arXiv subjects

Daniel Wolf Savin

Publications and source records attributed to Daniel Wolf Savin.

42 records · Page 3Linked to original sources

arXiv2009

New Discoveries in Stars and Stellar Evolution through Advances in Laboratory Astrophysics

As the Stars and Stellar Evolution (SSE) panel is fully aware, the next decade will see major advances in our understanding of these areas of research. To quote from their charge, these advances will occur in studies of the Sun as a star, stellar astrophysics, the structure and evolution of single and multiple stars, compact objects, SNe, gamma-ray bursts, solar neutrinos, and extreme physics on stellar scales. Central to the progress in these areas are the corresponding advances in laboratory astrophysics, required to fully realize the SSE scientific opportunities within the decade 2010-2020. Laboratory astrophysics comprises both theoretical and experimental studies of the underlying physics that produces the observed astrophysical processes. The 6 areas of laboratory astrophysics, which we have identified as relevant to the CFP panel, are atomic, molecular, solid matter, plasma, nuclear physics, and particle physics. In this white paper, we describe in Section 2 the scientific context and some of the new scientific opportunities and compelling scientific themes which will be enabled by advances in laboratory astrophysics. In Section 3, we discuss some of the experimental and theoretical advances in laboratory astrophysics required to realize the SSE scientific opportunities of the next decade. As requested in the Call for White Papers, Section 4 presents four central questions and one area with unusual discovery potential. Lastly, we give a short postlude in Section 5.

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arXiv2004

Rate Coefficient for H^+ + H_2(X ^1Sigma_g^+, nu=0, J=0) --> H(1s) + H_2^+ Charge Transfer and Some Cosmological Implications

Krstic has carried out the first quantum mechanical calculations near threshold for the charge transfer (CT) process H^+ + H_2(X ^1Sigma_g^+, nu=0, J=0) --> H(1s) + H_2^+. These results are relevant for models of primordial galaxy and first star formation that require reliable atomic and molecular data for obtaining the early universe hydrogen chemistry. Using the results of Krstic, we calculate the relevant CT rate coefficient for temperatures between 100 and 30,000 K. We also present a simple fit which can be readily implemented into early universe chemical models. Additionally, we explore how the range of previously published data for this reaction translates into uncertainties in the predicted gas temperature and H_2 relative abundance in a collapsing primordial gas cloud. Our new data significantly reduce these cosmological uncertainties that are due to the uncertainties in the previously published CT rate coefficients.

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arXiv2001

Rate Coefficients for D(1s) + H^+ <--> D^+ + H(1s) Charge Transfer and Some Astrophysical Implications

We have calculated the rate coefficients for D(1s) + H^+ <--> D^+ + H(1s) using recently published theoretical cross sections. We present results for temperatures T from 1 K to 2x10^5 K and provide fits to our data for use in plasma modeling. Our calculations are in good agreement with previously published rate coefficients for 25 <= T <= 300 K, which covers most of the limited range for which those results were given. Our new rate coefficients for T >~ 100 K are significantly larger than the values most commonly used for modeling the chemistry of the early universe and of molecular clouds. This may have important implications for the predicted HD abundance in these environments. Using our results, we have modeled the ionization balance in high redshift QSO absorbers. We find that the new rate coefficients decrease the inferred D/H ratio by <~ 0.4%. This is a factor of >~ 25 smaller than the current >~ 10% uncertainties in QSO absorber D/H measurements.

astro-ph↗

arXiv2001

Atomic Data Needs for Modeling Photoionized Plasmas

Many of the fundamental questions in astrophysics can be addressed using spectroscopic observations of photoionized cosmic plasmas. However, the reliability of the inferred astrophysics depends on the accuracy of the underlying atomic data used to interpret the collected spectra. In this paper, we review some of the most glaring atomic data needs for better understanding photoionized plasmas.

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arXiv2001

Uncertainties in Dielectronic Recombination Rate Coefficients: Effects on Solar and Stellar Upper Atmosphere Abundance Determinations

We have investigated how the relative elemental abundances inferred from the solar upper atmosphere are affected by uncertainties in the dielectronic recombination (DR) rate coefficients used to analyze the spectra. We find that the inferred relative abundances can be up to a factor of ~5 smaller or ~1.6 times larger than those inferred using the currently recommended DR rate coefficients. We have also found a plausible set of variations to the DR rate coefficients which improve the inferred (and expected) isothermal nature of solar coronal observations at heights of >~ 50 arcsec off the solar limb. Our results can be used to help prioritize the enormous amount of DR data needed for modeling solar and stellar upper atmospheres. Based on the work here, our list of needed rate coefficients for DR onto specific isoelectronic sequences reads, in decreasing order of importance, as follows: O-like, C-like, Be-like, N-like, B-like, F-like, Li-like, He-like, and Ne-like. It is our hope that this work will help to motivate and prioritize future experimental and theoretical studies of DR.

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arXiv1999

Chemical Abundances and the Metagalactic Radiation Field at High Redshift

We have carried out model calculations of the photoionized intergalactic medium (IGM) to determine the effects on the predicted ionic column densities due to uncertainties in the published dielectronic recombination (DR) rate coefficients. Based on our previous experimental work and a comparison of published theoretical DR rates, we estimate there is in general a factor of 2 uncertainty in existing DR rates used for modeling the IGM. We demonstrate that this uncertainty results in factors of ~1.9 uncertainty in the predicted N V and Si IV column densities, ~1.6 for O VI, and ~1.7 for C IV. We show that these systematic uncertainties translate into a systematic uncertainty of up to a factor of ~3.1 in the Si/C abundance ratio inferred from observations. The inferred IGM abundance ratio could thus be less than the solar Si/C ratio or greater than 3 times the solar ratio. If the latter is true, then it suggests the metagalactic radiation field is not due purely to active galactic nuclei, but includes a significant stellar component. Lastly, column density ratios of Si IV to C IV versus C II to C IV are often used to constrain the decrement in the metagalactic radiation field at the He II absorption edge. We show that the variation in the predicted Si IV to C IV ratio due to a factor of 2 uncertainty in the DR rates is almost as large as that due to a factor of 10 change in the decrement. Laboratory measurements of the relevant DR resonance strengths and energies are the only unambiguous method to remove the effects of these atomic physics uncertainties from models of the IGM.

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