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Silicates in D-Type Symbiotic Stars: An Infrared Space Observatory Overview
We investigate the IR spectral features of a sample of D-type symbioticstars in order to constrain the emitting properties of coupled dust-gasparticles across the whole system. In particular, by analyzingunexploited ISO Short Wavelength Spectrometer data, deriving the basicobservational parameters of dust bands, and comparing them with respectto those observed in other astronomical sources, we try to highlight theeffect of environment on grain chemistry and physics. We find strongamorphous silicate emission bands at ~10 and ~18 μm in a largefraction of the sample. The analysis of the ~10 μm band, along with adirect comparison with several astronomical sources, reveals thatsilicate dust in symbiotic stars shows features between thecharacteristic circumstellar environments and the interstellar medium.This indicates an increasing reprocessing of grains in relation tospecific symbiotic behavior of the objects. A correlation between thecentral wavelengths of the ~10 and ~18 μm dust bands is found. Bymodeling IR spectral lines we also investigate dust grain conditionswithin the shocked nebulae. Both the unusual depletion values and thehigh sputtering efficiency might be explained by the formation of SiOmolecules, which are known to be a very reliable shock tracer. Weconclude that the signature of dust chemical disturbance due tosymbiotic activity should be looked for in the outer, circumbinary,expanding shells where the environmental conditions for grain processingmight be achieved. Symbiotic stars are thus attractive targets for newmid-IR and millimeter observations.
| Unbinned maximum-likelihood estimators for low-count data. Applications to faint X-ray spectra in the Taurus molecular cloud
Traditional binned statistics such as χ2 suffer frominformation loss and arbitrariness of the binning procedure, which isespecially important at low count rates as encountered in the XMM-NewtonExtended Survey of the Taurus Molecular Cloud (XEST). We point out thatthe underlying statistical quantity (the log likelihood L) does notrequire any binning beyond the one implied by instrumental readoutchannels, and we propose to use it for low-count data. The performanceof L in the model classification and point estimation problems isexplored by Monte-Carlo simulations of Chandra and XMM-Newton X-rayspectra, and is compared to the performances of the binned Poissonstatistic (C), Pearson's χ2 and Neyman's χ^2_N, theKolmogorov-Smirnov, and Kuiper's statistics. It is found that theunbinned log likelihood L performs best with regard to the expectedchi-square distance between true and estimated spectra, the chance of asuccessful identification among discrete candidate models, the areaunder the receiver-operator curve of reduced (two-model) binaryclassification problems, and generally also with regard to the meansquare errors of individual spectrum parameters. The χ2(χ^2_N) statistics should only be used if more than 10 (15)predicted counts per bin are available. From the practical point ofview, the computational cost of evaluating L is smaller than for any ofthe alternative methods if the forward model is specified in terms of aPoisson intensity and normalization is a free parameter. The maximum-Lmethod is applied to 14 XEST observations, and confidence regions arediscussed. The unbinned results are compared to binned XSPEC results,and found to generally agree, with exceptions explained by instabilityunder re-binning and by background fine structures. In particular, HOTau is found by the unbinned method to be rather cool (kT ~ 0.2 keV),which may be a sign of shock emission. The maximum-L method has no lowerlimit on the available counts, and allows to treat weak sources whichare beyond the means of binned methods.
| Synthesis of CO and CO2 Molecules by UV Irradiation of Water Ice-covered Hydrogenated Carbon Grains
We present the results of UV irradiation with Lyα photons ofcarbon grains with a water ice cap at 11 K. Formation of CO andCO2 molecules takes place during irradiation. An estimationof the formation cross section of these molecules by Lyα photonshas been obtained from the intensity increase of their infraredstretching bands as a function of the photon fluence. The fraction ofcarbon in the grains converted to CO and CO2 by UV photons is0.06 and 0.05, respectively. The spectral profile of the CO stretchingfeature and that of the CO2 bending mode indicate a polarenvironment for these molecules. On the basis of the present laboratoryresults and those obtained in previous work on ion irradiation ofsimilar samples, it has been possible to estimate the contribution ofpolar CO and CO2 produced on carbon grains by energeticprocessing to the observed column densities of these molecules for denseclouds whose visual extinction is known. A significant amount of polarCO and CO2 is produced through the mechanism we have studied.Furthermore, we have found that the laboratory profile of the bendingband of CO2 produced on carbon grains is compatible with thatobserved toward the field star Elias 16.
