend from the core(Av 7) to the northern cloud edge and into the halo(Av 1). They are complemented by deeply integrated [CI]spectra made along a 1D cut of similar extent. Escape probability andphoton-dominated region (PDR) models are employed to interpret theobservations.Results.12CO and 13CO are detected inthe cloud and the halo, while [CI] is detected only toward the molecularcloud. This occurs even though the neutral carbon column density is5 times larger than the CO column density in the halo, but it canbe understood in terms of excitation. The [CI] excitation is governed bycollisions even at the low halo densities, while the CO excitation isdominated by the absorption of line photons emitted by the nearbymolecular cloud. The upper limit on the neutral carbon column density inthe halo is 6× 1015 cm-2. The PDR studiesshow that even small column densities of H2 and CO, such as those in theB5 halo, can significantly change the [CI] and COline emission (pre-shielding). Since this effect decreases the [CI]intensity and increases the CO intensity, the largest impact is notedfor the [CI]/CO line ratios. For the B5 cloud, a PDRmodel with a molecular hydrogen column density of ~6×1019 cm-2 in the halo matches the observed [CI]/COline ratios best. Models with no pre-shielding, in contrast, suggesthigh gas densities that are in conflict with independently deriveddensities. The PDR models with a χ<1 demonstrate that the [CI]/COratios cannot be attributed solely to a reduced FUV field.
| A search for fine structure inside high resolution profiles of weak diffuse interstellar bands
This paper presents a survey of the high-resolution profiles ofselected, moderately weak diffuse interstellar bands (DIBs) between 4725and 6730 Å. In very high signal-to-noise spectra, obtained as aresult of averaging several individual exposures of reddened, early-typestars that show Doppler splitting of <2 km s-1 ininterstellar gas lines, the profiles seem to have a substructure. Thissupports the molecular origin hypothesis for DIBs. We studied theprofiles of the diffuse interstellar bands at wavelengths of 4726.33,4963.85, 5418.89, 5541.74, 5544.95, 5546.46, 5762.73, 5766.05, 5769.09,6439.41, 6445.53, 6449.16, 6729.28 Å.
| Search for C2- in Diffuse Clouds
A search has been carried out for the C2- ion indiffuse clouds toward HD 23180, HD 24912, HD 24398, HD 46711, and HD50064 using the HIDES spectrometer on the Okayama 188-cm telescope. Anupper limit of 8.3 × 1010, 1.8 × 1012cm-2 was obtained for the C2- columndensity. The upper limit value (3.8 × 1011cm-2) toward HD 23180 is two orders of magnitude smaller thanthat of the C2 radical. Possible production mechanisms forC2- are discussed.
| Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction
We relate the equivalent widths of 11 diffuse interstellar bands,measured in the spectra of 49 stars, to different colour excesses in theultraviolet. We find that most of the observed bands correlatepositively with the extinction in the neighbourhood of the2175-Åbump. Correlation with colour excesses in other parts of theextinction curve is more variable from one diffuse interstellar band toanother; we find that some diffuse bands (5797, 5850 and 6376 Å)correlate positively with the overall slope of the extinction curve,while others (5780 and 6284 Å) exhibit negative correlation. Wediscuss the implications of these results on the links between thediffuse interstellar band carriers and the properties of theinterstellar grains.
| 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.
