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Far-ultraviolet scattering by dust in Orion
We have modelled diffuse far-ultraviolet (FUV) spectrum observed by theFar Ultraviolet Spectroscopic Explorer (FUSE) near M42 as the scatteringof the starlight from the Trapezium stars by dust in front of thenebula. The dust grains are known to be anomalous in Orion withRV= 5.5 and these are the first measurements of the FUVoptical properties of the grains outside of `normal' Milky Way dust. Wefind an albedo varying from 0.3 +/- 0.1 at 912 Å to 0.5 +/- 0.2 at1020 Åwhich is consistent with theoretical predictions.
| VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
We discuss the abundances of interstellar CH, CH+, and CN inthe Magellanic Clouds, derived from spectra of seven SMC and 13 LMCstars obtained (mostly) with the VLT UVES. CH and/or CH+ havenow been detected toward three SMC and nine LMC stars; CN is detectedtoward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly allthe optical detections of these molecular species in interstellar mediabeyond the Milky Way. In the LMC, the CH/H2 ratio iscomparable to that found for diffuse Galactic molecular clouds in foursight lines but is lower by factors of 2.5-4.0 in two others. In theSMC, the CH/H2 ratio is comparable to the local Galacticvalue in one sight line but is lower by factors of 10-15 in two others.The abundance of CH in the Magellanic Clouds thus appears to depend onlocal physical conditions and not just on metallicity. In both the SMCand the LMC, the observed relationships between the column density of CHand those of CN, CH+, Na I, and K I are generally consistentwith the trends observed in our Galaxy.Using existing data for the rotational populations of H2 inthese sight lines, we estimate temperatures, radiation field strengths,and local hydrogen densities for the diffuse molecular gas. The inferredtemperatures range from about 45 to 90 K, the radiation fields rangefrom about 1 to 900 times the typical local Galactic field, and thedensities (in most cases) lie between 100 and 600 cm-3.Densities estimated from the observed N(CH), under the assumption thatCH is produced via steady state gas-phase reactions, are considerablyhigher than those derived from H2. Much better agreement isfound by assuming that the CH is made via the (still undetermined)process(es) responsible for the observed CH+. A significantfraction of the CH and CH+ in diffuse molecular material inthe SMC and LMC may be produced in photon-dominated regions. Theexcitation temperature obtained from the populations of the two lowestCN rotational levels toward Sk -67 2 is quite consistent with thetemperature of the cosmic microwave background radiation measured withCOBE.Toward most of our targets, the UVES spectra also reveal absorption atvelocities corresponding to the Magellanic Clouds ISM from several ofthe strongest of the diffuse interstellar bands (DIBs; at 5780, 5797,and 6284 Å). On average, the three DIBs are weaker by factors of7-9 (LMC) and about 20 (SMC), compared to those typically observed inGalactic sight lines with similar N(H I), presumably due to the lowermetallicities and stronger radiation fields in the LMC and SMC. Thethree DIBs are also weaker (on average, but with some exceptions), byfactors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in theMagellanic Clouds. The detection of several of the so-calledC2 DIBs toward Sk 143 and Sk -67 2 with strengths similar tothose in comparable Galactic sight lines, however, indicates that nosingle, uniform scaling factor (e.g., one related to metallicity)applies to all DIBs (or for all sight lines) in the Magellanic Clouds.Based on observations collected at the European Southern Observatory,Chile, under programs 67.C-0281, 70.D-0164, 72.C-0064, 72.C-0682, and74.D-0109.
| 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.
| 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.
