arXiv subjects

A. Psaridi

Publications and source records attributed to A. Psaridi.

14 recordsLinked to original sources

arXiv2026

HD 148797: A bright F-type star with two moderate-period low-density sub-Jovian planets. Compact multi-planet architectures are common in the Neptunian savanna

We report the confirmation and characterisation of two moderate-period sub-Jovian planets transiting the bright F-type star HD 148797 (G=9.4 mag, Teff=6441 +/- 51 K). Using photometric time series from TESS and CHEOPS, we determine orbital periods of 42.1 d for HD 148797 b and 68.2 d for HD 148797 c, putting the period ratio very close to the golden mean at 1.619 and therefore near several strong harmonics, and measure planetary radii of 8.25 +/- 0.37 RE and 8.37 +/- 0.38 RE, respectively. We detect significant anti-correlated transit-timing variations for both planets, which contain enough harmonic information to yield photodynamical masses of 39.3 +13 -8.5 ME for HD 148797 b and 39.6 +/- 9.3 ME for HD 148797 c. The corresponding bulk densities, 0.39 +/- 0.12 and 0.377 +/- 0.084 g/cm3, place both planets among the low-density sub-Jovians of the Neptunian savanna. The architecture of HD 148797 is not unusual within this regime: we find that detected multi-system fractions in the savanna remain at ~70-90%, and that most savanna multi-planet systems contain at least one adjacent planet pair with Pout/Pin < 3. This pattern suggests that savanna sub-Jovians are commonly found in dynamically cold systems, consistent with smoother migration pathways such as disk-driven migration rather than disruptive high-eccentricity tidal migration. As a bright, co-evolved system hosting two warm savanna sub-Jovians with similar radii and masses, HD 148797 is also a promising target for comparative atmospheric characterisation.

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arXiv2026

TOI-159 b: an eccentric hot-Jupiter planet around a young, pulsating $\gamma$ Doradus star

Fast-rotating hot stars are challenging targets for exoplanet searches due to rotational broadening and stellar variability. Moreover, hot stars often exhibit pulsations, an additional source of scatter in both photometric and spectroscopic series. Because of these challenges, such stars remain a relatively unexplored environment for planetary architecture and evolution studies. In this study, we present the confirmation and preliminary atmospheric characterisation of a giant planet orbiting a young ($\approx$ 150 Myr), pulsating $\gamma$ Doradus star. TOI-159 b ($P_{\rm orb} \simeq 3.7$ d, $R_{\rm p} \simeq 1.6~R_{\rm J}$, $M_{\rm p} \simeq 3.5 M_{\rm J}$) is an S-type planet in a close binary system and is the hottest ($T_{\rm eq} \simeq 1900$ K) hot Jupiter with a significant eccentricity ($e = 0.24 \pm 0.04$) ever detected. Our joint modelling of radial velocities (HARPS and CORALIE), transits (\textit{TESS}), and spectro-photometry (IMACS) allows us to detect its Keplerian signal at high significance ($13 \sigma$), place strong constraints on its eccentricity ($6 \sigma$), disentangle the stellar rotational modulation and pulsation periods, and generate a low-resolution transmission spectrum, on which we conduct an exploratory analysis to constrain the presence of a planetary atmosphere using combined star-planet retrievals. Whilst our spectrum appears to display some modulation, the data is too coarse to allow for any conclusive detections at this stage. Higher-resolution observations are needed to confirm or refute these features and, if genuine, determine whether they originate from contamination from the star or a planetary atmosphere.