| The Calar Alto lunar occultation program: update and new results
We present an update of the lunar occultation program which is routinelycarried out in the near-IR at the Calar Alto Observatory. A total of 350events were recorded since our last report (Fors et al. 2004, A&A,419, 285). In the course of eight runs we have observed, among others,late-type giants, T-Tauri stars, and infrared sources. Noteworthy was apassage of the Moon close to the galactic center, which produced a largenumber of events during just a few hours in July 2004. Results includethe determinations of the angular diameter of RZ Ari,and the projected separations and brightness ratios for one triple and13 binary stars, almost all of which representing first time detections.Projected separations range from 0farcs09 to 0farcs007. We provide aquantitative analysis of the performance achieved in our observations interms of angular resolution and sensitivity, which reach about 0farcs003and K ≈8.5 mag, respectively. We also present a statisticaldiscussion of our sample, and in particular of the frequency ofdetection of binaries among field stars.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| On the Evolutionary Status of Class I Stars and Herbig-Haro Energy Sources in Taurus-Auriga
We present high-resolution (R~34,000) optical (6330-8750 Å)spectra obtained with the HIRES spectrograph on the W. M. Keck Itelescope of stars in Taurus-Auriga whose circumstellar environmentsuggests that they are less evolved than optically revealed T Tauristars. Many of the stars are seen only via scattered light. The sampleincludes 15 class I stars and all class II stars that power Herbig-Haroflows in this region. For 28 of the 36 stars observed, our measurementsare the first high-dispersion optical spectra ever obtained.Photospheric features are observed in all stars with detected continuum,11 of 15 class I stars (42% of known Taurus class I stars) and 21 of 21class II stars; strong emission lines (e.g., Hα) are detected inthe spectra of all stars. These spectra, in combination with previousmeasurements, are used to search for differences between stars thatpower Herbig-Haro flows and stars that do not and to reassess theevolutionary state of so-called protostars (class I stars) relative tooptically revealed T Tauri stars (class II stars). The stellar massdistribution of class I stars is similar to that of class II stars andincludes three spectroscopically confirmed class I brown dwarfs. Class Istars (and brown dwarfs) in Taurus are slowly rotating (vsini<35 kms-1) the angular momentum of a young star appears todissipate prior to the optically revealed T Tauri phase. The amount ofoptical veiling and the inferred mass accretion rates of class I starsare surprisingly indistinguishable from class II stars. Class I stars donot have accretion-dominated luminosities; the accretion luminosityaccounts for ~25% of the bolometric luminosity. The median massaccretion rate of class I and class II stars of K7-M1 spectral type is4×10-8Msolar yr-1, and the medianmass outflow rate is 5% of the mass accretion rate. The large ranges inmass accretion rate (~2 orders of magnitude), mass outflow rate (~3orders of magnitude), and the ratio of these quantities (~2 orders ofmagnitude) represent real dispersions in young accreting stars ofsimilar mass. We confirm previous results that find largerforbidden-line emission associated with class I stars than class IIstars. We suggest that this is caused by an orientation bias that allowsa more direct view of the somewhat extended forbidden emission lineregions than of the obscured stellar photospheres, rather than beingcaused by larger mass outflow rates. Overall, the similar masses,luminosities, rotation rates, mass accretion rates, mass outflow rates,and millimeter flux densities of class I stars and class II stars arebest explained by a scenario in which most class I stars are no longerin the main accretion phase and are much older than traditionallyassumed. Similarly, although stars that power Herbig-Haro flows appearto have larger mass outflow rates, their stellar and circumstellarproperties are generally indistinguishable from those of similar massstars that do not power these flows.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| A study of high velocity molecular outflows with an up-to-date sample
A statistical study of the properties of molecular outflows is performedbased on an up-to-date sample. 391 outflows were identified in publishedarticles or preprints before February 28, 2003. The parameters ofposition, morphology, mass, energy, outflow dynamics and central sourceluminosity are presented for each outflow source. Outflow lobe polarityis known for all the sources, and 84% are found to be bipolar. Thesources are divided into low mass and high mass groups according toeither the available bolometric luminosity of the central source or theoutflow mass. The pace of discovery of outflows over the past sevenyears has increased much more rapidly than in previous periods. Surveysfor outflows are still continuing. The number of high-mass outflowsdetected (139) has considerably increased, showing that they arecommonly associated with massive as well as low mass stars. Energeticmass ejection may be a common