Engineering topics

Schaefer, Bradley E.

Publications and source records attributed to Schaefer, Bradley E..

44 records · Page 3

NASA NTRS1988

Visibility of the lunar crescent

A model designed to predict whether or not the moon will be visible under any set of observing conditions is presented. It explicitly includes atmospheric clarity, which is calculated from the site's altitude, latitude, temperature, relative humidity, aerosol content, and time of the year. This mathematical model is combined with a lunar and solar ephemeris to yield a computer program which will predict the date of the first crescent sighting from any location. A list of 201 observations of lunar visibility was collected to test this model and other models. It is found that criteria involving the moonset lagtime and the moon's age have zones of uncertainty which typically cover the entire earth.

Schaefer, Bradley E.↗

NASA NTRS1988

Moonwatch - July 14, 1988

The Moonwatch program using amateur astronomers to obtain a wide range of naked-eye sightings of the new moon to test a mathematical model for predicting first visibilty of the new moon is discussed. The algorithm takes physical effects such as brightness of the twilight sky, clarity of the atmosphere at the observing site, and the physiology of the human eye into account. A 1987 test of visibility prediction obtained poor results due to weather conditions. Observers across North and Central America, the Caribbean, and northern South America are asked to record information about the time and location where they first see the crescent of the new moon appear on the horizon.

Doggett, Leroy E.↗

NASA NTRS1988

Large-amplitude photometric variations of Nereid

Photometric UVBRI and astrometric data on Nereid were collected in June, 1987 using the 0.9-m telescope and CCD camera at the Cerro Tololo Inter-American Observatory. Four astrometric positions and UVBRI reflectances spanning eight days were obtained. From these data, an unusual reflectance spectrum for Nereid was determined. Variability of Nereid was observed in all bandpasses, with an amplitude greater than 1.5 mag and a probable period of between 8 and 24 hr.

Schaefer, Martha W.↗

NASA NTRS1988

Light echoes - Novae

The sudden brilliance of a nova eruption will be reflected on surrounding dust grains to create a phantom nebula. Previous searches for these light echoes have used relatively short exposures with photograhic detectors. This paper reports on a search around eight recent novae with long exposures using a CCD camera. Despite an increase of sensitivity by over an order of magnitude, no light echoes were detected. It is found that the average grain density must be less than about 10 to the -9th per cu cm for distances from 0.1 pc to 1000 pc from the novae. The light echo around Nova Persei 1901 was caused by reflection off clouds with grain densities of several times 10 to the -9th per cu cm which are at distances between 0.1 pc and 10 pc. Echoes from dust in a circumstellar shell or ejected during a previous eruption will be effectively unobservable.

Schaefer, Bradley E.↗

NASA NTRS1987

Light echoes - Supernovae 1987A and 1986G

The sudden brilliance of a supernova (SN) eruption will be reflected on surrounding dust grains to create a phantom nebula. The paper presents a series of calculations in which the apparent brightness of this 'light echo' is predicted for a variety of situations where the dust is part of the interstellar medium (ISM). It is found that the supernova 1987 A will have a very bright echo off the ISM that may perhaps be visible with binoculars for many years. At a time of 400 days past maximum, the SN 1986G is found to be 2.7 mag brighter than would be predicted by an extrapolation of its light curve. This unique property has an easy explanation as a light echo off the dust in the dust lane of Cen A.

Schaefer, Bradley E.↗

NASA NTRS1987

Light echoes - Type II supernovae

Type II supernovae (SNs) light curves show a remarkable range of shapes. Data have been collected for the 12 Type II SNs that have light curve information for more than four months past maximum. Contrary to previous reports, it is found that (1) the decay rate after 100 days past maximum varies by almost an order of magnitude and (2) the light curve shapes are not bimodally distributed, but actually form a continuum. In addition, it is found that the extinctions to the SNs are related to the light curve shapes. This implies that the absorbing dust is local to the SNs. The dust is likely to be part of a circumstellar shell emitted by the SN progenitor that Dwek (1983) has used to explain infrared echoes. The optical depth of the shell can get quite large. In such cases, it is found that the photons scattered and delayed by reflection off dust grains will dominate the light curve several months after peak brightness. This 'light echo' offers a straightforward explanation of the diversity of Type II SN light curves.

Schaefer, Bradley E.↗

NASA NTRS1987

The Perseus Flasher and satellite glints

The Perseus Flasher (PF) is claimed to be an astrophysical source which frequently emits bright optical flashes (Katz et al. 1986). These flashes have all been detected by the naked eye, with the exception of one photographed flash for which an accurate position is measured. Notable properties of the PF are its large amplitude (greater than 19 mag), short duration (about 1 s), and frequent occurrence (a flash every 12 hr). Here, evidence is presented that the PF is not an astrophysical source but is merely the observation of glints of reflected sunlight from artificial earth satellites. This conclusion is supported by the following facts: (1) a total of 3400 hr of photographic, video, and CCD observations have detected no flashes in or near the small PF error box, despite the claim of one bright flash every 12 hr; (2) thirteen of the 26 flashes are shown to be nonastrophysical in origin; and (3) both the observational and theoretical glint rates indicate that most, if not all, PF observations are caused by satellite glints.

Schaefer, Bradley E.↗

NASA NTRS1987

Gamma-ray burster counterparts - Infrared

The discovery of an IR counterpart to a gamma-ray burster (GRB) would be a big step forward in understanding this enigmatic phenomenon. A program of searching for IR emission from the smallest GRB source regions has been initiated. Seven boxes have been searched with ground-based telescopes at a wavelength of 2.2 microns and 23 with the IRAS data base at wavelengths of 12, 25, 60, and 100 microns. No convincing candidates were identified. The most constraining result are for two error boxes where the K-band magnitude are found to be fainter than 19.03. This observation is compared with the flux predicted from either a companion star or an accretion disk. The new IR observations pose serious difficulties for several GRB models.

Schaefer, Bradley E.↗