Imaging the complex atmosphere of cool evolved stars Observing stars like the Sun

Imaging the complex atmosphere of cool evolved stars Observing stars like the Sun Keiichi Ohnaka Max Planck Institute for Radio Astronomy Bonn, Germany Graphics: L. Calçada (ESO)

Mass-loss in cool evolved stars: long-standing problem Driving force is not identified Mira AGB stars Large-amplitude pulsation + radiation pressure or scattering on dust (S. Höfner s talk on Thursday) Non-Mira AGB stars (semi-regular & irregular variables) Much smaller pulsation amplitude. Mass-loss rate comparable to (optically bright) Miras = 10-7 10-6 M 8 /yr Red supergiants Small pulsation amplitude. Mass-loss rate = 10-7 10-6 M 8 /yr Convection? MHD process? Something else? How to tackle this problem? Physical properties of the outer atmosphere Hints for the wind acceleration mechanism

Schematic view of the outer atmosphere Extended molecular outer atmosphere (MOLsphere) 1000 2000 K, 2 5 stellar radii Chromosphere Co-existence of multi-components MOLsphere (1000 2000 K) Chromosphere (~10 4 10 5 K) Possibly chromosphere confined to magnetic regions or shocks (McMurry & Jordan 2000; Harper & Brown 2006) Photosphere Dust formation

Spatially resolving stars like the Sun Stars with largest angular diameters (measured in 2.3 mm continuum) R Dor (AGB) : 47 mas Betelgeuse (RSG) : 43 mas Antares (RSG) : 37 mas Diffraction limit 25 mas (8m telescope @ 1 mm) 20 mas (HST @ 2400 Å) How to image detailed structures over the surface of stars like the Sun? Aperture-synthesis imaging by optical / infrared long-baseline interferometry

Spatially resolving stars like the Sun Very Large Telescope Interferometer (VLTI) Angular resolution = 2.3 mas (VLTI @ 1.6 mm) 4 Unit Telescopes (8.2m, Fixed) 4 Movable 1.8m telescopes Change the array configuration depending on object s size/shape & science cases

Imaging surface structures Betelgeuse Imaging of Betelgeuse (M1.5I) 1.64 mm IOTA array (Haubois et al. 2009) 2 spots detected over the surface 20 mas Comparison with 3-D models (A. Chiavassa s talk this morning, P. Cruzalèbes S1-01) T Per 1 mas RS Per 1 mas T Per (M2I) & RS Per (M4I) 1.6 mm CHARA array (Baron et al. 2014) Bright spot on T Per Dark regions on RS Per Contrast ~ 25%

2014 Interferometry Imaging Beauty Contest (Slide from J. Monnier) Imaging the outer atmosphere 1.61 mm 1.67 mm 1.73mm R Car 10 mas H 2 O+CO H 2 O (weak) H 2 O IR spectrum of R Phe, proxy to R Car (Lançon et al. 2000) Mira star: R Car, phase = 0.84, VLTI / PIONIER instrument 1 or 2 spots detected over the surface Extended emission more prominent at 1.61 and 1.73 mm CO & H 2 O bands Other Miras (Ragland et al. 2008; Pluzhnik et al. 2009; Le Bouquin et al. 2009) Comparison with dynamical models: Wittkowski et al. (2011) Talk on Thursday Hillen et al. (2012) Poster S1-06

Spatially resolving the dynamics of the atmosphere Dynamics of the inhomogeneous atmosphere Directly probe the wind acceleration mechanism High spatial and high spectral resolution needed Optical / IR interferometry with high spectral resolution

First velocity-resolved imaging of the surface of stars Betelgeuse: 1-D imaging in the CO lines VLTI / AMBER Spectral resolution up to 12000 Individual CO lines resolved Probing the outer atmosphere 1-D image in each CO line Spatial resolution = 9.8 mas Beam size = 1/4 stellar size Spectral resolution = 6000 Ohnaka et al. (2011) Movie available at http://www3.mpifr-bonn.mpg.de/staff/kohnaka/

First velocity-resolved imaging of the surface of stars Betelgeuse: Spatially resolved CO line spectrum 9.8 mas Spatially unresolved (= usual) spectrum Extended atmosphere appears as spikes

1-D imaging of Betelgeuse: Spectrum of the CO lines at each spatial position Movie available at http://www3.mpifr-bonn.mpg.de/staff/kohnaka/

1-D imaging of Betelgeuse: Spectrum of the CO lines at each spatial position Stellar astrophysics a few steps closer to solar physics Movie available at http://www3.mpifr-bonn.mpg.de/staff/kohnaka/

First velocity-resolved imaging of the surface of stars Betelgeuse: 1-D imaging in the CO lines Extended component up to 1.3 stellar radii First imaging of the outer atmosphere in CO lines Betelgeuse Blue wing: more extended, asymmetric Red wing: No extended component, symmetric Ohnaka et al. (2011)

First velocity-resolved imaging of the surface of stars Betelgeuse: Modeling the inhomogeneous velocity field Betelgeuse Ohnaka et al. (2011) First velocity-resolved imaging of the surface of a star Weak upwelling at 0 5 km/s & Strong downdrafts at 20 30 km/s Similar velocity field spatially resolved on Antares too (Ohnaka et al. 2013) No systematic outflow, random, turbulent motions within ~1.5 stellar radii

Origin of the inhomogeneous velocity field Convection unlikely Observationally estimated density ~ 10-14 g/cm 3 at 1.3 R 3-D convection model (Chiavassa et al. 2011) < 10-20 g/cm 3 at 1.2 R Driven by MHD processes? Magnetic field detected on Betelgeuse ~1 G (Aurière et al. 2010) But no self-consistent simulation yet for red supergiants Pulsation? But variability amplitude is small, DV = 1 1.5 mag

Velocity-resolved aperture-synthesis imaging M8 giant R Dor R Dor: M8 giant Diameter: 47 mas Beam = 6 x 10 mas Preliminary result: Ohnaka et al. (2014, in prep)

Velocity-resolved aperture-synthesis imaging M8 giant R Dor R Dor: M8 giant Diameter: 47 mas Beam = 6 x 10 mas Ohnaka et al. (2014, in prep) Extended emission stronger in the blue wing than in the red wing Possible detection of outward motions

Conclusions & Prospects Imaging of cool evolved stars reveals the complex atmosphere and the inhomogeneous velocity field Probing the velocity field at different heights using different molecular and atomic lines VLTI 2 nd generation instrument MATISSE (3 13 mm) SiO lines @ 4 mm, CO lines @4.7 mm (G. Perrin s talk on Friday) Spatially resolving the chromosphere and its dynamics in the visible lines (Ha, Ca II triplet)

Thank you for your attention!