Research ArticleASTRONOMY

Dust formation and wind acceleration around the aluminum oxide–rich AGB star W Hydrae

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Science Advances  01 Nov 2017:
Vol. 3, no. 11, eaao2149
DOI: 10.1126/sciadv.aao2149

Abstract

Dust grains, formed around asymptotic giant branch (AGB) stars, are accelerated by stellar radiation to drive stellar winds, which supply freshly synthesized nuclides to the Galaxy. Silicate is the dominant dust species in space, but ~40% of oxygen-rich AGB stars are thought to have comparable amounts of aluminum oxide dust. Dust formation and the wind-driving mechanism around these oxygen-rich stars, however, are poorly understood. We report on the spatial distributions of AlO and 29SiO molecules around an aluminum oxide–rich M-type AGB star, W Hydrae, based on observations obtained with the Atacama Large Millimeter/submillimeter Array. AlO molecules were only observed within three stellar radii (Rstar), whereas 29SiO was distributed in the accelerated wind beyond 5 Rstar without significant depletion. This strongly suggests that condensed aluminum oxide dust plays a key role in accelerating the stellar wind and in preventing the efficient formation of silicate dust around W Hydrae.

INTRODUCTION

Acceleration of dust grains by radiation pressure from stellar radiation has been proposed as a trigger for ejecting winds from low- to intermediate-mass stars (1). Since its discovery (2), broad emission at a wavelength of 11 to 12 μm has been observed for many oxygen-rich asymptotic giant branch (AGB) stars and is now widely interpreted to come from amorphous or metastable aluminum oxide (Al2O3) dust (3, 4). Aluminum oxide is one of the most refractory dust species, which form in oxygen-rich circumstellar environments, and has been found in meteorites in the form of presolar grains (5). It is surprising that circumstellar dust around ~40% of oxygen-rich AGB stars is rich in aluminum oxide (6), considering the low abundance ratio of Al to Si (Al/Si ~ 0.08) (7). It has remained a significant conundrum as to why aluminum oxide dust is abundant around oxygen-rich AGB stars.

W Hydrae (W Hya) is a semiregular variable star with a period of 389 days located at a distance of 78 pc (8, 9). Mid-infrared observations of W Hya have shown that 50 to 60% of its dust consists of amorphous silicate and one-third of the dust is amorphous aluminum oxide (10, 11). Observations obtained with a near-infrared polarimetric interferometer show that W Hya has a shell of 0.3-μm-radius dust grains at 1.6 to 2.0 Rstar at the variable phase (φ) of 0.2 (12). More recently, Ohnaka et al. (13) showed that W Hya, in its premaximum phase (φ = 0.9), has an optically thin dust shell with inner and outer radii of 1.9 to 2.0 Rstar and 3.0 ± 0.5 Rstar, respectively, based on visible polarimetric imaging. The dust size was estimated to be 0.1 to 0.5 μm in radius (13). Their follow-up observation at minimum light (φ = 0.5) confirmed the reality of the dust shell, and they also found newly formed dust at radii up to 10 Rstar, where 0.1 μm–sized dust is predominant (14). These observations suggest that the dust shell consists of aluminum oxide or iron-free silicate, which can exist close to the star, but the dust species has not been identified.

We observed the spatial distributions of AlO and SiO gas molecules around W Hya, because they represent the gas species from which aluminum oxide and silicate dust forms, and they can be useful proxies for dust formation in the stellar outflow. The major isotope of Si is 28Si, but 29SiO was observed to trace the distribution of SiO molecules because 28SiO is expected to be optically too thick near the star and significant isotopic fractionation between 28SiO and 29SiO is not expected during dust formation. Other Al-bearing species such as Al, AlOH, and Al2O are also important for aluminum oxide formation (15), but AlO can trace the dust formation site in the high-temperature region near the star and could be a basic molecule to form aluminum oxide clusters [for example, see the study of van Heijnsbergen et al. (16)]. So far, AlO molecules have been detected only around an oxygen-rich supergiant, VY Canis Majoris (15), Mira variables, specifically o Ceti (17, 18), and R Aquarii (18) at (sub)millimeter wavelengths.

