目录
图像
上传图像
DSS Images Other Images
相关文章
FUSE Measurements of Far-Ultraviolet Extinction. I. Galactic Sight Lines
We present extinction curves that include data down to far-ultravioletwavelengths (FUV; 1050-1200 Å) for nine Galactic sight lines. TheFUV extinction was measured using data from the Far UltravioletSpectroscopic Explorer. The sight lines were chosen for their unusualextinction properties in the infrared through the ultraviolet; that theyprobe a wide range of dust environments is evidenced by the large spreadin their measured ratios of total to selective extinction,RV=2.43-3.81. We find that extrapolation of the Fitzpatrick& Massa relationship from the ultraviolet appears to be a goodpredictor of the FUV extinction behavior. We find that predictions ofthe FUV extinction based on the Cardelli, Clayton, & Mathis (CCM)dependence on RV give mixed results. For the seven extinctioncurves well represented by CCM in the infrared through ultraviolet(x<8 μm-1), the FUV extinction is well predicted inthree sight lines, overpredicted in two sight lines, and underpredictedin two sight lines. A maximum entropy method analysis using a simplethree-component grain model shows that seven of the nine sight lines inthe study require a larger fraction of grain materials to be in dustwhen FUV extinction is included in the models. Most of the added grainmaterial is in the form of small (radii <~ 200 Å) grains.Based on observations with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| Small Magellanic Cloud-Type Interstellar Dust in the Milky Way
It is well known that the sight line toward HD 204827 in the clusterTrumpler 37 shows a UV extinction curve that does not follow the averageGalactic extinction relation. However, when a dust component, foregroundto the cluster, is removed, the residual extinction curve is identicalto that found in the SMC within the uncertainties. The curve is verysteep and has little or no 2175 Å bump. The position of HD 204827in the sky is projected onto the edge of the Cepheus IRAS bubble. Inaddition, HD 204827 has an IRAS bow shock, indicating that it may beembedded in dust swept up by the supernova that created the IRAS bubble.Shocks due to the supernova may have led to substantial processing ofthis dust. The HD 204827 cloud is dense and rich in carbon molecules.The 3.4 μm feature indicating a C-H grain mantle is present in thedust toward HD 204827. The environment of the HD 204827 cloud dust maybe similar to the dust associated with HD 62542, which lies on the edgeof a stellar wind bubble and is also dense and rich in molecules. Thissight line may be a Rosetta Stone if its environment can be related tothose in the SMC having similar dust.
| 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.
| The relation between far-UV and visible extinctions
For directions of sufficient reddening (/E(B-V)>~0.25), there is asimple relation between the slope of the extinction curve in the far-UVand /E(B-V). Regardless of direction, the far-UV extinction curve isproportional to 1/λn e-2E(B-V)/λ(/λ in μm, /n=4), in accordance with the idea that reddenedstars spectra are contaminated by scattered light (Zagury, 2001b). Thisrelation is not compatible with the standard theory of extinction whichstates that far-UV and visible extinctions are due to different classesof particle. In that model the two (far-UV and visible) extinctions varythus independently according to the proportion of each type of particle.In preceding papers I have shown that the standard theory cannot explainUV observations of nebulae, and is contradicted by the UV spectra ofstars with very low reddening: for how long shall the standard theory beconsidered as the interpretation of the extinction curve?
| Chemical Abundances of OB Stars with High Projected Rotational Velocities
Elemental abundances of carbon, nitrogen, oxygen, magnesium, aluminum,and silicon are presented for a sample of 12 rapidly rotating OB star(vsini>60kms-1) members of the Cep OB2, Cyg OB3, and CygOB7 associations. The abundances are derived from spectrum synthesis,using both LTE and non-LTE calculations. As found in almost all previousstudies of OB stars, the average abundances are slightly below solar, byabout 0.1 to 0.3 dex. In the case of oxygen, even with the recentlyderived low solar abundances, the OB stars are closer to, but stillbelow, the solar value. Results for the nine Cep OB2 members in thissample can be combined with results published previously for eight CepOB2 stars with low projected rotational velocities to yield the mostcomplete set of abundances, to date, for this particular association.These abundances provide a clear picture of both the general chemicaland individual stellar evolution that has occurred within thisassociation. By placing the Cep OB2 stars studied in an HR-diagram weidentify the presence of two distinct age subgroups, with both subgroupshaving quite uniform chemical abundances. Two stars are found in theolder subgroup that show significant N/O overabundances, with both starsbeing two of the most massive, the most evolved, and the most rapidlyrotating of the members studied in Cep OB2. These characteristics ofincreased N abundances being tied to high mass, rapid rotation, and anevolved phase are those predicted from models of rotating stars thatundergo rotationally driven mixing.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Dust properties in the direction of Trumpler 37.
