861 research outputs found
Interpreting the convergence of Lyman series absorption lines
Spectra of quasars at high z often show absorption at the Lyman limit from intervening gas systems at intermediate z having N(H) approx. greater than 10(exp 7) cm(-2). In some circumstances, N(H) can be determined by measuring the strength of the Lyman limit absorption or the damping wings of Lyman - alpha. With a spectrum taken at low wavelength resolution, say, lambda/delta lambda approx. 10(exp 3), it is usually not possible to distinguish individual Lyman series lines near the limit, yet one can still discern how rapidly the average intensity drops off as the limit is approached from the long wavelength side. The purpose here is to point out the information which is available from measurements of this series convergence
Observations of interstellar lines using the IUE satellite, covering the years 1978 - 1988
The study of absorption lines produced by dispersed gases in front of various sources in the sky were addressed. Data was used from the IUE satellite. The results of the projects are summarized
The interstellar cloud surrounding the Sun: a new perspective
Aims: We offer a new, simpler picture of the local interstellar medium, made
of a single continuous cloud enveloping the Sun. This new outlook enables the
description of a diffuse cloud from within and brings to light some unexpected
properties. Methods: We re-examine the kinematics and abundances of the local
interstellar gas, as revealed by the published results for the ultraviolet
absorption lines of MgII, FeII, and HI. Results: In contrast to previous
representations, our new picture of the local interstellar medium consists of a
single, monolithic cloud that surrounds the Sun in all directions and accounts
for most of the matter present in the first 50 parsecs around the Sun. The
cloud fills the space around us out to about 9 pc in most directions, although
its boundary is very irregular with possibly a few extensions up to 20 pc. The
cloud does not behave like a rigid body: gas within the cloud is being
differentially decelerated in the direction of motion, and the cloud is
expanding in directions perpendicular to this flow, much like a squashed
balloon. Average HI volume densities inside the cloud vary between 0.03 and 0.1
cm-3 over different directions. Metals appear to be significantly depleted onto
grains, and there is a steady increase in depletion from the rear of the cloud
to the apex of motion. There is no evidence that changes in the ionizing
radiation influence the apparent abundances. Secondary absorption components
are detected in 60% of the sight lines. Almost all of them appear to be
interior to the volume occupied by the main cloud. Half of the sight lines
exhibit a secondary component moving at about -7.2 km/s with respect to the
main component, which may be the signature of a shock propagating toward the
cloud's interior.Comment: Accepted for publication in Astronomy & Astrophysic
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