Contents
Images
Upload your image
DSS Images Other Images
Related articles
The Puzzle of the Metallic Line Stars
In the puzzle of the metallic line (Am) stars, there still seem to bemissing pieces. While the ``normal'' A stars have elemental abundancesclose to solar, the classical Am stars show stronger absorption linesfor most heavy elements in their spectra. Elements with ionizationpotentials that nearly agree with those of hydrogen or helium havereduced abundances. The Ca II and Sc II lines are especially weak. TheAm stars have no ultraviolet emission lines. They are binaries that,with very few exceptions, have rotational velocities vsini lower than100 km s-1. Of the main-sequence A stars, 20% to 30% are Amstars. Here we rediscuss previous suggestions that tried to explain thepeculiar line strengths in the Am star spectra. In particular, wecompare the well-studied properties of Hyades A and Am stars in order toidentify reasons that can or cannot explain the differences. We findthat accretion of interstellar material by A stars with distortedmagnetic fields, which are weaker than those in peculiar A (Ap) stars,has the best chance of explaining the main characteristics of thepeculiar heavy-element abundances in Am star photospheres.Charge-exchange reactions also seem to be important.
| Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| Properties of dust and detection of Hα emission in LDN 1780
We present ISOPHOT observations between 60 and 200μm and anear-infrared extinction map of the small intermediate-density cloudLynds Dark Nebula (LDN) 1780 (Galactic coordinates l = 359° and b =36.8°). For an angular resolution of 4arcmin, the visual extinctionmaximum is AV = 4.4mag. We have used the ISOPHOT datatogether with the 25-, 60- and 100-μm IRIS maps to disentangle thewarm and cold components of large dust grains that are observed intranslucent clouds and dense clouds. The warm and cold components in LDN1780 have different properties (temperature, emissivity) and spatialdistributions, with the warm component surrounding the cold component.The warm component is mainly in the illuminated side of the cloud facingthe Galactic plane and the Scorpius-Centaurus (Sco-Cen) OB association,as in the case of the HI excess emission. The cold component isassociated with the 13CO(J = 1-0) line integrated(W13), which trace molecular gas at densities of~103cm-3. The warm component has a uniform colourtemperature of 25 +/- 1K (assuming β = 2), and the colourtemperature of the cold component slightly varies between 15.8 and 17.3K(β = 2, ΔT = 0.5K). The ratio between the emission at200μm of the cold component [Icν(200)] andAV is Icν(200)/AV = 12.1+/- 0.7MJysr-1mag-1 and the average ratioτ200/AV = (2.0 +/- 0.2) ×10-4 mag-1. The far-infrared emissivity of thewarm component is significantly lower than that of the cold component.The Hα emission [Iν(Hα)] and AVcorrelate very well; a ratio Iν(Hα)/AV =2.2 +/- 0.1Rayleighmag-1 is observed. This correlation isobserved for a relatively large range of column densities and indicatesthe presence of a source of ionization that can penetrate deep into thecloud (reaching zones with optical extinctions AV of 2mag).Based on modelling predictions, we reject out a shock front as precursorof the observed Hα surface brightness although that process couldbe responsible of the formation of LDN 1780. Using the ratioIν(Hα)/AV, we have estimated anionization rate for LDN 1780 that results to be ~10-16γs-1. We interpret this relatively high value as due toan enhanced cosmic ray radiation rate of ~10 times the standard value.This is the first time such an enhancement is observed in a moderatelydense molecular cloud. The enhancement in the ionization rate could beexplained as the result of a confinement of low-energy (~100MeV) cosmicrays by self-generated magnetohydrodynamics waves in agreement with therecent modelling results of Padoan & Scalo. The origin of the cosmicrays could be from supernovae in the Sco-Cen OB association and/or therunaway ζ Ophiuchus. The observed low- 13CO abundanceand relatively high temperatures of the dust in LDN 1780 support theexistence of a heating source that can come in through the denserregions of the cloud.
| Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data*
The MOST (Microvariability and Oscillations of Stars) satellite obtainsultraprecise photometry from space with high sampling rates and dutycycles. Astronomical photometry or imaging missions in low Earth orbits,like MOST, are especially sensitive to scattered light from Earthshine,and all these missions have a common need to extract target informationfrom voluminous data cubes. They consist of upwards of hundreds ofthousands of two-dimensional CCD frames (or subrasters) containing fromhundreds to millions of pixels each, where the target information,superposed on background and instrumental effects, is contained only ina subset of pixels (Fabry Images, defocused images, mini-spectra). Wedescribe a novel reduction technique for such data cubes: resolvinglinear correlations of target and background pixel intensities. Thisstep-wise multiple linear regression removes only those targetvariations which are also detected in the background. The advantage ofregression analysis versus background subtraction is the appropriatescaling, taking into account that the amount of contamination may differfrom pixel to pixel. The multivariate solution for all pairs oftarget/background pixels is minimally invasive of the raw photometrywhile being very effective in reducing contamination due to, e.g. straylight. The technique is tested and demonstrated with both simulatedoscillation signals and real MOST photometry.
| Blueshifted diffuse interstellar bands in the spectrum of HD 34078
In this paper, we report the very first observation of diffuseinterstellar bands (DIBs) that, in the spectrum of HD 34078 (AE Aur),are blueshifted with respect to the normal position that they have inother objects, where the rest-wavelength velocity frame is determinedusing very sharp interstellar atomic lines or molecular features. Onlyreasonably narrow DIBs seemingly show this effect, which is absent inbroader ones. The result is confirmed independently using threedifferent spectrographs attached to two different telescopes.
| A multiwavelength investigation of the temperature of the cold neutral medium
We present measurements of the HI spin temperatures (Ts) ofthe cold neutral medium (CNM) towards radio sources that are closelyaligned with stars for which published H2 ortho-paratemperatures (T01) are available from ultraviolet (UV)observations. Our sample consists of 18 radio sources close to 16 nearbystars. The transverse separation of the lines of sight of thecorresponding UV and radio observations varies from 0.1 to 12.0 pc atthe distance of the star. The UV measurements do not have velocityinformation, so we use the velocities of low ionization species (e.g.NaI/KI/CI) observed towards these same stars to make a plausibleidentification of the CNM corresponding to the H2 absorption.We then find that T01 and Ts match withinobservational uncertainties for lines of sight with H2 columndensity above 1015.8cm-2, but deviate from eachother below this threshold. This is consistent with the expectation thatin the CNM Ts tracks the kinetic temperature due tocollisions and that T01 is driven towards the kinetictemperature by proton exchange reactions.
| A Model for Atomic and Molecular Interstellar Gas: The Meudon PDR Code
We present the revised ``Meudon'' model of photon-dominated region (PDR)code, available on the Web under the GNU Public License. Generalorganization of the code is described down to a level that should allowmost observers to use it as an interpretation tool with minimal helpfrom our part. Two grids of models, one for low-excitation diffuseclouds and one for dense highly illuminated clouds, are discussed, andsome new results on PDR modelization highlighted.
| The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
We present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA.
