Gigantic, red and full of spots
About eight percent of red giants are covered by sunspot-like, dark
areas; these stars rotate faster than others of their kind

Date:
July 13, 2020
Source:
Max Planck Institute for Solar System Research
Summary:
Starspots are more common among red giant stars than previously
thought.

Astronomers report that approximately eight percent of red giants
exhibit such spots. Although red giants are generally regarded
as slowly rotating stars, those with starspots are apparently an
exception. The new publication offers a comprehensive analysis of
the reasons for their short rotation periods.



FULL STORY ========================================================================== Among the Sun's most striking features are its sunspots, relatively
darker areas compared to the rest of the surface, some of which are
visible from Earth even without magnification. Numerous other stars,
which like the Sun are in the prime of their lives, are also covered
by spots. In red giants, on the other hand, which are in an advanced
stage of stellar evolution, such spots were previously considered to be
rare. The reason for this difference can be found deep in the interior
of stars. In a dynamo process, the interplay of electrically conductive
plasma currents and rotation generates a star's magnetic field that
is then washed up to its surface. In some places, particularly strong
magnetic fields prevent hot plasma from flowing upwards.

These regions appear dark and constitute starspots.


========================================================================== "Rotation and convection are both crucial ingredients for the formation
of surface magnetic fields and starspots," explains Dr. Federico Spada
of MPS, co- author of the new study. "Stars with outer convective layers
have the potential to generate surface magnetic fields via dynamo action,
but only when the star rotates fast enough the magnetic activity becomes detectable," he adds. Until now, researchers had assumed that almost
all red giants rotate rather slowly around their own axis. After all,
stars expand dramatically when they develop into red giants towards the
end of their lives. As a result their rotation slows down, like a figure
skater doing a pirouette with his arms stretched out.

The new study led by scientists from MPS and New Mexico State University
(USA) now paints a different picture. About eight percent of the observed
red giants rotate quickly enough for starspots to form.

The research team scoured the measurement data of about 4500 red giants recorded by NASA's Kepler space telescope from 2009 to 2013 for evidence
of spots. Such spots reduce the amount of light that a star emits
into space.

Since they usually change only slightly over several months, they
gradually rotate out of the telescope's field of view -- and then
reappear after some time. This produces typical, regularly recurring
brightness fluctuations.

In a second step, the scientists investigated the question why the
spotted giants rotate so quickly. How do they muster the necessary
energy? "To answer this question, we had to determine as many of the
stars' properties as possible and then put together an overall picture,"
says Dr. Patrick Gaulme, lead author of the publication. At the Apache
Point Observatory in New Mexico (USA), for example, the researchers
studied how the wavelengths of starlight from some of the stars change
over time. This allows conclusions about their exact movement.

The team also looked at rapid fluctuations in brightness, which
are superimposed on the slower ones caused by starspots. The faster fluctuations are the expression of pressure waves propagating through a
star's interior to its surface. They contain information on many internal properties such as the star's mass and age.

The analysis revealed that approximately 15 percent of the spotted
giants belong to close binary star systems, usually constituted of a
red giant with a small and less massive companion. "In such systems, the rotational speeds of both stars synchronize over time until they rotate
in unison like a pair of figure skaters," says Gaulme. The slower red
giant thus gains momentum and spins faster than it would have without
a companion star.

The other red giants with starspots, about 85 percent, are on their own
-- and yet they rotate quickly. Those with a mass roughly equal to that
of the Sun probably merged with another star or planet in the course of
their evolution and thus gained speed. The somewhat heavier ones, whose
masses are two to three times that of the Sun, look back on a different development. In the heyday of their lives before they became red giants,
their internal structure prevented the creation of a global magnetic
field that gradually carries particles away from the star. Unlike their magnetic counterparts, which therefore rotate slower and slower over time, their rotation has probably never slowed down significantly. Even as red giants, they still rotate almost as quickly as they did in their youth.

"In total, behind the common observational feature that some red giants
have spots, we find three groups of rapidly rotating stars, each of which
has a very different explanation. So it's no wonder that the phenomenon
is more widespread than we previously thought," says Gaulme.

Studies like the present research shed light, among other things, on the evolution of rotation and magnetic activity in stars, and their complex interplay, including the impact on the habitability of the planetary
systems they may host. These are among the prime objectives of ESA's PLATO mission, whose launch is expected by the end of 2026. "We look forward
to having the PLATO mission in space; with its unique long-duration observations we will be able to extend the study to other regions of
the Milky Way," concludes Spada.

This research was supported by the German Aerospace Center (DLR) under
PLATO Data Center grant 50OO1501.


========================================================================== Story Source: Materials provided by Max_Planck_Institute_for_Solar_System_Research. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Patrick Gaulme, Jason Jackiewicz, Federico Spada, Drew Chojnowski,
Benoi^t Mosser, Jean McKeever, Anne Hedlund, Mathieu Vrard, Mansour
Benbakoura, Cilia Damiani. Active red giants: Close binaries versus
single rapid rotators. Astronomy & Astrophysics, 2020; 639: A63
DOI: 10.1051/0004-6361/202037781 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200713125503.htm

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