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Register a new userLyman-alpha in the GJ 1132 System: Stellar Emission and Planetary Atmospheric Evolution
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GJ 1132b, which orbits an M dwarf, is one of the few known Earth-sized planets, and at 12 pc away it is one of the closest known transiting planets. Receiving roughly 19x Earths insolation, this planet is too hot to be habitable but can inform us about the volatile content of rocky planet atmospheres around cool stars. Using Hubble STIS spectra, we search for a transit in the Lyman-alpha line of neutral hydrogen (Ly-alpha). If we were to observe a deep Ly-alpha absorption signature, that would indicate the presence of a neutral hydrogen envelope flowing from GJ 1132b. On the other hand, ruling out deep absorption from neutral hydrogen may indicate that this planet does not have a detectable amount of hydrogen loss, is not losing hydrogen, or lost hydrogen and other volatiles early in the stars life. We do not detect a transit and determine a 2-sigma upper limit on the effective envelope radius of 0.36 R* in the red wing of the Ly-alpha line, which is the only portion of the spectrum we detect after absorption by the ISM. We analyze the Ly-alpha spectrum and stellar variability of GJ1132, which is a slowly-rotating 0.18 solar mass M dwarf with previously uncharacterized UV activity. Our data show stellar variabilities of 5-22%, which is consistent with the M dwarf UV variabilities of up to 41% found by citet{Loyd2014}. Understanding the role that UV variability plays in planetary atmospheres is crucial to assess atmospheric evolution and the habitability of cooler rocky exoplanets.
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The atmospheres of close-in planets are strongly influenced by mass loss driven by the high-energy (X-ray and extreme ultraviolet, EUV) irradiation of the host star, particularly during the early stages of evolution. We recently developed a framework to exploit this connection and enable us to recover the past evolution of the stellar high-energy emission from the present-day properties of its planets, if the latter retains some remnants of their primordial hydrogen-dominated atmospheres. Furthermore, the framework can also provide constraints on planetary initial atmospheric mass fractions. The constraints on the output parameters improve when more planets can be simultaneously analysed. This makes the Kepler-11 system, which hosts six planets with bulk densities between 0.66 and 2.45g cm^{-3}, an ideal target. Our results indicate that the star has likely evolved as a slow rotator (slower than 85% of the stars with similar masses), corresponding to a high-energy emission at 150 Myr of between 1-10 times that of the current Sun. We also constrain the initial atmospheric mass fractions for the planets, obtaining a lower limit of 4.1% for planet c, a range of 3.7-5.3% for planet d, a range of 11.1-14% for planet e, a range of 1-15.6% for planet f, and a range of 4.7-8.7% for planet g assuming a disc dispersal time of 1 Myr. For planet b, the range remains poorly constrained. Our framework also suggests slightly higher masses for planets b, c, and f than have been suggested based on transit timing variation measurements. We coupled our results with published planet atmosphere accretion models to obtain a temperature (at 0.25 AU, the location of planet f) and dispersal time of the protoplanetary disc of 550 K and 1 Myr, although these results may be affected by inconsistencies in the adopted system parameters.
The GJ 436 planetary system is an extraordinary system. The Neptune-size planet that orbits the M3 dwarf revealed in the Ly$alpha$ line an extended neutral hydrogen atmosphere. This material fills a comet-like tail that obscures the stellar disc for more than 10 hours after the planetary transit. Here, we carry out a series of 3D radiation hydrodynamic simulations to model the interaction of the stellar wind with the escaping planetary atmosphere. With these models, we seek to reproduce the $sim56%$ absorption found in Ly$alpha$ transits, simultaneously with the lack of absorption in H$alpha$ transit. Varying the stellar wind strength and the EUV stellar luminosity, we search for a set of parameters that best fit the observational data. Based on Ly$alpha$ observations, we found a stellar wind velocity at the position of the planet to be around [250-460] km s$^{-1}$ with a temperature of $[3-4]times10^5$ K. The stellar and planetary mass loss rates are found to be $2times 10^{-15}$ M$_odot$ yr$^{-1}$ and $sim[6-10]times10^9$ g s$^{-1}$, respectively, for a stellar EUV luminosity of $[0.8-1.6]times10^{27}$ erg s$^{-1}$. For the parameters explored in our simulations, none of our models present any significant absorption in the H$alpha$ line in agreement with the observations.
