GJ 3470b is a warm Neptune transiting an M-dwarf star at the edge of the evaporation desert. It offers the possibility of investigating how low-mass, close-in exoplanets evolve under the irradiation ...from their host stars. We observed three transits of GJ 3470b in the Lyman-α line with the Hubble Space Telescope (HST) as part of the Panchromatic Comparative Exoplanet Treasury (PanCET) program. Absorption signatures are detected with similar properties in all three independent epochs, with absorption depths of 35 ± 7% in the blue wing of the line, and 23 ± 5% in the red wing. The repeatability of these signatures, their phasing with the planet transit, and the radial velocity of the absorbing gas allow us to conclude that there is an extended upper atmosphere of neutral hydrogen around GJ 3470b. We determine from our observations the stellar radiation pressure and XUV irradiation from GJ 3470 and use them to perform numerical simulations of the upper atmosphere of GJ 3470b with the EVaporating Exoplanets (EVE) code. The unusual redshifted signature can be explained by the damping wings of dense layers of neutral hydrogen that extend beyond the Roche lobe and are elongated in the direction of the planet motion. This structure could correspond to a shocked layer of planetary material formed by the collision of the expanding thermosphere with the wind of the star. The blueshifted signature is well explained by neutral hydrogen atoms escaping at rates of about 1010 g s−1 that are blown away from the star by its strong radiation pressure and are quickly photoionized, resulting in a smaller exosphere than that of the warm Neptune GJ 436b. The stronger escape from GJ 3470b, however, may have led to the loss of about 4–35% of its current mass over its ~2 Gyr lifetime.
High resolution transit spectroscopy has proven to be a reliable technique for the characterization of the chemical composition of exoplanet atmospheres. Taking advantage of the broad spectral ...coverage of the CARMENES spectrograph, we initiated a survey aimed at characterizing a broad range of planetary systems. Here, we report our observations of three transits of GJ 3470 b with CARMENES in search of He (2
3
S) absorption. On one of the nights, the He
I
region was heavily contaminated by OH
−
telluric emission and, thus, it was not useful for our purposes. The remaining two nights had a very different signal-to-noise ratio (S/N) due to weather. They both indicate the presence of He (2
3
S) absorption in the transmission spectrum of GJ 3470 b, although a statistically valid detection can only be claimed for the night with higher S/N. For that night, we retrieved a 1.5 ± 0.3% absorption depth, translating into a
R
p
(
λ
)∕
R
p
= 1.15 ± 0.14 at this wavelength. Spectro-photometric light curves for this same night also indicate the presence of extra absorption during the planetary transit with a consistent absorption depth. The He (2
3
S) absorption is modeled in detail using a radiative transfer code, and the results of our modeling efforts are compared to the observations. We find that the mass-loss rate,
Ṁ
, is confined to a range of 3 × 10
10
g s
−1
for
T
= 6000 K to 10 × 10
10
g s
−1
for
T
= 9000 K. We discuss the physical mechanisms and implications of the He
I
detection in GJ 3470 b and put it in context as compared to similar detections and non-detections in other Neptune-size planets. We also present improved stellar and planetary parameter determinations based on our visible and near-infrared observations.
The He
I
λ
10833 Å triplet is a powerful tool for characterising the upper atmosphere of exoplanets and tracing possible mass loss. Here, we analysed one transit of GJ 1214 b observed with the ...CARMENES high-resolution spectrograph to study its atmosphere via transmission spectroscopy around the He
I
triplet. Although previous studies using lower resolution instruments have reported non-detections of He
I
in the atmosphere of GJ 1214 b, we report here the first potential detection. We reconcile the conflicting results arguing that previous transit observations did not present good opportunities for the detection of He
I
, due to telluric H
2
O absorption and OH emission contamination. We simulated those earlier observations, and show evidence that the planetary signal was contaminated. From our single non-telluric-contaminated transit, we determined an excess absorption of 2.10
−0.50
+0.45
% (4.6
σ
) with a full width at half maximum (FWHM) of 1.30
−0.25
+0.30
Å. The detection of He
I
is statistically significant at the 4.6
σ
level, but repeatability of the detection could not be confirmed due to the availability of only one transit. By applying a hydrodynamical model and assuming an H/He composition of 98/2, we found that GJ 1214 b would undergo hydrodynamic escape in the photon-limited regime, losing its primary atmosphere with a mass-loss rate of (1.5–18) × 10
10
g s
−1
and an outflow temperature in the range of 2900–4400 K. Further high-resolution follow-up observations of GJ 1214 b are needed to confirm and fully characterise the detection of an extended atmosphere surrounding GJ 1214 b. If confirmed, this would be strong evidence that this planet has a primordial atmosphere accreted from the original planetary nebula. Despite previous intensive observations from space- and ground-based observatories, our He
I
excess absorption is the first tentative detection of a chemical species in the atmosphere of this benchmark sub-Neptune planet.
