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PlanetPol: A Very High Sensitivity Polarimeter
We have built and used on several occasions an optical broadband stellarpolarimeter, PlanetPol, which employs photoelastic modulators andavalanche photodiodes and achieves a photon-noise-limited sensitivity ofat least 1 in 106 in fractional polarization. Observations ofa number of polarized standards taken from the literature show that theaccuracy of polarization measurements is ~1%. We have developed a methodfor accurately measuring the polarization of altitude-azimuth mountedtelescopes by observing bright nearby stars at different parallacticangles, and we find that the on-axis polarization of the WilliamHerschel Telescope is typically ~15 × 10-6, measuredwith an accuracy of a few parts in 107. The nearby stars(distance less than 32 pc) are found to have very low polarizations,typically a few ×10-6, indicating that very littleinterstellar polarization is produced close to the Sun and that theirintrinsic polarization is also low. Although the polarimeter can be usedfor a wide range of astronomy, the very high sensitivity was set by thegoal of detecting the polarization signature of unresolved extrasolarplanets.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| Anomalous extinction behaviour towards the Type Ia SN 2003cg
We present optical and near-infrared photometry and spectroscopy of theType Ia SN 2003cg, which exploded in the nearby galaxy NGC 3169. Theobservations cover a period between -8.5 and +414d post-maximum. SN2003cg is a normal but highly reddened Type Ia event. Its B magnitude atmaximum Bmax = 15.94 +/- 0.04 andΔm15(B)obs = 1.12 +/- 0.04[Δm15(B)intrinsic = 1.25 +/- 0.05]. AllowingRV to become a free parameter within the Cardelli et al.extinction law, simultaneous matches to a range of colour curves ofnormal SNe Ia yielded E(B - V) = 1.33 +/- 0.11, and RV = 1.80+/- 0.19. While the value obtained for RV is small, suchvalues have been invoked in the past, and may imply a grain size whichis small compared with the average value for the local interstellarmedium.
| VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
We discuss the abundances of interstellar CH, CH+, and CN inthe Magellanic Clouds, derived from spectra of seven SMC and 13 LMCstars obtained (mostly) with the VLT UVES. CH and/or CH+ havenow been detected toward three SMC and nine LMC stars; CN is detectedtoward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly allthe optical detections of these molecular species in interstellar mediabeyond the Milky Way. In the LMC, the CH/H2 ratio iscomparable to that found for diffuse Galactic molecular clouds in foursight lines but is lower by factors of 2.5-4.0 in two others. In theSMC, the CH/H2 ratio is comparable to the local Galacticvalue in one sight line but is lower by factors of 10-15 in two others.The abundance of CH in the Magellanic Clouds thus appears to depend onlocal physical conditions and not just on metallicity. In both the SMCand the LMC, the observed relationships between the column density of CHand those of CN, CH+, Na I, and K I are generally consistentwith the trends observed in our Galaxy.Using existing data for the rotational populations of H2 inthese sight lines, we estimate temperatures, radiation field strengths,and local hydrogen densities for the diffuse molecular gas. The inferredtemperatures range from about 45 to 90 K, the radiation fields rangefrom about 1 to 900 times the typical local Galactic field, and thedensities (in most cases) lie between 100 and 600 cm-3.Densities estimated from the observed N(CH), under the assumption thatCH is produced via steady state gas-phase reactions, are considerablyhigher than those derived from H2. Much better agreement isfound by assuming that the CH is made via the (still undetermined)process(es) responsible for the observed CH+. A significantfraction of the CH and CH+ in diffuse molecular material inthe SMC and LMC may be produced in photon-dominated regions. Theexcitation temperature obtained from the populations of the two lowestCN rotational levels toward Sk -67 2 is quite consistent with thetemperature of the cosmic microwave background radiation measured withCOBE.Toward most of our targets, the UVES spectra also reveal absorption atvelocities corresponding to the Magellanic Clouds ISM from several ofthe strongest of the diffuse interstellar bands (DIBs; at 5780, 5797,and 6284 Å). On average, the three DIBs are weaker by factors of7-9 (LMC) and about 20 (SMC), compared to those typically observed inGalactic sight lines with similar N(H I), presumably due to the lowermetallicities and stronger radiation fields in the LMC and SMC. Thethree DIBs are also weaker (on average, but with some exceptions), byfactors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in theMagellanic Clouds. The detection of several of the so-calledC2 DIBs toward Sk 143 and Sk -67 2 with strengths similar tothose in comparable Galactic sight lines, however, indicates that nosingle, uniform scaling factor (e.g., one related to metallicity)applies to all DIBs (or for all sight lines) in the Magellanic Clouds.Based on observations collected at the European Southern Observatory,Chile, under programs 67.C-0281, 70.D-0164, 72.C-0064, 72.C-0682, and74.D-0109.
