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Project Progress

Module 1 concerns the study of Astrochemistry and how it may have contributed to the development of living systems. This year has focused on following the chemistry of carbon, and of phosphorus. Studies of molecules in circumstellar envelopes have been conducted, in particular, investigating the composition of oxygen-rich vs. carbon-rich envelopes, and their carbon and phosphorus chemistry. For the first time, an in-depth study of an oxygen-rich envelope, VYCanis Majoris, has been conducted, resulting in the detection of phosphorus monoxide. Circumstellar envelopes evolved into planetary nebulae, and this module has been examining the chemical composition of such nebulae and establishing the extent of molecule survival in these objects, as well. Connections are being drawn between the molecules in these nebulae and in diffuse clouds. Finally, the chemical composition of dense clouds, which form from gravitational collapse of diffuse clouds, is being investigated via a confusion-limited spectral-line survey of Sgr B2(N). This survey is complemented by laboratory spectroscopy studies of small organic molecules. A comparison is being made between organic compounds found in these clouds and those in meteorites. This module combines laboratory spectroscopy, radio astronomy of interstellar molecules, gas-phase organic chemistry, meteoritic studies, and ab inito calculations of molecular reactivity.

This module addresses

• Goal 3 of the Astrobiology Roadmap, Understand how life originates from cosmic and planetary precursors, Objectives 3.1 (sources of prebiotic materials) and 3.2 (origins and evolution of functional biomolecules), and
• Goal 4 of the Astrobiology Roadmap, Understand how past life on Earth interacted with its changing planetary and Solar System environment, Objective 4.3 (effects of extraterrestrial events upon the Biosphere).

Highlighted Accomplishments

• Detected phosphorus monoxide, PO, in interstellar space. Phosphorus monoxide is the first interstellar molecule found to contain the P-O bond.
• Conducted studies of phosphorus-containing molecules in circumstellar gas. Detected PN and HCP in several circumstellar envelopes, including CRL2688.
• Completed analysis of rotational spectrum of lowest energy conformer of ethylamine, CH₃CH₂NH₂. The analysis of this species was particularly complex owing to a low-barrier inversion of the amine hydrogens and non-rigid bonds between the methyl and methylene carbons, as well as between the methylene carbon and the amine nitrogen.
  
• Further investigation with collaborators in Poland, Germany and The Ohio State University are underway to better characterize the spectrum of this interesting chemical system.
• Upgrades completed for pulsed molecular-beam Fourier transform microwave spectrometer, including development of magnetic shield, pulsed discharge nozzle, and higher frequency electronics.
• Conducted astronomical search for ethylamine towards the massive star forming region, Sgr B2(N) which yielded negative results. This finding contradicts the claim that ethylamine is a major organic component in the particulate matter collected by the Star Dust mission.
• Continued confusion-limited 1, 2 and 3 mm (70 — 170 GHz, 200 — 280 GHz) spectral-line survey of the molecular cloud Sgr B2(N), which is about 75% completed. Survey had shown that ethylamine, dihydroxyacetone (DHA), hydroxyacetone, lactonitrile, lactic acid, formyl cyanide, and methylene cyclopropene are not present in this cloud down to the confusion limit. In contrast, glycolaldehyde and acetamide are present. Several other species are still under investigation.
• Analysis of extensive survey of formamide (NH₂CHO), a possible pre-biotic solvent, near completed in molecular clouds. This species has been observed in six interstellar sources, Sgr B2(N), W51M , W51 e1/e2, G34.3, Orion-KL, and NGC7538, three of which are new discoveries. Abundances were found to widely vary among sources.
• Detected C3H2 and H2CO for the first time in the old, evolved planetary nebula, the Helix Nebula. Along with past results for CCH, these data suggest that organic molecules and carbon-carbon bonds survive at least until the very end of stellar evolution.
• Continued a broadband survey of VY CMa using the new ALMA Band 6 receiver system at the SMT on Mt. Graham. This survey has already yielded the new phosphorus containing species, PO, and has shown that the chemistry of carbon survives even in an O-rich environment.
• Performed over 1000 hours of observations using the Arizona Radio Observatory’s 12-meter telescope at Kitt Peak, Arizona and Sub-millimeter Telescope on Mt. Graham.
• Completed theoretical study showing that a gas-phase Formose reaction can produce glycolaldehyde from formaldehyde.

