arXiv subjects

M. Pinamonti

Publications and source records attributed to M. Pinamonti.

At least 55 records · Page 3Linked to original sources

arXiv2022

A quarter century of spectroscopic monitoring of the nearby M dwarf Gl 514. A super-Earth on an eccentric orbit moving in and out of the habitable zone

We investigated the presence of planetary companions around the nearby (7.6 pc) and bright ($V=9$ mag) early-type M dwarf Gl 514, analysing 540 radial velocities collected over nearly 25 years with the HIRES, HARPS, and CARMENES spectrographs. The data are affected by time-correlated signals at the level of 2-3 ms$^{-1}$ due to stellar activity, that we filtered out testing three different models based on Gaussian process regression. As a sanity cross-check, we repeated the analyses using HARPS radial velocities extracted with three different algorithms. We used HIRES radial velocities and Hipparcos-Gaia astrometry to put constraints on the presence of long-period companions, and we analysed TESS photometric data. We found strong evidence that Gl 514 hosts a super-Earth on a likely eccentric orbit, residing in the conservative habitable zone for nearly $34\%$ of its orbital period. The planet Gl 514 b has minimum mass $m_b\sin i_b=5.2\pm0.9$ $M_{\rm Earth}$, orbital period $P_b=140.43\pm0.41$ days, and eccentricity $e_b=0.45^{+0.15}_{-0.14}$. No evidence for transits is found in the TESS light curve. There is no evidence for a longer period companion in the radial velocities and, based on astrometry, we can rule out a $\sim0.2$ $M_{\rm Jup}$ planet at a distance of $\sim3-10$ au, and massive giant planets/brown dwarfs out to several tens of au. We discuss the possible presence of a second low-mass companion at a shorter distance from the host than Gl 514 b. Gl 514 b represents an interesting science case to study the habitability of planets on eccentric orbits. We advocate for additional spectroscopic follow-up to get more accurate and precise planetary parameters. Further follow-up is also needed to investigate sub \ms and shorter period signals.

astro-ph.EP↗

arXiv2022

HADES RV Programme with HARPS-N at TNG XV. Planetary occurrence rates around early-M dwarfs

We present the complete Bayesian statistical analysis of the HArps-n red Dwarf Exoplanet Survey (HADES), which monitored the radial velocities of a large sample of M dwarfs with HARPS-N at TNG, over the last 6 years. The targets were selected in a narrow range of spectral types from M0 to M3, $0.3$ M$_\odot < M_\star < 0.71$ M$_\odot$, in order to study the planetary population around a well-defined class of host stars. We take advantage of Bayesian statistics to derive an accurate estimate of the detectability function of the survey. Our analysis also includes the application of Gaussian Process approach to take into account stellar activity induced radial velocity variations, and improve the detection limits, around the most-observed and most-active targets. The Markov chain Monte Carlo and Gaussian process technique we apply in this analysis has proven very effective in the study of M-dwarf planetary systems, helping the detection of most of the HADES planets. From the detectability function we can calculate the occurrence rate of small mass planets around early-M dwarfs, either taking into account only the 11 already published HADES planets or adding also the 5 new planetary candidates discovered in this analysis, and compare them with the previous estimates of planet occurrence around M-dwarf or Solar-type stars: considering only the confirmed planets, we find the highest frequency for low-mass planets ($1$ M$_\oplus < m_p \sin i < 10$ M$_\oplus$) with periods $10$ d$ < P < 100$ d, $f_\text{occ} = 85^{+5}_{-19}\%$, while for short-period planets ($1$ d$ < P < 10$ d) we find a frequency of $f_\text{occ} = 10.3^{+8.4}_{-3.3}\%$, significantly lower than for later-M dwarfs. These results, and their comparison with other surveys focused on different stellar types, confirms the central role that stellar mass plays in the formation and evolution of planetary systems.

astro-ph.EP↗

arXiv2021

HADES RV programme with HARPS-N at TNG XIV. A candidate super-Earth orbiting the M-dwarf GJ 9689 with a period close to half the stellar rotation period

