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1.
This work is a continuation of our previous paper about brightening of Comet 17P/Holmes (Kossacki, K.J., Szutowicz, S. [2010]. Icarus 207, 320–340). In that paper we presented results of simulations indicating that the nonuniform crystallization of amorphous water ice itself is probably not sufficient for an explosion. In the present work we investigate the possibility that the explosion is caused by a rapid sublimation of the CO ice leading to the rise of gas pressure above the tensile strength of the nucleus. We simulated evolution of a model nucleus in the orbit of Comet 17P/Holmes. The nucleus is composed of water ice, carbon monoxide ice and dust and has the shape of an elongated ellipsoid. The simulations include crystallization of amorphous ice in the nucleus, changes of the dust mantle thickness, and sublimation of the CO ice. In our model CO is mantling grains composed of dust and amorphous water ice. Orientation of the nuclear spin axis in space is the same as derived in Moreno et al. (Moreno, F., Ortiz, J.L., Santos-Sanz, P., Morales, N., Vidal-Nunez, M.J., Lara, L.M., Gutierrez, P.J. [2008]. Astrophys. J. 677, L63–L66) for Comet Holmes during recent brightening event. Hence, the angle between the orbital and the equatorial planes of the comet is I = 95°, and the cometocentric solar longitude at perihelion is Φ = 210°. The calculations are performed for the south pole being the sub-solar point close to time of the outburst. Our computations indicate, that the CO pressure within the comet nucleus can rise to high values. When the layer between the dust mantle and the crystallization front of the amorphous water ice is very fine grained, few microns in radius, the CO pressure within the nucleus can exceed 10 kPa. This value is the lowest estimate for the tensile strength of the nucleus of Comet Holmes (Reach, W.T., Vaubaillon, J., Lisse, C.M., Holloway, M., Rho, J. [2010]. Icarus 208, 276–292). Hence, when the gas pressure reaches this value the nucleus may explode.  相似文献   

2.
We present an analysis of the long-term evolution of outbursts in the neutron star soft X-ray transient GRS 1747–312. Observations taken from ASM/RXTE, in the 1.5–12 keV passband, are utilized. We reveal a cyclic behavior in the residuals of the outburst recurrence time with respect to the mean value of TC = 136 ± 2 days. The profile of this cycle is approximately sinusoidal; the remaining cycle-to-cycle fluctuations possess a considerably smaller amplitude. We find that, although the peak flux of the outbursts displays a significant scatter at a given phase of the cycle, the most luminous outbursts occur after the longest TC. The fluence displays a large scatter for the individual outbursts and tends to decrease with time. We argue that although the cycle-length of ～5.4 yr is compatible with that of the presumed magnetic activity of the late-type donor, it cannot be explained by variations of the mass outflow from the donor to the disk. In our interpretation, the stellar activity is translated to variations of TC via interaction of the magnetic field of the spots on the donor with the magnetic field of the disk. This gives rise to a variable efficiency of the removal of the angular momentum from the quiescent disk during the activity cycle of the donor. This mechanism can be strengthened by accompanying variations of the radius of the optically thin advection-dominated accretion flow in quiescence. We show that the peak mass accretion rate onto the neutron star in the individual outbursts of GRS 1747–312 is considerably more stable than in two other similar systems with frequent outbursts, Aql X-1 and 4U 1608–52; this allows the cyclic modulation of TC to show itself in GRS 1747–312.  相似文献   

3.
Augusto Carballido 《Icarus》2011,211(1):876-884
Numerical magnetohydrodynamic (MHD) simulations of a turbulent solar nebula are used to study the growth of dust mantles swept up by chondrules. A small neighborhood of the solar nebula is represented by an orbiting patch of gas at a radius of 3 AU, and includes vertical stratification of the gas density. The differential rotation of the nebular gas is replaced by a shear flow. Turbulence is driven by destabilization of the flow as a result of the magnetorotational instability (MRI), whereby magnetic field lines anchored to the gas are continuously stretched by the shearing motion. A passive contaminant mimics small dust grains that are aerodynamically well coupled to the gas, and chondrules are modeled by Lagrangian particles that interact with the gas through drag. Whenever a chondrule enters a region permeated by dust, its radius grows at a rate that depends on the local dust density and the relative velocity between itself and the dust. The local dust abundance decreases accordingly. Compaction and fragmentation of dust aggregates are not included. Different chondrule volume densities ρc lead to varying depletion and rimmed-chondrule size growth times. Most of the dust sweep-up occurs within ～1 gas scale-height of the nebula midplane. Chondrules can reach their asymptotic radius in 10–800 years, although short growth times due to very high ρc may not be altogether realistic. If the sticking efficiency Q of dust to chondrules depends on their relative speed δv, such that Q < 10?2 whenever δv > vstick ≈ 34 cm/s (with vstick a critical sticking velocity), then longer growth times result due to the prevalence of high MRI-turbulent relative velocities. The vertical variation of nebula turbulent intensity results in a moderate dependence of mean rimmed-chondrule size with nebula height, and in a ～20% dispersion in radius values at every height bin. The technique used here could be combined with Monte Carlo (MC) methods that include the physics of dust compaction, in a self-consistent MHD-MC model of dust rim growth around chondrules in the solar nebula.  相似文献   

