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1.
The surface composition of Titan is of great importance for understanding both the internal evolution of Titan and its atmosphere. The Visual and Infrared Mapping Spectrometer (VIMS) investigation on Cassini is observing Titan from 0.35 to 5.11 μm with spatial resolution down to a few kilometers during each flyby of the spacecraft as it orbits Saturn. Our search for spectral diversity using seven methane transmission windows in the near infrared suggests that spectrally distinct units exist on the surface of Titan and that most of the surface can be modeled using only a few distinct spectral units: water frost, CO2 frost, atmospheric scattering, and an unknown material bright at 2 μm. A dark, spectrally neutral material is also implied. Use of an atmospheric scattering component with spectral mixing analysis may provide a method for partially removing atmospheric effects. In some locations, atmospheric scattering accounts for the majority of the signal. There are also small regions with unusual spectra that may be due to low signal and high noise and/or may be exotic materials of interest. Further, we searched within the methane windows for spectral features associated with Titan's surface. Only the 5-μm and, to a lesser extent, the 2-μm window provide a reasonable opportunity for this, as the shorter-wavelength windows are too narrow and the 2.8-μm window is cluttered with an unknown atmospheric constituent. We find evidence for only one spectral feature: near 4.92 μm for the 5-μm bright Tui Regio region. CO2 frost with grains smaller than about 10 μm is the best candidate we have found so far to explain this absorption as well as the feature's spectral contrast between the 2.7- and the 2.8-μm atmosphere subwindows. This suggested CO2 identification is supported by the presence of an endmember in the spectral mixture analysis that is consistent with CO2 frost with large grain sizes. We find no other absorption features that are statistically significant, including those reported earlier by others. These results are consistent with but greatly extend our early analysis that treated only the Ta data set [McCord, T.B., et al., 2006a. Planet. Space Sci. 54, 1524-1539]. In the spectral feature search process, we explored in detail the noise characteristics of the VIMS data within the 5-μm window, which has generally very low signal (4-20 DN), due to the measurement conditions and low illumination levels. We find noise of nearly Gaussian statistics except for some erratic darks and noise spikes, and the data set seems generally well behaved. We present examples of our attempt to improve on the standard VIMS pipeline data calibration.  相似文献   

2.
The dynamical evolution of six open star cluster models is analyzed using the correlation and spectral analysis of phase-space density fluctuations. The two-time and mutual correlation functions are computed for the fluctuations of the phase-space density of cluster models. The data for two-time and two-particle correlations are used to determine the correlation time for phase-space density fluctuations ((0.1–1) τ v.r., where τ v.r. is the violent relaxation time of the model) and the average phase velocities of the propagation of such fluctuations in cluster models. These velocities are 2–20 times smaller than the root mean square velocities of the stars in the cluster core. The power spectra and dispersion curves of phase-space density fluctuations are computed using the Fourier transform of mutual correlation functions. The results confirm the presence of known unstable phase-space density fluctuations due to homologous fluctuations of the cluster cores. The models are found to exhibit a number of new unstable phase-space density fluctuations (up to 32–41 pairs of fluctuations with different complex conjugate frequencies in each model; the e-folding time of the amplitude growth of such fluctuations is (0.4–10) τ v.r. and their phases are distributed rather uniformly). Astrophysical applications of the obtained results (irregular structure of open star clusters, formation and decay of quasi-stationary states in such clusters) are discussed.  相似文献   

3.
We present a fast solver for computing potential and linear force-free fields (LFFF) above the full solar disk with a synoptic magnetic map as input. The global potential field and the LFFF are dealt with in a unified way by solving a three-dimensional Helmholtz equation in a spherical shell and a two-dimensional Poisson equation on the solar surface. The solver is based on a combination of the spectral method and the finite-difference scheme. In the longitudinal direction the equation is transformed into the Fourier spectral space, and the resulting two-dimensional equations in the r?C?? plane for the Fourier coefficients are solved by finite differencing. The solver shows an extremely fast computing speed, e.g., the computation for a magnetogram with a resolution of 180(??)×360(?) is completed in less than 2 s. Even on a high-resolution 600×1200 grid, the solution can be obtained within only about one minute on a single CPU. The solver can potentially be applied directly to the original resolution of observed magnetograms from SDO/HMI for routinely analyzing daily full-disk data.  相似文献   

