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Limb polarization of Uranus and Neptune. II. Spectropolarimetric observations
Aims.We have detected a strong limb polarization for Uranus and Neptune.With spectropolarimetric observations we characterize the spectraldependence of this limb polarization and explore the diagnosticpotential for investigating the distribution and properties of thescattering particles. Methods: We present disk resolvedspectropolarimetry of Uranus and Neptune covering the wavelength rangefrom 5300 Å to 9300 Å and compare the spectropolarimetricsignal for different limb sections and the center of the planetary disk.As an additional benefit we obtained center to limb disk profiles forthe intensity and polarization for various wavelengths. Results: Ourdata show for both planets a strong linear limb polarization orientedperpendicular to the limb. The polarization at the center of theplanetary disks is essentially zero. Overall the limb polarizationincreases towards shorter wavelengths, and it is further enhanced in themethane absorption bands when compared to adjacent inter band regions.Qualitatively, the polarization signal is very similar for Uranus andNeptune. For Uranus the peak limb polarization of the methane absorptionbands reaches a maximum of 3% whereas in the nearby higher albedoregions it peaks at about 2%. The measured polarization for Neptune issignificantly lower because the signal is averaged down by the limitedspatial resolution of our Neptune data. The disk profiles for Uranus(center to limb profiles) show for the intensity a strong asymmetrybetween the bright South pole regions and the darker northern latitudes.The asymmetry is particularly strong in the methane bands. We measure asignificant limb brightening in the methane bands at the Uranian easternand western limbs. The intensity asymmetry between North and South andthe limb brightening display a tight anti-correlation with the albedo ofthe selected wavelength interval. From the polarization profiles ofUranus we determine slit integrated polarization values in order toderive intrinsic limb polarization parameters which are corrected forthe limited spatial resolution of our data. These parameters arecompared with previous imaging polarimetry. Conclusions: .Thepolarimetric measurements of Uranus and Neptune are compatible withexpectations for the limb polarization of scattering atmospherescontaining substantial layers of particles which scatter radiationsimilar to Rayleigh scattering. It seems that the limb polarizationcontains interesting diagnostic information, in particular on thescattering properties of the aerosol particles in the haze layers athigh altitudes.Based on observations obtained at the ESO 3.6 m Telescope at La Silla,Chile (ESO program 72.C-0498).
| Limb polarization of Uranus and Neptune. I. Imaging polarimetry and comparison with analytic models
Imaging polarimetry of Uranus and Neptune in the R, i, and z bands arepresented. In all observations a radial limb polarization on the orderof 1% was detected with a position angle perpendicular to the limb. Thepolarization is higher in both planets for the shorter wavelength bands.As a first approximation, the polarization seems to be equally strongalong the entire limb. This is unlike Jupiter and Saturn, wheresignificant limb polarization is only observed at the poles. Wedetermined flux-weighted averages of the limb polarization and radiallimb polarization profiles, and investigated the degradation andcancellation effects in the polarization signal due to theseeing-limited spatial resolution of our observations. Taking this intoaccount we derived corrected values for the limb polarization in Uranusand Neptune. The results are compared with analytic models for Rayleighscattering atmospheres for the semi-infinite case and finite layers withground albedo. The comparison shows that the detected polarization iscompatible with expectations. This indicates that limb-polarizationmeasurements offer a powerful diagnostic tool for investigating theproperties of scattering particles in the upper atmospheres of Uranusand Neptune, in particular if more sophisticated numerical modeling ofthe limb polarization becomes available. It is also concluded from theoverall strength of the limb polarization that the disk-integratedpolarization of Uranus and Neptune for large phase angles is high(p>20%). This is of interest for future polarimetric detections ofextra-solar planets with atmospheric properties similar to Uranus andNeptune.
| The NICMOS Polarimetric Calibration
An accurate knowledge of the absolute calibration of the polarizingelements in the Near Infrared Camera and Multi-Object Spectrometer(NICMOS) becomes especially important when conducting studies thatrequire measuring degrees of polarization of close to 1% in thenear-infrared. We present a comprehensive study of all previouslyobserved polarimetric standards using the NIC2 camera on NICMOS.Considering both pre- and post-NICMOS Cooling System observations, wefind variations in the polarimetry consistent with the effects ofsubpixel misalignments and the point-spread function. We also measurenonzero results from unpolarized standards, indicating an instrumentalpolarization of p~1.2%, θ~88deg. The lack of polarizedand unpolarized standard stars with which to perform a comprehensivecalibration study means that we cannot be confident that the currentcalibration will be effective for a number of recent large NICMOS GOprograms. Further observations of polarimetric standards are needed inorder to fully characterize the behavior of NICMOS at around p=1%.Based on observations made with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Incorporated,under NASA contract NAS 5-26555.
