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Young Stars in the Camelopardalis Dust and Molecular Clouds. I. The Cam OB1 Association
The distribution of dust and molecular clouds in the direction ofGalactic longitudes 132--158° and latitudes ± 12\degr\ isinvestigated. The maps of dust distribution in the area were plottedfrom the following surveys: the star counts in the DSS I database byDobashi et al. (2005), the survey of the average infrared color excessesby Froebrich et al. (2007) and the thermal dust emission survey at 100μ m by Schlegel et al. (1998). The distribution of molecular cloudswas taken from the whole sky CO survey by Dame et al. (2001). All thesesurveys show very similar cloud patterns in the area. Using the radialvelocities of CO, the distances to separate clouds are estimated. Arevised list of the Cam OB1 association members contains 43 stars andthe open cluster NGC 1502. 18 young irregular variable and Hαemission stars are identified in the area. All this proves that the starforming process in the Camelopardalis clouds is still in progress.
| Detection of optical linear polarization in the SN 2006aj/XRF 060218 non-spherical expansion
Aims.We have performed optical polarimetric observations of the SN2006aj associated to the X-ray flash (XRF) of February 18, 2006,XRF 060218 that provide information on its expansiongeometry. Methods: .The data were acquired in the R-band with the0.7 m telescope of Crimea, 2.5 m Nordic Optical Telescope and the 2.2 mof Calar Alto. Results: .We report the detection of linearpolarization between 3 and 39 days after the gamma-ray event (t-t_0).This represents the first polarization detection of a Ic supernova (SN)associated to an XRF. Our data exhibit a degree of linear polarization(P) around P˜4% at t-t0 ˜ 3{-}5 days, followed bya constant polarization phase with P ˜ 1.4% at 13.7 ⪉t-t0 ⪉ 39 days. Our data suggest a decay in P, and moreinterestingly, show a position angle (θ) rotation of 100°comparing data taken before and after the R-band lightcurve peak.Conclusions: .The reported polarization measurements can be explained bythe evolution of an asymmetric SN expansion. We discuss on severalingredients that could account for the observed θ rotation.
| FUSE Measurements of Far-Ultraviolet Extinction. II. Magellanic Cloud Sight Lines
We present an extinction analysis of nine reddened/comparison star pairsin the Large and Small Magellanic Clouds (LMC and SMC) based onFar-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date,just two LMC sight lines have probed dust grain composition and sizedistributions in the Magellanic Clouds using spectral data forwavelengths as short as 950 Å. We supplement these two with datafrom four regions distinguished by their IR through UV extinction curvesand grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinctcharacters of extinction in the Clouds and Milky Way, our results aregenerally analogous to those found for Galactic curves-namely, that theFUSE portions of each extinction curve are described reasonably well byFitzpatrick & Massa curves fitted only to longer wavelength data andlack any dramatic new extinction features, and any deviations from theCardelli, Clayton, & Mathis (CCM) formalism continue into FUVwavelengths. A maximum entropy method analysis of all of these curvessuggests that LMCAvg and SMC wing sight lines, whose extinctionparameters more closely resemble those for Galactic paths, require moresilicon and/or carbon in dust than current abundance measurements wouldindicate are available. The requirements for LMC2 and SMC bar sightlines do not fully tax the available reservoirs, in part because largegrains contribute less to the extinction in these directions. Anintermediate product of this extinction analysis is the measurement ofnew H2 abundances in the Magellanic Clouds. Collectivelyconsidering Cloud sight lines that possess significant H2column densities, E(B-V)/N(H I) ratios are reduced by significantfactors relative to the Galactic mean, whereas the correspondingE(B-V)/N(H2) values more closely resemble their Galacticcounterpart. These trends reflect the fact that among these sight linesf(H2) values are lower than those common in the Milky Way forpaths with similar degrees of reddening.Based on observations with the NASA-CNES-CSA Far-UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS-32985.