| The Asymmetrical Wind of the Candidate Luminous Blue Variable MWC 314
We present the results of long-term spectropolarimetric andspectroscopic monitoring of MWC 314, a candidate luminous blue variablestar. We detect the first evidence of Hα variability in MWC 314and find no apparent periodicity in this emission. The total R-bandpolarization is observed to vary between 2.21% and 3.00% at a positionangle consistently around ~0°, indicating the presence of atime-variable intrinsic polarization component, and hence anasymmetrical circumstellar envelope. We find suggestive evidence thatMWC 314's intrinsic polarization exhibits a wavelength-independentmagnitude varying between 0.09% and 0.58% at a wavelength-independentposition angle covering all four quadrants of the Stokes Q-U plane.Electron scattering off of density clumps in MWC 314's wind isconsidered as the probable mechanism responsible for these variations.
| BVI photometry and the spectroscopy of Nova Scuti 2005 N.2
Our CCD photometry of Nova Scuti 2005 N.2 (=V477 Sct) shows it to be avery fast nova, which is characterized by t2 = 3 andt3 = 6 days, affected by a EB-V ≥ 1.3 magreddening, and which peaked at V ~ 9.8 mag on ~Oct. 12.0 UT. The novawas probably entering a dust condensation episode or brightnessoscillations during the transition phase when it became unobservable forthe seasonal conjunction with the Sun. Absolute spectrophotometry showsit to belong to the He/N class. The emission line width at halfintensity is 2600 km s-1. At least five ripples areidentified in the high resolution emission lines profiles at radialvelocities ranging from -980 to +700 km s-1. The nova eruptedat a large distance from the Sun and at an appreciable height above theGalactic plane, suggesting an association with the Galactic bulge(unusual for a He/N nova). The progenitor was too faint to be recordedon DSS1/2 survey plates, when setting the outburst amplitude to ΔV ≥ 11 mag.
| Five WC9 stars discovered in the AAO/UKST Hα survey
We report the discovery of five massive Wolf-Rayet (WR) stars resultingfrom a programme of follow-up spectroscopy of candidate emission-linestars in the Anglo-Australian Observatory United Kingdom SchmidtTelescope (AAO/UKST) Southern Galactic Plane Hα survey. The6195-6775 Åspectra of the stars are presented and discussed. A WC9class is assigned to all five stars through comparison of their spectrawith those of known late-type WC stars, bringing the known total numberof Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibitnear-infrared (NIR) excesses characteristic of hot dust emission (asseen in the great majority of known WC9 stars), we find that two of thestars show no discernible evidence of such excesses. This increases thenumber of known WC9 stars without NIR excesses to seven. Reddenings anddistances for all five stars are estimated.
| Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H+31,
Using an absorption line from the metastable (J, K)=(3, 3) level ofH+3 together with other lines ofH+3 and CO observed along several sight lines, wehave discovered a vast amount of high-temperature (T~250 K) andlow-density (n~100 cm-3) gas with a large velocity dispersionin the central molecular zone (CMZ) of the Galaxy, i.e., within 200 pcof the center. Approximately three-fourths of theH+3 along the line of sight to the brightestsource we observed, the Quintuplet object GCS 3-2, is inferred to be inthe CMZ, with the remaining H+3 located inintervening spiral arms. About half of the H+3 inthe CMZ has velocities near ~-100 km s-1, indicating that itis associated with the 180 pc radius expanding molecular ring, whichapproximately forms the outer boundary of the CMZ. The other half, withvelocities of ~-50 and ~0 km s-1, is probably closer to thecenter. CO is not very abundant in these clouds. Hot and diffuse gas inwhich the (3, 3) level is populated was not detected toward severaldense clouds and diffuse clouds in the Galactic disk where large columndensities of colder H+3 have been reportedpreviously. Thus, the newly discovered environment appears to be uniqueto the CMZ. The large observed H+3 columndensities in the CMZ suggest an ionization rate much higher than in thediffuse interstellar medium in the Galactic disk. Our finding that theH+3 in the CMZ is almost entirely in diffuseclouds indicates that the reported volume filling factor (f>=0.1) forn>=104 cm-3 clouds in the CMZ is anoverestimate by at least an order of magnitude.Based on data collected at the United Kingdom Infrared Telescope, whichis operated by the Joint Astronomy Centre on behalf of the UK ParticlePhysics and Astronomy Research Council.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Asphericity and clumpiness in the winds of Luminous Blue Variables
We present the first systematic spectropolarimetric study of LuminousBlue Variables (LBVs) in the Galaxy and the Magellanic Clouds, in orderto investigate the geometries of their winds. We find that at least halfof our sample show changes in polarization across the strong Hαemission line, indicating that the light from the stars is intrinsicallypolarized and therefore that asphericity already exists at the base ofthe wind. Multi-epoch spectropolarimetry on four targets revealsvariability in their intrinsic polarization. Three of these, AG Car, HRCar and P Cyg, show a position angle (PA) of polarization which appearsrandom with time. Such behaviour can be explained by the presence ofstrong wind-inhomogeneities, or “clumps” within the wind.Only one star, R 127, shows variability at a constant PA, and henceevidence for axi-symmetry as well as clumpiness. However, if viewed atlow inclination, and at limited temporal sampling, such a wind wouldproduce a seemingly random polarization of the type observed in theother three stars. Time-resolved spectropolarimetric monitoring of LBVsis therefore required to determine if LBV winds are axi-symmetric ingeneral. The high fraction of LBVs (>50%) showing intrinsicpolarization is to be compared with the lower ~20-25% for similarstudies of their evolutionary neighbours, O supergiants and Wolf-Rayetstars. We anticipate that this higher incidence is due to the lowereffective gravities of the LBVs, coupled with their variabletemperatures within the bi-stability jump regime. This is alsoconsistent with the higher incidence of wind asphericity that we find inLBVs with strong Hα emission and recent (last ~10 years) strongvariability.
| Diffuse interstellar bands of unprecedented strength in the line of sight towards high-mass X-ray binary 4U 1907+09
High-resolution VLT/UVES spectra of the strongly reddened O supergiantcompanion to the X-ray pulsar 4U 1907+09 provide aunique opportunity to study the nature of the diffuse interstellar bands(DIBs) at unprecedented strength. We detect about 180 known DIBs, ofwhich about 25 were listed as tentative and are now confirmed. A dozennew DIB candidates longwards of 6900 Å are identified. We showthat the observed 5797 Å DIB strength is in line with the Galacticcorrelation with reddening, whereas the 5780 Å DIB strength isrelatively weak. This indicates the contribution of denser regions,where the UV penetration is reduced. The presence of dense cloud coresis supported by the detection of C2 rotational transitions. Members ofone DIB family (5797, 6379 Å and 6196, 6613 Å) behavecoherently, although one can make a distinction between the twocorrelated pairs. The broadened profiles of narrow DIBs are shown to beconsistent with the premise that each of the main clouds in the line ofsight discerned in the interstellar K I profile is contributingproportionally to the DIB profile. We complement and extend the relationof DIB strength with reddening {E}(B-V), as well as withneutral hydrogen column density N(H I), respectively, using stronglyreddened sightlines towards another four distant HMXBs. The 5780 ÅDIB, and tentatively also the 5797 and 6613 Å DIBs, are bettercorrelated to the gas tracer H I than to the dust tracer{E}(B-V). The resulting relationship can be applied to anyline of sight to obtain an estimate of the H I column density. In thesearch for the nature of the DIB carrier, this strongly reddened line ofsight is a complementary addition to single cloud line of sight studies.
