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  • 1.
    Brandeker, Alexis
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Heng, K.
    Lendl, M.
    Patel, Jayshil Ashokkumar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Morris, B. M.
    Broeg, C.
    Guterman, P.
    Beck, M.
    Maxted, P. F. L.
    Demangeon, O.
    Delrez, L.
    Demory, B-O
    Kitzmann, D.
    Santos, N. C.
    Singh, V
    Alibert, Y.
    Alonso, R.
    Anglada, G.
    Barczy, T.
    Barrado y Navascues, D.
    Barros, S. C. C.
    Baumjohann, W.
    Beck, T.
    Benz, W.
    Billot, N.
    Bonfils, X.
    Bruno, G.
    Cabrera, J.
    Charnoz, S.
    Collier Cameron, A.
    van Damme, C. Corral
    Csizmadia, Sz
    Davies, M. B.
    Deleuil, M.
    Deline, A.
    Ehrenreich, D.
    Erikson, A.
    Farinato, J.
    Fortier, A.
    Fossati, L.
    Fridlund, M.
    Gandolfi, D.
    Gillon, M.
    Guedel, M.
    Hoyer, S.
    Isaak, K. G.
    Kiss, L.
    Laskar, J.
    Lecavelier des Etangs, A.
    Lovis, C.
    Luntzer, A.
    Magrin, D.
    Nascimbeni, V
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ottensamer, R.
    Pagano, I
    Pallé, E.
    Peter, G.
    Piotto, G.
    Pollacco, D.
    Queloz, D.
    Ragazzoni, R.
    Rando, N.
    Rauer, H.
    Ribas, I
    Scandariato, G.
    Ségransan, D.
    Simon, A. E.
    Smith, A. M. S.
    Sousa, S. G.
    Steller, M.
    Szabó, G. M.
    Thomas, N.
    Udry, S.
    Van Grootel, V.
    Walton, N.
    Wolter, D.
    CHEOPS geometric albedo of the hot Jupiter HD 209458 b2022In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 659, article id L4Article in journal (Refereed)
    Abstract [en]

    We report the detection of the secondary eclipse of the hot Jupiter HD 209458 b in optical/visible light using the CHEOPS space telescope. Our measurement of 20.4−3.3+3.2 parts per million translates into a geometric albedo of Ag = 0.096 ± 0.016. The previously estimated dayside temperature of about 1500 K implies that our geometric albedo measurement consists predominantly of reflected starlight and is largely uncontaminated by thermal emission. This makes the present result one of the most robust measurements of Ag for any exoplanet. Our calculations of the bandpass-integrated geometric albedo demonstrate that the measured value of Ag is consistent with a cloud-free atmosphere, where starlight is reflected via Rayleigh scattering by hydrogen molecules, and the water and sodium abundances are consistent with stellar metallicity. We predict that the bandpass-integrated TESS geometric albedo is too faint to detect and that a phase curve of HD 209458 b observed by CHEOPS would have a distinct shape associated with Rayleigh scattering if the atmosphere is indeed cloud free.

  • 2.
    Brandeker, Alexis
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lendl, M.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Patel, Jayshil Ashokkumar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    55 Cancri e's occultation captured with CHEOPS2023In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 669, article id A64Article in journal (Refereed)
    Abstract [en]

    Past occultation and phase-curve observations of the ultra-short period super-Earth 55 Cnc e obtained at visible and infrared wavelengths have been challenging to reconcile with a planetary reflection and emission model. In this study, we analyse a set of 41 occultations obtained over a two-year timespan with the CHEOPS satellite. We report the detection of 55 Cnc e’s occultation with an average depth of 12 ± 3 ppm. We derive a corresponding 2σ upper limit on the geometric albedo of Ag < 0.55 once decontaminated from the thermal emission measured by Spitzer at 4.5 µm. CHEOPS’s photometric performance enables, for the first time, the detection of individual occultations of this super-Earth in the visible and identifies short-timescale photometric corrugations likely induced by stellar granulation. We also find a clear 47.3-day sinusoidal pattern in the time-dependent occultation depths that we are unable to relate to stellar noise, nor instrumental systematics, but whose planetary origin could be tested with upcoming JWST occultation observations of this iconic super-Earth.

