Kavli Affiliate: Robert F. Goeke
| First 5 Authors: Hugh P. Osborn, Grzegorz Nowak, Guillaume Hébrard, Thomas Masseron, J. Lillo-Box
| Summary:
HIP 9618 (HD 12572, TOI-1471, TIC 306263608) is a bright ($G=9.0$ mag) solar
analogue. TESS photometry revealed the star to have two candidate planets with
radii of $3.9 pm 0.044$ $R_oplus$ (HIP 9618 b) and $3.343 pm 0.039$
$R_oplus$ (HIP 9618 c). While the 20.77291 day period of HIP 9618 b was
measured unambiguously, HIP 9618 c showed only two transits separated by a
680-day gap in the time series, leaving many possibilities for the period. To
solve this issue, CHEOPS performed targeted photometry of period aliases to
attempt to recover the true period of planet c, and successfully determined the
true period to be 52.56349 d. High-resolution spectroscopy with HARPS-N, SOPHIE
and CAFE revealed a mass of $10.0 pm 3.1 M_oplus$ for HIP 9618 b, which,
according to our interior structure models, corresponds to a $6.8pm1.4%$ gas
fraction. HIP 9618 c appears to have a lower mass than HIP 9618 b, with a
3-sigma upper limit of $< 18M_oplus$. Follow-up and archival RV measurements
also reveal a clear long-term trend which, when combined with imaging and
astrometric information, reveal a low-mass companion ($0.08^{+0.12}_{-0.05}
M_odot$) orbiting at $26^{+19}_{-11}$ au. This detection makes HIP 9618 one of
only five bright ($K<8$ mag) transiting multi-planet systems known to host a
planet with $P>50$ d, opening the door for the atmospheric characterisation of
warm ($T_{rm eq}<750$ K) sub-Neptunes.
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