Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Observations of Rotationally Resolved C3 in Translucent Sight Lines
The rotationally resolved spectrum of theA1Πu<--X1Σ+g000-000 transition of C3, centered at 4051.6 Å, hasbeen observed along 10 translucent lines of sight. To interpret thesespectra, a new method for the determination of column densities andanalysis of excitation profiles involving the simulation and fitting ofobserved spectra has been developed. The populations of lower rotationallevels (J<=14) in C3 are best fitted by thermaldistributions that are consistent with the kinetic temperaturesdetermined from the excitation profile of C2. Just as in thecase of C2, higher rotational levels (J>14) ofC3 show increased nonthermal population distributions inclouds that have been determined to have total gas densities below ~500cm-3.
| Some Diffuse Interstellar Bands Related to Interstellar C2 Molecules
We have investigated the correlations between the equivalent widths of21 selected diffuse interstellar bands (DIBs) and the correspondinginterstellar column densities N(C2), N(CN), and N(CH), toward53 stars with color excesses 0.11<=E(B-V)<=1.99. The observationaldata were derived primarily from echelle spectra acquired at R=38,000 aspart of our extensive, continuing survey of the bands. All but six ofthe 53 final spectra show signal-to-noise ratios >=800 at 5780Å. The principal result presented here is that seven of the 21bands prove to be examples of ``the C2 DIBs,'' a class ofweak, narrow bands whose normalized equivalent widthsWλ(X)/Wλ (λ6196) are wellcorrelated specifically with N(C2)/E(B-V) via power laws. Incontrast, the similarly normalized equivalent widths of the 14 other,well-known DIBs analyzed here are uncorrelated, or weaklyanticorrelated, with N(C2)/E(B-V), to within theobservational uncertainties. Thus, the polyatomic molecule(s) presumedto cause these seven C2 DIBs may bear a direct chemicalrelation to C2 that is not shared by the polyatomic moleculesputatively responsible for the other 14 bands. The C2 DIBsalso show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) inour particular sample of light paths, although generally with shallowerslopes in the case of N(CN) and with greater scatter in the case ofN(CH). Eleven additional C2 DIBs are also identified but arenot analyzed here. Among the 18 C2 DIBs identified, fourapparently have not been previously detected. The λ4963 band isgenerally the strongest of the 18 C2 DIBs, while theλ4734 band shows the most sensitive correlation withN(C2).Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Observations of C3 in Translucent Sight Lines
The A1Πu<--X1Σ+g transition of the simplestpolyatomic carbon chain molecule, C3, at 4051.6 Å hasbeen searched for toward reddened stars where abundant C2 hadbeen reported and toward other stars with high color excess. Absorptionfrom C3 has been detected toward 15 stars with color excessE(B-V) from 0.33 to 1.12. The observed C3 column densities,ranging from 1012 to 1013 cm-2, arewell correlated with the corresponding C2 column densities,with N(C2)/N(C3)~40, indicating their closechemical relation. The carbon-rich sight line toward HD 204827 (forwhich no previous C2 observation had been reported) has byfar the highest C3 and C2 column densities. Thechemistry of formation of C3 from C2 is discussed.A search for the next strongest 020-000 vibronic band was unsuccessfulas a result of the low Franck-Condon factor and interference with astellar line. Searches for C4 and C5 werenegative.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447
| On the behavior of the Cii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars
With the aim of investigating the possible particular behavior of carbonin a sample of chemically peculiar stars of the main sequence withoutturning to modeling, we performed spectroscopic observations of threeimportant and usually prominent single ionized carbon lines: 4267.261,6578.052 and 6582.882 Å. In addition, we observed a large numberof standard stars in order to define a kind of normality strip, usefulfor comparing the observed trend for the peculiar stars. We paidparticular attention to the problem of the determination of fundamentalatmospheric parameters, especially for the chemically peculiar stars forwhich the abundance anomalies change the flux distribution in such a waythat the classical photometric methods to infer effective temperaturesand gravities parameter cannot be applied. Regarding CP stars, we founda normal carbon abundance in Hg-Mn, Si (with some exceptions) and Hestrong stars. He weak stars are normal too, but with a large spread outof the data around the mean value. A more complicated behavior has beennoted in the group of SrCrEu stars: four out of seven show a strongoverabundance, being the others normal.