| The Remarkable Be Star HD 110432 (BZ Crucis)
HD 110432 (B1e) has gained considerable recent attention because it is ahard, variable X-ray source with local absorption and also because itsoptical spectrum is affected by an extensive Be disk. From time-serialechelle data obtained over 2 weeks during 2005 January and February, wehave discovered several remarkable characteristics in the star's opticalspectrum. The line profiles show rapid variations on some nights, whichcan most likely be attributed to irregularly occurring and short-livedmigrating subfeatures. Such features have been found in spectra ofγ Cas and AB Dor, two stars for which it is believed magneticfields force circumstellar clouds to corotate over the star's surface.The star's optical spectrum also exhibits a number of mainly Fe II andHe I emission features with double-lobed profiles typical of anoptically thin circumstellar disk viewed nearly edge-on. Using spectralsynthesis techniques for the January data, we find that its temperatureand column density are close to 9800 K and roughly3×1022 cm-2, respectively. Its projecteddisk size covers remarkably large 100 stellar areas, and the emittingvolume resides at a surprisingly large distance of 1 AU from the star.Surprisingly, we also find that the absorption wings of the strongestoptical and UV lines in the spectrum extend to at least +/-1000 kms-1, even though the rotational velocity is 300-400 kms-1. We are unable to find a satisfactory explanation forthese extreme line broadenings. Otherwise, HD 110432 and γ Casshare similarly peculiar X-ray and optical characteristics. Theseinclude a high X-ray temperature, erratic X-ray variability ontimescales of a few hours, optical metallic emission lines, andsubmigrating features in optical line profiles. Because of thesesimilarities, we suggest that HD 110432 is a member of a select newclass of ``γ Cas analogs.''
| 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.
| The Discordance of Mass-Loss Estimates for Galactic O-Type Stars
We have determined accurate values of the product of the mass-loss rateand the ion fraction of P+4, M˙q(P+4), for asample of 40 Galactic O-type stars by fitting stellar wind profiles toobservations of the P V resonance doublet obtained with FUSE, ORFEUSBEFS, and Copernicus. When P+4 is the dominant ion in thewind [i.e., 0.5<~q(P+4)<=1], M˙q(P+4)approximates the mass-loss rate to within a factor of <~2. Theorypredicts that P+4 is the dominant ion in the winds of O7-O9.7stars, although an empirical estimator suggests that the range O4-O7 maybe more appropriate. However, we find that the mass-loss rates obtainedfrom P V wind profiles are systematically smaller than those obtainedfrom fits to Hα emission profiles or radio free-free emission bymedian factors of ~130 (if P+4 is dominant between O7 andO9.7) or ~20 (if P+4 is dominant between O4 and O7). Thesediscordant measurements can be reconciled if the winds of O stars in therelevant temperature range are strongly clumped on small spatial scales.We use a simplified two-component model to investigate the volumefilling factors of the denser regions. This clumping implies thatmass-loss rates determined from ``ρ2'' diagnostics havebeen systematically overestimated by factors of 10 or more, at least fora subset of O stars. Reductions in the mass-loss rates of this size haveimportant implications for the evolution of massive stars andquantitative estimates of the feedback that hot-star winds provide totheir interstellar environments.
| Measurements of the f-Values of the Resonance Transitions of Ni II at 1317.217 and 1370.132 Å
We have retrieved high-resolution UV spectra of 69 hot stars from theHST archive and determined the strengths of the interstellar Ni IIabsorption features at 1317.217 Å arising from the ground3d92D5/2 electronic state to the3d8(1G)4p2Fo5/2excited level. We then compared them to absorptions to either the3d8(3F)4p2Do5/2or3d8(3P)4p2Po3/2upper levels occurring at, respectively, λ=1741.553 Å(covered in the spectra of 21 of the stars) and 1370.132 Å (seenfor the remaining 48 stars). All spectra were recorded by the either theE140M, E140H, or E230H gratings of the Space Telescope ImagingSpectrograph. By comparing the strengths of the two lines in eachspectrum and evaluating a weighted average of all such comparisons, wehave found that the f-value of the 1317 Å line is 1.34+/-0.019times the one at 1741 Å, and 0.971+/-0.014 times the one at 1370Å. We adopt as a comparison standard an experimentally determinedf-value for the 1741 Å line (known to 10% accuracy), so thatf(1317 )=0.0571+/-0.006. It follows from this f-value and our measuredline-strength ratios that f(1370 )=0.0588+/-0.006. As an exercise tovalidate our methodology, we compared the 1317 Å transition toanother Ni II line at 1454.842 Å to the3d8(1D)4p2Do5/2level and arrived at an f-value for the latter that is consistent with apreviously measured experimental value to within the expected error.Based on observations from the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Extinction and metal column density of HI regions up to redshift z ≃ 2
