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Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope
We present the results of an infrared imaging survey of two clusters inthe Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPSinstruments on board the Spitzer Space Telescope. Our observations coverthe wavelength range from 3.6 to 24 μm, allowing us to detect diskemission over a typical range of radii ~0.1 to ~20 AU from the centralstar. In Tr 37, with an age of about 4 Myr, about 48% of the low-massstars exhibit detectable disk emission in the IRAC bands. Roughly 10% ofthe stars with disks may be ``transition'' objects, with essentiallyphotospheric fluxes at wavelengths <=4.5 μm but with excesses atlonger wavelengths, indicating an optically thin inner disk. The medianoptically thick disk emission in Tr 37 is lower than the correspondingmedian for stars in the younger Taurus region; the decrease in infraredexcess is larger at 6-8 μm than at 24 μm, suggesting that graingrowth and/or dust settling has proceeded faster at smaller disk radii,as expected on general theoretical grounds. Only about 4% of thelow-mass stars in the 10 Myr old cluster NGC 7160 show detectableinfrared disk emission. We also find evidence for 24 μm excessesaround a few intermediate-mass stars, which may represent so-called``debris disk'' systems. Our observations provide new constraints ondisk evolution through an important age range.
| H2 active jets in the near IR as a probe of protostellar evolution
We present an in-depth near-IR analysis of a sample of H2 outflows fromyoung embedded sources to compare the physical properties and coolingmechanisms of the different flows. The sample comprises 23 outflowsdriven by Class 0 and I sources having low-intermediate luminosity. Wehave obtained narrow band images in H2 2.12 μm and [Fe II] 1.64 μmand spectroscopic observations in the range 1-2.5 μm. From [Fe II]images we detected spots of ionized gas in ~74% of the outflows which insome cases indicate the presence of embedded HH-like objects. H2 lineratios have been used to estimate the visual extinction and averagetemperature of the molecular gas. Av values range from ~2 to~15 mag; average temperatures range between ~2000 and ~4000 K. Inseveral knots, however, a stratification of temperatures is found withmaximum values up to 5000 K. Such a stratification is more commonlyobserved in those knots which also show [Fe II] emission, while athermalized gas at a single temperature is generally found in knotsemitting only in molecular lines. Combining narrow band imaging (H2,2.12 μm and [Fe II], 1.64 μm) with the parameters derived from thespectroscopic analysis, we are able to measure the total luminosity ofthe H2 and [Fe II}] shocked regions (L_H2 and L[FeII]) in each flow. H2 is the major NIR coolant with an averageL_H_2/L[Fe II] ratio of ~102. We find that ~83% ofthe sources have a L_H_2/L_bol ratio ~0.04, irrespective of the Class ofthe driving source, while a smaller group of sources (mostly Class I)have L_H_2/L_bol an order of magnitude smaller. Such a separationreveals the non-homogeneous behaviour of Class I, where sources withvery different outflow activity can be found. This is consistent withother studies showing that among Class I one can find objects withdifferent accretion properties, and it demonstrates that the H2 power inthe jet can be a powerful tool to identify the most active sources amongthe objects of this class.
| On the diffuse bands related to the C2 interstellar molecule
The recently published idea that intensities of some weak diffuseinterstellar bands (DIBs) are related to the C2 molecule column densityhave been examined. We use a set of high quality echelle spectra ofheavily reddened stars, acquired at the Bohyunsan Optical AstronomicalObservatory (South Korea), with a resolution R=30 000. The high quality(high S/N ratio) of our spectra is proved by the fact that despite usingthe most widely used Phillips (2, 0) band of the C2 molecule (near 8760Å), we can trace the (3, 0) Phillips band (near 7725 Å) aswell. Equivalent widths of four (5176, 5542, 5546 and 5769 Å) outof 16 examined DIBs demonstrate relatively good correlation with C2column density. However, a majority of the studied DIBs, alreadyreported as "C2" ones, most likely are not related to this simplestcarbon molecule. A removal of peculiar objects like HD 34078 from theanalyzed sample does not substantially change the level of correlations.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| 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.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. II. Analysis
To complement the optical absorption line survey of diffuse moleculargas in Paper I, we obtained and analyzed far-ultraviolet H2and CO data on lines of sight toward stars in Cep OB2 and Cep OB3.Possible correlations between column densities of different species forindividual velocity components, not total columns along a line of sightas in the past, were examined and were interpreted in terms of cloudstructure. The analysis reveals that there are two kinds of CH indiffuse molecular gas: CN-like CH and CH+-like CH. Evidenceis provided that CO is also associated with CN in diffuse molecularclouds. Different species are distributed according to gas density inthe diffuse molecular gas. Both calcium and potassium may be depletedonto grains in high-density gas, but with different dependencies onlocal gas density. Gas densities for components where CN was detectedwere inferred from a chemical model. Analysis of cloud structureindicates that our data are generally consistent with the large-scalestructure suggested by maps of CO millimeter-wave emission. On smallscales, the gas density is seen to vary by factors greater than 5.0 overscales of ~10,000 AU. The relationships between column densities of COand CH with that of H2 along a line of sight show similarslopes for the gas toward Cep OB2 and Cep OB3, but the CO/H2and CH/H2 ratios tend to differ, which we ascribe tovariation in average density along the line of sight.
