Jupiter-mass planet orbiting giant star discovered

Observed RV variations and the best-fit orbital solutions
of HD 208897 with its residuals (bottom) to the best-fit-model.
The solid red, dotted, and green dashed lines indicate the best
Keplerian fit for TUG+OAO, OAO, and TUG, respectively. Dark
gray points and blue squares represent data from TUG and OAO,
respectively. Credit: Yilmaz et al., 2017.

(Phys.org)—An international team of astronomers has
discovered a Jupiter-mass alien world circling a giant star
known as HD 208897. The newly detected exoplanet was found as
a result of high-precision radial velocity measurements. The
discovery was detailed in a paper published Aug. 6 on the
arXiv preprint server.

HD 208897 is a giant star of spectral type K0, located some 210
light years away from the Earth. It is about five times larger
than our sun and has a mass of approximately 1.25 solar masses.
The star is metal rich and at the early phase of ascent along
the red giant branch. “Our stellar parameters indicate that HD
208897 is a metal-rich star that is at the base of RGB phase,”
the scientists wrote in the paper.

HD 208897 was one of the targets of an extensive observational
campaign carried out from 2007 to 2017, the main goal of which
is to search for substellar companions and planets around 50
evolved G and K-type . As a result of this
survey, a group of researchers led by Mesut Yilmaz of the
Ankara University, Turkey, found that 13 of the targets,
including HD 208897, show significant radial velocity
variations.

The researchers acquired 73 spectra of HD 208897 using the
Coude Echelle Spectrograph (CES) installed on the 1.5-meter
Russian-Turkish Telescope (RTT150) at TÜBİTAK National
Observatory (TUG) in Antalya, Turkey. They also conducted
follow-up spectroscopic observations of this star with the
1.88-meter telescope and the High Dispersion Echelle
Spectrograph (HIDES) high-efficiency fiberfeeding system
(HIDES-F) at the Okayama Astrophysical Observatory (OAO) in
Japan. Moreover, the team observed the star photometrically at
the Ankara University Kreiken Observatory (AUKR) to check
photometric variability or detect any transit phenomena.

The long-lasting observational campaign allowed the astronomers
to find a periodic signal, suggesting the presence of an unseen
and probably low-mass companion of HD 208897.

“In this work, we report the first planet discovery around a
HD 208897 in our planet search program
using the RTT150 and 1.88 m telescope at OAO,” the paper reads.

The researchers revealed that the newly found planet has a
minimum mass of 1.4 Jupiter masses. It is located at a distance
of about 1.05 AU from the star and circles its host every 353
days. Notably, the extrasolar world has a nearly circular
orbit.

According to the authors of the paper, their study demonstrates
that it is possible to detect such lower mass in the range of Jupiter, even around
giant stars, if long-term observations are carried out. They
also emphasize the importance of the finding for our
understanding of planet formation scenarios.

“This discovery will be important in understanding the planet
formation around metal-rich intermediate-mass stars and the
effect of stellar evolution on the planetary system
configuration,” the researchers concluded.

Explore further:

Four new short-period giant planets discovered

More information: M. Yilmaz et al. A Jupiter-mass planet
around the K0 giant HD 208897, Astronomy &
Astrophysics
(2017). DOI: 10.1051/0004-6361/201731184

A Jupiter-mass planet around the K0 giant HD 208897,
arXiv:1708.01895 [astro-ph.SR] arxiv.org/abs/1708.01895

Abstract
For over 10 years, we have carried out a precise radial
velocity (RV) survey to find substellar companions around
evolved G,K-type stars to extend our knowledge of planet
formation and evolution. We performed high precision RV
measurements for the giant star HD 208897 using an iodine (I2)
absorption cell. The measurements were made at T”UB.ITAK
National Observatory (TUG, RTT150) and Okayama Astrophysical
Observatory (OAO). For the origin of the periodic variation
seen in the RV data of the star, we adopted a Keplerian motion
caused by an unseen companion. We found that the star hosts a
planet with a minimum mass of m2sini=1.40MJ, which is
relatively low compared to those of known planets orbiting
evolved intermediate-mass stars. The planet is in a nearly
circular orbit with a period of P=353 days at about 1 AU
distance from the host star. The star is metal rich and located
at the early phase of ascent along the red giant branch. The
photometric observations of the star at Ankara University
Kreiken Observatory (AUKR) and the HIPPARCOS photometry show no
sign of variation with periods associated with the RV
variation. Neither bisector velocity analysis nor analysis of
the Ca II and Halpha lines shows any correlation with the RV
measurements.

Journal reference: Astronomy
& Astrophysics

© 2017 Phys.org

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