[Paper Review] SOPHIE velocimetry of Kepler transit candidates. I. Detection of the low-mass white dwarf KOI-74b

D. Ehrenreich, A.‐M. Lagrange+20 more|arXiv (Cornell University)|Sep 9, 2010

Stellar, planetary, and galactic studies37 references20 citations

TL;DR

This study uses high-precision radial velocity measurements from the SOPHIE spectrograph to validate the nature of KOI-74b, a transiting compact object around an A1V star. The radial velocity variation of 18.2 ± 1.7 km/s confirms a companion mass of 0.252 ± 0.025 M☉, strongly supporting its identification as a low-mass white dwarf, thus independently validating the photometric Doppler boosting method used in prior Kepler analyses.

ABSTRACT

The Kepler mission has detected transits and occultations of a hot compact object around an early-type star, the Kepler Object of Interest KOI 74. The mass of this transiting object was photometrically assessed in a previous study using the presence of the relativistic beaming effect (so-called `Doppler boosting') in the light curve. Our aim was to provide a spectroscopic validation of this pioneering approach. We measured the radial velocity variations of the A1V star KOI 74 with the SOPHIE spectrograph at the 1.93-m telescope of the Observatoire de Haute-Provence (France). Radial velocity measurements of this star are challenging because of the high level of stellar pulsations and the few available spectral lines. Using a technique dedicated to early-type main-sequence stars, we measured radial velocity variations compatible with a companion of mass 0.252+/-0.025 Msun, in good agreement with the value derived from the Kepler light curve. This work strengthens the scenario suggesting that KOI 74 is a blue straggler orbited by a stellar core despoiled of its envelope, the low-mass white dwarf KOI 74b.

Motivation & Objective

• To spectroscopically validate the nature of KOI-74b, a transiting compact object identified via Kepler photometry.
• To measure radial velocity variations of the A1V host star KOI-74, which is challenging due to rapid rotation and weak spectral lines.
• To test the reliability of the Doppler boosting method for deriving radial velocities from photometry alone, as proposed by van Kerkwijk et al. (2010).
• To provide independent confirmation of the mass estimate for KOI-74b derived from relativistic photometric effects in the Kepler light curve.
• To establish the system as a benchmark for low-mass white dwarf evolution and atmospheric modeling.

Proposed method

• Radial velocity measurements were obtained using the SOPHIE spectrograph on the 1.93-m telescope at Observatoire de Haute-Provence.
• A specialized radial velocity extraction technique tailored for early-type main-sequence stars was applied to overcome challenges from stellar pulsations and low line density.
• The radial velocity curve was fitted to a Keplerian orbit model to derive the orbital parameters and companion mass.
• The measured radial velocity semi-amplitude K₁ = 18.2 ± 1.7 km/s was used to compute the companion mass m₂ = 0.252 ± 0.025 M☉.
• The orbital inclination i = 88.8° was fixed based on prior Kepler light curve analysis.
• The results were compared with photometric mass estimates from Doppler boosting and ellipsoidal variations to validate consistency.

Experimental results

Research questions

• RQ1 Is the companion to KOI-74 a low-mass white dwarf, as suggested by photometric Doppler boosting analysis?
• RQ2 Can radial velocity measurements from a ground-based spectrograph confirm the mass of KOI-74b derived from Kepler photometry?
• RQ3 How reliable is the Doppler boosting method for estimating radial velocities in systems with massive, compact companions?
• RQ4 Does the radial velocity solution rule out alternative explanations such as stellar blends or grazing eclipses?
• RQ5 Can this system serve as a benchmark for testing white dwarf atmospheric models and binary evolution scenarios?

Key findings

• The radial velocity semi-amplitude of KOI-74 is measured as 18.2 ± 1.7 km/s, indicating a massive companion with a high orbital inclination.
• The derived companion mass is 0.252 ± 0.025 M☉, in excellent agreement with the photometric mass estimate of 0.22 ± 0.03 M☉ from Doppler boosting.
• The system's orbital inclination is consistent with edge-on geometry (i = 88.8°), supporting the validity of the photometric modeling.
• The companion's radius is 0.0393 ± 0.0013 R☉, corresponding to a high density of 5.86 g/cm³, consistent with a white dwarf.
• The surface gravity log g = 6.65 and temperature of ~13,000 K further support the identification as a low-mass white dwarf.
• This work provides the first independent spectroscopic confirmation of the Doppler boosting method, strengthening its use in exoplanet and compact object detection.