Spectral characterization of near-Earth asteroids on cometary orbits

A fraction of near-Earth asteroids has the orbital elements similar to those of comets, but a visual aspect as any other point-like source. These are potentially dormant comets nuclei who entered in a period of inactivity. Their study can provide a new understanding of the final state in which volatile-rich objects reside and of the existing organic material or water content distribution from the early Solar System.

Dynamically, cometary orbits can be filtered by their Tisserand parameter with respect to Jupiter (T_Jup). With few exceptions, comets have T_Jup < 3 while asteroids displays T_Jup > 3. Although the value of T_Jup can indicate whether or not the asteroid crosses the Jupiter's orbit, this is not enough to outline a cometary orbit. Tancredi (2014) had developed a method to classify asteroids on cometary orbits (ACOs), based only on orbital elements, which doesn't require any numerical time integration. Beside Tisserand criterion, this algorithm rejects all samples in mean-resonant motion, with large orbital uncertainties and with large minimum orbital intersection distances (MOID) among giant planets.

We seek to make a statistical analysis of the potentially dormant (extinct) comets from near-Earth objects population (NEACOs), using the spectral observations over the visible and near-infrared wavelength interval. The aim of this work is to constraint the fraction of dormant comets orbiting in the near-Earth space. For this study, we’ve compiled a catalog with 149 spectra of near-Earth asteroids (NEAs) with T_Jup < 3. This sample represents 10% out of all known asteroids which obey the T_Jup criterium (Fig. 1).

Fig 1. Absolute magnitude cumulative distributions of all NEACOs from Tancredi's list of ACOs with respect to all with known taxonomy, to those with T_Jup < 3.1 and to all known NEAs (as of January 30, 2020).

The data include new observations of 26 NEAs and 123 spectra retrieved from the literature. The new measurements were obtained with the 2.5 m Issac Newton Telescope and the Nordic Optical Telescope for the visible region, and with the 3.0 m NASA Infrared Facility Telescope for the NIR interval.

For a simplified analysis we’ve grouped all classes from Bus-DeMeo system into four compositional groups. The silicate-like spectra group is mainly consists of objects from Q / S complex (S-, Sr-, Sq-, Sv- types) and some end-members like O-, R- and A-types. In the carbonaceus-like spectra group we've gathered together C-, X- complexes and B-, D-, T- types. The last two groups consist of basaltic asteroids, corresponding to V-type and of relatively rare spectra (miscellaneous) like K- and L-type.

Figure 2. Taxonomic distribution of NEAs with T_Jup < 3.1

The dominant group is of bodies with carbonaceus-like composition, representing 47.5% (71 / 149): 26 C-complex, 22 X-complex, 17 D-type, 6 B-type. The silicate group represents 47% (70 / 149), with an effective of 66 Q/S-complex, 3 R-type and one pure olivine A-type. We report 3 extreme cases with silicate composition: 2 R-types (466130) 2012 FZ23, (394130) 2006 HY51 and one Sr-type (465402) 2008 HW1. Their T_Jup of 2.3, 2.39 and 2.4 respectively are too small for this compositional group. For (466130) we obtained a NIR spectrum. Also, (394130) have a low recorded albedo (0.071), reaching 980 K to perihelion. Taxonomic distribution of entire catalogue (149 samples), presented in Figure 2, displays a strong variation of compositional ratio between silicate and comet-like objects relative to T_Jup.

Within our sample, we could gather data only for 7 asteroids which obey the criteria of Tancredi (2014). All of them are in the Jupiter Familly Comets orbital class. Their spectra classifies them in the carboanceus-like group: 4 D-type (3552, 85490, 248590, 2001 UU92), 1 C-type (475665), 1 T-type (485652) and 1 Xc-type (506437). We conclude that these 7 bodies are dormant / extinct comets. It is important to note that no object with other taxonomy than carbonaceous chondrite, had passed this enhanced criterion, in agreement with the results of Licandro et al. 2018 (who included part of these objects).

References

[1] Tancredi G., 2014, Icarus, 234, 66
[2] Licandro J., et al. 2018, A&A, 618, A170

 

Acknowledgements

Part of the data utilized in this publication were obtained and made available by the MITHNEOS MIT-Hawaii Near-Earth Object Spectroscopic Survey. 
This work was developed in the framework of EURONEAR collaboration and of ESA P3NEOI projects. The work of M.P. was supported by a grant of the Romanian National Authority for Scientific Research - UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0371. M.P., J.dL. and J.L. acknowledge support from the AYA2015-67772-R (MINECO, Spain), and from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870403 (project NEOROCKS).