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arXiv2026

The atmosphere of the warm Neptune GJ 436 b probed with ESPRESSO

Aims. We aim to identify the presence of atomic and molecular species in the upper atmosphere of the warm Neptune-sized transiting planet GJ 436 b, which has a radiative equilibrium temperature of 690 K and a mass of 25.4 Earth masses. Methods. Using transmission spectroscopy, we observed two full transits of GJ 436 b with the ESPRESSO spectrograph, covering the wavelength range from 3800 to 7880 Angstrom. We searched for traces of atomic (H I, Li I, Na I, Mg I, V I, Cr I, Fe I, and Fe II) and molecular (TiO, VO) species by directly detecting planetary absorption features and by cross-correlating the planetary spectrum with theoretical spectra computed for each investigated species. Results. Our analysis reveals no strong planetary detection for any of the species, consistent with a featureless optical spectrum. We derived upper limits by combining all ESPRESSO observations. Post-transit stellar flares were detected on both nights, primarily affecting chromospheric lines. A tentative Fe I signal appears in the first transit (S/N = 3.4 +/- 0.2) at a wind velocity of about -18.6 km/s, which is unexpectedly large for a cool planet. This weak signal is not present in the second transit and, combined with its low significance, suggests an origin in noise. In the less probable scenario where the feature is suppressed during the second transit by the higher stellar activity state, the T1 tentative signal peaks at 1300 K, which is above the equilibrium temperature of GJ 436 b. Ultimately, this result would imply a neutral iron abundance comparable to or exceeding that of the host star.

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arXiv2026

Precise measurement of WASP-31 b's Rossiter-McLaughlin effect and characterization of the planet transmission spectra

Context. Hot Jupiters are ideal natural laboratories to investigate atmospheric composition and dynamics. However, high-resolution transmission spectroscopy is currently limited by our capability of removing planet-occulted line-distortion (POLD) contamination from the signal. Aims. In this paper, we aim to characterize the transmission spectrum of WASP-31 b from two and a half transits observed with the ESPRESSO spectrograph at the VLT. Methods. The Rossiter-McLaughlin (RM) signature was analyzed using the RM "revolutions" method. Before extracting the transmission spectrum of the planet, we corrected the dataset for telluric lines using molecfit and further modeled the POLD deformations using EvE. Results. We confirm the planet low sky-projected spin-orbit angle from previous studies and further refine its value to $\lambda = -0.09^{+0.31}_{-0.32}$ deg. We do not detect any species (including previously detected species such as K or CrH) in the planetary atmosphere. In most cases the non-detections are due to the strong POLDs contamination or lack of observable lines in the ESPRESSO wavelength range, and so previous detections cannot be ruled out. Conclusions. Planet-occulted line-distortion contamination continues to be the main limitation of high-resolution transmission spectroscopy for species present in both the star and the planet, hindering atmospheric detections even with state-of-the-art models, in particular for planets with a low sky-projected spin-orbit angle. Developing advanced techniques to isolate planetary signatures is of utmost importance in the advent of ELT-like observations.

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arXiv2025

A decade of transit photometry for K2-19: Revised system architecture

The star K2-19 hosts a pair of Neptunian planets deep inside the 3:2 resonance. They induce strong transit-timing variations with two incommensurate frequencies. Previous photodynamical modeling of 3.3 years of transit and radial velocity data produced mass estimates of 32.4 +/- 1.7 M_E and 10.8 +/- 0.6 M_E for planets b and c, respectively, and corresponding eccentricity estimates of 0.20 +/- 0.03 and 0.21 +/- 0.03. These high eccentricities raise questions about the formation origin of the system, and this motivated us to extend the observing baseline in an attempt to better constrain their values. We present a photodynamical analysis of 10 years of transit data that confirms the previous mass estimates (30.8 +/- 1.3 M_E and 11.1 +/- 0.4 M_E), but reduces the median eccentricities to 0.04 +/- 0.02 and 0.07 +/- 0.02 for b and c, respectively. These values are more consistent with standard formation models, but still involve nonzero free eccentricity. The previously reported high eccentricities appear to be due to a single transit for which measurements taken at twilight mimicked ingress. This resulted in a 12-minute error in the midtransit time. The data that covered 1.3 and 5 so-called super and resonant periods were used to match a Fourier analysis of the transit-timing variation signal with simple analytic expressions for the frequencies and amplitudes to obtain planet mass estimates within 2% of the median photodynamical values, regardless of the eccentricities. Theoretical details of the analysis are presented in a companion paper. Additionally, we identified a possible planet candidate situated exterior to the b-c pair. Finally, in contrast to a previous study, our internal structure modeling of K2-19 b yields a metal mass fraction that is consistent with core accretion.

