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Large Silicon Abundance in Photodissociation Regions
We have made one-dimensional raster scan observations of the ρ Ophand σ Sco star-forming regions with two spectrometers (SWS andLWS) on board the ISO. In the ρ Oph region, [Si II] 35 μm, [O I]63 μm, 146 μm, [C II] 158 μm, and the H2 purerotational transition lines S(0) to S(3) are detected, and thephotodissociation region (PDR) properties are derived as the radiationfield scaled by the solar neighborhood value G0~30-500, thegas density n~250-2500 cm-3, and the surface temperatureT~100-400 K. The ratio of [Si II] 35 μm to [O I] 146 μm indicatesthat silicon of 10%-20% of the solar abundance must be in the gaseousform in the PDR, suggesting that efficient dust destruction is ongoingeven in the PDR and that a fraction of the silicon atoms may becontained in volatile forms in dust grains. The [O I] 63 μm and [CII] 158 μm emissions are too weak relative to [O I] 146 μm to beaccounted for by standard PDR models. We propose a simple model, inwhich overlapping PDR clouds along the line of sight absorb the [O I] 63μm and [C II] 158 μm emissions, and show that the proposed modelreproduces the observed line intensities fairly well. In the σ Scoregion, we have detected three fine-structure lines, [O I] 63 μm, [NII] 122 μm, and [C II] 158 μm, and derived that 30%-80% of the [CII] emission comes from the ionized gas. The upper limit of the [Si II]35 μm is compatible with the solar abundance relative to nitrogen,and no useful constraint on the gaseous Si is obtained for the σSco region.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.
| On the origin of the X-ray emission from Herbig Ae/Be stars
Context: .Herbig Ae/Be stars are fully radiative and not expected tosupport dynamo action analogous to their convective lower-masscounterparts, the T Tauri stars. Alternative X-ray productionmechanisms, related to stellar winds or star-disk magnetospheres havebeen proposed, but their X-ray emission has remained a mystery. Aims: .A study of Herbig Ae/Be stars' global X-ray properties (such asdetection rate, luminosity, temperature, variability), helps toconstrain the emission mechanism by comparison to other types of stars,e.g. similar-age but lower-mass T Tauri stars, similar-mass but moreevolved main-sequence A- and B-type stars, and with respect to modelpredictions. Methods: .We performed a systematic search forChandra archival observations of Herbig Ae/Be stars. The superiorspatial resolution of this satellite with respect to previous X-rayinstrumentation has allowed us to also examine the possible role oflate-type companions in generating the observed X-rays. Results:.In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-rayemitting faint companions or other unrelated X-ray bright objects within10''. The detection fraction of Herbig Ae/Be stars is 76%, but decreasesto 35% if all emission is attributed to further known and unresolvedcompanions. The spectral analysis confirms the high X-ray temperatures( 20 MK) and large range of fractional X-ray luminosities(log{L_x/L_*}) of this class derived from earlier studies of individualobjects. Conclusions: .Radiative winds are ruled out as anemission mechanism on the basis of the high temperatures. The X-rayproperties of Herbig Ae/Be stars are not vastly different from those oftheir late-type companion stars (if such are known), nor from otheryoung late-type stars used for comparison. Therefore, either a similarkind of process takes place in both classes of objects, or there must beas yet undiscovered companion stars.
| The effects of metallicity, radiation field and dust extinction on the charge state of PAHs in diffuse clouds: implications for the DIB carrier
Context.The unidentified diffuse interstellar bands (DIB) are observedthroughout the Galaxy, the Local Group and beyond. Their carriers arepossibly related to complex carbonaceous gas-phase molecules, such as(cationic) polycyclic aromatic hydrocarbons and fullerenes. Aims.In order to reveal the identity of the DIB carrier we investigatethe effects of metallicity, radiation field and extinction curve on thePAH charge state distribution, and thus the theoretical emergent PAHspectrum, in diffuse interstellar clouds. This behaviour can then belinked to that of the DIB carrier, thus giving insight into itsidentity. Methods.We use radiative transfer and chemical models tocompute the physical and chemical conditions in diffuse clouds withGalactic and Magellanic Cloud types of interstellar dust and gas.Subsequently, the PAH charge state distributions throughout these cloudsare determined. Results.We find that the fraction of PAH cationsis much higher in the Magellanic Cloud environments than in the MilkyWay, caused predominantly by the respective lower metallicities, andmitigated by the steeper UV extinction curve. The fraction of anions ismuch lower in a low metallicity environment. The predicted DIB strengthof cationic PAH carriers is similar to that of the Milk Way for the LMCand 40% for the SMC due to the overall metallicity. Stronger DIBs couldbe expected in the Magellanic Clouds if they emanate from clouds thatare exposed to an average interstellar radiation field that issignificantly stronger than in the Milky Way, although photo-destructionprocesses could possibly reduce this effect, especially for the smallerPAHs. Our results show that the presence and absence of DIB carriers inthe Magellanic Cloud lines of sight can be tied to the PAH chargebalance which is driven by metallicity, UV radiation and dust extinctioneffects.
