Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Ultraviolet spectroscopy of the extended solar corona
The first observations of ultraviolet spectral line profiles andintensities from the extended solar corona (i.e., more than 1.5 solarradii from Sun-center) were obtained on 13 April 1979 when arocket-borne ultraviolet coronagraph spectrometer of theHarvard-Smithsonian Center for Astrophysics made direct measurements ofproton kinetic temperatures, and obtained upper limits on outflowvelocities in a quiet coronal region and a polar coronal hole. Followingthose observations, ultraviolet coronagraphic spectroscopy has expandedto include observations of over 60 spectral lines in coronal holes,streamers, coronal jets, and solar flare/coronal mass ejection (CME)events. Spectroscopic diagnostic techniques have been developed todetermine proton, electron and ion kinetic temperatures and velocitydistributions, proton and ion bulk flow speeds and chemical abundances.The observations have been made during three sounding rocket flights,four Shuttle deployed and retrieved Spartan 201 flights, and the Solarand Heliospheric Observatory (SOHO) mission. Ultraviolet spectroscopy ofthe extended solar corona has led to fundamentally new views of theacceleration regions of the solar wind and CMEs. Observations with theUltraviolet Coronagraph Spectrometer (UVCS) on SOHO revealedsurprisingly large temperatures, outflow speeds, and velocitydistribution anisotropies in coronal holes, especially for minor ions.Those measurements have guided theorists to discard some candidatephysical processes of solar wind acceleration and to increase and expandinvestigations of ion cyclotron resonance and related processes.Analyses of UVCS observations of CME plasma properties and the evolutionof CMEs have provided the following: temperatures, inflow velocities andderived values of resistivity and reconnection rates in CME currentsheets, compression ratios and extremely high ion temperatures behindCME shocks, and three dimensional flow velocities and magnetic fieldchirality in CMEs. Ultraviolet spectroscopy has been used to determinethe thermal energy content of CMEs allowing the total energy budget tobe known for the first time. Such spectroscopic observations are capableof providing detailed empirical descriptions of solar energetic particle(SEP) source regions that allow theoretical models of SEP accelerationto be tailored to specific events, thereby enabling in situ measurementsof freshly emitted SEPs to be used for testing and guiding the evolutionof SEP acceleration theory. Here we review the history of ultravioletcoronagraph spectroscopy, summarize the physics of spectral lineformation in the extended corona, describe the spectroscopic diagnostictechniques, review the advances in our understanding of solar windsource regions and flare/CME events provided by ultraviolet spectroscopyand discuss the scientific potential of next generation ultravioletcoronagraph spectrometers.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| Radial velocity measurements of B stars in the Scorpius-Centaurus association
We derive single-epoch radial velocities for a sample of 56 B-type starsmembers of the subgroups Upper Scorpius, Upper Centaurus Lupus and LowerCentaurus Crux of the nearby Sco-Cen OB association. The radial velocitymeasurements were obtained by means of high-resolution echelle spectravia analysis of individual lines. The internal accuracy obtained in themeasurements is estimated to be typically 2-3 km s-1, butdepends on the projected rotational velocity of the target. Radialvelocity measurements taken for 2-3 epochs for the targets HD 120307, HD142990 and HD 139365 are variable and confirm that they arespectroscopic binaries, as previously identified in the literature.Spectral lines from two stellar components are resolved in the observedspectra of target stars HD 133242, HD 133955 and HD 143018, identifyingthem as spectroscopic binaries.
