Astronomical Science

Catching Stellar Mergers at Work with the Very Large ­Telescope Interferometer

Florentin Millour1 ing the inner hundreds of astronomical The 2002 eruption has been interpreted 1 Eric Lagadec units, exactly where discs and com­ as the merger of a star of ~ 8 MA with a Orsola de Marco2 panions occur and are intimately related subsolar mass star (Tylenda & Soker, Dipankar P. K. Banerjee3 to the evolution of the stars. Interferome- 2006). Djamel Mékarnia1 try is ideally suited to resolve discs in the Alain Spang1 core of nebulae around stars of a range V838 Mon is embedded in a dense, large- Olivier Chesneau1† of masses from low to high. Here we pre- scale environment that was lit up by the sent a snapshot of two such objects — light echoes resulting from the 2002 erup- the R Coronae Borealis (RCB) star V838 tion (Bond et al., 2003), and potentially 1 Laboratoire Lagrange, UMR 7293, Univ. Cen and the eruptive variable V838 Mon contaminates observations of the central Nice Sophia-Antipolis, CNRS, Observa- — highlighting the seminal contribution star. High angular resolution studies with a toire de la Côte d’Azur, Nice, France of the astronomer Olivier Chesneau, who small field of view of a few arcseconds are 2 Department of Physics & Astronomy, passed away earlier this year. an asset in isolating the central regions Macquarie University, Sydney, Australia from the extended dusty cloud. A great 3 Astronomy & Astrophysics Division, The R Coronae Borealis stars are hydro- advantage of optical interferometry studies Physical Research Laboratory, gen-deficient carbon-rich supergiants, is thus to isolate the measurements from Navrangpura, India best known for their spectacular declines the extended environment. in brightness at irregular intervals (De Marco et al., 2002). Two evolutionary sce- The two Very Large Telescope Interfer- narios have been suggested for produc- Observations ometer (VLTI) instruments AMBER ing an RCB star: a double-degenerate and MIDI were used to study the close merger of two white dwarfs (WD) or a VLTI observations of both targets were environment of two post-merger stellar final helium-shell flash in a planetary neb- obtained with the low spectral resolution systems: the R Coronae Borealis star ula central star. V854 Cen has a large mode of AMBER (R = 35), the three-­ V854 Cen and the red nova V838 Mon. hydrogen content and polycyclic aromatic telescope combiner (Petrov et al., 2007). The observations reveal the presence hydrocarbons (PAHs) detected in the The observations were performed with of flattened dusty structures in the core mid-infrared, making it an unusual mem- the 1.8-metre Auxiliary Telescopes (ATs) of both objects, which are very likely ber of its class. This star also has a fast using the compact (baseline, B ≤ 35 m) discs. This finding confirms that the wind which reaches several hundreds and long configurations (B ≤ 140 m). Full merger of two stars can lead to the for- of km s–1 (Clayton et al., 2013; Kameswara details can be found in Chesneau et al. mation of a disc. Rao & Lambert, 1993). Kameswara Rao (2014a) for V854 Cen and Chesneau et al. & Lambert (1993) provided strong argu- (2014b) for V838 Mon. Near-infrared ments that V854 Cen may be surrounded observations were obtained with AMBER, With the advent of optical interferometry, by a bipolar nebula, extending up to about similar to those for R Corona Borealis, the close environments around many 2 arcseconds, as revealed by its C II ex­­ using three ATs and the compact base- evolved stars have been resolved (see tended emission (Clayton & Ayres, 2001). lines. Mid-infrared interferometric data Chesneau, 2011). Companions encom- were also obtained for V838 Mon with the passing a large range of mass, from stel- V838 Mon is a nova-like object that two-telescope recombiner MIDI (Leinert lar objects to Jovian-mass planets, are erupted in 2002 (reaching V = 6.8; Bond et al., 2004), which had a field of view suspected to strongly influence the ejecta et al., 2003). The eruption was unlike ~ 0.3 arcseconds, identical to the near- when a low- to intermediate-mass star classical novae because the effective infrared field of view, thanks to the use of reaches the asymptotic giant branch, or temperature of the object dropped and the Unit Telescopes. even as early as the red giant branch the spectral type evolved to that of a very (de Marco, 2009). This interaction can late L-type supergiant (Loebman et al., V854 Cen was also observed with the potentially influence dramatically the fate 2014). The mid-infrared flux of V838 Mon mid-infrared imager VISIR (Lagage of the star, leading to poorly known increased by a factor of two between et al., 2004) with a pixel scale of ­evolutionary paths, affecting the time- 2004 and 2007, suggesting that new dust 0.075 arcseconds and field of view of scales of the different stellar evolutionary was forming in the expanding ejecta of 19.2 by 19.2 arcseconds through the stages, to the extent that the product the outbursts (Wisniewski et al., 2008). SiC filter (λ = 11.65 μm, width 2.34 μm). bears little similarity to, for example, the The expanding ejecta engulfed a com- Figure 1 displays the VISIR image, timescale and chemistry involved for panion close to the central source (Bond, ­showing a clearly extended nebulosity the evolution of a single, naked star 2006), seen as a faint, blue component around the central star. (Frankowski & Jorissen, 2007). in the spectrum, and Tylenda et al. (2005) argued that the pre-outburst spectral Interferometry in the near- and mid-­ energy distribution (SED) is well matched Modelling the visibilities infrared is the best tool with which to by a pair of early main-sequence stars probe the close environment of evolved (B3V + B1.5V or B4V + A0.5V), making The interferometer data do not directly stars. Indeed, ultra-sharp images of V838 Mon a triple system (main star + provide an image of the sources, and these environments can be made, prob- subsolar merging star + B3V companion). we used an in-house tool called fitOmatic

