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At least 379 records · Page 21
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DOE OSTI2011

VERY LARGE ARRAY MONITORING OF 1720 MHz OH MASERS TOWARD THE GALACTIC CENTER

We present the first variability study of the 1720 MHz OH masers located in the Galactic center. Most of these masers are associated with the interaction between the supernova remnant Sgr A East and the interstellar medium, but a few masers are associated with the circumnuclear disk (CND). The monitoring program covered five epochs and a timescale of 20-195 days, during which no masers disappeared and no new masers appeared. All masers have previously been detected in a single-epoch observation about one year prior to the start of the monitoring experiment, implying relatively stable conditions for the 1720 MHz OH masers. No extreme variability was detected. The masers associated with the northeastern interaction region between the supernova remnant and the +50 km s{sup -1} molecular cloud show the highest level of variability. This can be explained with the +50 km s{sup -1} molecular cloud being located behind the supernova remnant and with a region of high OH absorbing column density along the line of sight. Possibly, the supernova remnant provides additional turbulence to the gas in this region, through which the maser emission must travel. The masers in the southern interaction region are located on the outermost edge of Sgr A East, the line of sight of which is not covered by either absorbing OH gas or a supernova remnant, in agreement with the much lower variability level observed. Similarly, the masers associated with the CND show little variability, consistent with those arising through collisions between relatively large clumps of gas in the CND and no significant amount of turbulent gas along the line of sight.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE STABILITY OF LOW SURFACE BRIGHTNESS DISKS BASED ON MULTI-WAVELENGTH MODELING

To investigate the structure and composition of the dusty interstellar medium (ISM) of low surface brightness (LSB) disk galaxies, we have used multi-wavelength photometry to construct spectral energy distributions for three low-mass, edge-on LSB galaxies (V{sub rot} = 88-105 km s{sup -1}). We use Monte Carlo radiation transfer codes that include the effects of transiently heated small grains and polycyclic aromatic hydrocarbon molecules to model and interpret the data. We find that, unlike the high surface brightness galaxies previously modeled, the dust disks appear to have scale heights equal to or exceeding their stellar scale heights. This result supports the findings of previous studies that low-mass disk galaxies have dust scale heights comparable to their stellar scale heights and suggests that the cold ISM of low-mass, LSB disk galaxies may be stable against fragmentation and gravitational collapse. This may help to explain the lack of observed dust lanes in edge-on LSB galaxies and their low current star formation rates. Dust masses are found in the range (1.16-2.38) x 10{sup 6} M{sub sun}, corresponding to face-on (edge-on), V-band, optical depths 0.034 {approx}< {tau}{sub face} {approx}< 0.106 (0.69 {approx}< {tau}{sub eq} {approx}< 1.99).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THERMAL REACTIONS OF OXYGEN ATOMS WITH ALKENES AT LOW TEMPERATURES ON INTERSTELLAR DUST

Laboratory experiments show that the thermal heterogeneous reactions of oxygen atoms may contribute to the synthesis of epoxides in interstellar clouds. The data set also indicates that the contribution of these pathways to epoxide formation, in comparison to non-thermal routes, is likely to be strongly temperature dependent. Our results indicate that an increased abundance of epoxides, relative to the corresponding aldehydes, could be an observational signature of a significant contribution to molecular oxidation via thermal O atom reactions with alkenes. Specifically surface science experiments show that both C{sub 2}H{sub 4}O and C{sub 3}H{sub 6}O are readily formed from reactions of ethene and propene molecules with thermalized oxygen atoms at temperatures in the range of 12-90 K. It is clear from our experiments that these reactions, on a graphite surface, proceed with significantly reduced reaction barriers compared with those operating in the gas phase. For both the C{sub 2}H{sub 4} + O and the C{sub 3}H{sub 6} + O reactions, the surface reaction barriers we determine are reduced by approximately an order of magnitude compared with the barriers in the gas phase. The modeling of our experimental results, which determines these reaction barriers, also extracts desorption energies and rate coefficients for the title reactions. Our results clearly show that the major product from the O + C{sub 2}H{sub 4} reaction is ethylene oxide, an epoxide.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

ON THE ELECTRONIC STRUCTURE OF ISOLATED MONO-DEHYDROGENATED POLYAROMATIC HYDROCARBON IONS AND THEIR ASTROPHYSICAL RELEVANCE