| The spectrum of the very massive binary system WR 20a (WN6ha + WN6ha): Fundamental parameters and wind interactions
We analyse the optical spectrum of the very massive binary system WR 20a(WN6ha + WN6ha). The most prominent emission lines, Hα and He iiλ 4686, display strong phase-locked profile variability. From thevariations of their equivalent widths and from a tomographic analysis,we find that part of the line emission probably arises in a windinteraction region between the stars. Our analysis of the opticalspectrum of WR 20a indicates a reddening of AV ≃ 6.0mag and a distance of 7.9 kpc, suggesting that the star actuallybelongs to the open cluster Westerlund 2. The location of the system at 1.1 pc from the cluster core could indicate that WR 20a was gentlyejected from the core via dynamical interactions. Using a non-LTE modelatmosphere code, we derive the fundamental parameters of each component:Teff = 43 000 ± 2000 K,log{Lbol/Lȯ} ≃ 6.0, dot{M} = 8.5× 10-6 Mȯ yr-1 (assuming aclumped wind with a volume filling factor f = 0.1). Nitrogen is enhancedin the atmospheres of the components of WR 20a, while carbon isdefinitely depleted. Finally, the position of the binary components inthe Hertzsprung-Russell diagram suggests that they are core hydrogenburning stars in a pre-LBV stage and their current atmospheric chemicalcomposition probably results from rotational mixing that might beenhanced in a close binary compared to a single star of same age.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| The Optical Spectrum of an LBV Candidate in the Cyg OB2 Association
We have obtained the first high-spectral-resolution (R=15000 and 60000)optical spectra for the extremely luminous star No. 12, identified withthe IR source IRAS 20308+4104 in the Cyg OB2 association. We haveidentified about 200 spectral features at 4552 7939 Å, includingthe interstellar NaI, KI lines and numerous DIBs, which are thestrongest absorption lines in the spectrum, along with the HeI, CII, andSiII lines. A two-dimensional spectral classification indicates that thestar's spectral type is B5±0.5 Ia+. Our analysis of theradial-velocity pattern shows the presence of a radial-velocity gradientin the stellar atmosphere, due to the infall of matter onto the star.The strong Hα emission line displays broad Thompson wings andtime-variable core absorption, providing evidence that the stellar windis inhomogeneous, and a slightly blue-shifted P Cygni absorptionprofile. We conclude that the wind is time-variable.
| The buried Balmer-edge signatures from quasars
In our previous paper, we reported the detection of a Balmer edgeabsorption feature in the polarized flux of one quasar (Ton202). We havenow found similar Balmer edge features in the polarized flux of fourmore quasars (4C09.72, 3C95, B21208+32, 3C323.1), and possibly a fewmore, out of 14 newly observed with the Very Large Telescope and theKeck Telescope. In addition, we also re-observed Ton202, but we did notdetect such a dramatic feature, apparently due to polarizationvariability (the two observations are 1 yr apart). The polarizationmeasurements of some quasars are affected by an interstellarpolarization in our Galaxy, but the measurements have been corrected forthis effect reasonably well.As the broad emission lines are essentially unpolarized and thepolarization is confined only to the continuum in the five quasarsincluding Ton202 in both epochs, the polarized flux is considered tooriginate interior to the broad emission-line region. The Balmer edgefeature seen in the polarized flux is most simply interpreted as anintrinsic spectral feature of the quasar ultraviolet/optical continuum,or the `Big Blue Bump' emission. In this case, the edge feature seen inabsorption indeed indicates the thermal and optically thick nature ofthe continuum emitted. However, we also discuss other possibleinterpretations.
