arXiv subjects

K. W. F. Lam

Publications and source records attributed to K. W. F. Lam.

At least 55 records · Page 3Linked to original sources

arXiv2023

Modelling the Light Curves of Transiting Exomoons: a Zero-order Photodynamic Agent Added to the Transit and Light Curve Modeller

Despite the ever-growing number of exoplanets discovered and the extensive analyses carried out to find their potential satellites, only two exomoon candidates, Kepler-1625b-i and Kepler-1708 b-i, have been discovered to date. A considerable amount of effort has been invested in the development of algorithms for modelling, searching, and detecting exomoons in exoplanetary light curves. In this work, we incorporate moon handling capabilities into the state-of-the-art and publicly available code, the Transit and Light Curve Modeller (TLCM). The code is designed for the analysis of transiting exoplanet systems with the inclusion of a wavelet-based noise handling algorithm. Here we present an updated version of TLCM that is capable of modelling a coplanar planet-moon system on an elliptical orbit around its host, accounting for mutual eclipses between the two bodies (and neglecting perturbative effects) -- a so-called photodynamic model. The key benefit of this framework is the ability for a joint analysis of multiple planet-moon transits. We demonstrate the necessity of this software on a case study of Kepler-1625b. Similarly to prior works, we conclude that there is no firm evidence of an exomoon in that system, by showing that temporally correlated noise can mimic apparent lunar transits.

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arXiv2023

No random transits in CHEOPS observations of HD 139139

HD 139139 (a.k.a. 'The Random Transiter') is a star that exhibited enigmatic transit-like features with no apparent periodicity in K2 data. The shallow depth of the events ($\sim$200 ppm -- equivalent to transiting objects with radii of $\sim$1.5 R$_\oplus$ in front of a Sun-like star), and their non-periodicity, constitutes a challenge for the photometric follow-up of this star. The goal of this study is to confirm with independent measurements the presence of shallow, non-periodic transit-like features on this object. We performed observations with CHEOPS, for a total accumulated time of 12.75 d, distributed in visits of roughly 20 h in two observing campaigns in years 2021 and 2022. The precision of the data is sufficient to detect 150 ppm features with durations longer than 1.5 h. We use the duration and times of the events seen in the K2 curve to estimate how many should have been detected in our campaigns, under the assumption that their behaviour during the CHEOPS observations would be the same as in the K2 data of 2017. We do not detect events with depths larger than 150 ppm in our data set. If the frequency, depth, and duration of the events were the same as in the K2 campaign, we estimate the probability of having missed all events due to our limited observing window would be 4.8 %. We suggest three different scenarios to explain our results: 1) Our observing window was not long enough, and the events were missed with the estimated 4.8 % probability. 2) The events recorded in the K2 observations were time critical, and the mechanism producing them was either not active in the 2021 and 2022 campaigns or created shallower events under our detectability level. 3) The enigmatic events in the K2 data are the result of an unidentified and infrequent instrumental noise in the original data set or its data treatment.

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arXiv2023

Constraining the reflective properties of WASP-178b using Cheops photometry

Multiwavelength photometry of the secondary eclipses of extrasolar planets is able to disentangle the reflected and thermally emitted light radiated from the planetary dayside. This leads to the measurement of the planetary geometric albedo $A_g$, which is an indicator of the presence of clouds in the atmosphere, and the recirculation efficiency $ε$, which quantifies the energy transport within the atmosphere. In this work we aim to measure $A_g$ and $ε$ for the planet WASP-178 b, a highly irradiated giant planet with an estimated equilibrium temperature of 2450 K.} We analyzed archival spectra and the light curves collected by Cheops and Tess to characterize the host WASP-178, refine the ephemeris of the system and measure the eclipse depth in the passbands of the two respective telescopes. We measured a marginally significant eclipse depth of 70$\pm$40 ppm in the Tess passband and statistically significant depth of 70$\pm$20 ppm in the Cheops passband. Combining the eclipse depth measurement in the Cheops (lambda_eff=6300 AA) and Tess (lambda_eff=8000 AA) passbands we constrained the dayside brightness temperature of WASP-178 b in the 2250-2800 K interval. The geometric albedo 0.1<$\rm A_g$<0.35 is in general agreement with the picture of poorly reflective giant planets, while the recirculation efficiency $ε>$0.7 makes WASP-178 b an interesting laboratory to test the current heat recirculation models.

