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DOE OSTI2015

Magnetized interstellar molecular clouds – I. Comparison between simulations and Zeeman observations

The most accurate measurements of magnetic fields in star-forming gas are based on the Zeeman observations analysed by Crutcher et al. Here, we show that their finding that the 3D magnetic field scales approximately as density 0.65 can also be obtained from analysis of the observed line-of-sight fields. We present two large-scale adaptive-mesh-refinement magnetohydrodynamic simulations of several thousand M ⊙ of turbulent, isothermal, self-gravitating gas, one with a strong initial magnetic field (Alfven Mach number $\mathcal{M}_{A,0}=1$) and one with a weak initial field ($\mathcal{M}_{A,0}=10$). We construct samples of the 100 most massive clumps in each simulation and show that they exhibit a power-law relation between field strength and density ($\bar{n}_H$) in excellent agreement with the observed one. Our results imply that the average field in molecular clumps in the interstellar medium (ISM) is $\langle B_{tot}(\bar{n}_H)\rangle ≈ 43\bar{n}^{0.65}_{H,4}μG$. Furthermore, the median value of the ratio of the line-of-sight field to density 0.65 in the simulations is within a factor of about (1.3, 1.7) of the observed value for the strong- and weak-field cases, respectively. The median value of the mass-to-flux ratio, normalized to the critical value, is 70 per cent of the line-of-sight value. This is larger than the 50 per cent usually cited for spherical clouds because the actual mass-to-flux ratio depends on the volume-weighted field, whereas the observed one depends on the mass-weighted field. Our results indicate that the typical molecular clump in the ISM is significantly supercritical (! factor of 3). The results of our strong-field model are in very good quantitative agreement with the observations of Li et al., which show a strong correlation in field orientation between small and large scales. Because there is a negligible correlation in the weak-field model, we conclude that molecular clouds form from strongly magnetized (although magnetically supercritical) gas, in agreement with the conclusion of Li et al.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2012

NO ICE HYDROGENATION: A SOLID PATHWAY TO NH{sub 2}OH FORMATION IN SPACE

Icy dust grains in space act as catalytic surfaces onto which complex molecules form. These molecules are synthesized through exothermic reactions from precursor radicals and, mostly, hydrogen atom additions. Among the resulting products are species of biological relevance, such as hydroxylamine-NH{sub 2}OH-a precursor molecule in the formation of amino acids. In this Letter, laboratory experiments are described that demonstrate NH{sub 2}OH formation in interstellar ice analogs for astronomically relevant temperatures via successive hydrogenation reactions of solid nitric oxide (NO). Inclusion of the experimental results in an astrochemical gas-grain model proves the importance of a solid-state NO+H reaction channel as a starting point for prebiotic species in dark interstellar clouds and adds a new perspective to the way molecules of biological importance may form in space.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1990

The formation and structure of circumstellar and interstellar dust

The intriguing abundance of long linear carbon chain molecules in some dark clouds and in circumstellar shells is still not well understood. Recent laboratory studies which have probed this problem indicate that when carbon vapor nucleates to form particles, linear chains and hollow cage molecules (fullerenes) also form at more-or-less the same time. The results have consequences for the formation, structures and spectroscopic properties of the molecular and dust components ejected from cool carbon-rich stars. A most interesting result of the experimental observations relates to the probability that a third character in addition to the chains and grains, the C(sub 60) molecule probably in the form of the ion C(sub 60)(sup +) in the less shielded regions, is present and perhaps responsible for some of the ubiquitously observed interstellar spectroscopic features such as the Diffuse Interstellar Features, the 2170A UV Absorption or perhaps some of the Unidentified Infrared Bands. Further study of small carbon particles which form in the gas phase has resulted in the discovery that they have quasi-icosahedral spiral shell structures. The role that such species may play in the interstellar medium as well as that played by C(sub 60) (or C sub 60 sup +) should soon be accessible to verification by a combination of laboratory experiment and astronomical spectroscopy.

Kroto, H. W.↗

NASA NTRS1990

A unifying picture of gas-phase formation and growth of PAH (Polycyclic Aromatic Hydrocarbons), soot, diamond and graphite

A variety of seemingly different carbon formation processes -- polycyclic aromatic hydrocarbons and diamond in the interstellar medium, soot in hydrocarbon flames, graphite and diamond in plasma-assisted-chemical vapor deposition reactors -- may all have closely related underlying chemical reaction mechanisms. Two distinct mechanisms for gas-phase carbon growth are discussed. At high temperatures it proceeds via the formation of carbon clusters. At lower temperatures it follows a polymerization-type kinetic sequence of chemical reactions of acetylene addition to a radical, and reactivation of the resultant species through H-abstraction by a hydrogen atom.

