[Paper Review] The CORALIE survey for southern extra-solar planets VII. Two short-period Saturnian companions to HD108147 and HD168746

F. Pepe, M. Mayor+6 more|ArXiv.org|Feb 25, 2002

Stellar, planetary, and galactic studies17 references357 citations

TL;DR

This paper reports the discovery of two low-mass exoplanets—HD 108147 b (1.34 M_Sat) and HD 168746 b (0.77 M_Sat)—using high-precision radial velocity measurements from the CORALIE spectrograph on the 1.2-m Euler telescope. The study introduces a weighted cross-correlation technique and a cleaned numerical mask to reduce photon noise and telluric residuals, improving radial velocity precision below 3 m s⁻¹, enabling the detection of sub-Saturnian planets with minimal mass down to 0.77 M_Sat.

ABSTRACT

We present the discovery of two Saturn-mass companions to HD108147 and HD168746. Both belong to the lightest ever discovered planets. The minimum mass of the companion to HD168746 is of only 0.77 the mass of Saturn and its orbital period is 6.4 days. The companion to HD108147 orbits its parent star in 10.9 days and its minimum mass is 1.34 that of Saturn. Its orbit is characterized by a high eccentricity, e=0.50, indicating possibly the presence of a second companion. The detection of Saturn-mass planets by means of the Doppler technique demands high radial-velocity measurement precision. The two new candidates were discovered by means of the CORALIE echelle spectrograph. The instrumental accuracy of CORALIE combined with the simultaneous ThAr-reference technique has reached a level better than 3m/s. On many observed objects the precision is now limited by photon noise. We present in this paper the weighted cross-correlation technique, which leads to an improvement in the photon noise of the computed radial velocity. We discuss as well a modification of the numerical cross-correlation mask which reduces significantly the residual perturbation effects produced by telluric absorption lines.

Motivation & Objective

• To detect low-mass exoplanets, particularly sub-Saturnian companions, using high-precision radial velocity measurements.
• To improve radial velocity precision by reducing photon noise and residual systematics in CORALIE data.
• To develop and implement a weighted cross-correlation method and a modified numerical cross-correlation mask to enhance signal detection.
• To confirm the planetary nature of low-amplitude radial velocity signals by ruling out stellar activity and instrumental effects.
• To demonstrate the capability of the CORALIE spectrograph for detecting lightest known exoplanets, including those below Saturn's mass.

Proposed method

• The weighted cross-correlation technique was applied to high-resolution spectra to reduce photon noise in radial velocity measurements.
• A cleaned numerical cross-correlation mask was developed to suppress residual perturbations from telluric absorption lines.
• Radial velocities were computed using the cross-correlation function (CCF) between observed spectra and a Doppler-shifted mask representing stellar absorption lines.
• The radial velocity zero-point was determined by fitting a Gaussian to the CCF minimum, with uncertainty derived from photon noise and instrumental stability.
• Orbital parameters were derived via Keplerian fitting to the radial velocity time series, with uncertainties propagated from measurement errors.
• The ThAr reference method was used to stabilize the spectrograph, enabling long-term radial velocity precision below 3 m s⁻¹.

Experimental results

Research questions

• RQ1 Can the detection of sub-Saturnian mass exoplanets be improved through enhanced radial velocity data reduction techniques?
• RQ2 To what extent does the weighted cross-correlation method reduce photon noise in radial velocity measurements?
• RQ3 How effectively does the cleaned numerical mask suppress telluric line residuals in radial velocity extraction?
• RQ4 What is the minimum detectable planetary mass using the CORALIE spectrograph with current data reduction improvements?
• RQ5 Can low-amplitude radial velocity signals from low-mass planets be distinguished from stellar activity or instrumental systematics?

Key findings

• The companion to HD 168746 has a minimum mass of 0.77 M_Sat, making it one of the lightest known exoplanets, with a radial velocity semi-amplitude of 27 ± 1 m s⁻¹ and an orbital period of 6.403 ± 0.001 days.
• The companion to HD 108147 has a minimum mass of 1.34 M_Sat, an orbital period of 10.9 days, and a high eccentricity of e = 0.50, suggesting possible dynamical interactions with a second companion.
• The weighted cross-correlation method reduced the photon noise contribution to radial velocity measurements by a factor of approximately 1.25, improving data precision.
• The use of a cleaned numerical mask significantly reduced residual perturbations from telluric absorption lines, enhancing signal fidelity.
• The radial velocity dispersion of the data for HD 168746 decreased from 11.2 m s⁻¹ to 9.8 m s⁻¹ after applying the improved data reduction, with a reduced χ² of 1.5.
• No photometric transits were detected for either planet, and radial velocity variations were confirmed to be stellar in origin, not due to activity or instrumental effects.