Universität zu Köln

Willis, E. R., Garrod, R. T., Belloche, A., Mueller, H. S. P., Barger, C. J., Bonfand, M. and Menten, K. M. (2020). Exploring molecular complexity with ALMA (EMoCA): complex isocyanides in Sgr B2(N). Astron. Astrophys., 636. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

Context. The Exploring Molecule Complexity with ALMA (EMoCA) survey is an imaging spectral line survey using the Atacama Large Millimeter/submillimeter Array (ALMA) to study the hot-core complex Sagittarius B2(N). Recently, EMoCA revealed the presence of three new hot cores in this complex (N3-N5), in addition to providing detailed spectral data on the previously known hot cores in the complex (N1 and N2). The present study focuses on N2, which is a rich and interesting source for the study of complex molecules whose narrow line widths ameliorate the line confusion problem.Aims. We investigate the column densities and excitation temperatures of cyanide and isocyanide species in Sgr B2(N2). We then use state-of-the-art chemical models to interpret these observed quantities. We also investigate the effect of varying the cosmic-ray ionization rate (zeta) on the chemistry of these molecules.Methods. We used the EMoCA survey data to search for isocyanides in Sgr B2(N2) and their corresponding cyanide analogs. We then used the coupled three-phase chemical kinetics code MAGICKAL to simulate their chemistry. Several new species, and over 100 new reactions have been added to the network. In addition, a new single-stage simultaneous collapse/warm-up model has been implemented, thus eliminating the need for the previous two-stage models. A variable, visual extinction-dependent zeta was also incorporated into the model and tested.Results. We report the tentative detection of CH3NC and HCCNC in Sgr B2(N2), which represents the first detection of both species in a hot core of Sgr B2. In addition, we calculate new upper limits for C2H5NC, C2H3NC, HNC3, and HC3NH+. Our updated chemical models can reproduce most observed NC:CN ratios reasonably well depending on the physical parameters chosen. The model that performs best has an extinction-dependent cosmic-ray ionization rate that varies from 2 x 10(-15) s(-1) at the edge of the cloud to 1 x 10(-16) s(-1) in the center. Models with higher extinction-dependent zeta than this model generally do not agree as well, nor do models with a constant zeta greater than the canonical value of 1.3 x 10(-17) s(-1) throughout the source. Radiative transfer models are run using results of the best-fit chemical model. Column densities produced by the radiative transfer models are significantly lower than those determined observationally. Inaccuracy in the observationally determined density and temperature profiles is a possible explanation. Excitation temperatures are well reproduced for the true hot core molecules, but are more variable for other molecules such as HC3N, for which fewer lines exist in ALMA Band 3.Conclusions. The updated chemical models do a very good job of reproducing the observed abundances ratio of CH3NC:CH3CN towards Sgr B2(N2), while being consistent with upper limits for other isocyanide/cyanide pairs. HCCNC:HC3N is poorly reproduced, however. Our results highlight the need for models with A(V)-depdendent zeta. However, there is still much to be understood about the chemistry of these species, as evidenced by the systematic overproduction of HCCNC. Further study is also needed to understand the complex effect of varying zeta on the chemistry of these species. The new single-stage chemical model should be a powerful tool in analyzing hot-core sources in the future.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Willis, E. R. UNSPECIFIED UNSPECIFIED UNSPECIFIED Garrod, R. T. UNSPECIFIED UNSPECIFIED UNSPECIFIED Belloche, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Mueller, H. S. P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Barger, C. J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Bonfand, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Menten, K. M. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-337256
DOI:	10.1051/0004-6361/201936489
Journal or Publication Title:	Astron. Astrophys.
Volume:	636
Date:	2020
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language SUBMILLIMETER-WAVE SPECTRUM; ROTATIONAL SPECTRA; ETHYL ISOCYANIDE; VINYL CYANIDE; DISSOCIATIVE RECOMBINATION; MICROWAVE-SPECTRUM; METHYL CYANIDE; N-PROPYL; SPECTROSCOPY; MILLIMETER Multiple languages Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/33725