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1.
The dependence of interstellar extinction on distance in the direction of the dark cloud south of Merope is determined using photoelectric photometry of 93 stars in the Vilnius photometric system. The cloud front edge is detected at 120–130 pc from the Sun and the distance of the Pleiades cluster is found to be 127 pc. Mean extinctionA V in the Merope cloud is of the order of 1.0 mag. There is no evidence of extinction at distances exceeding the Merope cloud distance. Variable extinction method yieldsR=A V/EB-V=3.6, while the maximum polarization wavelength method gives the value 3.4. Some Pleiades stars are suspected to be unresolved binaries.  相似文献   

2.
Photometric data inuvbyß system from 3713 stars is used to map the distribution of colour excesses up to 500 parsec from the Sun, producing diagrams for a series of layers aligned with the galactic plane. Individual clouds are detected, with a mean diameter of 6 parsecs, while cloud complexes have a mean diameter of 100 pc. From the maps it seems that the interstellar medium near the Sun presents regions of non-negligible extinction, while the Sun itself is enbedded in a diffuse cloud, or slightly off its border. The large scatter in the distribution ofE(b-) is confirmed, with a mean extinction ofA v=0 . m 38 up to 500 pc.  相似文献   

3.
Using the results of observations of the Carina nebula made with the space telescope Glazar, it is shown that the extinction law for the nebula is abnormal and that there is a single OB star complex within the nebula at a distance of about 2200 pc. It is suggested that the observed distribution of OB stars in the nebula and also the appearance of the nebula itself is due to a specific structure of the absorbing clouds within the nebula, and that the absorbing clouds may have such structure as a result of an explosion in the center of the nebula.  相似文献   

4.
Using a method which will be explained in detail by Steenman and Thé (1989) the extinction law towards several stars indicating the distance of the Chamaeleon dark cloud was determined. This has also been done for HD 97048 and HD 97300, which turn out to be anomalous with a ratio of total to selective extinctionR5.0. In studies involving the distance of the Chamaeleon dark cloud we propose to use 150±15 pc. The extinction-free spectral energy distribution of HD 97300 does not show the presence of a hot dust shell of 2750 K, which was found by Théet al. (1986) in a previous study.Based in part on observations made at the ESO, La Silla, Chile.  相似文献   

5.
Thirty stars in the direction of the open cluster NGC 6996 are measured in the Vilnius photometric system. Photometric spectral types, absolute magnitudes, interstellar reddenings, extinctions, and distances are determined for most of them. Fifteen stars are suspected to be cluster members. Their mean distance is 620±30 pc and mean extinctionA v is 1.74 mag. The extinction within the cluster is variable. The age of the cluster is of the order of 108 yr and this excludes the possibility that it is evolutionary related with the North America and Pelican Nebulae complex. The distance of this complex 550 pc is confirmed.  相似文献   

6.
Soft X-rays (0.1–0.8 keV) from the region including the Oph dark cloud were observed with the SAS-3 low-energy X-ray telescope. No X-ray absorption by the cloud was observed. This indicates that the diffuse component of soft X-rays in this region is mostly from the foreground of the Oph cloud which is located at a distance of 160–200 pc.  相似文献   

7.
Observations obtained with the Midcourse Space Experiment (MSX) satellite reveal for the first time the complex mid-infrared morphology of the entire Carina Nebula (NGC 3372). On the largest size scale of approximately 100 pc, the thermal infrared emission from the giant H ii region delineates one coherent structure: a (somewhat distorted) bipolar nebula with the major axis perpendicular to the Galactic plane. The Carina Nebula is usually described as an evolved H ii region that is no longer actively forming stars, clearing away the last vestiges of its natal molecular cloud. However, the MSX observations presented here reveal numerous embedded infrared sources that are good candidates for sites of current star formation. Several compact infrared sources are located at the heads of dust pillars or in dark globules behind ionization fronts. Because their morphology suggests a strong interaction with the peculiar collection of massive stars in the nebula, we speculate that these new infrared sources may be sites of triggered star formation in NGC 3372.  相似文献   

8.
The potential of mid-infrared imaging, at a resolution of 0.5 or better, as a powerful tool in unravelling the complex environments in the vicinity of active galactic nuclei is discussed in the light of new data on the Seyfert nucleus in NGC 1068. In particular, we examine the growing body of evidence that the extinction of the broad-line region in NGC 1068 has its origins in an extended molecular disk and is the result of obscuration at 100 pc from the AGN.  相似文献   

