תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
CN column densities and excitation temperatures
We analyse abundances and rotational temperatures of the interstellar CNmolecule. We have calculated the column densities and excitationtemperatures of the molecule along 73 lines of sight basing on ouroriginal measurements of the B 2?+ -X2?+ (0,0) vibrational band recorded in highsignal-to-noise ratio spectra and also for 88 directions based onmeasurements already available in literature. We compare the columndensities obtained from different bands of CN molecule available toground-based instruments. The obtained excitation temperatures in theanalysed directions show always an excess over the cosmic microwavebackground radiation (CMBR) temperature.
| Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
We carried out a comprehensive far-UV survey of 12CO andH2 column densities along diffuse molecular Galactic sightlines. This sample includes new measurements of CO from HST spectraalong 62 sight lines and new measurements of H2 from FUSEdata along 58 sight lines. In addition, high-resolution optical datawere obtained at the McDonald and European Southern Observatories,yielding new abundances for CH, CH+, and CN along 42 sightlines to aid in interpreting the CO results. These new sight lines wereselected according to detectable amounts of CO in their spectra andprovide information on both lower density (<=100 cm-3) andhigher density diffuse clouds. A plot of logN(CO) versuslogN(H2) shows that two power-law relationships are neededfor a good fit of the entire sample, with a break located atlogN(CO,cm-2)=14.1 and logN(H2)=20.4,corresponding to a change in production route for CO in higher densitygas. Similar logarithmic plots among all five diatomic molecules revealadditional examples of dual slopes in the cases of CO versus CH (breakat logN=14.1, 13.0), CH+ versus H2 (13.1, 20.3),and CH+ versus CO (13.2, 14.1). We employ both analytical andnumerical chemical schemes in order to derive details of the molecularenvironments. In the denser gas, where C2 and CN moleculesalso reside, reactions involving C+ and OH are the dominantfactor leading to CO formation via equilibrium chemistry. In thelow-density gas, where equilibrium chemistry studies have failed toreproduce the abundance of CH+, our numerical analysis showsthat nonequilibrium chemistry must be employed for correctly predictingthe abundances of both CH+ and CO.
| Hubble Space Telescope Survey of Interstellar 12CO/13CO in the Solar Neighborhood
We examine 20 diffuse and translucent Galactic sight lines and extractthe column densities of the 12CO and 13COisotopologues from their ultraviolet A-X absorption bands detected inarchival Space Telescope Imaging Spectrograph data withλ/Δλ>=46,000. Five more targets with GoddardHigh-Resolution Spectrograph data are added to the sample that more thandoubles the number of sight lines with published Hubble Space Telescopeobservations of 13CO. Most sight lines have12CO-to-13CO isotopic ratios that are notsignificantly different from the local value of 70 for12C/13C, which is based on millimeter-waveobservations of rotational lines in emission from CO and H2COinside dense molecular clouds, as well as on results from opticalmeasurements of CH+. Five of the 25 sight lines are found tobe fractionated toward lower 12C/13C values, whilethree sight lines in the sample are fractionated toward higher ratios,signaling the predominance of either isotopic charge exchange orselective photodissociation, respectively. There are no obvious trendsof the 12CO-to-13CO ratio with physical conditionssuch as gas temperature or density, yet 12CO/13COdoes vary in a complicated manner with the column density of either COisotopologue, owing to varying levels of competition between isotopiccharge exchange and selective photodissociation in the fractionation ofCO. Finally, rotational temperatures of H2 show that allsight lines with detected amounts of 13CO pass through gasthat is on average colder by 20 K than the gas without 13CO.This colder gas is also sampled by CN and C2 molecules, thelatter indicating gas kinetic temperatures of only 28 K, enough tofacilitate an efficient charge exchange reaction that lowers the valueof 12CO/13CO.
| The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration Based on Synthetic He I Lines
We derive projected rotational velocities (v sin i) for a sample of 156Galactic OB star members of 35 clusters, H II regions, and associations.The He I lines at 4026, 4388, and 4471 Å were analyzed in order todefine a calibration of the synthetic He I full widths at half-maximum(FWHMs) versus stellar v sin i. A grid of synthetic spectra of He I lineprofiles was calculated in non-LTE using an extensive helium model atomand updated atomic data. The v sin i values for all stars were derivedusing the He I FWHM calibrations, but also, for those target stars withrelatively sharp lines, v sin i values were obtained from best-fitsynthetic spectra of up to 40 lines of C II, N II, O II, Al III, Mg II,Si III, and S III. This calibration is a useful and efficient tool forestimating the projected rotational velocities of O9-B5 main-sequencestars. The distribution of v sin i for an unbiased sample of early Bstars in the unbound association Cep OB2 is consistent with thedistribution reported elsewhere for other unbound associations.
