HD 26172: RS CVn Binary & Stellar Magnetism

• HD 26172 is a detached RS CVn binary system consisting of a solar-type subgiant primary and a main-sequence secondary with a near-circular orbit.
• Joint TESS photometry and radial velocity analyses deliver precise orbital parameters and stellar masses, enabling detailed modeling of starspot activity.
• Its pronounced magnetic cycle and high-latitude starspots offer key empirical constraints for dynamo models in evolved, tidally interacting binaries.

HD 26172 is a detached, single-lined spectroscopic binary of the RS CVn class, consisting of a solar-type subgiant primary and a main-sequence secondary. Recent observational campaigns have provided the first comprehensive determination of its physical parameters, evolutionary status, and magnetic activity characteristics, firmly establishing its significance as a laboratory for stellar magnetism in evolved close binaries (Meng et al., 20 Jan 2026).

1. System Architecture and Orbital Parameters

HD 26172 is characterized by a short orbital period and pronounced tidal interactions. Joint analysis of TESS eclipse photometry and ground-based radial velocity measurements yields an orbital period

$P = 1.8592535(4)\;\mathrm{d}$

and a nearly circular orbit (eccentricity $e \approx 0$) with high inclination

$i = 87.97 \pm 0.98^\circ.$

The semi–major axis, derived from Kepler's third law and the observed masses, is

$a = 7.85 \pm 0.59\,R_\odot.$

The system's single-lined nature enables a determination of the primary's radial-velocity semi-amplitude

$K_1 = 71.20 \pm 2.77\,\mathrm{km\,s^{-1}}$

and a mass function

$f(m) \approx 0.044\,M_\odot.$

Three independent techniques (isochrone fitting, SED fitting, luminosity constraints) consistently yield a primary mass

$M_1 = 1.25 \pm 0.32\,M_\odot$

and, using the inferred mass ratio $q = 0.50 \pm 0.05$,

$M_2 = 0.63 \pm 0.11\,M_\odot.$

Parameter	Value (Primary)	Value (Secondary)
Mass ($M$)	$1.25 \pm 0.32\,M_\odot$	$0.63 \pm 0.11\,M_\odot$
Radius ($R$)	$2.08 \pm 0.16\,R_\odot$	$0.65 \pm 0.05\,R_\odot$
$T_\mathrm{eff}$ (K)	$5850 \pm 28$	$4129 \pm 20$
Luminosity ($L$)	$4.54 \pm 0.70\,L_\odot$	$0.11 \pm 0.02\,L_\odot$

2. Evolutionary Status and Field Membership

Despite its location near Taurus–Auriga, multiple lines of evidence exclude pre-main-sequence (PMS) status. High-resolution spectroscopy shows no lithium absorption near 6708 Å, inconsistent with PMS retention. The full SED from Gaia, APASS, 2MASS, and WISE exhibits no infrared excess, indicating no substantial circumstellar material. Isochrone fitting (PARSEC) to $(T_{\mathrm{eff}}, \log g, [\mathrm{Fe/H}])$ locates the primary on the subgiant branch at an age $\sim4.2$ Gyr, with

$$M_1 = 1.26^{+0.10}_{-0.12}\,M_\odot,\quad R_1 = 2.26^{+0.36}_{-0.29}\,R_\odot.$$

Bayesian assignment (BANYAN Σ) returns only 13.8% likelihood of Taurus membership and 86.2% for the field, supporting post–main–sequence evolution.

3. Photometric Variability and Starspot Modeling

TESS photometry across seven sectors (S05, 32, 42–44, 70–71) reveals deep eclipses and out-of-eclipse rotational modulations up to 0.1 mag, with strong indications of starspot activity. Analysis of eclipse timing (O–C variations) shows high anticorrelation ($r\approx -0.85$) with the O’Connell-effect ratio, confirming starspots rather than true orbital period variation as the dominant mechanism.

Wilson–Devinney light-curve modeling (detached mode, $q=0.5$, $T_1=5850$ K, $g_{1,2}=0.32$, $A_{1,2}=0.5$, van Hamme limb darkening) robustly reproduces light curve asymmetries via two cool spots on the primary. The spot temperature contrast is fixed at $T_{\mathrm{spot}}/T_1=0.85$, with sector-to-sector variations in spot position and size. The principal spots are located at high latitudes (near-polar), a hallmark of RS CVn subgiants and indicative of large-scale dynamo activity.

4. Magnetic Activity Cycles and Flaring Phenomena

Long-term KWS ($HJD\,2455516$–$2460635$) V-band photometry reveals a quasi-sinusoidal "activity cycle" of period

$$P_{\rm cycle} = 5635 \pm 366\,\mathrm{d} \approx 15.4\,\mathrm{yr}$$

with amplitude $A = 0.040 \pm 0.003$ Vmag. The modulation resembles the solar 11-year cycle and those found in other RS CVn systems.

Spectroscopy demonstrates persistent core emission in Ca II H & K, with moderate H$\alpha$ filling—characteristic of strong, spatially extended chromospheres. Ten optical flares are detected (TESS Sectors 43–44, 70–71), with amplitudes $\Delta m_{\rm max} \approx 7$–36 mmag, rise times 7–34 min, and decay times 45–178 min. Estimated energies range from $E_{\rm flare} \sim 3\times10^{34}$ erg to $2\times10^{35}$ erg, with occurrence rate $\sim0.4\%$. All flares are temporally clustered near long-term cycle brightness maxima, suggesting enhanced magnetic reconnection at cycle peak.

5. Theoretical Significance and Future Directions

The combination of a solar-type subgiant in a close, tidally locked binary with rapid rotation enables HD 26172 to exhibit the full suite of pronounced RS CVn phenomena: large, high-latitude starspots, a long-duration magnetic cycle, frequent and energetic flaring, and sustained chromospheric activity. The amplitude and timescale of its spot and cycle behavior provide critical constraints for dynamo models operating in evolved convective envelopes under tidal synchronization.

HD 26172 serves as a testbed for empirical calibration of dynamo timescales, spot properties and evolutionary transitions, direct comparison with dynamo amplification mechanisms, and the interplay between rotation, convection, and activity in post-main-sequence stars. Continued ultraviolet and X-ray monitoring and Doppler imaging are projected to refine inferences on spot distribution, cycle variability, and magnetic topology, with broader applicability to the understanding of active binaries and RS CVn archetypes (Meng et al., 20 Jan 2026).