An optical and ultraviolet survey of Algols

We present a-table of results from our survey of Algols, conducted with the Lick Observatory ITS scanners and the IUE spectrometer.We have determined the continuous flux distributions for a number of the Algol systems. Optical scans were made with the ITS scanners of Lick Observatory, while for the ultraviolet flux distributions, we used the IUE satellite spectrometer in the low-dispersion mode. The following table summarizes the results: {ie340-01}     

High energy neutrinos from quasars

We review and clarify the assumptions of our basic model for neutrino production in the cores of quasars, as well as those modifications to the model made subsequently by other workers. We also present a revised estimate of the neutrino background flux and spectrum obtained using more recent empirical studies of quasars and their evolution. We compare our results with other theoretical calculations and experimental upper limits on the AGN neutrino background flux. We also estimate possible neutrino fluxes from the jets of blazars detected recently by the EGRET experiment on the Compton Gamma Ray Observatory. We discuss the theoretical implications of these estimates.NAS-NRC Senior Research Associate, on leave from the Physics Department, University of Utah     

Infrared irradiance calibration

Infrared astronomical measurements are calibrated against reference sources, usually primary standard stars that are, in turn, calibrated either by direct or indirect means. A direct calibration compares the star with a certified source, typically a blackbody. Indirect methods extrapolate a direct measurement of the flux at one wavelength to the flux at another. Historically, α Lyr (Vega) has been used as the primary standard as it is bright, easily accessible from the northern hemisphere, and is well calibrated in the visual. Until recently, the direct absolute infrared calibrations of α Lyr and those derived from the absolute solar flux scaled to the observed spectral energy distributions of solar type stars increasingly diverged with wavelength from those obtained using a model atmosphere to extrapolate the absolute visual flux of Vega into the infrared. The exception is the direct calibration by the 1996/97 Midcourse Space Experiment of the absolute fluxes for a number of the commonly used infrared standard stars, including Vega.In the mid-1980s, the Air Force Geophysics Laboratory began a program that led to the establishment of a network of stars with which to calibrate infrared space-based sensors. α Lyr and a CMa were adopted as the fundamental references and the absolute 1.2 to 35 µm infrared spectral energy distributions for the 616 secondary standard stars in the network were derived through spectral and photometric comparisons with the primary standards. The stars are also used for calibration at ground-based infrared observatories. For applications in which the network stars may not be bright enough, particularly at the longer infrared wavelengths, planets and the larger asteroids are used. Planets and asteroids move and rather sophisticated thermal modeling of the bodies is required to predict the disk-integrated brightness at a specific time with reasonable accuracy. The Infrared Space Observatory applied such a sophisticated ‘thermo-physical’ model to the largest asteroids to support calibration of the sensors to a claimed accuracy of within 5%. The AFRL program also created a spectral atlas of the brightest stars in the sky that, although they are variable, may be used for calibration if the large(r) attendant uncertainties are acceptable.This revised version was published online in July 2005 with a corrected cover date.     

Infrared Irradiance Calibration

Infrared astronomical measurements are calibrated against reference sources, usually primary standard stars that are, in turn, calibrated either by direct or indirect means. A direct calibration compares the star with a certified source, typically a blackbody. Indirect methods extrapolate a direct measurement of the flux at one wavelength to the flux at another. Historically, α Lyr (Vega) has been used as the primary standard as it is bright, easily accessible from the northern hemisphere, and is well calibrated in the visual. Until recently, the direct absolute infrared calibrations of α Lyr and those derived from the absolute solar flux scaled to the observed spectral energy distributions of solar type stars increasingly diverged with wavelength from those obtained using a model atmosphere to extrapolate the absolute visual flux of Vega into the infrared. The exception is the direct calibration by the 1996/97 Midcourse Space Experiment of the absolute fluxes for a number of the commonly used infrared standard stars, including Vega.In the mid-1980s, the Air Force Geophysics Laboratory began a program that led to the establishment of a network of stars with which to calibrate infrared space-based sensors. α Lyr and a CMa were adopted as the fundamental references and the absolute 1.2 to 35 µm infrared spectral energy distributions for the 616 secondary standard stars in the network were derived through spectral and photometric comparisons with the primary standards. The stars are also used for calibration at ground-based infrared observatories. For applications in which the network stars may not be bright enough, particularly at the longer infrared wavelengths, planets and the larger asteroids are used. Planets and asteroids move and rather sophisticated thermal modeling of the bodies is required to predict the disk-integrated brightness at a specific time with reasonable accuracy. The Infrared Space Observatory applied such a sophisticated ‘thermo-physical’ model to the largest asteroids to support calibration of the sensors to a claimed accuracy of within 5%. The AFRL program also created a spectral atlas of the brightest stars in the sky that, although they are variable, may be used for calibration if the large(r) attendant uncertainties are acceptable.     

