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Ýlgili Makaleler
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.
| 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.
| H2 excitation in turbulent interstellar clouds
We discuss the observational differences between lines of sight thatintercept a group of turbulent dissipative structures and lines of sightthat cross less-active regions. Using time-dependent calculations weshow that the energy level distribution of the hydrogen molecule evolvesin time in response to the local thermal phase. We find that relativelysimple models can explain the observed properties of molecular hydrogenin diffuse interstellar clouds in terms of time evolution induced bycollisional excitation in a low-density, high-temperature gas.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| A far UV study of interstellar gas towards HD 34078: High excitation H2 and small scale structure
To investigate the presence of small scale structure in the spatialdistribution of H2 molecules we have undertaken repeated FUSEUV observations of the runaway O9.5V star, HD 34078. In this paper wepresent five spectra obtained between January 2000 and October 2002.These observations reveal an unexpectedly large amount of highly excitedH2>. Column densities for H2 levels from (v =0, J = 0) up to (v = 0, J = 11) and for several v = 1 and v = 2 levelsare determined. These results are interpreted in the frame of a modelinvolving essentially two components: i) a foreground cloud (unaffectedby HD 34078) responsible for the H2 (J = 0, 1), CI, CH,CH+ and CO absorptions; ii) a dense layer of gas (n ≃104 cm-3) close to the O star and stronglyilluminated by its UV flux which accounts for the presence of highlyexcited H2. Our model successfully reproduces theH2 excitation, the CI fine-structure level populations aswell as the CH, CH+ and CO column densities. We also examinethe time variability of H2 absorption lines tracing each ofthese two components. From the stability of the J = 0, 1 and 2 dampedH2 profiles we infer a 3σ upper limit on column densityvariations Δ N(H2)/N(H2) of 5% over scalesranging from 5 to 50 AU. This result clearly rules out any pronouncedubiquitous small scale density structure of the kind apparently seen inHI. The lines from highly excited gas are also quite stable (equivalentto Δ N/N ≤30%) indicating i) that the ambient gas throughwhich HD 34078 is moving is relatively uniform and ii) that the gas flowalong the shocked layer is not subject to marked instabilities.Based on observations performed by the FUSE mission and at the CFHTtelescope.
| Local interstellar medium kinematics towards the Southern Coalsack and Chamaeleon-Musca dark clouds
We present the results of a spectroscopic programme aiming toinvestigate the kinematics of the local interstellar medium componentstowards the Southern Coalsack and Chamaeleon-Musca dark clouds. Theanalysis is based upon high-resolution (R~ 60000) spectra of theinterstellar Na I D absorption lines towards 63 B-type stars (d<= 500pc) selected to cover these clouds and the connecting area defined bythe Galactic coordinates: 308°>=l>= 294° and-22°<=b<= 5°. The radial velocities, column densities,velocity dispersions, colour excess and photometric distances to thestars are used to understand the kinematics and distribution of theinterstellar cloud components. The analysis indicates that theinterstellar gas is distributed in two extended sheet-like structurespermeating the whole area, one at d<= 60 pc and another around120-150 pc from the Sun. The nearby feature is approaching the localstandard of rest with an average radial velocity of -7 kms-1, has low average column density logNNaI~ 11.2cm-2 and velocity dispersion b~ 5 km s-1. The moredistant feature has column densities between 12.3 <=logNNaI<= 13.2, average velocity dispersion b~ 2.5 kms-1 and seems associated with the dust sheet observed towardsthe Coalsack, Musca and Chamaeleon direction. Its velocity is centredaround 0 km s-1, but there is a trend for increasing from -3km s-1 near b= 1° to 3 km s-1 near b=-18°.The nearby low column density feature indicates a general outflow fromthe Sco-Cen association, in agreement with several independent lines ofdata in the general searched direction. The dust and gas feature around120-150 pc seem to be part of an extended large-scale feature of similarkinematic properties, supposedly identified with the interaction zone ofthe Local and Loop I Bubbles. Assuming that the interface and thering-like volume of dense neutral matter that would have been formedduring the collision of the two bubbles have similar properties, ourresults suggest that the interaction zone between the bubbles is twistedand folded.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 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/404/913
