We present data on the gas-phase abundances for 9 different elements in the interstellar medium of the Small Magellanic Cloud (SMC), based on the strengths of ultraviolet absorption features over ...relevant velocities in the spectra of 18 stars within the SMC. From this information and the total abundances defined by the element fractions in young stars in the SMC, we construct a general interpretation on how these elements condense into solid form onto dust grains. As a group, the elements Si, S, Cr, Fe, Ni, and Zn exhibit depletion sequences similar to those in the local part of our Galaxy defined by Jenkins. The elements Mg and Ti deplete less rapidly in the SMC than in the Milky Way, and Mn depletes more rapidly. We speculate that these differences might be explained by the different chemical affinities to different existing grain substrates. For instance, there is evidence that the mass fractions of polycyclic aromatic hydrocarbons in the SMC are significantly lower than those in the Milky Way. We propose that the depletion sequences that we observed for the SMC may provide a better model for interpreting the element abundances in low-metallicity Damped Lyman Alpha (DLA) and sub-DLA absorption systems that are recorded in the spectra of distant quasars and gamma-ray burst afterglows.
ABSTRACT We develop a model of dust evolution in a multiphase, inhomogeneous interstellar medium (ISM) using hydrodynamical simulations of giant molecular clouds in a Milky Way-like spiral galaxy. We ...improve the treatment of dust growth by accretion in the ISM to investigate the role of the temperature-dependent sticking coefficient and ion-grain interactions. From detailed observational data on the gas-phase Si abundances measured in the local Galaxy, we derive a relation between the average and the local gas density that we use as a critical constraint for the models. This relation requires a sticking coefficient that decreases with the gas temperature. The relation predicted by the models reproduces the slope of −0.5 for the observed relation in cold clouds, which is steeper than that for the warm medium and is explained by dust growth. We find that growth occurs in the cold medium for all adopted values of the minimum grain size amin from 1 to 5 nm. For the classical cutoff of , the Coulomb repulsion results in slower accretion and higher than the observed values. For , the Coulomb interactions enhance the growth rate, steepen the slope of the - relation, and provide a better match to observations. The rates of dust re-formation in the ISM by far exceed the rates of dust production by stellar sources. After the initial 140 Myr, the cycle of matter in and out of dust reaches a steady state, in which the dust growth balances the destruction on a similar timescale of 350 Myr.
When the neutral interstellar medium is exposed to extreme-ultraviolet and soft X-ray radiation, the argon atoms in it are far more susceptible to being ionized than the hydrogen atoms. We make use ...of this fact to determine the level of ionization in the nearby warm neutral medium. By analyzing Far-Ultraviolet Spectroscopic Explorer observations of ultraviolet spectra of 44 hot subdwarf stars a few hundred parsecs away from the Sun, we can compare column densities of Ar I to those of O I, where the relative ionization of oxygen can be used as a proxy for that of hydrogen. The measured deficiency Ar I/O I = -0.427 + or - 0.11 dex below the expectation for a fully neutral medium implies that the electron density n(e) approximately 0.04 cm super(-3) if n(H) = 0.5 cm super(-3). This amount of ionization is considerably larger than what we expect from primary photoionizations resulting from cosmic rays, the diffuse X-ray background, and X-ray emitting sources within the medium, along with the additional ionizations caused by energetic secondary photoelectrons, Auger electrons, and photons from helium recombinations. We favor an explanation that bursts of radiation created by previous, nearby supernova remnants that have faded by now may have elevated the ionization, and the gas has not yet recombined to a quiescent level. A different alternative is that the low-energy portion of the soft X-ray background is poorly shielded by the H I because it is frothy and has internal pockets of very hot, X-ray emitting gases.
