In recent years, the advent of a new generation of radial velocity instruments has allowed us to detect planets with increasingly lower mass and to break the one Earth-mass barrier. Here we report a ...new milestone in this context by announcing the detection of the lowest-mass planet measured so far using radial velocities: L 98-59 b, a rocky planet with half the mass of Venus. It is part of a system composed of three known transiting terrestrial planets (planets b–d). We announce the discovery of a fourth nontransiting planet with a minimum mass of 3.06
−0.37
+0.33
M
⊕
and an orbital period of 12.796
−0.019
+0.020
days and report indications for the presence of a fifth nontransiting terrestrial planet. With a minimum mass of 2.46
−0.82
+0.66
M
⊕
and an orbital period 23.15
−0.17
+0.60
days, this planet, if confirmed, would sit in the middle of the habitable zone of the L 98-59 system. L 98-59 is a bright M dwarf located 10.6ṗc away. Positioned at the border of the continuous viewing zone of the
James Webb
Space Telescope, this system is destined to become a corner stone for comparative exoplanetology of terrestrial planets. The three transiting planets have transmission spectrum metrics ranging from 49 to 255, which undoubtedly makes them prime targets for an atmospheric characterization with the
James Webb
Space Telescope, the
Hubble
Space Telescope, Ariel, or ground-based facilities such as NIRPS or ESPRESSO. With an equilibrium temperature ranging from 416 to 627 K, they offer a unique opportunity to study the diversity of warm terrestrial planets without the unknowns associated with different host stars. L 98-59 b and c have densities of 3.6
−1.5
+1.4
and 4.57
−0.85
+0.77
g cm
−3
, respectively, and have very similar bulk compositions with a small iron core that represents only 12 to 14% of the total mass, and a small amount of water. However, with a density of 2.95
−0.51
+0.79
g cm
−3
and despite a similar core mass fraction, up to 30% of the mass of L 98-59 d might be water.
ABSTRACT
We present a super-Earth orbiting close to the inner edge of the habitable zone of the cool dwarf star K2-286 (EPIC 249889081), detected with data from the K2 mission in its 15th campaign. ...The planet has radius of 2.1 ± 0.2 R⊕, near the 1.5–2.0 R⊕ gap in the radii distribution. The equilibrium temperature is $347^{+21}_{-11}$ K, cooler than most of the small planets with well-measured masses, and the orbital period is 27.359 ± 0.005 d. K2-286, located at a distance of 76.3 ± 0.3 pc, is an M0V star with estimated effective temperature of 3926 ± 100 K, less active than other M dwarf stars hosting exoplanets. The expected radial velocity semi-amplitude induced by the planet on the star is $1.9^{+1.3}_{-1.2}$ m s−1, and the amplitude of signals in transit transmission spectroscopy is estimated at 5.0 ± 3.0 ppm. Follow-up observations for mass measurements and transit spectroscopy should be desirable for this relatively bright target (mV = 12.76, mKs = 9.32) hosting a transiting super-Earth within the inner edge of the habitable zone.
Context.
The detection and characterization of exoplanet atmospheres is currently one of the main drivers pushing the development of new observing facilities. In this context, high-resolution ...spectrographs are proving their potential and showing that high-resolution spectroscopy will be paramount in this field.
Aims.
We aim to make use of ESPRESSO high-resolution spectra, which cover two transits of HD 209458b, to probe the broadband transmission optical spectrum of the planet.
Methods.
We applied the chromatic Rossiter–McLaughin method to derive the transmission spectrum of HD 209458b. We compared the results with previous HST observations and with synthetic spectra.
Results.
We recover a transmission spectrum of HD 209458b similar to the one obtained with HST data. The models suggest that the observed signal can be explained by only Na, only TiO, or both Na and TiO, even though none is fully capable of explaining our observed transmission spectrum. Extra absorbers may be needed to explain the full dataset, though modeling approximations and observational errors can also be responsible for the observed mismatch.
Conclusions.
Using the chromatic Rossiter–McLaughlin technique, ESPRESSO is able to provide broadband transmission spectra of exoplanets from the ground, in conjunction with space-based facilities, opening good perspectives for similar studies of other planets.
Context.
Comparing the properties of planets orbiting the same host star, and thus formed from the same accretion disc, helps in constraining theories of exoplanet formation and evolution. As a ...result, the scientific interest in multi-planetary systems is growing with the increasing number of detections of planetary companions.
