Context. M dwarfs are known to generate the strongest magnetic fields among main-sequence stars with convective envelopes, but we are still lacking a consistent picture of the link between the ...magnetic fields and underlying dynamo mechanisms, rotation, and activity. Aims. In this work we aim to measure magnetic fields from the high-resolution near-infrared spectra taken with the CARMENES radial-velocity planet survey in a sample of 29 active M dwarfs and compare our results against stellar parameters. Methods. We used the state-of-the-art radiative transfer code to measure total magnetic flux densities from the Zeeman broadening of spectral lines and filling factors. Results. We detect strong kG magnetic fields in all our targets. In 16 stars the magnetic fields were measured for the first time. Our measurements are consistent with the magnetic field saturation in stars with rotation periods P < 4 d. The analysis of the magnetic filling factors reveal two different patterns of either very smooth distribution or a more patchy one, which can be connected to the dynamo state of the stars and/or stellar mass. Conclusions. Our measurements extend the list of M dwarfs with strong surface magnetic fields. They also allow us to better constrain the interplay between the magnetic energy, stellar rotation, and underlying dynamo action. The high spectral resolution and observations at near-infrared wavelengths are the beneficial capabilities of the CARMENES instrument that allow us to address important questions about the stellar magnetism.
Context.
The properties, distribution, and evolution of inhomogeneities on the surface of active stars, such as dark spots and bright faculae, significantly influence the determination of the ...parameters of an orbiting exoplanet. The chromatic effect they have on transmission spectroscopy, for example, could affect the analysis of data from future space missions such as
James Webb
Space Telescope and Ariel.
Aims.
To quantify and mitigate the effects of those surface phenomena, we developed a modelling approach to derive the surface distribution and properties of active regions by modelling simultaneous multi-wavelength time-series observables.
Methods.
We present an upgraded version of the
StarSim
code, now featuring the capability to solve the inverse problem and derive the properties of the stars and their active regions by modelling time-series data. As a test case, we analyse ~600 days of
BV RI
multiband photometry from the 0.8-m
Joan Oró
(TJO) and 1.2-m STELLA telescopes of the K2 V exoplanet host star WASP-52. From the results, we further simulated the chromatic contribution of surface phenomena on the observables of its transiting planet.
Results.
Using
StarSim
we are able to determine the relevant activity parameters of WASP-52 and reconstruct the time-evolving longitudinal map of active regions. The star shows a heterogeneous surface composed of dark spots with a mean temperature of 575 ± 150 K lower than the photospheric value, with filling factors ranging from 3 to 14%. We used the results to study the chromatic effects on the depths of exoplanet transits obtained at different epochs and corresponding to different stellar spot distributions. In the case of WASP-52, which has peak-to-peak photometric variations of ~7% in the visible, the residual effects of dark spots on the measured transit depth of its giant planet, after applying the calculated corrections, are about 10
−4
at 550 nm and 3 × 10
−5
at 6
μ
m.
Conclusions.
We demonstrate that by using contemporaneous ground-based multiband photometry of an active star, it is possible to reconstruct the parameters and distribution of active regions over time, thus making it feasible to quantify the chromatic effects on the planetary radii measured with transit spectroscopy and mitigate them by about an order of magnitude.
Exoplanet Doppler surveys are currently the most efficient means to detect low-mass companions to nearby stars. Among these stars, the light M dwarfs provide the highest sensitivity to detect ...low-mass exoplanet candidates. GJ 163 is a nearby inactive M dwarf with abundant public observations obtained using the HARPS spectrograph. We obtain and analyse radial velocities from the HARPS public spectra of GJ 163 and investigate the presence of a planetary companions orbiting it. The architecture of GJ 163 system resembles a scaled-down Solar System in the sense that there are two low-mass planets on orbital periods of 8.7 and 25.6 days in the inner system, a possible slightly more massive companion on an intermediate orbit, and an outer sub-Saturnian companion at roughly 1 AU. The discovery of (yet) another planetary system with several low-mass companions around a nearby M-dwarf indicates that the high-multiplicity planetary systems found by the NASA Kepler mission around G and K dwarfs is also present around low-mass stars.