| Ion-by-Ion Differential Emission Measure Determination of Collisionally Ionized Plasma. II. Application to Hot Stars
In a previous paper we have described a technique to derive constraintson the differential emission measure (DEM) distribution, a measure ofthe temperature distribution, of collisionally ionized hot plasmas fromtheir X-ray emission line spectra. We apply this technique to theChandra HETGS spectra of all of the nine hot stars available to us atthe time that this project was initiated. We find that DEM distributionsof six of the seven O stars in our sample are very similar, but thatθ1 Ori C has an X-ray spectrum characterized by highertemperatures. The DEM distributions of both of the B stars in our samplehave lower magnitudes than those of the O stars, and one, τ Sco, ischaracterized by higher temperatures than the other, β Cru. Theseresults confirm previous work in which high temperatures have been foundfor θ1 Ori C and τ Sco and taken as evidence forchanneling of the wind in magnetic fields, the existence of which isrelated to the stars' youth. Our results demonstrate the utility of ourmethod for deriving temperature information for large samples of X-rayemission-line spectra.
| Velocity Dispersion of the High Rotational Levels of H2
We present a study of the high rotational bands (J>=2) ofH2 toward four early-type Galactic stars: HD 73882, HD192639, HD 206267, and HD 207538. In each case, the velocity dispersion,characterized by the spectrum fitting parameter b, increases with thelevel of excitation, a phenomenon that has previously been detected bythe Copernicus and IMAPS observatories. In particular, we show with 4σ confidence that for HD 192639 it is not possible to fit all Jlevels with a single b-value and that higher b-values are needed for thehigher levels. The amplitude of the line broadening, which can be ashigh as 10 km s-1, makes explanations such as inhomogeneousspatial distribution unlikely. We investigate a mechanism in which thebroadening is due to the molecules that are rotationally excited throughthe excess energy acquired after their formation on a grain(H2 formation pumping). We show that different dispersionswould be a natural consequence of this mechanism. We note, however, thatsuch a process would require a formation rate 10 times higher than whatwas inferred from other observations. In view of this result, and of thedifficulty in accounting for the velocity dispersion as thermalbroadening (T would be around 10,000 K), we conclude then that we aremost certainly observing some highly turbulent warm layer associatedwith the cold diffuse cloud. Embedded in a magnetic field, it could beresponsible for the high quantities of CH+ measured in thecold neutral medium.
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| Cepheus OB2: Disk Evolution and Accretion at 3-10 Myr
We present the results of MMT observations of young stars for our studyof protoplanetary disks at ages 1-10 Myr in two young clusters locatedin the Cepheus OB2 region: Trumpler 37 (embedded in the H II region IC1396) and NGC 7160. Using low-resolution optical spectra from theHectospec multifiber spectrograph, we have tripled the number of knownlow-mass cluster members, identifying ~130 new members in Tr 37 and ~30in NGC 7160. We use indicators of youth (Li absorption at 6707 Å)and accretion/chromospheric activity (Hα emission) to identify andclassify the low-mass cluster members. We derive spectral types for allthe low-mass candidates and calculate the individual extinctions and theaverage over the clusters. With the extended member samples, we estimatethe disk fraction in the clusters, finding that ~40% of the low-massstars in Tr 37 are actively accreting, whereas only 1 of the ~55low-mass stars in NGC 7160 shows indications of accretion. Opticalphotometry and theoretical isochrones are used to determine the age ofthe cluster members, confirming the estimates of ~4 Myr for Tr 37 and~10 Myr for NGC 7160. Accretion rates in Tr 37(~10-8Msolaryr-1 on average) arederived from U-band photometry. We find that only ~50% of the accretingstars have near-IR excesses (from 2MASS), which could be due to thegeometry of their disks or be an indication dust of settling/graingrowth. Finally, we study the high- and intermediate-mass members of theclusters. With the extended member list, we revise the spatialdistribution of stars with disks. Our results are crucial forinterpreting Spitzer Space Telescope studies of accretion disks at theages of planet formation (3-10 Myr).Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.
