ESA’s Herschel space observatory has detected a cool layer in the atmosphere of Alpha Centauri A, the first time this has been seen in a star beyond our own Sun. The finding is not only important for understanding the Sun’s activity, but could also help in the quest to discover protoplanetary systems around other stars.
Cool layer in a Sun-like star: One of the great curiosities in solar science is that our Sun’s outer atmosphere – the corona – is heated to millions of degrees when its visible surface is ‘only’ about 6000 degrees. Even stranger is a curious temperature minimum of 4000 degrees lying between the two layers, in the chromosphere. Now, using ESA’s Herschel space observatory, scientists have made the first discovery of an equivalent cool layer in the atmosphere of the Sun-like star, Alpha Centauri A. Image Credit: ESA
The Sun’s nearest neighbours are the three stars of the Alpha Centauri system. The faint red dwarf, Proxima Centauri, is nearest at just 4.24 light-years, with the tight double star, Alpha Centauri AB, slightly further away at 4.37 light-years.
Alpha Centauri B has recently been in the news after the discovery of an Earth-mass planet in orbit around it (see also the image below). But Alpha Centauri A is also very important to astronomers: almost a twin to the Sun in mass, temperature, chemical composition and age, it provides an ideal natural laboratory to compare other characteristics of the two stars.
One of the great curiosities in solar science is that the Sun’s wispy outer atmosphere – the corona – is heated to millions of degrees while the visible surface of the Sun is ‘only’ about 6000ºC. Even stranger, there is a temperature minimum of about 4000ºC between the two layers, just a few hundred kilometres above the visible surface in the part of Sun’s atmosphere called the chromosphere.
This artist’s impression shows a planet orbiting the star Alpha Centauri B, a member of the triple star system that is the closest to Earth. Alpha Centauri B is the most brilliant object in the sky and the other dazzling object is Alpha Centauri A. Our own Sun is visible to the upper right. Image Credit: ESO/L. Calçada/N. Risinger (skysurvey.org)
Both layers can be seen during a total solar eclipse (see image below), when the Moon briefly blocks the bright face of the Sun: the chromosphere is a pink-red ring around the Sun, while the ghostly white plasma streamers of the corona extend out millions of kilometers.
The heating of the Sun’s atmosphere has been a conundrum for many years, but is likely to be related to the twisting and snapping of magnetic field lines sending energy rippling through the atmosphere and out into space – possibly in the direction of Earth – as solar storms. Why there is a temperature minimum has also long been of interest to solar scientists.
Now, by observing Alpha Centauri A in far-infrared light with Herschel and comparing the results with computer models of stellar atmospheres, scientists have made the first discovery of an equivalent cool layer in the atmosphere of another star.
A total solar eclipse. Image Credit: Anik De Groof /ESA
“The study of these structures has been limited to the Sun until now, but we clearly see the signature of a similar temperature inversion layer at Alpha Centauri A,” says René Liseau of the Onsala Space Observatory, Sweden, and lead author of the paper presenting the results.
“Detailed observations of this kind for a variety of stars might help us decipher the origin of such layers and the overall atmospheric heating puzzle.”
Understanding the temperature structure of stellar atmospheres may also help to determine the presence of dusty planet-forming disks around other stars like the Sun.
This video shows an imaginary journey from Earth to the Alpha Centauri system. As we leave the Solar System we see the familiar constellation figures including the Southern Cross (Crux) and the bright stars Alpha and Beta Centauri. As we approach Alpha Centauri we pass a faint red star, this is Proxima Centauri, the closest star to Earth and the faintest component of a triple star system. The final part shows the bright double star Alpha Centauri A and B with the Sun visible in the background. Video Credit: ESO./L. Calçada/Nick Risinger (skysurvey.org)
“Although it is likely only a small effect, a temperature minimum region in other stars could result in us underestimating the amount of dust present in a cold debris disk surrounding it,” says Dr Liseau.
“But armed with a more detailed picture of how Alpha Centauri A shines, we can hope to make more accurate detections of the dust in potential planet-bearing systems around other Sun-like stars.”
“These observations are an exciting example of how Herschel can be used to learn more about processes in our own Sun, as well as in other Sun-like stars and the dusty discs that may exist around them,” says Göran Pilbratt, ESA’s Herschel Project Scientist.
The survey was conducted as part of the DUNES (Dust around Nearby Stars) Herschel Key Programme.
The paper “α Centauri A in the far infrared. First measurement of the temperature minimum of a star other than the Sun,” by R. Liseau et al. is published in Astronomy & Astrophysics 549, L7 (2013).