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Register a new userSolid State Astrophysics: Probing Interstellar Dust and Gas Properties with X-rays
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The abundances of gas and dust (solids and complex molecules) in the interstellar medium (ISM) as well as their composition and structures impact practically all of astrophysics. Fundamental processes from star formation to stellar winds to galaxy formation all scale with the number of metals. However, significant uncertainties remain in both absolute and relative abundances, as well as how these vary with environment, e.g., stellar photospheres versus the interstellar medium (ISM). While UV, optical, IR, and radio studies have considerably advanced our understanding of ISM gas and dust, they cannot provide uniform results over the entire range of column densities needed. In contrast, X-rays will penetrate gas and dust in the cold (3K) to hot (100,000,000K) Universe over a wide range of column densities (log NH=20-24 cm^-2), imprinting spectral signatures that reflect the individual atoms which make up the gas, molecule or solid. *X-rays therefore are a powerful and viable resource for delving into a relatively unexplored regime for determining gas abundances and dust properties such as composition, charge state, structure, and quantity via absorption studies, and distribution via scattering halos.*
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We have explored the capabilities of dust extinction and $gamma$ rays to probe the properties of the interstellar medium in the nearby anti-centre region. We have jointly modelled the $gamma$-ray intensity and the stellar reddening, E(B-V) as a combination of H$_{rm I}$-bright, CO-bright, and ionised gas components. The complementary information from dust reddening and $gamma$ rays is used to reveal the dark gas not seen, or poorly traced, by H$_{rm I}$, free-free, and $^{12}$CO emissions. We compare the total gas column densities, $N_{rm{H}}$, derived from the $gamma$ rays and stellar reddening with those inferred from a similar analysis (Remy et al. 2017) of $gamma$ rays and of the optical depth of the thermal dust emission, $tau_{353}$, at 353 GHz. We can therefore compare environmental variations in specific dust reddening, E(B-V)/$N_{rm H}$, and in dust emission opacity (dust optical depth per gas nucleon), $tau_{353}/N_{rm{H}}$. Over the whole anti-centre region, we find an average E(B-V)/$N_{rm H}$ ratio of $(2.02pm0.48)times$ $10^{-22}$~mag~cm$^2$, with maximum local variations of about $pm30%$ at variance with the two to six fold coincident increase seen in emission opacity as the gas column density increases. In the diffuse medium, the small variations in specific reddening, E(B-V)/$N_{rm H}$ implies a rather uniform dust-to-gas mass ratio in the diffuse parts of the anti-centre clouds. The small amplitude of the E(B-V)/$N_{rm H}$ variations with increasing $N_{rm{H}}$ column density confirms that the large opacity $tau_{353}/N_{rm{H}}$ rise seen toward dense CO clouds is primarily due to changes in dust emissivity. The environmental changes are qualitatively compatible with model predictions based on mantle accretion on the grains and the formation of grain aggregates.
Interstellar abundance determinations from fits to X-ray absorption edges often rely on the incorrect assumption that scattering is insignificant and can be ignored. We show instead that scattering contributes significantly to the attenuation of X-rays for realistic dust grain size distributions and substantially modifies the spectrum near absorption edges of elements present in grains. The dust attenuation modules used in major X-ray spectral fitting programs do not take this into account. We show that the consequences of neglecting scattering on the determination of interstellar elemental abundances are modest; however, scattering (along with uncertainties in the grain size distribution) must be taken into account when near-edge extinction fine structure is used to infer dust mineralogy. We advertise the benefits and accuracy of anomalous diffraction theory for both X-ray halo analysis and near edge absorption studies. An open source Fortran suite, General Geometry Anomalous Diffraction Theory (GGADT), is presented that calculates X-ray absorption, scattering, and differential scattering cross sections for grains of arbitrary geometry and composition.
The dense Galactic environment is a large reservoir of interstellar dust. Therefore, this region represents a perfect laboratory to study the properties of the cosmic dust grains. X-rays are the most direct way to detect the interaction of light with dust present in these dense environments. The interaction between the radiation and the interstellar matter imprints specific absorption features in the X-ray spectrum. We study them with the aim of defining the chemical composition, the crystallinity and structure of the dust grains which populate the inner regions of the Galaxy. We investigate the magnesium and the silicon K-edges detected in the Chandra/HETG spectra of eight bright X-ray binaries, distributed in the neighbourhood of the Galactic centre. We model the two spectral features using accurate extinction cross sections of silicates, that we have measured at the synchrotron facility Soleil, France. Near the Galactic centre magnesium and silicon show abundances similar to the solar ones and they are highly depleted from the gas phase ($delta_{rm{Mg}}>0.90$ and $delta_{rm{Si}}>0.96$). We find that amorphous olivine with a composition of $rm MgFeSiO_{4}$ is the most representative compound along all lines of sight according to our fits. The contribution of Mg-rich silicates and quartz is low (less than $10%$). On average we observe a percentage of crystalline dust equal to $11%$. For the extragalactic source LMC X-1, we find a preference for forsterite, a magnesium-rich olivine. Along this line of sight we also observe an underabundance of silicon $A_{rm Si}/A_{rm LMC} = 0.5pm0.2$.
The X-ray regime is a largely underused resource for constraining interstellar dust grain models and improving our understanding of the physical processes that dictate how grains evolve over their lifetimes. This is mostly due to current detectors relatively low sensitivity and high background, limiting the targets to the brightest sources. The improved sensitivity of the next generation of X-ray detectors will allow studies of much fainter sources, at much higher angular resolution, expanding our sampled sightlines in both quality and quantity.
We have analyzed the atomic and molecular gas using the 21 cm HI and 2.6/1.3 mm CO emissions toward the young supernova remnant (SNR) RCW 86 in order to identify the interstellar medium with which the shock waves of the SNR interact. We have found an HI intensity depression in the velocity range between $-46$ and $-28$ km s$^{-1}$ toward the SNR, suggesting a cavity in the interstellar medium. The HI cavity coincides with the thermal and non-thermal emitting X-ray shell. The thermal X-rays are coincident with the edge of the HI distribution, which indicates a strong density gradient, while the non-thermal X-rays are found toward the less dense, inner part of the HI cavity. The most significant non-thermal X-rays are seen toward the southwestern part of the shell where the HI gas traces the dense and cold component. We also identified CO clouds which are likely interacting with the SNR shock waves in the same velocity range as the HI, although the CO clouds are distributed only in a limited part of the SNR shell. The most massive cloud is located in the southeastern part of the shell, showing detailed correspondence with the thermal X-rays. These CO clouds show an enhanced CO $J$ = 2-1/1-0 intensity ratio, suggesting heating/compression by the shock front. We interpret that the shock-cloud interaction enhances non-thermal X-rays in the southwest and the thermal X-rays are emitted by the shock-heated gas of density 10-100 cm$^{-3}$. Moreover, we can clearly see an HI envelope around the CO cloud, suggesting that the progenitor had a weaker wind than the massive progenitor of the core-collapse SNR RX J1713.7$-$3949. It seems likely that the progenitor of RCW 86 was a system consisting of a white dwarf and a low-mass star with low-velocity accretion winds.
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