| An unbiased search for the signatures of protostars in the ρ Ophiuchi molecular cloud . II. Millimetre continuum observations
The dense cores which conceive and cradle young stars can be exploredthrough continuum emission from associated dust grains. We haveperformed a wide field survey for dust sources at 1.2 millimetres in theρ Ophiuchi molecular cloud, covering more than 1 square degree in anunbiased fashion. We detect a number of previously unknown sources,ranging from extended cores over compact, starless cores to envelopessurrounding young stellar objects of Class 0, Class I, and Class IItype. We analyse the mass distribution, spatial distribution and thepotential equilibrium of the cores. For the inner regions, the surveyresults are consistent with the findings of previous narrower surveys.The core mass function resembles the stellar initial mass function, withthe core mass function shifted by a factor of two to higher masses (forthe chosen opacity and temperature). In addition, we find no statisticalvariation in the core mass function between the crowded inner regionsand those in more isolated fields except for the absence of the mostmassive cores in the extended cloud. The inner region contains compactercores. This is interpreted as due to a medium of higher mean pressurealthough strong pressure variations are evident in each region. Thecores display a hierarchical spatial distribution with no preferredseparation scale length. However, the frequency distribution of nearestneighbours displays two peaks, one of which at 5000 AU can be the resultof core fragmentation. The orientations of the major axes of cores areconsistent with an isotropic distribution. In contrast, the relativeorientations of core pairs are preferentially in the NW-SE direction onall separation scales. These results are consistent with core productionand evolution in a turbulent environment. Finally, we report thediscovery of a new, low-mass Class 0 object candidate and its COoutflow.
| X-ray emission probing the limiting cases of stellar dynamos
Magnetic activity, driven by a dynamo, is a frequently observedphenomenon on solar-like stars and the Sun itself. Its manifestationsinclude chromospheric and coronal emission. Solar-like stars regenerateand accumulate their magnetic fields in the transition zone between theradiative core and the convective envelope. Consequently, stars in whichthis `overshoot' region is absent are not expected to display activity.In particular, the critical cases are (A) the fully radiativeintermediate-mass stars (M > 2 M_ȯ), and (B) the fullyconvective lowest mass stars (M < 0.3 M_ȯ) and brown dwarfs.Nevertheless, high-energy emission is observed from both classes ofobjects. I discuss recent efforts to constrain the emission mechanism inthese limiting regimes of the stellar dynamo by means of X-rayobservations with the Chandra satellite.
