Susanna L. Widicus Weaver

COMPLEX CHEMISTRY IN STAR-FORMING REGIONS : AN EXPANDED GAS-GRAIN WARM-UP CHEMICAL MODEL

Gas-phase processes were long thought to be the key formation mechanisms for complex organic molecules in star-forming regions. However, recent experimental and theoretical evidence has cast doubt on the efficiency of such processes. Grain-surface chemistry is frequently invoked as a solution, but until now there have been no quantitative models taking into account both the high degree of chemical complexity and the evolving physical conditions of star-forming regions. Here, we introduce a new gas-grain chemical network, wherein a wide array of complex species may be formed by reactions involving radicals. The radicals we consider (H, OH, CO, HCO, CH3, CH3O, CH2OH, NH, and NH2) are produced primarily by cosmic ray-induced photodissociation of the granular ices formed during the colder, earlier stages of evolution. The gradual warm up of the hot core is crucial to the formation of complex molecules, allowing the more strongly bound radicals to become mobile on grain surfaces. This type of chemistry is capable of reproducing the high degree of complexity seen in Sgr B2(N), and can explain the observed abundances and temperatures of a variety of previously detected complex organic molecules, including structural isomers. Many other complex species are predicted by this model, and several of these species may be detectable in hot cores. Differences in the chemistry of high- and low-mass star formation are also addressed; greater chemical complexity is expected where evolution timescales are longer.

Complex organic molecules in protoplanetary disks

Context. Protoplanetary disks are vital objects in star and planet formation, possessing all the material, gas and dust, which may form a planetary system orbiting the new star. Small, simple molecules have traditionally been detected in protoplanetary disks; however, in the ALMA era, we expect the molecular inventory of protoplanetary disks to significantly increase. Aims. We investigate the synthesis of complex organic molecules (COMs) in protoplanetary disks to put constraints on the achievable chemical complexity and to predict species and transitions which may be observable with ALMA. Methods. We have coupled a 2D steady-state physical model of a protoplanetary disk around a typical T Tauri star with a large gas-grain chemical network including COMs. We compare the resulting column densities with those derived from observations and perform ray-tracing calculations to predict line spectra. We compare the synthesised line intensities with current observations and determine those COMs which may be observable in nearby objects. We also compare the predicted grain-surface abundances with those derived from cometary comae observations. Results. We find COMs are efficiently formed in the disk midplane via grain-surface chemical reactions, reaching peak grain-surface fractional abundances ~10-6–10-4 that of the H nuclei number density. COMs formed on grain surfaces are returned to the gas phase via non-thermal desorption; however, gas-phase species reach lower fractional abundances than their grain-surface equivalents, ~10-12–10-7. Including the irradiation of grain mantle material helps build further complexity in the ice through the replenishment of grain-surface radicals which take part in further grain-surface reactions. There is reasonable agreement with several line transitions of H2CO observed towards T Tauri star-disk systems. There is poor agreement with HC3N lines observed towards LkCa 15 and GO Tau and we discuss possible explanations for these discrepancies. The synthesised line intensities for CH3OH are consistent with upper limits determined towards all sources. Our models suggest CH3OH should be readily observable in nearby protoplanetary disks with ALMA; however, detection of more complex species may prove challenging, even with ALMA “Full Science” capabilities. Our grain-surface abundances are consistent with those derived from cometary comae observations providing additional evidence for the hypothesis that comets (and other planetesimals) formed via the coagulation of icy grains in the Sun’s natal disk.

Contributions from Grain Surface and Gas Phase Chemistry to the Formation of Methyl Formate and Its Structural Isomers

Both grain surface and gas phase chemistry have been invoked to explain the disparate relative abundances of methyl formate and its structural isomers acetic acid and glycolaldehyde in the Sgr B2(N) star-forming region. While a network of grain surface chemistry involving radical-radical reactions during the warm-up phase of a hot core is the most chemically viable option proposed to date, neither qualitative nor quantitative agreement between modeling and observation has yet been obtained. In this study, we seek to test additional grain surface and gas phase processes to further investigate methyl formate-related chemistry by implementing several modifications to the Ohio State University gas/grain chemical network. We added two new gas phase chemical pathways leading to methyl formate, one involving an exothermic, barrierless reaction of protonated methanol with neutral formic acid; and one involving the reaction of protonated formic acid with neutral methanol to form both the cis and trans forms of protonated methyl formate. In addition to these gas phase processes, we have also investigated whether the relative product branching ratios for methanol photodissociation on grains influence the relative abundances of methyl formate and its structural isomers. We find that while the new gas phase formation pathways do not alter the relative abundances of methyl formate and its structural isomers, changes in the photodissociation branching ratios and adjustment of the overall timescale for warm-up can be used to explain their relative ratios in Sgr B2(N).

FIRST DETECTION OF GAS-PHASE METHANOL IN A PROTOPLANETARY DISK

The first detection of gas-phase methanol in a protoplanetary disk (TW Hya) is presented. In addition to being one of the largest molecules detected in disks to date, methanol is also the first disk organic molecule with an unambiguous ice chemistry origin. The stacked methanol emission, as observed with ALMA, is spectrally resolved and detected across six velocity channels (

A quantum cascade laser cw cavity ringdown spectrometer coupled to a supersonic expansion source

>3 \sigma

Spatial Distributions and Interstellar Reaction Processes

), reaching a peak signal-to-noise of

The Submillimeter Spectrum of Glycolaldehyde

5.5\sigma

Is HO2+ a detectable interstellar molecule?

, with the kinematic pattern expected for TW~Hya. Using an appropriate disk model, a fractional abundance of

Millimeter-Wave and Vibrational State Assignments for the Rotational Spectrum of Glycolaldehyde

3\times 10^{-12} - 4 \times 10^{-11}

1,3-Dihydroxyacetone in Sagittarius B2(N-LMH): The First Interstellar Ketose

(with respect to H