| T Tauri stellar magnetic fields: HeI measurements
We present measurements of the longitudinal magnetic field in thecircumstellar environment of seven classical T Tauri stars. Themeasurements are based on high-resolution circular spectropolarimetry ofthe HeIλ5876 emission line, which is thought to form in accretionstreams controlled by a stellar magnetosphere. We detect magnetic fieldsin BP Tau, DF Tau and DN Tau, and detect statistically significantfields in GM Aur and RW Aur A at one epoch but not at others. We detectno field for DG Tau and GG Tau, with the caveat that these objects wereobserved at one epoch only. Our measurements for BP Tau and DF Tau areconsistent, in terms of both sign and magnitude, with previous studies,suggesting that the characteristics of T Tauri magnetospheres arepersistent over several years. We observed the magnetic field of BP Tauto decline monotonically over three nights, and have detected a peakfield of 4 kG in this object, the highest magnetic field yet observed ina T Tauri star. We combine our observations with results from theliterature in order to perform a statistical analysis of themagnetospheric fields in BP Tau and DF Tau. Assuming a dipolar field, wedetermine a polar field of ~3 kG and a dipole offset of 40° for BPTau, while the field of DF Tau is consistent with a polar field of ~-4.5kG and a dipole offset of 10°. We conclude that many classical TTauri stars have circumstellar magnetic fields that are both strongenough and sufficiently globally ordered to sustain large-scalemagnetospheric accretion flows.
| Is It Round? Spectropolarimetry of the Type II-p Supernova 1999EM
We present the first multiepoch spectropolarimetry of a Type II plateausupernova (SN II-P), with optical observations of SN 1999em on days 7,40, 49, 159, and 163 after discovery. These data are used to probe thegeometry of the electron scattering atmosphere before, during, and afterthe plateau phase, which ended roughly 90 days after discovery. Weakcontinuum polarization with an unchanging polarization angle(θ~160deg) is detected at all epochs, with p~0.2% onday 7, p~0.3% on days 40 and 49, and p~0.5% in the final observations.Distinct polarization modulations across strong line features arepresent on days 40, 49, 159, and 163. Uncorrected for interstellarpolarization (which is believed to be quite small), polarization peaksare associated with strong P Cygni absorption troughs, and nearlycomplete depolarization is seen across the Hα emission profile.The temporal evolution of the continuum polarization and sharp changesacross lines indicate polarization intrinsic to SN 1999em. When modeledin terms of the oblate, electron scattering atmospheres of Höflich,the observed polarization suggests an asphericity of at least 7% duringthe period studied. The temporal polarization increase may indicategreater asphericity deeper into the ejecta. We discuss the implicationsof asphericity on the use of Type II-P supernovae as primaryextragalactic distance indicators through the expanding photospheremethod (EPM). If asphericity produces directionally dependent flux andpeculiar galaxy motions are characterized byσvrec=300 km s-1, it is shownthat the agreement between previous EPM measurements of SNe II anddistances to the host galaxies predicted by a linear Hubble lawrestricts mean SN II asphericity to values less than 30% (3 σ)during the photospheric phase.
| Speckle Interferometry of New and Problem Hipparcos Binaries. II. Observations Obtained in 1998-1999 from McDonald Observatory
The Hipparcos satellite made measurements of over 9734 known doublestars, 3406 new double stars, and 11,687 unresolved but possible doublestars. The high angular resolution afforded by speckle interferometrymakes it an efficient means to confirm these systems from the ground,which were first discovered from space. Because of its coverage of adifferent region of angular separation-magnitude difference(ρ-Δm) space, speckle interferometry also holds promise toascertain the duplicity of the unresolved Hipparcos ``problem'' stars.Presented are observations of 116 new Hipparcos double stars and 469Hipparcos ``problem stars,'' as well as 238 measures of other doublestars and 246 other high-quality nondetections. Included in these areobservations of double stars listed in the Tycho-2 Catalogue andpossible grid stars for the Space Interferometry Mission.
