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DOE OSTI2024

Cosmic-Ray Feedback on Bistable Interstellar Medium Turbulence

Abstract While cosmic rays ( E ≳ 1 GeV) are well coupled to a galaxy’s interstellar medium (ISM) at scales of L > 100 pc, adjusting stratification and driving outflows, their impact on small scales is less clear. Based on calculations of the cosmic-ray diffusion coefficient from observations of the grammage in the Milky Way, cosmic rays have little time to dynamically impact the ISM on those small scales. Using numerical simulations, we explore how more complex cosmic-ray transport could allow cosmic rays to couple to the ISM on small scales. We create a two-zone model of cosmic-ray transport, with the cosmic-ray diffusion coefficient set at the estimated Milky Way value in cold gas but smaller in warm gas. We compare this model to simulations with a constant diffusion coefficient. Quicker diffusion through cold gas allows more cold gas to form compared to a simulation with a constant, small diffusion coefficient. However, slower diffusion in warm gas allows cosmic rays to take energy from the turbulent cascade anisotropically. This cosmic-ray energization comes at the expense of turbulent energy which would otherwise be lost during radiative cooling. Finally, we show our two-zone model is capable of matching observational estimates of the grammage for some transport paths through the simulation.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1975

UV stars and the interstellar medium - A statistical time-dependent model

A statistical time-dependent model for the effects on the interstellar gas of the ionizing radiation from UV stars is presented. The radiation from the stars is assumed to be emitted as monochromatic bursts at random times and locations. The thermal and ionization history of the gas is followed in a Monte Carlo simulation. Probability densities as well as mean values for the ionization fraction, temperature, and other observable quantities are derived. The fluctuations in observed quantities due to the stochastic nature of the model are estimated. Models of the interstellar medium perpendicular to the galactic plane are developed and compared with observations. The action of the UV stars can explain the bulk properties of the interstellar gas toward the galactic poles and is consistent with the Copernicus satellite observations toward lambda Scorpii.

Lyon, J.↗

NASA NTRS1993

Detection of the Carbon Monoxide Ion (CO+) in the Interstellar Medium and a Planetary Nebula

We report detection of the carbon monoxide ion (CO+) in the interstellar medium (Ml7SW) and a planetary nebula (NGC 7027). These detections are based on observations of three millimeter and submillimeter transitions in M17SW and one in NGC 7027. Chemical models suggest that CO+ should be most abundant where complex molecules are least likely to be present. In our search for CO+ we therefore minimized the chance of confusion while maximizing the probability of detection by observing regions whose chemistry is dominated by the effects of ultraviolet radiation.

Latter, William B.↗

NASA NTRS1995

The Galactic Distribution of Aliphatic Hydrocarbons in the Diffuse Interstellar Medium

The infrared absorption feature near 2950/ cm (3.4 microns), characteristic of dust in the diffuse interstellar medium (ISM), is attributed to C-H stretching vibrations of aliphatic hydrocarbons. We show here that the strength of the band does not scale linearly with visual extinction everywhere, but instead increases more rapidly for objects near the center of the Galaxy, a behavior that parallels that of the Si-O stretching band due to silicate materials in the diffuse ISM. This implies that the grains responsible for the diffuse medium aliphatic C-H and silicate Si-O stretching bands are different from those responsible for much of the observed visual extinction. It also suggests that the distribution of the carbonaceous component of the diffuse ISM is not uniform throughout the Galaxy, but instead may increase in density toward the center of the Galaxy. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of silicate-core, organic-mantle grains. Several possible models of the distribution of this material are presented and it is demonstrated that the inner parts of the Galaxy has a carrier density that is 5 to 35 times higher than in the local ISM. Depending on the model used, the density of aliphatic material in the local ISM is found to be about 1 to 2 -CH3 groups /cc and about 2 to 5 -CH2- groups/cc. These densities are consistent with the strengths of the 2955 and 2925/ cm (3.38 and 3.42 microns) subfeatures (due to -CH3 and -CH2- groups, respectively) within the overall 2950/ cm (3.4 microns) band being described by the relations A(sub upsilon)/tau(sub 2925/cm) = 270 +/- 40 and A(sub upsilon)/tau(sub 2925/cm) = 250 +/- 40 in the local diffuse ISM.

Sandford, Scott A.↗

NASA NTRS1985

Probing the local interstellar medium

A sophisticated model of solar wind expansion is applied to deduce a range of parameters for the local interstellar medium that predicts a location for the heliospheric shock of about 30 AU. It is found that either the interstellar magnetic field is more than double the presently accepted value of 0.3 nT, or the pressure due to galactic cosmic rays with energies near 0.1 MeV is that obtained by simple extrapolation of the observed flux at higher energies inside the heliosphere. Alternatively, some combination of these two external effects yields an effective interstellar pressure approximately quadruple present estimates.