| Large-Scale CO and [C I] Emission in the ρ Ophiuchi Molecular Cloud
We present a comprehensive study of the ρ Ophiuchi molecular cloudthat addresses aspects of the physical structure and condition of themolecular cloud and its photodissociation region (PDR) by combiningfar-infrared and submillimeter-wave observations with a wide range ofangular scale and resolution. We present 1600 arcmin2 maps(2.3 pc2) with 0.1 pc resolution in submillimeter CO(4-->3) and [C I](3P1-->3P0) line emissionfrom the Antarctic Submillimeter Telescope and Remote Observatory(AST/RO) and pointed observations in the CO (7-->6) and [C I](3P2-->3P1) lines. Withinthe large-scale maps, smaller spectral line maps of 3000 AU resolutionover ~90 arcmin2 (0.2 pc2) of the cloud in CO, CS,HCO+, and their rare isotopomers are made at the HeinrichHertz Telescope (HHT) in Arizona. Comparison of CO, HCO+, and[C I] maps with far-infrared observations of atomic and ionic speciesfrom the Infrared Space Observatory (ISO) far-infrared and submillimetercontinuum emission and near-infrared H2 emission allowsclearer determination of the physical and chemical structure of theρ Oph PDR, since each species probes a different physical region ofthe cloud structure. Although a homogeneous plane-parallel PDR model canreproduce many of the observations described here, the excitationconditions needed to produce the observed HCO+ and [O I]emission imply inhomogeneous structure. Strong chemical gradients areobserved in HCO+ and CS; the former is ascribed to a localenhancement in the H2 ionization rate, and the latter isprincipally due to shocks. Under the assumption of a simpletwo-component gas model for the cloud, we find that [C II] and [C I]emission predominantly arises from the lower density envelopes(103-104 cm-3) that surround densercloud condensations, or ``clumps.'' The distribution of [C I] is verysimilar to that of C18O and is generally consistent withillumination from the ``far'' side of the cloud. A notable exception isfound at the western edge of the cloud, where UV photons create a PDRviewed ``edge-on.'' The abundance of atomic carbon is accurately modeledusing a radiation field that decreases with increasing projecteddistance from the exciting star HD 147889 and a total gas column densitythat follows that of C18O, decreasing toward the edges of thecloud. In contrast to the conclusions of other studies, we find that nononequilibrium chemistry is needed to enhance the [C I] abundance. Eachspectral line is traced to a particular physical component of the cloudand PDR. Although CO rotational line emission originates from both densecondensations and diffuse envelopes, the millimeter-wave transitionsmostly find their origins in envelope material, whereas the high-Jsubmillimeter lines stem more from the dense clumps. SubmillimeterHCO+ and infrared [O I] and [C II] emission indicate clumpsurface temperatures of 50-200 K, an ultraviolet radiation field withIUV~10-90, densities of 105-106cm-3, and interior temperatures of <=20 K. This studyhighlights the value of large-scale infrared and submillimeter mappingfor the interpretation of molecular cloud physical and chemicalstructure, and important future observations are highlighted.
| X-Ray Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| A multiplicity survey of the ρ Ophiuchi molecular clouds
We present a volume-limited multiplicity survey with magnitude cutoff({m}K≤ 10.5 mag) of 158 young stellar objects locatedwithin or in the vicinity of the ρ Ophiuchi Dark Cloud. Withexception of eleven already well observed objects, all sources have beenobserved by us in the K-band with 3.5 m telescopes by using speckletechniques. The separation range covered by our survey is0.13''≤θ≤ 6.4'', where the lower limit is given by thediffraction limit of the telescopes and the upper limit by confusionwith background stars. The multiplicity survey is complete for fluxratios ≥ 0.1 (Δ{m}K≤ 2.5) at the diffractionlimit. After taking the background density into account the degree ofmultiplicity is 29.1%± 4.3% and thus only marginally higher thanthe value 23.5%± 4.8 % derived for the given separation range forthe main-sequence solar-like stars in the solar neighbourhood (Duquennoy& Mayor 1991). We discuss the implications of these findings.
| Are PAHs precursors of small hydrocarbons in photo-dissociation regions? The Horsehead case
We present maps at high spatial and spectral resolution in emissionlines of CCH, c-C3H2, C4H, 12CO and C18O of theedge of the Horsehead nebula obtained with the IRAM Plateau de BureInterferometer (PdBI). The edge of the Horsehead nebula is aone-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on.All hydrocarbons are detected at high signal-to-noise ratio in the PDRwhere intense emission is seen both in the H2 ro-vibrational lines andin the PAH mid-infrared bands. C18O peaks farther away fromthe cloud edge. Our observations demonstrate that CCH, c-C3H2 and C4Hare present in UV-irradiated molecular gas, with abundances nearly ashigh as in dense, well-shielded molecular cores. PDR models i) need alarge density gradient at the PDR edge to correctly reproduce the offsetbetween the hydrocarbons and H2 peaks; and ii) fail to reproduce thehydrocarbon abundances. We propose that a new formation path of carbonchains, in addition to gas phase chemistry, should be considered inPDRs: because of intense UV-irradiation, large aromatic molecules andsmall carbon grains may fragment and feed the interstellar medium withsmall carbon clusters and molecules in significant amounts.
