SEARCH · Search NASA
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Explore the source record for details and available documents.
The Large Magellanic Cloud (LMC) is the nearest (~50 kpc) star-forming galaxy characterized by a low metallicity (Z~0.3-0.5 Z⨀) similar to galaxies during the early phases of their assembly. As a result, star formation studies in the LMC provide a stepping stone to understanding star formation at earlier epochs of the universe where these processes cannot be directly observed. N113 is one of the most prominent star-forming regions in the LMC hosting one of the most massive giant molecular clouds. N113 is small enough to be imaged in its entirety, but large enough to showcase many important phenomena such as multiple generations of stars, stellar feedback, and different environments. We present our findings from an investigation of the early stages of star formation in the N113 region using the Atacama Large Millimeter/submillimeter Array (ALMA) molecular line data probing a wide density range: 12CO, 13CO, and C18O (2-1), 13CO and C18O (1-0), HCN (1-0), HCO+ (1-0), H13CN (1-0) and (3-2), H13CO+ (1-0) and (3-2), CS (2-1) and (5-4), as well as 1.3 mm and 3 mm continuum. We used the Python package quickclump to identify molecular clumps. We utilized the multiline non-LTE fitting tool based on models from RADEX developed by Finn et al. (2021, ApJ, 917, 106) to construct the CO, HCN, HCO+, and CS temperature and column density, and the H2 density maps of N113. We constructed a catalog of molecular clumps including their physical properties, chemical abundances, sizes, velocities, and velocity dispersions. To establish the evolutionary status of the clumps, their positions were compared with previously identified young stellar objects (YSOs) from the Spitzer/SAGE and Herschel/HERITAGE surveys, as well as water and OH masers. We compared the properties of the clumps in N113 to those in the Galaxy and other regions in the LMC to assess the impact of the environment (e.g., metallicity, stellar feedback) on the star formation process.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Differential carrier lifetime (DCL) measurements were performed on c-plane In 𝑥 Ga 1−𝑥 N/GaN single-quantum-well (QW) light-emitting diodes (LEDs) with varying indium-content QWs (x = 13.5%, 16%, 22%), emitting with violet, blue, and green wavelengths. The recombination lifetimes of LEDs were found to increase with increasing indium composition, resulting in increased carrier densities n measured by DCL. Extraction of the A coefficients, which are assumed to not vary with n, and of the effective B(n) and C(n) coefficients of the ABC model of the internal quantum efficiency (IQE) of QWs showed no significant changes in the A coefficient with increasing indium content [In] in the In 𝑥 Ga 1−𝑥 N QW, and a reduction in the B(n) and C(n) coefficients with increasing [In]. When looking at the Shockley-Read-Hall (SRH) recombination rate An, the radiative recombination rate B(n)𝑛 2 , and the Auger-Meitner (AM) recombination rate C(n)𝑛 3 , we observed that at any given n, the SRH rate is the same for the three [In] measured, while the radiative rate decreases (by up to approximately 9 times) and the AM rate decreases (by up to approximately 7 times) with increasing indium content. This shows that the larger reduction in radiative recombination rates relative to nonradiative recombination rates with increasing [In] is the largest contributor to the decreased quantum efficiencies of LEDs (the “green gap”) at any given n. While some of the reduction in the effective recombination coefficients B(n) and C(n) can be explained by the reduced wave-function overlaps of the QW with increasing indium content, the larger reduction of B(n) relative to C(n) motivates further study of the intrinsic recombination coefficients 𝐵 0 and 𝐶 0 of bulk In 𝑥 Ga 1−𝑥 N alloys and how they are related to the effective recombination coefficients B(n) and C(n) in QWs. The relative contributions of nonradiative recombination are enhanced at any given current density due to the sublinear relationship between the carrier density n and the current density. Thus, the solution to the green gap, up to any intrinsic limitations set by the indium content of the QW, from an IQE perspective, requires the design of long-wavelength (In,Ga)N LEDs with improved wave-function overlaps that can operate with a lower carrier density at any given current density.