ABSTRACT We report new observations from a systematic, spectroscopic, ultraviolet absorption-line survey that maps the spatial and kinematic properties of the high velocity gas in the Galactic Center ...(GC) region. We examine the hypothesis that this gas traces the biconical nuclear outflow. We use an ultraviolet spectra of 47 background QSOs and halo stars projected inside and outside the northern Fermi Bubble from the Hubble Space Telescope to study the incidence of high velocity absorption around it. We use five lines of sight inside the northern Fermi Bubble to constrain the velocity and column densities of outflowing gas traced by O i, Al ii, C ii, C iv, Si ii, Si iii, Si iv, and other species. We find that all five lines of sight inside the northern Fermi Bubble exhibit blueshifted high velocity absorption components, whereas only 9 out of the 42 lines of sight outside the northern Fermi Bubble exhibit blueshifted high velocity absorption components. The observed outflow velocity profile decreases with Galactic latitude and radial distance (R) from the GC. The observed blueshifted velocities change from at R 2.3 kpc to at R 6.5 kpc. We derive the metallicity of the entrained gas along the 1H1613-097 sightline, one that passes through the center of the northern Fermi Bubble, finding O/H −0.54 0.15. A simple kinematic model, tuned to match the observed absorption component velocities along the five lines of sight inside the Bubble, constrains the outflow velocities to 1000-1300 , and the age of the outflow to be ∼6-9 Myr. We estimate a minimum mass outflow rate for the nuclear outflow to be . Combining the age and mass outflow rates, we determine a minimum mass of total UV-absorbing cool gas entrained in the Fermi Bubbles to be .
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 Mg II, Fe II, and H I. 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 H I 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-1 with respect to the main component, which may be the signature of a shock propagating toward the cloud’s interior.
Abstract
In a survey of archived ultraviolet spectra of 100 stars recorded by the echelle spectrograph of the Space Telescope Imaging Spectrograph on the
Hubble Space Telescope
, we measure the ...strengths of the weak absorption features of O
i
, Ge
ii
, and Kr
i
in the interstellar medium. Our objective is to undertake an investigation that goes beyond earlier abundance studies to see how these elements are influenced independently by three different environmental properties: (1) values of a generalized atomic depletion factor
F
*
due to condensations onto dust grains (revealed here by the abundances of Mg and Mn relative to H), (2) the fraction of H atoms in the form of H
2
f
(H
2
), and (3) the ambient intensity
I
of ultraviolet starlight relative to an average value in our part of the Galaxy
I
0
. As expected, the gas-phase abundances of all three elements exhibit negative partial correlations with
F
*
. The abundances of free O atoms show significant positive partial correlations with
and
, while Ge and Kr exhibit negative partial correlations with
at marginal levels of significance. After correcting for these trends, the abundances of O relative to H show no significant variations with location, except for the already-known radial gradient of light-element abundances in the Milky Way. A comparison of Ge and O abundances revealed no significant regional enhancements or deficiencies of neutron-capture elements relative to
α
-process ones.
Comparison of interstellar medium (ISM) absorption in the UV spectrum of LS 4825, a B1 Ib−II star d = 21 5 kpc from the Sun toward l = 1 67 and b = −6 63, with ISM absorption toward an aligned ...foreground star at d < 7.0 1.7 kpc, allows us to isolate and study gas associated with the Milky Way nuclear wind. Spectra from the Space Telescope Imaging Spectrograph show low-ionization absorption out to d < 7 kpc (e.g., O i, C ii, Mg ii, Si ii, Fe ii, S ii) only between 0 and 40 km s−1, while absorption at d > 7 kpc, ∼1 kpc below the Galactic plane, is complex and spans −290 to +94 km s−1. The intermediate and high ions Si iii, C iv, Si iv, and N v show extremely strong absorption with multiple components from −283 to 107 km s−1, implying that the ISM ∼1 kpc below the Galactic center has a substantial reservoir of plasma and more gas containing C iv and N v than in the Carina OB1 association at z = 0 kpc. Abundances and physical conditions are presented for many absorption components. The high ion absorption traces cooling transition temperature plasma probably driven by the outflowing hot gas, while the extraordinarily large thermal pressure, p/k ∼ 105 cm−3 K−1, in an absorption component at −114 km s−1 probably arises from the ram pressure of the outflowing hot gas. The observations are consistent with a flow whose ionization structure in the high ions can be understood through a combination of nonequilibrium radiative cooling and turbulent mixing.
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