Chemical depletion in the Large Magellanic Cloud: RV Tauri stars and the photospheric feedback from their dusty discs

Abstract

Aims. By studying the photospheric abundances of 4 RV Tauri stars in the LMC, we test whether the depletion pattern of refractory elements, seen in similar Galactic sources, is also common for extragalactic sources. Since this depletion process probably only occurs through interaction with a stable disc, we investigate the circumstellar environment of these sources. Methods. A detailed photospheric abundance study was performed using high-resolution UVES optical spectra. To study the circumstellar environment we use photometric data to construct the spectral energy distributions of the stars, and determine the geometry of the circumstellar environment, whereas low-resolution Spitzer-IRS infrared spectra are used to trace its mineralogy. Results. Our results show that, also in the LMC, the photospheres of RV Tauri stars are commonly affected by the depletion process, although it can differ significantly in strength from source to source. From our detailed disc modelling and mineralogy study, we find that this process, as in the Galaxy, appears closely related to the presence of a stable Keplerian disc. The newly studied extragalactic objects have similar observational characteristics as Galactic post-AGB binaries surrounded by a dusty disc, and are therefore also believed to be part of a binary system. One source shows a very small infrared excess, atypical for a disc source, but still has evidence for depletion. We speculate this could point to the presence of a very evolved disc, similar to debris discs seen around young stellar objects.

IRAS 11472−0800: an extremely depleted pulsating binary post-AGB star

“…Interferometric studies confirm the very compact nature of the circumstellar material around the disc sources (Deroo et al 2006(Deroo et al , 2007, and the infrared spectroscopic data show a very high processing of the circumstellar dust grains (Gielen et al 2007(Gielen et al , 2008(Gielen et al , 2009(Gielen et al , 2011. Our radial velocity programme is still ongoing, but we indeed confirmed the suspected high binary rate: for non-pulsating (or low-amplitude pulsating) objects, a binary rate of 100% was found ).…”

The optically bright post-AGB population of the LMC

“…disc and shell sources, or by mass-loss from the exterior. As in the Galaxy, some disc sources in the LMC show the effect of accretion (Reyniers & Van Winckel 2007;Gielen et al 2009), which increases the mass of the Hrich envelope and slows down the evolutionary timescale of the central star. Because the discs are stable, the IR lifetime of these objects is expected to be much longer than for shell objects (Van Winckel 2003), which allows strong grain processing to occur.…”

“…The discs are spatially resolved, but this is only detectable using interferometric techniques (Deroo et al 2006(Deroo et al , 2007, and they typically have a radius of only 50 AU at 10 μm. The high dust-grain processing is an observational indication that the compact discs are stable (De Ruyter et al 2005;Gielen et al 2008Gielen et al , 2009. Since the discovery of the binary nature of a small sample of the most depleted objects (Van Winckel et al 1995), this binary nature of the disc sources has been further confirmed ).…”

Eccentricity-pumping in post-AGB stars with circumbinary discs

“…from a CB disc, the envelope ejection could take much longer and extend the post-AGB lifetime. From the observed strongly depleted post-AGB stars in the LMC (Reyniers & van Winckel 2007;Gielen et al 2009), we can infer a dilution of their envelopes by a factor of about 10. As the envelope mass is about 10 −3 M (Bloecker 1995), assuming a fully convective envelope, 10 −2 M must have been accreted from a CB disc.…”