Astronomical Science DOI: doi.org/10.18727/0722-6691/5004 The Nearby Evolved Star L2 Puppis as a Portrait of the Future Solar System 1,2 Pierre Kervella Five billion years from now, the Sun will tudes in the visible, L2 Pup has experi- Miguel Montargès3 grow into a red giant star, more than a enced a remarkable, slow photometric Anita M. S. Richards4 hundred times larger than its current size. dimming over the last decades by more Ward Homan5 It will also experience intense mass loss than 2 magnitudes in the visible. Bedding Leen Decin5 in the form of a stellar wind. The end et al. (2002) interpreted this long-term Eric Lagadec6 product of its evolution, seven billion dimming as the consequence of the Stephen T. Ridgway7 years from now, will be a white dwarf star obscuration of the star by circumstellar Guy Perrin2 — about the size of the Earth and ex­­ dust. Iain McDonald4 tremely dense (density ~ 5 × 106 g cm–3). 8 Keiichi Ohnaka We observed L2 Pup on the night of This metamorphosis will have a dramatic 21 March 2013 with NAOS CONICA impact on the planets of the Solar Sys- (NACO) as part of a survey aimed at 1 Unidad Mixta Internacional Franco- tem, including the Earth. While Mercury imaging the circumstellar environments Chilena de Astronomía (CNRS UMI and Venus will be engulfed by the giant of selected nearby evolved stars (Kervella 3386), Departamento de Astronomía, star and destroyed, the fate of the Earth et al., 2014a). We used 12 narrow-band Universidad de Chile, Santiago, Chile is still uncertain. The brightening of the filters spread in wavelength between 1.04 2 LESIA (UMR 8109), Observatoire de Sun will make the Earth hostile to life in and 4.05 µm. We processed the image Paris, PSL Research University, CNRS, about one billion years. As an aside, cubes using a serendipitous imaging UPMC, Université Paris-Diderot, France assuming that life appeared on Earth approach (also known as “Lucky imag- 3 Institut de Radioastronomie Millimétri­­ 3.7 billion years ago (Ohtomo et al., 2014), ing”). Using 8400 very short exposures que, Saint-Martin d’Hères, France this implies that life on Earth has already (8 milliseconds each) in each filter, we 4 Jodrell Bank Centre for Astrophysics, exhausted ~ 80 % of its development were able to freeze the residual atmos- Dept. of Physics and Astronomy, Uni- time. However, we do not know whether pheric perturbations. After selecting the versity of Manchester, United Kingdom our then-lifeless rock will be destroyed by best 50 % of the series of images, we 5 Institute of Astronomy, Katholieke the burgeoning Sun, or survive in orbit recentred and averaged them to obtain Universiteit Leuven, Belgium around the white dwarf. the 12 final, diffraction-limited images. 6 Laboratoire Lagrange (UMR 7293), They were finally deconvolved using a Université de Nice-Sophia Antipolis, To address the question of the impact of point spread function (PSF) calibrator CNRS, Observatoire de la Côte d’Azur, the final phases of stellar evolution on star. The morphology of L2 Pup in these Nice, France planetary systems, hydrodynamical mod- images (Figure 1) was very surprising. 7 National Optical Astronomy Observa- els have been proposed (see, for exam- From a seemingly double source between tory, Tucson, USA ple, the recent review by Veras, 2016). 1.0 and 1.3 µm, the star became a single 8 Instituto de Astronomía, Universidad But observational constraints on the star- source with an east-west extension at Católica del Norte, Antofagasta, Chile planet interaction models are still rare. 2.1 µm, and exhibited a spectacular spiral Planets of asymptotic giant branch (AGB) loop at 4.0 µm! stars are embedded in complex circum- The impact of the dramatic terminal stellar envelopes and are vastly outshone We proposed (Kervella et al., 2014a) that phases of the lives of Sun-like stars on by their parent star. The observation of L2 Pup is surrounded by an equator-on their orbiting planets is currently uncer- this critical phase of planetary system dust disc. In this framework, the opacity tain. Observations with NAOS CONICA evolution thus presents considerable, and and thermal emission of the dust change and SPHERE/ZIMPOL in 2014–2015 as yet unsolved, technical challenges. As dramatically between 1 and 4 µm, provid- have revealed that the nearby red giant a result, there currently exists only indi- ing a natural explanation for the changing star L2 Puppis is surrounded by an rect evidence of the presence of planets aspect of the circumstellar envelope. At almost edge-on disc of dust and gas. orbiting AGB stars (Wiesemeyer et al., 1 µm, the dust efficiently scatters the light We have observed several remarkable 