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The article to be presented on Thursday, October 12th is

A Survey of Interstellar H I From Lyman Alpha Absorption Measurements II
R. C. BohlinB. D. Savage, and J. F. Drake
 The Astrophysical Journal , 224:132-142, 1978 August 15


This paper reports the results of a survey of Lyman alpha measurements towards 100 stars.  From the measured absorption, the authors determine the column density of H I along the lines of sight to the observed stars.  From the color excess, E(B - V), the authors can determine the amount of dust along the same lines of sight, and the galactic distribution of neutral hydrogen and dust are discussed.

Before I get started explaining the article, let's get some of the jargon out of the way.  These are the definitions that seem particularly important to me:

H I :                         Neutral atomic hydrogen

Lyman Lines :         These are the spectral lines of hydrogen that result from the electron entering or leaving the first energy level (n = 1).  Lyman alpha absorption comes from an electron absorbing a photon of energy 9.655 eV, forcing it into a n = 1 to n = 2 transition.

Column Density :    The number of atoms in a cylinder with a cross-sectional area of 1 cm2 and a height equal to the distance to the object being observed.  The units of column density are [cm-2].

Color Excess :         "The reddening or color excess E(X -Y) in some color X - Y is defined to be the difference between the observed color m(X) - m(Y) and the intrinsic color m0(X) - m0(Y):

E(X - Y) = [m(X) - m(Y)] - [m0(X) - m0(Y)] = AX - AY

Since colors are always defined such that the shorter waveband is on the left and the strength of interstellar extinction generally decreases from short to long wavelengths, color excesses are usually positive" (Binney and Merrifield, Galactic Astronomy).

What was measured

The authors find the column density of H I along the line of sight to 100 stars of known spectral type.  Using this value and the with the knowledge of the distance to the stars, they are able to determine the average number density of neutral atomic hydrogen.  Citing a previous paper, the authors also state the number density of molecular hydrogen (H2) along the same lines of sight.  The B and V magnitudes of the 100 stars were also measured, and from this the color excess and column density of dust can be determined.

The Ratio of Gas to Color Excess

        Let f be defined as the ratio of atoms of molecular hydrogen to the total number of atoms of neutral hydrogen along a particular line of sight.  If f < 0.01, then the line of sight falls entirely within the diffuse intercloud medium.  If f > 0.01, then the line of sight intersects a region of high density, which can shield molecular hydrogen from ionizing radiation.

        The ratio of gas to color excess, <N(H I + H2)/E(B - V)>, has a mean value of 5.8x1021 atoms cm-2 mag-1 and a small scatter.  There is a clear positive correlation between the two quantities, as is shown in the figures in the paper.

        When E(B - V) > 0.2, the ratio of molecular hydrogen to HI increases.  This is expected, since the formation of H2 is facilitated by the presence of dust.  In the densest clouds, there is sufficient shielding to protect the H2, and it is not suprising that the ratio of molecular hydrogen to atomic hydrogen increases as reddening increases.

        The gas to color excess seems to be the same independent of location in the sky, with the exception of the line of sight towards Rho Ophiuchi.  There is no evidence dust having a different scale height or radial distribution than gas.

        The star Rho Ophiuchi is the deviant member of this stellar sample.  It's gas to color excess ratio is 2.7 times larger than the mean despite the high quality data.  The authors attribute the difference to changes in the dust reddening efficiency along this line of sight.  This change is most likely due to an increase in the size of the dust grains, which is evidenced by the lack of a rapid rise toward short wavelengths in the far-ultraviolet extinction curve for Rho Ophiuchi.

The Galactic Distribution of HI

The authors estimate a mean density of 0.86 HI atoms cm-3.  The authors use ellipses to show the column density, distance, and space density of HI towards each of the stars.  From this plot, it is seen that the local distribution of HI is inhomogeneous, but points near each other are likely to have similar values of n(HI).  This suggests that the HI distribution varies smoothly across the sky, but also that the neutral intercloud medium may be clumpy.

Estimating n(HI) = 0.16 cm-3, it is possible to determine the z distribution of HI, which is the distribution of atomic hydrogen perpendicular to the plane of the disk.  The authors fit the data with an exponential decay: n(z) = 0.16 exp(-z2/h) cm-3 and estimate h = 350 pc.

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