The intersection between high-energy astrophysics and astrochemistry.

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Tree of cosmic ray astrochemistry

Cosmic Ray AstroChemistry

Since my PhD, I have been very active in investigating the role cosmic ray physics plays on the chemistry of molecular clouds. Cosmic rays initiate a diverse zoo of chemistry, as shown by the “Tree of Cosmic Ray Chemistry”. Much of my work has been investigating the impact of cosmic rays on the emission from species in the so-called “Carbon cycle” (C+/C/CO). Their emission is of fundamental importance, playing roles both in cooling the gas, and being important in observational studies of the interstellar medium. New and future telescopes will significantly open the access to lines from C+, C and high-J CO rotation transitions. Understanding their physics and chemistry is thus key to our understanding of the molecular universe.

  • There has been a variety of embedded sources of low-energy cosmic rays proposed. Astrochemical models have typically assumed either constant ionization rates or declining profiles with density or depth into the cloud. In my 2019 papers, we investigated how cosmic rays accelerated by clusters of embedded protostars may impact the chemistry. The models showed that embedded protoclusters, if they accelerate cosmic rays, may enhance the amount of atomic carbon in the gas phase and heat the gas substantially.

  • Using the public photo-dissociation region code, 3D-PDR, we have investigated the impact different assumptions about the cosmic ray ionization rate have on the gas-phase chemistry of molecular clouds. I have extended the code to include both a prescription for the ionization rate versus depth and a spectrum-resolved solver for the energy-loss equation in 1D and 3D. Using the prescribed profiles, we have found crucially that when modeling molecular clouds to interpret multiple emission lines, no single constant ionization rate can reproduce the more physical model including attenuation.

    These have been the first PDR models of molecular clouds including cosmic ray attenuation. They are now being extended to include a larger chemical network including Nitrogen, carbon chains and a wide range of hydrocarbons.

  • The low-energy cosmic rays, dominantly protons, which ionize the molecular gas cannot be directly detected. Therefore, a number of calibrators have been proposed to infer the cosmic ray ionization rate from molecular line observations. These calibrators are generally derived analytically from simple chemical networks. Using our PDR models which include a range of cosmic ray physics, we are actively evaluating current calibrators and investigating new potential calibrators.

Low-energy Cosmic Ray Acceleration

Low-energy cosmic rays play a vital role in the thermochemistry of the interstellar medium. However, they are tightly coupled to the gas, being significantly impacted by energy losses and transport along magnetic fields. There is a possible contradiction with models using only external ionization sources and the flatter ionization profiles observed. Further, there have been a number of resolved studies of the ionization rate which show it increasing towards star formation. Therefore, it is likely that there are multiple types of embedded sources of low-energy cosmic ray acceleration present. In 2018, I evaluated the possibility that accretion onto protostars could accelerate particles and found that protostar accretion may be able to accelerate protons up to 10s of GeV. The ionization rates predicted by the model match inferred rates towards protostellar regions such as OMC-2 FIR 4 and B335.

In 2021, I proposed a new source of acceleration: magnetic reconnection within the turbulent cascade in molecular clouds, dubbed CRAFT (Cosmic-Ray Acceleration From Turbulence). This source provides a near-homogenous, distributed source of ionization throughout molecular clouds. Further, since molecular clouds are in general turbulent, this mechanism would provide similar ionization rates in molecular clouds under similar conditions. Using the model, we could reproduce the inferred ionization rates both in nearby molecular clouds and in the galactic center.

HADES

Resolving protostar accretion to sub-mAU resolutions

How embedded protostars grow their mass is still a matter of debate. There have been high-resolution simulations of the earliest phases, of the first collapse to stellar densities, and the latter stages, like T-Tauri stars, when the protostar is exposed and the accretion has dropped significantly. Large-scale star formation simulations (0.1 - 10s of pc) can capture the main phases of protostar formation, but they cannot resolve the gas flowing down to the protostar surface. There is a gap in our knowledge from theory and observations in the underlying accretion physics for the main accretion phase.

The High-resolution Accretion Disks of Embedded protoStars (HADES) seek to fill this gap. These are sub-mAU resolution simulations of a Solar-mass protostar, modeled with a range of protostellar magnetic fields. The HADES simulations aim to uniquely trace the gas from an actively accreting protostar down to the protostellar surface.

