A&A 539, A152 (2012) Astronomy DOI: 10.1051/0004-6361/201118505 & c ESO 2012 Astrophysics The extra red plutino (55638) 2002 VE95 M. A. Barucci1, F. Merlin1,2,D.Perna3, A. Alvarez-Candal4,T.Müller5, M. Mommert6,C.Kiss7, S. Fornasier1,2, P. Santos-Sanz1, and E. Dotto8 1 LESIA-Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris-Diderot, 5 Place J. Janssen, 92195 Meudon Principal Cedex, France e-mail: [email protected] 2 Université Paris 7 Denis Diderot, Sorbonne Paris Cité, Paris, France 3 Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Capodimonte, Italy 4 ESO, Santiago, Chile 5 Max-Planck-Institute for Extraterrestrial Physics, Germany 6 Deutsches Zentrum für Luft-und Raumfahrt, Germany 7 Konkoly Observatory of the Hungarian Academy of Sciences, Hungary 8 Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Roma, Italy Received 23 November 2011 / Accepted 2 February 2012 ABSTRACT Aims. In the framework of a large program, we observed (55638) 2002 VE95 with the ESO-VLT telescope to better constrain its surface composition and to investigate the possible heterogeneity of the surface. Methods. We report new near-infrared observations performed in 2007 and 2008. Using the new constraints of the albedo obtained by Herschel Space Observatory observations, a surface model was computed using the complete set of spectra (from visible to the near-infrared) as well as those of the previous published data to investigate the surface composition properties of (55638) 2002 VE95. Results. The surface is heterogeneous. This red object is covered by different icy compounds, such as water (4–19%) and methanol (10–12%). Different organic compounds, such as titan and triton tholins seem also to be present in the surface. The amount of the components is different depending on the observed area. That methanol ice seems to be present mainly on very red surface favors the hypothesis that surfaces of very red objects are more primordial. Key words. techniques: spectroscopic – Kuiper belt objects: individual: (55638) 2002 VE95 1. Introduction (90377) Sedna, and different from the previous visible obser- vations reported by Barucci et al. (2006), for which the com- The transneptunian object (55638) 2002 VE95 has been observed puted slope was 27.9 ± 0.1%/(103 Å). The period is not well in the framework of a large program carried out at ESO-VLT us- known, Ortiz et al. (2006) reported a noisy lightcurve with a ing almost simultaneously the UT1, UT2, and UT4 telescopes small amplitude of about 0.08 with possible periodicities of (Cerro Paranal, Chile) in 2007 and 2008 (Barucci et al. 2010a). 6.76 or 9.47 hrs. Thirouin et al. (2010) reported new observa- The aim of this large program was to obtain high signal-to-noise / tions with a rotational period of 9.97 h and an amplitude of ratio (S N) simultaneous visible and near-IR spectroscopy (us- 0.05 ± 0.01 mag. Even though these authors detected the peri- ing FORS, ISAAC and SINFONI instruments) for almost all odicity with more than 99% confidence level, they concluded transneptunian objects observable within the VLT capability. A because of the smaller amplitude that their derivation is only total of 40 objects have been observed in the framework of this tentative and more observation data with smaller scatter will be program (Barucci et al. 2011). necessary to derive a completely reliable rotational period. New (55638) 2002 VE95 is a peculiar object of the dynamical photometric color data obtained by Perna et al. (2010) confirmed Plutino class (orbit in the 3:2 resonance with Neptune). It is the RR taxonomic group. among the reddest objects and belongs to the RR taxonomic Herschel group of the trans-Neptunian object (TNO) and Centaur popu- This object has been also observed by the Space lation (Fulchignoni et al. 2008). Barucci et al. (2006) already Observatory (Pilbratt et al. 2010) in the framework of the open time key program (OTKP) “TNOs are cool: a survey of the observed it spectroscopically and detected water ice and clear evidence of methanol in its surface. Fornasier et al. (2009)re- transneptunian region” (Müller et al. 2009) with the PACS in- ported new visible spectral observations with a steep visible strument (Poglitsch et al. 2010) and a geometric albedo has been derived for the first time. slope of 40 ± 0.1%/(103 Å), very similar to the extra red object In this paper we present new spectroscopic data of (55638) 2002 VE obtained with ISAAC (1.1–1.4 microns) in 2007 and Based on observations made with ESO Very Large Telescope at 95 the La Silla-Paranal Observatory under program ID 178.C-0036 (PI: SINFONI (1.5–2.4 microns) in 2007 and 2008 together with A. Barucci) and the Herschel Space Observatory. Herschel is an ESA all the available data (visible spectra and photometry) obtained space observatory with science instruments provided by European-led quasi simultaneous to the near-infrared observations. Using the Principal Investigator consortia and with important participation from new constraints of the albedo value, we model the complete NASA. set of new available spectra as well the previous published data Article published by EDP Sciences A152, page 1 of 6 A&A 539, A152 (2012) Table 1. Observational circumstances. Table 2. V-NIR photometric colors∗ . ∗ ∗ Date UT Exp Seeing Airmass Instrument Date V − RV− JV− HV− K ∗∗ s 2007-Dec.