Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected the chemical fingerprints of sodium chloride (NaCl), potassium chloride (KCl), and their 37Cl and 41K isotopologues, in a high-mass circumstellar disk around Orion Source I, a young, massive star about 1,500 light-years away.
An artist’s impression of Orion Source I, a young, massive star about 1,500 light-years away. The blue region — about 1/3 the way out from the center of the disk — represents the region where ALMA detected the millimeter-wavelength ‘glow’ from the salts. Image credit: NRAO / AUI / NSF / S. Dagnello.
To detect molecules in space, astronomers use radio telescopes to search for their chemical signatures — telltale spikes in the spread-out spectra of radio and millimeter-wavelength light.
Atoms and molecules emit these signals in several ways, depending on the temperature of their environments.
The new ALMA observations contain a bristling array of spectral signatures — or transitions, as astronomers refer to them — of the same molecules.
To create such strong and varied molecular fingerprints, the temperature differences where the molecules reside must be extreme, ranging anywhere from 100 to 4,000 K (about minus 175 degrees Celsius to 3,700 degrees Celsius).
An in-depth study of these spectral spikes could provide insights about how the star is heating the disk, which would also be a useful measure of the luminosity of the star.
“When we look at the information ALMA has provided, we see about 60 different transitions of molecules like sodium chloride and potassium chloride coming from the disk. That is both shocking and exciting,” said co-author Dr. Brett McGuire, a chemist at the National Radio Astronomy Observatory (NRAO).
“It’s amazing we’re seeing these molecules at all,” said lead author Dr. Adam Ginsburg, an astronomer at the NRAO.
“Since we’ve only ever seen these compounds in the sloughed-off outer layers of dying stars, we don’t fully know what our new discovery means. The nature of the detection, however, shows that the environment around this star is very unusual.”
ALMA image of the salty disk surrounding the young, massive star Orion Source I (blue ring). It is shown in relation to the Orion Molecular Cloud 1, a region of explosive starbirth. The background near infrared image was taken with the Gemini Observatory. Image credit: ALMA / NRAO / ESO / NAOJ / AUI / NSF / Gemini Observatory / AURA.
The team speculates that these salts come from dust grains that collided and spilled their contents into the surrounding disk.
Their observations confirm that the salty regions trace the location of the circumstellar disk.
“Usually when we study protostars in this manner, the signals from the disk and the outflow from the star get muddled, making it difficult to distinguish one from the other,” Dr. Ginsburg said.
“Since we can now isolate just the disk, we can learn how it is moving and how much mass it contains. It also may tell us new things about the star.”
The detection of salts around a young star is also of interest to scientists because some of constituent atoms of salts are metals — sodium and potassium.
This suggests there may be other metal-containing molecules in this environment. If so, it may be possible to use similar observations to measure the amount of metals in star-forming regions.
“This type of study is not available to us at all presently. Free-floating metallic compounds are generally invisible to radio astronomy,” Dr. McGuire said.
The findings were published in the Astrophysical Journal (arXiv.org preprint).
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Adam Ginsburg et al. 2019. Orion SrcI’s Disk is Salty. ApJ 872, 54; doi: 10.3847/1538-4357/aafb71