4.
We present the 2005–2010 outburst history of the SU UMa-type dwarf HS 0417+7445, along with a detailed analysis of extensive time-series photometry obtained in March 2008 during the second recorded superoutburst of the system. The mean outburst interval is 197 ± 59 d, with a median of 193 d. The March 2008 superoutburst was preceded by a precursor outburst, had an amplitude of 4.2 magnitudes, and the whole event lasted about 16 days. No superhumps were detected during the decline from the precursor outburst, and our data suggests instead that orbital humps were present during that phase. Early superhumps detected during the rise to the superoutburst maximum exhibited an unusually large fractional period excess of ? = 0.137 (Psh = 0.0856(88) d). Following the maximum, a linear decline in brightness followed, lasting at least 6 days. During this decline, a stable superhump period of Psh = 0.07824(2) d was measured. Superimposed on the superhumps were orbital humps, which allowed us to accurately measure the orbital period of HS 0417+7445, Porb = 0.07531(8) d, which was previously only poorly estimated. The fractional superhump period excess during the main phase of the outburst was ? = 0.037, which is typical for SU UMa dwarf novae with similar orbital period. Our observations are consistent with the predictions of the thermal-tidal instability model for the onset of superoutbursts, but a larger number of superoutbursts with extensive time-series photometry during the early phases of the outburst would be needed to reach a definite conclusion on the cause of superoutbursts.  相似文献   