| The Calar Alto lunar occultation program: update and new results
We present an update of the lunar occultation program which is routinelycarried out in the near-IR at the Calar Alto Observatory. A total of 350events were recorded since our last report (Fors et al. 2004, A&A,419, 285). In the course of eight runs we have observed, among others,late-type giants, T-Tauri stars, and infrared sources. Noteworthy was apassage of the Moon close to the galactic center, which produced a largenumber of events during just a few hours in July 2004. Results includethe determinations of the angular diameter of RZ Ari,and the projected separations and brightness ratios for one triple and13 binary stars, almost all of which representing first time detections.Projected separations range from 0farcs09 to 0farcs007. We provide aquantitative analysis of the performance achieved in our observations interms of angular resolution and sensitivity, which reach about 0farcs003and K ≈8.5 mag, respectively. We also present a statisticaldiscussion of our sample, and in particular of the frequency ofdetection of binaries among field stars.
| New optical polarization measurements of quasi-stellar objects. The data
New linear polarization measurements (mainly in the V band) arepresented for 203 quasi-stellar objects (QSOs). The sample is made up of94 QSOs located in the North Galactic Pole (NGP) region and of 109 QSOsin the South Galactic Pole (SGP) region. First time measurements havebeen obtained for 184 QSOs. Among them, 109 known radio-emitters, 42known Broad Absorption Line (BAL) QSOs, and 1 gravitationally lensedquasi-stellar object. We found high polarization levels (p > 3%) for12 QSOs, including the BAL QSO SDSS J1409+0048. For10 objects, measurements obtained at different epochs do exist. Two ofthem show evidence for variability: the highly polarized BL Laccandidate PKS 1216-010 and the radio sourcePKS 1222+037.Based on observations collected at the European Southern Observatory(ESO, La Silla and Paranal). Table 4 is only available in electronicform at the 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/433/757
| 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.
| Spectropolarimetry of the borderline Seyfert 1 galaxy ESO 323-G077
We report the detection of high linear polarization in the brightSeyfert 1 galaxy ESO 323-G077. Based on opticalspectropolarimetry with FORS1 at the VLT we find a continuumpolarization which ranges from 2.2% at 8300 Å to 7.5% at 3600Å. Similar amounts of linear polarization are found for the broademission lines, while the narrow lines are not polarized. The positionangle of the polarization (theta =84o) is independent of thewavelength and found to be perpendicular to the orientation of theextended [O III] emission cone of this galaxy. Within the standard modelof Seyfert nuclei the observations can be well understood assuming thatthis AGN is observed at an inclination angle where the nucleus ispartially obscured and seen mainly indirectly in the light scattered bydust clouds within or above the torus and the illuminated inner edge ofthe dust torus itself. Hence we conclude that ESO 323-G077 is aborderline Seyfert 1 galaxy which can provide important information onthe geometric properties of active nuclei.Based on observations obtained at the ESO VLT UT1 (Antu) at CerroParanal, Chile (ESO program 66.B-0063).
| VLT-spectropolarimetry of the high-polarization Seyfert 1 galaxy Fairall 51
We present high-precision optical spectropolarimetry of the Seyfert 1galaxy Fairall 51 (F51) taken with FORS1 at the VLT.The observed spectrum shows unpolarized and linearly polarizedcomponents. The AGN continuum and the broad lines show (after correctionfor the Galactic interstellar polarization and the light contribution ofthe F51 host galaxy) a practically identical amountof intrinsic polarization ranging from 5% in the red to 13% in the UV.The narrow lines are unpolarized or show only little intrinsicpolarization. The observed AGN continuum and the broad line radiationcan be explained by a combination of reddened (and attenuated) directlight and scattered light reflected from an optically thin dust region.Hence, within the framework of the unification scheme of AGN, theSeyfert 1 galaxy F51 appears to be an example of aborderline Seyfert 1/Seyfert 2 case where the nucleus is partiallyobscured like for other type 1 AGN with high intrinsic scatteringpolarization. It is found that the scattering region inF51 is located far from the BLR and the continuumsource. Thanks to this special scattering configuration, we were able tostudy the kinematics (line profiles) of the broad line region from twodifferent viewing angles, one along the line of sight (in total light)and one via the scattering region (in polarized light). The lineprofiles in polarized and total light are found to be indistinguishableto a very high accuracy, strongly indicating that the velocity field ofthe F51 BLR is essentially spherically symmetric.Based on observations obtained at the ESO VLT UT1 (Antu) at CerroParanal, Chile (ESO program 63.P-0074).
| Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface
Observations of interstellar linear polarization in the spectral range0.35-2.2 μm are presented for several stars reddened by dust in theTaurus region. Combined with a previously published study by Whittet etal., these results represent the most comprehensive data set availableon the spectral dependence of interstellar polarization in this nearbydark cloud (a total of 27 sight lines). Extinction data for these andother reddened stars in Taurus are assembled for the same spectralrange, combining published photometry and spectral classifications withphotometry from the Two Micron All Sky Survey. The polarization andextinction curves are characterized in terms of the parametersλmax (the wavelength of maximum polarization) andRV (the ratio of total to selective extinction),respectively. The data are used to investigate in detail the question ofwhether the optical properties of the dust change systematically as afunction of environment, considering stars observed throughprogressively more opaque (and thus progressively denser) regions of thecloud. At low visual extinctions (03, real changes in grainproperties occur, characterized by observed RVvalues in the range 3.5-4.0. A simple model for the development ofRV with AV suggests thatRV may approach values of 4.5 or more in thedensest regions of the cloud. The transition between ``normal''extinction and ``dense cloud'' extinction occurs at AV~3.2, avalue coincident with the threshold extinction above whichH2O-ice is detected on grains within the cloud. Changes inRV are thus either a direct consequence ofmantle growth or occur under closely similar physical conditions. Dustin Taurus appears to be in a different evolutionary state compared withother nearby dark clouds, such as ρ Oph, in which coagulation is thedominant physical process.
| Optical polarization of 47 quasi-stellar objects: The data
New broad-band linear polarization measurements are presented for asample of 47 QSOs which includes 27 broad absorption line QSOs and 2gravitational lens candidates. Tables 3 and 4 are also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html Based onobservations collected at the European Southern Observatory (ESO, LaSilla).
| Determining the extinction through dark clouds
We discuss the problem of a consistent determination of the visualextinction to objects in and behind a dark cloud. The use ofnear-infrared colours is discussed, in particular the H-K colours. Weconcentrate on the uncertainties, with emphasis on the extinction lawand on the intrinsic colours of the young stars embedded in a darkcloud. The cases of the two intensively studied nearby star-formingclouds, Taurus and Ophiuchus, are addressed.
| Ices and extinction through the Taurus and Ophiuchus clouds
A detailed intercomparison is made between published observations of H_2O and CO ices towards the Taurus and Ophiuchus dark clouds. The columndensities of the ices are intercompared, and each compared to the visualextinction through the clouds, A_v. It is neither clear that the twoclouds have different or well defined threshold extinctions for thesurvival of ice mantles, nor that, for each of the clouds, thethresholds for the survival of CO and H_2 O mantles are different. Theinclusion of new objects in Taurus (Teixeira et al. 1998) introduces alarge scatter in the relations between those quantities relative toresults obtained by previous authors. Lines-of-sight towards deeplyembedded Young Stellar Objects in Taurus appear to show an enhancementin the amount of water-ice relative to lines-of-sight towards fieldstars behind that cloud. While for A_v < 14 mag there is a tightcorrelation between the water-ice column density, N_s(H_2 O), and A_v,the inclusion of those new objects reveals a discontinuity in therelation between N_s(H_2 O) and A_v. The interpretation of thisdiscontinuity is discussed.
| The diffuse interstellar bands at 5797, 6379 and 6613 Angstroms. Ionization properties of the carriers
We present a study of the behaviour and ionization properties of threenarrow Diffuse Interstellar Bands (DIBs) at lambda lambda 5797, 6379 and6613 Angstroms. In all three DIBs substructures have recently beendetected, indicating large gaseous molecular carriers. Studying DIBs inregions with drastically different physical properties in terms of UVflux and density enables us to monitor the behaviour of the carriers andhence to constrain their nature. We observed these three DIBs along 40different lines-of-sight (35 program stars and 5 standard stars)consisting of HII regions, dark clouds, molecular clouds and reflectionnebulae. The DIB variations at low reddening are explained by a newmodel of photoionization equilibrium of the DIB carriers. This modeltakes into account the penetration depth of UV ionizing photonsthroughout the cloud. The slope of the variation of DIB strength as afunction of reddening thus allows us to estimate the effectiveionization potentials of the carriers. Following this new analysis, thecarriers of the lambda 5797 and lambda 6613 Angstroms DIBs would haveionization potentials above 10eV, reminiscent of large PAHs orfullerenes which have a single positive charge. The estimated ionizationpotential (7--9eV) of the lambda 6379 Angstroms DIB seems to indicate alarge neutral carrier. Based on observations with OHP 1.93m Telescopeand Elalie spectrograph.
