Browsing by Author "Perger, M."

Context. Precision radial velocity (RV) measurements continue to be a key tool for detecting and characterising extrasolar planets. While instrumental precision keeps improving, stellar activity remains a barrier to obtaining reliable measurements below 1–2 m s −1 accuracy. Aims. Using simulations and real data, we investigate the capabilities of a deep neural network approach to producing activity-free Doppler measurements of stars. Methods. As case studies we used observations of two known stars, ϵ Eridani and AU Microscopii, both of which have clear signals of activity-induced Doppler variability. Synthetic observations using the starsim code were generated for the observables (inputs) and the resulting Doppler signal (labels), and then they were used to train a deep neural network algorithm to predict Doppler corrections. We identified a relatively simple architecture, consisting of convolutional layers followed by fully connected layers, that is adequate for the task. The indices investigated are mean line-profile parameters (width, bisector, and contrast) and multi-band photometry. Results. We demonstrate that the RV-independent approach can drastically reduce spurious Doppler variability from known physical effects, such as spots, rotation, and convective blueshift. We identify the combinations of activity indices with the most predictive power. When applied to real observations, we observe a good match of the correction with the observed variability, but we also find that the noise reduction is not as good as in the simulations, probably due to a lack of detail in the simulated physics. Conclusions. We demonstrate that a model-driven machine learning approach is sufficient to clean Doppler signals from activity-induced variability for well-known physical effects. There are dozens of known activity-related observables whose inversion power remains unexplored, indicating that the use of additional indicators, more complete models, and more observations with optimised sampling strategies can lead to significant improvements in our detrending capabilities for new and existing datasets.

Context. Statistical analyses based on Kepler data show that most of the early-type M dwarfs host multi-planet systems consisting of Earth- to sub-Neptune-sized planets with orbital periods of up to ~250 days, and that at least one such planet is likely located within the habitable zone. M dwarfs are therefore primary targets to search for potentially habitable planets in the solar neighbourhood. Aims. 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 yr with the HIRES, HARPS, and CARMENES spectrographs. Methods. The data are affected by time-correlated signals at the level of 2–3 m s −1 due to stellar activity, which 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. Results. We find 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 514b has minimum mass m b sin i b = 5.2 ± 0.9 M ⊕ , orbital period P b = 140.43 ± 0.41 days, and eccentricity e b = 0.45 −0.14 +0.15 . 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 ~0.2 M Jup planet at a distance of ~3–10 astronomical units, and massive giant planets and brown dwarfs out to several tens of astronomical units. We discuss the possible presence of a second low-mass companion at a shorter distance from the host than Gl 514 b. Conclusions. Gl 514 b represents an interesting science case for studying 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 the presence of additional planetary signals of less than 1 m s −1 .

Context. M-dwarfs have proven to be ideal targets for planetary radial velocity (RV) searches due to their higher planet-star mass contrast, which favors the detection of low-mass planets. The abundance of super-Earth and Earth-like planets detected around this type of star motivates further such research on hosts without reported planetary companions. Aims. The HADES and CARMENES programs are aimed at carrying out extensive searches of exoplanetary systems around M-type stars in the northern hemisphere, allowing us to address, in a statistical sense, 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 that is compatible with a planetary companion. Methods. 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 simulations and Gaussian process regression to model the activity of the star. Results. We present the discovery of a short-period super-Earth with an orbital period of 2.37756 −0.00011 +0.00013 d and a minimum mass of 2.96 −0.48 +0.50 M ⊕ . We offer an update to the previously reported characterization of the magnetic cycle and rotation period of the star, obtaining values of P rot = 35.563 ± 0.071 d and P cycle = 2800 ± 150 d. Furthermore, the RV time series exhibits a possibly periodic long-term signal, which might be related to a Saturn-mass planet of ~100 M ⊕ .

Context. Targeted spectroscopic exoplanet surveys face the challenge of maximizing their planet detection rates by means of careful planning. For a large planet survey, the number of possible observation combinations, i.e., the sequence of observations night after night, both in total time and amount of targets, is enormous. Aims. Sophisticated scheduling tools and the improved understanding of the exoplanet population are employed to investigate an e cient and optimal way to plan the execution of observations. This is applied to the CARMENES instrument, which is an optical and infrared high-resolution spectrograph that has started a survey of about 300 M-dwarf stars in search of terrestrial exoplanets. Methods. We used evolutionary computation techniques to create an automatic scheduler that minimizes the idle periods of the telescope and distributes the observations among all the targets using configurable criteria.We simulated the case of the CARMENES survey with a realistic sample of targets, and we estimated the e ciency of the planning tool both in terms of telescope operations and planet detection. Results. Our scheduling simulations produce plans that use about 99% of the available telescope time (including overheads) and optimally distribute the observations among the di erent targets. Under such conditions, and using current planet statistics, the optimized plan using this tool should allow the CARMENES survey to discover about 65% of the planets with radial-velocity semi-amplitudes greater than 1 ms-1 when considering only photon noise. Conclusions. The simulations using our scheduling tool show that it is possible to optimize the survey planning by minimizing idle instrument periods and fulfilling the science objectives in an e cient manner to maximize the scientific return.

