Context.
The atmosphere of exoplanets has been studied extensively in recent years, making use of numerical models to retrieve chemical composition, dynamical circulation, or temperature from the ...data. One of the best observational probes in transmission is the sodium doublet thanks to its extensive cross-section. However, modelling the shape of planetary sodium lines has proven to be challenging. Models with different assumptions regarding the atmosphere have been employed to fit the lines in the literature, yet statistically-sound, direct comparisons of different models are needed to paint a clear picture.
Aims.
We aim to compare different wind and temperature patterns, as well as to provide a tool to distinguish them based on their best fit for the sodium transmission spectrum of the hot Jupiter HD 189733b. We parametrise different possible wind patterns that have already been tested the in literature and introduce the new option of an upwards-driven vertical wind.
Methods.
We construct a forward model where the wind speed, wind geometry, and temperature are injected into the calculation of the transmission spectrum. We embed this forward model in a nested sampling retrieval code to rank the models via their Bayesian evidence.
Results.
We retrieve a best-fit to the HD 189733b data for vertical upward winds |
v
ver
(mean)| = 40 ± 4 km s
−1
at altitudes above 10
−6
bar. With the current data from HARPS, we cannot distinguish wind patterns for higher-pressure atmospheric layers.
Conclusions.
We show that vertical upwards winds in the upper atmosphere provide a possible explanation for the broad sodium signature in hot Jupiters. We highlight other influences on the width of the doublet and we explore strong magnetic fields acting on the lower atmosphere as one possible origin of the retrieved wind speed.
KELT-9 b, the hottest known exoplanet, with
T
eq
~ 4400 K, is the archetype of a new planet class known as ultra-hot Jupiters. These exoplanets are presumed to have an atmosphere dominated by neutral ...and ionized atomic species. In particular, H
α
and H
β
Balmer lines have been detected in the KELT-9 b upper atmosphere, suggesting that hydrogen is filling the planetary Roche lobe and escaping from the planet. In this work, we detected
δ
Scuti-type stellar pulsation (with a period
P
puls
= 7.54 ± 0.12 h) and studied the Rossiter-McLaughlin effect (finding a spin-orbit angle
λ
= −85.01° ± 0.23°) prior to focussing on the Balmer lines (H
α
to H
ζ
) in the optical transmission spectrum of KELT-9 b. Our HARPS-N data show significant absorption for H
α
to H
δ
. The precise line shapes of the H
α
, H
β
, and H
γ
absorptions allow us to put constraints on the thermospheric temperature. Moreover, the mass loss rate, and the excited hydrogen population of KELT-9 b are also constrained, thanks to a retrieval analysis performed with a new atmospheric model. We retrieved a thermospheric temperature of
T
= 13 200
−720
+800
K and a mass loss rate of
Ṁ
= 10
12.8±0.3
g s
−1
when the atmosphere was assumed to be in hydrodynamical expansion and in local thermodynamic equilibrium (LTE). Since the thermospheres of hot Jupiters are not expected to be in LTE, we explored atmospheric structures with non-Boltzmann equilibrium for the population of the excited hydrogen. We do not find strong statistical evidence in favor of a departure from LTE. However, our non-LTE scenario suggests that a departure from the Boltzmann equilibrium may not be sufficient to explain the retrieved low number densities of the excited hydrogen. In non-LTE, Saha equilibrium departure via photo-ionization, is also likely to be necessary to explain the data.
The M dwarf GJ 436 hosts a transiting warm Neptune known to experience atmospheric escape. Previous observations revealed the presence of a giant hydrogen exosphere transiting the star for more than ...5 h, and absorbing up to 56% of the flux in the blue wing of the stellar Lyman-α line of neutral hydrogen (H i Lyα). The unexpected size of this comet-like exosphere prevented observing the full transit of its tail. In this Letter, we present new Lyα observations of GJ 436 obtained with the Space Telescope Imaging Spectrograph (STIS) instrument onboard the Hubble Space Telescope. The stability of the Lyα line over six years allowed us to combine these new observations with archival data sets, substantially expanding the coverage of the exospheric transit. Hydrogen atoms in the tail of the exospheric cloud keep occulting the star for 10–25 h after the transit of the planet, remarkably confirming a previous prediction based on 3D numerical simulations with the EVaporating Exoplanet code (EVE). This result strengthens the interpretation that the exosphere of GJ 436b is shaped by both radiative braking and charge exchanges with the stellar wind. We further report flux decreases of 15 ± 2% and 47 ± 10% in the red wing of the Lyα line and in the line of ionised silicon (Si iii). Despite some temporal variability possibly linked with stellar activity, these two signals occur during the exospheric transit and could be of planetary origin. Follow-up observations will be required to assess the possibility that the redshifted Lyα and Si iii absorption signatures arise from interactions between the exospheric flow and the magnetic field of the star.