Suess, S. T.↗

NASA NTRS1993

Hydrodynamical simulations of star-gas interactions in the interstellar medium with an external gravitational potential

We have calculated 2D models for the global structure of the interstellar medium in a disk galaxy where stars and gas are coupled through star formation, mass loss, stellar heating of the gas, and optically thin radiative cooling. In one set of models, the two dimensions are in the plane of a disk, and for one case, the total size of the region is 1 kpc square. Using a more accurate numerical approach, we confirm the results of Chiang and Bregman (1988) that H I forms into filaments or sheets separated by hotter, more diffuse gas. Although the filaments often have denser subregions, individual clouds of neutral gas are rare. We calculated the vertical distribution of the gas in another set of models where one dimension was in the plane and the other was perpendicular to it. A filament and hot gas network still occurs, although dense neutral complexes are more common and occur near the midplane. The hot gas often cools as it rises, leading to the highest velocity material having both positive and negative velocities. We find other features that are qualitatively similar to observations; for instance, H I 'worms' are produced.

Rosen, Alexander↗

NASA NTRS1995

The galactic distribution of aliphatic hydrocarbons in the diffuse interstellar medium

The infrared absorption feature near 2950(exp -1) (3.4 micron), characteristic of dust in the diffuse interstellar medium (ISM), is attributed to C-H stretching vibrations of aliphatic hydrocarbons. We show here that the strength of the band does not scale linearly with visual extinction everywhere, but instead increases more rapidly for objects near the center of the Galaxy, a behavior that parallels that of the Si-O stretching band due to silicate materials in the diffuse ISM. This implies that the grains responsible for the diffuse medium aliphatic C-H and silicate Si-O stretching bands are different from those responsible for much of the observed visual extinction. It also suggests that the distribution of the carbonaceous component of the diffuse ISM is not uniform throughout the Galaxy, but instead may increase in density toward the center of the Galaxy. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of silicate-core, organic-mantle grains. Several possible models of the distribution of this material are presented and it is demonstrated that the inner parts of the Galaxy has a carrier density that is 5 to 35 times higher than in the local ISM. Depending on the model used, the density of aliphatic material in the local ISM is found to be about 1 to 2 -CH3 groups m(exp -3) and about 2 to 5 -CH2- groups m(exp -3). These densities are consistent with the strengths of the 2955 and 2925 cm(exp -1) (3.4 micron) band being described by the relations A(sub nu)/tau(sub 2955 cm(exp -1)) = 270 +/- 40 and A(sub nu)/tau(sub 2925 cm(exp -1)) = 250 +/- 40 in the local diffuse ISM.

Sandford, Scott A.↗

NASA NTRS1987

Supernovae and the interstellar medium

Repeated supernovae from an OB association will, in a few 10s of Myr, create a cavity of coronal gas in the interstellar medium, with radius greater than 100 pc, surrounded by a dense expanding shell of cool interstellar gas. Such a cavity will likely burst through the gas layer of a disk galaxy. Such holes and 'supershells' have been observed in optical and H I radio emission maps of the Galaxy and other nearby galaxies. The gas swept up in supershell is likely to become gravitationally unstable, providing a mechanism for propagating star formation that may be particularly effective in irregular galaxies.

Mccray, Richard↗

NASA NTRS1982

Radio emission from supernova remnants in a cloudy interstellar medium

The van der Laan (1962) theory of SNR radio emission is modified in light of the inhomogeneity of the interstellar medium, and in order to allow for particle acceleration in shock fronts. It is proposed that most of the radio emission in 10-20 pc radius SNRs originates in cold interstellar clouds that have been crushed by the high pressure hot gas within the expanding remnant. Under these circumstances, simple reacceleration of ambient interstellar cosmic ray electrons can account for the surface brightness-diameter distribution of observed remnants, with the additional, relativistic particle energy compensating for the decreased filling factor of the radio-emitting regions. Warm interstellar gas, at about 8000 K, may also be compressed within very large SNRs (of radius of 30-100 pc) and account for both the giant radio loops, when these SNRs are seen individually, and the anomalously bright galactic nonthermal radio background, which may be the superposition of a number of such features.