| H2 Pure Rotational Lines in the Orion Bar
Photodissociation regions (PDRs), where UV radiation dominates theenergetics and chemistry of the neutral gas, contain most of the mass inthe dense interstellar medium of our Galaxy. Observations ofH2 rotational and rovibrational lines reveal that PDRscontain unexpectedly large amounts of very warm (400-700 K) moleculargas. Theoretical models have difficulty explaining the existence of somuch warm gas. Possible problems include errors in the heating andcooling functions or in the formation rate for H2. To date,observations of H2 rotational lines smear out the structureof the PDR. Only by resolving the hottest layers of H2 canone test the predictions and assumptions of current models. Using theTexas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission inthe H2 v=0-0 S(1) and S(2) lines toward the Orion Bar PDR at2'' resolution. We also observed H2 v=0-0 S(4) atselected points toward the front of the PDR. Our maps cover a12'' by 40'' region of the bar where H2rovibrational lines are bright. The distributions of H2 0-0S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail.The high spatial resolution (0.002 pc) of our observations allows us toprobe the distribution of warm gas in the Orion Bar to a distanceapproaching the scale length for FUV photon absorption. We use these newobservational results to set parameters for the PDR models described ina companion paper in preparation by Draine et al. The best-fit model canaccount for the separation of the H2 emission from theionization front and the intensities of the ground-state rotationallines, as well as the 1-0 S(1) and 2-1 S(1) lines. This model requiressignificant adjustments to the commonly used values for the dust UVattenuation cross section and the photoelectric heating rate.
| Profiles of Very Weak Diffuse Interstellar Bands around 6440 Å
Profiles of very weak diffuse interstellar bands (DIBs) between 6400 and6470 Å observed with high resolution and very high S/N aredemonstrated. We show that with the increasing quality of reddenedstellar spectra, the whole spectral range is covered with weak or veryweak DIBs-at least one every 2-3 Å. We also present the details ofthe profiles of a few stronger features; the presence of substructuresresembles the profiles of strong DIBs observed in high resolution duringthe last decade and supports the hypothesis of a molecular origin of atleast a majority of DIBs.
| Fourier Transform Emission Spectra of the (000)-(000) Band of the λ4051.6 Band of C3
The (000)-(000) band of the 4051.6 Å group(A˜1Πu-X˜1Σ+g)of C3 was recorded in emission with a Bruker IFS 120HRFourier transform (FT) spectrometer at the University of Waterloo. Theband was excited by a microwave discharge in isopropanol (less than afew mtorr) diluted in helium (2 torr). Our new FT data provide morereliable and accurately calibrated transition wavenumbers than thosefrom the grating spectra given by Gausset and coworkers. Analysis of ournew spectrum combined with the data by McCall and coworkers confirmedthat the lower J levels in the A˜ state were strongly perturbed,as reported by Gausset and coworkers. The unidentified lines observed byMcCall and coworkers could be attributed to extra transitions to anunknown perturbing state.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| Interstellar 12C/13C ratios through CH^+λλ 3957,4232 absorption in local clouds: incomplete mixing in the ISM
The 12C/13C isotope ratio is a tracer of stellaryields and the efficiency of mixing in the ISM.12CH+/13CH+ is not affectedby interstellar chemistry, and is the most secure way of measuring12C/13C in the diffuse ISM.R=12C/13C is 90 in the solar system. Previousmeasurements of 12CH+λλ3957.7,4232.3and 13CH+λλ3958.2,4232.0 absorptiontoward nearby stars indicate some variations in12C/13C, with values ranging from 40 to 90suggesting inefficient mixing. Except for the cloud toward ζOph,these R values are strongly affected by noise. With UVES on the VLT wehave improved on the previous interstellar 12C/13Cmeasurements. The weighted 12C/13C ratio in thelocal ISM is 78.27 ± 1.83, while the weighted dispersion of ourmeasurements is 12.7, giving a 6.9σ scatter. Thus we report on a6.9σ detection of 16.2% root-mean-square variations in the carbonisotopic ratio on scales of ~100 pc: R= 74.7 ± 2.3 in theζOph cloud, while R = 88.6 ± 3.0 toward HD 152235 in theLupus clouds, R = 62.2 ± 5.3 towards HD 110432 in the Coalsack,and R = 98.9 ± 10.1 toward HD 170740. The observed variations in13C/12C are the first significant detection ofchemical heterogeneity in the local ISM.
