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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 163 records · Page 9
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DOE OSTI2016

A machine-learning approach to measuring the escape of ionizing radiation from galaxies in the reionization epoch

Recent observations of galaxies at z≳7, along with the low value of the electron scattering optical depth measured by the Planck mission, make galaxies plausible as dominant sources of ionizing photons during the epoch of reionization. However, scenarios of galaxy-driven reionization hinge on the assumption that the average escape fraction of ionizing photons is significantly higher for galaxies in the reionization epoch than in the local universe. The NIRSpec instrument on the James Webb Space Telescope (JWST) will enable spectroscopic observations of large samples of reionization-epoch galaxies. While the leakage of ionizing photons will not be directly measurable from these spectra, the leakage is predicted to have an indirect effect on the spectral slope and the strength of nebular emission lines in the rest-frame ultraviolet and optical. Here, we apply a machine learning technique known as lasso regression on mock JWST/NIRSpec observations of simulated z = 7 galaxies in order to obtain a model that can predict the escape fraction from JWST/NIRSpec data. Barring systematic biases in the simulated spectra, our method is able to retrieve the escape fraction with a mean absolute error of Δf{sub esc}≈0.12 for spectra with signal-to-noise ratio ≈ 5 at a rest-frame wavelength of 1500 Å for our fiducial simulation. This prediction accuracy represents a significant improvement over previous similar approaches.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Galaxy properties and UV escape fractions during the epoch of reionization: results from the renaissance simulations

Cosmic reionization is thought to be primarily fueled by the first generations of galaxies. We examine their stellar and gaseous properties, focusing on the star formation rates and the escape of ionizing photons, as a function of halo mass, redshift, and environment using the full suite of the Renaissance Simulations with an eye to provide better inputs to global reionization simulations. This suite probes overdense, average, and underdense regions of the universe of several hundred comoving Mpc{sup 3}, each yielding a sample of over 3000 halos in the mass range of 10{sup 7}–10{sup 9.5} M{sub ⊙} at their final redshifts of 15, 12.5, and 8, respectively. In the process, we simulate the effects of radiative and supernova feedback from 5000 to 10,000 Population III stars in each simulation. We find that halos as small as 10{sup 7} M{sub ⊙} are able to host bursty star formation due to metal-line cooling from earlier enrichment by massive Population III stars. Using our large sample, we find that the galaxy-halo occupation fraction drops from unity at virial masses above 10{sup 8.5} M{sub ⊙} to ∼50% at 10{sup 8} M{sub ⊙} and ∼10% at 10{sup 7} M{sub ⊙}, quite independent of redshift and region. Their average ionizing escape fraction is ∼5% in the mass range of 10{sup 8}–10{sup 9} M{sub ⊙} and increases with decreasing halo mass below this range, reaching 40%–60% at 10{sup 7} M{sub ⊙}. Interestingly, we find that the escape fraction varies between 10%–20% in halos with virial masses of ∼3 × 10{sup 9} M{sub ⊙}. Taken together, our results confirm the importance of the smallest galaxies as sources of ionizing radiation contributing to the reionization of the universe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ULTRAVIOLET ESCAPE FRACTIONS FROM GIANT MOLECULAR CLOUDS DURING EARLY CLUSTER FORMATION

The UV photon escape fraction from molecular clouds is a key parameter for understanding the ionization of the interstellar medium and extragalactic processes such as cosmic reionization. We present the ionizing photon flux and the corresponding photon escape fraction ( f {sub esc}) arising as a consequence of star cluster formation in a turbulent, 10{sup 6} M {sub ⊙} giant molecular cloud, simulated using the code FLASH. We make use of sink particles to represent young, star-forming clusters coupled with a radiative transfer scheme to calculate the emergent UV flux. We find that the ionizing photon flux across the cloud boundary is highly variable in time and space due to the turbulent nature of the intervening gas. The escaping photon fraction remains at ∼5% for the first 2.5 Myr, followed by two pronounced peaks at 3.25 and 3.8 Myr with a maximum f {sub esc} of 30% and 37%, respectively. These peaks are due to the formation of large H ii regions that expand into regions of lower density, some of which reaching the cloud surface. However, these phases are short-lived, and f {sub esc} drops sharply as the H ii regions are quenched by the central cluster passing through high-density material due to the turbulent nature of the cloud. We find an average f {sub esc} of 15% with factor of two variations over 1 Myr timescales. Our results suggest that assuming a single value for f {sub esc} from a molecular cloud is in general a poor approximation, and that the dynamical evolution of the system leads to large temporal variation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Exosphere Modeling of Proxima b: A Case Study of Photochemical Escape with a Venus-like Atmosphere