5.
Hubble Space Telescope/Wide Field and Planetary Camera 2 (HST/WFPC2) images of Io obtained between 1995 and 2007 between 0.24 and 0.42 μm led to the detection of the Pele plume in reflected sunlight in 1995 and 1999; imaging of the Pele plume via absorption of jovian light in 1996 and 1999; detection of the Prometheus-type Pillan plume in reflected sunlight in 1997; and detection of the 2007 Pele-type Tvashtar plume eruption in reflected sunlight and via absorption of jovian light. Based on a detailed analysis of these observations we characterize and compare the gas and dust properties of each of the detected plumes. In each case, the brightness of the plumes in reflected sunlight is less at 0.26 μm than at 0.33 μm. Mie scattering analysis of the wavelength dependence of each plume’s reflectance signature suggests that range of particle sizes within the plumes is quite narrow. Assuming a normal distribution of particle sizes, the range of mean particle sizes is ～0.035–0.12 μm for the 1997 Pillan eruption, ～0.05–0.08 μm for the 1999 Pele and 2007 Tvasthar plumes, and ～0.05–0.11 μm for the 1995 Pele plume, and in each case the standard deviation in the particle size distribution is <15%. The Mie analysis also suggests that the 2007 Tvashtar eruption released ～109 g of sulfur dust, the 1999 Pele eruption released ～109 g of SO2 dust, the 1997 Pillan eruption released ～1010 g of SO2 dust, and the 1995 Pele plume may have released ～1010 g of SO2 dust. Analysis of the plume absorption signatures recorded in the F255W filter bandpass (0.24–0.28 μm) indicates that the opacity of the 2007 Tvashtar plume was 2× that of the 1996 and 1999 Pele plume eruptions. While the sulfur dust density estimated for the Tvashtar from the reflected sunlight data could have produced 61% of the observed plume opacity, <10% of the 1999 Pele F255W plume opacity could have resulted from the SO2 dust detected in the eruption. Accounting for the remaining F255W opacity level of the Pele and Tvasthar plumes based on SO2 and S2 gas absorption, the SO2 and S2 gas density inferred for each plume is almost equivalent corresponding to ～2–6 × 1016 cm?2 and 3–5 × 1015 cm?2, respectively, producing SO2 and S2 gas resurfacing rates ～0.04–0.2 cm yr?1 and 0.007–0.01 cm yr?1; and SO2 and S2 gas masses ～1–4 × 1010 g and ～2–3 × 109 g; for a total dust to gas ratio in the plumes ～10?1–10?2. The 2007 Tvashtar plume was detected by HST at ～380 ± 40 km in both reflected sunlight and absorbed jovian light; in 1999, the detected Pele plume altitude was 500 km in absorbed jovian light, but in reflected sunlight the detected height was ～2× lower. Thus, for the 1999 Pele plume, similar to the 1979 Voyager Pele plume observations, the most efficient dust reflections occurred in the region closest to the plume vent. The 0.33–0.42 μm brightness of the 1997 Pillan plume was 10–20× greater than the Pele or Tvashtar plumes, exceeding by a factor of 3 the average brightness levels observed within 200 km of 1979 Loki eruption vent. But, the 0.26 μm brightness of the 1997 Pillan plume in reflected sunlight was significantly lower than would be predicted by the dust scattering model. Presuming that the 0.26 μm brightness of the 1997 Pillan plume was attenuated by the eruption plume’s gas component, then an SO2 gas density ～3–6 × 1018 cm?2 is inferred from the data (for S2/SO2 ratios ?4%), comparable to the 0.3–2 × 1018 cm?2 SO2 density detected at Loki in 1979 (Pearl, J.C. et al. [1979]. Nature 280, 755; Lellouch et al., 1992), and producing an SO2 gas mass ～3–8 × 1011 g and an SO2 resurfacing rate ～8–23 cm yr?1. These results confirm the connection between high (?1017 cm?2) SO2 gas content and plumes that scatter strongly at nearly blue wavelengths, and it validates the occurrence of high density SO2 gas eruptions on Io. Noting that the SO2 gas content inferred from a spectrum of the 2003 Pillan plume was significantly lower ～2 × 1016 cm?2 (Jessup, K.L., Spencer, J., Yelle, R. [2007]. Icarus 192, 24–40); and that the Pillan caldera was flooded with fresh SO2 frost/slush just prior to the 1997 Pillan plume eruption (Geissler, P., McEwen, A., Phillips, C., Keszthelyi, L., Spencer, J. [2004a]. Icarus 169, 29–64; Phillips, C.B. [2000]. Voyager and Galileo SSI Views of Volcanic Resurfacing on Io and the Search for Geologic Activity at Europa. Ph.D. Thesis, Univ. of Ariz., Tucson); we propose that the density of SO2 gas released by this volcano is directly linked to the local SO2 frost abundance at the time of eruption.  相似文献   

6.
We investigate the method by which nearby supernovae – within a few tens of pc of the solar system – can penetrate the solar system and deposit live radioactivities on earth. The radioactive isotopic signatures that could potentially leave an observable geological imprint are in the form of refractory metals; consequently, it is likely they would arrive in the form of supernova-produced dust grains. Such grains can penetrate into the solar system more easily than the bulk supernova plasma, which gets stalled and deflected near the solar system due to the solar wind plasma pressure. We therefore examine the motion of charged grains as they decouple from the supernova plasma and are influenced by the solar magnetic, radiation, and gravitational fields. We characterize the dust trajectories with analytical approximations which display the roles of grain size, initial velocity, and surface voltage. These results are verified with full numerical simulations for wide ranges of dust properties. We find that supernova dust grains traverse the inner solar system nearly undeflected, if the incoming grain velocity – which we take to be that of the incident supernova remnant – is comparable to the solar wind speeds and much larger than the escape velocity at 1 AU. Consequently, the dust penetration to 1 AU has essentially 100% transmission probability and the dust capture onto the earth should have a geometric cross section. Our results cast in a new light the terrestrial deposition of radioisotopes from nearby supernovae in the geological past. For explosions beyond ～10 pc from earth, dust grains can still deliver supernova ejecta to earth, and thus the amount of supernova material deposited is set by the efficiency of dust condensation and survival in supernovae. Turning the problem around, we use observations of live 60Fe in both deep-ocean and lunar samples to infer a conservative lower bound iron condensation efficiency of Mdust,Fe/Mtot,Fe ? 4  × 10?4 for the supernova which apparently produced these species 2–3 Myr ago.  相似文献   

