First Doppler image and starspot-corrected orbit for λ Andromedae

A multifaceted activity analysis

¹ Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Institut für Physik und Astronomie, Universität Potsdam, 14476 Potsdam, Germany
³ HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Observatory, 1121 Budapest, Hungary
⁴ HUN-REN CSFK, MTA Centre of Excellence, 1121 Budapest, Hungary

^★ Corresponding author; oadebali@aip.de

Received: 19 November 2024
Accepted: 8 February 2025

Abstract

Context. Starspots on a rotating stellar surface impact the measured radial velocities and thereby limit the determination of precise orbital elements as well as astrophysical stellar parameters and even jeopardize the detection and characterization of (exo)planets.

Aims. We quantify the effect of starspots for the orbital elements of the spotted RS CVn binary λAnd and present an empirical correction. The aim is to obtain a more precise orbital solution that can be used to better study the system’s severe orbital-rotational asynchronism.

Methods. Phase-resolved high-resolution optical spectra were recorded over the course of 522 days in 2021–2022. We employed two facilities with medium and high resolution spectroscopy for the multiple activity analyses. Doppler imaging is used to reconstruct λ And’s starspots with a high resolution (R = 250 000) and high signal-to-noise ratio spectra. Optimized cross-correlation functions were used to measure precise radial velocities at a level of a few ten’s of m/s.

Results. The spot-corrected radial velocities enable, on average, a threefold increase in precision of the individual orbital elements. The residual velocity jitter with a full range of 500 m s⁻¹ is modulated by the rotation period of λAnd of 54.4±0.3 d. Our logarithmic gravity from spectrum synthesis of 2.8±0.2 together with the interferometrically determined stellar radius suggest a most-likely mass of the primary of ≈1.4 M_⊙. The small orbital mass function then implies a secondary mass of just ≈0.1 M_⊙, which is appropriate for an L-class brown dwarf. The Doppler image reconstructs a dominating cool spot with an umbral temperature difference of ≈1000 K with respect to the photosphere of 4660 K and is likely surrounded by a moat-like velocity field. Three more weaker spots add to the total surface spottedness, which is up to 25% of the visible surface. Seven optical chromospheric tracers show rotational modulation of their emission line fluxes in phase with the cool spots. This surface configuration appears to have been stable for the 522 days of our observations. We also redetermined the carbon isotope ratio to ¹²C/¹³C = 30 ± 5 and measured a contemporaneous disk-integrated mean longitudinal magnetic field of polarity Plus/Minus up to 2.70 ± 0.35 G from Stokes-V line profiles.