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arXiv2023

CHEOPS and TESS view of the ultra-short period super-Earth TOI-561 b

Ultra-short period planets (USPs) are a unique class of super-Earths with an orbital period of less than a day and hence subject to intense radiation from their host star. While most of them are consistent with bare rocks, some show evidence of a heavyweight envelope, which could be a water layer or a secondary metal-rich atmosphere sustained by an outgassing surface. Much remains to be learned about the nature of USPs. The prime goal of the present work is to study the bulk planetary properties and atmosphere of TOI-561b, through the study of its transits and occultations. We obtained ultra-precise transit photometry of TOI-561b with CHEOPS and performed a joint analysis of this data with four TESS sectors. Our analysis of TOI-561b transit photometry put strong constraints on its properties, especially on its radius, Rp=1.42 +/- 0.02 R_Earth (at ~2% error). The internal structure modelling of the planet shows that the observations are consistent with negligible H/He atmosphere, however requiring other lighter materials, in addition to pure iron core and silicate mantle to explain the observed density. We find that this can be explained by the inclusion of a water layer in our model. We searched for variability in the measured Rp/R* over time to trace changes in the structure of the planetary envelope but none found within the data precision. In addition to the transit event, we tentatively detect occultation signal in the TESS data with an eclipse depth of ~27 +/- 11 ppm. Using the models of outgassed atmospheres from the literature we find that the thermal emission from the planet can mostly explain the observation. Based on this, we predict that NIR/MIR observations with JWST should be able to detect silicate species in the atmosphere of the planet. This could also reveal important clues about the planetary interior and help disentangle planet formation and evolution models.

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arXiv2023

TOI-1416: A system with a super-Earth planet with a 1.07d period

TOI 1416 (BD+42 2504, HIP 70705) is a V=10 late G or early K-type dwarf star with transits detected by TESS. Radial velocities verify the presence of the transiting planet TOI-1416 b, with a period of 1.07d, a mass of $3.48 M_{Earth}$ and a radius of $1.62 R_{Earth}$, implying a slightly sub-Earth density of $4.50$ g cm$^{-3}$. The RV data also further indicate a tentative planet c with a period of 27.4 or 29.5 days, whose nature cannot be verified due to strong suspicions about contamination by a signal related to the Moon's synodic period of 29.53 days. The near-USP (Ultra Short Period) planet TOI-1416 b is a typical representative of a short-period and hot ($T_{eq} \approx$ 1570 K) super-Earth like planet. A planet model of an interior of molten magma containing a significant fraction of dissolved water provides a plausible explanation for its composition, and its atmosphere could be suitable for transmission spectroscopy with JWST. The position of TOI-1416 b within the radius-period distribution corroborates that USPs with periods of less than one day do not form any special group of planets. Rather, this implies that USPs belong to a continuous distribution of super-Earth like planets with periods ranging from the shortest known ones up to ~ 30 days, whose period-radius distribution is delimitated against larger radii by the Neptune desert and by the period-radius valley that separates super-Earths from sub-Neptune planets. In the abundance of small-short periodic planets against period, a plateau between periods of 0.6 to 1.4 days has however become notable that is compatible with the low-eccentricity formation channel. For the Neptune desert, its lower limits required a revision due to the increasing population of short period planets and new limits are provided. These limits are also given in terms of the planets' insolation and effective temperatures.