RESULTS

Long-baseline observations of W Hya with the Atacama Large Millimeter/submillimeter Array (ALMA) (Band 7) were performed on 30 November and 3 and 5 December 2015 (φ = 0.3) using 31, 33, and 41 of the 12-m antennas, respectively (see Materials and Methods). The AlO (N = 9–8) line originating from a rotational transition of AlO radicals in the ground electronic state was detected at 344.36 to 344.44 GHz with a frequency resolution of 488.3 kHz channel−1 (Fig. 1A). The broad emission feature is consistent with the presence of 12 hyperfine structures of the N = 9–8 transition (Fig. 1A) (19). With a beam size of 29 mas (milli–arc sec) × 38 mas, the distribution of AlO (N = 9–8) around W Hya was spatially resolved (Fig. 1B). The emission of the AlO (N = 9–8) transition sharply declined at 50 to 70 mas, and detection at the 3σ confidence level was limited only within ~70 mas (~3 Rstar) from the star (Fig. 1B). The AlO distribution near the star is surprisingly similar to those of <0.5 μm–sized dust inferred from optical polarized imaging at premaximum and minimum light (φ = 0.9 and 0.5; 150 days before and 110 days after the present observation, respectively) (Fig. 1C) (13, 14). This suggests that the dominant dust in the dust shell near the star (2 to 3 Rstar) consists of aluminum oxide.

Fig. 1 AlO N = 9–8 transition observed around W Hya.

(A) AlO (N = 9–8) line spectrum. Error bars represent the SD obtained in the sky region. The positions of the 12 hyperfine structures of the N = 9–8 transition (19) at the local standard-of-rest velocity of 40.4 km s−1 (20) are also indicated (vertical blue lines). (B) Continuum-subtracted and integrated AlO intensity map. The open ellipse in the bottom left-hand corner represents the synthesized beam size. The dashed circles show locations at 3 and 5 Rstar from the star. (C) Comparison of the AlO distribution (contours of brightness in units of Jy beam−1 km s−1) and a polarized intensity map at 645 nm reflecting the dust distribution (13).

The spatially resolved distribution of 29SiO (343.0 GHz, v = 0, J = 8–7) emission was also obtained (Fig. 2) with a beam size of 31 mas × 39 mas. The brightness of the 29SiO line decreased more gradually with distance from the star than that of the AlO emission and extended up to 0.9 arc sec (~40 Rstar) from the star at the 3σ confidence level, significantly beyond the AlO distribution (Fig. 2B). The observed SiO molecular distribution (Fig. 2A) covers the region of the newly detected dust at minimum light after our observation (14).

Fig. 2 29SiO line (v = 0, J = 8–7) observed around W Hya.

(A) Integrated intensity map of 29SiO. The open ellipse in the bottom left-hand corner represents the synthesized beam size. The dashed circles show locations at 3, 5, and 10 Rstar from the star. (B) Annular-averaged line profile of 29SiO at 39.5, 40.4, and 41.2 km s−1. The modeled brightness of 29SiO in the preaccelerated region at 0.07 arc sec (3 Rstar) is shown by a circle, and the modeled abundance profiles with different degrees of 29SiO depletion (10, 30, 50, 70, and 90%) owing to silicate formation are shown for comparison (see Materials and Methods).

Position-velocity diagrams of the 29SiO line (v = 0, J = 8–7) along the R.A. (right ascension) and Dec. (declination) axes (Fig. 3) show dark stripes at 35 to 35.8 km s−1 due to self-absorption. This suggests that 29SiO molecules are optically thick along the line of sight at this velocity, corresponding to 4.6 to 5.4 km s−1 of blueshift relative to the velocity center [40.4 km s−1; (20)]. This is most likely caused by an accelerated laminar flow of 29SiO molecules. The 4.6 to 5.4 km s−1 blueshift is consistent with the wind velocity of 5.5 km s−1 (11). Three small blobs with radial velocity components of ~43.5 to 45.3 km s−1, which are close to the accelerated expansion velocity of the wind, were also observed at 60 to 85 mas from the star (Fig. 3 and fig. S1).

Fig. 3 Position-velocity diagrams of 29SiO line (v = 0, J = 8–7) around W Hya.

(A) R.A. plane. (B) Dec. plane. White rectangles represent the spatial and velocity resolution.

