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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 1,675 records · Page 93
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DOE OSTI2016

ROTATIONAL DYNAMICS AND STAR FORMATION IN THE NEARBY DWARF GALAXY NGC 5238

We present new H i spectral-line images of the nearby low-mass galaxy NGC 5238, acquired with the Karl G. Jansky Very Large Array. Located at a distance of 4.51 ± 0.04 Mpc, NGC 5238 is an actively star-forming galaxy with widespread H α and ultraviolet (UV) continuum emission. The source is included in many ongoing and recent nearby galaxy surveys, but until this work the spatially resolved qualities of its neutral interstellar medium have remained unstudied. Our H i images resolve the disk on physical scales of ∼400 pc, allowing us to undertake a detailed comparative study of the gaseous and stellar components. The H i disk is asymmetric in the outer regions, and the areas of high H i mass surface density display a crescent-shaped morphology that is slightly offset from the center of the stellar populations. The H i column density exceeds 10{sup 21} cm{sup −2} in much of the disk. We quantify the degree of co-spatiality of dense H i gas and sites of ongoing star formation as traced by far-UV and H α emission. The neutral gas kinematics are complex; using a spatially resolved position–velocity analysis, we infer a rotational velocity of 31 ± 5 km s{sup −1}. We place NGC 5238 on the baryonic Tully–Fisher relation and contextualize the system among other low-mass galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

HST emission line galaxies at z ∼ 2: Comparing physical properties of Lyman alpha and optical emission line selected galaxies

We compare the physical and morphological properties of z ∼ 2 Lyα emitting galaxies (LAEs) identified in the HETDEX Pilot Survey and narrow band studies with those of z ∼ 2 optical emission line selected galaxies (oELGs) identified via HST WFC3 infrared grism spectroscopy. Both sets of galaxies extend over the same range in stellar mass (7.5<logM/M{sub ⊙}<10.5), size (0.5 < R < 3.0 kpc), and star formation rate (∼1<SFR<100 M{sub ⊙} yr{sup −1}). Remarkably, a comparison of the most commonly used physical and morphological parameters—stellar mass, half-light radius, UV slope, SFR, ellipticity, nearest neighbor distance, star formation surface density, specific SFR, [O iii] luminosity, and [O iii] equivalent width—reveals no statistically significant differences between the populations. This suggests that the processes and conditions which regulate the escape of Lyα from a z ∼ 2 star-forming galaxy do not depend on these quantities. In particular, the lack of dependence on the UV slope suggests that Lyα emission is not being significantly modulated by diffuse dust in the interstellar medium. We develop a simple model of Lyα emission that connects LAEs to all high-redshift star-forming galaxies where the escape of Lyα depends on the sightline through the galaxy. Using this model, we find that mean solid angle for Lyα escape is Ω{sub Lyα}=2.4±0.8 steradians; this value is consistent with those calculated from other studies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE SWIFT GRB HOST GALAXY LEGACY SURVEY. II. REST-FRAME NEAR-IR LUMINOSITY DISTRIBUTION AND EVIDENCE FOR A NEAR-SOLAR METALLICITY THRESHOLD

We present rest-frame near-IR (NIR) luminosities and stellar masses for a large and uniformly selected population of gamma-ray burst (GRB) host galaxies using deep Spitzer Space Telescope imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3, and we determine the effects of galaxy evolution and chemical enrichment on the mass distribution of the GRB host population across cosmic history. We find a rapid increase in the characteristic NIR host luminosity between z ∼ 0.5 and z ∼ 1.5, but little variation between z ∼ 1.5 and z ∼ 5. Dust-obscured GRBs dominate the massive host population but are only rarely seen associated with low-mass hosts, indicating that massive star-forming galaxies are universally and (to some extent) homogeneously dusty at high redshift while low-mass star-forming galaxies retain little dust in their interstellar medium. Comparing our luminosity distributions with field surveys and measurements of the high-z mass–metallicity relation, our results have good consistency with a model in which the GRB rate per unit star formation is constant in galaxies with gas-phase metallicity below approximately the solar value but heavily suppressed in more metal-rich environments. This model also naturally explains the previously reported “excess” in the GRB rate beyond z ≳ 2; metals stifle GRB production in most galaxies at z < 1.5 but have only minor impact at higher redshifts. The metallicity threshold we infer is much higher than predicted by single-star models and favors a binary progenitor. Our observations also constrain the fraction of cosmic star formation in low-mass galaxies undetectable to Spitzer to be small at z < 4.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

