The stellar cycles and the development of life

An international team publishes a paper that researches the activity cycles of a star similar to the young Sun. The study will help to understand whether the solar activity cycles could influence the development of life on Earth. The results of this research are published in the edition of May 6th in the magazine Astronomy & Astrophysics.

To know the behaviour of stars similar to our Sun is essential to better interpret its past and future history. But it is also helpful to understand, for instance, how its activity could influence the development of life on Earth.

The stars like our Sun have activity cycles that are noticed through phenomena in their outer layers: photosphere, chromosphere and corona. The solar cycle lasts 11 years on average, and it influences the Earth's climate, specially through the phenomena arising in the corona. The chromospheric cycle is currently known for many stars, but only three stars have a known coronal cycle similar to our Sun's. The three of them are old and have a low activity level.

However, there is a star that accomplish the requirements to be a young Sun and has also stellar activity cycles. The star is Iota Hor, in the constellation of Horologium ("The Clock"), and it has an age of about 600 million years (our Sun is 4500 million years old). A former work carried out in the visible wavelength range showed that its activity cycle in the chromosphere was the shortest observed to date.

According to Jorge Sanz-Forcada, researcher of INTA at the Centro de Astrobiologia (CSIC-INTA, Spain), who leads this work, "we did not know anything about the coronal cycles of the Sun at the time when life appeared on Earth. So we carried out X-ray observations of the corona of this star, with the purpose of testing its behavior during the chromospheric cycle".

The star was monitored with the X-ray satellite XMM-Newton during almost two years and both cycles, chromospheric and coronal, showed to be very similar in length. The monitoring during almost three cycles shows a peculiar behaviour: apparently the star completes a cycle, and the next cycle is interrupted before the maximum is reached, resulting then in a temporary chaotic behaviour. The cyclic behaviour in then resumed. Besides, this 1.6 yr cycle seems to be modulated by a longer duration cycle.

"This is the first time that an X-ray cycle is observed in an active star, and the first time that the coronal cycle is established in a star like our Sun in its youth. Iota Hor completes an activity cycle every 1.6 yr. We are probably observing the first cycles in the life of a star like the Sun", affirms Sanz-Forcada.

The development of life

These data are not only helpful to understand the enigmatic physics behind the activity cycles, but they also provide information on the relation between solar cycles and the climate, sheding light on astrobiological questions.

There is a series of ingredients that are believed to be involved in the origin of life on our planet: an environment in which many chemical reactions can take place (the water); the existence of an element that easily combines with others (such as the carbon); and a source of energy (like the UV radiation from the Sun, or electrical shocks from storms).

On the last of these ingredients, it is remarkable to notice that the young stars like iota Hor rotate faster than the Sun, thus producing a higher level of activity. This is in turn translated into a larger emission of ultraviolet and X-ray radiation. On top of the winds loaded with energetic particles typical of a young star, iota Hor also has a cycle with irregularities, and it has the same age of the Sun when life started on Earth.

Additional information
This work has been published in the paper "Iota Hor: the first coronal activity cycle in a young solar-like star", in the Astronomy & Astrophysics magazine. The authors are J. Sanz-Forcada (Departamento de Astrofisica, Centro de Astronbiologia - CSIC-INTA, Spain); B. Stelzer (INAF - Osservatorio Astronomico di Palermo, Italy), and T. S. Metcalfe (Space Science Institute, USA).

Figure Caption: Snapshots of the solar corona during its past cycle (1996-2006). Iota Hor completes a coronal cycle in just 1.6 yr, and its minimum coronal emission level is higher than the solar maximum. Credit: SOHO (ESA & NASA). Link to the original image

 

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