THE SPHERE VIEW OF BETELGEUSE

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1 Title : will be set by the publisher Editors : will be set by the publisher EAS Publications Series, Vol.?, 2013 THE SPHERE VIEW OF BETELGEUSE O. Chesneau 1, H.-M. Schmid 2, M. Carbillet 3, A. Chiavassa 1, L. Abe 1 and D. Mouillet 4 Abstract. SPHERE, the Spectro-Polarimetric High-contrast Exoplanet RZsearch instrument for the VLT is optimized towards reaching the highest contrast in a limited field of view and at short distances from the central star, thanks to an extreme AO system. SPHERE is very well suited to study the close environment of Betelgeuse, and has a strong potential for detecting the ejection activity around this key red supergiant. 1 SPHERE SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) is the secondgeneration VLT instrument devoted to direct imaging and characterization of faint objects close to a bright star (giant exoplanets at several AUs). The large number of observing modes that offers SPHERE, in addition to its extreme AO system SAXO, includes the dual-band imaging camera IRDIS, the integral field spectrograph IFS, both operating in the near-infrared, and the imaging polarimeter ZIMPOL, operating in the visible (Kasper et al., 2012). These instruments are equipped with various polarizing devices (linear light) and coronographs. SPHERE is foreseen to see its first sky light in late Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d Azur, Bvd de l Obs., BP4229 F NICE Cedex 4 2 Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093, Zurich, Switzerland 3 Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d Azur, Bt. Fizeau, Parc Valrose, Nice, France 4 Institut de Planétologie et Astrophysique de Grenoble, CNRS-UJF UMR 5571, 414 rue de la Piscine, St Martin d Hères, France c EDP Sciences 2013 DOI: (will be inserted later)

2 2 Title : will be set by the publisher 2 Observing Betelgeuse with SPHERE 2.1 Goals in the visible With an angular diameter in the visible of 43mas, the photosphere is theoretically resolved at 650nm by a 8.2m telescope with a diffraction limit of FWHM=15mas. The brightness of Betelgeuse is such that the extreme AO will work nominally providing even for the visible wavelengths range a Strehl ratio of up to 50% in the R-band. At visual wavelengths, the three chief ways to spatially resolve circumstellar material are with starlight scattered by dust, with large scale molecular lines forming region (in particular TiO bands around 717nm) with intrinsic emission lines formed in the hot chromosphere: Hα and also KI770nm Because the star is extremely bright one needs to take for the observations of the photosphere short integrations ( 1 sec) in narrow filters, possibly combined with neutral density filters. For the circumstellar environment one can use one out of many coronographic modes offered by the instrument. Polarized light is well suited to test the scattering properties of the dusty grains formed around Betelgeuse. It must be noted that little has been done at these wavelengths by the HST, the studies being focused to the UV, where many lines could be spatially resolved. On the ground, the focus was on the measurements of the star diameter in continuum and also in molecular bands by means of high angular resolution techniques such as optical interferometry or aperture masking (Young et al., 2000). Interesting studies of the dusty environment from polarized light investigation can also be reported (Le Borgne et al., 1986). 2.2 Goals in the near-ir The FWHM of an 8.2m at 2.1µm is 56mas, and the extreme AO system of SPHERE will provide a Strehl better than 0.9 for such a bright source. Important investigations were published in the last year with the NACO and VISIR instrument (Kervella et al., 2011, 2009) and with the VLTI (Ohnaka et al., 2011, 2009; Perrin et al., 2007). SPHERE observations should help to bridge the gap between theses studies by providing highly-contrasted images of the vicinity of Betelgeuse in the continuum and within the CO bands ( µm). Such high-quality images provide a large flexibility for the observing strategy: one can consider narrow band observations in polarized light but without coronograph. Techniques involving a PSF subtraction or even a deconvolution are possible. One can also consider the use of coronographs to probe the regions beyond the 0.2 arcsec limit.