| The Large Magellanic Cloud: diffuse interstellar bands, atomic lines and the local environmental conditions
The Large Magellanic Cloud (LMC) offers a unique laboratory to study thediffuse interstellar bands (DIBs) under conditions that are profoundlydifferent from those in the Galaxy. DIB carrier abundances depend onseveral environmental factors, in particular the local UV radiationfield. In this paper we present measurements of twelve DIBs in fivelines of sight to early-type stars in the LMC, including the 30Doradus region. From the high resolution spectra obtained withVLT/UVES we also derive environmental parameters that characterise thelocal interstellar medium (ISM) in the probed LMC clouds. These includethe column density components (including total column density) for theatomic resonance lines of Na I, Ca II, Ti II, K I. In addition, wederive the H I column density from 21 cm line profiles, thetotal-to-selective visual extinction RV and the gas-to-dustratio N(H I)/A_V. Furthermore, from atomic line ratios we derive theionisation balance and relative UV field strength in these environments.We discuss the properties of the LMC ISM in the context of DIB carrierformation. The behaviour of DIBs in the LMC is compared to that of DIBsin different local environmental conditions in the Milky Way. A keyresult is that in most cases the diffuse band strengths are weak (up tofactor 5) with respect to Galactic lines of sight of comparablereddening, EB-V. In the line of sight towards Sk -69223 the 5780 and 5797 Å DIBs are very similar instrength and profile to those observed towards HD144217, which is typical of an environment exposed to a strongUV field. From the velocity analysis we find that DIB carriers (towardsSk -69 243) are better correlated with the ionisedspecies like Ca II than with neutrals (like Na I and CO). The mostsignificant parameter that governs the behaviour of the DIB carrier isthe strength of the UV field.
| Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| Analysis of the Dynamic Stability of Selected Multiple Stars with Weak Hierarchy
The stability of multiple systems with known orbital elements and withsubsystems occupying adjacent hierarchy levels is analyzed using sixstability criteria and numerical simulations of their dynamicalevolution. All the stability criteria considered are in qualitativeagreement with the numerical computations. Of the 16 systems studied, 11are confirmed to be stable and five (HD 40887, HD 136176, HD 150680, HD217675, and HD 222326) may be unstable on time scales of 106 yr orless. The small dynamical ages of the unstable systems may indicate thatthey have captured components during encounters between close binariesand field or moving cluster stars. The instability could also resultfrom the perturbation of a stable system when it approaches a massiveobject (star, black hole, or molecular cloud). It is possible that someof the unstable systems are remnants of small clusters or stellargroups.
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| A Study of Far-Ultraviolet Extinction in the Upper Scorpius Cloud Using the SPINR Sounding Rocket Experiment
In this study, six new interstellar extinction curves in thefar-ultraviolet are presented using data from a sounding rocketexperiment. The sounding rocket data were combined with IUE data for sixlines of sight in the Upper Scorpius group to cover the wavelength rangeof 912-3030 Å. The extinction curves were produced using the paircomparison method with B stars of similar spectral types.Parameterizations from Fitzpatrick & Massa, Cardelli et al., andFitzpatrick were then fitted to the derived extinction curves. From thederived extinction curves, their corresponding fits, and the dust modelof Weingartner & Draine, it is concluded that the dust population inthe Upper Scorpius region exhibits a larger than average grainpopulation with a depletion of smaller grains.
| A new look at the position of the 1604 Supernova (V843 Ophiuchi)
The position of the supernova of 1604 (V843 Oph) is re-assessed, withrelevant discussion pertaining to the present-day remnant, 3C 358.
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| Diffuse interstellar bands of unprecedented strength in the line of sight towards high-mass X-ray binary 4U 1907+09
High-resolution VLT/UVES spectra of the strongly reddened O supergiantcompanion to the X-ray pulsar 4U 1907+09 provide aunique opportunity to study the nature of the diffuse interstellar bands(DIBs) at unprecedented strength. We detect about 180 known DIBs, ofwhich about 25 were listed as tentative and are now confirmed. A dozennew DIB candidates longwards of 6900 Å are identified. We showthat the observed 5797 Å DIB strength is in line with the Galacticcorrelation with reddening, whereas the 5780 Å DIB strength isrelatively weak. This indicates the contribution of denser regions,where the UV penetration is reduced. The presence of dense cloud coresis supported by the detection of C2 rotational transitions. Members ofone DIB family (5797, 6379 Å and 6196, 6613 Å) behavecoherently, although one can make a distinction between the twocorrelated pairs. The broadened profiles of narrow DIBs are shown to beconsistent with the premise that each of the main clouds in the line ofsight discerned in the interstellar K I profile is contributingproportionally to the DIB profile. We complement and extend the relationof DIB strength with reddening {E}(B-V), as well as withneutral hydrogen column density N(H I), respectively, using stronglyreddened sightlines towards another four distant HMXBs. The 5780 ÅDIB, and tentatively also the 5797 and 6613 Å DIBs, are bettercorrelated to the gas tracer H I than to the dust tracer{E}(B-V). The resulting relationship can be applied to anyline of sight to obtain an estimate of the H I column density. In thesearch for the nature of the DIB carrier, this strongly reddened line ofsight is a complementary addition to single cloud line of sight studies.