4.
High-resolution (∼0.22 Å) spectra of the north jovian aurora were obtained in the 905-1180 Å window with the Far Ultraviolet Spectroscopic Explorer (FUSE) on October 28, 2000. The FUSE instrument resolves the rotational structure of the H2 spectra and the spectral range allows the study of self-absorption. Below 1100 Å, transitions connecting to the v″?2 levels of the H2 ground state are partially or totally absorbed by the overlying H2 molecules. The FUSE spectra provide information on the overlying H2 column and on the vibrational distribution of H2. Transitions from high-energy H2 Rydberg states and treatment of self-absorption are considered in our synthetic spectral generator. We show comparisons between synthetic and observed spectra in the 920-970, 1030-1080, and 1090-1180 Å spectral windows. In a first approach (single-layer model ), the synthetic spectra are generated in a thin emitting layer and the emerging photons are absorbed by a layer located above the source. It is found that the parameters of the single-layer model best fitting the three spectral windows are 850, 800, and 800 K respectively for the H2 gas temperature and 1.3×1018, 1.5×1020, and 1.3×1020 cm−2 for the H2 self-absorbing vertical column respectively. Comparison between the H2 column and a 1-D atmospheric model indicates that the short-wavelength FUV auroral emission originates from just above the homopause. This is confirmed by the high H2 rovibrational temperatures, close to those deduced from spectral analyses of H+3 auroral emission. In a second approach, the synthetic spectral generator is coupled with a vertically distributed energy degradation model, where the only input is the energy distribution of incoming electrons (multi-layer model ). The model that best fits globally the three FUSE spectra is a sum of Maxwellian functions, with characteristic energies ranging from 1 to 100 keV, giving rise to an emission peak located at 5 μbar, that is ∼100 km below the methane homopause. This multi-layer model is also applied to a re-analysis of the Hopkins Ultraviolet Telescope (HUT) auroral spectrum and accounts for the H2 self-absorption as well as the methane absorption. It is found that no additional discrete soft electron precipitation is necessary to fit either the FUSE or the HUT observations.  相似文献   

5.
Since the original suggestion by Gillett et al. (1969) it has generally been assumed that the region of partial transparency near 5 μm in Jupiter's atmosphere (the 5-μm window) is bounded by the v4 NH3 at 6.1 μm and the v3 CH4 band at 3.3 μm. New measurements of Jupiter and of laboratory phosphine (PH3) samples show that PH3 is a significant contributor to the continuum opacity in the window and in fact defines its short-wavelength limit. This has important implications for the use of 5-mu;m observations as a means to probe the deep atmospheric structure of Jupiter. The abundance of PH3 which results from a comparison of Jovian and laboratory spectra is about 3 to 5 cm-am. This is five to eight times less than that found by Larson et al. [Astrophys. J. (1977) 211, 972–979] in the same spectral region, but is in good agreement with the result of Tokunaga et al. [Astrophys. J. (1979) 232, 603–615] from 10-μm observations.  相似文献   