Abstract
We present a study of four different formation scenarios that may be able to explain the origin of the recently announced planet (‘Proxima b’) orbiting Proxima Centauri. The aim is to ...examine how the formation scenarios differ in their predictions for the multiplicity of the Proxima system, the water/volatile content of Proxima b and its orbital eccentricity, so that these can be tested by future observations. A scenario of in situ formation via giant impacts from a locally enhanced disc of planetary embryos/planetesimals predicts that Proxima b will be in a multiplanet system with a measurably finite eccentricity. Assuming that the local solid enhancement needed to form a Proxima b analogue arises because of the inwards drift of solids in the form of small planetesimals/boulders, this scenario also results in Proxima b analogues being only moderately endowed with water/volatiles. A scenario in which multiple embryos form, migrate and mutually collide within a gas disc results in Proxima b being a member of a multiple system, possibly displaying mean-motion resonances, but where the constituent members are Ocean planets due to accretion occurring mainly outside of the snowline. A scenario in which a single accreting embryo forms outside the snowline, and migrates inwards while accreting planetesimals/pebbles results in Proxima b being an isolated Ocean planet on a circular orbit. A scenario in which Proxima b formed via pebble accretion interior to the snowline produces a dry planet on a circular orbit. Future observations that characterize the physical and orbital properties of Proxima b, and any additional planets in the system, will provide valuable insights into the formation history of this neighbouring planetary system.
We report the discovery of a second long-period giant planet orbiting HD 30177, a star previously known to host a massive Jupiter analog (HD 30177b: a = 3.8 0.1 au, m sin i = 9.7 0.5 MJup). HD 30177c ...can be regarded as a massive Saturn analog in this system, with a = 9.9 1.0 au and m sin i = 7.6 3.1 MJup. The formal best-fit solution slightly favors a closer-in planet at a ∼ 7 au, but detailed n-body dynamical simulations show that configuration to be unstable. A shallow local minimum of longer period, lower eccentricity solutions was found to be dynamically stable, and hence we adopt the longer period in this work. The proposed ∼32 year orbit remains incomplete; further monitoring of this and other stars is necessary to reveal the population of distant gas giant planets with orbital separations a ∼ 10 au, analogous to that of Saturn.
We report Atacama Large Millimeter/submillimeter Array and Very Large Array continuum observations that potentially identify the four youngest protostars in the Orion Molecular Clouds taken as part ...of the Orion VANDAM program. These are distinguished by bright, extended, irregular emission at 0.87 and 8 mm and are optically thick at 0.87 mm. These structures are distinct from the disk or point-like morphologies seen toward the other Orion protostars. The 0.87 mm emission implies temperatures of 41-170 K, requiring internal heating. The bright 8 mm emission implies masses of 0.5-1.2 M assuming standard dust opacity models. One source has a Class 0 companion, while another exhibits substructure indicating a companion candidate. Three compact outflows are detected, two of which may be driven by companions, with dynamical times of ∼300 to ∼1400 yr. The slowest outflow may be driven by a first hydrostatic core. These protostars appear to trace an early phase when the centers of collapsing fragments become optically thick to their own radiation and compression raises the gas temperature. This phase is thought to accompany the formation of hydrostatic cores. A key question is whether these structures are evolving on freefall times of ∼100 yr, or whether they are evolving on Kelvin-Helmholtz times of several thousand years. The number of these sources imply a lifetime of ∼6000 yr, in closer agreement with the Kelvin-Helmholtz time. In this case, rotational and/or magnetic support could be slowing the collapse.
Aims. We aim at detecting water vapor in the atmosphere of the hot Jupiter HD 209458 b and perform a multi-band study in the near infrared with CARMENES. Methods. The water vapor absorption lines ...from the atmosphere of the planet are Doppler-shifted due to the large change in its radial velocity during transit. This shift is of the order of tens of km s−1, whilst the Earth’s telluric and the stellar lines can be considered quasi-static. We took advantage of this shift to remove the telluric and stellar lines using SYSREM, which performs a principal component analysis including proper error propagation. The residual spectra contain the signal from thousands of planetary molecular lines well below the noise level. We retrieve the information from those lines by cross-correlating the residual spectra with models of the atmospheric absorption of the planet. Results. We find a cross-correlation signal with a signal-to-noise ratio (S/N) of 6.4, revealing H2O in HD 209458 b. We obtain a net blueshift of the signal of –5.2 −1.3+2.6 $^{+2.6}_{-1.3}$ −1.3+2.6 km s−1 that, despite the large error bars, is a firm indication of day- to night-side winds at the terminator of this hot Jupiter. Additionally, we performed a multi-band study for the detection of H2O individually from the three near infrared bands covered by CARMENES. We detect H2O from its 0.96–1.06 μm band with a S/N of 5.8, and also find hints of a detection from the 1.06–1.26 μm band, with a low S/N of 2.8. No clear planetary signal is found from the 1.26–1.62 μm band. Conclusions. Our significant H2O signal at 0.96–1.06 μm in HD 209458 b represents the first detection of H2O from this band individually, the bluest one to date. The unfavorable observational conditions might be the reason for the inconclusive detection from the stronger 1.15 and 1.4 μm bands. H2O is detected from the 0.96–1.06 μm band in HD 209458 b, but hardly in HD 189733 b, which supports a stronger aerosol extinction in the latter, in line with previous studies. Future data gathered at more stable conditions and with larger S/N at both optical and near-infrared wavelengths could help to characterize the presence of aerosols in HD 209458 b and other planets.