| Diffuse Interstellar Bands in NGC 1448
We present spectroscopic VLT/UVES observations of two emergingsupernovae, the Type Ia SN 2001el and the Type IISN 2003hn, in the spiral galaxy NGC1448. Our high resolution and high signal-to-noise spectradisplay atomic lines of Ca II, Na I, Ti II and K I in the host galaxy.In the line of sight towards SN 2001el, we alsodetect over a dozen diffuse interstellar bands (DIBs) withinNGC 1448. These DIBs have strengths comparable to lowreddening galactic lines of sight, albeit with some variations. Inparticular, a good match is found with the line of sight towards theσ type diffuse cloud (HD 144217). The DIBstowards SN 2003hn are significantly weaker, and thisline of sight has also lower sodium column density. The DIB centralvelocities show that the DIBs towards SN 2001el areclosely related to the strongest interstellar Ca II and Na I components,indicating that the DIBs are preferentially produced in the same cloud.The ratio of the λ 5797 and λ 5780 DIB strengths (r 0.14) suggests a rather strong UV field in the DIB environment towardsSN 2001el. We also note that the extinction estimatesobtained from the sodium lines using multiple line fitting agree withreddening estimates based on the colors of the Type Ia SN2001el.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO Programmes 67.D-0227 and 71.D-0033).Table \ref{tb:ISfit} and Figs. \ref{fig:IS_MW} and \ref{fig:6284} areonly available in electronic form at http://www.edpsciences.org
| 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.
| A re-analysis of the 1971 Beta Scorpii occultation by Jupiter: study of temperature fluctuations and detection of wave activity
Data from the 13 May 1971 β Scorpii occultation by the southernpolar region of Jupiter (Vapillon et al., 1973, Astron. Astrophys. 29,135-149) are re-analyzed with current methods. We correct the previousresults for an inacurrate background estimation and calculate newtemperature profiles, that are now consistent with the results of otherobservers of this occultation, as well as with the current knowledge ofthe jovian atmosphere. The characteristics of the profiles oftemperature gradient and the spectral behavior of the temperaturefluctuations are found to be similar to the results of previousinvestigations of planetary atmospheres and in agreement with thepresence of atmospheric propagating gravity waves in the jovianatmosphere. We use a wavelet analysis of the temperature profiles toidentify the dominant modes of wave activity and compare thereconstructed temperature fluctuations to model-generated gravity waves.
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| Reanalysis of Copernicus Measurements of Interstellar Carbon Monoxide
We used archival data acquired with Copernicus to reexamine CO columndensities, as self-consistent oscillator strengths are now available.Our focus is on lines of sight containing modest amounts of molecularspecies. Our resulting column densities are small enough thatself-shielding from photodissociation does not occur in the cloudsprobed by the observations. While our sample shows that the columndensities of CO and H2 are related, no correspondence withthe CH column density is evident. The case for the CH+ columndensity is less clear. Recent chemical models for these sight linessuggest that CH is mainly a by-product of CH+ synthesis inlow-density gas. The models are most successful in reproducing theamounts of CO in the densest sight lines. Thus, much of the COabsorption must arise from denser clumps along the line of sight toaccount for the trend with H2.
