ALMA Observations of the Massive Molecular Outflow G331.512-0.103. II. Physical Properties, Kinematics, and Geometry Modeling

Abstract

We present observations and analysis of the massive molecular outflow G331.512-0.103, obtained with ALMA band 7, continuing the work from Merello et al. (2013a). Several lines were identified in the observed bandwidth, consisting of two groups: lines with narrow profiles, tracing the emission from the core ambient medium; and lines with broad velocity wings, tracing the outflow and shocked gas emission. The physical and chemical conditions, such as density, temperature, and fractional abundances are calculated. The ambient medium, or core, has a mean density of ∼ 5 × 10 6 cm −3 and a temperature of ∼ 70 K. The SiO and SO 2 emission trace the very dense and hot part of the shocked outflow, with values of n H2 ∼ 10 9 cm −3 and T ∼ 160 − 200 K. The interpretation of the molecular emission suggests an expanding cavity geometry powered by stellar winds from a new-born UCHII region, alongside a massive and high-velocity molecular outflow. This scenario, along with the estimated physical conditions, is modeled using the 3D geometry radiative transfer code MOLLIE for the SiO(J= 8 − 7) molecular line. The main features of the outflow and the expanding shell are reproduced by the model.

Cyanoacetylene in the outflow/hot molecular core G331.512−0.103

“…The authors interpreted the emission like a high-velocity jet lying almost along the line of sight with an expanding shocked shell/bubble surrounding the driving source, which is observed in projection as a ring of ∼ 5 arcsec in size, at the systemic velocity (∼ -90 km s −1 ). The line J=38-37 of HC3N was also reported towards G331 with ALMA observations (Hervías-Caimapo et al 2019). The authors identified this line as a part of a group of lines (HC3N, SiO, S 18 O, HCO + , and H 13 CN) with broad velocity wings tracing the shocked gas and the outflow, for which a temperature of 160-200 K was derived.…”

“…This could in turn explain the narrower profile of the HC3N line in the group of lines with broad wings (see fig. 1 from Hervías-Caimapo et al 2019).…”

Methyl acetylene in G331.512-0.103: Looking at massive star formation through the lens of chemistry

“…Given the rather low temperatures derived from the rotational diagrams (T exc ∼ 40 K), the hypothesis of a compact emission arising from a region of 5 is unlikely. Rather, we infer that the CH 3 CCH emission is originated from a cooler gas in the outer envelope of G331, in accordance with other observational works toward star-forming regions (Andron et al, 2018;Hervías-Caimapo et al, 2019;Nagy et al, 2015;Churchwell and Hollis, 1983).…”

“…The molecular outflow/HMC G331.512-0.103 (henceforth G331) is one of the most distinctive HMCs known to date. It is associated with a very rich emission spectrum, including simple hydrocarbons, radical species, and a plethora of complex organic molecules (Merello et al, 2013a,b;Mendoza et al, 2018;Duronea et al, 2019;Hervías-Caimapo et al, 2019).…”

“…We have estimated the H 2 column density in G331 from the ALMA measurements of N tot (H 13 CO + )≈(1.5-3)×10 13 cm −2 (Merello et al, 2013a,b;Hervías-Caimapo et al, 2019), adopting an abundance ratio N tot (H 13 CO + )/N tot (H 2 )=3.3×10 −11 , as measured by Blake et al (1987) in Orion KL. Then, by dividing the estimated CH 3 CCH column density (in the extended-emission and optically-corrected scenarios) by the inferred N tot (H 2 ), we obtained a ratio of X(CH 3 CCH)≈(1.5-7.6)×10 −8 .…”