15 research outputs found
Interstellar Krypton Abundances: The Detection of Kiloparsec-scale Differences in Galactic Nucleosynthetic History
We present an analysis of Kr I 1236 line measurements from 50 sight lines in
the Hubble Space Telescope Space Telescope Imaging Spectrograph and Goddard
High Resolution Spectrograph data archives that have sufficiently high
resolution and signal-to-noise ratio to permit reliable krypton-to-hydrogen
abundance ratio determinations. The distribution of Kr/H ratios in this sample
is consistent with a single value for the ISM within 5900 pc of the Sun,
log(Kr/H) = -9.02+/-0.02, apart from a rough annulus from between about 600 and
2500 pc distant. The Kr/H ratio toward stars within this annulus is elevated by
approximately 0.11 dex, similar to previously noted elevations of O/H and Cu/H
gas-phase abundances beyond about 800 pc. A significant drop in the gas-phase
N/O ratio in the same region suggests that this is an artifact of
nucleosynthetic history. Since the physical scale of the annulus' inner edge is
comparable to the radius of the Gould Belt and the outer limit of heliocentric
distances where the D/H abundance ratio is highly variable, these phenomena may
be related to the Gould Belt's origins.Comment: 32 pages, including 6 figures; accepted by Ap
Far-ultraviolet Dust Extinction and Molecular Hydrogen in the Diffuse Milky Way Interstellar Medium
We aim to compare variations in the full-UV dust extinction curve (912-3000
Angstrom), with the HI/H/total H content along diffuse Milky Way
sightlines, to investigate possible connections between ISM conditions and dust
properties. We combine an existing sample of 75 UV extinction curves based on
IUE and FUSE data, with atomic and molecular column densities measured through
UV absorption. The H column density data are based on existing Lyman-Werner
absorption band models from earlier work on the extinction curves. Literature
values for the HI column density were compiled, and improved for 23 stars by
fitting a Ly profile to archived spectra. We discover a strong
correlation between the H column and the far-UV extinction, and the
underlying cause is a linear relationship between H and the strength of the
far-UV rise feature. This extinction does not scale with HI, and the total H
column scales best with instead. The carrier of the far-UV rise
therefore coincides with molecular gas, and further connections are shown by
comparing the UV extinction features to the molecular fraction. Variations in
the gas-to-extinction ratio correlate with the UV-to-optical
extinction ratio, and we speculate this could be due to coagulation or
shattering effects. Based on the H temperature, the strongest far-UV rise
strengths are found to appear in colder and denser sightlines.Comment: 24 pages, 9 figures, accepted for publication in ApJ; fix missing
text in generated pdf due to broken tex comman
Interstellar Carbon in Translucent Sightlines
We report interstellar C II column densities or upper limits determined from
weak absorption of the 2325.4029 A intersystem transition observed in six
translucent sightlines with STIS. The sightlines sample a wide range of
interstellar characteristics including total-to-selective extinction, R_{V} =
2.6 - 5.1; average hydrogen density along the sightline, = 3 - 14
cm^{-3}; and fraction of H in molecular form, 0 - 40%. Four of the sightlines,
those toward HD 37021, HD 37061, HD 147888 and HD 207198, have interstellar
gas-phase abundances that are consistent with the diffuse sightline ratio of
161 +/- 17 carbon atoms in the gas per million hydrogen nuclei. We note that
while it has a gas-phase carbon abundance that is consistent with the other
sightlines, a large fraction of the C II toward HD 37061 is in an excited
state. The sightline toward HD 152590 has a measured interstellar gas-phase
carbon abundance that is well above the diffuse sightline average; the column
density of C in this sightline may be overestimated due to noise structure in
the data. Toward HD 27778 we find a 3 sigma abundance upper limit of <108 C
atoms in the gas per million H, a substantially enhanced depletion of C as
compared to the diffuse sightline value. The interstellar characteristics
toward HD 27778 are otherwise not extreme among the sample except for an
unusually large abundance of CO molecules in the gas.Comment: Accepted for publication in the Astrophysical Journa
The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high resolution HST Space Telescope Imaging
Spectrograph (STIS) observations of Kr I 1236 Angstrom absorption in seven
sight lines that probe a variety of interstellar environments. In combination
with krypton and hydrogen column densities derived from current and archival
STIS and Far-Ultraviolet Spectroscopic Explorer data, the number of sight lines
with reliable Kr/H ISM abundance ratios has been increased by 50% to
26--including paths that sample a range of nearly 5 orders of magnitude in
f(H_2), over 2 orders of magnitude in , and extending up to 4.8 kpc in
length. For sight lines contained entirely within the local spiral arm (the
Orion Spur), the spread of Kr/H ratios about the mean of log_10[N(Kr)/N(H)]_ISM
= -9.02+/-0.02 is remarkably tight (0.06 dex), less than the typical datapoint
uncertainty. Intriguingly, the only two sight lines that extend through
neighboring structures, in particular gas associated with the
Carina/Sagittarius Arm, exhibit relatively large, near-solar krypton abundances
(log_10[N(Kr)/N(H)]_combined = -8.75+0.09_-0.11). Although these deviations are
only measured at the 2 sigma level, they suggest the possibility that krypton
abundances beyond the Orion Spur may differ from the local value.Comment: 15 page, including 3 figures and 2 tables, accepted for publication
in Ap
The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high resolution HST Space Telescope Imaging
Spectrograph (STIS) observations of O I 1356 and H I Lyman-alpha absorption in
36 sight lines that probe a variety of Galactic disk environments and include
paths that range over nearly 4 orders of magnitude in f(H_2), over 2 orders of
magnitude in mean sight line density, and that extend up to 6.5 kpc in length.
