A Systematic Study of the Correlations Between Meteorite Impacts and Soot Formation

A massive extinction of more than 50 percent of existing life forms on Earth occurred 65 million years (Ma) ago. This event is marked in the geological record by the Cretaceous-Tertiary (KT) boundary and corresponds to the Chicxulub meteorite impact in the Yucatan Peninsula. Since 1985, large quantities ofreduced elemental carbon in the form of characteristic spheroidal clusters of soot have been found in twelve KT boundary sites from across the globe. Because ofthe wide geographic distribution ofthese sites, the data was interpreted to indicate that deposition ofsoot was a global phenomenon. The source of this global soot layer is suspected to be eolian (airfall) deposition of fine-grained particles resulting from widespread wildfires. A global soot concentration of 2.2 ± 0.7 . mg/cm2 has been estimated from these studies. As a systematic study of the correlation between meteorite impact and soot formation, five sedimentary sequences ofmeteorite impact-related samples were analyzed for the presence ofelemental carbon in the form of soot. Two KT boundary sedimentary sequences from the Berwind, Colorado, and Madrid, Colorado sites were analyzed as continuations of previous KT boundary studies. Soot concentrations of 3000 ± 0 parts per million (ppm) and 780 ± 90 ppm, respectively, were found in the KT boundary sediments ofthese sequences. These concentrations are similar to those found in previous KT boundary studies and further refine the KT boundary global soot concentration value. Likewise, samples from Deep Sea Drilling Project (DSDP) Core 465-A were analyzed as a determination ofthe global nature ofthe soot layer. A soot concentration of 550 ± 750 ppm was found, again similar to those found in previous KT boundary studies. The mid-Pacific location of Core 465-A rules out the possibility ofsoot deposition from\u27 groundwater runoff and indicates that eolian transport is the most probable mecharllsm for soot deposition. Therefore, the presence of soot in boundary sediments in DSDP Core 465-A supports the theory that the KT boundary soot layer is global. In addition, samples related to two other impact events of differing size and age were analyzed for evidence ofwildfires. In the first study, samples were analyzed from the Sudbury, Ontario impact structure. This structure is the result of an impact similar in size to the Chicxulub event occurring 1850 Ma ago. These sediments were found to contain soot similar in concentrations to those found in KT boundary studies, ranging from 2300 ± 200 ppm soot to 3000 ± 300 ppm soot across the post-impact sedimentary sequence. The presence of soot in these sediments further strengthens the correlation between meteorite impact and soot formation. In a second study, samples were analyzed from the Gardnos impact structure, Norway. This structure is the result ofan impact at least one order ofmagnitude smaller than the Chicxulub event occurring between 900 and 400 Ma ago~ No appreciable amounts of soot were found in sediments directly related to the impact event. The absence ofsoot in Gardnos samples suggests that an impact event ofthis size is below the impact threshold required for ignition of soot-producing wildfires

MILLIMETER AND SUBMILLIMETER STUDIES OF O(1D) INSERTION REACTIONS TO FORM MOLECULES OF ASTROPHYSICAL INTEREST

While both the number of detected interstellar molecules and their chemical complexity continue to increase, understanding of the processes leading to their formation is lacking. Our research group combines laboratory spectroscopy, observational astronomy, and astrochemical modeling for an interdisciplinary examination of the chemistry of star and planet formation. This talk will focus on our laboratory studies of O($^1$D) insertion reactions with organic molecules to produce molecules of astrophysical interest. By employing these reactions in a supersonic expansion, we are able to produce interstellar organic reaction intermediates that are unstable under terrestrial conditions; we then probe the products using millimeter and submillimeter spectroscopy. We benchmarked this setup using the well-studied O($^1$D) + methane reaction to form methanol. After optimizing methanol production, we moved on to study the O($^1$D) + ethylene reaction to form vinyl alcohol (CH$_2$CHOH), and the O($^1$D) + methyl amine reaction to form aminomethanol (NH$_2$CH$_2$OH). Vinyl alcohol measurements have now been extended up to 450 GHz, and the associated spectral analysis is complete. A possible detection of aminomethanol has also been made, and continued spectral studies and analysis are underway. We will present the results from these experiments and discuss future applications of these molecular and spectroscopic techniques

A theoretical study of the conversion of gas phase methanediol to formaldehyde

Methanediol, or methylene glycol, is a product of the liquid phase reaction of water and formaldehyde and is a predicted interstellar grain surface species. Detection of this molecule in a hot core environment would advance the understanding of complex organic chemistry in the interstellar medium, but its laboratory spectroscopic characterization is a prerequisite for such observational searches. This theoretical study investigates the unimolecular decomposition of methanediol, specifically the thermodynamic and kinetic stability of the molecule under typical laboratory and interstellar conditions. Methanediol was found to be thermodynamically stable at temperatures of <100 K, which is the characteristic temperature range for interstellar grain mantles. The infinite-pressure RRKM unimolecular decomposition rate was found to be <10^(−18) s^(−1) at 300 K, indicating gas phase kinetic stability for typical laboratory and hot core temperatures. Therefore, both laboratory studies of and observational searches for this molecule should be feasible