IN-SYNC. IV. The young stellar population in the Orion A molecular cloud

We present the results of the Sloan Digital Sky Survey APOGEE INfrared Spectroscopy of Young Nebulous Clusters program (IN-SYNC) survey of the Orion A molecular cloud. This survey obtained high-resolution near-infrared spectroscopy of about 2700 young pre-main-sequence stars on a ∼6{sup ∘} field of view. We have measured accurate stellar parameters (T{sub eff}, logg, vsini) and extinctions and placed the sources in the Hertzsprung–Russel diagram (HRD). We have also extracted radial velocities for the kinematic characterization of the population. We compare our measurements with literature results to assess the performance and accuracy of the survey. Source extinction shows evidence for dust grains that are larger than those in the diffuse interstellar medium: we estimate an average R{sub V} = 5.5 in the region. Importantly, we find a clear correlation between HRD inferred ages and spectroscopic surface-gravity-inferred ages and between extinction and disk presence; this strongly suggests a real spread of ages larger than a few Myr. Focusing on the young population around NGC 1980/ι Ori, which has previously been suggested to be a separate, foreground, older cluster, we confirm its older (∼5 Myr) age and low A{sub V}, but considering that its radial velocity distribution is indistinguishable from Orion A’s population, we suggest that NGC 1980 is part of Orion A’s star formation activity. Based on their stellar parameters and kinematic properties, we identify 383 new candidate members of Orion A, most of which are diskless sources in areas of the region poorly studied by previous works.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A massive molecular gas reservoir in the z = 2.221 type-2 quasar host galaxy SMM J0939+8315 lensed by the radio galaxy 3C220.3

We report the detection of CO(J = 3 → 2) line emission in the strongly lensed submillimeter galaxy (SMG) SMM J0939+8315 at z = 2.221, using the Combined Array for Research in Millimeter-wave Astronomy. SMM J0939+8315 hosts a type-2 quasar, and is gravitationally lensed by the radio galaxy 3C220.3 and its companion galaxy at z = 0.685. The 104 GHz continuum emission underlying the CO line is detected toward 3C220.3 with an integrated flux density of S{sub cont} = 7.4 ± 1.4 mJy. Using the CO(J = 3 → 2) line intensity of I{sub CO(3-2)} = (12.6 ± 2.0) Jy km s{sup −1}, we derive a lensing- and excitation-corrected CO line luminosity of L{sub CO(1−0)}{sup ′} = (3.4 ± 0.7) × 10{sup 10} (10.1/μ{sub L}) K km s{sup −1} pc{sup 2} for the SMG, where μ{sub L} is the lensing magnification factor inferred from our lens modeling. This translates to a molecular gas mass of M{sub gas} = (2.7 ± 0.6) × 10{sup 10} (10.1/μ{sub L}) M{sub ⊙}. Fitting spectral energy distribution models to the (sub)-millimeter data of this SMG yields a dust temperature of T = 63.1{sub −1.3}{sup +1.1} K, a dust mass of M{sub dust} = (5.2 ± 2.1) × 10{sup 8} (10.1/μ{sub L}) M{sub ⊙}, and a total infrared luminosity of L{sub IR} = (9.1 ± 1.2) ×10{sup 12} (10.1/μ{sub L}) L{sub ⊙}. We find that the properties of the interstellar medium of SMM J0939+8315 overlap with both SMGs and type-2 quasars. Hence, SMM J0939+8315 may be transitioning from a starbursting phase to an unobscured quasar phase as described by the “evolutionary link” model, according to which this system may represent an intermediate stage in the evolution of present-day galaxies at an earlier epoch.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Exploring the origins of deuterium enrichments in solar nebular organics

