Radiative transfer modeling of the low-mass proto-binary system, IRAS 4A1 and 4A2

TL;DR

This work uses a 1D RATRAN radiative-transfer framework to interpret the low-mass protobinary IRAS4A, comprised of A1 and A2, by fitting CH$_3$OH and H$_2$CO lines across millimeter and centimeter wavelengths. The key finding is that the stark mm-line contrast between A1 (absorption) and A2 (emission) is driven primarily by dust opacity differences, with A1 being optically thick and A2 more transparent; the model also captures infall in A2 and, under high-opacity conditions, how cm-wavelength lines reveal emission and potential maser activity in methanol for certain transitions. The study derives an inferred infall velocity at 1000 au of $V_{1000}=0.55$ km s$^{-1}$ for A2, finds CH$_3$OH abundances roughly $X_{CH_3OH}\sim(8\times10^{-9})$ to $8\times10^{-8}$ depending on species and region, and demonstrates that including an outflow and foreground cloud is essential to reproduce H$_2$CO line profiles in A2. Overall, the work highlights the critical role of dust opacity and multi-wavelength data in diagnosing the kinematics and chemistry of closely spaced protostellar components, and it extends the interpretation to cm wavelengths to test the consistency of the inferred physical conditions.

Abstract

NGC 1333 IRAS4A is a well-studied low-mass sun-like proto-binary system. It has two components, A1 and A2, which are diverse according to their physical and chemical properties. We modeled this hot corino using the RATRAN radiative transfer code and explained different spectral signatures observed towards A1 and A2, specifically for CH3OH and H2CO. Our main goal is to understand the kinematical and chemical differences between A1 and A2 and to classify their dust emission and absorption properties. We considered a simple 1D spherical infalling envelope consisting of collimated outflow in the source. Recent high-resolution interferometric observations of ALMA shed new light on why the same molecular transitions towards A1 and A2 show different spectral profiles. The significant difference between spectral profiles observed towards A1 and A2 is mainly due to the dust opacity effect. Dust continuum emission toward A1 is optically thick, causing the transitions observed in absorption. Meanwhile, A2 is optically thin, leading to the observed emission profiles, and an inverse P-Cygni profile suggests the presence of an infalling envelope. Using high-resolution observations from ALMA and VLA, we expanded our model from the millimeter wavelength range to the centimeter wavelength range. This expansion demonstrates the opacity effect, which is reduced in the centimeter wavelength range, causing us to observe the lines in emission. Using our model, we reproduced the population inversion causing maser emission of methanol 44 GHz and 95 GHz transitions.

Figures (11)

• Figure 1: 0.85 mm continuum image of IRAS4A system with ALMA band 7 observation at 357.45 GHz. Peak intensity of the continuum is 300.149 mJy/beam. Contour levels are at 20%, 40%, 60%, and 80% of the peak flux. The synthesized beam is shown in the lower left-hand corner of the figure. The continuum peak position for IRAS4A1 and IRAS4A2 are marked in black cross symbol.
• Figure 2: Radial profile of the physical parameters; a. H$_2$ density (black line is for para H$_2$ and orange line is for ortho H$_2$, green line is for H$_2$ using dusty model obtained from kris12), b. infall velocity (blue line is for V$_{1000}$=1.1 and red line is for V$_{1000}$=0.55 km s$^{-1}$.), and c. dust temperature considered here are shown.
• Figure 3: A comparison between the observed CH$_3$OH lines (black) towards IRAS4A2 core, with modeled (red) line profiles for (a) e-CH$_3$OH 4-3 transition (b) a-CH$_3$OH 1-0 transition and (c) e-CH$_3$OH 9-10 transition using a constant abundance $8.0\times10^{-9}$ for both a-CH3OH and e-CH$_3$OH and an FWHM used is 1.5 km.s$^{-1}$ for a-CH$_3$OH and 1.67 km.s$^{-1}$ for e-CH$_3$OH.
• Figure 4: A comparison between the observed H$_2$CO lines (black) towards IRAS4A2 core, with modeled (red) line profiles for (a) o-H$_2$CO, $5_{1,5} \rightarrow 4_{1,4}$ transition (b) p-H$_2$CO, $5_{0,5} \rightarrow 4_{0,4}$ transition (c) p-H$_2$CO, $5_{2,4} \rightarrow 4_{2,3}$ transition (d) p-H$_2$CO, $5_{4,1} \rightarrow 4_{4,0}$ transition (e) o-H$_2$CO, $5_{3,3} \rightarrow 4_{3,2}$ transition (f) o-H$_2$CO, $5_{3,2} \rightarrow 4_{3,1}$ transition using a constant abundance $1\times10^{-10}$ and a FWHM value 1.17 km s$^{-1}$.
• Figure 5: The ratio of ortho to para H$_2$ as a function of temperature is shown. The violet line represents the ratio that is thermalized at kinetic temperature, as derived from equation \ref{['eqn:opr']}. The blue line indicates the ortho to para H$_2$ ratio obtained from mot13.