Context. Hot subdwarfs experienced strong mass loss on the red giant branch (RGB) and are now hot and small He-burning objects. These stars constitute excellent opportunities for addressing the ...question of the evolution of exoplanetary systems directly after the RGB phase of evolution. Aims. In this project we aim to perform a transit survey in all available light curves of hot subdwarfs from space-based telescopes (Kepler, K2, TESS, and CHEOPS) with our custom-made pipeline SHERLOCK in order to determine the occurrence rate of planets around these stars as a function of orbital period and planetary radius. We also aim to determine whether planets that were previously engulfed in the envelope of their red giant host star can survive, even partially, as a planetary remnant. Methods. For this first paper, we performed injection-and-recovery tests of synthetic transits for a selection of representative Kepler, K2, and TESS light curves to determine which transiting bodies in terms of object radius and orbital period we will be able to detect with our tools. We also provide estimates for CHEOPS data, which we analyzed with the pycheops package. Results. Transiting objects with a radius ≲1.0 R⊕ can be detected in most of the Kepler, K2, and CHEOPS targets for the shortest orbital periods (1 d and shorter), reaching values as low as ~0.3 R⊕ in the best cases. Sub-Earth-sized bodies are only reached for the brightest TESS targets and for those that were observed in a significant number of sectors. We also give a series of representative results for larger planets at greater distances, which strongly depend on the target magnitude and on the length and quality of the data. Conclusions. The TESS sample will provide the most important statistics for the global aim of measuring the planet occurrence rate around hot subdwarfs. The Kepler, K2, and CHEOPS data will allow us to search for planetary remnants, that is, very close and small (possibly disintegrating) objects.
Context
. TOI-732 is an M dwarf hosting two transiting planets that are located on the two opposite sides of the radius valley. Inferring a reliable demographics for this type of systems is key to ...understanding their formation and evolution mechanisms.
Aims
. By doubling the number of available space-based observations and increasing the number of radial velocity (RV) measurements, we aim at refining the parameters of TOI-732 b and c. We also use the results to study the slope of the radius valley and the density valley for a well-characterised sample of M-dwarf exoplanets.
Methods
. We performed a global Markov chain Monte Carlo analysis by jointly modelling ground-based light curves and CHEOPS and TESS observations, along with RV time series both taken from the literature and obtained with the MAROON-X spectrograph. The slopes of the M-dwarf valleys were quantified via a support vector machine (SVM) procedure.
Results
. TOI-732b is an ultrashort-period planet (
P
= 0.76837931
-0.00000042
+0.0000039
days) with a radius
R
b
= 1.325
-0.058
+0.057
R
⊕
, a mass
M
b
= 2.46 ± 0.19
M
⊕
, and thus a mean density
ρ
b
= 5.8
-0.8
+1.0
g cm
-3
, while the outer planet at
P
= 12.252284 ± 0.000013 days has
R
c
= 2.39
-0.11
+0.10
R
⊕
,
M
c
= 8.04
-0.48
+0.50
M
⊕
, and thus
ρ
c
= 3.24
-0.43
+0.55
g cm
-3
. Even with respect to the most recently reported values, this work yields uncertainties on the transit depths and on the RV semi-amplitudes that are smaller up to a factor of ~1.6 and ~2.4 for TOI-732 b and c, respectively. Our calculations for the interior structure and the location of the planets in the mass-radius diagram lead us to classify TOI-732 b as a super-Earth and TOI-732 c as a mini-Neptune. Following the SVM approach, we quantified d log
R
p,valley
/ d log
P
= -0.065
-0.013
+0.024
, which is flatter than for Sun-like stars. In line with former analyses, we note that the radius valley for M-dwarf planets is more densely populated, and we further quantify the slope of the density valley as d log ρ^
valley
/ d log
P
= -0.02
-0.04
+0.12
.
Conclusions
. Compared to FGK stars, the weaker dependence of the position of the radius valley on the orbital period might indicate that the formation shapes the radius valley around M dwarfs more strongly than the evolution mechanisms.
TOI-732 is an M dwarf hosting two transiting planets that are located on the two opposite sides of the radius valley. By doubling the number of available space-based observations and increasing the ...number of radial velocity (RV) measurements, we aim at refining the parameters of TOI-732 b and c. We also use the results to study the slope of the radius valley and the density valley for a well-characterised sample of M-dwarf exoplanets. We performed a global MCMC analysis by jointly modelling ground-based light curves and CHEOPS and TESS observations, along with RV time series both taken from the literature and obtained with the MAROON-X spectrograph. The slopes of the M-dwarf valleys were quantified via a Support Vector Machine (SVM) procedure. TOI-732 b is an ultrashort-period planet (\(P\sim0.77\) d) with a radius \(R_b=1.325_{-0.058}^{+0.057}\) \(R_{\oplus}\) and a mass \(M_b=2.46\pm0.19\) \(M_{\oplus}\) (mean density \(\rho_b=5.8_{-0.8}^{+1.0}\) g cm\(^{-3}\)), while the outer planet at \(P\sim12.25\) d has \(R_c=2.39_{-0.11}^{+0.10}\) \(R_{\oplus}\), \(M_c=8.04_{-0.48}^{+0.50}\) \(M_{\oplus}\), and thus \(\rho_c=3.24_{-0.43}^{+0.55}\) g cm\(^{-3}\). Also taking into account our interior structure calculations, TOI-732 b is a super-Earth and TOI-732 c is a mini-Neptune. Following the SVM approach, we quantified \(\mathrm{d}\log{R_{p,{\mathrm{valley}}}/\mathrm{d}\log{P}=-0.065_{-0.013}^{+0.024}\), which is flatter than for Sun-like stars. In line with former analyses, we note that the radius valley for M-dwarf planets is more densely populated, and we further quantify the slope of the density valley as \(\mathrm{d}\log{\hat{\rho}_{\mathrm{valley}}}/\mathrm{d}\log{P}=-0.02_{-0.04}^{+0.12}\). Compared to FGK stars, the weaker dependence of the position of the radius valley on the orbital period might indicate that the formation shapes the radius valley around M dwarfs more strongly than the evolution mechanisms.
