A study of gas-phase element abundances reported in the literature for 17 different elements sampled over 243 sight lines in the local part of our Galaxy reveals that the depletions into solid form (dust grains) are extremely well characterized by trends that employ only three kinds of parameters. One is an index that describes the overall level of depletion applicable to the gas in any particular sight line, and the other two represent linear coefficients that describe how to derive each element's depletion from this sight-line parameter. The information from this study reveals the relative proportions of different elements that are incorporated into dust at different stages of grain growth. An extremely simple scheme is proposed for deriving the dust contents and metallicities of absorption-line systems that are seen in the spectra of distant quasars or the optical afterglows of gamma-ray bursts. Contrary to presently accepted thinking, the elements sulfur and krypton appear to show measurable changes in their depletions as the general levels of depletions of other elements increase, although more data are needed to ascertain whether or not these findings are truly compelling. Nitrogen appears to show no such increase. The incorporation of oxygen into solid form in the densest gas regions far exceeds the amounts that can take the form of silicates or metallic oxides; this conclusion is based on differential measurements of depletion and thus is unaffected by uncertainties in the solar abundance reference scale.

https://iopscience.iop.org/article/10.1088/0004-637X/700/2/1299/pdf

A unified representation of gas-phase element depletions in the interstellar medium*

A study of gas-phase element abundances reported in the literature for 17 different elements sampled over 243 sight lines in the local part of our Galaxy reveals that the depletions into solid form (dust grains) are extremely well characterized by trends that employ only three kinds of parameters. One is an index that describes the overall level of depletion applicable to the gas in any particular sight line, and the other two represent linear coefficients that describe how to derive each element's depletion from this sight-line parameter. The information from this study reveals the relative proportions of different elements that are incorporated into dust at different stages of grain growth. An extremely simple scheme is proposed for deriving the dust contents and metallicities of absorption-line systems that are seen in the spectra of distant quasars or the optical afterglows of gamma-ray bursts. Contrary to presently accepted thinking, the elements sulfur and krypton appear to show measurable changes in their depletions as the general levels of depletions of other elements increase, although more data are needed to ascertain whether or not these findings truly compelling. Nitrogen appears to show no such increase. The incorporation of oxygen into solid form in the densest gas regions far exceeds the amounts that can take the form of silicates or metallic oxides; this conclusion is based on differential measurements of depletion and thus is unaffected by uncertainties in the solar abundance reference scale.

A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium

Context. Our position inside the Galaxy requires 3D-modelling to obtain the distribution of the Galactic magnetic field, cosmic- ray (CR) electrons and thermal electrons. Aims. Our intention is to find a Galactic 3D-model which agrees best with available radio observations. Methods. We constrain simulated all-sky maps in total intensity, lin ear polarization, and rotation measure (RM) by observations. For the simulated maps as a function of frequency we integrate in 15 ′ wide cones the emission along the line of sight calculated from Galactic 3D-models. We test a number of large-scale magnetic field configurations and take the properties of the warm inte rstellar medium into account. Results. From a comparison of simulated and observed maps we are able to constrain the regular large-scale Galactic magnetic field in the disk and the halo of the Galaxy. The local regular fi eld is 2� G and the average random field is about 3 � G. The known local excess of synchrotron emission originating either fr om enhanced CR electrons or random magnetic fields is able to e xplain the observed high-latitude synchrotron emission. The thermal electron model (NE2001) in conjunction with a proper fill ing factor accounts for the observed optically thin thermal emission and low frequency absorption by optically thick emission. A coupling factor between thermal electrons and the random magnetic field comp onent is proposed, which in addition to the small filling fact or of thermal electrons increases small-scale RM fluctuations an d thus accounts for the observed depolarization at 1.4 GHz. Conclusions. We conclude that an axisymmetric magnetic disk field configur ation with reversals inside the solar circle fits available observations best. Out of the plane a strong toroidal magnetic field with di fferent signs above and below the plane is needed to account for the observed high-latitude RMs. The large field strength is a consequence of the small thermal electron scale height of 1 kpc, which also limits the CR electron extent up to a height of 1 kpc not to contradict with the observed synchrotron emission out of the plane. Our preferred 3D-model fits the observed Galactic total intensi ty and polarized emission better than other models over a wide frequency range and also agrees with the observed RM from extragalactic sources.

