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1.
In a series of papers of You, Cheng, et al. (cf., for example, [2–4]) the Cherenkov mechanism of radiation was proposed as the explanation of the wide emission lines in the spectra of quasars. This mechanism acts because the refractive index of the plasma in a certain frequency range near a spectral line can exceed one. It has been found that the role of Cherenkov radiation becomes significant for an optically thick medium when the density of the packet of relativistic electrons ñe is 104–106 cm–3. However, the question of the formation of a spectral line as the result of ordinary (recombination and collision) processes under the same physical conditions has not been considered, and hence no comparison has been made between the intensities of lines formed by different mechanisms. Moreover, a large number of effects that may have significant influence on the profile of a line (multiple scattering in the medium, redistribution of radiation over frequencies within a line, redistribution of energy between a line and the continuous spectrum) have remained unexamined by these authors.The present work aims at sharpening some of the formulas used in [2–4] and filling up some of these gaps. We pose the classical problem of formation of a spectral line taking account of the action of the Cherenkov radiation mechanism and give an exact solution of it. The computations are carried out for the line L. It is shown that under the values presently accepted for the electron temperature for an average quasar (Te 1.5 · 104 K) the influence of the Cherenkov mechanism becomes noticeable only for implausibly high densities of relativistic electrons (1011 cm–3, The geometric model proposed in [4] for quasar radiation in a line encounters insuperable difficulties involving the energy of the quasar and the sizes of the radiating regions.Translated fromAstrofizika, Vol. 37, No. 3, 1994.  相似文献   

2.
The molecular phase of the ISM constitutes the main source of fuel for the activity in starburst and AGNs. The physical conditions and chemical constitution of the molecular gas will change with, and respond to, the evolution of the activity. This paper includes a short discussion of the 12CO/13CO 1–0 line intensity ratio as a diagnostic tool of the molecular gas properties of luminous galaxies – paired with examples of high-resolution studies of how the line ratio varies within galaxies. A possible connection between the OH megamasers and galaxies with unusually high 12CO/13CO 1–0 line intensity ratios are also briefly discussed.The relative intensities of the dense gas tracers HNC, HCN, HCO+ and CN are a result of both chemistry and starburst evolution. The discussion on the interpretation of HNC 1–0 emission includes the importance of ion-neutral chemistry in a luminous starburst region. Finally, simple cartoon ISM models and how they can be applied to LIRGs and ULIRGs, are presented.  相似文献   

3.
We present Monte Carlo radiative-transfer simulations for spiral galaxies modelled as a stellar disc and a two-phase clumpy dust distribution. We divide the volume occupied by the dust into a three-dimensional grid and assign each cell a clump or smooth medium status. Cell dimension, clump dust mass and spatial distribution are derived from the observed properties of giant molecular clouds and molecular gas in the Galaxy. We produce models for several values of the optical depth and fraction of the interstellar medium residing in clumps. As a general result, clumpy models are less opaque than the corresponding homogeneous models. For the adopted parameters, the increase in the fraction of energy that escapes the disc is moderate, resulting in surface-brightness profiles that are less than one magnitude brighter than those of the homogeneous models. The effects of clumping are larger for edge-on views of the disc. This is in contrast with previous preliminary results for clumping in the literature. We show how differences arise from the different parametrization and clump distribution adopted. We also consider models in which a fraction of the stellar radiation is emitted within the clumps. In this case, galaxies are less transparent than in the case when only dust is clumped. The opacity can be even higher than in the homogeneous case, depending on the fraction of embedded stellar emission. We point out the implications of the results for the determination of the opacity and dust mass of spiral galaxies.  相似文献   

4.
On the basis of Sobolev's method, the population of 30 levels of hydrogen atom is determined allowing for the radiative and collision processes of the heating and ionization of the medium with velocity gradient gradv=10–9–10–11s–1, electron temperatureT e=104 K-2×104 K and electron densityN e=1010 cm–3–1011 cm–3. The central source radiation is characterized by a power spectrum with spectral indices varying from 0 to 2. A region of possible physical conditions is found where the thermal diffuse radiation of the envelope exceeds the emission in the Balmer H line.  相似文献   

