60 research outputs found
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
H2 formation and excitation in the diffuse interstellar medium
We use far-UV absorption spectra obtained with FUSE towards three late B
stars to study the formation and excitation of H2 in the diffuse ISM. The data
interpretation relies on a model of the chemical and thermal balance in
photon-illuminated gas. The data constrain well the nR product between gas
density and H2 formation rate on dust grains: nR = 1 to 2.2 e-15 s-1. For each
line of sight the mean effective H2 density n, assumed uniform, is obtained by
the best fit of the model to the observed N(J=1)/N(J=0) ratio, since the
radiation field is known. Combining n with the nR values, we find similar H2
formation rates for the three stars of about R = 4 e-17 cm3/s.
Because the target stars do not interact with the absorbing matter we can
show that the H2 excitation in the J>2 levels cannot be accounted for by the UV
pumping of the cold H2 but implies collisional excitation in regions where the
gas is much warmer. The existence of warm H2 is corroborated by the fact that
the star with the largest column density of CH+ has the largest amount of warm
H2.Comment: 7 pages, 4 figures, to be published in Astronomy & Astrophysic
What is the Total Deuterium Abundance in the Local Galactic Disk?
Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer
(FUSE) satellite, together with spectra from the Copernicus and IMAPS
instruments, reveal an unexplained very wide range in the observed
deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk
beyond the Local Bubble. We argue that spatial variations in the depletion of
deuterium onto dust grains can explain these local variations in the observed
gas-phase D/H ratios. We present a variable deuterium depletion model that
naturally explains the constant measured values of D/H inside the Local Bubble,
the wide range of gas-phase D/H ratios observed in the intermediate regime (log
N(H I} = 19.2-20.7), and the low gas-phase D/H ratios observed at larger
hydrogen column densities. We consider empirical tests of the deuterium
depletion hypothesis: (i) correlations of gas-phase D/H ratios with depletions
of the refractory metals iron and silicon, and (ii) correlation with the
molecular hydrogen rotational temperature. Both of these tests are consistent
with deuterium depletion from the gas phase in cold, not recently shocked,
regions of the ISM, and high gas-phase D/H ratios in gas that has been shocked
or otherwise heated recently. We argue that the most representative value for
the total (gas plus dust) D/H ratio within 1 kpc of the Sun is >=23.1 +/- 2.4
(1 sigma) parts per million (ppm). This ratio constrains Galactic chemical
evolution models to have a very small deuterium astration factor, the ratio of
primordial to total (D/H) ratio in the local region of the Galactic disk, which
we estimate to be f_d <= 1.19 +/-0.16 (1 sigma) or <= 1.12 +/- 0.14 (1 sigma)
depending on the adopted light element nuclear reaction rates.Comment: 19 pages, 9 figure
Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample
We report total abundances and related parameters for the full sample of the
FUSE survey of molecular hydrogen in 38 translucent lines of sight. New results
are presented for the "second half" of the survey involving 15 lines of sight
to supplement data for the first 23 lines of sight already published. We assess
the correlations between molecular hydrogen and various extinction parameters
in the full sample, which covers a broader range of conditions than the initial
sample. In particular, we are now able to confirm that many, but not all, lines
of sight with shallow far-UV extinction curves and large values of the
total-to-selective extinction ratio, = / -- characteristic
of larger than average dust grains -- are associated with particularly low
hydrogen molecular fractions (). In the lines of sight with large
, there is in fact a wide range in molecular fractions, despite the
expectation that the larger grains should lead to less H formation.
However, we see specific evidence that the molecular fractions in this
sub-sample are inversely related to the estimated strength of the UV radiation
field and thus the latter factor is more important in this regime. We have
provided an update to previous values of the gas-to-dust ratio, (H)/, based on direct measurements of (H) and (H I).
Although our value is nearly identical to that found with Copernicus data, it
extends the relationship by a factor of 2 in reddening. Finally, as the new
lines of sight generally show low to moderate molecular fractions, we still
find little evidence for single monolithic "translucent clouds" with 1.Comment: 35 pages, 5 tables, 7 figures, accepted for publication in The
Astrophysical Journal Supplements Serie
The Far Ultraviolet Spectroscopic Explorer Survey of OVI Absorption in the Disk of the Milky Way
To probe the distribution and physical characteristics of interstellar gas at
temperatures T ~ 3e5 K in the disk of the Milky Way, we have used the Far
Ultraviolet Spectroscopic Explorer (FUSE) to observe absorption lines of OVI
toward 148 early-type stars situated at distances 1 kpc. After subtracting off
a mild excess of OVI arising from the Local Bubble, combining our new results
with earlier surveys of OVI, and eliminating stars that show conspicuous
localized X-ray emission, we find an average OVI mid-plane density n_0 = 1.3e-8
cm^-3. The density decreases away from the plane of the Galaxy in a way that is
consistent with an exponential scale height of 3.2 kpc at negative latitudes or
4.6 kpc at positive latitudes. Average volume densities of OVI along different
sight lines exhibit a dispersion of about 0.26 dex, irrespective of the
distances to the target stars. This indicates that OVI does not arise in
randomly situated clouds of a fixed size and density, but instead is
distributed in regions that have a very broad range of column densities, with
the more strongly absorbing clouds having a lower space density. Line widths
and centroid velocities are much larger than those expected from differential
Galactic rotation, but they are nevertheless correlated with distance and
N(OVI), which reinforces our picture of a diverse population of hot plasma
regions that are ubiquitous over the entire Galactic disk. The velocity
extremes of the OVI profiles show a loose correlation with those of very strong
lines of less ionized species, supporting a picture of a turbulent, multiphase
medium churned by shock-heated gas from multiple supernova explosions.Comment: Accepted for publication in ApJS. Preprint with full resolution
images and all 148 spectra available at
http://www.astro.princeton.edu/~dvb/o
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