aspect of the formation of high mass aswell as low mass stars. Outflow masses are correlated strongly withbolometric luminosity of the center sources, which was obtained for thefirst time. There are also correlations between the central sourceluminosity and the parameters of mechanical luminosity and the thrust orforce necessary to drive the outflow. The results show that flow mass,momentum and energy depend on the nature of the central source. Despitetheir similarity, there are differences between the high mass and lowmass outflows. Low mass outflows are more collimated than high massoutflows. On average, the mass of high mass sources can be more than twoorders of magnitude larger than those of low mass outflows. The relationbetween flow mass and dynamical time appears to differ for the two typesof outflows. Low mass sources make up 90% of outflows associated with HHobjects while high mass outflows make up 61% of the sources associatedwith H_2O masers. Sources with characteristics of collapse or infallcomprise 12% of the entire outflow sample. The spatial distribution ofthe outflow sources in the Galaxy is presented and the local occurrencerate is compared with the stellar birth rate.Tables 1a and 1b are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/503
| ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks
Observations of interstellar gas-phase and solid-state species in the2.4 200 m range obtained with the spectrometers on board the InfraredSpace Observatory (ISO) are reviewed. Lines and bands caused by ices,polycyclic aromatic hydrocarbons, silicates, and gas-phase atoms andmolecules (in particular H2, CO, H2O, OH, andCO2) are summarized and their diagnostic capabilitiesillustrated. The results are discussed in the context of the physicaland chemical evolution of star-forming regions, includingphoton-dominated regions, shocks, protostellar envelopes, and disksaround young stars.
| Very cold cores in the Taurus Molecular Ring as seen by ISO
Three prominent cold objects of the Taurus MolecularRing (TMR) were revealed by our ISOPHOT 200μm map of the south-eastern part of the Heiles Cloud2 (HCL 2) cloud complex. One corresponds tothe cyanopolyyne peak region of the TMC-1 ridge,(TMC-1 CP), one is the HCL2-Ecloud, and one which we call HCL2-ES lies south ofTMC-1. The 200/100 μm colour temperatures and column densities of thethree ISOPHOT cold clouds are ≈12 K, and 1.2 ± 0.7 ×1022 cm-2 respectively, as calculated fromISO/IRAS surface brightnesses. As Nagoya-4 m C18 (1-0)spectra show, these are dense molecular clouds with N(H)>1021 cm-2 column density peaks. The ISOPHOT 200μm surface brightness is well correlated with the C18Oline intensity (corr. coef.≈ 70%). The large dust particle emissivityis found to be increased in the prototypical very dense core TMC-1 CP.As the low linewidths (Δ v=0.8 ± 0.2 km s-1)indicate, the level of turbulent energy density is 50% lower in thesethree clouds than in other clouds of HCL2. Dense cores were identifiedinside the C18O clouds by NH3 measurements withthe Effelsberg-100 m telescope and Nobeyama-45 mH13CO+ data. The density of the dense cores isn≥1.1 × 105 cm-3, and their kinetictemperatures are ⪉10 K, in good agreement with the FIR results. Thetotal molecular gas mass in the gravitationally bound cloud cores ofTMC-1 CP and HCL2-E is about 21 Mȯ and 8Mȯ respectively. The cores, TMC-1 CPb and HCL2-Eab areassociated with 3 low mass YSO candidate 2MASS point sources, while 35other low mass YSO candidates are seen elsewhere in TMR south, which weconsider as evidence for ongoing low mass star formation.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| A quantitative analysis of OCN- formation in interstellar ice analogs
The 4.62 \mum absorption band, observed along the line-of-sight towardsvarious young stellar objects, is generally used as a qualitativeindicator for energetic processing of interstellar ice mantles. Thisinterpretation is based on the excellent fit with OCN-, whichis readily formed by ultraviolet (UV) or ion-irradiation of icescontaining H2O, CO and NH3. However, theassignment requires both qualitative and quantitative agreement in termsof the efficiency of formation as well as the formation of additionalproducts. Here, we present the first quantitative results on theefficiency of laboratory formation of OCN- from ices composedof different combinations of H2O, CO, CH3OH, HNCOand NH3 by UV- and thermally-mediated solid state chemistry.Our results show large implications for the use of the 4.62 \mum featureas a diagnostic for energetic ice-processing. UV-mediated formation ofOCN- from H2O/CO/NH3 ice matrices fallsshort in reproducing the highest observed interstellar abundances. Inthis case, at most 2.7% OCN- is formed with respect toH2O under conditions that no longer apply to a molecularcloud environment. On the other hand, photoprocessing and in particularthermal processing of solid HNCO in the presence of NH3 arevery efficient OCN- formation mechanisms, converting 60%-85%and ˜100%, respectively of the original HNCO. We propose thatOCN- is most likely formed thermally from HNCO given the easeand efficiency of this mechanism. Upper limits on solid HNCO and theinferred interstellar ice temperatures are in agreement with thisscenario.