The attribution of the unidentified infrared bands to polycyclic aromatic hydrocarbon (PAH) molecules is a key argument for their abundant occurrence in interstellar environments, which has important implications for interstellar chemistry. In contrast to terrestrial conditions, their transient forms are of importance in the low-density astrophysical environments. Here, the gas-phase IR spectra of three PAH molecules in a carbocation state (naphthyl{sup +}, C{sub 10}H{sub 7}{sup +}; phenanthryl{sup +}, C{sub 14}H{sub 9}{sup +}; and pyrenyl{sup +}, C{sub 16}H{sub 9}{sup +}) are investigated by action spectroscopy methods using an infrared free electron laser and an ion-trap mass spectrometer. The IR spectra of the mono-dehydrogenated PAH{sup +} (aryl) ions in the 6-18 {mu}m spectral range are compared to computed IR spectra for various structural isomers of the aryl ions; the comparison indicates that the most stable structures under isolated conditions have a triplet electronic configuration. Electronic structure calculations on systems as large as the mono-dehydrogenated circumcoronene cation (C{sub 54}H{sub 17}{sup +}) provide further evidence for the higher stability of a triplet state as compared with the singlet state. Moreover, the gas-phase IR spectra reveal that the IR signatures of a PAH cation before and after H-atom loss are very similar, in particular in the 6-9 {mu}m region involving the skeletal CC stretching modes, so that triplet mono-dehydrogenated PAH ions are similarly compliant with the general match between PAH mid-IR features and the interstellar unidentified infrared emissions. The establishment of a triplet electronic ground state suggests that interstellar scenarios should consider the possible influence of triplet aromatic chemistry as well as the possible influence of the altered optical properties of triplet PAH species.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

THE OBSERVATIONAL AND THEORETICAL TIDAL RADII OF GLOBULAR CLUSTERS IN M87

Globular clusters have linear sizes (tidal radii) which theory tells us are determined by their masses and by the gravitational potential of their host galaxy. To explore the relationship between observed and expected radii, we utilize the globular cluster population of the Virgo giant M87. Unusually deep, high signal-to-noise images of M87 are used to measure the effective and limiting radii of approximately 2000 globular clusters. To compare with these observations, we simulate a globular cluster population that has the same characteristics as the observed M87 cluster population. Placing these simulated clusters in the well-studied tidal field of M87, the orbit of each cluster is solved and the theoretical tidal radius of each cluster is determined. We compare the predicted relationship between cluster size and projected galactocentric distance to observations. We find that for an isotropic distribution of cluster velocities, theoretical tidal radii are approximately equal to observed limiting radii for R{sub gc} < 10 kpc. However, the isotropic simulation predicts a steep increase in cluster size at larger radii, which is not observed in large galaxies beyond the Milky Way. To minimize the discrepancy between theory and observations, we explore the effects of orbital anisotropy on cluster sizes, and suggest a possible orbital anisotropy profile for M87 which yields a better match between theory and observations. Finally, we suggest future studies which will establish a stronger link between theoretical tidal radii and observed radii.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CAUSE AND EFFECT OF FEEDBACK: MULTIPHASE GAS IN CLUSTER CORES HEATED BY AGN JETS

Multiwavelength data indicate that the X-ray-emitting plasma in the cores of galaxy clusters is not cooling catastrophically. To a large extent, cooling is offset by heating due to active galactic nuclei (AGNs) via jets. The cool-core clusters, with cooler/denser plasmas, show multiphase gas and signs of some cooling in their cores. These observations suggest that the cool core is locally thermally unstable while maintaining global thermal equilibrium. Using high-resolution, three-dimensional simulations we study the formation of multiphase gas in cluster cores heated by collimated bipolar AGN jets. Our key conclusion is that spatially extended multiphase filaments form only when the instantaneous ratio of the thermal instability and free-fall timescales (t{sub TI}/t{sub ff}) falls below a critical threshold of Almost-Equal-To 10. When this happens, dense cold gas decouples from the hot intracluster medium (ICM) phase and generates inhomogeneous and spatially extended H{alpha} filaments. These cold gas clumps and filaments 'rain' down onto the central regions of the core, forming a cold rotating torus and in part feeding the supermassive black hole. Consequently, the self-regulated feedback enhances AGN heating and the core returns to a higher entropy level with t{sub TI}/t{sub ff} > 10. Eventually, the core reaches quasi-stable global thermal equilibrium, and cold filaments condense out of the hot ICM whenever t{sub TI}/t{sub ff} {approx}< 10. This occurs despite the fact that the energy from AGN jets is supplied to the core in a highly anisotropic fashion. The effective spatial redistribution of heat is enabled in part by the turbulent motions in the wake of freely falling cold filaments. Increased AGN activity can locally reverse the cold gas flow, launching cold filamentary gas away from the cluster center. Our criterion for the condensation of spatially extended cold gas is in agreement with observations and previous idealized simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