Frenklach, Michael↗

DOE OSTI2013

PROBING THE INTERSTELLAR MEDIUM OF z {approx} 1 ULTRALUMINOUS INFRARED GALAXIES THROUGH INTERFEROMETRIC OBSERVATIONS OF CO AND SPITZER MID-INFRARED SPECTROSCOPY

We explore the relationship between gas, dust, and star formation in a sample of 12 ultraluminous infrared galaxies (ULIRGs) at high-redshift compared to a similar sample of local galaxies. We present new CO observations and/or Spitzer mid-IR spectroscopy for six 70 {mu}m selected galaxies at z {approx} 1 in order to quantify the properties of the molecular gas reservoir, the contribution of an active galactic nucleus (AGN) to the mid-IR luminosity, and the star formation efficiency (SFE = L{sub IR}/L{sup '}{sub CO}). The mid-IR spectra show strong polycyclic aromatic hydrocarbon (PAH) emission, and our spectral decomposition suggests that the AGN makes a minimal contribution (<25%) to the mid-IR luminosity. The 70 {mu}m selected ULIRGs, which we find to be spectroscopic close pairs, are observed to have high SFE, similar to local ULIRGs and high-redshift submillimeter galaxies, consistent with enhanced IR luminosity due to an ongoing major merger. Combined with existing observations of local and high-redshift ULIRGs, we further compare the PAH, IR, and CO luminosities. We show that the ratio L{sub PAH,6.2}/L{sub IR} decreases with increasing IR luminosity for both local and high-redshift galaxies, but the trend for high-redshift galaxies is shifted to higher IR luminosities; the average L{sub PAH,6.2}/L{sub IR} ratio at a given L{sub IR} is {approx}3 times higher at high-redshift. When we normalize by the molecular gas, we find this trend to be uniform for galaxies at all redshifts and that the molecular gas is correlated with the PAH dust emission. The similar trends seen in the [C II] to molecular gas ratios in other studies suggests that PAH emission, like [C II], continues to be a good tracer of photodissociation regions even at high-redshift. Together the CO, PAH, and far-IR fine structure lines should be useful for constraining the interstellar medium conditions in high-redshift galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1984

The laboratory simulation of unmagnetized supernova remnants Absence of a blast wave

Supernova remnants are experimentally simulated by irradiating spherical targets with eight-beam carbon dioxide laser in a chamber containing finite amounts of neutral gas, the gas being ionized by radiation from the hot target. The expansion velocities of the target plasmas are approximately the same as the expansion velocities of supernova ejecta and the experiment is successfully scaled to the case of a supernova remnant in an unmagnetized, low-density, interstellar medium. No sweep-up of the ambient plasma is detected, indicating that no hydrodynamic shock wave is formed to couple the target ejecta to the ambient gas. The experiment implies that if supernova ejecta couple to the interstellar medium, magnetic-field effects may be crucial to the physical description.

Borovsky, J. E.↗

NASA NTRS1986

Studies of interstellar vibrationally-excited molecules

Several molecules thus far have been detected in the ISM in vibrationally-excited states, including H2, SiO, HC3N, and CH3CN. In order for vibrational-excitation to occur, these species must be present in unusually hot and dense gas and/or where strong infrared radiation is present. In order to do a more thorough investigation of vibrational excitation in the interstellar medium (ISM), studies were done of several mm-wave transitions originating in excited vibrational modes of HCN, an abundant interstellar molecule. Vibrationally-excited HCN was recently detected toward Orion-KL and IRC+10216, using a 12 meter antenna. The J=3-2 rotational transitions were detected in the molecule's lowest vibrational state, the bending mode, which is split into two separate levels, due to l-type doubling. This bending mode lies 1025K above ground state, with an Einstein A coefficient of 3.6/s. The J=3-2 line mode of HCN, which lies 2050K above ground state, was also observed toward IRC+10216, and subsequently in Orion-KL. Further measurements of vibrationally-excited HCN were done using a 14 meter telescope, which include the observations of the (0,1,0) and (0,2,0) modes towards Orion-KL, via their J=3-2 transitions at 265-267 GHz. The spectrum of the J=3-2 line in Orion taken with the 14 meter telescope, is shown, along with a map, which indicates that emission from vibrationally-excited HCN arises from a region probably smaller than the 14 meter telescope's 20 arcsec beam.

