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Planet-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

Maciejewski G., Knutson H. A., Howard A. W., Isaacson H., Fernández-Lajús E., Di Sisto R. P., Migaszewski C.

Acta Astronomica

2020

Vol. 70, No. 1

1--18

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 yr 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 3.9×106 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.

Multiple Stellar Encounters on Protoplanetary Disks in Birth Clusters of Stars

Jiménez-Torres J. J.

Acta Astronomica

2020

Vol. 70, No. 1

53--74

We investigate the orbital modification of planetesimals in protoplanetary disks due to multiple stellar encounters in star formation clusters. We modeled multiple encounters with different flyby masses ranging from 0.02 M⊙ up to 0.5 M⊙, approach distances within 400 a.u. and virial velocities to simulate conditions of encounters in star formation regions. We propose an analysis of mass densities as a function of time and found that densities from 3×103 M⊙/pc3 to 1.4×104 M⊙/pc3 can produce from one up to three stellar encounters on a cross sectional area with a radius of 400 a.u. containing a 100 a.u. planetary disk in a period of time of 1×106 yr. We found that the orbital structure of observed transneptunian objects such as 2005 QU182 and the sednoid 2012 VP113 can simultaneously be produced with multiple encounters. We also found that the effect of multiple encounters with low mass stars (<0.5 M⊙) can reproduce similar orbital eccentricities and inclinations as produced by a single stellar encounter with a mass in the order of 1 M⊙. This corresponds to a realistic scenario considering that low mass stars represent the majority of the new born stars in a birth cluster of stars. We provide a relation between the number of stellar encounters on a cross sectional area and the stellar mass density. With interferometers such as ALMA, the influence of stellar encounters in star formation regions may become testable, which suggests the incorporation of different planetary disk conditions in future studies.

Planet-Star Interactions with Precise Transit Timing. I. The Refined Orbital Decay Rate for WASP-12 b and Initial Constraints for HAT-P-23 b, KELT-1 b, KELT-16 b, WASP-33 b and WASP-103 b

Maciejewski G., Fernández M., Aceituno F., Martín-Ruiz S., Ohlert J., Dimitrov D., Szyszka K., von Essen C., Mugrauer M., Bischoff R., Michel K.-U., Mallonn M., Stangret M., Moździerski D.

Acta Astronomica

2018

Vol. 68, No. 4

371--401

Theoretical calculations and some indirect observations show that massive exoplanets on tight orbits must decay due to tidal dissipation within their host stars. This orbital evolution could be observationally accessible through precise transit timing over a course of decades. The rate of planetary in-spiraling may not only help us to understand some aspects of evolution of planetary systems, but also can be used as a probe of the stellar internal structure. In this paper we present results of transit timing campaigns organized for a carefully selected sample of the Northern hemisphere hot Jupiter-like planets which were found to be the best candidates for detecting planet-star tidal interactions. Among them, there is the WASP-12 system which is the best candidate for possessing an in-falling giant exoplanet. Our new observations support the scenario of orbital decay of WASP-12 b and allow us to refine its rate. The derived tidal quality parameter of the host star Q'*=(1.82±0.32)×105 is in agreement with theoretical predictions for subgiant stars. For the remaining systems – HAT-P-23, KELT-1, KELT-16, WASP-33, and WASP-103 – our transit timing data reveal no deviations from the constant-period models, hence constraints on the individual rates of orbital decay were placed. The tidal quality parameters of host stars in at least four systems – HAT-P-23, KELT-1, WASP-33, and WASP-103 – were found to be greater than the value reported for WASP-12. This is in line with the finding that those hosts are main sequence stars, for which efficiency of tidal dissipation is predicted to be relatively weak.