We present the analysis of TESS optical photometry of WASP-121b, which reveals the phase curve of this transiting ultra-hot Jupiter. Its hotspot is located at the sub-stellar point, showing ...inefficient heat transport from the dayside (2870 ± 50 K) to the nightside (<2500 K at 3
σ
) at the altitudes probed by TESS. The TESS eclipse depth, measured at the shortest wavelength to date for WASP-121b, confirms the strong deviation from blackbody planetary emission. Our atmospheric retrieval on the complete emission spectrum supports the presence of a temperature inversion, which can be explained by the presence of VO and possibly TiO and FeH. The strong planetary emission at short wavelengths could arise from an H
−
continuum.
Context. The quiet M2.5 star GJ 436 hosts a warm Neptune that displays an extended atmosphere that dwarfs its own host star. Predictions of atmospheric escape in such planets state that H atoms ...escape from the upper atmosphere in a collisional regime and that the flow can drag heavier atoms to the upper atmosphere. It is unclear, however, what astrophysical mechanisms drive the process. Aims. Our objective is to leverage the extensive coverage of observations of the far-ultraviolet (FUV) spectrum of GJ 436 obtained with the Cosmic Origins Spectrograph (COS) to search for signals of metallic ions in the upper atmosphere of GJ 436 b, as well as study the activity-induced variability of the star. Methods. We analyzed flux time-series of species present in the FUV spectrum of GJ 436 and successfully performed geocoronal contamination removal in the COS Lyman-α profiles obtained near the Earth’s night-side. Results. GJ 436 displays flaring events with a rate of ~10 d−1. There is evidence for a possibly long-lived active region or longitude that modulates the FUV metallic lines of the star with amplitudes up to 20%. Despite the strong geocoronal contamination in the COS spectra, we detected in-transit excess absorption signals of ~50 and ~30% in the blue and red wings, respectively, of the Lyman-α line. We rule out a wide range of excess absorption levels in the metallic lines of the star during transit. Conclusions. The large atmospheric loss of GJ 436 b observed in Lyman-α transmission spectra is stable over the timescale of a few years, and the red wing signal supports the presence of a variable hydrogen absorption source besides the stable exosphere. The previously claimed in-transit absorption in the Si III line is likely an artifact resulting from the stellar magnetic cycle. The non-detection of metallic ions in absorption could indicate that the escape is not hydrodynamic or that the atmospheric mixing is not efficient in dragging metals high enough for sublimation to produce a detectable escape rate of ions to the exosphere.
ABSTRACT
The determination of the fundamental properties (mass, separation, age, gravity, and atmospheric properties) of brown dwarf companions allows us to infer crucial informations on their ...formation and evolution mechanisms. Spectroscopy of substellar companions is available to date only for a limited number of objects (and mostly at very low resolution, R < 50) because of technical limitations, i.e. contrast and angular resolution. We present medium resolution (R = 350), coronagraphic long-slit spectroscopic observations with SPHERE of two substellar companions, HD 1160 B and HD 19467 B. We found that HD 1160 B has a peculiar spectrum that cannot be fitted by spectra in current spectral libraries. A good fit is possible only considering separately the Y+J and the H spectral band. The spectral type is between M5 and M7. We also estimated a Teff of 2800–2900 K and a log g of 3.5–4.0 dex. The low surface gravity seems to favour young age (10–20 Myr) and low mass (∼20 MJup ) for this object. HD 19467 B is instead a fully evolved object with a Teff of ∼1000 K and log g of ∼5.0 dex. Its spectral type is T6 ± 1.
Context.
Small planets located at the lower mode of the bimodal radius distribution are generally assumed to be composed of iron and silicates in a proportion similar to that of the Earth. However, ...recent discoveries are revealing a new group of low-density planets that are inconsistent with that description.
Aims.
We intend to confirm and characterize the TESS planet candidate TOI-244.01, which orbits the bright (
K
= 7.97 mag), nearby (
d
= 22 pc), and early-type (M2.5 V) M-dwarf star GJ 1018 with an orbital period of 7.4 days.
Methods.
We used Markov chain Monte Carlo methods to model 57 precise radial velocity measurements acquired by the ESPRESSO spectrograph together with TESS photometry and complementary HARPS data. Our model includes a planetary component and Gaussian processes aimed at modeling the correlated stellar and instrumental noise.
Results.
We find TOI-244 b to be a super-Earth with a radius of
R
p
= 1.52 ± 0.12
R
⊕
and a mass of
M
p
= 2.68 ± 0.30
M
⊕
. These values correspond to a density of
ρ
= 4.2 ± 1.1 g cm
−3
, which is below what would be expected for an Earth-like composition. We find that atmospheric loss processes may have been efficient to remove a potential primordial hydrogen envelope, but high mean molecular weight volatiles such as water could have been retained. Our internal structure modeling suggests that TOI-244 b has a 479
−96
+128
km thick hydrosphere over a 1.17 ± 0.09
R
⊕
solid structure composed of a Fe-rich core and a silicate-dominated mantle compatible with that of the Earth. On a population level, we find two tentative trends in the density-metallicity and density-insolation parameter space for the low-density super-Earths, which may hint at their composition.
Conclusions.
With a 8% precision in radius and 12% precision in mass, TOI-244 b is among the most precisely characterized super-Earths, which, together with the likely presence of an extended hydrosphere, makes it a key target for atmospheric observations.
