arXiv subjects

N. Nari

Publications and source records attributed to N. Nari.

12 recordsLinked to original sources

arXiv2026

The GAPS Programme with HARPS-N at TNG LXXVII. Occurrence rates of small close-in planets in the presence of cold Jupiters

Context. The architecture of our Solar System, with inner small planets (ISPs) and outer giants, may or may not be common. Understanding whether a correlation exists between ISPs and outer cold giants is key to evaluating how common systems with a similar architecture to our own are. Aims. This study aims to build a large, homogeneous sample of systems hosting cold Jupiters (CJs, a > 1 au, msini > 0.1 M$_J$) detected via radial velocities (RVs), and to assess the presence of additional ISPs (P < 400 d, 3 < msini < 31.7 M$_{\oplus}$), studying the correlation between these two types of objects. Methods. We selected 137 stars known to host a CJ, including 23 which also harbor a hot Jupiter and were treated separately. Data from various instruments were compiled, including unpublished data gathered with HARPS-N within the GAPS program, and consistently fitted using PyORBIT. We derived RV detection maps and calculated occurrence rates for ISPs, cross-validating results with two independent codes. The sample was divided into subgroups to evaluate how system parameters influence planet occurrence. Results. We confirmed the 213 already known planets in the 137 systems and also identified six new candidates. We divided them, based on mass and period, into Neptunes (10 < m sin i < 31.7 M$_{\oplus}$) and Super-Earths (3 < m sin i < 10 M$_{\oplus}$), and into hot (1 < P < 10 d), warm (10 < P < 100 d), and cool (100 < P < 400 d). We found occurrences of 5%, 13%, and 12% for hot, warm, and cool Neptunes, respectively, and 11% and 16% for hot and warm Super-Earths, respectively. Systems with dynamically stable inner regions show higher rates of small planets. These findings are consistent with previous studies showing no strong correlation between ISPs and CJs at average stellar metallicity and mass, and suggest that HJs may be more commonly associated with external giants.

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arXiv2026

The RoPES project with HARPS and HARPS-N III. Two candidate planets orbiting the G-type star HD 161098

Context. The development of refined instruments and techniques for a detailed analysis of the radial velocities (RVs) of stars other than the Sun allows the presence of planetary signals of amplitude below 1 ms$^{-1}$ to be investigated. Long-term RV surveys allow the detection of Earth-like and super-Earth-like planets in the habitable zones of Sun-like stars, prime targets for future missions for the atmospheric characterization of exoplanets. Aims. We present the analysis of the nearby G8 V-type star HD 161098 (V = 7.68 mag, d = 29.75 pc). We searched for terrestrial planets in the habitable zone. Methods. We combined historical datasets with new data collected in an ongoing blind search program with HARPS, HARPS-N, and ESPRESSO. We utilized recently developed tools to extract RVs and to deal with the analysis of stellar activity. We performed a joint analysis of RVs and activity indicators to separate the planetary signals from those related to activity. Results. We detected two sub-ms$^{-1}$ signals that we claim as candidate planets. We are not able to confirm their nature with certainty. Candidate HD 161098 b has an orbital period of 72.578$_{-0.060}^{+0.059}$ d and a minimum mass of 3.63 $\pm$ 0.59 MEarth. HD 161098 c has an orbital period of 682.5$_{-9.9}^{+9.5}$ d and a minimum mass of 7.8$_{-1.4}^{+1.5}$ MEarth.If confirmed, candidate HD 161098 c would reside in the optimistic habitable zone of the star. We find a magnetic cycle of 4090$_{-130}^{+140}$ d period and a rotation period of 28.22$_{-0.35}^{+0.30}$ d. Our analysis sets the stage for future observing campaigns of the star, finalized for the confirmation of our results. Conclusions. We are entering the sub-ms$^{-1}$ era at long orbital periods with a combination of stellar activity treatment and long-term campaigns.