Ziurys, L. M.↗

NASA NTRS2006

Laboratory Spectroscopy of Large Carbon Molecules and Ions in Support of Space Missions

One of the major objectives of Laboratory Astrophysics is the optimization of data return from space missions by measuring spectra of atomic and molecular species in laboratory environments that mimic interstellar conditions (WhitePaper (2002, 2006)). Among interstellar species, PAHs are an important and ubiquitous component of carbon-bearing materials that represents a particularly difficult challenge for gas-phase laboratory studies. We present the absorption spectra of jet-cooled neutral and ionized PAHs and discuss the implications for astrophysics. The harsh physical conditions of the interstellar medium have been simulated in the laboratory. We are now, for the first time, in the position to directly compare laboratory spectra of PAHs and carbon nanoparticles with astronomical observations. This new phase offers tremendous opportunities for the data analysis of current and upcoming space missions geared toward the detection of large aromatic systems (HST/COS, FUSE, JWST, Spitzer).

Salana, Farid↗

NASA NTRS2005

Infrared Extinction and the Initial Conditions for Star and Planet Formation

This grant funded a research program to use infrared extinction measurements to probe the detailed structure of dark molecular clouds and investigate the physical conditions which give rise to star and planet formation. The goals of the this program were to: 1) acquire deep infrared and molecular-line observations of a carefully selected sample of nearby dark clouds, 2) reduce and analyze the data obtained in order to produce detailed extinction maps of the clouds, 3) use the results to measure and quantitatively describe the physical conditions of the dense gas and dust that produce stars and their accompanying planetary systems in molecular clouds. The goals of this project were met and exceeded as described below. 1) The infrared data for the project were obtained in a number of observing runs using the 3.5-meter NTT and 8-meter VLT telescopes of the European Southern Observatory in Chile and the 1.2-meter telescope of the Smithsonian Astrophysical Observatory in Arizona, the 1 0-meter Keck telescope in Hawaii, the 6.5-meter MMT of the Smithsonian Astrophysical Observatory in Arizona, and the NASA Hubble Space Telescope. The molecular-line data was obtained in three runs using the IRAM 30-meter telescope in Spain and one run with the ESO-15 meter millimeter-wave telescope in Chile. Millimeter-wave continuum measurements were obtained with the 15-meter JCMT in Hawaii. 2) Considerable effort was expended to reduce the infrared imaging observations including the development of custom software to produce high quality photometry and source astrometry. All the millimeter-line data was reduced using standard reduction routines. The highlights of the infrared analysis were the production of detailed extinction maps and the construction of profiles of the density structure of the B68, Coalsack, B335 and Lupus clouds. 3) The principal scientific accomplishments of this research program include the following: We were able to use our infrared observations to determine the density structure of the B68 cloud to an unprecedented level of precision. This lead to a major breakthrough in the study of molecular cloud structure. For the first time we have been able to characterize the structure of a dark cloud in a detail only exceeded by that known for a star. We determined that the cloud's structure is exquisitely well described by the equations of a Bonner-Ebert sphere (a pressure confined isothermal sphere). We were able to show that the cloud is very nearly in equilibrium with the internal thermal pressure of the cloud balancing gravity and the external pressure of the surrounding interstellar medium. We were able to determine for the first time the gas-to-dust ratio in a dense cloud core. We also demonstrated a new method to determine extremely precise distances to such clouds by combining knowledge of the properties of Bonner-Ebert Spheres with our infrared and millimeter-wave observations.

Lada, Charles J.↗

NASA NTRS1991

Energy deposition by X-ray photoelectrons into interstellar molecular clouds

The deposition of energy by 100-10,000 eV electrons into molecular hydrogen and helium gas of cosmic abundances and fractional ionization ranging from 10 to the -12th to 0.1 is considered. In order to account for energy deposition by both discrete and continuous degradation processes, the loss spectrum, a form of the Spencer-Fano degradation spectrum particularly suited to describing electron energy deposition in a partially ionized gas is considered. The calculated excitation and ionization yields drop as the fractional ionization increases. The dependences of these yields on fractional ionization are fitted to a form suitable for swift numerical computation.