| Far-infrared detection of methylene
We present a clear detection of CH2 in absorption towards themolecular cloud complexes Sagittarius B2 andW49 N using the ISO Long Wavelength Spectrometer.These observations represent the first detection of its low excitationrotational lines in the interstellar medium. Towards Sagittarius B2, wedetect both ortho and para transitions allowing a determination of thetotal CH2 column density ofN(CH2)=(7.5±1.1)×1014cm-2. We compare this with the related molecule, CH, todetermine [CH/CH2]=2.7±0.5. Comparison with chemicalmodels shows that the CH abundance along the line of sight is consistentwith diffuse cloud conditions and that the high [CH/CH2]ratio can be explained by including the effect of grain-surfacereactions.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands and the UK) with the participation of ISAS and NASA.
| Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
We present column density measurements of the HD molecule in theinterstellar gas toward 17 Galactic stars. The values for the seven mostheavily reddened sightlines, with E(B-V) = 0.38-0.72, are derived fromobservations with the Far Ultraviolet Spectroscopic Explorer (FUSE). Theother ten values are from a reanalysis of spectra obtained withCopernicus. In all cases, high-resolution ground-based observations of KI and/or the CH molecule were used to constrain the gas velocitystructure and to correct for saturation effects. Comparisons of thecolumn densities HD, CH, CN, and K I in these 17 sightlines indicatethat HD is most tightly correlated with CH. Stringent lower limits tothe interstellar D/H ratio, derived from the HD/2H2 ratio,range from 3.7 × 10-7 to 4.3 × 10-6.Our results also suggest that the HD/H2 ratio increases withthe molecular fraction f(H2) and that the interstellar D/Hratio might be obtained from HD by probing clouds with f(H2) 1. Finally, we note an apparent relationship between the molecularfractions of hydrogen and deuterium.
| A far UV study of interstellar gas towards HD 34078: High excitation H2 and small scale structure
To investigate the presence of small scale structure in the spatialdistribution of H2 molecules we have undertaken repeated FUSEUV observations of the runaway O9.5V star, HD 34078. In this paper wepresent five spectra obtained between January 2000 and October 2002.These observations reveal an unexpectedly large amount of highly excitedH2>. Column densities for H2 levels from (v =0, J = 0) up to (v = 0, J = 11) and for several v = 1 and v = 2 levelsare determined. These results are interpreted in the frame of a modelinvolving essentially two components: i) a foreground cloud (unaffectedby HD 34078) responsible for the H2 (J = 0, 1), CI, CH,CH+ and CO absorptions; ii) a dense layer of gas (n ≃104 cm-3) close to the O star and stronglyilluminated by its UV flux which accounts for the presence of highlyexcited H2. Our model successfully reproduces theH2 excitation, the CI fine-structure level populations aswell as the CH, CH+ and CO column densities. We also examinethe time variability of H2 absorption lines tracing each ofthese two components. From the stability of the J = 0, 1 and 2 dampedH2 profiles we infer a 3σ upper limit on column densityvariations Δ N(H2)/N(H2) of 5% over scalesranging from 5 to 50 AU. This result clearly rules out any pronouncedubiquitous small scale density structure of the kind apparently seen inHI. The lines from highly excited gas are also quite stable (equivalentto Δ N/N ≤30%) indicating i) that the ambient gas throughwhich HD 34078 is moving is relatively uniform and ii) that the gas flowalong the shocked layer is not subject to marked instabilities.Based on observations performed by the FUSE mission and at the CFHTtelescope.
| Internal Motions of Trapezium Systems
The separations of the various components of 44 trapezia as a functionof time are studied, taking advantage of many new, high precisionobservations for these objects. A previous study revealed that somesystems have components moving with velocities larger than the escapevelocity (Allen et al. 1974). The present work updates our previousstudy, extending the observations by about 30 years, and thus allowingan improved determination of the relative transverse motions of thecomponents. The analysis of the new data confirms the conclusions wereached in our previous work: most of the trapezia show the internalmotions expected for bound, virialized small clusters, but a few haveescaping components. The available observational material lends supportto the concept that trapezia are unstable systems with lifetimes of theorder of a few million years.
