The stellar content of the Orion OB1 association

We present a photometric investigation, using the VBLUW system, of the stellar content of Orion OB1. Physical parameters (logg, logT _eff) for the stars are derived with the aid of model atmospheres. From these, visual extinctions, absolute magnitudes and distance moduli are derived. The distance moduli are used to determine membership for the stars in each of the subgroups and distances to the subgroups are calculated. The ages of the subgroups are derived through isochrone fitting and the IMF is derived for each subgroup. The energy deposited into the ISM through stellar winds and supernovae is calculated and compared to observed large scale features in the ISM around Orion OB1.     

Attitude and Oribit Estimation Using Stars and Landmarks

An extended Kalman filter is used to process line-of-sight measurements to stars and known landmarks providing a statistical indication of performance in estimating spacecraft attitude, orbital ephemeris, and the bias drift of a set of three strapdown gyros. The landmark measurements were assumed to have been taken from the imagery of an Earth-observing multispectral scanner. It is shown that filtering of these noisy measurements results in highly accurate estimates of the above parameters. Results are given showing the sensitivity of performance to various system parameters such as star tracker accuracy, errors in the knowledge of landmark position, and number of stars and landmarks processed.     

Fourier spectroscopy in planetary research

The application of Fourier Transform Spectroscopy (FTS) to planetary research is reviewed. The survey includes FTS observations of the Sun, all the planets except Uranus and Pluto, the Galilean satellites and Saturn's rings. Instrumentation and scientific results are considered. The prospects and limitations of FTS for planetary research in the forthcoming years are discussed.     

Problems in the analysis of gamma-ray burst spectra

Spectral measurements by the Solar Maximum Mission have been used to confirm the cyclotron lines in gamma-ray bursts reported from the Konus experiment. We present ISEE-3 data for the same burst (GB800419) during the same period of time which show no line. We discuss various problems in the analysis of scintillator spectra and point out that unfolded spectra are not necessarily unique and that the position of a data point in a deconvolved spectrum may vary depending on the assumed overall shape of the spectrum. As a result, if the analysis assumes a soft spectrum (such as optically thin thermal bremsstrahlung) an absorption feature might appear, whereas a harder spectrum (such as a Comptonized blackbody) would not require the feature. Since the continuum shape probably changes during the duration of a typical burst, the nonuniqueness of the spectral unfolding, combined with the assumption that the continuum is optically thin thermal bremsstrahlung, could give rise to spurious “absorption” features which vary on a time scale of seconds. Despite these problems, there is still some evidence for $\text{narrow}$ spectral lines in the range 45 to 65 keV but not for the broad lines reported from the Konus experiment. Unfortunately, the range 45 to 65 keV is the most difficult spectral region to unfold.     

Nonlinear analysis of hydrodynamic instability in laminar flames—II. Numerical experiments

We consider a nonstationary nonlinear integrodifferential equation describing the evolution of a disturbed plane flame front. The following two-dimensional problems are solved numerically: (i) The formation of a wrinkled noncellular laminar flame front under conditions of spontaneous hydrodynamic instability. (ii) The formation of a turbulent cellular flame front under conditions of spontaneous diffusional-thermal instability. (iii) The formation of a turbulent wrinkled-cellular flame front under conditions of spontaneous hydrodynamic and diffusional-thermal instability. We compute the flame front velocity in laminar and turbulent propagation.     

The sensitivity of the earth's radiation budget to changes in cloudiness

Cloudiness modulates the radiation budget at the top of the Earth-atmosphere system. For radiation balance studies, for climate diagnostic studies, and for climate modeling studies, it is important to know the sensitivity of both the outgoing longwave radiation and the net (absorbed solar minus outgoing longwave) radiation of the system to changes in cloudiness on a global basis. Based on a 45 month series of NOAA satellite scanning radiometer observations, estimates of the global distribution of these sensitivity parameters are obtained.     

Probability of Anomaly Expressions for Random Waveform Registration

Registration by integral-square error correlation of one-dimensional discrete-time waveforms which are treated as random processes with specified autocorrelation functions is considered. An important design parameter for this class of problems is the probability of anomaly (a false dip in the correlation function) because it gives an indication of system immunity to gross registration errors. Explicit expressions for this parameter are not possible, so bounds and appoximations must be derived. Two upper bounds and an approximation for the probability of anomaly are derived here. The use of these expressions is illustrated by an example. The relative utility of these performance indicators is shown for the example by comparison with actual values of the probability of anomaly obtained by computer simulation.     

Changes in the loco-regional Cerebral Blood Flow (r.C.B.F.) during a simulation of weightlessness.

Experiments of prolonged bedrest in antiorthostatic position are conducted in order to simulated cardio-circulatory modifications observed in weightlessness. Until now, no studies of r.C.B.F. have been effected in these conditions. Six young, healthy volunteers (average age 23.8) were placed in strict bedrest and in antiorthostatic position -4 degrees for 7 days. The r.C.B.F. measurements were studied by 133Xe inhalation method using a 32 detectors system. Studies were made first in basal conditions, then between the 6th and 12th hr, and finally between the 72nd and the 78th hr after the beginning of the experiment. Three of the subjects received 0.450 mg of Clonidine daily during the experiment. In the subjects having taken no Clonidine, we observed a constant increase in r.C.B.F. (12, 17 and 16% respectively) in the first 12 hr; at the 72nd hour, all values had returned to basal state. This findings agrees with the well known notion of a rapid correction of hemodynamic disturbances observed in the first days of weightlessness. In the subjects treated with Clonidine, the increase of r.C.B.F. did not occur. Several mechanisms of action are possible; the Clonidine affecting either the heart by inhibiting volemic atrial receptors or the brain by direct vasoconstriction.