FERMI-LAT OBSERVATIONS OF THE DIFFUSE {gamma}-RAY EMISSION: IMPLICATIONS FOR COSMIC RAYS AND THE INTERSTELLAR MEDIUM

The {gamma}-ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse {gamma}-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X{sub CO} factor, the ratio between integrated CO-line intensity and H{sub 2} column density, the fluxes and spectra of the {gamma}-ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as {gamma}-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MINIMUM REQUIREMENTS FOR DETECTING A STOCHASTIC GRAVITATIONAL WAVE BACKGROUND USING PULSARS

We assess the detectability of a nanohertz gravitational wave (GW) background in a pulsar timing array (PTA) program by considering the shape and amplitude of the cross-correlation function summed over pulsar pairs. The distribution of correlation amplitudes is found to be non-Gaussian and highly skewed, which significantly influences detection and false-alarm probabilities. When only white noise combines with GWs in timing data, our detection results are consistent with those found by others. Contamination by red noise from spin variations and from any uncorrected interstellar plasma effects significantly increases the false-alarm probability. The number of arrival times (and thus the observing cadence) is important only as long as the residuals are dominated by white noise. When red noise and GWs dominate, the statistical significance of the correlation estimate can be improved only by increasing the number of pulsars. We characterize plausible detection regimes by evaluating the number of millisecond pulsars (MSPs) that must be monitored in a high-cadence, five-year timing program to detect a GW background spectrum h{sub c} (f) = A(f/f{sub 0}){sup -2/3} with f{sub 0} = 1 yr{sup -1} and A = 10{sup -15}. Our results indicate that a sample of 20 super-stable MSPs-those with rms timing residuals {sigma}{sub r} {approx}< 20 ns(A/10{sup -15}) from red-noise contributions over a five-year span-will allow detection of the GW background and study of its spectrum. However, a timing program on {approx}> 50-100 MSPs is likely needed for a complete PTA program, particularly if red noise is generally present in MSPs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A SEARCH FOR HYDROXYLAMINE (NH{sub 2}OH) TOWARD SELECT ASTRONOMICAL SOURCES

Observations of 14 rotational transitions of hydroxylamine (NH{sub 2}OH) using the NRAO 12 m telescope on Kitt Peak are reported toward IRC+10216, Orion KL, Orion S, Sgr B2(N), Sgr B2(OH), W3IRS5, and W51M. Although recent models suggest the presence of NH{sub 2}OH in high abundance, these observations resulted in non-detection. Upper limits are calculated to be as much as six orders of magnitude lower than those predicted by models. Possible explanations for the lower-than-expected abundance are explored.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EVIDENCE FOR ENVIRONMENTAL CHANGES IN THE SUBMILLIMETER DUST OPACITY

The submillimeter opacity of dust in the diffuse interstellar medium (ISM) in the Galactic plane has been quantified using a pixel-by-pixel correlation of images of continuum emission with a proxy for column density. We used multi-wavelength continuum data: three Balloon-borne Large Aperture Submillimeter Telescope bands at 250, 350, and 500 {mu}m and one IRAS band at 100 {mu}m. The proxy is the near-infrared color excess, E(J - K{sub s}), obtained from the Two Micron All Sky Survey. Based on observations of stars, we show how well this color excess is correlated with the total hydrogen column density for regions of moderate extinction. The ratio of emission to column density, the emissivity, is then known from the correlations, as a function of frequency. The spectral distribution of this emissivity can be fit by a modified blackbody, whence the characteristic dust temperature T and the desired opacity {sigma}{sub e}(1200) at 1200 GHz or 250 {mu}m can be obtained. We have analyzed 14 regions near the Galactic plane toward the Vela molecular cloud, mostly selected to avoid regions of high column density (N{sub H} > 10{sup 22} cm{sup -2}) and small enough to ensure a uniform dust temperature. We find {sigma}{sub e}(1200) is typically (2-4) Multiplication-Sign 10{sup -25} cm{sup 2} H{sup -1} and thus about 2-4 times larger than the average value in the local high Galactic latitude diffuse atomic ISM. This is strong evidence for grain evolution. There is a range in total power per H nucleon absorbed (and re-radiated) by the dust, reflecting changes in the strength of the interstellar radiation field and/or the dust absorption opacity. These changes in emission opacity and power affect the equilibrium T, which is typically 15 K, colder than at high latitudes. Our analysis extends, to higher opacity and lower temperature, the trend of increasing {sigma}{sub e}(1200) with decreasing T that was found at high latitudes. The recognition of changes in the emission opacity raises a cautionary flag because all column densities deduced from dust emission maps, and the masses of compact structures within them, depend inversely on the value adopted.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE GRAY NEEDLE: LARGE GRAINS IN THE HD 15115 DEBRIS DISK FROM LBT /PISCES/Ks AND LBTI /LMIRcam/L' ADAPTIVE OPTICS IMAGING

