An examination of the effect of the TESS extended mission on southern hemisphere monotransits

Abstract

Context. NASA recently announced an extended mission for TESS. As a result it is expected that the southern ecliptic hemisphere will be re-observed approximately two years after the initial survey. Aims. We aim to explore how TESS re-observing the southern ecliptic hemisphere will impact the number and distribution of monotransits discovered during the first year of observations. This simulation will be able to be scaled to any future TESS re-observations. Methods. We carry out an updated simulation of TESS detections in the southern ecliptic hemisphere. This simulation includes realistic Sector window-functions based on the first 11 sectors of SPOC 2 min SAP lightcurves. We then extend this simulation to cover the expected Year 4 of the mission when TESS will re-observe the southern ecliptic fields. For recovered monotransits we also look at the possibility of predicting the period based on the coverage in the TESS data. Results. We find an updated prediction of 339 monotransits from the TESS Year 1 southern ecliptic hemisphere, and that approximately 80% of these systems (266/339) will transit again in the Year 4 observations. The Year 4 observations will also contribute new monotransits not seen in Year 1, resulting in a total of 149 monotransits from the combined Year 1 and Year 4 data sets. We find that 75% (189/266) of recovered Year 1 monotransits will only transit once in the Year 4 data set. For these systems we will be able to constrain possible periods, but period aliasing due to the large time gap between Year 1 and Year 4 observations means that the true period will remain unknown without further spectroscopic or photometric follow-up.

CHEOPS observations of TESS primary mission monotransits

“…From this plot we see that the TESS monotransits period distribution is similar to those seen in Cooke et al (2018) and Cooke et al (2019). The distribution drops off with period due to the underlying occurrence rates used in this simulation, however we still find over 200 detections with P ≥ 25 days and ∼ 30 detections at P ≥ 50 days.…”

“…Continuing our simulated TESS observations into the extended mission using the same procedure as Cooke et al (2019) we find that each system will have an average of 35 aliases when accounting for the TESS coverage and the separation between transits (for full details of this period alias simulation see Cooke et al 2020, in prep.). 35 aliases is still too many for CHEOPS to reasonably target since most of these will be false and thus reveal no transit.…”

“…This produces a more realistic observing strategy than was used in 1 http://archive.stsci.edu/tess/tic_ctl.html Cooke et al (2018). The improvement between the method employed here and the Cooke et al (2019) implementation of this method is that data for more TESS sectors is available. We now have timestamps for sectors 1-15.…”

“…The Transiting Exoplanet Survey Satellite (TESS, Ricker et al 2015) is over a year into its primary mission having already moved into the northern ecliptic hemisphere. As part of its exoplanet yield TESS is expected to discover hundreds of monotransiting systems (Cooke et al 2018;Villanueva et al 2019;Cooke et al 2019). These systems will be known to host transiting exoplanets so will be valid targets for CHEOPS but will have no known period.…”

A long-period (P = 61.8 d) M5V dwarf eclipsing a Sun-like star from TESS and NGTS

“…The TOI catalogue is heavily biased towards short period systems that exhibit many transits within their remit of TESS data. However, TESS data provides an excellent hunting ground for single transit systems (Cooke et al 2018;Villanueva et al 2019;Cooke et al 2019). TESS single transit systems have, by necessity, periods of greater than ∼ 15 days.…”