| Dense Cores and Protostellar Collapse in Nearby Star-Forming Regions
We present the results of a complete survey for high densitycondensations in the Taurus star-forming region. The unbiased C{\msf18}O survey is based on {\msf 13}CO observationscovering the whole Taurus region with the 4 m telescope of NagoyaUniversity. The H{\msf 13}CO{\msf +} ({\msf J}=1-0) survey for dense molecular condensations inside the C{\msf18}O cores is based on observations with the 45 m telescopeat Nobeyama Radio Observatory. We detected 55 H{\msf13}CO{\msf +} condensations, of which 44 arestarless. These starless condensations are compact ({\msf R≤}0.1 pc)and have high densities ({\msf ≥105} cm{\msf-3}). The statistical analysis of the starless condensationsyields their lifetime of {\msf ˜} 4{\msf ×}10{\msf5} years which is several times longer than their free-falltime. One of the starless condensations, MC 27, exhibits fairly strongand narrow emission in the {\msf J} = 4-3, 3-2 and 1-0 transitions ofH{\msf 13}CO{\msf +}, as well as self-reversedprofiles of HCO{\msf +}, {\msf J} = 4-3 and 3-2. Astatistical analysis indicates for MC 27 a very short lifetime of {\msf˜}10{\msf 4} years, consistent with the free-fall timefor a density of {\msf 106} cm{\msf -3}. MC 27 isprobably very close to the moment of the formation of a protostellarcore.
| Galactic environment and the 10-μm silicate feature of young stellar objects
Disc and sphere dust models are used to fit 8-13 μm flux spectra of19 low-mass young stellar objects (YSOs) and five Herbig AeBe stars. The13 non-photospheric low-mass YSOs in quiescent environments and the fiveHerbig AeBe stars have mean disc temperature indices of 0.4, indicatingthat the emission arises from optically thin layers above a flaredoptically thick disc; 10 out of 14 of the low-mass YSO and four out offive of the Herbig AeBe features contain an optically thin silicateemission component. The radius of the peak 10-μm emission for nineout of the 13 low-mass YSOs is 10-130 au, and three out of the fiveHerbig AeBe stars are 10-30 au in size. In contrast, the five YSOs fromdisrupted molecular clouds that have been shaped by expanding supernovaremnants have temperature indices of between 0.3 and 0.8; four out ofthe five are optically thick and three out of the five have radii <~2au. The photosphere-like continuum of Taurus-Elias 18 could be fittedonly with truncated optically thick models, implying the presence of avoid between the >500 K and cold (<~100 K) foreground dust.Silicates surrounding low-mass YSOs in quiescent molecular clouds aresimilar to those in the Trapezium region of the Orion Nebula except whenAV<~ 2 mag. In the low-AV case and in low-massYSOs in disrupted molecular clouds the silicates are similar tocircumstellar dust around the evolved star μ Cephei.
