Search and characterization of exo-earths participation in the development of the espresso spectrograph for the VLT

The search for exo-Earths using Doppler spectroscopy faces nowadays two main difficulties. The first is a technical limitation. The precision of the instruments limits the measurable amplitudes to signals produced by super-Earths orbiting close to their host stars. Terrestrial planets orbiting solar-type stars induce variations smaller than 1 m s−1 except when orbiting very close to the star. The arrival of ESPRESSO and the Laser Frequency Comb as calibration unit will palliate this limitation and move the measurement precision to the range of 5 cm s−1 . So far the search in solar-Type stars is limited to super-Earths. The alternative is to find terrestrial planets in the habitable zone is the search around M-type stars. Their lower mass makes the signals of terrestrial planets detectable by today’s instruments. The second issue is the presence of radial velocity variations induced by the homogeneities of the stellar surface. The changes in the stellar surface and the stellar rotation induce periodic signals that go from less than 1 m s−1 in the quietest stars to dozens of m s−1 in active stars and might be easily mistaken for signals induced by rocky planets. In the case of M-dwarfs the need of performing the analysis of the stellar induced signals is even more important. Signals produced by early M-dwarfs have periods compatible with the habitable zone of those stars and amplitudes similar to those of super-Earths. The rise in precision expected for the next years will make this kind of signals visible even in the quietest stars making even more important to understand and correctly model them. Using chromospheric indicators from high resolution spectroscopic and photometric time series we study both long term and short term variability (solarlike cycles and rotation) of a sample of 176 low magnetic activity stars with spectral types going from late F-type to mid M-type stars. We provide new measurements of magnetic cycles for 105 stars, and rotation periods for 123 stars. We study the distribution of rotation periods, finding that there is an evolution in the rotation period lengths, getting longer for later type stars, that saturates when reaching K-type stars and begins to increase again at types later than M2. We study the distribution of magnetic cycles, finding no clear difference among different spectral types. We study the relationships between the measured periods and the chromospheric activity level of the stars, measured as the log10(R0 HK). To do so we extend the log10(R0 HK) calibration to cover stars up to B-V ∼ 2.0. We find a strong correlation between the measured rotation period and the mean level of chromospheric activity that allows to predict the rotation periods of main sequence stars from G-type to mid M-type with an accuracy of the ∼ 20% of the period. We analyse the radial velocity time series of 133 stars searching both for activity induce signals and planetary signals. We detect rotation induced signals in 46 stars, with amplitudes going from 0.3 to 16.4 ms−1 . We study the relationship between the amplitude of the induced signals and the activity indicators, finding a correlation between the mean level of chromospheric activity log10(R0 HK) and the measured amplitude of the signal. This relationship shows that FGK stars show a similar behaviour, which is different for M-type stars. We find also a relationship between the amplitude of the modulation in the Mount Wilson S index and the amplitude of the induced radial velocity signal. This relationship behaves differently for two populations of stars, with a breaking point around mid-K stars. After cleaning the radial velocity series form stellar activity induced signals we performed a search for dynamical signals in the same sample of stars. We recovered most of the previously published planet candidates, except for a few cases where we attribute a stellar origin to the previously assumed planetary companion. We present the discovery of six new planets around the stars HD 1581, HD 161098, HD 176986, GJ 536 and GJ 3998, with minimum masses in M⊕ units of 5.87 ± 0.99, 26.60 ± 4.39, 4.86 ± 0.66 and 5.47 ± 0.84, 2.35 ± 0.42 and 6.12 ± 1.11 respectively located at 0.321, 2.503, 0.061, 0.066, 0.029 and 0.089 au from their parent stars. The latter two have been reported by Affer et al. (2016). Five of them are presumably rocky planets. We suggest the presence of a seventh one in the star HD 161098 with a minimum mass of 5.30 ± 1.22 M⊕ located at 0.317 au of the star, which could be habitable given the appropriate atmospheric conditions. We suggest also the presence of five additional candidates that still require further analysis and observations.La búsqueda de exotierras por medio de espectroscopía doppler se enfrenta hoy en día a dos grandes dificultades. La primera es de carácter técnico. En estrellas de tipo solar la precisión de los instrumentos existentes limita la amplitud de las señales que se pueden medir a aquellas producidas por supertierras a separaciones