The Messenger 158 – December 2014 35 Astronomical Science Millour F. et al., Catching Stellar Mergers at Work with the VLTI

(closure phase and differential phases). In explained this way, and indeed it does. V854 Cen order to fit the interferometric data (i.e. However, such a model contains too minimise the least squares), it uses a sim- many components and can only be ulated annealing algorithm, leading to the loosely constrained. The position angle best-match solutions shown in this paper. (PA) of the outer contours of the VISIR 11.65 μm images is only marginally con- sistent with that derived with AMBER, V854 Cen so it is possible that the spatial structure The best-match model for the compact of the mass loss of V853 Cen is inhomo- array data has two components: an geneous and randomly variable. ­unresolved uniform disc (Θ < 2.5 milli­ arcseconds [mas], star component), and N a flattened Gaussian (shell component) V838 Mon 1ǥ with a full width at half maximum (FWHM) The MIDI dispersed visibilities were E of the minor axis of 8 ± 1 mas, and major ­translated into the simplest possible geo- axis 11 ± 3 mas. The orientation of the metrical ad hoc model — a Gaussian Figure 1. A VISIR 11.65 μm image is shown of V854 major axis is 126 ± 29° (Chesneau et al., brightness distribution, as described in Cen with enhanced contours of the outer regions. The 2014a). The long-array data shows that Leinert et al. (2004). The resulting fits extended nebulosity can be seen in blue and purple. the star is unresolved (Θ < 1.0 mas). Fig- shown in Figure 3 depict the general ure 2 shows the fit of the AMBER data by appearance of the mid-infrared structure. (Millour et al., 2009) to fit simple geo­ the geometrical model. The semimajor and semiminor axes of metric components. These fits provide the drawn ellipses represent the half- the poor man’s images displayed in this Given that the closure phase signal width at half maximum of the Gaussian. article (Figures 2 and 3), at an unmatched observed on V854 Cen is null within the The extent of that structure increases spatial resolution. The ­fitOmatic tool uses error bars, a companion star, if any, from a FWHM of 25 mas at 8 μm to a set of simple (uniform disc, Gaussian, would not have a flux exceeding 3 % of 70 mas at 13 μm, with a high flattening etc.) and less simple (“pinwheel”, rotating the total flux, i.e. well below the detection ratio. The major axis is oriented at a and expanding disc, etc.) chromatic limit down to a typical spatial resolution PA close to 10 ± 30°. The 2013 short-­ ­models that can be combined together to of 10 mas. A model with a fully clumped baseline AMBER visibilities are all very form more complex objects. The model is uniform shell that contained up to 30 close to unity in the band centres at matched to the data using least squares, clumps was used to determine whether 1.7 μm and 2.2 μm and are consistent with special care taken for the phases the high H-band visibilities could be with a completely unresolved object. The

