A&A 604, A95 (2017) Astronomy DOI: 10.1051/0004-6361/201630354 & c ESO 2017 Astrophysics “TNOs are Cool”: A survey of the trans-Neptunian region XII. Thermal light curves of Haumea, 2003 VS2 and 2003 AZ84 with Herschel/PACS? P. Santos-Sanz1, E. Lellouch2, O. Groussin3, P. Lacerda4, T. G. Müller5, J. L. Ortiz1, C. Kiss6, E. Vilenius5; 7, J. Stansberry8, R. Duffard1, S. Fornasier2; 9, L. Jorda3, and A. Thirouin10 1 Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008-Granada, Spain e-mail: [email protected] 2 LESIA-Observatoire de Paris, CNRS, UPMC Univ. Paris 6, Univ. Paris-Diderot, France 3 Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France 4 Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK 5 Max-Planck-Institut für extraterrestrische Physik (MPE), Garching, Germany 6 Konkoly Observatory of the Hungarian Academy of Sciences, Budapest, Hungary 7 Max-Planck-Institut für Sonnensystemforschung (MPS), Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany 8 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 9 Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris, France 10 Lowell Observatory, 1400 W Mars Hill Rd, Flagstaff, Arizona 86001, USA Received 24 December 2016 / Accepted 19 April 2017 ABSTRACT Context. Time series observations of the dwarf planet Haumea and the Plutinos 2003 VS2 and 2003 AZ84 with Herschel/PACS are presented in this work. Thermal emission of these trans-Neptunian objects (TNOs) were acquired as part of the “TNOs are Cool” Herschel Space Observatory key programme. Aims. We search for the thermal light curves at 100 and 160 µm of Haumea and 2003 AZ84, and at 70 and 160 µm for 2003 VS2 by means of photometric analysis of the PACS data. The goal of this work is to use these thermal light curves to obtain physical and thermophysical properties of these icy Solar System bodies. Methods. When a thermal light curve is detected, it is possible to derive or constrain the object thermal inertia, phase integral and/or surface roughness with thermophysical modeling. Results. Haumea’s thermal light curve is clearly detected at 100 and 160 µm. The effect of the reported dark spot is apparent at 100 µm. Different thermophysical models were applied to these light curves, varying the thermophysical properties of the surface within and outside the spot. Although no model gives a perfect fit to the thermal observations, results imply an extremely low thermal inertia (<0.5 J m−2 s−1=2 K−1, hereafter MKS) and a high phase integral (>0:73) for Haumea’s surface. We note that the dark spot region appears to be only weakly different from the rest of the object, with modest changes in thermal inertia and/or phase integral. The thermal light curve of 2003 VS2 is not firmly detected at 70 µm and at 160 µm but a thermal inertia of (2 ± 0:5) MKS can be derived from these data. The thermal light curve of 2003 AZ84 is not firmly detected at 100 µm. We apply a thermophysical model to the mean thermal fluxes and to all the Herschel/PACS and Spitzer/MIPS thermal data of 2003 AZ84, obtaining a close to pole-on orientation as the most likely for this TNO. Conclusions. For the three TNOs, the thermal inertias derived from light curve analyses or from the thermophysical analysis of the mean thermal fluxes confirm the generally small or very small surface thermal inertias of the TNO population, which is consistent with a statistical mean value Γmean = 2:5 ± 0:5 MKS. Key words. Kuiper belt objects: individual: Haumea – Kuiper belt objects: individual: 2003 VS2 – Kuiper belt objects: individual: 2003 AZ84 – submillimeter: planetary systems – techniques: photometric – infrared: planetary systems 1. Introduction (TNOs) and Centaurs can be shape-driven (i.e. Jacobi-shaped rotating body, such as Varuna, Jewitt & Sheppard 2002) or be The study of the visible photometric variation of solar system causes by albedo contrasts on the surface of a Maclaurin-shaped minor bodies enables us to determine optical light curves (flux rotating spheroid (e.g. the Pluto case). Combinations of shape or magnitude versus time), for which essential parameters are the and albedo effects are also possible and very likely occur within peak-to-peak amplitude and the rotational period of the object. TNOs (e.g. Makemake) and Centaurs. Contact-binaries can also Short-term photometric variability of trans-Neptunian objects produce short-term photometric variability within the TNO and Centaur populations. The largest amplitudes are usually associ- ? Herschel is an ESA space observatory with science instruments pro- ated with Jacobi shapes and the smallest ones with Maclaurin vided by European–led Principal Investigator consortia and with impor- shapes with highly variegated surfaces. Large