DISCUSSION

Our ALMA observations show that AlO molecules exist only at ≲3 Rstar from the star and that their distribution is consistent with the dust distribution observed at both near-maximum and minimum light (Fig. 1C), whereas SiO molecules are present beyond ~3 Rstar from the star. We obtained an order-of-magnitude estimate of the efficiency of silicate dust formation around W Hya based on a simple radiative transfer model involving the 29SiO line for different acceleration radii Racc (at 2.5 or 5 Rstar) and different degrees of SiO depletion to silicate dust at Rcond (at 2.5 or 5 Rstar) (see Materials and Methods, Fig. 2B, and fig. S2). The model for Racc = 2.5 Rstar showed that an abrupt drop of the 29SiO line intensity, by one order of magnitude, should be observed at 2.5 to 5 Rstar from the star owing to wind acceleration even without silicate dust formation, which is not the case for the present observation (fig. S2). The relatively flat line profile at <5 Rstar (Fig. 2B) can be reproduced for Racc = 5 Rstar (fig. S2). This comparison shows that the overall wind acceleration does not occur inside the dust shell observed within 3 Rstar but occurs beyond the dust shell.

The 29SiO line intensity decreases with distance from the star because of expansion, but it should further decrease if a fraction of SiO molecules condense as silicate dust. A comparison of the decrease of the observed line intensity and that modeled for different degrees of silicate formation (Rcond = Racc = 5 Rstar) suggests that the observed decrease between 70 and 500 mas (3 to 22 Rstar) can be explained only when <30% of SiO molecules are depleted from gas owing to silicate formation (see Fig. 2B and Materials and Methods). Although more detailed model analysis is necessary to obtain a more quantitative estimate, it is clear that silicate dust does not form effectively in W Hya’s accelerated wind and that SiO molecules remain in the gas phase, as observed in this study. The inefficient silicate formation is consistent with the silicate–to–aluminum oxide ratio estimated from mid-infrared dust emission, where only ~6% of the Si atoms should condense as silicate dust if all Al exists in the form of aluminum oxide (11).

Because the radial profile of the 29SiO line intensity is consistent with inefficient silicate formation, the dominant dust species in the dust shell at <3 Rstar observed at φ = 0.9 and 0.5 (13, 14) is most likely aluminum oxide. This has been proposed based on mid-infrared (8 to 13 μm) interferometric observations (21, 22). The well-matched spatial distribution of AlO with the dust shell (Fig. 1C) also supports the predominance of aluminum oxide dust in the high-temperature region near the star.

The dust mass in the dust shell within 3.0 Rstar, observed at different phases both before and after our observation, is estimated at 0.5 × 10−9 to 1.0 × 10−9 MSun if the dust is composed of aluminum oxide or silicate (11, 14, 23). If all Al atoms condense to dust, the mass of aluminum oxide formed within a year is ~2 × 10−11 MSun, assuming the solar elemental abundance and a total mass-loss rate of 1.3 × 10−7 MSun year−1 (21). This indicates that condensed aluminum oxide dust must pile up in the dust shell over several tens of pulsation cycles. The presence of aluminum oxide dust over multiple pulsation cycles would be sufficient to grow the dust to submicrometer size (24), which is comparable to the size estimated from dust scattering (13, 14) (Fig. 4). The survival and growth of aluminum oxide dust near the star is likely possible because of its refractory nature and transparency to the stellar flux (24). Once the aluminum oxide grains become larger than ~0.1 μm and their number density increases because of the pileup, they effectively gain momentum through absorption and scattering of stellar photons (12, 2426) and may contribute to wind acceleration.

Fig. 4 Dust formation and wind acceleration around W Hya.

Aluminum oxide dust, which grows and piles up in the inner dust shell, and partially condensed silicate dust trigger the acceleration of the wind, which suppresses silicate formation around an aluminum oxide–rich star like W Hya.