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue 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/cgi-bin/qcat?J/A+A/399/141
| Star Formation in Space and Time: Taurus-Auriga
To understand the formation of stellar groups, one must first documentcarefully the birth pattern within real clusters and associations. Inthis study of Taurus-Auriga, we combine pre-main-sequence ages from ourown evolutionary tracks with stellar positions from observationalsurveys. Aided by the extensive millimeter data on the molecular clouds,we develop a picture of the region's history. Star formation began, at arelatively low level and in a spatially diffuse manner, at least107 yr in the past. Within the last few million years, newstars have been produced at an accelerating rate, almost exclusivelywithin a confined group of striated cloud filaments. The gas both insideand around the filaments appears to be in force balance. Thus, theappearance of the filaments is due to global, quasi-static contractionof the parent cloud material. Gravity drives this contraction and shockdissipation mediates it, but the internal motion of the gas does notappear to be turbulent. The accelerating nature of recent star formationmeans that the condensation of cloud cores is a threshold phenomenon,requiring a minimum background density. Other, nearby cloud regions,including Lupus and Chamaeleon, contain some locales that have attainedthis density, and others that have not. In the latter, we find extensiveand sometimes massive molecular gas that is still devoid of young stars.
| Detection of abundant solid CO in the disk around CRBR 2422.8-3423
We present direct evidence for CO freeze-out in a circumstellar diskaround the edge-on class I object CRBR 2422.8-3423,observed in the M band with VLT-ISAAC at a resolving power R~ 10 000.The spectrum shows strong solid CO absorption, with a lower limit on thecolumn density of 2.2 x 1018 cm-2. The solid COcolumn is the highest observed so far, including high-mass protostarsand background field stars. Absorption by foreground cloud materiallikely accounts for only a small fraction of the total solid CO, basedon the weakness of solid CO absorption toward nearby sources and theabsence of gaseous C18O J=2-> 1 emission 30\arcsec south.Gas-phase ro-vibrational CO absorption lines are also detected with amean temperature of 50 +/- 10 K. The average gas/solid CO ratio is ~ 1along the line of sight. For an estimated inclination of 20degr +/-5°, the solid CO absorption originates mostly in the cold, shieldedouter part of the flaring disk, consistent with the predominance ofapolar solid CO in the spectrum and the non-detection of solidOCN-, an indicator of thermal/ultraviolet processing of theice mantle. By contrast, the warm gaseous CO likely originates closer tothe star. Based on observations obtained at the European SouthernObservatory (ESO), Paranal, Chile, within the observing program164.I-0605.}
| A comparison between the compositions of cometary and interstellar materials. I. Molecular abundances
Comets were created as a by-product of planet formation from pristineinterstellar material. A certain similarity between the composition ofboth cometary nuclei and interstellar clouds has actually be observed byseveral authors in few last decades. We summarize the quantitativemeasurements of abundances of observed molecules in both the aboveentities and discuss their similarities and differences. The foundvariety of the chemical composition of comets is practically within therange of observed molecular composition of relatively cold matter in theGalaxy. It proves that the gaseous and dusty components from which thecometary nuclei were built are primordial.
| Flows, Fragmentation, and Star Formation. I. Low-Mass Stars in Taurus
The remarkably filamentary spatial distribution of young stars in theTaurus molecular cloud has significant implications for understandinglow-mass star formation in relatively quiescent conditions. The largescale and regular spacing of the filaments suggests that small-scaleturbulence is of limited importance, which could be consistent with thedriving on large scales by flows that produced the cloud. The smallspatial dispersion of stars from gaseous filaments indicates that thelow-mass stars are generally born with small velocity dispersionsrelative to their natal gas, of the order of the sound speed or less.The spatial distribution of the stars exhibits a mean separation ofabout 0.25 pc, comparable to the estimated Jeans length in the densestgaseous filaments, and is consistent with roughly uniform density alongthe filaments. The efficiency of star formation in filaments is muchhigher than elsewhere, with an associated higher frequency of protostarsand accreting T Tauri stars. The protostellar cores are generallyaligned with the filaments, suggesting that they are produced bygravitational fragmentation, resulting in initially quasi-prolate cores.Given the absence of massive stars that could strongly dominate clouddynamics, Taurus provides important tests of theories of dispersedlow-mass star formation and numerical simulations of molecular cloudstructure and evolution.
