Astrochemistry Discussions are tentatively scheduled every other week beginning 22 April, 2020. The plan is for the Main Event to run 10:00 - 11:30 a.m. Eastern US Time.
A Google Calendar is provided here that you can add to your own, and should display in your local timezone (baring VPN issues - see the note at the bottom of the calendar to see what time zone it thinks you're in).
Things will certainly evolve with time as we figure out the system that works best for everyone.
Phosphorus Carriers in the Protosolar Analog B1-a
Phosphorus is a critical ingredient in terrestrial biochemistry, yet is rarely observed during the low-mass star formation sequence, and so its chemistry in protosolar analogs is poorly understood. We recently detected the phosphorus carriers PO and PN towards the low-mass protostar B1-a using the IRAM 30m telescope, representing the second detection of P-bearing molecules in a solar-type star forming region. I will discuss the inferred abundances and emission origin of PO and PN in this source, and implications for phosphorus partitioning between the volatile and refractory phases. Comparing B1-a to other dense ISM regions with PO and PN detections, the PO/PN ratio is surprisingly uniform in different environments, which I will discuss in the context of current astrochemical models. I will also introduce follow-up efforts to further characterize the phosphorus molecule emission in B1-a and additional low-mass protostars. Together these observations are advancing our understanding of how this key prebiotic element is inherited in planetary system progenitors.
PO and PN in Orion-KL: Insights to the Phosphorus Chemistry of Molecular Clouds
The PO molecule (X2r) has been identified for the first time in Orion-KL via its J = 2.5→1.5 transition, as part of a 3mm survey using the Arizona Radio Observatory 12m telescope. The J = 2.5→1.5 transition consists of two lambda doublets, both of which were detected. The PN (X1Σ) J = 2 →1 transition was also detected. Both PO and PN exhibited line profiles with ΔV1/2 ~ 21 - 25 km s-1, and VLSR ~ 10 km s-1, characteristic of the Orion Plateau region. The presence of both species in the Plateau region indicates that shocks are primarily responsible for their formation. Abundances of both species were calculated, with a PO/PN value of ~3, similar to other sources where both species have been observed. The implications of the PO/PN detections for chemical formation pathways will be discussed.
Co-Authors: Lucy M. Ziurys and Lilia Koelemay
The Interstellar Journey of Phosphorus
Phosphorus is a key chemical biogenic element for the development of life. But how it arrived on the early Earth is still a mystery. To track the interstellar journey of phosphorus from molecular clouds to planets, we have studied the phosphorus chemical reservoir in a large sample of star-forming regions in our Galaxy and in the comet 67P/Churyumov–Gerasimenko. In this talk I will present the main results of several observational campaigns carried out with single dish telescopes and ALMA, and measurements of the ESA Rosetta mission. Our findings, combined with newly developed chemical models, have allowed us to understand how phosphorus-bearing molecules are formed in star-forming regions, and how they could have been transferred to our own Solar System.
Wednesday, 27 January 2021 @ 10 a.m. Eastern US Time
Vibrational and rotational characterization of phenylpropiolonitrile to facilitate astrochemical surveys
Detection of new molecules in the interstellar medium has been ongoing for almost 100 years, with over 200 detected so far. To facilitate these searches, in depth characterization of molecules with potential to be detected is required. Choosing which species to study is based on what has been detected in the past guided by computational methods, or intuition on what molecules could be built from those pieces. With recent detection of benzonitrile in TMC-1, the characterization of other nitrogen containing aromatic rings is an intuitive path of investigation. In this talk, I'll present the work our team has performed on phenylpropiolonitrile (C6H5C3N) with microwave, mm-wave and infrared spectroscopies. Over 6200 transitions in the 8-200 GHz range have been assigned and used to derive rotational spectroscopic constants (rotational, centrifugal distortion, and nuclear hyperfine) for C6H5C3N. In addition, 14 fundamental vibrational bands were measured and identified with help from computational work. With the measurement of these parameters, an interstellar search for C6H5C3N can be performed with confidence as many of the lines have either been assigned, or can be calculated with high accuracy below 300 GHz.
