HD 26172: an active solar-type subgiant in a close binary system

Abstract

We present the first comprehensive photometric and spectroscopic analysis of the RS CVn system HD 26172, robustly determining the previously debated evolutionary state of its primary star. Since this system is a single-lined spectroscopic binary with spot-induced light curve modulations, we derived its physical parameters by combining the TESS light curves, the radial velocity curve from our observations, and the primary-star mass estimates based on three complementary methods.Our results reveal that HD 26172 is a detached binary system composed of a $1.25 \pm 0.32 M_{\odot}$ subgiant and a $0.63 \pm 0.11 M_{\odot}$ main-sequence star. The conclusion of subgiant primary is also supported by the absence of lithium absorption and no observed infrared excess. Using long-term photometry from the KWS survey, we detected a tentative stellar activity cycle of 5635 days with an amplitude of 0.04 mag in HD 26172. Additionally, we identified ten optical flare events exhibiting temporally clustered outburst behavior. The presence of a long-term activity cycle, pronounced starspot activity, and frequent optical flares makes HD 26172 a valuable laboratory for studying magnetic activity in subgiants within close binary systems.

Figures (11)

• Figure 1: TESS light curves of HD 26172 observed across seven sectors, shown on a common time scale (HJD - 2450000). Approximate observing dates are: Sector 05 - Nov–Dec 2018; Sector 32 - Nov–Dec 2020; Sectors 42–44 - Aug–Nov 2021; and Sectors 70–71 - Sep–Nov 2023. Out-of-eclipse variability and occasional flare events are apparent (see Section \ref{['section 2.1']} for data description).
• Figure 2: Top panel: O-C diagram after linear correction, with the revised orbital period of 1.8592535(4) days. Bottom panel: Variation of the O’Connell Effect Ratio (OER; quantifying the asymmetry of the light curve) with cycle number $E$ (see Section \ref{['section 2.2']}).
• Figure 3: O-C in phase as a function of the O’Connell Effect Ratio (OER; quantifying the asymmetry of the light curve) for all points shown in Fig. \ref{['OC-OER']}. The scatter plot illustrates the relationship between OER and the corresponding O-C values. The red line represents the fitted linear model, and the Pearson correlation coefficient is $r = -0.8798$ (see Section \ref{['section 2.2']}) .
• Figure 4: Top panel: fitted spectrum of HD 26172 around phase 0.5, where the secondary star is fully eclipsed and the spectrum represents that of the primary star. The red line denotes the fitted model. Bottom panels: zoomed-in regions sensitive to $T_{\text{eff }}$ and $\log g$ (see Section \ref{['section 3.2']}).
• Figure 5: Broadening functions (BFs) of HD 26172 obtained at three different orbital phases. Each BF shows a single, well-defined peak with no sign of secondary components, confirming that the spectra are single-lined (see Section \ref{['section 3.2']}).