Astronomical Science Molecular and Dusty Layers of Asymptotic Giant Branch Stars Studied with the VLT Interferometer Markus Wittkowski1 Low to intermediate mass stars, including and their effects on the mass­loss mech­ Iva Karovicova1 our Sun, evolve to red giant stars and anism are also not well understood. David A. Boboltz2 subsequently to asymptotic giant branch Because of the extension of the atmos­ Eric Fossat3 stars after the hydrogen and helium phere, temperatures are cool enough Michael Ireland4, 5 supplies in the core have been exhausted so that molecules form, leading to a sce­ Keiichi Ohnaka6 by nuclear fusion. An AGB star is in the nario where molecular layers lie above Michael Scholz7, 8 final stage of stellar evolution that is the continuum­forming photosphere. Francois van Wyk9 driven by nuclear fusion, where a degen­ Dust is formed and believed to be accel­ Patricia Whitelock9, 10 erate carbon–oxygen core is surrounded erated within this extended atmospheric Peter R. Wood11 by hydrogen­ and helium­burning layers, region with a certain condensation Albert A. Zijlstra12 a huge convective envelope and a very sequence, where dust species of higher extended and diluted stellar atmosphere. condensation temperatures form closer Mass loss becomes increasingly impor­ to the stellar surface and dust species 1 ESO tant during AGB evolution, both for stellar with lower condensation temperatures 2 United States Naval Observatory, evolution, and for the return of material form at larger distances. There is also a Washington, DC, USA to the interstellar medium. It reduces the notion that different dust species may 3 Laboratoire d’Université convective stellar envelope until the star prevail at different stages along the AGB, d’Astrophysique de Nice, France begins to shrink and evolves at constant along with increasing stellar luminosity 4 Dept. of Physics and Astronomy, luminosity toward the hotter post­AGB and increasing mass­loss rate. Macquarie University, Sydney, Australia and planetary nebula (PN) phases, and is 5 Australian Astronomical Observatory, thus the most important driver for the Optical interferometry at near­IR and mid­ Epping, Australia further stellar evolution (e.g., Habing & IR wavelengths has proved to be a pow­ 6 Max-Planck Institut für Radioastrono­ Olofsson, 2003). Mass loss from AGB erful tool to study the extended atmos­ mie, Bonn, Germany stars is also one of the most important phere and dust condensation zones of 7 Zentrum für Astronomie, University of sources for the chemical enrichment of AGB stars, because of its ability to spa­ Heidelberg, Germany the interstellar medium and of galaxies. tially resolve these regions. Indeed, red 8 School of Physics, University of giants and AGB stars have historically Sydney, Australia Depending on whether or not carbon has been prime targets for optical interfer­ 9 South African Astronomical Observa­ been dredged up from the core into the ometry, because of their brightness and tory, Cape Town, South Africa atmosphere, AGB stars appear in obser­ the match of their angular size to the typi­ 10 Astronomy Dept., University of Cape vations to have an oxygen-rich or a car­ cal spatial resolution of optical interfer­ Town, Rondebusch, South Africa bon­rich chemistry. A canonical model of ometers. However, despite the long his­ 11 Australian National University, the mass­loss process has been devel­ tory of such measurements, they still Canberra, Australia oped for the case of carbon­rich chemis­ continue to provide important new con­ 12 Jodrell Bank Centre for Astrophysics, try, where atmospheric carbon dust has straints on the open questions discussed University of Manchester, United a sufficiently large opacity to be radia­ above. In particular, the near­infrared Kingdom tively accelerated and driven out of the (NIR) instrument AMBER and the mid- gravitational potential of the star and infrared (MIR) instrument MIDI of the VLT where it drags the gas along. For the Interferometer (VLTI) have been shown Mass loss from asymptotic giant branch case of oxygen-rich chemistry, the details to be well suited for new measurements (AGB) stars is the most important driver of this process are not understood, and of AGB stars because of their unprec e­ for the evolution of low to intermediate are currently a matter of vigorous debate. dented ability to provide spectro­ mass stars towards planetary nebulae. Questions remain also for the carbon­rich interferometric observations with spectral It is also one of the most important case, such as regarding the formation resolutions of 35–12 000 (AMBER) and sources of chemical enrichment of the of the recently detected oxygen-bearing 30–230 (MIDI). Here, we report on very interstellar medium. The mass-loss pro- molecule water in the inner atmospheres recent results ( Karovicova et al., 2011; cess originates in the extended atmos- of carbon­rich AGB stars by the Herschel Wittkowski