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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

VERY LARGE TELESCOPE/X-SHOOTER SPECTROSCOPY OF THE CANDIDATE BLACK HOLE X-RAY BINARY MAXI J1659-152 IN OUTBURST

We present the optical to near-infrared spectrum of MAXI J1659-152 during the onset of its 2010 X-ray outburst. The spectrum was obtained with X-shooter on the ESO Very Large Telescope early in the outburst simultaneous with high-quality observations at both shorter and longer wavelengths. At the time of the observations, the source was in the low-hard state. The X-shooter spectrum includes many broad ({approx}2000 km s{sup -1}), double-peaked emission profiles of H, He I, and He II, characteristic signatures of a low-mass X-ray binary during outburst. We detect no spectral signatures of the low-mass companion star. The strength of the diffuse interstellar bands results in a lower limit to the total interstellar extinction of A{sub V} {approx_equal} 0.4 mag. Using the neutral hydrogen column density obtained from the X-ray spectrum we estimate A{sub V} {approx_equal} 1 mag. The radial velocity structure of the interstellar Na I D and Ca II H and K lines results in a lower limit to the distance of {approx}4 {+-} 1 kpc, consistent with previous estimates. With this distance and A{sub V} , the dereddened spectral energy distribution represents a flat disk spectrum. The two 10 minute X-shooter spectra show significant variability in the red wing of the emission-line profiles, indicating a global change in the density structure of the disk, though on a timescale much shorter than the typical viscous timescale of the disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MAGNETIC SUBSTRUCTURE IN THE NORTHERN FERMI BUBBLE REVEALED BY POLARIZED MICROWAVE EMISSION

We report a correspondence between giant, polarized microwave structures emerging north from the Galactic plane near the Galactic center and a number of GeV gamma-ray features, including the eastern edge of the recently discovered northern Fermi Bubble. The polarized microwave features also correspond to structures seen in the all-sky 408 MHz total intensity data, including the Galactic center Spur. The magnetic field structure revealed by the Wilkinson Microwave Anisotropy Probe polarization data at 23 GHz suggests that neither the emission coincident with the Bubble edge nor the Galactic center Spur are likely to be features of the local interstellar medium. On the basis of the observed morphological correspondences, similar inferred spectra, and the similar energetics of all sources, we suggest a direct connection between the Galactic center Spur and the northern Fermi Bubble.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

FIRST SCIENCE RESULTS FROM SOFIA/FORCAST: THE MID-INFRARED VIEW OF THE COMPACT H II REGION W3A

The massive star-forming region W3 was observed with the faint object infrared camera for the SOFIA telescope as part of the Short Science program. The 6.4, 6.6, 7.7, 19.7, 24.2, 31.5, and 37.1 {mu}m bandpasses were used to observe the emission of polycyclic aromatic hydrocarbon (PAH) molecules, very small grains, and big grains. Optical depth and color temperature maps of W3A show that IRS2 has blown a bubble devoid of gas and dust of {approx}0.05 pc radius. It is embedded in a dusty shell of ionized gas that contributes 40% of the total 24 {mu}m emission of W3A. This dust component is mostly heated by far-ultraviolet, rather than trapped Ly{alpha} photons. This shell is itself surrounded by a thin ({approx}0.01 pc) photodissociation region where PAHs show intense emission. The infrared spectral energy distribution (SED) of three different zones located at 8'', 20'', and 25'' from IRS2 shows that the peak of the SED shifts toward longer wavelengths, when moving away from the star. Adopting the stellar radiation field for these three positions, DUSTEM model fits to these SEDs yield a dust-to-gas mass ratio in the ionized gas similar to that in the diffuse interstellar medium (ISM). However, the ratio of the IR-to-UV opacity of the dust in the ionized shell is increased by a factor of {approx_equal}3 compared to the diffuse ISM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A WARM MODE OF GAS ACCRETION ON FORMING GALAXIES

