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Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| Nanodiamond as a Possible Carrier of Extended Red Emission
Diamond nanocrystals (size ~100 nm) emit bright luminescence at 600-800nm when exposed to green and yellow photons. The photoluminescence,arising from excitation of the nitrogen-vacancy defect centers createdby proton-beam irradiation and thermal annealing, closely resembles theextended red emission (ERE) bands observed in reflection nebulae andplanetary nebulae. The central wavelength of the emission is ~700 nm,and it blueshifts to ~660 nm as the excitation wavelength decreases from535 to 470 nm as the result of a combined excitation of two differentdetect centers [(N-V)- and (N-V)0]. Ourobservations lend support to the suggestion that nanodiamond is apossible carrier for the ERE band.
| Mass functions and structure of the young open cluster NGC 6611
We use J, H and KS 2MASS photometry to study colour-magnitude(CMDs) and colour-colour diagrams, structure and mass distribution inthe ionizing open cluster NGC 6611. Reddening variation throughout thecluster region is taken into account followed by field-stardecontamination of the CMDs. Decontamination is also applied to derivethe density profile and luminosity functions in the core, halo andoverall (whole cluster) regions. The field-star decontamination showedthat the lower limit of the main sequence (MS) occurs at ≈5 M_ȯ.Based on the fraction of KS excess stars in the colour-colourdiagram we estimate an age of 1.3±0.3 Myr which is consistentwith the presence of a large number of pre-main sequence (PMS) stars.The distance from the Sun was estimated from known O V stars in thecluster area and the turn-on stars connecting the PMS and MS, resultingin d_ȯ=1.8±0.5 kpc. The radial density distributionincluding MS and PMS stars is fitted by a King profile with a coreradius R_core=0.70±0.08 pc. The cluster density profile mergesinto the background at a limiting radius R_lim=6.5±0.5 pc. Fromthe field-star subtracted luminosity functions we derive the massfunctions (MFs) in the form φ(m)∝ m-(1+χ). Inthe halo and through the whole cluster the MFs have slopesχ=1.52±0.13 and χ=1.45±0.12, respectively, thusslightly steeper than Salpeter's IMF. In the core the MF is flat,χ=0.62±0.16, indicating some degree of mass segregation sincethe cluster age is a factor 2 larger than the relaxation time. Becauseof the very young age of NGC 6611, part of this effect appears to berelated to the molecular cloud-fragmentation process itself. We detect362±120 PMS stars. The total observed mass including detected MS(in the range 5{-}85 M_ȯ) and PMS stars amounts to 1600 M_ȯ,thus more massive than the Trapezium cluster. Compared to older openclusters of different masses, the overall NGC 6611 fits in the relationsinvolving structural and dynamical parameters. However, the core isatypical in the sense that it looks like an old/dynamically evolvedcore. Again, part of this effect must be linked to formation processes.
| To see or not to see a bow shock. Identifying bow shocks with Hα allsky surveys
OB-stars have the highest luminosities and strongest stellar winds ofall stars, which enables them to interact strongly with theirsurrounding ISM, thus creating bow shocks. These offer us an idealopportunity to learn more about the ISM. They were first detected andanalysed around runaway OB-stars using the IRAS allsky survey by vanBuren et al. (1995, AJ, 110, 2614). Using the geometry of such bowshocks information concerning the ISM density and its fluctuations canbe gained from such infrared observations. As to help to improve the bowshock models, additional observations at other wavelengths, e.g.Hα, are most welcome. However due to their low velocity these bowshocks have a size of 1°, and could only be observed as awhole with great difficulties. In the light of the new Hα allskysurveys (SHASSA/VTSS) this is no problem any more. We developeddifferent methods to detect bow shocks, e.g. the improved determinationof their symmetry axis with radial distance profiles. Using twoHα-allsky surveys (SHASSA/VTSS), we searched for bow shocks andcompared the different methods. From our sample we conclude, that thecorrelation between the direction of both proper motion and the symmetryaxis determined with radial distance profile is the most promisingdetection method. We found eight bow shocks around HD17505, HD 24430, HD48099, HD 57061, HD92206, HD 135240, HD149757, and HD 158186 from 37 candidatestaken from van Buren et al. (1995, AJ, 110, 2614). Additionally to thetraditional determination of ISM parameters using the standoff distanceof the bow shock, another approach was chosen, using the thickness ofthe bow-shock layer. Both methods lead to the same results, yieldingdensities ( 1 cm-3) and the maximal temperatures (104 K), that fit well to the up-to-date picture of the WarmIonised Medium.
