[Paper Review] Chemical complexity in high-mass star formation: An observational and modeling case study of the AFGL 2591 VLA 3 hot core

C. Gieser, D. Semenov+31 more|Kent Academic Repository (University of Kent)|Oct 11, 2019

Astrophysics and Star Formation StudiesPhysics and Astronomy135 references19 citations

TL;DR

This study combines high-resolution NOEMA observations and chemical modeling with MUSCLE to analyze the AFGL 2591 VLA 3 hot core, revealing complex molecular distributions and a physical structure with a kinetic temperature power-law index of q=0.41±0.08 and density index p=1.7±0.1. The model successfully reproduces 10 out of 14 observed molecular abundances at a chemical age of ~21,100 years, highlighting asymmetric chemistry in a high-mass star-forming environment.

ABSTRACT

We present a detailed observational and modeling study of the hot core VLA 3 in the high-mass star-forming region AFGL 2591, which is a target region of the NOrthern Extended Millimeter Array (NOEMA) large program CORE. Using NOEMA observations at 1.37 mm with an angular resolution of ~0."42 (1 400 au at 3.33 kpc), we derived the physical and chemical structure of the source. We modeled the observed molecular abundances with the chemical evolution code MUSCLE (MUlti Stage ChemicaL codE). Results. With the kinetic temperature tracers CH3CN and H2CO we observe a temperature distribution with a power-law index of q = 0.41+-0.08. Using the visibilities of the continuum emission we derive a density structure with a power-law index of p = 1.7+-0.1. The hot core spectra reveal high molecular abundances and a rich diversity in complex molecules. The majority of the molecules have an asymmetric spatial distribution around the forming protostar(s), which indicates a complex physical structure on scales < 1 400 au. Using MUSCLE, we are able to explain the observed molecular abundance of 10 out of 14 modeled species at an estimated hot core chemical age of ~21 100 years. In contrast to the observational analysis, our chemical modeling predicts a lower density power-law index of p < 1.4. Reasons for this discrepancy are discussed. Conclusions. Combining high spatial resolution observations with detailed chemical modeling allows us to derive a concise picture of the physical and chemical structure of the famous AFGL 2591 hot core. The next steps are to conduct a similar analysis for the whole CORE sample, and then use this analysis to constrain the chemical diversity in high-mass star formation to a much greater depth.

Motivation & Objective

• To understand the physical and chemical structure of the AFGL 2591 VLA 3 hot core at high angular resolution to explain molecular diversity in high-mass star formation.
• To determine the temperature and density structure using continuum and molecular line visibilities from NOEMA at 1.37 mm.
• To model observed molecular abundances using the MUSCLE chemical evolution code to infer chemical age and physical conditions.
• To investigate the origin of asymmetric molecular distributions in the inner 1,400 au around the protostar(s).
• To assess the limitations of spherical symmetry and static physical models in reproducing observed abundances.

Proposed method

• Conducted high-sensitivity, high-angular-resolution (0.′′42) interferometric observations with NOEMA at 1.37 mm to map molecular line and continuum emission.
• Used CH3CN and H2CO as kinetic temperature tracers to derive the temperature power-law index q from the radial temperature profile.
• Analyzed continuum visibilities under the assumption of optically thin, spherically symmetric emission to derive the density power-law index p.
• Employed the XCLASS software package to derive column densities and rotation temperatures for all detected molecular species.
• Applied the MUSCLE chemical evolution code, including gas-phase and grain-surface chemistry, to model molecular abundances over time.
• Fitted the model to observed abundances by adjusting the chemical age and density structure, assuming spherical symmetry and a warm-up phase.

Experimental results

Research questions

• RQ1 What is the physical structure (temperature and density profiles) of the AFGL 2591 VLA 3 hot core at scales <1,400 au?
• RQ2 How do the spatial distributions of complex organic molecules correlate with temperature and density structures in the inner core?
• RQ3 Can the MUSCLE chemical model reproduce the observed abundances of 14 key molecular species in the hot core?
• RQ4 What is the inferred chemical age of the hot core based on the modeled abundances?
• RQ5 Why does the model predict a lower density power-law index (p<1.4) than observed (p=1.7±0.1), and what physical processes might explain this discrepancy?

Key findings

• The kinetic temperature profile follows a power law with index q=0.41±0.08, indicating a strong radial gradient in the inner 1,400 au.
• The density structure derived from 1.37 mm continuum visibilities has a power-law index of p=1.7±0.1, consistent with a centrally concentrated envelope.
• The total gas mass within 10,000 au of the continuum peak is estimated at 6.9±0.5 M☉, though this is a lower limit due to missing large-scale flux.
• Complex molecules such as CH3OH, CH3OCHO, and CH3COCH3 exhibit asymmetric, ring-like emission peaking toward the north and northeast, indicating non-spherical chemistry.
• The MUSCLE model successfully reproduces the abundances of 10 out of 14 observed molecular species within observational uncertainties, assuming a chemical age of ~21,100 years.
• The model predicts a lower density power-law index (p_model=1.0) than observed, suggesting limitations in the spherical symmetry and static structure assumptions of the model.