Universität zu Köln

Borchert, E. M. A., Walch, S., Seifried, D., Clarke, S. D., Franeck, A. and Nuernberger, P. C. (2022). Synthetic CO emission and the X-CO factor of young molecular clouds: a convergence study. Mon. Not. Roy. Astron. Soc., 510 (1). S. 753 - 774. OXFORD: OXFORD UNIV PRESS. ISSN 1365-2966

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Abstract

The properties of synthetic CO emission from 3D simulations of forming molecular clouds are studied within the SILCC-Zoom project. Since the time-scales of cloud evolution and molecule formation are comparable, the simulations include a live chemical network. Two sets of simulations with an increasing spatial resolution (dx = 3.9 pc to dx = 0.06 pc) are used to investigate the convergence of the synthetic CO emission, which is computed by post-processing the simulation data with the radmc-3d radiative transfer code. To determine the excitation conditions, it is necessary to include atomic hydrogen and helium alongside H-2, which increases the resulting CO emission by similar to 7-26 per cent. Combining the brightness temperature of (CO)-C-12 and (CO)-C-13, we compare different methods to estimate the excitation temperature, the optical depth of the CO line and hence, the CO column density. An intensity-weighted average excitation temperature results in the most accurate estimate of the total CO mass. When the pixel-based excitation temperature is used to calculate the CO mass, it is over-/underestimated at low/high CO column densities where the assumption that (CO)-C-12 is optically thick while (CO)-C-13 is optically thin is not valid. Further, in order to obtain a converged total CO luminosity and hence X-CO factor, the 3D simulation must have dx less than or similar to 0.1 pc. The X-CO evolves over time and differs for the two clouds; yet pronounced differences with numerical resolution are found. Since high column density regions with a visual extinction larger than 3 mag are not resolved for dx greater than or similar to 1 pc, in this case the H-2 mass and CO luminosity both differ significantly from the higher resolution results and the local X-CO is subject to strong noise. Our calculations suggest that synthetic CO emission maps are only converged for simulations with dx less than or similar to 0.1 pc.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Borchert, E. M. A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Walch, S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Seifried, D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Clarke, S. D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Franeck, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Nuernberger, P. C. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-584030
DOI:	10.1093/mnras/stab3354
Journal or Publication Title:	Mon. Not. Roy. Astron. Soc.
Volume:	510
Number:	1
Page Range:	S. 753 - 774
Date:	2022
Publisher:	OXFORD UNIV PRESS
Place of Publication:	OXFORD
ISSN:	1365-2966
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language CARBON-MONOXIDE; CO-TO-H-2 CONVERSION; INTERSTELLAR-MEDIUM; VISUAL EXTINCTION; LINE-INTENSITIES; COLUMN DENSITY; SILCC-ZOOM; DARK GAS; FILAMENTS; IMPACT Multiple languages Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/58403