| A Complete Search for Dense Cloud Cores in Taurus
We present the results of an H13CO+ J=1-0 surveyfor dense molecular condensations in Taurus. The observations werecarried out with the 45 m telescope at Nobeyama on the basis of anextensive C18O survey made with the 4 m telescope at NagoyaUniversity. The present survey is based purely on molecular lineobservations and thus provides the most extensive and uniform sample ofhigh-density condensations. We detected 55 H13CO+condensations, of which 44 are starless. These starless condensationsare compact (R<~0.1 pc) and of high density (>~105cm-3) and thus are highly probable candidates forprotostellar condensations just before star formation. The strikingfeature of these starless compact condensations is the steep slope ofthe mass spectrum (dN/dM~M-2.5 for3.5Msolar
| A mineralogy of extrasolar silicate dust from 10-μm spectra
The 10- and 18-μm spectroscopy of a variety of galactic environmentsreveals smooth bands which have been associated with (respectively) Si-Ostretching and bending modes in amorphous silicates, since the spectraof crystalline silicates are narrow and highly structured. The standardapproach to the interpretation of astronomical spectra is to assume thatthe silicates are amorphous and then to add in crystalline components(usually a single olivine followed by a pyroxene) to match finestructures in the data. Conversely, in this analysis we match the grossproperties of the astronomical profiles - the full width at half-maximum(FWHM) and the wavelength of the peak (λc) - with amixture of crystalline silicates from different structural (and hencedifferent spectral) classes and add a component of amorphous silicateonly if there is too much structure in the simulation. We find that thenarrow bands of crystalline grains could blend to form the broad10-μm bands observed. For all the environments included herein, ifcrystalline silicates are included in the mixture, χ2/nof the fits improves significantly (by factors of 1.3-4.4) and thenumber of silicon atoms required to model the spectra decreases by 30-50per cent. Upper limits to the mass fraction of crystalline pyroxeneincreases with the FWHM of the profile from ~50 per cent in the sampledcircumstellar environments to ~80 per cent in the Taurus molecular cloud(TMC) and its embedded young stellar objects (YSOs). Fine structurescommon to both the averaged spectra of laboratory silicates andastronomical profiles suggest that <~10 per cent by mass of thesilicates in circumstellar and star-forming environments could bepartially crystalline hydrous (i.e. OH- containing) silicatessimilar to clays like talc and montmorillonite, but that these grainsare absent from the ultraviolet-rich diffuse medium towards Cyg OB2 no.12. In contrast, the relative abundance of submicrometre-sizedcrystalline olivine is insufficient (<~25 per cent) in thesecircumstellar, diffuse-medium, molecular-cloud and YSO spectra toproduce an 11.2-μm emission or absorption feature. Using this methodof spectral analysis, the mass fraction of amorphous silicate in thesespectra could be as low as 17 per cent in the TMC and 0 per cent in somecircumstellar environments.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| Solid Carbon Dioxide in Regions of Low-Mass Star Formation
We present high-resolution (R~1500-2000) spectra of the 4.27 μmasymmetric stretching feature of solid CO2 in eight lines ofsight observed with the Short Wavelength Spectrometer of the InfraredSpace Observatory. Two of the sources are field stars located behind theTaurus molecular cloud; the others are young stellar objects (YSOs) ofpredominantly low-to-intermediate mass. We find a significantsource-to-source variation in the solid CO2/H2Oabundance ratio in our sample: two lines of sight, Elias 18 and RAFGL989, have CO2 abundances of ~34%-37%, considerably higherthan in other lines of sight studied to date. In agreement with aprevious study of Elias 16, we confirm a substantial (~20%) abundance ofsolid CO2 relative to H2O in the quiescentintracloud medium. We compare the CO2 profiles withlaboratory spectra of interstellar ice analogs from the LeidenObservatory Laboratory database. Results show that the 4.27 μmprofiles toward field stars and embedded low-mass objects are remarkablysimilar to each other and seem to originate mostly in coldH2O-rich ice. In two higher mass YSOs (RAFGL 989 and S255IRS1), the profiles are clearly different, and at least the lattersource shows signs of thermal processing.