| X-Ray Emission from Early-Type Stars in the Orion Nebula Cluster
The X-ray properties of twenty ~1 Myr old O, B, and A stars of the OrionTrapezium are examined with data from the Chandra Orion UltradeepProject (COUP). On the basis of simple theories for X-ray emission, wedefine two classes separated at spectral type B4: hotter stars havestrong winds that may give rise to X-ray emission in small- orlarge-scale wind shocks, and cooler stars that should be X-ray dark dueto their weaker winds and absence of outer convection zones wheredynamos can generate magnetic fields. Emission by late-type magneticallyactive companions may be present in either class. Sixteen of the 20stars are detected with a wide range of X-ray luminosities,logLX (ergs s-1)~29-33, and X-ray efficiencies,log(LX/Lbol)~-4 to -8. Only two stars,θ1 Ori D (B0.5) and NU Ori (B1), show exclusively theconstant soft-spectrum emission at log(LX/Lbol)~-7expected from the standard model involving many small shocks in anunmagnetized radiatively accelerated wind. Most of the other massiveO7-B3 stars exhibit some combination of soft-spectrum wind emission,hard-spectrum flaring, and/or rotational modulation indicatinglarge-scale inhomogeneity. Magnetic confinement of winds withlarge-scale shocks can be invoked to explain these phenomena. This issupported in some cases by nonthermal radio emission and/or chemicalpeculiarities, or direct detection of the magnetic field(θ1 Ori C). Most of the stars in the weak-wind classexhibit X-ray flares and logLX<31 ergs s-1,consistent with magnetic activity from known or unseen low-masscompanions. In most cases, the X-ray spectra can be interpreted in termsof a two-temperature plasma model with a soft component of 3-10 MK and ahard component up to 40 MK. All nondetections belong to the weak-windclass. A group of stars exhibit hybrid properties-flarelike behaviorsuperimposed on a constant component with logLX~32 ergss-1-which suggest both magnetic activity and wind emission.
| Global X-Ray Properties of the Orion Nebula Region
Based on the Chandra Orion Ultradeep Project (COUP) observation, weestablish the global X-ray properties of the stellar populationassociated with the Orion Nebula. Three components contribute roughlyequally to the integrated COUP luminosity in the hard (2-8 keV) X-rayband: several OB stars, 822 lightly obscured cool stars in the OrionNebula Cluster (ONC), and 559 heavily obscured stars. ONC stars 0.5-2 pcfrom the center show a spatial asymmetry consistent with violentrelaxation in the stellar dynamics. The obscured COUP sourcesconcentrate around both OMC-1 molecular cores; these small-scalestructures indicate ages t<~0.1 Myr. The X-ray luminosity function(XLF) of the lightly obscured sample is roughly lognormal in shape. Theobscured population is deficient in lower luminosity stars, perhaps dueto localized circumstellar material. Mass-stratified XLFs show thatone-third of the Orion Nebula region hard-band emission is produced bythe bright O6 star θ1 Ori C and half is produced bylower mass pre-main-sequence stars with masses 0.3Msolar
| Chandra Orion Ultradeep Project: Observations and Source Lists
We present a description of the data reduction methods and the derivedcatalog of more than 1600 X-ray point sources from the exceptionallydeep 2003 January Chandra X-Ray Observatory (Chandra) observation of theOrion Nebula Cluster and embedded populations around OMC-1. Theobservation was obtained with Chandra's Advanced CCD ImagingSpectrometer (ACIS) and has been nicknamed the Chandra Orion UltradeepProject (COUP). With an 838 ks exposure made over a continuous period of13.2 days, the COUP observation provides the most uniform andcomprehensive data set on the X-ray emission of normal stars everobtained in the history of X-ray astronomy.