| Dust and Diffuse Interstellar Bands in the za = 0.524 Absorption System toward AO 0235+164
We present new Hubble Space Telescope (HST) Space Telescope ImagingSpectrograph (STIS) NUV-MAMA and STIS CCD observations of the BL Lacobject AO 0235+164 and the intervening damped Lyα (DLA) line atza=0.524. The line profile givesN(HI)=(5+/-1)×1021 cm-2 and, combined withthe H I 21 cm absorption data, leads to a spin temperature ofTs=220+/-60 K. Those spectra also show a strong, broadfeature at the expected position of the 2175 Å graphitic dustfeature at za=0.524. Assuming a Galactic-type dust extinctioncurve at za=0.524 gives a dust-to-gas ratio of 0.19 times theGalactic value, but the fit, assuming that the underlying, unreddenedspectrum is a single power law, is poor in the far-UV. A dust-to-gasratio of 0.19 times the Galactic value is similar to the LMC, but the AO0235+164 spectrum does not fit either the LMC extinction curve or theSMC extinction curve (which has practically no 2175 Å feature). Apossible interpretation includes dust similar to that in the Galaxy, butwith fewer of the small particles that produce the far-UV extinction.The metallicity of the za=0.524 absorber, estimated from theobserved N(HI) and excess X-ray absorption (beyond Galactic) derivedfrom contemporaneous and archival ASCA and ROSAT X-ray data, isZ=0.72+/-0.28 Zsolar, implying in turn a dust-to-metals ratioof 0.27 times the Galactic value. If the dust mass density is the samein the za=0.524 DLA system as in our Galaxy, only 14% (+/-6%)of the metals (by mass) are in dust, compared to 51%, 36%, and 46% forthe Galaxy, LMC, and SMC, respectively. Such a dusty za=0.524AO 0235+164 absorption system is a good example of the kind of DLAsystem that will be missed because of selection effects, which in turncan bias the measurement of the comoving density of interstellar gas (inunits of the closure density), Ωg, as a function of z.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| The Nonthermal Rotational Distribution of H+3
Although H+3 is nonpolar in the equilateraltriangle equilibrium structure, symmetry breakdown due to centrifugaldistortion causes a small dipole moment and hence rotationaltransitions. The spontaneous emission times are on the order of a fewweeks for low rotational levels and are comparable to collisionintervals in interstellar space. Moreover, there are metastablerotational levels such as J=K=3, from which spontaneous emissions arerigorously forbidden. A very nonthermal rotational distribution isproduced. We present a model calculation simulating the thermalizationof H+3. Since the lifetime ofH+3 in interstellar space is orders of magnitudelonger than the spontaneous emission time and collision intervals, asteady state approximation is assumed. Accurate theoretical values by abinitio theory are used for spontaneous emission rates. The rates ofcollision-induced transitions between rotational levels are calculatedon the assumption of completely random selection rules using anapproximate formula that satisfies the principle of detailed balancing.The results indicate that the observed high population ofH+3 in the (3, 3) metastable level toward theGalactic center (M. Goto and coworkers) signifies the presence of verylarge high-temperature (T>=300 K) and low-density[n(H2)<=70 cm-3] clouds. It is shown that otherhigher metastable levels may accommodate observableH+3 in such clouds and that the excitationtemperature determined from the observed relative populations of (1, 0)and (1, 1) should provide crucial information on the condition of suchclouds.
| LkHα 101 and the Young Cluster in NGC 1579
The central region of the dark cloud L1482 is illuminated by LkHα101, a heavily reddened (AV~10 mag) high-luminosity(>=8×103 Lsolar) star having an unusualemission-line spectrum plus a featureless continuum. About 35 muchfainter (mostly between R=16 and >21) Hα emitters have been foundin the cloud. Their color-magnitude distribution suggests a median ageof about 0.5 Myr, with considerable dispersion. There are also at leastfive bright B-type stars in the cloud, presumably of about the same age;none show the peculiarities expected of HAeBe stars. Dereddened, theirapparent V magnitudes lead to a distance of about 700 pc. Radioobservations suggest that the optical object LkHα 101 is in fact ahot star surrounded by a small H II region, both inside an opticallythick dust shell. The level of ionization inferred from the shape of theradio continuum corresponds to a Lyman continuum luminosity appropriatefor an early B-type zero-age main-sequence star. The V-I color isconsistent with a heavily reddened star of that type. However, theoptical spectrum does not conform to this expectation: the absorptionlines of an OB star are not detected. Also, the [O III] lines of an H IIregion are absent, possibly because those upper levels are collisionallydeexcited at high densities. There are several distinct contributors tothe optical spectrum of LkHα 101. The Hα emission line isvery strong, with wings extending to about +/-1700 km s-1,which could be produced by a thin overlying layer of hot electronscatterers. There is no sign of P Cygni type mass ejection. Lines of SiII are narrower, while the many Fe II lines are still narrower and aredouble with a splitting of about 20 km s-1. Lines of [Fe II],[O I], and [S II] are similarly sharp but are single, at the samevelocity as the Fe II average. Work by Tuthill et al. allowed theinference, from K-band interferometry, that the central source isactually a small horseshoe-shaped arc about 0.05" (35 AU) across. Atipped annulus of that size in rotation about a 15 Msolarstar would produce double spectrum lines having about the splittingobserved for Fe II. The totality of observational evidence encouragesthe belief that LkHα 101 is a massive star caught in an earlyevolutionary state.