| 3D continuum radiative transfer in complex dust configurations. II. 3D structure of the dense molecular cloud core ρ Oph D
Constraints on the density and thermal 3D structure of the densemolecular cloud core ρ Oph D are derived from a detailed 3Dradiative transfer modeling. Two ISOCAM images at 7 and 15 μm arefitted simultaneously by representing the dust distribution in the corewith a series of 3D Gaussian density profiles. Size, total density andposition of the Gaussians are optimized by simulated annealing to obtaina 2D column density map. The projected core density has a complexelongated pattern with two peaks. We propose a new method to calculatean approximate temperature in an externally illuminated complex 3Dstructure from a mean optical depth. This"Toverlineτ"-method is applied to a 1.3 mm map obtainedwith the IRAM 30m telescope to find the approximate 3D density andtemperature distribution of the core ρ Oph D. The spatial 3Ddistribution deviates strongly from spherical symmetry. The elongatedstructure is in general agreement with recent gravo-turbulent collapsecalculations for molecular clouds. We discuss possible ambiguities ofthe background determination procedure, errors of the maps, the accuracyof the Toverlineτ-method and the influence of the assumeddust particle sizes and properties.
| PAH charge state distribution and DIB carriers: Implications from the line of sight toward HD 147889
We have computed physical parameters such as density, degree ofionization and temperature, constrained by a large observational dataset on atomic and molecular species, for the line of sight toward thesingle cloud HD 147889. Diffuse interstellar bands(DIBs) produced along this line of sight are well documented and can beused to test the PAH hypothesis. To this effect, the charge statefractions of different polycyclic aromatic hydrocarbons (PAHs) arecalculated in HD 147889 as a function of depth forthe derived density, electron abundance and temperature profile. Asinput for the construction of these charge state distributions, themicroscopic properties of the PAHs, e.g., ionization potential andelectron affinity, are determined for a series of symmetry groups. Thecombination of a physical model for the chemical and thermal balance ofthe gas toward HD 147889 with a detailed treatment ofthe PAH charge state distribution, and laboratory and theoretical dataon specific PAHs, allow us to compute electronic spectra of gas phasePAH molecules and to draw conclusions about the required properties ofPAHs as DIB carriers. We find the following. 1) The variation of thetotal charge state distribution of each specific class (series) of PAHin the translucent cloud toward HD 147889 (and alsoof course for any other diffuse/translucent cloud) depends strongly onthe molecular symmetry and size (number of π electrons). This is dueto the strong effects of these parameters on the ionization potential ofa PAH. 2) Different wavelength regions in the DIB spectrum are populatedpreferentially by different PAH charge states depending on theunderlying PAH size distribution. 3) The PAH size distribution forHD 147889 is constrained by the observed DIB spectrumto be Gaussian with a mean of 50 carbon atoms. 4) For the given PAH sizedistribution it is possible to constrain the total small catacondensedPAH column density along the line of sight to HD147889 to 2.4×1014 cm-2 bycomparing the total observed UV extinction to the strong UV absorptionsof neutral PAHs in the 2000-3000 Å region. 5) Catacondensed PAHswith sizes above some 40 C-atoms are expected to show strong DIBSlongward of 10 000 Å. Large condensed PAHs in the series, pyrene,coronene, ovalene, .... , on the other hand, mainly absorb between 4000and 10 000 Å but extrapolation to even larger pericondensed PAHsin this series also shows strong absorptions longward of 10 000 Å.6) Only the weak DIBs in HD 147889 could bereproduced by a mix of small catacondensed PAHs (<50 C atoms) whilefor large pericondensed PAHs (50 < C atoms < 100) the intermediateDIBs are well reproduced. Small catacondensed PAHs cannot contributemore than 50% of the total observed equivalent width toward HD147889. Strong DIBs can only be reproduced by addition of veryspecific PAH molecules or homologue series to the sample set (i.e., asmall number of PAHs with high oscillator strength or a large number ofPAHs with a low oscillator strength). An outline is provided for a moregeneral application of this method to other lines of sight, which can beused as a pipeline to compute the spectroscopic response of a PAH orgroup of PAHs in a physical environment constrained by independent(non-DIB) observations.Based on observations made at the European Southern Observatory,Paranal, Chile (ESO programs 67.C-0281 and 64.H-0224). Appendices A-Care only available in electronic form at http://www.edpsciences.org
| Star-forming protoclusters associated with methanol masers