| H{2}O maser emission from bright rimmed clouds in the northern hemisphere
We report the results of a multi-epoch survey of water maserobservations at 22.2 GHz with the Medicina radiotelescope from 44 brightrimmed clouds (BRCs) of the northern hemisphere identified by Sugitaniet al. (1989, ApJ, 342, L87) as potential sites of star formation. Thedata span 16 years of observations and allow to draw conclusions aboutthe maser detection rate in this class of objects. In spite of therelatively high far-infrared luminosities of the embedded sources(L_FIR 102 L_ȯ), H2O maser emission was detectedtowards three globules only. Since the occurrence of water masers ishigher towards bright IRAS sources, the lack of frequent H2O maseremission is somewhat surprising if the suggestion of inducedintermediate- and high-mass star formation within these globules iscorrect. The maser properties of two BRCs are characteristic of excitingsources of low-mass, while the last one (BRC 38) is consistent with anintermediate-mass object. We argue that most BRCs host young stellarobjects of low-luminosity, likely in an evolutionary phase later thanthe protostellar Class 0 sources, and that a significant contribution tothe observd IRAS luminosity comes from warm dust heated by the radiationfrom the bright rim.
| Star formation in globules in IC 1396
We present a large-scale study of the IC 1396 regionusing new deep NIR and optical images, complemented by 2MASS data. Forten globules in IC 1396 we determine (H-K, J-H) colour-colour diagramsand identify the young stellar population. Five of these globulescontain a rich population of reddened objects, most of them probablyyoung stellar objects. Two new HH objects (HH 865 andHH 864) could be identified by means of [SII]emission, one of them a parsec-scale flow. Using star counts based on2MASS data we create an extinction map of the whole region. This map isused to identify 25 globules and to estimate their mass. The globulemasses show a significant increase with the distance from the excitingO6.5V star HD 206267. We explain this correlation bythe enhanced radiation pressure close to this star, leading toevaporation of the nearby clouds and hence smaller globule masses. Wesee evidence that the radiation from HD 206267 has a major impact on thestar formation activity in these globules.All Appendices are only available in electronic form athttp://www.edpsciences.org
| Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
We present column density measurements of the HD molecule in theinterstellar gas toward 17 Galactic stars. The values for the seven mostheavily reddened sightlines, with E(B-V) = 0.38-0.72, are derived fromobservations with the Far Ultraviolet Spectroscopic Explorer (FUSE). Theother ten values are from a reanalysis of spectra obtained withCopernicus. In all cases, high-resolution ground-based observations of KI and/or the CH molecule were used to constrain the gas velocitystructure and to correct for saturation effects. Comparisons of thecolumn densities HD, CH, CN, and K I in these 17 sightlines indicatethat HD is most tightly correlated with CH. Stringent lower limits tothe interstellar D/H ratio, derived from the HD/2H2 ratio,range from 3.7 × 10-7 to 4.3 × 10-6.Our results also suggest that the HD/H2 ratio increases withthe molecular fraction f(H2) and that the interstellar D/Hratio might be obtained from HD by probing clouds with f(H2) 1. Finally, we note an apparent relationship between the molecularfractions of hydrogen and deuterium.
| Doppler Tomography of the Massive Compact Binary Stars in the Multiple Star Systems δ Orionis and HD 206267
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| 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.
