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Register a new userNear-UV Absorption, Chromospheric Activity, and Star-Planet Interactions in the WASP-12 system
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We observed the extreme close-in hot Jupiter system WASP-12 with HST. Near-UV transits up to three times deeper than the optical transit of WASP-12b reveal extensive diffuse gas, extending well beyond the Roche lobe. The distribution of absorbing gas varies between visits. The deepest NUV transits are at wavelength ranges with strong photospheric absorption, implying the absorbing gas may have temperature and composition similar to the stellar photosphere. Our spectra reveal significantly enhanced absorption (greater than 3 sigma below the median) at ~200 wavelengths on each of two HST visits; 65 of these wavelengths are consistent between the two visits, using a strict criterion for velocity matching which excludes matches with velocity shifts exceeding ~20 km/s. Excess transit depths are robustly detected throughout the inner wings of the MgII resonance lines independently on both HST visits. We detected absorption in FeII 2586A, the heaviest species yet detected in an exoplanet transit. The MgII line cores have zero flux, emission cores exhibited by every other observed star of similar age and spectral type are conspicuously absent. WASP-12 probably produces normal MgII profiles, but the inner portions of these strong resonance lines are likely affected by extrinsic absorption. The required Mg+ column is an order of magnitude greater than expected from the ISM, though we cannot completely dismiss that possibility. A more plausible source of absorption is gas lost by WASP-12b. We show that planetary mass loss can produce the required column. Our Visit 2 NUV light curves show evidence for a stellar flare. We show that some of the possible transit detections in resonance lines of rare elements may be due instead to non-resonant transitions in common species. We present optical observations and update the transit ephemeris.
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We measured the chromospheric activity of the four hot Jupiter hosts WASP-43, WASP-51/HAT-P-30, WASP-72 & WASP-103 to search for anomalous values caused by the close-in companions. The Mount Wilson Ca II H&K S-index was calculated for each star using observations taken with the Robert Stobie Spectrograph at the Southern African Large Telescope. The activity level of WASP-43 is anomalously high relative to its age and falls among the highest values of all known main sequence stars. We found marginal evidence that the activity of WASP-103 is also higher than expected from the system age. We suggest that for WASP-43 and WASP-103 star-planet interactions (SPI) may enhance the Ca II H&K core emission. The activity levels of WASP-51/HAT-P-30 and WASP-72 are anomalously low, with the latter falling below the basal envelope for both main sequence and evolved stars. This can be attributed to circumstellar absorption due to planetary mass loss, though absorption in the ISM may contribute. A quarter of known short period planet hosts exhibit anomalously low activity levels, including systems with hot Jupiters and low mass companions. Since SPI can elevate and absorption can suppress the observed chromospheric activity of stars with close-in planets, their Ca II H&K activity levels are an unreliable age indicator. Systems where the activity is depressed by absorption from planetary mass loss are key targets for examining planet compositions through transmission spectroscopy.
From its discovery, the WASP-18 system with its massive transiting planet on a tight orbit was identified as a unique laboratory for studies on tidal planet-star interactions. In an analysis of Doppler data, which include five new measurements obtained with the HIRES/Keck-I instrument between 2012 and 2018, we show that the radial velocity signal of the photosphere following the planetary tidal potential can be distilled for the host star. Its amplitude is in agreement with both theoretical predictions of the equilibrium tide approximation and an ellipsoidal modulation observed in an orbital phase curve. Assuming a circular orbit, we refine system parameters using photometric time series from TESS. With a new ground-based photometric observation, we extend the span of transit timing observations to 28 years in order to probe the rate of the orbital period shortening. Since we found no departure from a constant-period model, we conclude that the modified tidal quality parameter of the host star must be greater than 3.9x10^6 with 95% confidence. This result is in line with conclusions drawn from studies of the population of hot Jupiters, predicting that the efficiency of tidal dissipation is 1 or 2 orders of magnitude weaker. As the WASP-18 system is one of the prime candidates for detection of orbital decay, further timing observations are expected to push the boundaries of our knowledge on stellar interiors.
