The Eclipsing Binary V1061 Cygni: Confronting Stellar Evolution Models for Active and Inactive Solar‐Type Stars

Abstract

We present spectroscopic and photometric observations of the eclipsing system V1061 Cyg (P ¼ 2:35 days). A third star is visible in the spectrum, and the system is a hierarchical triple. We combine the radial velocities for the three stars, times of eclipse, and intermediate astrometric data from the Hipparcos mission (abscissa residuals) to establish the elements of the outer orbit, which is eccentric and has a period of 15.8 yr. We determine accurate values for the masses, radii, and effective temperatures of the binary components: M Aa ¼ 1:282 AE 0:015 M , R Aa ¼ 1:615 AE 0:017 R , and T Aa eA ¼ 6180 AE 100 K for the primary (star Aa), and M Ab ¼ 0:9315 AE 0:0068 M , R Ab ¼ 0:974 AE 0:020 R , and T Ab eA ¼ 5300 AE 150 K for the secondary (Ab). The mass of the tertiary is determined to be M B ¼ 0:925 AE 0:036 M and its effective temperature is T B eA ¼ 5670 AE 150 K. Current stellar evolution models agree well with the properties of the primary but show a very large discrepancy in the radius of the secondary, in the sense that the predicted values are $10% smaller than observed (a $5 effect). In addition, the temperature is cooler than predicted, by some 200 K. These discrepancies are quite remarkable given that the star is only 7% less massive than the Sun, the calibration point of all stellar models. We identify the chromospheric activity as the likely cause of the effect. Inactive stars agree very well with the models, while active ones such as V1061 Cyg Ab appear systematically too large and too cool.

O-C Analysis of Selected 3-Body Systems

“…For V1061 Cyg the 3-body effect described by Torres et al (2006) is confirmed by recent observations. The period changes of XZ Per were described by Qian (2001) as the result of mass transfer, but recent observations and more complete historic data confirm a 3-body model of the system.…”

Absolute Properties of the Spotted Eclipsing Binary Star Cv Boötis

“…Spot coverage can produce similar effects. Theoretical and observational evidence for the conservation of the luminosity in these systems has been presented by Delfosse et al (2000), Mullan & MacDonald (2001), Torres & Ribas (2002), Ribas (2006), Torres et al (2006), Chabrier et al (2007), and others (see also Morales et al 2008). We do not see any obvious discrepancy in the temperature of CV Boo B compared to models, although our uncertainties are large enough that the effect may be masked.…”

“…However, the recent study by Torres et al (2006) pointed out that the problem is not confined to the lower-mass stars, but extends to active objects approaching 1 M , such as V1061 Cyg Ab, with M = 0.93 M . CV Boo B has an even larger mass of 0.968 M , and also appears to be oversized.…”

The effect of activity on stellar temperatures and radii

“…Using the available data and analysing the possible scenarios, Ribas (2006b) and Torres et al (2006) concluded that a plausible explanation for the observed discrepancies could be related to stellar activity. In this respect, the active components of eclipsing binaries appear to be larger and cooler than inactive single stars (i.e., those reproduced by theoretical models) while keeping similar luminosity.…”