Figure 2. Bellow: Geometrical models from fitOmatic 1.0 1.52892 2.49122 of the environment of V854 Cen at 2.5 μm. Upper right panel: Combined H- (blue) and K- (red) band interferometric visibilities obtained with AMBER compared with the signal from the polychromatic

V² 0.5 geometrical modelling. Lower right panel: Closure phases in the H- and K-bands plotted with the same colours. 0.0 10 )

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36 The Messenger 158 – December 2014  ƅL  

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$     ƅL      _L@R   m Figure 3. Left: 2D Gaussian fit to the whole MIDI  dataset for V838 Mon. The dashed lines represent 1DR  the direction of the observation baselines. Right: The MIDI observed visibilities (colours with error bars)  together with the best-fit visibilities derived from     model of the left panel (black lines). The lower panel shows the residuals on the fit. 2O@SH@KEQDPTDMBXBXBKDR@QBRDB  visibilities as a function of wavelength degenerate merger of two white dwarfs, the intermediate-luminosity red transient of the H- and K-bands decrease relative which may lead to an axis of symmetry (ILOT) V4332 Sgr (Kaminski & Tylenda to the band centres (1.7 μm and 2.2 μm) that promotes equatorially enhanced 2013). ILOTs are outbursts with energies indicative of an object whose shape mass-loss and discs, or the final helium- intermediate between those of novae and changes as a function of wavelength. shell flash in a planetary nebula central supernovae, examples of which are V838 star, which may retain a central symmetry Mon. If the merger scenario for the RCBs We again used the fitOmatic package to and promote no disc (Clayton et al., 2011). applies, then they too may be in the ILOT derive an angular diameter assuming a range if observed at the time of the uniform-disc model around 2.2 μm. A The evidence for distributed dust emis- merger. Two 0.5 MA WDs will deliver quite diameter of 1.15 ± 0.20 mas was found, sion suggests a disc that is possibly a substantial amount of gravitational significantly smaller than that published the relic of a past event, or that the energy (≤ 5 × 1049 ergs), more than would by Lane et al. (2005). Since the AMBER observed spatial distribution reflects a be the case for a main-sequence star closure phases are all equal to zero within field of randomly launched clumps in merger, therefore they may cluster in the the error bars, we can constrain the flux free fall around the central star, perhaps upper region of the ILOT locus on the of a hypothetical companion star in addi- constantly replenished by the central energy–time diagram. The stellar expan- tion to the central merger. Our conclusion star. Comparison can be made with the sion that results would make the object a was that there is a dif­ference of at least Be stars ­(Rivinius et al., 2013), for which giant, as is the case for ILOTs. 4 mag between the primary and the com- the fast rotation of the central star panion. The AMBER observations do together with its pulsational properties not exclude the presence of the compan- triggers the ­formation of a dense cir­ V838 Mon ion, but place important constraints on cumstellar disc. The RCB stars are The size of the dusty flattened structure its near-infrared contribution to the spec- known to be slow rotators, and the emis- discovered by MIDI around V838 Mon is tral energy distribution. sion lines of V854 Cen are unresolved distributed between 150 and 400 au from (≤ 20 km s–1; Kameswara Rao & Lambert, the star. This is also in fair agreement 1993). V854 Cen has a well-known with the spectropolarimetric measure- Implications ­single-pulsation period (43.2 days) whose ments reported by Wisniewski et al. phase is related to the formation of the (2003) interpreted as scattering by a disc V854 Cen dust (Crause et al., 2007). Convection with a major axis at a PA of 37°. The A moderately flattened dusty environment may also contribute to the launching pro- supergiant has decreased in angular size around V854 Cen was discovered from cess of the dust clumps, as in cool by ~ 40 % in around ten years since the the VLTI AMBER observations. This supergiant stars. measurements of Lane et al. (2005). The sheds new light on the evolutionary sce- linear radius is now estimated to be 750 ± narios that have been suggested for The polarisation properties of V854 Cen 200 RA (3.5 ± 1.0 au) and the diameter the production of RCB stars: the double- are similar to (albeit weaker than) those of decrease means the star’s photosphere