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arXiv2025

Detection and characterisation of a 106-day transiting Jupiter : TOI-2449 b / NGTS-36 b

Only a handful of transiting giant exoplanets with orbital periods longer than 100 days are known. These warm exoplanets are valuable objects as their radius and mass can be measured leading to an in-depth characterisation of the planet's properties. Thanks to low levels of stellar irradiation and large orbital distances, the atmospheric properties and orbital parameters of warm exoplanets remain relatively unaltered by their host star, giving new insights into planetary formation and evolution. We aim at extending the sample of warm giant exoplanets with precise radii and masses. Our goal is to identify suitable candidates in the Transiting Exoplanet Survey Satellite (TESS) data and perform follow-up observations with ground-based instruments. We use the Next Generation Transit Survey (NGTS) to detect additional transits of planetary candidates in order to pinpoint their orbital period. We also monitored the target with several high-resolution spectrographs to measure the planetary mass and eccentricity. We report the discovery of a 106-day period Jupiter-sized planet around the G-type star TOI-2449 / NGTS-36. We jointly modelled the photometric and radial velocity data and find that the planet has a mass of 0.70 Mj and a radius of 1.002 Rj. The planetary orbit has a semi-major axis of 0.449 au and is slightly eccentric. We detect an additional 3-year signal in the radial velocity data likely due to the stellar magnetic cycle. Based on the planetary evolution models considered here, we find that TOI-2449 b / NGTS-36 b contains 11 Me of heavy elements and has a marginal planet-to-star metal enrichment of 3.3. Assuming a Jupiter-like Bond albedo, TOI-2449 b / NGTS-36 b has an equilibrium temperature of 400 K and is a good target for understanding nitrogen chemistry in cooler atmospheres.

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arXiv2025

The ESPRESSO transmission spectrum of HD$\,$189733$\,$b : Extracting the planetary sodium and lithium signatures amid stellar contamination

While transmission spectroscopy has allowed us to detect many atomic and molecular species in exoplanet atmospheres, the improvement in resolution and signal-to-noise ratio enabled us to become sensitive to planet-occulted line distortions (POLDs) in the spectrum that are induced by center-to-limb variations and the Rossiter-McLaughlin effect. POLDs can bias the interpretation of the transmission spectrum, and it is difficult to correct for them with stellar models. We analyzed two ESPRESSO transits (R $\sim$ 140$\,$000) of the archetypal hot Jupiter HD$\,$189733$\,$b. The transmission spectrum of this aligned system is heavily affected by POLDs, stellar activity, and instrumental effects. It is therefore a challenging study case of how to account for these effects when the planetary signal is retrieved from chemical species through transmission spectroscopy. We confirm the previous detections of the sodium doublet signature in the upper atmosphere of HD$\,$189733$\,$b. When we accounted for POLDs and isolated the planetary signal from uncorrected stellar residuals, we found a shallower (0.432 $\pm$ 0.027 %) and more strongly blueshifted (-7.97 $\pm$ 0.28 km/s) signal. We attempted to reinterpret the other high-resolution sodium studies of this system in light of our results. We suggest that the POLDs and stellar activity are insufficiently corrected for in all analyses, including ours. We also detected a planetary lithium signature of 0.102 $\pm$ 0.016 % (6.4$\sigma$) at a blueshift of -2.4 $\pm$ 1.8 km/s.

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arXiv2024

Discovery of a cold giant planet and mass measurement of a hot super-Earth in the multi-planetary system WASP-132