7.
The modeling of UV and optical spectra emitted from the symbiotic system AG Dra, adopting collision of the winds, predicts soft X-ray bremsstrahlung from nebulae downstream of the reverse shock with velocities > 150 km s?1 and intensities comparable to those of the WD black body flux. At outbursts, the envelop of debris, which corresponds to the nebula downstream of the high velocity shocks (700–1000 km s?1) accompanying the blast wave, absorbs the black body soft X-ray flux from the WD, explains the broad component of the H and He lines, and leads to low optical-UV–X-ray continuum fluxes. The high optical-UV flux observed at the outbursts is explained by bremsstrahlung downstream of the reverse shock between the stars. The depletion of C, N, O, and Mg relative to H indicates that they are trapped into dust grains and/or into diatomic molecules, suggesting that the collision of the wind from the WD with the dusty shells, ejected from the RG with a ～1 year periodicity, leads to the U-band fluctuations during the major bursts.  相似文献   

8.
We present spectral and spatial information for major volatile species in Comet 10P/Tempel 2, based on high-dispersion infrared spectra acquired on UT 2010 July 26 (heliocentric distance Rh = 1.44 AU) and September 18 (Rh = 1.62 AU), following the comet’s perihelion passage on UT 2010 July 04. The total production rate for water on July 26 was (1.90 ± 0.12) × 1028 molecules s?1, and abundances of six trace gases (relative to water) were: CH3OH (1.58% ± 0.23%), C2H6 (0.39% ± 0.04%), NH3 (0.83% ± 0.20%), and HCN (0.13% ± 0.02%). A detailed analysis of intensities for water emission lines provided a rotational temperature of 35 ± 3 K. The mean OPR is consistent with nuclear spin populations in statistical equilibrium (OPR = 3.01 ± 0.18), and the (1σ) lower bound corresponds to a spin temperature >38 K. Our measurements were contemporaneous with a jet-like feature observed at optical wavelengths. The spatial profiles of four primary volatiles display strong enhancements in the jet direction, which favors release from a localized vent on the nucleus. The measured IR continuum is much more sharply peaked and is consistent with a dominant contribution from the nucleus itself. The peak intensities for H2O, CH3OH, and C2H6 are offset by ～200 km in the jet direction, suggesting the possible existence of a distributed source, such as the release of icy grains that subsequently sublimed in the coma. On UT September 18, no obvious emission lines were present in our spectra, nevertheless we obtained a 3σ upper limit Q(H2O) < 2.86 × 1027 molecules s?1.  相似文献   

9.
We analyze the intensity modulation in the final, very broad peak of the main outburst of the neutron star low-mass X-ray binary KS 1731-260. We use ASM/RXTE observations for a time-series analysis of the long-term variations. We also investigate the X-ray color (hardness ratio) changes in the 1.5–12 keV band. The modulation with the mean cycle-length of 37 days is transient and is detected only in several time segments. It underwent significant variations of both the cycle-length and the amplitude. This cycle cannot be caused by transitions of the outer disk region between the hot and cool state that gave rise to the subsequent series of the echo outbursts. Because of its high X-ray luminosity (LX ≈ 0.1 of the Eddington luminosity), KS 1731-260 is a promising candidate for having its accretion disk tilted and warped. The properties of the modulation can therefore be explained as due to variable (multimodal), but still detectable superorbital periods caused by a disk precession with mode switching and unstable warps. We find that a variable LX is not the sole parameter that governs the presence of the cycle. Variable absorption of X-rays cannot be dominant in producing the modulation of the ASM flux during the cycle. Variations of the stream impact on the tilted and warped disk, hence affecting the mass flow in the inner disk region, consequently the emission components, are a promising mechanism for the observed cycle. In this scenario, the true cycle-length can be twice as long because of the double-wave profile.  相似文献   