/pdf/radio-observational-constraints-on-galactic-3d-emission-245m3npx21.pdf

Radio observational constraints on Galactic 3D-emission models

Jeffries & Binks (2014) and Malo et al. (2014) have recently reported Li depletion boundary (LDB) ages for the {\beta} Pictoris moving group (BPMG) which are twice as old as the oft-cited kinematic age of $\sim$12 Myr. In this study we present (1) a new evaluation of the internal kinematics of the BPMG using the revised Hipparcos astrometry and best available published radial velocities, and assess whether a useful kinematic age can be derived, and (2) derive an isochronal age based on the placement of the A-, F- and G-type stars in the colour-magnitude diagram (CMD). We explore the kinematics of the BPMG looking at velocity trends along Galactic axes, and conducting traceback analyses assuming linear trajectories, epicyclic orbit approximation, and orbit integration using a realistic gravitational potential. None of the methodologies yield a kinematic age with small uncertainties using modern velocity data. Expansion in the Galactic X and Y directions is significant only at the 1.7{\sigma} and 2.7{\sigma} levels, and together yields an overall kinematic age with a wide range (13-58 Myr; 95 per cent CL). The A-type members are all on the zero age-main-sequence, suggestive of an age of $>$20Myr, and the loci of the CMD positions for the late-F- and G-type pre-main-sequence BPMG members have a median isochronal age of 22 Myr ($\pm$ 3 Myr stat., $\pm$ 1 Myr sys.) when considering four sets of modern theoretical isochrones. The results from recent LDB and isochronal age analyses are now in agreement with a median BPMG age of 23 $\pm$ 3 Myr (overall 1{\sigma} uncertainty, including $\pm$2 Myr statistical and $\pm$2 Myr systematic uncertainties).

On the age of the β Pictoris moving group

The Solar System is embedded in a flow of low-density, warm, and partially ionized interstellar material that has been sampled directly by in situ measurements of interstellar neutral gas and dust in the heliosphere. Absorption line data reveal that this interstellar gas is part of a larger cluster of local interstellar clouds, which is spatially and kinematically divided into additional small-scale structures indicating ongoing interactions. An origin for the clouds that is related to star formation in the Scorpius-Centaurus OB association is suggested by the dynamic characteristics of the flow. Variable depletions observed within the local interstellar medium (ISM) suggest an inhomogeneous Galactic environment, with shocks that destroy grains in some regions. Although photoionization models of the circumheliospheric ISM do an excellent job of reproducing the observed properties of the surrounding ISM, the unknown characteristics of the very low-density hot plasma filling the Local Bubble introduces uncertainty about the source of ionization and nature of cloud boundaries. Recent observations of small cold clouds provide new insight into the processes affecting the local ISM. A fuller understanding of the local ISM can provide insights into the past and future Galactic environment of the Sun, and deeper knowledge of the astrospheres of nearby stars.

/pdf/the-interstellar-medium-surrounding-the-sun-2jc6zidxuv.pdf

The Interstellar Medium Surrounding the Sun

We present anew unbiasedsearchand analysisofall B starsin the solarneighbourhood(within a volume of 400 pc diameter) using the Arivel data base to track down theremains of the OB associations, which hosted the supernovae responsible for the LocalBubble in the interstellar gas. We ﬁnd after careful dereddening and by comparisonwith theoretical isochrones, that besides the Upper Scorpius the Upper CentaurusLupus and Lower Centaurus Crux subgroups are the youngest stellar associations inthe solar neighbourhood with ages of 20 to 30 Myr, in agreement with previous work.In search for the “smoking gun” of the origin of the Local Bubble, we have traced thepaths of the associations back into the past and found that they entered the presentbubble region 10 to 15 Myr ago. We argue that the Local Bubble began to form thenand estimate that 14 to 20 supernovae have gone oﬀ since. It is shown that the impliedenergy input is suﬃcient to excavate a bubble of the presently observed size.Key words: solar neighbourhood - open clusters and associations: individual:ScoOB2 - ISM: individual: local bubble - Local ISM