We present a multiwavelength study of five methanol maser sites whichare not directly associated with a strong (>100 mJy) radio continuumsource: G 31.28+0.06, G 59.78+0.06, G 173.49+2.42 (S231, S233IR), G188.95+0.89 (S252, AFGL5180) and G 192.60-0.05 (S255IR). Theseradio-quiet methanol maser sites are often interpreted as precursors ofultra-compact H II regions or massive protostar sites. In this work, theenvironment of methanol masers is probed from mid-IR to millimetrewavelengths at angular resolutions of 8''-34''. Spectral energydistribution (SED) diagrams for each site are presented, together withmass and luminosity estimates. Each radio-quiet maser site is alwaysassociated with a massive (>50 Mȯ), deeply embedded(Av>40 mag) and very luminous (>104Lȯ) molecular clump, with Ltotal∝Mgas0.75. These physical properties characterisemassive star-forming clumps in earlier evolutionary phases than H IIregions. In addition, colder gas clumps seen only at mm-wavelengths arealso found near the methanol maser sites. These colder clumps mayrepresent an even earlier phase of massive star formation. These resultssuggest an evolutionary sequence for massive star formation from a coldclump, seen only at mm wavelengths, evolving to a hot molecular corewith a two-component SED with peaks at far-IR and mid-IR wavelengths, toan (ultra-compact) H II region. Alternatively, the cold clumps might beclusters of low-mass YSOs, in formation near the massive star-formingclusters. Finally, the values of the dust grain emissivity index(β) range between 1.6 and 1.9.
| 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| The Ultraviolet Extinction Curve of Intraclump Dust in Taurus (TMC-1): Constraints on the 2175 Å Bump Absorber
Ultraviolet extinction curves for three early-type stars that sampledust in the Taurus dark cloud in the vicinity of TMC-1 are analyzed toexamine the contribution of internal (intraclump) dust to the totalline-of-sight extinction. One of our selected targets samples only dustin the ``diffuse-screen'' component of the cloud, where the dust grainslack ice mantles and have optical properties similar to average``diffuse-ISM'' dust. Comparison with data for more heavily reddenedstars that also sample dust within TMC-1 (where grains are mantled)allows us to determine the intraclump extinction. Our principal resultis that the 2175 Å ``extinction bump'' is extremely weak or absentinside the clump. The feature present in the observed extinction curvesappears to arise almost entirely in the diffuse screen. This dramaticreduction in bump amplitude in the high-density environment isconsistent with the prediction of the graphite model that the feature issuppressed by hydrogenation. Amplitude reduction is accompanied by amore subtle change in the profile of the bump, specifically, an increasein width. Comparison of Taurus and other clouds strongly suggests thatthe increase in width results from accumulation of surface coatings onthe carrier grains rather than from particle clustering.
| Carbon budget and carbon chemistry in Photon Dominated Regions
We present a study of small carbon chains and rings in Photon DominatedRegions (PDRs) performed at millimetre wavelengths. Our sample consistsof the Horsehead nebula (B33), the ρ Oph L1688 cloud interface, andthe cometary-shaped cloud IC 63. Using the IRAM 30-m telescope, the SESTand the Effelsberg 100-m telescope, we mapped the emission ofC2H, c-C3H2 and C4H, andsearched for heavy hydrocarbons such as c-C3H,l-C3H, l-C3H2,l-C4H2 and C6H. The large scale mapsshow that small hydrocarbons are present until the edge of all PDRs,which is surprising as they are expected to be easily destroyed by UVradiation. Their spatial distribution reasonably agrees with thearomatic emission mapped in mid-IR wavelength bands. C2H andc-C3H2 correlate remarkably well, a trend alreadyreported in the diffuse ISM (Lucas & Liszt \cite{Lucas2000}). Theirabundances relative to H2 are relatively high and comparableto the ones derived in dark clouds such as L134N or TMC-1, known asefficient carbon factories. The heavier species are however onlydetected in the Horsehead nebula at a position coincident with thearomatic emission peak around 7 μm. In particular, we report thefirst detection of C6H in a PDR. We have run steady-state PDRmodels using several gas-phase chemical networks (UMIST95 and the NewStandard Model) and conclude that both networks fail in reproducing thehigh abundances of some of these hydrocarbons by an order of magnitude.The high abundance of hydrocarbons in the PDR may suggest that thephoto-erosion of UV-irradiated large carbonaceous compounds couldefficiently feed the ISM with small carbon clusters or molecules. Thisnew production mechanism of carbon chains and rings could overcome theirdestruction by the UV radiation field. Dedicated theoretical andlaboratory measurements are required to understand and implement theseadditional chemical routes.Appendix A is only available in electronic form athttp://www.edpsciences.org
| A Multiplicity Survey of the ρ Oph Dark Cloud
Not Available
| 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.
