首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 78 毫秒
1.
The vertical dust distribution of dust clouds around planets, resulting from electrostatic forces, is calculated as a function of dust and plasma parameters. Photoelectric charging is included and differences between clouds on the illuminated side and in the shadow zone are examined. We compute ring structures for conditions which may apply in the spoke-forming regions and study at what dust and plasma conditions the shadow has a significant effect on the vertical dust cloud structure.  相似文献   

2.
We use optical color indices (colors) from the SDSS database to study the effect of dust in starburst galaxies by mea‐suring the dependence of colors on galaxy inclination. Starburst galaxies with ongoing star formation, are rich with metals/dust and are, therefore, an excellent objects for studying the effect of dust in galaxies. They are selected using the [O III ]λ 5007/Hα vs. [N II ]λ 6584/Hβ diagram, that is, the BPT‐diagram. We use Kauffmann's empirical demarcation line in the BPT‐diagram to exclude galaxies with active galactic nuclei (AGN) from the sample because they have different physical and dust properties from normal galaxies. The sample is divided into bins according to galaxy stellar mass and 4000 Å break (which is a coarse measure of a galaxy star formation history; SFH) and the reddening with inclination is studied as a function of these two physical parameters. Assuming that the dust effect is negligible in the SDSS z ‐band, we derive the attenuation curves for these galaxies. We fit the attenuation curves with a simple power law and use power law index to interpret the relative distribution of dust and stars in the starburst galaxies (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
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   

4.
We present a multiwavlength infrared (IR) study of the nearby, edge-on, spiral galaxy NGC 891. We have examined 20 independent, spatially resolved IR images of this galaxy, 14 of which are newly reduced and/or previously unpublished images. These images span a wavelength regime from  λ 1.2 μ  m in which the emission is dominated by cool stars, through the mid-IR, in which emission is dominated by polycyclic aromatic hydrocarbons (PAHs), to λ 850 μm, in which emission is dominated by cold dust in thermal equilibrium with the radiation field. The changing morphology of the galaxy with wavelength illustrates the changing dominant components. We detect extraplanar dust emission in this galaxy, consistent with previously published results, but now show that PAH emission is also in the halo, to a vertical distance of   z ≥ 2.5 kpc  . We compare the vertical extents of various components and find that the PAHs (from λ 7.7 and 8 μm data) and warm dust (λ 24 μm) extend to smaller z heights than the cool dust (λ 450 μm). For six locations in the galaxy for which the signal-to-noise ratio was sufficient, we present spectral energy distributions (SEDs) of the IR emission, including two in the halo – the first time a halo SED in an external galaxy has been presented. We have modelled these SEDs and find that the PAH fraction, f PAH, is similar to Galactic values (within a factor of 2), with the lowest value at the galaxy's centre, consistent with independent results of other galaxies. In the halo environment, the fraction of dust exposed to a colder radiation field, f cold, is of the order of unity, consistent with an environment in which there is no star formation. The source of excitation is likely from photons escaping from the disc.  相似文献   

5.
The large amounts of dust detected in sub-millimeter galaxies and quasars at high redshift pose a challenge to galaxy formation models and theories of cosmic dust formation. At z>6 only stars of relatively high mass (>3 M⊙) are sufficiently short-lived to be potential stellar sources of dust. This review is devoted to identifying and quantifying the most important stellar channels of rapid dust formation. We ascertain the dust production efficiency of stars in the mass range 3–40 M⊙ using both observed and theoretical dust yields of evolved massive stars and supernovae (SNe) and provide analytical expressions for the dust production efficiencies in various scenarios. We also address the strong sensitivity of the total dust productivity to the initial mass function. From simple considerations, we find that, in the early Universe, high-mass (>3 M⊙) asymptotic giant branch stars can only be dominant dust producers if SNe generate ≲3×10−3 M⊙ of dust whereas SNe prevail if they are more efficient. We address the challenges in inferring dust masses and star-formation rates from observations of high-redshift galaxies. We conclude that significant SN dust production at high redshift is likely required to reproduce current dust mass estimates, possibly coupled with rapid dust grain growth in the interstellar medium.  相似文献   

