Matching Items (30)

141502-Thumbnail Image.png

Astrobiological Stoichiometry

Description

Chemical composition affects virtually all aspects of astrobiology, from stellar astrophysics to molecular biology. We present a synopsis of the research results presented at the “Stellar Stoichiometry” Workshop Without Walls

Chemical composition affects virtually all aspects of astrobiology, from stellar astrophysics to molecular biology. We present a synopsis of the research results presented at the “Stellar Stoichiometry” Workshop Without Walls hosted at Arizona State University April 11–12, 2013, under the auspices of the NASA Astrobiology Institute. The results focus on the measurement of chemical abundances and the effects of composition on processes from stellar to planetary scales. Of particular interest were the scientific connections between processes in these normally disparate fields. Measuring the abundances of elements in stars and giant and terrestrial planets poses substantial difficulties in technique and interpretation. One of the motivations for this conference was the fact that determinations of the abundance of a given element in a single star by different groups can differ by more than their quoted errors. The problems affecting the reliability of abundance estimations and their inherent limitations are discussed. When these problems are taken into consideration, self-consistent surveys of stellar abundances show that there is still substantial variation (factors of ∼2) in the ratios of common elements (e.g., C, O, Na, Al, Mg, Si, Ca) important in rock-forming minerals, atmospheres, and biology. We consider how abundance variations arise through injection of supernova nucleosynthesis products into star-forming material and through photoevaporation of protoplanetary disks. The effects of composition on stellar evolution are substantial, and coupled with planetary atmosphere models can result in predicted habitable zone extents that vary by many tens of percent. Variations in the bulk composition of planets can affect rates of radiogenic heating and substantially change the mineralogy of planetary interiors, affecting properties such as convection and energy transport.

Contributors

Agent

Created

Date Created
  • 2014-07-01

132898-Thumbnail Image.png

Analysis of Radiation Shielding for Spacecraft

Description

The intention of this report is to use computer simulations to investigate the viability of two materials, water and polyethylene, as shielding against space radiation. First, this thesis discusses some

The intention of this report is to use computer simulations to investigate the viability of two materials, water and polyethylene, as shielding against space radiation. First, this thesis discusses some of the challenges facing future and current manned space missions as a result of galactic cosmic radiation, or GCR. The project then uses MULASSIS, a Geant4 based radiation simulation tool, to analyze the effectiveness of water and polyethylene based radiation shields against proton radiation with an initial energy of 1 GeV. This specific spectrum of radiation is selected because it a component of GCR that has been shown by previous literature to pose a significant threat to humans on board spacecraft. The analysis of each material indicated that both would have to be several meters thick to adequately protect crew against the simulated radiation over a several year mission. Additionally, an analysis of the mass of a simple spacecraft model with different shield thicknesses showed that the mass would increase significantly with internal space. Thus, using either material as a shield would be expensive as a result of the cost of lifting a large amount of mass into orbit.

Contributors

Agent

Created

Date Created
  • 2019-05

133368-Thumbnail Image.png

Production of Short-Lived Radionuclides in Asymmetric Supernovae

Description

Supernovae are vital to supplying necessary elements to forming bodies in our solar systems. This project studies the creation of a subset of these necessary elements, called short-lived radionuclides (SLRs).

Supernovae are vital to supplying necessary elements to forming bodies in our solar systems. This project studies the creation of a subset of these necessary elements, called short-lived radionuclides (SLRs). SLRs are isotopes with relatively short half-lives and can serve as heat sources for forming planetary bodies, and their traces can be used to date stellar events. Computational models of asymmetric supernovae provide opportunities to study the effect of explosion geometry on the SLR yields. We are most interested in the production of \iso{Al}{26}, \iso{Fe}{60}, and \iso{Ca}{41}, whose decayed products are found in our own solar system. To study the effect of explosion asymmetries in supernovae, we use TYCHO stellar evolution code, SNSHP smooth particle hydrodynamics code for 3D explosion simulations, Burn code for nucleosythesis post-processing, and Python code written to analyze the output of the post-processing code.