10.
We have examined thermal emission from 240 active or recently-active volcanic features on Io and quantified the magnitude and distribution of their volcanic heat flow during the Galileo epoch. We use spacecraft data and a geological map of Io to derive an estimate of the maximum possible contribution from small dark areas not detected as thermally active but which nevertheless appear to be sites of recent volcanic activity. We utilize a trend analysis to extrapolate from the smallest detectable volcanic heat sources to these smallest mapped dark areas. Including the additional heat from estimates for “outburst” eruptions and for a multitude of very small (“myriad”) hot spots, we account for ～62 × 1012 W (～59 ± 7% of Io’s total thermal emission). Loki Patera contributes, on average, 9.6 × 1012 W (～9.1 ± 1%). All dark paterae contribute 45.3 × 1012 W (～43 ± 5%). Although dark flow fields cover a much larger area than dark paterae, they contribute only 5.6 × 1012 W (～5.3 ± 0.6%). Bright paterae contribute ～2.6 × 1012 W (～2.5 ± 0.3%). Outburst eruption phases and very small hot spots contribute no more than ～4% of Io’s total thermal emission: this is probably a maximum value. About 50% of Io’s volcanic heat flow emanates from only 1.2% of Io’s surface. Of Io’s heat flow, 41 ± 7.0% remains unaccounted for in terms of identified sources. Globally, volcanic heat flow is not uniformly distributed. Power output per unit surface area is slightly biased towards mid-latitudes, although there is a stronger bias toward the northern hemisphere when Loki Patera is included. There is a slight favoring of the northern hemisphere for outbursts where locations were well constrained. Globally, we find peaks in thermal emission at ～315°W and ～105°W (using 30° bins). There is a minimum in thermal emission at around 200°W (almost at the anti-jovian longitude) which is a significant regional difference. These peaks and troughs suggest a shift to the east from predicted global heat flow patterns resulting from tidal heating in an asthenosphere. Global volcanic heat flow is dominated by thermal emission from paterae, especially from Loki Patera (312°W, 12°N). Thermal emission from dark flows maximises between 165°W and 225°W. Finally, it is possible that a multitude of very small hot spots, smaller than the present angular resolution detection limits, and/or cooler, secondary volcanic processes involving sulphurous compounds, may be responsible for at least part of the heat flow that is not associated with known sources. Such activity should be sought out during the next mission to Io.  相似文献   

11.
Hipparcos photometric data for the massive O-type binary UW CMa were analysed within the framework of the Roche model. Photometric solutions were obtained for five mass ratios in the q = M2/M1 = 0.5–1.5 range. The system is found to be in a contact configuration. Independently of q, the best-fitting model solutions correspond to the orbital inclination i ～ 71° and the temperature of the secondary component T2 ～ 33500 K, at the fixed temperature of the primary T1 = 33750 K. Considering that the spectrum of the secondary is very weak, photometric solutions corresponding to the contact configuration favor the mass ratio q smaller than unity (in which case the luminosity of the secondary is smaller than that of the primary). The absolute parameters of the system are estimated for different values of the mass ratio.  相似文献   

12.
We obtained estimates of the Johnson V absolute magnitudes (H) and slope parameters (G) for 583 main-belt and near-Earth asteroids observed at Ond?ejov and Table Mountain Observatory from 1978 to 2011. Uncertainties of the absolute magnitudes in our sample are <0.21 mag, with a median value of 0.10 mag. We compared the H data with absolute magnitude values given in the MPCORB, Pisa AstDyS and JPL Horizons orbit catalogs. We found that while the catalog absolute magnitudes for large asteroids are relatively good on average, showing only little biases smaller than 0.1 mag, there is a systematic offset of the catalog values for smaller asteroids that becomes prominent in a range of H greater than ～10 and is particularly big above H ～ 12. The mean (Hcatalog ? H) value is negative, i.e., the catalog H values are systematically too bright. This systematic negative offset of the catalog values reaches a maximum around H = 14 where the mean (Hcatalog ? H) is ?0.4 to ?0.5. We found also smaller correlations of the offset of the catalog H values with taxonomic types and with lightcurve amplitude, up to ～0.1 mag or less. We discuss a few possible observational causes for the observed correlations, but the reason for the large bias of the catalog absolute magnitudes peaking around H = 14 is unknown; we suspect that the problem lies in the magnitude estimates reported by asteroid surveys. With our photometric H and G data, we revised the preliminary WISE albedo estimates made by Masiero et al. (Masired, J.R. et al. [2011]. Astrophys. J. 741, 68–89) and Mainzer et al. (Mainzer, A. et al. [2011b]. Astrophys. J. 743, 156–172) for asteroids in our sample. We found that the mean geometric albedo of Tholen/Bus/DeMeo C/G/B/F/P/D types with sizes of 25–300 km is pV = 0.057 with the standard deviation (dispersion) of the sample of 0.013 and the mean albedo of S/A/L types with sizes 0.6–200 km is 0.197 with the standard deviation of the sample of 0.051. The standard errors of the mean albedos are 0.002 and 0.006, respectively; systematic observational or modeling errors can predominate over the quoted formal errors. There is apparent only a small, marginally significant difference of 0.031 ± 0.011 between the mean albedos of sub-samples of large and small (divided at diameter 25 km) S/A/L asteroids, with the smaller ones having a higher albedo. The difference will have to be confirmed and explained; we speculate that it may be either a real size dependence of surface properties of S type asteroids or a small size-dependent bias in the data (e.g., a bias towards higher albedos in the optically-selected sample of asteroids). A trend of the mean of the preliminary WISE albedo estimates increasing with asteroid size decreasing from D ～ 30 down to ～5 km (for S types) showed in Mainzer et al. (Mainzer, A. et al. [2011a]. Astrophys. J. 741, 90–114) appears to be mainly due to the systematic bias in the MPCORB absolute magnitudes that progressively increases with H in the corresponding range H = 10–14.  相似文献   