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arXiv2022

Radial velocity confirmation of a hot super-Neptune discovered by TESS with a warm Saturn-mass companion

We report the discovery and confirmation of the planetary system TOI-1288. This late G dwarf harbours two planets: TOI-1288 b and TOI-1288 c. We combine TESS space-borne and ground-based transit photometry with HARPS-N and HIRES high-precision Doppler measurements, which we use to constrain the masses of both planets in the system and the radius of planet b. TOI-1288~b has a period of $2.699835^{+0.000004}_{-0.000003}$ d, a radius of $5.24 \pm 0.09$ R$_\oplus$, and a mass of $42 \pm 3$ M$_\oplus$, making this planet a hot transiting super-Neptune situated right in the Neptunian desert. This desert refers to a paucity of Neptune-sized planets on short period orbits. Our 2.4-year-long Doppler monitoring of TOI-1288 revealed the presence of a Saturn-mass planet on a moderately eccentric orbit ($0.13^{+0.07}_{-0.09}$) with a minimum mass of $84 \pm 7$ M$_\oplus$ and a period of $443^{+11}_{-13}$ d. The 5 sectors worth of TESS data do not cover our expected mid-transit time for TOI-1288 c, and we do not detect a transit for this planet in these sectors.

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arXiv2022

HD 20329b: An ultra-short-period planet around a solar-type star found by TESS

We used TESS light curves and HARPS-N spectrograph radial velocity measurements to establish the physical properties of the transiting exoplanet candidate found around the star HD 20329 (TOI-4524). We performed a joint fit of the light curves and radial velocity time series to measure the mass, radius, and orbital parameters of the candidate. We confirm and characterize HD 20329b, an ultra-short-period (USP) planet transiting a solar-type star. The host star (HD 20329, $V = 8.74$ mag, $J = 7.5$ mag) is characterized by its G5 spectral type with $\mathrm{M}_\star= 0.90 \pm 0.05$ M$_\odot$, $\mathrm{R}_\star = 1.13 \pm 0.02$ R$_\odot$, and $\mathrm{T}_{\mathrm{eff}} = 5596 \pm 50$ K; it is located at a distance $d= 63.68 \pm 0.29$ pc. By jointly fitting the available TESS transit light curves and follow-up radial velocity measurements, we find an orbital period of $0.9261 \pm (0.5\times 10^{-4})$ days, a planetary radius of $1.72 \pm 0.07$ $\mathrm{R}_\oplus$, and a mass of $7.42 \pm 1.09$ $\mathrm{M}_\oplus$, implying a mean density of $ρ_\mathrm{p} = 8.06 \pm 1.53$ g cm$^{-3}$. HD 20329b joins the $\sim$30 currently known USP planets with radius and Doppler mass measurements.

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arXiv2022

HD 191939 revisited: New and refined planet mass determinations, and a new planet in the habitable zone

HD 191939 (TOI-1339) is a nearby (d=54pc), bright (V=9mag), and inactive Sun-like star (G9 V) known to host a multi-planet transiting system. Ground-based spectroscopic observations confirmed the planetary nature of the three transiting sub-Neptunes (HD 191939 b, c, and d) originally detected by TESS and were used to measure the masses for planets b and c with 3$σ$ precision. These previous observations also reported the discovery of an additional Saturn-mass planet (HD 191939 e) and evidence for a further, very long-period companion (HD 191939 f). Here, we report the discovery of a new non-transiting planet in the system and a refined mass determination of HD 191939 d. The new planet, HD 191939 g, has a minimum mass of 13.5$\pm$2.0 M$_\oplus$ and a period of about 280 d. This period places the planet within the conservative habitable zone of the host star, and near a 1:3 resonance with HD 191939 e. The compilation of 362 radial velocity measurements with a baseline of 677 days from four different high-resolution spectrographs also allowed us to refine the properties of the previously known planets, including a 4.6$σ$ mass determination for planet d, for which only a 2$σ$ upper limit had been set until now. We confirm the previously suspected low density of HD 191939 d, which makes it an attractive target for attempting atmospheric characterisation. Overall, the planetary system consists of three sub-Neptunes interior to a Saturn-mass and a Uranus-mass planet plus a high-mass long-period companion. This particular configuration has no counterpart in the literature and makes HD 191939 an exceptional multi-planet transiting system with an unusual planet demographic worthy of future observation.

