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 31 毫秒
1.
Space infrared nulling interferometry has been identified as one of the most promising techniques for direct detection of Earthlike extrasolar planets and spectroscopic analysis of their atmospheres in the near future. After a review of various nulling interferometer schemes, we introduce the concept of internal modulation. As an illustration, we describe a two-dimensional array of telescopes that provides full internal modulation capabilities: the Mariotti space interferometer. It consists of six free-flying telescopes positioned on the sides of an equilateral triangle and grouped into three nulling interferometers. Their nulled outputs are suitably phase-shifted with respect to each other, coherently recombined, and detected. The phase shifts applied between the nullers are periodically changed, providing signal modulation at a frequency that can be selected to minimize instrumental and background noise. The frequency upper limit is set by the read-out noise of the detectors, and turns out to be 10−1-10−2 Hz for currently available Si:As BIB devices. This “fast” signal modulation allows much better monitoring of the background and detector drifts than when one relies solely on the external modulation provided by the slow rotation of the whole interferometer (at typical frequencies of 3×10−4-3×10−5 Hz). Mariotti internal modulation, also known as “phase chopping,” thus appears as a major step toward the feasibility of the Darwin and TPF space missions.  相似文献   

2.
Magnetic photon splitting γ → γγ, a quantum electrodynamic process that becomes important when magnetic field approaching the quantum critical value, B c = 4.413 × 1013 G, may have important effects on pulsar radio emission. According to the standard model, the pulsar radio emission is produced by coherent curvature radiation of a large amounts of e ± pairs, which are thought to be generated by the pair creation process γ + B → e ±. However, if the magnetic field is strong enough, the photon splitting may dominate the pair creation process, then the amounts of e ± pairs and the radio luminosity will be strongly suppressed and may be undetectable. Here we use the fitted analytical formula of the photon splitting attenuation coefficient to study the above process, and find that the photon splitting will strongly decrease the radio emission when B > 1013 G. We also note that the photon splitting can strongly but not totally suppress the creation of pairs due to the diminishing dependence of B in the attenuation coefficient. We find that the ratio of the probability of a photon being absorbed by photon splitting to that by pair creation is no more than about six. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

3.
In this paper, we improve the previous work on the MHD Alfvén wave oscillation model for the neutron star (NS) kHz quasi‐periodic oscillations (QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS X‐ray sources with the simultaneously detected twin kHz QPO frequencies, the stellar mass M and radius R constraints are given by means of the derived parameter A in the model, which is associated with the averaged mass density of the star as 〈ρ 〉 = 3M /(4πR3) ≃ 2.4 × 1014 (A /0.7)2 g/cm3, and we also compare the M ‐R constraints with the stellar equations of state. Moreover, we also discuss the theoretical maximum kHz QPO frequency and maximum twin peak separation, and some expectations on SAX J1808.4–3658 are mentioned, such as its highest kHz QPO frequency ∼ 870 Hz, which is about 1.4–1.5 times less than those of the other known kHz QPO sources. The estimated magnetic fields for both Z sources (about Eddington accretion rate ) and Atoll sources (∼ 1% ) are approximately ∼109 G and ∼108 G, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
G. Cremonese  M. Bruno  S. Marchi 《Icarus》2005,177(1):122-128
Meteoroid impact has been shown to be a source of sodium, and most likely of other elements, on the Moon. The same process could be also relevant for Mercury. In this work we calculate the vapor and neutral Na production rates on Mercury due to the impacts of meteoroids in the radius range of 10−8-10−1 m. We limit our calculations to this size range, because meteoroids with radius larger than 10−1 m have not to be found important for the daily production of the exosphere. This work is based on a new dynamical model of the meteoroid flux at the heliocentric distance of Mercury, regarding objects in the size range 10−2-10−1 m. This size range, never investigated before, is not affected by nongravitational forces, such as the Poynting-Robertson effect, which is dominant for particles smaller than 10−2 m. In order to evaluate the release of neutral sodium atoms also for smaller meteoroids we have used the distribution reported by M.J. Cintala [1992. Impact-induced thermal effects in the lunar and mercurian regoliths. J. Geophys. Res. 97, 947-973] calculated for particle size range 10−8-10−3 m. We have extrapolated this distribution up to 10−2 m and we have based the impact calculations on a new surface composition assuming 90% plagioclase and 10% pyroxene. The results of our model are that (i) the total mass of vapor produced by the impact of meteoroids in the size range 10−8-10−1 m is 4.752×108 g per year, and (ii) the production rate of neutral sodium atoms is 1.5×1022 s−1.  相似文献   