Aims.
We report the characterisation of a multi-planetary system composed of five exoplanets orbiting the K-dwarf HD 23472 (TOI-174).
Methods.
In addition to the two super-Earths that were previously confirmed, we confirm and characterise three Earth-size planets in the system using ESPRESSO radial velocity observations. The planets of this compact system have periods of
P
d
~
3.98,
P
e
~
7.90,
P
f
~
12.16,
P
b
~ 17.67, and
P
c
~ 29.80 days and radii of
R
d
~ 0.75 ,
R
e
~ 0.82,,
R
f
~ 1.13 ,
R
b
~ 2.01, and,
R
c
~
1.85
R
⊕
.Because of its small size, its proximity to planet d’s transit, and close resonance with planet d, planet e was only recently found.
Results.
The planetary masses were estimated to be
M
d
= 0.54 ± 0.22,
M
e
= 0.76 ± 0.30,
M
f
= 0.64
−0.39
+0.46
,
M
b
= 8.42
−0.84
+0.83
, and
M
c
= 3.37
−0.87
+0.92
M
⊕
. These planets are among the lightest planets, with masses measured using the radial velocity method, demonstrating the very high precision of the ESPRESSO spectrograph. We estimated the composition of the system’s five planets and found that their gas and water mass fractions increase with stellar distance, suggesting that the system was shaped by irradiation. The high density of the two inner planets (
ρ
d
= 7.5
−3.1
+3.9
and
ρ
e
= 7.5
−3.0
+3.9
g cm
−3
) indicates that they are likely to be super-Mercuries. This is supported by the modelling of the internal structures of the planets, which also suggests that the three outermost planets have significant water or gas content.
Conclusions.
If the existence of two super-Mercuries in the system is confirmed, this system will be the only one known to feature two super-Mercuries, making it an excellent testing bed for theories of super-Mercuries formation. Furthermore, the system is close to a Laplace resonance, and further monitoring could shed light on how it was formed. Its uniqueness and location in the continuous viewing zone of the
James Webb
space telescope will make it a cornerstone of future in-depth characterisations.
Context.
The study of exoplanet atmospheres is essential for understanding the formation, evolution, and composition of exoplanets. The transmission spectroscopy technique is playing a significant ...role in this domain. In particular, the combination of state-of-the-art spectrographs at low- and high-spectral resolution is key to our understanding of atmospheric structure and composition.
Aims.
We observed two transits of the close-in sub-Saturn-mass planet, WASP-127b, with ESPRESSO in the frame of the Guaranteed Time Observations Consortium. We aim to use these transit observations to study the system architecture and the exoplanet atmosphere simultaneously.
Methods.
We used the Reloaded Rossiter-McLaughlin technique to measure the projected obliquity
λ
and the projected rotational velocity
v
eq
⋅sin(
i
*
). We extracted the high-resolution transmission spectrum of the planet to study atomic lines. We also proposed a new cross-correlation framework to search for molecular species and we applied it to water vapor.
Results.
The planet is orbiting its slowly rotating host star (
v
eq
⋅sin(
i
*
) = 0.53
−0.05
+0.07
km s
−1
) on a retrograde misaligned orbit (
λ
= −128.41
−5.46
+5.60
°). We detected the sodium line core at the 9-
σ
confidence level with an excess absorption of 0.34 ± 0.04%, a blueshift of 2.74 ± 0.79 km s
−1
, and a full width at half maximum of 15.18 ± 1.75 km s
−1
. However, we did not detect the presence of other atomic species but set upper limits of only a few scale heights. Finally, we put a 3-
σ
upper limit on the average depth of the 1600 strongest water lines at equilibrium temperature in the visible band of 38 ppm. This constrains the cloud-deck pressure between 0.3 and 0.5 mbar by combining our data with low-resolution data in the near-infrared and models computed for this planet.
Conclusions.
WASP-127b, with an age of about 10 Gyr, is an unexpected exoplanet by its orbital architecture but also by the small extension of its sodium atmosphere (~7 scale heights). ESPRESSO allows us to take a step forward in the detection of weak signals, thus bringing strong constraints on the presence of clouds in exoplanet atmospheres. The framework proposed in this work can be applied to search for molecular species and study cloud-decks in other exoplanets.
