Edwin S. Barker

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. II, An intensive study of NGC 5548 at optical wavelengths

A large, international program of ground-based optical spectroscopy and photometry of the variable Seyfert 1 galaxy NGC 5548 undertaken in support of an IUE monitoring campaign is described. This contribution presents the data base and describes the methods used to correct for systematic differences between spectra from different sources. Optical continuum and H-beta emission-line light curves are derived from the spectra. The behavior of the optical continuum is qualitatively the same as the behavior of the ultraviolet continuum. Cross-correlation of the ultraviolet and optical continuum measurements does not reveal any significant lag between them. The h-beta emission-line variations show the same basic pattern as seen in the continuum and ultraviolet emission lines, with H-beta lagging behind the continuum by about 20 days. This is significantly larger than the about 10 day lag deduced for Ly-alpha. 15 refs.

Mars: Detection of Atmospheric Water Vapor during the Southern Hemisphere Spring and Summer Season

Water vapor was found to reappear in the atmosphere of Mars during its southern hemisphere spring and summer season, with a maximum vertical column abundance of 45 to 50 microns of precipitable water averaged over the entire planet. Although the spring-summer seasons for each hemisphere are generally symmetrical with respect to the appearance of water vapor, the data suggest that water vapor may appear later in the season and in slightly larger amounts during the southern hemisphere spring-summer.

Comparison of ground-based and Viking Orbiter measurements of Martian water vapor - Variability of the seasonal cycle

Abstract Earth-based observations of Mars atmospheric water vapor, made from McDonald Observatory, are presented for the 1975–1976, 1977–1978, and 1983 apparitions. Comparisons are made with near-simultaneous spacecraft measurements made from the Viking Orbiter MarsAtmospheric Water Detection experiment during 1976–1978 and with previous Earth-based measurements (made since 1964). Differences occur between the behavior in the different years, and may be related to the Mars climate. Measurements during the southern summer in 1969 indicate a factor of three times as much water as is present at this same season in other years.This difference may have resulted from the sublimation of water from the south polar residual cap upon removal of most or all of the CO 2 ice present; sublimation of all the CO 2 ice during some years could be a result of a greater thermal load being placed on the cap due to the presence of differing amounts of atmospheric dust. If substantiated, the water vapor variability will turn out to be a very sensitive indicator of yearly variability in the Martian climate.

The McDonald Observatory faint comet survey: gas production in 17 comets

Abstract We present spectral data from 152 observations of 17 comets obtained with an Intensified Dissector Scanner spectrograph at McDonald Observatory. We present the details of the observations and reduction of these data to production rates. These production rates are then analyzed to show that comets seem to be reasonably homogeneous as a group. The ratios of production rates remain constant with activity level and with heliocentric distance, with the exception of Q(NH2)/Q(CN), which shows a heliocentric distance dependence.

Absolute spectrophotometry of Titan, Uranus, and Neptune: 30,500–10,500 Å

Absolute measurements of the geometric albedo spectra of Titan, Uranus, and Neptune from 3500 to 10,500 A are reported. The measurements have spectral resolution of about 7 A and high signal-to-noise ratio owing to the superb efficiency of the spectrograph and Reticon detector used to acquire them. Agreement is excellent with albedo measurements of Uranus G. W. Lockwood, B. L. Lutz, D. T. Thompson, and A. Warnock (1983, Astrophys. J., 266, 402–414). The high precision and spectral resolution of the data make possible quantitative measurements of the effects of Raman scattering by H2 in the atmospheres of Uranus and Neptune.

Spectrophotometry of the continuum in 18 comets

Abstract We have studied the continuum emission in spectra of 18 comets. We find that the gas-to-dust ratio (GDR) increases with increasing activity (as measured by the production of CN). This may be explained by having dust production depend on the gas production raised to the 0.55 ± 0.17 power (1σ error). We interpret this to indicate that cometary gas production in the range of heliocentric distances (and gas production rates) for which we have data, is primarily through a porous mantle. Approximately equal numbers of comets show changes in continuum brightness with distance from the optocenter equal to, or significantly slower than, the canonical (distance)−1 falloff. All continua were red compared to the solar spectrum, with an average reddening of 22% per 1000 A (between 4400 and 5700 A). There is some evidence for a change in the slope of the continuum with distance from the optocenter in some comets. Trends in these observables throughout the apparition of each comet are discussed, and comparisons among comets are made.