Detection of Phosphorus Monoxide in VY CMa: The First Identification of the P-O Bond in Space

In terms of mass, phosphorus is the fifth most important biogenic element after carbon, hydrogen, oxygen and nitrogen. It plays a central role in biochemistry, particularly when bonded to oxygen in phosphate esters. The P-O bond is in fact a fundamental unit in DNA, RNA, and ATP, and thus phosphorus is relevant to both replication and metabolism in living systems. Phosphorus is a relatively abundant element cosmically, with P/H ~ 3 × 10^-7. Therefore, one might expect that it should play a role in interstellar chemistry. Yet, to date, only one phosphorus-bearing molecule has been identified in molecular clouds: PN. In circumstellar gas, PN and the free radical CP have been identified, but only in one source, the carbon-rich envelope of IRC+10216.

During this past year, Tenenbaum, Woolf and Ziurys detected a new interstellar phosphorus-bearing molecule, PO, towards the envelope of the oxygen-rich supergiant star, VY Canis Majoris (VY CMa), using the Submillimeter Telescope (SMT) of the Arizona Radio Observatory. The J = 5.5- 4.5 and J = 6.5- 5.5 rotational transitions of PO at 240 GHz and 284 GHz were observed, each consisting of well-defined lambda-doublets (see Figure 1.1). The line profiles are roughly parabolic in shape, analogous to PN, and suggest that this species arises from the spherical wind in VY CMa, as opposed to the collimated blue and red-shifted outflows. Comparison of line intensities indicates that PO arises from a confined source roughly 1’‘ in extent, with a column density of N_tot ~ 2.8 × 10¹⁵ cm^-2, which corresponds to a fractional abundance of f ~ 9 × 10^-8, relative to H₂. Consequently, PO and PN have similar concentrations in VY CMa, a result not predicted by either LTE or kinetic models of circumstellar chemistry. These phosphorus compounds may arise from shock-induced reactions in this active envelope. Phosphorus monoxide is the first interstellar molecule detected that contains a P-O bond.

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Laboratory Spectroscopic Studies of Gas- phase Organic Precursors of Relevant Pre-biotic Molecules: Ethyl Amine

The success in finding new molecules in space relies on precise laboratory measurements. Gas-phase spectra of even small asymmetric organic molecules become very complicated because they have three unique axis of rotation with usually large dipole moments along those axis, resulting in intense spectra, soft internal motions and vibrations – all of which act to increase the number of lines present in room temperature experiments. For this reason, we built a Fourier transform microwave (FTMW) spectrometer particularly well suited for recording spectra of organic molecules. This machine measures rotationally cooled spectra in a molecular beam, which cools to about 3 K. The cold gas ensures that we are recording the lowest energy states of the molecules, which will be most relevant to interstellar searches. The low frequency work is then extended up to as high as 800 GHz using one of our millimeter wave spectrometers. This low to high approach suppresses confusion, which leads to an accurate assignment. So far we have measured the spectra of three molecules relevant to astrobiology, hydroxyacetone, lactonitrile and ethylamine. It is noteworthy that the spectra of these molecules consist of thousands of transitions of which as many as possible need to be accounted for before an accurate and unambiguous assignment can be made in the laboratory owing to high line densities. The Hamiltonians used to fit these spectra are “soft” enough to be able conform to several incorrectly assigned lines. Even one misplaced line could poison an analysis to the point where it has no predictable power even though at face value it seems adequate. The clear danger is an astronomer armed with tabulated frequencies of incorrect laboratory determinations who will never be able to assign the space spectrum accurately. It is of utmost importance that the laboratory determinations be made thoroughly and completely — this unfortunately takes time and effort.

Star Dust mission scientists have recently reported the detection of large amounts of methylamine and ethylamine derivatives believed to be of cometary origin. These reports are preliminary and somewhat controversial, but not at all surprising. Methylamine has long been known as a major organic constituent of the Sgr B2(N) star forming region, but no radio astronomical search had ever been conducted on ethylamine because the radio spectrum prior to our work had not been adequately characterized in the laboratory. The spectrum of ethylamine is quite complex owing to the existence of several low-lying conformers, each with three large amplitude motions. The ground state of the lowest lying conformer known as anti-ethylamine is particularly perturbed owing to mixing of an inversion of the amine hydrogens with the end-over-end rotation of the molecule (see Figure 1.2). The mixing is so severe that a rigid rotor Hamilitonian is inadequate to fully characterize the spectrum. Even though we are unable to fully analyze ethylamine, we have directly measured and assigned an adequate number (more than 600 transitions) of this species, which allowed us to search for this species in interstellar gas clouds.