The fundamental parameters of the central stars of eight WR ring nebulae

We describe work that has recently been completed on deriving the fundamental parameters of eight WR stars through the photoionization modelling of their surrounding nebulae using non-LTE WR flux distributions. The resulting effective temperatures range from 57 000–71 000 K for the WN4-5 stars and <30 000–42 000 K for the WN6-8 stars. The derived stellar parameters are compared with those obtained from stellar emission line modelling. We find good agreement for the hot early WN stars, indicating that the non-LTE WR flux distributions have essentially the correct shape in the crucial far-UV region. We find lower temperatures for the four cooler late WN stars, particularly for the two WN6 stars. For the nebulae surrounding these stars, we find that the model flux distributions produce too much nebular ionization. We suggest that these discrepancies arise because of the lack of line-blanketing in the WR atmospheres. For the WO1 central star of G2.4+1.4, with strong nebular He II 4686 A emission, we derive a temperature of 105 000 K, somewhat less than previous estimates. The positions of our eight WR stars on the H-R diagram are compared with the evolutionary tracks of Maeder (1990) for solar metallicity. In common with previous workers, we find that our derived luminosities are too low, giving an initial mass range of 25–40 M, below that expected for the majority of WR stars.     

Microflares and the Statistics of X-ray Flares

This review surveys the statistics of solar X-ray flares, emphasising the new views that RHESSI has given us of the weaker events (the microflares). The new data reveal that these microflares strongly resemble more energetic events in most respects; they occur solely within active regions and exhibit high-temperature/nonthermal emissions in approximately the same proportion as major events. We discuss the distributions of flare parameters (e.g., peak flux) and how these parameters correlate, for instance via the Neupert effect. We also highlight the systematic biases involved in intercomparing data representing many decades of event magnitude. The intermittency of the flare/microflare occurrence, both in space and in time, argues that these discrete events do not explain general coronal heating, either in active regions or in the quiet Sun.     

Optical observations of the X-ray source 2S0921-630

We report photometric observations of the optical counterpart of the X-ray source 2S0921-630. The data, obtained at the South African Astronomical Observatory during 3 weeks in 1980 and 1981, are consistent with a 17.9 day periodic modulation of the flux in the B band. Correlated variability of the (B-V) and (U-B) colour indices with the B mag. is demonstrated and quantified. The observed B mag. and colours at maximum and minimum light are used to compute some of the system parameters. A model of 2S0921-630 is proposed in terms of a binary system in which the variable inclination of a luminous accretion disc produces the long-term modulation of the optical flux.     

X-ray Diffraction Gratings for Astrophysics

Over the past year, we have celebrated the tenth anniversary of the Chandra and XMM-Newton X-ray observatories. Both carry powerful, novel diffraction grating spectrometers, which have opened true X-ray spectroscopy for astrophysics. I will describe the design and operation of these instruments, as the background to some of the beautiful results they have produced. But these designs do not exhaust the versatility and essential simplicity of diffraction grating spectrometers, and I will discuss applications for the International X-ray Observatory IXO.     

Statistical characteristics of spray autoignition of transient kerosene jet in cross flow

To study statistical characteristics of the random spray autoignition, aviation kerosene was injected transiently into non-vitiated air crossflow in a flow reactor with optical accesses. The operating conditions were relevant to gas turbine combustor: the air crossflow pressure and temperature were in the range of 1.4–1.7 MPa and 830–947 K, respectively, and the jet-to-crossflow momentum flux ratios were 20, 50 and 80. Statistical distributions of random ignition delay times with adequate convergence were estimated based on histograms. The dependences of the distributions on reactor pressure, temperature, and jet-to-crossflow momentum flux ratio were studied. The results show that the resulting distributions appear more concentrated with the increase of air temperature or jet-to-crossflow momentum flux ratio. And then the correlations for the mean and standard deviation of the ignition delay time sample data were developed based on the present results. Compared with the correlations of ignition delay time of homogeneous premixed gas-phase kerosene/air mixture reported in the literature, the results show a greater significance pressure dependence and lower temperature sensitivity of the ignition delay time of non-premixed kerosene spray.     