| On Ultrasmall Silicate Grains in the Diffuse Interstellar Medium
The abundance of both amorphous and crystalline silicates in very smallgrains is limited by the fact that the 10 μm silicate emissionfeature is not detected in the diffuse interstellar medium (ISM). On thebasis of the observed IR emission spectrum for the diffuse ISM, theobserved ultraviolet extinction curve, and the 10 μm silicateabsorption profile, we obtain upper limits on the abundances ofultrasmall (a<~15 Å) amorphous and crystalline silicate grains.Contrary to previous work, as much as ~10% of interstellar Si could bein a<~15 Å silicate grains without violating observationalconstraints. Not more than ~5% of the Si can be in crystalline silicates(of any size).
| Observational Constraints on the Abundance and Evolution of ``XCN'' in Interstellar Grain Mantles
The 4.62 μm ``XCN'' absorption feature, attributed to CN-bearingmolecules in solids, is potentially an important diagnostic of theevolution of organic matter in the interstellar medium and the envelopesof newly formed stars. We report quantitative limits on the strength ofthis feature in the diffuse interstellar medium toward the reddenedB-type star Cyg OB2 No. 12 and in the dark cloud toward the youngstellar object R CrA IRS 2. On the basis of an assumed band strength forthe carrier species, we estimate that less than 0.3% and less than 0.1%of the elemental nitrogen is in CN bonds along these lines of sight,respectively; if they are typical of diffuse and dense environments, itfollows that the carrier of XCN is no more than a trace constituent ofeither organic-refractory or icy interstellar grain mantles. AppreciableXCN abundances seem to occur only in the envelopes of certain youngstellar objects (YSOs), most notably the high-mass objects W33A and AFGL7009S. We confirm the presence of XCN in the spectrum of the low-massYSO R CrA IRS 7. The strengths of the XCN absorptions in R CrA IRS 7 andother low-mass YSOs indicate mean XCN concentrations relative toH2O in the ices of ~1%, comparable with the abundance ofCN-bearing species in comets. Based on observations with Infrared SpaceObservatory, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, the Netherlands, and theUnited Kingdom) and with the participation of ISAS and NASA.
| X-ray emission of multiple T Tauri stars in Taurus
We present a study of X-ray emission of known multiple T Tauri stars(TTS) in Taurus based on ROSAT observations. We used the ROSAT All-SkySurvey (RASS) detection rates of single classical (cTTS) and weak-lineTTS (wTTS) to investigate statistically the TTS nature (classical orweak-line) of the components in multiple TTS, which are too close forspatially resolved spectroscopy so far. Because single wTTS show ahigher RASS detection rate than single cTTS, the different binary TTS(cTTS-cTTS, cTTS-wTTS, wTTS-wTTS) should also have different detectionrates. We find that the observed RASS detection rates of binary wTTS,where the nature of the secondary is unknown, are in agreement with thesecondaries being wTTS rather than cTTS, and mixed pairs are very rare.Furthermore we analyse the X-ray emission of TTS systems resolvable bythe ROSAT HRI. Among those systems we find statistical evidence thatprimaries show larger X-ray luminosity than secondaries, and that thesamples of primary and secondary TTS are similar concerning the X-rayover bolometric luminosity ratios. Furthermore, primaries always emitharder X-rays than secondaries. In all cases where rotational velocitiesand/or periods are known for both companions, it is always the primarythat rotates faster. Hence, the stronger X-ray emission of the primariesmay be due to higher bolometric luminosity and/or faster rotation.
| A Direct Measurement of the CO Depletion in the Elias 18 Circumstellar Disk
In this Letter we present low-resolution 2.3 μm and high-resolution4.6 μm spectra of Elias 18 (IRAS 04369+2539), a low-mass youngstellar object (YSO) in the Taurus molecular cloud. The 2.3 μmspectrum shows CO band head emission (v=2-0) indicating a circumstellardisk. The 4.6 μm spectrum shows very weak CO absorption in the lowestJ levels. Comparing preexisting CO ice observations to our dataindicates that there is at least 30 times less gas-phase CO thansolid-state CO in the outer, cold (T<20 K) regions of the disk. Highdepletions of CO in YSO disks have been inferred from millimeter-wave COemission and detailed disk models, but Elias 18 is the first case forwhich both gas- and ice-phase data have been directly compared.