5.
Unique among neutron stars, 1E 1207.4-5209 is an X-ray pulsar with a spin period of 424 ms that contains at least two strong absorption features in its energy spectrum. This neutron star is positionally coincident with the supernova remnant PKS 1209-51/52 and has been identified as a member of the growing class of radio-quiet compact central objects in supernova remnants. From previous observations with Chandra and XMM-Newton, it has been found that the 1E 1207.4-5209 is not spinning down monotonically as is common for young, isolated pulsars. The spin frequency history requires either strong, frequent glitches, the presence of a fall-back disk, or a binary companion. Here, we report on a sequence of seven XMM-Newton observations of 1E 1207.4-5209 performed during a 40 day window between 2005 June 22 and July 31. Due to unanticipated variance in the phase measurements during the observation period that was beyond the statistical uncertainties, we could not identify a unique phase-coherent timing solution. The three most probable timing solutions give frequency time derivatives of +0.9, ?2.6, and +1.6×10?12 Hz s?1 (listed in descending order of significance). We conclude that the local frequency derivative during our XMM-Newton observing campaign differs from the long-term spin-down rate by more than an order of magnitude. This measurement effectively rules out glitch models for 1E 1207.4-5209. If the long-term spin frequency variations are caused by timing noise, the strength of the timing noise in 1E 1207.4-5209 is much stronger than in other pulsars with similar period derivatives. Therefore, it is highly unlikely that the spin variations are caused by the same physical process that causes timing noise in other isolated pulsars. The most plausible scenario for the observed spin irregularities is the presence of a binary companion to 1E 1207.4-5209. We identified a family of orbital solutions that are consistent with our phase-connected timing solution, archival frequency measurements, and constraints on the companions mass imposed by deep IR and optical observations.  相似文献   

6.
Annie Baglin 《Solar physics》1991,133(1):155-160
High precision photometry from space is the only technique to detect the nonradial oscillations of a significant set of stars with different masses, ages, angular momentum, chemical composition, as already observed for the Sun. During an observation run of several days, a reasonably small telescope (10 cm) will collect enough photons to detect periodic signals of relative amplitude as low as 10 –6, coherent over this period. The frequency resolution necessary to distinguish the rotational splitting (0.5 H) will be achieved in increasing the observation run to 20 days.  相似文献   

7.
We have analyzed spectra of CO recorded with the instrument PFS onboard Mars Express in the (1-0) band. The dataset we used ranges in time from January until June 2004 (LS=331°.17 until LS=51°.61; end of Mars Year 26, beginning of Mars Year 27). The aim of this work was to determine the amplitude of the CO mixing ratio departures from the mean globally averaged value currently admitted (8±3×10-4) [Kaplan, L.D., Connes, J., Connes, P., 1969. Carbon monoxide in the martian atmosphere. Astron. J. 157, L187-L192] as a function of season, local time and location on the planet. We therefore processed the data from 90 calibrated orbits. The globally averaged CO mixing ratio value we derive from our dataset, 11.1×10-4, is compatible with the range found by Kaplan et al. [1969. Carbon monoxide in the martian atmosphere. Astron. J. 157, L187-L192], although somewhat higher than the “standard” value. However, the CO mixing ratio we retrieve exhibits large variations (roughly between 3×10-4 and 18×10-4). Such relative variations have been used on a statistical basis to derive main trends as a function of latitude for three LS ranges: 331-360°, 0-30° and 30-52°. For the first LS range, we seem to have an enhancement of the CO mixing ratio towards the northern latitudes, probably linked to the CO2 condensation in winter on the north polar cap. The situation for the two other LS ranges is not so clear, mainly as we lack data on the southern hemisphere. We roughly agree with the work of Krasnopolsky [2007. Long-term spectroscopic observations of Mars using IRTF/CSHELL: mapping of O2 dayglow, CO and search for CH4. Icarus 190, 93-102] for LS=331-360°, thus confirming the effect of seasonal condensation of CO2 on the north polar cap, but we have no agreement for other seasons.  相似文献   