The Infrared Space Observatory (ISO)

The Infrared Space Observatory (ISO), a fully approved and funded project of ESA, will operate at wavelengths from 3–200 microns. The satellite essentially consists of a large cryostat containing about 2300 litres of superfluid helium to maintain the telescope (primary mirror diameter of 60 cm) and the scientific instruments at temperatures between 2K and 8K. A pointing accuracy of a few arc seconds is provided by a three-axis-stabilisation system. ISO's instrument complement consists of four instruments, namely: an imaging photo-polarimeter (3–200 microns), a camera (3–17 microns), a short wavelength spectrometer (3–45 microns) and a long wavelength spectrometer (45–180 microns). ISO's scheduled launch date is May 1993 and it will be operational for at least 18 months. In keeping with ISO's role as an observatory, two-thirds of its observing time will be made available to the general astronomical community via several Calls for Observing Proposals.     

Deducing Electron Properties from Hard X-ray Observations

X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, photoelectric absorption and Compton backscatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons and improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data in the future.     

Martian Radiation EnvIronment Experiment (MARIE)

Measurements of radiation levels at Mars including the contributions of protons, neutrons, and heavy ions, are pre-requisites for human exploration. The MARIE experiment on the Mars-01 Odyssey spacecraft consists of a spectrometer to make such measurements in Mars orbit. MARIE is measuring the galactic cosmic ray energy spectra during the maximum of the 24^th solar cycle, and studying the dynamics of solar particle events and their radial dependence in orbit of Mars. The MARIE spectrometer is designed to measure the energy spectrum from 15 to 500 MeV/n, and when combined other space based instruments, such as the Advanced Composition Explorer (ACE), would provide accurate GCR spectra. Similarly, observations of solar energetic particles can be combined with observations at different points in the inner heliosphere from, for example, the Solar Heliospheric Observatory (SOHO), to gain information on the propagation and radial dependence in the Earth-Mars space. Measurements can be compared with the best available radiation environment and transport models in order to improve these models for subsequent use, and to provide key inputs for the engineering of spacecraft to better protect the human crews exploring Mars.     

Active Region Dynamics

New methods of local helioseismology and uninterrupted time series of solar oscillation data from the Solar and Heliospheric Observatory (SOHO) have led to a major advance in our understanding of the structure and dynamics of active regions in the subsurface layers. The initial results show that large active regions are formed by repeated magnetic flux emergence from the deep interior, and that their roots are at least 50 Mm deep. The active regions change the temperature structure and flow dynamics of the upper convection zone, forming large circulation cells of converging flows. The helioseismic observations also indicate that the processes of magnetic energy release, flares and coronal mass ejections, might be associated with strong (1–2 km/s) shearing flows, 4–6 Mm below the surface.     

Gamma rays and neutrinos as complementary probes in astrophysics

Electrons are more susceptible to energy losses in magnetic fields and photon fields than protons. Hence, photons at various wavelengths, including gamma rays, bring more readily information on high-energy electrons than on protons. Neutrinos provide a unique tracer for protons. Furthermore, at high energies the neutrino flux can considerably exceed the gamma-ray flux, as gamma rays above ～1 MeV are degraded by γ-γ interactions in compact high-intensity sources. Active galactic nuclei (AGN) with outputs >10⁴⁵ ergs s^?1 and dimensions ～10¹⁴ cm would constitute such sources. If the AGN are powered by ultra-massive black holes, then these numerical conditions are satisfied, and at high energies the flux J _v >J _γ . Berezinsky and Ginzburg have pointed out that the photon intensity around spinars is not sufficient to cause gamma-ray degradation. These authors have demonstrated that the measurement of neutrino flux, combined with the measurement (or upper limit) of gamma-ray flux would show whether the active galactic nuclei are powered by massive black holes or spinars. We estimate that gamma rays would be degraded at spinars, too, at energies >1 GeV.     

Observations of substorm-associated particle-flux variations at 6⩽L⩽8 with GEOS-1

Measurements made with the charged particle spectrometer of the Max-Planck-Institut für Aeronomie onboard GEOS-1 were used to investigate the behaviour of energetic electrons and ions in the dusk sector of the magnetosphere. During substorms the integral ion flux ( 24 KeV) increased whereas the integral electron flux ( 20 KeV) first decreased and later on also increased. The dependences of these flux variations on pitch-angle and particle energy are described and discussed in terms of particle drift in the geomagnetic and geoelectric fields and adiabatic energy variations. The results also provide some information on the source region of the drifting particles.     

