Abstract
Context. Asymptotic giant branch (AGB) stars are characterised by complex stellar surface dynamics that affect the measurements and amplify the uncertainties on stellar parameters. The uncertainties in observed absolute magnitudes have been found to originate mainly from uncertainties in the parallaxes. The resulting motion of the stellar photocentre could have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the convection-related surface structure in AGBs on the photocentric variability. We quantify these effects to characterise the observed parallax errors and estimate fundamental stellar parameters and dynamical properties. Methods. We use three-dimensional (3D) radiative hydrodynamics simulations of convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G band [325–1030 nm]. From those maps, we calculate the intensity-weighted mean of all emitting points tiling the visible stellar surface (i.e. the photocentre) and evaluate its motion as a function of time. We extract the parallax error from Gaia data-release 2 (DR2) for a sample of semi-regular variables in the solar neighbourhood and compare it to the synthetic predictions of photocentre displacements. Results. AGB stars show a complex surface morphology characterised by the presence of few large-scale long-lived convective cells accompanied by short-lived and small-scale structures. As a consequence, the position of the photocentre displays temporal excursions between 0.077 and 0.198 AU (≈5 to ≈11% of the corresponding stellar radius), depending on the simulation considered. We show that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error of our sample of semi-regular variables. Finally, we present evidence for a correlation between the mean photocentre displacement and the stellar fundamental parameters: surface gravity and pulsation. We suggest that parallax variations could be exploited quantitatively using appropriate radiation-hydrodynamics (RHD) simulations corresponding to the observed star.
Highlights
Low- to intermediate-mass stars evolve to red giant and asymptotic giant branch increasing the mass-loss during this evolution
We show that the convection-related variability accounts for a substantial part of the Gaia data-release 2 (DR2) parallax error of our sample of semi-regular variables
We suggest that parallax variations could be exploited quantitatively using appropriate radiation-hydrodynamics (RHD) simulations corresponding to the observed star
Summary
Low- to intermediate-mass stars evolve to red giant and asymptotic giant branch increasing the mass-loss during this evolution They are characterised: (i) by large-amplitude variations in radius, brightness, and temperature of the star; and (ii) by a strong mass-loss rate driven by an interplay between pulsation, dust formation in the extended atmosphere, and radiation pressure on the dust (Höfner & Olofsson 2018). Their complex dynamics affect the measurements and amplify the uncertainties on stellar parameters. Important information about stellar properties, such as the fundamental stellar parameters, may be hidden behind the Gaia measurement uncertainty
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