We present diffraction-limited Ks band and L' adaptive optics images of the edge-on debris disk around the nearby F2 star HD 15115, obtained with a single 8.4 m primary mirror at the Large Binocular Telescope. At the Ks band, the disk is detected at signal-to-noise per resolution element (SNRE) {approx} 3-8 from {approx}1 to 2.''5 (45-113 AU) on the western side and from {approx}1.''2 to 2.''1 (63-90 AU) on the east. At L' the disk is detected at SNRE {approx} 2.5 from {approx}1 to 1.''45 (45-90 AU) on both sides, implying more symmetric disk structure at 3.8 {mu}m. At both wavelengths the disk has a bow-like shape and is offset from the star to the north by a few AU. A surface brightness asymmetry exists between the two sides of the disk at the Ks band, but not at L'. The surface brightness at the Ks band declines inside 1'' ({approx}45 AU), which may be indicative of a gap in the disk near 1''. The Ks - L' disk color, after removal of the stellar color, is mostly gray for both sides of the disk. This suggests that scattered light is coming from large dust grains, with 3-10 {mu}m sized grains on the east side and 1-10 {mu}m dust grains on the west. This may suggest that the west side is composed of smaller dust grains than the east side, which would support the interpretation that the disk is being dynamically affected by interactions with the local interstellar medium.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EVIDENCE FOR LOW EXTINCTION IN ACTIVELY STAR-FORMING GALAXIES AT z > 6.5

We present a search for the [C II] 158 {mu}m fine structure line (a main cooling line of the interstellar medium) and the underlying far-infrared (FIR) continuum in three high-redshift (6.6 < z < 8.2) star-forming galaxies using the IRAM Plateau de Bure Interferometer. We targeted two Ly{alpha}-selected galaxies (Ly{alpha} emitters, LAEs) with moderate UV-based star formation rates (SFRs {approx} 20 M{sub Sun} yr{sup -1}; Himiko at z = 6.6 and IOK-1 at z = 7.0) and a gamma-ray burst (GRB) host galaxy (GRB 090423 at z {approx} 8.2). Based on our 3{sigma} rest-frame FIR continuum limits, previous (rest-frame) UV continuum measurements and spectral energy distribution (SED) fitting, we rule out SED shapes similar to highly obscured galaxies (e.g., Arp 220, M 82) and less extreme dust-rich nearby spiral galaxies (e.g., M 51) for the LAEs. Conservatively assuming an SED shape typical of local spiral galaxies we derive upper limits for the FIR-based star formation rates (SFRs) of {approx}70 M{sub Sun} yr{sup -1}, {approx}50 M{sub Sun} yr{sup -1}, and {approx}40 M{sub Sun} yr{sup -1} for Himiko, IOK-1, and GRB 090423, respectively. For the LAEs these limits are only a factor {approx}3 higher than the published UV-based SFRs (uncorrected for extinction). This indicates that the dust obscuration in the z > 6 LAEs studied here is lower by a factor of a few than what has recently been found in some LAEs at lower redshift (2 < z < 3.5) with similar UV-based SFRs. A low obscuration in our z > 6 LAE sample is consistent with recent rest-frame UV studies of z {approx} 7 Lyman break galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