6.
The Cassini Huygens mission provides a unique opportunity to combine ground-based and spacecraft investigations to increase our understanding of chemical and dynamical processes in Titan’s atmosphere. Spectroscopic measurements from both vantage points enable retrieving global wind structure, temperature structure, and atmospheric composition. An updated analysis of Titan data obtained with the NASA Goddard Space Flight Center’s Infrared Heterodyne Spectrometer (IRHS) and Heterodyne Instrument for Planetary Wind and Composition (HIPWAC) prior to and during the Cassini Huygens mission is compared to retrievals from measurements with the Cassini Composite Infrared Spectrometer (CIRS). IRHS/HIPWAC results include the first direct stratospheric wind measurements on Titan, constraints on stratospheric temperature, and the study of atmospheric molecular composition. These results are compared to CIRS retrievals of wind and temperature profile from thermal mapping data and ethane abundance at 10-15° South latitude, near the equatorial region. IRHS/HIPWAC wind results are combined with other direct techniques, stellar occultation measurements, and CIRS results to explore seasonal variability over nearly one Titan year and to provide an empirical altitude profile of stratospheric winds, varying from ∼50 to 210 m/s prograde. The advantage of fully resolved line spectra in species abundance measurements is illustrated by comparing the possible effect on retrieved ethane abundance by blended spectral features of other molecular constituents, e.g., acetylene (C2H2), ethylene (C2H4), allene (C3H4), and propane (C3H8), which overlap the ν9 band of ethane, and are not resolved at lower spectral resolution. IR heterodyne spectral resolution can discriminate weak spectral features that overlap the ν9 band of ethane, enabling ethane lines alone to be used to retrieve abundance. Titan’s stratospheric mean ethane mole fraction (8.6±3 ppmv) retrieved from IRHS/HIPWAC emission line profiles (resolving power λ?Δλ∼106) is compared to past values obtained from lower resolution spectra and from CIRS measurements (resolving power λ?Δλ∼2×103) and more compatible recent analysis. Results illustrate how high spectral resolution ground-based studies complement the spectral and spatial coverage and resolution of moderate spectral resolution space-borne spectrometers.  相似文献   

7.
Study of curve of growth abundance determinations is performed for the method of weighting functions. We evaluated abundances of iron for 15 stars changing their effective temperatures and gravities within the mean error of determination of these parameters, i.e.θ eff ± 0.05θ eff and logg ± 0.5. Wide range of spectral types of stars taken into account, permits analysis of accuracy of abundance determinations for large part of H-R diagram. The results prove, that in spite of using the most accurate method of curve of growth analysis, the accuracy of abundance determination (due to errors inT eff and log g) is sometimes very small (in extreme cases not better than 0.4 dex) and the value of accuracy is connected with of spectral types of stars.  相似文献   

8.
We perform the correlation and spectral analysis of phase-space density and potential fluctuations in a model of an open star cluster for various values of the smoothing parameter ? of the force functions in the equations of motion of cluster stars, and compute the mutual correlation functions for the fluctuations of potential U and phase-space density f of the cluster model at different clustercentric distances. We use the Fourier transform of the mutual correlation functions to compute the power spectra and dispersion curves of the potential and phase-space density fluctuations. The spectrum of potential fluctuations proves to be less complex than that of phase-space density fluctuations. The most powerful potential fluctuations are associated with phase-space density fluctuations, and their spectrum lies in the domain of low frequencies ν < 3/τ v.r.; at intermediate and high frequencies (ν > 3/τ v.r.), the contribution of potential fluctuations to those of the phase-space density is small or equal to zero (here τ v.r. is the violent relaxation time scale of the cluster). We find a number of unstable potential fluctuations in the core of the cluster model (up to 30 pairs of fluctuations with different complex conjugate frequencies). We also find and analyze the dependences of the spectra and dispersion curves of phase-space density and potential fluctuations on ?. We find a “repeatability” (significant correlation) of the spectra at some values of parameter ?. The form of the dispersion curve is unstable against small variations of ?. We discuss the astrophysical applications of our results: the break-up in the cluster core of the phase-space density wave running from the cluster periphery toward its center into several waves with frequencies commensurable to that of the external (tidal) influence; emission and reflection of phase-space and potential waves near the cluster core boundary; possible wavelength and phase discretization of the phase-space and potential waves in the cluster model.  相似文献   

