The mystery of the pulsating blue stars

The mystery of the pulsating blue stars

Credit: Paweł Pietrukowicz

In the middle of the large Chilean Atacama desert, a team of
Polish astronomers are patiently monitoring millions of
celestial bodies night after night with the help of a modern
robotic telescope. In 2013, the team was surprised when they
discovered, in the course of their survey, stars that
pulsated much faster than expected. In the following years,
the team that included Dr. Marilyn Latour, an astronomer from
the Dr. Remeis-Sternwarte Bamberg, the astronomical institute
of Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU),
studied these stars in more detail and concluded that they
had stumbled upon a new class of variable star.

Many classes of star exhibit variations in brightness. Unlike
our Sun, these stars are not stable; their surface oscillates,
meaning that the surface expands and shrinks by a few percent.
This is what happens in the case of the more familiar Cepheids
and RR Lyrae stars, which have oscillation periods that extend
over a few hours to hundreds of days.

The researchers discovered a dozen stars that seemed at first
sight to show variations that were very similar to those of the
Cepheids and RR Lyrae stars but have much shorter (20-40
minutes) oscillation periods and, at the same time, are much
bluer in colour. This indicates that the newly identified stars
are hotter and more compact. It was because of these
characteristics that it was proposed to give this new class of
variable stars the acronym BLAPS, i.e. Blue Large-Amplitude
Pulsators. What kind of stars these were, however, remained an
enigma.

The nature of the newly discovered stars

For the astronomers, these new stars posed a riddle. At first,
they assumed that BLAPs could be hot dwarf stars since they
have similar oscillation periods. Hot dwarf stars are old stars
approaching the end of their lives. They generate their energy
by means of the thermonuclear fusion of helium to form carbon.
The Sun, being in an earlier phase of its life, is currently
converting hydrogen to helium.

In order to find out whether BLAPs are actually hot dwarfs, the
astronomers used two of their largest telescopes to make
observations. They were able to capture suitable spectra of
some BLAPs using the large Gemini and Magellan telescopes, both
located in the Chilean Atacama desert. Latour analysed these
spectra using sophisticated physical-numerical models. She was
able to show that the variations in luminosity are attributable
to temperature changes on the surface of the stars. The
temperature of the BLAPs turned out to be five times greater
than that of the Sun – something that is characteristic of hot
dwarfs.

However, the BLAPs are significantly bigger than hot dwarfs,
meaning that they form a new class of that are similar to hot dwarfs but have a
more bloated envelope than the latter. Why BLAPs oscillate like
Cepheids and why they are bloated remain puzzles, as does their
origin. Further investigations need to be undertaken to solve
the mystery of how BLAPs come into being.

Explore further:

Milky Way could be home to 100 billion ‘failed stars’

More information: Paweł Pietrukowicz et al, Blue
large-amplitude pulsators as a new class of variable stars,
Nature Astronomy (2017). DOI: 10.1038/s41550-017-0166

Journal reference: Nature
Astronomy

Provided by:
University of Erlangen-Nuremberg