Strong atmospheric escape has been detected in several close-in exoplanets. As these planets consist mostly of hydrogen, observations in hydrogen lines, such as Ly-alpha and H-alpha, are powerful diagnostics of escape. Here, we simulate the evolution of atmospheric escape of close-in giant planets and calculate their associated Ly-alpha and H-alpha transits. We use a one-dimensional hydrodynamic escape model to compute physical properties of the atmosphere and a ray-tracing technique to simulate spectroscopic transits. We consider giant (0.3 and 1M_jup) planets orbiting a solar-like star at 0.045au, evolving from 10 to 5000 Myr. We find that younger giants show higher rates of escape, owing to a favourable combination of higher irradiation fluxes and weaker gravities. Less massive planets show higher escape rates (1e10 -- 1e13 g/s) than those more massive (1e9 -- 1e12 g/s) over their evolution. We estimate that the 1-M_jup planet would lose at most 1% of its initial mass due to escape, while the 0.3-M_jup planet, could lose up to 20%. This supports the idea that the Neptunian desert has been formed due to significant mass loss in low-gravity planets. At younger ages, we find that the mid-transit Ly-alpha line is saturated at line centre, while H-alpha exhibits transit depths of at most 3 -- 4% in excess of their geometric transit. While at older ages, Ly-alpha absorption is still significant (and possibly saturated for the lower mass planet), the H-alpha absorption nearly disappears. This is because the extended atmosphere of neutral hydrogen becomes predominantly in the ground state after ~1.2 Gyr.
HD 21749 is a bright ($V=8.1$ mag) K dwarf at 16 pc known to host an inner terrestrial planet HD 21749c as well as an outer sub-Neptune HD 21749b, both delivered by TESS. Follow-up spectroscopic observations measured the mass of HD 21749b to be $22.7pm2.2 M_{oplus}$ with a density of $7.0^{+1.6}_{-1.3}$ g~cm$^{-3}$, making it one of the densest sub-Neptunes. However, the mass measurement was suspected to be influenced by stellar rotation. Here we present new high-cadence PFS RV data to disentangle the stellar activity signal from the planetary signal. We find that HD 21749 has a similar rotational timescale as the planets orbital period, and the amplitude of the planetary orbital RV signal is estimated to be similar to that of the stellar activity signal. We perform Gaussian Process (GP) regression on the photometry and RVs from HARPS and PFS to model the stellar activity signal. Our new models reveal that HD 21749b has a radius of $2.86pm0.20 R_{oplus}$, an orbital period of $35.6133pm0.0005$ d with a mass of $M_{b}=20.0pm2.7 M_{oplus}$ and a density of $4.8^{+2.0}_{-1.4}$ g~cm$^{-3}$ on an eccentric orbit with $e=0.16pm0.06$, which is consistent with the most recent values published for this system. HD 21749c has an orbital period of $7.7902pm0.0006$ d, a radius of $1.13pm0.10 R_{oplus}$, and a 3$sigma$ mass upper limit of $3.5 M_{oplus}$. Our Monte Carlo simulations confirm that without properly taking stellar activity signals into account, the mass measurement of HD 21749b is likely to arrive at a significantly underestimated error bar.
Aims: Our objective is the optical and near-infrared spectroscopic characterisation of 2MASS J0249-0557 c, a recently discovered young planetary mass companion to the $beta$ Pictoris member 2MASS J0249-0557. Methods: Using the Visible and Infrared Survey Telescope for Astronomy (VISTA) Hemisphere Survey (VHS) and the Two Micron All Sky Survey (2MASS) data, we independently identified the companion 2MASS J0249-0557 c. We obtained low-resolution optical spectroscopy of this object using the Optical System for Imaging and low-intermediate-Resolution Integrated Spectroscopy (OSIRIS) spectrograph at the Gran Telescopio Canarias (GTC), and near-infrared spectroscopy using the Son of Isaac (SofI) spectrograph on the New Technology Telescope (NTT). Results: We classified 2MASS J0249-0557 c with a spectral type of L2.5$pm$0.5 in the optical and L3$pm$1 in the near-infrared. We identified spectroscopic indicators of youth that are compatible with the age of the $beta$ Pictoris moving group. We also detect a strong H$alpha$ emission, with a pEW of -90$^{+20}_{-40}$A, which seems persistent in time. This indicates strong chromospheric activity or disk accretion. Although many M-type brown dwarfs have strong H$alpha$ emission, this target is one of the very few L-type planetary mass objects in which this strong H$alpha$ emission has been detected. Lithium absorption at 6708 A is observed with pEW $lesssim$ 5A. We also computed the binding energy of 2MASS J0249-0557 c and obtained an (absolute) upper limit of $U=(-8.8pm4.4) 10^{32}$ J. Conclusions: Similarly to other young brown dwarfs and isolated planetary mass objects, strong H$alpha$ emission is also present in young planetary mass companions at ages of some dozen million years. We also found that 2MASS J0249-0557 c is one of the wide substellar companions with the lowest binding energy known to date.
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