| 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.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| 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
| β Cep stars from a spectroscopic point of view
In this review we present the current status of line-profile-variationstudies of β Cep stars. Such studies have been performed for 26bright members of this class of pulsating stars in the past 25 years. Wedescribe all these currently available data and summarize theinterpretations based on them in terms of the excited pulsation modes.We emphasize that line-profile variations offer a much more detailedpicture of the pulsational behaviour of pulsating stars compared toground-based photometric data. The latter, however, remain necessary tounravel the often complex frequency pattern and to achieve unambiguousmode identification for multiperiodic β Cep stars and also toderive the pulsational properties of the faint members of the class. Wehighlight the statistical properties of the sample of 26 stars for whichaccurate spectroscopic studies are available and point out some futureprospects.
| 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.
| The 10 October 1999 HIP 9369 occultation by the northern polar region of Jupiter: ingress and egress lightcurves analysis
The occultation of bright star HIP9369 by the northern polar region ofJupiter was observed from four locations in North and South America,providing four data sets for ingress and egress. The inversion of theeight occultation lightcurves provides temperature profiles at differentlatitudes ranging from 55°N to 73.2°N. We estimate the errors onthe profiles due to the uncertainties of the inversion method andcompare the value of the temperature at the deepest level probed (~ 50μbar) with previous observations. The shape of the temperaturegradient profile is found similar to previous investigations ofplanetary atmospheres with propagating and breaking gravity waves. Weanalyze the small scale structures in both lightcurves and temperatureprofiles using the continuous wavelet transform. The calculated powerspectra of localized fluctuations in the temperature profiles showslopes close to -3 for all eight profiles. We also isolate andreconstruct the three-dimensional geometry of a single wave mode withvertical and horizontal wavelengths of respectively 3 and 70 km. Theidentified wave is consistent with the gravity wave regime, with ahorizontal phase speed nearly parallel to the planetary meridian.Nevertheless, the dissipation of the corresponding wave in Jupiter'sstratosphere should preclude its detection at the observed levels and anacoustic wave cannot be ruled out.
| 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.
| Abundances and Physical Conditions in the Interstellar Gas toward HD 185418
We present a study of the abundances and physical conditions in theinterstellar gas toward the moderately reddened B0.5 V star HD 185418.This star is located at (l,b)=(53deg,-2.2d) and has anestimated distance of 790 pc and a reddening E(B-V)=0.50.This work is based on the analysis of Far Ultraviolet SpectroscopicExplorer (FUSE) and Hubble Space Telescope/Space Telescope ImagingSpectrograph (HST/STIS) spectra covering the range from 912 to 1361Å. We used both sets of far-ultraviolet data, together withhigh-resolution optical data, to derive accurate gas-phase columndensities for important interstellar species such as C I, C I*, C I**, NI, O I, O I*, Mg II, S I, S III, Cl I, Mn II, Fe II, Ni II, Cu II, GeII, Kr I, and CO. Numerous lines of H2 are present in theFUSE spectra, with a kinetic temperature for the lowest rotationallevels T01=100+/-15 K. Analysis of the C I fine-structureexcitation at that temperature implies an average local density ofhydrogen nH=6.3+/-2.5 cm-3. Chemical argumentsbased on analyses of CH, CH+, and C2 indicate thata fraction of the CH (>=20%) is formed as a by-product ofnonequilibrium CH+ synthesis. The electron densityne, derived under the assumption of photoionizationequilibrium, ranges from 0.03 to 0.32 cm-3 for the sixdifferent neutral/first-ion ratios considered. The relatively complexcomponent structure seen in high-resolution spectra of K I, Na I, and CaII, the relatively low average volume densities, the modest molecularfraction f(H2)=0.44, and the measured depletions all indicateno evidence for the presence of translucent components in the lightpath. The line of sight rather contains a mixture of cold and warmneutral diffuse gas. The detection of Si III and S III indicates thepresence of some ionized gas (at a 1% level) along the line of sight. Wecould not determine unambiguously whether this ionized gas wasphysically related to the neutral components.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the data archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555. Theobservations are associated with proposal 8241. Based in part onobservations obtained with the Apache Point Observatory 3.5 m telescope,which is owned and operated by the Astrophysical Research Consortium.