We used the photometric database of the Sloan Digital Sky Survey (SDSS)to estimate the reddening of 13 SDSS quasars selected on the basis ofthe presence of zinc absorption lines in an intervening Damped Lyα (DLA) system. In 5 of these quasars the reddening is detected at2σ confidence level in two independent color indices of theSDSS ugriz photometric system. A detailed analysis of the data supportsan origin of the reddening in the intervening absorbers. We used theserare measurements of extinction in DLA systems to probe the relationbetween extinction and metal column density in the interval ofabsorption redshift 0.7 z 2.0. We find that the meanextinction in the V band per atom of iron in the dust is remarkablysimilar to that found in interstellar clouds of the Milky Way. Thisresult lends support to previous estimates of the dust obscurationeffect in DLA systems based on a Milky Way extinction/metal columndensity relation. We propose a simple mechanism, based on dust graindestruction/accretion properties, which may explain the approximateconstancy of the extinction per atom of iron in the dust.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| The effects of metallicity, radiation field and dust extinction on the charge state of PAHs in diffuse clouds: implications for the DIB carrier
Context.The unidentified diffuse interstellar bands (DIB) are observedthroughout the Galaxy, the Local Group and beyond. Their carriers arepossibly related to complex carbonaceous gas-phase molecules, such as(cationic) polycyclic aromatic hydrocarbons and fullerenes. Aims.In order to reveal the identity of the DIB carrier we investigatethe effects of metallicity, radiation field and extinction curve on thePAH charge state distribution, and thus the theoretical emergent PAHspectrum, in diffuse interstellar clouds. This behaviour can then belinked to that of the DIB carrier, thus giving insight into itsidentity. Methods.We use radiative transfer and chemical models tocompute the physical and chemical conditions in diffuse clouds withGalactic and Magellanic Cloud types of interstellar dust and gas.Subsequently, the PAH charge state distributions throughout these cloudsare determined. Results.We find that the fraction of PAH cationsis much higher in the Magellanic Cloud environments than in the MilkyWay, caused predominantly by the respective lower metallicities, andmitigated by the steeper UV extinction curve. The fraction of anions ismuch lower in a low metallicity environment. The predicted DIB strengthof cationic PAH carriers is similar to that of the Milk Way for the LMCand 40% for the SMC due to the overall metallicity. Stronger DIBs couldbe expected in the Magellanic Clouds if they emanate from clouds thatare exposed to an average interstellar radiation field that issignificantly stronger than in the Milky Way, although photo-destructionprocesses could possibly reduce this effect, especially for the smallerPAHs. Our results show that the presence and absence of DIB carriers inthe Magellanic Cloud lines of sight can be tied to the PAH chargebalance which is driven by metallicity, UV radiation and dust extinctioneffects.
| A search for fine structure inside high resolution profiles of weak diffuse interstellar bands
This paper presents a survey of the high-resolution profiles ofselected, moderately weak diffuse interstellar bands (DIBs) between 4725and 6730 Å. In very high signal-to-noise spectra, obtained as aresult of averaging several individual exposures of reddened, early-typestars that show Doppler splitting of <2 km s-1 ininterstellar gas lines, the profiles seem to have a substructure. Thissupports the molecular origin hypothesis for DIBs. We studied theprofiles of the diffuse interstellar bands at wavelengths of 4726.33,4963.85, 5418.89, 5541.74, 5544.95, 5546.46, 5762.73, 5766.05, 5769.09,6439.41, 6445.53, 6449.16, 6729.28 Å.
