Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Relation between CH cation and neutral/molecular hydrogen
Observations of interstellar absorption bands of CH+ moleculeat 3957 and 4232 Å were applied to determine column densitiestoward 53 stars. The targets were selected because the atomic andmolecular hydrogen column densities are published. The data onCH+ were acquired using four echelle spectrographs situatedin both the Northern and Southern hemispheres. Spatial relations betweencolumn densities of CH+ and those of molecular, atomic andtotal hydrogen show large scatter, suggesting there is no relationbetween abundances of methylidyne cation and hydrogen in any form.
| Can the intrinsic polarization of stellar radiation correspond to the Serkowski law?
We demonstrate a situation where the wavelength dependence of theintrinsic linear polarization of stellar radiation matches that of theinterstellar linear polarization described by the Serkowski law. Such asituation can arise when the radiation from a star with a dipolemagnetic field is scattered in a circumstellar plasma shell with auniform electron density distribution. As a result, we have estimatedthe magnetic field strength at the photospheric phase of Supernova1999gi. We show that the existence of intrinsic polarization in Galacticstars disguised as interstellar polarization is possible in principle.
| First Detection of Polarized Scattered Light from an Exoplanetary Atmosphere
We report the first direct detection of an exoplanet in the visiblepolarized light. The transiting planet HD 189733b is one of the very hotJupiters with shortest periods and, thus, smallest orbits, which makesthem ideal candidates for polarimetric detections. We obtainedpolarimetric measurements of HD 189733 in the B band well distributedover the orbital period and detected two polarization maxima nearplanetary elongations with a peak amplitude of ~2 ×10-4. Assuming Rayleigh scattering, we estimated theeffective size of the scattering atmosphere (Lambert sphere) to be 1.5+/- 0.2 RJ, which is 30% larger than the radius of the opaquebody previously inferred from transits. If the scattering matter fillsthe planetary Roche lobe, the lower limit of the geometrical albedo canbe estimated as 0.14. The phase dependence of polarization indicatesthat the planetary orbit is oriented almost in a north-south directionwith a longitude of ascending node Ω = (16° or 196°) +/-8°. We obtain independent estimates of the orbit inclination i =98° +/- 8° and eccentricity e = 0.0 (with an uncertainty of0.05), which are in excellent agreement with values determinedpreviously from transits and radial velocities. Our findings clearlydemonstrate the power of polarimetry and open a new dimension inexploring exoplanetary atmospheres even for systems without transits.
| Broad-band optical polarimetric study of IC 1805
We present the BVR broad-band polarimetric observations of 51 starsbelonging to the young open cluster IC 1805. Along with the photometricdata from the literature, we have modelled and subtracted the foregrounddust contribution from the maximum polarization (Pmax) andcolour excess [E (B - V)]. The mean value of the Pmax forintracluster medium and the foreground are found to be 5.008 +/- 0.005and 4.865 +/- 0.022 per cent, respectively. Moreover, the mean value ofthe wavelength of maximum polarization (λmax) forintracluster medium is 0.541 +/- 0.003μ m, which is quite similar asthe general interstellar medium (ISM). The resulting intracluster dustcomponent is found to have negligible polarization efficiency ascompared to interstellar dust. Some of the observed stars in IC 1805have shown the indication of intrinsic polarization in theirmeasurements.
| Spectropolarimetry and IR photometry of magnetic white dwarfs: Vacuum polarization or rydberg states in their magnetic fields?
We suggest two mechanisms to explain IR photometric andspectropolarimetric observations of magnetic white dwarfs: vacuumpolarization and the existence of Rydberg atomic states with largedipole moments arising due to atomic collisions in the strong magneticfield of the white dwarf (so-called magnetic collision-inducedabsorption, or magnetic CIA). Both mechanisms can explain the observedrotations of the polarization ellipses and the depression of the IRspectral energy distribution. We present the results ofspectropolarimetric observations of several magnetic white dwarfs withthe Special Astrophysical Observatory 6-m telescope, together withphotometric observations in the near-IR obtained with theRussian-Italian AZT-24 Telescope at Campo Imperatore.
| 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.