We present results from high-resolution cosmological hydrodynamical simulations of a Milky-Way-sized halo, aimed at studying the effect of feedback on the nature of gas accretion. Simulations include a model of interstellar medium and star formation, in which supernova (SN) explosions provide effective thermal feedback. We distinguish between gas accretion onto the halo, which occurs when gas particles cross the halo virial radius, and gas accretion onto the central galaxy, which takes place when gas particles cross the inner one-tenth of the virial radius. Gas particles can be accreted through three different channels, depending on the maximum temperature value, T{sub max}, reached during the particles' past evolution: a cold channel for T{sub max} < 2.5 Multiplication-Sign 10{sup 5} K, a hot one for T > 10{sup 6} K, and a warm one for intermediate values of T{sub max}. We find that the warm channel is at least as important as the cold one for gas accretion onto the central galaxy. This result is at variance with previous findings that the cold mode dominates gas accretion at high redshift. We ascribe this difference to the different SN feedback scheme implemented in our simulations. While results presented so far in the literature are based on uneffective SN thermal feedback schemes and/or the presence of a kinetic feedback, our simulations include only effective thermal feedback. We argue that observational detections of a warm accretion mode in the high-redshift circumgalactic medium would provide useful constraints on the nature of the feedback that regulates star formation in galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EXPLORING THE TIME DISPERSION OF THE IBEX-HI ENERGETIC NEUTRAL ATOM SPECTRA AT THE ECLIPTIC POLES

The Interstellar Boundary Explorer (IBEX) has observed energetic neutral atom (ENA) hydrogen emissions from the edge of the solar system for more than three years. The observations span energies from 0.01 to 6 keV FWHM. At energies greater than 0.5-6 keV, and for a travel distance of {approx}100 AU, the travel time difference between the slowest and the fastest ENA is more than a year. Therefore, we construct spectra including the effect that slower ENAs left the source at an earlier time than faster ones. If the source produces a steady rate of ENAs and the extinction does not vary, then we expect that the spectral shape would be time independent. However, while the extinction of ENAs has been fairly constant during the first two and a half years, the source appears to have changed, and thus the spectra at a single time may not represent the conditions at the source. IBEX's viewing allows continuous sampling of the ecliptic poles where fluxes can be continuously monitored. For a given source distance we construct spectra assuming that the measured ENAs left the source at roughly the same time. To accomplish this construction, we apply time lag corrections to the signal at different ENA energies that take into account the travel time difference. We show that the spectral shape at the poles exhibits a statistically significant change with time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

IS IGR J11014-6103 A PULSAR WITH THE HIGHEST KNOWN KICK VELOCITY?

We report on Chandra X-ray and Parkes radio observations of IGR J11014-6103, which is a possible pulsar wind nebula with a complex X-ray morphology and a likely radio counterpart. With the superb angular resolution of Chandra, we find evidence that a portion of the extended emission may be related to a bow shock due to the putative pulsar moving through the interstellar medium. The inferred direction of motion is consistent with IGR J11014-6103 having been born in the event that produced the supernova remnant (SNR) MSH 11-61A. If this association is correct, then previous constraints on the expansion of MSH 11-61A imply a transverse velocity for IGR J11014-6103 of 2400-2900 km s{sup -1}, depending on the SNR model used. This would surpass the kick velocities of any known pulsars and rival or surpass the velocities of any compact objects that are associated with SNRs. While it is important to confirm the nature of the source, our radio pulsation search did not yield a detection.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

DOUBLE BOW SHOCKS AROUND YOUNG, RUNAWAY RED SUPERGIANTS: APPLICATION TO BETELGEUSE

A significant fraction of massive stars are moving supersonically through the interstellar medium (ISM), either due to disruption of a binary system or ejection from their parent star cluster. The interaction of their wind with the ISM produces a bow shock. In late evolutionary stages these stars may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks. Recent three-dimensional simulations of the bow shock produced by the nearby runaway red supergiant (RSG) Betelgeuse, under the assumption of a constant wind, indicate that the bow shock is very young (<30, 000 years old), hence Betelgeuse may have only recently become an RSG. To test this possibility, we have calculated stellar evolution models for single stars which match the observed properties of Betelgeuse in the RSG phase. The resulting evolving stellar wind is incorporated into two-dimensional hydrodynamic simulations in which we model a runaway blue supergiant (BSG) as it undergoes the transition to an RSG near the end of its life. We find that the collapsing BSG wind bubble induces a bow shock-shaped inner shell around the RSG wind that resembles Betelgeuse's bow shock, and has a similar mass. Surrounding this is the larger-scale retreating bow shock generated by the now defunct BSG wind's interaction with the ISM. We suggest that this outer shell could explain the bar feature located (at least in projection) just in front of Betelgeuse's bow shock.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SOFT X-RAY IRRADIATION OF H{sub 2}S ICE AND THE PRESENCE OF S{sub 2} IN COMETS