| The Large Magellanic Cloud: diffuse interstellar bands, atomic lines and the local environmental conditions
The Large Magellanic Cloud (LMC) offers a unique laboratory to study thediffuse interstellar bands (DIBs) under conditions that are profoundlydifferent from those in the Galaxy. DIB carrier abundances depend onseveral environmental factors, in particular the local UV radiationfield. In this paper we present measurements of twelve DIBs in fivelines of sight to early-type stars in the LMC, including the 30Doradus region. From the high resolution spectra obtained withVLT/UVES we also derive environmental parameters that characterise thelocal interstellar medium (ISM) in the probed LMC clouds. These includethe column density components (including total column density) for theatomic resonance lines of Na I, Ca II, Ti II, K I. In addition, wederive the H I column density from 21 cm line profiles, thetotal-to-selective visual extinction RV and the gas-to-dustratio N(H I)/A_V. Furthermore, from atomic line ratios we derive theionisation balance and relative UV field strength in these environments.We discuss the properties of the LMC ISM in the context of DIB carrierformation. The behaviour of DIBs in the LMC is compared to that of DIBsin different local environmental conditions in the Milky Way. A keyresult is that in most cases the diffuse band strengths are weak (up tofactor 5) with respect to Galactic lines of sight of comparablereddening, EB-V. In the line of sight towards Sk -69223 the 5780 and 5797 Å DIBs are very similar instrength and profile to those observed towards HD144217, which is typical of an environment exposed to a strongUV field. From the velocity analysis we find that DIB carriers (towardsSk -69 243) are better correlated with the ionisedspecies like Ca II than with neutrals (like Na I and CO). The mostsignificant parameter that governs the behaviour of the DIB carrier isthe strength of the UV field.
| 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.
| α Eridani: rotational distortion, stellar and circumstellar activity
We explore the geometrical distortion and the stellar and circumstellaractivity of α Eri (HD 10144), the brightest Be star in the sky. Wepresent a thorough discussion of the fundamental parameters of theobject for an independent determination of its rotational distortion. Weused stellar atmosphere models and evolutionary tracks calculated forfast rotating early-type stars. If the star is a rigid rotator, itsangular velocity rate is Ω/Ωc ≃ 0.8, sothat its rotational distortion is smaller than the one inferred fromrecent interferometric measurements. We then discuss the stellar surfaceactivity using high resolution and high S/N spectroscopic observationsof He i and Mg ii lines, which concern a period of Hα lineemission decline. The variations in the He i lines are interpreted asdue to non-radial pulsations. Time series analysis of variations wasperformed with the cleanest algorithm, which enabled us to detect thefollowing frequencies: 0.49, 0.76, 1.27 and 1.72 c/d and pulsationdegrees ℓ (3{-}4) for ν = 0.76 c/d; ℓ (2{-}3) forν = 1.27 c/d and ℓ (3{-}4) for ν = 1.72 c/d. The studyof the absolute deviation of the He i λ6678 Å spectral linerevealed mass ejection between 1997 and 1998. We conclude that thelowest frequency found, ν = 0.49 c/d, is due to the circumstellarenvironment, which is present even at epochs of low emission in thewings of He i λ6678 Å and Mg ii λ4481 Å lineprofiles, as well as during nearly normal aspects of the Hα line.This suggests that there may be matter around the star affecting somespectral regions, even though the object displays a B-normal like phase.The long-term changes of the Hα line emission in α Eri arestudied. We pay much attention to the Hα line emission at theepoch of interferometric observations. The Hα line emission ismodeled and interpreted in terms of varying structures of thecircumstellar disc. We conclude that during the epoch of interferometricmeasurements there was enough circumstellar matter near the star toproduce λ 2.2 μm flux excess, which could account for theoverestimated stellar equatorial angular diameter. From the study of thelatest BrightleftharpoonsBe phase transition of α Eri we concludedthat the Hα line emission formation regions underwent changes sothat: a) the low Hα emission phases are characterized by extendedemission zones in the circumstellar disc and a steep outward matterdensity decline; b) during the strong Hα emission phases theemitting regions are less extended and have a constant densitydistribution. The long-term variations of the Hα line in αEri seem to have a 14-15 year cyclic BrightleftharpoonsBe phasetransition. The disc formation time scales, interpreted as the periodsduring which the Hα line emission increases from zero to itsmaximum, agree with the viscous decretion model. On the other hand, thetime required for the disc dissipation ranges from 6 to 12 years whichquestions the viscous disc model.