13.
Vertical distributions and spectral characteristics of Titan’s photochemical aerosol and stratospheric ices are determined between 20 and 560 cm?1 (500–18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined.In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ～3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC3N), as inferred from a composite emission feature at ～160 cm?1, appear to be located over a narrow altitude range in the stratosphere centered at ～90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S.There is some evidence of a second ice cloud layer at ～60 km altitude at 58°S associated with an emission feature at ～80 cm?1. We speculate that the identify of this cloud may be due to C2H6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH4) in the stratosphere of Titan.Unlike the highly restricted range of altitudes (50–100 km) associated with organic condensate clouds, Titan’s photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3–2.4 at about 160 km to 1.1–1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ～140 cm?1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics.Finally, volume extinction coefficients NχE derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios NχE(1.583 μm)/NχE(62.5 μm) of roughly 70 and 20, respectively, for Titan’s aerosol and stratospheric ices. The inferred particle cross-section ratios χE(1.583 μm)/χE(62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.  相似文献   

14.
《Planetary and Space Science》2007,55(9):1050-1057
The ESA Rosetta Spacecraft, launched on March 2, 2004 with the ultimate destination being Comet 67P/Churyumov–Gerasimenko, carries a relatively small and lightweight millimeter–submillimeter spectrometer instrument, the first of its kind launched into deep space. The instrument, named Microwave Instrument for the Rosetta Orbiter (MIRO), consists of a 30-cm diameter, offset parabolic reflector telescope, which couples energy in the millimeter and submillimeter bands to two heterodyne receivers. Center-band operating frequencies are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband, total power continuum measurements can be made in both bands. A 4096-channel spectrometer with 44 kHz resolution is connected to the submillimeter receiver. The spectral resolution is sufficient to observe individual, thermally broadened spectral lines (T⩾10 K). The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species—CO, CH3OH, NH3 and three isotopes of water, H216O, H217O and H218O. The MIRO experiment will use these species as probes of the physical conditions within the nucleus and coma. The basic quantities measured by MIRO are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This information will be used to infer coma structure and outgassing processes, including the nature of the nucleus/coma interface.  相似文献   

15.
We suggest that planets, brown dwarfs, and even low mass stars can be formed by fragmentation of protoplanetary disks around very massive stars (M ? 100 M⊙). We discuss how fragmentation conditions make the formation of very massive planetary systems around very massive stars favorable. Such planetary systems are likely to be composed of brown dwarfs and low mass stars of ～0.1–0.3 M⊙, at orbital separations of ～ few × 100–104 AU. In particular, scaling from solar-like stars suggests that hundreds of Mercury-like planets might orbit very massive stars at ～103 AU where conditions might favor liquid water. Such fragmentation objects can be excellent targets for the James Webb Space Telescope and other large telescopes working in the IR bands. We predict that deep observations of very massive stars would reveal these fragmentation objects, orbiting in the same orbital plane in cases where there are more than one object.  相似文献   

16.
Ocean wave growth on Titan is considered. The classic Sverdrup–Munk theory for terrestrial wave growth is applied to Titan, and is compared with a simple energy balance model that exposes the effect of Titan’s environmental parameters (air density, gravity, and fluid density). These approaches are compared with the only previously-published (semi-empirical) model (Ghafoor, N.A.-L., Zarnecki, J.C., Challenor, P., Srokosz, M.A. [2000] J. Geophys. Res. 105, 12,077–12,091, hereafter G2k), and allow the impact of various parameters such as atmospheric density to be transparently explored.Our model, like G2k, suggests fully-developed significant wave heights on Titan Hs = 0.2 U2, where U is the windspeed (SI units): in dimensionless terms this is rather close to Hs = 0.2 U2/g, a rule of thumb previously noted for terrestrial waves (we find various datasets where the prefactor varies by ～2). It is noted that liquid and air densities affect the growth rate of waves, but not their fully-developed height: for 1 m/s winds wave amplitude reaches 0.15 m (75% of fully-developed) with a fetch of only 1 km, rather faster than predicted by G2k. Liquid viscosity has no major effect on gravity wave growth, but does influence the threshold windspeed at which gravity–capillary waves form in the first place.The model is used to develop predicted ranges for wave height to guide the design of the Titan Mare Explorer (TiME), a proposed Discovery-class mission to float a capsule on Ligeia Mare in 2023. For the expected maximum 1 m/s winds, a significant wave height of 0.2 m and wavelength of ～4 m can be expected. Assuming that wave heights follow Rayleigh statistics as they do on Earth, then given the wave period of ～4 s, individual waves of ～0.6 m might be encountered over a 3 month period.For predicted Titan winds at Kraken Mare, significant wave heights may reach ～0.6 m in the peak of summer but do not exceed the tidal amplitude at its northern end, consistent with the area around Mayda Insula being a tidal flat, while elsewhere on Kraken and Ligeia and at Ontario Lacus, shorelines may be wave- or tidally-dominated, depending on the specific location.  相似文献   

