Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered the first ‘hot molecular core’ — a cocoon of complex molecules around a newborn star — in a galaxy other than our own.
This figure shows observations of the first hot molecular core to be found outside the Milky Way Galaxy with ALMA and a view of the region of sky in infrared light. Left: distributions of molecular line emission from a hot molecular core in the LMC observed with ALMA. Emissions from dust, sulfur dioxide (SO2), nitric oxide (NO), and formaldehyde (H2CO) are shown as examples. Right: an infrared image of the surrounding star-forming region (based on data from NASA’s Spitzer Space Telescope). Image credit: T. Shimonishi / Tohoku University / ALMA / ESO / NAOJ / NRAO.
“This is the first detection of an extragalactic hot molecular core, and it demonstrates the great capability of new generation telescopes to study astrochemical phenomena beyond the Milky Way,” said Dr. Takashi Shimonishi, an astronomer at Tohoku University, Japan.
“The observations have suggested that the chemical compositions of materials that form stars and planets are much more diverse than we expected.”
Dr. Shimonishi and co-authors have used ALMA’s resolving power to observe a young, massive star known as ST11 (full name 2MASS J05264658-6848469) in the Large Magellanic Cloud (LMC), a neighboring dwarf galaxy some 160,000 light-years away.
“LMC is an excellent target to study interstellar and circumstellar chemistry in different metallicity environments owing to its proximity and low metallicity (about one third of the solar neighborhood),” the astronomers explained.
“The low dust content in the galaxy results in harsh radiation environment, and thus photoprocessing of interstellar medium should be more effective in the LMC than in our Galaxy.”
“Furthermore, according to gamma-ray observations, the cosmic-ray density in the LMC is estimated to be lower than the Galactic typical values by a factor of four.”
“It is therefore highly anticipated that these environmental differences should affect chemical processes, and hot cores in the LMC should provide us key information to understand chemistry, particularly those of complex molecules, in low metallicity environments.”
Emission from a number of molecular gases in ST11 was detected. These indicated that the astronomers had discovered a concentrated region of hot and dense molecular gas.
This was evidence that they had found something never before seen outside of our Galaxy — a hot molecular core.
“Hot molecular cores are small, with a diameter of less than 0.3 light-years. These objects have a density over a thousand billion molecules per m3 (far lower than the Earth’s atmosphere, but high for an interstellar environment) and warm in temperature, at over minus 173 degrees Celsius. This makes them at least 80 degrees Celsius warmer than a standard molecular cloud, despite being of similar density,” the scientists said.
“These hot cores form early on in the evolution of massive stars and they play a key role in the formation of complex chemicals in space.”
The ALMA observations revealed that this newly discovered core in the LMC has a very different composition to similar objects found in the Milky Way.
The most prominent chemical signatures in ST11 include familiar molecules such as sulfur dioxide, nitric oxide, and formaldehyde — alongside the ubiquitous dust. But several organic compounds, including methanol, had remarkably low abundance.
In contrast, cores in the Milky Way have been observed to contain a wide assortment of complex organic molecules, including methanol and ethanol.
The LMC has a low abundance of elements other than hydrogen or helium. The astronomers suggest that this very different galactic environment has affected the molecule-forming processes taking place surrounding ST11. This could account for the observed differences in chemical compositions.
The team’s findings were published in the Astrophysical Journal (arXiv.org preprint) on August 9, 2016.
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Takashi Shimonishi et al. 2016. The Detection of a Hot Molecular Core in the Large Magellanic Cloud with ALMA. ApJ 827, 72; doi: 10.3847/0004-637X/827/1/72