9.
We study the hypothetical conditions for interstellar clouds dense enough to produce glaciations on the Earth. A simple differential formula (adequate to give lower limits to dust absorption) is used to relate mean temperatures and visual albedos today and during the glacial eras. For this, the geological and oceanographical records of the Pleistocene are used. The temperature decays are associated to an absorption of the solar light in visual magnitudes m v. As the effective albedo, integrated in all wavelengths are lower than the corresponding visual value, the adoption of a visual scale leads to an underestimation of the actual amount of dust. A minimum dust absorption m v= 0.02 mag, necessary to start a glacial era is then obtained. This should mean interstellar clouds with dust densities of 4100 mag. pc -1 and sizes of 0.3 pc or more, taking into account the time span of the glacial eras and the mean velocity of the Sun with respect to the LSR. Such clouds were never observed and are uncompatible with what is known from the interstellar medium: the glaciation clouds should be clouds with densities 50–100 times above the tolerable value for gravitational stability; on the other hand, the necessary number of clouds per cubic parsec should produce the collapse of the galactic disc as a whole. From a comparative analysis of the temperatures of the other planets it seems to be that the actual thermal temperatures in their surfaces depend less than one expects from the visual albedos. From this it is raised the suspicion that the cause of the ice ages was the Sun itself.  相似文献   

10.
To investigate the kinematics of the neutral material around the Gum nebula, emission from hydroxyl at 1667 MHz was observed at many positions over the region. Fitting models of expanding shells to these data together with previously published molecular line data shows that the diffuse molecular clouds and cometary globules form a single expanding shell centred on G261−2.5. The mean angular radius is 10.5° and its maximum radius is 14°. The models show that the distance range to the expansion centre is from 200 pc to 500 pc.
The path of the runaway O-star ζ Puppis passed within <0.5° of the expansion centre of the neutral shell ∼1.5 Myr ago. The supernova of the erstwhile binary companion of ζ Puppis is the probable origin of the Gum nebula and the swept up expanding neutral shell. The 500-pc distance to the supernova is adopted as the distance to the expansion centre of the neutral shell. At this distance the energy required to produce the observed expansion could have been met with a single supernova. The radii of the front and back faces of the shell are 130 and 70 pc respectively. The front face is expanding faster than the back face, at 14 and 8.5 km s−1 respectively.
The extent of the neutral shell matches the radio continuum and H α emission of the Gum nebula well. The photoionized gas in the nebula is probably primarily ionized by ζ Puppis, which is still within the neutral shell. No evidence was found for the IRAS -Vela shell as a separate expanding shell.  相似文献   

11.
The star LSS 3027, embedded in a parabolic-shaped dark nebula, and several stars in its vicinity, were observed photometrically and spectroscopically to investigate whether they are candidates of very young Herbig Ae/Be stars. Only the star LSS 3027B is without doubt a very good candidate. Its H-emission is very strong, indicating an extended atmosphere around the star, and its near- and far-infrared spectral energy distributions exhibit a strong excess radiation, which most probably is caused by a thick dust shell in the close environment of the star. The ratio of total to selective extinction is found to be equal toR-5.1. It can be shown that it is embedded in the dark cloud mentioned above. From its properties we believe that the remarkable object LSS 3027B is a typical Hergig Be pre-Main-Sequence object.Based on data obtained at the ESO, La Silla, Chile and with the Infrared Astronomical Satellite (IRAS).  相似文献   

12.
Gravitational settling of dust grains in dark clouds has been considered. It has been shown that such a process gives rise to a modification of the grain size distribution. Starting with a simple model of uniform spherical cloud and normal interstellar grain size distribution for the dust we derive expressions for the modified grain size distribution function, average grain size and extinction as functions of distance from the cloud's center and the age of the cloud. The mean grain size increases towards the center of the cloud as does the extinction. Results of the numerical evaluation of these quantities have been discussed with their implications for the observations of anomalous reddening and polarization within dark clouds and Bok globules.  相似文献   

13.
The correlation betweenE(B-V) reddenings of high latitude radio quasars (from Teerikorpi, 1981) and star/galaxy counts from the PSA prints was investigated. With the resolution down to 5×5, the reddenings and the counts show the relationship expected if it is assumed that most of the high latitude stars (B<21 mag) are situated above the dust clouds. The dust causing the reddenings is probably connected with the population of molecular clouds observed at high galactic latitudes. The present study underlines the importance of simple star/galaxy counts (all images counted) for the study of high latitude variable extinction. The existence of variable extinction shows that also the average extinction must be significant.  相似文献   

14.
We present a submillimetre continuum survey for accretion discs around seven embedded protostars in the Perseus and Serpens molecular clouds. Observations were made at frequencies between 339 and 357 GHz using the James Clerk Maxwell Telescope–Caltech Submillimeter Observatory single-baseline interferometer on Mauna Kea, Hawaii. All the objects in our survey show compact dust emission on scales ≲1 arcsec, assumed to arise in a circumstellar accretion disc. We compare the properties of this compact component with evolutionary indicators, such as the ratio of compact to extended emission, and bolometric temperature. We find that discs of mass ∼0.01 M⊙ have formed by the Class 0 stage, and that similar mass discs are observed in Class I and Class II sources. A trend is observed whereby the ratio of compact to extended emission in our sources increases from Class 0 to Class II sources. For three of the objects in the survey, NGC 1333 IRAS2:CR1 and SVS13 in Perseus, and FIRS1 in Serpens, the signal-to-noise ratio is sufficient to allow us to model the brightness distributions with elliptical Gaussian and power-law disc models. The Gaussian fits give semimajor half-power radii of approximately 90 to 140 au, at the assumed distance of 350 pc to the Perseus and Serpens clouds.  相似文献   