The 130 – 370 GHz Rotational Spectra of Four Astronomically Relevant Cyanobutadiene Isomers (C5H5N)
Co-Authors: P. Matisha Dorman, Vanessa L. Orr, Andrew N. Owen, Samuel M. Kougias, Aatmik R. Patel, Brian J. Esselman, R. Claude Woods, and Robert J. McMahon
Finding Unknown Unknowns for Astrochemistry with Broadband Rotational Spectroscopy
A large portion of molecular discoveries in the interstellar medium have been motivated and guided by exhaustive laboratory studies. These experiments provide, for instance, precise rest frequencies and reactive branching ratios in typically targeted studies focusing on specific molecules of interest. With the advent of broadband chirped-pulse microwave spectroscopy, we are now able to collect rotational spectra over many gigahertz—closer to the capabilities of many radio telescopes—covering multiple transitions for potentially many molecules at high signal-to-noise within only a few hours of integration. With these capabilities, we have access to rapid experimental assays covering different precursors and other experimental parameters; the potential discovery space is enormous.
In this talk, I will present exhaustive studies of unbiased assays of benzene in electrical discharges, conducted at the Center for Astrophysics | Harvard & Smithsonian, which highlight just how much new data we can obtain in single experiments. Moreover, these experiments demonstrate how automation of spectroscopic analysis is highly necessary where high quality data can be obtained at a higher cadence than can be regularly analyzed; I will discuss the open-source tools (PySpecTools, https://github.com/laserkelvin/pyspectools) we have recently developed that simultaneously decreases the amount of human oversight required, thereby improving the overall reproducibility and collaborative nature of spectral analysis. Overall, these improvements allow the focus of analysis to be shifted towards unknown unknowns—potentially new molecules and states that are previously unknown and unsought that are of interest to molecular astrophysics.
The Search for New Molecular Species in the Atmosphere of Titan
Saturn’s largest moon, Titan, harbors a dense, organic-rich atmosphere primarily composed of N2 and CH4. A wealth of additional hydrocarbon, O- and N-bearing species make up the remaining atmospheric composition, formed through the photodissociation of nitrogen and methane, interactions with the Saturnian magnetosphere and galactic cosmic rays, or from exogenic sources such as Enceladus. While in orbit around Saturn, the Cassini mission detected a vast array of previously unidentified positive and negative ion species in Titan’s upper atmosphere, revealing the complexity of the moon’s naturally occurring atmospheric chemistry. The extent of Titan’s neutral atmospheric inventory, the incorporation of these molecular species into its stratospheric haze layer, and their participation in Titan’s methane-based meteorological cycle are all important processes that enhance our understanding of Titan’s global climate and the connections between the atmosphere, organic regolith and hydrocarbon lakes.
Though many early detections of molecular species in Titan’s atmosphere were made by the Voyager 1 and Cassini missions and ground-based telescopes in the infrared, we now harness the power of cutting-edge (sub)millimeter facilities, such as ALMA, to search for additional trace compounds. Here, I detail recent spectral identifications of molecular species found for the first time in Titan’s atmosphere - or, in fact, any planet’s atmosphere thus far. In addition to increasing our understanding of Titan’s atmospheric and surface properties, new molecular discoveries help to further characterize Titan’s atmospheric photochemistry and assess its impact on Titan’s potential for habitability.
