WASP-South detection of HD219666b transits provides an accurate ephemeris
DDraft version October 17, 2019
Typeset using L A TEX
RNAAS style in AASTeX62
WASP-South detection of HD 219666b transits provides an accurate ephemeris
C. Hellier, D. R. Anderson,
1, 2
S. Gill, and R. G. West Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
Keywords:
ExoplanetsThe hot-Neptune HD 219666b (= TOI-118.01; Esposito et al. 2019) was an early discovery from Sector 1 of the
TESS transit survey (Ricker et al. 2016). Being a rare “Neptune desert” planet, and transiting a bright, V = 9.9 star,it is a prime target for further study including atmospheric characterisation. Targeting exoplanet transits with majorfacilities such as HST and the imminent
JWST depends on an accurate ephemeris. The transit period presented byEsposito et al (2019) derives primarily from 4 transits spanning 18 days and has an uncertainty that will amount toseveral hours by the time that
TESS re-observes this region of sky, likely to be in 2021.Here we report a recovery of the transit in WASP-South data, enabling an ephemeris with a period uncertainty thatis a factor of 60 smaller. The 8-camera, WASP-South transit survey (Pollacco et al. 2006) has produced 150 transitingexoplanets (e.g. Hellier et al. 2019a). The transit depth of HD 219666b, at only 0.17%, is shallower than that of anyWASP planet (for example the bright-star, super-Neptune WASP-166b has a transit depth of 0.28%; Hellier et al.2019b). Nevertheless, at V = 9.9, and being relatively isolated on the sky, HD 219666 was in the “sweet spot” forWASP-South, both when using 200-mm lenses (observing HD 219666 from 2010–2012), and when using 85-mm lenses(observing 2012–2014). A total of 148 000 photometric data points were obtained on this star.The standard WASP transit-search code (Collier Cameron et al. 2007) successfully finds the transit, from coverageof 24 partial or full transits over the period 2010 July to 2014 Oct. The code reports the dip as 0.2% deep and 2-hrwide, values consistent with those from TESS (0.17% deep and 2.5-hr wide). The resulting transit ephemeris is:WASP (JD TDB) = (245 5788 . ± . N × (6 . ± . . ± . N × (6 . ± . . The
TESS /WASP period ratio is 1.00027 and thus the chances of a period match to this accuracy being spurious are1-in-2000, though bear in mind that we have also been looking for period matches with several hundred other
TESS candidates where a WASP match is feasible.At the epoch of the
TESS observation the WASP ephemeris predicts the transit to an accuracy of 0.03 day, or 0.005in phase. Thus the chances of a spurious match within this window are 1-in-100. The
TESS epoch actually locatesat 420.99999 cycles on the WASP ephemeris. Thus the combination of the period match and the phase match giveconfidence that the WASP detection is real, with a likelihood of being spurious below 1%.Taking the difference between the epochs as exactly 421 cycles then leads to the more accurate ephemeris:Combined (JD TDB) = (245 8329 . ± . N × (6 . ± . . The combined ephemeris has a drift of only one minute per year, and should prove useful to those schedulingobservations of HD 219666b. The early WASP timing can also contribute to long-term monitoring of any periodchanges. The revised period is 2.5
TESS error bars from the
TESS period, where we note that the last of the 4
TESS transits was at a time of excess noise in the
TESS data, which may have affected that timing.
Corresponding author: C. [email protected] a r X i v : . [ a s t r o - ph . E P ] O c t Figure 1.
Portions of the 200-mm WASP-South data on HD 219666. The orange lines mark transit times as found by thesearch algorithm. There are likely detections of the shallow, 0.17%-deep transit at midpoint times: 245 5396.45, 245 5420.59& 245 5426.62, and these will be the earliest transits of HD 219666b recorded. While the individual transit events are notnecessarily convincing to the eye, the match in both period and phase of the search-algorithm output gives confidence that thedetection is real.