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Mapping the contours of the Local bubble: preliminary results
We present preliminary results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,taking advantage of Hipparcos stellar distances. Equivalent widths ofthe NaI D-line doublet at 5890 Å are presented for thelines-of-sight towards some 143 new target stars lying within 300 pc ofthe Sun. Using these data which were obtained at the Observatoire deHaute Provence, together with previously published NaI absorptionmeasurements towards a further 313 nearby targets, we present absorptionmaps of the distribution of neutral gas in the local interstellar mediumas viewed from 3 different galactic projections. In particular, thesemaps reveal the Local Bubble region as a low neutral densityinterstellar cavity in the galactic plane with radii between 65-250 pcthat is surrounded by a (dense) neutral gas boundary (or ``wall''). Wehave compared our iso-column contours with the contours derived bySnowden et al. (\cite{snowden98}) using ROSAT soft X-ray emission data.Consistency in the global dimensions derived for both sets of contoursis found for the case of a million degree hot LB plasma of emissivity0.0023 cm(-6) pc with an electron density of 0.005 cm(-2) . We havedetected only one relatively dense accumulation of cold, neutral gaswithin 60 pc of the Sun that surrounds the star delta Cyg, and note thatthe nearest molecular cloud complex of MBM 12 probably resides at thevery edge of the Local Bubble at a distance of ~ 90 pc. Our observationsmay also explain the very different physical properties of the columnsof interstellar gas in the line-of-sight to the two hot stars epsilonCMa and beta CMa as being due to their locations with respect to theBubble contours. Finally, in the meridian plane the LB cavity is foundto be elongated perpendicularly to the Gould's Belt plane, possiblybeing ``squeezed'' by the expanding shells of the Sco-Cen andPerseus-Taurus OB associations. Tables 1 and 2 are also available inelectronic form at the CDS (Strasbourg) via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The structure of the interstellar gas towards stars in the globular cluster M 55
Observations are presented of high resolution interstellar Na D lineprofiles towards 10 stars in the globular cluster M 55 and 6 foregroundstars which lie within 3 deg from the cluster. The spectral regioncontaining the K I (lambda 7699 A) line is simultaneously recorded butthis line is detected only towards one of the foreground stars as acomponent having a relatively high K I/Na I abundance ratio. Twoprincipal components are observed towards the cluster stars atvelocities of approximately +1 and +15 km/s and weaker higher velocitygas is detected at approximately -21, +33 and +55 km/s. The +1 km/scomponent is at a distance of approximately less than 100 pc and wesuggest that this is part of an extensive region of diffuse gas whichconnects the molecular and dark clouds observed closer to the galacticplane. The +15 km/s component lies at a distance between 130 to 300 pcand is identified with an anomalous velocity arc observed in HI. Thehigher velocity gas components are at a distance approximately greaterthan 700 pc. From a comparison with our previous observations in thegeneral direction of the Carina-Sagittarius arm we suggest that thesefeatures have an origin in spiral arm gas, in which case their detectiontowards M 55 places them some 600 to 800 pc from the galactic plane.
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Observation and Astrometry data
| Constellation: | Sagittaire |
| Right ascension: | 19h45m24.34s |
| Declination: | -30°54'10.8" |
| Apparent magnitude: | 6.816 |
| Distance: | 234.192 parsecs |
| Proper motion RA: | 5.4 |
| Proper motion Dec: | -16.2 |
| B-T magnitude: | 6.964 |
| V-T magnitude: | 6.829 |
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INHABITED SKY