orbitales cercanas a sus estrellas. Los planetas de tipo terrestre orbitando alrededor de estrellas de tipo solar producen variaciones muy inferiores a 1 m/s excepto a distancias muy cercanas a la estrella. La llegada de ESPRESSO y el Laser Frequency Comb como unidad de calibración paliarán esta limitación al llevar la precisión de las medidas hasta el rango de los 5 cm/s. Hasta ese momento la búsqueda en estrellas de tipo solar queda limitada a supertierras. La alternativa en estos momentos está en la búsqueda alrededor de estrellas de tipo M, ya que su baja masa hace que las señales de los planetas de tipo terrestre sean detectables con los instrumentos actuales. La segunda dificultad está en las variaciones en velocidad radial producidas por las inhomogeneidades de la superficie de las estrellas. Los cambios en las regiones activas de la superficie en las estrellas y su rotación provocan variaciones periódicas en la velocidad radial que pueden ir desde menos de 1 m/s en estrellas tranquilas hasta varias decenas de m/s en las más activas y pueden ser fácilmente confundidas con la variación provocada por planetas rocosos. En el caso de las estrellas M la necesidad de realizar este análisis de las señales originadas por la actividad estelar resulta aun más importante. Las señales inducidas por las estrellas M tempranas tienen periodos compatibles con los de las órbitas de los planetas en la zona habitable de estas estrellas, y amplitudes compatibles con las de las supertierras. Con el aumento de precisión previsto para el futuro estas señales aparecerán hasta en las estrellas menos activas conocidas, resultando cada vez más importante entenderlas y modelarlas correctamente. Usando series temporales de indicadores cromosféricos de espectros de alta resolución y curvas fotométricas estudiamos la variabilidad de 176 estrellas de baja actividad magnética de tipos espectrales desde F tardío hasta M intermedio, buscando tanto actividad a largo plazo (ciclos similares al ciclo solar) como a corto plazo (rotación). Presentamos nuevas medidas de ciclos magnéticos para 105 estrellas, y periodos de rotación para 123 estrellas. Estudiamos la distribución de periodos de rotación, donde encontramos una evolución hacia mayores valores en la duración del periodo en los tipos espectrales más tardíos. Esa evolución se detiene al llegar a las estrellas de tipo K temprano, y se reanuda al cruzar la frontera de las estrellas M2. Estudiamos así mismo la distribución de ciclos magnéticos, sin encontrar diferencias evidentes entre las estrellas de diferentes tipos espectrales. Estudiamos la relación entre el periodo de rotación medido y el nivel medio de actividad cromosférica, medido como el log(Rhk). Para ello extendemos la calibración del log(Rhk) para cubrir estrellas de B-V hasta ~2.0. Encontramos una fuerte correlación entre el periodo de rotación medido y el nivel de actividad cromosférica, que permite predecir el periodo de rotación de estrellas desde estrellas de tipo G hasta estrellas de tipo M intermedio de secuencia principal con una precisión del ~23\%. Analizamos las curvas de velocidad radial de 133 estrellas buscando tanto señales de velocidad radial inducidas por actividad como señales inducidas por planetas. Detectamos la señal inducida por rotación en 46 estrellas, con amplitudes desde 0.3 hasta 16.4 m/s. Estudiamos la relación entre la amplitud de las señales inducidas y los indicadores de actividad cromosférica, encontrando una vez más una correlación entre el nivel medio de actividad log(Rhk) y la amplitud de la señal inducida. Esta relación nos enseña que las estrellas FGK siguen un comportamiento similar, mientras que para las estrellas M las amplitudes inducidas son mayores. Encontramos también una relación entre la amplitud de la modulación rotacional del índice Smw y la amplitud de la señal en velocidad radial inducida por la rotación. En este caso encontramos dos poblaciones bien diferenciadas, divididas por las estrellas de tipo K intermedio. Tras limpiar las curvas de velocidad radial de señales inducidas por actividad llevamos a cabo una búsqueda de señales dinámicas en la misma muestra de estrellas. Recuperamos la mayor parte de los candidatos a planetas previamente publicados, salvo unos pocos casos donde atribuimos a la señal un origen intrínseco a la estrella. Presentamos el descubrimiento de seis nuevos planetas alrededor de las estrellas HD 1581, HD 161098, HD 176986, GJ 536 y GJ 3998, con masas mínimas de 5.87 +- 0.99, 26.60 +- 4.39, 4.86 +- 0.66 y 5.47 +- 0.84 Me, 2.35 +- 0.42 Me y 6.12 +- 1.11 Me respectivamente, situados a 0.321, 2.503, 0.061, 0.066, 0.029 y 0.089 au de sus estrellas. Los dos últimos publicados ya por Affer et al. 2016. Cinco de ellos probablemente sean planetas rocosos. Sugerimos también la presencia de un séptimo en la estrella HD 161098 con una masa mínima de 5.30 +-1.22 Me orbitando a una distancia de 0.317 au, que podría ser habitable si se diesen las condiciones atmosféricas apropiadas. Sugerimos por último la presencia de otros cinco candidatos que aún requieren observaciones adicionales y un análisis detallado

