A HIFI view on circumstellar H<sub>2</sub>O in M-type AGB stars: radiative transfer, velocity profiles, and H<sub>2</sub>O line cooling

Maercker, M.; Danilovich, T.; Olofsson, H.; Beck, E. De; Justtanont, K.; Lombaert, R.; Royer, P.

doi:10.1051/0004-6361/201628310

Cited by 52 publications

(103 citation statements)

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“…R Dor is the closest AGB star to our Sun with a gas massloss rate value of ∼0.9−2 × 10 −7 M /yr and an expansion velocity of only ∼5.7 km s −1 (Schöier et al 2004;Khouri 2014;Maercker et al 2016;Van de Sande et al 2017). This star is a semi-regular variable with two pulsation modes that have periods of 332 days and about 175 days; the latter mode has a much smaller amplitude (Bedding et al 1998).…”

Section: Target Selectionmentioning

confidence: 91%

“…IK Tau is a high mass-loss rate Mira-type AGB star with a gas mass-loss rate of ∼4.5×10 −6 M /yr, an expansion velocity of 17.7 km s −1 (Decin et al 2010a;Maercker et al 2016), and a pulsation period of ∼470 days (Wing & Lockwood 1973). The angular diameter is estimated around 20 mas (Decin et al 2010a).…”

Section: Target Selectionmentioning

confidence: 99%

“…The main input parameters for R Dor and IK Tau are listed in Table 3, following Maercker et al (2016). These properties are simplified estimates, for example the velocity structure inside the dust formation radius is complex, but are adequate for 1D modelling in the regions where these Al compounds have been observed.…”

Section: Abundances Of Aluminium-bearing Moleculesmentioning

confidence: 99%

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Study of the aluminium content in AGB winds using ALMA

Decin

Richards

Waters

et al. 2017

A&A

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Context. The condensation of inorganic dust grains in the winds of evolved stars is poorly understood. As of today, it is not yet known which molecular clusters form the first dust grains in oxygen-rich (C/O < 1) asymptotic giant branch (AGB) winds. Aluminium oxides and iron-free silicates are often put forward as promising candidates for the first dust seeds. Aims. We aim to constrain the dust formation histories in the winds of oxygen-rich AGB stars. Methods. We obtained Atacama Large Millimeter/sub-millimeter array (ALMA) observations with a spatial resolution of 120 × 150 mas tracing the dust formation region of the low mass-loss rate AGB star, R Dor, and the high mass-loss rate AGB star, IK Tau. We detected emission line profiles of AlO, AlOH, and AlCl in the ALMA data and used these line profiles to derive a lower limit of atomic aluminium incorporated in molecules. This constrains the aluminium budget that can condense into grains. Results. Radiative transfer models constrain the fractional abundances of AlO, AlOH, and AlCl in IK Tau and R Dor. We show that the gas-phase aluminium chemistry is completely different in both stars with a remarkable difference in the AlO and AlOH abundance stratification. The amount of aluminium locked up in these three molecules is small, ≤1.1 × 10 −7 w.r.t. H 2 , for both stars, i.e. only ≤2% of the total aluminium budget. An important result is that AlO and AlOH, which are the direct precursors of alumina (Al 2 O 3 ) grains, are detected well beyond the onset of the dust condensation, which proves that the aluminium oxide condensation cycle is not fully efficient. The ALMA observations allow us to quantitatively assess the current generation of theoretical dynamical-chemical models for AGB winds. We discuss how the current proposed scenario of aluminium dust condensation for low mass-loss rate AGB stars within a few stellar radii from the star, in particular for R Dor and W Hya, poses a challenge if one wishes to explain both the dust spectral features in the spectral energy distribution (SED) in interferometric data and in the polarized light signal. In particular, the estimated grain temperature of Al 2 O 3 is too high for the grains to retain their amorphous structure. We advocate that large gas-phase (Al 2 O 3 ) n clusters (n > 34) can be the potential agents of the broad 11 µm feature in the SED and in the interferometric data and we propose potential formation mechanisms for these large clusters.