Deuterium-to-hydrogen (D/H) enrichments in molecular species provide clues about their original formation environment. The organic materials in primitive solar system bodies generally have higher D/H ratios and show greater D/H variation when compared to D/H in solar system water. We propose this difference arises at least in part due to (1) the availability of additional chemical fractionation pathways for organics beyond that for water, and (2) the higher volatility of key carbon reservoirs compared to oxygen. We test this hypothesis using detailed disk models, including a sophisticated, new disk ionization treatment with a low cosmic-ray ionization rate, and find that disk chemistry leads to higher deuterium enrichment in organics compared to water, helped especially by fractionation via the precursors CH{sub 2}D{sup +}/CH{sub 3}{sup +}. We also find that the D/H ratio in individual species varies significantly depending on their particular formation pathways. For example, from ∼20–40 au, CH{sub 4} can reach D/H∼2×10{sup −3}, while D/H in CH{sub 3}OH remains locally unaltered. Finally, while the global organic D/H in our models can reproduce intermediately elevated D/H in the bulk hydrocarbon reservoir, our models are unable to reproduce the most deuterium-enriched organic materials in the solar system, and thus our model requires some inheritance from the cold interstellar medium from which the Sun formed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The dense matter equation of state from neutron star radius and mass measurements

We present a comprehensive study of spectroscopic radius measurements of twelve neutron stars obtained during thermonuclear bursts or in quiescence. We incorporate, for the first time, a large number of systematic uncertainties in the measurement of the apparent angular sizes, Eddington fluxes, and distances, in the composition of the interstellar medium, and in the flux calibration of X-ray detectors. We also take into account the results of recent theoretical calculations of rotational effects on neutron star radii, of atmospheric effects on surface spectra, and of relativistic corrections to the Eddington critical flux. We employ Bayesian statistical frameworks to obtain neutron star radii from the spectroscopic measurements as well as to infer the equation of state from the radius measurements. Combining these with the results of experiments in the vicinity of nuclear saturation density and the observations of ∼2 M{sub ⊙} neutron stars, we place strong and quantitative constraints on the properties of the equation of state between ≈2--8 times the nuclear saturation density. We find that around M=1.5M{sub ⊙}, the preferred equation of state predicts radii between 10.1 and 11.1 km. When interpreting the pressure constraints in the context of high density equations of state based on interacting nucleons, our results suggest a relatively weak contribution of the three-body interaction potential.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

TRACING THE EVOLUTION OF HIGH-REDSHIFT GALAXIES USING STELLAR ABUNDANCES

This paper presents the first results from a model for chemical evolution that can be applied to N-body cosmological simulations and quantitatively compared to measured stellar abundances from large astronomical surveys. This model convolves the chemical yield sets from a range of stellar nucleosynthesis calculations (including asymptotic giant branch stars, Type Ia and II supernovae, and stellar wind models) with a user-specified stellar initial mass function (IMF) and metallicity to calculate the time-dependent chemical evolution model for a “simple stellar population” (SSP) of uniform metallicity and formation time. These SSP models are combined with a semianalytic model for galaxy formation and evolution that uses merger trees from N-body cosmological simulations to track several α- and iron-peak elements for the stellar and multiphase interstellar medium components of several thousand galaxies in the early (z ≥ 6) universe. The simulated galaxy population is then quantitatively compared to two complementary data sets of abundances in the Milky Way stellar halo and is capable of reproducing many of the observed abundance trends. The observed abundance ratio distributions are best reproduced with a Chabrier IMF, a chemically enriched star formation efficiency of 0.2, and a redshift of reionization of 7. Many abundances are qualitatively well matched by our model, but our model consistently overpredicts the carbon-enhanced fraction of stars at low metallicities, likely owing to incomplete coverage of Population III stellar yields and supernova models and the lack of dust as a component of our model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ISM masses and the star formation law at Z = 1 to 6: ALMA observations of dust continuum in 145 galaxies in the cosmos survey field

