Best ever image of a star's surface, atmosphere

To the unaided eye the famous, bright star Antares shines with a strong red tint in the heart of the constellation of Scorpius (The Scorpion). It is a huge and comparatively cool red supergiant star in the late stages of its life, on the way to becoming a supernova.

Using ESO's Very Large Telescope Interferometer astronomers have constructed this remarkable image of the red 
supergiant star Antares. This is the most detailed image ever of this object, or any other star apart from the sun 
[Credit: ESO/K. Ohnaka]

A team of astronomers, led by Keiichi Ohnaka, of the Universidad Católica del Norte in Chile, has now used ESO's Very Large Telescope Interferometer (VLTI) at the Paranal Observatory in Chile to map Antares's surface and to measure the motions of the surface material. This is the best image of the surface and atmosphere of any star other than the Sun.

Using ESO’s Very Large Telescope Interferometer astronomers have constructed this remarkable map of the motions of 
material on the surface of the red supergiant star Antares. This is the first such velocity map of any star other than 
the Sun. In red regions the material is moving away from us and in the blue areas the material is approaching. The 
empty region around the star is not a real feature, but shows where velocity measurements were not possible 
[Credit: ESO/K. Ohnaka]

The VLTI is a unique facility that can combine the light from up to four telescopes, either the 8.2-metre Unit Telescopes, or the smaller Auxiliary Telescopes, to create a virtual telescope equivalent to a single mirror up to 200 metres across. This allows it to resolve fine details far beyond what can be seen with a single telescope alone.

This artist’s impression shows the red supergiant star Antares in the constellation of Scorpius. Using ESO’s Very Large 
Telescope Interferometer astronomers have constructed the most detailed image ever of this, or any star other than
 the Sun. Using the same data they have also made the first map of the velocities of material the atmosphere
 of a star other than the Sun [Credit: ESO/M. Kornmesser]

"How stars like Antares lose mass so quickly in the final phase of their evolution has been a problem for over half a century," said Keiichi Ohnaka, who is also the lead author of the Nature paper. "The VLTI is the only facility that can directly measure the gas motions in the extended atmosphere of Antares -- a crucial step towards clarifying this problem.The next challenge is to identify what's driving the turbulent motions."

Using the new results the team has created the first two-dimensional velocity map of the atmosphere of a star other than the Sun. They did this using the VLTI with three of the Auxiliary Telescopes and an instrument called AMBER to make separate images of the surface of Antares over a small range of infrared wavelengths. The team then used these data to calculate the difference between the speed of the atmospheric gas at different positions on the star and the average speed over the entire star. This resulted in a map of the relative speed of the atmospheric gas across the entire disc of Antares -- the first ever created for a star other than the Sun..

The astronomers found turbulent, low-density gas much further from the star than predicted, and concluded that the movement could not result from convection, that is, from large-scale movement of matter which transfers energy from the core to the outer atmosphere of many stars. They reason that a new, currently unknown, process may be needed to explain these movements in the extended atmospheres of red supergiants like Antares.

"In the future, this observing technique can be applied to different types of stars to study their surfaces and atmospheres in unprecedented detail. This has been limited to just the Sun up to now," concludes Ohnaka. "Our work brings stellar astrophysics to a new dimension and opens an entirely new window to observe stars."

Source: ESO [August 23, 2017]