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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7
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DOE OSTI2013

DETECTION OF A LUMINOUS HOT X-RAY CORONA AROUND THE MASSIVE SPIRAL GALAXY NGC 266

The presence of luminous hot X-ray coronae in the dark matter halos of massive spiral galaxies is a basic prediction of galaxy formation models. However, observational evidence for such coronae is very scarce, with the first few examples having only been detected recently. In this paper, we study the large-scale diffuse X-ray emission associated with the massive spiral galaxy NGC 266. Using ROSAT and Chandra X-ray observations we argue that the diffuse emission extends at least {approx}70 kpc, whereas the bulk of the stellar light is confined to within {approx}25 kpc. Based on X-ray hardness ratios, we find that most of the diffuse emission is released at energies {approx}< 1.2 keV, which indicates that this emission originates from hot X-ray gas. Adopting a realistic gas temperature and metallicity, we derive that in the (0.05-0.15)r{sub 200} region (where r{sub 200} is the virial radius) the bolometric X-ray luminosity of the hot gas is (4.3 {+-} 0.8) Multiplication-Sign 10{sup 40} erg s{sup -1} and the gas mass is (9.1 {+-} 0.9) Multiplication-Sign 10{sup 9} M{sub Sun }. These values are comparable to those observed for the two other well-studied X-ray coronae in spiral galaxies, suggesting that the physical properties of such coronae are similar. This detection offers an excellent opportunity for comparison of observations with detailed galaxy formation simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PERICENTER PASSAGE OF THE GAS CLOUD G2 IN THE GALACTIC CENTER

We have further followed the evolution of the orbital and physical properties of G2, the object currently falling toward the massive black hole in the Galactic Center on a near-radial orbit. New, very sensitive data were taken in 2013 April with NACO and SINFONI at the ESO VLT. The ''head'' of G2 continues to be stretched ever further along the orbit in position-velocity space. A fraction of its emission appears to be already emerging on the blueshifted side of the orbit, past pericenter approach. Ionized gas in the head is now stretched over more than 15,000 Schwarzschild radii R{sub S} around the pericenter of the orbit, at Almost-Equal-To 2000 R{sub S} Almost-Equal-To 20 light hours from the black hole. The pericenter passage of G2 will be a process stretching over a period of at least one year. The Brackett-{gamma} luminosity of the head has been constant over the past nine years, to within {+-}25%, as have the line ratios Brackett-{gamma}/Paschen-{alpha} and Brackett-{gamma}/Helium-I. We do not see any significant evidence for deviations of G2's dynamical evolution due to hydrodynamical interactions with the hot gas around the black hole from a ballistic orbit of an initially compact cloud with moderate velocity dispersion. The constant luminosity and the increasingly stretched appearance of the head of G2 in the position-velocity plane, without a central peak, is not consistent with several proposed models with continuous gas release from an initially bound zone around a faint star on the same orbit as G2.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THERMAL INSTABILITY BEHIND A SHOCK WAVE IN H I AND MOLECULAR CLOUDS

We performed one-dimensional hydrodynamic simulations with detailed cooling, heating, and chemical processes to examine the thermal stability of shocked gas in cold neutral medium (CNM) and molecular clouds. We find that both CNM and molecular clouds can be thermally unstable in the cooling layer behind the shock wave. The characteristic wavelength of the thermal instability ranges from 10{sup –5} pc to 0.1 pc in the CNM, and from 10{sup –7} pc to 0.1 pc in the molecular clouds. This coincides with the size of observed tiny scale structures in the CNM and molecular clouds, indicating that the thermal instability in the shocked gas could be a formation mechanism of these tiny structures in the interstellar medium. We have also calculated the e-folding number of the thermal instability to estimate the amplification of the density fluctuation in the shocked gas. Density perturbations in the CNM grow by a factor of exp (5) ≅ 150, whereas the perturbations in the molecular clouds grow only by a factor of a few behind a high Mach number shock. The amplification factor is larger at lower densities and higher velocities. Formation of very small scale structures by thermal instability in shocked gas is more effective in lower densities.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