Contributors

Agent

Created

Date Created
  • 2018-05

Interstellar Landscapes: Landscapes of Carbon and Magnesium Planets

Description

How do we visualize environments outside our solar system? I have researched two very alien planets and their compositions with the goal of finding out how those differences would affect

How do we visualize environments outside our solar system? I have researched two very alien planets and their compositions with the goal of finding out how those differences would affect the way a planet appears on its surface. The first is a planet orbiting the nearby G type star Tau Ceti. This star has Mg/Si ratio of 1.78, compared to 1.2 found on the Earth. A planet formed around this star could have a very active surface, covered in volcanoes. The other planet is a hypothetical carbon planet that could orbit the star HD 144899. This star has a C/O ratio of 0.8, compared to 0.5 in the Sun. A planet formed here might be comprised mostly of carbides, with a hydrocarbon atmosphere. It would likely be geologically dead, the main forces shaping its surface being meteorites. Both planets, due to their extremes, would likely be barren and lifeless. The results of this project are two digital paintings showcasing my vision of these planets.

Contributors

Agent

Created

Date Created
  • 2015-05

148230-Thumbnail Image.png

The Effect of Varying Mass Loss Rate on the Initial-Final Mass Relation of Massive Stars

Description

Stellar mass loss has a high impact on the overall evolution of a star. The amount<br/>of mass lost during a star’s lifetime dictates which remnant will be left behind and

Stellar mass loss has a high impact on the overall evolution of a star. The amount<br/>of mass lost during a star’s lifetime dictates which remnant will be left behind and how<br/>the circumstellar environment will be affected. Several rates of mass loss have been<br/>proposed for use in stellar evolution codes, yielding discrepant results from codes using<br/>different rates. In this paper, I compare the effect of varying the mass loss rate in the<br/>stellar evolution code TYCHO on the initial-final mass relation. I computed four sets of<br/>models with varying mass loss rates and metallicities. Due to a large number of models<br/>reaching the luminous blue variable stage, only the two lower metallicity groups were<br/>considered. Their mass loss was analyzed using Python. Luminosity, temperature, and<br/>radius were also compared. The initial-final mass relation plots showed that in the 1/10<br/>solar metallicity case, reducing the mass loss rate tended to increase the dependence of final mass on initial mass. The limited nature of these results implies a need for further study into the effects of using different mass loss rates in the code TYCHO.

Contributors

Agent

Created

Date Created
  • 2021-05

129168-Thumbnail Image.png

A CATALOG OF STELLAR EVOLUTION PROFILES AND THE EFFECTS OF VARIABLE COMPOSITION ON HABITABLE SYSTEMS

Description

We present stellar evolution models for 0.5 - 1.2 M[subscript ⊙] at scaled metallicities of 0.1 - 1.5 Z[subscript ⊙] and O/Fe values of 0.44 - 2.28 O/Fe[subscript ⊙]. The

We present stellar evolution models for 0.5 - 1.2 M[subscript ⊙] at scaled metallicities of 0.1 - 1.5 Z[subscript ⊙] and O/Fe values of 0.44 - 2.28 O/Fe[subscript ⊙]. The time dependent evolution of habitable zone boundaries are calculated for each stellar evolution track based on stellar mass, effective temperature, and luminosity parameterizations. The rate of change of stellar surface quantities and the surrounding habitable zone position are strong functions of all three quantities explored. The range of orbits that remain continuously habitable, or habitable for at least 2 Gyr, are provided. The results show that the detailed chemical characterization of exoplanet host stars and a consideration of their evolutionary history are necessary to assess the likelihood that a planet found in the instantaneous habitable zone has had sufficient time to develop a biosphere capable of producing detectable biosignatures. This model grid is designed for use by the astrobiology and exoplanet communities to efficiently characterize the time evolution of host stars and their habitable zones for planetary candidates of interest.

