A pair of Sub-Neptunes transiting the bright K-dwarf TOI-1064 characterised with CHEOPS

We report the discovery and characterisation of a pair of sub-Neptunes
transiting the bright K-dwarf TOI-1064 (TIC 79748331), initially detected in
TESS photometry. To characterise the system, we performed and retrieved CHEOPS,
TESS, and ground-based photometry, HARPS high-resolution spectroscopy, and
Gemini speckle imaging. We characterise the host star and determine $T_{\rm
eff, \star}=4734\pm67$ K, $R_{\star}=0.726\pm0.007$ $R_{\odot}$, and
$M_{\star}=0.748\pm0.032$ $M_{\odot}$. We present a novel detrending method
based on PSF shape-change modelling and demonstrate its suitability to correct
flux variations in CHEOPS data. We confirm the planetary nature of both bodies
and find that TOI-1064 b has an orbital period of $P_{\rm b}=6.44387\pm0.00003$
d, a radius of $R_{\rm b}=2.59\pm0.04$ $R_{\oplus}$, and a mass of $M_{\rm
b}=13.5_{-1.8}^{+1.7}$ $M_{\oplus}$, whilst TOI-1064 c has an orbital period of
$P_{\rm c}=12.22657^{+0.00005}_{-0.00004}$ d, a radius of $R_{\rm
c}=2.65\pm0.04$ $R_{\oplus}$, and a 3$\sigma$ upper mass limit of 8.5 ${\rm
M_{\oplus}}$. From the high-precision photometry we obtain radius uncertainties
of $\sim$1.6%, allowing us to conduct internal structure and atmospheric escape
modelling. TOI-1064 b is one of the densest, well-characterised sub-Neptunes,
with a tenuous atmosphere that can be explained by the loss of a primordial
envelope following migration through the protoplanetary disc. It is likely that
TOI-1064 c has an extended atmosphere due to the tentative low density, however
further RVs are needed to confirm this scenario and the similar radii,
different masses nature of this system. The high-precision data and modelling
of TOI-1064 b are important for planets in this region of mass-radius space,
and it allows us to identify a trend in bulk density-stellar metallicity for
massive sub-Neptunes that may hint at the formation of this population of
planets.