The K-giant XX Tri

Robotic astronomy: a 16-year movie of the spotted surface of the K-giant XX Tri

by Strassmeier et al. (2023)

Abstract

Sunspots are the most well-known manifestations of solar magnetic fields and exhibit a range of phenomena related to the interior dynamo. Among the observables from the spatially resolved solar disk are the number, size, and morphology of sunspots, their growth and decay, and their migration in latitude and longitude. Starspots are the direct analogs of sunspots on other stars but with the big observational restriction that we usually can not resolve other star's surfaces. In this paper we employ an indirect imaging technique called Doppler imaging. Typically, only occasional snapshots of spots on stellar surfaces are ob-tained while it is well-known that spots systematically change with time and, like on the Sun, only then tell us about the interior dynamo and structure of the target in question. Here we present a 16-year long time series of Doppler images for one of the most spotted stars in the sky. Creating this unique times series was only possible thanks to the continuous operation of the STELLA robotic telescope on Tenerife and its high-resolution echelle spectrograph SES. The robot has observed the star on every available clear night since July 2006, acquiring a total of over 2000 high-resolution, high signal-to-noise-ratio (S/N) spectra that are used to create 99 independent Doppler images. We combine these images into a movie visualizing XX Tri's surface spot evolution for the past 16 years. Stellar-disk photocenter displacements of up to 24 μas, or ~10% of the stellar disk radius, are reconstructed for XX Tri, but do not conclusively show the typical solar-like periodic behavior that could be interpreted as an activity cycle. A period of ~4.1 yr is retrieved only from the effective temperature time series but could not be clearly confirmed from other surface tracers. It suggests a mostly chaotic, likely unperiodic, dynamo. The spot-induced stellar photocenter variations pose a natural limit for astrometric exoplanet catches. The images also indicate missing (blocked) flux due to cool spots of up to 10% of the total flux.

Data available download

HD012545_VaCp002.joh, HD012545_CkCp002.joh : differential photoelectric photometry data obtained with T7, from Feb. 1996 to Dec. 2018. Measurements in Johnson-Cousins system, V and I, until Sep. 1997 also in R. Comparison star (VaCp) was HD 12478, check star BD+35 385.

HD012545_VaCp002.str , HD012545_CkCp002.str: differential photoelectric photometry data obtained with T6, from Sep. 1997 to Sep. 2009. Measurements in Stromgren b & y, same comparison and check stars.

xxtrionlyspectra.tgz: All 1876 high-res spectra in plain ASCII, wavelength in Angstrom vs. intensity and error

xxtri_filling_factor_new_SB.txt: HJD vs. derived filling factor for 99 rotations

xxtri_lc_1to99_c.txt: reconstructed light curves and photocenter shifts from the Doppler-images. phase is from 0 - 2pi

xxtri_v2b-filtered.txt: For all spectra, HJD, radial velocity, Teff (plus error) log(g) (plus error), metallicity (plus error), vsin(i) (plus error), microturbulence (plus error), quality of fit

xx_tri/xxtri_vhelio.txt: HJD vs. radial velocity and its error.