| An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards
In this paper we present a new method for deriving UV through IRextinction curves, based on the use of stellar atmosphere models toprovide estimates of the intrinsic (i.e., unreddened) stellar spectralenergy distributions (SEDs), rather than unreddened (or lightlyreddened) standard stars. We show that this ``extinction withoutstandards'' technique greatly increases the accuracy of the derivedextinction curves and allows realistic estimations of the uncertainties.An additional benefit of the technique is that it simultaneouslydetermines the fundamental properties of the reddened stars themselves,making the procedure valuable for both stellar and interstellar studies.Given the physical limitations of the models we currently employ, thetechnique is limited to main-sequence and mildly evolved B stars.However, in principle, it can be adapted to any class of star for whichaccurate model SEDs are available and for which the signatures ofinterstellar reddening can be distinguished from those of the stellarparameters. We demonstrate how the extinction without standards curvesmake it possible to (1) study the uniformity of curves in localizedspatial regions with unprecedented precision, (2) determine therelationships between different aspects of curve morphology, (3) producehigh-quality extinction curves from low color excess sight lines, and(4) derive reliable extinction curves for mid to late B stars, therebyincreasing spatial coverage and allowing the study of extinction in openclusters and associations dominated by such stars. The application ofthis technique to the available database of UV through IR SEDs, and tofuture observations, will provide valuable constraints on the nature ofinterstellar grains and on the processes that modify them, and it willenhance our ability to remove the multiwavelength effects of extinctionfrom astronomical energy distributions.
| On the highly reddened members in six young galactic star clusters - a multiwavelength study
The spectral and reddening properties of 211 highly reddenedproper-motion members with V < 15 mag in six young galactic starclusters are investigated using low-resolution spectroscopic, broad-bandUBVRIJHK and mid-infrared (IR) data. We report emission features in CaIIHK and HI lines for a sample of 29 stars including 11 stars reported forthe first time and also provide either a new or more reliable spectralclass for a sample of 24 stars. CaII triplet width measurements are usedto indicate the presence of an accretion disc for a dozen stars and toindicate luminosity for a couple of stars. On the basis of spectralfeatures, near-IR excesses, dereddened colour-colour diagrams and mid-IRspectral indices we identify a group of 28 pre-main-sequence clustermembers including five highly probable Herbig Ae/Be and six classical TTauri stars. A total of 25 non-emission main-sequence (MS) stars,amounting to ~10 per cent early-type MS members, appears to showVega-like characteristics or are precursors to such a phenomenon. Thevarious membership indicators suggest that ~16 per cent of theproper-motion members are non-members. A significant fraction (>70per cent) of programme stars in NGC 1976, NGC 2244, NGC 6530 and NGC6611 show anomalous reddening with RV= 4.78 +/- 0.10, 3.54+/- 0.04, 3.87 +/- 0.05 and 3.56 +/- 0.02, respectively, indicating thepresence of grain size dust larger than that typical of the diffusemedium. A small number of stars in NGC 1976, NGC 2244 and NGC 6611 alsoshow normal behaviour while the cluster NGC 6823 appears to have normalreddening. Three highly luminous late-type giants, one in NGC 2244 andtwo in NGC 6530, appear to be members and are inpost-hydrogen-core-burning stages, suggesting a prolonged duration (~25Myr) of star formation.
| Anomalous dust-to-gas ratios in the Galaxy
Lines of sight with E(B-V)/N(HI) considerably smaller than the averagevalue for the solar neighbourhood have been selected from the catalogueof Diplas & Savage. In order to develop quantitative considerations,estimates of the molecular hydrogen column density were obtained usingthe relation of Savage et al. extended at E(B-V) > 0.4 with therecent data of Rachford et al. Contrary to the prevailing opinion in theliterature for sightlines with similar behaviour, we found that only 22per cent of our sample was characterized by both an average gas densitylarger than 1 cm-3 and a value of RV larger thanthat in the diffuse interstellar medium. By computing extinction models,we were able to fit the E(B-V)/N(HI) by changing the value ofRV only for some sightlines. For the remaining ones, aρd/ρH ratio different from the averageGalactic value must be invoked. The application of the Kramers-Kronigrelation to the observed extinction curves confirmed this possibility.Moreover, attempts to fit such curves with models having grain volumescorresponding to the standard ρd/ρH ratiofailed.We find a linear relation between ρd/ρHand E(B-V)/N(H) for our sightlines. The average Galactic value marks theseparation into two groups characterized by lower abundances of C and Sitrapped into the grains when E(B-V)/N(H) is smaller than the Galacticvalue, and by larger abundances when E(B-V)/N(H) is greater.