In order to maximize efficiency a solar cell must: (1) absorb as much light as possible in electron-hole production, (2) transport as large a fraction as possible of the electrons to the n-type terminal and holes to the p-type terminal without their first recombining, and (3) produce as high as possible terminal voltage. Step (1) is largely fixed by the spectrum of sunlight and the fundamental absorption characteristics of silicon, although some improvements are possible through texturizing induced light trapping and back surface reflectors. Steps (2) and (3) are, however, dependent on the recombination mechanisms of the cell. The recombination, on the contrary, is strongly influenced by cell processing and design. Some of the lessons during the development of point-contact-cell are discussed. Cell dependence on recombination, surface recombination, and contact recombination are discussed. Results show the overwhelming influence of contact recombination on the operation of the cell when the other sources of recombination are reduced by careful processing.
A detailed analysis of current-voltage characteristics of N(+)-P/P solar cells indicate that there is a combination of different mechanisms which results in an enhancement in the dark current and in turn deteriorates the photovoltaic performance of the solar cells after 1 MeV e(-) irradiation. The increase in the dark current is due to three effects, i.e., bulk recombination, space charge recombination by deep traps and space charge recombination through shallow traps. It is shown that the increase in bulk recombination current is about 2 to 3 orders of magnitude whereas space charge recombination current due to shallow traps increases only by an order or so and no space charge recombination through deep traps was observed after irradiation. Thus, in order to improve the radiation hardness of these devices, bulk properties should be preserved.
Minimum recombination and low injection level are essential for high efficiency. Twenty percent AM1 efficiency requires a dark recombination current density of 2 x 10 to the minus 13th power A/sq cm and a recombination center density of less than 10 to the 10th power /cu cm. Recombination mechanisms at thirteen locations in a conventional single crystalline silicon cell design are reviewed. Three additional recombination locations are described at grain boundaries in polycrystalline cells. Material perfection and fabrication process optimization requirements for high efficiency are outlined. Innovative device designs to reduce recombination in the bulk and interfaces of single crystalline cells and in the grain boundary of polycrystalline cells are reviewed.
MWC349 is an emission-line star found by Merrill, Humason and Burwell (1932). Braes, Habing and Schoenmaker (1972) discovered that it is a strong radio source. The radio emission originates in a massive ionized wind that is expanding with a velocity of about 50 km s(-1). Its continuum spectrum fits well a nu(0.6) power law from the cm wavelengths to the far-IR. Radio recombination line emission from the envelope of MWC349 was first detected by Altenhoff, Strittmatter and Wendker (1981). We have obtained good signal-to-noise ratio, Very Large Array observations of the H76 alpha radio recombination line from the ionized wind of MWC349. Our data reveal that the profile is markedly asymmetric, with a steep rise on the blue side. This asymmetry could be due to non-LTE effects in the formation and transfer of the line or to intrinsic asymmetries in the envelope. Our analysis suggests that most probably the peculiar profile is caused by a non-LTE enhancement of the line emission from the side of the envelope nearer to the observer. This asymmetry has the opposite sense than that observed in optical and IR recombination lines, where a different effect (absorption of the stellar continuum by the gas in the wind between the star and the observer) is known to be dominant, leading to the classic P Cygni profile. We propose that the profiles of the radio recombination lines from ionized stellar winds will have this characteristic shape, while optical and IR recombination lines are characterized by P Cygni-like profiles. Unfortunately, at present the detection of radio recombination lines from ionized stellar winds is only feasible for MWC349 and a few other objects.