HADES RV programme with HARPS-N at TNG Suárez Mascareño, A.; Rebolo, R.; González Hernández, J. I. ...
Astronomy and astrophysics (Berlin),
04/2018, Letnik:
612
Journal Article
Recenzirano
We aim to investigate the presence of signatures of magnetic cycles and rotation on a sample of 71 early M-dwarfs from the HADES RV programme using high-resolution time-series spectroscopy of the Ca ...II H&K and H
α
chromospheric activity indicators, the radial velocity series, the parameters of the cross correlation function and the
V
-band photometry. We used mainly HARPS-N spectra, acquired over 4 yr, and add HARPS spectra from the public ESO database and ASAS photometry light-curves as support data, extending the baseline of the observations of some stars up to 12 yr. We provide log
10
(
R
′
HK
) measurements for all the stars in the sample, cycle length measurements for 13 stars, rotation periods for 33 stars and we are able to measure the semi-amplitude of the radial velocity signal induced by rotation in 16 stars. We complement our work with previous results and confirm and refine the previously reported relationships between the mean level of chromospheric emission, measured by the log
10
(
R
′
HK
), with the rotation period, and with the measured semi-amplitude of the activity induced radial velocity signal for early M-dwarfs. We searched for a possible relation between the measured rotation periods and the lengths of the magnetic cycle, finding a weak correlation between both quantities. Using previous
v
sin
i
measurements we estimated the inclinations of the star’s poles to the line of sight for all the stars in the sample, and estimate the range of masses of the planets GJ 3998 b and c (2.5–4.9 and 6.3–12.5
M
⊕
), GJ 625 b (2.82
M
⊕
), GJ 3942 b (7.1–10.0
M
⊕
) and GJ 15A b (3.1–3.3
M
⊕
), assuming their orbits are coplanar with the stellar rotation.
Context.
The ability to detect and characterise an increasing variety of exoplanets has been made possible by the continuous development of stable, high-resolution spectrographs and the Doppler ...radial velocity (RV) method. The cross-correlation function (CCF) method is one of the traditional approaches used to derive RVs. More recently, template matching has been introduced as an advantageous alternative for M-dwarf stars.
Aims.
We describe a new implementation of the template matching technique for stellar RV estimation within a semi-Bayesian framework, providing a more statistically principled characterisation of the RV measurements and associated uncertainties. This methodology, named the Semi-Bayesian Approach for RVs with Template matching, S-BART, can currently be applied to HARPS and ESPRESSO data. We first validate its performance with respect to other template matching pipelines using HARPS data. We then apply S-BART to ESPRESSO observations, comparing the scatter and uncertainty of the derived RV time series with those obtained using the CCF method. We leave a full analysis of the planetary and activity signals present in the considered datasets for future work.
Methods.
In the context of a semi-Bayesian framework, a common RV shift is assumed to describe the difference between each spectral order of a given stellar spectrum and a template built from the available observations. Posterior probability distributions are obtained for the relative RV associated with each spectrum using the Laplace approximation, after marginalization with respect to the continuum. We also implemented, for validation purposes, a traditional template matching approach, where a RV shift is estimated individually for each spectral order and the final RV estimate is calculated as a weighted average of the RVs of the individual orders.
Results.
The application of our template-based methods to HARPS archival observations of Barnard’s star allowed us to validate our implementation against other template matching methods. Although we find similar results, the standard deviation of the RVs derived with S-BART is smaller than that obtained with the HARPS-TERRA and SERVAL pipelines. We believe this is due to differences in the construction of the stellar template and the handling of telluric features. After validating S-BART, we applied it to 33 ESPRESSO GTO targets, evaluating its performance and comparing it to the CCF method as implemented in ESO’s official pipeline. We find a decrease in the median RV scatter of ~10 and ~4% for M- and K-type stars, respectively. Our semi-Bayesian framework yields more precise RV estimates than the CCF method, in particular in the case of M-type stars where S-BART achieves a median uncertainty of ~15 cm s
−1
over 309 observations of 16 targets. Further, with the same data we estimated the nightly zero point (NZP) of the instrument, finding a weighted NZP scatter of below ~0.7 m s
−1
. Given that this includes stellar variability, photon noise, and potential planetary signals, it should be taken as an upper limit on the RV precision attainable with ESPRESSO data.