The fourteen brightest OB stars of the Trumpler 37 Cluster have beenobserved in the V,R,I,J,H,K photometric bands. These have been used toderive accurate measurements of the quantity R_V_=A_V_/E(B-V). Differentfrom previous studies it is found that in Trumpler 37 isclose to the mean interstellar value of 3.1. Most of the reddeningtowards Trumpler 37 is found to be of foreground origin. This isconsistent with the lack of variations, observed by us, of R_V_ with theprojected distance from the center of the cluster. In two highlyreddened stars of the cluster, whose reddening is certainly mostlylocal, the observed R_V_ is lower than the mean interstellar value of3.1. This is to be compared with the behaviour of the Trapezium stars inOrion, whose R_V_ is quite above such value whereas the age of thecluster is similar to the one of Trumpler 37.
| Diffuse interstellar bands: resolved rotational band structure at 5850A.
The only candidate of what may be a resolved rotational vibrational bandin the DIB Survey of Jenniskens & Desert (1994), the triplet ofbands at 5844, 5850 and 5852 A, is examined in detail in order toestablish the nature of these DIBs. We find that superposed on the broadλ5844 are four weak features, reminiscent of substructure foundon top of the broad λ5778. The relative band strength of DIBsλ5844 and λ5850 correlates with the rotational temperaturein the J=0 and J=1 levels of molecular hydrogen. However, we do not findvariations in band shape that are consistent with substructure of aresolved rotational band profile. Instead, the narrow λ5850 isfound to have a shoulder on the red and blue side of the main band,which can imply that this band itself is an unresolved rovib band withP, Q and R branches.
| The structure of the IC1396 region.
We have made an extensive study of the molecular clouds associated withthe Hii region IC1396 in the rotational transitions J=1-0 and J=2-1 of^12^CO and ^13^CO and J=3-2 of ^12^CO with an average spatial resolutionof 2.5' and an average sampling of about 2.0', in order to getinformation on its structure and evolution. On the basis of ourobservations, which cover an area of more than 6deg^2^, we can classifythe molecular clouds into those directly associated with the ionizingO6.5V star HD 206267, producing the bright-rimmed clouds, and the coldgas along the line of sight, which is mainly foreground material. Thebright-rimmed clouds show the presence of warmer molecular gas throughhigher ^12^CO (2-1)/(1-0), ^13^CO (2-1)/(1-0) and ^12^CO (3-2)/(2-1)line ratios than the cold foreground gas. The warm clouds form roughly ashell-like arrangement with a diameter of 25 to 40pc around HD206267(though most are slightly closer to the Sun than the O star), and theyseem to be the remainder of the now dispersing molecular cloud whichgave birth to the O6.5 star and the star cluster Tr37 associated withit. All bright-rimmed clouds show internal structure on all size scales,including bipolar outflows. Optical, FIR (IRAS 12 to 100μm) and COmaps are in close agreement over the whole region, especially for thebright-rimmed clouds: Exceptions are some optically bright (ionized)regions, which show FIR, but no CO emission, and the cold foregroundclouds, which are very weak at FIR wavelengths. The entire mass of themapped IC1396 region is estimated to be 12000Msun_, which iscomposed of molecular (4000Msun_), atomic(5000Msun_), and ionized material (3000Msun_) innearly equal amounts. The masses of the bright-rimmed clouds range froma few to several 100Msun_.
| Diffuse interstellar bands and UV extinction curves. The missing link.