5.
Oxygen and carbon isotope ratios in the martian CO2 are key values to study evolution of volatiles on Mars. The major problems in spectroscopic determinations of these ratios on Mars are uncertainties associated with: (1) equivalent widths of the observed absorption lines, (2) line strengths in spectroscopic databases, and (3) thermal structure of the martian atmosphere during the observation. We have made special efforts to reduce all these uncertainties. We observed Mars using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope. While the oxygen and carbon isotope ratios on Mars were byproducts in the previous observations, our observation was specifically aimed at these isotope ratios. We covered a range of 6022 to 6308 cm−1 with the highest resolving power of ν/δν=3.5×105 and a signal-to-noise ratio of 180 in the middle of the spectrum. The chosen spectral range involves 475 lines of the main isotope, 184 lines of 13CO2, 181 lines of CO18O, and 119 lines of CO17O. (Lines with strengths exceeding 10−27 cm at 218 K are considered here.) Due to the high spectral resolution, most of the lines are not blended. Uncertainties of retrieved isotope abundances are in inverse proportion to resolving power, signal-to-noise ratio, and square root of the number of lines. Laboratory studies of the CO2 isotope spectra in the range of our observation achieved an accuracy of 1% in the line strengths. Detailed observations of temperature profiles using MGS/TES and data on temperature variations with local time from two GCMs are used to simulate each absorption line at various heights in each part of the instrument field of view and then sum up the results. Thermal radiation of Mars' surface and atmosphere is negligible in the chosen spectral range, and this reduces errors associated with uncertainties in the thermal structure on Mars. Using a combination of all these factors, the highest accuracy has been achieved in measuring the CO2 isotope ratios: 13C/12C = 0.978 ± 0.020 and 18O/16O = 1.018 ± 0.018 times the terrestrial standards. Heavy isotopes in the atmosphere are enriched by nonthermal escape and sputtering, and depleted by fractionation with solid-phase reservoirs. The retrieved ratios show that isotope fractionation between CO2 and oxygen and carbon reservoirs in the solid phase is almost balanced by nonthermal escape and sputtering of O and C from Mars.  相似文献   

6.
Molecular clouds are clumpy on length scales down to the limits of observational resolution. At least some ultracompactHii regions (UCHiiR) may result from the interaction of a young early type star and this type of cloud. The clumps can act as reservoirs of ionized gas distributed within theHii region. These models reproduce the relatively long lifetimes implied by the population statistics of UCHiiR. We present line profile and emission measure plots based on the simplest case where the flow remains supersonic through to a recombination front. The morphology agrees with the shell-like UCHiiR as classified by Churchwell. The predicted line profiles are broad and double peaked with a separation of about 50 km s–1 for the example given.  相似文献   

7.
The spatial distribution of the molecule CO in the internal layer of the circumstellar shell of the cool carbon-rich star TX Psc is presented. The main source of the photodissociation of CO in the internal layer of the shell is chromospheric UV radiation. The abundance of CO is determined by the photodissociation through absorption of UV photons by line processes in the wavelength range of 1099 to 913.5 Å. We have used a weak extended and a strong extended chromosphere. While in a homogeneous shell, the molecule CO is not photodissociated, in a clumpy envelope, it is destroyed by the chromospheric UV radiation field, depending of the degree of clumpiness of the shell.  相似文献   

8.
A numerical scheme is proposed for the solution of the three-dimensional (3D) radiative transfer equation with variable optical depth. We show that time-dependent ray tracing is an attractive choice for simulations of astrophysical ionization fronts, particularly when one is interested in covering a wide range of optical depths within a three-dimensional clumpy medium. Our approach combines the explicit advection of radiation variables with the implicit solution of local rate equations given the radiation field at each point. Our scheme is well suited to the solution of problems for which line transfer is not important, and could, in principle, be extended to those situations also. This scheme allows us to calculate the propagation of supersonic ionization fronts into an inhomogeneous medium. The approach can be easily implemented on a single workstation and should also be fully parallelizable.  相似文献   

9.
We have performed the first observation of the CO(3-2) spectral line in the atmosphere of Saturn with the James Clerk Maxwell Telescope. We have used a transport model of the atmosphere of Saturn to constrain the origin of the observed CO. The CO line is best-fit when the CO is located at pressures less than (15±2) mbar with a mixing ratio of (2.5±0.6)×10-8 implying an external origin. By modeling the transport in Saturn’s atmosphere, we find that a cometary impact origin with an impact 200-350 years ago is more likely than continuous deposition by interplanetary dust particles (IDP) or local sources (rings/satellites). This result would confirm that comet impacts are relatively frequent and efficient providers of CO to the atmospheres of the outer planets. However, a diffuse and/or local source cannot be rejected, because we did not account for photochemistry of oxygen compounds. Finally, we have derived an upper limit of ∼1×10-9 on the tropospheric CO mixing ratio.  相似文献   