Aminomethanol water elimination: Theoretical examination

The mechanism for the formation of hexamethylenetetraamine predicts the formation of aminomethanol from the addition of ammonia to formaldehyde. This molecule subsequently undergoes unimolecular decomposition to form methanimine and water. Aminomethanol is the predicted precursor to interstellar glycine, and is therefore of great interest for laboratory spectroscopic study, which would serve as the basis for observational searches. The height of the water loss barrier is therefore useful in the determination of an appropriate experimental approach for spectroscopic characterization of aminomethanol. We have determined the height of this barrier to be 55 kcal/mol at ambient temperatures. In addition, we have determined the infinite-pressure Rice-Ramsperger-Kassel-Marcus unimolecular decomposition rate to be < 10^(-25) s^(-1) at 300 K, indicating gas-phase kinetic stability for typical laboratory and hot core temperatures. Therefore, spectroscopic characterization of and observational searches for this molecule should be straightforward provided an efficient formation mechanism can be found

A CSO Search for ll-C3_3H+^+: Detection in the Orion Bar PDR

The results of a Caltech Submillimeter Observatory (CSO) search for $l$-C$_3$H$^+$, first detected by Pety et al. (2012) in observations toward the Horsehead photodissociation region (PDR), are presented. A total of 39 sources were observed in the 1 mm window. Evidence of emission from $l$-C$_3$H$^+$ is found in only a single source - the Orion Bar PDR region, which shows a rotational temperature of 178(13) K and a column density of 7(2) x $10^{11}$ cm$^{-2}$. In the remaining sources, upper limits of ~10$^{11} - 10^{13}$ cm$^{-2}$ are found. These results are discussed in the context of guiding future observational searches for this species.Comment: 9 pages, 8 figures, 4 table

A search for ortho-benzyne (o-C6H4) in CRL 618

Polycyclic aromatic hydrocarbons (PAHs) have been proposed as potential carriers of the unidentified infrared bands (UIRs) and the diffuse interstellar bands (DIBs). PAHs are not likely to form by gas-phase or solid-state interstellar chemistry, but rather might be produced in the outflows of carbon-rich evolved stars. PAHs could form from acetylene addition to the phenyl radical (C6H5), which is closely chemically related to benzene (C6H6) and ortho-benzyne (o-C6H4). To date, circumstellar chemical models have been limited to only a partial treatment of benzene-related chemistry, and so the expected abundances of these species are unclear. A detection of benzene has been reported in the envelope of the proto-planetary nebula (PPN) CRL 618, but no other benzene-related species has been detected in this or any other source. The spectrum of o-C6H4 is significantly simpler and stronger than that of C6H5, and so we conducted deep Ku-, K- and Q-band searches for o-C6H4 with the Green Bank Telescope. No transitions were detected, but an upper limit on the column density of 8.4x10^13 cm^-2 has been determined. This limit can be used to constrain chemical models of PPNe, and this study illustrates the need for complete revision of these models to include the full set of benzene-related chemistry.Comment: 13 pages, 4 figures, to be published in The Astrophysical Journal Letter

Complex Organic Molecules at High Spatial Resolution Toward Orion-KL I: Spatial Scales

Here we present high spatial resolution (<1 arcsecond) observations of molecular emission in Orion-KL conducted using the Combined Array for Research in Millimeter-Wave Astronomy (CARMA). This work was motivated by recent millimeter continuum imaging studies of this region conducted at a similarly high spatial resolution, which revealed that the bulk of the emission arises from numerous compact sources, rather than the larger-scale extended structures typically associated with the Orion Hot Core and Compact Ridge. Given that the spatial extent of molecular emission greatly affects the determination of molecular abundances, it is important to determine the true spatial scale for complex molecules in this region. Additionally, it has recently been suggested that the relative spatial distributions of complex molecules in a source might give insight into the chemical mechanisms that drive complex chemistry in star-forming regions. In order to begin to address these issues, this study seeks to determine the spatial distributions of ethyl cyanide [C2H5CN], dimethyl ether [(CH3)2O], methyl formate [HCOOCH3], formic acid [HCOOH], acetone [(CH3)2CO], SiO, methanol [CH3OH], and methyl cyanide [CH3CN] in Orion-KL at \lambda = 3 mm. We find that for all observed molecules, the molecular emission arises from multiple components of the cloud that include a range of spatial scales and physical conditions. Here we present the results of these observations and discuss the implications for studies of complex molecules in star-forming regions.Comment: Accepted for publication in the Astrophysical Journal Supplement; Part 1 of a 2 paper series; 37 page