Archivi tag: stellar atmospheres

Dig Sites of Stellar Archeology: Giant Stars in the Milky Way

Elemental abundances are the fingerprints of the stellar evolution; they provide critical information about the history of Galactic chemical evolution. Low and intermediate mass stars play the most important role in Galactic chemical enrichment and depending on many parameters such as initial mass, metallicity, and mass loss rate at red giant phase, age, helium abundance and rotation, they follow different paths on the HR-diagram.

Evolved giants (RHB and Red Clump stars), especially of lower masses than 2.5 M, are the best laboratories to investigate the extra-mixing mechanisms observed right after the luminosity function bump of red giant branch. Carbon and nitrogen (C/N) and 12C/13C ratios are the best indicators of deep mixing. The results show different ratios depending on the initial mass, metallicity and the stellar evolutionary stages. However, unambiguous relationships among these parameters have not been found yet. New observational and theoretical efforts are needed to unveil the relation between extra-mixing processes and the observed elemental abundances. Such extra-mixing is not explained by standard stellar evolution and therefore brings new challenges in theoretical studies.

Other evolutionary stages that witness drastic chemical changes in stellar atmospheres are AGB and post-AGB. Such stars are of critical importance to understand the last evolutionary stages of low and intermediate mass stars. Because of their fairly fast evolution, not many are known to date; these stars are still largely shrouded in mystery. A major influence of the third dredge-up phenomenon takes place in the post-AGB atmospheres which mainly alters the chemical abundance of 13C at the surface. This phenomenon also triggers the slow neutron-capture reactions (the s-process) in the stellar interiors. Some post-AGB stars are known to be enriched by the s-process elements, although some others do not show a single sign. The variety of the Galactic post-AGB population especially in chemical domain makes them even more difficult objects to investigate. The linkage between the post-AGBs enriched and non-enriched by certain elements remains unknown.

Giant stars are the ideal objects to dig down the ancient history of the Milky Way. The aim of this workshop is to bring up the current struggles and new challenges in both observational and theoretical studies on chemical evolution of the evolved stars, and discuss the new observational and theoretical approaches to improve our understanding on the Galactic chemical evolution.

Topics will include:

  • What happens after main sequence: the evolution of low and intermediate mass stars
  • Abundance alterations in red giants: extra-mixing processes
  • He-core burning stars in the field and stellar clusters: RHB vs. Red Clumps
  • AGB and Post-AGB stars: the third dredge-up and the complex atmospheres

Hot spots in the outer atmosphere of Betelgeuse

Astronomers have released a new image of the outer atmosphere of Betelgeuse, one of the nearest red supergiants to Earth, revealing the detailed structure of the matter being thrown off the star. The new image, taken by the e-MERLIN radio telescope array operated from the Jodrell Bank Observatory in Cheshire, also shows regions of surprisingly hot gas in the star’s outer atmosphere and a cooler arc of gas weighing almost as much as the Earth. Betelgeuse is easily visible to the unaided eye as the bright, red star on the shoulder of Orion the Hunter. The star itself is huge, 1,000 times larger than our Sun, but at a distance of about 650 light years it still appears as a tiny dot in the sky, so special techniques combining telescopes in arrays are required to see details of the star and the region around it.

The new e-MERLIN image of Betelgeuse shows its atmosphere extends out to five times the size of the visual surface of the star. It reveals two hot spots within the outer atmosphere and a faint arc of cool gas even farther out beyond the radio surface of the star. The hot spots are separated by roughly half the visual diameter of the star and have a temperature of about 4,000-5,000 Kelvin, much higher than the average temperature of the radio surface of the star (about 1,200 Kelvin) and even higher than the visual surface (3,600 Kelvin).  The arc of cool gas lies almost 7.4 billion kilometres away from the star, about the same distance as the farthest Pluto gets from the Sun. It is estimated to have a mass almost two thirds that of the Earth and a temperature of about 150 Kelvin. Anita Richards, from The University of Manchester, said that it was not yet clear why the hot spots are so hot: “One possibility is that shock waves, caused either by the star pulsating or by convection in its outer layers, are compressing and heating the gas. Another is that the outer atmosphere is patchy and we are seeing through to hotter regions within. The arc of cool gas is thought to be the result of a period of increased mass loss from the star at some point in the last century but its relationship to structures like the hot spots, which lie much closer in, within the star’s outer atmosphere, is unknown”. The mechanism by which supergiant stars like Betelgeuse lose matter into space is not well understood despite its key role in the lifecycle of matter, enriching the interstellar material from which future stars and planets will form. Detailed high-resolution studies of the regions around massive stars like the ones presented here are essential to improving our understanding. “Betelgeuse produces a wind equivalent to losing the mass of the Earth every three years, enriched with the chemicals that will go into the next generation of star and planet formation.  The full detail of how these cool, evolved stars launch their winds is one of the remaining big questions in stellar astronomyThis is the first direct image showing hot spots so far from the centre of the star. We are continuing radio and microwave observations to help decide which mechanisms are most important in driving the stellar wind and producing these hot spots. This won’t just tell us how the elements that form the building blocks of life are being returned to space, it will also help determine how long it is before Betelgeuse explodes as a supernova”. Future observations planned with e-MERLIN and other arrays, including ALMA and VLA, will test whether the hotspots vary in concert due to pulsation, or show more complex variability due to convection. If it is possible to measure a rotation speed this will identify in which layer of the star they originate.

University of Manchester: Mysterious hot spots observed in a cool red supergiant
arXiv: e-MERLIN resolves Betelgeuse at wavelength 5 cm