10.
We report measurements of line intensities and line widths for three quiescent prominences observed with the Naval Research Laboratory slit spectrograph on ATM/Skylab. The wavelengths of the observed lines cover the range 1175 Å to 1960 Å. The measured intensities have been calibrated to within approximately a factor 2 and are average intensities over a 2 arc sec by 60 arc sec slit. We derive nonthermal velocities from the measured line widths. The nonthermal velocity is found to increase with temperature in the prominence transition zone. Electron densities and pressures are derived from density sensitive line ratios. Electron pressures for two of the prominences are found to lie in the range 0.04–0.08 dyn cm–2, while values for the third and most intense and active of the three prominences are in the range 0.07–0.22 dyn cm–2.  相似文献   

11.
The first generation of X-ray telescopes to use Charge-Coupled Devices (CCDs) is being launched this decade. With a read noise of a few electrons, CCDs provide Fano-limited spectral resolution across the soft X-ray band (0.1 – 10 keV). However, degradation of resolution due to charge transfer losses becomes noticeable as Charge Transfer Inefficiency (CTI) increases to 10–5. In this paper, we present a model which calculates the effects of radiation damage in low Earth orbit in order to predict CCD lifetimes over which good charge transfer is maintained. The model presented here considers damage mechanisms within the CCD, environmental conditions in which the CCD operates, and experiment shielding. We find that the predicted CTI approaches 10–5 after a one to two year mission for the flight instruments considered here.  相似文献   

12.
We present a detailed study of the bremsstrahlung gamma-ray emissivity of the galactic disk. We show that there are large uncertainties in the production spectrum of photons in the medium energy range (10–100 MeV) due to our lack of knowledge of the interstellar electron spectrum below a few hundred MeV. In fact, gamma-ray observations can be of great help in determining this spectrum. At present, the spectral shape of the local gamma-ray emissivity above 30 MeV is available, thanks to the SAS-II and the COS-B satellites. Comparing it to our calculations, we determine the local interstellar electron flux in the 50–500 MeV range; the corresponding integrated gamma-ray emissivity above 100 MeV is equal to 2.4×10–25 photons s–1 (H-atom)–1, 60% higher than previously accepted values.  相似文献   

13.
Cosmic gamma-ray burst spectroscopy   总被引：1，自引：0，他引：1  
A review is given of the gamma-ray burst energy spectrum measurements on Venera 11 and Venera 12 space probes. The gamma burst continuum approximates in shape thermal brems-strahlung emission of a hot plasma. The radiation temperature varies over a broad range, 50–1000 keV, for different events. Spectra of many bursts contain cyclotron absorption and/or redshifted annihilation lines. Strong variability is typically observed in both continuum and line spectra. These spectral data provide convincing evidence for the gamma-ray bursts being generated by neutron stars with superstrong magnetic fields 1012–1013 G.  相似文献   

14.
We quantitatively scrutinize the effects of the radiation drag arising from the radiation fields in a galactic bulge in order to examine the possibility that the radiation drag could be an effective mechanism to extract angular momentum in a spheroidal system like a bulge and allow plenty of gas to accrete on to the galactic centre. For this purpose, we numerically solve the relativistic radiation hydrodynamical equation coupled with accurate radiative transfer, and quantitatively assess the radiation drag efficiency. As a result, we find that in an optically thick regime the radiation drag efficiency is sensitively dependent on the density distributions of the interstellar medium (ISM). The efficiency drops according to     in an optically thick uniform ISM, where τ T is the total optical depth of the dusty ISM , whereas the efficiency remains almost constant at a high level if the ISM is clumpy . Hence, if bulge formation begins with a star formation event in a clumpy ISM, the radiation drag will effectively work to remove the angular momentum and the accreted gas may form a supermassive black hole. As a natural consequence, this mechanism reproduces a putative linear relation between the mass of a supermassive black hole and the mass of a galactic bulge, although further detailed modelling for stellar evolution is required for a more precise prediction.  相似文献   