8.
Meléndez  J.L.  Sawant  H.S.  Fernandes  F.C.R.  Benz  A.O. 《Solar physics》1999,187(1):77-88
Here we report the statistical analysis of 160 isolated decimetric type III bursts, a majority of them (74%) having central frequency above 1000 MHz, observed in 8 flares by the spectrometer Phoenix. The most important finding of the detailed analysis is: 67% of all the bursts are reverse slope, 38% normal and 5% bi-directional. Also, we obtained the following results: (a) the best fits for the average half power duration and for the average drift rate as a function of frequency are given by t1/2=1.7×104f–0.60and /df/dt|=(0.09±0.03)f(1.35 ± 0.10), respectively; (b) the frequency range of most of the type III bursts is less than 250 MHz; (c) the number of bursts decreases with increasing starting frequency and flux; (d) peak flux decreases with increasing frequency. The relations obtained fit well for higher frequency observations. Assuming an improved density model and type III emission at 2ndharmonic, the beam parameters of type III bursts are determined.  相似文献   

9.
We have compared the frequency distribution of the dynamical observedquantity log (V z 2 r p), for a sample of 46 pairs of elliptical galaxies, to the distribution of this quantity obtained from numerical simulations of pairs of galaxies. From such an analysis, where we have considered the structure of the galaxies and its influence in the orbital evolution of the system, we have obtained the characteristic mass and the mass-luminosity ratio for the sample. Our results show that the hypothesis of point-mass in elliptical orbits is, for this sample, an approximation as good as the model that takes into account the structure of the galaxies. The statistical method used here gives an estimate of a more reliable mass, it minimizes the contamination of spurious pairs and it considers adequately the contribution of the physical pairs. We have obtained a characteristic mass to the 46 elliptical pairs of 1.68 × 1012 ± 7.01 ×1011 M⊙ with M/L = 17.6 ± 7.3 (H 0 = 60 km s-1Mpc-1). This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

10.
We are currently performing a new analysis of the close environment of star-forming dwarf galaxies and its effects on their star-forming activity. For a sample of 142 thoroughly studied objects, we search the NASA Extragalactic Database for possible companion galaxies of any apparent luminosity and angular size. From a first analysis of a part of our sample, we find only about 50% of our objects to be isolated in the sense that no companion object was detected within a redshift difference of 500 km s-1 and an angular separation corresponding to a projected distance of 1 Mpc, assuming H 0=75km s-1 Mpc-1. By splitting the sample into different redshift intervals, we demonstrate an observational bias against low-luminosity companion objects which implies that a large fraction of star-forming dwarf galaxies, although lacking bright companions, might possess dwarf companion objects. Similarly to other authors, we find no evidence for a difference in star-forming activity due to the presence or absence of possible companions. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
We obtained spectra of comet C/1999 S4 (LINEAR) with the UAGS spectrograph(long slit and CCD) installed on the 1-m Zeiss reflector of the SAO of the RAS(Northern Caucuses, Nizhny Arkhyz) on July 23/24, 26/27 and 27/28, 2000. OnJuly 22/23, before the splitting of the cometary nucleus, several emission lines,such as C2, C3, CN, NH, CH, NH2, CO+, H2O+ wereclearly identified in the spectra. The inspections of the CCD spectra obtainedon July 27/28, 2000 reveals only very weak emission lines superimposed on thesolar reflection spectrum. From analyzing the surface brightness profile of C2 along the slit the velocity of separation of two secondary fragments (V = 10 km/h) and the energy of the fragment separation (E = 8.7 × 1015 erg) were estimated. A luminescence cometary continuum of 26% of the total continuum level is detected in the spectra of the comet at 5000 Å. Possible mechanisms of nucleus splitting are discussed.  相似文献   