We present three transit observations of HD 189733 b obtained with the high-resolution spectrograph CARMENES at Calar Alto. A strong absorption signal is detected in the near-infrared He I triplet at ...10830 Å in all three transits. During mid-transit, the mean absorption level is 0.88 ± 0.04% measured in a ±10 km s−1 range at a net blueshift of − 3.5 ± 0.4 km s−1 (10829.84–10830.57 Å). The absorption signal exhibits radial velocities of + 6.5 ± 3.1 km s−1 and − 12.6 ± 1.0 km s−1 during ingress and egress, respectively; all radial velocities are measured in the planetary rest frame. We show that stellar activity related pseudo-signals interfere with the planetary atmospheric absorption signal. They could contribute as much as 80% of the observed signal and might also affect the observed radial velocity signature, but pseudo-signals are very unlikely to explain the entire signal. The observed line ratio between the two unresolved and the third line of the He I triplet is 2.8 ± 0.2, which strongly deviates from the value expected for an optically thin atmospheres. When interpreted in terms of absorption in the planetary atmosphere, this favors a compact helium atmosphere with an extent of only 0.2 planetary radii and a substantial column density on the order of 4 × 1012 cm−2. The observed radial velocities can be understood either in terms of atmospheric circulation with equatorial superrotation or as a sign of an asymmetric atmospheric component of evaporating material. We detect no clear signature of ongoing evaporation, like pre- or post-transit absorption, which could indicate material beyond the planetary Roche lobe, or radial velocities in excess of the escape velocity. These findings do not contradict planetary evaporation, but only show that the detected helium absorption in HD 189733 b does not trace the atmospheric layers that show pronounced escape signatures.
Chromospheric Ca II activity cycles are frequently found in late-type stars, but no systematic programs have been created to search for their coronal X-ray counterparts. The typical time scale of Ca ...II activity cycles ranges from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. The
XMM-Newton
satellite has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like star
ɛ
Eridani, previously observed on two occasions: in 2003 and in early 2015, both by
XMM-Newton
. With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available for
ɛ
Eridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-term
XMM-Newton
lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity is 2 × 10
28
erg s
−1
, with an amplitude that is only a factor of 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of
ɛ
Eridani in terms of solar magnetic structures: active regions, cores of active regions, and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can only describe the observed X-ray emission measure of
ɛ
Eridani if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of
ɛ
Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of
ɛ
Eridani. Our analysis also revealed that the X-ray cycle of
ɛ
Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of
ɛ
Eridani is thus explained by the high percentage of magnetic structures on its surface.
Context.
HD 209458 b is an exoplanet with an upper atmosphere undergoing blow-off escape that has mainly been studied using measurements of the Ly
α
absorption. Recently, high-resolution measurements ...of absorption in the He
I
triplet line at 10 830 Å of several exoplanets (including HD 209458 b) have been reported, creating a new opportunity to probe escaping atmospheres.
Aims.
We aim to better understand the atmospheric regions of HD 209458 b from where the escape originates.
Methods.
We developed a 1D hydrodynamic model with spherical symmetry for the HD 209458 b thermosphere coupled with a non-local thermodynamic model for the population of the He
I
triplet state. In addition, we performed high-resolution radiative transfer calculations of synthetic spectra for the helium triplet lines and compared them with the measured absorption spectrum in order to retrieve information about the atmospheric parameters.
Results.
We find that the measured spectrum constrains the H/H
+
transition altitude occurring in the range of 1.2
R
P
–1.9
R
P
. Hydrogen is almost fully ionised at altitudes above 2.9
R
P
. We also find that the X-ray and extreme ultraviolet absorption takes place at effective radii from 1.16 to 1.30
R
P
, and that the He
I
triplet peak density occurs at altitudes from 1.04 to 1.60
R
P
. Additionally, the averaged mean molecular weight is confined to the 0.61–0.73 g mole
−1
interval, and the thermospheric H/He ratio should be larger than 90/10, and most likely approximately 98/2. We also provide a one-to-one relationship between mass-loss rate and temperature. Based on the energy-limited escape approach and assuming heating efficiencies of 0.1–0.2, we find a mass-loss rate in the range of (0.42–1.00) ×10
11
g s
−1
and a corresponding temperature range of 7125–8125 K.
Conclusions.
The analysis of the measured He
I
triplet absorption spectrum significantly constrains the thermospheric structure of HD 209458 b and advances our knowledge of its escaping atmosphere.