| 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 Angular Momentum Evolution of 0.1-10 Msolar Stars from the Birth Line to the Main Sequence
Projected rotational velocities (vsini) have been measured for a sampleof 145 stars with masses between 0.4 and greater than 10Msolar (median mass 2.1 Msolar) located in theOrion star-forming complex. These measurements have been supplementedwith data from the literature for Orion stars with masses as low as 0.1Msolar. The primary finding from analysis of these data isthat the upper envelope of the observed values of angular momentum perunit mass (J/M) varies as M0.25 for stars on convectivetracks having masses in the range ~0.1 to ~3 Msolar. Thispower law extends smoothly into the domain of more massive stars (3-10Msolar), which in Orion are already on the zero-age mainsequence. This result stands in sharp contrast to the properties ofmain-sequence stars, which show a break in the power law and a sharpdecline in J/M with decreasing mass for stars with M<2Msolar. A second result of our study is that this break isseen already among the pre-main-sequence stars in our Orion sample thatare on radiative tracks, even though these stars are only a few millionyears old. A comparison of rotation rates seen for stars on either sideof the convective-radiative boundary shows that stars do not rotate assolid bodies during the transition from convective to radiative tracks.As a preliminary demonstration of how observations can be used toconstrain the processes that control early stellar angular momentum, weshow that the broad trends in the data can be accounted for by simplemodels that posit that stars (1) lose angular momentum before they aredeposited on the birth line, plausibly through star-disk interactions;(2) undergo additional braking as they evolve down their convectivetracks; and (3) are subject to core-envelope decoupling during theconvective-radiative transition.
| Total to Selective Extinction Ratios and Visual Extinctions from Ultraviolet Data
We present determinations of the total to selective extinction ratio R_Vand visual extinction A_V values for Milky Way stars using ultravioletcolor excesses. We extend the analysis of Gnacinski and Sikorski (1999)by using non-equal weights derived from observational errors. We presenta detailed discussion of various statistical errors. In addition, weestimate the level of systematic errors by considering differentnormalization of the extinction curve adopted by Wegner (2002). Ourcatalog of 782 R_V and A_V values and their errors is available in theelectronic form on the World Wide Web.
| 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.
| The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope SpaceTelescope Imaging Spectrograph (STIS) observations of Kr I λ1236absorption in seven sight lines that probe a variety of interstellarenvironments. In combination with krypton and hydrogen column densitiesderived from current and archival STIS and Far-Ultraviolet SpectroscopicExplorer data, the number of sight lines with reliable Kr/H ISMabundance ratios has been increased by 50% to 26-including paths thatsample a range of nearly 5 orders of magnitude in f(H2) andover 2 orders of magnitude in , and extend up to4.8 kpc in length. For sight lines contained entirely within the localspiral arm (the Orion spur), the spread of Kr/H ratios about the mean oflog10[N(Kr)/N(H)]ISM=-9.02+/-0.02is remarkably tight (0.06 dex), less than the typical data-pointuncertainty. Intriguingly, the only two sight lines that extend throughneighboring structures, in particular gas associated with theCarina/Sagittarius arm, exhibit relatively large, near-solar kryptonabundances (log10[N(Kr)/N(H)]combined=-8.75+0.09-0.11).Although these deviations are only measured at the 2 σ level, theysuggest the possibility that krypton abundances beyond the Orion spurmay differ from the local value.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 NAS-32985.
| Observations of Rotationally Resolved C3 in Translucent Sight Lines
The rotationally resolved spectrum of theA1Πu<--X1Σ+g000-000 transition of C3, centered at 4051.6 Å, hasbeen observed along 10 translucent lines of sight. To interpret thesespectra, a new method for the determination of column densities andanalysis of excitation profiles involving the simulation and fitting ofobserved spectra has been developed. The populations of lower rotationallevels (J<=14) in C3 are best fitted by thermaldistributions that are consistent with the kinetic temperaturesdetermined from the excitation profile of C2. Just as in thecase of C2, higher rotational levels (J>14) ofC3 show increased nonthermal population distributions inclouds that have been determined to have total gas densities below ~500cm-3.
