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The ultraviolet energy distributions of the B stars in the LMC cluster NGC 2100 have been studied. The ultraviolet extinction law was studied for the LMC cluster. The T(eff)-luminosity diagrams were determined. The brightest stars in the cluster are evolved stars on the 'horizontal' supergiant branch. No P Cygni profiles indicating mass loss were recognized for these supergiants.
O and B stars in the LMC and SMC have been observed with the IUE in order to compare their winds and mass loss rates with those of their galactic counterparts. It is found that while the wind terminal velocities are generally about 20 percent lower than in galactic stars of the same spectral type, it is not clear that the mass loss rates are significantly lower. This conclusion differs from the conclusions of other studies, and the reasons are discussed. The most important reason is that a less luminous set of stars with new spectral types determined from slit spectra is being examined. The discussion of stellar winds rests heavily on a comparison of relevant wind parameters observed in O stars using both high and low dispersion IUE spectra.
The properties of ultraviolet extinction in the LMC are reexamined using some of the existing LMC data from the International Ultraviolet Explorer (IUE) satellite and a large number of new observations obtained during the IUE Sixth Episode. The main goal is to determine if the LMC extinction properties are regionally modulated and, if so, to examine the type and range of the variations. Among the new IUE observations presented is a large grid of unreddened LMC flux standards, which will eliminate spectral mismatch effects as complications in the extinction analysis. Eight new reddened stars were observed. The properties of the program stars are summarized; the method for producing the extinction curves (including the criteria for pairing reddened and unreddened stars) is discussed; and the curves themselves are presented and discussed.
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Twelve planetary nebulae of the 15 surveyed by IUE in the Megellanic Clouds were analyzed. Chemical abundances and other nebular parameters were determined, along with masses for the central stars. The latter are clustered in the range 0.58 to 0.71 solar masses, contrary to preliminary finding for 3 of the stars. This difference is attributed to the adoption of stellar atmosphere models that better represent the emergent flux distributions below the Lyman limit.
LMC regions located more than 0.5 deg from the center of the 30 Doradus nebula have an average UV extinction curve that is about 2 units higher in the far UV than the average Galactic curve. Within the 30 Doradus region, the normalized far-UV extinction is much steeper than the average Galactic or average LMC curves, yet has been considered to be representative of the LMC in general in the past. The present results show typical LMC extinction properties to have much more in common with those in the Milky Way.
Preliminary results are reported for an investigation comparing theoretical models of the sudden appearance of an extended RGB (and its effects on the spectral energy distributions of stellar populations) with data from ESO CCD observations of clusters in the LMC and SMC. Isochrones for the entire RGB are being constructed on the basis of 100 new evolutionary sequences (calculated using the evolution code of Sweigart and Gross, 1976 and 1978) to permit determination of synthetic colors and spectral energy distributions. The observations so far indicate a main sequence about 0.1 mag redder than that predicted by the present models or by the isochrones of VandenBerg and Bell (1985), and fail to show a B-V color difference at the RGB phase transition.
Data from the HEAO 1 low energy detector was used to map the diffuse 1/4 keV X-ray background around the LMC. Data was collected over 9 mos in 1977-1978 using detectors sensitive in the range 0.18-2.8 keV. Diffuse emission, rather than absorption, was observed from the LMC, as were two large emission (2 million K) regions with very low surface brightness. The regions coincided with the Shapley III supergiant shell of optical nebulosity and the LMC bar. Higher spatial resolution imagery is required if the emission features are to be confirmed as the same type of phenomena as the X-ray superbubble in Cygnus.
Deep Einstein IPC imagery of about 50 percent of the main body and 'wing' of the SMC has been obtained and analyzed. The four X-ray images have exposure times between 12,000 and 24,000 s and reveal a total of 25 X-ray sources. Twelve of these sources are new detections. Two, possibly three, sources with soft spectra may be newly discovered supernova remnants. The rest of the sources have harder spectra and intrinsic X-ray luminosities in the range log L(x) = 34.0-35.0 and are most likely stellar objects in the SMC. These luminosities are much lower than that found for typical Galactic X-ray binaries, and a factor of 100 larger than the range for 'normal' galactic O and B stars.
Nebular and central star parameters and elemental abundances of C, N, O, Ne, S, and Ar are presented for the planetary nebulae N2, N5, N43, N54, and N67 in the SMC and P2, P7, P9, P25, P33, and P40 in the LMC. The nebular chemical compositions are affected by nuclear processes in the precursor stars, which may not have been sufficiently massive to synthesize Ne, S, or Ar, which appear to be deficient with respect to their solar abundances by factors of roughly four and five for the LMC and SMC, respectively. Even after excluding nebulae formed by stars in which O apparently was destroyed by nuclear processes, O depletion in the LMC and SMC nebulae is significantly greater than in galactic planetaries. The estimated masses of the 12 remnant central stars range from 0.58 to 0.71 solar mass.
Linearized, absolutely calibrated VUV images of the LMC with a resolution of about 50 arcsec are presented. The images were made by a sounding rocket payload in two bandpasses with effective wavelengths for hot stars near 1500 A and 1930 A. The flux in each bandpass is measured for the associations in the list of Lucke and Hodge (1970). The results are discussed and their relationship to the overall characteristics of star formation in the LMC are discussed. A simple model for propagating star formation in the LMC is presented whose results closely resemble the distribution of associations revealed by the VUV images.
