Estudos fotométricos de objetos transNetunianos

Abstract

This work reports the results of photometric studies of transNeptunian objetcs (TNOs). Absolute magnitudes and phase coefficient from TNOs were computed (H V and β V for 124 TNOs and H R and β R for 123 TNOs). We computed what we defined as absolute colours H V − H R and relative phase coefficients ∆β = β V − β R for 117 TNOs. Observational data was obtained from several telescopes in different countries: Chile, Brasil, Spain and United kingdom. We used our own data togheter with data from literature to compute phase curves. For this, we assumed a linear trend of reduced magnitudes and the phase angles. We find that the objects have a wide range of H V − H R ranging from blue to very red, while the relative phase coefficients have a unimodal distribution. We have observed that the phase curves in the same phase angle range show different behaviors in different filters R and V , some of them have negative values for β λ . We used the Spearman parameter to test correlations between H V − H R and ∆β with physical and orbital parameters, in order to test correlations that are debated in literature. We did not find any correlation with orbital paremeters, by considering the complete sample nor considering different dynamical groups as suggested by other authors (Tegler & Romanishin, 2000; Peixinho et al., 2012). We find that there is an anti-correlation between H V − H R and ∆β, which indicates that the reddest objects have more sloped phase curves in the R filter, while bluer objects have more sloped phase curves in the V filter. The correlation holds by considering all the objects of the sample as well as, considering different dynamical groups also any other type of group, i.e. groups divided by H or diameter D. Therefore, we suggest that the anti-correlation depends on a property of the whole population, which does not depend on the object location, their surface temperature, composition (and/or albedo) or size. We suggest that the anti-correlation may be due to the microscopic properties of the surfaces. We find a correlation of H V − H R and the geometric albedo p V when we separate the sample by groups according to H (as an approximation to the D). For small TNOs with H V > 4.5 mag the correlation has r S = -0.7912 and p-value = 3.34 × 10 −19 , while for the larger TNOs H V < 4.5, r S = -0.8986 and p-value = 3.15× 10−8 , we consider that this ratio is due to composition differences, possibly a smaller amount of volatile compounds in the minor ones. In order to test our hypothesis we used the Lumme & Bowell (1981) model which states that the phase curves can be characterized by one single parameter, the multiple scattering factor Q. Since it depends of two components that describe roughness and porosity, it was useful to corroborate if there was evidence of different shadowing effect between the V and R data. We found that the model does not reproduce the observed phase curves. The phenomenon known as opposition effect is an increase of the aparent magnitude of the Solar System objects observed at small phase angles (Gehrels, 1956; Harris & Young, 1989). It is produced by the decrease of the shadowing effect (Hapke et al., 1998) and by the coherent backscattering of light phenomenon (Muinonen, 1989). Considering that TNOs are all observed at small phase angles < 8 ◦ , it is expected that coherent backscatter will be the dominant source that contributes to the opposing effect. Thus, our results indicate that the TNOs show opposite behavior to what was expected for multiple scattering, and therefore we suggest that in the TNOs the dominant source of OE is the single scattering.