| 13CO (J = 1 -- 0) Survey of Molecular Clouds toward the Monoceros and Canis Major Region
rough a large-scale 13CO(J=1-0) survey toward a region inMonoceros and Canis Major, we present following results: (1) Weidentified in total 115 clouds. (2) GroupI (CMaOB1 and G220.8 ‑1.7) may physically connected with GroupII (Mon R2 and NGC2149). (3)496YSO candidates were chosen from IRAS point source catalog (IPC). (4)The mass spectrum of the Local clouds shows a power-law relation with anindex of ‑1.55 ± 0.09. (5) The number fraction ofstar-forming clouds increases as the cloud mass increases. Especially,massive clouds of Mcloud≥103.5Modot are all star-forming clouds. (6) Theline width-cloud radius relation is best fitted asΔVcomp ∝ rcloud0.47(correlation coefficient = 0.87). (7) The distribution of(Mvir / Mcloud) shows that star-forming cloudstend to be more virialized than no star-forming clouds. (8) Starformation in GroupI is more active than that in GroupII. (9) Unexpectedsmall number of low-mass clouds in GroupI may be due to strong UVradiation from O-type stars in the vicinity of the clouds. (10) Starformation in GroupII might occurred by self-gravitational, except forthe most massive cloud.
| 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.
| 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.
| Effects of Metallicity on the Rotational Velocities of Massive Stars
Recent theoretical predictions for low-metallicity massive stars predictthat these stars should have drastically reduced equatorial winds (massloss) while on the main sequence, and so should retain most of theirangular momentum. Observations of both the Be/(B+Be) ratio and theblue-to-red supergiant ratio appear to have a metallicity dependencethat may be caused by high rotational velocities. We have analyzed 39archival Hubble Space Telescope Imaging Spectrograph (STIS),high-resolution, ultraviolet spectra of O-type stars in the MagellanicClouds to determine their projected rotational velocities Vsini. Ourmethodology is based on a previous study of the projected rotationalvelocities of Galactic O-type stars using International UltravioletExplorer (IUE) short-wavelength prime (SWP) camera high-dispersionspectra, which resulted in a catalog of Vsini values for 177 O-typestars. Here we present complementary Vsini values for 21 LargeMagellanic Cloud and 22 Small Magellanic Cloud O-type stars based onSTIS and IUE UV spectroscopy. The distribution of Vsini values forO-type stars in the Magellanic Clouds is compared to that of GalacticO-type stars. Despite the theoretical predictions and indirectobservational evidence for high rotation, the O-type stars in theMagellanic Clouds do not appear to rotate faster than their Galacticcounterparts.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| 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
| Bright OB stars in the Galaxy. I. Mass-loss and wind-momentum rates of O-type stars: A pure H\alpha analysis accounting for line-blanketing
We study mass-loss and wind momentum rates of 29 Galactic O-type starswith luminosity classes I, III and V by means of a pure H\alpha profileanalysis and investigate to what extent the results compare to thoseoriginating from a state-of-the-art, complete spectral analysis. Ourinvestigation relies on the approximate method developed by\citet{Puls96} which we have modified to account for the effects ofline-blanketing. Effective temperatures and gravities needed to obtainquantitative results from such a simplified approach have been derivedby means of calibrations based on most recent spectroscopic NLTEanalyses and models of Galactic stars by \citet{Repo03} and\citet{Martins02}. Comparing (i) the derived wind-densities to thosedetermined by \citet{Repo03} for eleven stars in common and (ii) theWind-momentum Luminosity Relationship (WLR) for our sample stars tothose derived by other investigations, we conclude that our approximateapproach is actually able to provide consistent results. Additionally,we studied the consequences of ``fine tuning'' some of the direct andindirect parameters entering the WLR, especially by accounting fordifferent possible values of stellar reddening and distances. Combiningour data set with the corresponding data provided by \citet{Herrero02}and \citet{Repo03} we finally study the WLR for the largest sample ofGalactic O-type stars gathered so far, including an elaborate errortreatment. The established disagreement between the theoreticalpredictions and the ``observed'' WLRs being a function of luminosityclass is suggested to be a result of wind clumping. Different strategiesto check this hypothesis are discussed, particularly by comparing theH\alpha mass-loss rates with the ones derived from radio observations.
| 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.
| 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.