| Low- and Medium-Dispersion Spectropolarimetry of Nova V475 Scuti (Nova Scuti 2003): Discovery of an Asymmetric High-Velocity Wind in a Moderately Fast Nova
We present low-resolution (R~90) and medium-resolution (R~2500)spectropolarimetry of nova V475 Sct taken with the HBS instrument,mounted on the 0.91 m telescope at the Okayama AstrophysicalObservatory, and taken with the Faint Object Camera and Spectrograph,mounted on the 8.2 m Subaru telescope. We estimated the interstellarpolarization toward the nova from the steady continuum polarizationcomponents and Hα line emission components. After subtracting theinterstellar polarization component from the observations, we found thatthe Hα emission seen on 2003 October 7 was clearly polarized. Inthe polarized flux spectrum the Hα emission had a distinct redwing extending to ~+4900 km s-1 and a shoulder around +3500km s-1, showing a constant position angle (P.A.) of linearpolarization θ*~=155deg+/-15deg.This suggests that the nova had an asymmetric outflow with a velocity ofvwind~=3500 km s-1 or more, which is 6 timeshigher than the expansion velocity of the ionized shell at the sameepoch. Such a high-velocity component has not previously been reportedfor a nova in the ``moderately fast'' speed class. Our observationssuggest the occurrence of violent mass-loss activity in the nova binarysystem even during the common-envelope phase. The P.A. of thepolarization in the Hα wing is in good agreement with that of thecontinuum polarization found on 2003 September 26(p*~=0.4%-0.6%), which disappeared within the following 2days. The uniformity of the P.A. between the continuum polarization andthe wing polarization on October 7 suggests that the axis of thecircumstellar asymmetry remained nearly constant during the period ofour observations.Based on data obtained at the Subaru telescope and the OkayamaAstrophysical Observatory, which are operated by the NationalAstronomical Observatory of Japan.
| Polarized standard stars
Not Available
| Spectropolarimetry of the Peculiar Type Ia Supernova 2005hk
We present Keck spectropolarimetry of the unusual Type Ia supernova (SNIa) 2005hk several days before maximum light. An analysis of the highsignal-to-noise ratio total-flux spectrum shows the object's extremesimilarity to the peculiar SN 2002cx. SN 2005hk has an optical spectrumdominated by Fe III lines and only weak lines of intermediate-masselements, unlike a normal SN Ia at this epoch. The photospheric velocitymeasured from the minima of strong absorption lines is very low for anSN Ia (~6000 km s-1), solidifying the connection to SN2002cx. The spectrum-synthesis code SYNOW was used to identify thepresence of iron-peak elements, intermediate-mass elements, and possiblyunburned carbon at similar velocities in the outer ejecta of SN 2005hk.Many weak spectral features remain unidentified. The spectropolarimetryshows a low level of continuum polarization (~0.4%) after correction forthe interstellar component, and only a weak Fe III line feature isdetected. The level of continuum polarization is normal for an SN Ia,implying that the unusual features of SN 2005hk cannot be readilyexplained by large asymmetries.
| An Extreme Case of a Misaligned Highly Flattened Wind in the Wolf-Rayet Binary CX Cephei
CX Cep (WR 151) is the WR+O binary (WN5+O5 V) with the second shortestperiod known in our Galaxy. To examine the circumstellar matterdistribution and to better constraint the orbital parameters andmass-loss rate of the W-R star, we obtained broadband and multiband(i.e., UBVRI) linear polarization observations of the system. Ouranalysis of the phase-locked polarimetric modulation confirms the highorbital inclination of the system (i.e., i=65deg). Using theorbital solution of Lewis et al. (1993), we obtain masses of 33.9 and23.9 Msolar for the O and W-R stars, respectively, whichagree with their spectral types. A simple polarimetric model accountingfor finite stellar size effects allowed us to derive a mass-loss ratefor the W-R star of (0.3-0.5)×10-5 Msolaryr-1. This result was remarkably independent of the model'sinput parameters and favors an earlier spectral type for the W-Rcomponent (i.e., WN4). Finally, using our multiband observations, wefitted and subtracted from our data the interstellar polarization. Theresulting constant intrinsic polarization of 3%-4% is misaligned inrelation to the orbital plane (i.e.,θCIP=26deg vs. Ω=75deg)and is the highest intrinsic polarization ever observed for a W-R star.This misalignment points toward a rotational (or magnetic) origin forthe asymmetry and contradicts the most recent evolutionary models formassive stars (Meynet & Maeder 2003) that predict sphericallysymmetric winds during the W-R phase (i.e., CIP=0%).