| Protostars in the Elephant Trunk Nebula
The optically dark globule IC 1396A is revealed using Spitzer SpaceTelescope images at 3.6, 4.5, 5.8, 8, and 24 μm to be infrared-brightand to contain a set of previously unknown protostars. The mid-infraredcolors of the 24 μm detected sources indicate several very young(Class I or 0) protostars and a dozen Class II stars. Three of the newsources (IC 1396A:γ, 1396A:δ, and 1396A:ɛ) emit over90% of their bolometric luminosities at wavelengths greater than 3μm, and they are located within ~0.02 pc of the ionization front atthe edge of the globule. Many of the sources have spectra that are stillrising at 24 μm. The two previously known young stars LkHα 349aand 349c are both detected, with component c harboring a massive diskand component a being bare. On the order of 5% of the mass of materialin the globule is presently in the form of protostars in the105-106 yr age range. This high star formationrate was likely triggered by radiation from a nearby O star.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. I. Data and Component Structure
We present high-resolution optical spectra (at ~0.6-1.8 kms-1) of interstellar CN, CH, CH+, Ca I, K I, andCa II absorption toward 29 lines of sight in three star-forming regions,ρ Oph, Cep OB2, and Cep OB3. The observations and data reduction aredescribed. The agreement between earlier measurements of the totalequivalent widths and our results is quite good. However, our higherresolution spectra reveal complex structure and closely blendedcomponents in most lines of sight. The velocity component structure ofeach species is obtained by analyzing the spectra of the six species fora given sight line together. The tabulated column densities and Dopplerparameters of individual components are determined by using the methodof profile fitting. Total column densities along lines of sight arecomputed by summing results from profile fitting for individualcomponents and are compared with column densities from the apparentoptical depth method. A more detailed analysis of these data and theirimplications will be presented in a companion paper.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| Low-Mass Stars and Accretion at the Ages of Planet Formation in the Cepheus OB2 Region
We present the first identification of low-mass (spectral types K-M)stars in the young clusters Tr 37 and NGC 7160, members of the CepOB2association. This is part of a program to follow the evolution ofprotoplanetary accretion disks through the ages thought to be crucial tounderstanding disk dissipation and planet formation (~3-10 Myr).Combining optical photometry and optical spectroscopy, we haveidentified ~40 members in Tr 37 and ~15 in NGC 7160, using severalindependent tests for determining the membership (optical colors,optical variability, Hα emission, and Li λ6707 absorption).We confirm previous age estimates of 1-5 Myr for Tr 37 and 10 Myr forNGC 7160. We find active accretion in some of the stars in Tr 37, withaverage accretion rates of ~10-8 Msolaryr-1, derived from their U-band excesses. These resultsexpand the existing samples of accreting stars and are consistent withthe models of viscous accretion disk evolution. No signs of activeaccretion have been detected so far in the older cluster NGC 7160,suggesting that disk accretion ends before the age of 10 Myr. Theseresults are consistent with those from other populations and are a clearsign of disk evolution within the CepOB2 region. We also investigate thespatial asymmetries in Tr 37 and the possible presence of youngerpopulations triggered by Tr 37 itself, and we outline an efficientmethod to detect and study the rest of the clusters members and theircharacteristics.This research has made use of the WIYN Observatory facilities. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory.
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| 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
| High Resolution Spectra of Stellar Winds in Very Young Massive Stars
We present a series of spectra from hot stars obtained with the highenergy transmission grating spectrometer (HETGS) onboard the ChandraX-ray Observatory. Most of the selected stars are located at or near thecenters of young star-forming regions, which include IC 1396, NGC 2362,and Orion. The median ages of these clusters range from 0.3 to 12 Myr.The X-ray properties of these young, hot stars are then compared to ahighly resolved X-ray spectrum of the more evolved field star zeta Pup.Cluster stars with ages larger than 3 Myr show moderate temperatures andbroad asymmetric lines. These properties are quite similar to what hasbeen observed in zeta Pup and are consistent with X-rays produced byshock instabilities in a radiation-driven wind. Stars with agessignificantly less than 1 Myr, as found in the Orion Trapezium, showcompletely different characteristics. Here most of the X-ray flux showsvery high temperatures and the X-ray lines are narrow and unresolved.This strongly indicates that the X-ray properties are dominated bymagnetic field effects.
| A Method of Correcting Near-Infrared Spectra for Telluric Absorption
We present a method for correcting near-infrared medium-resolutionspectra for telluric absorption. The method makes use of a spectrum ofan A0 V star, observed near in time and close in air mass to the targetobject, and a high-resolution model of Vega, to construct a telluriccorrection spectrum that is free of stellar absorption features. Thetechnique was designed specifically to perform telluric corrections onspectra obtained with SpeX, a 0.8-5.5 μm medium-resolutioncross-dispersed spectrograph at the NASA Infrared Telescope Facility,and uses the fact that for medium resolutions there exist spectralregions uncontaminated by atmospheric absorption lines. However, it isalso applicable (in a somewhat modified form) to spectra obtained withother near-infrared spectrographs. An IDL-based code that carries outthe procedures is available for downloading via the World Wide Web.Based on observations obtained with the Infrared Telescope Facility,which is operated by the University of Hawaii under contract to theNational Aeronautics and Space Administration.
| 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.
