An optical spectral classiÐcation study of 106 OB stars within the 30 Doradus Nebula has sharpened the description of the spatial and temporal structures among the associated clusters. Five distinct stellar groups are recognized : (1) the central early-O (Carina phase) concentration, which includes Radcli †e 136 (R136) ; (2) a younger (Orion phase) population to the north and west of R136, containing heavily embedded early-O dwarfs and IR sources, the formation of which was likely triggered by the central concentration ; (3) an older population of late-O and early-B supergiants (Scorpius OB1 phase) throughout the central Ðeld, whose structural relationship, if any, to the younger groups is unclear ; (4) a previously known, even older compact cluster 3@ northwest of R136, containing A-and M-type supergiants (h and s Persei phase), which has evidently a †ected the nebular dynamics substantially ; and (5) a newly recognized Sco OB1-phase association, surrounding the recently discovered luminous blue variable (LBV) R143, in the southern part of the Nebula. The intricacy of this region and the implications for the interpretation of more distant starbursts are emphasized. The evidence indicates that the formation of the 30 Dor stellar content was neither instantaneous nor continuous, but rather that the stars formed in discrete events at di †erent epochs.The average di †erence between the derived and calibration absolute visual magnitudes of the stars is 0.05, indicating that the classiÐcation, calibration, and adopted distance modulus are (V 0 [ M V \ 18.6) accurate. For 70 of the stars, either the absolute value of that di †erence is ¹0.6 mag, or they are subluminous dwarfs or superluminous supergiants. Many astrophysically interesting objects have been isolated for further investigation. Surprisingly, in view of the presence of several O3 supergiants, the mid-Of star R139 is identiÐed as the most massive object in this sample ; it is located well along the 120 M _ track, very near the Humphreys-Davidson limit, and it is probably an immediate LBV precursor. This work can and should be extended in three ways : (1) higher resolution and higher S/N observations of many of the stars with larger ground-based telescopes for quantitative analysis, (2) ground-based spectral classiÐcation of the numerous additional accessible stars in the Ðeld, and (3) spatially resolved spectral classiÐcation of compact multiple systems with the Hubble Space T elescope.

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We have used STIS aboard HST to search for Lyα absorption lines in the outer regions of eight nearby galaxies using background QSOs and AGN as probes. Lyα lines are detected within a few hundred km s −1 of the systemic velocity of the galaxy in all cases. We conclude that a background line of sight which passes within 26 − 200 h −1 kpc of a foreground galaxy is likely to intercept low column density neutral hydrogen with log N(H I) > ∼ 13.0. The ubiquity of detections implies a covering factor of ≃ 100 % for low N(H I) gas around galaxies within 200 h −1 kpc. We find, however, that the Lyα lines are usually composed of individual components spread out in velocity over ranges of 300 − 900 km s −1 . Two sightlines show components which are unusually broad for low-redshift Lyα systems, with Doppler parameters ∼ 150 km s −1 . These may arise in intragroup gas at temperatures of 1 − 2 × 10 6 K.We discuss the difficulty in trying to associate individual absorption components with the selected galaxies and their neighbors, but show that by degrading our STIS data to lower resolutions, we are able to reproduce the anti-correlation of Lyα equivalent width and impact parameter found at higher redshift. The anti-correlation does not improve by correcting for the absolute magnitude of a galaxy in the same way as found at higher-z. We also show that the equivalent width and column density of Lyα complexes (when individual components are summed over ∼ 1000 km s −1 ) correlate well with a simple estimate of the volume density of galaxies brighter than M B = −17.5 at the same redshift as a Lyα complex. We do not reject the hypothesis that the selected galaxies are directly responsible for the observed Lyα lines, but our analysis indicates that absorption by clumpy intragroup gas is an equally likely explanation.Subject headings: quasars:absorption lines-galaxies:halos -intergalactic medium -large-scale structure of universe

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A sample of 27 Planetary Nebulae (PNs) in the Small Magellanic Clouds (SMC) have been observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) to determine their morphology, size, and the spatial variation of the ratios of bright emission lines. The morphologies of SMC PNs are similar to those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs are asymmetric, compared to half in the LMC. The low metallicity environment of the SMC seems to discourage the onset of bipolarity in PNs. We measured the line intensity, average surface brightness (SB), and photometric radius of each nebula in Hα , Hβ , [Oiii] λ4959 and 5007, [Nii] λ6548 and 6584, [Sii] λ6716 and 5731, He I λ6678, and [Oi] λ6300 and 6363. We show that the surface brightness to radius relationship is the same as in LMC PNs, indicating its possible use as a distance scale indicator for Galactic PNs. We determine the electron densities and the ionized masses of the nebulae where the [Sii] lines were measured accurately, and we find that the SMC PNs are denser than the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3 M ⊙ . We also found that the median [Oiii] /Hβ intensity ratio in the SMC is about half than the corresponding LMC median. We use Cloudy to model the dependence

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