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On the MBM 12 Young Association
I present a comprehensive study of the MBM 12 young association (MBM12A). By combining infrared (IR) photometry from the Two-Micron All-SkySurvey (2MASS) survey with new optical imaging and spectroscopy, I haveperformed a census of the MBM 12A membership that is complete to 0.03Msolar (H~15) for a 1.75d×1.4d field encompassing theMBM 12 cloud. I find five new members with masses of 0.1-0.4Msolar and a few additional candidates that have not beenobserved spectroscopically. From an analysis of optical and IRphotometry for stars in the direction of MBM 12, I identify M dwarfs inthe foreground and background of the cloud. By comparing the magnitudesof these stars to those of local field dwarfs, I arrive at a distancemodulus 7.2+/-0.5 (275 pc) to the MBM 12 cloud; it is not the nearestmolecular cloud and is not inside the local bubble of hot ionized gas ashad been implied by previous distance estimates of 50-100 pc. I havealso used Li strengths and H-R diagrams to constrain the absolute andrelative ages of MBM 12A and other young populations; these dataindicate ages of 2+3-1 Myr for MBM 12A and ~10 Myrfor the TW Hya and η Cha associations. MBM 12A may be a slightlyevolved version of the aggregates of young stars within the Taurus darkclouds (~1 Myr) near the age of the IC 348 cluster (~2 Myr).
| 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
| Distances to the High Galactic Latitude Molecular Clouds G192-67 and MBM 23-24
We report on distance determinations for two high-Galactic latitudecloud complexes, G192-67 and MBM 23-24. No distance determination existsin the literature for either cloud. Thirty-four early-type stars wereobserved toward the two clouds, more than half of which have parallaxesmeasured by the Hipparcos satellite. For the remaining stars, we havemade spectroscopic distance estimates. The data consist ofhigh-resolution echelle spectra centered on the Na I D lines and wereobtained over six nights at the Coudé Feed telescope at Kitt PeakNational Observatory. Interstellar absorption lines were detected towardsome of the stars, which enabled estimates of the distances to theclouds of 109+/-14 pc for G192-67 and 139+/-33 pc for MBM 23-24. Wediscuss the relationship of these clouds to other ISM features such asthe Local Hot Bubble and the local cavity in neutral hydrogen.
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