ALMA Cycle 2 observations of long-wavelength dust emission in 145 star-forming galaxies are used to probe the evolution of the star-forming interstellar medium (ISM). We also develop a physical basis and empirical calibration (with 72 low-z and z ∼ 2 galaxies) for using the dust continuum as a quantitative probe of ISM masses. The galaxies with the highest star formation rates (SFRs) at 〈z〉 = 2.2 and 4.4 have gas masses up to 100 times that of the Milky Way and gas mass fractions reaching 50%–80%, i.e., gas masses 1-4× their stellar masses. We find a single high-z star formation law: SFR=35 M{sub mol}{sup 0.89}×(1+z){sub z=2}{sup 0.95}×(sSFR){sub MS}{sup 0.23} M{sub ⊙} yr{sup −1}—an approximately linear dependence on the ISM mass and an increased star formation efficiency per unit gas mass at higher redshift. Galaxies above the main sequence (MS) have larger gas masses but are converting their ISM into stars on a timescale only slightly shorter than those on the MS; thus, these “starbursts” are largely the result of having greatly increased gas masses rather than an increased efficiency of converting gas to stars. At z > 1, the entire population of star-forming galaxies has ∼2–5 times shorter gas depletion times than low-z galaxies. These shorter depletion times indicate a different mode of star formation in the early universe—most likely dynamically driven by compressive, high-dispersion gas motions—a natural consequence of the high gas accretion rates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SINGLE AND DOUBLE PHOTOIONIZATION AND PHOTODISSOCIATION OF TOLUENE BY SOFT X-RAYS IN A CIRCUMSTELLAR ENVIRONMENT

The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl derivatives mainly occurs in the dust shells of asymptotic giant branch (AGB) stars. The bands at 3.3 and 3.4 μm, observed in infrared emission spectra of several objects, are attributed C–H vibrational modes in aromatic and aliphatic structures, respectively. In general, the feature at 3.3 μm is more intense than that at 3.4 μm. Photoionization and photodissociation processes of toluene, the precursor of methylated PAHs, were studied using synchrotron radiation at soft X-ray energies around the carbon K edge using time-of-flight mass spectrometry. Partial ion yields of a large number of ionic fragments were extracted from single and 2D-spectra, where electron-ion coincidences have revealed the doubly charged parent molecule and several doubly charged fragments containing seven carbon atoms with considerable abundance. Ab initio calculations based on density functional theory were performed in order to elucidate the chemical structure of these stable dicationic species. The survival of the dications subjected to hard inner shell ionization suggests that they could be observed in the interstellar medium, especially in regions where PAHs are detected. The ionization and destruction of toluene induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB star. In this context, a minimum photodissociation radius and the half-life of toluene subjected to the incidence of the soft X-ray flux emitted from a companion white dwarf star were determined.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

The optical–infrared extinction curve and its variation in the Milky Way

The dust extinction curve is a critical component of many observational programs and an important diagnostic of the physics of the interstellar medium. Here we present new measurements of the dust extinction curve and its variation toward tens of thousands of stars, a hundred-fold larger sample than in existing detailed studies. We use data from the APOGEE spectroscopic survey in combination with ten-band photometry from Pan-STARRS1, the Two Micron All-Sky Survey, and Wide-field Infrared Survey Explorer. We find that the extinction curve in the optical through infrared is well characterized by a one-parameter family of curves described by R(V). The extinction curve is more uniform than suggested in past works, with σ(R(V))=0.18, and with less than one percent of sight lines having R(V)>4. Our data and analysis have revealed two new aspects of Galactic extinction: first, we find significant, wide-area variations in R(V) throughout the Galactic plane. These variations are on scales much larger than individual molecular clouds, indicating that R(V) variations must trace much more than just grain growth in dense molecular environments. Indeed, we find no correlation between R(V) and dust column density up to E(B−V)≈2. Second, we discover a strong relationship between R(V) and the far-infrared dust emissivity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

PSR B0329+54: STATISTICS OF SUBSTRUCTURE DISCOVERED WITHIN THE SCATTERING DISK ON RADIOASTRON BASELINES OF UP TO 235,000 km