Exoplanets orbiting M dwarfs within habitable zones are exposed to stellar environments more extreme than that terrestrial planets experience in our solar system, which can significantly impact the atmospheres of the exoplanets and affect their habitability and sustainability. This study provides the first prediction of hot oxygen corona structure and the associated photochemical loss from a 1 bar CO 2 -dominated atmosphere of a Venus-like rocky exoplanet, where dissociative recombination of O 2 + ions is assumed to be the major source reaction for the escape of neutral O atoms and formation of the hot O corona (or exospheres) as on Mars and Venus. We employ a 3D Monte Carlo code to simulate the exosphere of Proxima Centauri b (PCb) based on the ionosphere simulated by a 3D magnetohydrodynamic model. Our simulation results show that variability of the stellar wind dynamic pressure over one orbital period of PCb does not affect the overall spatial structure of the hot O corona but contributes to the change in the global hot O escape rate that varies by an order of magnitude. The escape increases dramatically when the planet possesses its intrinsic magnetic fields as the ionosphere becomes more extended with the presence of a global magnetic field. Furthermore, the extended hot O corona may lead to a more extended H exosphere through collisions between thermal H and hot O, which exemplifies the importance of considering nonthermal populations in exospheres to interpret future observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Substructure at High Speed. I. Inferring the Escape Velocity in the Presence of Kinematic Substructure

Abstract The local escape velocity provides valuable inputs to the mass profile of the galaxy, and requires understanding the tail of the stellar speed distribution. Following Leonard & Tremaine, various works have since modeled the tail of the stellar speed distribution as ∝ ( v esc − v ) k , where v esc is the escape velocity, and k is the slope of the distribution. In such studies, however, these two parameters were found to be largely degenerate and often a narrow prior is imposed on k in order to constrain v esc . Furthermore, the validity of the power-law form can breakdown in the presence of multiple kinematic substructures or other mis-modeled features in the data. In this paper, we introduce a strategy that for the first time takes into account the presence of kinematic substructure. We model the tail of the velocity distribution as a sum of multiple power laws as a way of introducing a more flexible fitting framework. Using mock data and data from FIRE simulations of Milky Way-like galaxies, we show the robustness of this method in the presence of kinematic structure that is similar to the recently discovered Gaia Sausage. In a companion paper, we present the new measurement of the escape velocity and subsequently the mass of the Milky Way using Gaia eDR3 data.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The impact of unresolved turbulence on the escape fraction of Lyman Continuum photons

We investigate the relation between the turbulent Mach number (M) and the escape fraction of Lyman continuum photons (f{sub esc}) in high-redshift galaxies. Approximating the turbulence as isothermal and isotropic, we show that the increase in the variance in column densities from M=1 to M=10 causes f{sub esc} to increase by ≈25%, and the increase from M=1 to M=20 causes f{sub esc} to increases by ≈50% for a medium with opacity τ≈1. At a fixed Mach number, the correction factor for escape fraction relative to a constant column density case scales exponentially with the opacity in the cell, which has a large impact for simulated star-forming regions. Furthermore, in simulations of isotropic turbulence with full atomic/ionic cooling and chemistry, the fraction of HI drops by a factor of ≈2.5 at M≈10 even when the mean temperature is ≈5×10{sup 3} K. If turbulence is unresolved, these effects together enhance f{sub esc} by a factor >3 at Mach numbers above 10. Such Mach numbers are common at high redshifts where vigorous turbulence is driven by supernovae, gravitational instabilities, and merger activity, as shown both by numerical simulations and observations. These results, if implemented in the current hydrodynamical cosmological simulations to account for unresolved turbulence, can boost the theoretical predictions of the Lyman Continuum photon escape fraction and further constrain the sources of reionization.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

High Lyman continuum escape fraction in a lensed young compact dwarf galaxy at z = 2.5