Hot, rocky and warm, puffy super-Earths orbiting TOI-402 (HD 15337)

Context: The Transiting Exoplanet Survey Satellite (TESS) is revolutionising the search for planets orbiting bright and nearby stars. In sectors 3 and 4, TESS observed TOI-402 (TIC-120896927), a bright V = 9.1 K1 dwarf also known as HD 15337, and found two transiting signals with periods of 4.76 and 17.18 days and radii of 1.90 and 2.21 R⊕, respectively. This star was observed prior to the TESS detection as part of the radial-velocity (RV) search for planets using the HARPS spectrometer, and 85 precise RV measurements were obtained before the launch of TESS over a period of 14 yr. Aims: In this paper, we analyse the HARPS RV measurements in hand to confirm the planetary nature of these two signals. Methods: HD 15337 happens to present a stellar activity level similar to the Sun, with a magnetic cycle of similar amplitude and RV measurements that are affected by stellar activity. By modelling this stellar activity in the HARPS radial velocities using a linear dependence with the calcium activity index log(RHK′), we are able, with a periodogram approach, to confirm the periods and the planetary nature of TOI-402.01 and TOI-402.02. We then derive robust estimates from the HARPS RVs for the orbital parameters of these two planets by modelling stellar activity with a Gaussian process and using the marginalised posterior probability density functions obtained from our analysis of TESS photometry for the orbital period and time of transit. Results: By modelling TESS photometry and the stellar host characteristics, we find that TOI-402.01 and TOI-402.02 have periods of 4.75642 ± 0.00021 and 17.1784 ± 0.0016 days and radii of 1.70 ± 0.06 and 2.52 ± 0.11 R⊕ (precision 3.6 and 4.2%), respectively. By analysing the HARPS RV measurements, we find that those planets are both super-Earths with masses of 7.20 ± 0.81 and 8.79 ± 1.68 M⊕ (precision 11.3 and 19.1%), and small eccentricities compatible with zero at 2σ. Conclusions: Although having rather similar masses, the radii of these two planets are very different, putting them on different sides of the radius gap. By studying the temporal evolution under X-ray and UV (XUV) driven atmospheric escape of the TOI-402 planetary system, we confirm, under the given assumptions, that photo-evaporation is a plausible explanation for this radius difference. Those two planets, being in the same system and therefore being in the same irradiation environment are therefore extremely useful for comparative exoplanetology across the evaporation valley and thus bring constraints on the mechanisms responsible for the radius gap

