Disc-binary interactions in depleted post-AGB binaries

Binary post-asymptotic giant branch (post-AGB) stars have orbital periods in the range of 100--2500 days in eccentric orbits. They are surrounded by circumbinary dusty discs. They are the immediate result of unconstrained binary interaction processes. Their observed orbital properties do not correspond to model predictions: Neither the periods nor the high eccentricities are expected. Our goal is to investigate if interactions between a binary and its circumbinary disc during the post-AGB phase can result in their eccentric orbits, while simultaneously explaining the chemical anomaly known as depletion. For this paper, we selected three binaries (EP Lyr, RU Cen, HD 46703) with well-constrained orbits, luminosities, and chemical abundances. We used the MESA code to evolve post-AGB models, while including the accretion of metal-poor gas. This allows us to constrain the evolution of the stars and study the impact of circumbinary discs on the orbital properties of the models. We investigate the effect of torques produced by gas inside the binary cavity and the effect of Lindblad resonances on the orbit, while also including the tidal interaction following the equilibrium tide model. We find that none of our models are able to explain the high orbital eccentricities of the binaries in our sample. The accretion torque does not significantly impact the binary orbit, while Lindblad resonances can pump the eccentricity up to only $e \approx 0.2$. At higher eccentricities, the tidal interaction becomes too strong, so the high observed eccentricities cannot be reproduced. However, even if we assume tides to be ineffective, the eccentricities in our models do not exceed $\approx 0.25$. We conclude that either our knowledge of disc-binary interactions is still incomplete, or the binaries must have left their phase of strong interaction in an eccentric orbit.Comment: 17 pages + 8 pages appendix, 12 figures, accepted for publication in Astronomy and Astrophysic

Orbital properties of binary post-AGB stars

Binary post-asymptotic giant branch (post-AGB) stars are thought to be the products of a strong but poorly-understood interaction during the AGB phase. The aim of this contribution is to update the orbital elements of a sample of galactic post-AGB binaries observed in a long-term radial-velocity monitoring campaign. Radial velocities are computed from high signal-to-noise spectra by use of a cross-correlation method. The radial-velocity curves are fitted by using both a least-squares algorithm and a Nelder-Mead simplex algorithm. We use a Monte Carlo method to compute uncertainties on the orbital elements. The resulting mass functions are used to derive a companion mass distribution by optimising the predicted to the observed cumulative mass-function distributions, after correcting for observational bias. As a result, we derive and update orbital elements for 33 galactic post-AGB binaries, among which 3 are new orbits. The orbital periods of the systems range from 100 to about 3000 days. Over 70 percent (23 out of 33) of our binaries have significant non-zero eccentricities ranging over all periods. Their orbits are non-circular despite the fact that the Roche-lobe radii are smaller than the maximum size of a typical AGB star and tidal circularisation should have been strong when the objects were on the AGB. We derive a distribution of companion masses that is peaked around 1.09 $M_\odot$ with a standard deviation of 0.62 $M_\odot$. The large spread in companion masses highlights the diversity of post-AGB binary systems. Furthermore, we find that only post-AGB stars with high effective temperatures (> 5500 K) in wide orbits are depleted in refractory elements, suggesting that re-accretion of material from a circumbinary disc is an ongoing process. It appears, however, that chemical depletion is inefficient for the closest orbits irrespective of the actual surface temperature.Comment: 21 pages total, 3 appendices, 8 figures excluding appendix figures, 3 table

Bias correction of OMI HCHO columns based on FTIR and aircraft measurements and impact on top-down emission estimates