| Dust extinction and absorption: the challenge of porous grains
In many models of dusty objects in space the grains are assumed to becomposite or fluffy. However, the computation of the optical propertiesof such particles is still a very difficult problem. We analyze how theincrease of grain porosity influences basic features of cosmic dust -interstellar extinction, dust temperature, infrared bands and millimeteropacity. It is found that an increase of porosity leads to an increaseof extinction cross sections at some wavelengths and a decrease atothers depending on the grain model. However, this behaviour issufficient to reproduce the extinction curve in the direction of thestar σ Sco using current solar abundances. In the case of the starζ Oph our model requires larger amounts of carbon and iron in thedust-phase than is available. Porous grains can reproduce the flatextinction across the 3 - 8 μm wavelength range measured for severallines of sight by ISO and Spitzer. Porous grains are generally coolerthan compact grains. At the same time, the temperature of very porousgrains becomes slightly larger in the case of the EMT-Mie calculationsin comparison with the results found from the layered-sphere model. Thelayered-sphere model predicts a broadening of infrared bands and a shiftof the peak position to larger wavelengths as porosity grows. In thecase of the EMT-Mie model variations of the feature profile are lesssignificant. It is also shown that the millimeter mass absorptioncoefficients grow as porosity increases with a faster growth occurringfor particles with Rayleigh/non-Rayleigh inclusions. As a result, forvery porous particles the coefficients given by two models can differ bya factor of about 3.
| Looking for Discrete UV Absorption Features in the Early-Type Eclipsing Binaries μ1 Scorpii and AO Cassiopeiae
A search for discrete absorption components in the ultraviolet spectraof the early-type binaries μ1 Scorpii and AO Cassiopeiaehas been undertaken by analyzing material secured with the InternationalUltraviolet Explorer satellite during an exclusively assigned intervalof nearly 50 hr. While the spectra of μ1 Sco definitely donot show the presence of such lines, the spectra of AO Cas do confirmthem and permit us to draw some conclusions about where they may beformed.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88m/HIDES Spectra
The LTE abundances of sulfur (S) of 21 metal-poor stars and one normalstar were explored in the metallicity range of -3 < [Fe/H] ≤ 0,based on the equivalent widths of the S I (1) 9212, 9237Å and S I(6) 8693, 9894Å lines measured on high-resolution spectra, whichwere observed by the OAO 1.88-m telescope equipped with HIDES. Our mainresults are: (1) The abundances derived from the S I (6) lines areconsistent with those from the S I (1) lines among our sample stars inthe range of [Fe/H] > -2 with an average difference of +0.03 ±0.05 dex, whereas a significant discrepancy is observed in the range of[Fe/H] ≤ -2. (2) The behavior of [S(6)/Fe], versus [Fe/H] of ourhalo sample stars exhibits a nearly flat trend with an average of +0.62± 0.09 dex in the range of -3 < [Fe/H] < -1.25, and shows adistribution around +0.29 dex in -1.25 ≤ [Fe/H] ≤ -0.7. Oursample stars with -1.25 ≤ [Fe/H] ≤ -0.5 follow an increasingtrend with decreasing [Fe/H]. The behavior of [S(1)/Fe] of our samplestars also shows essentially the same trend as [S(6)/Fe], though it isquantitatively different. (3) The S behavior in the range of -3 <[Fe/H] ≤ 0 inferred from the abundances of multiplets 6 and 1 arequalitatively consistent with each other, and may be represented by acombination of a nearly flat trend and a linearly increasing trend withdecreasing [Fe/H]. A transition of the trend is likely to occur at[Fe/H] -1.5 dex.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction
We relate the equivalent widths of 11 diffuse interstellar bands,measured in the spectra of 49 stars, to different colour excesses in theultraviolet. We find that most of the observed bands correlatepositively with the extinction in the neighbourhood of the2175-Åbump. Correlation with colour excesses in other parts of theextinction curve is more variable from one diffuse interstellar band toanother; we find that some diffuse bands (5797, 5850 and 6376 Å)correlate positively with the overall slope of the extinction curve,while others (5780 and 6284 Å) exhibit negative correlation. Wediscuss the implications of these results on the links between thediffuse interstellar band carriers and the properties of theinterstellar grains.