17.
We derive astrophysical and structural parameters of the poorly studied open clusters NGC 6866, NGC 7062, and NGC 2360 based on filtered 2MASS (J, J ? H) diagrams, and stellar radial density profiles. The field star decontamination technique is utilised for selecting high-probability cluster members. The E(B ? V) reddening values of the three clusters derived from 2MASS JHKs agree with those inferred from UBV and uvby ? β photometries. We find that the core mass function slopes are flatter than the halo’s for the three clusters. The large core and cluster radii of NGC 6866 and NGC 2360 indicate an expanded core, which may suggest the presence of stellar mass black-holes. NGC 2360 is located in the third quadrant (? = 229°.80), where Giant Molecular Clouds are scarce that, together with its relatively large mass (～1800 m⊙), might explain its longevity (～1.8 Gyr) in the Galaxy.  相似文献   

18.
This paper presents new CCD Bessell BVRI light curves and photometric analysis of the Algol-type binary star TX Her. The CCD observations were carried out at Çanakkale Onsekiz Mart University Observatory in 2010. New BVRI light curves from this study and radial velocity curves from Popper (1970) were solved simultaneously using modern light and radial velocity curves synthesis methods. The general results show that TX Her is a well-detached eclipsing binary, however, both component stars fill at least half of their Roche lobes. A significant third light contribution to the total light of the system could not be determined. Using O–C residuals formed by the updated minima times, an orbital period study of the system was performed. It was confirmed that the tilted sinusoidal O–C variation corresponds to an apparent period variation caused by the light travel time effect due to an unseen third body. The following absolute parameters of the components were derived: M1 = 1.62 ± 0.04 M⊙, M2 = 1.45 ± 0.03 M⊙, R1 = 1.69 ± 0.03 R⊙, R2 = 1.43 ± 0.03 R⊙, L1 = 8.21 ± 0.90 L⊙ and L2 = 3.64 ± 0.60 L⊙. The distance to TX Her was calculated as 155 ± 10 pc, taking into account interstellar extinction. The position of the components of TX Her in the HR diagram are also discussed. The components are young stars with an age of ～500 Myr.  相似文献   

19.
The European Space Agency’s Rosetta spacecraft is the first Solar System mission to include instrumentation capable of measuring planetary thermal fluxes at both near-IR (VIRTIS) and submillimeter–millimeter (smm–mm, MIRO) wavelengths. Its primary mission is a 1 year reconnaissance of Comet 67P/Churyumov–Gerasimenko beginning in 2014. During a 2010 close fly-by of Asteroid 21 Lutetia, the VIRTIS and MIRO instruments provided complementary data that have been analyzed to produce a consistent model of Lutetia’s surface layer thermal and electrical properties, including a physical model of self-heating. VIRTIS dayside measurements provided highly resolved 1 K accuracy surface temperatures that required a low thermal inertia, I < 30 J/(K m2 s0.5). MIRO smm and mm measurements of polar night thermal fluxes produced constraints on Lutetia’s subsurface thermal properties to depths comparable to the seasonal thermal wave, yielding a model of I < 20 J/(K m2 s0.5) in the upper few centimeters, increasing with depth in a manner very similar to that of Earth’s Moon. Subsequent MIRO-based model predictions of the dayside surface temperatures reveal negative offsets of ～5–30 K from the higher VIRTIS-measurements. By adding surface roughness in the form of 50% fractional coverage of hemispherical mini-craters to the MIRO-based thermal model, sufficient self-heating is produced to largely remove the offsets relative to the VIRTIS measurements and also reproduce the thermal limb brightening features (relative to a smooth surface model) seen by VIRTIS. The Lutetia physical property constraints provided by the VIRTIS and MIRO data sets demonstrate the unique diagnostic capabilities of combined infrared and submillimeter/millimeter thermal flux measurements.  相似文献   