N+2 and CO+ in Comets 122P/1995 S1 (deVico) and C/1995 O1 (Hale–Bopp)

We observed Comets 122P/1995 S1 (deVico) and C/1995 O1 (Hale–Bopp) with high spectral resolving power in order to determine the ratio of N+2/CO+ in their comae. While we clearly detected the CO+ in both of these comets, no N+2 was detected in either comet. From these spectra, we derive sensitive upper limits for N+2/CO+. These upper limits are substantially below other reported detections of N+2/CO+ in other comets. We discuss the prior N+2 detections and compare them with our observations. The abundance of N2 in comets is important to our understanding of the condensation of ices in the solar nebula. In addition, N2 is a tracer of Ar so the study of N2 allows an understanding of the role of comets for delivering volatiles to the terrestrial planets. It appears that many, if not most, comets are depleted in N2 and it will be necessary to search for a mechanism for depleting this molecule in order to be consistent with current models of the solar nebula.

Martian Atmospheric Water Vapor Observations: 1972-74 Apparition

The patrol of Martian water vapor carried out with the echelle-coude scanner at McDonald Observatory during the 1972–1974 apparition has produced 469 individual photoelectric scans of Doppler-shifted Martian H2O lines. Almost an entire Martian year was covered during the 1972–1974 period (Ls = 118−269° and 301−80°). Three types of coverage have been obtained: (1) regular—the slit placed pole to pole on the central meridian; (2) latitudinal—the slit placed parallel to the Martian equator at various latitudes; (3) diurnal—the slit placed parallel to the terminator at several times during a Martian day measured from local noon. Both the seasonal and diurnal effects seem to be controlled by the insolation and not the local topography with respect to the 6.1 mb surface. A slight negative correlation with elevation was noted which improved during the seasons of greater H2O content. The previous seasonal behavior has been confirmed and amplified. The following are the primary conclusions: (1) The planetwide abundance is low (5−15 μm of ppt H2O) during both equinoctical periods. (2) The maximum abundance of about 40 μm occurs in each hemisphere after solstice at about 40° latitude in that hemisphere. (3) The latitude of the maximum amount in the N-S distribution precedes the latitude of maximum insolation by 10–20° of latitude. (4) During the “drier” seasons (5–20 μm) near the equinoxes on Mars, the atmospheric water vapor changes by a factor of 2–3x over a diurnal cycle with the maximum near local noon. (5) The effects of the 1973 dust storm during the southern summer reduced the amount of water vapor over the southern hemisphere regions to 3–8 μm.

Spectrophotometric observations of P/Schwassmann-Wachmann 1 during outburst

Spectra of Comet Schwassmann-Wachmann 1 were obtained on November 26-27, 1978 and February 15, 1979 using the Cassegrain Digicon spectrograph, and on February 21-23, 1979 using the IDS spectrograph of McDonald Observatory. All spectra were obtained while the comet was in an extended series of outburst (12th-13th mag). The spectra were predominantly reflected solar light, indicating the outburst was very rich in dust. Weak emission between 4000 and 4050 A, and between 4240 and 4290 A was detected in November 1978 and is probably due to CO(+). These outbursts, and the detection of CO(+), indicate that comets are not inactive at distances as great as 6 AU.

Discovery of a main-belt asteroid resembling ordinary chondrite meteorites

Although ordinary chondrite material dominates meteorite falls, the identification of a main-belt asteroid source has remained elusive. From a new survey of more than 80 small main-belt asteroids comes the discovery of one having a visible and near-infrared reflectance spectrum similar to L6 and LL6 ordinary chondrite meteorites. Asteroid 3628 Božněmcov� has an estimated diameter of 7 kilometers and is located in the vicinity of the 3:1 Jovian resonance, a predicted meteorite source region. Although the discovery of a spectral match may indicate the existence of ordinary chondrite material within the main asteroid belt, the paucity of such detections remains an unresolved problem.