DISCOVERY OF CRYSTALLIZED WATER ICE IN A SILHOUETTE DISK IN THE M43 REGION

We present the 1.9-4.2 {mu}m spectra of the five bright (L {<=} 11.2) young stars associated with silhouette disks with a moderate to high inclination angle of 39 Degree-Sign -80 Degree-Sign in the M42 and M43 regions. The water ice absorption is seen toward d121-1925 and d216-0939, while the spectra of d182-316, d183-405, and d218-354 show no water ice feature around 3.1 {mu}m within the detection limits. By comparing the water ice features toward nearby stars, we find that the water ice absorption toward d121-1925 and d216-0939 most likely originates from the foreground material and the surrounding disk, respectively. The angle of the disk inclination is found to be mainly responsible for the difference of the optical depth of the water ice among the five young stars. Our results suggest that there is a critical inclination angle between 65 Degree-Sign and 75 Degree-Sign for the circumstellar disk where the water ice absorption becomes strong. The average density at the disk surface of d216-0939 was found to be 6.38 Multiplication-Sign 10{sup -18} g cm{sup -3}. The water ice absorption band in the d216-0939 disk is remarkable in that the maximum optical depth of the water ice band is at a longer wavelength than detected before. It indicates that the primary carrier of the feature is purely crystallized water ice at the surface of the d216-0939 disk with characteristic size of {approx}0.8 {mu}m, which suggests grain growth. This is the first direct detection of purely crystallized water ice in a silhouette disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

DYNAMO ACTION IN THERMALLY UNSTABLE INTERSTELLAR FLOWS

Numerous studies have investigated the role of thermal instability in regulating the phase transition between the cold cloudy and warm diffuse medium of the interstellar medium. Considerable interest has also been devoted to investigating the properties of turbulence in thermally unstable flows, with a special emphasis on molecular clouds and the possibility of star formation. In this study, we investigate another setting in which this instability may be important, namely its effect on dynamo action in interstellar flows. The setup we consider is a three-dimensional periodic cube of gas with an initially weak magnetic field, subject to heating and cooling, the properties of which are such that thermal instability is provoked in a certain temperature regime. Dynamo action is established through external forcing on the flow field. By comparing the results with a cooling function with exactly the same net effect but no thermally unstable regime, we find the following. Reference runs with non-helical forcing were observed to produce no small-scale dynamo action below the Reynolds number 97. Therefore, we expect the magnetic fields generated in the helical runs to be purely due to the action of a large-scale dynamo mechanism. The critical Reynolds number for the onset of the large-scale dynamo was observed to roughly double between the thermally stable versus unstable runs, the conclusion being that the thermal instability makes large-scale dynamo action more difficult. Whereas density and magnetic fields were observed to be almost completely uncorrelated in the thermally stable cases investigated, the action of thermal instability was observed to produce a positive correlation of the form B{proportional_to}{rho}{sup 0.2}. This correlation is rather weak, and in addition it was observed to break down at the limit of highest densities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE NATURE OF DUST IN COMPACT GALACTIC PLANETARY NEBULAE FROM SPITZER SPECTRA

We present the Spitzer/Infrared Spectrograph (IRS) spectra of 157 compact Galactic planetary nebulae (PNe). These young PNe provide insight on the effects of dust in early post-asymptotic giant branch evolution, before much of the dust is altered or destroyed by the hardening stellar radiation field. Most of the selected targets have PN-type IRS spectra, while a few turned out to be misclassified stars. We inspected the group properties of the PN spectra and classified them based on the different dust classes (featureless or F, carbon-rich dust or CRD, oxygen-rich dust or ORD, mixed-chemistry dust or MCD) and subclasses (aromatic and aliphatic, and crystalline and amorphous). All PNe are characterized by dust continuum and more than 80% of the sample shows solid-state features above the continuum, in contrast with the Magellanic Cloud sample where only {approx}40% of the entire sample displays solid-state features; this is an indication of the strong link between dust properties and metallicity. The Galactic PNe that show solid-state features are almost equally divided among the CRD, ORD, and MCD. We analyzed dust properties together with other PN properties and found that (1) there is an enhancement of MCD PNe toward the Galactic center, in agreement with studies of Galactic bulge PNe; (2) CRD PNe could be seen as defining an evolutionary sequence, contrary to the ORD and MCD PNe, which are scattered in all evolutionary diagrams; (3) carbon-rich and oxygen-rich grains retain different equilibrium temperatures, as expected from models; and (4) ORD PNe are highly asymmetric, i.e., bipolar or bipolar core, and CRD PNe highly symmetric, i.e., round or elliptical; point symmetry is statistically more common in MCD than in other dust class PNe. By comparing the sample of this paper to that of Magellanic Cloud PNe, we find that the latter sample does not include MCD PNe, and the other dust classes are differently populated, with continuity of the fraction of F, CRD, ORD, and MCD populations from high to low metallicity environments. We also find similar sequences for CRD PNe in the Galactic disk and the Magellanic Clouds, except that the Magellanic Cloud PNe seem to attain higher dust temperatures at similar evolutionary stages, in agreement with the observational findings of smaller dust grains (i.e., lower radiation efficiency) in low metallicity interstellar environments.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE CHEMISTRY OF INTERSTELLAR OH{sup +}, H{sub 2}O{sup +}, AND H{sub 3}O{sup +}: INFERRING THE COSMIC-RAY IONIZATION RATES FROM OBSERVATIONS OF MOLECULAR IONS