Voit, G. M.↗

NASA NTRS2000

Vertical Shear of the Galactic Interstellar Medium

The detection of UV absorption, 21 cm, H alpha and other diffuse optical emission lines from gas up to ten kiloparsecs above the plane of the Milky Way and other galaxies provides the first, opportunity to probe the rotational properties of the ionized "atmospheres" of galaxies. This rotation has implications for our understanding of the Galactic gravitational potential, angular momentum transport in the Galactic disk, and the maintenance of a Galactic dynamo. The available evidence indicates that gas rotates nearly cylindrically up to a few kiloparsecs. This is in contrast to the expectation that there should be a significant gradient in rotation speed as a function of height assuming a reasonable mass model for the Galaxy. For example, for a vertical cut at galactocentric radius R = 5 kpc in NGC 891 by Rand, the rotation speed is observed to drop by approximately 30 kilometers per second from z = 1 to 5 kpc and is expected to drop by 80 kilometers per second. Magnetic tension forces may resolve this discrepancy. Other possibilities will be examined in the near future.

Benjamin, Robert A.↗

NASA NTRS2002

13-kV Ion-Extraction System Being Developed for Inert Gas Ion Engines

A high-voltage ion optics design was chosen for an assumed outer planet or interstellar precursor mission that would require a long-life, high-power, high-specific-impulse krypton ion engine. Such an engine could support energetic space missions to the outer planets or beyond. Detailed performance and lifetime analyses and several inexpensive subscale grid tests were conducted at the NASA Glenn Research Center and at the Colorado State University under a NASA Glenn grant. A subscale grid set of the selected geometry shown was tested at voltages up to 13,000 V. This yielded a krypton ion beam current that would, when scaled to a full-size 50-cm diameter, produce an ion beam with a power of 30 kW at a specific impulse over 14,000 sec. The operational ion beam focusing limits, as a function of ion current per hole, were found to impose requirements of high uniformity on the discharge chamber plasma density. A full-size set of two-grid, 50-cm-diameter titanium ion optics has been fabricated and awaits testing.

Rawlin, Vincent K.↗

DOE OSTI2012

HOT AND COLD GALACTIC GAS IN THE NGC 2563 GALAXY GROUP

The role of environmentally induced gas stripping in driving galaxy evolution in groups remains poorly understood. Here we present extensive Chandra and Very Large Array mosaic observations of the hot and cold interstellar medium within the members of the nearby, X-ray bright NGC 2563 group, a prime target for studies of the role of gas stripping and interactions in relatively small host halos. Our observations cover nearly all group members within a projected radius of 1.15 Mpc ({approx}1.4 R{sub vir}) of the group center, down to a limiting X-ray luminosity and H I mass of 3 Multiplication-Sign 10{sup 39} erg s{sup -1} and 2 Multiplication-Sign 10{sup 8} M{sub Sun }, respectively. The X-ray data are consistent with efficient ram pressure stripping of the hot gas halos of early-type galaxies near the group core, but no X-ray tails are seen and the limited statistics preclude strong conclusions. The H I results suggest moderate H I mass loss from the group members when compared to similar field galaxies. Six of the 20 H I-detected group members show H I evidence of ongoing interactions with other galaxies or with the intragroup medium. Suggestive evidence is further seen for galaxies with close neighbors in position-velocity space to show relatively low H I content, consistent with tidal removal of H I. The results thus indicate removal of both hot and cold gas from the group members via a combination of ram pressure stripping and tidal interactions. We also find that 16 of the 20 H I detections occur on one side of the group, reflecting an unusual morphological segregation whose origin remains unclear.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

A chemical dynamics study on the gas-phase formation of triplet and singlet C 5 H 2 carbenes

Significance Carbenes represent key reactive intermediates in molecular mass growth processes leading to carbonaceous nanostructures in the interstellar medium and in combustion systems. However, due to their short lifetimes and tendency for dimerization, carbenes represent one of the foremost obscured classes of reactive intermediates, with the preparation of carbenes in the gas phase remaining largely elusive. By merging molecular beams with electronic structure and quasi-classical trajectory calculations and exploiting triplet pentadiynylidene (HCCCCCH) and singlet ethynylcyclopropenylidene (c-C 5 H 2 ) carbene as benchmarks, we present a versatile protocol to unravel the dynamics leading to exotic carbenes under single-collision conditions. These elementary mechanisms are of significance to our understanding of the fundamental processes leading to unsaturated organic transients in extreme, hydrocarbon-rich environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Magnetic flux density from the relative circular motion of stars and partially ionized gas in the galaxy mid-plane vicinity