| Inferring Physical Conditions in Interstellar Clouds of H2
We have developed a code that models the formation, destruction,radiative transfer, and vibrational/rotational excitation ofH2 in a detailed fashion. We discuss generally how suchcodes, together with Far Ultraviolet Spectrographic Explorer (FUSE)observations of H2 in diffuse and translucent lines of sight,may be used to infer various physical parameters. We illustrate theeffects of changes in the major physical parameters (UV radiation field,gas density, metallicity), and we point out the extent to which changesin one parameter may be mirrored by changes in another. We provide ananalytic formula for the molecular fraction, fH2,as a function of cloud column density, radiation fields, and grainformation rate of H2. Some diffuse and translucent lines ofsight may be concatenations of multiple distinct clouds viewed together.Such situations can give rise to observables that agree with the data,complicating the problem of uniquely identifying one set of physicalparameters with a line of sight. Finally, we illustrate the applicationof our code to an ensemble of data, such as our FUSE survey ofH2 in the Large and Small Magellanic Clouds, in order toconstrain the elevated UV radiation field intensity and reduced grainformation rate of H2 in those low-metallicity environments.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| H2 formation and excitation in the diffuse interstellar medium
We use far-UV absorption spectra obtained with FUSE towards three late Bstars to study the formation and excitation of H2 in thediffuse ISM. The data interpretation relies on a model of the chemicaland thermal balance in photon-illuminated gas. The data constrain wellthe n R product between gas density and H2 formation rate ondust grains: n R = 1 to 2.2 x 10-15 s-1. For eachline of sight the mean effective H2 density n, assumeduniform, is obtained by the best fit of the model to the observedN(J=1)/N(J=0) ratio, since the radiation field is known. Combining nwith the n R values, we find similar H2 formation rates forthe three stars of about R = 4 x 10-17 cm3s-1. Because the target stars do not interact with theabsorbing matter we can show that the H2 excitation in theJ> 2 levels cannot be accounted for by the UV pumping of the coldH2 but implies collisional excitation in regions where thegas is much warmer. The existence of warm H2 is corroboratedby the fact that the star with the largest column density ofCH+ has the largest amount of warm H2.
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 is 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
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| Proper motions of faint ROSAT WTT stars in the Chamaeleon region
We present proper motions of 59 stars of the ROSAT All-Sky Survey (RASS)located in direction of the Chamaeleon star forming region (SFR) in themagnitude range B=5.1 - 17 mag. Proper motions of the fainter stars werenewly derived utilizing survey Schmidt plates from the GSC II platearchive and from a set of special plates taken with the ESO Schmidttelescope. The vector point diagram (VPD) indicates that the certifiedWTT stars cluster away from the region occupied by the brighterpre-main-sequence stars (PMS) in Cha I. The distance to this newassociation is estimated at ~ 100 pc, sensibly smaller than the 150 pcgenerally assumed for the SFR. This yields an upper limit of 2 km s(-1)for the velocity dispersion of this new kinematic group. The de-reddenedCM diagram of the group members suggest the WTT stars are still PMSobjects, but older (3-30 Myr) and less massive than previousdeterminations. These revised age estimates, the newly derived grouppeculiar velocity, and current distance estimates to the Cha I/II/IIIcomplex would favour in-situ formation against that predicted by highvelocity cloud models. Finally, based on a redetermination of thepeculiar motions of stars and gas, we speculate that the whole SFRoriginated from the local Orion spur as a result of more classicalmechanisms like interactions with the spiral arms. Based on observationscollected at the European Southern Observatory (Chile) and on data fromthe Hipparcos astrometry satellite. Table~1 is available only inelectronic form at the CDS via anonymous ftp at 130.79.128.5.