9.
The observational data from radio to X-ray wavebands were collected from the SSDC (Italian Space Agency Science Data Center) for 68 Fermi blazars, and their spectral energy distributions (SEDs) were calculated by means of the least square fitting with a log-parabolic function. Based on the SED fitting parameters, the correlations of the synchrotron peak frequency, curvature and effective spectral index were discussed, and an empirical formula was also proposed to estimate the synchrotron peak frequencies by using the effective spectral indexes. The main results are as follows: (1) From the linear correlation between the synchrotron peak frequency (lg νp) and the curvature (k), we find that the result supports the energy-dependent acceleration probability model for all BL Lac objects, while the result for the BL Lac objects of lg νp > 15.3 is consistent with the model of fluctuation of fractional acceleration gain. (2) For the sources of nearly same lg νp, a significant correlation between the effective spectral index αro of the radio-optical waveband and the curvature is detected, while the effective spectral index αox of the optical-x-ray waveband is not correlated with the curvature. According to the effective spectral index αro, a relation between the synchrotron peak frequency and the curvature can be defined.  相似文献   

10.
Analysis of over 15 years of V-band and Wing three filter near-IR photometry of the bright M5Ib-II supergiant has been carried out. Wavelet analysis of these data reveals that the star pulsates with several complicated oscillation modes. Different time scales of variability are identified, and with the aid of discrete Fourier analysis, depending on the filter, up to seven significant pulsation modes are identified and their frequencies and amplitudes extracted. The Long Secondary Period (LSP) with a mean period of ～1343 d has been identified, as well as other periods of the order of ～125 d. The longer period appears to be attributed to the radial pulsational mode, while the various peaks near ～125 d appear to arise from stochastically excited p-modes. After removing the light contribution of the 5th magnitude binary companion and calibrating the intermediate-band photometry to the Wing photometric system, TiO (719 nm) and near-IR (B-C) Wing color indices were formed. These indices have been calibrated with T eff, while the Wing-C bandpass (1025 nm) serves as a proxy for bolometric magnitude and was transformed to approximate m bol. Finally, the derivation of the variations in the star’s temperature, luminosity and radius is straightforward.  相似文献   

11.
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12.
We examine the effects of NH3 ice particle clouds in the atmosphere of Jupiter on outgoing thermal radiances. The cloud models are characterized by a number density at the cloud base, by the ratio of the scale height of the vertical distribution of particles (Hp) to the gas scale height (Hg), and by an effective particle radius. NH3 ice particle-scattering properties are scaled from laboratory measurements. The number density for the various particle radius and scale height models is inferred from the observed disk average radiance at 246 cm?1, and preliminary lower limits on particle sizes are inferred from the lack of apparent NH3 absorption features in the observed spectral radiances as well as the observed minimum flux near 2100 cm?1. We find lower limits on the particle size of 3 μm if Hp/Hg = 0.15, or 10μmif Hp/Hg = 0.50 or 0.05. NH3 ice particles are relatively dark near the far-infrared and 8.5-μm atmospheric windows, and the outgoing thermal radiances are not very sensitive to various assumptions about the particle-scattering function as opposed to radiances at 5 μm, where particles are relatively brighter. We examined observations in these three different spectral window regions which provide, in principle, complementary constraints on cloud parameters. Characterization of the cloud scale height is difficult, but a promising approach is the examination of radiances and their center-to-limb variation in spectral regions where there is significant opacity provided by gases of known vertical distribution. A blackbody cloud top model can reduce systematic errors due to clouds in temperature sounding to the level of 1K or less. The NH3 clouds provide a substantial influence on the internal infrared flux field near the 600-mbar level.  相似文献   

13.
New Photometric data in R and V passband filters of the classical Algol system U Cep were analyzed. The orbital and physical parameters of the system were obtained via light curve solutions, using Wilson-Devinney code. The absolute dimensions are also estimated. The O-C curve (only photometric data) is analyzed through Kalimeris method. Fourier analysis of P(E) function gave at least two modulation periods, i.e., P 1=16.38 yr and P 2=9.77 yr, the first was attributed to magnetic cycle effects and the second to mean time interval of occasional mass out bursts.  相似文献   

