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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.
Wolf-Rayet (W-R) stars, which are the descendants of massive O-type stars, can be subdivided into three groups depending on their spectral appearance. These groups include the nitrogen class (WN), the carbon class (WC), and the oxygen class (WO). The present paper is concerned with the WC stars. The assignment of WC subtypes has been based on visual inspections of photographic plates. One of the aims of this study is related to the quantification of the visual estimates. The measured ratios of equivalent widths and the FWHM of the 4650 A line for Galactic and LMC stars are presented, and the reclassification of some stars is proposed on this basis. In particular, it is shown that the majority of the LMC WC stars should logically be classified WC4 instead of WC5. Comments on individual stars are provided, and terminal velocities are discussed. It is attempted to give a complete overview of the most important spectroscopic features of the WC stars in the optical region.
Attention is given to the Ca II K absorption spectra of 48 early-type supergiants in the SMC and LMC, with a view to the Clouds' possible fragmentary nature and the distance of the gas responsible for the absorption lines at 60 and 120 km/sec. The comparison of the spectra obtained with H I measurements toward the Clouds indicates that in both Clouds, the velocity components in the neutral hydrogen distribution lie at different distances along the line of sight. Good agreement is found between the radial velocity shown in absorbing material and the radial velocity spread predicted by Murai and Fujimoto's (1980) dynamical model of the formation of the Magellanic Stream by tidal stripping. It is speculated that both the SMC and LMC are extended and fragmented along the line of sight.
The Infrared Astronomy Satellite (IRAS) detected several hundred individual regions of star formation in the Large and Small Magellanic Clouds. Nearly two dozen of the brightest such sources were searched for from the ground at 10 microns; most of these were located and measured at wavelengths from 0.6 to 20 microns. Three principle results emerge from this study: First, the IRAS data show that star formation is considerably less active in the SMC than in the LMC, relative either to mass, luminosity, or H I content. The reduced activity in the SMC is consistent with the smaller amount of molecular material inferred from CO observations. Second, the sizes of the objects range from less than a few arcsecs (objects which look like extremely compact HII regions, with little or no extended radio, optical, or infrared emission) to some tens of arcsecs across (giant HII regions, of which the largest and brightest is 30 Doradus). Third, there are no obvious differences in the characteristics of the central portions of the LMC and SMC sources; all look like Galactic H II regions of similar luminosity.
Evidence and arguments concerning the origin and location of the gamma-ray burst (GRB) of March 5, 1979 are reviewed. This GRB has been positionally identified with the SNR 49 in the LMC. Such an association would fix the GRB's distance at 55 kpc, and the observed flux from the GRB would require prodigious energy and luminosity, casting doubt on the GRB's distance and its association with the LMC. Some Kosmos 856 observations which may provide more direct evidence on the energy released are discussed.
The observed light echoes around SN 1987A are modeled by dust in planar clouds roughly 120 and 330 pc in front of the supernova. The expected echo profiles depend on the observed light curve and on the cloud thicknesses. The surface brightness and colors of the echoes are modeled using a mix of silicate and graphite grains appropriate to the Large Magellanic Cloud (LMC). The model colors approximately agree with the observed ones. The observed surface brightness suggests that the dust column density in the echo clouds is comparable to the total dust column density in the LMC toward SN 1987A. The model predicts the presence of a narrow ultraviolet echo outside the visual echo; it should be detectable with future ultraviolet observatories and will give information on the supernova near the phase of shock breakout.
The earliest IUE high-resolution spectra of SN 1987A have been studied and reveal the presence of several clouds in the line of sight to the LMC. In particular, there are two clouds with radial velocities of about 130 km/s and about 180 km/s. These clouds' velocities are between those of Galactic clouds at 0-80 km/s and those of LMC gas at about 270 km/s. Chemical-abundance determinations may help to determine the origin and location of these clouds. Curve-of-growth analysis and 21-cm observations show that they may be underabundant in heavy elements by about a factor of 2 as compared to solar abundances. No depletion indicative of grain formation can be seen.
Images obtained by a rocket-borne UV imaging telescope are used here to determine near-UV fluxes for 17 sources in M31 that are optical globular-cluster candidates and for the bright open cluster vdB0 in M31. Far-UV fluxes or flux limits are determined for the same clusters. The m(NUV)-V colors for M31 clusters are similar to those of Galactic clusters, except for the high-metallicity M31 cluster Bo 171. Four of the detected clusters have optical, m(NUV) - V, and m(FUV) - V colors indicating ages of about 100 million years. These four clusters are probably similar to the so-called 'blue globular' clusters of the LMC. The existence of young LMC-type blue globulars and the possible existence of middle-aged metal-rich globulars may indicate that M31 has continued to form globular clusters throughout its life.
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.
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.