The connection between diffuse interstellar bands and components of theinterstellar ultraviolet extinction curve, decomposed according to theFitzpatrick & Massa scheme, is analysed from new observations ofseveral DIBs in the line of sight to 28 stars measured by IUE. We findthat the strength of the 217.5nm bump positively correlates with DIBstrength, whereas correlations with a negative slope exist with the FUVnon-linear rise and the width of the bump. There is no correlation withlinear rise. The relation of bump height versus DIB strength does notpass through zero. The results are interpreted within the polycyclicaromatic hydrocarbon (PAH) framework. They strengthen the hypothesisthat the FUV non-linear rise is produced by neutral PAHs, whereas DIBcarriers are found among some ionised or radical PAHs.
| Far-Ultraviolet (912--1900 Angstrom ) Energy Distribution in Early-Type Main-Sequence Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...449..280C&db_key=AST
| Tracing the Roots of Interstellar Mid Infrared Emission
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...275..549J&db_key=AST
| Complex Structure in Two Diffuse Interstellar Bands
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...274..465J&db_key=AST
| Environment Dependence of Interstellar Extinction Curves
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...274..439J&db_key=AST
| SAO stars with infrared excess in the IRAS Point Source Catalog
We have undertaken a search for SAO stars with infrared excess in theIRAS Point Source Catalog. In contrast to previous searches, the entireIRAS (12)-(25)-(60) color-color diagram was used. This selection yieldeda sample of 462 stars, of which a significant number are stars withcircumstellar material. The stars selected can be identified aspre-main-sequence stars, Be stars, protoplanetary systems, post-AGBstars, etc. A number of objects are (visual) binary stars.Characteristic temperatures and IR excesses are calculated and theirrelations to spectral type are investigated.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| 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.
| Infrared dust and millimeter-wave carbon monoxide emission in the Orion region
The far-infrared dust emission seen by the IRAS satellite in the Orionregion is analyzed as a function of the local radiation field intensity,and the dust temperature and opacity are compared with (C-12)O and(C-13)O emission. The infrared radiation is interpreted within theframework of a single-component large grain model and a multicomponentgrain model consisting of subpopulations of grains with size-dependenttemperatures. A strong dependence of the 100-micron optical depthderived is found using the large grain model on the averageline-of-sight dust temperature and radiation field. In the hotenvironment surrounding high-luminosity sources and H II regions, alldust along the line-of-sight radiates at 100 microns, and thedust-to-gas ratio, based on the 100-micron opacity and I(/C-13/O),appears to be in agreement with the standard value, about 1 percent bymass. A relationship is found between the inferred dust-to-gas ratio andthe radiation field intensity responsible for heating the dust which canbe used to estimate the gas column density from the dust opacity derivedfrom the 60- and 100-micron IRAS fluxes.
| Photometry of the young open cluster Trumpler 37
Photoelectric UBV observations of 120 stars in the young open clusterTrumpler 37 are presented, primarily in the magnitude range 10.0 - 13.5.An analysis of the color-magnitude diagram of the cluster yields an ageof 6.7 million yr and reveals the presence of a number of possiblepre-main-sequence stars in the cluster.
| The interstellar extinction law towards the open cluster TR-37
Near-infrared photometry of OB stars in the young open cluster TR-37 hasbeen obtained. These data have been combined with previous visual andultraviolet observations of the same sample and a detailed extinctioncurve has been derived, ranging from the near-infrared to thefar-ultraviolet. The extinction towards this sample of stars seems to beanomalous, consistent with a shift in the grain-size distribution withrespect to an average galactic extinction curve.
| An analysis of the shapes of ultraviolet extinction curves. II - The far-UV extinction
In this paper the properties of interstellar extinction in thefar-ultraviolet region are examined utilizing IUE extinction curves fora primary data sample of 45 reddened Milky Way OB stars. These resultsare combined with those derived for the 2175 A bump in a previous study.It is found that IUE extinction curves can be represented by linearcombinations of a Lorentzian-like 2175 A bump profile, a well-determinedFUV curvature term, and an underlying linear component. The parametersof the linear component are strongly correlated and therefore only fivefree parameters are required to fit all of the curves in our sample.Three parameters describe the 2175 A bump, one parameter describes thelinear background, and one parameter describes the strength of the FUVcurvature term. The shape of the FUV curvature is found to be identical,to within the observational errors, for all the curves in the sample -which spans virtually the entire range of extinction curve morphologiesobserved in the Milky Way.