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| A New Spectropolarimeter at the Dodaira Observatory
We report a newly developed spectropolarimeter with a low-dispersionresolution (R=40-200) and a wide coverage in wavelength (400-900 nm). Itis mainly used along with the 0.91 m telescope at the DodairaObservatory of the National Astronomical Observatory, Japan. Thisinstrument and the related reduction software are described briefly. Thepresent polarimetric accuracy (rms) is estimated to be[(P/50)^2+(0.05)^2]^1/2%, where P is the linear polarization degree inpercent. The spectropolarimeter has the advantage of being most usefulin the polarimetric studies of both the continuum and the strongemission and/or absorption lines. It is mainly used for the short- orlong-term monitoring of variable circumstellar polarization and ofpolarization in comets, and for studies on interstellar polarization.
| Ultraviolet Interstellar Linear Polarization. V. Analysis of the Final Data Set
Using recent measurements of ultraviolet interstellar polarization, wehave examined its relationship to ultraviolet extinction and topolarization and extinction measurements in the visible and infrared.The relationship between the relative amount of ultraviolet polarizationand the parameter lambda_max, determined using only visible data, isconfirmed and strengthened, for example, by a tight correlation betweenp(6 mum^-1)/p_max and lambda^-1_max. A good fit to the wavelengthdependence of the polarization from the infrared to the ultraviolet canbe achieved with a five-parameter function combining a power law in theinfrared and a Serkowski-like function in the ultraviolet. Thepolarization efficiency (ratio of polarization to extinction) is less inthe ultraviolet than in the visual, and the ratio of these efficienciesincreases systematically with lambda^-1_max. We relate these effects tosystematic changes in the underlying aligned grain size distribution.The polarization efficiency of the grains causing the 2175 Åextinction bump along most sight lines is so (unusually) small that nostatistically significant polarization feature is detectable in thatwavelength region. Only two of 28 sight lines show a definitepolarization feature. The environments of the two bump sight lines aresimilar but not unique, and the mechanism for producing the polarizationfeature along only these two sight lines is still not known.
| A search for optical polarization from the narrow-line Seyfert 1 galaxy RE J1034+396
We present spectropolarimetry data on RE J1034+396, an ultrasoft X-raynarrow-line Seyfert 1 galaxy, and find an upper limit of about 0.4 tothe linear polarization in the optical band. This suggests that there isno synchrotron emission in this AGN, and thus it is unlikely that REJ1034+396 is related to BL Lac objects. Furthermore, any otherpolarization arising from transmission through dust, or reflection fromdust and electrons, must be cancelled out by geometrical effects ordiluted to a high degree by unpolarized radiation.
| The obscured BLR in the radio galaxy 3C 234
We present spectropolarimetric observations of the radio galaxy 3C 234at optical and, for the first time, near-IR wavelengths. In agreementwith previous observations we detect broad H-alpha in total andpolarized flux, consistent with the scenario in which the central activenucleus is hidden from view and observed via scattered radiation. Wemodel the flux and polarization properties at optical and near-IRwavelengths, which result in a point-source, cone-based scatteringgeometry in the optical and an extended source scattering in thenear-IR, with a dichroic view to the emission regions, which becomesimportant only at the longer wavelengths. From this model we calculatean intrinsic H-alpha luminosity of 4.9 x 10 exp 44 erg/s, and anextinction to the near-IR emission region of 60 mag for A(V). Thescattered radiation also undergoes extinction, this time by 1 mag forA(V), assuming a source function of a typical stellar-subtracted Seyfert1 galaxy. We also require that the scatterers are in bulk radial outflowat approximately 600 km/s to explain an observed increase in theintrinsic polarization of the scattered broad H-alpha line in the redwing, and blueshifted narrowline components in polarized flux.