We present the HST WFC3/F275W UV imaging observations of A2218-Flanking, a lensed compact dwarf galaxy at redshift z≈2.5. The stellar mass of A2218-Flanking is log(M{sub ∗}/M{sub ⊙})=9.14{sub −0.04}{sup +0.07} and SFR is 12.5{sub −7.4}{sup +3.8} M{sub ⊙} yr{sup −1} after correcting the magnification. This galaxy has a young galaxy age of 127 Myr and a compact galaxy size of r{sub 1/2}=2.4 kpc. The HST UV imaging observations cover the rest-frame Lyman continuum (LyC) emission (∼800 Å) from A2218-Flanking. We firmly detect (14σ) the LyC emission in A2218-Flanking in the F275W image. Together with the HST F606W images, we find that the absolute escape fraction of LyC is f{sub abs,esc}>28%--57% based on the flux density ratio between 1700 and 800 Å (f{sub 1700}/f{sub 800}). The morphology of the LyC emission in the F275W images is extended and follows the morphology of the UV continuum morphology in the F606W images, suggesting that the f {sub 800} is not from foreground contaminants. We find that the region with a high star formation rate surface density has a lower f{sub 1700}/f{sub 800} (higher f{sub 800}/f{sub 1700}) ratio than the diffused regions, suggesting that LyC photons are more likely to escape from the region with the intensive star-forming process. We compare the properties of galaxies with and without LyC detections and find that LyC photons are easier to escape in low-mass galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Lyman Continuum Escape Fraction of Emission Line-selected z ∼ 2.5 Galaxies Is Less Than 15%

Recent work suggests that strong emission line, star-forming galaxies (SFGs) may be significant Lyman continuum leakers. We combine archival Hubble Space Telescope broadband ultraviolet and optical imaging (F275W and F606W, respectively) with emission line catalogs derived from WFC3 IR G141 grism spectroscopy to search for escaping Lyman continuum (LyC) emission from homogeneously selected z ∼ 2.5 SFGs. We detect no escaping Lyman continuum from SFGs selected on [O ii] nebular emission ( N = 208) and, within a narrow redshift range, on [O iii]/[O ii]. We measure 1 σ upper limits to the LyC escape fraction relative to the non-ionizing UV continuum from [O ii] emitters, f {sub esc} ≲ 5.6%, and strong [O iii]/[O ii] > 5 ELGs, f {sub esc} ≲ 14.0%. Our observations are not deep enough to detect f {sub esc} ∼ 10% typical of low-redshift Lyman continuum emitters. However, we find that this population represents a small fraction of the star-forming galaxy population at z ∼ 2. Thus, unless the number of extreme emission line galaxies grows substantially to z ≳ 6, such galaxies may be insufficient for reionization. Deeper survey data in the rest-frame ionizing UV will be necessary to determine whether strong line ratios could be useful for pre-selecting LyC leakers at high redshift.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Safety in earth orbit study. Volume 3: Analysis of tumbling spacecraft, escape and rescue

Detailed and supporting analyses of the hazards of a disabled spacecraft in a tumbling mode and of crew escape, rescue, and survivability are presented for earth orbital operations of the space shuttle program. Tumbling conditions, arresting tumbling by external means, and escape from a tumbling vehicle are discussed. Details on design concepts for the modular space station, integral tank orbiter, drop tank orbiter, and a small space vehicle are included. Results of the crew safety study indicate that the shuttle orbiter should be the primary vehicle for dealing with emergencies and one vehicle should be available for rapid emergency rescue. An Apollo command module should be carried in the orbiter cargo bay in case an orbiter is not available. Approaches for a quicker escape or rescue capability are also recommended.

Source record↗
NASA NTRS1974

Technical evaluation of the Aerospace Medical Panel Specialists Meeting on Escape Problems and Manoeuvres in Combat Aircraft

A technical evaluation of the papers presented at a conference on escape systems for helicopters and V/STOL aircraft was made. The subjects discussed include the following: (1) bioengineering aspects of spinal injury during ejection, (2) aerodynamic forces acting on crewman during escape, (3) operational practicality of fly away ejection seats, (4) helicopter survivability requirements, (5) ejection experience from V/STOL aircraft, and (6) research projects involving escape and retrieval systems.

Jones, W. L.↗
NASA NTRS1975

The escape of magnetic flux from a turbulent body of gas

Topological conditions on the surface of a turbulent body of highly conductive fluid under which escape of magnetic force lines would be impossible (Drobyshevski, Yuferev 1974) are studied with exact calculation of intermediate field configurations as a possible explanation of the decay of stellar magnetic fields via turbulent mixing. It is shown that upward escape of magnetic force lines in the regions between convective cells is possible through rapid reconnection of the force lines (studied as neutral point annihilation) such as occurs continually in rapidly evolving sunspot groups. It is concluded that there exists no topological barrier to the escape of magnetic flux from the sun or from the local galaxy.

