[Paper Review] Extrasolar planets and brown dwarfs around A-F type stars. II. A planet found with ELODIE around the F6V star HD 33564

F. Galland, A.‐M. Lagrange+5 more|ArXiv.org|Sep 6, 2005

Stellar, planetary, and galactic studies15 references246 citations

TL;DR

This paper presents the detection of a massive exoplanet around the F6V star HD 33564 using radial velocity measurements from the ELODIE spectrograph at the Haute-Provence Observatory. With a minimum mass of 9.1 M_Jup and an orbital period of 388 days at 1.1 AU, it is the first planet confirmed around a star earlier than F7, extending radial velocity planet searches to more massive A-F type stars and challenging assumptions about planetary formation around such hosts.

ABSTRACT

We present here the detection of a planet orbiting around the F6V star HD 33564. The radial velocity measurements, obtained with the ELODIE echelle spectrograph at the Haute-Provence Observatory, show a variation with a period of 388 days. Assuming a primary mass of 1.25 Mo, the best Keplerian fit to the data leads to a minimum mass of 9.1 MJup for the companion.

Motivation & Objective

• To extend radial velocity surveys for extrasolar planets and brown dwarfs to early A-F type main-sequence stars, which were previously considered inaccessible due to weak and broad spectral lines.
• To test whether planetary formation and migration processes differ around more massive stars compared to solar-type stars.
• To investigate the potential correlation between stellar mass and minimum planetary mass in the context of planet formation theories.
• To rule out stellar activity and instrumental effects as the cause of radial velocity variations in HD 33564, confirming the planetary origin of the signal.
• To contribute to the understanding of planetary system architecture and metallicity trends in early-type stars, particularly given HD 33564's low metallicity.

Proposed method

• Acquired 15 high-S/N (≈150) ELODIE echelle spectroscopic observations of HD 33564 over 417 days using the simultaneous thorium-argon reference method to correct for instrumental drift.
• Measured radial velocities using a cross-correlation technique based on the method of Chelli (2000), with an average uncertainty of 11 m s⁻¹.
• Fitted the radial velocity data to a Keplerian orbital model to determine orbital parameters, including period, eccentricity, and minimum planetary mass.
• Assessed the impact of orbital eccentricity on mass uncertainty by testing a range of eccentricity values (0.28–0.40), yielding a 6% uncertainty in the minimum mass.
• Compared the bisector span of the cross-correlation function and Ca II H & K line emission with those of an active F7V star (HD 25998) to rule out stellar activity as the source of variability.
• Evaluated the star's chromospheric activity using the log(R'_HK) index, confirming it as inactive (log(R'_HK) = -4.95), inconsistent with activity-induced radial velocity signals.

Experimental results

Research questions

• RQ1 Can radial velocity techniques detect planets around early A-F type stars, despite their weak and broad spectral lines?
• RQ2 Is the observed radial velocity variation in HD 33564 due to a planetary companion or stellar activity?
• RQ3 What is the minimum mass and orbital period of the companion around HD 33564, and how does it compare to known planets around similar stars?
• RQ4 Does the low metallicity of HD 33564 contradict known trends in planet-hosting stars, and what does this imply for planet formation models?
• RQ5 Does the mass of the host star correlate with the minimum mass of its planets, particularly in the context of gravitational instability versus core accretion formation mechanisms?

Key findings

• A planet with a minimum mass of 9.1 M_Jup was detected around the F6V star HD 33564 using radial velocity measurements from the ELODIE spectrograph.
• The orbital period of the companion is 388 ± 3 days, corresponding to a semi-major axis of 1.1 AU, assuming a primary mass of 1.25 M_☉.
• The orbital solution has an eccentricity of 0.34, and the residuals from the Keplerian fit have a dispersion of 7 m s⁻¹, indicating a good fit.
• The radial velocity variations are not caused by stellar activity, as confirmed by a flat bisector span and low chromospheric activity (log(R'_HK) = -4.95), which is inconsistent with spot-induced signals.
• The companion is not detectable with the CORAVEL spectrograph due to its low amplitude (less than 250 m s⁻¹), confirming the need for high-precision instruments like ELODIE.
• HD 33564 is the earliest spectral type star (F6V) known to host a planet with a minimum mass in the planetary domain (≥ 0.4 M_Jup), marking a significant extension of radial velocity planet searches to more massive stars.