GJ 9827 is a bright, nearby K7V star orbited by two super-Earths and one mini-Neptune on close-in orbits. The system was first discovered using K2 data and then further characterized by other ...spectroscopic and photometric instruments. Previous literature studies provide several mass measurements for the three planets, however, with large variations and uncertainties. To better constrain the planetary masses, we added high-precision radial velocity measurements from ESPRESSO to published datasets from HARPS, HARPS-N, and HIRES and we performed a Gaussian process analysis combining radial velocity and photometric datasets from K2 and TESS. This method allowed us to model the stellar activity signal and derive precise planetary parameters. We determined planetary masses of M b = 4.28 −0.33 +0.35 M ⊕ , M c = 1.86 −0.39 +0.37 M ⊕ , and M d = 3.02 −0.57 +0.58 M ⊕ , and orbital periods of 1.208974 ± 0.000001 days for planet b, 3.648103 −0.000010 +0.000013 days for planet c, and 6.201812 ± 0.000009 days for planet d. We compared our results to literature values and found that our derived uncertainties for the planetary mass, period, and radial velocity amplitude are smaller than the previously determined uncertainties. We modeled the interior composition of the three planets using the machine-learning-based tool ExoMDN and conclude that GJ 9827 b and c have an Earth-like composition, whereas GJ 9827 d has an hydrogen envelope, which, together with its density, places it in the mini-Neptune regime.
The high-energy X-ray to ultraviolet (XUV) irradiation of close-in planets by their host star influences their evolution and might be responsible for the existence of a population of ultra-short ...period planets eroded to their bare core. In orbit around a bright, nearby G-type star, the super-Earth 55 Cnc e offers the possibility to address these issues through transit observations at UV wavelengths. We used the Hubble Space Telescope to observe the transit in the far-ultraviolet (FUV) over three epochs in April 2016, January 2017, and February 2017. Together, these observations cover nearly half of the orbital trajectory in between the two quadratures, and reveal significant short- and long-term variability in 55 Cnc chromospheric emission lines. In the last two epochs, we detected a larger flux in the C III, Si III, and Si IV lines after the planet passed the approaching quadrature, followed by a flux decrease in the Si IV doublet. In the second epoch these variations are contemporaneous with flux decreases in the Si II and C II doublets. All epochs show flux decreases in the N V doublet as well, albeit at different orbital phases. These flux decreases are consistent with absorption from optically thin clouds of gas, are mostly localized at low and redshifted radial velocities in the star rest frame, and occur preferentially before and during the planet transit. These three points make it unlikely that the variations are purely stellar in origin, yet we show that the occulting material is also unlikely to originate from the planet. We thus tentatively propose that the motion of 55 Cnc e at the fringes of the stellar corona leads to the formation of a cool coronal rain. The inhomogeneity and temporal evolution of the stellar corona would be responsible for the differences between the three visits. Additional variations are detected in the C II doublet in the first epoch and in the O I triplet in all epochs with a different behavior that points toward intrinsic stellar variability. Further observations at FUV wavelengths are required to disentangle definitively between star-planet interactions in the 55 Cnc system and the activity of the star.
Context. The study of high-contrast imaged brown dwarfs and exoplanets depends strongly on evolutionary models. To estimate the mass of a directly imaged substellar object, its extracted photometry ...or spectrum is used and adjusted with model spectra together with the estimated age of the system. These models still need to be properly tested and constrained. HD 4747B is a brown dwarf close to the H burning mass limit, orbiting a nearby (d = 19.25 ± 0.58 pc), solar-type star (G9V); it has been observed with the radial velocity method for over almost two decades. Its companion was also recently detected by direct imaging, allowing a complete study of this particular object. Aims. We aim to fully characterize HD 4747B by combining a well-constrained dynamical mass and a study of its observed spectral features in order to test evolutionary models for substellar objects and to characterize its atmosphere. Methods. We combined the radial velocity measurements of High Resolution Echelle Spectrometer (HIRES) and CORALIE taken over two decades and high-contrast imaging of several epochs from NACO, NIRC2, and SPHERE to obtain a dynamical mass. From the SPHERE data we obtained a low-resolution spectrum of the companion from Y to H band, and two narrow band-width photometric measurements in the K band. A study of the primary star also allowed us to constrain the age of the system and its distance. Results. Thanks to the new SPHERE epoch and NACO archival data combined with previous imaging data and high-precision radial velocity measurements, we were able to derive a well-constrained orbit. The high eccentricity (e = 0.7362 ± 0.0025) of HD 4747B is confirmed, and the inclination and the semi-major axis are derived (i = 47.3 ± 1.6°, a = 10.01 ± 0.21 au). We derive a dynamical mass of mB = 70.0 ± 1.6 MJup, which is higher than a previous study but in better agreement with the models. By comparing the object with known brown dwarfs spectra, we derive a spectral type of L9 and an effective temperature of 1350 ± 50 K. With a retrieval analysis we constrain the oxygen and carbon abundances and compare them with the values from the HR 8799 planets.
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