In addition, we have completed several upgrades of the FTMW spectrometer. First, a dual purpose shield was installed into the vacuum chamber. This shield is constructed of a mu-metal, which shields the interaction region of the cavity from the effects of the Earth’s magnetic field. The reduction of the magnetic field allows for the measurement of radicals (i.e. molecules with unpaired electrons). Also, the shield is thermally isolated from the chamber walls, which allows for the cryopump to cool more efficiently. Several radical species have now been observed with this new shield in place (ZnCN, FeCO, ZnCl, and CrCl). As part of this project, a pulsed discharge nozzle was developed to create these radicals. Thirdly, electronics for a frequency upgrade were integrated into the system in August 2006. The design incorporates two frequency bands, 3.5 – 18.5 GHz and 17.5 – 40 GHz. The bands are easily switched using computer logic and mechanical broadband microwave switches. Antennas were developed to give excellent performance from about 9 GHz – 40 GHz without having to adjust or replace the antennas. In fact, we believe our system works best between 26 and 40 GHz where others in the field have difficulty. Mode launching at higher frequencies is common owing to delivery of the molecules through a large (1/4”) hole near the center of one of the mirrors. We have avoided this problem by placing the nozzle at an optimal point between the mirrors instead of through the mirrors; thus there are no “extra” holes in our mirrors.

Furthermore, we are in the design stages of adding a third microwave band (either 40 — 60 or 50 — 75 GHz) to the system. This one is more complex because it requires waveguide components. The new band will be added to the existing 18 — 40 GHz system by way of a doubler/amplifier. All waveguide components will be attached to the back of the mirrors and automatically switched on using electromechanical microwave switches. It is anticipated that after the upgrade is complete, we will achieve contiguous coverage from 4 — 60 GHz (or nearly contiguous coverage through 75 GHz) with equal sensitivity in each band.

Radio Astronomy Searches for Prebiotic Molecules

A true understanding of the chemical processes of the interstellar medium requires a complete and accurate molecular inventory. Many of the recent claims of gas phase interstellar molecules have been clouded with controversy, specifically glycine, dihydroxyacetone, ethylmethylether and glycolaldehyde, to mention a few. The controversy in these detections is owing to the large number of spectral features arising from primarily asymmetric organic molecules. The line density is so high in some star formation cores that astronomers are no longer system noise limited, but instead limited by signal confusion. In other words, there are more lines present than can comfortably fit in the spectral region. In this confusion limit, the chances for false identifications become high. One remedy to this confusion is extremely broadband spectral coverage, which effectively covers the entire spectrum of a particular molecule. Because physics requires that the spectrum of gas phase molecules be self-consistent and exhibit a sensible temperature and abundance, this broadband approach allows us to assign the spectral features as a collection of molecules instead of molecules individually.

During the first 31/2 years of the grant period, we have used this approach to study important, as well as practical, biologically relevant precursors that can be formed from the abundant interstellar molecules formaldehyde, hydrogen cyanide and water. These simple compounds will react to form a myriad of complex organic molecules from simple sugars to hydroxy acids to amino acids and potentially even nucleobases.

The richest known molecular source is near the galactic center in a region known as Sgr B2(N). Past studies of this object have lacked the necessary sensitivity to identify the most recently detected molecule and the coverage has been sparse. We are continuing a major observing effort to cover from 70 – 170 GHz and 210 – 270 GHz using the Arizona Radio Observatory’s telescopes. Thus far, we have covered about 75% of these regions at or near the confusion limit and have recorded thousands of spectral features of which about half are unidentifiable. The most significant results from this year are the confirmation of acetamide in this source. Over a hundred different transitions of this species were found in Sgr B2(N).

Additionally, we have nearly completed our survey of the molecule formamide towards a number of interstellar sources. In this survey, we found that formamide is a common constituent in warm molecular gas, presumably associated with star formation regions. So far it has been detected in Sgr B2, Orion-KL, G34.3, W51M, and W51e1e2. A large abundance of formamide could be important in life processes on planets at early times because of its polar aprotic solvent properties.

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Chemical Studies of Late-type Carbon and Oxygen-Rich Stellar Envelopes