-5 4:17 2400 1.05 1.210 FORS 2007-Dec.-5-6 0.72 ± 0.05 2.20 ± 0.06 2.53 ± 0.06 2.57 ± 0.06 2007-Dec.-6 3:16 2160 1.18 1.208 ISAAC 2008-Nov.-22-23 0.75 ± 0.10 2.24 ± 0.09 2.60 ± 0.09 2.65 ± 0.11 2007-Dec.-6 0:31 5100 1.05 1.297 SINFONI 2008-Nov.-22 5:18 4800 0.90 1.289 SINFONI References. (∗) From Perna et al. (2010). Notes. The observational date, universal time UT of the beginning of the exposure, the total exposure time (second), the seeing, and the airmass. 3. Herschel/PACS observations and data reduction (∗) Median seeing and airmass values during the observations. (∗∗) The visible spectrum is reported in Fornasier et al. (2009). 2002 VE95 was also observed by the Herschel Space Observatory in the framework of the OTKP “TNOs are cool!” (Müller et al. 2009). According to the OTKP observation strat- egy, the target was observed with the PACS detector (Poglitsch to investigate the surface composition properties and search for et al. 2010)oftheHerschel Space Observatory (Pilbratt et al. surface variation. 2010) in mini-scanmap mode at two epochs, separated by a time interval that corresponds to a movement of ∼30 of the target, allowing for an optimal background subtraction. At each epoch the target was observed twice in the “blue” (nominal wavelegths 2. ESO-VLT observations and data reduction of 70 μm) and in the “green” (100 μm) band, using two different μ Spectroscopy in the J band (1.1–1.4 μm) was obtained us- scan position angles (red channel – 160 m – data are always ing the SW mode of the ISAAC instrument (equipped with a taken in parallel, either with the blue or the green channel). This Rockwell Hawaii 1024 × 1024 pixel Hg:Cd:Te array), mounted forms a series of four measurements in the blue and green bands, at the unit 1 (Antu) of the VLT. The spectral resolution is about and a series of eight measurements in the red band for a specific 500 with a 1 slit. The observations were executed by nodding target. the object along the slit by 10 between two positions A and B. (55638) 2002 VE95 was observed on August 12, 2010 at two epochs separated by ∼16 h, following the OTKP observation We firstly used the ESO ISAAC pipeline (which runs through / EsoRex, the “ESO Recipe Execution Tool”) for flat-fielding, strategy. The Herschel PACS data were reduced with the mod- ified and optimized version of the standard mini-scanmap data wavelength calibration, correction for spatial and spectral axis 2 distortion, and shifting and adding of the frames. The resulting reduction pipeline in HIPE (Version 8.0.1761). The reliabilitly combined spectrum of the object was then extracted using ESO- of the OTKP pipeline was checked by reducing standard star measurements, and comparing their obtained fluxes to the cor- MIDAS. The reflectivity of (55638) 2002 VE95 was obtained by dividing its spectrum by that of the solar analog star Land93-101, responding model predictions. A more detailed description of observed at similar airmass. The spectrum was finally smoothed the data reduction pipeline and the related data products will be with a median filter technique, with a box of 9 Å in the spectral published in Kiss et al. (in prep.). direction and a threshold of 15%. We obtained the following fluxes for (55638) 2002 VE95 in the 70, 100 and 160 μm PACS bands: F(70) = 10.4 ± 0.8 mJy, Spectroscopy in the H and K bands was obtained using the F(100) = 8.5 ± 1.1 mJy, F(160) = 6.9 ± 1.6 mJy. SINFONI instrument in the unit 4 (Yepun) of the VLT1. All these data were reduced using the SINFONI pipeline version 2.0.0, re- leased by ESO. The reduction followed that described in Barucci 4. Spectral analysis et al. (2011). Particular attention was given to the sky-subtraction by adopting a performed procedure (Davies 2007) to improve The visible spectrum, carried out in 2007, was used to gener- the spectra. The reduction process produces data-cubes, in which ate a complete spectrum from 0.4 to 2.3 μm with the two near- the spatial information lies in the x-y space and the spectral one infrared spectra obtained in 2007 and 2008.