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1.
本文首次提出了从观测得到的具有星周尘埃壳层的恒星的能谱分布求取星周尘埃云的消光,并由此可通过改正星周消光改正后的星际消光法求得恒星距离的方法。  相似文献   

2.
We present the results of our photometric (V RI) and spectroscopic observations of the young variable star V730 Cep (MisV1147) classified by Uemura et al. (2004) as a Herbig Be star. Our photometry confirms the conclusion of the above authors that this star has a complex pattern of variability including periodic or quasi-periodic brightness variations with a period of about 14 days and deep Algol-like minima typical for UX Ori stars. Our spectroscopy shows that the classification of V730 Cep as a Herbig Be star is wrong. Actually, this star has a much lower temperature and belongs to the family of T Tauri stars. This allows us to explain the nature of the unusual photometric activity of V730 Cep based on a combination of two well-known models of variable circumstellar extinction applied to young stars: AA Tau- and UX Oritype variability. It follows from our observations that the color tracks on the V ?(V ?I) color–magnitude diagram for these models slightly differ: the AA Tau-type variability of circumstellar extinction is caused by larger grains than the UX Ori-type variability. Such a difference can be due to an increase in the characteristic sizes of circumstellar dust as the star is approached and has a simple explanation: small dust grains evaporate faster than large ones.  相似文献   

3.
Photoelectric Vilnius photometry of the B-type stars HD 29 647 and HDE 283 809 in the direction of the Taurus molecular cloud indicates their brightness and energy distribution to be constant within 1–2%. The interstellar extinction law is determined for the star HDE 283 809 from the photometry data in the Vilnius andUBVRJHKL systems, which yield the ratioR=A V/EB-V=3.5 and grain sizes exceeding the average by approximately 10%. The interstellar extinction law for the two stars is found to be the same in the infrared, however, it is very different in the near ultraviolet. The new spectra of HDE 283 809 confirm the earlier classification and indicate an absence of emission in the hydrogen lines. The interstellar band at 443 nm is observed but its intensity is a half of what is expected forE B-V=1.61. The observed peculiarities of the energy distribution in the spectrum of HDE 283 809 apparently originate in interstellar or circumstellar dust, not in the star itself.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.  相似文献   

4.
The very large brightness decrease of late-type Herbig Ae/Be stars is believed to be caused by obscuring dust clouds orbiting in the outer parts of their circumstellar disks. The distances of the dust clouds to the central stars have been estimated using the wavelength at maximum flux of the excess near-IR radiation, Wien's displacement law, and a formula derived by Rowan-Robinson (1980). The critical masses of these clouds were calculated employing Chandrasekhar's (1943) formula. The minimum size of the dust grains in the obscuring clouds was estimated using Aumannet al.'s (1984) formula they had applied to the star Lyr. However, it can be about ten times smaller if the dust grains are situated at the back of the cloud. The average size of these grains has been determined by assuming a size distribution similar to that in the asteroidal belt (Dohnanyi, 1969) and in the interstellar medium (Mathiset al., 1977). Their number density was determined by means of the extinction power of the dust cloud at theV pass-band. The results of our calculations show that above parameters are similar to those in our solar system. Therefore, we believe that most probably (a) the formation of planetesimals in the circumstellar disks of Herbig Ae/Be stars is on-going; and (b) the obscuring clouds will, in the long run, become planet-like objects.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.  相似文献   

5.
Summary Cosmic dust grains play an important role for the thermal, dynamical, and chemical structure of the interstellar medium. This is especially true for the star formation process and the late stages of stellar evolution. Dust grains determine the spectral appearance of protostars, very young stellar objects with disk-like structures as well as of evolved stars with circumstellar envelopes.In this review, we will demonstrate that solid particles in interstellar space are both agent and subject of galactic evolution. We will especially discuss the different dust populations in circumstellar envelopes, the diffuse interstellar medium, and the molecular clouds with strong emphasis on the evolutionary aspects and the metamorphosis of these populations.  相似文献   