| M33's Variable A: A Hypergiant Star More Than 35 YEARS in Eruption
Variable A in M33 is a member of a rare class of highly luminous,evolved stars near the upper luminosity boundary that show sudden anddramatic shifts in apparent temperature due to the formation ofoptically thick winds in high mass loss episodes. Recent optical andinfrared spectroscopy and imaging reveal that its ``eruption,'' begun in~1950, has ended, having lasted ~45 yr. Our current observations showmajor changes in its wind from a cool, dense envelope to a much warmerstate surrounded by low-density gas with rare emission lines of Ca II,[Ca II], and K I. Its spectral energy distribution has unexpectedlychanged, especially at the long wavelengths, with a significant decreasein its apparent flux, while the star remains optically obscured. Weconclude that much of its radiation is now escaping out of our line ofsight. We attribute this to the changing structure and distribution ofits circumstellar ejecta, corresponding to the altered state of its windas the star recovers from a high mass loss event.
| The Gas Phase Spectrum of Cyclic C18 and the Diffuse Interstellar Bands
The gas phase spectrum of the cyclic C18 molecule recorded inthe laboratory at a temperature typical of diffuse interstellar cloudsis compared with absorption features toward ζ Oph and HD 204827 inthe 5730-5934 Å region. For the origin band at 5928.5 Å anupper limit to the column density of <=1.8×1011cm-2 is inferred. The origin band pattern in the laboratoryspectrum changes on lowering the internal temperature in the 100-20 Krange and bears a striking resemblance to the observed structure of anumber of DIBs at other wavelengths. This suggests that platelikemolecules or ions, comprising a couple of dozen to hundred carbon atoms,could be responsible for some of the latter absorptions.
| On the diffuse bands related to the C2 interstellar molecule
The recently published idea that intensities of some weak diffuseinterstellar bands (DIBs) are related to the C2 molecule column densityhave been examined. We use a set of high quality echelle spectra ofheavily reddened stars, acquired at the Bohyunsan Optical AstronomicalObservatory (South Korea), with a resolution R=30 000. The high quality(high S/N ratio) of our spectra is proved by the fact that despite usingthe most widely used Phillips (2, 0) band of the C2 molecule (near 8760Å), we can trace the (3, 0) Phillips band (near 7725 Å) aswell. Equivalent widths of four (5176, 5542, 5546 and 5769 Å) outof 16 examined DIBs demonstrate relatively good correlation with C2column density. However, a majority of the studied DIBs, alreadyreported as "C2" ones, most likely are not related to this simplestcarbon molecule. A removal of peculiar objects like HD 34078 from theanalyzed sample does not substantially change the level of correlations.
