Session S39: Molecular Astrophysics: From Stars to Planets: the Trail of Water and Organics

Water in protoplanetary disks: from Spitzer to JWST

Water is a fundamental ingredient of planet formation, both as a key constituent of planetary atmospheres, and as a mass catalyst of giant planet cores. It is ubiquitous in our own solar system, but its distribution is not uniform, with only trace amounts being part of the terrestrial planets, including the Earth. In recent decades, we have found abundant water vapor and ice in protoplanetary disks using infrared telescopes such as Spitzer and Herschel, and most recently JWST. With these facilities, we have developed a growing understanding of the trail of water from the interstellar medium to young planets. I will review our current knowledge of the disposition of planet-forming water and discuss its role in determining the architectures and make-up of exoplanetary systems.   

Deuterated Water in Comets: Importance, Current Status, and Future Directions

Comets are comprised of icy planetesimals left over from planet formation, and contain the least processed remnant material from solar system formation. This makes them ideal for constraining the origin and thermal evolution of water in the solar nebula. D/H ratios in water may depend on distance of formation from the proto-sun, and on when the icy material incorporated into comets was last subjected to the gas-phase D/H exchange driven by vertical mixing between the planetary disk midplane and photosphere. 

Reports of cometary HDO/H₂O ratios to date suggest an intrinsic dispersion spanning approximately 1 to 3 times that of Earth’s ocean water (Vienna Standard Mean Ocean Water, or VSMOW). However, this is based on a relatively small sample of measured comets (fewer than 20). Therefore, the majority of reported HDO measurements are susceptible to systematic biases and uncertainties inherent to the diverse observational and analysis techniques used. Furthermore, with the exception of space-based measurements of two comets, HDO and H₂O have yet to be sampled simultaneously, which makes them vulnerable to potential time-varying water production for individual comets. 

In this talk, the evolution of deuterium in water will be traced from the cold dense pre-solar molecular cloud core, through the shielded proto-solar disk midplane to existing measurements of cometary HDO. Prospects for future measurement of multiple HDO lines simultaneously with other water isotopes (H₂¹⁶O, as well as H₂¹⁷O and H₂¹⁸O) will also be discussed.   

On the Formation of Complex Organic Molecules in Astrophysically-Relevant Laboratory Ices

Extraterrestrial ices, such as those found on interstellar icy grains, comets, and Kuiper Belt Objects (KBOs), serve as incubators for the formation of complex organic molecules (COMs). The accretion of common reactants like water, carbon monoxide, carbon dioxide, methanol, and ammonia into these ices along with an external source of energy such as solar wind, cosmic rays, or ultraviolet photons to process these ices can lead to a variety of COMs, some of which may have biological significance. To better understand the possible inventory of COMs and their formation mechanisms inside the ices, laboratory experiments are necessary to simulate their chemical, energetic, and temperature conditions. This presentation describes an ultrahigh vacuum machine used to simulate these conditions to monitor and isomer-selectively detect key organic molecules capable of formation inside extraterrestrial ices. By utilizing tunable laser photon energies coupled with reflectron time-of-flight mass spectrometry (ReTOF-MS), techniques such as selective photoionization (PI), resonance enhanced multiphoton ionization (REMPI), photoionization efficiency (PIE) curves, and selective photolysis, which are all supported by theoretical chemical calculations, specific isomers of COMs can be identified. These experiments demonstrate how COMs can be synthesized via non-equilibrium chemistry initiated by high energy ions and photons inside extraterrestrial ices.   

The OSIRIS-REx mission and organic-rich materials from asteroid Bennu

The OSIRIS-REx asteroid sample return mission successfully returned samples from asteroid Bennu to Earth on Sept 24, 2023.  The returned samples are currently undergoing a preliminary examination by a team of scientists from around the world.  The analyses being done address multiple aspects of the samples including their chemical and mineralogical composition, petrology, spectral characteristics, isotopic systematics, and physical characteristics.  One key focus of these analyses is to determine the abundance and nature of organics within the samples, as materials like those returned from Bennu may be representative of those that delivered the original biogenic elements to the early Earth that played a role in the origin of life.  This talk will provide a brief overview of the overall OSIRIS-REx mission and summarize some of the things that are being learned from study of the returned samples.   

Tracing the Path of Iron: Detection of FeC (X 3Δi) in the Circumstellar Envelope of IRC+10216