| High velocity gas and dust evolution in Chamaeleon clouds
We report on GHRS observations which reveal conspicuous differences inthe absorption spectra of two nearby stars, close to each other. Thestar HD102065 lies behind a cloud in Chamaeleon with unusually strongmid-IR emission, indicating a large abundance of very small dustparticles. Along this line of sight, 5% of the gas (about 6 10(19)cm(-2) ) is at large velocities (up to -50 km s(-1) ) compared to themain absorption component at vlsr ~ 0 km s(-1) . The highvelocity gas is very excited and has an unusually large siliconabundance. The other star HD96675 lies behind a cloud with standardmid-IR emission. Along this line of sight, high velocity gas is alsodetected, but to somewhat smaller offset velocities and with a muchlower excitation. In particular, the SiII(*) lines are not observed.From the excitation of Si(+) and C(+) in the direction of HD102065, weinfer that the high velocity gas has a temperature higher than several100 K and an electron density of at least 10 cm(-3) . These results,together with the lack of an ionizing star in the neigborhood, suggestthat a large amount of kinetic energy is being deposited in this gas.The collision of an infalling cloud and a local cloud is a plausiblesource of energy. The peculiarity of the dust size distribution inferredfrom the IRAS data is likely to be related to the processes whichdissipate the kinetic energy and heat the gas. A shock seems to berequired to produce the excitation and ionization degrees. But thecarbon ionization ratio combined with the electron density inferred fromthe silicon and carbon excitation implies that carbon is not inionization equilibrium and should recombine extremely quickly. Futurehigher resolution observations might help solving this incoherency.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| On the distance to the Chamaeleon I and II associations
Constraints on the distances to the dark clouds Chamaeleon I and II areinvestigated in detail. A compilation of photometric data, spectraltypes and absolute magnitudes for field stars towards each cloud ispresented, and results are used to examine the distribution of reddeningwith distance along each line of sight. The distances to starsassociated with reflection nebulae in each cloud are examined in detail.On the basis of these results, we deduce the most probable distance ofCha I to be 160+/-15pc, and that of Cha II to be 178+/-18pc. Anexamination of the mean fluxes of T Tauri stars in each cloud providesindependent evidence to suggest that Cha II is significantly moredistant than Cha I. Both clouds appear to be embedded in a macroscopicsheet-like structure extending over much of the Chamaeleon-Musca-Cruxregion. The Chamaeleon III and DC\ts 300.2--16.9 clouds are probablypart of the same structure, with probable distances ~ 140--160pc.
| Optical polarimetry, high--resolution spectroscopy and IR analysis of the Chamaeleon I dark cloud
We present optical polarimetry and high resolution spectroscopy of asample of stars toward the Chamaeleon I dark cloud. We use ourpolarimetry which includes 33 stars to study the wavelength dependenceof the degree and position angle of polarization. From fits to thenormalized wavelength dependence of interstellar polarization, we deriveestimates of lambdaMax ranging from 4500 \AA to 6700 \AA, andPMax ranging from 3 to 8%. The values of lambdaMaxwere found to be well correlated with the IRAS 100 micron intensity,while PMax was found to increase with E_{B-V}. Highresolution spectra of the Ca II, CH, and CH^{+} lines were obtained for10 stars, which show two components of Ca II in absorption at 3.0
| An improved HR diagram for Chamaeleon I pre-main-sequence stars.
Abstract image available at:http://adsabs.harvard.edu/abs/1996MNRAS.280.1071L
| A study of the Chamaeleon star forming region from the ROSAT all-sky survey. I. X-ray observations and optical identifications.
We present the observations of the ROSAT all-sky survey (RASS) in thedirection of the Chamaeleon cloud complex, as well as the spectroscopicidentifications of the detected X-ray sources. The main purpose of thisidentification program was the search for low mass pre-main sequencestars. Sixteen previously known PMS stars were detected with highconfidence by ROSAT. Eight are classical T Tauri stars and eight areweak-line T Tauri stars, Seventy-seven new weak-line T Tauri stars wereidentified on the basis of the presence of strong Li λ 6707absorption, spectral type later than F0 and chromospheric emission. Wegive coordinates and count rates of the X-ray sources, and presentoptical spectra and finding charts for the sources identified opticallyas new pre-main sequence stars. Optical UBV(RI)_c_ and near-infraredJHKLM photometry for this sample of stars is also provided. In addition,6 new dKe-dMe candidates are found among the RASS sources.