amplitudes can tant participation from NASA. PACS: The Photodetector Array Camera also be associated with contact-binaries and small amplitudes and Spectrometer is one of Herschel’s instruments. Article published by EDP Sciences A95, page 1 of 19 A&A 604, A95 (2017) with objects with rotational axes close to pole-on. If we know techniques applied are detailed in Sect.3. Data are analyzed, the rotational properties (i.e. rotation period and amplitude) of modeled, and interpreted in Sect.4. Finally, the major conclu- a Jacobi shaped object, it is possible to derive the axes ratio of sions of this work are summarized and discussed in Sect.5. the ellipsoid (i.e. a shape model) and also a lower limit for the density (Jewitt & Sheppard 2002; Sheppard et al. 2008, and ref- 2. Observations erences therein), assuming the object is in hydrostatic equilib- rium (Chandrasekhar 1987) with a certain aspect angle. On the The observations presented here are part of the project other hand, if we suspect that the object has a Maclaurin shape “TNOs are Cool”: a survey of the trans-Neptunian region, we can derive a shape model from the rotational period, but it a Herschel Space Observatory open time key programme is needed to assume a realistic density in this case. The majority (Müller et al. 2009). This programme used ∼372 h of Her- of the TNOs/Centaurs (∼70%) present shallow light curves (am- schel time (plus ∼30 h within the SDP) to observe 130 plitudes less than 0.15 mag), which indicates that most of them TNOs/Centaurs, plus two giant planet satellites (Phoebe and are Maclaurin-shaped bodies (Duffard et al. 2009; Thirouin et al. Sycorax), with Herschel/PACS; 11 of these objects were also 2010). For a couple of special cases (only for Centaurs until now) observed with Herschel/SPIRE at 250, 350, and 500 µm (see where more information is available (i.e. long-term changes in Fornasier et al. 2013), with the main goal of obtaining sizes, light curve amplitudes), the position of the rotational axis can be albedos, and thermophysical properties of a large set of ob- derived or at least constrained (Tegler et al. 2005; Duffard et al. jects that are representative of the different dynamical popu- 2014a; Ortiz et al. 2015; Fernandez-Valenzuela et al. 2017). lations within the TNOs. For PACS measurements, we used Complementary to optical light curves, thermal light curves a range of observation durations from about 40 to 230 min are a powerful tool to obtain additional information about physi- based on flux estimates for each object. Results of the PACS cal and, in particular, thermal properties of these bodies. At first and SPIRE measurements to date have been published in order, immediate comparison of the thermal and optical light Müller et al.(2010), Lellouch et al.(2010), Lim et al.(2010), curves enables us to differentiate between shape-driven light Santos-Sanz et al.(2012), Mommert et al.(2012), Vilenius et al. curves (the thermal light curve is then correlated with the op- (2012), Pál et al.(2012), Fornasier et al.(2013), Lellouch et al. tical one) and light curves that are the result of albedo markings (2013), Vilenius et al.(2014), Du ffard et al.(2014b) 1. In addi- (the two light curves are anti-correlated). Furthermore, quantita- tion, four bright objects (Haumea, 2003 VS2, 2003 AZ84 and tive modeling of the thermal light curve enables us to constrain Varuna) were re-observed long enough to search for thermal the surface energetic and thermal properties, namely its bolomet- emission variability related to rotation (i.e. thermal light curve). ric albedo, thermal inertia, and surface roughness (e.g. Pluto, see This paper presents results for the first three. Other objects Lellouch et al. 2016, and references therein). In a more intuitive (Pluto, Eris, Quaoar, Chiron) were also observed outside of the way, in the case of positively correlated thermal and optical light “TNOs are Cool” programme to search for their thermal light curves, it is also possible to constrain the thermal inertia using curve. the delay between the thermal and optical light curves and their The general strategy we used to detect a thermal light curve relative amplitudes. with Herschel/PACS was to perform a long observation covering Until recently, only a few thermal light curves of outer most of the expected light curve duration, followed by a shorter Solar System minor bodies (or dwarf planets) have been ob- follow-on observation, which enabled us to clean the images’ tained. Pluto’s thermal light curve was detected with ISO/PHOT, backgrounds, as explained in Sect. 3.1. Spitzer/MIPS, and /IRC at a variety of wavelengths long- Dwarf planet (136108) Haumea was observed twice with wards of 20 µm (Lellouch et al.