[Paper Review] Planets around evolved intermediate-mass stars. I. Two substellar companions in the open clusters NGC 2423 and NGC 4349

C. Lovis, M. Mayor|ArXiv.org|Jun 14, 2007

Stellar, planetary, and galactic studies50 references147 citations

TL;DR

This study presents the first radial velocity detections of a giant planet (10.6 M_Jup) and a brown dwarf (19.8 M_Jup) around evolved intermediate-mass stars in open clusters NGC 2423 and NGC 4349, using high-precision spectroscopy with HARPS and Coralie. The findings reveal a higher frequency of massive planets around stars with masses between 1.5 and 4 M☉, suggesting planet formation efficiency scales with stellar mass and protoplanetary disk mass.

ABSTRACT

Context. Many efforts are being made to characterize extrasolar planetary systems and unveil the fundamental mechanisms of planet formation. An important aspect of the problem, which remains largely unknown, is to understand how the planet formation process depends on the mass of the parent star. In particular, as most planets discovered to date orbit a solar-mass primary, little is known about planet formation around more massive stars. Aims. To investigate this point, we present first results from a radial velocity planet search around red giants in the clump of intermediate-age open clusters. We choose clusters harbouring red giants with masses between 1.5 and 4 M_sun, using the well-known cluster parameters to accurately determine the stellar masses. We are therefore exploring a poorly-known domain of primary masses, which will bring new insights into the properties of extrasolar planetary systems. Methods. We are following a sample of about 115 red giants with the Coralie and HARPS spectrographs to obtain high-precision radial velocity (RV) measurements and detect giant planets around these stars. We use bisector and activity index diagnostics to distinguish between planetary-induced RV variations and stellar photospheric jitter. Results. We present the discoveries of a giant planet and a brown dwarf in the open clusters NGC 2423 and NGC 4349, orbiting the 2.4 M_sun-star NGC2423 No3 (TYC 5409-2156-1) and the 3.9 M_sun-star NGC4349 No127 (TYC 8975-2606-1). These low-mass companions have orbital periods of 714 and 678 days and minimum masses of 10.6 and 19.8 M_jup, respectively. Combined with the other known planetary systems, these detections indicate that the frequency of massive planets is higher around intermediate-mass stars, and therefore probably scales with the mass of the protoplanetary disk.

Motivation & Objective

• Investigate planet formation around intermediate-mass stars (1.5–4 M☉), a poorly explored domain compared to solar-type stars.
• Overcome uncertainties in stellar mass estimation for field red giants by using well-constrained cluster parameters to derive accurate masses.
• Detect and characterize giant planets and brown dwarfs around evolved intermediate-mass stars using high-precision radial velocity measurements.
• Assess the dependence of planet frequency and mass distribution on the mass of the host star, particularly for massive planets.
• Distinguish planetary signals from stellar activity and photospheric jitter using bisector and activity index diagnostics.

Proposed method

• Conducted high-precision radial velocity (RV) monitoring of ~115 red giants in intermediate-age open clusters using the HARPS and Coralie spectrographs.
• Employed stellar cluster parameters (age, metallicity, distance) to determine accurate masses for red giants in the clump, with uncertainties below ±0.5 M☉.
• Applied bisector span and chromospheric activity index diagnostics to disentangle planetary-induced RV variations from intrinsic stellar jitter.
• Used a consistent detection threshold for planetary companions, including objects up to 40 M_Jup to avoid bias in the census of massive planets and brown dwarfs.
• Defined planet candidates as objects with minimum masses ≥ 13 M_Jup and orbital periods < 1000 days, excluding close binary systems.
• Computed planet frequency and average planetary system mass per stellar mass bin to assess trends across the stellar mass spectrum.

Experimental results

Research questions

• RQ1 What is the frequency of massive planets and brown dwarfs around evolved intermediate-mass stars (1.5–4 M☉) in open clusters?
• RQ2 How does the occurrence rate of giant planets depend on the mass of the host star, particularly compared to solar-type and low-mass stars?
• RQ3 Can high-precision radial velocity surveys detect planetary signals in evolved intermediate-mass stars despite intrinsic stellar variability?
• RQ4 Does the mass distribution of planetary systems scale with the mass of the host star, and what does this imply for planet formation models?
• RQ5 Are the observed trends consistent with the core-accretion or disk-instability scenario of planet formation?

Key findings

• A giant planet with a minimum mass of 10.6 M_Jup and an orbital period of 714 days was detected around the 2.4 M☉ red giant NGC2423 No3.
• A brown dwarf with a minimum mass of 19.8 M_Jup and an orbital period of 678 days was discovered around the 3.9 M☉ red giant NGC4349 No127.
• The planet frequency among intermediate-mass stars (2.5% among 200 targets) is significantly higher than the 0.5% frequency observed for solar-type stars, with a 0.3% probability of this occurring by chance.
• The average mass of planetary systems increases with host star mass, suggesting a scaling of total planetary mass with stellar mass, consistent with higher disk masses around more massive stars.
• The results challenge predictions from some core-accretion models that suggest decreasing giant planet frequency beyond 1 M☉, indicating higher formation efficiency for massive planets around intermediate-mass stars.
• The detection of massive companions in cluster environments supports the viability of radial velocity surveys for evolved intermediate-mass stars, provided stellar activity is carefully monitored and corrected.