Context. It is now well-established that small, rocky planets are common around low-mass stars. However, the detection of such planets is challenged by the short-term activity of the host stars. Aims. The HArps-N red Dwarf Exoplanet Survey (HADES) program is a long-term project at the Telescopio Nazionale Galileo aimed at the monitoring of nearby, early-type, M dwarfs, using the HARPS-N spectrograph to search for small, rocky planets. Methods. A total of 174 HARPS-N spectroscopic observations of the M0.5V-type star GJ 9689 taken over the past seven years have been analysed. We combined these data with photometric measurements to disentangle signals related to the stellar activity of the star from possible Keplerian signals in the radial velocity data. We run an MCMC analysis, applying Gaussian Process regression techniques to model the signals present in the data. Results. We identify two periodic signals in the radial velocity time series, with periods of 18.27 d, and 39.31 d. The analysis of the activity indexes, photometric data, and wavelength dependency of the signals reveals that the 39.31 d signal corresponds to the stellar rotation period. On the other hand, the 18.27 d signal shows no relation to any activity proxy or the first harmonic of the rotation period. We, therefore, identify it as a genuine Keplerian signal. The best-fit model describing the newly found planet, GJ 9689 b, corresponds to an period P$_{\rm b}$ = 18.27 $\pm$ 0.01 d, and a minimum mass M$_{\rm P}\sin i$ = 9.65 $\pm$ 1.41 M$_{\oplus}$.

astro-ph.SR↗

arXiv2021

Three Years of HARPS-N High-Resolution Spectroscopy and Precise Radial Velocity Data for the Sun

The solar telescope connected to HARPS-N has been observing the Sun since the summer of 2015. Such high-cadence, long-baseline data set is crucial for understanding spurious radial-velocity signals induced by our Sun and by the instrument. On the instrumental side, this data set allowed us to detect sub-\ms\,systematics that needed to be corrected for. The goal of this manuscript is to i) present a new data reduction software for HARPS-N, ii) demonstrate the improvement brought by this new software on the first three years of the HARPS-N solar data set, and iii) release all the obtained solar products, from extracted spectra to precise radial velocities. To correct for the instrumental systematics observed in the data reduced with the current version of the HARPS-N data reduction software (DRS version 3.7), we adapted the newly available ESPRESSO DRS (version 2.2.3) to HARPS-N and developed new optimized recipes for the spectrograph. We then compared the first three years of HARPS-N solar data reduced with the current and new DRS. The most significant improvement brought by the new DRS is a strong decrease in the day-to-day radial-velocity scatter, from 1.27 to 1.07\ms; this is thanks to a more robust method to derive wavelength solutions, but also to the use of calibrations closer in time. The newly derived solar radial-velocities are also better correlated with the chromospheric activity level of the Sun on the long-term, with a Pearson correlation coefficient of 0.93 compared to 0.77 before, which is expected from our understanding of stellar signals. Finally, we also discuss how HARPS-N spectral ghosts contaminate the measurement of the calcium activity index, and present an efficient technique to derive an index free of instrumental systematics. This paper presents a new data reduction software for HARPS-N, and demonstrates its improvements [...]

astro-ph.SR↗

arXiv2021

A super-Earth on a close-in orbit around the M1V star GJ 740. A HADES and CARMENES collaboration

M-dwarfs have proven to be ideal targets for planetary radial velocity (RV) searches due to their higher planet-star mass contrast. The HADES and CARMENES programs aim to carry out extensive searches of exoplanetary systems around this type of stars in the northern hemisphere, allowing us to address statistically the properties of the planets orbiting these objects. In this work, we perform a spectroscopic and photometric study of one of the program stars (GJ 740), which exhibits a short-period RV signal compatible with a planetary companion. We carried out a spectroscopic analysis based on 129 HARPS-N spectra taken over a time-span of 6 yr combined with 57 HARPS spectra taken over 4 yr, as well as 32 CARMENES spectra taken during more than 1 yr, resulting in a dataset with a time coverage of 10 yr. We also relied on 459 measurements from the public ASAS survey with a time-coverage of 8 yr along with 5 yr of photometric magnitudes from the EXORAP project taken in the $V$, $B$, $R$, and $I$ filters to carry out a photometric study. Both analyses were made using Markov Chain Monte Carlo (MCMC) simulations and Gaussian Process regression to model the activity of the star. We present the discovery of a short-period super-Earth with an orbital period of 2.37756$^{+0.00013}_{-0.00011}$ d and a minimum mass of 2.96$^{+0.50}_{-0.48}$ M$_{\oplus}$. We offer an update to the previously reported characterization of the magnetic cycle and rotation period of the star, obtaining values of $P_{\rm rot}$=35.563$\pm$0.071 d and $P_{\rm cycle}$=2800$\pm$150 d. Furthermore, the RV time-series exhibits a possibly periodic long-term signal which might be related to a Saturn-mass planet of $\sim$ 100 M$_{\oplus}$.