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arXiv2022

Precise mass determination for the keystone sub-Neptune planet transiting the mid-type M dwarf G 9-40

Context. Despite being a prominent subset of the exoplanet population discovered in the past three decades, the nature and provenance of sub-Neptune-sized planets are still one of the open questions in exoplanet science. Aims. For planets orbiting bright stars, precisely measuring the orbital and planet parameters of the system is the best approach to distinguish between competing theories regarding their formation and evolution. Methods. We obtained 69 new radial velocity observations of the mid-M dwarf G 9-40 with the CARMENES instrument to measure for the first time the mass of its transiting sub-Neptune planet, G 9-40 b, discovered in data from the K2 mission. Results. Combined with new observations from the TESS mission during Sectors 44, 45, and 46, we are able to measure the radius of the planet to an uncertainty of 3.4% (Rb = 1.900 +- 0.065 Re) and determine its mass with a precision of 16% (Mb = 4.00 +- 0.63 Me). The resulting bulk density of the planet is inconsistent with a terrestrial composition and suggests the presence of either a water-rich core or a significant hydrogen-rich envelope. Conclusions. G 9-40 b is referred to as a keystone planet due to its location in period-radius space within the radius valley. Several theories offer explanations for the origin and properties of this population and this planet is a valuable target for testing the dependence of those models on stellar host mass. By virtue of its brightness and small size of the host, it joins L 98-59 d as one of the two best warm (Teq ~ 400 K) sub-Neptunes for atmospheric characterization with JWST, which will probe cloud formation in sub-Neptune-sized planets and break the degeneracies of internal composition models.

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arXiv2022

The HD 93963 A transiting system: A 1.04d super-Earth and a 3.65 d sub-Neptune discovered by TESS and CHEOPS

We present the discovery of two small planets transiting HD 93963A (TOI-1797), a G0\,V star (M$_*$=1.109\,$\pm$\,0.043\,M$_\odot$, R$_*$=1.043\,$\pm$\,0.009\,R$_\odot$) in a visual binary system. We combined TESS and CHEOPS space-borne photometry with data from MuSCAT 2, `Alopeke, PHARO, TRES, FIES, and SOPHIE. We validated and spectroscopically confirmed the outer transiting planet HD 93963 Ac, a sub-Neptune with an orbital period of P$_c \approx$ 3.65 d, reported as a TESS object of interest (TOI) shortly after the release of Sector 22 data. HD 93963 Ac has a mass of M$_c = 19.2 \pm 4.1$ M$_{\oplus}$ and a radius of R$_c = 3.228 \pm 0.059$ R$_{\oplus}$, implying a mean density of $ρ_c=3.1\pm0.7$ gcm$^{-3}$. The inner object, HD 93963 Ab, is a validated 1.04 d ultra-short period (USP) transiting super-Earth that we discovered in the TESS light curve and that was not listed as a TOI, owing to the low significance of its signal (TESS signal-to-noise ratio $\approx$ 6.7, TESS $+$ CHEOPS combined transit depth D$_b=141.5 \pm 8.5$ ppm). We intensively monitored the star with CHEOPS by performing nine transit observations to confirm the presence of the inner planet and validate the system. HD 93963 Ab is the first small (R$_b = 1.35 \pm 0.042$ R$_{\oplus}$) USP planet discovered and validated by TESS and CHEOPS. Unlike planet c, HD 93963 Ab is not significantly detected in our radial velocities (M$_b = 7.8 \pm 3.2$ M$_{\oplus}$). We also discovered a linear trend in our Doppler measurements, suggesting the possible presence of a long-period outer planet. With a V-band magnitude of 9.2, HD 93963 A is among the brightest stars known to host a USP planet, making it one of the most favourable targets for precise mass measurement via Doppler spectroscopy and an important laboratory to test formation, evolution, and migration models of planetary systems hosting ultra-short period planets.

