Hi&#8217;iaka&#8217;s physical and dynamical properties using long-term photometric data
Authors: Fernández-Valenzuela Estela , Ortiz José Luis , Morales Nicolás , Jehin Emmanuel , Burdanov Artem , de Wit Julien , Ferrais Marin , Vara-Lubiano Mónica , Morales Rafael , Kretlow Mike , Santos-Sanz Pablo , Alvarez-Candal Alvaro , Duffard René , Pál András , Kiss Csaba , Szakáts Róbert 
Published: September 2022

&lt;p&gt;Hi&amp;#8217;iaka is the largest satellite of the dwarf planet Haumea, with an estimated area-equivalent diameter of 300 km (Fern&amp;#225;ndez-Valenzuela et al., 2021). It is the best studied satellite in the trans-Neptunian region. Its rotational light-curve was observed with Hubble, for which an approximate rotation period of 9.8 h was obtained (Hastings et al. 2016). The system is very peculiar because it stands out from all other TNO-binary systems. While all other known satellites are thought to be synchronous, Hi&amp;#8217;iaka&amp;#8217;s rotation period is fast compared to the 49 days that takes to complete an orbit around Haumea. Therefore, the study of Haumea-Hi&amp;#8217;iaka system yields important information about the formation processes of the whole Haumea&amp;#8217;s system, which includes another moon (Brown et al. 2006), a ring (Ortiz et al. 2017) and a family of objects (Brown et al. 2007).&lt;/p&gt;
&lt;p&gt;Our group has been observing Haumea since its discovery, compiling a large database of images since around 20 years ago. Using this set of images we have obtained high accuracy astrometric measurements of the photocenter of the Haumea-Hi&amp;#8217;iaka system. We have applied a similar procedure as in Ortiz et al. (2017) to disentangle the position of Haumea from the contribution of Hi'iaka, but for a much larger time span as mentioned above. Therefore, we have been able to determine more accurate orbits for Haumea and Hi'iaka.&lt;/p&gt;
&lt;p&gt;Additionally, we have carried out two specific observational runs of several days in order to obtain the rotational phase of Hi&amp;#8217;iaka at that moment of the two stellar occultations that occurred last year (in April 2021). We used the 1.23-m telescope at Calar Alto Observatory, the Artemis telescope at Teide Observatory and the 1.5-m telescope at Sierra Nevada Observatory to acquire images of the unresolved system. The resulting photometry of these images give rise two rotational light-curves of Haumea in 2021 and 2022. We fitted a fourth-order Fourier function, which represents Haumea&amp;#8217;s body-shape contribution to the rotational light-curves. From this fit, we took the residuals of the observational data and searched for periodicities within them. We obtained a rotation period in agreement with the estimations in Hastings et al. (2016), but much more accurate. These residuals, when folded to the resulting period, provide Hi&amp;#8217;iaka&amp;#8217;s rotational light-curve. The amplitude obtained for Hi&amp;#8217;iaka&amp;#8217;s rotational light-curve is 0.015 mag, which agrees with the expected signal induced in Haumea&amp;#8217;s rotational light-curve when accounting for a variable source as that produced by Hi&amp;#8217;iaka, i.e., considering the rotational light-curve obtained in Hastings et al. (2016). We have not detected a change in the amplitude of Hi&amp;#8217;iaka&amp;#8217;s rotational light-curve when comparing our data, taken in 2021 and 2022, with those from Hastings et al. (2016), taken in 2010. This means that the obliquity of Hi&amp;#8217;iaka must be close to 90&amp;#186; in its orbit around Haumea.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
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