The Messenger 158 – December 2014 37 Astronomical Science Millour F. et al., Catching Stellar Mergers at Work with the VLTI

has shrunk during the intervening period ­presence of dense material close to the the Calibration Group. Olivier organised at an approximate rate of 1 km s–1. equatorial plane of the systems decel­ the VLTI school in Porquerolles in April erates a wind or a fireball in these direc- 2010 and was a co-organiser of the inter- The most likely hypothesis is that the tions, leading to a bipolar outflow. These ferometry school in Barcelonnette in ­flattened structure is simply the relic of disc-like structures could also affect the autumn of 2013. His enthusiasm, the large dust formation event that was dust evolution, as dust evolves differently curiosity, and passion will be missed by a consequence of the merging of the in a cooler and denser environment (disc) many. two stars (Wisniewski et al., 2008). The than the surroundings. ejecta velocity was low at the time of ­outburst — less than 200 km s–1 — as References derived from P-Cygni profiles by Lynch Remembering Olivier Chesneau Bond, H. E. et al. 2003, Nature, 422, 405 et al. (2004). In the ten years since the Bond, H. E. 2006, ATel, 966, 1 outburst, ejected material would not have The work described here forms the final Chesneau, O. 2011, in Asymmetric Planetary travelled beyond 400 au. The picture published work of Olivier before he Nebulae 5 Conference, arXiv: 1010:1081 revealed by interferometry of V838 Mon passed away. He wrote the two papers Chesneau, O. et al. 2014a, A&A, 569, 4 Chesneau, O. et al. 2014b, A&A, 569, L3 as of today is the following: it is now describing the VLTI observations of Clayton, G. C. & Ayres, T. R. 2001, ApJ, 560, 986 slowly becoming an anonymous red V854 Cen and V838 Mon while he was in Clayton, G. C. et al. 2011, ApJ, 743, 44 supergiant, surrounded by a flattened, hospital. Clayton, G. C., Geballe, T. R. & Zhang, W. 2013, probably transitory, dusty environment AJ, 146, 23 Crause, L. A., Lawson, W. A. & Henden, A. A. 2007, extending up to several hundreds of au. Olivier Chesneau made wide-ranging MNRAS, 375, 301 contributions to the VLTI and high resolu- De Marco, O. et al. 2002, AJ, 123, 3387 tion astronomy. His results were widely De Marco, O. 2009, PASP, 121, 316 Conclusions publicised and he was awarded the 2012 Frankowski, A. & Jorissen, A. 2007, Baltic Astron., 16, 104 Michelson Prize of the International Astro- Kameswara Rao, N. & Lambert, D. L. 1993, AJ, The VLTI is the ideal tool with which to nomical Union and the Mount Wilson 105, 1915 study such objects, thanks to its very Institute for major contributions in stellar Kaminski, T. & Tylenda, R. 2013, A&A, 558, A82 high angular resolution. These observa- astrophysics made with long-baseline Lagage, P. O. et al. 2004, The Messenger, 117, 12 Lane, B. F. et al. 2005, ApJ, 622, L137 tions have enabled us to study the close interferometry. He was directly involved in Leinert, C. et al. 2004, A&A, 423, 537 circumstellar environment of post-merger the MIDI instrument and, through many Loebman, S. R. et al. 2014, ApJ, submitted stellar systems. The observations of observing programmes, with AMBER. He Lynch, D. K. et al. 2004, ApJ, 607, 460 V838 Mon and V854 Cen confirm, for the was part of the VLT SPHERE Guaranteed Millour, F. et al. 2009, A&A, 507, 317 Petrov, R. G. et al. 2007, A&A, 464, 1 first time directly, that the merger of two Time Observing programme and was Rivinius, T., Carciofi, A. C. & Martayan, C. 2013, stars can lead to the formation of a disc. involved with the second generation VLTI A&A Rev., 21, 69 MATISSE instrument consortium. He Tylenda, R. & Soker, N. 2006, A&A, 451, 223 This work has implications for the forma- also worked in the Jean-Mariotti Center Wisniewski, J. P. et al. 2003b, ApJ, 588, 486 Wisniewski, J. P. et al. 2008, ApJ, 683, L171 tions of bipolar nebulae. Indeed the (JMMC) as Principal Investigator of NASA and The Hubble Heritage (AURA/STScI) Team

Hubble Space Telescope Advanced Camera for Surveys (ACS) image of the light echoes around V838 Mon observed in 2004, two years after the outburst. Broadband ACS images in F435W (B), F606W (V), F814W (I) were combined for this colour­ composite. See heic0405a for details.

38 The Messenger 158 – December 2014