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We report on the results of a continuation of a large monitoring program of optical spectroscopy of the Seyfert 1 galaxy NGC 5548. The new observations presented here were obtained between 1990 December and 1992 October, and extend the existing database to nearly 1400 days, dating back to 1988 December. The continuum variations are generally smooth and well-resolved, except during the third year of this 4 year project, when the variations were apparently more rapid and of lower amplitude than observed at other times. The broad Hβ emission line is found to vary in response to the continuum variations with a lag of about 18 days, but with some changes from year to year. The Hβ transfer functions for each of the 4 yr and for the entire 4 yr database are derived by using a maximum entropy method.

The Seyfert 1 galaxy NGC 3783 was intensely monitored in several bands between 1991 December and 1992 August. This paper presents the results from the ground-based observations in the optical and near-IR bands, which complement the data set formed by the International Ultraviolet Explorer (IUE) spectra, discussed elsewhere. Spectroscopic and photometric data from several observatories were combined in order to obtain well-sampled light curves of the continuum and of H(beta). During the campaign the source underwent significant variability. The light curves of the optical continuum and of H(beta) display strong similarities to those obtained with the IUE. The near-IR flux did not vary significantly except for a slight increase at the end of the campaign. The cross-correlation analysis shows that the variations of the optical continuum have a lag of 1 day or less with respect to those of the UV continuum, with an uncertainty of is less than or equal to 4 days. The integrated flux of H(beta) varies with a delay of about 8 days. These results confirm that (1) the continuum variations occur simultaneously or with a very small lag across the entire UV-optical range, as in the Seyfert galaxy NGC 5548; and (2) the emission lines of NGC 3783 respond to ionizing continuum variations with less delay than those of NGC 5548. As observed in NGC 5548, the lag of H(beta) with respect to the continuum is greater than those of the high-ionization lines.

We report on the analysis of 154 hours of early continuous high-speed photometry on the pulsating DB white dwarf (DBV) GD 358, obtained during the Whole Earth Telescope (WET) run of 1990 May. The power spectrum of the light curve is dominated by power in the range from 1000 to 2400 μHz, with more than 180 significant peaks in the total spectrum. We identify ali of the triplet frequencies as degree l = 1, and from the details of their spacings we derive the total stellar mass as 0.61 ± 0.03 Mʘ , the mass of the outer helium envelope as 2.0 ± 1.0 x 10-6 M*, the absolute luminosity as 0.050 ± 0.012 Lʘ and the distance as 42 ± 3 pc. We find strong evidence for differential rotation in the radial direction−the outer envelope is rotating at least 1.8 times faster than the core−and we detect the presence of a weak magnetic field with a strength of 1300 ± 300 G. We also find a significant power at the sums and differences of the dominant frequencies, indi­ cating nonlinear processes are significant, but they have a richness and complexity that rules out resonant mode coupling as a major cause.

The Seyfert 1 galaxy NGC 3783 was intensely monitored in several bands between 1991 December and 1992 August. This paper presents the results from the ground-based observations in the optical and near-IR bands, which complement the data set formed by the International Ultraviolet Explorer (IUE) spectra, discussed elsewhere. Spectroscopic and photometric data from several observatories were combined in order to obtain well-sampled light curves of the continuum and of Hβ. During the campaign the source underwent significant variability. The light curves of the optical continuum and of Hβ display strong similarities to those obtained with the IV E. The near-IR flux did not vary significantly except for a slight increase at the end of the campaign. The cross-correlation analysis shows that the variations of the optical continuum have a lag of 1 day or Iess with respect to those of the UV continuum, with an uncertainty of ≤4 days. The integrated flux of Hβ varies with a delay of about 8 days. These results confirm that (1) the continuum variations occur simultaneously or with a very small lag across the entire UV-optical range, as in the Seyfert galaxy NGC 5548; and (2) the emission !ines of NGC 3783 respond to ionizing continuum variations with less delay than those of NGC 5548. As observed in NGC 5548, the lag of Hβ with respect to the continuum is greater than those of the high-ionization !ines.