Nightside condensation of iron in an ultra-hot giant exoplanet

Ultra-hot giant exoplanets receive thousands of times Earth's insolation. Their high-temperature atmospheres (>2,000 K) are ideal laboratories for studying extreme planetary climates and chemistry. Daysides are predicted to be cloud-free, dominated by atomic species and substantially hotter than nightsides. Atoms are expected to recombine into molecules over the nightside, resulting in different day-night chemistry. While metallic elements and a large temperature contrast have been observed, no chemical gradient has been measured across the surface of such an exoplanet. Different atmospheric chemistry between the day-to-night ("evening") and night-to-day ("morning") terminators could, however, be revealed as an asymmetric absorption signature during transit. Here, we report the detection of an asymmetric atmospheric signature in the ultra-hot exoplanet WASP-76b. We spectrally and temporally resolve this signature thanks to the combination of high-dispersion spectroscopy with a large photon-collecting area. The absorption signal, attributed to neutral iron, is blueshifted by -11+/-0.7 km s-1 on the trailing limb, which can be explained by a combination of planetary rotation and wind blowing from the hot dayside. In contrast, no signal arises from the nightside close to the morning terminator, showing that atomic iron is not absorbing starlight there. Iron must thus condense during its journey across the nightside.Comment: Published in Nature (Accepted on 24 January 2020.) 33 pages, 11 figures, 3 table

A short-period sub-Earth orbiting Proxima Centauri

The discovery of a planetary candidate orbiting the habitable zone of our closest neighbor, Proxima Centauri (Gl 551), shook the planetary community as few other discoveries have done in recent years. It not only showed that the nearest star to the Sun could host a planetary system but also that, given the right conditions, it could host a habitable rocky planet. 2020 brought back the attention to Proxima with the announcement of a second planetary candidate in the system, the confirmation of Proxima b, using ESPRESSO, and the suggestion of a third, very low mass. Now, after an intense observational campaign, we can confidently say there’s a third companion with just one quarter of the mass of the Earth. Proxima d is one of the lightest exoplanets known to date. It orbits its parent star at 0.029 AU, with a period of 5.12 days. It’s induced RV semi-amplitude, of just 39 cm/s on a V-mag 11 star, highlights the capabilities of the ESPRESSO spectrograph, installed at the VLT telescope array of the Paranal observatory. The signal has an amplitude that is just 1/5 of the amplitude of the activity-induced radial velocity signal, which shows that, under the right conditions, current techniques can detect planetary signals much smaller than activity signals. The discovery of Proxima d opens the door to the characterization of the population of very low mass planets of the solar neighborhood. It shows that radial velocity studies are now capable of detecting exoplanets with masses similar to the Earth and much smaller

V1298 Tau - Extracting planetary signals in extreme stellar activity

Current theories predict that very young giant planets have large radii and very low densities before they slowly contract to reach their final size after several hundred million years. Open stellar clusters and young moving groups of stars offer a unique opportunity to study exoplanets at their infant stages because of the well-constrained ages. The extreme stellar activity of young stars makes measuring the masses of their planets a very challenging task. Planet-induced radial velocity signals are typically smaller in amplitude and at lower frequencies than stellar-induced signals. These situations place very demanding requirements for RV observation campaigns and puts us in a worst-case scenario for the standard methods of stellar activity modelling. With an estimated age of 20 million years, V1298 Tau is one of the youngest solar-type stars known to host transiting planets; it harbours a system composed of four planets, two Neptune-sized, one Saturn-sized and one Jupiter-sized. Using more than 200 radial velocity measurements, condensed into a 4-month period, we were able to measure the radial velocity amplitude induced by the planet V1298 Tau b, and to pick up a second signal that was suspected to be linked to V1298 Tau e. The analysis required the combination of radial velocities, ground-based photometry contemporary to the RV campaign, and K2 photometry. The result obtained indicated that V1298 Tau might have contracted much faster than predicted by theoretical models, highlighting the importance of attempting these difficult cases.Slides presented at the Oxford GPRV workshop (https://sites.google.com/view/gprv-workshop-2022). Recording available at: https://www.youtube.com/@user-vu3rd3jj2l/video

