WASP-4 is Accelerating Toward the Earth. (arXiv:2004.00637v1 [astro-ph.EP])

WASP-4 is Accelerating Toward the Earth. (arXiv:2004.00637v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bouma_L/0/1/0/all/0/1">L. G. Bouma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Winn_J/0/1/0/all/0/1">J. N. Winn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Howard_A/0/1/0/all/0/1">A. W. Howard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Howell_S/0/1/0/all/0/1">S. B. Howell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Isaacson_H/0/1/0/all/0/1">H. Isaacson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Knutson_H/0/1/0/all/0/1">H. Knutson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Matson_R/0/1/0/all/0/1">R. A. Matson</a>

The orbital period of the hot Jupiter WASP-4b appears to be decreasing at a
rate of $-8.64 pm 1.26$ msec/yr, based on transit-timing measurements spanning
12 years. Proposed explanations for the period change include tidal orbital
decay, apsidal precession, and acceleration of the system along the line of
sight. To investigate further, we performed new radial velocity measurements
and speckle imaging of WASP-4. The radial-velocity data show that the system is
accelerating towards the Sun at a rate of $-0.0422pm 0.0028$ m/s/day. The
associated Doppler effect should cause the apparent period to shrink at a rate
of $-5.94 pm 0.39$ msec/yr, comparable to the observed rate. Thus, the
observed change in the transit period is mostly or entirely produced by the
line-of-sight acceleration of the system. This acceleration is probably caused
by a wide-orbiting companion of mass 10-300$,M_{rm Jup}$ and orbital distance
10-100$,$AU, based on the magnitude of the radial-velocity trend and the
non-detection of any companion in the speckle images. We expect that the
orbital periods of 1 out of 3 hot Jupiters will change at rates similar to
WASP-4b, based on the hot-Jupiter companion statistics of Knutson et al.
(2014). Continued radial velocity monitoring of hot Jupiters is therefore
essential to distinguish the effects of tidal orbital decay or apsidal
precession from line-of-sight acceleration.

The orbital period of the hot Jupiter WASP-4b appears to be decreasing at a
rate of $-8.64 pm 1.26$ msec/yr, based on transit-timing measurements spanning
12 years. Proposed explanations for the period change include tidal orbital
decay, apsidal precession, and acceleration of the system along the line of
sight. To investigate further, we performed new radial velocity measurements
and speckle imaging of WASP-4. The radial-velocity data show that the system is
accelerating towards the Sun at a rate of $-0.0422pm 0.0028$ m/s/day. The
associated Doppler effect should cause the apparent period to shrink at a rate
of $-5.94 pm 0.39$ msec/yr, comparable to the observed rate. Thus, the
observed change in the transit period is mostly or entirely produced by the
line-of-sight acceleration of the system. This acceleration is probably caused
by a wide-orbiting companion of mass 10-300$,M_{rm Jup}$ and orbital distance
10-100$,$AU, based on the magnitude of the radial-velocity trend and the
non-detection of any companion in the speckle images. We expect that the
orbital periods of 1 out of 3 hot Jupiters will change at rates similar to
WASP-4b, based on the hot-Jupiter companion statistics of Knutson et al.
(2014). Continued radial velocity monitoring of hot Jupiters is therefore
essential to distinguish the effects of tidal orbital decay or apsidal
precession from line-of-sight acceleration.

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