We report the discovery of two planetary systems, namely G 264–012, an M 4.0 dwarf with two terrestrial planets ( M b sin i =2.50 −0.30 +0.29 M ⊕ and M c sin i =3.75 −0.47 +0.48 M ⊕ ), and Gl 393, a bright M 2.0 dwarf with one terrestrial planet ( M b sin i = 1.71 ± 0.24 M ⊕ ). Although both stars were proposed to belong to young stellar kinematic groups, we estimate their ages to be older than about 700 Ma. The two planets around G 264–012 were discovered using only radial-velocity (RV) data from the CARMENES exoplanet survey, with estimated orbital periods of 2.30 d and 8.05 d, respectively.Photometric monitoring and analysis of activity indicators reveal a third signal present in the RV measurements, at about 100 d,caused by stellar rotation. The planet Gl 393 b was discovered in the RV data from the HARPS, CARMENES, and HIRES instruments. Its identification was only possible after modelling, with a Gaussian process (GP), the variability produced by the magnetic activity of the star. For the earliest observations, this variability produced a forest of peaks in the periodogram of the RVs at around the 34 d rotation period determined from Kepler data, which disappeared in the latestepochs. After correcting for them with this GP model, a significant signal showed at a period of 7.03 d. No significant signals in any of our spectral activity indicators or contemporaneous photometry were found at any of the planetary periods. Given the orbital and stellar properties, the equilibrium temperatures of the three planets are all higher than that for Earth. Current planet formation theories suggest that these two systems represent a common type of architecture. This is consistent with formation following the core accretion paradigm.

Context. The interaction between Earth-like exoplanets and the magnetic field of low-mass host stars are considered to produce weak emission signals at radio frequencies. A study using LOFAR data announced the detection of radio emission from the mid M-type dwarf GJ 1151 that could potentially arise from a close-in terrestrial planet. Recently, the presence of a 2.5- M ⊕ planet orbiting GJ 1151 with a 2-day period has been claimed using 69 radial velocities (RVs) from the HARPS-N and HPF instruments. Aims. We have obtained 70 new high-precision RV measurements in the framework of the CARMENES M-dwarf survey and use these data to confirm the presence of the claimed planet and to place limits on possible planetary companions in the GJ 1151 system. Methods. We analysed the periodicities present in the combined RV data sets from all three instruments and calculated the detection limits for potential planets in short-period orbits. Results. We cannot confirm the recently announced candidate planet and conclude that the 2-day signal in the HARPS-N and HPF data sets is most probably produced by a long-term RV variability, possibly arising from an outer planetary companion that has yet to be constrained. We calculate a 99.9% significance detection limit of 1.50 m s −1 in the RV semi-amplitude, which places upper limits of 0.7 M ⊕ and 1.2 M ⊕ on the minimum masses of potential exoplanets with orbital periods of 1 and 5 days, respectively.

Context. Detecting a planetary companion in a short-period orbit through radio emission from the interaction with its host star is a new prospect in exoplanet science. Recently, a tantalising signal was found close to the low-mass stellar system GJ 1151 using LOFAR observations. Aims. We studied spectroscopic time-series data of GJ 1151 in order to search for planetary companions, investigate possible signatures of stellar magnetic activity, and to find possible explanations for the radio signal. Methods. We used the combined radial velocities measured from spectra acquired with the CARMENES, HARPS-N, and HPF instruments, extracted activity indices from those spectra in order to mitigate the impact of stellar magnetic activity on the data, and performed a detailed analysis of Gaia astrometry and all available photometric time series coming from the MEarth and ASAS-SN surveys. Results. We found a M > 10.6 M ⊕ companion to GJ 1151 in a 390d orbit at a separation of 0.57 au. Evidence for a second modulation is also present; this could be due to long-term magnetic variability or a second (substellar) companion. The star shows episodes of elevated magnetic activity, one of which could be linked to the observed LOFAR radio emission. We show that it is highly unlikely that the detected GJ 1151 c, or any additional outer companion is the source of the detected signal. We cannot firmly rule out the suggested explanation of an undetected short-period planet that could be related to the radio emission, as we establish an upper limit of 1.2 M ⊕ for the minimum mass.