The star formation history in the nucleus of late spiral galaxies is compared between a sample in a high galaxy density medium (HDS) and a control sample (CS) of isolated galaxies. We have observed 20 HDS and 18 CS galaxies from a larger list generated by the application of a group-finding algorithm to the SSRS survey. Using equivalent widths of absorption lines and the continuum distribution, we determined the nuclear stellar population types, from those dominated by old population to those containing star formation bursts of different ages and intensities. The HDS and CS stellar population type histograms are similar, suggesting that environmental influences, at least for the present sample, do not affect substantially the nuclear stellar population. However, the nuclear emission lines indicate that, in the BPT diagnostic diagrams, there is an excess of HDS galaxies located within or close to the AGN loci. For six HDS and two CS galaxies, it was possible to determine oxygen (0/H) and nitrogen (N/H) abundances. The samples present similar 0/H values, but in the CS galaxies the N/0 ratio is lower at equal galaxy luminosity.

We report the recent appearance of a very broad component in the Hα and Hβ emission lines of the weakly active nucleus of the Sersic-Pastoriza galaxy NGC 1097. Subtraction of the contribution of the stellar population from our spectra reveals that the FWZI of the broad component is about 21,000 km s-ˡ and that its profile is double-peaked. The data also suggest the presence of a blue, featureless continuum in the nucleus. The Hα/Hβ flux ratio indicates that the broad-line region (BLR) is not significantly reddened. The broad component was first observed in Ho: in 1991 November 2, and confirmed 11 months later. The Hα profile and flux did not change in this time interval. Comparison with previously published spectral data indicates that the broad lines have only recently appeared. Together with the relatively high X-ray luminosity and the compact nuclear radio source, our results characterize the presence of a Seyfert 1 nucleus in a galaxy which had previously shown only LINER characteristics. The simplest interpretation is that obscuring material along our line of sight to the nucleus has recently cleared, permitting a direct view of the active nucleus. We discuss two possible structures for the broad line region-biconical outflow and an accretion disk-that could give rise to the observed profile. We also speculate on the relationship between the broad line region and the previously known optical jets, which extend to tens of kpc from the nucleus.

We have discovered that R143 in the Large Magellanic Cloud is a luminous blue variable (LBV), the first and perhaps the lone LBV in the central cluster of 30 Doradus, and only the sixth known LMC LBV. Photo­ metric and spectroscopic observations over the past 40 yr indicate that during that time R143 moved redward (changing from an F5 to F8 supergiant), then blueward (possibly becoming as early as 09.5), and is now moving back to the red (currently appearing as a late B supergiant). Similarly, the V magnitude of the star has changed by at least 1.4 mag. lmages of R143 show very unusual filaments of nebulosity extending from the star to a shell at a distance of 3.5 pc, perhaps due to a similar ejection mechanism that created the spiral jets and shell associated with AG Car, another LBV

We report the discovery of a new ZZ Ceti star observed on the assumption that most DA stars with temperature near 11,000 K are pulsators and on a temperature determination using lUE data. Even though BPM 37093 had already been claimed as a nonvariable, it pulsates with a semi-amplitude of 4 mmag and a period of ~600 s, being the smallest amplitude ZZ Ceti star known to date.

We present narrow-band images centered on [O III] λ5007, Hα, and adjacent continua of the Seyfert 1 galaxy NGC 3783. The continuum images evidence that the bar emission is mainly due to old stars. The [O III] λ5007 image shows that the high-excitation gas is very concentrated while the Hα emission is more extended and elongated perpendicularly to the direction of the bar. Ten new optical spectra, obtained between 1987 and 1990, are used to study the variability of the contin­ uum and emission lines in this galaxy. The contribution of the bulge stellar population to the nuclear spec­ trum is determined for each epoch and subtracted. The remaining optical continuum is then decomposed in two variable components, and after these components have been subtracted, the resultant spectra, constituted by pure line emission, are analyzed. A multiple Gaussian components model is found in order to describe the different emission lines on the various spectra. With the results of this decomposition the structure and physi­ cal conditions of the emission-line regions are determined. The variations in the continuum and emission-line fluxes are then analyzed, and the relations between them are pointed out.

We report the discovery, in the course of a survey for high-latitude, ultraviolet-excess objects in the southern hemisphere, of a new white dwarf star belonging to the PG 1159 spectroscopic class. A variety of spectroscopic observations show this object to be similar to the prototype PG 1159-035 in exhibiting He u, C IV, and O VI features in its optical spectrum, as well as a rich absorption spectrum in the ultraviolet. In addition, the slope of its energy distribution appears as steep as that of PG 1159-035 and is consistent with the Rayleigh-Jeans tail of a very hot ('I;, 100,000 K) star. White-light, high-speed photometry reveals that this object does not exhibit periodic variations in the range 10-2000 s with semiamplitudes larger than 0.002 mag. The importance of such objects for current models of the spectral evolution of white dwarfs is emphasized.