12.
Diffractive and refractive magnetospheric scintillations may allow direct probing of the plasma inside the pulsar light cylinder. The unusual electrodynamics of the strongly magnetized electron-positron plasma allows separation of the magnetospheric and interstellar scattering. The most distinctive feature of the magnetospheric scintillations is their independence of frequency. Diffractive scattering due to small scale inhomogeneities produces a scattering angle that may be as large as 0.1radians, and a typical decorrelation time of 10-8 seconds. Refractive scattering due to large scale inhomogeneities is also possible, with atypical angle of 10-3 radians and a correlation time of the order of10-4 seconds. Some of the magnetospheric propagation effects may have already been observed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

13.
In order to investigate the formation of martian gullies and the stability of fluids on Mars, we examined about 120 gully images. Twelve HiRISE images contained a sufficient number of Transverse Aeolian Ridges (TARs) associated with the gullies to make the following measurements: overall gully length, length of the alcove, channel and apron, and we also measured the frequency of nearby TARs. Six of the 12 images examined showed a statistically significant negative correlation between overall gully length (alcove, channel and apron length) and TAR frequency. Previous experimental work from our group has shown that at temperatures below ∼200 K, evaporation rate increases by about an order of magnitude as wind speed increases from 0 to ∼15 m/s. Thus the negative correlations we observe between gully length and dune frequency can be explained by formation at temperatures below ∼200 K where wind speed/evaporation is a factor governing gully length. In these cases evaporation of the fluid carving the gully was a constraint on their dimensions. Cases where there is no correlation between gully length and TAR frequency, can be explained by formation at temperatures >200 K. The temperatures are consistent with Global Circulation Model and Thermal Emission Spectrometer (TES) data for these latitudes. The temperatures suggested by these trends are consistent with the fluid responsible for gully formation being a strong brine, such as Fe2(SO4)3 which has a eutectic temperature of ∼200 K. We also find that formation timescales for gullies are 105-106 years.  相似文献   

14.
Keiji Ohtsuki 《Icarus》2006,183(2):384-395
We examine rotation rates of gravitating particles in low optical depth rings, on the basis of the evolution equation of particle rotational energy derived by Ohtsuki [Ohtsuki, K., 2006. Rotation rate and velocity dispersion of planetary ring particles with size distribution. I. Formulation and analytic calculation. Icarus 183, 373-383]. We obtain the rates of evolution of particle rotation rate and velocity dispersion, using three-body orbital integration that takes into account distribution of random velocities and rotation rates. The obtained stirring and friction rates are used to calculate the evolution of velocity dispersion and rotation rate for particles in one- and two-size component rings as well as those with a narrow size distribution, and agreement with N-body simulation is confirmed. Then, we perform calculations to examine equilibrium rotation rates and velocity dispersion of gravitating ring particles with a broad size distribution, from 1 cm up to 10 m. We find that small particles spin rapidly with 〈ω2〉1/2/Ω?102-103, where ω and Ω are the particle rotation rate and its orbital angular frequency, respectively, while the largest particles spin slowly, with 〈ω2〉1/2/Ω?1. The vertical scale height of rapidly rotating small particles is much larger than that of slowly rotating large particles. Thus, rotational states of ring particles have vertical heterogeneity, which should be taken into account in modeling thermal infrared emission from Saturn's rings.  相似文献   

15.
The interacting of two winds model and a nonspherical density functionin three dimensions is introduced to study the dynamical structure ofplanetary nebulae. A fast wind with a mechanical energy interacts with asuper wind mass-loss rate of 2 × 10-10 M yr and avelocity of 10 km s-1. As a result it produces a dense and luminosmedium.Taking into account the above assumptions, we introduce the code(DS3D),and numerically we calculate the following physical quantities:the shell velocity, the shell radious and thickness, and other physicalquantities throughout the entire nebula.  相似文献   

16.
Regarding new bipolar magnetic regions as sources of flux, we have simulated the evolution of the radial component of the solar photospheric magnetic field during 1976–1984 and derived the corresponding evolution of the line-of-sight polar fields as seen from Earth. The observed timing and strength of the polar-field reversal during cycle 21 can be accounted for by supergranular diffusion alone, for a diffusion coefficient of 800 km2 s-1. For an assumed 300 km2 s-1 rate of diffusion, on the other hand, a poleward meridional flow with a moderately broad profile and a peak speed of 10 m s-1 reached at about 5° latitude is required to obtain agreement between the simulated and observed fields. Such a flow accelerates the transport of following-polarity flux to the polar caps, but also inhibits the diffusion of leading-polarity flux across the equator. For flows faster than about 10 m s-1 the latter effect dominates, and the simulated polar fields reverse increasingly later and more weakly than the observed fields.Laboratory for Computational Physics and Fluid Dynamics.E. O. Hulburt Center for Space Research.  相似文献   