We model the production of OH{sup +}, H{sub 2}O{sup +}, and H{sub 3}O{sup +} in interstellar clouds, using a steady-state photodissociation region code that treats the freezeout of gas species, grain surface chemistry, and desorption of ices from grains. The code includes polycyclic aromatic hydrocarbons (PAHs), which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at A{sub V} {approx}< 1 and one at A{sub V} {approx} 3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far-ultraviolet flux on the cloud ({chi} {approx}< 1000; {chi} = 1 = local interstellar value), the columns of OH{sup +} and H{sub 2}O{sup +} scale roughly as the cosmic-ray primary ionization rate {zeta}{sub crp} divided by the hydrogen nucleus density n. The H{sub 3}O{sup +} column is dominated by the second peak, and we show that if PAHs are present, N(H{sub 3}O{sup +}) {approx}4 Multiplication-Sign 10{sup 13} cm{sup -2} independent of {zeta}{sub crp} or n. If there are no PAHs or very small grains at the second peak, N(H{sub 3}O{sup +}) can attain such columns only if low-ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH{sup +})/N(H) and N(OH{sup +})/N(H{sub 2}O{sup +}) can be used to estimate {zeta}{sub crp}/n, {chi}/n and A{sub V} in them. We compare our models to Herschel observations of these two ions, and estimate {zeta}{sub crp} {approx}4-6 Multiplication-Sign 10{sup -16}(n/100 cm{sup -3}) s{sup -1} and {chi}/n = 0.03 cm{sup 3} for diffuse foreground clouds toward W49N.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

TOWARD UNDERSTANDING THE ORIGIN OF TURBULENCE IN MOLECULAR CLOUDS: SMALL-SCALE STRUCTURES AS UNITS OF DYNAMICAL MULTI-PHASE INTERSTELLAR MEDIUM

In order to investigate the origin of the interstellar turbulence, detailed observations in the CO J = 1-0 and 3-2 lines have been carried out in an interacting region of a molecular cloud with an H II region. As a result, several 1000-10,000 AU scale cloudlets with small velocity dispersion are detected, whose systemic velocities have a relatively large scatter of a few km s{sup -1}. It is suggested that the cloud is composed of small-scale dense and cold structures and their overlapping effect makes it appear to be a turbulent entity as a whole. This picture strongly supports the two-phase model of a turbulent medium driven by thermal instability proposed previously. On the surface of the present cloud, the turbulence is likely to be driven by thermal instability following ionization shock compression and UV irradiation. Those small-scale structures with line widths of {approx}0.6 km s{sup -1} have a relatively high CO line ratio of J 3-2 to 1-0, 1 {approx}< R{sub 3-2/1-0} {approx}< 2. The large velocity gradient analysis implies that the 0.6 km s{sup -1} width component cloudlets have an average density of 10{sup 3}-10{sup 4} cm{sup -3}, which is relatively high at cloud edges, but their masses are only {approx}< 0.05 M{sub Sun }.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

OBSERVATIONS OF ISOTROPIC INTERSTELLAR PICK-UP IONS AT 11 AND 17 AU FROM NEW HORIZONS

We report new observations by the Solar Wind Around Pluto (SWAP) instrument on the New Horizons spacecraft of an energy-per-charge (E/q) spectrum of interstellar pick-up ions (PUIs) from an unprecedented heliocentric distance of 17 AU. This E/q spectrum is fit well by an isotropic PUI distribution function combined with the detailed response of the SWAP instrument. In contrast to earlier work, we are also able to fit an isotropic PUI model to an E/q spectrum measured by SWAP at 11.3 AU by explicitly including two additional effects. These are (1) the E/q-dependent geometric factor of SWAP, which increases with decreasing E/q owing to effects associated with the post-acceleration of particles exiting the electrostatic analyzer portion of the instrument; and (2) a solar wind distribution, the model spectrum of which contributes significantly to the low-E/q part of the overall model owing, presumably, to secondary particles produced within the instrument.

79 ASTRONOMY AND ASTROPHYSICS↗