Stellar activity signatures such as spots and plages can significantly limit the search for extrasolar planets. Current models of activity-induced radial velocity (RV) signals focus on the impact of ...temperature contrast in spots according to which they predict the signal to diminish toward longer wavelengths. The Zeeman effect on RV measurements counteracts this: the relative importance of the Zeeman effect on RV measurements should grow with wavelength because the Zeeman displacement itself grows with λ, and because a magnetic and cool spot contributes more to the total flux at longer wavelengths. In this paper, we model the impact of active regions on stellar RV measurements including both temperature contrast in spots and line broadening by the Zeeman effect. We calculate stellar line profiles using polarized radiative transfer models including atomic and molecular Zeeman splitting over large wavelength regions from 0.5 to 2.3 μm. Our results show that the amplitude of the RV signal caused by the Zeeman effect alone can be comparable to that caused by temperature contrast; a spot magnetic field of ~1000 G can produce a similar RV amplitude as a spot temperature contrast of ~1000 K. Furthermore, the RV signal caused by cool and magnetic spots increases with wavelength, in contrast to the expectation from temperature contrast alone. We also calculate the RV signal caused by variations in average magnetic field strength from one observation to the next, for example due to a magnetic cycle, but find it unlikely that this can significantly influence the search for extrasolar planets. As an example, we derive the RV amplitude of the active M dwarf AD Leo as a function of wavelength using data from the HARPS spectrograph. Across this limited wavelength range, the RV signal does not diminish at longer wavelengths but shows evidence for the opposite behavior, consistent with a strong influence of the Zeeman effect. We conclude that the RV signal of active stars does not vanish at longer wavelength but sensitively depends on the combination of spot temperature and magnetic field; in active low-mass stars, it is even likely to grow with wavelength.
Context. The K2.5 dwarf HD 40307 has been reported to host three super-Earths. The system lacks massive planets and is therefore a potential candidate for having additional low-mass planetary ...companions. Aims. We re-derive Doppler measurements from public HARPS spectra of HD 40307 to confirm the significance of the reported signals using independent data analysis methods. We also investigate these measurements for additional low-amplitude signals. Methods. We used Bayesian analysis of our radial velocities to estimate the probability densities of different model parameters. We also estimated the relative probabilities of models with differing numbers of Keplerian signals and verified their significance using periodogram analyses. We investigated the relation of the detected signals with the chromospheric emission of the star. As previously reported for other objects, we found that radial velocity signals correlated with the S-index are strongly wavelength dependent. Results. We identify two additional clear signals with periods of 34 and 51 days, both corresponding to planet candidates with minimum masses a few times that of the Earth. An additional sixth candidate is initially found at a period of 320 days. However, this signal correlates strongly with the chromospheric emission from the star and is also strongly wavelength dependent. When analysing the red half of the spectra only, the five putative planetary signals are recovered together with a very significant periodicity at about 200 days. This signal has a similar amplitude as the other new signals reported in the current work and corresponds to a planet candidate with Msini ~ 7 M⊕ (HD 40307 g). Conclusions. We show that Doppler measurements can be filtered for activity-induced signals if enough photons and a sufficient wavelength interval are available. If the signal corresponding to HD 40307 g is a genuine Doppler signal of planetary origin, this candidate planet might be capable of supporting liquid water on its surface according to the current definition of the liquid water habitable zone around a star and is not likely to suffer from tidal locking. Also, at an angular separation of ~46 mas, HD 40307 g would be a primary target for a future space-based direct-imaging mission.
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