The thermoradiative cell is a new method for converting heat energy to electrical power, first detailed by Strandberg in 2015. The cell is a p-n junction semiconductor device, similar to a photovoltaic cell but thermodynamically operating in the reverse direction, converting the thermal dark current into electrical power while radiating waste heat to space. The power and efficiency can be calculated as a function of bandgap in the Shockley-Queisser detailed-balance limit, in which the thermal emissivity of the cell is due to the recombination of electron-hole pairs, and all other recombination losses are ignored. The current produced is directly proportional to the recombination radiation. The fundamental loss mechanism for the thermoradiative cell is the energy carried by the infrared radiation into space from band-to-band recombination of carriers injected across the junction. In an ideal cell, to maximize the efficiency, the emission energy of these photons would precisely equal the bandgap. This can be achieved, for example, using dielectric filters or meta-material filters to recycle emission at other wavelengths back into the cell. The voltage is proportional to the external bias. These two constraints allow optimization of the optimum bias point for maximum power. Unlike photovoltaic cells, the maximum power operating point is not the same as the maximum efficiency point, and higher efficiency can be achieved at a higher (negative) bias in the ideal case. Incorporating non-ideal losses, however, shifts the maximum efficiency point toward lower bias. Unlike in photovoltaic cells, non-radiative recombination (e.g., Auger losses) will reduce the output current, but will not reduce the conversion efficiency, since the recombination energy is retained in the cell in the form of heat. Since a thermoradiative cell operates by radiating directly to space, the current produced by themoradiative cells will increase as Stefan-Boltzmann radiation; roughly the fourth power of the temperature. Thus, the power produced is highest at high operating temperatures, and, unlike conventional thermal conversion, increasing radiator temperature increases, the efficiency. Thus, the choice of technology will be toward semiconductors resistant to degradation at high temperature.
We investigate the defect-dependent electronic structure and gas-sensing potential of cubic α-CsPbI 3 using first-principles density functional theory and nonadiabatic molecular dynamics. Among the intrinsic defects, interstitials, vacancies, antisites, and switches studied, the I Pb and Pb I antisite defects exhibit transition energy levels near the middle of the band gap, thus functioning as deep traps. Short-term adsorption of ammonia selectively modifies the electronic structure, coordinating with Pb at Pb I sites and Cs at I Pb sites, significantly altering recombination pathways. Detailed analysis reveals that NH 3 reduces anharmonicity at I Pb defects, enabling enhanced recombination at elevated temperatures, while trap-assisted recombination dominates at room temperature. Other analytes, including CH 3 NH 2 and NO 2 , show negligible impact on the band gap or recombination dynamics, highlighting the potential selectivity of NH 3 interactions. Ab initio nonadiabatic molecular dynamics simulations at 300 K and 600 K further demonstrate temperature-dependent modulation of carrier lifetimes, with NH 3 accelerating recombination at ambient conditions and suppressing certain pathways at higher temperatures. These findings suggest that α-CsPbI 3 can serve as a selective and sensitive ammonia sensor over a broad temperature range and offer insights for ammonia detection under industrially relevant conditions.
Allelic loss is an important mutational mechanism in human carcinogenesis. Loss of heterozygosity (LOH) at an autosomal locus is one outcome of the repair of DNA double-strand breaks (DSBs) and can occur by deletion or by mitotic recombination. We report that mitotic recombination between homologous chromosomes occurred in human lymphoid cells exposed to densely ionizing radiation. We used cells derived from the same donor that express either normal TP53 (TK6 cells) or homozygous mutant TP53 (WTK1 cells) to assess the influence of TP53 on radiation-induced mutagenesis. Expression of mutant TP53 (Met 237 Ile) was associated with a small increase in mutation frequencies at the hemizygous HPRT (hypoxanthine phosphoribosyl transferase) locus, but the mutation spectra were unaffected at this locus. In contrast, WTK1 cells (mutant TP53) were 30-fold more susceptible than TK6 cells (wild-type TP53) to radiation-induced mutagenesis at the TK1 (thymidine kinase) locus. Gene dosage analysis combined with microsatellite marker analysis showed that the increase in TK1 mutagenesis in WTK1 cells could be attributed, in part, to mitotic recombination. The microsatellite marker analysis over a 64-cM region on chromosome 17q indicated that the recombinational events could initiate at different positions between the TK1 locus and the centromere. Virtually all of the recombinational LOH events extended beyond the TK1 locus to the most telomeric marker. In general, longer LOH tracts were observed in mutants from WTK1 cells than in mutants from TK6 cells. Taken together, the results demonstrate that the incidence of radi-ation-induced mutations is dependent on the genetic background of the cell at risk, on the locus examined, and on the mechanisms for mutation available at the locus of interest.
