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מאמרים קשורים
Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
We have used archival far-ultraviolet spectra from observations made byHST STIS and FUSE to determine the column densities and rotationalexcitation temperatures for carbon monoxide and molecular hydrogen,respectively, along 23 sight lines to Galactic O and B stars. Thereddening values range from E(B-V)=0.07 to 0.62, sampling the diffuse totranslucent interstellar medium (ISM). We find that the H2column densities range from 5×1018 to8×1020 cm-2 and the CO from upper limitsaround 2×1012 cm-2 to detections as high as1.4×1016 cm-2. CO increases with increasingH2, roughly following a power law of factor ~2. TheCO/H2 column density ratio is thus not constant, ranging from10-7 to 10-5, with a mean value of3×10-6. The sample segregates into ``diffuse'' and``translucent'' regimes, the former with molecular fraction <~0.25and AV/d<1 mag kpc-1. The mean CO/H2for these two regimes are 3.6×10-7 and9.3×10-6, respectively, significantly lower than thecanonical dark cloud value of 10-4. Six sight lines show theisotopic variant 13CO, and the isotopic ratio we observe(~50-70) is consistent with, if perhaps a little below, the average12C/13C for the ISM at large. The averageH2 rotational excitation temperature is 74+/-24 K, agreeingwell with previous studies, and the average CO temperature is 4.1 K,with some sight lines showing temperatures as high as 6.4 K. The higherexcitation CO is observed with higher column densities, consistent withthe effects of photon trapping in clouds with densities in the 20-100cm-3 range. We discuss the implications for the structure ofthe diffuse/translucent regimes of the ISM and the estimation ofmolecular mass in galaxies.
| A Model for Atomic and Molecular Interstellar Gas: The Meudon PDR Code
We present the revised ``Meudon'' model of photon-dominated region (PDR)code, available on the Web under the GNU Public License. Generalorganization of the code is described down to a level that should allowmost observers to use it as an interpretation tool with minimal helpfrom our part. Two grids of models, one for low-excitation diffuseclouds and one for dense highly illuminated clouds, are discussed, andsome new results on PDR modelization highlighted.
| H2 excitation in turbulent interstellar clouds
We discuss the observational differences between lines of sight thatintercept a group of turbulent dissipative structures and lines of sightthat cross less-active regions. Using time-dependent calculations weshow that the energy level distribution of the hydrogen molecule evolvesin time in response to the local thermal phase. We find that relativelysimple models can explain the observed properties of molecular hydrogenin diffuse interstellar clouds in terms of time evolution induced bycollisional excitation in a low-density, high-temperature gas.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| 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.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 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/404/913
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| H2 formation and excitation in the diffuse interstellar medium
We use far-UV absorption spectra obtained with FUSE towards three late Bstars to study the formation and excitation of H2 in thediffuse ISM. The data interpretation relies on a model of the chemicaland thermal balance in photon-illuminated gas. The data constrain wellthe n R product between gas density and H2 formation rate ondust grains: n R = 1 to 2.2 x 10-15 s-1. For eachline of sight the mean effective H2 density n, assumeduniform, is obtained by the best fit of the model to the observedN(J=1)/N(J=0) ratio, since the radiation field is known. Combining nwith the n R values, we find similar H2 formation rates forthe three stars of about R = 4 x 10-17 cm3s-1. Because the target stars do not interact with theabsorbing matter we can show that the H2 excitation in theJ> 2 levels cannot be accounted for by the UV pumping of the coldH2 but implies collisional excitation in regions where thegas is much warmer. The existence of warm H2 is corroboratedby the fact that the star with the largest column density ofCH+ has the largest amount of warm H2.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 is 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/Abstract.html
| The role of the geometry and dust environment in stationary models of translucent interstellar clouds
A full radiative transfer model is presented for the ultraviolet (UV)radiation impinging on an interstellar cloud of spherical or finiteplane-parallel slab geometry containing gas and dust. The penetration ofthe UV photons is coupled to detailed chemical processes.Photodestruction rates of atomic and molecular species are calculatedfrom the corresponding cross-sections. We show that CO line intensitiesare quite sensitive to geometrical effects and to the extinction curvein the far-UV.
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| High velocity gas and dust evolution in Chamaeleon clouds
We report on GHRS observations which reveal conspicuous differences inthe absorption spectra of two nearby stars, close to each other. Thestar HD102065 lies behind a cloud in Chamaeleon with unusually strongmid-IR emission, indicating a large abundance of very small dustparticles. Along this line of sight, 5% of the gas (about 6 10(19)cm(-2) ) is at large velocities (up to -50 km s(-1) ) compared to themain absorption component at vlsr ~ 0 km s(-1) . The highvelocity gas is very excited and has an unusually large siliconabundance. The other star HD96675 lies behind a cloud with standardmid-IR emission. Along this line of sight, high velocity gas is alsodetected, but to somewhat smaller offset velocities and with a muchlower excitation. In particular, the SiII(*) lines are not observed.From the excitation of Si(+) and C(+) in the direction of HD102065, weinfer that the high velocity gas has a temperature higher than several100 K and an electron density of at least 10 cm(-3) . These results,together with the lack of an ionizing star in the neigborhood, suggestthat a large amount of kinetic energy is being deposited in this gas.The collision of an infalling cloud and a local cloud is a plausiblesource of energy. The peculiarity of the dust size distribution inferredfrom the IRAS data is likely to be related to the processes whichdissipate the kinetic energy and heat the gas. A shock seems to berequired to produce the excitation and ionization degrees. But thecarbon ionization ratio combined with the electron density inferred fromthe silicon and carbon excitation implies that carbon is not inionization equilibrium and should recombine extremely quickly. Futurehigher resolution observations might help solving this incoherency.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| High-Velocity Gas in GHRS Spectra of Three OB Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...468..290T&db_key=AST
| Dust Metamorphosis in the Galaxy
Not Available
| Hyades and Sirius supercluster members brighter than magnitude (V) 7.1. II - Right ascension six to twelve hours
The present star sample is contained in the Bright Star Catalogue andits Supplement, augmented with a further supplement of 788 stars foundduring various observing programs over the past 40 years. Accurate,four-color and H-beta, or (RI), photometry is available for most of thesupercluster members. The criteria for membership are the comparisons ofthe proper motion, radial velocity, and luminosity obtained from thesupercluster parameters with the observed motions and the luminosityderived from the photometric parameters. New proper motions, based onall available catalogs, have been derived for the additional 788 starsdiscussed here, as well as all supercluster members.
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| קבוצת-כוכבים: | זיקית |
| התרוממות ימנית: | 11h43m36.95s |
| סירוב: | -80°29'00.5" |
| גודל גלוי: | 6.607 |
| מרחק: | 168.067 פארסק |
| תנועה נכונה: | -30.3 |
| תנועה נכונה: | -7 |
| B-T magnitude: | 6.697 |
| V-T magnitude: | 6.615 |
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