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.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/410/905
| The embedded star clusters in the nebulae vdB-RN 92 and Gy 3-7 in Canis Majoris R1
We employed J, H and Ks photometry from the 2MASS PointSource Catalogue to study the embedded star clusters in the nebulaevdB-RN 92 and Gy 3-7, which are located in the molecular cloud CanisMajoris R1. We employed colour-colour and colour-magnitudes diagramstogether with theoretical pre-main sequence isochrones to derive theirfundamental parameters. Ages are based on the fraction of stars withanomalous colours, supposedly of types Herbig AeBe and T Tauri. ThevdB-RN 92 cluster has an age of 5-7 Myr and the Gy 3-7 cluster ~2 Myr.The average reddening values are AV=4.4 andAV=6.3, respectively for vdB-RN 92 and Gy 3-7. The distancefound for vdB-RN 92 was 1.5 kpc which is, within uncertainties,compatible with previous estimates for the complex. Gy 3-7 also appearsto be related to the CMaR1 complex. We conclude that CMaR1 is a veryactive star-forming site, now with four star clusters in earlyevolutionary stages.
| 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
| The Photophysics of the Carrier of Extended Red Emission
Interstellar dust contains a component that reveals its presence byemitting a broad unstructured band of light in the 540-950 nm wavelengthrange, referred to as extended red emission (ERE). The presence ofinterstellar dust and ultraviolet photons are two necessary conditionsfor ERE to occur. This is the basis for suggestions that attribute EREto an interstellar dust component capable of photoluminescence. In thisstudy, we have collected all published ERE observations withabsolute-calibrated spectra for interstellar environments, where thedensity of ultraviolet photons can be estimated reliably. In each case,we determined the band-integrated ERE intensity, the wavelength of peakemission in the ERE band, and the efficiency with which absorbedultraviolet photons are contributing to the ERE. The data show thatradiation is not only driving the ERE, as expected for aphotoluminescence process, but is modifying the ERE carrier, asmanifested by a systematic increase in the ERE band's peak wavelengthand a general decrease in the photon conversion efficiency withincreasing densities of the prevailing exciting radiation. The overallspectral characteristics of the ERE and the observed high quantumefficiency of the ERE process are currently best matched by the recentlyproposed silicon nanoparticle (SNP) model. Using the experimentallyestablished fact that ionization of semiconductor nanoparticles quenchestheir photoluminescence, we proceeded to test the SNP model bydeveloping a quantitative model for the excitation and ionizationequilibrium of SNPs under interstellar conditions for a wide range ofradiation field densities. With a single adjustable parameter, the crosssection for photoionization, the model reproduces the observations ofERE intensity and ERE efficiency remarkably well. The assumption thatabout 50% of the ERE carriers are neutral under radiation conditionsencountered in the diffuse interstellar medium leads to a prediction ofthe single-photon ionization cross section of SNPs with average diameter3.5 nm of <=3.4×10-15 cm2. The shift ofthe ERE band's peak wavelength toward larger values with increasingradiation density requires a change of the size distribution of theactively luminescing ERE carriers through a gradual removal of thesmaller particles by size-dependent photofragmentation. We propose thatheat-assisted Coulomb decay of metastable, multiply charged SNPs is sucha process, which selectively removes the smaller components of anexisting SNP size distribution.
| Interstellar CN toward CH+-forming regions
Measurements on interstellar CN absorption are presented for stars inthree southern OB associations, NGC 2439, Vela OB1, and Cen OB1. CN isdetected in 21 out of 31 stars observed. The doublet ratio for the R(1)and P(1) lines of the (0, 0) band of the B 2Sigma+-X 2Sigma + violet system and acomparison of violet system data with measurements of the (1, 0) and (2,0) bands of the A 2Pi -X 2Sigma + redsystem are used to derive Doppler parameters and total column densities.Inferred CN column densities vary by more than an order of magnitude forlines of sight with similar CH column densities. Observations of the (0,0) band of the CH B 2Sigma --X 2Pisystem are used to revise previously published CH column densitiestoward the lines of sight studied in CN. Together with earlier resultson CH, CH+, and C2, the CN data presented hereprovide a homogeneous set of column densities and radial velocities ofdiatomic molecules in three individual translucent clouds. We use thesedata to study CN production via chemical models. Gas densities areinferred from models based on production via CH and C2 incool gas. Most sightlines in our sample test densities typical fordiffuse molecular gas (a few hundred cm-3 ) when theultraviolet flux permeating the gas is between 1 and 5 times the averageinterstellar flux. A few lines of sight indicate that CN is producedunder dark cloud conditions because relatively large densities areobtained or because this simple chemical scheme is unable to reproducethe observed CN columns. Low densities are indicated for directions withupper limits on CN. We add an ad hoc component of a number oflow-velocity (<10 km s-1) criss-crossing MHD shocks toexplain observed column densities of interstellar CH+. Theseshocks also produce about 10 to 30% of the total CH column along theline of sight.
