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Morphology and Kinematics of the Gas Envelope of the Mira Binary W Aquilae ⋆ RAA 2016 Vol. 16 No. 7, 106 (8pp) doi: 10.1088/1674–4527/16/7/106 Research in http://www.raa-journal.org http://iopscience.iop.org/raa Astronomy and Astrophysics Morphology and kinematics of the gas envelope of the Mira binary W Aquilae ⋆ Do Thi Hoai, Pham Tuyet Nhung, Pham Ngoc Diep, Nguyen Thi Phuong, Pham Tuan-Anh, Nguyen Thi Thao and Pierre Darriulat Department of Astrophysics, Vietnam National Satellite Center, VAST, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam; [email protected] Received 2015 November 16; accepted 2016 February 1 Abstract We analyse ALMA observations of the 12CO(3-2) emission of the circumstellar envelope (CSE) of the Mira variable binary star W Aql. These provide, for the first time, spatially resolved Doppler velocity spectra of the CSE up to angular distances to the central star of ∼ 5′′ (meaning some 2000 AU). The exploratory nature of the observations (only five minutes in each of two different configurations) does not allow for a detailed modelling of the properties of the CSE but provides important qualitative information on its morphology and kinematics. Emission is found to be enhanced along an axis moving from east/west to north-east/south-west when the angular distance from the central star projected on the plane of the sky increases from zero to four arcseconds. In parallel, the Doppler velocity distribution displays asymmetry along an axis moving from east/west to north-west/south-east. The results are discussed in the context of earlier observations, in particular of the dust morphology. Key words: stars: AGB and post-AGB — (stars:) circumstellar matter — stars: individual (W Aql) — stars: mass-loss — radio lines: stars 1 INTRODUCTION star with an effective temperature of ∼6000 K and a mass of ∼1 M⊙ (Danilovich et al. 2015; Mayer et al. 2013). W Aql is a Mira variable star of spectral type S6,6, namely The distance from Earth is poorly known, with values in the transition between being oxygen and carbon dom- between 230 pc and 610 pc being quoted in the litera- inated, with a period of 490 days, a mean luminosity of ture (Guandalini & Busso 2008; Wallerstein et al. 2011; ∼7000 solar luminosities and an effective temperature of Knapp et al. 1998; Loup et al. 1993; Keenan & Boeshaar ∼2000 K (Sloan & Price 1998; Kukarkin et al. 1971; Feast 1980; Danilovich et al. 2014; Ramstedt et al. 2009, 2011; & Whitelock 2000; Alfonso-Garz´on et al. 2012; Ramstedt Ramstedt & Olofsson 2014), indirectly obtained from the et al. 2009; Danilovich et al. 2014). luminosity-period relation (Whitelock et al. 1994, 2008). It is one of the few AGB stars for which direct evi- Retaining a distance of ∼400 pc, the projected distance be- dence for the presence of a companion has been obtained tween the central star and its companion is of the order of (Ramstedt et al. 2011). A high-resolution Hubble Space ∼200AU. The real distance may be much largeras nothing Telescope B-band image1 taken in 2004 at 435 nm (Fig. 1) is known of the inclination of the orbital plane on the sky. displays a clearly resolved binary system, with the com- As two HST images separated by 11 years do not reveal panion being located in the south-west direction at about any displacement between star and companion, the orbital 56◦ from west and at a projected distance of ∼ 0.47′′ period is estimated to exceed ∼1000 years (Mayer et al. from the central star. The companion is a main sequence 2013). ⋆ This paper makes use of the following ALMA data: The star has been extensively observed in the visible, ADS/JAO.ALMA#<2012.1.00524.S>. ALMA is a partnership of infrared, down to far infrared, and radio. Ramstedt et al. ESO (representing its member states), NSF (USA) and NINS (Japan), (2011) have recently detected polarised light in the visi- together with NRC (Canada) and NSC and ASIAA (Taiwan), and KASI ble, namely essentially light from the star scattered from (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. the dust in its circumstellar envelope (CSE). As polariza- ◦ The data are retrieved from the JVO portal (http://jvo.nao.ac.jp/portal) tion is strongly peaked around a scattering angle of 90 , operated by the NAOJ. they observe the circumstellar dust in a slice containing 1 Based on observations made with the NASA/ESA Hubble Space the central star and parallel to the sky plane. Their ob- Telescope, obtained at the data archive at the Space Telescope Science servations reveal a clear enhancement of the emission in Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5–26555. HST the south-west direction up to an angular distance of some ′′ Program 10185, PI: Raghvendra Sahai. 