The HD108236 system was first announced with the detection of four small planets based on TESS data. Shortly after, the transit of an additional planet with a period of 29.54d was serendipitously ...detected by CHEOPS. In this way, HD108236 (V=9.2) became one of the brightest stars known to host five small transiting planets (R\(_p\)<3R\(_{\oplus}\)). We characterize the planetary system by using all the data available from CHEOPS and TESS space missions. We use the flexible pointing capabilities of CHEOPS to follow up the transits of all the planets in the system, including the fifth transiting body. After updating the host star parameters by using the results from Gaia eDR3, we analyzed 16 and 43 transits observed by CHEOPS and TESS, respectively, to derive the planets physical and orbital parameters. We carried out a timing analysis of the transits of each of the planets of HD108236 to search for the presence of transit timing variations. We derived improved values for the radius and mass of the host star (R\(_{\star}\)=0.876\(\pm\)0.007 R\(_{\odot}\) and M\(_{\star}\)=0.867\(_{-0.046}^{+0.047}\) M\(_{\odot}\)). We confirm the presence of the fifth transiting planet f in a 29.54d orbit. Thus, the system consists of five planets of R\(_b\)=1.587\(\pm\)0.028, R\(_c\)=2.122\(\pm\)0.025, R\(_d\)=2.629\(\pm\)0.031, R\(_e\)=3.008\(\pm\)0.032, and R\(_f\)=1.89\(\pm\)0.04 R\(_{\oplus}\). We refine the transit ephemeris for each planet and find no significant transit timing variations for planets c, d, and e. For planets b and f, instead, we measure significant deviations on their transit times (up to 22 and 28 min, respectively) with a non-negligible dispersion of 9.6 and 12.6 min in their time residuals. We confirm the presence of planet f and find no significant evidence for a potential transiting planet in a 10.9d orbital period, as previously suggested. Full abstract in the PDF file.
Stellar occultation is a powerful technique that allows the determination of some physical parameters of the occulting object. The result depends on the photometric accuracy, the temporal resolution, ...and the number of chords obtained. Space telescopes can achieve high photometric accuracy as they are not affected by atmospheric scintillation. Using ESA's CHEOPS space telescope, we observed a stellar occultation by the Transneptunian object (50000) Quaoar. We compare the obtained chord with previous occultations by this object and determine its astrometry with sub-milliarcsecond precision. Also, we determine upper limits to the presence of a global methane atmosphere on the occulting body. We predicted and observed a stellar occultation by Quaoar using the CHEOPS space telescope. We measured the occultation light curve from this data-set and determined the dis- and re-appearance of the star behind the occulting body. Furthermore, a ground-based telescope in Australia was used to constrain Quaoar's limb. Combined with results from previous works, these measurements allow us to obtain a precise position of Quaoar at the occultation time. We present results obtained from the first stellar occultation by a Transneptunian object (TNO) using space telescope orbiting Earth. It was the occultation by Quaoar observed on 2020 June 11. We used the CHEOPS light curve to obtain a surface pressure upper limit of 85 nbar for the detection of a global methane atmosphere. Also, combining this observation with a ground-based observation we fit Quaoar's limb to determine its astrometric position with an uncertainty below 1.0 mas. This observation is a first of its kind, and it shall be considered as a proof of concept of stellar occultation observations of Transneptunian objects with space telescopes orbiting Earth. Moreover, it shows significant prospects for the James Webb Space Telescope.
Context. Hot subdwarfs experienced strong mass loss on the Red Giant Branch (RGB) and are now hot and small He-burning objects. Aims. In this project we aim to perform a transit survey in all ...available light curves of hot subdwarfs from space-based telescopes (Kepler, K2, TESS, and CHEOPS), with our custom-made pipeline SHERLOCK, in order to determine the occurrence rate of planets around these stars, as a function of orbital period and planetary radius. Methods. In this first paper, we perform injection-and-recovery tests of synthetic transits for a selection of representative Kepler, K2 and TESS light curves, to determine which transiting bodies, in terms of object radius and orbital period, we will be able to detect with our tools. We also provide such estimates for CHEOPS data, which we analyze with the pycheops package. Results. Transiting objects with a radius \(\lesssim\) 1.0 \(R_{\Earth}\) can be detected in most of Kepler, K2 and CHEOPS targets for the shortest orbital periods (1 d and below), reaching values as small as \(\sim\)0.3 \(R_{\Earth}\) in the best cases. Reaching sub-Earth-sized bodies is achieved only for the brightest TESS targets, and the ones observed during a significant number of sectors. We also give a series of representative results for farther and bigger planets, for which the performances strongly depend on the target magnitude, the length and the quality of the data. Conclusions. The TESS sample will provide the most important statistics for the global aim of measuring the planet occurrence rate around hot subdwarfs. The Kepler, K2 and CHEOPS data will allow us to search for planetary remnants, i.e. very close and small (possibly disintegrating) objects, which would have partly survived the engulfment in their red giant host.
PDF