6.
The observational determination of coma outflow velocity for gaseous species is fairly straightforward using high-resolution spectroscopy. The determination of the outflow speed of the dust is much more difficult. Most sources cite Bobrovnikoff (1954). This brief report is not a strictly refereed publication, however, and mixes data from different comets.We present here a simple analysis of some data from the International Halley Watch (IHW) archive. Differences between continuum images from successive nights show dust jets and shells clearly. Their motion is apparent to first order from the edges of the features. The component of the dust outflow velocity perpendicular to the observer's line of sight may thus be determined. This is of course a lower limit on the dust outflow velocity. Many measurements, at different heliocentric distances (R), allow determination of the heliocentric dependence of the dust outflow velocity.We find that the dust outflow velocity in comet P/Halley varied as R –0.41 . If data from an outburst at 14 AU (Sekanina et al. 1992) is included in the fit, this dependence becomes R –0.55. This confirms the canonical (e.g. Delsemme 1982) inverse-square-root law, and supports the conclusion of Storrs et al. (1992) on the variability of cometary gas-to-dust ratios.Space Telescope Science Institute  相似文献   

7.
Belheouane  S.  Zaslavsky  A.  Meyer-Vernet  N.  Issautier  K.  Mann  I.  Maksimovic  M. 《Solar physics》2012,281(1):501-506

Most in situ measurements of cosmic dust have been carried out with dedicated dust instruments. However, dust particles can also be detected with radio and plasma wave instruments. The high velocity impact of a dust particle generates a small crater on the spacecraft, and the dust particle and the crater material are vaporised and partly ionised. The resulting electric charge can be detected with plasma instruments designed to measure electric waves. Since 2007 the STEREO/WAVES instrument has recorded a large number of events due to dust impacts. Here we will concentrate on the study of those impacts produced by dust grains originating from the local interstellar cloud. We present these fluxes during five years of the STEREO mission. Based on model calculations, we determine the direction of arrival of interstellar dust. We find that the interstellar dust direction of arrival is ～260°, in agreement with previous studies.

  相似文献   

8.
Our aim is to explore the relation between gas, atomic and molecular, and dust in spiral galaxies. Gas surface densities are from atomic hydrogen and CO line emission maps. To estimate the dust content, we use the disk opacity as inferred from the number of distant galaxies identified in twelve HST/WFPC2 fields of ten nearby spiral galaxies. The observed number of distant galaxies is calibrated for source confusion and crowding with artificial galaxy counts and here we verify our results with sub‐mm surface brightnesses from archival Herschel ‐SPIRE data. We find that the opacity of the spiral disk does not correlate well with the surface density of atomic (H I) or molecular hydrogen (H2) alone implying that dust is not only associated with the molecular clouds but also the diffuse atomic disk in these galaxies. Our result is a typical dust‐to‐gas ratio of 0.04, with some evidence that this ratio declines with galactocentric radius, consistent with recent Herschel results. We discuss the possible causes of this high dust‐to‐gas ratio; an over‐estimate of the dust surface‐density, an under‐estimate of the molecular hydrogen density from CO maps or a combination of both. We note that while our value of the mean dust‐to‐gas ratio is high, it is consistent with the metallicity at the measured radii if one assumes the Pilyugin & Thuan (2005) calibration of gas metallicity. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
The Infra-Red Astronomical Satellite (IRAS) observations of the zodiacal dust emission are used to fit the dust grain composition and distribution in the ecliptical plane. We obtain a good fit to the data for a density distribution of black-body grains given by p = pr 0.66/log(1.7r/R) for r < 0.87R and r < 3oR  相似文献   

10.
We discuss the Brans-Dicke field equations for the non-flat (k 2=1) Friedmann-Robertson-Walker models filled with dust. It is shown that the problem of constructing the general dust solutions can be reduced to the problem of solving a first-order differential equation.  相似文献   

11.
Abstract— Photometric observations of the twilight sky were carried out during Leonids 1998. The obtained vertical distributions of aerosol between 20 and 140 km demonstrate the processes of the intrusion of fine meteor dust and its subsequent intra-atmospheric dynamics. The characteristic radii of two fractions of the meteor dust particles were estimated by their sedimentation velocities. They varied within rp = 0.006–0.06 μm and rp = 19–81 μm limits depending on an assumed particle density within ρp = 0.4–4.0 g cm?3. The assumption of ρp = 2.0 g cm?3 gave radii of the two fractions to be 0.01 and 30 μm, respectively.  相似文献   

