Sep 18

"Negative Molecular Ions in the Laboratory and in Space"
Harshal Gupta, Department of Chemistry and Biochemistry, University of Texas at Austin

"I present my thesis research on the theoretical, laboratory, and astronomical spectroscopy of negative molecular ions (anions), starting with the laboratory detection of the large carbon chain anion C6H- in the radio band, and its identification in two well-known astronomical sources: the molecular envelope of the carbon star IRC+10216 and in the cold dark molecular cloud TMC-1. In IRC+10216 the identification solved the long standing problem of the unidentified series of lines with rotational constant 1377 MHz first observed by K. Kawaguchi et al. Rotational spectra of the structurally similar anions -- CCH-, C4H-, C8H-, CN-, and C3N- -- have been detected in the laboratory, and three of these anions --C4H-, C8H-, and C3N- -- have now been conclusively identified in space. I discuss radio astronomical observations of the four larger anions toward TMC-1 done with the 100 m Green Bank Telescope (GBT), comparing the surprisingly high C6H-/C6H and C8H-/C8H ratios of a few percent with the several orders of magnitude smaller upper limits derived for the C4H-/C4H and C3N-/C3N ratios. I present the results from a survey of C6H- and the related carbon chain radicals, C4H and C6H, done with the GBT toward 24 galactic molecular sources. The C6H- anion was newly detected in two dark clouds -- L1544 and L1521F -- at fractional abundances (relative to C6H) comparable to that in TMC-1. The C4H radical was detected in 11 of the 12 dark clouds surveyed -- in six for the first time; the C6H radical was newly found in five dark clouds and one translucent cloud. The observed C6H-/ C6H (1 % - 4 %) and C6H/C4H (0.2 %-1 %) ratios in these sources suggest that C6H -- may be close to detection in many other dark clouds. A close correlation between the column densities of C6H and C4H, similar to that previously observed for the cyanopolyynes HC3N and HC5N, and between C4H and the cyanopolyynes, suggests that long carbon chains probably form by similar processes in interstellar clouds."

Sep 25

"Spitzer Observations of an Extreme Protostellar Outflow"
Joel Green, University of Texas at Austin

"GGD 37 (Cepheus A West) is a collection of Herbig Haro objects -- small emission nebulae that signify the interaction between both broad and collimated outflows from young stellar objects and the ambient molecular cloud material. Although it has been well-studied at millimeter and longer wavelengths, the relationship of this flow to its companion Cepheus A East is not well understood. The Outflows GO program utilized the Spitzer Space Telescope to map several outflow regions, including GGD 37 in the mid-infrared. With this data we have spatially resolved weaker shocks (less than ~ 10000 K) that excite the molecular hydrogen into various rotational states detectable at IRS wavelengths, from strong shocks that completely destroy the molecules and illuminate the ions. The dataset contains a rich array of chemical species, allowing us to analyze the energetics of a multi-phase shock as it impacts the surrounding molecular cloud. How does the instability of driving sources of Herbig Haro jets affect their surrounding medium? By studying the pre-and post-shock gas, we can determine whether outflows from young stars have greater clumping or dispersive effects on their environment. Do outflows trigger or suppress star formation in the neighborhood? We present evidence that powerful flows such as HH 168 can unbind a protostar in its early stages of development, opening a 10000 AU cavity of ionized material. We provide evidence that the progenitor source of the HH 168 flow is at the radio source W2, rather than the more distant HW3c, and is among the hottest and most intense protostellar outflows. We calculate outflow parameters and highlight the unusual nature of this flow. As a result we now have the opportunity to observe a high mass protostar in its initial stages of formation."