AN AZIMUTHAL ASYMMETRY IN THE LkHα 330 DISK

Theory predicts that giant planets and low mass stellar companions shape circumstellar disks by opening annular gaps in the gas and dust spatial distribution. For more than a decade it has been debated whether this is the dominant process that leads to the formation of transitional disks. In this paper, we present millimeter-wave interferometric observations of the transitional disk around the young intermediate mass star LkHα 330. These observations reveal a lopsided ring in the 1.3 mm dust thermal emission characterized by a radius of about 100 AU and an azimuthal intensity variation of a factor of two. By comparing the observations with a Gaussian parametric model, we find that the observed asymmetry is consistent with a circular arc, that extends azimuthally by about 90° and emits about 1/3 of the total continuum flux at 1.3 mm. Hydrodynamic simulations show that this structure is similar to the azimuthal asymmetries in the disk surface density that might be produced by the dynamical interaction with unseen low mass companions orbiting within 70 AU from the central star. We argue that such asymmetries might lead to azimuthal variations in the millimeter-wave dust opacity and in the dust temperature, which will also affect the millimeter-wave continuum emission. Alternative explanations for the observed asymmetry that do not require the presence of companions cannot be ruled out with the existing data. Further observations of both the dust and molecular gas emission are required to derive firm conclusions on the origin of the asymmetry observed in the LkHα 330 disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

GIANT MOLECULAR CLOUD FORMATION IN DISK GALAXIES: CHARACTERIZING SIMULATED VERSUS OBSERVED CLOUD CATALOGS

We present the results of a study of simulated giant molecular clouds (GMCs) formed in a Milky Way-type galactic disk with a flat rotation curve. This simulation, which does not include star formation or feedback, produces clouds with masses ranging between 10{sup 4} M{sub ☉} and 10{sup 7} M{sub ☉}. We compare our simulated cloud population to two observational surveys: the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey and the BIMA All-Disk Survey of M33. An analysis of the global cloud properties as well as a comparison of Larson's scaling relations is carried out. We find that simulated cloud properties agree well with the observed cloud properties, with the closest agreement occurring between the clouds at comparable resolution in M33. Our clouds are highly filamentary—a property that derives both from their formation due to gravitational instability in the sheared galactic environment, as well as to cloud-cloud gravitational encounters. We also find that the rate at which potentially star-forming gas accumulates within dense regions—wherein n{sub thresh} ≥ 10{sup 4} cm{sup –3}—is 3% per 10 Myr, in clouds of roughly 10{sup 6} M{sub ☉}. This suggests that star formation rates in observed clouds are related to the rates at which gas can be accumulated into dense subregions within GMCs via filamentary flows. The most internally well-resolved clouds are chosen for listing in a catalog of simulated GMCs—the first of its kind. The cataloged clouds are available as an extracted data set from the global simulation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