| Interstellar Ca II Line Intensities and the Distances of the OB stars
We show that the equivalent widths of the well-known interstellar Ca IIH and K lines can be used to determine the distances to OB stars in ourGalaxy. The equivalent widths, measured in the spectra of 147 early-typestars, are strongly related to the Hipparcos parallaxes of thoseobjects. The lines fitted to the parallax-equivalent width data aregiven by the formulae π=1/[2.78EW(K)+95] and π=1/[4.58EW(H)+102],where π is in arcseconds and EW is in milliangstroms. The form of theformulae, yielding a finite parallax even for zero absorption, showsthat space within ~100 pc of the Sun contains very little Ca II, whichis in agreement with the known dimensions of the Local Bubble. Using CaII lines for distance determination does not require the knowledge ofthe absolute magnitude of the object; it is thus well suited for targetsfor which the absolute calibration is either not precise (OBsupergiants) or not available at all (peculiar objects). We alsodemonstrate that neither the reddening E(B-V) nor the equivalent widthsof interstellar K I and CH lines are suitable candidates for distanceestimation, their relation with parallaxes being far less tight than forCa II.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. II. Analysis
To complement the optical absorption line survey of diffuse moleculargas in Paper I, we obtained and analyzed far-ultraviolet H2and CO data on lines of sight toward stars in Cep OB2 and Cep OB3.Possible correlations between column densities of different species forindividual velocity components, not total columns along a line of sightas in the past, were examined and were interpreted in terms of cloudstructure. The analysis reveals that there are two kinds of CH indiffuse molecular gas: CN-like CH and CH+-like CH. Evidenceis provided that CO is also associated with CN in diffuse molecularclouds. Different species are distributed according to gas density inthe diffuse molecular gas. Both calcium and potassium may be depletedonto grains in high-density gas, but with different dependencies onlocal gas density. Gas densities for components where CN was detectedwere inferred from a chemical model. Analysis of cloud structureindicates that our data are generally consistent with the large-scalestructure suggested by maps of CO millimeter-wave emission. On smallscales, the gas density is seen to vary by factors greater than 5.0 overscales of ~10,000 AU. The relationships between column densities of COand CH with that of H2 along a line of sight show similarslopes for the gas toward Cep OB2 and Cep OB3, but the CO/H2and CH/H2 ratios tend to differ, which we ascribe tovariation in average density along the line of sight.
| Evidence for Spectropolarimetric Diversity in Type Ia Supernovae
We present single-epoch, postmaximum spectropolarimetry of four Type Iasupernovae (SNe Ia) that span a range of spectral and photometricproperties: SN 2002bf and SN 2004dt exhibit unusually high-velocity (HV)absorption lines. SN 1997dt is probably somewhat subluminous, and SN2003du is slightly overluminous. We detect polarization modulationsacross strong lines in all four objects, demonstrating that all areintrinsically polarized. However, the nature and degree of thepolarization varies considerably. Including all SNe Ia studied thus far,the following order emerges in terms of increasing strength ofline-polarization features: ordinary/overluminous
| An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards
In this paper we present a new method for deriving UV through IRextinction curves, based on the use of stellar atmosphere models toprovide estimates of the intrinsic (i.e., unreddened) stellar spectralenergy distributions (SEDs), rather than unreddened (or lightlyreddened) standard stars. We show that this ``extinction withoutstandards'' technique greatly increases the accuracy of the derivedextinction curves and allows realistic estimations of the uncertainties.An additional benefit of the technique is that it simultaneouslydetermines the fundamental properties of the reddened stars themselves,making the procedure valuable for both stellar and interstellar studies.Given the physical limitations of the models we currently employ, thetechnique is limited to main-sequence and mildly evolved B stars.However, in principle, it can be adapted to any class of star for whichaccurate model SEDs are available and for which the signatures ofinterstellar reddening can be distinguished from those of the stellarparameters. We demonstrate how the extinction without standards curvesmake it possible to (1) study the uniformity of curves in localizedspatial regions with unprecedented precision, (2) determine therelationships between different aspects of curve morphology, (3) producehigh-quality extinction curves from low color excess sight lines, and(4) derive reliable extinction curves for mid to late B stars, therebyincreasing spatial coverage and allowing the study of extinction in openclusters and associations dominated by such stars. The application ofthis technique to the available database of UV through IR SEDs, and tofuture observations, will provide valuable constraints on the nature ofinterstellar grains and on the processes that modify them, and it willenhance our ability to remove the multiwavelength effects of extinctionfrom astronomical energy distributions.
| Profiles of Very Weak Diffuse Interstellar Bands around 6440 Å
Profiles of very weak diffuse interstellar bands (DIBs) between 6400 and6470 Å observed with high resolution and very high S/N aredemonstrated. We show that with the increasing quality of reddenedstellar spectra, the whole spectral range is covered with weak or veryweak DIBs-at least one every 2-3 Å. We also present the details ofthe profiles of a few stronger features; the presence of substructuresresembles the profiles of strong DIBs observed in high resolution duringthe last decade and supports the hypothesis of a molecular origin of atleast a majority of DIBs.
