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Register a new userDisentangling the planet from the star in late type M dwarfs: A case study of TRAPPIST-1g
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The atmospheres of late M stars represent a significant challenge in the characterization of any transiting exoplanets due to the presence of strong molecular features in the stellar atmosphere. TRAPPIST-1 is an ultra-cool dwarf, host to seven transiting planets, and contains its own molecular signatures which can potentially be imprinted on planetary transit lightcurves due to inhomogeneities in the occulted stellar photosphere. We present a case study on TRAPPIST-1g, the largest planet in the system, using a new observation together with previous data, to disentangle the atmospheric transmission of the planet from that of the star. We use the out-of-transit stellar spectra to reconstruct the stellar flux based on one-, two-, and three-temperature components. We find that TRAPPIST-1 is a 0.08 M$_*$, 0.117 R$_*$, M8V star with a photospheric effective temperature of 2400 K, with ~35% 3000 K spot coverage and a very small fraction, <3%, of ~5800 K hot spot. We calculate a planetary radius for TRAPPIST-1g to be Rp = 1.124 R$_oplus$ with a planetary density of $rho_p$ = 0.8214 $rho_oplus$. Based on the stellar reconstruction there are eleven plausible scenarios for the combined stellar photosphere and planet transit geometry; in our analysis we are able to rule out 8 of the 11 scenarios. Using planetary models we evaluate the remaining scenarios with respect to the transmission spectrum of TRAPPIST-1g. We conclude that the planetary transmission spectrum is likely not contaminated by any stellar spectral features, and are able to rule out a clear solar H2/He-dominated atmosphere at greater than 3-sigma.
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We present the discovery of WASP-39b, a highly inflated transiting Saturn-mass planet orbiting a late G-type dwarf star with a period of $4.055259 pm 0.000008$,d, Transit Epoch T$_{0}=2455342.9688pm0.0002$,(HJD), of duration $0.1168 pm 0.0008$,d. A combined analysis of the WASP photometry, high-precision follow-up transit photometry, and radial velocities yield a planetary mass of $mpl=0.28pm0.03,mj$ and a radius of $rpl=1.27pm0.04,rj$, resulting in a mean density of $0.14 pm 0.02,rhoj$. The stellar parameters are mass $mstar = 0.93 pm 0.03,msun$, radius $rstar = 0.895pm 0.23,rsun$, and age $9^{+3}_{-4}$,Gyr. Only WASP-17b and WASP-31b have lower densities than WASP-39b, although they are slightly more massive and highly irradiated planets. From our spectral analysis, the metallicity of WASP-39 is measured to be feh,$= -0.12pm0.1$,dex, and we find the planet to have an equilibrium temperature of $1116^{+33}_{-32}$,K,. Both values strengthen the observed empirical correlation between these parameters and the planetary radius for the known transiting Saturn-mass planets.
Cool M dwarfs outnumber sun-like G stars by ten to one in the solar neighborhood. Due to their proximity, small size, and low mass, M-dwarf stars are becoming attractive targets for exoplanet searches via almost all current search methods. But what planetary systems can form around M dwarfs? Following up on the Cool Stars~16 Splinter Session Planet Formation Around M Dwarfs, we summarize here our knowledge of protoplanetary disks around cool stars, how they disperse, what planetary systems might form and can be detected with current and future instruments.
The CARMENES exoplanet survey of M dwarfs has obtained more than 18 000 spectra of 329 nearby M dwarfs over the past five years as part of its guaranteed time observations (GTO) program. We determine planet occurrence rates with the 71 stars from the GTO program for which we have more than 50 observations. We use injection-and-retrieval experiments on the radial-velocity (RV) time series to measure detection probabilities. We include 27 planets in 21 planetary systems in our analysis. We find 0.06+0.04-0.03 giant planets (100 M_Earth < M_pl sin i < 1000 M_Earth) per star in periods of up to 1000 d, but due to a selection bias this number could be up to a factor of five lower in the whole 329-star sample. The upper limit for hot Jupiters (orbital period of less than 10 d) is 0.03 planets per star, while the occurrence rate of planets with intermediate masses (10 M_Earth < M_pl sin i < 100 M_Earth) is 0.18+0.07-0.05 planets per star. Less massive planets with 1 M_Earth < M_pl sin i < 10 M_Earth are very abundant, with an estimated rate of 1.32+0.33-0.31 planets per star for periods of up to 100 d. When considering only late M dwarfs with masses M_star < 0.34 M_sol, planets more massive than 10 M_Earth become rare. Instead, low-mass planets with periods shorter than 10 d are significantly overabundant. For orbital periods shorter than 100 d, our results confirm the known stellar mass dependences from the Kepler survey: M dwarfs host fewer giant planets and at least two times more planets with M_pl sin i < 10 M_Earth than G-type stars. In contrast to previous results, planets around our sample of very low-mass stars have a higher occurrence rate in short-period orbits of less than 10 d. Our results demonstrate the need to take into account host star masses in planet formation models.
There have recently been detections of radio emission from low-mass stars, some of which are indicative of star-planet interactions. Motivated by these exciting new results, here we present stellar wind models for the active planet-hosting M dwarf AU Mic. Our models incorporate the large-scale photospheric magnetic field map of the star, reconstructed using the Zeeman-Doppler Imaging method. We use our models to assess if planet-induced radio emission could be generated in the corona of AU Mic, through a mechanism analogous to the sub-Alfvenic Jupiter-Io interaction. In the case that AU Mic has a mass-loss rate of 27 times that of the Sun, we find that both planets b and c in the system can induce radio emission from 10 MHz to 3 GHz in the corona of the host star for the majority of their orbits, with peak flux densities of 10 mJy. Our predicted emission bears a striking similarity to that recently reported from GJ 1151 by Vedantham et al. (2020), which is indicative of being induced by a planet. Detection of such radio emission would allow us to place an upper limit on the mass-loss rate of the star.
There have recently been detections of radio emission from low-mass stars, some of which are indicative of star-planet interactions. Motivated by these exciting new results, in this paper we present Alfven wave-driven stellar wind models of the two active planet-hosting M dwarfs Prox Cen and AU Mic. Our models incorporate large-scale photospheric magnetic field maps reconstructed using the Zeeman-Doppler Imaging method. We obtain a mass-loss rate of $0.25~dot{M}_{odot}$ for the wind of Prox Cen. For the young dwarf AU Mic, we explore two cases: a low and high mass-loss rate. Depending on the properties of the Alfven waves which heat the corona in our wind models, we obtain mass-loss rates of $27$ and $590~dot{M}_{odot}$ for AU Mic. We use our stellar wind models to assess the generation of electron cyclotron maser instability emission in both systems, through a mechanism analogous to the sub-Alfvenic Jupiter-Io interaction. For Prox Cen we do not find any feasible scenario where the planet can induce radio emission in the stars corona, as the planet orbits too far from the star in the super-Alfvenic regime. However, in the case that AU Mic has a stellar wind mass-loss rate of $27~dot{M}_{odot}$, we find that both planets b and c in the system can induce radio emission from $sim10$ MHz to 3 GHz in the corona of the host star for the majority of their orbits, with peak flux densities of $sim10$ mJy. Detection of such radio emission would allow us to place an upper limit on the mass-loss rate of the star.
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