astro-ph.EP↗

arXiv2021

HADES RV Programme with HARPS-N at TNG XIII. A sub-Neptune around the M dwarf GJ 720 A

Context. The high number of super-Earth and Earth-like planets in the habitable zone (HZ) detected around M-dwarf stars in the last years has revealed these stellar objects to be the key for planetary radial velocity (RV) searches. Aims. Using the HARPS-N spectrograph within The HArps-n red Dwarf Exoplanet Survey (HADES) we reach the precision needed to detect small planets with a few Earth masses using the RV technique. Methods. We obtained 138 HARPS-N RV measurements between 2013 May and 2020 September of GJ 720 A, classified as an M0.5V star located at a distance of 15.56 pc. To characterize the stellar variability and to discern the periodic variation due to the Keplerian signals from those related to stellar activity, the HARPS-N spectroscopic activity indicators and the simultaneous photometric observations were analyzed. The combined analysis of HARPS-N RVs and activity indicators let us to address the nature of the periodic signals. The final model and the orbital planetary parameters were obtained by fitting simultaneously the stellar variability and the Keplerian signal using a Gaussian process regression and following a Bayesian criterion. Results. The HARPS-N RV periodic signals around 40 d and 100 d have counterparts at the same frequencies in HARPS-N activity indicators and photometric light curves. Then we attribute these periodicities to stellar activity the former period being likely associated with the stellar rotation. GJ 720 A shows the most significant signal at 19.466$\pm$0.005 d with no counterparts in any stellar activity indices. We hence ascribe this RV signal, having a semiamplitude of 4.72$\pm$0.27 m/s , to the presence of a sub-Neptune mass planet. The planet GJ 720 Ab has a minimum mass of 13.64$\pm$0.79 M$_{\oplus}$, it is in circular orbit at 0.119$\pm$0.002 AU from its parent star, and lies inside the inner boundary of the HZ around its parent star.

astro-ph.EP↗

arXiv2020

K2-111: an old system with two planets in near-resonance

This paper reports on the detailed characterisation of the K2-111 planetary system with K2, WASP, and ASAS-SN photometry as well as high-resolution spectroscopic data from HARPS-N and ESPRESSO. The host, K2-111, is confirmed to be a mildly evolved ($\log g=4.17$), iron-poor ([Fe/H]$=-0.46$), but alpha-enhanced ([$α$/Fe]$=0.27$), chromospherically quiet, very old thick disc G2 star. A global fit, performed by using PyORBIT shows that the transiting planet, K2-111b, orbits with a period $P_b=5.3518\pm0.0004$ d, and has a planet radius of $1.82^{+0.11}_{-0.09}$ R$_\oplus$ and a mass of $5.29^{+0.76}_{-0.77}$ M$_\oplus$, resulting in a bulk density slightly lower than that of the Earth. The stellar chemical composition and the planet properties are consistent with K2-111b being a terrestrial planet with an iron core mass fraction lower than the Earth. We announce the existence of a second signal in the radial velocity data that we attribute to a non-transiting planet, K2-111c, with an orbital period of $15.6785\pm 0.0064$ days, orbiting in near-3:1 mean-motion resonance with the transiting planet, and a minimum planet mass of $11.3\pm1.1$ M$_\oplus$. Both planet signals are independently detected in the HARPS-N and ESPRESSO data when fitted separately. There are potentially more planets in this resonant system, but more well-sampled data are required to confirm their presence and physical parameters.

astro-ph.EP↗

arXiv2020

An unusually low density ultra-short period super-Earth and three mini-Neptunes around the old star TOI-561

