MAGellanic Outflow and chemistry Survey (MAGOS): Hot cores in the LMC

TL;DR

This study presents MAGOS, a uniform ALMA Band 7 survey of ~30 massive protostars in the LMC at ~0.1 pc resolution, identifying 9 hot cores and 1 hot-core candidate among 36 continuum sources. It reveals extensive molecular inventories including CH$_3$OH, CH$_3$OCH$_3$, HCOOCH$_3$, and SO$_2$, with CH$_3$OH exhibiting large source-to-source abundance variations, while SO$_2$ is detected in all hot cores and correlates with rotational temperature. A newly identified hot core outside the LMC bar (Lm10) shows COMs including CH$_3$OCH$_3$, marking the first such detection outside the bar in this environment. The high-excitation SO line proves to be a robust tracer of hot-core gas, suggesting a practical route for large-sample hot-core identification in low-metallicity systems. The authors argue that a combination of low metallicity, active nearby star formation, and high protostellar luminosity likely drives COM-poor hot-core chemistry in the LMC, with metallicity alone being insufficient to explain the observed diversity.

Abstract

The Large Magellanic Cloud (LMC) provides a key laboratory for exploring the diversity of star formation and interstellar chemistry under subsolar metallicity conditions. We present the results of a hot core survey toward 30 massive protostellar objects in the LMC using the Atacama Large Millimeter/submillimeter Array (ALMA) at 350 GHz. Continuum imaging reveals 36 compact sources in total, among which line analyses identify 9 hot cores and 1 hot-core candidate, including two newly identified sources. We detect CO, HCO+, H13CO+, HC15N, HC3N, SiO, SO, SO+, NS, SO2, 34SO2, 33SO2, CH3OH, 13CH3OH, HCOOH, HCOOCH3, CH3OCH3, C2H5OH, H2CCO (tentative), and hydrogen recombination lines from hot cores. CH3OCH3, a complex organic molecule larger than CH3OH, is detected for the first time in a hot core outside the LMC bar region. All hot cores show stronger emission in the high-excitation SO line compared to non-hot-core sources, suggesting that its strong detection will be useful for identifying hot-core candidates in the LMC. Chemical analysis reveals a spread of more than two orders of magnitude in CH3OH abundances, with some sources deficient in COMs. In contrast, SO2 is detected in all hot cores, and its abundance shows a good correlation with rotational temperature. The hot cores without CH3OH detections are all located outside the LMC bar region and are characterized by either high luminosity or active star formation in their surroundings. A combination of locally low metallicity, active star formation in the vicinity, and high protostellar luminosity may jointly trigger the COM-poor hot core chemistry observed in the LMC.

Figures (28)

• Figure 1: Spatial distribution of the observed massive protostars in the LMC. Hot cores with CH$_3$OH detection are indicated by magenta squares, those without CH$_3$OH detection are by cyan squares (including one hot core candidate), and the remaining sources are shown in white (see Section \ref{['sec_disc_distribution']} for details). The background is a two-color image: light blue represents Spitzer/IRAC 3.6 $\mu$m data and red represents Herschel/PACS 160 $\mu$m data Mei06Mei10. The 3.6 $\mu$m emission mainly traces the stellar distribution, while the 160 $\mu$m emission traces the distribution of the ISM. The location of the 30 Dor and N11 regions are labeled in yellow. North is up, and east is to the left.
• Figure 2: Integrated intensity distributions of the CH$_3$OH emission. High-excitation ($E_{u}$$>$100 K) lines are stacked to reduce the noise level. Gray contours represent the 850 $\mu$m continuum distribution and the contour levels are 5$\sigma$, 10$\sigma$, 25$\sigma$, 100$\sigma$, 500$\sigma$ (only for Lh09) of the rms noise (0.1-0.2 mJy/beam). The spectra discussed in the text are extracted from the region indicated by the black open circle/ellipse. The blue cross represents the peak positions of continuum emission. The synthesized beam size of 0$\farcs$40 (0.1 pc) is shown by the gray filled circle in each panel.
• Figure 3: The same as in Figure \ref{['img1']}, but for the high-excitation ($E_{u}$$>$80 K) SO$_2$ emission.
• Figure 4: The same as in Figure \ref{['img1']}, but for the SO (8$_{8}$--7$_{7}$) line.
• Figure 5: Results of rotation diagram analyses for CH$_3$OH. The solid lines represent the fitted straight line. Derived column densities and rotation temperatures are shown in each panel.