Iron is one of the more abundant refractory elements with a cosmic abundance of 3.2 × 10^-5. It is thought to play a pivotal role in the formation of rocky, Earth-like exoplanets. It is also common in meteorites and in interstellar dust grains, as pure iron and in minerals such as schreibersite, (Fe,Ni)P₃. The connection between solid-state and gas-phase iron in the interstellar medium (ISM) is not well-understood, and the progression to Fe-containing planetary bodies is uncertain. Gas-phase atomic iron is known to be present in stellar photospheres, star-forming regions, and planetary nebulae, but molecular forms of iron remain elusive. For over 50 years, only one iron-bearing molecule has been conclusively identified in the ISM: FeCN. The species was found in the circumstellar envelope of the late-type carbon star IRC+10216. Here we present the interstellar identification of a second iron-containing molecule, FeC (X ³Δ_i). The radical was also observed in the envelope of IRC+10216, using the  Arizona Radio Observatory (ARO) Submillimeter Telescope (SMT) at 1.3 mm and the ARO 12-m telescope at 2 mm. The J = 4→3, J = 5→4, and J = 6→5 rotational transitions of FeC were detected; these are the three lowest energy lines in the ground state spin-orbit ladder, Ω = 3. FeC was found to have a fractional abundance, relative to H₂, of 6 × 10^-11, with a shell distribution extending out to 500 R_* (~ 10ʺ). In contrast, FeCN has an abundance of 8 × 10^-11 in IRC+10216. These data suggest that much of the iron is sequestered into dust grains in circumstellar shells, setting the stage for condensation into larger objects.   

The Organic Molecular Continuum: Bridging the Gap at Planetary Nebulae

The circumstellar envelopes of asymptotic giant branch (AGB) stars are known to be rich in carbon-bearing molecules. It has been thought that as these objects transition into planetary nebulae (PNe), the molecular material undergoes significant photodissociation. As a result, chemical content within subsequent diffuse clouds generally have to be rebuilt from atomic constituents, potentially delaying the emergence of organic molecules. Recent observations, however, contest this notion. The Submillimeter Telescope and 12m antenna of the Arizona Radio Observatory in conjunction with the Institut de radioastronomie millimétrique (IRAM) 30m telescope have been used to study planetary nebulae. Evidence from such sources demonstrate a persistent molecular presence (HC₃N, c-C₃H₂, and CCH, among others) over the lifespan of PNe, suggesting that they seed the interstellar medium with molecular material. The detections suggest a chemical link between evolved stars and diffuse clouds, in which the later inherit essential carbon-carbon bonds. In this talk, the current state of molecular chemistry in PNe will be presented, along with new results for select objects (e.g. M4-17 and M1-59). Spectra and abundances will be shown, illuminating the uninterrupted trajectory of interstellar molecules.    

Metal Chemistry in the Unusual Outflows of the Hypergiant Star VY Canis Majoris (VY CMa): A Focus on Salt

VY Canis Majoris (VY CMa), a hypergiant star, offers an intriguing platform for studying chemistry in the ejecta of massive stars. It experiences intense mass loss, generating asymmetric outflows that create intricate structures comprised of dust and gas, including arcs, knots, and clumps. The material lost in their outflows populate the diffuse interstellar medium. The interplay between gas and dust is important in the stellar ejecta. To explore this phenomenon, ALMA at Band 6 was used to image the refractory molecules NaCl, AlO, AlOH, TiO₂, and, notably, KCl, with resolutions of 0.25" to 1". We combined this data with single-dish observations from the Arizona Radio Astronomy Observatory Submillimeter Telescope to capture total flux. NaCl, common table salt, displays emission closely that surrounds the central star and also traces an ejected cloudlet of gas called the SW Clump, located at a distance of about 180 R_*. Their presence in the SW Clump suggests direct but intact expulsion from the star's photosphere. KCl, a novel discovery in VY CMa, exhibits a similar distribution. Conversely, emission from AlO, AlOH, and TiO₂ exist within 60 R_* of the star, indicating rapid condensation onto the dust grain.  The persistence of the alkali halides is a chemical puzzle. NaCl is found in solar system objects such as Mars, Europa, and Io and in protoplanetary disks. Alkali halides, in particular salt, appear to have a unique chemistry that avoids complete condensation, unlike other metal-bearing molecules.   

Investigation of Fe2O3 Clusters: a first-principles study

Fe2O3 clusters exhibit a broad range of configurations with unique geometric arrangements and surface terminations, making them suitable for different purposes. The stability and reactivity of (Fe2O3)n clusters, ranging from n=2 to 6, are studied here using molecular dynamics simulations, genetic algorithms, and density functional theory (DFT) to understand their atomic-scale details. In particular, the distribution of the magnetic moment is investigated in detail. The identified clusters possess distinctive electronic properties that make them promising candidates for various applications such as catalysis, sensor technology, and energy storage.   

Panel 2: A panel discussion of solar system bodies

Solar system bodies house an exotic mixture of organic molecules that span from the simplest of organic molecules to amazingly complex having hundreds of atoms. The organic inventory of solar system bodies from meteorotes, asteroids, satellites and planets will be discussed by this panel. The broad topics of the discussion session will include Hydrocarbon worlds: Titan and beyond, OSIRES-Rex and the Organic inventory of asteroids and Organic inventory of Enceladus etc.

Some topics and questions that will be posed to the panel include:

1. How can we enhance our understanding of the chemistry and physics of solar system bodies to improve the interpretation of observational data especially in the JWST era?

2. What are the primary challenges and opportunities in the field of molecular spectroscopic measurements and ab initio studies for solar system studies?

3. How can we create an inclusive workplace environment that encompasses a broad range of perspectives and backgrounds?