Universität zu Köln

Tanha, Nassim (2024). Coupling dynamics to chemical modeling: The effects of episodic accretion and episodic outflow on the chemistry of protostellar cores. PhD thesis, Universität zu Köln.

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Abstract

Low mass proto-stars commonly go through a phase of episodic accretion. These systems exhibit a rich and diverse chemistry. In this thesis, I present a study of the spatio-temporal evolution of interstellar complex organic molecules (iCOMs) in an episodic accretion scenario, with the aim of understanding the processes involved in the early phases of star formation and the coeval chemical evolution. I post-process smoothed particle hydrodynamics (SPH) simulations of low-mass star formation in collapsing, turbulent prestellar cores. The included sub-grid episodic accretion model efficiently heats the proto-stellar accretion disk and drives episodic outflows (Rohde et al., 2021). I extract the density and temperature evolution of a subset of SPH particles and apply the astro-chemistry code Saptarsy (Choudhury et al., 2015). Saptarsy is a rate-equation-based 1D astro-chemical code which includes gas-phase reactions, gas-grain interactions and surface chemistry, as well as multilayered dust chemistry.  I investigate the time evolution of different gas phase tracers and look at the effects of the episodic flares on the chemistry surrounding the proto-stars. The flares produce an abrupt temperature change with a lifetime of a few decades. I find that different species react differently, a group of species are elevated in the interior e.g. CH3OH abundance changes by 3 orders of magnitude within 1500 AU, whereas some others like HCO+ get destroyed, decreases abundance by 2 orders of magnitude within 1000 AU. The response time to the flares varies considerably and some molecules are not affected at all.  I focus further on methanol as the smallest and the predecessor to other iCOMs, whose abundance is systematically underestimated in most existing chemical models. The spread of abundance systematically increases in the smaller radii and reaches above 10 orders of magnitude within 400 AU, because accretion mixes material with different thermal history onto the disk. Therefore the inhomogeneity of the chemistry systematically increases with decreasing radius. Furthermore, I show outflows entrain methanol rich material as far as 10000 AU.  I use a visualization tool, Saptalizer (Schaefer, 2017) to analyze the detail of chemistry and show methanol is destroyed in temperatures above 500 K in gas phase, by reacting with atomic hydrogen. It is produced on dust via hydrogenation of frozen out formaldehyde in temperatures around 80 K, while in lower temperatures of 30 K it is produced by hydrogenation of frozen out carbon monoxide. The difference lies in different desorption temperatures of dust species.  This work suggests that dynamic physical modeling has non-linear and non-negligible effect on the chemistry. One can use this behavior to build a dynamic chemical clock for protostellar cores.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Tanha, Nassim tanha@ph1.uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-722701
Date:	2024
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Natural sciences and mathematics Physics
Uncontrolled Keywords:	Keywords Language Star formation English Astrochemistry English Astrophysics English Core collapse English Dynamic chemical modeling English
Date of oral exam:	24 October 2023
Referee:	Name Academic Title Tanha, Nassim Prof. Dr. Fuller, Gary Prof.
Funders:	SFB 956
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/72270