Based on HARPS-N radial velocities (RVs) and TESS photometry, we present a full characterisation of the planetary system orbiting the late G dwarf TOI-561. After the identification of three transiting candidates by TESS, we discovered two additional external planets from RV analysis. RVs cannot confirm the outer TESS transiting candidate, which would also make the system dynamically unstable. We demonstrate that the two transits initially associated with this candidate are instead due to single transits of the two planets discovered using RVs. The four planets orbiting TOI-561 include an ultra-short period (USP) super-Earth (TOI-561 b) with period $P_{\rm b} = 0.45$ d, mass $M_{\rm b} =1.59 \pm 0.36$ M$_\oplus$ and radius $R_{\rm b}=1.42 \pm 0.07$ R$_\oplus$, and three mini-Neptunes: TOI-561 c, with $P_{\rm c} = 10.78$ d, $M_{\rm c} = 5.40 \pm 0.98$ M$_\oplus$, $R_{\rm c}= 2.88 \pm 0.09$ R$_\oplus$; TOI-561 d, with $P_{\rm d} = 25.6$ d, $M_{\rm d} = 11.9 \pm 1.3$ M$_\oplus$, $R_{\rm d} = 2.53 \pm 0.13$ R$_\oplus$; and TOI-561 e, with $P_{\rm e} = 77.2$ d, $M_{\rm e} = 16.0 \pm 2.3$ M$_\oplus$, $R_{\rm e} = 2.67 \pm 0.11$ R$_\oplus$. Having a density of $3.0 \pm 0.8$ g cm$^{-3}$, TOI-561 b is the lowest density USP planet known to date. Our N-body simulations confirm the stability of the system and predict a strong, anti-correlated, long-term transit time variation signal between planets d and e. The unusual density of the inner super-Earth and the dynamical interactions between the outer planets make TOI-561 an interesting follow-up target.

astro-ph.EP↗

arXiv2020

HADES RV Programme with HARPS-N at TNG XII. The abundance signature of M dwarf stars with planets

Most of our current knowledge on planet formation is still based on the analysis of main-sequence, solar-type stars. Conversely, detailed chemical studies of large samples of M-dwarf planet hosts are still missing. We develop for the first time a methodology to determine stellar abundances of elements others than iron for M dwarf stars from high-resolution, optical spectra. Our methodology is based on the use of principal component analysis and sparse Bayesian's methods. We made use of a set of M dwarfs orbiting around an FGK primary with known abundances to train our methods. We applied our methods to derive stellar metalliticies and abundances of a large sample of M dwarfs observed within the framework of current radial velocity surveys. We then used a sample of nearby FGK stars to cross-validate our technique by comparing the derived abundance trends in the M dwarf sample with those found on the FGK stars. The metallicity distribution of the different subsamples shows that M dwarfs hosting giant planets show a planet-metallicity correlation as well as a correlation with the stellar mass. M dwarfs hosting low-mass planets do not seem to follow the planet-metallicity correlation. We also found that the frequency of low-mass planets does not depend on the mass of the stellar host. These results seem in agreement with previous works. However, we note that for giant planet hosts our metallicities predict a weaker planet metallicity correlation but a stronger mass-dependency than photometric values. We show, for the first time, that there seems to be no differences in the abundance distribution of elements different from iron between M dwarfs with and without known planets. Our data shows that low-mass stars with planets follow the same metallicity, mass, and abundance trends than their FGK counterparts.

astro-ph.SR↗

arXiv2020

The GAPS Programme at TNG XXVII. Reassessment of a young planetary system with HARPS-N: is the hot Jupiter V830 Tau b really there?

Detecting and characterising exoworlds around very young stars (age$<$10 Myr) are key aspects of exoplanet demographic studies, especially for understanding the mechanisms and timescales of planet formation and migration. However, detection using the radial velocity method alone can be very challenging, since the amplitude of the signals due to magnetic activity of such stars can be orders of magnitude larger than those induced even by massive planets. We observed the very young ($\sim$2 Myr) and very active star V830 Tau with the HARPS-N spectrograph to independently confirm and characterise the previously reported hot Jupiter V830 Tau b ($K_{\rm b}=68\pm11$ m/s; $m_{\rm b}sini_{\rm b}=0.57\pm0.10$ $M_{jup}$; $P_{\rm b}=4.927\pm0.008$ d). Due to the observed $\sim$1 km/s radial velocity scatter clearly attributable to V830 Tau's magnetic activity, we analysed radial velocities extracted with different pipelines and modelled them using several state-of-the-art tools. We devised injection-recovery simulations to support our results and characterise our detection limits. The analysis of the radial velocities was aided by using simultaneous photometric and spectroscopic diagnostics. Despite the high quality of our HARPS-N data and the diversity of tests we performed, we could not detect the planet V830 Tau b in our data and confirm its existence. Our simulations show that a statistically-significant detection of the claimed planetary Doppler signal is very challenging. Much as it is important to continue Doppler searches for planets around young stars, utmost care must be taken in the attempt to overcome the technical difficulties to be faced in order to achieve their detection and characterisation. This point must be kept in mind when assessing their occurrence rate, formation mechanisms and migration pathways, especially without evidence of their existence from photometric transits.