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arXiv2022

TOI-1268b: the youngest, hot, Saturn-mass transiting exoplanet

We report the discovery of TOI-1268b, a transiting Saturn-mass planet from the TESS space mission. With an age of less than one Gyr, derived from various age indicators, TOI-1268b is the youngest Saturn-mass planet known to date and contributes to the small sample of well characterised young planets. It has an orbital period of $P\,=\,8.1577080\pm0.0000044$ days, and transits an early K dwarf star with a mass of $M_\star$ = $ 0.96 \pm 0.04$ $M_{\odot}$, a radius of $R_\star$ = $ 0.92 \pm 0.06$ $R_{\odot}$, an effective temperature of $T_\mathrm{eff}\,=\,5300\pm100$ K, and a metallicity of $0.36\pm0.06$ dex. By combining TESS photometry with high-resolution spectra acquired with the Tull spectrograph at McDonald observatory, and the high-resolution spectrographs at Tautenburg and Ondrejov observatories, we measured a planetary mass of $M_\mathrm{p}\,=\,96.4 \pm 8.3\,M_{\oplus}$ and a radius of $R_\mathrm{p}\,=\,9.1 \pm 0.6\,R_{\oplus}$. TOI-1268 is an ideal system to study the role of star-planet tidal interactions for non-inflated Saturn-mass planets. We used system parameters derived in this paper to constrain the planet tidal quality factor to the range of $10^{4.5-5.3}$. When compared with the sample of other non-inflated Saturn-mass planets, TOI-1268b is one of the best candidates for transmission spectroscopy studies.

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arXiv2021

K2-99 revisited: a non-inflated warm Jupiter, and a temperate giant planet on a 522-d orbit around a subgiant

We report new photometric and spectroscopic observations of the K2-99 planetary system. Asteroseismic analysis of the short-cadence light curve from K2's Campaign 17 allows us to refine the stellar properties. We find K2-99 to be significantly smaller than previously thought, with $R_{\star} = 2.55\pm0.02$ $\mathrm{R_\odot}$. The new light curve also contains four transits of K2-99b, which we use to improve our knowledge of the planetary properties. We find the planet to be a non-inflated warm Jupiter, with $R_\mathrm{b} = 1.06 \pm 0.01$ $\mathrm{R_{Jup}}$. Sixty new radial velocity measurements from HARPS, HARPS-N, and HIRES enable the determination of the orbital parameters of K2-99c, which were previously poorly constrained. We find that this outer planet has a minimum mass $M_\mathrm{c} \sin i_\mathrm{c} = 8.4\pm0.2$ $\mathrm{M_{Jup}}$, and an eccentric orbit ($e_\mathrm{c} = 0.210 \pm 0.009$) with a period of $522.2\pm1.4$ d. Upcoming TESS observations in 2022 have a good chance of detecting the transit of this planet, if the mutual inclination between the two planetary orbits is small.

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arXiv2021

37 New Validated Planets in Overlapping K2 Campaigns

We analysed 68 candidate planetary systems first identified during Campaigns 5 and 6 (C5 and C6) of the NASA \textit{K2} mission. We set out to validate these systems by using a suite of follow-up observations, including adaptive optics, speckle imaging, and reconnaissance spectroscopy. The overlap between C5 with C16 and C18, and C6 with C17, yields lightcurves with long baselines that allow us to measure the transit ephemeris very precisely, revisit single transit candidates identified in earlier campaigns, and search for additional transiting planets with longer periods not detectable in previous works. Using \texttt{vespa}, we compute false positive probabilities of less than 1\% for 37 candidates orbiting 29 unique host stars and hence statistically validate them as planets. These planets have a typical size of $2.2R_{\oplus}$ and orbital periods between 1.99 and 52.71 days. We highlight interesting systems including a sub-Neptune with the longest period detected by \textit{K2}, sub-Saturns around F stars, several multi-planetary systems in a variety of architectures. These results show that a wealth of planetary systems still remains in the \textit{K2} data, some of which can be validated using minimal follow-up observations and taking advantage of analyses presented in previous catalogs.

