Digging into the chemical complexity in the outer Galaxy: A hot molecular core in Sharpless 2-283

TL;DR

This study uses ALMA Band 7 observations to characterize the outer-Galaxy hot core Sh 2-283-1a SMM1 at $D_{\rm{GC}} = 15.7$ kpc and $Z \sim 0.3 Z_\odot$, revealing a compact, warm, and dense region ($T_{\rm rot} \sim 50$–$150$ K, $n_{\rm H_2} \sim 4\times10^6$ cm$^{-3}$) rich in COMs. Molecular inventories show that CH$_3$OH abundances, after metallicity correction, are similar to inner-Galaxy cores, while SO$_2$ is significantly depleted, suggesting a weaker cosmic-ray–driven radical environment in the outer Galaxy; COMs like CH$_3$OCH$_3$ and C$_2$H$_5$OH are moderately underabundant relative to CH$_3$OH, consistent with environmental effects on radical formation. Deuterium fractionation, via CH$_2$DOH/CH$_3$OH of about 1.5%, is comparable to other outer-Galaxy and some inner-Galaxy sources, indicating efficient early cold chemistry during CH$_3$OH formation. Overall, the outer Galaxy hosts chemically rich hot cores whose composition is governed by environmental conditions beyond metallicity, motivating larger surveys to map chemical diversity across low-metallicity star-forming regions.

Abstract

The outer Galaxy (galactocentric distance $\gtrsim$13.5 kpc) serves as an excellent laboratory for investigating the chemical complexity in low-metallicity environments. Here, we present the chemical analyses for the outer Galactic hot core Sh 2-283-1a SMM1 ($D_\mathrm{GC}$ = 15.7 kpc and $Z$ $\sim$0.3 $Z_\odot$), recently detected by Ikeda et al. (2025) using ALMA. Toward this source, a variety of molecular species, including complex organic molecules (COMs: CH$_3$OH, $^{13}$CH$_3$OH, CH$_2$DOH, and CH$_3$OCH$_3$) are detected. The molecular abundances relative to CH$_3$OH are similar to those of another outer Galactic hot core, demonstrating that chemically rich hot cores exist in different regions of the outer Galaxy. We also compared molecular abundances among hot cores in the inner Galaxy, outer Galaxy, and Magellanic Clouds. This comparison revealed that the metallicity-corrected $N$(SO$_2$)/$N$(H$_2$) ratios of outer Galactic hot cores are significantly lower than those of the inner Galactic ones, while their $N$(CH$_3$OH)/$N$(H$_2$) ratios are similar. The Magellanic hot cores show different trends despite having metallicities similar to those of the outer Galaxy, indicating that the chemical complexity of hot cores is governed by environmental conditions (e.g., cosmic ray intensity and dust temperature) rather than simple metallicity scaling. These environmental differences would also affect the production efficiency of COMs derived from CH$_3$OH, as the $N$(CH$_3$OCH$_3$)/$N$(CH$_3$OH) and $N$(C$_2$H$_5$OH)/$N$(CH$_3$OH) ratios in the outer Galactic sources are moderately lower than those of inner Galactic sources. The $N$(CH$_2$DOH)/$N$(CH$_3$OH) ratio of Sh 2-283-1a SMM1 is 1.5$^{+3.9}_{-1.2}$$\%$, comparable to that of inner Galactic high-mass sources.

Figures (10)

• Figure 1: ALMA Band 7 spectra of Sh 2-283-1a SMM1 extracted from the elliptical region (0$\farcs$89 $\times$ 0$\farcs$70) centered at RA = 6$^\mathrm{h}$38$^\mathrm{m}$29$\fs$660 and Dec = 0$\arcdeg$44$\arcmin$40$\farcs$75 (ICRS), which corresponds to the emission peak of the 0.87 mm continuum and molecular emissions. The lines with different colors represent the results of Gaussian fitting for each molecular line (Table \ref{['tab_line']}). The systemic velocity of 53.2 km s$^{-1}$ is assumed to identify the molecular lines.
• Figure 2: The integrated intensity maps of the detected molecular lines. Grey contours show the emission distribution of 0.87 mm continuum, and the contour levels are 5$\sigma$, 15$\sigma$, 25$\sigma$, and 100$\sigma$ of the rms noise level (1$\sigma$ = 0.07 mJy beam$^{-1}$). The blue cross indicates the peak position of 0.87 mm continuum. The black open circle represents the extraction region of the spectra used for the discussion in the text. For molecular species with multiple line detections, lines are stacked in order to reduce the noise level. The synthesized beam size is shown in the bottom left corner, as a gray ellipse.
• Figure 3: The results of the rotational diagram analysis for SO$_2$, CH$_3$OH, and CH$_2$DOH. The solid lines indicate the results of the linear regression. The derived column densities and rotational temperatures are shown in each panel. For SO$_2$, the left line is fitted with using the transitional lines with E$_u$$<$100 K, while the right solid line is fitted with using the transitional lines with E$_u$$\geq$100 K. SO$_2$(13$_{2,12}$--12$_{1,11}$) is removed in the fitting since this line is significantly blended with H$^{13}$CN (4--3). See Section \ref{['sec_rd']} for the details.
• Figure 4: Comparison of the molecular abundances normalized by CH$_3$OH column density between the outer Galactic hot cores. The dotted lines represent the abundance ratio of 2:1 and 1:2 for Sh 2-283-1a SMM1, while the solid line represents that of 1:1. The molecular abundances of WB89-789 SMM1 are taken from Shimonishi2021.
• Figure 5: The $N$(CH$_3$OH)/$N$(H$_2$) (a) and $N$(SO$_2$)/$N$(H$_2$) (b) ratios as a function of luminosities for the inner Galactic sources Fuente2014Qin2022Gelder2022LSantos2024Chen2025ATMsLi2025, the outer Galactic sources Shimonishi2021, the LMC sources Shimonishi2016Shimonishi2020Shimonishi2021Shimonishi_magosSewilo2022Golshan2024Broadmeadow2025, and the SMC sources Shimonishi2023 . The open marks in white show the values corrected for metallicity for each low-metallicity source (e.g., multiplied by a factor of 3 for $Z = 1/3$$Z_\odot$). The subscript arrows in each plot represent the upper limits.