| 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.
| A Deep Chandra X-Ray Observation of the Embedded Young Cluster in NGC 2024
We present results of a sensitive 76 ks Chandra observation of the youngstellar cluster in NGC 2024, lying at a distance of ~415 pc in the OrionB giant molecular cloud. Previous infrared observations have shown thatthis remarkable cluster contains several hundred embedded young stars,most of which are still surrounded by circumstellar disks. Thus, itpresents a rare opportunity to study X-ray activity in a large sample ofoptically invisible protostars and classical T Tauri stars (CTTSs)undergoing accretion. Chandra detected 283 X-ray sources, of which 248were identified with counterparts at other wavelengths, mostly in thenear-infrared. Astrometric registration of Chandra images against theTwo Micron All Sky Survey (2MASS) resulted in positional offsets of~0.25" near field center, yielding high confidence identifications ofinfrared counterparts. The Chandra detections are characterized by hardheavily absorbed spectra and spectacular variability. Spectral analysisof more than 100 of the brightest X-ray sources yields a mean extinction~10.5 mag and typical plasma energies ~3keV. The range of variability includes rapid impulsive flares andpersistent low-level fluctuations indicative of strong magneticactivity, as well as slow rises and falls in count rate whose origin ismore obscure. Some slowly evolving outbursts reached sustainedtemperatures of kT~6-10 keV. Chandra detected all but one of a subsampleof 27 CTTSs identified from previous near- and mid-infrared photometry,and their X-ray and bolometric luminosities are correlated. We alsoreport the X-ray detection of IRS 2b, which is thought to be a massiveembedded late O or early B star that may be the ionizing source of NGC2024. Seven millimeter-bright cores (FIR 1-7) in NGC 2024 that may beprotostellar were not detected, with the possible exception of faintemission near the unusual core FIR 4.
| A Search for Close Binaries in the ρ Ophiuchi Star-forming Region
We have carried out a new, near-infrared speckle imaging survey of 19members of the young stellar population in the nearby (d=140 pc) ρOph cloud core. Results for four binary and one newly discovered triplesystem are reported. Data for all known multiple systems among thepre-main-sequence population of ρ Oph are tabulated. We define arestricted binary fraction, Fb,r, and a restricted companionfraction, Fc,r, as counting only those systems mostdetectable in the present and previous high-resolution near-infraredimaging surveys having separations between 0.1" and ~1.1" and K-bandmagnitude differences ΔK<3. Analysis of all the availablemultiplicity data results in updated values of Fb,r=24%+/-11%and Fc,r=24%+/-11% for the Ophiuchus pre-main-sequencepopulation. These values are consistent with the values in the Taurusstar-forming region, and Fc,r is in excess by a factor of 2relative to the main sequence at the 1 σ level.
| A Test Case for the Organic Refractory Model of Interstellar Dust
The organic refractory model of interstellar dust postulates that thegrains primarily responsible for visual extinction consist of silicatecores coated with organic refractory matter (ORM) supposed to form byphotochemical processing of ice mantles. We identify the line of sightto the early-type star Elias 25, in the ρ Oph dark cloud, as a testfor local production of ORM in an environment where mantled grains areexposed to a substantial ultraviolet radiation field. Infrared spectraof Elias 25 in the 2.4-4.0 μm region were used to examine the 3 μmice profile and to search for the CH bond absorption feature near 3.4μm that occurs in laboratory-synthesized ORM. We find that the icefeature in Elias 25 has an unusually deep 3.1-3.7 μm wing, but nodetectable 3.4 μm feature is present in the wing profile. An upperlimit on the τ3.4/AV ratio is set that is afactor of 4 lower than that determined from the feature detected in thediffuse interstellar medium toward Cygnus OB2 No. 12. Our results areconsistent with other evidence indicating that the carrier of the 3.4μm feature is destroyed, rather than produced, inside dense clouds.