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arXiv2021

The power of wavelets in analysis of transit and phase curves in presence of stellar variability and instrumental noise I. Method and validation

Stellar photometric variability and instrumental effects, like cosmic ray hits, data discontinuities, data leaks, instrument aging etc. cause difficulties in the characterization of exoplanets and have an impact on the accuracy and precision of the modelling and detectability of transits, occultations and phase curves. This paper aims to make an attempt to improve the transit, occultation and phase-curve modelling in the presence of strong stellar variability and instrumental noise. We invoke the wavelet-formulation to reach this goal. We explore the capabilities of the software package Transit and Light Curve Modeller (TLCM). It is able to perform a joint radial velocity and light curve fit or light curve fit only. It models the transit, occultation, beaming, ellipsoidal and reflection effects in the light curves (including the gravity darkening effect, too). The red-noise, the stellar variability and instrumental effects are modelled via wavelets. The wavelet-fit is constrained by prescribing that the final white noise level must be equal to the average of the uncertainties of the photometric data points. This helps to avoid the overfit and regularizes the noise model. The approach was tested by injecting synthetic light curves into Kepler's short cadence data and then modelling them. The method performs well over a certain signal-to-noise (S/N) ratio. In general a S/N ratio of 10 is needed to get good results but some parameters requires larger S/N, some others can be retrieved at lower S/Ns. We give limits in terms of signal-to-noise ratio for every studied system parameter which is needed to accurate parameter retrieval. The wavelet-approach is able to manage and to remove the impacts of data discontinuities, cosmic ray events, long-term stellar variability and instrument ageing, short term stellar variability and pulsation and flares among others. (...)

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arXiv2021

Hot planets around cool stars -- two short-period mini-Neptunes transiting the late K-dwarf TOI-1260

We present the discovery and characterization of two sub-Neptunes in close orbits, as well as a tentative outer planet of a similar size, orbiting TOI-1260 - a low metallicity K6V dwarf star. Photometry from TESS yields radii of $R_{\rm b} = 2.33 \pm 0.10$ $R_{\oplus}$ and $R_{\rm c} = 2.82 \pm 0.15$ $R_{\oplus}$, and periods of 3.13 and 7.49 days for TOI-1260b and TOI-1260c, respectively. We combined the TESS data with a series of ground-based follow-up observations to characterize the planetary system. From HARPS-N high-precision radial velocities we obtain $M_{\rm b} = 8.61_{ - 1.46 } ^ { + 1.36 }$ $M_{\oplus}$ and $M_{\rm c} = 11.84_{ - 3.23 } ^ { + 3.38 }$ $M_{\oplus}$. The star is moderately active with a complex activity pattern, which necessitated the use of Gaussian process regression for both the light curve detrending and the radial velocity modelling, in the latter case guided by suitable activity indicators. We successfully disentangle the stellar-induced signal from the planetary signals, underlining the importance and usefulness of the Gaussian Process approach. We test the system's stability against atmospheric photoevaporation and find that the TOI-1260 planets are classic examples of the structure and composition ambiguity typical for the $2-3$ $R_{\oplus}$ range.

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arXiv2021

Masses and compositions of three small planets orbiting the nearby M dwarf L231-32 (TOI-270) and the M dwarf radius valley

We report on precise Doppler measurements of L231-32 (TOI-270), a nearby M dwarf ($d=22$ pc, $M_\star = 0.39$ M$_\odot$, $R_\star = 0.38$ R$_\odot$), which hosts three transiting planets that were recently discovered using data from the Transiting Exoplanet Survey Satellite (TESS). The three planets are 1.2, 2.4, and 2.1 times the size of Earth and have orbital periods of 3.4, 5.7, and 11.4 days. We obtained 29 high-resolution optical spectra with the newly commissioned Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) and 58 spectra using the High Accuracy Radial velocity Planet Searcher (HARPS). From these observations, we find the masses of the planets to be $1.58 \pm 0.26$, $6.15 \pm 0.37$, and $4.78 \pm 0.43$ M$_\oplus$, respectively. The combination of radius and mass measurements suggests that the innermost planet has a rocky composition similar to that of Earth, while the outer two planets have lower densities. Thus, the inner planet and the outer planets are on opposite sides of the `radius valley' -- a region in the radius-period diagram with relatively few members, which has been interpreted as a consequence of atmospheric photo-evaporation. We place these findings into the context of other small close-in planets orbiting M dwarf stars, and use support vector machines to determine the location and slope of the M dwarf ($T_\mathrm{eff} < 4000$ K) radius valley as a function of orbital period. We compare the location of the M dwarf radius valley to the radius valley observed for FGK stars, and find that its location is a good match to photo-evaporation and core-powered mass loss models. Finally, we show that planets below the M dwarf radius valley have compositions consistent with stripped rocky cores, whereas most planets above have a lower density consistent with the presence of a H-He atmosphere.

