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Register a new userThe wind of W Hya as seen by Herschel. II. The molecular envelope of W Hya
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The evolution of low- and intermediate-mass stars on the asymptotic giant branch (AGB) is mainly controlled by the rate at which these stars lose mass in a stellar wind. Understanding the driving mechanism and strength of the stellar winds of AGB stars and the processes enriching their surfaces with products of nucleosynthesis are paramount to constraining AGB evolution and predicting the chemical evolution of galaxies. In a previous paper we have constrained the structure of the outflowing envelope of W Hya using spectral lines of the $^{12}$CO molecule. Here we broaden this study by modelling an extensive set of H$_{2}$O and $^{28}$SiO lines observed by the three instruments on board Herschel using a state-of-the-art molecular excitation and radiative transfer code. The oxygen isotopic ratios and the $^{28}$SiO abundance profile can be connected to the initial stellar mass and to crucial aspects of dust formation at the base of the stellar wind, respectively. The modelling of H$_{2}$O and $^{28}$SiO confirms the properties of the envelope model of W Hya derived from $^{12}$CO lines. We find an H$_2$O ortho-to-para ratio of 2.5,$^{+2.5}_{-1.0}$, consistent with what is expected for an AGB wind. The O$^{16}$/O$^{17}$ ratio indicates that W Hya has an initial mass of about 1.5 M$_odot$. Although the ortho- and para-H$_{2}$O lines observed by HIFI appear to trace gas of slightly different physical properties, a turbulence velocity of $0.7pm0.1$ km s$^{-1}$ fits the HIFI lines of both spin isomers and those of $^{28}$SiO well. The ortho- and para-H$_2^{16}$O and $^{28}$SiO abundances relative to H$_{2}$ are $(6^{+3}_{-2}) times 10^{-4}$, $(3^{+2}_{-1}) times 10^{-4}$, and $(3.3pm 0.8)times 10^{-5}$, respectively. Assuming a solar silicon-to-carbon ratio, the $^{28}$SiO line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles.
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Asymptotic giant branch (AGB) stars lose their envelopes by means of a stellar wind whose driving mechanism is not understood well. Characterizing the composition and thermal and dynamical structure of the outflow provides constraints that are essential for understanding AGB evolution, including the rate of mass loss and isotopic ratios. We modeled the CO emission from the wind of the low mass-loss rate oxygen-rich AGB star W Hya using data obtained by the HIFI, PACS, and SPIRE instruments onboard the Herschel Space Observatory and ground-based telescopes. $^{12}$CO and $^{13}$CO lines are used to constrain the intrinsic $^{12}$C/$^{13}$C ratio from resolved HIFI lines. The acceleration of the outflow up to about 5.5 km/s is quite slow and can be represented by a beta-type velocity law with index 5. Beyond this point, acceleration up the terminal velocity of 7 km/s is faster. Using the J=10-9, 9-8, and 6-5 transitions, we find an intrinsic $^{12}$C/$^{13}$C ratio of $18pm10$ for W Hya, where the error bar is mostly due to uncertainties in the $^{12}$CO abundance and the stellar flux around 4.6 $mu$m. To match the low-excitation CO lines, these molecules need to be photo-dissociated at about 500 stellar radii. The radial dust emission intensity profile measured by PACS images at 70 $mu$m shows substantially stronger emission than our model predicts beyond 20 arcsec. The initial slow acceleration of the wind implies inefficient wind driving in the lower part of the envelope. The final injection of momentum in the wind might be the result of an increase in the opacity thanks to the late condensation of dust species. The derived intrinsic isotopologue ratio for W Hya is consistent with values set by the first dredge-up and suggestive of an initial mass of 2 M$_odot$ or more. However, the uncertainty in the main-sequence mass derived based on this isotopologic ratio is large.
Infrared spectroscopy is a powerful tool to probe the inventory of solid state and molecular species in circumstellar ejecta. Here we analyse the infrared spectrum of the Asymptotic Giant Branch star W Hya, obtained by the Short and Long Wavelength Spectrometers on board of the Infrared Satellite Observatory. These spectra show evidence for the presence of amorphous silicates, aluminum oxide, and magnesium-iron oxide grains. We have modelled the spectral energy distribution using laboratory measured optical properties of these compounds and derive a total dust mass loss rate of 3E-10 Msol/yr. We find no satisfactory fit to the 13 micron dust emission feature and the identification of its carrier is still an open issue. We have also modelled the molecular absorption bands due to H2O, OH, CO, CO2, SiO, and SO2 and estimated the excitation temperatures for different bands which range from 300 to 3000K. It is clear that different molecules giving rise to these absorption bands originate from different gas layers. We present and analyse high resolution Fabry-Perot spectra of the three CO2 bands in the 15 micron region. In these data, the bands are resolved into individual Q-lines in emission, which allows the direct determination of the excitation temperature and column density of the emitting gas. This reveals the presence of a warm (about 450K) extended layer of CO2, somewhere between the photosphere and the dust formation zone. The gas in this layer is cooler than the 1000K CO2 gas responsible for the low-resolution absorption bands at 4.25 and 15 micron. The rotational and vibrational excitation temperatures derived from the individual Q-branch lines of CO2 are different (450K and 150K, respectively) so that the CO2 level population is not in LTE.