15.
Clumpiness of the interstellar medium may play an important role in the transfer of infrared continuum radiation in star forming regions (Boisse, 1990). For example, in homogeneous models, C II emission should be confined to the cloud edge (Viala, 1986). However, in star formation regions (such as M17SW, M17 and W51), it is observed to extend deep into the molecular cloud (Stutzki et al., 1988; Keene et al., 1985). One plausible interpretation of these observations is that, due to their clumpiness, the clouds are penetrated by UV radiation far deeper than expected from simple homogeneous models. The interaction of H II regions around young massive stars with a clumpy medium is another area of interest. Molecular clouds are well established to be clumpy on length scales down to the limits of observational resolution. Clumps can act as localized reservoirs of gas which can be injected into the surroundings by photoionization and/or hydrodynamic ablation (Dyson et al., 1995; Mathis et al., 1998). The calculation of radiation transport in hot, clumpy materials is a challenging problem. Approximate, statistical treatments of this problem have been developed by several workers, but their application has not been tested in detail. We describe laboratory experiments, using the Omega laser to test modelling of radiation transport through clumpy media in the form of inhomogeneous plasmas.  相似文献   

16.
We observed the H2CO(110–111) absorption lines and H110α radio recombination lines (RRL) toward 180 NH3 sources using the Nanshan 25-m radio telescope. In our observation, 138 sources were found to have H2CO lines and 36 have H110α RRLs. Among the 138 detected H2CO sources, 38 sources were first detected. The detection rates of H2CO have a better correlation with extinction than with background continuum radiation. Line center velocities of H2CO and NH3 agree well. The line width ratios of H2CO and NH3 are generally larger than unity and are similar to that of 13CO. The correlation between column densities of H2CO and extinction is better than that between NH3 and extinction. These line width relation and column density relation indicate H2CO is distributed on a larger scale than that of NH3, being similar to the regions of 13CO. The abundance ratios between NH3 and H2CO were found to be different in local clouds and other clouds.  相似文献   

17.
Following the work of Whipple and Stefanik, radioactive heating by uranium, thorium and pottasium of a cometary nucleus is discussed. The assumed composition is that of interstellar medium. If thermal diffusivity is 10–4 cm2s–1, the central temperature of a nucleus with radius 10 km can be above 50 K, while if the thermal diffusivity is 5 × 10–3, the central temperature can be only 25 K or so. Volatile gases such as N2 and CO will flow toward the outer part of the nucleus and are lost in their first several approaches to the sun. This mechanism appears capable of explaining the depletion of N2 and CO relative to the interstellar abundance. It is argued that unfamiliar activity of comet Bowell could be explained by sublimation mainly of N2 and CO.  相似文献   

18.
We have made spectral line analysis of CO and its isotopic lines from dark cloud Lynds 183 (L183). Our dataset incorporates 12CO(1-0), 13CO(1-0) and 13CO(2-1) lines using NRAO-12 m and 12CO(3-2), 13CO(3-2) lines using CSO-10 m telescopes, respectively. Observations suggest steep north-south (direction with respect to the offset position (0, 0)) temperature gradient in the cloud. These are likely to be caused by non-uniform, Inter Stellar Radiation Field (ISRF) illumination due to the shadow of nearby L134 cloud complex. As the emission of radiation depends on local properties like density and kinetic temperature, the present study attempts to deduce the irradiation contrast (and the resulting temperature difference) using 1D Monte Carlo radiative transfer code RATRAN. The model results accord with the observed data and shows a temperature difference of ∼7 K mainly within the cloud envelope. This results in a non-uniform intensity distribution of both CO and its species.  相似文献   

19.
Wilhelm  K.  Lemaire  P.  Curdt  W.  Schühle  U.  Marsch  E.  Poland  A. I.  Jordan  S. D.  Thomas  R. J.  Hassler  D. M.  Huber  M. C. E.  Vial  J.-C.  Kühne  M.  Siegmund  O. H. W.  Gabriel  A.  Timothy  J. G.  Grewing  M.  Feldman  U.  Hollandt  J.  Brekke  P. 《Solar physics》1997,170(1):75-104
SUMER – the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) – observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 Å (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 Å. The second-order spectra of detectors A and B cover 330 to 805 Å and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 × 106 K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mÅ is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona.  相似文献   

20.
This study is based on a set of ten solar rocket spectra well exposed for photometry photographed on July 27, 1966 by Purcell, Snider, and Tousey.The photometry of the far UV continuum illustrates the transition of the solar temperature minimum at 1700 Å in the solar spectrum - (a) the continuum intensity decreases by 30–50% between 1700 Å and the1 D limit of silicon at 1682 Å, and (b) the equivalent brightness temperature shows minimum values throughout the spectral range 1540–1682 Å, which average just under 4700 ± 100K.The minimum UV brightness temperature is 300K higher than the far infrared measurement of the solar minimum temperature, and possible reasons for this are discussed.Brightness temperatures measured in prominent CO band heads and in the aluminum 1937 Å auto-ionization line also are given.  相似文献   

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