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arXiv2020

A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776

We report the discovery and characterization of two transiting planets around the bright M1 V star LP 961-53 (TOI-776, J = 8.5 mag, M = 0.54+-0.03 Msun) detected during Sector 10 observations of the Transiting Exoplanet Survey Satellite (TESS). Combining the TESS photometry with HARPS radial velocities, as well as ground-based follow-up transit observations from MEarth and LCOGT telescopes, we measured for the inner planet, TOI-776 b, a period of 8.25 d, a radius of 1.85+-0.13 Re, and a mass of 4.0+-0.9 Me; and for the outer planet, TOI-776 c, a period of 15.66 d, a radius of 2.02+-0.14 Re, and a mass of 5.3+-1.8 Me. The Doppler data shows one additional signal, with a period of 34 d, associated with the rotational period of the star. The analysis of fifteen years of ground-based photometric monitoring data and the inspection of different spectral line indicators confirm this assumption. The bulk densities of TOI-776 b and c allow for a wide range of possible interior and atmospheric compositions. However, both planets have retained a significant atmosphere, with slightly different envelope mass fractions. Thanks to their location near the radius gap for M dwarfs, we can start to explore the mechanism(s) responsible for the radius valley emergence around low-mass stars as compared to solar-like stars. While a larger sample of well-characterized planets in this parameter space is still needed to draw firm conclusions, we tentatively estimate that the stellar mass below which thermally-driven mass loss is no longer the main formation pathway for sculpting the radius valley is between 0.63 and 0.54 Msun. Due to the brightness of the star, the TOI-776 system is also an excellent target for the James Webb Space Telescope, providing a remarkable laboratory to break the degeneracy in planetary interior models and to test formation and evolution theories of small planets around low-mass stars.

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arXiv2020

The TOI-763 system: sub-Neptunes orbiting a Sun-like star

We report the discovery of a planetary system orbiting TOI-763 (aka CD-39 7945), a $V=10.2$, high proper motion G-type dwarf star that was photometrically monitored by the TESS space mission in Sector 10. We obtain and model the stellar spectrum and find an object slightly smaller than the Sun, and somewhat older, but with a similar metallicity. Two planet candidates were found in the light curve to be transiting the star. Combining TESS transit photometry with HARPS high-precision radial velocity follow-up measurements confirm the planetary nature of these transit signals. We determine masses, radii, and bulk densities of these two planets. A third planet candidate was discovered serendipitously in the radial velocity data. The inner transiting planet,TOI-763 b, has an orbital period of $P_\mathrm{b}$ = 5.6~days, a mass of $M_\mathrm{b}$ = $9.8\pm0.8$ $M_\oplus$, and a radius of $R_\mathrm{b}$ = $2.37\pm0.10$ $R_\oplus$. The second transiting planet,TOI-763 c, has an orbital period of $P_\mathrm{c}$ = 12.3~days, a mass of $M_\mathrm{c}$ = $9.3\pm1.0$ $M_\oplus$, and a radius of $R_\mathrm{c}$ = $2.87\pm0.11$ $R_\oplus$. We find the outermost planet candidate to orbit the star with a period of $\sim$48~days. If confirmed as a planet it would have a minimum mass of $M_\mathrm{d}$ = $9.5\pm1.6$ $M_\oplus$. We investigated the TESS light curve in order to search for a mono transit by planet~d without success. We discuss the importance and implications of this planetary system in terms of the geometrical arrangements of planets orbiting G-type stars.

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