* Astronomy

Blobrana · L

RE: Red giants

Blobrana · L

Astronomers reveal a rapidly spinning core inside old stars

An international team of astronomers led by PhD student Paul Beck from Leuven University in Belgium have managed to look deep inside some old stars and discovered that their cores spin at least ten times as fast as their surfaces. The result appeared today in the renowned journal nature. It has been known for a long time that the surfaces of these stars spin slowly, taking about a whole year to complete one rotation. The team has now discovered that the cores at the heart of the stars spin much faster with about one rotation per month. The discovery was made possible because of the ultra high precision of the data from NASA's Kepler space telescope.
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Blobrana · L

Red giants stars

Blobrana · L

RE: Red giants

Blobrana · L

Title: Granulation in Red Giants: observations by the Kepler mission and 3D convection simulations
Authors: S. Mathur, S. Hekker, R. Trampedach, J. Ballot, T. Kallinger, D. Buzasi, R. A. Garcia, D. Huber, A. Jimenez, B. Mosser, T. R. Bedding, Y. Elsworth, C. Regulo, D. Stello, W. J. Chaplin, J. De Ridder, S. J. Hale, K. Kinemuchi, H. Kjeldsen, F. Mullally, S. E. Thompson

The granulation pattern that we observe on the surface of the Sun is due to hot plasma from the interior rising to the photosphere where it cools down, and descends back into the interior at the edges of granules. This is the visible manifestation of convection taking place in the outer part of the solar convection zone. Because red giants have deeper convection zones and more extended atmospheres than the Sun, we cannot a priori assume that granulation in red giants is a scaled version of solar granulation. Until now, neither observations nor 1D analytical convection models could put constraints on granulation in red giants. However, thanks to asteroseismology, this study can now be performed. The resulting parameters yield physical information about the granulation. We analyse ~1000 red giants that have been observed by Kepler during 13 months. We fit the power spectra with Harvey-like profiles to retrieve the characteristics of the granulation (time scale tau_gran and power P_gran). We also introduce a new time scale, tau_eff, which takes into account that different slopes are used in the Harvey functions. We search for a correlation between these parameters and the global acoustic-mode parameter (the position of maximum power, nu_max) as well as with stellar parameters (mass, radius, surface gravity (log g) and effective temperature (T_eff)). We show that tau_eff nu_max^{-0.89} and P_gran nu_max^{-1.90}, which is consistent with the theoretical predictions. We find that the granulation time scales of stars that belong to the red clump have similar values while the time scales of stars in the red-giant branch are spread in a wider range. Finally, we show that realistic 3D simulations of the surface convection in stars, spanning the (T_eff, log g)-range of our sample of red giants, match the Kepler observations well in terms of trends.

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Blobrana · L

Red Giant Stars

Blobrana · L

RE: Red giants

Blobrana · L

A comprehensive study of the vibrations - or 'starquakes' - of thousands of distant stars is giving astronomers new insight into how stars work.
A team of scientists from Australia, Europe and the US, this week presents the results of a study using NASA's space-based Kepler telescope which looks in detail at the way stars vibrate.
The Kepler mission has "revolutionised" the study of star interiors, according to lead author on one of the studies, graduate student Daniel Huber from the University of Sydney.
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Blobrana · L

Astronomers recorded a chorus of immense red giant stars --- each hundreds of times larger than the Sun --- using NASA's Kepler space probe. They made the recording public this week during a press conference at Aarhus University in Denmark.
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Blobrana · L

In about 5 billion years, our Sun will expand to become a red giant star. This evolutionary stage is common to all stars that resemble the Sun once they have burned their fuel, hydrogen, present in the core. These stars are particularly interesting to study to test our theories representing the advanced stages of the stars. Precision photometric data obtained by the CoRoT space mission, performed by a national team of researchers; at the Observatoire de Paris, researchers analysis allowed to highlight the richness of the spectrum of these stars oscillations thus paving the way the use of techniques based on the seismology to probe the Interior of these stars.

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