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Section: Target Selectionmentioning

confidence: 91%

Section: Target Selectionmentioning

confidence: 99%

Section: Abundances Of Aluminium-bearing Moleculesmentioning

confidence: 99%

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Study of the aluminium content in AGB winds using ALMA

Decin

Richards

Waters

et al. 2017

A&A

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“…Maercker et al 2016). We do not detect emission from the N = 29−28 transition in the HIFI spectrum of either TX Cam or R Cas (Fig.…”

Section: Overview Of the Samplementioning

confidence: 99%

“…Maercker et al 2016). The APEX and HIFI observations together cover six transitions of AlO towards R Dor: N = 5−4, .…”

Section: Overview Of the Samplementioning

confidence: 99%

Search for aluminium monoxide in the winds of oxygen-rich AGB stars

Beck

Decin

Ramstedt

et al. 2017

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Context. Aluminium monoxide (AlO) is likely efficiently depleted from the gas around oxygen-rich evolved stars to form alumina (Al 2 O 3 ) clusters and dust seeds. The presence of AlO gas in the extended atmospheres of evolved stars has been derived from optical spectroscopy. More recently, AlO gas was also detected at long wavelengths around the supergiant VY CMa and the oxygenrich asymptotic giant branch star o Cet (Mira A). The central role aluminium might play in dust formation and wind driving, in combination with these first detections of AlO at long wavelengths, shows the need for a wider search for this molecule in the winds of evolved stars. Aims. The detection at long wavelengths of emission in rotational transitions of AlO towards asymptotic giant branch stars can help constrain the presence and location of AlO gas in the outflows and ultimately also the efficiency of the depletion process. Methods. In search of AlO, we mined data obtained with APEX, the IRAM 30 m telescope, Herschel/HIFI, SMA, and ALMA, which were primarily aimed at studying other species around asymptotic giant branch stars. We report here on observations of AlO towards a sample of eight oxygen-rich asymptotic giant branch stars in different rotational transitions, up to seven for some stars.Results. We present definite detections of one rotational transition of AlO for o Cet and R Aqr, and tentative detections of one transition for R Dor and o Cet and two transitions for IK Tau and W Hya. The presented spectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R Aqr, and IK Tau, we find that the AlO (N = 9−8) emission likely traces the inner parts of the wind, out to only a few tens of AU, where the gas has not yet been accelerated to its terminal velocity. This is in agreement with recently published results from a detailed study on o Cet. Conclusions. The conclusive detections of AlO emission in the case of o Cet and R Aqr confirm the presence of AlO in the gas phase in outflows of asymptotic giant branch stars. The tentative detections further support this. Since most of the observations presented in this study were obtained with stronger emission from other species than AlO in mind, observations with higher sensitivity in combination with high angular resolution will improve our understanding of the presence and behaviour of AlO. From the current data sets we cannot firmly conclude whether there is a direct correlation between the wind properties and the detection rate of AlO emission. We hope that this study can serve as a stimulus to perform sample studies in search of AlO in oxygen-rich outflows.

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Mass loss of stars on the asymptotic giant branch

Höfner

Olofsson

2018

Astron Astrophys Rev

440

376

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As low-and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newlyformed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angularresolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their windforming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angularresolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building B Susanne Höfner

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A HIFI view on circumstellar H₂O in M-type AGB stars: radiative transfer, velocity profiles, and H₂O line cooling

Cited by 52 publications

References 53 publications

Study of the aluminium content in AGB winds using ALMA

Study of the aluminium content in AGB winds using ALMA

Search for aluminium monoxide in the winds of oxygen-rich AGB stars

Mass loss of stars on the asymptotic giant branch

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A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling

Cited by 52 publications

References 53 publications

Study of the aluminium content in AGB winds using ALMA

Study of the aluminium content in AGB winds using ALMA

Search for aluminium monoxide in the winds of oxygen-rich AGB stars

Mass loss of stars on the asymptotic giant branch

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Product

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A HIFI view on circumstellar H₂O in M-type AGB stars: radiative transfer, velocity profiles, and H₂O line cooling