15.
The efficiency of the anti-proton (pBAR) production in interstellar clouds is increased by a factor 50 when the engendering cosmic-ray protons (CRs) experience diffusive acceleration by shocks. One can represent the pBAR/p observations by a conventional multi-phase model of the interstellar medium in which the volume-filling factor for shocked clouds is 1 % of that for all clouds. The pBAR/p ratio will reach a maximum value at an energy which depends upon a typical cloud size; the present observations give this size to be 1 pc. if individual cloud sizes do not get very much larger then thepBAR/p value may peak at a low energy, slightly above 10 GeV. The porosity factor of Cox and Smith (1974) is limited (Q0.2). Standard CR confinement models are examined using a diffusion equation approach. Most of the production ofpBAR's by any old CR population will occur in shocked clouds near the galactic plane. One does not need to assume an old CR population exists in order to explain thepBAR observations.  相似文献   

16.
This paper gives the result of photographic photography of the central star AGK3-0° 695 of the planetary nebula NGC 2346 made from 1981 through 1987. From it we see that after the large-amplitude eclipses which had started at the end of 1981 had continued for several years, the amplitude began to decrease rapidly in 1986 to ˜1.1 mag from ˜4 mag in 1984. In 1987, only irregular fluctuations with amplitudes ˜0.4 mag were present in the light curve and not longer any clear periodicity. A preliminary analysis of this phenomenon is carried out here.  相似文献   

17.
We report results concerning the distribution and properties of galactic extinction at high galactic latitudes derived from stellar statistics using the Sloan Digital Sky Survey (SDSS). We use the classical Wolf diagram method to identify regions with extinction, and derive the extinction and the extinction law of the dust using all five SDSS spectral bands. We estimate the distance to the extinguishing medium using simple assumptions about the stellar populations in the line of sight.
We report the identification of three extinguishing clouds, each a few tens of pc wide, producing 0.2–0.4 mag of g '-band extinction, located 1–2 kpc away or 0.5–1 kpc above the Galactic plane. All clouds exhibit grey extinction, i.e. almost wavelength independent in the limited spectral range of the SDSS. We discuss the implication of this finding on general astrophysical questions.  相似文献   

18.
The stellar field centred close to theh and Per double cluster is one of the 123 fields recorded in the galactic plane at 2000 Å by the balloon-borne stratospheric gondola of the SCAP-2000 programme. The analysis of the frame allows us to determine an ultraviolet colour indexU 1-V for more than 600 stars. Among these are stars belonging to theh and Per and Tr 2 clusters and to the PER OB1 association. The prevailing extinction law is found to produce greater extinction at 1965 Å than predicted by the mean extinction law. Moreover, the clouds responsible for the extinction are situated in the local arm and distributed in two layers with a very transparent interval. The comparison of theA v extinction and theHI and CO abundances leads us to assume the presence of a H2 cloud in front ofh and Per, in the second absorbing layer and, therefore, in the local arm. The two absorbing layers and the molecular cloud are perhaps in the plane of the Gould belt and associated with the expanding gas detected by Lindblad. A group of hot stars centred at the same distance as this molecular cloud has been detected and could form an association of OB stars in the local arm. Other, much more distant OB stars belonging to the Perseus arm of Efremov's list. Several stars which must have a very hot companion are detected in the field.  相似文献   

19.
Dense molecular clouds within the Taurus and NGC 2264 regions have undergone gravitational collapse and fragmentation to form groups of low mass (1M ) T-Tauri stars which are still embedded within the clouds and which are kinematically associated with them. Molecular column densities on the order of 1014 cm–2 are inferred from the emission lines of OH and NH3. Emission line widths are 2 km s–1 and the antenna beamwidths include linear extents of order 0.1 pc. The OH emission appears to be in a condition of local thermodynamic equilibrium, and it cannot arise from circumstellar sheils similar to those surrounding the masing infrared stars. The OH and NH3 emission occurs in clouds of 1 pc in extent with optical depths of 0.1 to 1.0 and excitation temperatures of the order of 10 K. The molecular clouds have radii of 0.5 pc, molecular hydrogen densities of 4000 cm–3, masses of 100 solar masses, and kinetic temperatures of 20 K. The observed data are not inconsistent with the molecular clouds being in a state of hydrostatic equilibrium.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.  相似文献   

20.
Observations of the ionized hydrogen region NGC 1499 have been carried out with the radio telescope UTR-2 at frequencies 12.6, 14.7, 16.7, 20 and 25 MHz. The half-power resolution of the instrument to zenith is 28×34 at 25 MHz. The average volume density of the non-thermal radio emission between the Sun and the nebula (1.75×10–40 W m–3 Hz–1 ster–1 at 25 MHz), the electron temperature of the HII nebula (T e =4400 K), the measure of emission (ME=1500 cm–6 pc) and other parameters have been obtained. Maps of brightness distribution over the source are presented for each observation frequency. The results are compared with previously obtained data.  相似文献   

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