| On the Periods of the Magnetic CP Stars
An HR diagram annotated to show several ranges of photometericallydetermined periods has been constructed for the magnetic CP stars whoseperiods have been determined by the author and his collaborators. Thedistribution of periods reflects both the initial conditions as well asthe subsequent stellar histories. Since the stellar magnetic field doesnot penetrate the convective core, eventually a shear zone near thecore-radiative envelope boundary may develop which produces turbulenceand modifies the field. Many, but not all, of the most rapidly rotatingmCP stars are close to the ZAMS and some of the least rapidly rotatingmCP stars are the furthest from the ZAMS.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| uvby photometry of the magnetic CP stars 20 Eridani, HR 1297, 12 Canis Majoris, and HD 134214
Differential Strömgren uvby observations from the Four CollegeAutomated Photoelectric Telescope (FCAPT) are presented for the mCPstars 20 Eri, HR 1297, 12 CMa, and HD 134214. Adelman & Boyce foundsuggestions that the light curves of 20 Eri change between observingseasons. Observations for two additional seasons confirm this behaviorand lead to an improved period of 1.928890 days. For HR 1297 a newperiod of 15.7490 days was derived. The uvby light curves now appear tobe the same for different observing seasons. 12 CMa is found to be aphotometric variable with the 2.18010 day period determined frommagnetic measurements by Bohlender et al. Observations of the rapidlyoscillating Ap star HD 134214 confirm the lack of photometricvariability which is not associated with the rapid oscillations. Tables2, 3, 4 and 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| Variability of the uvby Light Curves of the Magnetic CP Star 108 Aquarii
Light curves based on recent Strömgren uvby observations of the mCPstar 108 Aqr show definite changes with respect to published uvby dataof this star. This behavior is interpreted as being due to theprecession of the rotational axis changing slowly and moderately theobserved hemisphere of this star at a given rotational phase.
| CO and C_2 Absorption toward W40 IRS 1A
The H II region W40 harbors a small group of young, hot stars behindroughly 9 mag of visual extinction. We have detected gaseous carbonmonoxide (CO) and diatomic carbon (C_2) in absorption toward the starW40 IRS 1a. The J=2-0 R0, R1, and R2 lines of ^12CO at 2.3 μm weremeasured using the CSHELL on the NASA Infrared Telescope Facility (withupper limits placed on R3, R4, and R5) yielding an N_CO of(1.1+/-0.2)x10^18 cm^-2. Excitation analysis indicates T_kin>7 K. ThePhillips system of C_2 transitions near 8775 Å was measured usingthe Kitt Peak 4 m telescope and echelle spectrometer. Radiative pumpingmodels indicate a total C_2 column density of (7.0+/-0.4)x10^14 cm^-2,two excitation temperatures (39 and 126 K), and a total gas density ofn~250 cm^-3. The CO ice band at 4.7 μm was not detected, placing anupper limit on the CO depletion of delta<1%. We postulate that thesight line has multiple translucent components and is associated withthe W40 molecular cloud. Our data for W40 IRS 1a, coupled with othersight lines, shows that the ratio of CO/C_2 increases from diffusethrough translucent environs. Finally, we show that the hydrogen-to-dustratio seems to remain constant from diffuse to dense environments,whereas the CO-to-dust ratio apparently does not.
| Variability of the Hei5876 { Angstroms} line in early type chemically peculiar stars
Chemically peculiar stars present spectral and photometric variabilitywith a single period. In the oblique rotator model, the non homogeneousdistribution of elements on the stellar surface is at the origin of theobserved variations. As to helium weak stars, it has been suggested thatphotometric and helium line equivalent width variations are out ofphase. To understand the behaviour of helium in CP stars, we haveobtained time resolved spectra of the Hei5876\ { Angstroms} line for asample of 16 chemically peculiar stars in the spectral range B3 - A1 andbelonging to different sub-groups. The Hei5876 { Angstroms} line is tooweak to be measured in the spectra of the stars HD 24155, HD 41269, andHD 220825. No variation of the equivalent width of the selected He linehas been revealed in the stars HD 22920, HD 24587, HD 36589, HD 49606,and HD 209515. The equivalent width variation of the Hei5876 {Angstroms} line is in phase with the photometric variability for thestars HD 43819, HD 171247 and HD 176582. On the contrary it is out ofphase for the stars HD 28843, HD 182255 and HD 223640. No clear relationhas been found for the stars HD 26571 and HD 177003.
