Lucy M. Ziurys

Regents Professor, CBC and Astronomy

Research

At present, more than 170 different chemical species have been detected in interstellar space, primarily in giant gas clouds located throughout our Galaxy and in gaseous material surrounding old stars, but also in other unusual environments, as illustrated in Figure 1.  Despite the extreme conditions of the so-called interstellar medium (ISM), which is typically quite cold (T ~ 10-50 K) and very diffuse (n ~ 10³ - 10⁶ particles/cc), chemistry flourishes, producing many common, but sometimes exotic, compounds, including reactive radicals and molecular ions, ranging in size from simple diatomics like CO to complex species like C₆₀. It is now recognized that we live in a molecular universe, and that interstellar molecular material is continuously recycled in the ISM. The life cycle is shown in Figure 1. Molecules form in dying stars, and then are passed on theough the planetary nebula stage and into the diffuse ISM. This matter subsequently collapses to form dense clouds, the sites of star and solar systems. The stars eventually die, and the cycle repeats. As solar systems form, some of the pristine molecular material from the nascent molecular cloud is delivered to young planet surfaces via comets, meteorites, and interplanetary dust particles. This material is thought to have influenced early biochemistry, thus tracing life's originss back to interstellar space.

Image

Figure 1: The interstellar molecular life cycle, tracing molecular formation from the ejecta of dying stars into the diffuse, and then dense ISM, and subsequently into regions of star and solar system formation via the protoplanetary disk stage. Some of the pristine interstellar molecular matter is the delivered to planet surfaces via bombardment by comets, meterorites, and interplanetary dust particles.

The primary objective of research in the Ziurys group is to understand the molecular life cycle and study the chemistry occurring in the vast regions of space via an interdisciplinary approach that involves high resolution molecular spectroscopy in the laboratory and radio astronomical observations. The University of Arizona is unique in this regard as it has its own radio telescopes, operated by the Arizona Radio Observatory (ARO),  which can be used for graduate research.  Radio telescopes are used to measure rotational molecular spectra and identify new chemical compounds by their "fingerprint" signatures, which are known from laboratory measurements. We are interested in discovering what chemical compounds exist in space and in which types of interstellar sources, how they are formed, and how this impacts the origins of solar systems and planets, and ultimately life. As shown in Figure 2, many interstellar objects are have very dense and complicated radio spectra, illustrating the complexity of their chemistry. Displayed in this figure is a "spectrum" of the cloud Sagitarrius B2N,  which is located near the center of our Galaxy. Over the wavelngth region 68 - 280 GHz, over 15,000 spectral lines are present, a large fraction which are unidentified. Possible carriers of these unidentified lines are smaller species containing metals such as iron, magnesium, and chromium;  (also significant for organometallic chemistry),  organic molecules related to sugars and nucleic acids, and phosphorus-bearing compounds.

Image

Figure 2: The millimeter-wave spectrum (68 - 280 GHz) of the molecular cloud Sagittarius B2N, showing over 15,000 individual spectral emission and absorption lines, many which are unidentified. These data were obtained with the telescopes of the Arizona Radio Observatory

Laboratory studies focus on the measurement of gas-phase rotational spectrum of species of astrophysical interest in the microwave, millimeter and THz regions of the electromagnetic spectrum (~3 GHz - 1.5 THz). This goal requires design and construction of our own spectrometer systems, as shown in Figure 3. Currently, there are four working instruments in the Ziurys group: two mm/sub-mm direct absorption systems, a velocity-modulation spectrometer specifically designed to study molecular ions, and a pulsed, Fourier transform microwave/mm-wave (FTMmmW) machine. Part of the laboratory work also concerns developing exotic synthetic techniques for creating these transient species in detectable concentrations including electrical discharges and laser ablation. We have succeeded in recording the spectra of a wide range of metal-bearing species, in particular radicals and, more recently, ions, such as AlC₂, FeCN, FeCO⁺,CrCCH,  FeO⁺, and CrP. Many of these species have unpaired electrons, and thus their spectra exhibit complex fine and hyperfine splittings (see Figure 4). Analysis of such data requires a detailed knowledge of quantum mechanics. Other investigations include possible pre-biotic species such as EtNH₂ methylisocyanate and hydroxyacetone. The fingerprints measured in the laboratory enable such species to be identified in space.

Image

Figure 3: Two of the spectrometers of the Ziurys group: Left: Fourier transform microwave/mm-wave system, with the laser ablation source. Right: Millimeter wave direct absorption spectrometer.

Image

Figure 4 LEFT: Laboratory Fourier transform microwave spectra of the AlC₂radical in its ²A₁ electronic ground state, showing aluminum hyperfine components in three rotational transitions (Halfen and Ziurys 2018). RIGHT: Millimeter-wave direct absorption spectra of the CrP radical (X⁴Σ^-), displaying the quartet fine structure splittings in two rotational transitions (Burton et al. 2018).

Observational studies in the Ziurys group include the identification of new interstellar species, such as FeCN, AlO, AlOH, CCN,  and PO, which is usually conducted in conjunction with laboratory work. The newest molecule found by the Ziurys Group is CH₃NCO, which was identified in the spectral survey data of SgrB2N.  Other projects involve elucidating the chemistry associated with evolved stars, such as VY Canis Majoris, and the survival of molecules in planetary nebulae such as the Helix. Observations are also currently being conducted to trace the history of carbon and organic chemistry through the molecular life cycle, as shown in Figure 1.

Image

Figure 5: The Submillimeter Telescope (SMT) of the Arizona Radio Observatory, located at Mt. Graham, AZ (left), and the new ALMA prototype 12-meter telescope at Kitt Peak (right).

Interstellar molecules are primarily studied using the telescopes of the Arizona Radio Observatory (ARO), as well as the Atacama Large Millimeter Array (ALMA) and the IRAM 30 m. ARO operates the Submillimeter Telescope (SMT) on Mount Graham, AZ, and the new 12-meter (12m) Telescope at Kitt Peak (see Figure 4). The new 12m is built with the most advanced technology for radio telescopes. It was one of the three prototype antennas built and tested for ALMA, and became property of the UA on March 23, 2013. The surface is made of rhodium coated panels, while the structure is formed from carbon fiber.