“Secular light curves of 25 members of the themis family of asteroids, suspected of low level cometary activity”

Abstract

From 1996 to 2015 sixteen main belt asteroids were discovered exhibiting cometary activity (less than one per year), all of them during searches at the telescope. In this work we will explore another way to discover them. We reduced 192016 magnitude observations of 165 asteroids of the Themis family, using data from the astrometric-photometric database of the Minor Planet Center, MPCOBS, and measuring the absolute magnitudes from the phase plots. 25 objects of 165 (15.2%), exhibited bumps or enhancements in brightness that might indicate low level cometary activity. Since activity repeats at the same place in different orbits and in many occasions is centered at perihelion, activity might be due to water ice sublimation. As of September 2016, there are 717768 asteroids listed in the Minor Planet files. If we assume that we do not have any false positives and the above percentage can be extrapolated to the whole Main Belt, the number of potentially active asteroid gets to the very large number of ~111.000. This number is much larger than the ones predicted in previous surveys and indicates one of three scenarios: A) there are many false positives in our detections and the real number of active asteroid is much smaller than we found. This would be tantamount to saying that the MPC astrometric-photometric database is only astrometric and not photometric, invalidating absolute magnitude determinations based on this database. B) The location of active asteroids is restricted to the Themis family and an extrapolation to the whole belt is not possible. Or C) there are few false positives in our candidates and the main belt actually contains many low level active asteroids undetected by current surveys. Case C) would imply that the main belt is not a field of bare rocks but a graveyard of extinct comets, changing our current paradigm of the main belt. So it is of the outmost importance to verify observationally our candidates, and determine which of these scenarios is valid.

Secular light curves of NEAs 2201 Oljato, 3200 Phaethon, 99942 Apophis, 162173 Ryugu, 495848 and 6063 Jason

“…The definition is valid if (a) the object is spherical; (b) the polar axis is perpendicular to the ecliptic; (c) the aspect angle (polar axis vs perpendicular to the orbit) is small. Otherwise the aspect angle produces a wavy line on the SLC (Figure 3), with two maxima and two minima that is easily recognized in our plots (Ferrín et al, 2017). None of the objects discussed in this work exhibit such a signature, thus concluding that the aspect angle does not influence the results arrived at in this manuscript.…”

“…There are many physical effects that affect comet observations such as twilight, moon light, haze, cirrus clouds, dirty optics, old mirror coating, lack of dark adaptation, excess magnification, poor collimation, and in the case of CCDs, sky background too bright, insufficient exposure time, insufficient CCD measurement aperture, and in one case we know, the photometry tool has a maximum aperture smaller than the coma's diameter. All these factors diminish the captured photons coming from the comet, and the observer makes an Asteroids with large obliquity (aspect angle) show a well-defined signature in their SLCs, consisting of a wavy line with two maxima and two minima (Ferrín et al, 2017). We show two cases where this signature is very strong.…”

“…Figure3. Asteroids with large obliquity (aspect angle) show a well-defined signature in their SLCs, consisting of a wavy line with two maxima and two minima(Ferrín et al, 2017). We show two cases where this signature is very strong.…”

Variation of mineralogy and organic material during the early stages of aqueous activity recorded in Antarctic micrometeorites

“…Because aqueous alteration in comet nuclei is problematic (e. g. Wooden, 2008 and references therein), icy asteroids (P-and D-type asteroids) may be more plausible parent bodies of the Fluffy Fg MMs as well as some CP MMs like D10IB004. Because Themis family (C-type asteroids) is water ice-bearing (Campins et al, 2010;Rivkin et al, 2010) and because about 15 % of them may have a low level cometary activity (Ferrín et al 2017), even C-type asteroids may be also one of the plausible parent bodies of the MMs. These asteroids should have microporous structures that is different from parent bodies of hydrated carbonaceous chondrites except for CI and Tagish Lake, which may have compact structure, as suggested low porosities of the most carbonaceous chondrites (15-20%, Macke et al, 2011).…”

Thermophysical Modeling of 20 Themis Family Asteroids with WISE/NEOWISE Observations

“…We find that both thermal inertia and geometric albedo of 133P/Elst-Pizarro are within the range of Themistians we investigated. Using the data from MPC, Ferrín et al (2017) reduced 192016 magnitude observations of 165 Themis family asteroids, among which 25 (15.2%) of them exhibit bumps or enhancements in brightness that might suggest lowlevel cometary activity. Besides, the activity of aster- oid might be triggered by water-ice sublimation, but only a small portion of the Themistians are discovered to have cometary activities.…”