| UVBY Photometry of the Magnetic Chemically Peculiar Stars HR 1297, 36 Aurigae, and HR 2722 and the Nonmagnetic Chemically Peculiar Stars HR 1576 and alpha CANCRI
Differential Strömgren uvby photometric observations from the FourCollege Automated Photoelectric Telescope are presented for the magneticCP stars HR 1297, 36 Aur, and HR 2722 and the nonmagnetic CP stars HR1576 and alpha Cnc. Both the moderately rotating HR 1576, amercury-manganese star, and alpha Cnc, a metallic-line star, arenonvariable. For HR 1297 we refined Winzer's period to 1.06457 days. Ourperiod of 14.366 days for 36 Aur is an alias of Winzer's period and isin keeping with the sharp-lined nature of this star. We found a moreaccurate period of 2.31523 days for the low-amplitude variable HR 2722.
| On the HIPPARCOS photometry of chemically peculiar B, A, and F stars
The Hipparcos photometry of the Chemically Peculiar main sequence B, A,and F stars is examined for variability. Some non-magnetic CP stars,Mercury-Manganese and metallic-line stars, which according to canonicalwisdom should not be variable, may be variable and are identified forfurther study. Some potentially important magnetic CP stars are noted.Tables 1, 2, and 3 are available only in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The observed periods of AP and BP stars
A catalogue of all the periods up to now proposed for the variations ofCP2, CP3, and CP4 stars is presented. The main identifiers (HD and HR),the proper name, the variable-star name, and the spectral type andpeculiarity are given for each star as far as the coordinates at 2000.0and the visual magnitude. The nature of the observed variations (light,spectrum, magnetic field, etc.) is presented in a codified way. Thecatalogue is arranged in three tables: the bulk of the data, i.e. thosereferring to CP2, CP3, and CP4 stars, are given in Table 1, while thedata concerning He-strong stars are given in Table 2 and those foreclipsing or ellipsoidal variables are collected in Table 3. Notes arealso provided at the end of each table, mainly about duplicities. Thecatalogue contains data on 364 CP stars and is updated to 1996, October31. This research has made use of the SIMBAD database, operated at CDS,Strasbourg, France.
| Magnesium abundance in main sequence B-type and magnetic chemically peculiar stars.
The abundance of magnesium for a sample of 19 main sequence B-type and41 magnetic chemically peculiar stars has been derived by spectrumsynthesis analysis of the MgII448.1nm line under the LTE assumption. Thelogarithm of the average Mg abundance for the main sequence stars is log(N(MgII)/N(Tot))=-4.28+/-0.19. Comparing magnetic chemically peculiar(Cp) and main sequence stars with equal effective temperature andgravity, one finds that the magnesium abundance tends to be lower inpeculiar stars with the exception of helium rich stars where thiselement can be overabundant. In Cp stars with effective temperature ofabout 14000K, the magnesium abundance does not depend on gravity,microturbulent velocity or rotational period. There appears to exist acorrelation between the magnesium abundance and the surface magneticfield, with the stars poorest in magnesium presenting the strongestmagnetic fields. In accord with the theory of magnetically controlleddiffusion - which predicts a non-homogeneous distribution of magnesiumover the stellar surface and stratification in the photosphere - somepeculiar stars show evidence of spectral variability with the rotationalphase; the respective magnesium abundances of the HgMn stars HD49606 andHD78316 depend on optical depth.
| The Distribution of Dust Clouds in the Interstellar Medium
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...457..764D&db_key=AST
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The local distribution of NA I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambdaapproximately 75,000) of the interstellar Na I D lines at 5890 A toward80 southern hemisphere early-type stars located in the localinterstellar medium (LISM). Combining these results with other sodiummeasurements taken from the literature, we produce galactic maps of thedistribution of neutral sodium column density for a total of 293 starsgenerally lying within approximately 250 pc of the Sun. These mapsreveal the approximate shape of the mid-plane contours of the rarefiedregion of interstellar space termed the Local Bubble. Its shape is seenas highly asymmetric, with a radius ranging from 30 to 300 pc, and withan average radius of 60 pc. Similar plots of the Galactic mid-planedistribution of sources emitting extreme ultraviolet radiation show thatthey also trace out similar contours of the Local Bubble derived from NaI absorption measurements. We conclude that the Local Bubble absorptioninterface can be represented by a hydrogen column density,NuETA = 2 x 1019 cm-2, which explainsboth the local distribution of Na I absorption and the observed galacticdistribution of extreme ultraviolet sources. The derived mid-planecontours of the Bubble generally reproduce the large-scale featurescarved out in the interstellar medium by several nearby galactic shellstructures.