Photochemistry in Exoplanet Atmospheres: Insight from Lab Experiments
More than 4000 exoplanets have been discovered in last two decades, including a sample of terrestrial planets in the habitable zone of their host stars. New observing capabilities coming online over the next few years will provide opportunities for characterizing their atmospheres and assessing their potential habitability. Photochemistry could play an important role in their potential atmospheres and lead to the formation of haze that could obscure the detection of major atmospheric constituents. However, the photochemical processes are poorly understood in these exoplanets because their atmospheric parameters are different from that in Solar System bodies. We conducted a series of laboratory experiments that simulate photochemistry in a broad range of temperate (<800 K) exoplanet atmospheres. We investigated three types of atmospheric metallicities (100, 1,000, or 10,000 times solar) at four temperatures (300, 400, 600, and 800 K) using the PHAZER chamber at JHU with one of two energy sources (AC glow plasma and UV photons). We studied the size distributions (He et al. 2018a, 2018b) and the production rates (He et al. 2018a; Hörst et al. 2018a) of solid haze particles that formed, as well as the gas and solid phase chemistry (He et al. 2019, Moran et al. 2020, Vuitton et al. 2020). We find that the particle size is dependent on the experimental conditions, while the production rates are sensitive to atmospheric compositions (He et al. 2018a, b; 2020a, b; Hörst et al. 2018). Even a small amount of H2S can enrich the photochemistry and enhance haze production rate significantly (He et al. 2020a). We observed the photochemical formation of O2, sulfur and organic products in the gas phase, which have been considered as potential biosignatures but are produced abiotically in our experiments. Organic molecules are detected in both gas and solid phases, including potential prebiotic precursors (HCHO and HCN) and compounds with prebiotic molecular formulas (sugars, amino acids, and nucleobases), which could provide a source of organic materials for life to arise.
High-Temperature Non-LTE and LTE Cavity Ringdown Spectroscopy using SMAUG experimental setup
The SMAUG apparatus (Spectroscopy of Molecules Accelerated in Uniform Gas flows) was developed to produce high-resolution infrared spectra of polyatomic molecules of interest for hot astrophysical atmospheres, like for example those that surround hot Jupiters or cool carbon evolved giant stars, reaching up to 2500 K. In particular, high-temperature infrared spectroscopic data is needed to retrieve temperature and concentration profiles from observed telescope infrared spectra.
SMAUG can operate according to two complementary working regimes: non-LTE (vibrationally hot and rotationally cold) and LTE conditions, to interpret the complex pattern of highly-excited vibrational states. Two different gases, carbon monoxide (CO) and methane (CH4) were used as test molecules. Using non-LTE conditions a rotational temperature of 30(3) K was measured for CH4, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with TIvib = 894(47) K, while the population distribution within a given polyad was modelled correctly by TIIvib = 54(4) K, testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad. Using a "post-shock CRDS" technique CO and CH4 LTE spectra were recorded at 950 K and 1400 K.
Mapping CO Gas in Protoplanetary Disks
CO was the first molecule detected in protoplanetary disks. It is widely used as a tracer of the bulk gas and is a major carrier or volatile carbon. The CO snowline has been posited to play an important role in planet formation and in setting the overall chemical composition of the disk. However, basic properties such as the location of the midplane CO snowline and the CO abundance relative to the total gas mass remain difficult to constrain. I will discuss how ALMA observations over the last decade have provided new insights into the distribution of CO gas in protoplanetary disks, as well as raised new questions about volatile carbon chemistry.
The elemental composition of planet feeding gas
The elemental composition of proto-planetary disk gas and ice is a critical parameter for understanding the physics and chemistry in the disk. Furthermore as this gas and ice is incorporated into planets, it is also important for in understanding the composition of planets. I will show how C2H emission is used to constrain the C/O ratio of the gas and apply this to the (possibly) planet containing disks of AS 209, MWC 480 and HD 163296. Observations of these disks suggest that planets forming in the gaps of these disks will accrete gas that has a high C/O but sub-solar C/H. Implying that the atmospheres of pebble- or core-accretion formed planets might have a very perculiar chemical composition.
Connecting astrochemistry and planet formation: A C/O main sequence for warm and hot giant planets
Understanding how planets form is an integral part of understanding the ever growing population of exoplanets as well as our own solar system. Astrochemistry offers a unique probe of the underlying physics of planet formation because the formation history (mass accretion / migration rates) of a giant planet is at least partially encoded in the bulk chemical structure of its atmosphere. In this talk I will outline my ongoing effort to link the planet formation process with protoplanetary disk astrochemistry in order to understand the chemical properties of exoplanetary atmospheres. One particular discovery has been a tight correlation between the fraction of mass brought in by solid accretion and the final bulk carbon-to-oxygen ratio (C/O) in the atmospheres of giant planets. This ‘C/O main sequence’ naturally leads to the empirically derived mass-metallicity relation that has been found for our solar system giant planets, and beyond. Furthermore, this main sequence has shown that understanding the physics of solid accretion into the atmospheres of giant planets will play an important role in understanding the chemical properties of planetary atmospheres as it can dominate the delivery of carbon and oxygen over gas accretion for certain planetary masses.