| The embedded star clusters in the nebulae NGC 2327 and BRC 27 in Canis Majoris R1
We employed J, H and K_s photometry from the 2MASS Catalogue to studythe embedded star clusters in the nebulae NGC 2327 and BRC 27 locate inthe molecular cloud Canis Majoris R1. We made use of colour-colour andcolour-magnitudes diagrams of the sample objects in conjunction withthose of the Trapezium cluster which was used as a template, togetherwith theoretical isochrones. The fundamental parameters were derived forthe clusters. The distances found for NGC 2327 and BRC 27 were 1.2 kpcwhich are compatible with previous distances for the complex. Theestimated ages are based mostly on the number of stars with anomalouscolours, supposedly of types Herbig AeBe and T Tauri. Both embeddedclusters have ages of ~1.5 Myr. The results suggest the birth of aphysical pair of star clusters in CMaR1.
| A first detailed study of the colliding wind WR+O binary WR 30a
We present a detailed, extensive investigation of the photometric andspectroscopic behaviour of WR 30a. This star is definitely a binarysystem with a period around 4.6d. We propose the value P=4.619d(σ=0.002d). The identification of the components as WO4+O5((f))indicates a massive evolved binary system; the O5 component is amain-sequence or, more likely, a giant star. The radial velocities ofthe O star yield a circular orbit with an amplitude KO=29.9(σ=2.1)kms-1 and a mass function of 0.013(σ=0.003)Msolar. The spectrum of WR 30a exhibits strongprofile variations of the broad emission lines that are phase-lockedwith the orbital period. We report the detection of the orbital motionof the WO component with KWO=189kms-1, but thisshould be confirmed by further observations. If correct, it implies amass ratio MWO /MO=0.16. The star exhibitssinusoidal light variations of amplitude 0.024mag peak-to-peak with theminimum of light occurring slightly after the conjunction with the Ostar in front. On the basis of the phase-locked profile variations ofthe Civ λ4658 blend in the spectrum of the WO, we conclude that awind-wind collision phenomenon is present in the system. We discuss somepossibilities for the geometry of the interaction region.
| The stellar composition of the star formation region CMa R1 - II. Spectroscopic and photometric observations of nine young stars
We present new high- and low-resolution spectroscopic and photometricdata of nine members of the young association CMa R1. All the stars havecircumstellar dust at some distance, as could be expected from theirassociation with reflection nebulosity. Four stars (HD52721, HD53367,LkHα220 and LkHα218) show Hα emission and we arguethat they are Herbig Be stars with discs. Our photometric andspectroscopic observations of these stars reveal new characteristics oftheir variability. We present first interpretations of the variabilityof HD52721, HD53367 and the two LkHα stars in terms of a partiallyeclipsing binary, a magnetic activity cycle and circumstellar dustvariations, respectively. The remaining five stars show no clearindications of Hα emission in their spectra, although theirspectral types and ages are comparable with those of HD52721 andHD53367. This indicates that the presence of a disc around a star in CMaR1 may depend on the environment of the star. In particular we find thatall Hα emission stars are located at or outside the arc-shapedborder of the Hii region, which suggests that the stars inside the archave lost their discs through evaporation by UV photons from nearby Ostars, or from the nearby (<25pc) supernova, about 1Myr ago.
| Crystalline silicon nanoparticles as carriers for the Extended Red Emission
In an attempt to determine the carrier of the Extended Red Emission(ERE), we have investigated a series of amorphous and crystallinematerials: natural coal, amorphous hydrogenated carbon, amorphoushydrogenated silicon carbide, porous silicon, and crystalline siliconnanoparticles. The photoluminescence (PL) behavior of various samples ofthese materials upon excitation with UV light was studied at roomtemperature focusing on both the wavelength dependence of thephotoluminescence and the PL yield. For some samples the yield is by fartoo low, other samples do not comply with the characteristic wavelengthrange of ERE. Only the samples of nanocrystalline silicon (poroussilicon and silicon nanoparticles) reveal PL properties that arecompatible with the astronomical observations. Besides this experimentalevidence, we will supply additional arguments leading to the conclusionthat silicon nanoparticles should be seriously considered as anattractive carrier for the Extended Red Emission.
