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Register a new userPlanet-star interactions with precise transit timing. II. The radial-velocity tides and a tighter constraint on the orbital decay rate in the WASP-18 system
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Added by
Gracjan Maciejewski
Publication date
2020
fields
Physics
and research's language is
English
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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.
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Motivated by the previously reported high orbital decay rate of the planet WASP-43b, eight newly transit light curves are obtained and presented. Together with other data in literature, we perform a self-consistent timing analysis with data covering a timescale of 1849 epochs. The results give an orbital decay rate dP/dt = -0.02890795pm 0.00772547 sec/year, which is one order smaller than previous values. This slow decay rate corresponds to a normally assumed theoretical value of stellar tidal dissipation factor. In addition, through the frequency analysis, the transit timing variations presented here are unlikely to be periodic, but could be signals of a slow orbital decay.
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.
Transits in the planetary system WASP-4 were recently found to occur 80s earlier than expected in observations from the TESS satellite. We present 22 new times of mid-transit that confirm the existence of transit timing variations, and are well fitted by a quadratic ephemeris with period decay dP/dt = -9.2 +/- 1.1 ms/yr. We rule out instrumental issues, stellar activity and the Applegate mechanism as possible causes. The light-time effect is also not favoured due to the non-detection of changes in the systemic velocity. Orbital decay and apsidal precession are plausible but unproven. WASP-4b is only the third hot Jupiter known to show transit timing variations to high confidence. We discuss a variety of observations of this and other planetary systems that would be useful in improving our understanding of WASP-4 in particular and orbital decay in general.
Transit Timing Variations, or TTVs, can be a very efficient way of constraining masses and eccentricities of multi-planet systems. Recent measurements of the TTVs of TRAPPIST-1 led to an estimate of the masses of the planets, enabling an estimate of their densities. A recent TTV analysis using data obtained in the past two years yields a 34% and 13% increase in mass for TRAPPIST-1b and c, respectively. In most studies to date, a Newtonian N-body model is used to fit the masses of the planets, while sometimes general relativity is accounted for. Using the Posidonius N-body code, we show that in the case of the TRAPPIST-1 system, non-Newtonian effects might be also relevant to correctly model the dynamics of the system and the resulting TTVs. In particular, using standard values of the tidal Love number $k_2$ (accounting for the tidal deformation) and the fluid Love number $k_{2f}$ (accounting for the rotational flattening) leads to differences in the TTVs of TRAPPIST-1b and c similar to the differences caused by general relativity. We also show that relaxing the values of tidal Love number $k_2$ and the fluid Love number $k_{2f}$ can lead to TTVs which differ by as much as a few 10~s on a $3-4$-year timescale, which is a potentially observable level. The high values of the Love numbers needed to reach observable levels for the TTVs could be achieved for planets with a liquid ocean, which, if detected, might then be interpreted as a sign that TRAPPIST-1b and TRAPPIST-1c could have a liquid magma ocean. For TRAPPIST-1 and similar systems, the models to fit the TTVs should potentially account for general relativity, for the tidal deformation of the planets, for the rotational deformation of the planets and, to a lesser extent, for the rotational deformation of the star, which would add up to 7x2+1 = 15 additional free parameters in the case of TRAPPIST-1.
Transit timing analysis may be an effective method of discovering additional bodies in extrasolar systems which harbour transiting exoplanets. The deviations from the Keplerian motion, caused by mutual gravitational interactions between planets, are expected to generate transit timing variations of transiting exoplanets. In 2009 we collected 9 light curves of 8 transits of the exoplanet WASP-10b. Combining these data with published ones, we found that transit timing cannot be explained by a constant period but by a periodic variation. Simplified three-body models which reproduce the observed variations of timing residuals were identified by numerical simulations. We found that the configuration with an additional planet of mass of $sim$0.1 $M_{rm{J}}$ and orbital period of $sim$5.23 d, located close to the outer 5:3 mean motion resonance, is the most likely scenario. If the second planet is a transiter, the estimated flux drop will be $sim$0.3 per cent and can be observable with a ground-based telescope. Moreover, we present evidence that the spots on the stellar surface and rotation of the star affect the radial velocity curve giving rise to spurious eccentricity of the orbit of the first planet. We argue that the orbit of WASP-10b is essentially circular. Using the gyrochronology method, the host star was found to be $270 pm 80$ Myr old. This young age can explain the large radius reported for WASP-10b.
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