X-RAY SCALING RELATION IN EARLY-TYPE GALAXIES: DARK MATTER AS A PRIMARY FACTOR IN RETAINING HOT GAS

We have revisited the X-ray scaling relations of early-type galaxies (ETG) by investigating, for the first time, the L{sub X,Gas}-M{sub Total} relation in a sample of 14 ETGs. In contrast to the large scatter (a factor of 10{sup 2}-10{sup 3}) in the L{sub X,Total}-L{sub B} relation, we found a tight correlation between these physically motivated quantities with an rms deviation of a factor of three in L{sub X,Gas} = 10{sup 38}-10{sup 43} erg s{sup –1} or M{sub Total} = a few × 10{sup 10} to a few × 10{sup 12} M{sub ☉}. More striking, this relation becomes even tighter with an rms deviation of a factor of 1.3 among the gas-rich galaxies (with L{sub X,Gas} > 10{sup 40} erg s{sup –1}). In a simple power-law form, the new relation is (L{sub X,Gas}/10{sup 40} erg s{sup –1}) = (M{sub Total}/3.2 × 10{sup 11} M{sub ☉}){sup 3}. This relation is also consistent with the steep relation between the gas luminosity and temperature, L{sub X,Gas} ∼ T{sub Gas} {sup 4.5}, identified by Boroson et al., if the gas is virialized. Our results indicate that the total mass of an ETG is the primary factor in regulating the amount of hot gas. Among the gas-poor galaxies (with L{sub X,Gas} < a few × 10{sup 39} erg s{sup –1}), the scatter in the L{sub X,Gas}-M{sub Total} (and L{sub X,Gas}-T{sub Gas}) relation increases, suggesting that secondary factors (e.g., rotation, flattening, star formation history, cold gas, environment, etc.) may become important.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ALMA OBSERVATIONS OF THE MOLECULAR GAS IN THE DEBRIS DISK OF THE 30 Myr OLD STAR HD 21997

The 30 Myr old A3-type star HD 21997 is one of the two known debris dust disks having a measurable amount of cold molecular gas. With the goal of understanding the physical state, origin, and evolution of the gas in young debris disks, we obtained CO line observations with the Atacama Large Millimeter/submillimeter Array (ALMA). Here, we report on the detection of {sup 12}CO and {sup 13}CO in the J = 2-1 and J = 3-2 transitions and C{sup 18}O in the J = 2-1 line. The gas exhibits a Keplerian velocity curve, one of the few direct measurements of Keplerian rotation in young debris disks. The measured CO brightness distribution could be reproduced by a simple star+disk system, whose parameters are r{sub in} < 26 AU, r{sub out} = 138 ± 20 AU, M{sub *}=1.8{sup +0.5}{sub -0.2} M{sub ☉}, and i = 32.°6 ± 3.°1. The total CO mass, as calculated from the optically thin C{sup 18}O line, is about (4-8) × 10{sup –2} M{sub ⊕}, while the CO line ratios suggest a radiation temperature on the order of 6-9 K. Comparing our results with those obtained for the dust component of the HD 21997 disk from ALMA continuum observations by Moór et al., we conclude that comparable amounts of CO gas and dust are present in the disk. Interestingly, the gas and dust in the HD 21997 system are not colocated, indicating a dust-free inner gas disk within 55 AU of the star. We explore two possible scenarios for the origin of the gas. A secondary origin, which involves gas production from colliding or active planetesimals, would require unreasonably high gas production rates and would not explain why the gas and dust are not colocated. We propose that HD 21997 is a hybrid system where secondary debris dust and primordial gas coexist. HD 21997, whose age exceeds both the model predictions for disk clearing and the ages of the oldest T Tauri-like or transitional gas disks in the literature, may be a key object linking the primordial and the debris phases of disk evolution.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EVIDENCE OF CONTRIBUTION OF INTERVENING CLOUDS TO GAMMA-RAY BURST'S X-RAY COLUMN DENSITY

The origin of excess of X-ray column density with respect to optical extinction in gamma-ray bursts (GRBs) is still a puzzle. A proposed explanation of the excess is the photoelectric absorption due to the intervening clouds along a GRB's line of sight. Here, we test this scenario by using the intervening Mg II absorption as a tracer of the neutral hydrogen column density of the intervening clouds. We identify a connection between the large X-ray column density (and large column density ratio of log (N{sub H,X}/N{sub H{sub I}})∼0.5) and large neutral hydrogen column density probed by the Mg II doublet ratio (DR). In addition, GRBs with large X-ray column density (and large ratio of log (N{sub H,X}/N{sub H{sub I}})>0) tend to have multiple saturated intervening absorbers with DR < 1.2. These results therefore indicate an additional contribution from the intervening system to the observed X-ray column density in some GRBs, although the contribution from the host galaxy alone cannot be excluded based on this study.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EARLY STAR-FORMING PROCESSES IN DENSE MOLECULAR CLOUD L328; IDENTIFICATION OF L328-IRS AS A PROTO-BROWN DWARF

