Donna M. Pierce’s research while affiliated with Mississippi State University and other places

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Publications (6)


The spatial distribution of C2, C3, and nh in comet 2P/ENCKE
  • Article

December 2013

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30 Reads

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10 Citations

The Astrophysical Journal

Garrett Dorman

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Donna M. Pierce

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Anita L. Cochran

We examine the spatial distribution of C2, C3, and NH radicals in the coma of comet Encke in order to understand their abundances and distributions in the coma. The observations were obtained from 2003 October 22-24, using the 2.7 m telescope at McDonald Observatory. Building on our original study of CN and OH, we have used our modified version of the vectorial model, which treats the coma as one large cone, in order to reproduce Encke's highly aspherical and asymmetric coma. Our results suggest that NH can be explained by the photodissociation of NH2, assuming that NH2 is produced rapidly from NH3 in the innermost coma. Our modeling of C2 and C3 suggests a multi-generational photodissociation process may be required for their production. Using the results of our previous study, we also obtain abundance ratios with respect to OH and CN. Overall, we find that Encke exhibits typical carbon-chain abundances, and the results are consistent with other studies of comet Encke.


Jet Morphology and Coma Analysis of 103P/Hartley 2

October 2012

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10 Reads

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6 Citations

The Astronomical Journal

We have observed comet 103P/Hartley 2 using the George and Cynthia Mitchell Spectrograph (formerly VIRUS-P) on the 2.7 m telescope at McDonald Observatory (Hill et al. 2008). Data for CN, C2, C3, and NH2 were collected over six nights from 2010 July 15 to November 10. The data were processed to form images of the coma for each of the observed species. We have performed azimuthal average division on each of the coma images to examine jet morphology and have investigated the nature of the production of the radical species using our modified vectorial model (Ihalawela et al. 2011). This work enhances the ongoing investigation of the chemistry and outgassing behavior of Hartley 2 as studied by the EPOXI flyby mission.


Table 1 Times of Comet Observations
Table 2 CN/OH Ratios a by Position Angle
The Spatial Distribution of OH and CN Radicals in the Coma of Comet Encke
  • Article
  • Full-text available

October 2011

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60 Reads

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10 Citations

The Astrophysical Journal

Multiple potential parent species have been proposed to explain CN abundances in comet comae, but the parent has not been definitively identified for all comets. This study examines the spatial distribution of CN radicals in the coma of comet Encke and determines the likelihood that CN is a photodissociative daughter of HCN in the coma. Comet Encke is the shortest orbital period (3.3 years) comet known and also has a low dust-to-gas ratio based on optical observations. Observations of CN were obtained from 2003 October 22 to 24, using the 2.7 m telescope at McDonald Observatory. To determine the parent of CN, the classical vectorial model was modified by using a cone shape in order to reproduce Encke's highly aspherical and asymmetric coma. To test the robustness of the modified model, the spatial distribution of OH was also modeled. This also allowed us to obtain CN/OH ratios in the coma. Overall, we find the CN/OH ratio to be 0.009 ± 0.004. The results are consistent with HCN being the photodissociative parent of CN, but we cannot completely rule out other possible parents such as CH3CN and HC3N. We also found that the fan-like feature spans ~90°, consistent with the results of Woodney et al..

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Damping of the dipole vortex

April 2011

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368 Reads

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8 Citations

When a circular electric dipole moment, rotating in the x – y plane, is embedded in a material with relative permittivity ε r and relative permeability μ r , the field lines of energy flow of the emitted radiation are dramatically influenced by the surrounding material. For emission in free space, the field lines swirl around the z axis and lie on a cone. The direction of rotation of the field lines around the z axis is the same as the direction of rotation of the dipole moment. We found that when the real part of ε r is negative, the rotation of the field lines changes direction, and hence the energy counter-rotates the dipole moment. When there is damping in the material, due to an imaginary part of ε r , the cone turns into a funnel, and the density of the field lines diminishes near the location of the source. In addition, all radiation is emitted along the z axis and the x – y plane, whereas for emission in free space, the radiation is emitted in all directions. It is also shown that the displacement of the dipole image in the far field depends on the material parameters and that the shift can be much larger than the shift of the image in free space.


Field lines of the Poynting vector of the radiation emitted by a dipole in free space, oscillating along the z axis, are straight lines in a radially outward direction.
Field lines of the Poynting vector for a dipole oscillating along the z axis and embedded in a material with ε r = 1.7 + 0.06 i and μ r = 1 . These are the values for water at 3   μm . The index of refraction is n = 1.3 + 0.023 i .
Larger view of the field lines in Fig. 2. Far away from the dipole the field lines run approximately in a radial direction, except near the z axis. The curving close to the z axis is a near-field effect that persists in the far field.
Redistribution of energy flow in a material due to damping

January 2011

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27 Reads

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7 Citations

The field lines of energy flow of the radiation emitted by a linear dipole in free space are straight lines, running radially outward from the source. When the dipole is embedded in a medium, the field lines are curves when the imaginary part of the relative permittivity is finite. It is shown that due to the damping in the material all radiation is emitted in directions perpendicular to the dipole axis, whereas for a dipole in free space the radiation is emitted in all directions except along the dipole axis. It is also shown that some field lines in the near field form semiloops. Energy flowing along these semiloops is absorbed by the material and does not contribute to the radiative power in the far field.


