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Reddening and the Extinction Law at High Galactic Latitude
We present near-infrared (JHKL) photometry of 103 southern stars locatedbehind translucent interstellar clouds at high Galactic latitude. Ourdata are combined with visual photometry and spectral type informationfrom the literature in a detailed analysis of the wavelength dependenceof interstellar extinction by dust in these high-latitude clouds. Weinvestigate the shape of the near-infrared extinction curve and evaluatethe total line-of-sight extinction (AV) and ratio oftotal-to-selective extinction (RV) in each line of sight.Sources of uncertainty in RV arising from photometric errorsand spectral classification errors are carefully assessed and quantifiedas functions of the line-of-sight reddening. We detect appreciabledifferences in our results compared with the average extinction curvefor dust in the diffuse interstellar medium (ISM) close to the Galacticplane. Assuming a power-law form in the near-infrared, we find the meanfor high-latitude clouds to be well described by~λ-2.3, somewhat steeperthan that for the diffuse ISM(~λ-1.8). Our sampleincludes a substantial number of lines of sight with low RVvalues (47% with RV<2.8) relative to the diffuse ISMaverage of 3.05+/-0.15. We conclude that many high-latitude clouds haveenhanced abundances of relatively small grains.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447
| The local distribution of NA I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambdaapproximately 75,000) of the interstellar Na I D lines at 5890 A toward80 southern hemisphere early-type stars located in the localinterstellar medium (LISM). Combining these results with other sodiummeasurements taken from the literature, we produce galactic maps of thedistribution of neutral sodium column density for a total of 293 starsgenerally lying within approximately 250 pc of the Sun. These mapsreveal the approximate shape of the mid-plane contours of the rarefiedregion of interstellar space termed the Local Bubble. Its shape is seenas highly asymmetric, with a radius ranging from 30 to 300 pc, and withan average radius of 60 pc. Similar plots of the Galactic mid-planedistribution of sources emitting extreme ultraviolet radiation show thatthey also trace out similar contours of the Local Bubble derived from NaI absorption measurements. We conclude that the Local Bubble absorptioninterface can be represented by a hydrogen column density,NuETA = 2 x 1019 cm-2, which explainsboth the local distribution of Na I absorption and the observed galacticdistribution of extreme ultraviolet sources. The derived mid-planecontours of the Bubble generally reproduce the large-scale featurescarved out in the interstellar medium by several nearby galactic shellstructures.
| Optical spectroscopy of the high-latitude cloud LYNDS 1569
The interstellar material toward L1569 is studied in detail usingcombined optical and IR data. The cloud associated with the 9.0 km/scomponent of CO emission is seen to have N(CH) = (3.5 + or - 0.3) x 10to 13th/sq cm and two components of atomic Na I absorption at LSRvelocities of 9/0 and 22 km/s with column densities of N(Na I) = (7.3 +or - 1.3) x 10 to the 12th and (2.0 + or - 0.5) x 10 to the 11th/sq cm,respectively. Observed Ca II absorption in one direction is seen atvelocities of 9.6 and 21.6 km/s, and the 21.6 km/s component has a valueof N(Ca II)/N(Na I) greater than 64. Evidence for cloud heating and foran angular size of 30 arcmin for the L1569 cloud associated with theabsorption toward HD 25154 is discussed. This angular size, combinedwith a distance estimate of 150 pc, is used to derive approximate valuesof d = 1.2 pc and M = 50 solar masses. The CH column density isapproximately double that for dark clouds of comparable extinction inthe Galactic plane.
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