| Some Diffuse Interstellar Bands Related to Interstellar C2 Molecules
We have investigated the correlations between the equivalent widths of21 selected diffuse interstellar bands (DIBs) and the correspondinginterstellar column densities N(C2), N(CN), and N(CH), toward53 stars with color excesses 0.11<=E(B-V)<=1.99. The observationaldata were derived primarily from echelle spectra acquired at R=38,000 aspart of our extensive, continuing survey of the bands. All but six ofthe 53 final spectra show signal-to-noise ratios >=800 at 5780Å. The principal result presented here is that seven of the 21bands prove to be examples of ``the C2 DIBs,'' a class ofweak, narrow bands whose normalized equivalent widthsWλ(X)/Wλ (λ6196) are wellcorrelated specifically with N(C2)/E(B-V) via power laws. Incontrast, the similarly normalized equivalent widths of the 14 other,well-known DIBs analyzed here are uncorrelated, or weaklyanticorrelated, with N(C2)/E(B-V), to within theobservational uncertainties. Thus, the polyatomic molecule(s) presumedto cause these seven C2 DIBs may bear a direct chemicalrelation to C2 that is not shared by the polyatomic moleculesputatively responsible for the other 14 bands. The C2 DIBsalso show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) inour particular sample of light paths, although generally with shallowerslopes in the case of N(CN) and with greater scatter in the case ofN(CH). Eleven additional C2 DIBs are also identified but arenot analyzed here. Among the 18 C2 DIBs identified, fourapparently have not been previously detected. The λ4963 band isgenerally the strongest of the 18 C2 DIBs, while theλ4734 band shows the most sensitive correlation withN(C2).Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| 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.
| Deep Near-Infrared Observations and Identifications of Chandra Sources in Orion Molecular Clouds 2 and 3
We conducted deep near-infrared (NIR) imaging observations of Orionmolecular clouds 2 and 3 using the Quick Infrared Camera on the 88 inch(2.2 m) telescope of the University of Hawaii. Our purposes were (1) togenerate a comprehensive NIR source catalog of these star-forming cloudsand (2) to identify the NIR counterpart of the Chandra X-ray sourcesthat have no counterpart in the Two Micron All Sky Survey (2MASS)catalog. Our J-, H-, and K-band observations are ~2 mag deeper thanthose of 2MASS and match the current Chandra observation well. Wedetected 1448 NIR sources, for which we derived the position, the J-,H-, and K-band magnitude, and the 2MASS counterpart. Using this catalog,we identified the NIR counterpart for ~42% of the 2MASS unidentifiedChandra sources. The nature of these Chandra sources are discussed usingtheir NIR colors and spatial distributions, and a dozen protostar andbrown dwarf candidates are identified.
| Determination of the gas-to-dust ratio in nearby dense clouds using X-ray absorption measurements
We present a comparison of the gas and dust properties of the denseinterstellar matter in six nearby star-forming regions (d<500 pc):rho Oph, Cha I, R CrA, IC 348, NGC 1333, and Orion. We measure fromChandra and XMM-Newton observations the X-ray absorption toward pre-mainsequence stars (PMS) without accretion disks (i.e., Class III sources)to obtain the total hydrogen column density NH,X. For thesesources we take from the literature the corresponding dust extinction inthe near-infrared, AJ, or when unavailable we derive it fromSED fitting using the available DENIS, 2MASS, ISOCAM and other data. Wethen compare NH,X and AJ for each object, up tounprecedently high extinction. For the rho Oph dark cloud with arelatively large sample of 20 bona-fide Class III sources, we probe theextinction up to AJ <~ 14 (AV <~ 45), andfind a best-fit linear relation NH,X/AJ = 5.6 (+/-0.4) x 1021 cm-2 mag-1, adoptingstandard ISM abundances. The other regions reveal a large dispersion inthe NH,X/AJ ratio for each source but for lack ofadequate IR data these studies remain limited to moderate extinctions(AJ <~ 1.5 or AV <~ 5). For rho Oph, theNH,X/AJ ratio is significantly lower (ga 2sigma )than the galactic value, derived using the standard extinction curve(RV = 3.1). This result is consistent with the recentdownwards revision of the metallicity of the Sun and stars in the solarvicinity. We find that the rho Oph dense cloud has the same metallicitythan the local ISM when assuming that the galactic gas-to-dust ratioremains unchanged. The difference between galactic and local values ofthe gas-to-dust ratio can thus be attributed entirely to a difference inmetallicity.