Consequently, we have undertaken the study of gas-phase O/H abundance ratio
homogeneity using the current sample and previously published Goddard
High-Resolution Spectrograph (GHRS) results. Two distinct trends are identified
in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance
with increasing mean sight line density and a gap between the mean O/H ratio
for sight lines shorter and longer than about 800 pc. The first effect is a
smooth transition between two depletion levels associated with large mean
density intervals; it is centered near a density of 1.5 cm^-3 and is similar to
trends evident in gas-phase abundances of other elements. Paths less dense than
the central value exhibit a mean O/H ratio of log_10 (O/H) = -3.41+/-0.01 (or
390+/-10 ppm), which is consistent with averages determined for several long,
low-density paths observed by STIS (Andre et al. 2003) and short low-density
paths observed by FUSE (Moos et al. 2002). Sight lines of higher mean density
exhibit an average O/H value of log_10 (O/H) = -3.55+/-0.02 (284+/-12 ppm). The
datapoints for low-density paths are scattered more widely than those for
denser sight lines, due to O/H ratios for paths shorter than 800 pc that are
generally about 0.10 dex lower than the values for longer ones.Comment: 33 pages, including 8 figures and 4 tables; accepted for publication
in ApJ, tentatively in Oct 200
Interstellar Iron and Silicon Depletions in Translucent Sight Lines
We report interstellar FeII and SiII column densities toward six translucent
sight lines (A_V >~ 1) observed with the Space Telescope Imaging Spectrograph
(STIS). The abundances were determined from the absorption of SiII] at 2335
Angstroms, and several weak Fe transitions including the first reported
detections of the 2234 Angstrom line. We derive an empirical f-value for the
FeII 2234 Angstrom transition of log(f lambda) = -1.54 +/- 0.05. The observed
sight lines sample a variety of extinction characteristics as indicated by
their R_V values, which range from 2.6 - 5.8. The dust-phase abundances of both
Si and Fe are positively correlated with the small-grain population (effective
radii smaller than a few hundred micron) toward the targets. The physical
conditions along the sight lines suggest that this relationship may be due to
differences in the survival of small particles in some interstellar
environments. The chemical composition of the small grains could either
resemble dust mantles or be silicate rich.Comment: accepted for publication in Ap
FUSE Measurements of Far Ultraviolet Extinction. I. Galactic Sight Lines
We present extinction curves that include data down to far ultraviolet
wavelengths (FUV; 1050 - 1200 A) for nine Galactic sight lines. The FUV
extinction was measured using data from the Far Ultraviolet Spectroscopic
Explorer. The sight lines were chosen for their unusual extinction properties
in the infrared through the ultraviolet; that they probe a wide range of dust
environments is evidenced by the large spread in their measured ratios of
total-to-selective extinction, R_V = 2.43 - 3.81. We find that extrapolation of
the Fitzpatrick & Massa relationship from the ultraviolet appears to be a good
predictor of the FUV extinction behavior. We find that predictions of the FUV
extinction based upon the Cardelli, Clayton & Mathis (CCM) dependence on R_V
give mixed results. For the seven extinction curves well represented by CCM in
the infrared through ultraviolet, the FUV extinction is well predicted in three
sight lines, over-predicted in two sight lines, and under-predicted in 2 sight
lines. A Maximum Entropy Method analysis using a simple three component grain
model shows that seven of the nine sight lines in the study require a larger
fraction of grain materials to be in dust when FUV extinction is included in
the models. Most of the added grain material is in the form of small (radii <
200 A) grains.Comment: Accepted for publication in the Astrophysical Journal. 31 pages with
7 figure