| Hubble Space Telescope Measurements of Vacuum Ultraviolet Lines of Interstellar CH
Three interstellar absorption lines near 1370 Å seen toward ζOph have been assigned by Watson to Rydberg transitions in the G-X (or3d-X) band of CH. Our survey of a dozen diffuse interstellar lines ofsight shows that the three absorption lines are consistent with theknown column densities of CH, by deriving the following oscillatorstrengths: f(1368.74)=0.019+/-0.003, f(1369.13)=0.030+/-0.005, andf(1370.87)=0.009+/-0.001. We also determined intrinsic line widths thatcorrespond to decay rates of (1.5+/-0.6)×1011,(3.8+/-0.7)×1011, and (1.1+/-0.6)×1010s-1 for λλ1368, 1369, and 1370, respectively.These rates are significantly higher than those associated withradiative decays and, thus, are readily attributable to predissociationof the Rydberg state. A fourth interstellar line near 1271 Å hasbeen conjectured by Watson to be the strongest transition in the 4d-XRydberg band of CH. We detected this line along four sight lines and ourspectrum syntheses show that with f(1271.02)=0.007+/-0.002, it is alsoconsistent with the known column densities of CH. In addition, weconducted a search for the F-X band of CH near 1549 Å, andsuccessfully discovered two of its absorption features along four sightlines. The astronomical oscillator strengths derived for these featuresare f(1549.05)=0.021+/-0.006 and f(1549.62)=0.013+/-0.003. Finally, theX Per sight line provided us with an astronomical detection of anotherCH band via two D-X features near 1694 Å. Comparisons with resultsof available theoretical calculations for the four CH bands arepresented.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| 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.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| Chemical Abundances of OB Stars in the Cepheus OB2 Association
LTE and non-LTE abundances of carbon, nitrogen, oxygen, and silicon, aswell as LTE abundances of Fe, are presented from analyses of C II, N II,O II, Si III, and Fe III lines in the spectra of eight main-sequence OBstars in the Cep OB2 association. We find that the chemical compositionof the studied stars in this association is very homogeneous: theabundances of all elements analyzed do not show significant variations,and the observed abundance dispersions can be explained in terms of theuncertainties in the analysis. The abundance results indicate that thisassociation is slightly metal poor ([Fe/H]=-0.3) with the abundances ofall studied elements being subsolar by roughly the same amount.
| Origin and Evolution of the Cepheus Bubble
We have imaged a 10 deg x 10 deg region of the Cepheus bubble in the J =1-0 line of CO and the 21 cm line of atomic hydrogen. The CO emissiondefines a giant expanding shell 120 pc in diameter, which is similar tothat seen in the IRAS sky maps. We estimate the total gas mass in theregion to be ~4 x 10^5 M_ȯ. The total kinetic energy from theobserved spread of velocities of the molecular clouds is ~10^51 ergs. Wesuggest that the members of earlier generations of massive stars in NGC7160 are responsible for the origin of the Cepheus bubble. These starscreated an expanding compressed shell of gas that became gravitationallyunstable at an age of ~7 Myr. The members of the Cepheus OB2 associationcomprise the second, intermediate generation of stars in this regionthat formed as a consequence of this instability. The numerous colorselected IRAS point sources represent the third and youngest generationof stars in this region. Our observations suggest the great importanceof sequentially triggered star formation in the region of the Cepheusbubble.
| 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 HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.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.
| Age variation of helium abundance in He-rich stars.
Not Available
| A new list of effective temperatures of chemically peculiar stars. II.
Not Available
| Galactic OB associations in the northern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg
The literature on all OB associations was reviewed, and their IRAS pointsource content was studied, between galactic longitude 55 and 150 deg.Only one third of the 24 associations listed by Ruprecht et al. (1981)have been the subject of individual studies designed to identify thebrightest stars. Distances to all of these were recomputed using themethod of cluster fitting of the B main sequence stars, which makes itpoossible to reexamine the absolute magnitude calibration of the Ostars, as well as for the red supergiant candidate stars. Also examinedwas the composite HR diagram for these associations. Associations withthe best defined main sequences, which also tend to contain very youngclusters, referred to here as OB clusters, have extremely few evolved Band A or red supergiants. Associations with poorly defined mainsequences and few OB clusters have many more evolved stars. They alsoshow an effect in the upper HR diagram referred to as a ledge byFitzpatrick and Garmany (1990) in similar data for the Large MagellanicCloud. It is suggested that the differences in the associations are notjust observational selection effects but represent real differences inage and formation history.
| Mid-ultraviolet and optical photometry of helium stars
Mid-ultraviolet and optical photometric analysis of helium stars arepresented. A linear relation exists between the effective temperaturederived from model atmospheres and (1965-V)0 index. The effectivetemperatures derived from (1965-V)0 index are somewhat higher than thatof MK spectral type estimates especially for late B-type helium objects.
| On dependence of helium abundance upon magnetic field in He-r stars.