| Grey extinction in the solar neighbourhood?
Some of the close O and B dwarfs appear to be fainter than indicated bytheir Hipparcos distances, intrinsic absolute magnitudes attributed totheir spectral types, and estimated selective interstellar extinction.This discrepancy is explained in the paper by the grey (neutral)interstellar extinction in the visual range of spectrum. The measure ofsuch an effect is related to discrete features of the interstellarmatter.
| A Study of the Energy Sources of Herbig-Haro Objects
We make a statistical study of the energy sources of high-velocityphenomena, Herbig-Haro (HH) objects. IRAS counterparts of HH objects areidentified. Their colors, brightness, geometric relation to the HHobjects and SED are analysed. The sources are found to be concentratedin a band-shaped region in the IRAS color- color diagram. We suggest anexplanation of thick surrounding material for this distribution. Wepropose a new method for identifying the energy sources based on colorand brightness. This method is applied to more than 200 HH objects whoseenergy sources are still unknown. Finally, a group of very young stellarobject candidates which have similar properties to the HH energy sourcesis picked out. Their large-scale distribution is discussed.
| X-Ray-emitting Young Stars in the Orion Nebula
The Orion Nebula Cluster and the molecular cloud in its vicinity havebeen observed with the ACIS-I detector on board the Chandra X-rayObservatory with 23 hr exposure in two observations. We detect 1075X-ray sources, most with subarcsecond positional accuracy. Ninety-onepercent of the sources are spatially associated with known stellarmembers of the cluster, and an additional 7% are newly identified deeplyembedded cloud members. This provides the largest X-ray study of apre-main-sequence stellar population and covers the initial massfunction from brown dwarfs up to a 45 Msolar O star. Sourceluminosities span 5 orders of magnitude from logLX~=28.0 to33.3 ergs s-1 in the 0.5-8 keV band, plasma energies rangefrom 0.2 to >10 keV, and absorption ranges from logNH<20.0to ~23.5 cm-2. Comprehensive tables providing X-ray andstellar characteristics are provided electronically. We examine here theX-ray properties of Orion young stars as a function of mass; otherstudies of astrophysical interest will appear in companion papers.Results include: (a) the discovery of rapid variability in the O9.5 31Msolar star θ2A Ori, and several early Bstars, inconsistent with the standard model of X-ray production in smallshocks distributed throughout the radiatively accelerated wind; (b)support for the hypothesis that intermediate-mass mid-B through A typestars do not themselves produce significant X-ray emission; (c)confirmation that low-mass G through M type T Tauri stars exhibitpowerful flaring but typically at luminosities considerably below the``saturation'' level; (d) confirmation that the presence or absence of acircumstellar disk has no discernable effect on X-ray emission; (e)evidence that T Tauri plasma temperatures are often very high withT>=100 MK, even when luminosities are modest and flaring is notevident; and (f) detection of the largest sample of pre-main-sequencevery low-mass objects showing flaring levels similar to those seen inmore massive T Tauri stars and a decline in magnetic activity as theyevolve into L and T type brown dwarfs.
| X-Ray Properties of Young Stellar Objects in OMC-2 and OMC-3 from the Chandra X-Ray Observatory
We report X-ray results of the Chandra observation of OMC-2 and OMC-3. Adeep exposure of ~100 ks detects ~400 X-ray sources in the field of viewof the ACIS array, providing one of the largest X-ray catalogs in astar-forming region. Coherent studies of the source detection, timevariability, and energy spectra are performed. We classify the X-raysources into Class I, Class II, and Class III+MS based on the J-, H-,and K-band colors of their near-infrared counterparts and discuss theX-ray properties (temperature, absorption, and time variability) alongthese evolutionary phases.