| Interstellar Polarization in the Taurus Dark Clouds: Wavelength-dependent Position Angles and Cloud Structure near TMC-1
Systematic variations with wavelength in the position angle ofinterstellar linear polarization of starlight may be indicative ofmultiple cloud structure along the line of sight. We use polarimetricobservations of two stars (HD 29647 and HD 283809) in the generaldirection of TMC-1 in the Taurus dark cloud to investigate grainproperties and cloud structure in this region. We show the data to beconsistent with a simple two-component model in which generalinterstellar polarization in the Taurus cloud is produced by a widelydistributed cloud component with relatively uniform magnetic fieldorientation; light from stars close to TMC-1 suffers additionalpolarization arising in one (or more) subcloud(s) with larger averagegrain size and magnetic field directions different from the generaltrend. Toward HD 29647 in particular, we show that the unusually lowdegree of visual polarization relative to extinction is due todepolarization associated with the presence of distinct cloud componentsin the line of sight with markedly different magnetic fieldorientations. Stokes parameter calculations allow us to separate thepolarization characteristics of the individual components. Results arefitted with the Serkowski empirical formula to determine the degree andwavelength of maximum polarization. Whereas lambda max values in thewidely distributed material are similar to the average (0.55 mu m) forthe diffuse interstellar medium, the subcloud in the line of sight to HD283809, the most heavily reddened star in our study, has lambda max ~0.73 mu m, indicating the presence of grains ~30% larger than thisaverage. Our model also predicts detectable levels of circularpolarization toward both HD 29647 and HD 283809.
| Optical and near-infrared spectropolarimetry of the infrared-luminous galaxy IRAS 23060+0505
We present optical and near-IR spectropolarimetry of the type 2infrared-luminous galaxy IRAS 23060+0505, which clearly shows both broadHα and Paalpha in polarized flux. The full widths at half maximum(FWHM) of these lines are consistent with that of Paalpha in total fluxas measured by Hines. From the ratio of the polarized and total broadHα fluxes, we calculate the intrinsic polarization to be ~25 percent, assuming that the scattering geometry is the same for both thecontinuum and broad-line radiation. We model the flux density andpolarization characteristics as a combination of scattered continuum anda dichroic view of the near-IR-emitting regions through the postulatedtorus, which obscures the type 1 nucleus from the line of sight. Theextinction along the path of the scattered radiation is A_V~2.9 mag,whilst that of the direct view to the near-IR emission region is 20 mag,and to the BLR is ~30 mag, substantially higher than previous estimates.The presence of high extinction along the scattering path may havestrong implications for the observability of scattered optical broadlines in other objects. The calculated broad Hα luminosity is(2.3+/-0.3)x10^44 erg s^-1, consistent with that of QSOs.
| Grain Alignment in the Taurus Dark Cloud
Variations in the polarization efficiency (p/A) of interstellar grainsas a function of environment place vital constraints on models for themechanism of alignment. In this Letter, polarimetric observations ofbackground field stars are used to investigate alignment in the TaurusDark Cloud for extinctions in the magnitude range 0 < AK < 2.5 (0< AV < 25). Results show a strong systematic trend in polarizationefficiency with extinction, well represented by a power law p/A ~A-0.56. A number of possible interpretations of this result arediscussed. Assuming magnetic alignment of the grains, the observed trendmay be influenced by such factors as small-scale magnetic fieldstructure, variation of magnetic field strength and coupling of gas anddust temperatures as functions of density, and systematic changes in theefficiency of suprathermal spin as a function of grain surfaceproperties and H/H2 fraction within the dark cloud.