| Discovery of a WO star in the Scutum-Crux arm of the inner Galaxy
We report the discovery of only the fourth massive WO star to be foundin the Milky Way, and only the seventh identified within the LocalGroup. This has resulted from the first observations made in a programmeof follow-up spectroscopy of candidate emission-line stars from theAAO/UKST Southern Galactic Plane Hα Survey. The optical spectrumof this star, to become WR 93b in the Catalogue of Galactic Wolf-Rayetstars, is presented and described. WR 93b is classified as WO3 and isshown to be highly reddened [E(B-V) = 2.1 +/- 0.1]. A recombination-lineanalysis of the emission lines yields the abundance ratios C/He = 0.95and O/He = 0.13 (by number). Comparisons at near-infrared wavelengths ofreddening-corrected photometry between WR 93b and both of Sand 2 (WO3,D= 49 kpc) and Sand 5 (WO2, D= 1.75 kpc) yield a consistent distance toWR 93b of 3.4 kpc. Positioned at Galactic coordinates , the star is mostlikely located in the Scutum-Crux arm of the inner Milky Way. We notethat none of the four Galactic WO stars lies significantly beyond thesolar circle (with two well inside).Estimation of the wind terminal velocity in WR 93b at 5750 kms-1 makes this star the current wind-speed record holderamong all non-degenerate stars.
| HD 183143: A Hypergiant
We present spectroscopic evidence that the luminosity of HD 183143 ishigher by one magnitude than thought previously. The star is yet anotherB6-8 Ia-0 white hypergiant of the Galaxy. Its absolute visual magnitudeis close to -8 mag, and its distance is close to 2 kpc. We describespectroscopic manifestations of the nonstationary behavior of itsatmosphere and wind.
| 10-μm absorption spectra of silicates for two new diffuse interstellar medium sightlines
We present low resolution 10-μm spectra of the newly identifieddiffuse interstellar medium (ISM) sightlines towards StRS 217 and StRS371 and a new spectrum of Cyg OB2 no. 12 obtained at the UK InfraredTelescope (UKIRT) with the Michelle imager/spectrometer. As previously,the μ Cephei emissivity better represents the profile of diffusemedium silicates than does the broader Trapezium emissivity which ischaracteristic of dust in molecular clouds. These spectra together withseven spectra published by Roche & Aitken comprise the majority ofthe silicates data set for the diffuse ISM. The observed sample includessources in the first galactic quadrant at heliocentric distances ofabout 1.0-4.2 kpc. We find the silicate optical depth,csil=AV/(19.2 +/- 0.6) and the ratio ofcsil to heliocentric distance, d, is in the range 0.1<~csil/d<~ 0.7 kpc-1 for StRS 217 the ratiocould be as high as csil/d<~ 1.2 kpc-1.However, this ratio is dominated by uncertainties in the distanceestimates and does not indicate the degree of clumping in the diffuseinterstellar medium.
| Grey extinction in the solar neighbourhood?
Some of the close O and B dwarfs appear to be fainter than indicated bytheir Hipparcos distances, intrinsic absolute magnitudes attributed totheir spectral types, and estimated selective interstellar extinction.This discrepancy is explained in the paper by the grey (neutral)interstellar extinction in the visual range of spectrum. The measure ofsuch an effect is related to discrete features of the interstellarmatter.
| 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.
