No planet around the K giant star 42 Draconis^★

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
² Department of Earth & Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel
³ Astronomical Institute, Czech Academy of Sciences, 251 65 Ondrejov, Czech Republic
⁴ McDonald Observatory, The University of Texas at Austin, Austin, TX 78712, USA

^★★ Corresponding author: artie@tls-tautenburg.de

Received: 20 February 2025
Accepted: 7 May 2025

Abstract

Context. Published radial-velocity (RV) measurements of the K giant star 42 Dra taken over a three-year time span reveal variations consistent with a 3.9 Jupiter-mass companion in a 479-d orbit.

Aims. This exoplanet can be confirmed if these variations are long-lived and coherent. Continued monitoring may also reveal additional companions.

Methods. We acquired additional RV measurements of 42 Dra; the data now span 15 years. Standard periodogram analyses were used to investigate the stability of the planet’s RV signal. We also investigated variations in the spectral line shapes using the bisector velocity span as well as infrared photometry from the COBE mission.

Results. The recent RV measurements do not follow the published planet orbit. An orbital solution using the 2004–2011 data yields a period and eccentricity consistent with the published values, but the RV amplitude decreased by a factor of four since the earlier measurements. Including some additional RV measurements taken between 2014 and 2018 reveals a second period at 530 d. The interference (beating) of this period with the one at 479 d may account for the observed amplitude variations. The planet hypothesis is conclusively ruled out by COBE/DIRBE 1.25 μ photometry, which shows variations with the planet orbital period as well as a 170 d period.

Conclusions. The RV of 42 Dra shows significant amplitude variations, which along with the COBE/DIRBE photometry firmly established that there is no giant planet around this star. The presence of multi-periodic variations suggests that we are seeing stellar oscillations in this star, most likely oscillatory convection modes. These oscillations may account for some of the long-period RV variations attributed to planets around K giant stars that may skew the statistics of planet occurrence around intermediate-mass stars. Long-term monitoring with excellent sampling is required to exclude amplitude variations in the long periods found in the RV of K giant stars.