^1,2Lisseth Gavilan,³Phay J. Ho,^1,4Uma Gorti,⁵Hirohito Ogasawara,⁶Cornelia Jäger, ¹Farid Salama
The Astrophysical Journal 925, 86 Open Access Link to Article [DOI 10.3847/1538-4357/ac3dfd]
¹NASA Ames Research Center, Space Science & Astrobiology Division, Moffett Field, CA 94035, USA
²Universities Space Research Association (USRA), Columbia, MD 21046, USA
³Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA
⁴SETI Institute, Carl Sagan Center, Mountain View, CA 94035, USA
⁵Stanford Synchrotron Radiation Laboratory, P.O. Box 20450, Stanford, CA 94309, USA
⁶Laboratory Astrophysics and Cluster Physics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University & Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena, Germany

We present the results of an integrated laboratory and modeling investigation into the impact of stellar X-rays on cosmic dust. Carbonaceous grains were prepared in a cooled (<200 K) supersonic expansion from aromatic molecular precursors, and were later irradiated with 970 eV X-rays. Silicate (enstatite) grains were prepared via laser ablation, thermally annealed, and later irradiated with 500 eV X-rays. Infrared spectra of the 3.4 μm band of the carbon sample prepared with benzene revealed 84% ± 5% band area loss for an X-ray dose of 5.2 ×10²³ eV.cm⁻². Infrared spectra of the 8–12 μm Si–O band of the silicate sample revealed band area loss up to 63% ± 5% for doses of 2.3 × 10²³ eV.cm⁻². A hybrid Monte Carlo particle trajectory approach was used to model the impact of X-rays and ensuing photoelectrons, Auger and collisionally ionized electrons through the bulk. As a result of X-ray ionization and ensuing Coulomb explosions on surface molecules, the calculated mass loss is 60% for the carbonaceous sample and 46% for the silicate sample, within a factor of 2 of the IR band loss, supporting an X-ray induced mass-loss mechanism. We apply the laboratory X-ray destruction rates to estimate the lifetimes of dust grains in protoplanetary disks surrounding 1 M_⊙ and 0.1 M_⊙ G and M stars. In both cases, X-ray destruction timescales are short (a few million years) at the disk surface, but are found to be much longer than typical disk lifetimes (≳10 Myr) over the disk bulk.

¹Marco Fatuzzo,^2,3Fred C. Adams
The Astrophysical Journal 925 56 Open Access Link to Article [DOI 10.3847/1538-4357/ac38a7]
¹Department of Physics, Xavier University, Cincinnati, OH 45207, USA; fatuzzo@xavier.edu
²Department of Physics, University of Michigan, MI 48109, USA
³Department of Astronomy, University of Michigan, MI 48109, USA; fca@umich.edu

Short-lived radioactive nuclei (half-life τ_1/2 ∼ 1 Myr) influence the formation of stars and planetary systems by providing sources of heating and ionization. Whereas many previous studies have focused on the possible nuclear enrichment of our own solar system, the goal of this paper is to estimate the distributions of short-lived radionuclides (SLRs) for the entire population of stars forming within a molecular cloud. Here we focus on the nuclear species ⁶⁰Fe and ²⁶Al, which have the largest impact due to their relatively high abundances. We construct molecular-cloud models and include nuclear contributions from both supernovae and stellar winds. The resulting distributions of SLRs are time dependent with widths of ∼3 orders of magnitude and mass fractions ρ_SLR/ρ_* ∼ 10⁻¹¹–10⁻⁸. Over the range of scenarios explored herein, the SLR distributions show only modest variations with the choice of cloud structure (fractal dimension), star formation history, and cluster distribution. The most important variation arises from the diffusion length scale for the transport of SLRs within the cloud. The expected SLR distributions are wide enough to include values inferred for the abundances in our solar system, although most of the stars are predicted to have smaller enrichment levels. In addition, the ratio of ⁶⁰Fe/²⁶Al is predicted to be greater than unity, on average, in contrast to solar system results. One explanation for this finding is the presence of an additional source for the ²⁶Al isotope.