| Diffuse Interstellar Bands Toward HD 62542
Diffuse interstellar bands (DIBs) have been detected for the first timealong the peculiar translucent line of sight toward HD 62542, whichpasses through a diffuse cloud core. Although only a small fraction (18out of more than 300) of generally weak DIB features have been shown tocorrelate with C2 and C3 (the ``C2DIBs''), it is predominantly these DIBs that are observed toward HD62542. The typically strong DIBs λλ5780 and 5797 aredetected but are significantly weaker than toward other lines of sightwith similar reddening. Other commonly observed DIBs (such asλλ4430, 6270, and 6284) remain noticeably absent. Theseobservations further support the suggestion that the line of sighttoward HD 62542 crosses only the core of a diffuse cloud and show thatthe correlation between the C2 DIBs and small carbon chainsis maintained in environments with very large fractions of molecularhydrogen, fH2>0.8. A comparison of CH, CN,C2, and C3 column densities and C2 DIBstrengths toward HD 62542, HD 204827, and HD 172028 suggests that theline of sight toward HD 204827 passes through a diffuse cloud coresimilar to that seen toward HD 62542, as well as what might be referredto as a diffuse cloud envelope. This indicates that the bare core towardHD 62542 may not have significantly different relative chemicalabundances from other diffuse cloud cores and that the C2DIBs may serve as a diagnostic of such cores.
| First Ever Polarimetric Detection of a Wind-Wind Interaction Region and a Misaligned Flattening of the Wind in the Wolf-Rayet Binary CQ Cephei
In this paper we present unfiltered and multiband (i.e., UBVRI)polarimetric observations of the short-period Wolf-Rayet binary CQ Cep.Using the basic assumptions of an optically thin, corotating envelopeand pointlike sources (i.e., BME78 assumptions), we determined theorbital parameters of the system (i.e.,i=99deg+/-1deg andΩ=76deg+/-2deg at the 2 σ level) withan accuracy many times better than any previous work. Residual non-BME78variability around phase 0.0 was present in our data, which we associatewith the polarimetric eclipse of the dense central parts of theWolf-Rayet (W-R) wind by the orbiting O star. We attribute the observedphase lag of -0.15 between our residuals and those expected for astandard polarimetric eclipse to a wind-wind interaction (WWI) regiondistorted by Coriolis forces using the model presented by Marchenko etal. This model was also able to explain the strong wavelength dependenceof the polarimetric amplitudes in our multiband observations. Ouranalysis also reveals important epoch-dependent departures of the matterdistribution from spherical symmetry that were not related to theorbital plane and therefore cannot be the result of tidal interaction.We conclude that binarity is not playing an important role in drivingthe wind of the W-R star in CQ Cep and contributing to the observednonspherical matter distribution. On the other hand, this asymmetrycould be explained by a rotationally induced disk misaligned with theorbital plane.
| Ultraviolet Extinction Properties in the Milky Way
We have assembled a homogeneous database of 417 ultraviolet (UV)extinction curves for reddened sight lines having InternationalUltraviolet Explorer spectra. We have combined these with optical andTwo Micron All Sky Survey photometry, allowing estimates of the ratio oftotal-to-selective extinction, RV, for the entire sample.Fitzpatrick-Massa (FM) parameters have also been found for the entiresample. This is the largest study of parameterized UV extinction curvesyet published, and it covers a wide range of environments, from densemolecular clouds to the diffuse interstellar medium, with extinctionsAV ranging from 0.50 to 4.80. It is the first to extend farbeyond the solar neighborhood and into the Galaxy at large, with 30sight lines having distances greater than 5 kpc. Previously, the longestsight lines with FM parameters and RV extended ~1 kpc. Wefind that (1) the Cardelli, Clayton, & Mathis (CCM) extinction lawapplies for 93% of the sight lines, implying that dust processing in theGalaxy is efficient and systematic, (2) the central wavelength of the2175 Å bump is constant, and (3) the 2175 Å bump width isdependent on environment. Only four sight lines show systematicdeviations from CCM: HD 29647, 62542, 204827, and 210121. These sightlines all sample dense, molecule-rich clouds. The new extinction curvesand values of RV allow us to revise the CCM law.