astro-ph.EP↗

arXiv2020

u-RANIA: a neutron detector based on μ-RWELL technology

In the framework of the ATTRACT-uRANIA project, funded by the European Community, we are developing an innovative neutron imaging detector based on micro-Resistive WELL ($μ$ -RWELL) technology. The $μ$ -RWELL, based on the resistive detector concept, ensuring an efficient spark quenching mechanism, is a highly reliable device. It is composed by two main elements: a readout-PCB and a cathode. The amplification stage for this device is embedded in the readout board through a resistive layer realized by means of an industrial process with DLC (Diamond-Like Carbon). A thin layer of B$_4$C on the copper surface of the cathode allows the thermal neutrons detection through the release of $^7$Li and $α$ particles in the active volume. This technology has been developed to be an efficient and convenient alternative to the $^3$He shortage. The goal of the project is to prove the feasibility of such a novel neutron detector by developing and testing small planar prototypes with readout boards suitably segmented with strip or pad read out, equipped with existing electronics or readout in current mode. Preliminary results from the test with different prototypes, showing a good agreement with the simulation, will be presented together with construction details of the prototypes and the future steps of the project.

physics.ins-det↗

arXiv2020

The GAPS Programme at TNG -- XXIV. An eccentric Neptune-mass planet near the inner edge of the BD-11 4672 habitable zone

With the growth of comparative exoplanetology, it is increasingly clear that investigating the relationships between inner and outer planets plays a key role in discriminating between competing formation and evolution models. To do so, it is important to probe the inner region of systems hosting long-period giants in search for undetected lower-mass planetary companions. In this work we present our results on the K-dwarf star BD-11~4672, already known to host a long-period giant planet, as the first output of a subsample of the GAPS programme specifically aimed at assessing the impact of inefficient migration of planets formed beyond the snowline by searching for Neptune-mass and super-Earths planetary companions of known longer-period giants. The high-precision HARPS-N observations of BD-11~4672 are used in conjunction with literature time series in order to search for additional inner planetary signals to be fitted using differential evolution Markov chain Monte Carlo. The long-term stability of the new orbital solutions is tested by N-body dynamical simulations. We report the detection of BD-11~4672~c, a new Neptune-mass planet with an orbital period of $74.20_{-0.08}^{+0.06}\,\rm{d}$, eccentricity $0.40_{-0.15}^{+0.13}$, semimajor axis $0.30\pm0.01\,\rm{au}$ and minimum mass $15.37_{-2.81}^{+2.97}\,M_\oplus$ orbiting slightly outside the inner edge of the optimistic circumstellar habitable zone. In order to assess its impact on the dynamical stability of the habitable zone we compute the angular momentum deficit of the system, showing that planet c has a severe negative impact on the stability of possible additional lower-mass temperate planets. The BD-11~4672 system is notable for its architecture, hosting both a long-period giant planet and an inner lower-mass planet, the latter being also among the most eccentric Neptune-mass planets known at similar periods.

astro-ph.EP↗

arXiv2020

The GAPS Programme at TNG -- XXIII. HD 164922 d: a close-in super-Earth discovered with HARPS-N in a system with a long-period Saturn mass companion