Detection of exoplanets with high precision radial velocities

Since the discovery in 1995 of 51 Peg b, radial velocity searches have proven to be one of the most effective ways of finding new extrasolar planets, in particular extrasolar planets orbiting nearby stars, where transits are often not detected. Back when the first exoplanet orbiting a Sun-like star was discovered, it was possible to detect the presence of a Jupiter-like planet in a close orbit to its star. Today, high-precision radial velocity measurements, obtained using high-resolution spectrographs, allow us to find Earth-mass planets that orbit inside, or close to, the habitable zone of their host stars. In some cases, with new-generation instruments such as ESPRESSO, it is possible to detect the presence of planets much lighter than the Earth. At these levels of precision, signals induced by stellar activity in the RV curves become the most important limiting factor, even in the case of magnetically quiet stars. Stellar activity can induce apparent Doppler shifts of the stellar spectrum, which cause periodic signals that range from less than one to dozens of meters per second. The correct detection and characterization of the different star-induced signals and their effect in the RVs is one of the most important steps to detect and properly characterize low-mass exoplanets, and its importance increases with increased precision as, even in the case of the quietest stars, these activity-induced signals can be the dominant signals in the data. Here, will embark in a short journey on the history of the radial velocity technique as a mean to detect extrasolar planets. From the early concept of spectrographs dedicated to the measurement of stellar velocities, to the detection of a planet with 1/4 the mass of the Earth orbiting Proxima Centauri

Revisiting Proxima with ESPRESSO

The discovery of Proxima b marked one of the most important milestones in exoplanetary science in recent years. Yet the limited precision of the available radial velocity data and the difficulty in modelling the stellar activity calls for a confirmation of the Earth-mass planet. We aim to confirm the presence of Proxima b using independent measurements obtained with the new ESPRESSO spectrograph, and refine the planetary parameters taking advantage of its improved precision. Methods: We analysed 63 spectroscopic ESPRESSO observations of Proxima (Gl 551) taken during 2019. We obtained radial velocity measurements with a typical radial velocity photon noise of 26 cm s-1. We combined these data with archival spectroscopic observations and newly obtained photometric measurements to model the stellar activity signals and disentangle them from planetary signals in the radial velocity (RV) data. We ran a joint Markov chain Monte Carlo analysis on the time series of the RV and full width half maximum of the cross-correlation function to model the planetary and stellar signals present in the data, applying Gaussian process regression to deal with the stellar activity signals. We confirm the presence of Proxima b independently in the ESPRESSO data and in the combined ESPRESSO+ HARPS+UVES dataset. The ESPRESSO data on its own shows Proxima b at a period of 11.218 ± 0.029 days, with a minimum mass of 1.29 ± 0.13 M⊕. In the combined dataset we measure a period of 11.18427 ± 0.00070 days with a minimum mass of 1.173 ± 0.086 M⊕. We get a clear measurement of the stellar rotation period (87 ± 12 d) and its induced RV signal, but no evidence of stellar activity as a potential cause for the 11.2 days signal. We find some evidence for the presence of a second short-period signal, at 5.15 days with a semi-amplitude of only 40 cm s-1. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 ± 0.08 M⊕. We find that forthe case of Proxima, the full width half maximum of the cross-correlation function can be used as a proxy for the brightness changes and that its gradient with time can be used to successfully detrend the RV data from part of the influence of stellar activity. The activity-induced RV signal in the ESPRESSO data shows a trend in amplitude towards redder wavelengths. Velocities measured using the red end of the spectrograph are less affected by activity, suggesting that the stellar activity is spot dominated. This could be used to create differential RVs that are activity dominated and can be used to disentangle activity-induced and planetary-induced signals. The data collected excludes the presence of extra companions with masses above 0.6 M⊕ at periods shorter than 50 days.Seminar at the University of Birmingha

The HADES RV Programme with HARPS-N@TNG GJ 3998: An early M-dwarf hosting a system of Super-Earths