| Chemical transitions for interstellar C2 and CN in cloud envelopes
Observations were made of absorption from CH, C2, and CN towardmoderately reddened stars in Sco, OB2, Ceo OB3, and Taurus/Auriga. Forthese directions, most of the reddening is associated with a singlecloud complex, for example, the rho Ophiuchus molecular cloud, and as aresult, the observations probe moderately dense material. When combinedwith avaliable data for nearby directions, the survey provides the basisfor a comprehensive analysis of the chemistry for these species. Thechemical transitions affecting C2 and CN in cloud envelopes wereanalyzed. The depth into a cloud at which a transition takes place wascharacterized by tauuv, the grain optical depth at 1000 A.One transition at tauuv approx. = 2, which arises from, theconversion of C(+) into CO, affects the chemistries for both moleculesbecause of the key role this ion plays. A second one involvingproduction terms in the CN chemistry occurs at tauuv ofapprox. = 3; neutral reactions which C2 and CH is more important atlarger values for tauuv. The transition fromphotodissociation to chemical destruction takes place attauuv approx. = 4.5 for C2 and CN. The observational data forstars in Sco OB2, Cep OB3, and Taurus/Auriga were studied with chemicalrate equations containing the most important production and destructionmechanisms. Because the sample of stars in Sco OB2 includes sight lineswith Av ranging from 1-4 mag, sight lines dominated byphotochemistry could be analyzed separately from those controlled bygas-phase destruction. The analysis yielded values for two poorly knownrate constants for reactions involved in the production of CN; thereactions are C2 + N yields CN + C and C(+) + NH yields all products.The other directions were analyzed with the inferred values. Thepredicted column densities for C2 and CN agree with the observed valuesto better than 50%, and in most instances 20%. When combining theestimates for density and temperature derived from chemical modeling andmolecular excitation for a specific cloud, such as the rho Ophiuchusmolecular cloud, the portion of the cloud envelope probed by C2 and CNabsorption was found to be in pressure equilibrium.
| Millimetre observations of southern translucent clouds
Millimetre emission line observations of ^12^CO J=1-0 are presented fora sample of translucent molecular clouds previously studied by opticalabsorption lines towards background stars. The CO maps provideinformation on the extent, structure, mass and kinematics of theregions. In some clouds, the emission is characterized by a singlecomponent which does not vary much across the region. In other cases,multiple close-lying components or sheets are found, which may vary in aself-similar manner across the cloud. None of the clouds is close tosimple virial equilibrium. Sensitive searches for ^13^CO and C^18^OJ=1-0 emission have been made towards the stellar positions. C^18^O hasbeen detected in only 5 clouds, most of which have extinctions well inexcess of 2 mag. The inferred ^13^CO/C^18^O abundance ratios range from7-25, and the lower limits from >13 to >35. These values are asmuch as five times larger than the overall interstellar([^13^C].[^16^O])/([^12^C] .[^18^O]) ratio, suggesting thatisotope-selective photodissociation plays a role in at least some of theclouds. Searches for other molecules at millimetre wavelengths have beenmade for a few of the best characterized clouds. Surprisingly, noemission was detected from the C_2_H or C_3_H_2_ molecules, even thoughthe abundances of diatomic C_2_ and CH are quite large. On the otherhand, the abundance of HCO^+^ appears comparable to that found in denserclouds, and the abundance of HCN may be up to an order of magnitudelarger than the predictions of models in two clouds.
| CN rotational excitation
We report the results of a search for new lines of sight in which tostudy the CN excitation and a statistical analysis of all the excitationtemperatures measured using interstellar CN. This data set stronglyconfirms that the cosmic background radiation (CBR) is the dominantcontributor to the excitation of CN, and demonstrates the homogeneity ofthe CBR. Thirty-five observations is a sufficiently large sample to lookfor the presence of systematic effects in the CN excitation. Theweighted average of the CN excitation temperatures exceeds the T(CBR)obtained by COBE and the Canadian rocket by 82 +/- 30 mK. With the aimof looking at the origin of this difference, we have considered indetail the known mechanisms that could contribute to exciting the CNmolecule. None of the data necessary to quantify these mechanisms are ofsufficient quality to provide a clean explanation of the observeddifference.