In the framework of the Global Architecture of Planetary Systems (GAPS) project we collected more than 300 spectra with HARPS-N at the TNG for the bright G9V star HD164922. This target is known to host one gas giant planet in a wide orbit (Pb~1200 days, semi-major axis ~2 au) and a Neptune-mass planet with a period Pc ~76 days. We searched for additional low-mass companions in the inner region of the system. We compared the radial velocities (RV) and the activity indices derived from the HARPS-N time series to measure the rotation period of the star and used a Gaussian process regression to describe the behaviour of the stellar activity. We exploited this information in a combined model of planetary and stellar activity signals in an RV time-series composed of almost 700 high-precision RVs, both from HARPS-N and literature data. We performed a dynamical analysis to evaluate the stability of the system and the allowed regions for additional potential companions. Thanks to the high sensitivity of the HARPS-N dataset, we detect an additional inner super-Earth with an RV semi-amplitude of 1.3+/-0.2 m/s, a minimum mass of ~4+/-1 M_E and a period of 12.458+/-0.003 days. We disentangle the planetary signal from activity and measure a stellar rotation period of ~42 days. The dynamical analysis shows the long term stability of the orbits of the three-planet system and allows us to identify the permitted regions for additional planets in the semi-major axis ranges 0.18-0.21 au and 0.6-1.4 au. The latter partially includes the habitable zone of the system. We did not detect any planet in these regions, down to minimum detectable masses of 5 and 18 M_E, respectively. A larger region of allowed planets is expected beyond the orbit of planet b, where our sampling rules-out bodies with minimum mass > 50 M_E.

astro-ph.EP↗

arXiv2020

The GAPS Programme at TNG XXI -- A GIARPS case-study of known young planetary candidates: confirmation of HD 285507 b and refutation of AD Leo b

The existence of hot Jupiters is still not well understood. Two main channels are thought to be responsible for their current location: a smooth planet migration through the proto-planetary disk or the circularization of an initial high eccentric orbit by tidal dissipation leading to a strong decrease of the semimajor axis. Different formation scenarios result in different observable effects, such as orbital parameters (obliquity/eccentricity), or frequency of planets at different stellar ages. In the context of the GAPS Young-Objects project, we are carrying out a radial velocity survey with the aim to search and characterize young hot-Jupiter planets. Our purpose is to put constraints on evolutionary models and establish statistical properties, such as the frequency of these planets from a homogeneous sample. Since young stars are in general magnetically very active, we performed multi-band (visible and near-infrared) spectroscopy with simultaneous GIANO-B + HARPS-N (GIARPS) observing mode at TNG. This helps to deal with stellar activity and distinguish the nature of radial velocity variations: stellar activity will introduce a wavelength-dependent radial velocity amplitude, whereas a Keplerian signal is achromatic. As a pilot study, we present here the cases of two already claimed hot Jupiters orbiting young stars: HD285507 b and AD Leo b. Our analysis of simultaneous high-precision GIARPS spectroscopic data confirms the Keplerian nature of HD285507's radial velocities variation and refines the orbital parameters of the hot Jupiter, obtaining an eccentricity consistent with a circular orbit. On the other hand, our analysis does not confirm the signal previously attributed to a planet orbiting AD Leo. This demonstrates the power of the multi-band spectroscopic technique when observing active stars.

astro-ph.EP↗

arXiv2019

Photometric rotation periods for 107 M dwarfs from the APACHE survey

We present rotation period measurements for 107 M dwarfs in the mass range $0.15-0.70 M_\odot$ observed within the context of the APACHE photometric survey. We measure rotation periods in the range 0.5-190 days, with the distribution peaking at $\sim$ 30 days. We revise the stellar masses and radii for our sample of rotators by exploiting the Gaia DR2 data. For $\sim 20\%$ of the sample, we compare the photometric rotation periods with those derived from different spectroscopic indicators, finding good correspondence in most cases. We compare our rotation periods distribution to the one obtained by the Kepler survey in the same mass range, and to that derived by the MEarth survey for stars in the mass range $0.07-0.25 M_\odot$. The APACHE and Kepler periods distributions are in good agreement, confirming the reliability of our results, while the APACHE distribution is consistent with the MEarth result only for the older/slow rotators, and in the overlapping mass range of the two surveys. Combining the APACHE/Kepler distribution with the MEarth distribution, we highlight that the rotation period increases with decreasing stellar mass, in agreement with previous work. Our findings also suggest that the spin-down time scale, from fast to slow rotators, changes crossing the fully convective limit at $\approx0.3 M_\odot$ for M dwarfs. The catalogue of 107 rotating M dwarfs presented here is particularly timely, as the stars are prime targets for the potential identification of transiting small planets with TESS and amenable to high-precision mass determination and further atmospheric characterization measurements.

astro-ph.SR↗

arXiv2019

Biases in retrieving planetary signals in the presence of quasi-periodic stellar activity