Contributors

Agent

Created

Date Created
  • 2015-05-10

132092-Thumbnail Image.png

Attempting to Produce Helium White Dwarfs From Low-Metallicity Solar Mass Stars

Description

I examine the effects of metallicity on solar mass stellar evolution, trying to replicate a previous result in Windhorst et.al., 2018, in which a zer metallicity solar mass star did

I examine the effects of metallicity on solar mass stellar evolution, trying to replicate a previous result in Windhorst et.al., 2018, in which a zer metallicity solar mass star did not reach the AGB, and thus may turn into a helium white dwarf. In trying to replicate this result, I used the M.E.S.A. stellar evolution code and was unable to reproduce this result. While M.E.S.A has undergone several updates since the previous result was obtained, more current evidence suggests that this may have been a one-time occurrence, as no helium white dwarfs were produced for low-metallicity models. Nonetheless, interesting results were obtained, including a lowest metallicity value for which CNO burning does not significantly contribute during the main sequence, 1 −10 Z , which produces noticeable effects on post main sequence evolution. All models are run with no rotation, one solar mass, and a series of MESA parameters kept constant, with the only exception being metallicity. Any metallicity value listed as Nd −10 is an absolute mass fraction, and Z is relative to solar metallicity, 2d*10 −2 .

Contributors

Agent

Created

Date Created
  • 2019-12

Systematics of giant impacts in late-stage planet formation and active neutron experiments on the surface of Mars

Description

Part I – I analyze a database of Smoothed Particle Hydrodynamics (SPH) simulations of collisions between planetary bodies and use the data to define semi-empirical models that reproduce remant masses.

Part I – I analyze a database of Smoothed Particle Hydrodynamics (SPH) simulations of collisions between planetary bodies and use the data to define semi-empirical models that reproduce remant masses. These models may be leveraged when detailed, time-dependent aspects of the collision are not paramount, but analytical intuition or a rapid solution is required, e.g. in ‘N-body simulations’. I find that the stratification of the planet is a non-negligible control on accretion efficiency. I also show that the absolute scale (total mass) of the collision may affect the accretion efficiency, with larger bodies more efficiently disrupting, as a function of gravitational binding energy. This is potentially due to impact velocities above the sound speed. The interplay of these dependencies implies that planet formation, depending on the dynamical environment, may be separated into stages marked by differentiation and the growth of planets more massive than the Moon.

Part II – I examine time-resolved neutron data from the Dynamic Albedo of Neutrons (DAN) instrument on the Mars Science Laboratory (MSL) Curiosity rover. I personally and independently developed a data analysis routine (described in the supplementary material in Chapter 2) that utilizes spectra from Monte Carlo N-Particle Transport models of the experiment and the Markov-chain Monte Carlo method to estimate bulk soil/rock properties. The method also identifies cross-correlation and degeneracies. I use data from two measurement campaigns that I targeted during remote operations at ASU. I find that alteration zones of a sandstone unit in Gale crater are markedly elevated in H content from the parent rock, consistent with the presence of amorphous silica. I posit that these deposits were formed by the most recent aqueous alteration events in the crater, since subsequent events would have produced matured forms of silica that were not observed. I also find that active dunes in Gale crater contain minimal water and I developed a Monte Carlo phase analysis routine to understand the amorphous materials in the dunes.