Pacific Day - 7 Oct @ 20:00 Eastern US Time
*Please Note the Time Change From Usual!
From Hot-corinos to our chemical heritage; the case of NGC1333 IRAS 4A
Studying the physical and chemical properties of low mass star systems is very important as it is directly related to the Solar-like system. Hot-corinos are low mass young stellar objects (YSOs) where the temperature is high enough (>100 K) to release ice-phase complex organic molecules into the gas environment and these COMs can be detected using observation in millimeter wavelength. Some of the COMs are prebiotic and may be related to our chemical heritage. In this talk, I will focus on a protobinary system NGC1333 IRAS 4A and will discuss the presence of hot-corino and an upper limit calculation of a glycine isomer, methyl carbamate. Glycine is a prebiotic molecule, in this connection, I will also discuss how possibly our chemical heritage can be maintained from the early stage of a low mass star formation to planetary objects like our Sun-Earth system.
Molecular spectroscopic line lists
The rapid rise in interest in exoplanetary atmospheres has led to an explosion of interest by astronomers in high accuracy and comprehensive small molecule spectroscopy data. This data is needed to understand and characterise these exoplanetary atmospheres and their host stars (including soon the scientific search for the molecules of life). In this talk, I will discuss in depth a key type of data desired by astronomers - the molecular spectroscopic line list - and how this data is produced by myself and others based on laboratory spectroscopy experiments and high level ab-initio electronic structure calculations, focusing on the key example of diatomic TiO.
The demands for accuracy and breadth of spectral coverage are substantial. New high-resolution techniques now mean that astronomers need these line lists to have strong lines frequencies to an accuracy of < 0.3 cm-1. Simultaneously, very weak lines must be considered to ensure accurate opacities can be calculated, leading to databases of millions to billions of spectral lines per molecule.
New solid-state environments for the formation of complex organics
Spacecraft exploration of icy outer Solar System bodies such as Titan has alluded to a rich cyanide chemistry. Here, the recombination of radicals, atoms, and ions - generated by photolysis and energetic particle interactions with methane and molecular nitrogen - generate a wide variety complex organic molecules. Such chemistry has been the inspiration for many laboratory investigations over the past decades where cyanides have been a particular focus as they present as carriers of C-N bonds required for species of biological interest such as N-bearing heterocycles.
This short talk will focus on structural and spectroscopic characterisation of cyanide-bearing molecular co-crystals. Perhaps most relevant to the aerosols of Titan’s atmosphere - where fractional condensation of nitriles and hydrocarbons are expected to yield ices of mixed composition - these particles could generate higher order organics via interactions with cosmic rays that penetrate through to the surface. In our preliminary studies on cyanide co-crystals, we have used periodic-DFT to predict the ternary-phase 2C6H6-C2H2-HCN crystal; a system that shows competitive thermochemistry under Titan conditions. These co-crystal compositions, where polar nitrile species are completely mixed with hydrocarbon material in stable molecular arrangements, provide new environments to explore efficient solid-state chemistry. We discuss our recent endeavours in this pursuit.