17.
Millisecond pulsars have a very high rotation stability, which can be applied to many research fields, such as the establishment of the pulsar time standard, the detection of gravitational wave, the spacecraft navigation by using X-ray pulsars and so on. In this paper, we employ two millisecond pulsars PSR J0437-4715 and J1713+0743, which are observed by the International Pulsar Timing Array (IPTA), to analyze the precision of pulsar clock parameter and the prediction accuracy of pulse time of arrival (TOA). It is found that the uncertainty of spin frequency is 10?15 Hz, the uncertainty of the first derivative of spin frequency is 10?23 s?2, and the precision of measured rotational parameters increases by one order of magnitude with the accumulated observational data every 4～5 years. In addition, the errors of TOAs within 4.8 yr which are predicted by the clock model established by the 10 yr data of J0437-4715 are less than 1 μs. Therefore, one can use the pulsar time standard to calibrate the atomic clock, and make the atomic time deviate from the TT (Terrestrial Time) less than 1 μs within 4.8 yr.  相似文献   

18.
Analysis of the RXTE slew data in October 1996 revealed a weak X-ray burst from the millisecond pulsar SAX J 1808.4-3658. The 3–20-keV energy spectrum of the source can be described by a power law with an index of 2.0 and a(3-to 20 keV) luminosity of ～1.4×1035 erg s?1 (the distance to the source was taken to be 2.5 kpc). Because of the short exposure time, we failed to detect weak pulsations at a frequency of 401 Hz in the source. The (2σ) upper limit of the pulse fraction is ～13%.  相似文献   

19.
A wide-band digital decimetric spectroscope is in the process of development at INPE (Brazil), in conjunction with a 9-m diameter polar mounted antenna. Initially, this spectroscope was operating over a narrow-band 1.6 ± 0.05 MHz in an analogue mode. Here we report a slow drifting 5.0 MHz s-1, narrow-band 1630-1580 MHz, split frequency solar burst lasting for about 15 s, observed on 15 June, 1991. The separation between the split components is 30 MHz, and the upper split frequency component is more intense than the lower split frequency component. These observed characteristics favour the hypothesis of conversion of plasma waves by combinational scattering on upgoing ion-sound waves in a magnetic loop. Existing strong electron density gradients across the magnetic field in the source region will reduce the free-free absorption of radiation at the fundamental frequency. In order to explain the observed temporal characteristics of the upper and lower split frequency components, the radiation has to propagate at some angle to the electron density gradient in the source region. Estimated physical parameters of the source and exciters are as follows: (i) maximum source size (height) 2 × 107 cm; (ii) velocity and length of ion-sound pulse 3.1 × 106 cm s-1 and 5 × 107 cm, respectively, and (iii) velocity of the exciter of plasma waves (electron or proton beam) 5 × 108 cm s-1.On leave from the Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia.  相似文献   

20.
The abundances of PH3, CH3D, and GeH4 are derived from the 2100- to 2250-cm?1 region of the Voyager 1 IRIS spectra. No evidence is seen for large-scale variations of the phosphine abundance over Jovian latitudes between ?30 and +30°. In the atmospheric regions corresponding to 170–200°K, the derived PH3/H2 value is (4.5 ± 1.5) × 10?7 or 0.75 ± 0.25 times the solar value. This result, compared with other PH3 determinations at 10 μm, suggests than the PH3/H2 ratio on Jupiter decreases with atmospheric pressure. In the 200–250°K region, we derive, within a factor of 2, CH3D/H2 and GeH4/H2 ratios of 2.0 × 10?7 and 1.0 × 10?9, respectively. Assuming a C/H value of 1.0 × 10?3, as derived from Voyager, our CH3D/H2 ratio implies a D/H ratio of 1.8 × 10?5, in reasonable agreement with the interstellar medium value.  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