| Nonthermal Chemistry in Diffuse Clouds with Low Molecular Abundances
High-quality archival spectra of interstellar absorption from C I towardnine stars, taken with the Goddard High Resolution Spectrograph on theHubble Space Telescope, were analyzed. Our sample was supplemented bytwo sight lines, 23 Ori and β1 Sco, for which the C Imeasurements of Federman, Welty, & Cardelli were used. Directionswith known CH+ absorption, but only upper limits onabsorption from C2 and CN, were considered for our study.This restriction allows us to focus on regions where CH+chemistry dominates the production of carbon-bearing molecules. Profilesynthesis of several multiplets yielded column densities and Dopplerparameters for the C I fine-structure levels. Equilibrium excitationanalyses, using the measured column densities as well as the temperaturefrom H2 excitation, led to values for gas density. Thesedensities, in conjunction with measurements of CH, CH+,C2, and CN column densities, provided estimates for theamount of CH associated with CH+ production, which in turnset up constraints on the present theories for CH+ formationin this environment. We found for our sample of interstellar clouds thaton average 30%-40% of the CH originates from CH+ chemistry,and in some cases it can be as high as 90%. A simple chemical model forgas containing nonequilibrium production of CH+ was developedfor the purpose of predicting column densities for CH, CO,HCO+, CH+2, andCH+3 generated from large abundances ofCH+. Again, our results suggest that nonthermal chemistry isnecessary to account for the observed abundance of CH and probably thatof CO in these clouds.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| Scintillation Caustics in Planetary Occultation Light Curves
We revisit the GSC 5249-01240 light curve obtained during itsoccultation by Saturn's north polar region. In addition to refractivescintillations, the power spectrum of intensity fluctuations shows anenhancement of power between refractive and diffractive regimes. Weidentify this excess power as due to high-amplitude spikes in the lightcurve and suggest that these spikes are due to caustics associated withray-crossing situations. The flux variation in individual spikes followsthe expected caustic behavior including diffraction fringes, which wehave observed for the first time in a planetary occultation light curve.The presence of caustics in scintillation light curves requires an innerscale cutoff to the power spectrum of underlying density fluctuationsassociated with turbulence. Another possibility is the presence ofgravity waves in the atmosphere. While occultation light curvespreviously showed the existence of refractive scintillations, acombination of small projected stellar size and a low relative velocityduring the event have allowed us to identify caustics in thisoccultation. This has led us to reexamine previous data sets, in whichwe have also found likely examples of caustics.
| SPINR-A Wide-Field Ultraviolet Spectral Imaging System
We present the design, data analysis strategy, and flight results of asystem for recording spectral images of astrophysical sources atultraviolet wavelengths. The system, called the Spectrograph forPhotometric Imaging with Numeric Reconstruction (SPINR), is acombination of a new optical design with novel observing and dataprocessing strategies to record a full three-dimensional (two spatialand one spectral dimension) data cube. The SPINR technique allows theuse of higher throughput spectrographs than conventional push-broomtechniques and as a result delivers higher sensitivity.
| Apsidal Motion in Binaries: Rotation of the Components
A sample of 51 separated binary systems with measured apsidal periodsand rotational velocities of the components is examined. The ranges ofthe angles of inclination of the equatorial planes of the components tothe orbital plane are estimated for these systems. The observed apsidalvelocities can be explained by assuming that the axes of rotation of thestars are nonorthogonal to the orbital plane in roughly 47% of thesystems (24 of the 51) and the rotation of the components is notsynchronized with the orbital motion in roughly 59% of the systems (30of 51). Nonorthogonality and nonsynchrony are defined as deviations from90° and a synchronized angular velocity, respectively, at levels of1 or more.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Шкорпија |
| Ректацензија: | 16h05m26.20s |
| Deклинација: | -19°48'20.0" |
| Apparent магнитуда: | 2.62 |
| Даљина: | 162.602 parsecs |
| Proper motion RA: | 0 |
| Proper motion Dec: | 0 |
| B-T magnitude: | 2.519 |
| V-T magnitude: | 2.592 |
Каталог и designations:
|
INHABITED SKY