Low- and intermediate-mass stars go through a period of intense mass-loss at the end of their lives in a phase known as the asymptotic giant branch (AGB). During the AGB a significant fraction of their initial mass is expelled in a stellar wind. This process controls the final stages of their evolution and contributes to the chemical evolution of galaxies. However, the wind-driving mechanism of AGB stars is not yet well understood, especially so for oxygen-rich sources. Characterizing both the present-day mass-loss and wind structure and the evolution of the mass-loss rate of such stars is paramount to advancing our understanding of this processes. We modelled the dust envelope of W Hya using an advanced radiative transfer code. The dust model was analysed in the light of a previously calculated gas-phase wind model and compared to measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. We find that the dust spectrum of W Hya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Al$_2$O$_3$ emission. The composition of the large ($sim$,0.3,$mu$m) grains needed to explain the scattered light cannot be constrained, but probably is dominated by silicates. Silicate emission in the thermal infrared was found to originate from beyond 40 AU from the star and we find that they need to have substantial near-infrared opacities to be visible at such large distances. The increase in near-infrared opacity of the dust at these distances roughly coincides with a sudden increase in expansion velocity as deduced from the gas-phase CO lines. Finally, the recent mass loss of W Hya is confirmed to be highly variable and we identify a strong peak in the mass-loss rate that occurred about 3500 years ago and lasted for a few hundred years.
We present visible polarimetric imaging observations of the well-studied AGB star W Hya taken with VLT/SPHERE-ZIMPOL as well as high spectral resolution long-baseline interferometric observations with the AMBER instrument of the Very Large Telescope Interferometer (VLTI). We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820 nm), in the Halpha line at 656.3 nm, and in the TiO band at 717 nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 micron with a spectral resolution of 12000. Taking advantage of the polarimetric imaging capability of SPHERE-ZIMPOL combined with the superb adaptive optics performance, we have succeeded in spatially resolving three clumpy dust clouds located at ~50 mas (~2 Rstar) from the central star, revealing dust formation very close to the star. The AMBER data in the individual CO lines suggest a molecular outer atmosphere extending to ~3 Rstar. Furthermore, the SPHERE-ZIMPOL image taken over the Halpha line shows emission with a radius of up to ~160 mas (~7 Rstar). We found that dust, molecular gas, and Halpha-emitting hot gas are coexisting within 2--3 Rstar. Our modeling suggests that the observed polarized intensity maps can reasonably be explained by large (0.4--0.5 micron) grains of Al2O3 or Mg2SiO4 or MgSiO3 in an optically thin shell with an inner boundary radius of 1.9--2.0 Rstar. The observed clumpy structure can be reproduced by a density enhancement by a factor of 4 +/- 1. The grain size derived from our polarimetric images is consistent with the prediction of the hydrodynamical models for the mass loss driven by the scattering due to micron-sized grains. The detection of the clumpy dust clouds close to the star lends support to the dust formation induced by pulsation and large convective cells as predicted by the 3-D simulations for AGB stars.
This is the final photometric study of TW Hya based on new MOST satellite observations. During 2014 and 2017 the light curves showed stable 3.75 and 3.69 d quasi-periodic oscillations, respectively. Both values appear to be closely related with the stellar rotation period, as they might be created by changing visibility of a hot-spot formed near the magnetic pole directed towards the observer. These major light variations were superimposed on a chaotic, flaring-type activity caused by hot-spots resulting from unstable accretion - a situation reminiscent of that in 2011, when TW Hya showed signs of a moderately stable accretion state. In 2015 only drifting quasi-periods were observed, similar to those present in 2008-2009 data and typical for magnetised stars accreting in a strongly unstable regime. A rich set of multi-colour data was obtained during 2013-2017 with the primary aim to characterize the basic spectral properties of the mysterious occultations in TW Hya. Although several possible occultation-like events were identified, they are not as well defined as in the 2011 MOST data. The new ground-based and MOST data show a dozen previously unnoticed flares, as well as small-amplitude, 11 min - 3 hr brightness variations, associated with accretion bursts. It is not excluded that the shortest 11-15 min variations could also be caused by thermal instability oscillations in an accretion shock.
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