Ennis, C.; Cable, M.; Hodyss, R. & Maynard-Casely, H. “Mixed Hydrocarbon and Cyanide Ice Compositions for Titan’s Atmospheric Aerosols: A Ternary-Phase Co-crystal Predicted by Density Functional Theory” ACS Earth Space Chem (2020). DOI: 10.1021/acsearthspacechem.0c00130
Making a Habitable Planet
23 September, 2020 - 14:00 UTC
Department of Astronomy, University of Michigan
Today we stand on the cusp of characterizing potentially living worlds, or habitable planets, planets with orbits where liquid water would exist on a rocky surface. This concept of habitability implicitly assumes water in some form is present on rocky planets - but is it? More broadly are rocky planets generally "chemically habitable" - do they contain the elements needed for life, such as carbon and nitrogen, on their surfaces? In this talk, I will review what we know about the chemical habitability of forming planets. I will follow the most abundant volatile elements needed for life (C, H, O, N) from the vast cold and low pressure environs of the interstellar space to their presence on planetary surfaces of worlds such as our own. I will outline our current understanding of star and planetary birth while discussing the fate of primary carriers of life's elements, both volatile species (e.g. H2O, CO, CO2) and more refractory materials (e.g. silicates, aliphatic/aromatic hydrocarbons). Some of these carriers can be characterized via existing facilities (e.g. the Atacama Large Millimeter Array), planned facilities (the James Webb Space Telescope), while others require future options (such as the Origins Space Telescope). Finally, within a young planet, the ultimate fate of delivered material is not set until the hot young terrestrial world solidifies and core formation ceases - thus the process of planet formation and evolution itself influences whether a mature planet is habitable or uninhabitable. Looking forward, the astrochemical study of life's materials in space, and the astronomical characterization of terrestrial exoplanets, must be intimately linked to grounding knowledge from the planetary sciences. A fascinating interdisciplinary future awaits, where we will seek to ascertain the origin of our own biosphere and provide crucial chemical context for habitability.
Black in Astrochem Workshop and Networking
When: 15 July 2020; 14:00 UTC
Ashley Walker, Ayanna Jones, Bryne Hadnott, Kathleen Rink, and William Jackson
Theoretical Tools For Astrochem Tutorial/Workshop - 8 July, 2020
Using reaction rate theory (MESMER) to provide robust kinetic parameters for astrochemical modelling
Experiments provide kinetic parameters, rate coefficients and product yields, over a limited range of temperatures and pressures. Often this experimental temperature range is not sufficient to cover the temperatures required for astrochemical modelling. Traditionally, simple analytical forms are used to represent rate coefficients, e.g.
k = A Tn exp(-Ea/RT).
However, this approach can result in extrapolated rate coefficients with an order of magnitude uncertainty, especially at very low temperatures.
A more robust approach is to use a theoretical model to fit to the experimental data before extrapolation. For a given reaction, the reaction rate theory code MESMER calculates rate parameters from the information provided by ab initio structure calculations. In addition, MESMER allows the theoretical parameters to be adjusted to best fit the data, and thus provide a robust description of the reaction. The reaction between the hydroxyl radical, OH, and formaldehyde, H2CO, will be used to illustrate how MESMER can provide robust rate coefficients down to 5 K.
Simulation of Reactivity on the Surface of Dust Grains: Pitfalls and Opportunities
Gas-phase reaction rates can often be predicted with high accuracy because only a few atoms are involved. Compared to that, the simulation of reactions happening on the surface of a dust grain is much more involved, which often requires compromises concerning accuracy. They are very valuable, however, since the corresponding experiments also face the problem of many parameters that have to be controlled. I will give an overview of techniques to model adsorption and surface reactivity, their estimated accuracy, and their computational costs. It turns out that even an "active" surface, like ASW, often has rather little influence on the reaction barriers. Nevertheless, such surfaces influence the reactivity - but can be treated by a very efficient implicit surface model.
Astrochemical Snow Day! - 24 June, 2020
Interstellar Comets: A New Window into the Diversity of Protoplanetary Disk Midplane Chemistry
Comets contain a crucial record of the chemistry that occurred in the proto-Solar accretion disk during the epoch of formation of our planets. The recent discovery of interstellar comets provides a unique opportunity to measure the composition of planetary materials originating around other stars. Consequently, we are entering a new era of Galactic astronomy, with the ability to directly investigate the chemistry that occurred in the disk midplanes of extrasolar planetary systems. This talk will summarise our knowledge of the recently-detected interstellar objects 1I/'Oumumua and 2I/Borisov, focusing on our ALMA observations of gas-phase CO and HCN in 2I/Borisov. While the HCN abundance relative to water (~0.11%) was similar to that found in typical Solar System comets, the CO abundance (~68%) was among the highest observed in any comet within 2 au of the Sun, revealing a likely origin in a CO-enriched environment. The importance of this finding in the context our understanding of cometary and protoplanetary disk chemistry - in particular, the ice chemistry occurring close to the CO snowline - will be discussed.