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| Multiplicity of Massive Stars
We discuss the observed multiplicity of massive stars and implicationson theories of massive star formation. After a short summary of theliterature on massive star multiplicity, we focus on the O- and B-typestars in the Orion Nebula Cluster, which constitute a homogenous sampleof very young massive stars. 13 of these stars have recently been thetargets of a bispectrum speckle interferometry survey for companions.Considering the visual and also the known spectroscopic companions ofthese stars, the total number of companions is at least 14.Extrapolation with correction for the unresolved systems suggests thatthere are at least 1.5 and perhaps as much as 4 companions per primarystar on average. This number is clearly higher than the mean number of~0.5 companions per primary star found for the low-mass stars in thegeneral field population and also in the Orion Nebula cluster. Thissuggests that a different mechanism is at work in the formation ofhigh-mass multiple systems in the dense Orion Nebula cluster than forlow-mass stars.
| Space Telescope Imaging Spectrograph Observations of Interstellar Oxygen and Krypton in Translucent Clouds
We have obtained high-resolution Hubble Space Telescope (HST) SpaceTelescope Imaging Spectrograph (STIS) observations of O I λ1356and Kr I λ1236 absorption in 11 sight lines characterized by highextinction, large H I column densities, and/or long path lengths.Previous Goddard High Resolution Spectrograph (GHRS) measurements ofthese weak features in seven relatively nearby diffuse clouds have shownno evidence for density-dependent depletion of either oxygen or kryptonand have yielded a weighted mean gas-phase abundance ratio oflog[N(O)/N(Kr)]GHRS=5.56+/-0.04. Our STIS measurements yielda lower weighted mean of log[N(O)/N(Kr)]STIS=5.48 thedifference is due primarily to several translucent sight lines in theSTIS data set that diverge from the GHRS value. These translucent cloudsight lines pass near dense, star-forming regions, notably the ρOph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas,should not be depleted much into grains, these cases suggest a trendtoward the enhanced oxygen depletion predicted for denser ISM clouds.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.
| Profiles of the λ6196 and λ6379 Diffuse Interstellar Bands
We have looked for rotational structure in the sharp λ6196 andλ6379 diffuse interstellar bands (DIBs) at a resolution ~120,000in seven stars where the interstellar λ7699 K I line isunresolved. The λ6196 DIB is bell-shaped with a flat core anddiffers slightly in width from star to star. It is accompanied by a weakDIB at λ6194.7, with which it does not maintain a constant depthratio. The λ6379 DIB is asymmetric with a sharp double core, butthe profile hardly varies between the stars apart from beingundetectable for HD 37061. Weak features connect it to the weakerλ6376 DIB, with which it varies in unison. Simple rotationalmodels do not fit the observed profiles of λ6196 and λ6376at all well because of more prominent branch structure in the models. Weachieve an acceptable fit by arbitrarily convolving the modeled profileswith Gaussians (0.2 to 0.3 cm-1). The Gaussians correspond toan unexpected or anomalous broadening process that cannot be explainedby the interstellar velocity distribution or the instrumentalpoint-spread function. The fits give upper limits to the ratio of therotational temperature to the moment of inertia of the molecularcarriers. If the former lies in the range 10-100 K, then the moleculesmust be large, with moments of inertia comparable with that of thefullerene C60. We present evidence that the anomalousbroadening of the rotational profiles is intramolecular in origin, butit is not easily explained by broadening processes previously invoked inconnection with the DIBs. We suggest that some other process such as azero-point vibrational isotope shift may be involved that could becharacteristic of many of the narrow bands.
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