| High-Resolution Studies of Solid CO in the Taurus Dark Cloud: Characterizing the Ices in Quiescent Clouds
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...455..234C
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| Imaging polarimetry of the comet P/Swift-Tuttle
We develop a novel method of imaging polarimetry based on simultaneousrecording of two-dimensional polarization information and compensationfor atmospheric effects. We present wide-angle imaging polarimetry ofthe comet P/Swift-Tuttle at one pre-perihelion date near phase 45 deg.8.Our images show extended activity in the near-nucleus region, resultingprobably from a single dusty jet which emerges from the rotatingnucleus. We find that the polarization is higher than in typical `dusty'comets and interpret this as the contribution of fresh dust injectedinto the coma from the nucleus. We derive an independent value of 660 ms^-1 for the ejection velocity and compare it with that from molecularradio spectroscopy.
| Near-IR spectropolarimetry and modelling of NGC 1068
We present new spectropolarimetric data for NGC 1068, including, for thefirst time, coverage in the near-IR. Using these, and other previouslypublished data, we develop a scattering model for NGC 1068, using NGC5548 for the underlying Seyfert 1 continuum spectrum and M31 for thestellar contribution. Although electron scattering is the dominantprocess, if NGC 5548 is typical of Seyfert 1s in the near-IR thenelectron scattering alone cannot be soley responsible for the observedpolarization of NGC 1068 at these wavelengths, and a contribution fromdichroic absorption of the radiation from the IR-emitting regions of theactive nucleus, through the postulated torus, is required. Theextinction in this region is A_V>=45 mag, although we note that theextinction to broad-line region and nucleus is much higher.
| A new optical extinction law and distance estimate for the Taurus-Auriga molecular cloud
This paper presents optical spectrophotometry of field stars projectedon the Taurus-Auriga molecular cloud. We derive extinction laws forheavily reddened stars in this sample and show that the reddening lawthrough the dark cloud is nearly identical to the standard law forlambda-lambda(3600-6100) and AV approximately less than 3mag. Our spectroscopic parallaxes suggest a distance of 140 +/- 10 pcfor the northern portion of the cloud and show no compelling evidencefor a substantial variation in distance across the leading edge of thecloud.
| Narrow-band imaging of fields around optically-thick absorption systems: The line-of-sight towards Q 0000-2619
In this paper we describe deeper observations, both imaging andspectroscopy, of the most distant radio-quiet Ly-alpha galaxy known sofar, at redshift of z = 3.428 (galaxy G2) and of other faint objects inthe field around the QSO 0000 - 2619. Galaxy G2, which we discovered inthe field around the damped absorber of the QSO with narrow-band imagingtuned to detect Ly-alpha emission at the redshift of the absorptionsystem, is the best primaeval galaxy candidate identified so far. Herewe report measurements of its restframe UV spectral energy distributionin the interval lambda lambda 1000, 1700 A, in particular the depressionof the continuum due to the blanketing of the intervening Ly-alphaforest and the Ly-alpha emission line, and give better upper limits forthe high-ionization emission lines, such as NV and CIV. We conclude thatthis object is very likely a primaeval galaxy observed during a majorepisode of star formation and in which dust does not extinguish theLy-alpha emission. We also present Ly-alpha narrow-band imaging of thegalaxy which causes the damped absorber in the spectrum of the QSO. Noemission is detected, down to a 3 sigma upper limit of 1.2 x10-17 erg/s/sq cm in an aperture of 2 arcsec in diameter. Acorresponding upper limit to the rest-frame equivalent width is 6 A.Similar limits apply also to other 13 flat-spectrum z greater than 3galaxy candidates identified in the same field by Steidel &Hamilton, if their redshift is such that their Ly-alpha emission fallswithin our narrow-band filter. However, from their rest-frame UVcontinuum luminosity, we estimate that the star formation rates forthese galaxies are in the range 30 to 100 solar mass/yr. This wouldsuggest that radioquiet primaeval galaxies, though in a star-formingphase, are not characterized by the intense Ly-alpha emission and largestar formation rates (SFRs) typical of radio-galaxies. If thesecandidates are confirmed to be at similar redshifts than those of G2 andthe galaxy responsible for the damped system of the QSO, then thisassociation of galaxies would be the most distant cluster observed sofar, with a total stellar mass of the order of 1013 solarmass (throughout this paper we assume H0 = 50 km/s/Mpc andOmega = 0, unless differently stated).