  • 3.
    Patel, Jayshil Ashokkumar
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    The geometric albedo of the hot Jupiter HD 189733b measured with CHEOPS2023In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 672, article id A24Article in journal (Refereed)
    Abstract [en]

    Context. Measurements of the occultation of an exoplanet at visible wavelengths allow us to determine the reflective properties of a planetary atmosphere. The observed occultation depth can be translated into a geometric albedo. This in turn aids in characterising the structure and composition of an atmosphere by providing additional information on the wavelength-dependent reflective qualities of the aerosols in the atmosphere.

    Aims. Our aim is to provide a precise measurement of the geometric albedo of the gas giant HD 189733b by measuring the occultation depth in the broad optical bandpass of CHEOPS (350–1100 nm).

    Methods. We analysed 13 observations of the occultation of HD 189733b performed by CHEOPS utilising the Python package PyCHEOPS. The resulting occultation depth is then used to infer the geometric albedo accounting for the contribution of thermal emission from the planet. We also aid the analysis by refining the transit parameters combining observations made by the TESS and CHEOPS space telescopes.

    Results. We report the detection of an 24.7 ± 4.5 ppm occultation in the CHEOPS observations. This occultation depth corresponds to a geometric albedo of 0.076 ± 0.016. Our measurement is consistent with models assuming the atmosphere of the planet to be cloud-free at the scattering level and absorption in the CHEOPS band to be dominated by the resonant Na doublet. Taking into account previous optical-light occultation observations obtained with the Hubble Space Telescope, both measurements combined are consistent with a super-stellar Na elemental abundance in the dayside atmosphere of HD 189733b. We further constrain the planetary Bond albedo to between 0.013 and 0.42 at 3σ confidence.

    Conclusions. We find that the reflective properties of the HD 189733b dayside atmosphere are consistent with a cloud-free atmosphere having a super-stellar metal content. When compared to an analogous CHEOPS measurement for HD 209458b, our data hint at a slightly lower geometric albedo for HD 189733b (0.076 ± 0.016) than for HD 209458b (0.096 ± 0.016), or a higher atmospheric Na content in the same modelling framework. While our constraint on the Bond albedo is consistent with previously published values, we note that the higher-end values of ~0.4, as derived previously from infrared phase curves, would also require peculiarly high reflectance in the infrared, which again would make it more difficult to disentangle reflected and emitted light in the total observed flux, and therefore to correctly account for reflected light in the interpretation of those phase curves. Lower reported values for the Bond albedos are less affected by this ambiguity.

  • 4.
    Patel, Jayshil Ashokkumar
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Walton, N. A.
    CHEOPS and TESS view of the ultra-short-period super-Earth TOI-561 b2023In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 679, article id A92Article in journal (Refereed)
    Abstract [en]

    Context. Ultra-short-period planets (USPs) are a unique class of super-Earths with an orbital period of less than a day, and hence they are subject to intense radiation from their host star. These planets cannot retain a primordial H/He atmosphere, and most of them are indeed consistent with being bare rocky cores. A few USPs, however, show evidence for a heavyweight envelope, which could be a water layer resilient to evaporation or a secondary metal-rich atmosphere sustained by outgassing of the molten volcanic surface. Much thus remains to be learned about the nature and formation of USPs. Aims. The prime goal of the present work is to refine the bulk planetary properties of the recently discovered TOI-561 b through the study of its transits and occultations. This is crucial in order to understand the internal structure of this USP and to assess the presence of an atmosphere. Methods. We obtained ultra-precise transit photometry of TOI-561 b with CHEOPS, and performed a joint analysis of these data along with three archival visits from CHEOPS and four TESS sectors. Results. Our analysis of TOI-561 b transit photometry put strong constraints on its properties. In particular, we restrict the uncertainties on the planetary radius at similar to 2% retrieving R-p = 1.42 +/- 0.02 R-circle plus. This result informs our internal structure modelling of the planet, which shows that the observations are consistent with a negligible H/He atmosphere; however, other lighter materials are required, in addition to a pure iron core and a silicate mantle, to explain the observed density. We find that this can be explained by the inclusion of a water layer in our model. Additionally, we ran a grid of forward models with a water-enriched atmosphere to explain the transit radius. We searched for variability in the measured R-p/R-star over time, which could trace changes in the structure of the planetary envelope. However, no temporal variations are recovered within the present data precision. In addition to the transit event, we tentatively detect an occultation signal in the TESS data with an eclipse depth L = 27.40(-11.35)(+10.87) ppm. We use models of outgassed atmospheres from the literature to explain this eclipse signal. We find that the thermal emission from the planet can mostly explain the observation. Based on this, we predict that near- to mid-infrared observations with the James Webb Space Telescope 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.