Merel van't Hoff
Imaging the H2O and CO snowlines around young stars
Planets form in disks of gas and dust around young stars. With the discovery of more than 4000 exoplanets and the ability to study circumstellar disks in great detail we can begin to address the question of how the composition of a planet is linked to its birth environment. Key aspects of circumstellar-disk chemistry are snowlines: radii at which molecular species freeze out from the gas onto dust grains. The temperature at which a molecule freezes out is species dependent. The radial temperature profile in the disk therefore results in radial gradients in the chemical composition, for example, the elemental C/O-ratio of both the gas and ice. The bulk composition of planets may therefore be regulated by their formation location with respect to major snowlines. In this talk I will focus on how we can determine the locations of the two most important snowlines, the H2O and CO snowlines, observationally.
Infrared Resonant Vibrationally Induced Restructuring of Amorphous Solid Water
Amorphous solid water (ASW) is abundantly present in the interstellar medium, where it forms a mantle on interstellar dust particles and it is the precursor for cometary ices. In space, ASW acts as substrate for interstellar surface chemistry leading to complex molecules and it is postulated to play a critical role in proton-transfer reactions. Although ASW is widely studied and is generally well characterized by different techniques, energetically induced structural changes, such as ion, electron and photon irradiation, in these materials are less well understood. Selective pumping of specific infrared (IR) vibrational modes can aid in understanding the role of vibrations in restructuring of hydrogen bonding networks. In this talk, I will present the first experimental results on hydrogen bonding changes in ASW induced by the intense, nearly monochromatic mid-IR free-electron laser (FEL) radiation of the FELIX-2 beamline at the FELIX Laboratory at the Radboud University in Nijmegen, the Netherlands. Experiments are complimented with Molecular Dynamics simulations to constrain the effect at the molecular level.
Co-Authors: Jennifer A. Noble, Herma M. Cuppen, Stephane Coussan, Britta Redlich
Shocks! - Postponed
A themed special session on the astrochemistry and astrophysics of interstellar shocks.
Confirmed Invited Speakers
Women in Astrochemistry Meet & Greet - 20 May 2020
Ewine F. van Dishoeck
Ewine F. van Dishoeck is professor of molecular astrophysics at Leiden University, the Netherlands and external scientific member at MPE Garching. She graduated at Leiden in 1984 and held positions at Harvard, Princeton and Caltech before returning to Leiden in 1990. The work of her group innovatively combines the world of chemistry with that of physics and astronomy to study the molecular trail from star-forming clouds to planet-forming disks. She has mentored several dozens of students and postdocs and has been heavily involved in planning new observational facilities such as the Herschel Space Observatory, the Atacama Large Millimeter Array and the James Webb Space Telescope.
Ewine has been fortunate to receive many awards, including the 2000 Dutch Spinoza award, the 2015 Albert Einstein World Award of Science, and the 2018 Kavli Prize for Astrophysics. She is a Member or Foreign Associate of several academies, including that of the Netherlands, USA, Germany, Norway and Russia. Since 2007, she is the scientific director of the Netherlands Research School for Astronomy (NOVA). She was secretary and president of the IAU Working Group on Astrochemistry from 1991-2011, and co-responsible for four IAU Symposia in the field. As of 2018, Ewine serves as the president of the International Astronomical Union (IAU).
Leah Dodson is a brand new Assistant Professor in the Department of Chemistry & Biochemistry at the University of Maryland College Park. Her interests lie in using experimental physical chemistry techniques to investigate the chemical reactions that occur in the interstellar medium and in planetary atmospheres. Her group uses cryogenic and vacuum technology, spectroscopy, and mass spectrometry to study the kinetics and dynamics of astrochemical reactions under the low-temperature conditions relevant to these environments.
