The measurement of the Rossiter-McLaughlin effect for transiting exoplanetsplaces constraints on the orientation of the orbital axis with respect to the stellar spin axis, which can shed light on the mechanisms shaping the orbital configuration of planetary systems. Here we present the interesting case of the Saturn-mass planet HAT-P-18b, which orbits one of the coolest stars for which the Rossiter-McLaughlin effect has been measured so far. We acquired a spectroscopic time-series, spanning a full transit, with the HARPS-N spectrograph mounted at the TNG telescope. The very precise radial velocity measurements delivered by the HARPS-N pipeline were used to measure the Rossiter-McLaughlin effect. Complementary new photometric observations of another full transit were also analysed to obtain an independent determination of the star and planet parameters. We find that HAT-P18b lies on a counter-rotating orbit, the sky-projected angle between the stellar spin axis and the planet orbital axis being λ=132 ± 15 deg. By joint modelling of the radial velocity and photometric data we obtain new determinations of the star (M* = 0.770 ± 0.027 MSun; R* = 0.717 ± 0.026 RSun; Vsin(I*) = 1.58 ± 0.18 km s-1) and planet (Mp = 0.196 ± 0.008 MJ ; Rp = 0.947 ± 0.044 RJ) parameters. Ourspectra provide for the host star an effective temperature Teff = 4870 ± 50 K, a surface gravity of log g* = 4.57± 0.07 cm s-2 , and an iron abundance of [Fe/H] = 0.10 ± 0.06. HAT-P-18b is one of the few planets known to transit a star with Teff <6250 K on a retrograde orbit. Objects such as HAT-P-18b (low planet mass and/orrelatively long orbital period) most likely have a weak tidal coupling with their parent stars, therefore their orbits preserve any original misalignment. As such, they are ideal targets to study the causes of orbital evolution in cool main-sequence stars.}
Advertised on
References
It may interest you
-
The cosmic evolution of the barred galaxy population provides key information about the secular evolution of galaxies and the settling of rotationally dominated discs. We study the bar fraction in the SMACSJ0723.37323 (SMACS0723) cluster of galaxies at z = 0.39 using the Early Release Observations obtained with the NIRCam instrument mounted on the JWST telescope. We visually inspected all cluster member galaxies using the images from the NIRCam F200W filter. We classified the galaxies into ellipticals and discs and determine the presence of a bar. The cluster member selection was based on aAdvertised on
-
The development of the latest generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) over recent decades has led to the discovery of new extreme astrophysical phenomena in the very-high-energy (VHE, E > 100 GeV) gamma-ray regime. Time-domain and multi-messenger astronomy are inevitably connected to the physics of transient VHE emitters, which show unexpected (and mostly unpredictable) flaring or exploding episodes at different timescales. These transients often share the physical processes responsible for the production of the gamma-ray emission, through cosmic-ray accelerationAdvertised on
-
The formation and evolution of the disk of our Galaxy, the Milky Way, remains an enigma in astronomy. In particular, the relationship between the thick disk and the thin disk —two key components of the Milky Way— is still unclear. Understanding the chemical and dynamical properties of the stars within these disks is crucial, especially in the parameter spaces where their characteristics overlap, such the metallicity regime around [Fe/H] ~ -0.7, which marks the metal-poor end of the thin disk, higher than that of the thick disk. This is often interpreted as an indication that the thin diskAdvertised on