| Masses and other parameters of massive binaries
Binary stars provide us with the means to measure stellar mass. Here Ipresent several lists of known O-type stars with reliable mass estimatesthat are members of eclipsing, double-lined spectroscopic binaries. Themasses of young, unevolved stars in binaries are suitable for testingthe predictions of evolutionary codes, and there is good agreementbetween the observed and predicted masses (based upon temperature andluminosity) if the lower temperature scale from line-blanketed modelatmospheres is adopted. A final table lists masses for systems in awide variety of advanced evolutionary stages.
| 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.
| Small Magellanic Cloud-Type Interstellar Dust in the Milky Way
It is well known that the sight line toward HD 204827 in the clusterTrumpler 37 shows a UV extinction curve that does not follow the averageGalactic extinction relation. However, when a dust component, foregroundto the cluster, is removed, the residual extinction curve is identicalto that found in the SMC within the uncertainties. The curve is verysteep and has little or no 2175 Å bump. The position of HD 204827in the sky is projected onto the edge of the Cepheus IRAS bubble. Inaddition, HD 204827 has an IRAS bow shock, indicating that it may beembedded in dust swept up by the supernova that created the IRAS bubble.Shocks due to the supernova may have led to substantial processing ofthis dust. The HD 204827 cloud is dense and rich in carbon molecules.The 3.4 μm feature indicating a C-H grain mantle is present in thedust toward HD 204827. The environment of the HD 204827 cloud dust maybe similar to the dust associated with HD 62542, which lies on the edgeof a stellar wind bubble and is also dense and rich in molecules. Thissight line may be a Rosetta Stone if its environment can be related tothose in the SMC having similar dust.
| X-Ray Modeling of Very Young Early-Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications
The Orion Trapezium is one of the youngest and closest star-formingregions within our Galaxy. With a dynamic age of ~3×105yr, it harbors a number of very young hot stars, which likely are on thezero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectrain the wavelength range of 1.5-25 Å of three of itsX-ray-brightest members (Θ1 Ori A, C, and E) obtainedwith the High Energy Transmission Grating Spectrometer (HETGS) on boardthe Chandra X-Ray Observatory. We measured X-ray emission lines,calculated differential emission measure distributions (DEMs), andfitted broadband models to the spectra. The spectra from all three starsare very rich in emission lines, specifically from highly ionized Fe,which includes emission from Fe XVII to Fe XXV ions. A complete linelist is included. This is a mere effect of high temperatures rather thanan overabundance of Fe, which in fact turns out to be underabundant inall three Trapezium members. Similarly there is a significantunderabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appearclose to solar. The DEM derived from over 80 emission lines in thespectrum of Θ1 Ori C indicates three peaks located at7.9, 25, and 66 MK. The emission measure varies over the 15.4 day windperiod of the star. For the two phases observed, the low-temperatureemission remains stable, while the high-temperature emission showssignificant differences. The line widths seem to show a similarbifurcation, where we resolve some of the soft X-ray lines withvelocities up to 850 km s-1 (all widths are stated ashalf-width at half-maximum), whereas the bulk of the lines remainunresolved with a confidence limit of 110 km s-1. Thebroadband spectra of the other two stars can be fitted with severalcollisionally ionized plasma model components within a temperature rangeof 4.3-46.8 MK for Θ1 Ori E and 4.8-42.7 MK forΘ1 Ori A. The high-temperature emissivity contributesover 70% to the total X-ray flux. None of the lines are resolved forΘ1 Ori A and E with a confidence limit of 160 kms-1. The influence of the strong UV radiation field on theforbidden line in the He-like triplets allows us to set an upper limiton distance of the line-emitting region from the photosphere. The bulkof the X-ray emission cannot be produced by shock instabilities in aradiation-driven wind and are likely the result of magnetic confinementin all three stars. Although confinement models cannot explain all theresults, the resemblance of the unresolved lines and of the DEM withrecent observations of active coronae in II Peg and AR Lac during flaresis quite obvious. Thus we speculate that the X-ray production mechanismin these stars is similar, with the difference that the Orion stars maybe in a state of almost continuous flaring driven by the wind. Weclearly rule out major effects due to X-rays from a possible companion.The fact that all three stars appear to be magnetic and are near zeroage on the main sequence also raises the issue of whether the Orionstars are simply different or whether young massive stars enter the mainsequence carrying significant magnetic fields. The ratiologLX/Lbol using the ``wind'' component of thespectrum is -7 for the Trapezium stars, consistent with the expectationfrom O stars. This suggests that massive ZAMS stars generate their X-rayluminosities like normal O stars and magnetic confinement provides anadditional source of X-rays.
| 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.
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