Observations suggest a slower stellar rotation relative to gas rotation in the outer part of the Milky Way Galaxy. This difference could be attributed to an interaction with the interstellar magnetic field. In a simple model, fields of order 10 μG are then required, consistently with the observed values. This coincidence suggests a tool for estimating magnetic fields in spiral galaxies. A north–south asymmetry in the rotation of gas in the Galaxy could be of magnetic origin too.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1994

Gas kinematics and ionization along the extended sight line to HD 116852

We present Goddard High-Resolution Spectrograph intermediate observations of the interstellar medium toward HD 116852, a low halo star at a distance of 4.8 kpc (z = -1.3 kpc) in the direction l = 304. deg 9, b = 16.deg 1. The small science aperture observations have signal-to-noise ratios ranging from 30 to 90 and resolutions of 11 to 18 km/s (FWHM). We confirm the optical MK classification of this star through an analysis of its ultraviolet photosperic and stellar wind profiles. We detect interstellar lines of Al III, Si IV, C IV, and N V together with lines of C I, C I*, C I**, Si II, Ge II, P II, and Ni II. We convert the Mg II, P II, S II, Al III, Si IV, C IV, and N V profiles into measure of apparent column density as a function of LSR velocity. Gas scale height, velocity dispersion, and differential Galactic rotation effects govern the profile shapes. A simple computer model of the expected sight line column density profiles for the low and high ion species indicates that the gas velocity dispersions and scale heights increase as the ionization level of the gas increases. We find scale heigts H greater than or = 1 kpc for the high ions, which are comparable to the z-distance of the star, whereas we find H approximatley = 0.6 to 0.7 kpc for A1 III and H approximatley 0.1 kpc for P II and Ge II. An enhancement in the Al II profile near -15 km/s accounts for approximately 25% of the A1 III column along the sight line and probably arises within gas located approximately 500 pc below the Sagittarius-Carina spiral arm link. Portions of the broad underlying A1 III distribution are associated with the higher ionization lines, perhaps in conductive interfaces. The presence of N V and the column density ratios of Si IV, C IV, and N V favor the interpretation that much of the high ion absorption is produced by collisional ionization in gas with T = 1-3 x 10(exp 5) K. An enhancement near -35 km/s in both the Si IV and C IV profiles may be due to an outflow from the Norma spiral arm at a z-distance of about -1 kpc. The smooth decrease of the N(C IV)/N(Si IV) ratio at negative velocities may be due to an ISM ionization structure for hot gas that changes distance from the Galactic plane or to a two phase gas distribution in which the relative contribution from each phase changes with distance from the Galactic plane.

Sembach, Kenneth R.↗

DOE OSTI2017

On the formation of nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) in circumstellar and interstellar environments

The chemical evolution of extraterrestrial environments leads to the formation of polycyclic aromatic hydrocarbons (PAHs) via gas phase radical mediated aromatization reactions. Here, we review that these de facto barrierless reactions are capable of forming prebiotic molecules such as nitrogen substituted PAHs (NPAHs), which represent the missing link between nitrogen bearing acyclic molecules and prebiotic nucleobases along with vitamins found in meteorites. Crucial routes leading to the incorporation of nitrogen atoms into the aromatic ring have been exposed. Pyridine can be formed from the reaction of abundant vinyl cyanide and its radical or via cyano radicals reacting with 1,3-butadiene. The NPAHs 1,4-dihydro(iso)quinoline and (iso)quinoline can be synthesized through reaction of pyridyl radicals with 1,3-butadiene or sequentially with two acetylene molecules, respectively. The inclusion of nitrogen into an aromatic system and their growth can fill the mechanistic gaps missing leading from acyclic nitrogen-bearing molecules via pyridine to NPAH-type molecules in the interstellar medium.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1988

Aerodynamic properties of fractal grains - Implications for the primordial solar nebula

Gas-grain and grain-grain interactions have a significant role in the evolution of the primordial solar nebula. There is also compelling evidence for larger grains in dense interstellar clouds than in the diffuse interstellar medium. In the usual treatments of grain collisions, where it is assumed that collisions produce compact bodies with a constant density, the gas drag decreases rapidly with size. The first step toward a quantitative analysis of the aerodynamic behavior of fractal aggregates is reported here, and it is shown that for fractals drag decreases very slowly.

Meakin, P.↗