14.
W.W. Mullins 《Icarus》1978,33(3):624-629
A previous analysis of a stochastic model of lunar-type impact cratering is extended to utilize geological age data by defining a more general statistic Ωi(t) to be the number of equivalent whole craters of original diameter di and age ≤t in an observational area A; each crater is taken to be equivalent to the fraction of its rim (or area) that is in A and not occluded by later craters. By integration of a new gain-loss differential equation, a generalization of the previous basic equation is obtained that relates the expected value ωi(t) = E[Ωi(t)] to the process functions specifying the size distribution and flux of craters (primary or secondary) as they form. The results are specialized to the plausible case in which the cratered body can be subdivided into geological provinces of increasing ages t1, t2, …, ti … and the size probability distribution can be approximated as constant within each of the periods ti+1 - ti. It is shown that use of the Ωi permits, in principle, a reconstruction of the historical values of the process functions and correctly compensates for the effect of overlap by removing the false bias favoring large craters that results from the usual method of crater counting. Possible generalizations of the gain-loss equation are indicated.  相似文献   

15.
We present radiometrically calibrated spectrophotometric images of Mars taken at the NASA Infrared Telescope Facility (IRTF) near and during the 1995 and 1999 oppositions. Absolute intensity and radiance factor (rF = I/F) values have been calculated for approximately 95% of the surface over all longitudes between −70° to 90° latitude in the 1.5- to 4.1-μm spectral region at a spectral resolution (Δλ/λ) of 1.5%. Values of radiance factor range from rF = 0.4 to 0.6 at 2.2 μm for the bright regions such as Moab and Arabia to rF = 0.12 to 0.3 at 2.2 μm around the dark regions Syrtis Major and Acidalia Planitia. Variations are seen due to seasonal dust and/or condensate cloud cover and viewing geometry. Our results are generally consistent with the few reported previous radiance factor determinations for Mars. These data are unique among ground-based data in their relatively high spatial resolution (?200 km/pixel at the sub-Earth point) and coverage combined with their spectral resolution and coverage. These radiometrically calibrated observations can be used as input to studies focusing on spectral unmixing of surface and atmospheric components, radiative transfer modeling of disk and limb radiances, and photometric modeling of the martian phase function.  相似文献   

16.
The Herschel SPIRE Fourier transform spectrometer (FTS) performs spectral imaging in the 447–1546 GHz band. It can observe in three spatial sampling modes: sparse mode, with a single pointing on sky, or intermediate or full modes with 1 and 1/2 beam spacing, respectively. In this paper, we investigate the uncertainty and repeatability for fully sampled FTS mapping observations. The repeatability is characterised using nine observations of the Orion Bar. Metrics are derived based on the ratio of the measured intensity in each observation compared to that in the combined spectral cube from all observations. The mean relative deviation is determined to be within 2 %, and the pixel-by-pixel scatter is ～ 7 %. The scatter increases towards the edges of the maps. The uncertainty in the frequency scale is also studied, and the spread in the line centre velocity across the maps is found to be ～ 15 km s ? 1. Other causes of uncertainty are also discussed including the effect of pointing and the additive uncertainty in the continuum.  相似文献   

17.
Absolute spectrophotometry of the coma of Comet Kohoutek 1973f is discussed for the nights of January 24 and 26, 1974. Specific intensities are measured for spectral features and a continuum band in the wavelength region λλ3460–6062Å. The (0, 0) band of the Δν = 0 sequence of the violet system of the cyanogen molecule is analyzed and column densities of 1.7 × 1015m?2 and 3.4 × 1014m?2 are found for January 24 and 26, 1974, respectively. The analysis of the bands of C2 will be reported in a second paper of this series.  相似文献   

18.
To redetermine the Galactic spiral density wave parameters, we have performed a spectral (Fourier) analysis of the radial velocities for 44 masers with known trigonometric parallaxes, proper motions, and line-of-sight velocities. The masers are distributed in awide range of Galactocentric distances (3.5 kpc < R < 13.2 kpc) and are characterized by a wide scatter of position angles ?? in the Galactic XY plane. This has required an accurate allowance for the dependence of the perturbation phase both on the logarithm of the Galactocentric distances and on the position angles of the objects. To increase the significance of the extraction of periodicities from data series with large gaps, we have proposed and implemented a spectrum reconstruction method based on a generalized maximum entropy method. As a result, we have extracted a periodicity describing a spiral density wave with the following parameters from the maser radial velocities: the perturbation amplitude f R = 7.7 ?1.5 +1.7 km s?1, the perturbation wavelength ?? = 2.2 ?0.1 +0.4 kpc, the pitch angle of the spiral density wave i = ?5 ?0.9° +0.2° , and the phase of the Sun in the spiral density wave ?? ?? = ?147 ?17° +3° .  相似文献   

