Multi-frequency mapping of the S255IR region at a wavelength of 1~mm

TL;DR

This study presents a wide-band interferometric survey of the S255IR region with the SMA in the $210$–$250$ GHz window, detecting 53 molecules including several COMs. The authors construct maps and spectra for SMA1 and SMA2, revealing a dichotomy: a hot-core dominated by Group I species confined to SMA1 and a ring-like structure west of SMA1 populated by Group II species, likely tracing cavity walls carved by high-velocity outflows. Rotation diagrams for multiple COMs in SMA1 show $T_{rot}$ mostly in the $10^2$ K range, with some species indicating hotter or colder components and evidence for non-LTE effects; optical depths are substantial for methanol in SMA1, while many COM lines remain optically thin. The ring, linked to outflows and 2015 outburst activity, suggests shock-enhanced chemistry along cavity walls, with future work planned to detect O-COMs in the ring via line stacking and to refine abundance estimates. Overall, the work sheds light on chemical differentiation and feedback-driven structuring in a forming high-mass star-forming region.

Abstract

The results of interferometric observations of the star-forming region S255IR in the frequency range 210--250 GHz are presented. The observations were carried out with the antenna array SMA (Hawaii, USA). Fifty-three molecules were detected, including complex organic molecules (COMs) such as CH$_3$CHO, CH$_3$CN, CH$_3$CH$_2$CN, and many others. Typical rotational temperatures in the hot core SMA1 fall in the range 100--200 K. Optical depths in the lines of methanol and some other molecules in the cores SMA1 and SMA2 were estimated. In SMA1, the optical depth of one of the strongest methanol lines, $5_{-1}-4_{-1}E$, proved to be $23.8 \pm 1.5$. Based on this value, one can assume that the lines of other oxygen-containing COMs, such as CH$_3$OCHO, CH$_3$OCH$_3$, CH$_3$CH$_2$OH, which are typically much less abundant in hot cores than methanol, are optically thin in SMA1. Most of the detected molecules can be roughly divided into two groups. The molecules of the first group emit exclusively toward the hot core SMA1, while some or all lines of the molecules of the second group, in addition to SMA1, can be seen toward a ring-like structure to the west of SMA1. This structure is most likely associated with the walls of a cavity formed by high-velocity outflows driven by young stellar objects (YSOs) in molecular cores SMA1, SMA2, and possibly SMA3. The gas temperature and density in the cavity walls were estimated using methanol lines. The temperature was found to be about 50--60 K, and the density about $10^7-10^8$ cm$^{-3}$. The column density of methanol near the brightness peaks in the lines of this molecule is about $5\times 10^{15}$~cm$^{-2}$. The column densities of other COMs in the ring-like structure will be determined in future studies with increased sensitivity achieved by spectral line stacking.

Figures (10)

• Figure 1: Map of the S255IR region in the $5_{0}-4_{0}A^{+}$ methanol line at a frequency of 241791.367 MHz. Triangular markers indicate the dense cores SMA1 (coincident with NIRS3), SMA2, and SMA3 (coincident with NIRS1)
• Figure 2: Spectrum toward SMA1
• Figure 3: Spectrum toward SMA2
• Figure 4: Maps in the $12_{0,12}-11_{0,11}E$ ethanal (CH$_3$CHO) line, in the $22_{0,22}-21_{0,21} \quad A$ methyl formate (CH$_3$OCHO) line, in the $6_{3,4}-5_{2,3}$ ethanol (t-CH$_3$CH$_2$OH) line, and in the $25_{4,21}-24_{4,20}$ propionitrile (CH$_3$CH$_2$CN) line.
• Figure 5: Samples of spectral lines of the first group molecules. The figure shows the $11_{1,10}-10_{1,9}$ CH$_2$CO line, the $12_4-11_4$ CH$_3$CN line, the $12_{0,12}-11_{0,11}E$ CH$_3$CHO line, the $19_{3,17}-18_{2,16}E$ CH$_3$OCHO line, the $24_{3,22}-23_{3,21}$ CH$_3$CH$_2$CN line, the $10_{1,10}-9_{1,9}$ HNCO line, and the $18-17$ OCS line. Antenna temperature of the CH$_3$CH$_2$CN line is multiplied by six.