| Canada's Little Space Telescope That Could: Another Year of Scientific Surprises From the MOST Microsatellite
At CASCA 2004 in Winnipeg, I announced the first scientific results fromthe MOST (Microvariability & Oscillations of STars) mission,launched in summer 2003. These included the controversial null detectionof acoustic oscillations in the light output of Procyon, at odds withtheory and groundbased spectroscopy, and the first direct measurement ofdifferential rotation in a star other than the Sun, kappa 1 Ceti. A yearlater, by CASCA 2005 in Montreal, I'll be able to share even moreexciting astrophysics, including: ultraprecise photometric studies ofexoplanetary systems like 51 Pegasi, tau Bootis, and HD 209458; newperspectives on massive stars like zeta Oph and the Wolf-Rayet star WR123; the definitive eigenfrequency spectrum of a pulsating chemicallypeculiar star (HR 1217), which provides an acid test for theories ofmagneto-acoustic coupling; and seismology of pulsating protostars in theopen cluster NGC 2264. I'll also return to Procyon, shedding more lighton the oscillation controversy by showing how MOST observations compareto 3-D hydrodynamical simulations of granulation in that star. Andbetween February and May, MOST will monitor a G dwarf and a K dwarf tosearch for true analogues of the solar 5-min oscillations. Too much tofit into 50 minutes, but I'll try to convey a broad flavour of thediverse MOST science (and, as usual, I'll talk really, really fast).
| A near-IR stellar spectral library in the H band using the Mt. Abu telescope
We present an H band near-infrared (NIR) spectral library of 135 solartype stars covering spectral types O5-M3 and luminosity classes I-V asper MK classification. The observations were carried out with 1.2 meterGurushikhar Infrared Telescope (GIRT), at Mt. Abu, India using a NICMOS3HgCdTe 256 x 256 NIR array based spectrometer. The spectra have amoderate resolution of 1000 (about A) at the H band and have beencontinuum shape corrected to their respective ective temperatures. Adetailed paper describing all the aspects of this spectral library hasbeen published in Ranade et. al. (2004).
| A Medium Resolution Near-Infrared Spectral Atlas of O and Early-B Stars
We present intermediate-resolution (R~8000-12,000) high signal-to-noise(S/N) H- and K-band spectroscopy of a sample of 37 optically visiblestars, ranging in spectral type from O3 to B3 and representing mostluminosity classes. Spectra of this quality can be used to constrain thetemperature, luminosity, and general wind properties of OB stars, whenused in conjunction with sophisticated atmospheric model codes. Mostimportant is the need for moderately high resolutions (R>=5000) andvery high signal-to-noise (S/N>=150) spectra for a meaningful profileanalysis. When using near-infrared spectra for a classification system,moderately high signal-to-noise (S/N~100) is still required, though theresolution can be relaxed to just a thousand or two. In the Appendix weprovide a set of very high-quality near-infrared spectra of Brackettlines in six early-A dwarfs. These can be used to aid in the modelingand removal of such lines when early-A dwarfs are used for telluricspectroscopic standards.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | へびつかい座 |
| Right ascension: | 16h37m09.50s |
| Declination: | -10°34'02.0" |
| Apparent magnitude: | 2.56 |
| Distance: | 140.449 parsecs |
| Proper motion RA: | 0 |
| Proper motion Dec: | 0 |
| B-T magnitude: | 2.575 |
| V-T magnitude: | 2.546 |
Catalogs and designations:
|
INHABITED SKY