Leah received her B.S. in Chemistry in 2010 from Case Western Reserve University and her Ph.D. in Chemistry in 2016 from California Institute of Technology, working with Mitchio Okumura. Her thesis work focused on studying hydrocarbon oxidation and radical chemistry relevant to atmospheric and combustion chemistry and included collaborations with the Combustion Research Facility at Sandia National Labs and the Advanced Light Source at Lawrence Berkeley National Lab. She completed her postdoctoral work with Mathias Weber at NIST/JILA/University of Colorado Boulder before joining the UMD faculty in 2019.
Cosmic Rays Day! 6 May 2020
The Effects of Cosmic Rays on Carbon Chemistry
Cosmic rays (CRs) are energetic charged particles accelerated in extreme environments. Galactic CRs are accelerated primarily through supernovae. However, it has been recently proposed that protostars may be able to accelerate CRs. These cosmic rays will substantially alter the carbon chemistry in molecular gas. I will present astrochemical calculations of the impact of different CR populations and physics on carbon chemistry. The calculations were performed on a modified version of the publicly available astrochemistry code, 3D-PDR, in which CR attenation has been included in-situ. As such, we're to directly study the impact of different external and internal CR spectra. We find embedded sources of CRs significantly impact the carbon chemistry in relation to the HI-to-Hw transition. The dense gas is heated above 50 K and bright in [CII] emission. Embedded CR sources allow for atomic carbon to exist in dense gas where traditional models have carbon locked into carbon monoxide. Finally, I will show that the CO-to-H2 conversion factor used in extragalactic studies is relatively stable to significant enhancements of the CR ionization rate. However, the CI-to-H2 conversion factor is sensitive to the CR ionization rate and external environment.
Probing Galactic Cosmic Rays with Small Molecules
In the century following their discovery by Victor Hess in 1912, cosmic rays have been recognized as an important constituent of the Galaxy. With a total energy density somewhat larger than that of starlight, cosmic rays are the dominant source of ionization for the cold neutral medium (CNM) within the Galactic ISM. In starless molecular cloud cores, they are also the dominant source of heating. Thus, cosmic rays play a central role in astrochemistry by initiating a rich ion-neutral chemistry that operates within the CNM, and the cosmic-ray ionization rate (CRIR) is a key parameter in models for the chemistry of the ISM. In this talk, I will discuss recent estimates for the cosmic-ray ionization rate in the Galactic disk, obtained by using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret previously-published measurements of the abundance of four molecular ions: ArH+, OH+, H2O+ and H3+. The CRIR estimates thereby obtained show a remarkably small dispersion from one interstellar cloud to another. At the Galactocentric distance of the Sun, the primary CRIR per H nucleus is ~ 2x10-16 s-1 in both diffuse atomic clouds and diffuse molecular clouds. I will also discuss a recently-selected SOFIA Joint Legacy Program, HyGAL, which (among other things) will greatly expand the number of sight-lines on which ArH+, OH+, and H2O+ have been observed.
Cosmic Rays and Grain Chemistry in Star- and Planet-Forming Regions
Interstellar matter is subjected to bombardment by several types of ionizing radiation including cosmic rays, stellar winds, x-rays, and gamma-rays. It is known that such radiation can have a significant physicochemical impact on interstellar environments, and a large body of experimental work has shown that the interaction between such energetic particles and low-temperatures ices can result in the formation of complex - even prebiotic - molecules. Even so, modeling the chemical effects of cosmic ray collisions with interstellar dust grain ice mantles has proven challenging due to the complexity and variety of the underlying physical processes. In this talk, recent work on this subject by us is reviewed and the possible applications to better understanding the chemistry in star and planet-forming regions is highlighted.
From Clouds to Planets, The Astrochemical Link
22 April, 2020 - 14:00 UTC
All ingredients to make stars like our Sun and planets like our Earth are present in the dense (~100,000 H2 molecules per cc) and cold (~10 K) interstellar clouds. In these "stellar-system precursors" an active chemistry is already at work, as demonstrated by the presence of a rich variety of organic molecules in the gas phase and icy mantles encapsulating the sub-micrometer dust grains, the building blocks of planets. Here, I’ll present a journey from the earliest phases of star formation to protoplanetary disks, with links to our Solar System, highlighting the crucial role of astrochemistry as powerful diagnostic tool of the various steps present in the journey.