A non-LTE radiative transfer model to study ionized outflows and disks. The case of MWC349A

TL;DR

This paper introduces a detailed non-LTE 3D radiative transfer model for MWC349A, revealing the structure and kinematics of its ionized disk and outflow, and explaining diverse radio observations across frequencies.

Contribution

It presents the first comprehensive model of MWC349A that explains extensive radio data, including maser and stimulated emission processes, with detailed physical and kinematic insights.

Findings

MWC349A's ionized region includes a Keplerian rotating disk and an expanding outflow.

Maser amplification dominates for RRLs with n<41, stimulated emission for n>41.

The model matches observational data across multiple frequencies and resolutions.

Abstract

Context. The best example of a massive star with an ionized outflow launched from its photoevaporating disk is MWC349A. The large amount of reported radio-continuum and radio-recombination line observations toward this galactic UC-HII region offers a unique possibility to build a model of the ionized envelope of this source. Aims. To understand the physical conditions and kinematics of the ionized region of the circumstellar disk and also of the outflow of MWC349A. Methods. We compared the bulk of radio-continuum maps, radio-recombination line profiles, and the H30alpha centroid map published to date with the predictions of our non-LTE 3D radiative transfer model, MORELI (MOdel for REcombination LInes), which we describe here in detail. Results. Our non-LTE 3D radiative transfer model provides new evidence that the UC-HII region of MWC349A is composed of an ionized circumstellar disk rotating in Keplerian fashion around a star of 38 solar mass, and an ionized outflow expanding with a terminal velocity of 60 km/s and rotating in the same sense as the disk. The model shows that while maser amplification is the dominant process involved for Hn\alpha radio-recombination line (RRL) emission with quantum numbers n<41, stimulated emission is relevant for the emission of RRLs with n>41 up at least the H76alpha line. Conclusions. For the first time, we present a model of MWC349A which satisfactorily explains the vast amount of reported observational data for a very wide range of frequencies and angular resolutions.