12.
The problem of nonlinear localized dust acoustic (DA) is addressed in a plasma comprising positive ions, negative ions, and mobile negatively charged dust grains. We first consider the case when the grain charge remains constant and discuss later the case when the charge variations are self-consistently included. It is found that a relative increase of the positive ion density favors the propagation of the DA solitary waves, in the sense that the domain of their admissible Mach numbers enlarges. Furthermore, electronegativity makes the dust acoustic solitary structure more spiky. When the dust grain charge Q d is allowed to fluctuate, the latter is expressed in terms of the Lambert function and we take advantage of this transcendental function to investigate the variable charge DA solitary wave. Q d adopts a localized profile and becomes more negative as the number of charges Z (−) of the negative ion increases. The dust grains are found to be highly localized. This localization (accumulation) caused by a balance of the electrostatic forces acting on the dust grains becomes more effective for lower values of Z (−). An increase of Z (−) may lead to a local depletion of the negative ions from the region of the soliton’s localization. The results are useful to understand the salient features of localization of large amplitude dust acoustic waves in cosmic plasmas such as the ionospheric D-region and the mesosphere.  相似文献   

13.
There are several observations showing an enhancement of infrared emission and optical polarization at a distance of 4R , (R is the angular radius of the Sun) implying a ring of dust in near-ecliptic orbit about the Sun; but there is an almost equal number of observations, which do not show any such enhancement. We plotted the observational results for the detection and the non-detection of the circum-solar dust on a diagram for the variation of the sun-spot number with time, and found that its detection and non-detection occurred near the solar minimum and maximum phases, respectively. We present the possibility that this phenomena is caused by an additional process, i.e., the Lorentz force acting on a charged dust particle. Since, at the maximum phase a dust particle in a near-solar region acquires a higher positive potential, and the solar magnetic field is very strong, the Lorentz force becomes dominant and can affect the orbit of the dust particle.  相似文献   

14.
The LIDAR instrument operating from the surface of Mars on the Phoenix Mission measured vertical profiles of atmospheric dust and water ice clouds at temperatures around −65 °C. An equivalent lidar system was utilized for measurements in the atmosphere of Earth where dust and cloud conditions are similar to Mars. Coordinated aircraft in situ sampling provided a verification of lidar measurement and analysis methods and also insight for interpretation of lidar derived optical parameters in terms of the dust and cloud microphysical properties. It was found that the vertical distribution of airborne dust above the Australian desert is quite similar to what is observed in the planetary boundary layer above Mars. Comparison with the in situ sampling is used to demonstrate how the lidar derived optical extinction coefficient is related to the dust particle size distribution. The lidar measurement placed a constraint on the model size distribution that has been used for Mars. Airborne lidar measurements were also conducted to study cirrus clouds that form in the Earth’s atmosphere at a similar temperature and humidity as the clouds observed with the lidar on Mars. Comparison with the in situ sampling provides a method to derive the cloud ice water content (IWC) from the Mars lidar measurements.  相似文献   

15.
S.M. Metzger  M.C. Towner 《Icarus》2011,214(2):766-772
In situ (mobile) sampling of 33 natural dust devil vortices reveals very high total suspended particle (TSP) mean values of 296 mg m−3 and fine dust loadings (PM10) mean values ranging from 15.1 to 43.8 mg m−3 (milligrams per cubic meter). Concurrent three-dimensional wind profiles show mean tangential rotation of 12.3 m s−1 and vertical uplift of 2.7 m s−1 driving mean vertical TSP flux of 1689 mg m−3 s−1 and fine particle flux of ∼1.0 to ∼50 mg m−3 s−1. Peak PM10 dust loading and flux within the dust column are three times greater than mean values, suggesting previous estimates of dust devil flux might be too high. We find that deflation rates caused by dust devil erosion are ∼2.5-50 μm per year in dust devil active zones on Earth. Similar values are expected for Mars, and may be more significant there where competing erosional mechanisms are less likely.  相似文献   