Mass loss from stars usually occurs in the later stages of stellar evolution, particularly on the red giant and asymptotic giant branch stages. The material lost from these stars creates a circumstellar shell, which in certain cases have been found to exhibit complex gas-phase chemistry. The degree of chemical richness in the shell appears to depend on the elemental carbon to oxygen ratio, which is established by nucleosynthesis and the degree of convective mixing in a given star. Observations have suggested that carbon-rich circumstellar shells (C > O) exhibit a richer molecular content than their oxygen-rich analogs. In order to test this theory, we have conducted a very sensitive spectral-line molecular survey of the O-rich shell of the red supergiant VY Canis Majoris (VY CMa). VY CMa is one of the most luminous objects in the sky in the infrared, with a large and sporadic mass loss rate. These observations have resulted in the identification of 18 different chemical compounds in the envelope of VY CMa, including NaCl, PN, CS and SiS (see Figure 1.3). The line profiles exhibited by the molecules vary dramatically among the detected species, indicating that there are many distinct, layered chemical regions around the star, controlled in part by dust formation. These results suggest that oxygen-rich circumstellar shells are more complex chemical environments than previously thought. Further studies of O-rich shells, particularly of red supergiant stars such as VY CMa, may prove that this chemical richness is a more general phenomenon. In addition, we have been searching for phosphorus-bearing compounds in carbon-rich circumstellar shells. We have detected PN and HCP in the C-rich photoplanetary nebula, CRL2688, and confirmed the presence of PN and HCP in IRC+10216. Combined with the detection of PO and PN in VY CMa, these results suggest that phosphorus chemistry is more active in circumstellar gas than previously thought.

Chemical Studies in Planetary Nebulae

Molecules are commonly found in circumstellar envelopes of evolved stars, but what becomes of them as the objects mature into planetary nebulae “ What becomes of the molecules thereafter” Of particular interest are species with carbon-carbon bonds, which are quite abundant in C-rich circumstellar shells. In order to address these questions, a survey of the ethynyl radical, CCH, was conducted towards evolved planetary nebulae (PNe) using the Arizona Radio Observatory’s 12m antenna and the Submillimeter Telescope (SMT). This species has been detected in NGC 6720 (Ring Nebula), NGC 6781, and NGC 7293 (Helix Nebula) via the two hyperfine components of the N = 1 → 0, J = 3/2 → 1/2 transition near 87.3 GHz. The column densities obtained for CCH in the evolved planetary nebulae are in the range of 0.3-1.4 × 10¹³ cm², corresponding to fractional abundances of f(CCH/H₂) ~ 10⁷ – in reasonable agreement with certain chemical models. As a continuation of this study, this past year we also detected C3H2 and H2CO towards the Helix Nebula, see Figure 1.4. The abundances derived for these molecules are roughly comparable to those found in the protoplanetary nebula CRL 2688 and CRL 618, but are less than those obtained in diffuse clouds. Other species found in evolved PNe, such as HCN, CN, and HCO⁺, are also present in diffuse gas at decreased concentrations, suggesting that there may be a connection between the molecular content of these objects. C-rich circumstellar shells may be providing the carbon-carbon backbones that lead to the organic chemistry of dense clouds via planetary nebulae. The dispersion of these nebulae may be a major source of molecular enrichment to the ISM.

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Theoretical Calculations

Work conducted by L. Adamowicz, A. Jalbout, and students has centered on the study of structures and stabilities of some biologically-relevant species, in particular acetol and lactonitrile. This group has done ab initio calculations for these species, which have established the lower energy conformers. Their study has aided spectroscopic investigations in the laboratory. The group has now completed an investigation of the mechanism of glycolaldehyde formation, performing calculations on the Nazarov-type reaction concerning addition of two aldehydes mediated by a hydronium cation. This is a possible reaction contributing to the process of the gas-phase formation of simple sugars. The work has appeared in Astrobiology.

Public Outreach

Ziurys and Halfen participated in a symposium sponsored by the American Chemical Society to bring interstellar molecules into the classroom (high school, undergraduate college) as a means to teach general chemistry. The talk by Ziurys was videotaped for use in chemical education. Ziurys is currently recording material on her research for Pulse of the Planet, an NPR radio show. Tenenbaum and Milam have given talks at high schools on astrochemistry. The Ziurys group sponsored a high school student, Danna Qasim, who participated in a research project on carbon isotope ratios in circumstellar gas.

Acronyms

ARO: Arizona Radio Observatory, Tucson, Arizona

ALMA: Atacama Large Millimeter/Sub millimeter Array

BILMA: Berkeley-Illinois-Maryland-Associations

DHA: Dihydroxyacetone

FTMW: Fourier Transform Microwave Spectrometer

IRAM: Instituto de Radio Astronomía Milimétrica, Granda, Spain

ISM: InterStellar Medium

LAPLACE: Life And Planets Astrobiology Center, Tucson, Arizona

NASA: National Aeronautics and Space Administration

PI: Principal Investigator

SMT: Submillimeter Telescope, Mt. Graham, Arizona

SETI: Search for Extra Terrestrial Intelligence

VY CMa: VY Canis Majoris

Related Web site Links

Arizona Radio Observatory – http://aro.as.arizona.edu/

Ziurys Research Group – http://www.chem.arizona.edu/ziurys/ziur-group.html