| The ultraviolet and visible spectrum of the polycyclic aromatic hydrocarbon C10H8(+) - Possible contributions to the diffuse interstellar bands and to the ultraviolet-visible extinction
The properties of the cation of the PAH naphthalene (C10H8(+)) isolatedin inert gas matrices under conditions relevant to astrophysicalenvironments are described. The band at 6741 A is the strongest andfalls close to the weak 6742 A diffuse interstellar bands (DIBs). Fiveother weaker bands also fall remarkably close to the positions of knownDIBs. A very intense and broad continuum extended from the UV to thevisible, which seems to be associated with the ion, is reported. Themolar absorption coefficient at the peak of the continuum is 2.0 x 10exp 6 cu dm/mol cm. If a continuum is a general property of PAH cations,this characteristic will have a strong impact on the understanding ofhow PAHs convert interstellar UV and visible radiation into IRradiation.
| The Hyades supercluster in the FK5
The members of the Hyades supercluster brighter than about M(V) = + 4mag and contained in the FK5, or having nearly FK5 quality propermotions, show a convergent point of (A,D) = (6h, + 6.5 deg). The Hyadescluster stars in the FK5 have a mean distance of 46.7 pc. Thesupercluster, as well as the Hyades and Praesepe cluster, populationsrepresent at least three age groups. Standard models indicate ages of 3to 4, 6, and 8 x 10 exp 8 yr, whereas model ages with convectiveovershoot are nearly twice this. Most of the Am and USPC stars in thesupercluster are of the same age. The Ap stars mark the onset of shellhydrogen burning. The photometry of the red giants confirms the agespread and indicates a weakening of CN strength with age. Attention iscalled to the need for further study of NGC 2423 as an effectiveprolusion to understanding the evolution of the supercluster.
| Star formation in Taurus-Auriga - The high-mass stars
The question of whether the star formation in the Tau-Aur cloudsproduces low-mass stars exclusively is considered. The paper identifies29 B stars as possible kinematic members of the T association, and it isargued that the data are consistent with the existence of a singleinitial mass function (IMF) generating the low and high-mass stars. Itis concluded that the Tau-Aur T association is related to the Cas-Tau OBassociation, and there is no evidence that the association IMF differsfrom the field star IMF.
| Electron densities and the excitation of CN in molecular clouds
In molecular clouds of modest density and relatively high fractionalionization, the rotational excitation of CN is controlled by acompetition among electron impact, neutral impact and the interactionwith the cosmic background radiation. The degree of excitation can bemeasured through optical absorption lines and millimeter-wave emissionlines. The available, accurate data on CN in diffuse and translucentmolecular clouds are assembled and used to determine electron densities.The derived values, n(e) = roughly 0.02 - 0.5/cu cm, imply modestneutral densities, which generally agree well with determinations byother techniques. The absorption- and emission-line measurements of CNboth exclude densities higher than n(H2) = roughly 10 exp 3.5/cu cm onscales varying from 0.001 to 60 arcsec in these clouds.
| CO J = 3-2 observations of translucent and high-latitude molecular clouds
Measurements were carried out on the CO J = 3-2 emission line at 345 GHzfrom a number of translucent and high-latitude molecular clouds, as wellas on the J = 2-1 and J = 1-0 lines of both the (C-12)O and (C-13)O. Itis shown that the physical conditions in the high-latitude clouds arevery similar to those in the translucent clouds. The densities derivedfrom measured (C-12)O 1-0/3-2 ratios were often found to be higher thanthose based on the C2 excitation, while densities derived from measured3-2/(C-13)O 1-0 ratios were similar to those based on C2.
| The optical interstellar spectrum of HD 26571
High-resolution observations of interstellar Na I, Ca II, CH, CH(+) andCN towards the moderately reddened star HD 26571 are presented. Twovelocity components are identified in the atomic lines at heliocentricvelocities of about + 12 and + 20 km/s. Lines due to CH and CN werefound to associated with the latter component, which was found to have alarger N(Na I)/N(Ca II) ratio. CH(+) was not detected. A simple chemicalmodel suggests that densities in excess of 1000 particles/cu cm may berequired to account for the CN column density. The similarity of thevelocity of the molecular lines with those of the Taurus dark clouds,which lie close to the line-of-sight, suggests that this diffuse cloudis associated with the Taurus molecular cloud complex.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Tauro |
| Ascensión Recta: | 04h12m51.20s |
| Declinación: | +22°24'49.0" |
| Magnitud Aparente: | 6.12 |
| Distancia: | 316.456 parsecs |
| Movimiento Propio en Ascensión Recta: | -3.1 |
| Movimiento Propio en Declinación: | -12.2 |
| B-T magnitude: | 6.35 |
| V-T magnitude: | 6.162 |
Catálogos y designaciones:
|
INHABITED SKY