模型燃烧室紊流燃烧的大涡模拟

采用了两种不同的亚网格尺度燃烧模型对带 V型稳定器的模型燃烧室紊流化学反应流动进行了大涡模拟 ,用 k-ε方程亚网格尺度模型确定亚网格紊流粘性 ,为了考虑热辐射对燃烧室壁温和气流温度的影响 ,运用热流法辐射模型估算热辐射通量 ,用 SIMPLE算法和混合差分求解大涡模拟各守恒方程 ,通过对两种不同亚网格尺度燃烧模型数值模拟结果与实验值的比较表明 ,两种燃烧模型都与实验值较吻合 ,但 G方程小火焰模型要比亚网格 EBU燃烧模型符合得更好些      

A swept fin-induced flow field with different height mounting gaps

In order to apply the air fin successfully and ensure the maneuverability of hypersonic vehicle, a key problem to be studied urgently is the heat flux brought by the fin mounting gap. The appearance of mounting gap and fin shaft can induce many complex flow structures which need more attentions to be investigated. Under Ma 6, Nano-tracer-based Planar Laser Scattering (NPLS) and Temperature Sensitive Paints (TSP) were applied to visualize and measure transient flow structures and heat flux distribution of a swept fin-induced flow field with different height mounting gaps. Complementarily, Reynolds-averaged N-S equations were solved with k-ω SST turbulent model. The heat flux distribution results of numerical simulation and TSP observed the change of high heat flux region with different mounting gap, both in position and magnitude. The streamlines based on Computational Fluid Dynamics (CFD) and flow visualization results obtained by NPLS revealed the cause of high heat flux region. The high heat flux region in this flow field is mainly related to the reattachment of vortex and flow stagnation. The increase of gap height can lead to stronger gap overflow and shaft-induced horseshoe vortex, which are source of the high heat flux around the fin. The case with the highest mounting gap (4 mm) en-counters the most severe aerodynamic heating, both on the surface of fin and plate. Thus, under the premise of ensuring the flexibility of the fin, the gap should be set as small as possible.     

格林函数法求解含移动介质轴向导热的稳态温度场

用格林函数法求解了考虑移动介质轴向导热影响的稳态温度场,推导了第1类和第2类非齐次边界条件下求解稳态温度场的数学表达式,利用特征值和特征函数获得了格林函数解,证实格林函数法可以解决非齐次边界条件下的稳态传热问题.以平行板间和圆管内均匀速度移动介质的稳态传热为例,获得了在半无限长壁面和在有限长壁面有均匀热流密度两种情况下所对应的温度场的无量纲解析解,分析了移动介质轴向导热与Pe和x/H之间的关系.结果表明:在所分析的两种热流密度边界条件下,Pe越大无量纲温度越小;在半无限大区域有均匀热流密度条件下,无壁面热流密度区域温度变化越剧烈,有均匀壁面热流密度的区域无量纲温度随x/H的变化几乎不受Pe大小的影响;在有限区域有均匀热流密度条件下,与半无限大区域均匀加热的情况有显著差异,在全计算域内温度变化越剧烈.     

数值模拟纵向隔热屏加力室热态流场

数值研究纵向隔热屏加力燃烧室的在三维贴体坐标系下燃烧整体流场,所用的数学模型有:k-ε模型、EBU-Arrehenius和二阶矩紊流燃烧模型及六通量模型等.数值分析两种不同几何形状、进口气流参数分布及紊流燃烧模型等对加力室内各气流参数、隔热屏和加力室筒体壁面温度分布的影响,计算结果与试验数据比较表明:不同几何形状对加力室紊流燃烧流场的影响要比进口气流参数分布大些,二阶矩模型更适用模拟紊流燃烧流动.      

多喷流干扰级间热环境风洞试验研究

运载火箭级间热分离过程中,级间段受高温高压喷流的影响,所处环境恶劣,研究级间热环境中压力、温度和热流分布规律对级间段结构的优化具有重要意义。在Ф1m高超声速风洞中,采用以微型固体火箭燃气为喷流介质的热喷流模拟技术,模拟了运载火箭二级主发动机和四个游动发动机同时工作多喷流干扰条件下的级间热环境,并对级间压力、温度和热流测量试验技术进行了研究,获得了不同级间距、不同排燃窗开口数量情况下的二级底封头和一级前封头表面的热流、温度及压力分布特性。试验结果表明,级间距越小,分离环境越恶劣,压力、温度、热流分布越不均匀;总排燃面积保持不变,排燃窗开口数量变化,对一级前封头上的压力、温度、热流影响不大,但对二级底封头影响较为明显,随着开口数量的减少,二级底封头上压力、温度、热流值均有所增大。本项试验采用同轴热电偶测量了级问区域的热流,热流结果精准度的提高以及热流模拟准则还需进一步探索和研究。