Hot Jupiters generally do not have nearby planet companions, as they may have cleared out other planets during their inward migration from more distant orbits. This gives evidence that hot Jupiters more often migrate inward via high-eccentricity migration due to dynamical interactions between planets rather than more dynamically cool migration mechanisms through the protoplanetary disk. Here we further refine the unique system of WASP-132 by characterizing the mass of the recently validated 1.0-day period super-Earth WASP-132c (TOI-822.02), interior to the 7.1-day period hot Jupiter WASP-132b. Additionally, we announce the discovery of a giant planet at a 5-year period (2.7 AU). We also detected a long-term trend in the radial velocity data indicative of another outer companion. Using over nine years of CORALIE radial velocities (RVs) and over two months of highly sampled HARPS RVs, we determined the masses of the planets from smallest to largest orbital period to be M$_{\rm{c}}$ = $6.26^{+1.84}_{-1.83}$ $M_{\oplus}$, M$_{\rm{b}}$ = $0.428^{+0.015}_{-0.015}$ $M_{\rm{Jup}}$, and M$_{\rm{d}}\sin{i}$ = $5.16^{+0.52}_{-0.52}$ $M_{\rm{Jup}}$, respectively. Using TESS and CHEOPS photometry data, we measured the radii of the two inner transiting planets to be R$_{\rm{c}}$ = $1.841^{+0.094}_{-0.093}$ $R_{\oplus}$ and R$_{\rm{b}}$ = $0.901^{+0.038}_{-0.038}$ $R_{\rm{Jup}}$. We find a bulk density of $\rho_{\rm{c}}$ = $5.47^{+1.96}_{-1.71}$ g cm$^{-3}$ for WASP-132 c, which is slightly above the Earth-like composition line on the mass-radius diagram. WASP-132 is a unique multi-planetary system in that both an inner rocky planet and an outer giant planet are in a system with a hot Jupiter. This suggests it migrated via a rarer dynamically cool mechanism and helps to further our understanding of how hot Jupiter systems form and evolve.

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arXiv2024

The atmospheric composition of the ultra-hot Jupiter WASP-178 b observed with ESPRESSO

We search for atmospheric constituents for the UHJ WASP-178 b with two ESPRESSO transits using the narrow-band and cross-correlation techniques, focusing on the detections of NaI, H$\alpha$, H$\beta$, H$\gamma$, MgI, FeI and FeII. Additionally, we show parallel photometry used to obtain updated and precise stellar, planetary and orbital parameters. We report the resolved line detections of NaI (5.5 and 5.4 $\sigma$), H$\alpha$ (13 $\sigma$), H$\beta$ (7.1 $\sigma$), and tentatively MgI (4.6 $\sigma$). In cross-correlation, we confirm the MgI detection (7.8 and 5.8 $\sigma$) and additionally report the detections of FeI (12 and 10 $\sigma$) and FeII (11 and 8.4 $\sigma$), on both nights separately. The detection of MgI remains tentative, however, due to the differing results between both nights, as well as compared with the narrow-band derived properties. None of our resolved spectral lines probing the mid- to upper atmosphere show significant shifts relative to the planetary rest frame, however H$\alpha$ and H$\beta$ exhibit line broadenings of 39.6 $\pm$ 2.1 km/s and 27.6 $\pm$ 4.6 km/s, respectively, indicating the onset of possible escape. WASP-178 b differs from similar UHJ with its lack of strong atmospheric dynamics in the upper atmosphere, however the broadening seen for FeI (15.66 $\pm$ 0.58 km/s) and FeII (11.32 $\pm$ 0.52 km/s) could indicate the presence of winds in the mid-atmosphere. Future studies on the impact of the flux variability caused by the host star activity might shed more light on the subject. Previous work indicated the presence of SiO cloud-precursors in the atmosphere of WASP-178 b and a lack of MgI and FeII. However, our results suggest that a scenario where the planetary atmosphere is dominated by MgI and FeII is more likely. In light of our results, we encourage future observations to further elucidate these atmospheric properties.