| Cross Identification in the Region of Cluster NGC 1502
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114..222W&db_key=AST
| Final Astro-1 Calibration of the Hopkins Ultraviolet Telescope: A Test of White Dwarf Model Atmospheres as Far-Ultraviolet Calibration Standards
We present the final calibration of the Hopkins Ultraviolet Telescope(HUT) for its flight during the Astro-1 space shuttle mission in 1990December. The calibration is defined by a comparison between anobservation obtained in flight of the hot DA white dwarf G191-B2B andthe model atmosphere for this star calculated by P. Bergeron. We alsopresent the postflight laboratory calibration of the HUT sensitivity,which we find to be in excellent agreement with the white dwarfcalibration. The ratio of the white dwarf to the laboratory effectivearea calibration has a mean value of 1.0044 over the wavelength range912--1840 A, a slope of 7.61 x 10-6 A-1 (which corresponds to a relativedifference of only 0.35% at either end of the spectrum), andfluctuations about the mean value with an rms amplitude of 3%. Theuncertainties in the laboratory calibration are 4% for the overallnormalization, 5% relative to the mean on wavelength scales of 50 toseveral hundred A, and an uncertainty in the overall slope thatcorresponds to 1% at 1840 A and that increases smoothly to 3% at 912 A.These results validate the use of white dwarf models as absolute fluxstandards in the far-ultraviolet, especially for wavelengths shortwardof Ly alpha , where previous experiments often disagreed by largefactors. In particular, the normalization and wavelength dependence ofthe sensitivity calibration adopted by the Hubble Space Telescope FaintObject Spectrograph is confirmed to within the uncertainties of ourlaboratory calibration, while the normalization of the IUE finalcalibration is found to be low by an amount just exceeding our 1 sigmauncertainty. Fits to the spectra of both G191-B2B and the hot DA whitedwarf HZ 43 yield effective temperatures and surface gravities in goodagreement with values derived from Balmer line profiles. We also presenta comparison with Voyager spectra for the stars G191-B2B, HZ 43, BD +28deg4211, pi Aqr, and HD 25443. When averaged over broad bands (912--1050A, 1050--1180 A), fluxes from the two instruments typically agree towithin 10%.
| Ultraviolet Interstellar Polarization of Galactic Starlight.I.Observations by the Wisconsin Ultraviolet Photo Polarimeter Experiment
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2726A&db_key=AST
| Ultraviolet Imaging Telescope Near-Ultraviolet Bright Object Catalog
We present a photometric catalog of 2244 objects detected by theUltraviolet Imaging Telescope in the near-ultraviolet (NUV; 1650A <λ < 2900 A) during the Astro Space Shuttle mission. Sources inthe catalog are as faint as m_nuv_ ~ 18.8, or f_nuv_ ~ 1.1 x 10^16^ ergss^-1^ cm^-2^ A^-1^, but the survey is not complete to this level.Optical catalogs were used to cross identify sources and derive NUV - Vcolors. A majority of the objects (88%) do indeed have proposed opticalidentifications from catalogs, and most are stars. Our purpose increating the catalog is to form a database useful for identifying veryblue objects and performing Galactic UV stellar population studies.
| A Medium-Resolution Search for Polarmetric Structure: Moderate Y Reddening Sightlines
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....111..856W&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The Visual and Ultraviolet Polarimetric Data of alpha Camelopardalis and kappa Cassiopeia: Evidence of Shocked Regions
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...447..889F&db_key=AST
| Ultraviolet interstellar linear polarization. 2: The wavelength dependence
We present new ultraviolet (UV) polarimetry of the well-studiedinterstellar line of sight toward HD 204827 obtained with the FaintObject Spectrograph on Hubble Space Telescope. HD 204827 is of greatinterest because the dust along this line of sight has extremely lowvalues of both lambdamax and RV. Its far-UVextinction is very large, reflecting its small RV value. Inaddition, we reexamine the entire sample of 14 interstellar lines ofsight for which there are now UV polarization data. We find that thepreviously suggested relationship between lambdamax and thewavelength dependence of the polarization in the UV is stronglysupported by the data for this larger sample including HD 204827. Sevenstars with lambdamax greater than or equal to 0.54 micronagree well with an extrapolation of the Serkowski relation into the UVwhile seven stars with lambdamax less than or equal to 0.53micron show polarization in excess of the Serkowski extrapolation(super-Serkowski). However, the division of the observed lines of sightinto Serkowski and super-Serkowski categories is artificial. In fact,the amount of polarization in the UV is correlated with a singleparameter, lambdamax. This may indicate that there is a meaninterstellar polarization law analogous to the mean interstellarextinction law of Cardelli, Calyton, & Mathis which is based onRV. The data are consistent with a linear relationshipbetween 1/lambdamax and rho(UV)/rhomax but moredata are needed to define the functional form. We suggest that theSerkowski and super-Serkowski designations be replaced by high and lowlambdamax which are more physically descriptive. At the sametime, we note that all seven super-Serkowski (low lambdamax)stars lie in a relatively small region of the sky between lII= 90 deg - 150 deg and b = -5 deg - 15 deg. These stars all lie in orbehind a spur of the local Orion spiral arm. Similarly, most of theSerkowski (high lambdamax stars lie in or near theScorpio-Centaurus OB Association. So lines of sight covering largerareas of the sky are needed to test the universality of thelambdamax/UV polarization relationship. The recent discoveryof warm dust near HD 197770 suggests the possibility that a mechanismother than the traditional alignment to the Galactic magnetic field maybe invoked to explain its 2175 A polarization bump.
