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Interstellar NaI and CaII absorption observed towards the Cygnus Loop SNR
We present high resolution spectra (R ~ 5 km s-1) of theinterstellar NaI and CaII absorption lines observed towards 9 early-typestars with distances ranging from 250 to 2300 pc in the line-of-sighttowards the Cygnus Loop Supernova Remnant (SNR). All but one of theseabsorption profiles can be fit using a combination of one or more ofthree absorption components with average best-fit (lsr) velocities ofV1 = + 0.8 km s-1, V2 = +9.0 kms-1 and V3 = +19.7 km s-1. Anadditional velocity component at V4 = +29.7 km s-1is required in order to fit the NaI profile recorded towards the star HD198946, whose distance of 794 pc places it well in excess of the nominal440 pc distance to the SNR. The NaI/CaII column density ratios for thethree higher velocity components are typically <1.0, which aresimilar to values found for high-velocity gas components detectedtowards other evolved SNRs. Even though we have detected the threehigher velocity components solely along the sight-lines towards starswith distance estimates greater than that of the Cygnus Loop, we areunable to definitely associate these components with an interactionbetween the expansion of the SN shock wave and the ambient interstellarmedium. We suggest a more likely origin for these absorption componentsis that of an old pre-cursor SN neutral gas shell, within whoseinterstellar cavity the Cygnus Loop supernova explosion occured some 20000 years ago.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
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