| Correlations between diffuse interstellar bands and atomic lines
We present and discuss correlations between strengths of the well-known,strong interstellar atomic lines of KI and CaII, and four selected,strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850and 6614. In order to analyse a homogeneous sample of echellehigh-resolution spectra it has been chosen to use measurements fromTerskol Observatory in Northern Caucasus plus a selected number ofhigher resolution observations performed using other instruments. Wedemonstrate that the strength of certain DIBs correlate well withneutral potassium lines and to a much lower degree with ionized calciumlines. This fact suggests that the degree of irradiation of a cloud withUV photons, capable to ionize interstellar atoms, plays a crucial rolein the formation/maintenance of certain molecular species: possiblecarriers of DIBs.
| Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability
We present the results of a long-term monitoring campaign of theHα line in a sample of bright OB supergiants (O7.5-B9) which aimsat detecting rotationally modulated changes potentially related to theexistence of large-scale wind structures. A total of 22 objects weremonitored during 36 nights spread over six months in 2001-2002.Coordinated broad-band photometric observations were also obtained forsome targets. Conspicuous evidence for variability in Hα is foundfor the stars displaying a feature contaminated by wind emission. Mostchanges take place on a daily time-scale, although hourly variations arealso occasionally detected. Convincing evidence for a cyclical patternof variability in Hα has been found in two stars: HD 14134 and HD42087. Periodic signals are also detected in other stars, butindependent confirmation is required. Rotational modulation is suggestedfrom the similarity between the observed recurrence time-scales (in therange 13-25 d) and estimated periods of stellar rotation. We callattention to the atypical case of HD 14134, which exhibits a clear12.8-d periodicity, both in the photometric and in the spectroscopicdata sets. This places this object among a handful of early-type starswhere one may observe a clear link between extended wind structures andphotospheric disturbances. Further modelling may test the hypothesisthat azimuthally-extended wind streams are responsible for the patternsof spectral variability in our target stars.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| Interstellar Carbon in Translucent Sight Lines
We report interstellar C II column densities or upper limits determinedfrom weak absorption of the 2325.4029 Å intersystem transitionobserved in six translucent sight lines (AV>~1) with theSpace Telescope Imaging Spectrograph (STIS). The sight lines sample awide range of interstellar characteristics, including total to selectiveextinction, RV=2.6-5.1 average hydrogen density along thesight line, =3-14 cm-3 and fraction of H inmolecular form, 0-~40%. Four of the sight lines, those toward HD 37021,HD 37061, HD 147888, and HD 207198, have interstellar gas-phaseabundances that are consistent with the diffuse sight line ratio of161+/-17 carbon atoms in the gas per million hydrogen nuclei. We notethat while it has a gas-phase carbon abundance that is consistent withthe other sight lines, a large fraction of the C II toward HD 37061 isin an excited state. The sight line toward HD 152590 has a measuredinterstellar gas-phase carbon abundance that is well above the diffusesight line average; the column density of C in this sight line may beoverestimated because of noise structure in the data. Toward HD 27778 wefind a 3 σ abundance upper limit of <=108 C atoms in the gasper million H, a substantially enhanced depletion of C as compared tothe diffuse sight line value. The interstellar characteristics toward HD27778 are otherwise not extreme among the sample, except for anunusually large abundance of CO molecules in the gas.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| The Interstellar Rubidium Isotope Ratio toward ρ Ophiuchi A
The isotope ratio 85Rb/87Rb places constraints onmodels of the nucleosynthesis of heavy elements, but there is no precisedetermination of the ratio for material beyond the solar system. Wereport the first measurement of the interstellar Rb isotope ratio. Ourmeasurement of the Rb I line at 7800 Å for the diffuse gas towardρ Oph A yields a value of 1.21+/-0.30 (1 σ) that differssignificantly from the meteoritic value of 2.59. The Rb/K elementalabundance ratio for the cloud also is lower than that seen inmeteorites. Comparison of the 85Rb/K and 87Rb/Kratios with meteoritic values indicates that the interstellar85Rb abundance in this direction is lower than the solarsystem abundance. We attribute the lower abundance to a reducedcontribution from the r-process. Interstellar abundances for Kr, Cd, andSn are consistent with much less r-process synthesis for the solarneighborhood compared to the amount inferred for the solar system.
