The fields targeted by our proof-of-concept mini-survey were observed at relatively high airmasses (see Section 2). Therefore, it can be argued that our null result could be due to enhanced extinction caused by observing at such low elevations. However, this is likely not the case because the same EURONEAR collaboration carried out a nearly concurrent mini-survey looking for Atira and Vatira asteroids (see e.g. Greenstreet, Ngo & Gladman 2012) using the same instrumental setup and similar automated reduction pipeline. Atiras or Interior Earth Objects (IEOs) have aphelion distances 0.983 au and can only be observed at low solar elongations (often 70○); Vatiras have aphelia 0.718 au, are observed at very low solar elongations (typically 40○–45○), and just one such object is currently known (Bolin et al. 2020; de la Fuente Marcos & de la Fuente Marcos 2020; Greenstreet 2020), 594913 ‘Ayloi’chaxnim (2020 AV2). Although the nearly concurrent EURONEAR small-scale survey (12 nights with an average time of 1.5 h per night devoted to searching for Vatira and Atira asteroids towards the evening and early morning skies) did not find any new Vatiras or IEOs, it did observe at elevations in the range 15○–30○ and recovered known objects at apparent magnitudes close to or above 23 – for example 2017 HO20, a main belt asteroid that was observed near conjunction. Furthermore, Fig. 2, top panel, shows the r′-magnitude distribution of the detected stars from one representative pointing out of the 77 (×5) pointings intended to recover the TESS candidate in Cepheus. The histogram was produced using the matplotlib library (Hunter 2007) with sets of bins computed using numpy (van der Walt, Colbert & Varoquaux 2011; Harris et al. 2020) by applying the Freedman and Diaconis rule (Freedman & Diaconis 1981). Fig. 2, bottom panel, shows the expected S/N for WHT prime-focus imaging as a function of the exposure time from the exposure time calculator (ETC)11 for a similar camera (Red + 4, PFIP) for observations carried out under a seeing of 0${_{.}^{\prime\prime}}$9 at an airmass of 2.3 in dark time and R = 23.2 mag. TNOs have colour index V − R in the range 0.2–1.2 (see e.g. Peixinho, Delsanti & Doressoundiram 2015). Consistently, we are confident that we reached a limiting magnitude r′= 23.0 mag, which actually corresponds to $V\, \gt $23.0 mag (in fact, it could be 23.2–24.2 mag). Therefore, our distant TNO candidate survey could have detected candidates 9 and 11 in table 2 of Rice & Laughlin (2020) unless both candidates are much fainter than predicted or they are false positives in TESS data. In any case, Rice & Laughlin (2020) stated that many if not most of the high signal significances reported in their table 2 could be the result of unmodelled systematic errors. On the other hand, in the fields observed towards Cepheus, there are a few bright stars that might have hidden a putative moving object with the properties of candidate 9; we estimate that the probability of having missed the candidate as a result of it moving projected towards one of those bright stars is 1 per cent (considering the area affected by the bright stars and associated diffraction spikes).
Citation: de la Fuente Marcos & de la Fuente Marcos 2020
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