Hi’iaka’s physical and dynamical properties using long-term photometric data

<p>Hi&#8217;iaka is the largest satellite of the dwarf planet Haumea, with an estimated area-equivalent diameter of 300 km (Fern&#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&#8217;iaka&#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&#8217;iaka system yields important information about the formation processes of the whole Haumea&#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).</p> <p>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&#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.</p> <p>Additionally, we have carried out two specific observational runs of several days in order to obtain the rotational phase of Hi&#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&#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&#8217;iaka&#8217;s rotational light-curve. The amplitude obtained for Hi&#8217;iaka&#8217;s rotational light-curve is 0.015 mag, which agrees with the expected signal induced in Haumea&#8217;s rotational light-curve when accounting for a variable source as that produced by Hi&#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&#8217;iaka&#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&#8217;iaka must be close to 90&#186; in its orbit around Haumea.</p> <p>&#160;</p> <p>Brown et al. (2006), The Astrophysical Journal, Volume 639, Issue 1, pp. L43-L46.</p> <p>Brown et al. (2007), Nature, Volume 446, Issue 7133, pp. 294-296.</p> <p>Fern&#225;ndez-Valenzuela et al. (2021), AAS Division of Planetary Science meeting #53, id. 503.05. Bulletin of the American Astronomical Society, Vol. 53, No. 7 e-id 2021n7i503p05.</p> <p>Hastings et al. (2016), The Astronomical Journal, Volume 152, Issue 6, article id. 195, 12 pp.</p> <p>Ortiz et al. (2017), Nature, Volume 550, Issue 7675, pp. 219-223.</p>