Little is known about the effects of X-rays in interstellar ices. To understand the sulfur depletion in dense clouds and the presence of S{sub 2} in comets, we simulated experimentally the soft X-ray processing (0.3 keV) of H{sub 2}S ice for the first time. Experiments were performed under ultrahigh vacuum conditions at 8 K using infrared and quadrupole mass spectrometry to monitor the solid and gas phases, respectively. A UV irradiation experiment using a similar dose was made for comparison. After X-ray irradiation, an infrared absorption appears near 4.0 {mu}m which is attributed to H{sub 2}S{sub 2} formation in the ice. This identification is also supported by the desorption at 133 K of m/z 66, 65, 64, corresponding to the mass fragments of H{sub 2}S{sub 2}. The H{sub 2}S{sub 2} species is expected to be present in interstellar and cometary ices that were processed by X-rays. Further irradiation leads to dissociation of this molecule forming S{sub 2} and larger S-molecules up to S{sub 8}, which may explain the depletion of sulfur in dense clouds. CS{sub 2} was so far the parent molecule proposed for S{sub 2} formation in comets. But the abundance of H{sub 2}S{sub 2}, formed by irradiation of pure H{sub 2}S or H{sub 2}S in an H{sub 2}O-ice matrix, should be larger than that of CS{sub 2} in the ice, the latter requiring a carbon source for its formation. Based on our experimental results, we propose that S{sub 2} in comets could be formed by dissociation of H{sub 2}S{sub 2} in the ice.

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

RADIO SEARCH FOR H{sub 2}CCC TOWARD HD 183143 AS A CANDIDATE FOR A DIFFUSE INTERSTELLAR BAND CARRIER

To clarify the authenticity of a recently proposed identification of H{sub 2}CCC (linear-C{sub 3}H{sub 2}) as a diffuse interstellar band (DIB) carrier, we searched for the rotational transition of H{sub 2}CCC at a frequency of 103 GHz toward HD 183143 using the 45 m telescope at the Nobeyama Radio Observatory. Although rms noise levels of 32 mK in the antenna temperature were achieved, detection of H{sub 2}CCC was unsuccessful, producing a 3{sigma} upper limit corresponding to a column density of 2.0 Multiplication-Sign 10{sup 13} cm{sup -2}. The upper limit indicates that the contribution of H{sub 2}CCC to the DIB at 5450 Angstrom-Sign is less than 1/25; thus, it is unlikely that the laboratory bands of the B{sup 1} B{sub 1}-X{sup 1} A{sub 1} transition of H{sub 2}CCC and the DIBs at 5450 Angstrom-Sign (and also 4881 Angstrom-Sign ) toward HD 183143 are related.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE ABUNDANCE OF C{sub 3}H{sub 2} AND OTHER SMALL HYDROCARBONS IN THE DIFFUSE INTERSTELLAR MEDIUM

Hydrocarbons are ubiquitous in the interstellar medium, observed in diverse environments ranging from diffuse to molecular dark clouds and strong photon-dominated regions near H II regions. Recently, two broad diffuse interstellar bands (DIBs) at 4881 A and 5450 A were attributed to the linear version of propynylidene l-C{sub 3}H{sub 2}, a species whose more stable cyclic conformer c-C{sub 3}H{sub 2} has been widely observed in the diffuse interstellar medium at radio wavelengths. This attribution has already been criticized on the basis of indirect plausibility arguments because the required column densities are quite large, N(l-C{sub 3}H{sub 2})/E{sub B-V} =4 Multiplication-Sign 10{sup 14} cm{sup -2} mag{sup -1}. Here we present new measurements of N(l-C{sub 3}H{sub 2}) based on simultaneous 18-21 GHz Very Large Array absorption profiles of cyclic and linear C{sub 3}H{sub 2} taken along sight lines toward extragalactic radio-continuum background sources with foreground Galactic reddening E{sub B-V} = 0.1-1.6 mag. We find that N(l-C{sub 3}H{sub 2})/N(c-C{sub 3}H{sub 2}) Almost-Equal-To 1/15-1/40 and N(l-C{sub 3}H{sub 2})/E{sub B-V} Almost-Equal-To (2 {+-} 1) Multiplication-Sign 10{sup 11} cm{sup -2} mag{sup -1}, so that the column densities of l-C{sub 3}H{sub 2} needed to explain the DIBs are some three orders of magnitude higher than what is observed. We also find N(C{sub 4}H)/E{sub B-V} <1.3 Multiplication-Sign 10{sup 13} cm{sup -2} mag{sup -1} and N(C{sub 4}H{sup -})/E{sub B-V} <1 Multiplication-Sign 10{sup 11} cm{sup -2} mag{sup -1} (3{sigma}). Using available data for CH and C{sub 2}H we compare the abundances of small hydrocarbons in diffuse and dark clouds as a guide to their ability to contribute as DIB carriers over a wide range of conditions in the interstellar medium.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