This paper presents the results of millimeter to sub-millimeter observations of CO, HCN, N{sub 2}H{sup +}, and HCO{sup +} lines in the dense molecular cloud L328, which harbors L328-IRS, a Very Low Luminosity Object (VeLLO). Our analysis of the line width finds that {sup 13}CO and N{sub 2}H{sup +} lines are broadened right over the smallest sub-core S2 where L328-IRS is located, while they are significantly narrower in other regions of L328. Thus, L328-IRS has a direct association with the sub-core. CO observations show a bipolar outflow from this VeLLO with an extent of ∼0.08 pc. The outflow momentum flux and efficiency are much less than those of low-mass protostars. The most likely mass accretion rate (∼3.6 × 10{sup –7} M{sub ☉} yr{sup –1}) inferred from the analysis of the CO outflow is an order of magnitude smaller than the canonical value for a protostar. If the main accretion lasts during the typical Class 0 period of a protostar, L328-IRS will accrete the mass of a brown dwarf, but not that of a star. Given that its envelope mass is small (∼0.09 M{sub ☉}) and 100% star formation rate is unlikely, we suggest that L328-IRS is likely a proto-brown dwarf. Inward motions are found in global scale in the L328 cloud and its sub-cores with a typical infall speed found in starless cores. L328 is found to be fairly well isolated from other nearby clouds and seems to be forming three sub-cores simultaneously through a gravitational fragmentation process. Altogether, these all leave L328-IRS as the best example supporting the idea that a brown dwarf forms like a normal star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Star formation relations in nearby molecular clouds

We test some ideas for star formation relations against data on local molecular clouds. On a cloud by cloud basis, the relation between the surface density of star formation rate and surface density of gas divided by a free-fall time, calculated from the mean cloud density, shows no significant correlation. If a crossing time is substituted for the free-fall time, there is even less correlation. Within a cloud, the star formation rate volume and surface densities increase rapidly with the corresponding gas densities, faster than predicted by models using the free-fall time defined from the local density. A model in which the star formation rate depends linearly on the mass of gas above a visual extinction of 8 mag describes the data on these clouds, with very low dispersion. The data on regions of very massive star formation, with improved star formation rates based on free-free emission from ionized gas, also agree with this linear relation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Synthetic observations of the evolution of starless cores in a molecular cloud simulation: Comparisons with JCMT data and predictions for alma

Interpreting the nature of starless cores has been a prominent goal in star formation for many years. In order to characterize the evolutionary stages of these objects, we perform synthetic observations of a numerical simulation of a turbulent molecular cloud. We find that nearly all cores that we detect are associated with filaments and eventually form protostars. We conclude that observed starless cores that appear Jeans unstable are only marginally larger than their respective Jeans masses (within a factor of three). We note single dish observations such as those performed with the James Clerk Maxwell Telescope appear to miss significant core structure on small scales because of beam averaging. Finally, we predict that interferometric observations with Atacama Large Millimeter Array Cycle 1 will resolve the important small-scale structure, which has so far been missed by millimeter wavelength observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Comparing simulated emission from molecular clouds using experimental design

We propose a new approach to comparing simulated observations that enables us to determine the significance of the underlying physical effects. We utilize the methodology of experimental design, a subfield of statistical analysis, to establish a framework for comparing simulated position-position-velocity data cubes to each other. We propose three similarity metrics based on methods described in the literature: principal component analysis, the spectral correlation function, and the Cramer multi-variate two-sample similarity statistic. Using these metrics, we intercompare a suite of mock observational data of molecular clouds generated from magnetohydrodynamic simulations with varying physical conditions. Using this framework, we show that all three metrics are sensitive to changing Mach number and temperature in the simulation sets, but cannot detect changes in magnetic field strength and initial velocity spectrum. We highlight the shortcomings of one-factor-at-a-time designs commonly used in astrophysics and propose fractional factorial designs as a means to rigorously examine the effects of changing physical properties while minimizing the investment of computational resources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The PdBI arcsecond whirlpool survey (PAWS): Environmental dependence of giant molecular cloud properties in M51

