Astronomy & Astrophysics manuscript no. main c ESO 2018 October 22, 2018 High-contrast observations of 136108 Haumea? A crystalline water-ice multiple system C. Dumas1, B. Carry2;3, D. Hestroffer4, and F. Merlin2;5 1 European Southern Observatory. Alonso de Cordova´ 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile e-mail: [email protected] 2 LESIA, Observatoire de Paris, CNRS. 5 place Jules Janssen, 92195 Meudon CEDEX, France e-mail: [email protected] 3 European Space Astronomy Centre, ESA. P.O. Box 78, 28691 Villanueva de la Canada,˜ Madrid, Spain 4 IMCCE, Observatoire de Paris, CNRS. 77, Av. Denfert-Rochereau, 75014 Paris, France e-mail: [email protected] 5 Universit Paris 7 Denis Diderot. 4 rue Elsa Morante, 75013 Paris, France e-mail: [email protected] Received 2010 May 18 / Accepted 2011 January 6 ABSTRACT Context. The trans-neptunian region of the Solar System is populated by a large variety of icy bodies showing great diversity in orbital behavior, size, surface color and composition. One can also note the presence of dynamical families and binary systems. One surprising feature detected in the spectra of some of the largest Trans-Neptunians is the presence of crystalline water-ice. This is the case for the large TNO (136 108) Haumea (2003 EL61). Aims. We seek to constrain the state of the water ice of Haumea and its satellites, and investigate possible energy sources to maintain the water ice in its crystalline form. Methods. Spectro-imaging observations in the near infrared have been performed with the integral field spectrograph SINFONI mounted on UT4 at the ESO Very Large Telescope. The spectra of both Haumea and its larger satellite Hi’iaka are analyzed. Relative astrometry of the components is also performed, providing a check of the orbital solutions and equinox seasons. Results. We describe the physical characteristics of the crystalline water-ice present on the surface of Haumea and its largest satellite Hi’iaka and analyze possible sources of heating to maintain water in crystalline state: tidal dissipation in the system components vs radiogenic source. The surface of Hi’iaka appears to be covered by large grains of water ice, almost entirely in its crystalline form. Under some restricted conditions, both radiogenic heating and tidal forces between Haumea and Hi’iaka could provide the energy sufficient to maintain the ice in its crystalline state. Key words. Kuiper belt objects: individual: Haumea – Methods: observational – Techniques: high angular resolution – Techniques: imaging spectroscopy – Infrared: planetary systems 1. Introduction (2003 EL61), (50 000) Quaoar, (90 482) Orcus, etc [Noll et al. 2008]. Thanks to their binary nature, the total mass of these sys- The planetesimals orbiting beyond Neptune, the transneptunian tems can be inferred, which, when combined to spectroscopy objects (TNOs), are the remnant of the Solar System forma- and radiometric sizes, provides a valuable tool to characterize tion in its outer part. They are thought to be among the most their surface and internal physical properties. pristine objects of our solar system, although their outer sur- Haumea is the largest member of a TNO family, likely face layers have been altered by irradiation and collisions over the outcome of a collision [Brown et al. 2007; Ragozzine & the age of the solar system. Currently the TNOs population ac- Brown 2007; Schaller & Brown 2008; Rabinowitz et al. 2008; counts for ∼1300 known objects, which are difficult to observe Snodgrass et al. 2010]. Here we report spectro-imaging observa- due to their extreme heliocentric distances and relatively small tions of all three components of the Haumea system performed arXiv:1101.2102v1 [astro-ph.EP] 11 Jan 2011 size. Our knowledge of their physical characteristics is for now in 2007 at the ESO Very Large Telescope. Our data and related limited to studying the few largest and brightest objects, which compositional modeling show that the surface of the outer satel- still reveal that this population displays a large fraction of binary lite Hi’iaka is mostly coated with crystalline water-ice, as in the and multiple systems when compared to other small solar sys- case of the central body Haumea [Trujillo et al. 2007; Merlin tem bodies such as main belt asteroids [e.g. Noll et al. 2008]. et al. 2007; Pinilla-Alonso et al. 2009]. We also discuss the ef- Trans-neptunian binaries can be found as gravitationally bound fects of tidal torques as a possible source of energy responsible systems with similar mass components, but systems harboring for the crystalline state of the water-ice of Hi’iaka. smaller moons, which are by definition harder to detect, have also been discovered around (134 340) Pluto, (136 108) Haumea 2. Observations and data-reduction ? Based on observations collected at the European Southern Haumea was observed in H and K bands on March 15 UT 2007, Observatory, Paranal, Chile - 60.A-9235 using the laser guide star facility (LGSF) and the SINFONI 2 C. Dumas et al.: High-contrast observations of 136108 Haumea instrument (Spectrograph for INtegral Field Observations in the Near Infrared), both installed at the 8m “Yepun” unit of the ESO Very Large Telescope. The use of SINFONI for the observations of the large TNOs Haumea and Eris has been described in earlier papers [Merlin et al. 2007; Dumas et al. 2007] and more information about this instrument can be found in Eisenhauer et al.