The wind of W Hydrae as seen by<i>Herschel</i>

Khouri, T.; Koter, A. de; Decin, L.; Waters, L. B. F. M.; Lombaert, R.; Royer, P.; Swinyard, Bruce; Barlow, M. J.; Alcolea, J.; Blommaert, J. A. D. L.; Cernicharo, J.; Groenewegen, M. A. T.; Justtanont, K.; Kerschbaum, F.; Maercker, M.; Marston, A. P.; Matsuura, M.; Melnick, Gary J.; Menten, K. M.; Olofsson, H.; Planesas, P.; Polehampton, E. T.; Posch, Th.; Schmidt, M.; Szczerba, R.; Vandenbussche, B.; Yates, J.

doi:10.1051/0004-6361/201322578

Cited by 45 publications

(23 citation statements)

References 53 publications

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“…In line with this, we calculate consistently smaller values for r 1/2 . This is consistent with observational data for W Hya (Khouri et al 2014), TX Cam (Ramstedt et al 2008), and R Dor (Maercker et al 2016) for example. Models of these objects reproduce the observed line emission better when they assume a smaller CO envelope than is predicted by MGH88, in line with the results of this paper.…”

Section: Comparison With Literaturesupporting

confidence: 92%

Photodissociation of CO in the outflow of evolved stars

Saberi

Vlemmings

Beck

2019

A&A

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Context. Ultraviolet (UV) photodissociation of carbon monoxide (CO) controls the abundances and distribution of CO and its photodissociation products. This significantly influences the gas-phase chemistry in the circumstellar material around evolved stars. A better understanding of CO photodissociation in outflows also provides a more precise estimate of mass-loss rates. Aims. We aim to update the CO photodissociation rate in an expanding spherical envelope assuming that the interstellar radiation field (ISRF) photons penetrate through the envelope. This will allow us to precisely estimate the CO abundance distributions in circumstellar envelope around evolved stars. Methods. We used the most recent CO spectroscopic data to precisely calculate the depth dependency of the photodissociation rate of each CO dissociating line. We calculated the CO self-and mutual-shielding functions in an expanding envelope. We investigated the dependence of the CO profile on the five fundamental parameters mass-loss rate, the expansion velocity, the CO initial abundance, the CO excitation temperature, and the strength of the ISRF. Results. Our derived CO envelope size is smaller than the commonly used radius derived by Mamon et al. 1988. The difference between results varies from 1% to 39% and depends on the H 2 and CO densities of the envelope. We list two fitting parameters for a large grid of models to estimate the CO abundance distribution. We demonstrate that the CO envelope size can differ between outflows with the same effective content of CO, but different CO abundance, mass-loss rate, and the expansion velocity as a consequence of differing amounts of shielding by H 2 and CO. Conclusions. Our study is based on a large grid of models employing an updated treatment of the CO photodissociation, and in it we find that the abundance of CO close to the star and the outflow density both can have a significant effect on the size of the molecular envelope. We also demonstrate that modest variations in the ISRF can cause measurable differences in the envelope extent.

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Section: Comparison With Literaturesupporting

confidence: 92%

Photodissociation of CO in the outflow of evolved stars

Saberi

Vlemmings

Beck

2019

A&A

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“…The mass-loss rate of IK Tau is about 20-35 times higher than that of W Hya (Ṁ ∼1.3-2.3×10 −7 M yr −1 , Khouri et al 2014;Richards et al 2012). Also, the variability amplitude and pulsation period of IK Tau (up to 6 mag over 460 days) are higher and longer than those of W Hya (3 mag over 389 days).…”

Section: Amber Spectro-interferometric Observations Of the Central Starmentioning

confidence: 88%

Exploring the innermost dust formation region of the oxygen-rich AGB star IK Tauri with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Adam