16.
The secular variation of the thermal structure of the Martian atmosphere during the dissipation phase of the 1971 dust storm is examined, using temperatures obtained by the infrared spectroscopy investigation on Mariner 9. For the latitude range ?20° to ?30°, the mean temperature at the 2mbar level is found to decrease from approximately 220 K in mid-December 1971 to about 190 K by June 1972 while for the 0.3mbar level a decrease from 203 K to 160 K is observed. Over the same period, the amplitude of the diurnal temperature wave also decreased. Assuming a simplified radiative heating model, the dust optical depth is found to decrease approximately exponentially with an e-folding time of about 60 days at both the 0.3 and 2mbar levels. Stokes-Cunningham settling alone cannot account for this behavior. Sedimentation models which include both gravitational settling and vertical mixing are developed in an effort to explain the time evolution of the dust. Within the framework of a model which assumes an effective vertical diffusivity K independent of height, a mean dust particle diameter of ～2 μm is inferred. To provide the necessary vertical mixing, K ? 107 cm2sec?1 is required in the lower atmosphere.  相似文献   

17.
《Planetary and Space Science》2007,55(10):1346-1357
This work presents a review of the observations acquired by the planetary Fourier spectrometer (PFS) in the region of the Hellas basin.Taking advantage of the high spectral resolution of PFS, the vertical air temperature profile can be investigated with a previously unexperienced vertical resolution. Extensive comparisons with the expectations of EMCD 4.0 database highlight moderate discrepancies, strongly dependant on season. Namely, the morning observations acquired around Ls=45° show a series of temperature deficiencies with recurrent spatial patterns in different observations, correlated with the topography profile.Trends of integrated dust loads as a function of the field of view (FOV) elevation are also described. Values are consistent with the retrieval hypothesis of a dust scale height equal to the gas one, even far from the season of main dust storms.  相似文献   

18.
We consider the mechanisms of the formation of dust ejected from craters produced by large-meteoroid impacts on the Martian surface, as well as the mechanisms of the elevation of dust that already existed on the surface, due to impulsed aeolian processes. Detailed numerical calculations of the dust injection, the shock wave propagation, and the formation and evolution of the dust cloud are carried out for vertical impacts of meteoroids with sizes from 1 m to 100 m. The results of these calculations show that dust raised by a 1-m impactor is sufficient to produce a local dust storm, while the mass of dust formed in impacts of large bodies is comparable to the mass of a regional or even a global dust storm. The impact detection rates for 1-, 5-, 20-, and 100-m-sized meteoroids are estimated to be a few impact events per year, one event in every 5–6 years, one event in every 300–800 years, and one event in every 5000–20000 years, respectively. In the last case, the thickness of the global layer of precipitated dust and small fragments, which has been formed through impacts over a period of 107–108 years, is comparable to the thickness of the global dust layer on the Martian surface. In the first case, the mass of raised dust is greater than that for typical dust devils. The speed of impulsed wind at large distances from the impact site is shown to exceed the critical speed at which the blowing-off of dust from the surface begins. Some factors that may enhance the dust ejection have been previously ignored in numerical calculations. We discuss here the role of these factors. The second part of our study deals with the determination of the impact-induced radiation impulse and the estimation of its effect on the rise of dust.  相似文献   

19.
The initiation and evolution of a planet-encircling dust storm on Mars have been observed in unprecedented detail by the Thermal Emission Spectrometer (TES) on board the Mars Global Surveyor. A local dust storm began to expand quickly on 26 June 2001 (areo-centric longitude, Ls=185°), becoming a planet-encircling dust storm by 11 July 2001. The dust storm had dust optical depth >2 over wide areas and caused atmospheric temperatures over a large vertical range to warm by >40 K from 20°N latitude to the south pole.  相似文献   

20.
The broad range of optical–infrared colours of radio QSOs,1 < B-K < 6, has been cited as evidence for several mag of dust extinction (Webster et al.. If such large extinctions are typical, the implications for our understanding of the space density of optically selected QSOs are profound. We have previously found that the host galaxies of several of the reddest B3 QSOs are readily detectable in K-band images. This suggests contamination of the K apparent magnitudes by starlight, i.e. the redness in B-Kmay be due to excess light in K, rather than to dust extinction of the B light. We have now imaged the B3 QSOs inUBVR, and we use the range of observed optical and optical–IR colours to place an upper limit on the amount of dust extinction present, rest-frame A V < 1.5 mag. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号