Aims. One of the main scientific drivers for ESPRESSO, Échelle SPectrograph, is the detection and characterization of Earth-class exoplanets. With this goal in mind, the ESPRESSO guaranteed time ...observations (GTO) Catalog identifies the best target stars for a blind search for the radial velocity (RV) signals caused by Earth-class exoplanets. Methods. Using the most complete stellar catalogs available, we screened for the most suitable G, K, and M dwarf stars for the detection of Earth-class exoplanets with ESPRESSO. For most of the stars, we then gathered high-resolution spectra from new observations or from archival data. We used these spectra to spectroscopically investigate the existence of any stellar binaries, both bound or background stars. We derived the activity level using chromospheric activity indexes using log (R′HK) $(R' _{\textrm{HK}})$ (RHK′) , as well as the projected rotational velocity v sin i. For the cases where planet companions are already known, we also looked at the possibility that additional planets may exist in the host’s habitable zone using dynamical arguments. Results. We estimated the spectroscopic contamination level, v sin i, activity, stellar parameters and chemical abundances for 249 of the most promising targets. Using these data, we selected 45 stars that match our criteria for detectability of a planet like Earth. The stars presented and discussed in this paper constitute the ESPRESSO GTO catalog for the RV blind search for Earth-class planets. They can also be used for any other work requiring a detailed spectroscopic characterization of stars in the solar neighborhood.
Abstract
Transiting planets at young ages are key targets for improving our understanding of the evolution of exo-atmospheres. We present results of a new X-ray observation of V 1298 Tau with ...XMM-Newton, aimed to determine more accurately the high-energy irradiation of the four planets orbiting this pre-main-sequence star, and the possible variability due to magnetic activity on short and long timescales. Following the first measurements of planetary masses in the V 1298 Tau system, we revise early guesses of the current escape rates from the planetary atmospheres, employing our updated atmospheric evaporation models to predict the future evolution of the system. Contrary to previous expectations, we find that the two outer Jupiter-sized planets will not be affected by any evaporation on Gyr timescales, and the same occurs for the two smaller inner planets, unless their true masses are lower than ∼40
M
⊕
. These results confirm that relatively massive planets can reach their final position in the mass–radius diagram very early in their evolutionary history.
Context.
The 20 Myr old star V1298 Tau hosts at least four planets. Since its discovery, this system has been a target of intensive photometric and spectroscopic monitoring. To date, the ...characterisation of its architecture and planets’ fundamental properties has been very challenging.
Aims.
The determination of the orbital ephemeris of the outermost planet V1298 Tau e remains an open question. Only two transits have been detected so far by
Kepler
/K2 and TESS, allowing for a grid of reference periods to be tested with new observations, without excluding the possibility of transit timing variations. Observing a third transit would allow for better constraints to be set on the orbital period and would also help in determining an accurate radius for V1298 Tau e because the previous transits showed different depths.
Methods.
We observed V1298 Tau with the CHaracterising ExOPlanet Satellite (CHEOPS) to search for a third transit of planet e within observing windows selected to test three of the shortest predicted orbital periods. We also collected ground-based observations to verify the result found with CHEOPS. We reanalysed
Kepler
/K2 and TESS light curves to test how the results derived from these data are affected by alternative photometric extraction and detrending methods.
Results.
We report the CHEOPS detection of a transit-like signal that could be attributed to V1298 Tau e. If so, that result would imply that the orbital period calculated from fitting a linear ephemeris to the three available transits is close to ~45 days. Results from the ground-based follow-up marginally support this possibility. We found that i) the transit observed by CHEOPS has a longer duration compared to that of the transits observed by
Kepler
/K2 and TESS; and ii) the transit observed by TESS is >30% deeper than that of
Kepler
/K2 and CHEOPS, and it is also deeper than the measurement previously reported in the literature, according to our reanalysis.
Conclusions.
If the new transit detected by CHEOPS is found to be due to V1298 Tau e, this would imply that the planet experiences TTVs of a few hours, as deduced from three transits, as well as orbital precession, which would explain the longer duration of the transit compared to the
Kepler
/K2 and TESS signals. Another and a priori less likely possibility is that the newly detected transit belongs to a fifth planet with a longer orbital period than that of V1298 Tau e. Planning further photometric follow-up to search for additional transits is indeed necessary to solve the conundrum, as well as to pin down the radius of V1298 Tau e.
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