2009). In addition, the masses of AGB from the star, which therefore appears features in L2 Pup: plumes, spirals, and stars are notoriously difficult to estimate masked behind the dust band. The dust a secondary source (L2 Pup B) which from observations, preventing accurate becomes progressively more transparent is embedded in the disc at a projected determinations of their evolutionary as the wavelength increases. At 2.2 µm, separation of 2 au. ALMA observations states. the thermal emission from the hot inner have allowed us to measure a mass of rim of the disc is observable through the 0.659 ± 0.043 M⊙ for the central star. dust. A colour composite image of This indicates that L2 Pup is a close The discovery of the disc of L2 Puppis L2 Pup in the near infrared is presented analogue of the future Sun at an age with NACO in Figure 2. It shows the intrinsically red of 10 Gyr. We also estimate the mass of colour of the equatorial dust band due L2 Pup B to be 12 ± 16 MJup, implying At a distance of only 64 pc (van Leeuwen, to the stronger scattering at shorter that it is likely a planet or a brown 2007), L2 Pup is the second nearest wavelengths. dwarf. L2 Pup therefore offers us a AGB star after R Doradus. In addition to remarkable preview of the distant future its regular pulsation with a period of Our hypothesis of an edge-on dust disc of our Solar System. 141 days and an amplitude of ~ 2 magni- was initially relatively fragile. But a 3D 20 The Messenger 167 – March 2017 1.04 μm 1.26 μm 2.12 μm 4.05 μm N E 50 mas 5 au Figure 1. NACO deconvolved images of L2 Puppis elongated, and the subtraction of the Sparks et al. (2008) and Kervella et al. at a range of wavelengths from 1 to 4 μm. From central star revealed the presence of a (2014b). Kervella et al. (2014a). second source, L2 Pup B, at a separation of 32 milliarcseconds (mas) from the star Apart from the scattered light on the radiative transfer model using the to the west (Figure 3). disc’s upper and lower surfaces, a very RADMC-3D code (Dullemond, 2012) striking signature in the pL map (Figure 3, confirmed that this interpretation of the The polarimetric imaging capability of right panel) comes from the two plumes NACO images is consistent with the ZIMPOL has been particularly important emerging from the disc. Their high observations, reproducing convincingly in revealing the structure of the envelope degree of polarisation (~ 30 %) indicates both the spectral energy distribution and of L2 Pup (Figure 3, right). As the star that they contain dust and that the scat- the morphology of the disc (Kervella et is embedded in a dust-rich environment, tering angle is large at θ ~ 50°. These thin al., 2014a). the scattering of the starlight by the dust plumes, whose transverse diameter is grains induces a linear polarisation of smaller than 1 au, have a length of more the photons. For small dust grains, the than 10 au. The large scattering angle Stunning features from SPHERE polari- degree of linear polarisation pL is a implies that they emerge from the disc metric imaging smooth function of the scattering angle, close to perpendicular, and propagate in with a maximum pL value obtained for the northern cone cavity that is otherwise We took advantage of the Science Verifi- ~ 90° scattering. Knowing the degree of essentially devoid of dust. cation of the Spectro-Polarimetric High- polarisation therefore allowed us to esti- contrast Exoplanet REsearch instrument mate the scattering angle θ over the The map of the degree of polarisation (SPHERE) to observe L2 Pup in visible envelope. Knowing θ and the projected also shows well-defined local maxima at light imaging polarimetry with the Zurich position of the dust relative to the star in a radius of 6 au, symmetrically east and IMaging POLarimeter (ZIMPOL) camera the image, then allowed us to retrieve the west of the star. The degree of polarisa- (Kervella et al., 2015). The observation 3D distribution of the scattering material. tion reached at these positions is very was carried out on the night of 7 Decem- This polarimetric tomography technique high, up to pL = 60 % in the R-band, cor- ber 2014. The combination of the high was previously employed, for example, by responding to scattering of the light at brightness of L2 Pup and good seeing resulted in spectacular image quality. The Strehl ratio produced by the SPHERE adaptive optics reached more than 40 % at a wavelength of 646 nm for an atmos- pheric seeing of ~ 0.7 arcseconds. In one hour of telescope time, the observation of L2 Pup and a PSF calibrator star (β Col) confirmed unambiguously our hypothesis of an edge-on circumstellar dust disc (Figure 3, upper left).