| Dust Metamorphosis in the Galaxy
Not Available
| A Study of the Chamaeleon-I Dark Cloud and T-Association - Part Six - Interstellar Polarization Grain Alignment and Magnetic Field
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.268....1W&db_key=AST
| ROSAT X-Ray Study of the Chamaeleon I Dark Cloud. I. The Stellar Population
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...416..623F&db_key=AST
| The type of variability of Herbig Ae/Be stars
The paper presents a list of 23 Herbig Ae/Be stars and Herbig Ae/Be starcandidates, compiled on the basis of 2 to 8 years of observations in theStromgren photometric system. Results of the study show that spectraltype A0 separates the stars showing large variations in their brightnessfrom stars showing small variations, with the later spectral type starsbeing the more variable. The behavior of the stars in thecolor-magnitude diagram could be divided into three classes: (1) classR, in which there is a monotonic dependence of the color index on visualbrightness, (2) class CR, in which this dependence is not monotonic, and(3) class RB, in which there is no dependence. It was also found thatthe behavior of a star in the color-magnitude diagram is dependent noton the spectral type but on the size of the variation in brightness.
| Long-term photometry of variables at ESO. I - The first data catalogue (1982-1986)
This paper presents the catalog of photometric data in the Stromgrensystem obtained during the first four years (October 1982 - September1986) of the Long-Term Photometry of Variables (LTPV) program at ESO.The data are available in computer-readable form.
| Instrumental effects and the Stroemgren photometric system
The extent to which the use of different photometers can affect resultson stellar color indices was investigated by simultaneously observing asample of widely different stars with several uvby photometric systemsavailable at the ESO, La Silla, Chile. In one instance, the ESO 50-cmtelescope and the four-channel photometer at the Danish 50-cm telescopewere used simultaneously. The reductions were performed separately oneach data set, using a linear color transformation procedure. Theresults agree with theoretical investigations which showed thatsubstantial errors can arise from the nonconformity of passbands. It isemphasized that it is necessary to use separate color transformationsfor various stellar types and classes and for different interstellarreddenings.
| A study of the Chamaeleon dark cloud and T-association. I - Extinction, distance and membership
The results of an observing program to investigate the nature and degreeof reddening of a number of stars in the field towards the dark cloudand T-association in Chamaeleon are presented. The observations compriseoptical (UBVRI) and infrared (JHK) photometry, and optical spectroscopy.New and previously published data are combined to assess membershipcriteria for 110 stars, and separate catalogues of association membersand field stars are presented. Optical identifications are listed for 27IRAS point sources in the region. The reddening/distance relation forfield stars indicates a distance to the cloud of 140 + or - 12 pc. Thisresult confirms that the extinction law is anomalous towards some starsin the cloud, notably the embedded A0 ZAMS star HD 97300, with values ofup to 5.5 for the ratio of total-to-selective extinction. A comparisonof 21-cm data with the reddening of background field stars indicatesthat the ratio N(H I)/E(B-V) is significantly less than the interstellarmean, indicating that most of the hydrogen in the cloud is molecular.
| The ratio of total-to-selective extinction in the Chamaeleon T1 and R Coronae Australis dark clouds
The ratio of total-to-selective extinction, R = A(V)/E(B-V), in the ChaT1 and R CrA dark clouds is investigated via observations of backgroundfield stars seen through these clouds. The observations consist of UBVRIand JHKL photometry as well as classification spectrograms and yieldindividual R values by the color difference method. R is found to beessentially normal in the outer parts of the Cha T1 dark cloud, butseveral stars near the opaque core of this cloud show evidence ofanomalous extinction with R greater than 4. In the R CrA dark cloud, Rincreases with depth into the cloud and reaches clearly anomalous valuesof about 5 for lines of sight with A(V) = 3 mag. The results providefurther evidence for anomalous interstellar extinction laws in somedense interstellar clouds.
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| Sað Açýklýk: | 11h05m57.81s |
| Yükselim: | -76°07'48.9" |
| Görünürdeki Parlaklýk: | 7.685 |
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