Blandford, R. D.↗

NASA NTRS1997

The Diffuse Interstellar Medium

A major component of this research is to extend existing models of thermal processes in the local interstellar medium to the inner and outer Galaxy. In completing this goal we have calculated the thermal equilibrium gas temperature of the neutral diffuse gas and constructed phase diagrams (gas pressure versus density) for gas at galactic radii between 3 and 18 kpc. An important ingredient in this computation is the far-ultraviolet (FUV) radiation field in the Galactic disk. This radiation is important since photoelectric heating via FUV radiation on dust grains is expected to be a dominant heating process in the diffuse via gas. As a check of our calculated FUV field, we compared the infrared luminosity predicted by our theory with the observations of the COBE DIRBE space satellite and found the two to be in quite good agreement. Using our phase diagrams we have predicted the thermal pressure in the Galactic plane for which a multiphase equilibrium can be maintained. In addition, by using the maximum thermal pressure allowed by the observations, we have constrained the Galactic radii over which both cold and warm gas must exist, may exist, and cannot exist.

Hollenbach, David↗

NASA NTRS2011

Initiating Molecular Growth in the Interstellar Medium via Dimeric Complexes of Observed Ions and Molecules

A feasible initiation step for particle growth in the interstellar medium (ISM) is simulated by means of ab quantum chemistry methods. The systems studied are dimer ions formed by pairing nitrogen containing small molecules known to exist in the ISM with ions of unsaturated hydrocarbons or vice versa. Complexation energies, structures of ensuing complexes and electronic excitation spectra of the encounter complexes are estimated using various quantum chemistry methods. Moller-Plesset perturbation theory (MP2, Z-averaged perturbation theory (ZAP2), coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)), and density functional theory (DFT) methods (B3LYP) were employed along with the correlation consistent cc-pVTZ and aug-cc-pVTZ basis sets. Two types of complexes are predicted. One type of complex has electrostatic binding with moderate (7-20 kcal per mol) binding energies, that are nonetheless significantly stronger than typical van der Waals interactions between molecules of this size. The other type of complex develops strong covalent bonds between the fragments. Cyclic isomers of the nitrogen containing complexes are produced very easily by ion-molecule reactions. Some of these complexes show intense ultraviolet visible spectra for electronic transitions with large oscillator strengths at the B3LYP, omegaB97, and equations of motion coupled cluster (EOM-CCSD) levels. The open shell nitrogen containing carbonaceous complexes especially exhibit a large oscillator strength electronic transition in the visible region of the electromagnetic spectrum.

Bera, Partha P.↗

NASA NTRS1984

Future Studies of the Local Interstellar Medium with Space Telescope and Columbus

The spectrographs aboard Space Telescope and Columbus which will provide important new information about the interstellar medium in the immediate vicinity of the sun are described. The space telescope's highest resolution is adequate to define the multicomponent nature of interstellar absorption lines and to measure thermal line widths exceeding 3 km/s. The Columbus spacecraft will contain spectrographs capable of resolutions of 3 x 10 to the 4th power between 912 and 1200 A and 500 between 100 and 900 A. In the short wavelength region, lines of He I and II, are observable. If the 3 x 10 to the 4th power resolution spectrograph provides extended wavelength coverage to 770 A, lines of Ne VIII which are expected from 8 x 10 to the 5th power K gas are accessible. The ST HRS and Columbus spectrographs enable the study of a wide range of problems relating to cold, warm, and hot gas in the local ISM.

Savage, B. D.↗

NASA NTRS1996

An MHD Study of the Interaction Between the Solar Wind and the Interstellar Medium

The overall objective of this research program is to obtain a better understanding of the interaction between the solar wind and the interstellar medium through the use of numerical solutions of the time-dependent magnetohydrodynamic (MHD) equations. The simulated results have been compared with observations where possible and with the results from previous analytic and numerical studies. The primary accomplishment of this project has been the development of codes for 2-D models in both spherical and cylindrical coordinates and the application of the codes to the solar wind/interstellar medium interaction. Computations have been carried out for both a relatively simple gas-dynamic interaction and a flow-aligned interstellar magnetic field. The results have been shown to compare favorably with models that use more approximations and to modify and extend the previous results as would be expected. The simulations have also been used along with a data analysis study to provide a quantitative estimate of the distance to the termination and bow shocks. Some of the specific topics that have been studied are: (1) gas dynamic models of the solar wind/interstellar medium interaction, (2) termination shock response to large-scale solar wind fluctuations, and (3) distances to the termination shock and heliopause. The main results from each of these studies are summarized. The results were published in three papers which are included as attachments.

Steinolfson, R. S.↗

NASA NTRS1994

Interstellar medium continuum, autoionization, and line absorption in the extreme ultraviolet

We provide a new estimate of the effective absorption cross section of the interstellar medium at extreme ultraviolet wavelengths. The cross section for neutral helium is discussed in detail. Two spectral features of interstellar neutral helium, the 504 A ionization edge and the autoionization resonance feature at 206 A, are detectable with existing instrumentation. Measurements of the autoionization feature in continuum sources will provide a direct estimate of the intervening neutral helium column; in many cases this will be the only method to obtain this parameter. Although continuum metal opacities are negligible at wavelengths greater than 50 A we find that metal line features should be detectable in continuum sources with high signal to noise observations. A selection of some of the most prominent lines expected is provided.