A GAS-PHASE FORMATION ROUTE TO INTERSTELLAR TRANS-METHYL FORMATE

The abundance of methyl formate in the interstellar medium has previously been underpredicted by chemical models. Additionally, grain surface chemistry cannot account for the relative abundance of the cis- and trans-conformers of methyl formate, and the trans-conformer is not even formed at detectable abundance on these surfaces. This highlights the importance of studying formation pathways to methyl formate in the gas phase. The rate constant and branching fractions are reported for the gas-phase reaction between protonated methanol and formic acid to form protonated trans-methyl formate and water as well as adduct ion: Rate constants were experimentally determined using a flowing afterglow-selected ion flow tube apparatus at 300 K and a pressure of 530 mTorr helium. The results indicate a moderate overall rate constant of (3.19 {+-} 0.39) Multiplication-Sign 10{sup -10} cm{sup 3} s{sup -1} ({+-} 1{sigma}) and an average branching fraction of 0.05 {+-} 0.04 for protonated trans-methyl formate and 0.95 {+-} 0.04 for the adduct ion. These experimental results are reinforced by ab initio calculations at the MP2(full)/aug-cc-pVTZ level of theory to examine the reaction coordinate and complement previous density functional theory calculations. This study underscores the need for continued observational studies of trans-methyl formate and for the exploration of other gas-phase formation routes to complex organic molecules.

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

BL LAC OBJECT PKS B1144-379: AN EXTREME SCINTILLATOR

Rapid variability in the radio flux density of the BL Lac object PKS B1144-379 has been observed at four frequencies, ranging from 1.5 to 15 GHz, with the Very Large Array and the University of Tasmania's Ceduna antenna. Intrinsic and line-of-sight effects were examined as possible causes of this variability, with interstellar scintillation best explaining the frequency dependence of the variability timescales and modulation indices. This scintillation is consistent with a compact source 20-40 {mu}as or 0.15-0.3 pc in size. The inferred brightness temperature for PKS B1144-379 (assuming that the observed variations are due to scintillation) is 6.2 Multiplication-Sign 10{sup 12} K at 4.9 GHz, with approximately 10% of the total flux in the scintillating component. We show that scintillation surveys aimed at identifying variability timescales of days to weeks are an effective way to identify the active galactic nuclei with the highest brightness temperatures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MAGNETIC FIELD TRANSPORT FROM DISK TO HALO VIA THE GALACTIC CHIMNEY PROCESS IN NGC 6946

The interstellar medium (ISM) in galaxies is directly affected by the mass and energy outflows originating in regions of star formation. Magnetic fields are an essential ingredient of the ISM, but their connection to the gaseous medium and its evolution remains poorly understood. Here, we present the detection of a gradient in Faraday rotation measure (RM), co-located with a hole in the neutral hydrogen (H I) distribution in the disk of the nearby spiral galaxy NGC 6946. The gas kinematics in the same location show evidence for infall of cold gas. The combined characteristics of this feature point to a substantial vertical displacement of the initially plane-parallel-ordered magnetic field, driven by a localized star formation event. This reveals how the large-scale magnetic field pattern in galaxy disks is directly influenced by internal energetic phenomena. Conversely, magnetic fields are observed to be an important ingredient in disk-halo interactions, as predicted in MHD simulations. Turbulent magnetic fields at smaller spatial scales than the observed RM gradient will also be carried from the disk and provide a mechanism for the dynamo process to amplify the ordered magnetic field without quenching. We discuss the observational biases and suggest that this is a common feature of star-forming galaxies with active disk-halo flows.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ISOTOPIC ANOMALIES IN PRIMITIVE SOLAR SYSTEM MATTER: SPIN-STATE-DEPENDENT FRACTIONATION OF NITROGEN AND DEUTERIUM IN INTERSTELLAR CLOUDS

Organic material found in meteorites and interplanetary dust particles is enriched in D and {sup 15}N. This is consistent with the idea that the functional groups carrying these isotopic anomalies, nitriles and amines, were formed by ion-molecule chemistry in the protosolar nebula. Theoretical models of interstellar fractionation at low temperatures predict large enrichments in both D and {sup 15}N and can account for the largest isotopic enrichments measured in carbonaceous meteorites. However, more recent measurements have shown that, in some primitive samples, a large {sup 15}N enrichment does not correlate with one in D, and that some D-enriched primitive material displays little, if any, {sup 15}N enrichment. By considering the spin-state dependence in ion-molecule reactions involving the ortho and para forms of H{sub 2}, we show that ammonia and related molecules can exhibit such a wide range of fractionation for both {sup 15}N and D in dense cloud cores. We also show that while the nitriles, HCN and HNC, contain the greatest {sup 15}N enrichment, this is not expected to correlate with extreme D enrichment. These calculations therefore support the view that solar system {sup 15}N and D isotopic anomalies have an interstellar heritage. We also compare our results to existing astronomical observations and briefly discuss future tests of this model.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

