[Paper Review] Binarity of Transit Host Stars - Implications on Planetary Parameters

S. Daemgen, Felix Hormuth+5 more|Feb 12, 2009

Stellar, planetary, and galactic studiesPhysics and Astronomy34 references68 citations

TL;DR

This study identifies previously undetected stellar companions to three transiting exoplanet hosts—WASP-2, TrES-2, and TrES-4—using high-resolution Lucky Imaging at the Calar Alto Observatory. Revised planetary parameters, including radius and orbital distance, differ by up to 2σ from prior single-star assumptions, suggesting binarity significantly impacts exoplanet characterization and potentially planet formation processes.

ABSTRACT

Straight-forward derivation of planetary parameters can only be achieved in transiting planetary systems. However, planetary attributes such as radius and mass strongly depend on stellar host parameters. Discovering a transit host star to be multiple leads to a necessary revision of the derived stellar and planetary parameters. Based on our observations of 14 transiting exoplanet hosts, we derive parameters of the individual components of three transit host stars (WASP-2, TrES-2, and TrES-4) which we detected to be binaries. Two of these have not been known to be multiple before. Parameters of the corresponding exoplanets are revised. High-resolution "Lucky Imaging" with AstraLux at the 2.2m Calar Alto telescope provided near diffraction limited images in i' and z' passbands. These results have been combined with existing planetary data in order to recalibrate planetary attributes. Despite the faintness (delta mag ~ 4) of the discovered stellar companions to TrES-2, TrES-4, and WASP-2, light-curve deduced parameters change by up to more than 1sigma. We discuss a possible relation between binary separation and planetary properties, which - if confirmed - could hint at the influence of binarity on the planet formation process.

Motivation & Objective

• Investigate the multiplicity of transiting exoplanet host stars to assess the impact of undetected companions on derived planetary parameters.
• Address the risk of parameter bias in exoplanet characterization when assuming single-star systems despite potential close, faint stellar companions.
• Use high-angular-resolution imaging to detect and characterize previously unknown binary companions to known transit hosts.
• Re-evaluate planetary and stellar parameters—such as radius, mass, and orbital distance—by incorporating binary system effects.
• Explore potential correlations between binary separation and planetary system class (e.g., Safronov number) to assess dynamical influences on planet formation.

Proposed method

• Employed high-resolution Lucky Imaging with the AstraLux instrument on the 2.2 m telescope at Calar Alto Observatory to achieve near-diffraction-limited imaging in i′ and z′ bands.
• Analyzed astrometry and photometry of detected companions to determine their magnitude, separation, and spectral type (K7 to M0.5), assuming physical binding.
• Combined new binary parameters with existing transit light curves and spectroscopic data to recalibrate planetary and stellar parameters.
• Used the Safronov number (Θ) and equilibrium temperature (Teq) to classify planetary systems and assess correlations with binary separation.
• Evaluated parameter uncertainties (σ) to quantify shifts in planetary parameters (e.g., Rp/R∗, a/R∗, b, ρ∗) due to binarity.
• Compared revised parameters to prior single-star assumptions, identifying shifts exceeding 2σ in several cases.

Experimental results

Research questions

• RQ1 To what extent do undetected stellar companions in transit host stars bias the derived planetary parameters such as radius and orbital distance?
• RQ2 How do revised stellar and planetary parameters change when binarity is accounted for in systems like WASP-2, TrES-2, and TrES-4?
• RQ3 Is there a correlation between binary separation and planetary system classification (e.g., Class I vs. Class II based on Safronov number)?
• RQ4 Can high-resolution imaging techniques like Lucky Imaging detect faint, close companions that remain undetected in radial velocity or photometric surveys?
• RQ5 Does the presence of a stellar companion influence the formation or dynamical evolution of close-in exoplanets?

Key findings

• Three previously unknown stellar companions were detected to WASP-2, TrES-2, and TrES-4, with projected separations between 100 and 750 AU and magnitudes differing by ∼4 mag.
• Planetary parameters such as Rp/R∗, a/R∗, impact parameter b, and stellar density ρ∗ were revised, with changes exceeding 2σ in several cases.
• The exoplanet around TrES-4b, previously classified as Class II (Θ ≈ 0.04), remains in Class II, while WASP-2b and TrES-2b remain in Class I (Θ ≈ 0.07), despite revised parameters.
• A tentative correlation was found between binary separation and planetary class: systems with separations of 110–230 AU host Class I planets, while wider binaries (≥750 AU) host Class II planets.
• The Safronov number (Θ) of the three newly resolved binaries ranges from 0.05 to 0.12, with no significant shift in classification despite parameter updates.
• The study confirms that undetected companions can significantly alter planetary parameter estimates, emphasizing the need for multiplicity surveys in exoplanet host stars.