We discovered fine-scale structure within the scattering disk of PSR B0329+54 in observations with the RadioAstron ground–space radio interferometer. Here we describe this phenomenon, characterize it with averages and correlation functions, and interpret it as the result of decorrelation of the impulse-response function of interstellar scattering between the widely separated antennas. This instrument included the 10 m Space Radio Telescope, the 110 m Green Bank Telescope, the 14 × 25 m Westerbork Synthesis Radio Telescope, and the 64 m Kalyazin Radio Telescope. The observations were performed at 324 MHz on baselines of up to 235,000 km in 2012 November and 2014 January. In the delay domain, on long baselines the interferometric visibility consists of many discrete spikes within a limited range of delays. On short baselines it consists of a sharp spike surrounded by lower spikes. The average envelope of correlations of the visibility function shows two exponential scales, with characteristic delays of τ{sub 1}=4.1±0.3 μs and τ{sub 2}=23±3 μs, indicating the presence of two scales of scattering in the interstellar medium. These two scales are present in the pulse-broadening function. The longer scale contains 0.38 times the scattered power of the shorter one. We suggest that the longer tail arises from highly scattered paths, possibly from anisotropic scattering or from substructure at large angles.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

PRODUCTION AND RECOIL LOSS OF COSMOGENIC NUCLIDES IN PRESOLAR GRAINS

Presolar grains are small particles that condensed in the vicinity of dying stars. Some of these grains survived the voyage through the interstellar medium (ISM) and were incorporated into meteorite parent bodies at the formation of the Solar System. An important question is when these stellar processes happened, i.e., how long presolar grains were drifting through the ISM. While conventional radiometric dating of such small grains is very difficult, presolar grains are irradiated with galactic cosmic rays (GCRs) in the ISM, which induce the production of cosmogenic nuclides. This opens the possibility to determine cosmic-ray exposure (CRE) ages, i.e., how long presolar grains were irradiated in the ISM. Here, we present a new model for the production and loss of cosmogenic {sup 3}He, {sup 6,7}Li, and {sup 21,22}Ne in presolar SiC grains. The cosmogenic production rates are calculated using a state-of-the-art nuclear cross-section database and a GCR spectrum in the ISM consistent with recent Voyager data. Our findings are that previously measured {sup 3}He and {sup 21}Ne CRE ages agree within the (sometimes large) 2 σ uncertainties and that the CRE ages for most presolar grains are smaller than the predicted survival times. The obtained results are relatively robust since interferences from implanted low-energy GCRs into the presolar SiC grains and/or from cosmogenic production within the meteoroid can be neglected.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Bright and faint ends of Lyα luminosity functions at z = 2 determined by the Subaru survey: implications for AGNs, magnification bias, and ISM H I evolution

We present the Lyα luminosity functions (LFs) derived by our deep Subaru narrowband survey that identifies a total of 3137 Lyα emitters (LAEs) at z = 2.2 in five independent blank fields. This sample of LAEs is the largest to date and covers a very wide Lyα luminosity range of logL{sub Lyα}=41.7--44.4 erg s{sup −1}. We determine the Lyα LF at z = 2.2 with unprecedented accuracy and obtain the best-fit Schechter parameters of L{sub Lyα}{sup ∗}=5.29{sub −1.13}{sup +1.67}×10{sup 42} erg s{sup −1}, ϕ{sub Lyα}{sup ∗}=6.32{sub −2.31}{sup +3.08}×10{sup −4} Mpc{sup −3}, and α=−1.75{sub −0.09}{sup +0.10}, showing a steep faint-end slope. We identify a significant hump at the LF bright end (logL{sub Lyα}>43.4 erg s{sup −1}). Because all of the LAEs in the bright-end hump have a bright counterpart(s) in either the X-ray, UV, or radio data, this bright-end hump is not made by gravitational lensing magnification bias but by active galactic nuclei (AGNs). These AGNs allow us to derive the AGN UV LF at z ∼ 2 down to the faint magnitude limit of M {sub UV} ≃ −22.5 and to constrain the faint-end slope of the AGN UV LF, α {sub AGN} = −1.2 ± 0.1, which is flatter than those at z > 4. Based on the Lyα and UV LFs from our and previous studies, we find an increase of Lyα escape fraction f{sub esc}{sup Lyα} from z ∼ 0 to 6 by two orders of magnitude. This large f{sub esc}{sup Lyα} increase can be explained neither by the evolution of stellar population nor by outflow alone, but by the evolution of neutral hydrogen H i density in the interstellar medium that enhances dust attenuation for Lyα by resonance scattering. Our uniform expanding shell models suggest that the typical H i column density decreases from N{sub HI}∼7×10{sup 19} (z ∼ 0) to ∼1 × 10{sup 18} cm{sup −2} (z ∼ 6) to explain the large f{sub esc}{sup Lyα} increase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

