A low-density hot Jupiter in a near-aligned, 4.5-day orbit around a $V$ = 10.8, F5V star

Abstract

We report the independent discovery and characterisation of a hot Jupiter in a 4.5-d, transiting orbit around the star TYC 7282-1298-1 ($V$ = 10.8, F5V). The planet has been pursued by the NGTS team as NGTS-2b and by ourselves as WASP-179b. We characterised the system using a combination of photometry from WASP-South and TRAPPIST-South, and spectra from CORALIE (around the orbit) and HARPS (through the transit). We find the planet's orbit to be nearly aligned with its star's spin. From a detection of the Rossiter-McLaughlin effect, we measure a projected stellar obliquity of $\lambda =-19\pm 6^{\circ }$. From line-profile tomography of the same spectra, we measure $\lambda =-11\pm 5^{\circ }$. We find the planet to have a low density ($M_{\mathrm{P}}$ = 0.67 $\pm$ 0.09 $M_{\mathrm{Jup}}$, $R_{\mathrm{P}}$ = 1.54 $\pm$ 0.06 $R_{\mathrm{Jup}}$), which, along with its moderately bright host star, makes it a good target for transmission spectroscopy. We find a lower stellar mass ($M_{\ast }$ = $1.30\pm 0.07$ $M_{\odot }$) than reported by the NGTS team ($M_{\ast }$ = $1.64\pm 0.21$ $M_{\odot }$), though the difference is only $1.5$ $\sigma$.

Submitted to AJ. 9 pages, 6 figures, 5 tables