MSc thesis defense by Gergely Friss

Title: Chemical effects of episodic accretion

Abstract: An important problem that molecular astrophysics faces is to understand where, when and how complex organic and prebiotic molecules emerge. To form the basic building blocks of life - for example amino acids or sugars - molecules have to go through many complex chemical reactions between many different species. The origins of these biomolecular precursors are an on-going research field, however observations show that they already arise in star forming regions. Such molecules, including methanol (CH3OH), methyl formate (HCOOCH3), formamide (NH2CHO) and other complex organic molecules, are formed through gas- and solid-state chemistry. For instance, methanol is formed from carbon monoxide (CO) on grain surfaces through hydrogenation steps. Keeping CO in the solid state to form more complex species, however, requires a cold environment (<~20K). Therefore, temperature and other physical parameters might have an impact on the chemistry in star forming regions.

Over the last years a picture has emerged where young stars accrete gas and dust in a highly episodic manner with, e.g., strong bursts of accretion related to the formation of disks and binaries. This may strongly affect the chemistry as the luminosity of the protostar and thus the temperature in the envelope and disk surrounding the young star will vary significantly compared to what one would expect from simple classical infall models. In this project we characterise the chemical effects of a changing environment around a protostar. In our simulations, we couple the chemistry with an underlying physical model of episodic accretion. We explore the significance of, for example, the duration, frequency and magnitude of bursts and the density of the envelope on the formation of chemically interesting molecules. We quantify uncertainty values on the abundances of each species using the uncertainty of the chemical reaction rates and the Monte Carlo method.

We find that in case of most species, the effects of episodic accretion on the abundances are beyond the corresponding uncertainties. Comparing our results to observations of various species, we can constrain the histories of different protostars and reveal the importance of episodic accretion in regulating their chemistries.