WASP-12 is a hot Jupiter system with an orbital period of $P= 1.1textrm{ day}$, making it one of the shortest-period giant planets known. Recent transit timing observations by Maciejewski et al. (2016) and Patra et al. (2017) find a decreasing period with $P/|dot{P}| = 3.2textrm{ Myr}$. This has been interpreted as evidence of either orbital decay due to tidal dissipation or a long term oscillation of the apparent period due to apsidal precession. Here we consider the possibility that it is orbital decay. We show that the parameters of the host star are consistent with either a $M_ast simeq 1.3 M_odot$ main sequence star or a $M_ast simeq 1.2 M_odot$ subgiant. We find that if the star is on the main sequence, the tidal dissipation is too inefficient to explain the observed $dot{P}$. However, if it is a subgiant, the tidal dissipation is significantly enhanced due to nonlinear wave breaking of the dynamical tide near the stars center. The subgiant models have a tidal quality factor $Q_astsimeq 2times10^5$ and an orbital decay rate that agrees well with the observed $dot{P}$. It would also explain why the planet survived for $simeq 3textrm{ Gyr}$ while the star was on the main sequence and yet is now inspiraling on a 3 Myr timescale. Although this suggests that we are witnessing the last $sim 0.1%$ of the planets life, the probability of such a detection is a few percent given the observed sample of $simeq 30$ hot Jupiters in $P<3textrm{ day}$ orbits around $M_ast>1.2 M_odot$ hosts.
The single-lined spectroscopic binary $ u$ Octantis provided evidence of the first conjectured circumstellar planet demanding an orbit retrograde to the stellar orbits. The planet-like behaviour is now based on 1437 radial velocities (RVs) acquired from 2001 to 2013. $ u$ Octs semimajor axis is only 2.6 AU with the candidate planet orbiting $ u$ Oct A about midway between. These details seriously challenge our understanding of planet formation and our decisive modelling of orbit reconfiguration and stability scenarios. However, all non-planetary explanations are also inconsistent with numerous qualitative and quantitative tests including previous spectroscopic studies of bisectors and line-depth ratios, photometry from Hipparcos and the more recent space missions TESS and GAIA (whose increased parallax classifies $ u$ Oct A closer still to a subgiant ~ K1 IV). We conducted the first large survey of $ u$ Oct As chromosphere: 198 Ca II H-line and 1160 H $alpha$ indices using spectra from a previous RV campaign (2009-2013). We also acquired 135 spectra (2018-2020) primarily used for additional line-depth ratios, which are extremely sensitive to the photospheres temperature. We found no significant RV-correlated variability. Our line-depth ratios indicate temperature variations of only $pm$ 4 K, as achieved previously. Our atypical Ca II analysis models the indices in terms of S/N and includes covariance significantly in their errors. The H $alpha$ indices have a quasi-periodic variability which we demonstrate is due to telluric lines. Our new evidence provides further multiple arguments realistically only in favor of the planet.
WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age ($<$1 Gyr), the star presents an anomalously low stellar activity level: the measured logR$_{rm HK}$ activity parameter lies slightly below the basal level; there is no significant time-variability in the logR$_{rm HK}$ value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of Hubble Space Telescope aimed at explaining this anomaly. From the stars spectral energy distribution, we infer the extinction (E(B-V) $approx$ 0.01 mag) and then the interstellar medium (ISM) column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (CII, CIII, CIV, SiIV). Comparing the CII/CIV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old ($>$5 Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the SiIV line, yields an XUV integrated flux at the planet orbit of 10.2 erg cm$^{-2}$ s$^{-1}$. We employ the rescaled XUV solar fluxes to models of the planetary upper atmosphere, deriving an extremely low thermal mass-loss rate of 10$^{-20}$ $M_{rm J}$ Gyr$^{-1}$. For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.
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