/pdf/the-search-for-the-origin-of-the-local-bubble-redivivus-5dbn7mqmqr.pdf

The search for the origin of the Local Bubble redivivus

Context. The Local and Loop I superbubbles are the closest and best investigated supernova (SN) generated bubbles and serve as test laboratories for observations and theories of the interstellar medium. Aims. Since the morphology and dynamical evolution of bubbles depend on the ambient density and pressure distributions, a realistic modelling of the galactic environment is crucial for a detailed comparison with observations. Methods. We have performed 3D high resolution (down to 1.25 pc on a kpc-scale grid) hydrodynamic simulations of the Local Bubble (LB) and the neighbouring Loop I (L1) superbubble in a realistically evolving inhomogeneous background ISM, disturbed already by SN explosions at the Galactic rate for 200 Myr before the LB and L1 are generated. The LB is the result of 19 SNe occurring in a moving group, which passed through the present day local Hi cavity. Results. We can reproduce (i) the Ovi column density in absorption within the LB in agreement with Copernicus and recent FUSE observations, giving NOV I < 2 × 10 13 cm 2 and NOV I < 7 × 10 12 cm 2 , respectively, (ii) the observed sizes of the Local and Loop I superbubbles, (iii) the interaction shell between LB and L1, discovered with ROSAT, (iv) constrain the age of the LB to be 14.5± 0.7 0.4 Myr, (v) predict the merging of the two bubbles in about 3 Myr, when the interaction shell starts to fragment, (vi) the generation of blobs like the Local Cloud as a consequence of a dynamical instability. Conclusions. We find that evolving superbubbles strongly deviate from idealised self-similar solutions due to ambient pressure and density gradients, as well as due to turbulent mixing and mass loading. Hence, at later times the hot interior can break through the surrounding shell, which may also help to explain the puzzling energy “deficit” observed in LMC bubbles.

/pdf/the-history-and-future-of-the-local-and-loop-i-bubbles-5ayotx47dt.pdf

The History and Future of the Local and Loop I Bubbles

Until recently the dynamical evolution of the interstellar medium (ISM) was simulated using collisional ionization equilibrium (CIE) conditions. However, the ISM is a dynamical system, in which the plasma is naturally driven out of equilibrium due to atomic and dynamic processes operating on different timescales. A step forward in the field comprises a multi-fluid approach taking into account the joint thermal and dynamical evolutions of the ISM gas.

ISM Simulations: An Overview of Models

So far our understanding of the evolution of the ISM has been scanty, because of the inherent nonlinearity of all the processes involved. Modelling the interstellar medium in galaxies in a self-consistent way requires an approach that must take into account the relevant scales, which may cover several orders of magnitude ( e.g. , from kpc to less than 1 pc). This is a difficult task that requires the use of sophisticated numerical codes, adequate computing power, and precision input data from observations. The large range of scales can be resolved by means of adaptive mesh refinement (AMR), that is, the grids reproducing the computational domain are refined on the fly such that a minimum number of cells is needed to provide the most complete description of the flow. In this paper we review the most recent results on 3D modeling of the interstellar medium and disk-halo interaction in a section of the Milky Way that includes the Galactic magnetic field, background heating due to starlight, self-gravity and allows for the establishment of the duty-cycle between the disk and halo (commonly known as galactic fountain) by using a grid that extends up to 10 kpc on either side of the midplane. Our simulations capture both the largest structures ( e.g. , superbubbles) together with the smaller ones ( e.g. , filaments and eddies) down to 0.625 pc. We investigate, among other things, the variability of the magnetic field in the Galactic disk and its correlation with the density, the role of ram pressure in the dynamics of disk gas and the relative weight of the ram, thermal and magnetic pressures, the mass distribution and the volume filling factors of the different temperature regimes in the ISM, as well as the scales at which energy is injected into the interstellar turbulence and we give an estimate for the dimension of the most dissipative structures.

Modelling the Turbulent Interstellar Medium in Disk Galaxies Including the Disk-Halo Circulation

The observational and theoretical state of Galactic and extragalactic bubbles are reviewed. Observations of superbubbles are discussed, with some emphasis on nearby bubbles such as the Local Bubble (LB) and the Loop I superbubble (LI). Analytical bubble theory is revisited, and similarity solutions, including the time-dependent energy input by supernova explosions according to a Galactic initial mass function (IMF), are studied. Since the agreement with observations is not convincing in case of the LB, we present high resolution 3D AMR simulations of the LB and LI in an inhomogeneous background medium. It is demonstrated that both the morphology and recently published FUSE data on Ovi absorption line column densities can be well understood, if the LB is the result of about 20 supernova explosions from a moving group, and the LB age is about 14.7 Myrs.

M. A. de Avillez

Papers

The search for the origin of the Local Bubble redivivus

The History and Future of the Local and Loop I Bubbles

ISM Simulations: An Overview of Models

Modelling the Turbulent Interstellar Medium in Disk Galaxies Including the Disk-Halo Circulation

Keynote Lecture: Galactic and Extragalactic Bubbles