Stellar flares are a frequent occurrence on young low-mass stars around which many detected exoplanets orbit. Flares are energetic, impulsive events, and their impact on exoplanetary atmospheres ...needs to be taken into account when interpreting transit observations. We have developed a model to describe the upper atmosphere of extrasolar giant planets (EGPs) orbiting flaring stars. The model simulates thermal escape from the upper atmospheres of close-in EGPs. Ionisation by solar radiation and electron impact is included and photo-chemical and diffusive transport processes are simulated. This model is used to study the effect of stellar flares from the solar-like G star HD 209458 and the young K star HD 189733 on their respective planets, HD 209458b and HD 189733b. The Sun is used as a proxy for HD 209458, and ϵ Eridani, as a proxy for HD 189733. A hypothetical HD 209458b-like planet orbiting the very active M star AU Microscopii is also simulated. We find that the neutral upper atmosphere of EGPs is not significantly affected by typical flares on HD 209458 and HD 189733. Therefore, stellar flares alone would not cause large enough changes in planetary mass loss to explain the variations in HD 189733b transit depth seen in previous studies, although we show that it may be possible that an extreme stellar proton event could result in the required mass loss. Our simulations do however reveal an enhancement in electron number density in the ionosphere of these planets, the peak of which is located in the layer where stellar X-rays are absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flare levels and enhanced electron densities last from about 3 to 10 h after the onset of the flare, depending on the composition of the ionospheric layer. The strength of the flare and the width of its spectral energy distribution affect the range of altitudes in the ionosphere that see enhancements in ionisation. A large broadband continuum component in the XUV portion of the flaring spectrum in very young flare stars, such as AU Mic, results in a broad range of altitudes affected in planets orbiting this star. Indeed, as well as the X-ray absorption layer, the layer in which EUV photons are absorbed is also strongly enhanced.
Abstract
Atmospheric mass loss plays a major role in the evolution of exoplanets. This process is driven by the stellar high-energy irradiation, especially in the first hundreds of millions of years ...after dissipation of the proto-planetary disk. A major source of uncertainty in modeling atmospheric photoevaporation and photochemistry is due to the lack of direct measurements of the stellar flux at extreme-UV (EUV) wavelengths. Several empirical relationships have been proposed in the past to link EUV fluxes to emission levels in X-rays, but the stellar samples employed for this aim are heterogeneous, and the available scaling laws provide significantly different predictions, especially for very active stars. We present new far-UV and X-ray observations of V1298 Tau with Hubble Space Telescope/Cosmic Origins Spectrograph and XMM-Newton, aimed to determine more accurately the high-energy emission of this solar-mass pre-main-sequence star, which hosts four exoplanets. Spectroscopic data were employed to derive the plasma emission measure distribution versus temperature, from the chromosphere to the corona, and the possible variability of this irradiation on short and year-long timescales, due to magnetic activity. As a side result, we have also measured the chemical abundances of several elements in the outer atmosphere of V1298 Tau. We employ our results as a new benchmark point for the calibration of the X-ray to EUV scaling laws, and hence to predict the time evolution of the irradiation in the EUV band, and its effect on the evaporation of exo-atmospheres.
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.
Characterising the atmospheres of exoplanets is key to understanding their nature and provides hints about their formation and evolution. High resolution measurements of the helium triplet absorption ...of highly irradiated planets have been recently reported, which provide a new means of studying their atmospheric escape. In this work we study the escape of the upper atmospheres of HD 189733 b and GJ 3470 b by analysing high resolution He
I
triplet absorption measurements and using a 1D hydrodynamic spherically symmetric model coupled with a non-local thermodynamic model for the He
I
triplet state. We also use the H density derived from Ly
α
observations to further constrain their temperatures, mass-loss rates, and H/He ratios. We have significantly improved our knowledge of the upper atmospheres of these planets. While HD 189733 b has a rather compressed atmosphere and small gas radial velocities, GJ 3470 b, on the other hand with a gravitational potential ten times smaller, exhibits a very extended atmosphere and large radial outflow velocities. Hence, although GJ 3470 b is much less irradiated in the X-ray and extreme ultraviolet radiation, and its upper atmosphere is much cooler, it evaporates at a comparable rate. In particular, we find that the upper atmosphere of HD 189733 b is compact and hot, with a maximum temperature of 12 400
−300
+400
K, with a very low mean molecular mass (H/He = (99.2/0.8) ± 0.1), which is almost fully ionised above 1.1
R
P
, and with a mass-loss rate of (1.1 ± 0.1) × 10
11
g s
−1
. In contrast, the upper atmosphere of GJ 3470 b is highly extended and relatively cold, with a maximum temperature of 5100 ± 900 K, also with a very low mean molecular mass (H/He = (98.5/1.5)
−1.5
+1.0
), which is not strongly ionised, and with a mass-loss rate of (1.9 ± 1.1) × 10
11
g s
−1
. Furthermore, our results suggest that upper atmospheres of giant planets undergoing hydrodynamic escape tend to have a very low mean molecular mass (H/He ≳ 97/3).
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