  • 5.
    Patel, Jayshil Ashokkumar
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Espinoza, Néstor
    Empirical Limb-darkening Coefficients and Transit Parameters of Known Exoplanets from TESS2022In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 163, no 5, p. 1-24, article id 228Article in journal (Refereed)
    Abstract [en]

    Although the main goal of the Transiting Exoplanet Survey Satellite (TESS) is to search for new transiting exoplanets, its data can also be used to study already-known systems in further detail. The TESS bandpass is particularly interesting to study the limb-darkening effect of the stellar host that is imprinted in transit light curves, as the widely used phoenix and atlas stellar models predict different limb-darkening profiles. Here we study this effect by fitting the transit light curves of 176 known exoplanetary systems observed by TESS, which allows us to extract empirical limb-darkening coefficients (LDCs) for the widely used quadratic law but also updated transit parameters (including ephemeride refinements) as a by-product. Comparing our empirically obtained LDCs with theoretical predictions, we find significant offsets when using tabulated TESS LDCs. Specifically, the u2 coefficients obtained using phoenix models show the largest discrepancies depending on the method used to derive them, with offsets that can reach up to Δu2 ≈ 0.2, on average. Most of those average offsets disappear, however, if one uses the SPAM algorithm introduced by Howarth to calculate the LDCs instead. Our results suggest, however, that for stars cooler than about 5000 K, no methodology is good enough to explain the limb-darkening effect; we observe a sharp deviation between measured and predicted LDCs on both quadratic LDCs of order Δu1, Δu2 ≈ 0.2 for those cool stars. We recommend caution when assuming LDCs as perfectly known, in particular for these cooler stars when analyzing TESS transit light curves.

  • 6. Singh, V.
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Patel, Jayshil Ashokkumar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zingales, T.
    CHEOPS observations of KELT-20 b/MASCARA-2 b: An aligned orbit and signs of variability from a reflective day side2024In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 683, article id A1Article in journal (Refereed)
    Abstract [en]

    Context. Occultations are windows of opportunity to indirectly peek into the dayside atmosphere of exoplanets. High-precision transit events provide information on the spin-orbit alignment of exoplanets around fast-rotating hosts.

    Aims. We aim to precisely measure the planetary radius and geometric albedo of the ultra-hot Jupiter (UHJ) KELT-20 b along with the spin-orbit alignment of the system.

    Methods. We obtained optical high-precision transits and occultations of KELT-20 b using CHEOPS observations in conjunction with simultaneous TESS observations. We interpreted the occultation measurements together with archival infrared observations to measure the planetary geometric albedo and dayside temperatures. We further used the host star’s gravity-darkened nature to measure the system’s obliquity.

    Results. We present a time-averaged precise occultation depth of 82 ± 6 ppm measured with seven CHEOPS visits and from the analysis of all available TESS photometry. Using these measurements, we precisely constrain the geometric albedo of KELT-20 b to 0.26 ± 0.04 and the brightness temperature of the dayside hemisphere to K. Assuming Lambertian scattering law, we constrain the Bond albedo to along with a minimal heat transfer to the night side (ϵ = ). Furthermore, using five transit observations we provide stricter constraints of 3 9 ± 1 1 deg on the sky-projected obliquity of the system.