Not Available
| The Relation Between Magnetic Field Strength and Helium Abundance in Helium Rich Stars
Not Available
| Empirical temperature calibrations for early-type stars
Three temperature calibrations of suitable photometric quantities havebeen derived for O and B stars. A sample of 120 stars with reliableT(eff.) determinations has been used for establishing each calibration.The different calibrations have been critically discussed and compared.Temperature determinations for 1009 program stars have been obtainedwith an accuracy of the order of 10 percent.
| On the Effective Temperatures of Chemically Peculiar Stars
Not Available
| Observational problems of investigation of stars with anomalous helium lines.
Not Available
| Catalog of observed periods for the AP stars
A preliminary catalog of the Ap stars, taken in the broad sense toinclude many Bp stars, is presented. The HD and HR numbers,variable-star names, spectral types, and proposed periods are listed;the nature of the periodic variability is indicated; and references tothe observations are provided. Companion stars, confirmed spectroscopicbinaries, and anomalies are mentioned in notes.
| Spectrophotometric investigation of OB stars in the region of the association CEP OB2
The results are given of spectrophotometric investigation of 39 OB starsin the region of the association Cep OB2. The blue (4000-4800 A)absolute spectrophotometric gradients, the color excesses in the UBVsystem, and the distances of these stars have been determined. Analysisof the results suggests that the absorbing material is basicallyconcentrated within the association itself.
| Space Distribution of Stars in a Region of Cepheus around NGC 7160
Objective prism spectral types and photographic U, B, V magnitudes havebeen determined for 1810 stars in a region of 19.5 square degrees ofCepheus centered on the galactic cluster NGC 7160. A catalogue of datatogether with identification charts is given. The magnitude limit isapproximately V= 12.5. The field contains a part of the supposed oldersubsystem of the association Cepheus OB2. The interstellar absorption inthe region is smooth. The visual absorption is 1.0 mag. at the distanceof the association for the northern part of the field, and is about 1.4mag. for the southern part. Space densities of the stars have beencomputed as a function of spectral type and distance from the sun.Concentrations of B and early A type stars are found at 800 pc from thesun. Concentrations of B and early A type stars are found at 800 pc, andan attempt was made to derive the shape of the luminosity function ofthe association subgroup.
| Search for light variability among eight stars with anomalous helium content
A search for periodic light variability was carried out among eightstars with anomalous helium content. No variability was detected duringthe observation period for HD 178993, HD 207538, and HD 209339. Theperiod obtained for HD 184927 by Bond and Levato (1977) was confirmed.For HD 191980, HD 208266, and HD 217833, photometric variability wasdetected and periods were obtained.
| Helium abundance in atmospheres of stars appearing in OB associations and young clusters
An improved version of Liubimkov's (1974) method is used to determinethe relative helium abundance, N(He)/N(H), in the atmospheres of 80stars of spectral types O6.5 to B2 belonging to OB associations andyoung clusters. Specific results are presented for Mon OB1 and OB2, CepOB2 and OB3, Sco OB2, Lac OB1, Ori OB1, and Cas OB6. It is shown thatthe mean value of N(He)/N(H) increases with increasing stellar age andincreasing cluster or association size. A relative helium abundance of0.09 to 0.10 is found to be appropriate for stars that are severalmillion years old, and a relative abundance of 0.13 is obtained forstars older than 10 million years. It is noted that the latter value isalso characteristic of hot field stars.
| Photoelectric photometry of luminous B-type stars in a Milky Way field in Cepheus
Not Available
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | קפיאוס |
| התרוממות ימנית: | 21h53m00.35s |
| סירוב: | +60°38'32.7" |
| גודל גלוי: | 8.147 |
| מרחק: | 10000000 פארסק |
| תנועה נכונה: | -1.7 |
| תנועה נכונה: | -0.5 |
| B-T magnitude: | 8.411 |
| V-T magnitude: | 8.169 |
קטלוגים וכינוים:
|
INHABITED SKY
