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Using the ALMA telescope array a team of astronomers has spotted the tell-tale signs of sodium chloride (NaCl) – normal table salt – in the dust disc surrounding Orion Source I, a massive, young star in a dusty cloud behind the Orion Nebula.

A paper detailing the extraordinary discovery – which included other similar salty compounds – has been accepted for publication in the Astrophysical Journal.

Lead author, Adam Ginsburg, a Jansky Fellow of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, says: “It’s amazing we’re seeing these molecules at all.

“W e’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.”

In order to detect molecules in space, astronomers use radio telescopes to observe what wavelengths of light are being emitted and absorbed by material like gas and dust clouds and even stars. As these are chemical fingerprints are distinctive amongst compounds and elements, the spectral spikes tell them exactly what molecules are present.

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. ( ALMA (NRAO/ESO/NAOJ); NRAO/AUI/NSF; Gemini Observatory/AURA )

The new ALMA ( Atacama Large Millimeter/submillimeter Array) 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 kelvin to 4,000 Kelvin (about -175C to 3700C).



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.

Brett McGuire, a chemist at the NRAO in Charlottesville, Virginia, and co-author on the paper, says: “When we look at the information ALMA has provided, we see about 60 different transitions – or unique fingerprints – of molecules like sodium chloride and potassium chloride coming from the disk. That is both shocking and exciting.”

The paper’s authors speculate that these salts come from dust grains that collided and spilt their contents into the surrounding disk. Their observations confirm that the salty regions trace the location of the circumstellar disk.

John Bally, an astronomer at the University of Colorado and co-author on the paper, elaborates on how this star could have ended up with such an unusual environment: “This star was ejected from its parent cloud with a speed of about 10 kilometres per second around 550 years ago.



“It is possible that solid grains of salt were vaporized by shock waves as the star and its disc were abruptly accelerated by a close encounter or collision with another star. It remains to be seen if the salt vapour is present in all disks surrounding massive protostars, or if such vapour traces violent events like the one we observed with ALMA.”



Ginsburg continues: “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.

“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 astronomers and astrochemists because some of the 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 number of metals in star-forming regions.



As McGuire notes: “This type of study is not available to us at all presently. Free-floating metallic compounds are generally invisible to radio astronomy.”

The salty signatures were found about 30 to 60 astronomical units (AU, or the average distance between the Earth and the Sun) from the host stars. Based on their observations, the astronomers infer that there may be as much as one sextillion (a one with 21 zeros after it) kilograms of salt in this region, which is roughly equivalent to the entire mass of Earth’s oceans.

Ginsburg explains what the future of this research could hold: “Our next step in this research is to look for salts and metallic molecules in other regions. This will help us understand if these chemical fingerprints are a powerful tool to study a wide range of protoplanetary disks, or if this detection is unique to this source

“In looking to the future, the planned Next Generation VLA would have the right mix of sensitivity and wavelength coverage to study these molecules and perhaps use them as tracers for planet-forming disks.”

Header image: Artist impression of Orion Source I, a young, massive star about 1,500 light-years away. New ALMA observations detected a ring of salt — sodium chloride, ordinary table salt — surrounding the star. This is the first detection of salts of any kind associated with a young star. The blue region (about 1/3 the way out from the centre of the disk) represents the region where ALMA detected the millimetre-wavelength “glow” from the salts. (NRAO/AUI/NSF; S. Dagnello)

Original research: http://dx.doi.org/10.3847/1538-4357/aafb71





















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