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arXiv2024

NGTS-30 b/TOI-4862 b: An 1 Gyr old 98-day transiting warm Jupiter

Long-period transiting exoplanets bridge the gap between the bulk of transit- and Doppler-based exoplanet discoveries, providing key insights into the formation and evolution of planetary systems. The wider separation between these planets and their host stars results in the exoplanets typically experiencing less radiation from their host stars; hence, they should maintain more of their original atmospheres, which can be probed during transit via transmission spectroscopy. Although the known population of long-period transiting exoplanets is relatively sparse, surveys performed by the Transiting Exoplanet Survey Satellite (TESS) and the Next Generation Transit Survey (NGTS) are now discovering new exoplanets to fill in this crucial region of the exoplanetary parameter space. This study presents the detection and characterisation of NGTS-30 b/TOI-4862 b, a new long-period transiting exoplanet detected by following up on a single-transit candidate found in the TESS mission. Through monitoring using a combination of photometric instruments (TESS, NGTS, and EulerCam) and spectroscopic instruments (CORALIE, FEROS, HARPS, and PFS), NGTS-30 b/TOI-4862 b was found to be a long-period (P = 98.29838 day) Jupiter-sized (0.928 RJ; 0.960 MJ) planet transiting a 1.1 Gyr old G-type star. With a moderate eccentricity of 0.294, its equilibrium temperature could be expected to vary from 274 K to 500 K over the course of its orbit. Through interior modelling, NGTS-30 b/TOI-4862 b was found to have a heavy element mass fraction of 0.23 and a heavy element enrichment (Zp/Z_star) of 20, making it metal-enriched compared to its host star. NGTS-30 b/TOI-4862 b is one of the youngest well-characterised long-period exoplanets found to date and will therefore be important in the quest to understanding the formation and evolution of exoplanets across the full range of orbital separations and ages.

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arXiv2023

An ESPRESSO view of HD 189733 system. Broadband transmission spectrum, differential rotation, and system architecture

The development of state-of-the-art spectrographs has ushered in a new era in the detection and characterization of exoplanetary systems. Our objective is to utilize the high-resolution and precision capabilities of the ESPRESSO instrument to detect and measure the broad-band transmission spectrum of HD 189733b's atmosphere. Additionally, we aim to employ an improved Rossiter-McLaughlin model to derive properties related to the velocity fields of the stellar surface and to constrain the orbital architecture. Our results demonstrate a high degree of precision in fitting the observed radial velocities during transit using the improved modeling of the Rossiter-McLaughlin effect. We tentatively detect the effect of differential rotation with a confidence level of $93.4 \%$ when considering a rotation period within the photometric literature values, and $99.6\%$ for a broader range of rotation periods. For the former, the amplitude of differential rotation ratio suggests an equatorial rotation period of $11.45\pm 0.09$ days and a polar period of $14.9\pm 2$. The addition of differential rotation breaks the latitudinal symmetry, enabling us to measure the true spin-orbit angle $ \psi \approx 13.6 \pm 6.9 ^\circ$ and the stellar inclination axis angle $ i_{\star} \approx 71.87 ^{+6.91^\circ}_{-5.55^\circ}$. Moreover, we determine a sub-solar amplitude of the convective blueshift velocity $V_{CB}$ $\approx$ $-211 ^{+69} _{-61}$ m$\,$s$ ^{-1}$, which falls within the expected range for a K-dwarf host star and is compatible with both runs. Finally, we successfully retrieved the transmission spectrum of HD 189733b from the high-resolution ESPRESSO data. We observe a significant decrease in radius with increasing wavelength, consistent with the phenomenon of super-Rayleigh scattering.

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arXiv2023

TOI-5678 b: A 48-day transiting Neptune-mass planet characterized with CHEOPS and HARPS

A large sample of long-period giant planets has been discovered thanks to long-term radial velocity surveys, but only a few dozen of these planets have a precise radius measurement. Transiting gas giants are crucial targets for the study of atmospheric composition across a wide range of equilibrium temperatures and for shedding light on the formation and evolution of planetary systems. Indeed, compared to hot Jupiters, the atmospheric properties and orbital parameters of cooler gas giants are unaltered by intense stellar irradiation and tidal effects. We identify long-period planets in the Transiting Exoplanet Survey Satellite (TESS) data as duo-transit events. To solve the orbital periods of TESS duo-transit candidates, we use the CHaracterising ExOPlanet Satellite (CHEOPS) to observe the highest-probability period aliases in order to discard or confirm a transit event at a given period. We also collect spectroscopic observations with CORALIE and HARPS in order to confirm the planetary nature and measure the mass of the candidates. We report the discovery of a warm transiting Neptune-mass planet orbiting TOI-5678. After four non-detections corresponding to possible periods, CHEOPS detected a transit event matching a unique period alias. Joint modeling reveals that TOI-5678 hosts a 47.73 day period planet. TOI-5678 b has a mass of 20 (+-4) Me and a radius of 4.91 (+-0.08 Re) . Using interior structure modeling, we find that TOI-5678 b is composed of a low-mass core surrounded by a large H/He layer with a mass of 3.2 (+1.7, -1.3) Me. TOI-5678 b is part of a growing sample of well-characterized transiting gas giants receiving moderate amounts of stellar insolation (11 Se). Precise density measurement gives us insight into their interior composition, and the objects orbiting bright stars are suitable targets to study the atmospheric composition of cooler gas giants.

