Map
of the sky towards the central bulge of the Milky Way, with the
positions of the binary candidates indicated as red circles. The
background image is based on near-infrared observations obtained in the
course of the Vista Variables in the Vía Láctea (VVV) ESO Public Survey.
The scale is approximately 9.7 by 5.2 degrees. The centre of our galaxy
can be seen at the top of the figure, slightly to the right of the
centre. Credit: ESO/Márcio Catelan. JPG Image
Animation
showing the light-travel time effect. The upper plot shows the
brightness of a binary RR Lyrae star (OGLE-RRLYR-06498) as a function of
time. Grey symbols indicate all the available individual measurements,
whereas the red symbols show an selection of the data points as they
were collected over time. The bottom plot shows the same data, but
folded according to the pulsation period. The main brightness variations
show up as a sawtooth-like distribution corresponding to the pulsation
of the RR Lyrae variable, whilst the back-and-forth movement shown by
the red dots is the signal that is brought about by the presence of a
binary companion. Gif Image
Same
as above, but the RR Lyrae star OGLE-RRLYR-06545. This star has a very
similar light curve shape as OGLE-RRLYR-06498, but it does not display
signs of the light-travel time effect, indicating that it lacks a binary
companion. Gif Image
Many, perhaps most, stars in the Universe live their lives with
companions by their sides – in so-called binary systems. Until recently,
however, the ancient RR Lyrae stars appeared, for mysterious reasons,
to live their lives all alone. A recent study led by Chilean astronomers
shows that RR Lyrae stars may not be as lonely as previously thought.
The new research is published in Monthly Notices of the Royal Astronomical Society Letters.
Stars are very often found not in isolation, but rather in pairs. In
these so-called binary systems, two stars orbit around their common
centre of gravity. Suitable binary systems are of extreme importance in
astrophysics, as their properties can be inferred with unparalleled
accuracy from detailed analysis of their orbital properties.
Puzzlingly, however, an overwhelming majority of the known members of
a very important family of stars, known to astronomers as RR Lyrae
variables, have for long appeared to live their lives all alone. These
stars, being among the oldest known in the cosmos, contain precious
information about the origin and evolution of the stellar systems that
harbour them, such as the Milky Way itself.
However, the lack of RR Lyrae stars in binary systems has made a
direct assessment of some of their key properties difficult. Most often,
theory had to be invoked to fill the gap.
This apparent solitude has always intrigued astronomers. Now, however, an international research team led by experts of the Millennium Institute of Astrophysics (MAS) and the Pontificia Universidad Católica de Chile's Institute of Astrophysics (IA-PUC) have found evidence that these stars may not abhor companionship so thoroughly after all. In a Letter published in the journal Monthly Notices of the Royal Astronomical Society,
the team reports on the identification of as many as 20 candidate
RR Lyrae binaries – an increase of up to 2000% with respect to previous
tallies. Twelve of those candidates have enough measurements to conclude
with high confidence that they do indeed consist of two stars orbiting
each other.
"In the solar neighbourhood, about every second star is in a binary.
The problem with RR Lyrae variables is that for a long time only one of
them was known to be in a long-period binary system. The fact that among
100,000 known RR Lyrae stars only one of them had been seen to have
such a companion was something really intriguing for astronomers,"
explains Gergely Hajdu, IA-PUC Ph.D. student, MAS researcher, and lead
author of the study.
In their paper, the authors used a method that astronomers call the
"light-travel time effect," which exploits subtle differences in the
time it takes starlight to reach us.
"The RR Lyrae stars pulsate regularly, significantly increasing, and
then decreasing, their sizes, temperatures, and brightness, in a matter
of just a few hours. When a pulsating star is in a binary system, the
changes in brightness perceived by us can be affected by where exactly
the star is in the course of its orbit around the companion. Thus, the
starlight takes longer to reach us when it is at the farthest point
along its orbit, and vice-versa. This subtle effect is what we have
detected in our candidates," according to Hajdu.
"Our measurements were based on data published by the Polish OGLE Project. The OGLE team have obtained their data using the 1.3m Warsaw telescope, located in Las Campanas Observatory,
northern Chile, repeatedly observing the same patches of the sky for
many years. Our 20 candidates were found analysing the roughly 2000 best
observed RR Lyrae stars towards the central parts of the Milky Way.
That's about 5% of the known ones. It was only thanks to the high
quality of the OGLE data and the long timespan of these observations
that we could finally find signs of companions around so many of these
stars," says Hajdu.
Indeed, the systems detected by Hajdu et al. have orbital periods of
several years, which indicates that the companions, though bound
together by gravity, are not very close to one another. "Binaries with
even longer periods may also exist, but the current data do not extend
long enough for us to reach strong conclusions in this respect," he
adds.
For co-author Márcio Catelan, MAS Associate Researcher, IA-PUC
astrophysicist and Hajdu's thesis advisor, these results have
significant implications for astrophysics. "These are extremely old
stars, which have witnessed the formation of galaxies like our own Milky
Way, and survived to tell us the story. Besides, they are easy to
identify, since they show characteristic, cyclical brightness
variations, which make them excellent distance indicators for the nearby
Universe. However, a lot of what we know about them relies on
theoretical modelling. We can now exploit the orbital information
contained in these binary systems – and there are quite a few of them
now – in order to directly measure their physical properties, especially
their masses but possibly also their diameters, thus opening new doors
to discoveries that until recently seemed impossible.", he says.
This is just the first step towards achieving these goals, however:
according to Catelan, more data will be needed, particularly follow-up
observations of the binary candidates with sophisticated techniques like
spectroscopy and astrometry. The rewards awaiting at the end of the
road seem well worth the long journey, and the RR Lyrae will happily
traverse that path with their companions firmly by their sides.
Further information
The new work appears in G. Hajdu, "New RR Lyrae variables in binary systems", Monthly Notices of the Royal Astronomical Society, vol. 449, pp. L113-L117, 2015, published by Oxford University Press.
Science contact
Gergely Hajdu
PhD student
Instituto de Astrofísica, Pontificia Universidad Católica de Chile & Millennium Institute of Astrophysics
Tel: +56 223541540
ghajdu@astro.puc.cl
Media contact
Dr Keith Smith
Deputy Press Officer
Royal Astronomical Society
Tel: +44 (0)20 77344582 x209
kts@ras.org.uk
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