AE AURIGAE: FIRST DETECTION OF NON-THERMAL X-RAY EMISSION FROM A BOW SHOCK PRODUCED BY A RUNAWAY STAR

Runaway stars produce shocks when passing through interstellar medium at supersonic velocities. Bow shocks have been detected in the mid-infrared for several high-mass runaway stars and in radio waves for one star. Theoretical models predict the production of high-energy photons by non-thermal radiative processes in a number sufficiently large to be detected in X-rays. To date, no stellar bow shock has been detected at such energies. We present the first detection of X-ray emission from a bow shock produced by a runaway star. The star is AE Aur, which was likely expelled from its birthplace due to the encounter of two massive binary systems and now is passing through the dense nebula IC 405. The X-ray emission from the bow shock is detected at 30'' northeast of the star, coinciding with an enhancement in the density of the nebula. From the analysis of the observed X-ray spectrum of the source and our theoretical emission model, we confirm that the X-ray emission is produced mainly by inverse Compton upscattering of infrared photons from dust in the shock front.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

FIRST INTERSTELLAR HCO{sup +} MASER

A previously unseen maser in the J = 1-0 transition of HCO{sup +} has been detected by the Combined Array for Millimeter-wave Astronomy (CARMA). A subarcsecond map was produced of the 2 arcmin{sup 2} region around DR21(OH), which has had previous detections of OH and methanol masers. This new object has remained undetected until now due to its extremely compact size. The object has a brightness temperature of >2500 K and an FWHM linewidth of 0.497 km s{sup -1}, both of which suggest non-thermal line emission consistent with an unsaturated maser. This object coincides in position and velocity with the methanol maser named DR21(OH)-1 by Plambeck and Menten. No compact HCO{sup +} emission was present in the CARMA data toward the other methanol masers described in that Letter. These new results support the theory introduced in Plambeck and Menten that these masers likely arise from strong outflows interacting with low mass, high density pockets of molecular gas. This is further supported by recent observations of a CO outflow by Zapata et al. that traces the outflow edges and confirms that the maser position lies along the edge of the outflow where interaction with molecular tracers can occur.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

STAR FORMATION LAWS AND THRESHOLDS FROM INTERSTELLAR MEDIUM STRUCTURE AND TURBULENCE

We present an analytical model of the relation between the surface density of gas and star formation rate in galaxies and clouds, as a function of the presence of supersonic turbulence and the associated structure of the interstellar medium (ISM). The model predicts a power-law relation of index 3/2, flattened under the effects of stellar feedback at high densities or in very turbulent media, and a break at low surface densities when ISM turbulence becomes too weak to induce strong compression. This model explains the diversity of star formation laws and thresholds observed in nearby spirals and their resolved regions, the Small Magellanic Cloud, high-redshift disks and starbursting mergers, as well as Galactic molecular clouds. While other models have proposed interstellar dust content and molecule formation to be key ingredients to the observed variations of the star formation efficiency, we demonstrate instead that these variations can be explained by ISM turbulence and structure in various types of galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE COLUMN DENSITY VARIANCE IN TURBULENT INTERSTELLAR MEDIA: A FRACTAL MODEL APPROACH

Fractional Brownian motion structures are used to investigate the dependency of column density variance ({sigma}{sup 2}{sub lnN}) in the turbulent interstellar medium on the variance of three-dimensional density ({sigma}{sup 2}{sub ln{rho}}) and the power-law slope of the density power spectrum. We provide quantitative expressions to infer the three-dimensional density variance, which is not directly observable, from the observable column density variance and spectral slope. We also investigate the relationship between the column density variance and sonic Mach number (M{sub s}) in the hydrodynamic (HD) regime by assuming the spectral slope and density variance to be functions of sonic Mach number, as obtained from the HD turbulence simulations. They are related by the expression {sigma}{sup 2}{sub lnN} = A{sigma}{sub ln{rho}} {sup 2} = Aln (1 + b {sup 2} M{sup 2}{sub s}), suggested by Burkhart and Lazarian for the magnetohydrodynamic case. The proportional constant A varies from Almost-Equal-To 0.2 to Almost-Equal-To 0.4 in the HD regime as the turbulence forcing parameter b increases from 1/3 (purely solenoidal forcing) to 1 (purely compressive forcing). It is also discussed that the parameter A is lowered in the presence of a magnetic field.

79 ASTRONOMY AND ASTROPHYSICS↗