Many efforts to detect Earth-like planets around low-mass stars are presently devoted in almost every extra-solar planet search. M dwarfs are considered ideal targets for Doppler radial velocity searches because their low masses and luminosities make low-mass planets orbiting in their habitable zones more easily detectable than those around higher mass stars. Nonetheless, the statistics of frequency of low-mass planets hosted by low mass stars remains poorly constrained. Our M-dwarf radial velocity monitoring with HARPS-N within the GAPS (Global architectures of Planetary Systems) – ICE (Institut de Ciències de l’Espai/CSIC-IEEC) – IAC (Instituto de Astrofísica de Canarias) project can provide a major contribution to the widening of the current statistics through the in-depth analysis of accurate radial velocity observations in a narrow range of spectral sub-types (79 stars, between dM0 to dM3). Spectral accuracy will enable us to reach the precision needed to detect small planets with a few earth masses. Our survey will bring a contribute to the surveys devoted to the search for planets around M-dwarfs, mainly focused on the M-dwarf population of the northern emisphere, for which we will provide an estimate of the planet occurrence. We present here a long duration radial velocity monitoring of theM1 dwarf star GJ 3998 with HARPS-N to identify periodic signals in the data. Almost simultaneous photometric observations were carried out within the APACHE and EXORAP programs to characterize the stellar activity and to distinguish from the periodic signals those due to activity and to the presence of planetary companions. We run an MCMC simulation and use Bayesian model selection to determine the number of planets in this system, to estimate their orbital parameters and minimum masses and for a proper treatment of the activity noise. The radial velocities have a dispersion in excess of their internal errors due to at least four superimposed signals, with periods of 30.7, 13.7, 42.5 and 2.65 days. Our data are well described by a 2-planet Keplerian (13.7 d and 2.65 d) and 2 sinusoidal functions (stellar activity, 30.7 d and 42.5 d) fit. The analysis of spectral indices based on Ca II H & K and Hα lines demonstrates that the periods of 30.7 and 42.5 days are due to chromospheric inhomogeneities modulated by stellar rotation and differential rotation. This result is supported by photometry and is consistent with the results on differential rotation of M stars obtained with Kepler. The shorter periods of 13.74 ± 0.02 d and 2.6498 ± 0.0008 d are well explained with the presence of two planets, with minimum masses of 6.26 ± 0.79 M⊕ and 2.47 ± 0.27 M⊕ and distances of 0.089 AU and 0.029 AU from the host, respectively

Fundamental physics with ESPRESSO: Constraining a simple parametrisation for varying

Context. The spectrograph ESPRESSO recently obtained a limit on the variation of the fine-structure constant, α, through measurements along the line of sight of a bright quasar with a precision of 1.36 ppm at 1σ level. This imposes new constraints on cosmological models with a varying α. We assume such a model where the electromagnetic sector is coupled to a scalar field dark energy responsible for the current acceleration of the Universe. We parametrise the variation of α with two extra parameters, one defining the cosmological evolution of the quintessence component and the other fixing the coupling with the electromagnetic field. Aims. The objective of this work is to constrain these parameters with both astrophysical and local probes. We also carried out a comparative analysis of how each data probe may constrain our parametrisation. Methods. We performed a Bayesian analysis by comparing the predictions of the model with observations. The astrophysical datasets are composed of quasar spectra measurements, including the latest ESPRESSO data point, as well as Planck observations of the cosmic microwave background. We combined these with local results from atomic clocks and the MICROSCOPE experiment. Results. The constraints placed on the quintessence parameter are consistent with a null variation of the field, and are therefore compatible with a ΛCDM cosmology. The constraints on the coupling to the electromagnetic sector are dominated by the Eötvös parameter local bound. Conclusions. More precise measurements with ESPRESSO will be extremely important to study the cosmological evolution of α as it probes an interval of redshift not accessible to other types of observations. However, for this particular model, current available data favour a null variation of α resulting mostly from the strong MICROSCOPE limits