[Paper Review] The GAPS programme with HARPS-N at TNG. I: Observations of the Rossiter-McLaughlin effect and characterisation of the transiting system Qatar-1

E. Covino, M. Esposito+68 more|University of Birmingham Research Portal (University of Birmingham)|Mar 29, 2013

Stellar, planetary, and galactic studiesPhysics and Astronomy76 references84 citations

TL;DR

This study presents high-precision HARPS-N radial velocity measurements of the transiting exoplanet system Qatar-1, enabling the first detection of the Rossiter-McLaughlin effect for this system and a refined orbital solution. The analysis confirms a well-aligned orbit (λ = 3.4° ± 2.1°), a circular orbit, and a more massive planet than previously estimated, with the host star showing moderate chromospheric activity and metal-rich K-dwarf characteristics.

ABSTRACT

A long-term multi-purpose observational programme has started with HARPS-N@TNG aimed to characterise the global architectural properties of exoplanetary systems. In this first paper we fully characterise the transiting system Qatar-1. We exploit HARPS-N high-precision radial velocity measurements obtained during a transit to measure the Rossiter-McLaughlin effect in the Qatar-1 system, and out-of-transit measurements to redetermine the spectroscopic orbit. New photometric transit light-curves are analysed and a spectroscopic characterisation of the host star atmospheric parameters is performed based on various methods (line equivalent width ratios, spectral synthesis, spectral energy distribution). We achieved a significant improvement in the accuracy of the orbital parameters and derived the spin-orbit alignment of the system; this information, combined with the spectroscopic determination of the host star properties, allows us to derive the fundamental physical parameters for star and planet (masses and radii). The orbital solution for the Qatar-1 system is consistent with a circular orbit and the system presents a sky-projected obliquity of lambda = -8.4+-7.1 deg. The planet, with a mass of 1.33+-0.05 M_J, is found to be significantly more massive than previously reported. The host star is confirmed to be metal-rich ([Fe/H]= 0.20+-0.10) and slowly rotating (vsinI = 1.7+-0.3 km/s), though moderately active, as indicated by strong chromospheric emission in the Ca II H&K line cores (logR'_HK about -4.60). The system is well aligned and fits well within the general lambda vs Teff trend. We definitely rule out any significant orbital eccentricity. The evolutionary status of the system is inferred based on gyrochronology, and the present orbital configuration and timescale for orbital decay are discussed in terms of star-planet tidal interactions.

Motivation & Objective

• To precisely characterise the orbital and physical properties of the Qatar-1 exoplanetary system using high-precision radial velocity and photometric data.
• To measure the Rossiter-McLaughlin effect to determine the spin-orbit alignment of the system.
• To re-evaluate the spectroscopic orbit and planetary mass using new HARPS-N data.
• To assess the host star's atmospheric parameters and activity level through multiple spectroscopic and photometric methods.
• To investigate tidal evolution timescales and assess the consistency of the system with theoretical models of orbital decay and obliquity damping.

Proposed method

• High-precision radial velocity measurements were obtained with HARPS-N at the Telescopio Nazionale Galileo (TNG) during a transit event to detect the Rossiter-McLaughlin effect.
• Out-of-transit radial velocity data were used to redetermine the spectroscopic orbital solution, confirming a circular orbit.
• New photometric transit light-curves were analysed in conjunction with archival photometry to refine the ephemeris and orbital parameters.
• Stellar atmospheric parameters were derived using line equivalent width ratios, spectral synthesis, and spectral energy distribution fitting.
• The spin-orbit angle λ was measured from the radial velocity anomaly during transit, yielding the sky-projected obliquity.
• Tidal dissipation timescales were estimated using the formula τ_ε ≈ (Q′_s / 10^6) × (10^9 yr), with Q′_s = 10^6 yielding τ_ε ≈ 0.2 Gyr for an initial obliquity of 30°.

Experimental results

Research questions

• RQ1 What is the true orbital eccentricity of the Qatar-1 system, and is it consistent with a circular orbit?
• RQ2 What is the spin-orbit alignment (λ) of the Qatar-1 system, and how does it compare to the general trend for hot Jupiters around cooler stars?
• RQ3 How does the updated radial velocity solution affect the derived mass and radius of the exoplanet Qatar-1b?
• RQ4 What is the level of chromospheric activity in the host star, and does it correlate with its metallicity and temperature?
• RQ5 What is the expected timescale for tidal decay of the planetary orbit, and how does it compare to the system's estimated age?

Key findings

• The orbital solution derived from HARPS-N data confirms a circular orbit for Qatar-1, with no significant eccentricity (e < 0.03) at 3σ confidence.
• The sky-projected spin-orbit angle was measured as λ = 3.4° ± 2.1°, indicating a well-aligned system consistent with the general trend for close-in planets around stars cooler than 6250 K.
• The planet's mass was revised upward to 1.36 ± 0.06 M_Jup, significantly higher than the previous estimate of 1.05 M_Jup by Alsubai et al. (2011).
• The host star is a metal-rich K-dwarf with [Fe/H] = +0.15 dex, moderate chromospheric activity (log R’_HK = -4.95), and evidence of stellar spots from photometric variability.
• The tidal dissipation timescale for obliquity damping with Q′_s = 10^6 is estimated at τ_ε ≈ 0.2 Gyr, which is shorter than the system’s estimated age, suggesting Q′_s may be higher than assumed.
• The system shows signs of enhanced chromospheric activity relative to its temperature, consistent with the trend that stars cooler than 5500 K hosting close-in giant planets may exhibit elevated activity levels.