According to the predictions of line-driven stellar wind theory, the mass loss from luminous stars should be proportional to some power of the metallicity, yet previous studies have not shown this effect clearly. The authors have observed 22 main-sequence and giant O-type stars n the SMC with the low-resolution mode of the IUE in order to study the stellar winds in a particularly metal-poor galaxy. These stars are compared with carefully selected Galactic counterparts for wind terminal velocities and line strengths of C IV and N v. While the wind velocities in the SMC stars are confirmed to be systematically lower than in Galactic stars, as theory predicts, the mass-loss rates which are inferred from these data are comparable to those in the Galactic stars. The possible reasons for this result are discussed, including systematic effects in the data, assumed metal abundance, different ionization structure, and different physical parameters for the SMC stars.
The stellar and interstellar environments of the 32 known SNRs in the LMC have been examined to determine the relative numbers of population I and II progenitors. At least 2/3 of the LMC SNRs are associated with pop I environments. The data suggest that the existing SNR surveys are biased against the detection of SNRs located in evolved star-forming regions, such as supershells produced by OB asociations, and in the cores of luminous H II regions. Therefore, the fraction of SNRs in pop I environments may be even higher. The results are compared to previous studies of SN statistics in late-type galaxies.
Papers concerning SN 1987A are presented, covering topics such as images and spectrograms of the SN 1987A progenitor, a linear polarimetric study of SN 1987A, the energetics, nature, and uniqueness of the supernova, comparison of the SN 1987A light curve with other type II supernovae, P-Cygni features and photospheric velocities, the neutrino burst from SN 1987A, mass determination of neutrinos, neutrino transport, energetics, and oscillations. Additional papers discuss supernovae light echoes, the UV interstellar spectrum of SN 1987A, theoretical models of SN 1987A, circumstellar and interstellar interaction, the supernova as a stripped asymptotic-branch giant in a binary system, pulsar formation and the fall back mass fraction, the signals of particle acceleration at SN 1987A, the effects of the mixing of the ejecta on the hard X-ray emissions from the supernova, possible s-process gamma-ray lines in supernovae, detectability of early thermal radiation from a neutron star in SN 1987A, NASA studies of the supernova, and information exchange for SN 1987A. Observational studies presented include optical, IR, radio, and UV observations, IR speckle-interferometry, coded mask X-ray observations, broad band X-ray imaging spectrophotometry, gamma-ray and thermal X-ray observations, and reports from several observatories.
Ultraviolet and optical spectra data are presented for the oxygen-rich supernova remnant 1E 0102-7219 in the SMC. These UV data are the first to seriously constrain the UV emission from an oxygen-rich remnant. Emission lines of O I, forbidden O II, semiforbidden O III, semiforbidden O IV, C IV, forbidden Ne IV, and Mg II have been unambiguously detected in the UV. Shock models for material with abundances appropriate to massive stellar ejecta are calculated, and a number of differences between these models and the observations are found. The UV lines are observed to be weaker relative to optical lines than expected from models. Models of photoionization by the remnant's X-ray emission are calculated, and these provide a better qualitative match to the observed spectra. Approximate abundances in the ejecta are derived and the implications these have for the precursor star are discussed.
Comparison of high- and low-resolution IUE spectra of HD 269665 (Sanduleak -68 deg 110) show that the star is a subdwarf with a temperature in the range 30,000-50,000 K. The visual spectrum shows Balmer lines and weak He II 4686 A lines. Balmer-line width and a fit of the UV-visual flux distribution using the Blackwell-Shallis (1977) algorithm yields an effective temperature between 30,000 and 38,000 K and log g = 5-6. A table is presented of photospheric UV spectral lines in HD 269665. The radial velocity of the star is found to be 5.8 + or - 0.5 km/s.
Spectra of the SNR 0540-69.3 in the LMC are presented that cover the wavelength interval 3500-9800 A. An optical continuum is observed over this entire wavelength range from synchrotron emission near the 50 ms pulsar inside the remnant. The spectrum contains emission lines shifted by + 370 km/s from the local LMC rest velocity. The line widths average 2735 + or - 200 km/s, indicating an expansion age of 762 + or - 50 yr. Lines of forbidden O I, forbidden O II, forbidden O III, forbidden S II, forbidden S III, forbidden Ar III, forbidden Ni II, forbidden Fe II, forbidden Fe III, and H-alpha are detected, and forbidden Fe V and forbidden Fe VII may be detected. It is very likely that the object is the young SNR following core collapse of a massive star akin to the progenitor of SN 1987A.
The radial velocities of 11 to 37 stars have been measured in each of three rich young star clusters in the LMC: NGC 1866, NGC 2164, and NGC 2214. A thorough analysis of the observational errors and contamination by field stars is presented along with a new method to assign confidence limits to the velocity dispersions. Limits are set to the central densities, total masses, and mass-to-light ratios of the clusters, and the question of whether they have unbound halos is addressed. From the small velocity dispersion and large radial extent of NGC 1866, it is inferred that the cluster is not yet tidally limited by the LMC.