Universität zu Köln

Offner, Stella S. R., Gaches, Brandt A. L. and Holdship, Jonathan R. (2019). Impact of Cosmic-Ray Feedback on Accretion and Chemistry in Circumstellar Disks. Astrophys. J., 883 (2). BRISTOL: IOP PUBLISHING LTD. ISSN 1538-4357

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Abstract

We use the gas-grain chemistry code UCLCHEM to explore the impact of cosmic-ray feedback on the chemistry of circumstellar disks. We model the attenuation and energy losses of the cosmic rays as they propagate outward from the star and also consider ionization due to stellar radiation and radionuclides. For accretion rates typical of young stars of M-* similar to 10(-9)-10(-6) M-circle dot yr(-1), we show that cosmic rays accelerated by the stellar accretion shock produce an ionization rate at the disk surface zeta greater than or similar to 10(-1)5 s(-1), at least an order of magnitude higher than the ionization rate associated with the Galactic cosmic-ray background. The incident cosmic-ray flux enhances the disk ionization at intermediate to high surface densities (Sigma > 10 g cm(-2)), particularly within 10 au of the star. We find that the dominant ions are C+, S+, and Mg+ in the disk surface layers, while the H-3(+) ion dominates at surface densities above 1.0 g cm(-2). We predict the radii and column densities at which the magnetorotational instability (MRI) is active in T Tauri disks and show that ionization by cosmic-ray feedback extends the MRI-active region toward the disk midplane. However, the MRI is only active at the midplane of a minimum-mass solar nebula disk if cosmic rays propagate diffusively (zeta proportional to r(-1)) away from the star. The relationship between accretion, which accelerates cosmic rays, the dense accretion columns, which attenuate cosmic rays, and the MRI, which facilitates accretion, creates a cosmic-ray feedback loop that mediates accretion and may produce variable luminosity.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Offner, Stella S. R. UNSPECIFIED UNSPECIFIED UNSPECIFIED Gaches, Brandt A. L. UNSPECIFIED UNSPECIFIED UNSPECIFIED Holdship, Jonathan R. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-133025
DOI:	10.3847/1538-4357/ab3e02
Journal or Publication Title:	Astrophys. J.
Volume:	883
Number:	2
Date:	2019
Publisher:	IOP PUBLISHING LTD
Place of Publication:	BRISTOL
ISSN:	1538-4357
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language MAGNETOROTATIONAL INSTABILITY; PROTOPLANETARY DISKS; IONIZATION STATE; MAGNETIC-FIELDS; DUST; MASS; DIFFUSION; EMISSION Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/13302