[Paper Review] Extrasolar planets and brown dwarfs around A-F type stars VI. High precision RV survey of early type dwarfs with HARPS

A.‐M. Lagrange, M. Desort+3 more|ArXiv.org|Sep 26, 2008

Stellar, planetary, and galactic studies40 references261 citations

TL;DR

This study presents the first high-precision radial velocity survey of early-type A–F dwarfs using HARPS, demonstrating that despite intrinsic stellar variability, giant planets can still be detected around these stars—especially those with low projected rotational velocity and low activity—offering new pathways to study planet formation around massive stars.

ABSTRACT

(Abridged) Aims: Systematic surveys to search for exoplanets have been mostly dedicated to solar-type stars sofar. We developed in 2004 a method to extend such searches to earlier A-F type dwarfs and started spectroscopic surveys to search for planets and quantify the detection limit achievable when taking into account the stars properties and their actual levels of intrinsic variations. We give here the first results of our southern survey with HARPS. Results: 1) 64% of the 170 stars with enough data points are found to be variable. 20 are found to be binaries or candidate binaries (with stars or brown dwarfs). More than 80% or the latest type stars (once binaries are removed) are intrinsically variable at a 2 m/s precision level. Stars with earlier spectral type (B-V <= 0.2) are either variable or associated to levels of uncertainties comparable to the RV rms observed on variable stars of same B-V. 2) We have detected one long-period planetary system around an F6IV-V star. 3) We have quantified the jitter due to stellar activity and we show that taking into account this jitter in addition to the stellar parameters, it is still possible to detect planets with HARPS with periods of 3 days (resp. 10 days and 100 days) on 91% (resp. 83%, 61%) of them. We show that even the earliest spectral type stars are accessible to this type of search, provided they have a low vsini and low levels of activity. 4) Taking into account the present data, we compute the actually achieved detection limits for 107 targets and discuss the limits as a function of B-V. Given the data at hand, our survey is sensitive to short-period (few days) planets and to longer ones (100 days) at a lower extent (latest type stars). We derive first constrains on the presence of planets around A-F stars for these ranges of periods.

Motivation & Objective

• To extend radial-velocity exoplanet surveys beyond solar-type stars to early-type A–F dwarfs, which are less explored despite theoretical expectations of higher planet masses.
• To quantify the impact of stellar activity (spots, pulsations) and rotational broadening on radial velocity measurement precision in A–F dwarfs.
• To determine the achievable detection limits for planets around these stars, considering both instrumental noise and intrinsic stellar jitter.
• To identify the most favorable targets among A–F dwarfs for future planet searches based on low activity and low vsini.
• To provide first statistical constraints on the occurrence of short-period planets (3–100 days) around early-type main-sequence stars.

Proposed method

• Conducted a high-precision radial velocity survey of 185 A–F type main-sequence stars (B–V in [−0.1; 0.6]) using the HARPS spectrograph at ESO.
• Applied dedicated software to analyze radial velocity variations, using multiple diagnostics: bisector span, amplitude, and timescale of variations to distinguish between stellar activity and planetary companions.
• Quantified stellar jitter due to spots and pulsations by analyzing the intrinsic variability of stars with sufficient data points (170 stars with adequate coverage).
• Defined detection limits as 3×rms of radial velocity scatter, accounting for both instrumental noise and intrinsic stellar variability.
• Computed actual detection limits for 107 targets based on their individual RV scatter and observational baseline, using the 3×rms threshold as a proxy for detectability.
• Excluded known pulsating stars (δ Scuti, γ Doradus) from statistical analysis to isolate non-planet-induced variability.

Experimental results

Research questions

• RQ1 Can giant planets be detected around A–F type main-sequence stars using high-precision radial velocity techniques despite high intrinsic stellar variability?
• RQ2 What is the impact of stellar activity (spots, pulsations) and projected rotational velocity on the detectability of planetary signals in early-type stars?
• RQ3 What fraction of A–F dwarfs are suitable for planet searches, given their intrinsic jitter and rotational properties?
• RQ4 What are the actual detection limits for planets with periods of 3, 10, and 100 days in this sample, and how do they vary with B–V color and stellar type?
• RQ5 How does the achieved detection sensitivity compare to the theoretical limits, and what fraction of stars are sensitive to planets of 0.5–1 M_Jup?

Key findings

• 64% of the 170 stars with sufficient data points exhibit radial velocity variability, primarily due to stellar activity or intrinsic pulsations.
• 20 stars were identified as binaries or candidate binaries, including one long-period planetary system (periods >100 days) around an F6IV–V star.
• For 3-day period planets, 91% of A–F dwarfs are detectable with HARPS if they have low vsini and low activity; for 10-day and 100-day periods, detection rates are 83% and 61%, respectively.
• Even the earliest-type stars (B–V ≤ 0.2) are accessible to planet detection if they have low projected rotational velocity and low activity levels.
• For stars with B–V ≥ 0.4, less than 5% host planets with periods of 3 days and masses ≥1 M_Jup, indicating low occurrence of hot Jupiters around late-type A–F dwarfs.
• Achieved detection limits are set by the 3×rms threshold in 107 stars with sufficient data, confirming that the method reliably captures the noise floor of each target.