Contributors

Agent

Created

Date Created
  • 2019

150209-Thumbnail Image.png

Uranium isotope variations in nature: mechanisms, applications, and implications

Description

Historically, uranium has received intense study of its chemical and isotopic properties for use in the nuclear industry, but has been largely ignored by geoscientists despite properties that make it

Historically, uranium has received intense study of its chemical and isotopic properties for use in the nuclear industry, but has been largely ignored by geoscientists despite properties that make it an intriguing target for geochemists and cosmochemists alike. Uranium was long thought to have an invariant 238U/235U ratio in natural samples, making it uninteresting for isotopic work. However, recent advances in mass spectrometry have made it possible to detect slight differences in the 238U/235U ratio, creating many exciting new opportunities for U isotopic research. Using uranium ore samples from diverse depositional settings from around the world, it is shown that the low-temperature redox transition of uranium (U6+ to U4+) causes measurable fractionation of the 238U/235U ratio. Moreover, it is shown experimentally that a coordination change of U can also cause measurable fractionation in the 238U/235U ratio. This improved understanding of the fractionation mechanisms of U allows for the use of the 238U/235U ratio as a paleoredox proxy. The 238U/235U ratios of carbonates deposited spanning the end-Permian extinction horizon provide evidence of pronounced and persistent widespread ocean anoxia at, or immediately preceding, the extinction boundary. Variable 238U/235U ratios correlated with proxies for initial Cm/U in the Solar System's earliest objects demonstrates the existence of 247Cm in the early Solar System. Proof of variable 238U/235U ratios in meteoritic material forces a substantive change in the previously established procedures of Pb-Pb dating, which assumed an invariant 238U/235U ratio. This advancement improves the accuracy of not only the Pb-Pb chronometer that directly utilizes the 238U/235U ratio, but also for short-lived radiometric dating techniques that indirectly use the 238U/235U ratio to calculate ages of Solar System material.

Contributors

Agent

Created

Date Created
  • 2011

150214-Thumbnail Image.png

Luminosity function of Lyman-alpha emitters at the reionization epoch: observations & theory

Description

Galaxies with strong Lyman-alpha (Lya) emission line (also called Lya galaxies or emitters) offer an unique probe of the epoch of reionization - one of the important phases when most

Galaxies with strong Lyman-alpha (Lya) emission line (also called Lya galaxies or emitters) offer an unique probe of the epoch of reionization - one of the important phases when most of the neutral hydrogen in the universe was ionized. In addition, Lya galaxies at high redshifts are a powerful tool to study low-mass galaxy formation. Since current observations suggest that the reionization is complete by redshift z~ 6, it is therefore necessary to discover galaxies at z > 6, to use their luminosity function (LF) as a probe of reionization. I found five z = 7.7 candidate Lya galaxies with line fluxes > 7x10-18 erg/s/cm/2 , from three different deep near-infrared (IR) narrowband (NB) imaging surveys in a volume > 4x104Mpc3. From the spectroscopic followup of four candidate galaxies, and with the current spectroscopic sensitivity, the detection of only the brightest candidate galaxy can be ruled out at 5 sigma level. Moreover, these observations successfully demonstrate that the sensitivity necessary for both, the NB imaging as well as the spectroscopic followup of z~ 8 Lya galaxies can be reached with the current instrumentation. While future, more sensitive spectroscopic observations are necessary, the observed Lya LF at z = 7.7 is consistent with z = 6.6 LF, suggesting that the intergalactic medium (IGM) is relatively ionized even at z = 7.7, with neutral fraction xHI≤ 30%. On the theoretical front, while several models of Lya emitters have been developed, the physical nature of Lya emitters is not yet completely known. Moreover, multi-parameter models and their complexities necessitates a simpler model. I have developed a simple, single-parameter model to populate dark mater halos with Lya emitters. The central tenet of this model, different from many of the earlier models, is that the star-formation rate (SFR), and hence the Lya luminosity, is proportional to the mass accretion rate rather than the total halo mass. This simple model is successful in reproducing many observable including LFs, stellar masses, SFRs, and clustering of Lya emitters from z~ 3 to z~ 7. Finally, using this model, I find that the mass accretion, and hence the star-formation in > 30% of Lya emitters at z~ 3 occur through major mergers, and this fraction increases to ~ 50% at z~7.

Contributors

Agent

Created

Date Created
  • 2011