Spaceborne formaldehyde (HCHO) measurements constitute an excellent proxy for the sources of non-methane volatile organic compounds (NMVOCs). Past studies suggested substantial overestimations of NMVOC emissions in state-of-the-art inventories over major source regions. Here, the QA4ECV (Quality Assurance for Essential Climate Variables) retrieval of HCHO columns from OMI (Ozone Monitoring Instrument) is evaluated against (1) FTIR (Fourier-transform infrared) column observations at 26 stations worldwide and (2) aircraft in situ HCHO concentration measurements from campaigns conducted over the USA during 2012–2013. Both validation exercises show that OMI underestimates high columns and overestimates low columns. The linear regression of OMI and aircraft-based columns gives Ω$_{OMI}$=0,651 Ω$_{airc}$+2,95 x 10$^{15}$, molec. cm$^{-2}$ , with Ω$_{OMI}$ and Ω$_{airc}$ the OMI and aircraft-derived vertical columns, whereas the regression of OMI and FTIR data gives Ω$_{OMI}$= 6,59 Ω$_{FTIR}$ + 2.02 x 10$^{15}$, molec. cm$^{-2}$ . Inverse modelling of NMVOC emissions with a global model based on OMI columns corrected for biases based on those relationships leads to much-improved agreement against FTIR data and HCHO concentrations from 11 aircraft campaigns. The optimized global isoprene emissions ($\sim$ 445 Tgyr$^{-1}$) are 25 % higher than those obtained without bias correction. The optimized isoprene emissions bear both striking similarities and differences with recently published emissions based on spaceborne isoprene columns from the CrIS (Cross-track Infrared Sounder) sensor. Although the interannual variability of OMI HCHO columns is well understood over regions where biogenic emissions are dominant, and the HCHO trends over China and India clearly reflect anthropogenic emission changes, the observed HCHO decline over the southeastern USA remains imperfectly elucidated

Evolutionary properties of post-AGB binaries

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Orbital properties of binary post-AGB stars

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Modelling depletion by re-accretion of gas from a dusty disc in post-AGB stars

Many disc-type post-asymptotic giant branch (post-AGB) stars are chemically peculiar, showing underabundances of refractory elements in their photospheres that correlate with condensation temperature. The aim of this paper is to investigate how accretion from a circumbinary disc can cause this phenomenon of depletion and how this impacts the evolution of post-AGB stars. We used the MES

Disc-binary interactions in depleted post-AGB binaries

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Modelling depletion by re-accretion of gas from a dusty disc in post-AGB stars

status: publishe

Orbital properties of binary post-AGB stars

© ESO 2018. Binary post-Asymptotic giant branch (post-AGB) stars are thought to be the products of a strong but poorly understood interaction during the AGB phase. The aim of this contribution is to update the orbital elements of a sample of galactic post-AGB binaries observed in a long-Term radial-velocity monitoring campaign by analysing these systems in a homogeneous way. Radial velocities are computed from high signal-To-noise spectra via a cross-correlation method. The radial-velocity curves are fitted by using both a least-squares algorithm and a Nelder-Mead simplex algorithm. We use a Monte Carlo method to compute uncertainties on the orbital elements. The resulting mass functions are used to derive a companion mass distribution by optimising the predicted to the observed cumulative mass-function distributions, after correcting for observational bias. As a result, we derive and update orbital elements for 33 galactic post-AGB binaries, among which 3 are new orbits. The orbital periods of the systems range from 100 to about 3000 days. Over 70% (23 out of 33) of our binaries have significant non-zero eccentricities ranging over all periods. Their orbits are non-circular even though the Roche-lobe radii are smaller than the maximum size of a typical AGB star and tidal circularisation should have been strong when the objects were on the AGB. We derive a distribution of companion masses that is peaked around 1.09 Mâ with a standard deviation of 0.62 Mâ. The large spread in companion masses highlights the diversity of post-AGB binary systems. Post-AGB binaries are often chemically peculiar, showing in their photospheres the result of an accretion process of circumstellar gas devoid of refractory elements. We find that only post-AGB stars with high effective temperatures (> 5500 K) in wide orbits are depleted in refractory elements, suggesting that re-Accretion of material from a circumbinary disc is an ongoing process. It appears, however, that depletion is inefficient for the closest orbits irrespective of the actual surface temperature.status: publishe

Spectroscopic binaries RV Tauri and DF Cygni

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