Reading the Stellar Spots

Nikolai Piskunov of the Dynamics and Structure Formation in Stars and Stellar Environment project, explains why stellar and sun spots not only hold revealing clues to the internal workings of the stars but also may be key to regulating the effects for sustaining life on near planets.

Historically, astronomy has been a continuous race to see deeper and to resolve more details in celestial objects. This remains true for all fields of modern observational astronomy, from studies of the larger scale structure of the Universe to the objects in the Solar system, but the two goals are not easily combined: going deeper makes it harder to resolve details.

Galileo described the spots on our sun’s surface some four centuries ago, and today we have indirect evidence of spots on other stars yet no modern telescope is capable of resolving spots on stars other than the Sun. The origin of the spots and their evolution are not fully understood and proposed models are incapable of predicting the presence and the properties of spots for a given group of stars.

While spots occupy only a fraction of the stellar surface, their role is important. For example, the total energy irradiated by the Sun in the form of light is amazingly constant while we clearly see correlation between the number of spots (changing throughout the 11 year cycle) and the mean temperature on Earth. How exactly the presence of spots affects our atmosphere remains to be discovered. By studying younger solar analogues we see that on average these stars produce less light but have more spots than the Sun today.

Perhaps a balance between these properties of the Sun was crucial for creating stable conditions on the surface of the Earth allowing life to evolve. In the search for life, astronomers look also for planets around ‘less massive’ and cooler stars – the so-called M dwarfs – which are easier to detect but they have to be closer to their parent stars to have liquid water on their surfaces. The ejections of charged particles associated with stellar spots could be a decisive factor for life to survive or die.

Higher-resolution at large distances can be achieved using larger telescopes: they collect more photons and have ‘sharper’ vision. This path remains the mainstream development today with a number of 30-40 metre telescope projects (including the European Extremely Large Telescope) nearing construction. Alternatively, interferometry – combining the signals from many smaller telescopes – improves the resolution sacrificing the sensitivity compared to a large single-mirror telescope. Many large international radio-astronomy projects under development are following this path (e.g. Atacama Large Millimetre Array and Square Kilometre Array), as interferometry is technically easier to implement at longer wavelengths.

Building larger facilities isn’t cheap: while the resolution improves only proportional to the size, the price tag grows much faster. In the case of stellar spots astronomers came up with an elegant way of studying structures on stellar surfaces, which combines high spatial resolution and (virtually) no dependence on the distance. This second property means that existing four to eight metre telescopes can be used to reach a large number of targets. The method known as Doppler Imaging (DI) uses a spot on the stellar surface, which causes the spectrum to change as the star rotates. The DI thus reconstructs the map of the stellar surface from the observed time variations of its spectral lines.

Today, nearly all stars have inhomogeneous surfaces but the nature of those structures depends on their temperature, mass, rotation velocity and age. Hotter stars have very weak convective motions in their outer layers. This lack of mixing permits stratification of chemical elements creating ‘spots’ of peculiar chemical composition giving the name to the whole group of Chemically Peculiar (CP) stars. Cooler stars, including our Sun, have rather violent convective flows reaching the surface and in places where these flows are weaker they develop dark cool spots.

Early stellar evolution stages, when young stars are still surrounded by a disk of gas and dust – the birthplace of planets – material from the disk gradually drifts inward eventually falling onto the stellar surface. The falling material is believed to follow a few isolated flow ‘tubes’ that connect to the stellar surface and may live from a few hours to several days before the geometry of the flow changes. At the end of its fall the disk material comes to an abrupt stop creating hot and bright shock zones that have been traced with DI techniques.

Stellar spots not only affect the climate on the close-in planets but they also trace the important physical processes occurring inside stars. For example, the spots are often associated with strong magnetic fields, which is the case for both hotter CP stars and cooler solar-type stars. Magnetic fields have also been detected in flows around young stars. Large-scale numerical simulations indicate that magnetic fields play a crucial role in breaking symmetry in the circumstellar disk, facilitating the growth of planetary building blocks and eventually planetary cores. We do not know if magnetic fields are always involved in creating inhomogeneities in stars or stellar neighborhoods but in a recent discovery we have seen the “clouds” of mercury drift on the surface of a CP star Alpheratz (an Andromedae) where no magnetic fields have been found (yet).

Finally, we can measure the polarisation properties of light and, combining them with the magnetic version of the DI, we can reconstruct the distribution of magnetic fields across stellar surfaces. This novel technique and so far it has only been applied to a handful of CP stars but already those have revealed a fascinating complexity of field structures. As observation quality improves, magnetic DI will tell us more about the evolution and history of solar cycles.

To find out more about the project, contact Professor Nikolai Piskunov at piskunov@fyast.uu.se

Published: Monday, 26th April 2010 by Tom Henry

Comment on this article

Comments

No comments for this article, be the first to comment