6.
Multicolor photometry from the Tycho-2 and 2MASS catalogues for 11 990 OB and 30 671 K-type red giant branch stars is used to detect systematic large-scale variations of the interstellar extinction law within the nearest kiloparsec. The characteristic of the extinction law, the total-to-selective extinction ratio Rv, which also characterizes the size and other properties of interstellar dust grains, has been calculated for various regions of space by the extinction law extrapolation method. The results for the two classes of stars agree: the standard deviation of the “red giants minus OB” Rv differences within 500 pc of the Sun is 0.2. The detected Rv variations between 2.2 and 4.4 not only manifest themselves in individual clouds but also span the entire space near the Sun, following Galactic structures. In the Local Bubble within about 100 pc of the Sun, Rv has a minimum. In the inner part of the Gould Belt and at high Galactic latitudes, at a distance of about 150 pc from the Sun, Rv reaches a maximum and then decreases to its minimum in the outer part of the Belt and other directions at a distance of about 500 pc from the Sun, returning to its mean values far from the Sun. The detected maximum of Rv at high Galactic latitudes is important when allowance is made for the interstellar extinction toward extragalactic objects. In addition, a monotonic increase in Rv by 0.3 per kpc toward the Galactic center has been found near the Galactic equator. It is consistent with the result obtained by Zasowski et al. (2009) for much of the Galaxy. Ignoring the Rv variations and traditionally using a single value for the entire space must lead to systematic errors in the calculated distances reaching 10%.  相似文献   

7.
Diffraction-limited array images of the Trapezium/Ney Allen infrared nebula have been obtained at six wavelengths between 7.8 and 12.4 microns, including the 9.7 micron silicate feature. Extended emission from warm dust shows significant differences in structure around each of the four Trapezium stars. The most dramatic infrared source is associated with 1 Orionis D, where the bright mid-infrared emission is found to be a distinct crescent-shaped ridge or shell, concentric with the O star. This unambiguous relationship between a known type stellar luminosity source and a distinct circumstellar dust cloud of known distance and dimensions provides a unique opportunity to test the predictions of dust grain emission models for circumstellar infrared sources.  相似文献   

8.
The observed relation between the interstellar linear polarization curve parameters K and λ max characterizing the width and the wavelength of the polarization maximum, respectively, is interpreted quantitatively. We have considered 57 stars located in four dark clouds with evidence of star formation: in Taurus, Chamaeleon, around the stars ρ Oph and R CrA. In our modeling we have used the spheroidal dust grain model applied previously to simultaneously interpret the interstellar extinction and polarization curves in a wide wavelength range. The observed trend K ≈ 1.7λ max is shown to be most likely related to the growth of dust grains due to coagulation rather than accretion. The relationship of the interstellar polarization curve parameters K and λ max to the mean dust grain size is discussed.  相似文献   

9.
The surface distribution of M stars is studied by differentiating them according to whether they show a circumstellar dust shell (CS) or not. Analysis shows that galactic latitudinal and longitudinal distributions are not determined by spectral subclasses alone. The study also indicates that M-type stars with CS have higher luminosities in the K band than those without CS. The M stars used in the study are obtained from theTwo-Micron Sky-Survey Catalogue (IRC) which is a most unbiased sample with respect to the interstellar extinction. The CS feature is identified by the ratio of flux densities at 12 and 25 m in the IRAS point source catalog.  相似文献   

10.
Observations of circumstellar dust clouds of Wolf-Rayet stars made with the Short Wavelength Spectrometer on ISO reveal a subtle variety of spectral energy distributions not evident from ground-based observations. The modelling of these using the radiative transfer code "Dusty" by Ivezić, Nenkova & Elitzur is reported. The results are used to examine the contributions by different possible grain types to the emission and of circumstellar and interstellar components to the reddening. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