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arXiv2023

WASP-131 b with ESPRESSO I: A bloated sub-Saturn on a polar orbit around a differentially rotating solar-type star

In this paper, we present observations of two high-resolution transit datasets obtained with ESPRESSO of the bloated sub-Saturn planet WASP-131~b. We have simultaneous photometric observations with NGTS and EulerCam. In addition, we utilised photometric lightcurves from {\tess}, WASP, EulerCam and TRAPPIST of multiple transits to fit for the planetary parameters and update the ephemeris. We spatially resolve the stellar surface of WASP-131 utilising the Reloaded Rossiter McLaughlin technique to search for centre-to-limb convective variations, stellar differential rotation, and to determine the star-planet obliquity for the first time. We find WASP-131 is misaligned on a nearly retrograde orbit with a projected obliquity of $λ= 162.4\substack{+1.3 \\ -1.2}^{\circ}$. In addition, we determined a stellar differential rotation shear of $α= 0.61 \pm 0.06$ and disentangled the stellar inclination ($i_* = 40.9\substack{+13.3 \\ -8.5}^{\circ}$) from the projected rotational velocity, resulting in an equatorial velocity of $v_{\rm{eq}} = 7.7\substack{+1.5 \\ -1.3}$~km s$^{-1}$. In turn, we determined the true 3D obliquity of $ψ= 123.7\substack{+12.8 \\ -8.0}^{\circ}$, meaning the planet is on a perpendicular/polar orbit. Therefore, we explored possible mechanisms for the planetary system's formation and evolution. Finally, we searched for centre-to-limb convective variations where there was a null detection, indicating that centre-to-limb convective variations are not prominent in this star or are hidden within red noise.

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arXiv2022

Two long-period transiting exoplanets on eccentric orbits: NGTS-20 b (TOI-5152 b) and TOI-5153 b

Long-period transiting planets provide the opportunity to better understand the formation and evolution of planetary systems. Their atmospheric properties remain largely unaltered by tidal or radiative effects of the host star, and their orbital arrangement reflects a different, and less extreme, migrational history compared to close-in objects. The sample of long-period exoplanets with well determined masses and radii is still limited, but a growing number of long-period objects reveal themselves in the TESS data. Our goal is to vet and confirm single transit planet candidates detected in the TESS space-based photometric data through spectroscopic and photometric follow up observations with ground-based instruments. We use the Next Generation Transit Survey (NGTS) to photometrically monitor the candidates in order to observe additional transits. We report the discovery of two massive, warm Jupiter-size planets, one orbiting the F8-type star TOI-5153 and the other orbiting the G1-type star NGTS-20 (=TOI-5152). From our spectroscopic analysis, both stars are metal-rich with a metallicity of 0.12 and 0.15, respectively. Follow-up radial velocity observations were carried out with CORALIE, CHIRON, FEROS, and HARPS. TOI-5153 hosts a 20.33 day period planet with a planetary mass of 3.26 (+-0.18) Mj, a radius of 1.06 (+-0.04) Rj , and an orbital eccentricity of 0.091 (+-0.026). NGTS-20 b is a 2.98 (+-0.16) Mj planet with a radius of 1.07 (+-0.04) Rj on an eccentric (0.432 +- 0.023) orbit with an orbital period of 54.19 days. Both planets are metal-enriched and their heavy element content is in line with the previously reported mass-metallicity relation for gas giants. Both warm Jupiters orbit moderately bright host stars making these objects valuable targets for follow-up studies of the planetary atmosphere and measurement of the spin-orbit angle of the system.

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