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arXiv2026

The RoPES project with HARPS and HARPS-N II. A third planet in the multi-planet system HD 176986

Earth-like planets orbiting in the habitable zone of K- to G-type stars create an RV effect in amplitude of less than 1 \si{\meter\per\second} and have orbital periods of hundreds of days. Only long-term RV surveys with sub-meter per second precision instruments can explore the outer regions of Sun-like stars and look for Earth-like planets and super-Earths. We present the analysis of the K-type star HD 176986. It has a brightness of V=8.45 mag and a distance from the Sun of d = 27.88 pc. This star hosts a known planetary system of two super-Earths. We joined historical datasets with new data collected in an ongoing blind search program. We took advantage of recently developed tools for RV extraction and stellar activity filtering. The analysis of activity indicators permits us to determine the period of the magnetic cycle of the star alongside its rotation period. We performed a joint analysis of RVs and activity indicators through multidimensional GPs to better constrain the activity model in RVs and avoid overfitting. We detected a new planet orbiting the star and retrieved the two known planets. HD 176986 b has an orbital period of 6.49164$^{+0.00030}_{-0.00029}$ d and a minimum mass of 5.36 $\pm$ 0.44 M$\oplus$. HD 176986 c has an orbital period of P$_c$ = 16.8124 $\pm$ 0.0015 d and a minimum mass of 9.75$_{-0.64}^{+0.65}$ M$\oplus$. HD 176986 d has an orbital period of 61.376$^{+0.051}_{-0.049}$ d and a minimum mass of 6.76$_{-0.92}^{+0.91}$ M$\oplus$. From the analysis of activity indicators, we find evidence of a magnetic cycle with a period of 2432$_{-59}^{+64}$ d, along with a rotation period of 36.05 $_{-0.71}^{+0.67}$ d. We discover a new planet in the multi-planet system orbiting the K-type star HD 176986. All the planets have minimum masses compatible with super-Earths or mini-Neptunes.

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arXiv2025

A second low-mass planet orbiting the nearby M-dwarf GJ 536

GJ 536 is a low-mass star, located 10 pc away from the Sun, that hosts a low-mass planet orbiting with a period of 8.71 days. Based on an analysis of the radial velocity (RV) time series obtained from the available data of the spectrographs HARPS, HARPS-N, CARMENES and HIRES, we announce the discovery of a second low-mass planet orbiting the star. We performed a RV global analysis on RV, spectroscopic activity indicators, and ASAS photometry, within the multidimensional Gaussian process framework, updated the parameters of GJ 536 b, and found significant evidence of the presence of a second planet. GJ 536 c is a low-mass planet ($m_{p} \sin i$ = 5.89 $\pm$ 0.70 M$_{\oplus}$), orbiting with a period of 32.761 $\pm$ 0.015 days, at a distance of 0.1617 $\pm$ 0.0028 au from its parent star. It induces an RV semi-amplitude of 1.80 $\pm$ 0.20 m$\cdot$s$^{-1}$. Given its distance to the star, it receives a flux of 1.692 $\pm$ 0.069 F$_{\oplus}$, for an equilibrium temperature of 290.5 $\pm$ 9.5 K. We update the mass of the planet GJ 536 b to $m_{p} \sin i$ = 6.37 $\pm$ 0.38 M$_{\oplus}$. The orbits of both planets are consistent with circular. We explored the use of statistical Doppler imaging on the photometric and RV data, and find a tentative projected obliquity of the stellar rotation axis of 58$^{+16}_{-19}$ deg. Current evidence does not support the presence of additional planets with masses > 5 M$_{\oplus}$for orbital periods up to 100 days, or > 10 M$_{\oplus}$ for periods up to 1000 days.