We analyze the International Ultraviolet Explorer (IUE) UV and the optical spectra of 39 starburst and blue compact galaxies in order to study the average properties of dust extinction in extended regions of galaxies. The optical spectra have been obtained using an aperture which matches that of IUE, so comparable regions within each galaxy are sampled. The data from the 39 galaxies are compared with five models for the geometrical distribution of dust, adopting as extinction laws both the Milky Way and the Large Magellanic Cloud laws. The commonly used uniform dust screen is included among the models. We find that none of the five models is in satisfactory agreement with the data. In order to understand the discrepancy between the data and the models, we have derived an extinction law directly from the data in the UV and optical wavelength range. The resulting curve is characterized by an overall slope which is more gray than the Milky Way extinction law's slope, and by the absence of the 2175 A dust feature. Remarkably, the difference in optical depth between the Balmer emission lines H(sub alpha) and H(sub beta) is about a factor of 2 larger than the difference in the optical depth between the continuum underlying the two Balmer lines. We interpret this discrepancy as a consequence of the fact that the hot ionizing stars are associated with dustier regions than the cold stellar population is. The absence of the 2175 A dust feature can be due either to the effects of the scattering and clumpiness of the dust or to a chemical composition different from that of the Milky Way dust grains. Disentangling the two interpretations is not easy because of the complexity of the spatial distribution of the emitting regions. The extinction law of the UV and optical spectral continua of extended regions can be applied to the spectra of medium- and high-redshift galaxies, where extended regions of a galaxy are, by necessity, sampled.

We present narrow-band images in the lines [O III] λ5007 and Hα(+ [N II] λλ6548, 6584 and long-slit spec­ troscopy along the bar of the Seyfert 2 galaxy NGC 5643. The continuum subtracted [O III] and Hα( images show that the emission of the high excitation gas is elongated along the bar direction and extends to ≈1 kpc to each side of the nucleus. Farther out, there is low-excitation emission due to H II regions along the bar. Our data confirm previous suggestions of a biconical morphology for the high-excitation gas and indicate that the cone axis is slightly tilted relative to the bar. We study the reddening in the central regions and find that the most obscured region is located at ≈3" W from the peak of the continuum, which is also the region with the highest gas density. We speculate that the active nucleus is hidden and located there. The available data is consistent with the nuclear obscuration being due to the presence of a dusty torus, which would also be responsible for collimating the ionizing radiation and producing the biconical morphology. The stellar population derived from the spectra is moderately old in the central region and young in the H II regions along the bar. Within 10" of the nucleus we find that the absorption lines are diluted by a blue continuum. We discuss the origin of this continuum. The fluxes of the emission lines are obtained and plots showing the distribution of the line fluxes and ratios along the bar are constructed. The ionic and total abundances of the H II regions are calculated. The oxygen and nitrogen abundances increase toward the center reaching, respectively, solar and 2 times solar values. Photoionization model calculations for the high-excitation extended narrow-line region gas confirm these values and indicate that the sulphur has the same abundance relative to solar as nitrogen.