An Analysis of CO Production in Cometary Comae: Contributions from Gas-phase Phenomena

June 2010

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13 Reads

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11 Citations

The Astrophysical Journal

Understanding the sources of CO in cometary comae is important for understanding comet chemistry and the roles comets have played in the development of the solar system. Among comets sampled to date, the CO abundances vary widely and no direct correlation of CO abundance with other known comet properties has been identified. The picture is complicated further by the discovery of CO production in the comae of some comets, most notably comets Halley and Hale-Bopp. In this study, we investigate the conditions under which CO can be produced in the coma via gas-phase phenomena. We include photochemistry of several parent molecules, as well as two-body chemical reactions that involve the parents and their photodissociative daughter and granddaughter products. We also consider the level of excitation of "hot" hydrogen (H*) and O(1D) in the network, because the level of excitation of these reactants strongly influences reaction rates. Our results suggest that the dominant gas-phase contributor to CO formation is the photodissociation of H2CO. Even though typical abundances of H2CO are at ~1% relative to water in the coma, it produces more CO than other processes due to its relatively short photodissociation lifetime. Because other studies have shown H2CO to have a distributed source as well, it suggests that at least some CO formation in the coma is connected to the H2CO distributed source. We take the time to examine the CO2/CO ratio and note that while the CO2/CO ratio in comets Halley, Hale-Bopp, and Hyakutake are noticeably different when only native CO is considered, the CO2/CO ratios show greater similarity when total CO is considered. Although this sample is relatively small, should the relatively similar CO2/COTotal ratio of ~0.25 indeed be constant for comets with distributed CO sources, it suggests that the extended CO source of these comets is tied directly to the overall C, H, O chemistry of comets, as is likely to happen if hydrogenation of CO occurred on icy grains.

Citations (6)


... For many reasons, the electronic ground state of imidogen radicals NH is one of the attractive radicals for both experimentalists and theoreticians, for its role in flames [14], diffuse molecular clouds [15,16], and comets [17]. Astronomical observations have already reported that the abundance of NH strongly depends on the environment in which the species are detected [18]. ...

Reference:

Collision of C 2 with NH and deuterated analogs in the interstellar medium. Scattering calculations
The spatial distribution of C2, C3, and nh in comet 2P/ENCKE
  • Citing Article
  • December 2013

The Astrophysical Journal

... In principle, the OH filter is also effective for such studies, but it suffers from severe atmospheric extinction, and many telescopes have very low throughput at the relevant wavelengths. With recent improvements in integral field unit (IFU) spectroscopy, it is becoming possible to study coma morphology in gas "images" constructed from spectra (Vaughan et al. 2017;Opitom et al. 2019), with the added possibility of studying lines which are too spread out for conventional narrowband filters, like NH 2 . Coma morphology techniques are not limited to near-UV and visible wavelengths. ...

Jet Morphology and Coma Analysis of 103P/Hartley 2
  • Citing Article
  • October 2012

The Astronomical Journal

... The likely precursor of CO + , CO, sublimates readily beyond several AU where water sublimation becomes increasingly inefficient (Womack, Sarid & Wierzchos 2017 ), and the neutral CO distribution may be caused by one or more production mechanism(s) (Pierce & A'Hearn 2010 ). Once produced, CO + may serve as a proxy for CO in such distant comets if the emission and production mechanisms can be characterized (Fortenberry, Bodewits & Pierce 2021 ). ...

An Analysis of CO Production in Cometary Comae: Contributions from Gas-phase Phenomena
  • Citing Article
  • June 2010

The Astrophysical Journal

... with time and is independent of how the nucleus is oriented, consistent with a water vapor contribution from icy grains pushed antisunward by solar radiation pressure. The Sun is to the right, and Ecliptic North is down. is persistent toward the sunward direction (e.g., 9P/Tempel 1, 2P/Encke, and 67P/Churyumov-Gerasimenko; L. M. Feaga et al. 2007;C. A. Ihalawela et al. 2011;U. Fink et al. 2016). Notably, Hartley 2's asymmetry in H 2 O production is similar to what was observed around comet 46P/Wirtanen, which is thought to be a hyperactive comet like Hartley 2 and likely also contains icy grains in the coma, even though no direct detections of H 2 O ice were made during its 2018 apparition (B. P. Bonev et a ...

The Spatial Distribution of OH and CN Radicals in the Coma of Comet Encke

The Astrophysical Journal

... For example, in free space, the field lines of energy flow run radially outwards except for the dipole axis. In a material medium, the energy flow lines are perpendicular to the dipole axis due to damping [3]. Secondly, the experimentally measurable physical quantity is not the phase shift but rather the intensity of radiation. ...

Redistribution of energy flow in a material due to damping