High-angular-resolution ALMA imaging of the inhomogeneous dynamical atmosphere of the asymptotic giant branch star W Hya

Abstract

We present high-angular-resolution imaging of the asymptotic giant branch star W Hya with the Atacama Large Millimeter/submillimeter Array (ALMA) to probe the dynamics and chemistry in the atmosphere and inner wind. W Hya was observed with the longest baselines of ALMA at 250-268 GHz with an angular resolution of ~17x20 mas. ALMA's high angular resolution allowed us to resolve the stellar disk of W Hya, along with clumpy, irregularly shaped emission extending to ~100 mas: plume in the north-northwest, a tail in the south-southwest, and the extended atmosphere elongated in the east-northeast--west-southwest direction, with semimajor and semiminor axes of ~70 and 40 mas (~3.4 and 1.9 Rstar), respectively. We identified 57 lines, which include SiO, H2O, SO2, SO, HCN, AlO, AlOH, TiO, TiO2, OH, and some of their isotopologues. The molecular line images show spatially inhomogeneous molecular formation. Our ALMA data taken at phase 0.53 (minimum light) indicate global, accelerating infall within ~75 mas (3.6 Rstar) but also outflow at up to ~10 km/s in deeper layers. While 38 of the detected lines appear in absorption against the continuum stellar disk as expected, we detect nonthermal emission on top of the continuum over the stellar disk in 19 lines of SiO, H2O, SO2, and AlO. The emission of SiO, AlO, TiO, TiO2, SO, and SO2 coincides well with the clumpy dust cloud distribution obtained from contemporaneous visible polarimetric imaging in addition to H2O reported in our previous work. This lends support to the idea that SiO, H2O, and AlO are directly involved in grain nucleation. The overlap of SO/SO2 (possibly also TiO/TiO2) with the dust clouds suggests the formation of these molecules and dust behind shocks induced by pulsation and/or convection. We detect HCN emission close to the star, down to ~30 mas (~1.4 Rstar), which is consistent with shock-induced chemistry.

Figures (19)

• Figure 1: Continuum-subtracted maps of W Hya observed at the systemic velocity in the different molecular lines presented in the main text. The images of two H$_2$O lines (#6 and #7) are from Paper I. The color scale shown above each panel corresponds to mJy/beam except for #3, where it is given in Jy/beam. The black circles represent the ellipse fit to the continuum image. The identification number of each line in Table \ref{['linelist']} is shown in the upper left corner. The restoring beam size is shown in the lower left corner of each panel. North is up, and east is to the left.
• Figure 2: Continuum spectrum of W Hya at 250--268 GHz. The black dots represent the observed continuum fluxes, while the solid red line represents a power-law fit with $F_{\nu} \propto \nu^{2.08}$. The horizontal bars show the bandwidth of the spectral windows, not errors.
• Figure 3: Continuum-subtracted channel maps of W Hya observed in the $^{29}$SiO $\varv=3$$J$ = 6 -- 5 line at 251.930123 GHz. The black circles represent the ellipse fit to the continuum image. In the upper left corner of each panel, the LSR velocity and the relative velocity $V_{\rm rel}$ = $V_{\rm LSR}$ - $V_{\rm sys}$, $V_{\rm sys}$ = 40.4 km s$^{-1}$ are shown. The restoring beam size is shown in the lower left corner of each panel. North is up, and east is to the left.
• Figure 4: Spatially resolved spectra of four SiO lines detected in our ALMA observations. a: Continuum-subtracted intensity map of the Si$^{17}$O line ($\varv=0$, $J$ = 6 -- 5) integrated from $V_{\rm rel}$ = $-10$ to 10 km s$^{-1}$. The crosses and numbers represent the positions where the spatially resolved spectra shown in panels b--p were derived. b-- f: Spatially resolved spectra observed at five positions over the stellar disk. The black, red, green, and blue lines represent the spectra of the $^{29}$SiO $\varv=3$$J$ = 6 -- 5, $^{30}$SiO $\varv=2$$J$ = 6 -- 5, $^{30}$SiO $\varv=1$$J$ = 6 -- 5, and Si$^{17}$O $\varv=0$$J$ = 6 -- 5 lines, respectively. The spectra were obtained from the data cube with the continuum emission. g-- k: SiO spectra obtained at positions 5--8 at a radial distance of 50 mas from the stellar disk center. These spectra were extracted from the continuum-subtracted data cube. The spectra measured at the disk center are shown in panel g to facilitate comparison between the absorption and emission spectra. l-- p: SiO spectra obtained at positions 9--12 at a radial distance of 90 mas from the stellar disk center, shown in the same manner as in panels g-- k.
• Figure 5: Continuum-subtracted channel maps of W Hya obtained in the $^{30}$SiO $\varv=2$$J$ = 6 -- 5 line at 250.727751 GHz, shown in the same manner as Fig. \ref{['channelmap_29sio_251']}. The maps at $\hbox{$V_{\rm rel}$} \le -10.5$ km s$^{-1}$ are severely affected by the Si$^{17}$O line at 250.745 GHz (presented in Fig. \ref{['channelmap_si17o_250']}).