The Type Ia supernova SN 1992A in the SO galaxy NGC 1380 was observed as a target of opportunity by the International Ultraviolet Explorer (IUE) and with great alacrity by the Hubble Space Telescope (HST). Here we present the HST and IUE spectra and photometry that we obtained, as well as optical spectra obtained at the Cerro Tololo Inter-American Observatory (CTIO). The HST Faint Object Spectrograph (FOS) spectra, from 5 and 45 days past maximum light, are the best UV spectra of a Type Ia supernova and reveal for the first time with good signal-to-noise ratio the Type Ia spectral region blueward of ~2650 Å. The UV photometry taken in the F175W, F275W, and F342W bands defined by the HST filters shows light curves that resemble the Type Ia template U light curve. Using data from SN 1992A and SN 1990N, we have constructed a Type Ia template light curve for the flux region near 2750 Å that is quite detailed from 14 days before maximum light to 22 days after maximum light and that extends to 77 days after maximum light. This light curve also resembles the template U light curve. A high-resolution HST Goddard High Resolution Spectrograph (GHRS) spectrum of SN 1992A shows no evidence for any interstellar !ines attributable to NGC 1380 or to Galactic high-velocity clouds; low-velocity Galactic gas gives rise to relatively weak Mg 11 UV resonance absorption !ines. These results are consistent with low extinction along the line of sight to SN 1992A. We have done a parameterized LTE analysis of the SN 1992A spectra using a slightly modified version of Woosley's delayed-detonation model DD4. We find that the features in the region blueward of ~2650 Å in the HST spectra are P Cygni absorptions due to blends of iron peak element multiplets and the Mg II resonance multiplet. Newly synthesized magnesium, sulfur, and silicon probably extend to velocities at least as high as ~19,000 km s-ˡ. Newly synthesized Ni-Co may dominate the iron peak element abundances out to ~13,000 km s-ˡ in the ejecta of SN 1992A as in model DD4. However, further investigation of the extent of Ni-Co material in SN 1992A and other Type Ia supernovae is needed; this investigation could greatly help constrain Type Ia models. An analysis of the O I λ27773 line in SN 1992A and other Type Ia supernovae implies that the oxygen-rich layer in typical Type Ia's extends over a velocity range of at least ~11,000-19,000 km s-ˡ. None of the explosion models we considered, including model DD4, has an oxygen-rich layer that completely extends over this range. Model DD4, however, is promising, and further investigation of delayed-detonation models of its kind is merited. Spectropolarimetric observations taken by Spyromilio & Bailey of SN 1992A are consistent with the SN 1992A being spherically symmetric. We discuss these observations and the importance of taking spectropolarimetry of other Type Ia supernovae

We analyze background fields along the bulge major axis away from the Galactic center by 2°.5, 13° and 31°. Our aim is to study the spatial distribution of different steiiar populations. A similar component of young disk main-sequence population is present in ali fields. The bulge component prevails in the innermost field, decreases for the 13° one, and is essentiaiiy absent in the 31° one. Evidence for a weak old metal-poor component is seen in the three fields. In the 13° field we detect a peculiar new feature which could be interpreted as an intermediate-age component, and traces of this population in the 2°.5 field are also found. This might be an old disk component.

The characteristics of the nuclear spectrum of six Seyfert 2 and LINER nuclei with low activity and strong lines of ionized nitrogen are investigated. From the equivalent widths of the absorption lines, the age and metallicity of the stellar population are derived. A comparison of the observed emission-line intensities with photoionization and shock model calculations leads to the conclusion that these nuclei are photoionized by a power-law continuum, and that the only way to reproduce the large forbidden line N II/H-alpha ratio for at least four of them is with an overabundance of nitrogen relative to the other heavy elements. 30 refs.

From B and V photographic surface photometry we have obtained photometric parameters, mean­ luminosity profiles, and B- V color indices for the barred spiral galaxy NGC 4507. The decomposition of the mean-luminosity B profile shows the existence of an r 1/4 bulge, an exponential disk, a bar, and possibly a lens. The dimensions and colors of these structures are derived. The mean B- V distribution shows a color gra­ dient of 0.4 mag between the nucleus and arms region. A reddening of E(B- V) = 0.2-0.3 of the southern region can be detected when we compare its B- V values with those of the northern region.

The nuclei of two galaxies which show intense [O III] emission lines, NGC 3732, an Sa, and IC 4662, an irregular with a very bright nucleus, are analyzed spectroscopically. The physical condition of the emitting gas and the characteristics and re1ative degree of evolution of the ionizing sources are discussed. It was found that the principal differences between the line spectra of both objects are due to abundance effects, although there is some evidence suggesting a different degree of evolution of the ionizing stars.

Spectrophotometric and photographic observations of IC 4329 A show that this Seyfert galaxy has reduced dimensions D x d = (13.6 x 7.6) kpc anda high luminosity Mph = -20.2. The nucleus is stratified in three zones: first, a region where the wide components of the Balmer !ines are formed having Ne ~10 9 cm-³ and a velocity dispersion of ~5000 km s-ˡ; second, a region where the forbidden !ines and the shorter wavelength peak of the Balmer !ines are formed, characterized by Te ~12,000 K and Ne ~10 6 cm-³; finally, the longer wavelength peak of the Balmer !ines originates in a gas cloud receding from the nucleus with V= 2100 km s-ˡ.