| Total to Selective Extinction Ratios and Visual Extinctions from Ultraviolet Data
We present determinations of the total to selective extinction ratio R_Vand visual extinction A_V values for Milky Way stars using ultravioletcolor excesses. We extend the analysis of Gnacinski and Sikorski (1999)by using non-equal weights derived from observational errors. We presenta detailed discussion of various statistical errors. In addition, weestimate the level of systematic errors by considering differentnormalization of the extinction curve adopted by Wegner (2002). Ourcatalog of 782 R_V and A_V values and their errors is available in theelectronic form on the World Wide Web.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| Correlations between diffuse interstellar bands and atomic lines
We present and discuss correlations between strengths of the well-known,strong interstellar atomic lines of KI and CaII, and four selected,strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850and 6614. In order to analyse a homogeneous sample of echellehigh-resolution spectra it has been chosen to use measurements fromTerskol Observatory in Northern Caucasus plus a selected number ofhigher resolution observations performed using other instruments. Wedemonstrate that the strength of certain DIBs correlate well withneutral potassium lines and to a much lower degree with ionized calciumlines. This fact suggests that the degree of irradiation of a cloud withUV photons, capable to ionize interstellar atoms, plays a crucial rolein the formation/maintenance of certain molecular species: possiblecarriers of DIBs.
| Subaru Spectropolarimetry of Markarian 573: The Hidden High-Ionization Nuclear Emission-Line Region inside the Dusty Torus
We report the results of our high-quality spectropolarimetricobservation of the narrow-line region in a nearby Seyfert 2 galaxy, Mrk573, with the Subaru Telescope. The polarized flux spectrum of Mrk 573shows not only prominent scattered broad Hα emission but alsovarious narrow forbidden emission lines. We find that the measuredpolarization degree of the observed forbidden emission lines ispositively correlated with the ionization potential of the correspondingions and the critical density of the corresponding transitions. Wediscuss some possible origins of these correlations, and then we pointout that the correlations are caused by the obscuration of thestratified narrow-line region in Mrk 573 by the optically andgeometrically thick dusty torus, matching findings of a previous studyof NGC 4258.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan (NAOJ).
| Detection of the Polarized Broad Emission Line in the Seyfert 2 Galaxy Markarian 573
We report the discovery of the scattered emission from a hiddenbroad-line region (BLR) in a Seyfert 2 galaxy, Mrk 573, based on ourrecent spectropolarimetric observation performed at the SubaruTelescope. This object has been regarded as a type 2 active galacticnucleus (AGN) without a hidden BLR by previous observations. However,our high-quality spectrum of the polarized flux of Mrk 573 showsprominent broad (~3000 km s-1) Hα emission, broad weakHβ emission, and subtle Fe II multiplet emission. Our new detectionof these indications for the presence of the hidden BLR in the nucleusof Mrk 573 is thought to be because of the high signal-to-noise ratio ofour data, but the possibility of a time variation of the scattered BLRemission is also mentioned. Some diagnostic quantities such as the IRAScolor, the radio power, and the line ratio of the emission from thenarrow-line region of Mrk 573 are consistent with the distributions ofsuch quantities of type 2 AGNs with a hidden BLR. Mrk 573 is thought tobe an object whose level of the AGN activity is the weakest among thetype 2 AGNs with a hidden BLR. In terms of the systematic differencesbetween the type 2 AGNs with and without a hidden BLR, we brieflycomment on an interesting Seyfert 2 galaxy, Mrk 266 SW, which maypossess a hidden BLR but has been treated as a type 2 AGNs without ahidden BLR.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
| Polarimetry of the Protoplanetary Nebula AFGL 2688
Our spectroscopic (with resolution R=75 000) and spectropolarimetric(R=15 000) observations with the 6-m telescope of the SpecialAstrophysical Observatory have enabled us to distinguish photosphericand circumstellar features in the optical spectrum of the bipolarprotoplanetary nebula AFGL 2688 for the first time. The linearpolarization of the radiation was measured at 5000 6600 Å. Theemission in the lines of the sodium doublet and the Swan bands is formedin the envelope, and the mechanism exciting the transitions is resonancefluorescence. We conclude that the circumstellar envelope has a lowdensity. Features of the structure of the nebula are discussed based onpublished high-angular-resolution photometric and polarimetric (HSTNICMOS) data.