Gaussian process regression is a widespread tool used to mitigate stellar correlated noise in radial velocity time series. It is particularly useful to search for and determine the properties of signals induced by small-size, low-mass planets ($R_p<4R_{\rm \oplus}$, $m_p<10M_{\rm \oplus}$). By using extensive simulations based on a quasi-periodic representation of the stellar activity component, we investigate the ability in retrieving the planetary parameters in 16 different realistic scenarios. We analyse systems composed by one planet and host stars having different levels of activity, focusing on the challenging case represented by low-mass planets, with Doppler semi-amplitudes in the range 1-3 $m s^{-1}$. We consider many different configurations for the quasi-periodic stellar activity component, as well as different combinations of the observing epochs. We use commonly-employed analysis tools to search for and characterize the planetary signals in the datasets. The goal of our injection-recovery statistical analysis is twofold. First, we focus on the problem of planet mass determination. Then, we analyse in a statistical way periodograms obtained with three different algorithms, in order to explore some of their general properties, as the completeness and reliability in retrieving the injected planetary and stellar activity signals with low false alarm probabilities. This work is intended to provide some understanding of the biases introduced in the planet parameters inferred from the analysis of radial velocity time series that contain correlated signals due to stellar activity. It also aims to motivate the use and encourage the improvement of extensive simulations for planning spectroscopic follow-up observations.

astro-ph.EP↗

arXiv2019

The HADES RV Programme with HARPS-N at TNG XI. GJ 685 b: a warm super-Earth around an active M dwarf

Small rocky planets seem to be very abundant around low-mass M-type stars. Their actual planetary population is however not yet precisely understood. Currently several surveys aim to expand the statistics with intensive detection campaigns, both photometric and spectroscopic. We analyse 106 spectroscopic HARPS-N observations of the active M0-type star GJ 685 taken over the past five years. We combine these data with photometric measurements from different observatories to accurately model the stellar rotation and disentangle its signals from genuine Doppler planetary signals in the RV data. We run an MCMC analysis on the RV and activity indexes time series to model the planetary and stellar signals present in the data, applying Gaussian Process regression technique to deal with the stellar activity signals. We identify three periodic signals in the RV time series, with periods of 9, 24, and 18 d. Combining the analyses of the photometry of the star with the activity indexes derived from the HARPS-N spectra, we identify the 18 d and 9 d signals as activity-related, corresponding to the stellar rotation period and its first harmonic respectively. The 24 d signals shows no relations with any activity proxy, so we identify it as a genuine planetary signal. We find the best-fit model describing the Doppler signal of the newly-found planet, GJ 685\,b, corresponding to an orbital period $P_b = 24.160^{+0.061}_{-0.047}$ d and a minimum mass $M_P \sin i = 9.0^{+1.7}_{-1.8}$ M$_\oplus$. We also study a sample of 70 RV-detected M-dwarf planets, and present new statistical evidence of a difference in mass distribution between the populations of single- and multi-planet systems, which can shed new light on the formation mechanisms of low-mass planets around late-type stars.

astro-ph.EP↗

arXiv2019

HADES RV Programme with HARPS-N at TNG. X. The non-saturated regime of the stellar activity-rotation relationship for M-Dwarfs

Aims. We aim to extend the relationship between X-ray luminosity (Lx) and rotation period (Prot) found for main-sequence FGK stars and test whether it also holds for early-M dwarfs, especially in the non-saturated regime (Lx α P_{rot}^{-2}) which corresponds to slow rotators. Methods. We use the luminosity coronal activity indicator (Lx) of a sample of 78 early-M dwarfs with masses in the range from 0.3 to 0.75 Msun from the HArps-N red Dwarf Exoplanet Survey (HADES) radial velocity (RV) programme collected from ROSAT and XMM-Newton. The determination of the rotation periods (P_{rot}) was done by analysing time-series of high-resolution spectroscopy of the Ca ii H & K and Hα activity indicators. Our sample principally covers the slow rotation regime with rotation periods from 15 to 60 days. Results. Our work extends to the low mass regime the observed trend for more massive stars showing a continuous shift of the Lx/Lbol vs. Prot power-law towards longer rotation period values and includes the determination, in a more accurate way, of the value of the rotation period at which the saturation occurs (P_{sat}) for M dwarf stars. Conclusions. We conclude that the relations between coronal activity and stellar rotation for FGK stars also hold for early-M dwarfs in the non-saturated regime, indicating that the rotation period is sufficient to determine the ratio Lx/Lbol.

astro-ph.SR↗