Porcine astrovirus 4 (PoAstV4) has been recently associated with respiratory disease in pigs. In order to understand the scope of PoAstV4 infections and to support the development of a vaccine to combat PoAstV4 disease in pigs, we designed and produced a recombinant PoAstV4 capsid spike protein for use as an antigen in serological assays and for potential future use as a vaccine antigen. Structural prediction of the full-length PoAstV4 capsid protein guided the design of the recombinant PoAstV4 capsid spike domain expression plasmid. The recombinant PoAstV4 capsid spike was expressed in Escherichia coli, purified by affinity and size-exclusion chromatography, and its crystal structure was determined at 1.85 Å resolution, enabling structural comparisons to other animal and human astrovirus capsid spike structures. The recombinant PoAstV4 capsid spike protein was also used as an antigen for the successful development of a serological assay to detect PoAstV4 antibodies, demonstrating that the recombinant PoAstV4 capsid spike retains antigenic epitopes found on the native PoAstV4 capsid. These studies lay a foundation for seroprevalence studies and the development of a PoAstV4 vaccine for swine.
Galaxy clustering constrains the baryon fraction Omega_b/Omega_m through the amplitude of baryon acoustic oscillations and the suppression of perturbations entering the horizon before recombination. This produces a different pre-recombination distribution of baryons and dark matter. After recombination, the gravitational potential responds to both components in proportion to their mass, allowing robust measurement of the baryon fraction. This is independent of new-physics scenarios altering the recombination background (e.g. Early Dark Energy). The accuracy of such measurements does, however, depend on how baryons and CDM are modeled in the power spectrum. Previous template-based splitting relied on approximate transfer functions that neglected part of information. We present a new method that embeds an extra parameter controlling the balance between baryons and dark matter in the growth terms of the perturbation equations in the CAMB Boltzmann solver. This approach captures the baryonic suppression of CDM prior to recombination, avoids inconsistencies, and yields a clean parametrization of the baryon fraction in the linear power spectrum, separating out the simple physics of growth due to the combined matter potential. We implement this framework in an analysis pipeline using Effective Field Theory of Large-Scale Structure with HOD-informed priors and validate it against noiseless LCDM and EDE cosmologies with DESI-like errors. The new scheme achieves comparable precision to previous splitting while reducing systematic biases, providing a more robust way to baryon-fraction measurements. In combination with BBN constraints on the baryon density and Alcock-Paczynski estimates of the matter density, these results strengthen the use of baryon fraction measurements to derive a Hubble constant from energy densities, with future DESI and Euclid data expected to deliver competitive constraints.
The ultraviolet oxygen emissions at 1304 and 1356 A in the tropical nightglow seen from Ogo 4 by Hicks and Chubb (1970) and Barth and Schaffner (1970) are accompanied by emission at 7774 and 4368 A, which have been studied from the ground by looking through the beam of an ionosonde operated under the Appleton anomaly ionization peaks. Simultaneous forbidden O I 6300-A measurements were also made. A theoretical value for the partial rate coefficient for 7774 emission by radiative recombination has been obtained, and from the ionospheric data and a model atmosphere the expected rates of radiative recombination and ion-ion recombination were calculated. The time variations and absolute intensity of the calculated and observed intensities agree reasonably well, when the uncertainties involved are considered. It is concluded that radiative recombination is the major source of the tropical oxygen permitted line emissions, accompanied by a small contribution from ion-ion recombination.