Parker, E. N.↗
NASA NTRS1975

Implications of atmospheric Ar-40 escape on the interior structure of the moon

Radiogenic Ar-40 escapes from the lunar atmosphere at a rate of about 2 x 10 to the 21st atoms/sec. This amounts to 8% of the rate of argon production in the entire moon by potassium decay. A curious feature of the argon escape rate is a variability with time scale of several months. It is shown that the variation in argon loss correlates with high-frequency lunar teleseismic events. The only apparent region of the moon which could possibly supply the amount of argon needed for escape via a plausible temporal mechanism is a semimolten asthenosphere which may be entirely primitive unfractionated lunar material, or an Fe-FeS core that is enriched in potassium. A core that is devoid of potassium is not compatible with the atmospheric argon measurements.

Hodges, R. R., Jr.↗
NASA NTRS1978

The diurnal and solar cycle variation of the charge exchange induced hydrogen escape flux

On the basis of ion temperature and density data at specific points and times in June 1969 provided by the OGO 6 satellite, and altitude profiles of the ion and electron temperature and concentration provided by the Arecibo radar facility over the period February 1972-April 1974, the diurnal and solar cycle variation of the charge-exchange-induced hydrogen escape flux was investigated. It was calculated that for low to moderate solar activity at Arecibo, the diurnal ratio of the maximum-to-minimum charge-exchange-induced hydrogen escape flux was approximately 6 with a peak around noon and a minimum somewhere between 0100 and 0300 h LT. This study of a limited amount of OGO 6 and Arecibo data seems to indicate that the charge-exchange-induced hydrogen escape flux increases as the F(10.7) flux increases for low to moderate solar activity.

Maher, L. J.↗
NASA NTRS1978

Gravitational and radiative effects on the escape of helium from the moon

On the moon, and probably on Mercury and other similar regolith-covered bodies with tenuous atmosphere, the dominant gas is He-4. It arises as the radiogenic product of the decay of uranium and thorium within any planet, but its major source appears to be the alpha particle flux of the solar wind. The moon intercepts solar wind helium at an average rate of 1.1 times 10 to the 24th atom/sec, and loses it at the same rate. Some helium may escape directly as the result of the process of solar wind soil bombardment which may release previously trapped helium at superthermal speeds. Atmospheric models have been calculated with the total helium influx as source. Subsequent comparison of model and measured helium concentrations indicates that the fraction of helium escaping via the atmosphere may range from 20% to 100% of the solar wind influx. Of the escaping atmosphere, most of the helium (about 93%) becomes trapped in earth orbit, while about 5% gets trapped in satellite orbits about the moon. Owing to a 6 month lifetime for helium in solar radiation, the satellite atoms form a lunar corona that exceeds the lunar atmosphere in total abundance by a factor of 4 to 5.

Hodges, R. R., Jr.↗
NASA NTRS1980

The latitudinal variation of the charge exchange induced atomic hydrogen escape flux

Using plasma data from the Isis 2 spacecraft and Arecibo radar, diffusive equilibrium models of the ionosphere were constructed for equinox conditions. These plasmaspheric models were combined with models of the neutral atmosphere to calculate the atomic hydrogen escape flux due to charge exchange between thermal protons and to calculate cooler hydrogen and oxygen atoms as a function of dipole latitude and local time. These calculations showed that the daytime escape flux increases as the absolute value of the dipole latitude decreases, reaching its maximum value at the magnetic equator. At 15 hours local time (LT) on March 23, 1972, the calculated escape flux varied from an insignificant amount at 55 deg dipole latitude, to 3 x 10 to the 8th atoms/sq cm sec at the magnetic equators.

Maher, L. J., Jr.↗
NASA NTRS1981

Theoretical quasar emission line ratios. IV - General asymptotic escape probabilities and the effects of linear Stark broadening

Established techniques permitting the evaluation of exact asymptotic forms of photon escape probabilities for a wide range of absorption coefficient profile types are used for QSO radiative transfer effect studies, and it is shown that the exact formulae are easily evaluated. It is found that some of the commonly used expressions overestimate the true value by a factor of about two, and a computationally expedient approximate expression is derived for the asymptotic form of the photon escape probability when linear Stark broadening contributes to the line absorption coefficient profile in hydrogenic lines. Attention is given to Lyman-alpha, for the case in which partial redistribution, over the linear Stark component of the line absorption coefficient profile, dominates single-flight photon escape.

Puetter, R. C.↗
NASA NTRS1982

Thermal and nonthermal escape mechanisms for terrestrial bodies

A survey is given of the classical thermal escape (Jeans or blowoff) and some eight nonthermal processes involving ions. Diffusional, energetic, and other limitations on the escape flux are discussed. Isotopic fractionation, due to differences in thermal escape rates or diffusive separation, is considered. The dominant processes acting at the five terrestrial planets and six major satellites are summarized. In general, nonthermal processes dominate over the classic thermal ones.

Hunten, D. M.↗