| Cloud Structure and Physical Conditions in Star-forming Regions from Optical Observations. I. Data and Component Structure
We present high-resolution optical spectra (at ~0.6-1.8 kms-1) of interstellar CN, CH, CH+, Ca I, K I, andCa II absorption toward 29 lines of sight in three star-forming regions,ρ Oph, Cep OB2, and Cep OB3. The observations and data reduction aredescribed. The agreement between earlier measurements of the totalequivalent widths and our results is quite good. However, our higherresolution spectra reveal complex structure and closely blendedcomponents in most lines of sight. The velocity component structure ofeach species is obtained by analyzing the spectra of the six species fora given sight line together. The tabulated column densities and Dopplerparameters of individual components are determined by using the methodof profile fitting. Total column densities along lines of sight arecomputed by summing results from profile fitting for individualcomponents and are compared with column densities from the apparentoptical depth method. A more detailed analysis of these data and theirimplications will be presented in a companion paper.
| On the Possible Role of Carbon Chains as Carriers of Diffuse Interstellar Bands
Because the laboratory gas-phase electronic spectra of only the threepolyatomic bare carbon chains C3, C4 andC5 are available, we have made a further attempt to detectthe origin bands of C4 (3789 Å) and C5 (5109Å) in the diffuse cloud toward ζ Oph. The measurementsprovide an improved 3 σ limit to their column densities:N(C4)<=5×1011 cm-2 andN(C5)<=1×1011 cm-2, with asignal-to-noise ratio (S/N) of 42,000 and 31,000 Å-1,respectively, at a resolution of 110,000. The limits to N(C4)and N(C5) are lower than predictions from the chemical modelused by Roueff and coworkers for such diffuse clouds. In conjunctionwith observations of related, hydrogen-containing polar chains in themillimeter region and laboratory studies of the electronic transitionsof a number of homologous series, these results lead to interestingconclusions about the role of carbon chains as potential carriers of thediffuse interstellar bands (DIBs). First, the abundance and oscillatorstrength of smaller chains, say up to 10 atoms, are too small to accountfor the stronger DIBs. Second, because of the electronic configurationsof these open-shell species, the lowest energy π-π transition doesnot have a large oscillator strength but the higher energy one in the UVdoes, and the chains would have to be prohibitively long for theseabsorptions to shift into the DIB 4000-9000 Å wavelength region.The exceptions are closed-shell systems such as the odd-numbered barecarbon chains, and the ones in the C15-C31 sizerange that have their very strong transitions in this region. Thesespecies should be a major goal for laboratory and subsequentastronomical studies.
| Polarimetric Survey of Low-Mass X-ray Binaries
In many low-mass X-ray binaries the optical light is dominated by anaccretion disk. The bulk of the accretion disk is ionized, and in manymodels the surface layers are threaded by a magnetic field. Polarimetricobservations can give information on the scattering properties of thedisk surface. Polarimetric observations of three low-mass X-raybinaries, Sco X-1, Her X-1 and XTE J 1118+480, in optical wavelenghtsare presented. A strong interstellar polarization component dominatesthe polarization of Sco X-1, but we also detected its weak intrinsiclinear polarization. Marginally variable polarization is possibly seenin XTE J 1118+480. The circular polarization of Sco X-1 and XTEJ1118+480 is consistent with zero. Contrary to previous claims, thelinear polarization of Her X-1 seems to be constant. The observedpolarization can be explained with a combination of interstellarpolarization and electron scattering in the ionized accretion disk. Wealso derive the upper limit for the large-scale ordered magnetic fieldin the accretion disk. Some linear and circular polarimetry for thehigh-mass X-ray binary SS 433 is also presented.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cefeo |
| Ascensione retta: | 21h28m57.76s |
| Declinazione: | +58°44'23.2" |
| Magnitudine apparente: | 8.01 |
| Distanza: | 1030.928 parsec |
| Moto proprio RA: | 0.2 |
| Moto proprio Dec: | -6.7 |
| B-T magnitude: | 8.872 |
| V-T magnitude: | 8.082 |
Cataloghi e designazioni:
|