STICKING OF MOLECULES ON NONPOROUS AMORPHOUS WATER ICE

Accurate modeling of physical and chemical processes in the interstellar medium (ISM) requires detailed knowledge of how atoms and molecules adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H{sub 2}, D{sub 2}, N{sub 2}, O{sub 2}, CO, CH{sub 4}, and CO{sub 2} on nonporous amorphous solid water. The sticking coefficient was measured over a wide range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient—the King–Wells method—leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient, we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas–grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Bayesian analysis of cosmic ray propagation: evidence against homogeneous diffusion

We present the results of the most complete scan of the parameter space for cosmic ray (CR) injection and propagation. We perform a Bayesian search of the main GALPROP parameters, using the MultiNest nested sampling algorithm, augmented by the BAMBI neural network machine-learning package. This is the first study to separate out low-mass isotopes (p, p-bar , and He) from the usual light elements (Be, B, C, N, and O). We find that the propagation parameters that best-fit p, p-bar , and He data are significantly different from those that fit light elements, including the B/C and {sup 10}Be/{sup 9}Be secondary-to-primary ratios normally used to calibrate propagation parameters. This suggests that each set of species is probing a very different interstellar medium, and that the standard approach of calibrating propagation parameters using B/C can lead to incorrect results. We present posterior distributions and best-fit parameters for propagation of both sets of nuclei, as well as for the injection abundances of elements from H to Si. The input GALDEF files with these new parameters will be included in an upcoming public GALPROP update.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Explaining the stellar initial mass function with the theory of spatial networks

The distributions of stars and prestellar cores by mass (initial and dense core mass functions, IMF/DCMF) are among the key factors regulating star formation and are the subject of detailed theoretical and observational studies. Results from numerical simulations of star formation qualitatively resemble an observed mass function, a scale-free power law with a sharp decline at low masses. However, most analytic IMF theories critically depend on the empirically chosen input spectrum of mass fluctuations which evolve into dense cores and, subsequently, stars, and on the scaling relation between the amplitude and mass of a fluctuation. Here we propose a new approach exploiting techniques from the field of network science. We represent a system of dense cores accreting gas from the surrounding diffuse interstellar medium (ISM) as a spatial network growing by preferential attachment and assume that the ISM density has a self-similar fractal distribution following the Kolmogorov turbulence theory. We effectively combine gravoturbulent and competitive accretion approaches and predict the accretion rate to be proportional to the dense core mass: dM/dt∝M. Then we describe the dense core growth and demonstrate that the power-law core mass function emerges independently of the initial distribution of density fluctuations by mass. Our model yields a power law solely defined by the fractal dimensionalities of the ISM and accreting gas. With a proper choice of the low-mass cut-off, it reproduces observations over three decades in mass. We also rule out a low-mass star dominated “bottom-heavy” IMF in a single star-forming region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Spinning dust emission from ultra-small silicates: emissivity and polarization spectrum

Anomalous microwave emission (AME) is an important Galactic foreground of cosmic microwave background radiation. It is believed that AME arises from rotational emission by spinning polycyclic aromatic hydrocarbons in the interstellar medium (ISM). In this paper, we suppose that a new population of ultra-small silicate grains perhaps exists in the ISM, and quantify the rotational emissivity from these tiny particles and its polarization spectrum. We find that spinning silicate nanoparticles can produce strong rotational emission when the tiny grains follow a log-normal size distribution. The polarization fraction of spinning dust emission from tiny silicates increases with decreasing dipole moment per atom (β) and can reach P∼20% for β∼0.1 D at a grain temperature of 60 K. We identify a parameter space (β,Y{sub Si}), with Y{sub Si} being the fraction of Si abundance in nanoparticles, in which its rotational emission can adequately reproduce both the observed AME and the polarization of the AME, without violating the observational constraints of ultraviolet extinction and polarization of starlight. Our results reveal that rotational emission from spinning silicate may be an important source of AME.

79 ASTRONOMY AND ASTROPHYSICS↗