| The size distribution of interstellar dust particles as determined from polarization: Infinite cylinders
To extract the size distribution of polarizing dust grains from thewavelength dependence of interstellar linear polarization as objectivelyas possible, we have used the maximum-entropy method (MEM), as in anearlier study of size distributions based on extinction (Kim, Martin,& Hendry). There are additional complications using polarizationdata since polarization depends on shape and alignment. In this firstinvestigation, we adopted infinite cylinders with perfect spinningalignment. To constrain a wide range of sizes it is necessary to usedata from the infrared to the far-ultraviolet. Much of our analysis isbased on bare silicate grains. The modified Serkowski law representsinterstellar polarization quite well for the wavelength range 0.3 to 2micrometers using one parameter, lambdamax, the wavelength atwhich the polarization is maximum. Recent ultraviolet polarimetricobservations of eight stars of differing lambdamax indicatethat extrapolation of the modified Serkowski curve into the ultravioletproduces a reasonable approximation for larger lambdamax(greater than or approximately 0.55 micrometer), but for smallerlambdamax there is an excess of polarization observed.Therefore, we have investigated how the size distribution of thepolarizing grains changes with lambdamax simply by fittingthe modified Serkowski curve evaluated for lambdamax = 0.55,0.615, and 0.68 micrometers. But for HD 25443 (lambdamax =0.49 micrometer) which shows super-Serkowski behavi or, and for HD197770 (lambdamax = 0.51 micrometers) which might exhibit a2175 A polarization bump, we combined the modified Serkowski curve inthe infrared and optical with the actual far-ultraviolet data. The sizedistributions found for silicates bear little resemblance to a powerlaw. Instead, when expressed as contributions to the total mass, theypeak roughly at 0.14 micrometer and are skewed, with the relative rateof decrease to larger and smaller sizes depending onlambdamax. For the particles larger than 0.1 micrometer, thesize distribution does not change much with lambdamax becausethe infrared polarization is fairly invariant; on the other hand thereis a remarkable change for the smaller particles--the drop-off getsfaster as lambdamax increases, explaining the decreasingultraviolet polarization. In terms of the evolution of the sizedistribution, there is nothing to signal the transition from Serkowskito super-Serkowski behavior; this distinction is probably artificial. Wehave explored size distributions and polarization fitting usinghomogeneous grains of organic refractory material and other more highlyabsorbing materials like iron, magnetite, and an artificial oneintroduced by Chlewicki & Greenberg. Among three organicrefractories studied, two give a fairly good fit to the averagepolarization curve; but in detail, one of these gives excesspolarization in the infrared and the other predicts unobserved spectralstructure in the ultraviolet. The third more processed organicrefractory, thought to be most characteristic of interstellar grains,does not have features in the ultraviolet; however it produces a veryflat polarization curve in the far ultraviolet, unlike that observed. Onthe whole, these organic refractories appear less satisfactory thansilicates. Iron and magnetite have features in the visible andnear-infrared which do not agree with the polarization data.