In the near future, high energy (E greater than 20 MeV) gamma ray astronomy offers the promise of a new means of examining the closest galaxies. Two and possibly three local galaxies, the Small and Large Magellanic Clouds and M31, should be visible to the high energy gamma ray telescope on the Gamma Ray Observatory, and the first should be seen by GAMMA-1. With the assumptions of adequate cosmic ray production and reasonable magnetic field strengths, both of which should likely be satisfied, specific predictions of the gamma ray emission can be made separating the concepts of the galactic and universal nature of cosmic rays. A study of the synchrotron radiation from the Large Magellanic Cloud (LMC) suggests that the cosmic ray density is similar to that in the local region of our galaxy, but not uniform. It is hoped the measurements will be able to verify this independent of assumptions about the magnetic fields in the LMC.
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.
The core radii of 18 rich star clusters in the LMC with ages from 10 Myr to 1 Gyr. Data for an additional 17 clusters with ages from 1 Myr to 10 Gyr are available in the literature. The combined sample shows that the core radii increase from about 0 to about 5 pc between about 1 Myr and 1 Gyr, and then begin to decrease again. The expansion of the cores is probably driven by mass loss from evolving stars. Models of cluster evolution show that the rate of increase in core radius is sensitive to the slope of the initial mass function. The observed core radius-age relation for the LMC clusters favors an intial mass function with slope slightly flatter than the Salpeter value.
The projected ellipticities of 34 populous SMC star clusters have been derived by means of PDS 1010A scans and a computer interactive method of reduction implemented on an Apollo 570 workstation. A pair of J and R plates taken with the 1.2 m UK Schmidt telescope in Australia were used. Radial ellipticity variations within individual globular clusters seem to be a common phenomenon for the SMC clusters, similar to that observed in the LMC clusters where the innerparts are more elliptical than the outer ones in 95 percent of the cases. The derived ellipticities which correspond to the innermost part of the cluster at radial distances near to half-mass radii have been found to be statistically more elliptical than those of the LMC, known to be more elliptical than those of the Galaxy. The dynamical masses of the clusters seem to correlate with ellipticities supporting the hypothesis that, either the gravitational field of the parent galaxy being a dominant factor affect slower the shape of the high mass clusters and/or the most massive clusters, being dynamically younger, retain their original shape.
A collection of International Ultraviolet Explorer (IUE) light curves and sample spectra for the galactic recurrent novae observed in the ultraviolet are presented. These data are compared to that of the Nova LMC (Large Magellanic Cloud) 1990 number 2 which is shown to also be a recurrent novae. Based on the analysis of the outburst of N LMC 1990 number 2 it is suggested that all of the recurrent novae observed in the ultraviolet have luminosities at maximum light which exceed the Eddington limit. V745 Sco and RS Oph, the two recurrent novae with late type giant companions, show strong Mg II throughout the outburst. Those with low luminosity companions, U Sco and V394 CrA do not show Mg II but do show strong He II 1640 earlier in the outburst. Typical emission line velocities are greater than those observed in many of the classical novae. These characteristics suggest that the white dwarfs in recurrent novae systems are close to the Chandrasekhar limit.
New stellar evolution calculations for the SN 1987 A progenitor, including a small but appropriate amount of semiconvection and mixing induced by differential rotation, yield good agreement with many observational constraints, as the HRD position of the progenitor star, a previous red supergiant phase, and CNO surface abundances close to the values obtained with the IUE satellite. The HRD track and surface abundances in different evolutionary stages are found to reflect many general properties of massive stars in the LMC. The results indicate that the SN 1987 A progenitor may have been an average massive star in the LMC, and that blue supergiants may be common SN II progenitors in low metallicity galaxies.
A true distance modulus to the nearby spiral galaxy M33 has been determined based on CCD photometry obtained at BVRI wavelengths. M33 is presently one of five nearby galaxies used in the calibration of the IR Tully-Fisher relation, and thereby in the determination of the Hubble constant. Using period-luminosity relations at several wavelengths offers the advantage that the distance moduli derived can be corrected for the effects of interstellar extinction. These data indicate that there is internal reddening affecting the Cepheid photometry in M33 which must be accounted for if a true distance modulus is to be obtained for this galaxy. Adopting a true distance modulus to the LMC of 18.5 mag, the new CCD data yield a true distance to M33 of 24.64 + or - 0.09 mag, corresponding to a linear distance of 840 kpc. A mean value of the total color excess (foreground and internal) for the Cepheids in M33 is estimated to be E(B - V) = 0.10 + or - 0.09 mag, assuming a value for the total mean LMC Cepheid color excess of 0.10 mag.
The 158 micron 2P3/2-2P1/2 fine-structure transition of C(+) at selected locations in the LMC. The C II emission is most intense toward far-infrared continuum peaks and generally is not seen in positions exhibiting strong CO J = 2-1 radiation. Where both C II and CO emission are detected, the V(LSR) centroids are similar but the C II line is wider. The differences in spatial distribution and spectral shape suggest a more pronounced physical separation between the predominantly neutral atomic and molecular gas regions than is the case in the Galaxy. In the LMC, the intense and extended C II emission near 30 Dor implies a total amount of C(+) several times greater than that of Galactic molecular cloud complexes. An attempt was made to detect the 289 micron J = 9-8 transition of (C-12)O in a few locations. The observed upper intensity limit for N159 implies that moderate density molecular gas fills less than 5 percent of the beam and that most of the low J CO emission comes from lower density gas.