| Discs and the 10-μm silicate spectra of young stellar objects with non-photospheric continua
Dust emission in the non-photospheric 10-μm continua of HL Tau andTaurus-Elias 7 (Haro6-10, GV Tau) is distinguished from foregroundsilicate absorption using a simple disc model with radial power-lawtemperature and mass-density distributions based on the IR-submm modelof T Tauri stars by Adams, Lada & Shu with foreground extinction.The resulting 10-μm absorption profiles are remarkably similar tothose of the field star Taurus-Elias 16 obtained by Bowey, Adamson &Whittet. The fitted temperature indices are 0.44 (HL Tau) and 0.33(Elias 7) in agreement with Boss's theoretical models of the 200-300Kregion, but lower than those of IR-submm discs (0.5-0.61 Mannings &Emerson); a significant fraction of the modelled 10-μm emission of HLTau is optically thin, whilst that of Elias 7 is optically thick. Wesuggest that HL Tau's optically thin component arises from silicate dustwithin low-density layers above an optically thick disc.
| ROCN Species Produced by Ion Irradiation of Ice Mixtures: Comparison with Astronomical Observations
We have investigated the effects induced by ion bombardment of mixturescontaining nitrogen-bearing compounds at low temperatures. The resultsshow the formation of a band at 2080 cm-1 in binary mixtures,NH3:CH4 and N2:CH4, which weattribute to HCN embedded in the organic residue formed by ionirradiation. In addition to this band, ternary mixtures containing anoxygen-bearing species (i.e., H2O) form a compound with aprominent absorption band at about 2165 cm-1 (4.62 μm). Weascribe this band to a nitrile compound containing O that is bonded tothe organic residue. A detailed comparison of the laboratory resultswith astronomical data of the 4.62 μm absorption band in protostellarspectra shows good agreement in peak position and profile. Ourexperimental studies show that N2, which is a more likelyinterstellar ice component than NH3, can be the molecularprogenitor of the carrier of the interstellar band. This is analternative to the pathway by which UV photolysis ofNH3-containing ices produces the 4.62 μm band and impliesthat ion bombardment may well play an important role in the evolution ofinterstellar ices. Here, we discuss the implications of our studies forthe chemical route by which the carrier of the 4.62 μm band is formedin these laboratory experiments.
| The distribution of OH in Taurus Molecular Cloud-1
The column density of the OH radical in Taurus Molecular Cloud-1(TMC-1), reaches its maximum close to the cyanopolyyne peak. Acomparison with previously published maps of other molecules shows thatOH has a similar distribution as HC_3N and CCS, but differs largely fromCS, SO, NH_3 and N_2H+. The OH/C18O column densityratio is, however, almost constant along the dense ridge of TMC-1,suggesting that the fractional OH abundance does not change much, thederived value being about 10-7. This high number confirmsthat in dense gas OH is mainly formed by the electron recombination ofH_3O+. The approximate constancy of the fractional OHabundance is in accordance with a flat density distribution as has beenderived recently (\cite{pratap}). Previous SO/CS maps together with somerecent modelling results seem to be in conflict with the idea that thecyanopolyyne peak in the southeastern part of the cloud would bechemically less evolved than the ammonia maximum in the northwest.Therefore we discuss the possibility that the OH maximum represents theso called `radical peak', which occurs when freezing on to grainsurfaces starts to be the dominant factor controlling the chemicalcomposition and reactions (\cite{pdbrown}). It turns out that thegreater part of the data accumulated so far, including the present OHobservations, fit the `old' picture where a slightly higher density anda more advanced chemical state prevail in the northwestern part of theTMC-1 ridge. The `satellite' lines towards two locations in the cloudshow enhanced 1612 MHz emission. We suggest that this is due tonon-thermal excitation by far-infrared radiation from dust, heated bythe embedded young stars in the neighbourhood of the TMC-1 ridge.
| Interpreting the 10-micron astronomical silicate feature
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| Determining the extinction through dark clouds
We discuss the problem of a consistent determination of the visualextinction to objects in and behind a dark cloud. The use ofnear-infrared colours is discussed, in particular the H-K colours. Weconcentrate on the uncertainties, with emphasis on the extinction lawand on the intrinsic colours of the young stars embedded in a darkcloud. The cases of the two intensively studied nearby star-formingclouds, Taurus and Ophiuchus, are addressed.
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