| Evolution of macromolecular dust: Far-ultraviolet spectral dust extinction and gas absorption of stellar light as measured with the Hopkins Ultraviolet Telescope
From far-ultraviolet (FUV) spectra of B stars taken with the HopkinsUltraviolet Telescope (HUT) and Voyager space observations, we determinethe far-ultraviolet extinction by Galactic dust and hydrogen absorptionin the local spiral-arm clouds towards HD 25443 (B0.5 III), HD 37903(B1.5 V), and HD 200775 (B3 Ve). We find that the (n (H)) = 1.3 cu cmand number fraction (f = 0.31) of H atoms in molecular hydrogen relativeto the total hydrogen indicates a mostly diffuse medium toward HD 25443.We also determine f = 0.56 for the gas in the photodissociation region(PDR) in front of HD 37903 and f approximately equals 0.36 for the HD200775 PDR. The inferred ratios of N(H2)/I(CO) = 2.4 to 2.8 x1020/K(km/s)/sq cm for cool gas in dark clouds agree withprevious canonical estimates of N(H2)/I(CO) = 2 to 3 x1020/K(km/s)/sq cm and support the use of the ratio fordetermining masses of molecular clouds. We find that the shape andstrength of the HD 25443 FUV extinction is the same as a diffuse-mediummean extinction (Rnu = 2.95) extrapolated from mid-UVwavelengths (normal). The HD 37903 FUV extinction through a bright H2photodissociation region is higher than a mean FUVextinction(Rnu = 4.11), as is the HD 200775 extinction.Another star from the literature rho Oph, probes the dense medium andexhibits an FUV extinction of normal mean strength (Rnu =4.55) but steeper shape. The normal FUV -extinction of HD 25443 impliesthat the small FUV-extinction dust in the diffuse medium forms in p artfrom larger grains as the grains shatter under shocks. The normalstrength of the rho Oph FUV-extinction indicates that the minute dustcondenses onto or coagulates into larger grains in the dense medium. Thehigh extinction deviation of the HD 37903 PDR (and HD 200775 PDR) showsthat FUV radiation from hot stars with Teff approximatelyequals 20,000 to 23,000 K in PDRs can evaporate some of theFUV-extinction dust from grain surfaces. In contrast, previouslymeasured extinctions in Galactic H(+) (H II) regions shows that FUVradiation at Teff greater than 25,000 K can destroy some ofthe dust in H(+) regions. Thus, some of the FUV-extinction dust ishighly volatile. We find that the dust carrier of the variableFUV-extinction is correlated (r = 0.99) with the mid-IR (12 micrometers)radiation from hydrocarbon dust. The volatile FUV-extinction componentcould thus be due to minute hydrocarbon dust particles ormacromolecules, probably PAHs, which are known to condense onto largergrains in the dense medium. Larger grains compsed of PAHs clustered intoa solid could thus produce the steep FUV extinction of the rho Oph densemedium. After exposure to FUV photons wihtin approximately 1 pc of HD37903, PAHs with about 30 carbon atoms would evaporate off the largergrains, increasing gaseous PAH abundance by approximately 14%. Also,after receiving shocks in the diffuse medium toward HD 25443, the largerhydrocarbon grains would shatter into PAH molecules. In summary, as theGalactic medium cycles betwee n the diffuse and dense medium,hydrocarbon dust cycles into and out of larger grains under theinfluence of shocks, radiation, and condensation.
| A reassessment of some polarization standard stars
There have been recent reports suggesting that several polarizedstandard stars exhibit variability. In the investigation here, weconcentrate on the work of Bastien et al. (AJ, 495, 900 (1988)) whoclaim that the majority of their measurements reveal instability of thecataloged standards (11 stars out of 13). We criticize their statisticalassessments and comment on the shortcomings in their data and analyses.Firstly, it is demonstrated that their test based on comparisons ofinternal and external variances does not give rise to criteria forassessing variability. Secondly, their normality tests based on kurtosisand skewness are also shown to be fallacious and that in any case theylack rigor. Problems with the homogeneity of the observations arediscussed with respect to dead-time corrections, instrumentalpolarization and modulation efficiency, all of which may give rise tovariable systematic errors within the pooled data. For four stars whichhave sufficient homogeneous measurements, tests previously used oncatalogs of unpolarized stars have been adopted to investigate the dataand only one star (HD 111613) provides firm evidence for variability.