3 O. Chesneau et al.: The SPHERE view of Betelgeuse 3 3 The different structures probed 3.1 The hot chromosphere probed by emission lines in the visible The Hα line is well suited to study the hot chromosphere of Betelgeuse since the photosphere is too cool to generate the observed emission. The observed absorption line requires a large column of sufficiently hot plasma in front of the stellar disc. Moreover, the detection of temporal variations in the filling factor of the absorption line is a serious argument in favor of transient (patchy?) chromospheric activity. Yet, the SPHERE OA observations remain challenging, since the Hα emission is expected in the close vicinity of the photosphere and the Strehl ratio will be limited at such a short wavelengths. The discovery of circumstellar emission through resonance line scattering provided one of the first hint on the physical conditions around Betelgeuse. Observations of the KI770nm fluorescent emission line were performed on the ground as early as 1975 (Bernat & Lambert, 1975). Subsequent observations extended the detections of the KI770nm emission to 55 arcsec from the star and provided evidence for structures in the shell (Plez & Lambert, 2002). In particular, numerous clumps were observed unresolved both spectrally ( 2km s 1 ) and spatially (seeing-limited). Extended emission in the Na D lines (in particular Na I 588nm) and in the calcium triplet (854nm) were also detected from the ground. 3.2 The dust: a polarized light view Multicolor linear polarimetry of Betelgeuse was one of the first way used to investigate the properties of the dust formed in the vicinity. It confirmed in a qualitative way the ideas about large-scale slowing varying structures. High angular resolution observations performed in burst mode (short exposures) using VISIR (N band) and NACO (J, H, K bands) provided an unprecedented view of the plumes (Kervella et al., 2011, 2009). The dust grains remain to be characterized. This can be performed with SPHERE with broadband observations in polarized light from the visible to the near-ir. Moreover, the SPHERE images should be compared to the NACO ones to detect any hint of temporal changes. 3.3 The MOLsphere: Bringing the gap between OA and optical interferometry The presence of a MOLsphere i.e. an extended molecular envelop around Betelgeuse, first hypothesized by Tsuji (2000) and confirmed with interferometry (Perrin et al., 2007) has potentially a deep impact on the dust formation process. However, the interplay between dust and molecules is complex around such an extended supergiant that does not pulse but may exhibit significant local variability due to convection (Chiavassa et al., 2010) or chromospheric activity. In the K band, the CO ro-vibrationnal band ( µm) is the best suited band to study the molecular environment and its inhomogeneities. It was investigated for instance with the VLTI/AMBER instrument (Ohnaka et al., 2011, and

4 4 Title : will be set by the publisher reference therein). The diffraction-limited spatial resolution of SPHERE is 56mas at this wavelength, and the Strelh ratio will be maximum (i.e. 0.95) ensuring that good quality Point-Spread Function (PSF) can be recorded. Moreover, the dual-band capabilities of SPHERE provide a direct comparison on the continuum versus CO band appearance of the source. Therefore, PSF subtraction and deconvolution are expected to be very efficient. The more extended environment will be studied with coronographic masks. In the visible, the TiO band is the best suited. It was investigated extensively with optical interferometry, revealing some interesting holes (Young et al., 2000). SPHERE should provide a direct link between these holes and the nearby circumstellar medium. References Bernat, A. P., & Lambert, D. L. 1975, ApJL, 201, L153 Chiavassa, A., Haubois, X., Young, J. S., et al. 2010, A&A, 515, A12 Kasper, M., Beuzit, J.-L., Feldt, M., et al. 2012, The Messenger, 149, 17 Kervella, P., Perrin, G., Chiavassa, A., et al. 2011, A&A, 531, A117 Kervella, P., Verhoelst, T., Ridgway, S. T., et al. 2009, A&A, 504, 115 Le Borgne, J. F., Mauron, N., & Leroy, J. L. 1986, A&A, 168, 211 Lobel, A. 2010, Hot and Cool: Bridging Gaps in Massive Star Evolution, 425, 162 Lobel, A., & Dupree, A. K. 2001, ApJ, 558, 815 Ohnaka, K., Weigelt, G., Millour, F., et al. 2011, A&A, 529, A163 Ohnaka, K., Hofmann, K.-H., Benisty, M., et al. 2009, A&A, 503, 183 Plez, B., & Lambert, D. L. 2002, A&A, 386, 1009 Perrin, G., Verhoelst, T., Ridgway, S. T., et al. 2007, A&A, 474, 599 Tsuji, T. 2000, ApJ, 538, 801 Young, J. S., Baldwin, J. E., Boysen, R. C., et al. 2000, MNRAS, 315, 635

5 O. Chesneau et al.: The SPHERE view of Betelgeuse 5 Fig. 1. Illustration of the possibilities of observations with SPHERE. The sketch of the environment of Betelgeuse is a courtesy from Alex Lobel. Right: in the near-ir (K band) the instrumental response (PSF) is clean with a large strehl ratio and a FWHM of 56mas. A coronographic mask with a diameter of 145mas can be inserted, centered on the star or even shifted to study the regions close to the photosphere. Left: The PSF is theoretically sharper in the visible (FWHM of about 20mas) but the AO perform with less efficiency due to the turbulent atmosphere and the strehl ratio is decreased. The smaller coronographic mask is 93mas.