| Infrared and visual interstellar absorption features towards heavily reddened field stars
We present measurements of the 3.4-μm hydrocarbon dust absorptionfeature, and four visual diffuse interstellar bands, for 12 highlyreddened [9.0 < A(V) < 15.8], early-type stars identified from theStephenson catalogue, covering a range of Galactic longitude. Thehydrocarbon feature is detected in 11 sightlines with optical depths ofup to 0.05, and corresponding column densities are derived. The featureis weaker per unit A(V) than previously reported, further underliningearlier observations of its enhancement in the Galactic Centre. Acomparison of the 3.4-μm feature substructure indicates organicmaterial similar to that seen in earlier diffuse medium studies,suggesting a uniform formation history across the Galactic disc. Theprofile is well matched by both meteoritic material and severallaboratory-prepared analogues. The diffuse interstellar bands (DIBs)measured were λλ6177, 6203, 6284 and 6614; no strongcorrelations were detected, either between these bands themselves orbetween the DIBs and the 3.4-μm feature, althoughλλ6284 and 6614 appear to vary together. If the carriersof the DIBs are organic and molecular in nature and have cosmic carbonabundance requirements similar to those of the C-H stretch, theseresults imply that there is little direct carbon exchange between themand the aliphatic dust component. Measurements of the extinction tothese objects suggest luminosity anomalies similar to that already seenin Cyg OB2 no 12.
| 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.
| 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.
| 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.
| An atlas of spectra of B6-A2 hypergiants and supergiants from 4800 to 6700 Å
We present an atlas of spectra of 5 emission-line stars: thelow-luminosity luminous blue variables (LBVs) HD 168625 and HD 160529,the white hypergiants (and LBV candidates) HD 168607 and AS 314, and thesupergiant HD 183143. The spectra were obtained with 2 echellespectrometers at the 6-m telescope of the Russian Academy of Sciences inthe spectral range 4800 to 6700 Å, with a resolution of 0.4Å. We have identified 380 spectral lines and diffuse interstellarbands within the spectra. Specific spectral features of the objects aredescribed.The complete atlas and Table 2 are only available in electronic form atthe CDS via anonymous ftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/1035
| 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.
| The Intrinsic Profile of the 4428 Å Diffuse Interstellar Band
This paper presents high-quality CCD spectra (obtained at spectralresolving power λ/Δλ~2000) of the region surroundingthe diffuse interstellar band (DIB) centered near 4428 Å, in some35 O and early B stars located in the Cyg OB2 association. Thereddenings of the stars range from E(B-V)~1.0 to 2.5 mag, allowing us tostudy the properties of the feature at high extinction, as compared withprevious studies of DIBs in diffuse clouds. To our knowledge this is thefirst significant survey of the 4428 Å DIB profile to be based onCCD-quality spectra, allowing us to probe the properties of this featurewith high accuracy. Among our conclusions are (1) the profile ofλ4428 is symmetric and invariant within the uncertainties; (2)there is no clear-cut dependence of band properties on stellar spectraltype; (3) the equivalent width of the 4428 Å DIB as a function ofincreasing extinction appears to level off at around E(B-V)~1.0 (4)there is no evidence for emission wings associated with λ4428 and(5) within the uncertainties the profile of the feature is fittedperfectly by a Lorentzian function, strongly suggesting that the band isbroadened by natural damping. We conclude that the feature is created byrapid internal conversion in a molecular carrier.
| Detection of Diffuse Interstellar Bands in the Magellanic Clouds
With the Ultraviolet Visual Echelle Spectrograph mounted at the VeryLarge Telescope, we have observed at unprecedented spectral resolutionthe absorption spectrum toward reddened stars in the Magellanic Cloudsover the wavelength range of 3500-10500 Å. This range covers thestrong transitions associated with neutral and charged large carbonmolecules of varying sizes and structures. We report the first detectionof diffuse interstellar bands (DIBs) at 5780 and 5797 Å in theSmall Magellanic Cloud and the variation of the 6284 Å DIB towardseveral targets in the Large Magellanic Cloud. The variation of DIBs inthe Magellanic Clouds compared with Galactic targets may be governed bya combination of the different chemical processes prevailing inlow-metallicity regions and the local environmental conditions. Based onobservations made at the European Southern Observatory, Paranal, Chile(ESO program 67.C-0281).
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Observation and Astrometry data
| Constellation: | Sagitta |
| Right ascension: | 19h27m26.57s |
| Declination: | +18°17'45.2" |
| Apparent magnitude: | 6.912 |
| Distance: | 370.37 parsecs |
| Proper motion RA: | 1.9 |
| Proper motion Dec: | -6.2 |
| B-T magnitude: | 8.206 |
| V-T magnitude: | 7.019 |
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