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arXiv2025

Stellar-activity analysis of the nearby M dwarf GJ 526. Multi-dimensional Gaussian-process modelling of RV, FWHM and S-index

M dwarfs are the most abundant stars in the Galaxy, and their low masses make them natural favourites for exoplanet radial-velocity (RV) searches. Nevertheless, these stars often demonstrate strong stellar activity that is yet to be fully understood. We use high-precision ESPRESSO, HIRES, and HARPS spectroscopy to perform a stellar-activity analysis on the nearby early M dwarf GJ 526 ($d=5.4$ pc). We carry out joint modelling of: (i) stellar rotation in RV, FWHM, and Mount Wilson S-index through Gaussian processes, (ii) long-term trends in these three physical quantities, (iii) RV planetary signals. We constrain the stellar-rotation period to $P_\text{rot}=48.7\pm 0.3$ d, and discover a weak sinusoidal signature in RV, FWHM and S-index of period $P_\text{cyc}=1680^{+50}_{-40}$ d. We propose phase-space trajectories between RV and activity indicators as a novel means to visualise and interpret stellar activity. Current evidence does not support planetary companions of GJ 526.

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arXiv2025

HADES RV Programme with HARPS-N at TNG. XVI. A super-Earth in the habitable zone of the GJ 3998 multi-planet system

The low masses of M dwarfs create attractive opportunities for exoplanet radial-velocity (RV) detections. These stars, however, exhibit strong stellar activity that may attenuate or mimic planetary signals. We present a velocimetric analysis on one such M dwarf, GJ 3998 ($d=18.2\,\text{pc}$), with two published short-period super-Earths: GJ 3998 b and GJ 3998 c. We use additional data from the HARPS-N spectrograph to confirm these two planets and to look for more. We carry out joint modelling of: (i) RV planetary signals, (ii) stellar rotation in RV and activity indicators through Gaussian processes, (iii) long-term trends in RV and activity indicators. We constrain the rotational period of GJ 3998 to $P_\text{rot}=30.2\pm 0.3\,\text{d}$ and discover long-term sinusoidal imprints in RV and FWHM of period $P_\text{cyc}=316^{+14}_{-8}\,\text{d}$. We confirm GJ 3998 b and GJ 3998 c, and detect a third planet: GJ 3998 d, whose signal had been previously attributed to stellar activity. GJ 3998 d has an orbital period of $41.78\pm 0.05\,\text{d}$, a minimum mass of $6.07^{+1.00}_{-0.96}\,\text{M}_\oplus$ and a mean insolation flux of $1.2^{+0.3}_{-0.2}\,\Phi_\oplus$. This makes it one of the few known planets receiving Earth-like insolation flux.

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arXiv2025

Revisiting the multi-planetary system of the nearby star HD 20794: Confirmation of a low-mass planet in the habitable zone of a nearby G-dwarf

Close-by Earth analogs and super-Earths are of primary importance because they will be preferential targets for the next generation of direct imaging instruments. Bright and close-by G-to-M type stars are preferential targets in radial velocity surveys to find Earth analogs. We present an analysis of the RV data of the star HD 20794, a target whose planetary system has been extensively debated in the literature. The broad time span of the observations makes it possible to find planets with signal semi-amplitudes below 1 m/s in the habitable zone. We monitored the system with ESPRESSO. We joined ESPRESSO data with the HARPS data, including archival data and new measurements from a recent program. We applied the post-processing pipeline YARARA to HARPS data to correct systematics, improve the quality of RV measurements, and mitigate the impact of stellar activity. Results. We confirm the presence of three planets, with periods of 18.3142 +/- 0.0022 d, 89.68 +/- 0.10 d, and 647.6 +/- 2.6 d, along with masses of 2.15 +/- 0.17 MEarth, 2.98 +/- 0.29 MEarth, and 5.82 +/- 0.57 MEarth respectively. For the outer planet, we find an eccentricity of 0.45 +/- 0.10, whereas the inner planets are compatible with circular orbits. The latter is likely to be a rocky planet in the habitable zone of HD 20794. From the analysis of activity indicators, we find evidence of a magnetic cycle with a period around 3000 d, along with evidence pointing to a rotation period around 39 d. We have determined the presence of a system of three planets orbiting the solar-type star HD 20794. This star is bright (V=4.34 mag) and close (d = 6.04 pc), and HD 20794 d resides in the stellar habitable zone, making this system a high-priority target for future atmospheric characterization with direct imaging facilities.