    Conclusions. The aligned orbit of KELT-20 b is in contrast to previous CHEOPS studies that have found strongly inclined orbits for planets orbiting other A-type stars. The comparably high planetary geometric albedo of KELT-20 b corroborates a known trend of strongly irradiated planets being more reflective. Finally, we tentatively detect signs of temporal variability in the occultation depths, which might indicate variable cloud cover advecting onto the planetary day side.

  • 7. Zakhozhay, Olga
    et al.
    Launhardt, Ralf
    Müller, Andre
    Brems, Stefan S.
    Eigenthaler, Paul
    Gennaro, Mario
    Hempel, Angela
    Hempel, Maren
    Henning, Thomas
    Kennedy, Grant M.
    Kim, Sam
    Kürster, Martin
    Lachaume, Régis
    Manerikar, Yashodhan
    Patel, Jayshil A.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pavlov, Alexey
    Reffert, Sabine
    Trifonov, Trifon
    Radial Velocity Survey for Planets around Young stars (RVSPY) Target characterisation and high-cadence survey2022In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 667, article id A63Article in journal (Refereed)
    Abstract [en]

    Context. The occurrence rate and period distribution of (giant) planets around young stars is still not as well constrained as for older main-sequence stars. This is mostly due to the intrinsic activity-related complications and the avoidance of young stars in many large planet search programmes. Yet, dynamical restructuring processes in planetary systems may last significantly longer than the actual planet formation phase and may well extend long into the debris disc phase, such that the planet populations around young stars may differ from those observed around main-sequence stars.

    Aims. We introduce our Radial Velocity Survey for Planets around Young stars (RVSPY), which is closely related to the NaCo-ISPY direct imaging survey, characterise our target stars, and search for substellar companions at orbital separations smaller than a few au from the host star.

    Methods. We used the FEROS spectrograph, mounted to the MPG/ESO 2.2 m telescope in Chile, to obtain high signal-to-noise spectra and time series of precise radial velocities (RVs) of 111 stars, most of which are surrounded by debris discs. Our target stars have spectral types between early F and late K, a median age of 400 Myr, and a median distance of 45 pc. During the initial reconnaissance phase of our survey, we determined stellar parameters and used high-cadence observations to characterise the intrinsic stellar activity, searched for hot companions with orbital periods of up to 10 days, and derived the detection thresholds for longer-period companions. In our analysis we, have included archival spectroscopic data, spectral energy distribution, and data for photometric time series from the TESS mission.

    Results. For all target stars we determined their basic stellar parameters and present the results of the high-cadence RV survey and activity characterisation. We have achieved a median single-measurement RV precision of 6 m s−1 and derived the short-term intrinsic RV scatter of our targets (median 23 m s−1), which is mostly caused by stellar activity and decays with an age from >100 m s−1 at <20 Myr to <20 m s−1 at >500 Myr. We analysed time series periodograms of the high-cadence RV data and the shape of the individual cross-correlation functions. We discovered six previously unknown close companions with orbital periods between 10 and 100 days, three of which are low-mass stars, and three are in the brown dwarf mass regime. We detected no hot companion with an orbital period <10 days down to a median mass limit of ~1 MJup for stars younger than 500 Myr, which is still compatible with the established occurrence rate of such companions around main-sequence stars. We found significant RV periodicities between 1.3 and 4.5 days for 14 stars, which are, however, all caused by rotational modulation due to starspots. We also analysed the data for TESS photometric time series and found significant periodicities for most of the stars. For 11 stars, the photometric periods are also clearly detected in the RV data. We also derived stellar rotation periods ranging from 1 to 10 days for 91 stars, mostly from the TESS data. From the intrinsic activity-related short-term RV jitter, we derived the expected mass-detection thresholds for longer-period companions, and selected 84 targets for the longer-term RV monitoring.

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