| A possible sets of diffuse bands originating at the same carrier
This paper discusses measurements of eight selected diffuse interstellarbands (DIBs): lambda lambda 5793, 5809, 5819, 5828, 6196, 6397, 6614 and6660 performed in high resolution, high S/N spectra of 41 reddenedstars. Central depths, considered less error-prone than equivalentwidths, are measured and mutual correlations between the selected DIBsare analyzed. Tight correlations between the DIBs: 5809, 6196, 6614 and6660 may suggest their common origin despite their widths differing by afactor of up to 2. The performed simulations prove that this fact doesnot preclude a common, molecular carrier of such features.
| The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way
The picture of the young stellar groups in the Canis Major-Puppis-Vela(215 deg
| The stellar composition of the star formation region CMa R1 - I. Results from new photometric and spectroscopic classifications
A new photometric and spectroscopic survey of the star formation region(SFR) CMa R1 is described. In a sample of 165 stars brighter than 13thmag, 88 stars were found to be probable members of the SFR. They aredefined as early-type stars with E(B-V)>=0.16mag, which correspondsto a distance of about 1kpc. 74 of the probable members are B stars. 19stars are possibly associated with an IRAS point source. We derive amost probable distance of 1050+/-150pc to the association. It appearsthat about 80 candidate members are pre-main-sequence stars with ageslower than 6 million years, while the main sequence extends over6.0-7.6mag, which is consistent with star formation starting about 8million years ago and continuing until at least half a million yearsago. Two bright B stars in the association (GU CMa and FZ CMa) seem tobe much older and probably do not originate from the same star formationepisode. The star formation efficiency appears to increase roughlymonotonically with time up to half a million years ago. From our data,we conclude that only a minor fraction of the stars has been createdthrough the scenario suggested by Herbst & Assousa, in which themembers of CMa R1 form by compression of ambient material by a supernovashock wave. An extensive search for candidate members with Hαemission did not reveal new Herbig Ae/Be candidates, so that the numberof stars in this class seems to be limited to four: Z CMa, LkHα218, LkHα 220 and possibly HD 53367.
| 2175 Å and 3.4 Micron Absorption Bands and Carbon Depletion in the Diffuse Interstellar Medium
Using a theoretical model previously developed to study the UVextinction produced by small carbon particles, we present results thatshow that the observed strength, profile and central wavelength of theinterstellar 2175 Å absorption band can be reproduced with atwo-component mixture of these particles. These consist of smalldehydrogenated coronene molecules together with a size distribution oflarger particles having a range of hydrogenation. Observed values of tau(2175 Å)/A_v and tau (3.4 μm)/A_v can be obtained ifn(C)/n(H)~80 ppm is contained in this mixture. The 2175 Å peak ininterstellar spectra can be simulated as the superposition of a centralpeak having gamma=0.9 mum^-1 and lambda^-1_0=4.58 mum^-1 arising fromdehydrogenated coronene and a somewhat broader peak (gamma~1.25 mum^-1,lambda^-1_0~4.60 mum^-1) due to larger particles. Other solutions,incorporating as little carbon as n(C)/n(H)=28 ppm are also possible.Photoluminescence from these particles is also discussed in relation tothe extended red emission spectrum.
| Extended Red Emission from Carbon Clusters in Interstellar Clouds
We have simulated extended red emission (ERE) spectra using a model inwhich this emission arises as photoluminescence from small carbonparticles of mixed sp^2/sp^3 hybridized bonding characteristics. Theemission efficiency from such particles can be highly efficient whentheir size is such that geminate recombination of photoexcited electronhole pairs is enhanced. The amplitude and emission profile of the EREemission from the diffuse interstellar medium and nebular sources suchas NGC 2327 and NGC 7027 can be reproduced with a range of averageparticle size and size distribution. These carbon particles arecomponents of the same mixtures that yield an accurate fit to theinterstellar 2175 Å extinction peak and suggest that ERE emission,the 2175 Å absorption, and infrared absorption at 3.4 μm mayarise from the same carbonaceous components of interstellar matter.
| Biofluorescence and The Extended Red Emission in Astrophysical Sources
A broad emission band over the wavelength range ~ 6000-7500 Å insubmicron dust in the galaxy and in M82 can be explained by fluorescencephenomena in low-temperature chloroplasts and bacterial pigments.Alternative explanations do not appear to be promising.
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| קבוצת-כוכבים: | חד קרן |
| התרוממות ימנית: | 07h09m20.30s |
| סירוב: | -10°20'50.0" |
| גודל גלוי: | 6.21 |
| מרחק: | 1785.714 פארסק |
| תנועה נכונה: | -3.1 |
| תנועה נכונה: | 3 |
| B-T magnitude: | 6.203 |
| V-T magnitude: | 6.224 |
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