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.}
It may interest you
-
The standard cosmological model states that massive galaxies contain a large fraction of dark matter. Dark matter is a transparent substance that does not interact through regular baryonic matter and is only detected through its gravitational pull over the stars and the gas. NGC 1277 is known as the prototype of a relic galaxy, that is, a galaxy that has not accreted other galaxies since it formed. Relic galaxies are extremely rare and are the untouched remains of the giant galaxies that populated the early Universe. Since relic galaxies are very important to understand the conditions in the
Advertised on -
CaII Kgrains, i.e., intermittent, short-lived (about 1 minute), periodic (2-4 minutes), pointlike chromospheric brightenings, are considered to be the manifestations of acoustic waves propagating upward from the solar surface and developing into shocks in the chromosphere. After the simulations of Carlsson and Stein, we know that hot shocked gas moving upward interacting with the downflowing chromospheric gas (falling down after having been displaced upward by a previous shock) nicely reproduces the spectral features of the CaII K profiles observed in such grains, i.e., a narrowband emission
Advertised on -
Stellar ages are key to several fields of astrophysics such as exoplanet research, galactic-archeology, and of course stellar physics. Obtaining the ages of stars is however not straightforward and requires stellar modeling. The most widely used technique only requires stellar colors or temperature and surface gravity, but the uncertainties are quite large. This technique is most efficient for stars belonging to clusters, as they were born from the same molecular cloud and share the same ages. In the last decades, based on the study of stellar acoustic waves, asteroseismology became the most
Advertised on