| The Size Distribution of Interstellar Dust Particles as Determined from Polarization
To extract the sd of polarizing dust grains we have used the MaximumEntropy Method (MEM). In this first investigation we adopted infinitecylinders with perfect spinning alignment. Only bare silicate particleswere considered. The modified Serkowski law represents interstellarpolarization quite well for the wavelength range 0.3 si 2 micron usingone parameter, lmax, the wavelength at which the polarization ismaximum. For large lmax it extrapolates reasonably into the ultravioletand so we have investigated how the sd changes with lmax by fitting themser curve evaluated for lmax = 0.55, 0.60, and 0.65 micron. For HD25443 which shows sser behavior and for HD 197770 which might exhibit a2200 Angstroms polarization bump, we combined the mser curve in theoptical with the actual far-ultraviolet data. The sd s found bear littleresemblance to a power law. Instead they peak at si 0.1 micron and areskewed, with the relative rate of decrease to larger and smaller sizesdepending on lmax. For the particles larger than 0.1 micron, the sd doesnot change much with lmax; on the other hand there is a remarkablechange for the smaller ones -- the drop-off gets faster as lmaxincreases. Compared to the sd based on extinction, there is a similarityfor large particles, but not for small ones: to fit the polarizationcurve it is not necessary to have as many small particles. This meansthat the (excess) smaller particles which contribute to the extinctionare either not as well aligned or rounder. Mathis (1986) developed analignment theory based on superparamagnetic inclusions in coagulatedgrains; the probability of a grain having such an inclusion (andtherefore being aligned) decreases for smaller grains. While this isqualitatively correct, our MEM results indicate even fewer smallparticles than predicted. It might be that smaller particles tend to berounder, becoming more aspherical as they grow (by coagulation).
| Ultraviolet interstellar linear polarization. I - Applicability of current dust grain models
UV spectropolarimetric observations yielding data on thewavelength-dependence of interstellar polarization along eight lines ofsight facilitate the evaluation of dust grain models previously used tofit the extinction and polarization in the visible and IR. These modelspertain to bare silicate/graphite grains, silicate cores with organicrefractory mantles, silicate cores with amorphous carbon mantles, andcomposite grains. The eight lines-of-sight show three differentinterstellar polarization dependences.
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| The first spectropolarimetric study of the wavelength dependence of interstellar polarization in the ultraviolet
The first UV spectropolarimetry along six lines of sight withsignificant interstellar polarization is reported. The observations wereobtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment(WUPPE) during the Astro-1 mission. HD 37903, HD 62542 and HD 99264 showa wavelength dependence which follows the Serkowski relationextrapolated into the UV. HD 25443 and Alpha Cam have UV polarizationwell in excess of the Serkowski extrapolation. HD 197770 clearly shows apolarization bump which closely matches the 2175 A extinction feature.This bump polarization can be fitted by small aligned graphite disks.The differences along various lines of sight might be the result ofdifferences in the environments which affect the size and alignment ofthe grains.
| Fifth fundamental catalogue. Part 2: The FK5 extension - new fundamental stars
The mean positions and proper motions for 3117 new fundamental starsessentially in the magnitude range about 4.5 to 9.5 are given in thisFK5 extension. Mean apparent visual magnitude is 7.2 and is on average2.5 magnitudes fainter then the basic FK5 which has a mean magnitude of4.7. (The basic FK5 gives the mean positions and proper motions for theclassical 1535 fundamental stars). The following are discussed: theobservational material, reduction of observations, star selection, andthe system for the FK5 extension. An explanation and description of thecatalog are given. The catalog of 3117 fundamental stars for the equinoxand epoch J2000.0 and B1950.0 is presented. The parallaxes and radialvelocities for 22 extension stars with large forecasting effects aregiven. Catalogs used in the compilation of the FK5 fundamental catalogare listed.
| An atlas of Hubble Space Telescope photometric, spectrophotometric, and polarimetric calibration objects
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990AJ.....99.1243T&db_key=AST
| Empirical temperature calibrations for early-type stars
Three temperature calibrations of suitable photometric quantities havebeen derived for O and B stars. A sample of 120 stars with reliableT(eff.) determinations has been used for establishing each calibration.The different calibrations have been critically discussed and compared.Temperature determinations for 1009 program stars have been obtainedwith an accuracy of the order of 10 percent.
| The variability of polarized standard stars
The linear polarization of 11 out of a total of 13 standard polarizedstars has been found to be variable. This implies that these 11 stars,mostly giants and supergiants but also four early-type main-sequencestars, exhibit some level of small, variable, intrinsic polarization.Therefore, they are not suitable as polarized standards for accuratepolarimetry. The problem of finding suitable candidates for standardpolarized stars is therefore nontrivial and basically still unsolved.The general increase in the level of polarization variability with meanpolarization may be partly interstellar in origin, analogous toradio-flux scintillations seen in extragalactic radio sources.
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