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RISS 인기검색어
- 검색키워드
- 저자 : Turner-Evans, D (검색결과 3 건)
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Yu, Mo,Evans II, Neal J.,Dodson-Robinson, Sarah E.,Willacy, Karen,Turner, Neal J. American Astronomical Society 2017 The Astrophysical journal Vol.850 No.2
<P>Turbulence is the leading candidate for angular momentum transport in protoplanetary disks and therefore influences disk lifetimes and planet formation timescales. However, the turbulent properties of protoplanetary disks are poorly constrained observationally. Recent studies have found turbulent speeds smaller than what fullydeveloped MRI would produce (Flaherty et al.). However, existing studies assumed a constant CO/H-2 ratio of 10(-4) in locations where CO is not frozen-out or photo-dissociated. Our previous studies of evolving disk chemistry indicate that CO is depleted by incorporation into complex organic molecules well inside the freeze-out radius of CO. We consider the effects of this chemical depletion on measurements of turbulence. Simon et al. suggested that the ratio of the peak line flux to the flux at line center of the CO J = 3-2 transition is a reasonable diagnostic of turbulence, so we focus on that metric, while adding some analysis of the more complex effects on spatial distribution. We simulate the emission lines of CO based on chemical evolution models presented in Yu et al., and find that the peak-to-trough ratio changes as a function of time as CO is destroyed. Specifically, a CO-depleted disk with high turbulent velocity mimics the peak-to-trough ratios of a non-CO-depleted disk with lower turbulent velocity. We suggest that disk observers and modelers take into account the possibility of CO depletion when using line profiles or peak-to-trough ratios to constrain the degree of turbulence in disks. Assuming that CO/H-2 = 10(-4) at all disk radii can lead to underestimates of turbulent speeds in the disk by at least 0.2 km s(-1).</P>
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Disk Masses around Solar-mass Stars are Underestimated by CO Observations
Yu, Mo,Evans II, Neal J.,Dodson-Robinson, Sarah E.,Willacy, Karen,Turner, Neal J. American Astronomical Society 2017 The Astrophysical journal Vol.841 No.1
<P>Gas in protostellar disks provides. the raw material for giant planet formation and controls the dynamics of the planetesimal-building dust grains. Accurate gas mass measurements help map the observed properties of planet-forming disks onto the formation environments of known exoplanets. Rare isotopologues of carbon monoxide (CO) have been used as gas mass tracers for disks in the Lupus star-forming region, with an assumed interstellar CO/H-2 abundance ratio. Unfortunately, observations of T-Tauri disks show that CO abundance is not interstellar, a finding reproduced by models that show CO abundance decreasing both with distance from the star and as a function of time. Here, we present radiative transfer simulations that assess the accuracy of CO-based disk mass measurements. We find that the combination of CO chemical depletion in the outer disk and optically thick emission from the inner disk leads observers to underestimate gas mass by more than an order of magnitude if they use the standard assumptions of interstellar CO/H-2 ratio and optically thin emission. Furthermore, CO abundance changes on million-year timescales, introducing an age/mass degeneracy into observations. To reach a. factor of a few accuracy for CO-based disk mass measurements, we suggest that observers and modelers adopt the following strategies: (1) select. low-J transitions; (2) observe multiple CO isotopologues and use either intensity ratios or normalized line profiles to diagnose CO chemical depletion; and (3) use spatially resolved observations to measure the CO-abundance distribution.</P>
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Electrical characterization of single GaN nanowires
Stern, E,Cheng, G,Cimpoiasu, E,Klie, R,Guthrie, S,Klemic, J,Kretzschmar, I,Steinlauf, E,Turner-Evans, D,Broomfield, E,Hyland, J,Koudelka, R,Boone, T,Young, M,Sanders, A,Munden, R,Lee, T,Routenberg, D IOP Pub 2005 Nanotechnology Vol.16 No.12
<P>In this paper a statistically significant study of 1096 individual GaN nanowire (NW) devices is presented. We have correlated the effects of changing growth parameters for hot-wall chemically-vapour-deposited (HW-CVD) NWs fabricated via the vapour–liquid–solid mechanism. We first describe an optical lithographic method for creating Ohmic contacts to NW field effect transistors with both top and bottom electrostatic gates to characterize carrier density and mobility. Multiprobe measurements show that carrier modulation occurs in the channel and is not a contact effect. We then show that NW fabrication runs with nominally identical growth parameters yield similar electrical results across sample populations of >50 devices. By systematically altering the growth parameters we were able to decrease the average carrier concentration for these as-grown GaN NWs ∼10-fold, from 2.29 × 10<SUP>20</SUP> to 2.45 × 10<SUP>19</SUP> cm<SUP>−3</SUP>, and successfully elucidate the parameters that exert the strongest influence on wire quality. Furthermore, this study shows that nitrogen vacancies, and not oxygen impurities, are the dominant intrinsic dopant in HW-CVD GaN NWs.</P>
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