Using data from the PdBI Arcsecond Whirlpool Survey (PAWS), we have generated the largest extragalactic giant molecular cloud (GMC) catalog to date, containing 1507 individual objects. GMCs in the inner M51 disk account for only 54% of the total {sup 12}CO(1-0) luminosity of the survey, but on average they exhibit physical properties similar to Galactic GMCs. We do not find a strong correlation between the GMC size and velocity dispersion, and a simple virial analysis suggests that ∼30% of GMCs in M51 are unbound. We have analyzed the GMC properties within seven dynamically motivated galactic environments, finding that GMCs in the spiral arms and in the central region are brighter and have higher velocity dispersions than inter-arm clouds. Globally, the GMC mass distribution does not follow a simple power-law shape. Instead, we find that the shape of the mass distribution varies with galactic environment: the distribution is steeper in inter-arm region than in the spiral arms, and exhibits a sharp truncation at high masses for the nuclear bar region. We propose that the observed environmental variations in the GMC properties and mass distributions are a consequence of the combined action of large-scale dynamical processes and feedback from high-mass star formation. We describe some challenges of using existing GMC identification techniques for decomposing the {sup 12}CO(1-0) emission in molecule-rich environments, such as M51's inner disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The CO-to-H{sub 2} conversion factor across the Perseus molecular cloud

We derive the CO-to-H{sub 2} conversion factor, X {sub CO} = N(H{sub 2})/I {sub CO}, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H{sub 2}) data with the I {sub CO} data from the COMPLETE Survey. We estimate an average X {sub CO} ∼ 3 × 10{sup 19} cm{sup –2} K{sup –1} km{sup –1} s and find a factor of ∼3 variations in X {sub CO} between the five sub-regions in Perseus. Within the individual regions, X {sub CO} varies by a factor of ∼100, suggesting that X {sub CO} strongly depends on local conditions in the interstellar medium. We find that X {sub CO} sharply decreases at A{sub V} ≲ 3 mag but gradually increases at A{sub V} ≳ 3 mag, with the transition occurring at A{sub V} where I {sub CO} becomes optically thick. We compare the N(H I), N(H{sub 2}), I {sub CO}, and X {sub CO} distributions with two models of the formation of molecular gas, a one-dimensional photodissociation region (PDR) model and a three-dimensional magnetohydrodynamic (MHD) model, tracking both the dynamical and chemical evolution of gas. The PDR model based on the steady state and equilibrium chemistry reproduces our data very well but requires a diffuse halo to match the observed N(H I) and I {sub CO} distributions. The MHD model matches our data reasonably well, suggesting that time-dependent effects on H{sub 2} and CO formation are insignificant for an evolved molecular cloud like Perseus. However, we find interesting discrepancies, including a broader range of N(H I), likely underestimated I {sub CO}, and a large scatter of I {sub CO} at small A{sub V} . These discrepancies most likely result from strong compressions and rarefactions and density fluctuations in the MHD model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The role of turbulence in star formation laws and thresholds