| H3+ and other species in the diffuse cloud towards ζ Persei: A new detailed model
McCall et al. have recently shown that a large column density for themolecular ion H3+ of ≈8 × 1013cm-2 exists in the classical diffuse cloud towards ζPersei. They have used this observation to infer that the cosmic rayionization rate ζ for this source is approximately 40 times largerthan previously assumed. But, although the value of ζ they infer(≈1.2 × 10-15 s-1) can explain theabundance of H3+, it is not at all clear that sucha high ionization rate is consistent with the many other detailed atomicand molecular observations made along the same line of sight. Inparticular, the abundances of the species OH and HD were previously usedto determine a much lower ionization rate. In this paper, we report adetailed chemical model of the diffuse cloud towards ζ Persei whichappears to fit to a reasonable extent both the older atomic andmolecular observations and the new detection ofH3+. We consider two phases - a long (4 pc)diffuse region at 60 K and a tiny (≈100 AU) dense region at 20 K,both with an ionization rate ζ in between the standard value andthat advocated by McCall et al. The model reproduces almost allabundances, including that of H3+, to within afactor of three or better. To reproduce the CH+ abundance andthose of the excited rotational populations of H2, weconsider the addition of shocks. This phase has little effect on ourcalculated abundance for H3+.
| On the relation between diffuse bands and column densities of H2, CH and CO molecules
Mutual relations between column densities of H2, CH and COmolecules as well as between the latter and strengths of the major 5780and 5797 diffuse bands are presented and discussed. The CH radical seemsto be a good H2 tracer, possibly better than CO. It is alsodemonstrated that the molecular fraction of the H2 moleculeis correlated with an intensity ratio of 5797 and 5780 DIBs, suggestingthe possible formation of narrow DIB carriers in denser clouds,dominated by molecular hydrogen and reasonably shielded from ionizing UVradiation by small dust grains.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/949
| On the Origin of the Dispersion in the 7Li/6Li Ratio in the ISM
Measurements of the 7Li/6Li ratio in the localinterstellar medium (ISM) towards o Per and ζ Per by Knauth et al.(2000, AAA 073.131.129) show clouds with a range from near 2 (a valuewhich points to GCR production by spallation or alpha + alpha to near 11(the solar system value). These values can be very well accounted forwithin our model of general chemical enrichment of the Galaxy in which akey role is played by the continuing infall of metal-poor extragalacticgas to the Galactic plane, in the form of high-velocity clouds. Withinthis model, light nuclides are produced in the ISM, but destroyed indense hot interstellar regions around massive stars, the process ofwhich yields a differential depletion of 6Li relative to7Li, which can vary on quite short-distance scales.
| Very high resolution profiles of four diffuse interstellar bands
Ultra-high-resolution (R~ 300000) profiles of four diffuse interstellarbands (DIBs) are presented. The λλ 5797-, 5850-, 6196- and6379-Å DIBs were observed towards the reddened supergiant HD24398, a line of sight free of Doppler splitting; thus the observedprofiles can be considered as intrinsic to the DIB carriers. Three ofthe profiles show substructure which supports the hypothesis of amolecular origin for these DIBs.
| Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Perseo |
| Ascensione retta: | 03h54m07.90s |
| Declinazione: | +31°53'01.0" |
| Magnitudine apparente: | 2.85 |
| Distanza: | 301.205 parsec |
| Moto proprio RA: | 5.2 |
| Moto proprio Dec: | -8.4 |
| B-T magnitude: | 2.955 |
| V-T magnitude: | 2.858 |
Cataloghi e designazioni:
|
INHABITED SKY