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arXiv2024

A sub-Earth-mass planet orbiting Barnard's star

Barnard's star is a primary target within the ESPRESSO guaranteed time observations (GTO) as it is the second closest neighbour to our Sun after the $\alpha$ Centauri stellar system. We present here a large set of 156 ESPRESSO observations of Barnard's star carried out over four years with the goal of exploring periods of shorter than 50 days, thus including the habitable zone (HZ). Our analysis of ESPRESSO data using Gaussian process (GP) to model stellar activity suggests a long-term activity cycle at 3200d and confirms stellar activity due to rotation at 140d as the dominant source of radial velocity (RV) variations. These results are in agreement with findings based on publicly available HARPS, HARPS-N, and CARMENES data. ESPRESSO RVs do not support the existence of the previously reported candidate planet at 233d. After subtracting the GP model, ESPRESSO RVs reveal several short-period candidate planet signals at periods of 3.15d, 4.12d, 2.34d, and 6.74d. We confirm the 3.15d signal as a sub-Earth mass planet, with a semi-amplitude of $55 \pm 7$cm/s, leading to a planet minimum mass $m_p \sin i$ of $0.37 \pm 0.05$Mearth, which is about three times the mass of Mars. ESPRESSO RVs suggest the possible existence of a candidate system with four sub-Earth mass planets in circular orbits with semi-amplitudes from 20 to 47cm/s, thus corresponding to minimum masses in the range of 0.17-0.32Mearth. The sub-Earth mass planet at $3.1533 \pm 0.0006$d is in a close-to circular orbit with a semi-major axis of $0.0229 \pm 0.0003$AU, thus located inwards from the HZ of Barnard's star, with an equilibrium temperature of 400K. Additional ESPRESSO observations would be required to confirm that the other three candidate signals originate from a compact short-period planet system orbiting Barnard's star inwards from its HZ.

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arXiv2024

A sub-Earth-mass planet orbiting Barnard's star: No evidence of transits in TESS photometry

A sub-Earth-mass planet orbiting Barnard's star, designated as Barnard b, has been recently announced. At almost the same time, the first photometric data of Barnard's star by the Transit Exoplanet Survey Satellite (TESS) was released in Sector 80. We explore the possibility of emergent transits of Barnard b in TESS photometry. The detrended 2 min light curve appears to be flat, with a flux root mean square of 0.411 parts per thousand. Attempts of blind and informed transit-curve model inference suggest no evidence of transiting Barnard b, or any other body. This provides a 3$\sigma$ upper bound of 87.9 degrees for the orbital inclination of Barnard b.

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arXiv2024

Characterisation of TOI-406 as showcase of the THIRSTEE program: A 2-planet system straddling the M-dwarf density gap

The exoplanet sub-Neptune population currently poses a conundrum, as to whether small-size planets are volatile-rich cores without an atmosphere, or rocky cores surrounded by a H-He envelope. To test the different hypotheses from an observational point of view, a large sample of small-size planets with precise mass and radius measurements is the first step. On top of that, much more information will likely be needed, including atmospheric characterisation and a demographic perspective on their bulk properties. We present here the concept and strategy of the THIRSTEE project, which aims to shed light on the composition of the sub-Neptune population across stellar types by increasing their number and improving the accuracy of bulk density measurements, as well as investigating their atmospheres and performing statistical, demographic analysis. We report the first results of the program, characterising a new two-planet system around the M-dwarf TOI-406. We analyse TESS and ground-based photometry, together with ESPRESSO and NIRPS/HARPS RVs to derive the orbital parameters and investigate the internal composition of the 2 planets orbiting TOI-406, which have radii and masses of $R_c = 1.32 \pm 0.12 R_{\oplus}$, $M_c = 2.08_{-0.22}^{+0.23} M_{\oplus}$ and $R_b = 2.08_{-0.15}^{+0.16} R_{\oplus}$, $M_b = 6.57_{-0.90}^{+1.00} M_{\oplus}$, and periods of $3.3$ and $13.2$ days, respectively. Planet c is consistent with an Earth-like composition, while planet b is compatible with multiple internal composition models, including volatile-rich planets without H/He atmospheres. The 2 planets are located in 2 distinct regions in the mass-density diagram, supporting the existence of a density gap among small exoplanets around M dwarfs. With an T$_{\rm eq}$ of only 368 K, TOI-406 b stands up as a particularly interesting target for atmospheric characterisation with JWST in the low-temperature regime.