The Schmidt-Kennicutt relation links the surface densities of gas to the star formation rate in galaxies. The physical origin of this relation, and in particular its break, i.e., the transition between an inefficient regime at low gas surface densities and a main regime at higher densities, remains debated. Here, we study the physical origin of the star formation relations and breaks in several low-redshift galaxies, from dwarf irregulars to massive spirals. We use numerical simulations representative of the Milky Way and the Large and Small Magellanic Clouds with parsec up to subparsec resolution, and which reproduce the observed star formation relations and the relative variations of the star formation thresholds. We analyze the role of interstellar turbulence, gas cooling, and geometry in drawing these relations at 100 pc scale. We suggest in particular that the existence of a break in the Schmidt-Kennicutt relation could be linked to the transition from subsonic to supersonic turbulence and is independent of self-shielding effects. With this transition being connected to the gas thermal properties and thus to the metallicity, the break is shifted toward high surface densities in metal-poor galaxies, as observed in dwarf galaxies. Our results suggest that together with the collapse of clouds under self-gravity, turbulence (injected at galactic scale) can induce the compression of gas and regulate star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

H{sub 2} formation in diffuse clouds: A new kinetic Monte Carlo study

We used the continuous-time random-walk Monte Carlo technique to study anew the formation of H{sub 2} on the surfaces of interstellar dust grains in diffuse interstellar clouds. For our study, we considered three different grain materials, olivine (a polycrystalline silicate), amorphous silicate, and amorphous carbon, as well as a grain temperature that depends on granular size. For some runs, we included temperature fluctuations. Four different granular surfaces were used, one 'flat' with one type of binding site due to physisorption, one 'rough' with five different types of physisorption binding sites due to lateral forces, and two with sites for chemisorption, one in which chemisorption sites are entered through precursor physisorption sites, and one in which chemisorption is direct but occurs with a barrier for the adsorption of the first hydrogen atom. We found that on flat and rough olivine surfaces, molecular hydrogen is formed at low efficiencies, with smaller grains contributing very little despite their large numbers due to high temperatures. For flat amorphous carbon and amorphous silicate surfaces, the efficiency increases, reaching unity for the largest grains. For models with barrierless chemisorption, the efficiency of formation of H{sub 2} is near unity at all grain sizes considered, while for direct chemisorption via a barrier, we found efficiencies of 0.13-0.6 depending upon the barrier, but independent of grain size. Treating the flat olivine and amorphous silicate surfaces with temperature fluctuations increases the efficiency of H{sub 2} formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Observational evidence of dust evolution in galactic extinction curves

Although structural and optical properties of hydrogenated amorphous carbons are known to respond to varying physical conditions, most conventional extinction models are basically curve fits with modest predictive power. We compare an evolutionary model of the physical properties of carbonaceous grain mantles with their determination by homogeneously fitting observationally derived Galactic extinction curves with the same physically well-defined dust model. We find that a large sample of observed Galactic extinction curves are compatible with the evolutionary scenario underlying such a model, requiring physical conditions fully consistent with standard density, temperature, radiation field intensity, and average age of diffuse interstellar clouds. Hence, through the study of interstellar extinction we may, in principle, understand the evolutionary history of the diffuse interstellar clouds.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A large catalog of accurate distances to molecular clouds from PS1 photometry

Distance measurements to molecular clouds are important but are often made separately for each cloud of interest, employing very different data and techniques. We present a large, homogeneous catalog of distances to molecular clouds, most of which are of unprecedented accuracy. We determine distances using optical photometry of stars along lines of sight toward these clouds, obtained from PanSTARRS-1. We simultaneously infer the reddenings and distances to these stars, tracking the full probability distribution function using a technique presented in Green et al. We fit these star-by-star measurements using a simple dust screen model to find the distance to each cloud. We thus estimate the distances to almost all of the clouds in the Magnani et al. catalog, as well as many other well-studied clouds, including Orion, Perseus, Taurus, Cepheus, Polaris, California, and Monoceros R2, avoiding only the inner Galaxy. Typical statistical uncertainties in the distances are 5%, though the systematic uncertainty stemming from the quality of our stellar models is about 10%. The resulting catalog is the largest catalog of accurate, directly measured distances to molecular clouds. Our distance estimates are generally consistent with available distance estimates from the literature, though in some cases the literature estimates are off by a factor of more than two.

79 ASTRONOMY AND ASTROPHYSICS↗