The SiO blobs we observed (Fig. 3) could be the evidence of wind acceleration by aluminum oxide dust because of the following reasons. (i) The blobs are located 2.3 to 3.3 Rstar away from the star if we take the face value of the redshift of 3.1 to 4.9 km s−1, whereas they are located 4.3 to 6.1 Rstar away from the star if the actual velocity of the blobs is the same as the wind expansion velocity of 5.5 km s−1 (11). These two cases give the lower and upper limits of the blob locations, respectively, and the estimated blob locations (2.3 to 6.1 Rstar from the star) are within the dust shell or slightly farther beyond the dust shell. (ii) The mass and spatial distribution of the dust shell did not change significantly before and after our observation (13, 14), and the predominance of aluminum oxide dust in the dust shell requires dust accumulation in a relatively stationary extended stellar atmosphere. These features of the dust shell suggest that the extended atmosphere of W Hya is quasi-stationary. (iii) The relatively large velocities of the blobs (3.1 to 5.5 km s−1) should thus be the result of partial acceleration of gas within or near the dust shell, and the most likely driver of acceleration is the stellar radiation on aluminum oxide dust probably because of the locally enhanced dust density (Fig. 4). The present observation therefore strongly suggests that aluminum oxide dust can be a wind accelerator, contrary to the widely accepted view (26).

Besides the local wind acceleration near the dust shell, the overall wind acceleration occurs beyond the aluminum oxide dust shell (Fig. 2), which suggests that silicate dust condensing outside the aluminum oxide dust shell also contributes to the acceleration. The contribution of silicate dust to wind acceleration in this star, however, should not be fully significant because the observed SiO distribution implies inefficient silicate formation. Because larger and more concentrated aluminum oxide dust caused by pileup is present in the silicate-forming environment, a small addition of silicate dust to the preexisting aluminum oxide dust may be enough to trigger the overall wind acceleration (Fig. 4). The wind acceleration decreases the gas density, which would suppress further formation of silicate dust in the accelerated wind and explain the aluminum oxide dust–rich nature of W Hya (Fig. 4).

MATERIALS AND METHODS

Observation and data reduction

Observations of W Hya, located at R.A. = 13h49m01s.9981, Dec. = −28°22′03.488″ (J2000), were carried out for a total of ~4 hours over three separate scheduling blocks executed between 30 November and 5 December 2015 with ALMA in Cycle 3 (Project ID: 2015.1.01446.S). The number of antennas used during each observation block was 31, 33, and 41, respectively, in an extended array configuration. The baselines were 15 m to 8.3 km. Data obtained during these 3 days were calibrated and combined by running calibration scripts provided by the observatory and flagging some additional antennas. A quasar, J1337-1257, was used as a calibrator for flux and bandpass calibration. A second quasar, J1351-2912, was also observed for phase calibration. Because the three observations were performed within a week of each other, the flux of the calibrator was assumed to be constant, and flux equalization was applied to the fluxes on the second observation day, which was characterized by the lowest precipitable water vapor, PWV = 0.63 mm.

Three frequency ranges were covered within ALMA Band 7: 330.4 to 331.3 GHz, 342.4 to 343.3 GHz, and 344.0 to 345.9 GHz. The frequency resolutions were 976.562 kHz channel−1 for the first two frequency ranges and 488.281 kHz channel−1 for the third range. The 29SiO (342.98 GHz, J = 8–7) and AlO (344.45 GHz, N = 9–8) lines were covered by the second and third frequency ranges, respectively.

Images of AlO and 29SiO were obtained by using CLEAN tasks in CASA version 4.5.0. The standard CLEAN method (27) with a circular clean mask with a diameter of 2 arc sec centered at the star was applied to the AlO data, whereas the multiscale CLEAN method (28) was applied to clean the extended 29SiO emission. A circular clean mask with a diameter of 3.5 arc sec was applied, and multiscale parameters, which define the full widths at half maximum of clean components in the multiscale CLEAN method, were set to 0, 50, 100, 300, and 600 mas. For visibility weighting, the Briggs mode with a robustness parameter of 0.5 was applied to both data sets. The AlO and 29SiO data were cleaned to flux levels of 5 and 10 millijansky (mJy) beam−1, respectively (background noise levels were ~2 mJy beam−1). The synthesized pixel size was 10 mas pixel−1, and the resulting beam sizes were 29 mas × 38 mas and 31 mas × 39 mas for the AlO and 29SiO images, respectively. The channel map of the 29SiO emission is shown in fig. S1. Continuum subtraction was applied to the AlO image to derive the faint AlO emission around the bright central star. Integrated intensity (moment 0) maps of AlO and 29SiO were obtained by integrating them over 8.5 to 76 km s−1 (Figs. 1B and 2A). The noise level for the AlO image was evaluated at 40 mJy beam−1 km s−1, corresponding to the root mean square value in a blank sky region, and that for the 29SiO image was 32 mJy beam−1 km s−1. Systematic uncertainties of the brightness in the cleaned maps caused by the CLEAN analysis were also evaluated by comparing the images cleaned by different ways. For the AlO image, a comparison with an image cleaned with interactive masking was made, and the 29SiO image was compared with that synthesized by the standard CLEAN method. On the basis of the brightness difference of these images, the systematic brightness uncertainty was estimated to be around 5%.