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arXiv2024

TESS and ESPRESSO discover a super-Earth and a mini-Neptune orbiting the K-dwarf TOI-238

The number of super-Earth and mini-Neptune planet discoveries has increased significantly in the last two decades thanks to transit and radial velocity surveys. When it is possible to apply both techniques, we can characterise the internal composition of exoplanets, which in turn provides unique insights on their architecture, formation and evolution. We performed a combined photometric and radial velocity analysis of TOI-238 (TYC 6398-132-1), which has one short-orbit super-Earth planet candidate announced by NASA's TESS team. We aim to confirm its planetary nature using radial velocities taken with the ESPRESSO and HARPS spectrographs, to measure its mass and to detect the presence of other possible planetary companions. We carried out a joint analysis by including Gaussian processes and Keplerian orbits to account for the stellar activity and planetary signals simultaneously. We detected the signal induced by TOI-238 b in the radial velocity time-series, and the presence of a second transiting planet, TOI-238 c, whose signal appears in RV and TESS data. TOI-238 b is a planet with a radius of 1.402$^{+0.084}_{-0.086}$ R$_{\oplus}$ and a mass of 3.40$^{+0.46}_{-0.45}$ M$_{\oplus}$. It orbits at a separation of 0.02118 $\pm$ 0.00038 AU of its host star, with an orbital period of 1.2730988 $\pm$ 0.0000029 days, and has an equilibrium temperature of 1311 $\pm$ 28 K. TOI-238 c has a radius of 2.18$\pm$ 0.18 R$_{\oplus}$ and a mass of 6.7 $\pm$ 1.1 M$_{\oplus}$. It orbits at a separation of 0.0749 $\pm$ 0.0013 AU of its host star, with an orbital period of 8.465652 $\pm$ 0.000031 days, and has an equilibrium temperature of 696 $\pm$ 15 K. The mass and radius of planet b are fully consistent with an Earth-like composition, making it likely a rocky super-Earth. Planet c could be a water-rich planet or a rocky planet with a small H-He atmosphere.

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arXiv2024

The compact multi-planet system GJ 9827 revisited with ESPRESSO

GJ 9827 is a bright, nearby K7V star orbited by two super-Earths and one mini-Neptune on close-in orbits. The system was first discovered using K2 data and then further characterized by other spectroscopic and photometric instruments. Previous literature studies provide several mass measurements for the three planets, however, with large variations and uncertainties. To better constrain the planetary masses, we added high-precision radial velocity measurements from ESPRESSO to published datasets from HARPS, HARPS-N, and HIRES and we performed a Gaussian process analysis combining radial velocity and photometric datasets from K2 and TESS. This method allowed us to model the stellar activity signal and derive precise planetary parameters. We determined planetary masses of $M_b = 4.28_{-0.33}^{+0.35}$ M${_\oplus}$, $M_c = 1.86_{-0.39}^{+0.37}$ M${_\oplus}$, and $M_d = 3.02_{-0.57}^{+0.58}$ M${_\oplus}$, and orbital periods of $1.208974 \pm 0.000001$ days for planet b, $3.648103_{-0.000010}^{+0.000013}$ days for planet c, and $6.201812 \pm 0.000009$ days for planet d. We compared our results to literature values and found that our derived uncertainties for the planetary mass, period, and radial velocity amplitude are smaller than the previously determined uncertainties. We modeled the interior composition of the three planets using the machine-learning-based tool ExoMDN and conclude that GJ 9827 b and c have an Earth-like composition, whereas GJ 9827 d has an hydrogen envelope, which, together with its density, places it in the mini-Neptune regime.

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