10 from the central star. Close to the star, up to a dis- 106–2 D. T. Hoai et al. tance of ∼ 1′′, the emission is clearly elongated around the Figure 2 shows the distributions of the line flux inte- north-east/south-west direction. While this direction corre- grated over Doppler velocities and averaged over position −1 z sponds to the line joining the central star to its companion angle ϕ = tan ( y )+π (=0 west and positive clockwise) in Figure 1, a causal relation between the two directions is as a function of R = py2 + z2, the projected angular dis- far from obvious as the inclination of the orbit on the sky tance from the star. It gives evidence at ∼ 4′′ for a short plane is unknown. The dust mass of the south-west feature spacing cut-off in the ALMA data, namely the flux filtered is estimated to be (assuming optically thin dust-scattering) −6 out by the interferometer, and for extended emission in the of the order of 10 M⊙. ACA data extending up to at least 22′′ at a typical level of Radio single dish observations in the millimetre/sub- ∼ 32 mJy km s−1 arcsec−2 (a constant level in R means millimetre range include several molecular lines (Nyman a 1/r dependence in space). Accordingly, in what follows, et al. 1992, Knapp et al. 1998; Ramstedt et al. 2009; we restrict the analysis of the ALMA data to angular dis- Wallerstein et al. 2011 and Ramstedt & Olofsson 2014). ′′ −1 tances from the central star not exceeding 4 (meaning ∼ The associated line profiles cover typically a ±20 kms 1600 AU in projected distance). Doppler velocity interval, giving evidence for significant Figure 3 displays the dependence of the line flux den- winds. Danilovich et al. (2014) have recently modelled the ′′ 12 13 sity on Doppler velocity averaged over the central 1.6 × line profiles of CO, CO, SiO, H2O, HCN and NH3, ′′ −6 −1 1.6 (left panel) and over a Gaussian circular beam having and obtained a mass loss rate of 4.0×10 M⊙ yr , of an FWHM of 18′′ (central panel). The latter corresponds the same order of magnitude as previous estimates that −6 −6 −1 to the APEX single dish observation of Ramstedt et al. range between 2.2×10 and 9.4×10 M⊙ yr (Knapp (2009) which is also displayed in the figure (right panel). et al. 1998;Ramstedt et al. 2009; De Beck et al. 2010). The Significant differences between the ALMA and ACA data profiles are approximately symmetric with an occasional are the effect of the important short spacing problem in slight excess emission on the blue-shifted side, suggesting the former. In particular, the data integrated over the large an asymmetric outflow. Continuum emission has also been Gaussian beam show directly that more than half of the to- observed and the dust properties have been studied up to tal flux is contributed by the large distance environment of large distances from the central star (Mayer et al. 2013). the star, at distances exceeding 4′′. While the line profile The only interferometer observations that have been made observed by ACA is in good agreement with that observed are from the Berkeley Infrared Spatial Interferometer (ISI) in the single dish APEX data, evidence for an important at 11 µm and the Plateau de Bure Interferometer on CO slope in the central region is obtained; the red-shifted side molecular lines. The former use crude interferometry from is enhanced with respect to the blue-shifted side by ∼ 2% three aligned antennae and find an excess of emission in per kms−1. The slope is also apparent in the ACA data the eastern half-plane for small distances from the central ′′ when they are restricted to the central region (here a square star, reaching 0.5 (Tatebe et al. 2006); the latter are part of 1.6′′ × 1.6′′, significantly smaller than the beam size). of the COSAS programme (Castro-Carrizo et al. 2010) but Figure 4 displays the sky maps of the detected fluxes have not been published yet. over the whole fields of view. In the ACA data, a slight Recently, ALMA observations of the CO(3-2) emis- offset of the central star with respect to the origin of coor- sion of the circumstellar envelope have been made pub- dinates, 0.4′′ north and 1.3′′ west, has been corrected for licly available (ALMA 0100334[3/5]). The present work in what follows. presents an analysis of these data, which provide for the Figure 5 compares the ALMA sky maps of the cen- first time spatially resolved Doppler velocity spectra. tral region for Doppler velocities respectively smaller and larger than −14 km s−1.
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