19.
We present individual spectra 0.8-2.5 μm of the leading and trailing hemispheres of Enceladus obtained with the CorMASS spectrograph on the 1.8 m Vatican Advanced Technology Telescope (VATT) at the Mount Graham International Observatory. While the absorption bands of water ice dominate the spectrum of both hemispheres, most of these bands are stronger on the leading hemisphere than the trailing hemisphere. In addition, longward of 1 μm, the continuum slope is greater on the leading hemisphere than the trailing hemisphere. These differences could be produced by the presence of particles on the trailing side that are smaller and/or microstructurally more complex than those on the leading side, consistent with the preferential erosion or structural degradation of regolith particle grains on the trailing side by magnetospheric sweeping. We also explore compositional differences between the two hemispheres by applying Hapke spectrophotometric mixture models to the spectra whose components include water ice and ammonia hydrate (1% NH3⋅H2O). We find that spectral models which include as much as 25% by weight ammonia hydrate intimately mixed with water ice and covering 80% of the illuminated area of the satellite fit the observed spectrum of both the leading and trailing hemispheres. Areal (checkerboard) mixing models of ammonia hydrate and water ice fit the leading hemisphere with 15% of the surface comprised of ammonia hydrate and the trailing hemisphere with 10% ammonia hydrate. Therefore, while these spectral data do not contain an unambiguous detection of ammonia hydrate on Enceladus, our spectral models do not preclude the presence of a modest amount of 1% NH3⋅H2O on both hemispheres. We examine spectral differences and similarities between both hemispheres and the tenuous E ring within which Enceladus orbits. The spectral resolution (R=λ/Δλ) of these CorMASS data (R∼300) is comparable to but nevertheless higher than that of the Visual-Infrared Mapping Spectrometer (VIMS) (R=225) onboard the Cassini spacecraft.  相似文献   

20.
The Sunyaev-Zel’dovich (SZ) effect represents a small spectral distortion to the cosmic microwave background (CMB) radiation, caused by the Compton scattering of CMB photons by the hot gas of galaxy clusters. In an early stage of universe, the SZ effect generates \(\mu\)-type of distortions for the CMB spectrum. A \(\mu\)-type distortion is created between the double Compton scattering decoupling (\(z \sim 10^{6}\)) and the thermalization decoupling by the Compton scattering (\(z \sim 10^{5}\)). In this case, to describe the small spectral distortion of the CMB spectrum, we use the Bose-Einstein (\(\mu\)-type) distribution with a non-zero chemical potential. At present, it is interesting to investigate the effect of this spectral distortion on the integral characteristics of the Bose-Einstein (\(\mu\)-type) spectrum. The thermal radiative and thermodynamic functions are such integral characteristics. These functions are as follows: a) the total radiation power per unit area; b) total energy density; c) number density of photons; d) grand potential density; e) Helmholtz free energy density; f) entropy density; g) heat capacity at constant volume; h) enthalpy density; and i) pressure. Precise analytical expressions are obtained for the temperature dependences of these functions. Using the observational data obtained by the COBE FIRAS, PIXIE, PRISM, and Planck missions, the thermal radiative and thermodynamic functions are calculated. A comparative analysis of the results obtained with the results for the same functions of the CMB spectrum at \(T = 2.72548~\mbox{K}\) is carried out. Very small distortions are observed for the thermal radiative and thermodynamic functions. In the redshift range \(10^{5} < z < 3 \times10^{6}\), these functions are calculated. The expressions are obtained for new astrophysical parameters, such as the entropy density/Boltzmann constant and number density, created by the Bose-Einstein (\(\mu\)-type) spectrum.  相似文献   

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