Radiative transfer model

We carried out a simple radiative transfer calculation of 29SiO emission in the envelope of W Hya to assess the effects of wind acceleration and SiO depletion to silicate dust. A stellar radius of Rstar = 3 × 1011 m and a distance of 78 pc were adopted (9). A spherical envelope with a constant mass-loss rate was assumed. The silicate condensation radius was set at Rcond = 2.5 or 5 Rstar, where the number density of 29SiO was depleted to different degrees (10, 30, 50, 70, and 90%). The wind acceleration location was independently set at Racc = 2.5 or 5 Rstar. A random turbulent velocity of 1.4 km s−1 was assumed for the gas inside Racc, whereas a terminal expansion velocity of 5.5 km s−1 with a random motion of 1.4 km s−1 was adopted for the gas beyond Racc (21). The mass-loss rate was set at 1.3 × 10−7 MSun year−1 (21). The outer edge of the SiO envelope was set at 100 Rstar (29). The temperature profiles of the dust and gas were given by Tdust(r) = 2500 × (r/Rstar)–0.4 and Tgas(r) = 2500 × (r/Rstar)–0.65, respectively. A local thermodynamical equilibrium was assumed to calculate the SiO intensity profile. Line frequency and dipole moment of SiO were taken from Glenar et al. (30) and Langhoff and Bauschlicher (31), respectively, and the warm silicate opacity by Ossenkopf et al. (32) was adopted for dust continuum. The result was expressed as a two-dimensional channel map and converted to the observational image by a simple convolution with a Gaussian profile, which has the same size and position angle as the observation beam. The resolve-out effect is expected to be insignificant because the size of the 29SiO shell [~5.1 arc sec in diameter; (29)] is smaller than the maximum recoverable scale of our observations (7.2 arc sec).

Comparison of the radial distributions of the observed and modeled 29SiO emission (fig. S2) shows that regardless of the depletion degree, the models with Racc = 2.5 Rstar cannot reproduce the relatively flat pattern of the brightness observed at <5 Rstar. The comparison also indicates that the degree of 29SiO depletion should be less than 30%, irrespective of Racc and Rcond, to reproduce the observed brightness at 0.5 to 0.8 arc sec (~10−2 Jy beam−1).

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/11/eaao2149/DC1

fig. S1. Channel map of the 29SiO for different velocities.

fig. S2. Radial distribution of modeled 29SiO emission.

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REFERENCES AND NOTES

Acknowledgments: We thank K. Ohnaka and the ALMA star working group for valuable discussions, and F. Egusa, A. Kawamura, and the ALMA staff for their support of the observations and data analysis. Data analysis was carried out on the open use data analysis computer system at the Astronomy Data Center of the National Astronomical Observatory of Japan (NAOJ). Funding: This work was supported by Japan Society for the Promotion of Science KAKENHI (grants JP12J02495 and JP15K17787 to A.T.). Author contributions: A.T. designed and directed the study. T.K. carried out the data reduction. I.Y. made the radiative transfer model. S.T. discussed the data. A.T. wrote the manuscript. All authors commented on the paper. Competing interests: The authors declare that they have no competing interests. Data and materials availability: This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.01446.S. ALMA is a partnership of European Southern Observatory (ESO) (representing its member states), NSF (United States), and National Institutes of Natural Sciences (Japan), together with National Research Council (Canada), Ministry of Science and Technology and Academia Sinica Institute of Astronomy and Astrophysics (Taiwan), and Korea Astronomy and Space Science Institute (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities, Inc./National Radio Astronomy Observatory, and NAOJ. The data used in this study are available from the ALMA Science Archive (https://almascience.nrao.edu/aq/). Additional data related to this paper may be requested from the authors.
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