Ohnaka

2019

A&A

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Context. Low-and intermediate-mass stars at the asymptotic giant branch (AGB) are known to be prevalent dust providers to galaxies, replenishing the surrounding medium with molecules and dust grains. However, the mechanisms responsible for the formation and acceleration of dust in the cool extended atmospheres of AGB stars are still open to debate. Aims. We present visible polarimetric imaging observations of the oxygen-rich AGB star IK Tau obtained with the high-resolution polarimetric imager VLT/SPHERE-ZIMPOL at post-maximum light (phase 0.27) as well as high-spectral resolution long-baseline interferometric observations with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). We aim to spatially resolve the dust and molecule formation regions, and to investigate their physical and chemical properties within a few stellar radii of IK Tau. Methods. IK Tau was observed with VLT/SPHERE-ZIMPOL at three wavelengths in the pseudo-continuum (645, 748, and 820 nm), in the Hα 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 µm with a spectral resolution of 12 000. Results. The excellent polarimetric imaging capabilities of SPHERE-ZIMPOL have allowed us to spatially resolve clumpy dust clouds at 20-50 mas from the central star, which corresponds to 2-5 R when combined with a central star's angular diameter of 20.7±1.53 mas measured with VLTI/AMBER. The diffuse, asymmetric dust emission extends out to ∼73 R . We find that the TiO emission extends to 150 mas (15 R ). The AMBER data in the individual CO lines also suggest a molecular outer atmosphere extending to ∼1.5 R . The results of our 2-D Monte Carlo radiative transfer modelling of dust clumps suggest that the polarized intensity and degree of linear polarization can be reasonably explained by small-sized (0.1 µm) grains of Al 2 O 3 , MgSiO 3 , or Mg 2 SiO 4 in an optically thin shell (τ 550 nm =0.5±0.1) with an inner and outer boundary radius of 3.5 R and 25 R , respectively. The observed clumpy structures can be reproduced by a density enhancement of a factor of 3.0±0.5. However, the model still predicts the total intensity profiles to be too narrow compared to the observed data, which may be due to the TiO emission and/or grains other than homogeneous, filled spheres. Conclusions. IK Tau's mass-loss rate is 20 to 50 times higher than the previously studied AGB stars W Hya, R Dor, and o Cet. Nevertheless, our observations of IK Tau revealed that clumpy dust formation occurs close to the star as seen in those low mass-rate AGB stars.

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“…Its massloss rate is estimated to be ∼ 10 −7 M yr −1 , based on modelling of CO lines (e.g. Maercker et al 2008;Khouri et al 2014a), while its main-sequence mass has been constrained to be between 1.0 and 1.5 M (Khouri et al 2014b;Danilovich et al 2017), based on measured oxygen isotopic ratios. Over a period of ∼ 388 days, the visual magnitude of W Hya varies between ∼ 6 mag and 9 mag (Zhao-Geisler et al 2011).…”

Section: W Hyamentioning

confidence: 99%

Resolving the extended stellar atmospheres of asymptotic giant branch stars at (sub)millimetre wavelengths

Vlemmings

Khouri

Olofsson

2019

A&A

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Context. The initial conditions for mass loss during the asymptotic giant branch (AGB) phase are set in their extended atmospheres, where, among others, convection and pulsation driven shocks determine the physical conditions. Aims. High resolution observations of AGB stars at (sub)millimetre wavelengths can now directly determine the morphology, activity, density, and temperature close to the stellar photosphere. Methods. We used Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution observations to resolve the extended atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor, and R Leo. We interpreted the observations using a parameterised atmosphere model. Results. We resolve all four AGB stars and determine the brightness temperature structure between 1 and 2 stellar radii. For W Hya and R Dor we confirm the existence of hotspots with brightness temperatures > 3000 to 10000 K. All four stars show deviations from spherical symmetry. We find variations on a timescale of days to weeks, and for R Leo we directly measure an outward motion of the millimetre wavelength surface with a velocity of at least 10.6 ± 1.4 km s −1 . For all objects but W Hya we find that the temperatureradius and size-frequency relations require the existence of a (likely inhomogeneous) layer of enhanced opacity. Conclusions. The ALMA observations provide a unique probe of the structure of the extended AGB atmosphere. We find highly variable structures of hotspots and likely convective cells. In the future, these observations can be directly compared to multi-dimensional chromosphere and atmosphere models that determine the temperature, density, velocity, and ionisation structure between the stellar photosphere and the dust formation region. However, our results show that for the best interpretation, both very accurate flux calibration and near-simultaneous observations are essential.

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The wind of W Hydrae as seen byHerschel

Cited by 45 publications

References 53 publications

Photodissociation of CO in the outflow of evolved stars

Photodissociation of CO in the outflow of evolved stars

Exploring the innermost dust formation region of the oxygen-rich AGB star IK Tauri with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Resolving the extended stellar atmospheres of asymptotic giant branch stars at (sub)millimetre wavelengths

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