11.
《Icarus》1987,69(1):70-82
It is shown that the dense, turbulent, decelerating shells produced by protostellar flows around young stars are a probable site for rapid grain growth by coalescing collisions. The growth of grains occurs in a thin dust layer at the leading edge of the gas shell until a critical grain size on the order of 1−10 μm is reached. Grains larger than this decouple from the turbulence and eventually reach sizes of ≈100 μm. These large grains form a thin dust shell with low-velocity dispersion, in which ultimately local gravitational instability takes place. This causes the accumulation of comet-sized aggregations of dust, assuming that the dust velocity dispersion is on the order of 10−2 m sec−1. It is proposed that the mechanism could lead to a high space density of comets in molecular clouds. The efficient formation of “giant” grains, and even comet nuclei, in the regions around young stars has important implications both for cometary astronomy and for understanding the dynamical and chemical evolution of molecular clouds and the interstellar medium.  相似文献   

12.
It has been shown that two concentric ionization zones of interstellar magnesium must exist around each star: internal, with a radius coinciding with that of the zone of hydrogen ionizationS H; and external, with a radius greater thanS H, by one order. Unlike interstellar hydrogen, interstellar magnesium is ionized throughout the Galaxy. It also transpires that the ionizing radiation of ordinary hot stars cannot provide for the observed high degree of ionization of interstellar magnesium. The discrepance can be eliminated by assuming the existence of circumstellar clouds or additional ionization sources of interstellar magnesium (X-ray background radiation, high-energy particles, etc.). Stars of the B5 and B0 class play the main role in the formation of ionization zones of interstellar magnesium; the contribution of O class stars is negligible (<1%).  相似文献   

13.
We present results of near infrared observation of 21 molecular outflow sources and two non-outflow sources with compact cores. Combined with IRAS and other surface station observations we analyse their spectra and find that the outflow sources have, on average, steeper spectral gradients than the non-outflow sources in the range 2.28–25 μ. Most of the bipolar outflow sources have gradients greater than 2.0. Using a revised blackbody photosphere model we calculate the contributions to the JHK fluxes by the central young star and the circumstellar envelope. For the sources with known bolometric luminosity we derive the photospheric temperature of the central star and the circumstellar extinction. Results show that most of the young stars associated with molecular outflows are probably T Tauri stars (5000–7000 K) or emission line stars (9000–26000 K). The circumstellar extinction in JHK is around 10 to 20 magnitudes. These facts show that molecular outflow sources are young objects still embedded deep inside or around the interior of compact cores. Fitting the 3.5– 25 μ and 60–100 μ spectra with a λ-1 dust emission model to five source gives a negative power law for the temperature profile of the circumstellar shell with exponents between 0.39 and 0.48, close to the theoretical results for molecular clouds associated with HI I regions.  相似文献   

14.
The transfer of polarized radiation in inhomogeneous circumstellar shells with a spheroidal spatial distribution of porous dust particles is computed. The grains are modeled by an MRN mixture of silicate and graphite particles. The optical properties of porous particles (considered separately in the Appendix) are computed by using effective medium theory and Mie theory. The following observational characteristics have been computed for WW Vul, a typical Herbig Ae star with Algol-like minima: the spectral energy distribution from the ultraviolet to the far infrared, the color-magnitude diagrams, the wavelength dependence of linear polarization, and the shell brightness distribution. The effect of grain porosity on the results is considered. It has been found that only moderate particle porosity (the volume fraction of matter is f ～0.5) can explain available observational data in terms of the approach used. Since radiation pressure must rapidly sweep submicron-sized grains out of the vicinity of Herbig Ae/Be stars, we briefly discuss how particle porosity can affect this process.  相似文献   