Protostellar X-rays in Star Forming Regions

The gas that falls onto protostars during accretion shock heats to millions of degrees becoming X-ray bright. While many observational campaigns have demonstrated that point-like and diffuse X-ray emission are prevalent in star-forming regions, there have been no star formation simulations to date that explicitly include this X-ray emission. X-rays can couple to the gas, both heating and ionizing the molecular material. In 2023, I presented the radiation transfer module XRayTheSpot, an extension of TreeRay, which enables the inclusion of X-ray emission from arbitrary sources. This module was used to do a fiducial simulation of a star-forming molecular cloud with radiation feedback from the infrared to X-ray. These initial results found that the X-ray emission leads to gas which is still largely molecular but hundreds to thousands of degrees Kelvin. Simulation work is on going to expand the previous work to higher resolution.

Star formation simulation, showing the density, temperatures and radiation fields

Publications

  • Padovani, Gaches, Cosmic Rays: Physics, Chemistry, and Computational Challenges. Chapter in Astrochemical Modelling: Practical aspects of microphysics in numerical simulations, 2023, Elsevier. Editors: Stefano Bovino and Tommaso Grassi, ISBN: 9780323917469

    Gaches, Bialy, Bisbas, Padovani, Seifried, Walch, A&A, 2022, 664, A150, Cosmic-ray-induced H2 line emission: Astrochemical modeling and implications for JWST observations.

    Gaches, Bisbas, Bialy, A&A, 2022, 658, A151, The impact of cosmic-ray attenuation on the carbon cycle emission in molecular clouds.

    Gaches, Offner & Bisbas, 2019, ApJ, 883, 190, The Astrochemical Impact of Cosmic Rays in Protoclusters II: CI-to-H2 and CO-to-H2 Conversion Factors

    Offner, Gaches, Holdship, 2019, ApJ, 883, 121, Impact of Cosmic-Ray Feedback on Accretion and Chemistry in Circumstellar Disks

    Gaches, Offner & Bisbas, 2019, ApJ, 878, 105, The Astrochemical Impact of Cosmic Rays in Protoclusters I: Molecular Cloud Chemistry

  • Gaches, Walch, Lazarian, ApJL, 2021, 917, L39, CRAFT (Cosmic Ray Acceleration From Turbulence) in Molecular Clouds

    Fitz Axen, Offner, Gaches, Fryer, Hungerford, Silsbee, 2021, ApJ, 915, 43, Trans- port of Protostellar Cosmic Rays in Turbulent Dense Cores

    Gaches & Offner, 2018, ApJ, 861, 87, Exploration of Cosmic-ray Acceleration in Protostellar Accretion Shocks and a Model for Ionization Rates in Embedded Protoclusters

  • Gaches, Tan, Rosen & Kuiper, 2024, A&A Accepted, The High-resolution Accre- tion Disks of Embedded protoStars (HADES) simulations. I. Impact of Protostar Magnetization on the Accretion Modes. arXiv:2410.14777

    Gaches & M. Grudić, 2024, A&A Accepted, A Probabilistic Model to Estimate Number Densities from Column Densities in Molecular Clouds

    Panessa, Seifried, Walch, Gaches, Barnes, Bigiel, Neumann, 2023, MNRAS, 523, 6138, The evolution of HCO+ in molecular clouds using a novel chemical post- processing algorithm

    Yun, Lee, J., Evans, Offner, Heyer, Cho, Gaches, Yang, Chen, Choi, Y., Lee, Y., Baek, Choi, M., Kim, Kang, Lee, S., Tetematsu, ApJ Accepted, 2021, TIMES II: Investigating the Relation Between Turbulence and Star-forming Environments in Molecular Clouds

    Yun, Lee, J. , Choi, Evans, Offner, Heyer, Gaches, Lee, Y-H., Baek, Choi, Kang, Lee, S. , Tatematsu, Yang, Chen, Lee, Y., Jung, Lee, C., Cho, 2021, ApJ, 256, 16 , TIMES I: a Systematic Observation in Multiple Molecular Lines Toward the Orion A and Ophiuchus Clouds

    Gaches, Walch, Offner & Münker, 2020, ApJ, 898, 79, Aluminum-26 Enrichment in the Surface of Protostellar Disks Due to Protostellar Cosmic Ray, Featured in Sky & Telescope Magazine.

    Gaches, & Offner, 2018, ApJ, 854, 156, A Model for the CO-H2 Conversion Factor of Molecular Clouds with Embedded Star Clusters

    Gaches, Offner, Rosolowsky, & Bisbas 2015, ApJ, 799, 235, Astrochemical Correlations in Molecular Clouds

  • Gaches, Grassi, Vogt-Geisse, Bovolenta, Vallance, Heathcote, Padovani, Bovino, Gorai, 2024, A&A, 684, A41, The Astrochemistry Low-energy Electron Cross- Section (ALeCS) database I. Semi-empirical electron-impact ionization cross-section calculations and ionization rates