| Atlas of the wavelength dependence of ultraviolet extinction in the Galaxy
The paper presents a collection of 115 extinction curves derived fromlow-dispersion IUE spectra. The spectra have been reduced with the useof techniques designed to reduce the effects of random noise and theinfluence of residual spectral features due to classification mismatch.The magnitudes of other instrumental and interpretational uncertaintiesare estimated. The extinction curves are presented with normalization toE(B-V) = 1 and the FUV portion (below 170 nm) is also shown for E(13-17)= 1. The atlas includes examples of extinction originating in thediffuse medium and several major nebulae and dense clouds.
| Observational constraints on the carriers of the ultraviolet extinction bump
Data from the Infrared Astronomical Satellite is used to investigate theorigin of the ultraviolet (UV) extinction bump. Correlations of the dusttemperature with the parameters of the UV bump, as derived from IUEobservations, are presented. Significant correlations of the 60/100brightness ratio with the area under the UV bump and the height of theUV bump are found. These last results suggest that the particlesresponsible for the UV bump are very sensitive to the strength of theradiation field. However, no significant correlation is found betweenthe dust temperature and the width of the UV bump, as would have beenexpected on the basis of recent physical models.
| Anomalous dust in the open cluster Trumpler 37
New IUE data were obtained towards stars in the open cluster Trumpler37. Extinction curves derived from these data show that the anomalousdust detected towards HD 204827 and HD 210072 is not unique to thoselines of sight but occurs generally in that region of the sky. The dustis apparently associated with the Trumpler 37 cluster, which forms thecenter of Cepheus OB2. This is a complex region of star formation wherethe evolution of dust grains through shock waves could be consideredlikely. A supernova in a slightly older star-forming region of Cep OB2located next to Trumpler 37 is a possible source of such a shock wave.
| An analysis on the shapes of ultraviolet extinction curves. I - The 2175 A bump
IUE data are used to investigate the properties of the 2175 Ainterstellar extinction bump toward 45 reddened Milky Way stars. Usinganalytic fitting functions, parameters are derived which describe thecentral position, FWHM, and strength of the bump. The bump position isvery stable, with an extreme deviation which is still significantlylarger than the measurement uncertainty, indicating true variability.The bump FWHM has a large range of intrinsic values, 0.77/micron to1.25/micron. The normalized bump strength varies by more than a factorof three. There are no convincing correlations among the three bumpparameters, but the width of the bump is strongly correlated with thedust grain environments. Dense quiescent regions yield broad bumps,while diffuse interstellar medium and regions of recent star formationyield narrower bumps. The absence of any correlation between bump widthand central position argues strongly against graphite grains in the sizerange prescribed by the Mathis, Rumpl, and Nordsieck (1977) model ascarriers of the bump.
| The binary system MWC 349
The ionized zone surrounding MWC 349 has been resolved using the VLA. Onthe basis of a radio continuum map, it is concluded that there is aphysical interaction between the two stellar components of MWC 349, andthat the system is therefore a binary. The B component can be classifiedas a B0 III star, yielding a distance of about 1.2 kpc for the binary.The low ratio of infrared to short-wavelength luminosity in the Acomponent suggests a circumstellar dusty region of special geometry. Itis proposed that an appreciable dust and gas disk surrounds the star,modifying the symmetry of its H II region, and yielding the low infraredluminosity and perhaps its extra circumstellar extinction.
| Ultraviolet extinction and diffuse band strength correlations
Correlations between UV extinction parameters and diffuse band strengthsare south, using a data base of 50 stars with recently measuredextinction curves. A novel aspect of this study is that the basicdependence of most interstellar parameters on E(B-V) (or, nearlyequivalently, on the total hydrogen column density) has been largelycanceled out. After this cancellation, a weak correlation is foundbetween the 4430 A diffuse band and the 2175 A extinction feature. Noother correlations are found. These results support a molecular originfor the 5780 A and 6284 A bands. It is speculated that the correlationpattern found for the 4430 A band can be explained if this bandoriginates in reactions on disordered grain surfaces.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
