The environment of massive stars is salty!
In 2019, a team of astronomers led by Drs. Adam Ginsburg and Brett McGuire detected the chemical fingerprints of table salt - Sodium Chloride (NaCl) - and other similar salty compounds, about 1500 light-years from Earth, in the dusty disk surrounding a young star in the Orion Nebula. Since then, eight more salty disks have been detected, suggesting that salt emission is not rare in star-forming environments.
The chemical signatures of NaCl and KCl (see Figure 1) were identified in the spectral data delivered by the Atacama Large Millimeter/submillimeter Array (ALMA), a powerful telescope made up of 66 radio antennas located on the Chajnantor plateau in Chile. NaCl and KCl are found in the gas phase near the surface of the disk surrounding the central star, called Orion Source I, which is 12 times more massive than our Sun.
Figure 1: Example of the spectral data delivered by the ALMA telescope. This spectrum represents the chemical signature of the disk around the young massive star Orion Source I. It reveals all the molecular species responsible for the emission coming from the disk, seen as “spikes” in the spectrum at different frequencies, and of which the intensities are measured in units of “Jansky per beam” (power per unit area, unit frequency, and unit solid angle). In total, about 60 different transitions of KCl (in blue) and NaCl (in red) were identified, confirming their detection. Image credit: The Astrophysical Journal, Ginsburg, A., et al. (2019), ApJ, 872, 54.
Figure 2: 2D schematic view of a forming star, that accretes material from its surrounding spherical envelope, through its accretion disk.
Massive stars, such as Orion Source I, form by accreting material from their surrounding envelope through their accretion disk (see Figure 2). These envelopes are made of interstellar gas and dust, which are microscopic silicate and carbon grains. When observed with radio telescopes, the accretion disk around the star is highly obscured by the material in the envelope that has not been accreted to form the star yet. NaCl and KCl may be the only molecules that specifically trace the disks around massive stars and not the surrounding envelopes. For this reason, they are powerful probes to study how the disk is moving and how much mass it contains.
The blue ring shows the ALMA image of the salty disk surrounding the young massive star Orion Source I (Ginsburg, A., et al. 2019, ApJ, 872, 54). The narrow vertical extent of the salt emission indicates that NaCl is off the dust grains following their destruction, nearly immediately after being raised from the surface of the disk. The background image, taken from the Gemini Observatory, shows the Orion cloud, a region of explosive starbirth, located about 1500 light-years from Earth. Image credit: ALMA (NRAO/ESO/NAOJ); NRAO/AUI/NSF; Gemini Observatory/AURA.
NaCl and KCl were previously detected in the environment of old stars, as they are carried away with the left-over stellar envelope, blown up by the dying star. These two molecular species are then able to survive in the gas phase only for a short time, before being condensed onto interstellar dust grains. Since NaCl and KCl were recently observed in the gas phase in several disks around massive stars, it suggests that there is a zone where dust grains collide and spill their content into the gas that composes the disk. This grain sputtering phenomenon happens in case of strong “interstellar shocks”, that occur when some gas is rapidly ejected from the central star into the surrounding gas.
The detection of salts towards multiple astronomical objects suggests that salt emission is not rare, but it is also not ubiquitous. Several other similar disks around young massive stars have been investigated and did not show any sign of salt emission. Future observations, supported by theoretical calculations and chemical simulations will allow astronomers to better understand the processes that lead to the production of salt in space.
This article made use of the following publications:
Ginsburg, A., et al. 2019, ApJ, 872, 54
Tanaka , K., E., I et al 2020, ApJ, 900, L2