[2003] and Bonnet et al.[2004]. In a nutshell, SINFONI is an integral field spectrometer working in the [1.0-2.5] µm range, also equipped with an adaptive optics (AO) system with Natural Guide Star (NGS) and Laser Guide Star (LGS) channels. While our previous published observations were obtained in non-AO mode (seeing limited), the results presented in this paper make use of the AO system and the LGS facility. The laser produces Fig. 1. Comparison of H+K band SINFONI images of Haumea ∼ an artificial visible-light star of Rmag 13.4 in the line of sight of obtained under similar conditions but in seeing limited obser- ∼ Haumea (Vmag 17.4), returning thus a gain of 4 magnitudes for vations (A, left) and LGS-AO corrected mode (B, right). The characterizing the higher orders of the wavefront in comparison spatial and intensity scales are similar and the intensity is given to non-laser observations. Haumea itself was used as a reference in ADU. The improved contrast and spatial resolution of the AO source for the tip-tilt, delivering optimal correction by the image (B) is readily apparent in comparison to the no-AO im- AO-LGS system. The atmospheric conditions were extremely age (A), making possible the detection of the two faint satellites: good during the observations, with an uncorrected seeing 00 Namaka (faintest, just below Haumea) and Hi’iaka (brightest, varying between 0.5 and 0.6 . On 3/15/2007, between 6h34 bottom of image). The images were obtained by summing all UT and 7h24 UT, six exposures of 300s each were obtained the slices of our data-cube to produce the equivalent of a broad on Haumea (total integration time of 1h), inter-spaced by 3 H+K band image. exposures of 300s to record the sky background. We used the H+K spectral grating (spectral resolution of ∼ 1500) covering both H and K bands simultaneously, and a plate scale of 100 Absorption band Band depth (Primary) Band depth (Satellite) 00 00 mas/spaxel (3 × 3 total field). Calibrations to correct our 1.50 µm 0.36 ± 0.05 0.53 ±0.25 spectra from the solar response and telluric absorption features 1.65 µm 0.24 ±0.04 0.54 ±0.25 were obtained immediately after Haumea by observing the local 2.00 µm 0.55 ± 0.05 0.72 ± 0.35 telluric standard HD 142093 (Vmag ∼ 7:3, G2V) in NGS mode Table 1. Depth of the water ice absorption bands in the spectra at similar airmass and with the same instrumental setting. of Haumea and its outermost satellite Hi’iaka. The data (science target and telluric standard) were mainly reduced using the ESO pipeline 1.9.3 [Modigliani et al. 2006] We first corrected all raw frames from the noise pattern of dark and bright horizontal lines introduced when reading the the solar analog had the effect of introducing a small artifact in detector. We then used the ESO pipeline to produce all master the spectrum of Haumea in the [1.65-1.8] µm range. This par- files needed by the data reduction such as the badpixel masks, ticular feature was due to the contamination of our spectra by a master darks and flats, as well as the wavelength and distortion faint background object within the close vicinity of the standard calibration files, which respectively associate a wavelength star. We characterized the impact of the contaminant by divid- value to each pixels, and reconstruct the final image cubes. Each ing our spectrum of HD 142093 with the spectrum of good solar object frame was subtracted from the sky frame recorded closest analog, HD 11532 (Vmag ∼ 9:7, G5) used by our ESO Large in time and the quality of the sky subtraction was improved Program (Prog.ID 179.C-0171, PI: Barucci) and obtained with a by enabling the correction of sky residuals in the pipeline, i.e. similar setup and airmass (∆airmass ∼ 0:03). We then applied cor- by subtracting the median value of each image slice in the rection to our final spectra of Haumea and Hi’iaka by dividing reconstructed, sky-corrected, spectro-image cube. both of them by the relative response of the two telluric standard Fig.1 shows two H +K -band images of Haumea obtained in stars over the [1.65-1.8] µm range.