We have produced a catalog of 378 Galactic O stars with accurate spectral classifications that is complete for V < 8 but includes many fainter stars. The catalog provides cross-identifications with other sources; coordinates (obtained in most cases from Tycho-2 data); astrometric distances for 24 of the nearest stars; optical (Tycho-2, Johnson, and Strömgren) and NIR photometry; group membership, runaway character, and multiplicity information; and a Web-based version with links to on-line services.

Supernova explosions are one of the most energetic-and potentially lethal-phenomena in the Universe. Scientists have speculated for decades about the possible consequences for life on Earth of a nearby supernova, but plausible candidates for such an event were lacking. Here we show that the Scorpius-Centaurus OB association, a group of young stars currently located at ∼ 130 parsecs from the Sun, has generated 20 SN explosions during the last 11 Myr, some of them probably as close as 40 pc to our planet. We find that the deposition on Earth of 60 Fe atoms produced by these explosions can explain the recent measurements of an excess of this isotope in deep ocean crust samples. We propose that ∼ 2 Myr ago, one of the SNe exploded close enough to Earth to seriously damage the ozone layer, provoking or contributing to the Pliocene-Pleistocene boundary marine extinction.

We present new Hα and [O III] λ5007 narrow band images of the starbursting dwarf galaxy NGC 4214, obtained with the Wide Field and Planetary Camera (WFPC2) onboard the Hubble Space Telescope (HST), together with VLA observations of the same galaxy. The HST images resolve features down to physical scales of 2 − 5 pc, revealing several young (< 10 Myr) star forming complexes of various ionized gas morphologies (compact knots, complete or fragmentary shells) and sizes (∼ 10 − 200 pc). Our results are consistent with a uniform set of evolutionary trends: The youngest, smaller, filled regions that presumably are those just emerging from dense star forming clouds, tend to be of high excitation and are highly obscured. Evolved, larger shell-like regions have lower excitation and are less extincted due of the action of stellar winds and supernovae. In at least one case we find evidence for induced star formation which has led to a two-stage starburst. Age estimates based on W (Hα) measurements do not agree with those inferred from wind-driven shell models of expanding H II regions. The most likely explanation for this effect is the existence of an ≈ 2 Myr delay in the formation of superbubbles caused by the pressure exerted by the high density medium in which massive stars are born. We report the detection of a supernova remnant embedded in one of the two large H II complexes of NGC 4214. The dust in NGC 4214 is not located in a foreground screen but is physically associated with the warm ionized gas. OBSERVATIONS AND DATA REDUCTION HST/WFPC2 imagingWe obtained deep, high resolution, multiwavelength imaging of NGC 4214 with the WFPC2 instrument aboard HST (prop. ID 6569) on 1997 July 22. In Figure 1 we show a graphic representation of the WFPC2 field superimposed on a 13. ′ 6 × 13. ′ 6 greyscale reproduction of the Digitized Sky Survey Image 6 . The equatorial coordinates of the intersection point of the four camera fields are α = 12 h 15 m 40. s 53, δ = 36 • 19 ′ 37. ′′ 6 (J2000). This location was chosen in order to minimize Charge Transfer Efficiency (CTE) effects, since in this way no area of interest is separated from its collecting point by low signal areas (Whitmore et al. 1999). At a distance of 4.1 Mpc, 1 ′′ = 20 pc, so that one WF pixel corresponds to 2.0 pc and one PC pixel to 0.91 pc. Spatial resolution on these physical scales is within a factor of 2 − 4 of the best resolution that can be achieved from the ground for the LMC. Thus, the high spatial resolution, high sensitivity, wide dynamical range and faint magnitude limit (roughly 3 magnitudes fainter than typical ground-based limits, ∼ 26 in V) of WFPC2 permits one to identify regions structurally similar to those in the LMC (e.g. 30 Doradus), with ground-based resolution quality, for objects at nearly 80 times the distance to the LMC. Throughout the remainder of this paper, all conversions between pixels and arcseconds/physical scales are based on the plate scale of the WF chips, where most of NGC 4214 was imaged.We summarize the WFPC2 data set analyzed ...

New observations with the Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2), combined with prior archival data, provide nearly complete coverage of the inner 30 Doradus Nebula in both nebular lines and continuum at O'.'l resolution. The developing windblown cavity surrounding the massive firstgeneration, central cluster and its interface with the remanent molecular clouds, the site of second-generation, triggered star formation, are imaged in their entirety. Dithered Planetary Camera observations with 0703 res olution of Knots 1-3 reveal further structural details of the large dust pillars oriented toward the central clus ter and the newborn, massive multiple systems of the second generation in those fields. The new data also provide the first WFPC2 coverage of the fields of two interesting, luminous infrared sources observed in our previous Near Infrared Camera and Multi-Object Spectrometer program; comparisons of the highresolution optical and IR images are illuminating. In addition, we have obtained Space Telescope Imaging Spectrograph optical, long-slit observations of seven O-type multiple systems in the region, including those in Knots 1-3 and four representatives of the older generation. The spatially resolved spectrograms and classi fications of the close pairs, with separations ranging from 0'.'09 to 1718, are presented, as well as WFPC2 photometry of the individual components and the compact clusters of fainter stars associated with them. These new observations, and planned further analysis of them, offer significant new information about the intricate structure and evolution of the two-stage starburst in 30 Dor.