et al., 2011) from our ongoing phere, whose structure is affected by space mission (Decin et al., 2010). programme to characterise molecular stellar pulsations, and where molecular and dusty layers of AGB stars using the and dusty layers are formed. Optical AGB stars experience stellar pulsations, VLTI instruments. interferometry resolves the extended from semi­regular variable stars (SRVs) atmospheres of AGB stars and thereby on the early AGB to large­amplitude long­ enables us to obtain measurements period variable stars on the more evolved Recent VLTI observations of the intensity profile across this region. AGB, including Mira variables and more We present an overview of recent results dust­enshrouded stars at the tip of the VLTI observations of AGB stars using from our spectro-interferometric obser- AGB. Pulsations and the induced shock the instruments MIDI and AMBER started vations of AGB stars using the near- fronts are expected to play a crucial role with the measurements described by and mid-infrared instruments AMBER for the structure of the extended atmos­ Ohnaka et al. (2005) and Wittkowski et and MIDI of the VLT Interferometer. pheres, but the details of these processes al. (2008), respectively. These studies 24 The Messenger 145 – September 2011 demonstrated that the spectro­interfero­ Stellar cycle/phase 01234 metric capabilities of MIDI and AMBER 6 are well suited to characterise the dusty and molecular layers of AGB stars. AAVSO We obtained MIDI data of the Mira varia­ 8 AFOEV ble AGB stars S Ori, GX Mon, RR Aql, R Cnc, and X Hya between 2004 and 2009. Some of these observations were 10 aimed at performing a mid­infrared moni­ toring of AGB stars in order to investi­ gate intracycle and cycle­to­cycle varia­ g bility, and others were designed to com­ ma 12 plement AMBER observations to provide V a more complete picture of the dusty and molecular layers at the same time. Mid- 14 infrared interferometric monitoring of the sources listed above has been performed by Iva Karovicova during her PhD thesis, MIDI 16 together with theoretical simulations of VLBA the expected variability. For example, the observations of RR Aql described in Karovicova et al. (2011) include 52 obser­ 18 3000 3500 4000 4500 vations obtained at 13 epochs between Julian Date – 2450000 April 2004 and July 2007 covering three pulsation cycles (Figure 1). Figure 1. (Top) Visual light curve of RR Aql with our Figure 2. (Bottom) Uniform disc diameter as a func­ epochs of MIDI observations indicated in green. tion of wavelength obtained from VLTI/AMBER Epochs of Very Long Baseline Array (VLBA) observa­ observations of the Mira variable AGB star R Cnc. Using the AMBER instrument, we se­­ tions are shown in blue. From Karovicova et al. The red line denotes the best-fit prediction from the cured data of the Mira variables R Cnc, (2011). recent dynamical atmosphere model series CODEX. X Hya, W Vel, RW Vel, and RR Aql From Wittkowski et al. (2011). between 2008 and 2010 using the medium reso lution mode with a spectral resolution of 1500. First results from 22 R Cnc these campaigns have been presented VLTI/AMBER 1.6 by Wittkowski et al. (2011). ) 20 E0-G0-H0 as er Some of these observations were coor­ (m er 18 1.4 dinated with concurrent VLBA observa­ diamet tions of the SiO, H2O, and OH maser UD diamet emission (observation epochs shown in 16 1.2 ed Figure 1), which provide complementary disk information on the kinematics and geom­ m 14 etry of the molecular layers in which the or 1.0 Normaliz maser emissions originate. Unif 12 H2O CO 0.8 10 12C16O 12C16O 12C16O 12C16O 12C16O 13C16O Structure of molecular layers 2–0 3–1 4–2 5–3 6–4 5–3 2000 2100 2200 2300 2400 The medium resolution (R ~ 1500) near­ Wavelength (nm) infrared AMBER observations of the Mira variables of our sample confirmed a char­ so that the source appears larger and the study and to quantification of this effect acteristic wavelength­dependent shape of visibility smaller, and lower at other wave­ because of its wide wavelength coverage the visibility function that is consistent with lengths so that the source appears smaller with high spectral resolution. In our earlier low resolution AMBER observa­ and the visibility larger. Our observations measurements, the corresponding wave­ tions of S Ori and that can be understood confirm this effect, which has previously length­dependent uniform disc (UD) within the molecular layer scenario. In this been detected by interferometric obser­ diameters show a minimum near the scenario, the opacity of molecular layers, vations using a few narrowband filters. near-continuum bandpass at 2.25 μm. at NIR wavelengths, most importantly H2O Hereby, the AMBER instrument has shown They then increase by up to 30% toward and CO, is larger at certain wavelengths to be particularly well suited to further the H2O band at 2.0 μm and by up to The Messenger 145 – September 2011 25 Astronomical Science Wittkowski M.