| Distribution of the [C I] Emission in the ρ Ophiuchi Dark Cloud
The 3P1-3P0 fine-structureline of the neutral carbon atom ([C I]) has been mapped over the1.8d×1.3d area of the L1688 cloud in the ρ Ophiuchi regionwith the Mount Fuji submillimeter-wave telescope. The3P2-3P1 line of [C I] hasalso been observed toward two representative positions to evaluate theexcitation temperature of the [C I] lines. The overall extent of the [CI] distribution generally resembles that of the 13COdistribution. The [C I] distribution has two major peaks; one (peak I)is at ρ Oph A, and the other (peak II) is toward the east side ofthe C18O core in the southern part of L1688. Peak II islocated beyond the C18O core with respect to the excitingstar HD 147889. The C0 column density is5.0×1017 cm-2 toward peak II. The spatialdistribution of the [C I] emission is compared with plane-parallelphotodissociation region (PDR) models, which suggest that peak II isassociated with a lower density PDR front, adjacent to the dense cloudcores observed in the C18O line emission. Alternatively, peakII is in the early stage of chemical evolution, where C0 hasnot been completely converted to CO. In this case, the difference in the[C I] and C18O distributions represents an evolutionarysequence. This is consistent with a picture of a shock-compressedformation of the dense cores in this region due to influences from theSco OB2 association.
| Dust Grain Size Distributions from MRN to MEM
Employing the maximum entropy method (MEM) algorithm, we fitinterstellar extinction measurements that span the wavelength range0.125-3 μm. We present a uniform set of MEM model fits, all using thesame grain materials, optical constants, and abundance constraints. Inaddition, we are taking advantage of improved UV and IR data and betterestimates of the gas-to-dust ratio. The model fits cover the entirerange of extinction properties that have been seen in the Galaxy and theMagellanic Clouds. The grain models employed for this presentation arethe simplistic homogeneous sphere models (i.e., those of Mathis, Rumpl,& Nordsieck in 1977) with two (graphite, silicate) or three(graphite, silicate, amorphous carbon) components. Although such usageis only a first step, the results do provide interesting insight intothe use of grain size as a diagnostic of dust environment. We find thatthe SMC bar extinction curve cannot be fitted using carbon grains alone.This is a challenge to the recent observational result indicating littlesilicon depletion in the SMC.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| 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.
| Observations of C3 in Translucent Sight Lines
The A1Πu<--X1Σ+g transition of the simplestpolyatomic carbon chain molecule, C3, at 4051.6 Å hasbeen searched for toward reddened stars where abundant C2 hadbeen reported and toward other stars with high color excess. Absorptionfrom C3 has been detected toward 15 stars with color excessE(B-V) from 0.33 to 1.12. The observed C3 column densities,ranging from 1012 to 1013 cm-2, arewell correlated with the corresponding C2 column densities,with N(C2)/N(C3)~40, indicating their closechemical relation. The carbon-rich sight line toward HD 204827 (forwhich no previous C2 observation had been reported) has byfar the highest C3 and C2 column densities. Thechemistry of formation of C3 from C2 is discussed.A search for the next strongest 020-000 vibronic band was unsuccessfulas a result of the low Franck-Condon factor and interference with astellar line. Searches for C4 and C5 werenegative.
| 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.
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Observation and Astrometry data
| Constellation: | Οφιούχος |
| Right ascension: | 16h25m24.32s |
| Declination: | -24°27'56.6" |
| Apparent magnitude: | 7.957 |
| Distance: | 135.87 parsecs |
| Proper motion RA: | -1.4 |
| Proper motion Dec: | -25 |
| B-T magnitude: | 8.877 |
| V-T magnitude: | 8.033 |
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