| Diffuse interstellar bands in the Taurus dark clouds
This paper presents strengths and profiles of seven diffuse interstellarbands measured in a group of field stars situated behind the Taurus darkcloud complex. Comparison with the current 'band family' theories showsthat, within a single complex, the relative proportions of differentfamily carriers can vary in rather close unions: although correlationswith extinction are poor in all cases, some pairs of bands arenonetheless strongly interrelated. Band strengths relative to reddeningare investigated and found to be reasonably well fitted by a model inwhich all of the DIB production takes place in a surface layer of thecloud. These results are compared with existing data for the Rho Ophcloud. Band production efficiency varies in the same way in bothregions, falling rapidly with increasing E(B-V) to a limiting value afactor of 3-10 below the norm for the diffuse ISM. Comparison of grainproperties and the UV radiation field in the two cases shows that therelative weakness of the diffuse bands in dark clouds is driven bychanging grain properties rather than by species or other ionizedmolecules.
| 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
| Infrared spectroscopy of dust in the Taurus dark clouds - Ice and silicates
Low-resolution spectra of the 3 micron water-ice features of 22 starsand the 10 micron silicate dust features of 16 stars are presented forstars in the direction of the extensive dark cloud complex in Taurus,including both dust-embedded objects and background field stars seenthrough the cloud. A very close linear correlation is found between thepeak optical depth in the 3 micron feature and visual extinction, A(v),for field stars. The detection of ice in all cases where A(v) exceeds athreshold value of 3.3 + or - 0.1 mag serves as a basis for models ofvolatile mantle growth on grains in the dark cloud environment. Thesilicate feature is poorly correlated with A(v), and its relativeweakness per unit A(v) in the spectra of field stars with strong icebands may reflect the dilution of silicates as a fraction of total grainmass as mantles accrete in the dark cloud.
| Magnetic field structure in the Taurus dark cloud
Optical and infrared polarimetry of sources in the direction of theTaurus cloud are obtained in order to study the magnetic field in thiscloud and its possible role in the cloud's evolution. Most of the starsare background giant stars whose light shines through the cloud and ispolarized by the cloud material. The transverse component of themagnetic field, as delineated by the polarization vectors, is generallyperpendicular to the galactic plane, and the stratified structure of thecloud could be due to the effect of the magnetic field during the earlystages of collapse. Three of the 13 embedded stars are stronglypolarized with position angles nearly perpendicular to those of nearbyfield stars. The polarization of these stars is most likely intrinsic,and the direction of polarization indicates that the materialsurrounding these stars may be magnetic i.e., that the magnetic field isfrozen in this material.
| On interstellar reddening law in the Taurus dark clouds
The interstellar reddening law in the Taurus dark clouds is investigatedfrom visual, infrared, and ground based ultraviolet photometricobservations of heavily reddened stars HD 29647, HDE 283701, and HDE283812. The star HD 29647 located in the center of a very dense darkcloud Khavtassi 278 shows a reddening law which is anomalous in thewavelengths shorter than 400 nm. When combined with observations by Snowand Seab (1980) from the IUE it can be represented by a single straightline from 1 micron to 250 nm. The two other stars situated in less denseparts of the Taurus dark clouds show normal reddening law.
| U, B, V, R, I stellar photometry in the field of the Taurus dark clouds
A program of photoelectric stellar photometry has been carried out inthe field of the Taurus dark clouds with a view to establishing thedistance of the clouds and the absorption law in that region of the sky.It is found that the distance of the dark clouds accompanying the Tau T1and Tau T3 associations is 132 plus or minus 10 pc. Within theabsorption region 0-4 m, the Taurus dark clouds are characterized by anearly normal absorption law (to within 5-10%). In the field of the darkclouds a group of F0-G0 stars close to the main sequence is found whosespace density is three to eight times that in the solar neighborhood.
| Interstellar extinction in the dark Taurus clouds. I
The results of photoelectric photometry of 74 stars in the Vilniusseven-color system in the area of Taurus dark clouds with coordinates(1950) 4h20m-4h48m +24.5 deg to +27 deg are presented. Photometricspectral types, absolute magnitudes, color excesses, interstellarextinctions and distances of the stars are determined. The dark cloudKhavtassi 286, 278 and the surrounding absorbing nebulae are found toextend from 140 to 175 pc from the sun. The average interstellarextinction on both sides of the dark cloud is of the order of 1.5m. Noevidence of the existence of several absorbing clouds situated atvarious distances is found.
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Наблюдательные данные и астрометрия
| Созвездие: | Телец |
| Прямое восхождение: | 04h44m24.92s |
| Склонение: | +25°31'42.7" |
| Видимая звёздная величина: | 9.55 |
| Собственное движение RA: | 12 |
| Собственное движение Dec: | -19.2 |
| B-T magnitude: | 10.377 |
| V-T magnitude: | 9.619 |
Каталоги и обозначения:
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