15.
Photometry from the Tycho-2, 2MASS, andWISE catalogues for clump and branch giants at a distance up to 25 kpc toward the Galactic poles has allowed the variations of various characteristics of the infrared interstellar extinction law with distance to be analyzed. The results obtained by the extinction law extrapolation method are consistent for different classes of stars and different characteristics as well as with previous studies. The conventional extinction law with a low infrared extinction is characteristic of only a thin layer no farther than 100 pc from the Galactic plane and of two thin layers near Z = ?600 and +500 pc. Far from the Galactic plane, in the Galactic halo, the infrared extinction law is different: the extinction in the Ks, W1, W2, W3, and W4 bands is, respectively, 0.17, 0.16, 0.16, 0.07, and 0.03 of the extinction in the V band. The accuracy of these coefficients is 0.03. If the extinction law reflects primarily the grain size distribution, then the fraction of large dust grains far from the Galactic plane is greater than that in the circumsolar interstellar medium.  相似文献   

16.
We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field.  相似文献   

17.
Interstellar extinction curves obtained from the ‘extinction without standard’ method were used to constrain the dust characteristics in the mean ISM (R V = 3.1), along the lines of sight through a high latitude diffuse molecular cloud towards HD 210121 (R V = 2.1) and in a dense interstellar environment towards the cluster NGC 1977 (R V = 6.42). We have used three-component dust models comprising silicate, graphite and very small carbonaceous grains (polycyclic aromatic hydrocarbons) following the grain size distributions introduced by Li & Draine in 2001. It is shown that oxygen, carbon and silicon abundances derived from our models are closer with the available elemental abundances for the dust grains in the ISM if F & G type stars atmospheric abundances are taken for the ISM than the solar. The importance of very small grains in modelling the variation of interstellar extinction curves has been investigated. Grain size distributions and elemental abundances locked up in dust are studied and compared at different interstellar environments using these three extinction curves. We present the albedo and the scattering asymmetry parameter evaluated from optical to extreme-UV wavelengths for the proposed dust models.  相似文献   

18.
High linear polarization (up to 5–8%) discovered by our group in the deep minima of isolated AE-Herbig stars is discussed in the framework of the model of zodiacal light produced by scattering matter in a circumstellar dust envelope (probably in protoplanetary discs). The numerical simulations of polarizational and colourimetrical properties of the scattered light based on the Mie theory permit to obtain from the observations some important parameters of circumstellar dust: the size distribution of grains, their rough chemical composition (silicate/graphite), the flatness of the dust envelope.Moreover, the position angle of polarization in deep minima may be used for determination of the symmetry axis projection of circumstellar disc on the sky relative to the direction of local interstellar magnetic field. The latest is important in order to understand the role of the magnetic field at the initial phase of gravitational collapse of protostellar clouds.Finally, the component of linear polarization which is due to the alignment of nonspherical circumstellar grains may be separate from the observed polarization under certain conditions.Paper presented at the 12th European Regional Astronomical Meetings of the IAU European Astronomers Look to the Future, held 8–11 October, 1990 in Davos, Switzerland.  相似文献   

19.
TheIRAS colours of some B stars with anomalous UV extinction confirm that they have circumstellar dust.  相似文献   

20.
This paper presents a semi-empirical model for variations of interstellar polarization curves based upon the Serkowski-Wilking law for optical and near-infrared wavebands. The model assumes that nonspherical dust grains producing interstellar polarization are core-mantle particles shaped like oblate spheroids. The physical picture is one in which large (a 0 0.1µm) particles in the dense cloud phase are deposited into the diffuse cloud medium and thereafter undergo mantle processing by galactic shocks and UV starlight. It is shown that polarization curves vary their widths mainly as a consequence of the nonthermal sputtering of mantles by low-velocity shocks. Mantle sputtering by shocks in low density clouds tends to broaden the curves, whereas mantle sputtering by shocks in denser clouds produce narrow curves. Hence, shock processing of grain mantles can explain the observed correlation between the width of polarization curves and the dust grain environment.  相似文献   

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