Time-resolved spectroscopy of BD+46°442: Gas streams and jet creation in a newly discovered evolved binary with a disk

Abstract

Context. Previous studies have shown that many post-asymptotic giant branch (AGB) stars with dusty disks are associated with single-lined binary stars. The inferred orbital separations are too small to accommodate a fully grown AGB star, hence these systems represent a new evolutionary channel that bypasses a full AGB evolution. Aims. We wish to verify the binarity hypothesis for a larger sample establish the nature of the companions, and probe the disk structure and eventually the disk formation mechanisms in binary stars. To achieve these aims, we started a high-resolution spectral monitoring of ∼40 field giants whose binarity had been suspected based on either a light curve, an infrared excess, or a peculiar chemical composition. Methods. Starting from the spring of 2009, we monitored the programme stars with the fibre echelle spectrometer HERMES. We measure their radial velocities (RVs) with a precision of ∼0.2 km s −1 , perform detailed photospheric abundance analyses, and analyse the time-resolved high-resolution spectra to search for line-profile variability.Results. Here we report on the discovery of periodic RV variations in BD+46 • 442, a high Galactic latitude F giant with a disk. We infer that the variations are caused by the motion around a faint companion, and deduce the orbital parameters P orb = 140.77 ± 0.02 d , e = 0.083± 0.002, and a sin i = 0.31 AU. We find that it is a moderately metal-poor star ([M/H] = −0.7) without a strong depletion pattern in its photospheric abundances. Interestingly, many lines indeed show periodic changes with the orbital phase: Hα switches between a double-peak emission line and a P Cyg-like profile, while strong metal lines appear to be split at the maximum redshift. Similar effects are likely visible in the spectra of other post-AGB binaries, but their regularity is not always apparent owing to sporadic observations. We propose that these features result from an ongoing mass transfer from the evolved giant to the companion. In particular, the blueshifted absorption in Hα, which occurs only at superior conjunction, may result from a jet originating in the accretion disk around the companion and that is seen in absorption towards the luminous primary.

Why is the Red Rectangle Unique?

“…This is illustrated in Fig. 4 which shows time-resolved H α profiles of a binary (Gorlova et al 2012). At conjunction, a blue absorption appears when the jet obscures the background luminous star and continuum photons are scattered outside the line-of-sight.…”

“…This is similar to what is observed in the Red Rectangle , in a system which is seen under a different inclination. Data from Gorlova et al (2012). sources are being found (van Aarle et al 2011, Kamath, Wood & Van Winckel 2013 and they represent about half of the optically bright post-AGB candidates in these galaxies.…”

On the eclipsing binary ELHC 10 with occulting dark disc in the Large Magellanic Cloud

“…Contrary to ELHC 10, this last system shows strong orbital variability at the Hα emission; this line has been interpreted as formed in the disc around the secondary and the Hα blue-shifted absorption in terms of a disc jet (Gorlova et al 2012). …”

“…Evidence for interaction has been observed in the Galactic post-AGB star BD+46 o 442, an evolved binary with gas streams, jets and disc (Gorlova et al 2012). Contrary to ELHC 10, this last system shows strong orbital variability at the Hα emission; this line has been interpreted as formed in the disc around the secondary and the Hα blue-shifted absorption in terms of a disc jet (Gorlova et al 2012).…”

The Mercator telescope: relevance, status, and future

“…So far, this resulted in more than 140 scientific publications, (partly) based on HERMES data. Highlights of the HERMES exploitation range from the discovery of the extended overshoot of the convective core of a massive B-star [9], deriving the internal rotation profile of the core of a red giant [10], the surprising discovery of long-period eccentric binaries among sub-dwarf B stars [11,12], the mode identification of single and binary pulsators [13,14,15], the discovery of the binary properties of red giant pulsators in the Kepler field and also often in eccentric systems [16,17], the circumstellar gas flows in binary stars with interacting winds or accretion flows [18,19], to the first discovery of the long sought wide binaries among central stars of Planetary Nebulae [20]. These are only some illustrative example of science programmes that require the unique monitoring capabilities of HERMES on the Mercator telescope.…”