Rumph, Todd↗

NASA NTRS1972

Low intensity H-beta emission from the interstellar medium

Pulsar pulse dispersions and low-frequency absorption of galactic and extragalactic radio sources strongly suggest that the interstellar medium is much more ionized than previously assumed. The search was confined to directions near pulsars because of the additional information provided by the dispersion measure which gives the total number of electrons along the line of sight to the pulsar. Of the four directions in which observations were made, an emission line appears to be present in at least two and possibly three directions. The data are shown from a low galactic latitude direction near the Crab Nebula pulsar. The observing direction was about 9 arcmin off the Crab with the field of view of about 1.5 arcmin. The number of counts was plotted versus the local standard of rest velocity and local standard wavelength of H-beta.

Reynolds, R. J.↗

NASA NTRS1985

Local interstellar medium and gamma-ray astronomy

The recent improvement of the calibration of the galaxy counts used as an interstellar absorption tracer modifies significantly the picture of the local interstellar medium (ISM). Consequently, previous analyses of the gamma ray emission from the local ISM involving galaxy counts have to be revised. The implications regarding the cosmic ray (CR) density in the local ISM are considered and in particular within Loop I, a nearby supernova remnant (SNR).

Lebrun, F.↗

NASA NTRS2002

Observations of O (VI) Emission from the Diffuse Interstellar Medium Toward l =113 deg b = 71 deg

Emission from 0 VI (lambda lambda 1032, 1038) in the diffuse interstellar medium was observed in two, long Fur Ultra-violet Spectroscopic Explorer (FUSE) observations at l = approximately equals 1l3 degrees .0 and b approximately equals 70 degrees .7 (near the quasar HS 1307+4617). The observed intensities are 2580 +/- 380 (random) +/- 360 (systematic) and 1100 +/- 160 (random) +/- 160 (systematic) photons cm(exp -2)/s s/r in the 1032 and 1038 A emission lines, respectively. The electron density, thermal pressure, and depth of the emitting gas are calculated from the observed intensities. The velocity of the emitting material is about +10 km/s relative to the local standard of rest and less than + 30 km/s relative to the H I along the line of sight. The similarity suggests that the highly ionized gas is quiescent and not recently shock heated. Emission from the C 11 3 s(exp 2) S(sub 1/2) to 2p(exp 2) P(sub 3/2) transition at 1037 A is also observed, and upper limits are placed on the intensities of ultraviolet line emission from C I, C 111, N I, N 11, Mg 11, Si 11, S 11, S 111. S IV, S VI. Fe 11, and Fe 111.

Shelton, R. L.↗

NASA NTRS2013

Photoionization Modeling of Oxygen K Absorption in the Interstellar Medium: The Chandra Grating Spectra of XTE J1817-330

We present detailed analyses of oxygen K absorption in the interstellar medium (ISM) using four high-resolution Chandra spectra towards the X-ray low-mass binary XTE J1817-330. The 11-25 A broadband is described with a simple absorption model that takes into account the pileup effect and results in an estimate of the hydrogen column density. The oxygen K-edge region (21-25 A) is fitted with the physical warmabs model, which is based on a photoionization model grid generated with the XSTAR code with the most up-to-date atomic database. This approach allows a benchmark of the atomic data which involves wavelength shifts of both the K lines and photoionization cross sections in order to fit the observed spectra accurately. As a result we obtain: a column density of N(sub H) = 1.38 +/- 0.01 x 10(exp 21) cm(exp -2); ionization parameter of log xi = .2.70 +/- 0.023; oxygen abundance of A(sub O) = 0.689(exp +0.015./-0.010); and ionization fractions of O I/O = 0.911, O II/O = 0.077, and O III/O = 0.012 that are in good agreement with previous studies. Since the oxygen abundance in warmabs is given relative to the solar standard of Grevesse and Sauval (1998), a rescaling with the revision by Asplund et al. (2009) yields A(sub O) = 0.952(exp +0.020/-0.013, a value close to solar that reinforces the new standard. We identify several atomic absorption lines.K-alpha , K-beta, and K-gamma in O I and O II; and K-alpha in O III, O VI, and O VII--last two probably residing in the neighborhood of the source rather than in the ISM. This is the first firm detection of oxygen K resonances with principal quantum numbers n greater than 2 associated to ISM cold absorption.

Gatuzz, E.↗