20.
《Planetary and Space Science》2007,55(10):1328-1345
The planetary fourier spectrometer (PFS) for the Mars express mission (MEX) is an infrared spectrometer operating in the wavelength range from 1.2 to 45 μm by means of two spectral channels, called SWC (short wavelength channel) and LWC (long wavelength channel), covering, respectively, 1.2–5.5 and 5.5–45 μm.The middle-spring Martian north polar cap (Ls∼40°) has been observed by PFS/MEX in illuminated conditions during orbit 452. The SWC spectra are here used to study the cap composition in terms of CO2 ice, H2O ice and dust content. Significant spectral variation is noted in the cap interior, and regions of varying CO2 ice grain sizes, water frost abundance, CO2 ice cover and dust contamination can be distinguished. In addition, we correlate the infrared spectra with an image acquired during the same orbit by the OMEGA imaging spectrometer and with the altimetry from MOLA data. Many of the spectra variations correlate with heterogeneities noted in the image, although significant spectral variations are not discernible in the visible. The data have been divided into five regions with different latitude ranges and strong similarities in the spectra, and then averaged. Bi-directional reflectance models have been run with the appropriate lighting geometry and used to fit the observed data, allowing for CO2 ice and H2O ice grain sizes, dust and H2O ice contaminations in the form of intimate granular mixtures and spatial mixtures.A wide annulus of dusty water ice surrounds the recessing CO2 seasonal cap. The inner cap exhibits a layered structure with a thin CO2 layer with varying concentrations of dark dust, on top of an H2O ice underneath ground. In the best-fits, the ices beneath the top layer have been considered as spatial mixtures. The results are still very good everywhere in the spectral range, except where the CO2 ice absorption coefficients are such that even a thin layer is enough to totally absorb the incoming radiation (i.e. the band is saturated). This only happens around 3800 cm−1, inside the strong 2.7-μm CO2 ice absorption band. The effect of finite snow depth has been investigated through a layered albedo model. The thickness of the CO2 ice deposits increases with latitude, ranging from 0.5–1 g cm−2 within region II to 60–80 g cm−2 within the highest-latitude (up to 84°N) region V.Region I is at the cap edge and extends from 65°N to 72°N latitude. No CO2 ice is present in this region, which consists of relatively large grains of water ice (20 μm), highly contaminated by dust (0.15 wt%). The adjacent region II is a narrow region [76–79°N] right at the edge of the north residual polar cap. This region is very distinct in the OMEGA image, where it appears to surround the whole residual cap. The CO2 ice features are barely visible in these spectra, except for the strong saturated 2.7 μm band. It basically consists of a thin layer of 5-mm CO2 ice on top of an H2O ice layer with the same composition as region I. A third interesting region III is found all along the shoulder of the residual cap [79–81°N]. It extends over 1.5 km in altitude and over only 2° of latitude and consists of CO2 ice with a large dust content. It is an admixture of CO2 ice (3–4 mm), with several tens of ppm by mass of water ice and more than 2 ppt by mass of dust. The surface temperatures have been retrieved from the LWC spectra for each observation. We found an increase in the surface temperature in this region, indicating a spatial mixture of cold CO2 ice and warmer dust/H2O ice. Region IV is close to the top of the residual cap [81–84°N]; it is much brighter than region III, with a dust content 10 times lower than the latter. The CO2 grain size is 3 mm and strong CO2 ice features are present in the data, indicating a thicker CO2 ice layer than in region II (1–2 g cm−2). The final region V is right at the top of the residual cap (⩾84°N). It is “pure” CO2 ice (no dust) of 5 mm grain sizes, with 30 ppm by weight of water ice. The CO2 ice features are very pronounced and the 2.7 μm band is saturated. The optical thickness is close to the semi-infinite limit (30–40 g cm−2). Assuming a snowpack density of 0.5 g cm−3, we get a minimum thickness of 1–2 cm for the top-layer of regions II and III, 4–10 cm for region IV, and ⩾60–80 cm thickness for region V. These values are in close agreement with several recent results for the south seasonal polar cap.These results should provide new, useful constraints in models of the Martian climate system and volatile cycles.  相似文献   

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