TY - JOUR A1 - Sandin, C. A1 - Steffen, M. A1 - Schoenberner, D. A1 - Rühling, Ute T1 - Hot bubbles of planetary nebulae with hydrogen-deficient winds I. Heat conduction in a chemically stratified plasma JF - Frontiers in psychology N2 - Heat conduction has been found a plausible solution to explain discrepancies between expected and measured temperatures in hot bubbles of planetary nebulae (PNe). While the heat conduction process depends on the chemical composition, to date it has been exclusively studied for pure hydrogen plasmas in PNe. A smaller population of PNe show hydrogen-deficient and helium-and carbon-enriched surfaces surrounded by bubbles of the same composition; considerable differences are expected in physical properties of these objects in comparison to the pure hydrogen case. The aim of this study is to explore how a chemistry-dependent formulation of the heat conduction affects physical properties and how it affects the X-ray emission from PN bubbles of hydrogen-deficient stars. We extend the description of heat conduction in our radiation hydrodynamics code to work with any chemical composition. We then compare the bubble-formation process with a representative PN model using both the new and the old descriptions. We also compare differences in the resulting X-ray temperature and luminosity observables of the two descriptions. The improved equations show that the heat conduction in our representative model of a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent description; a lower value on the diffusion coefficient is compensated by a slightly steeper temperature gradient. The bubble becomes somewhat hotter with the improved equations, but differences are otherwise minute. The observable properties of the bubble in terms of the X-ray temperature and luminosity are seemingly unaffected. KW - conduction KW - hydrodynamics KW - planetary nebulae: general KW - stars: AGB and post-AGB KW - stars: Wolf-Rayet KW - X-rays: stars Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201527357 SN - 1432-0746 VL - 586 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Keles, Engin A1 - Kitzmann, Daniel A1 - Mallonn, Matthias A1 - Alexoudi, Xanthippi A1 - Fossati, Luca A1 - Pino, Lorenzo A1 - Seidel, Julia Victoria A1 - Caroll, Thorsten A. A1 - Steffen, M. A1 - Ilyin, Ilya A1 - Poppenhäger, Katja A1 - Strassmeier, Klaus G. A1 - von Essen, Carolina A1 - Nascimbeni, Valerio A1 - Turner, Jake D. T1 - Probing the atmosphere of HD189733b with the Na i and K i lines JF - Monthly Notices of the Royal Astronomical Society N2 - High spectral resolution transmission spectroscopy is a powerful tool to characterize exoplanet atmospheres. Especially for hot Jupiters, this technique is highly relevant, due to their high-altitude absorption, e.g. from resonant sodium (Na i) and potassium (K i) lines. We resolve the atmospheric K i absorption on HD189733b with the aim to compare the resolved K i line and previously obtained high-resolution Na i-D line observations with synthetic transmission spectra. The line profiles suggest atmospheric processes leading to a line broadening of the order of ∼10 km/s for the Na i-D lines and only a few km/s for the K i line. The investigation hints that either the atmosphere of HD189733b lacks a significant amount of K i or the alkali lines probe different atmospheric regions with different temperature, which could explain the differences we see in the resolved absorption lines. Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa2435 VL - 498 IS - 1 SP - 1033 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Steffen, M. A1 - Hubrig, Swetlana A1 - Todt, Helge Tobias A1 - Schoeller, M. A1 - Hamann, Wolf-Rainer A1 - Sandin, Christer A1 - Schönberner, Detlef T1 - Weak magnetic fields in central stars of planetary nebulae? JF - Astronomy and astrophysics : an international weekly journal N2 - Context. It is not yet clear whether magnetic fields play an essential role in shaping planetary nebulae (PNe), or whether stellar rotation alone and/or a close binary companion, stellar or substellar, can account for the variety of the observed nebular morphologies. Aims. In a quest for empirical evidence verifying or disproving the role of magnetic fields in shaping planetary nebulae, we follow up on previous attempts to measure the magnetic field in a representative sample of PN central stars. Methods. We obtained low-resolution polarimetric spectra with FORS2 installed on the Antu telescope of the VLT for a sample of 12 bright central stars of PNe with different morphologies, including two round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets are Wolf-Rayet type central stars. Results. For the majority of the observed central stars, we do not find any significant evidence for the existence of surface magnetic fields. However, our measurements may indicate the presence of weak mean longitudinal magnetic fields of the order of 100 Gauss in the central star of the young elliptical planetary nebula IC 418 as well as in the Wolf-Rayet type central star of the bipolar nebula Hen 2-113 and the weak emission line central star of the elliptical nebula Hen 2-131. A clear detection of a 250 G mean longitudinal field is achieved for the A-type companion of the central star of NGC 1514. Some of the central stars show a moderate night-to-night spectrum variability, which may be the signature of a variable stellar wind and/or rotational modulation due to magnetic features. Conclusions. Since our analysis indicates only weak fields, if any, in a few targets of our sample, we conclude that strong magnetic fields of the order of kG are not widespread among PNe central stars. Nevertheless, simple estimates based on a theoretical model of magnetized wind bubbles suggest that even weak magnetic fields below the current detection limit of the order of 100 G may well be sufficient to contribute to the shaping of the surrounding nebulae throughout their evolution. Our current sample is too small to draw conclusions about a correlation between nebular morphology and the presence of stellar magnetic fields. KW - planetary nebulae: general KW - stars: magnetic field KW - stars: AGB and post-AGB KW - binaries: close KW - techniques: polarimetric Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201423842 SN - 0004-6361 SN - 1432-0746 VL - 570 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Guerrero, Martín A. A1 - Ruiz, N. A1 - Hamann, Wolf-Rainer A1 - Chu, Y.-H. A1 - Todt, Helge Tobias A1 - Schönberner, Detlef A1 - Oskinova, Lida A1 - Gründl, R. A. A1 - Steffen, M. A1 - Blair, William P. A1 - Toala, Jesús Alberto T1 - Rebirth of X-Ray emission from the born-again planetary Nebula A30 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The planetary nebula A30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch Hubble Space Telescope images, we have detected the angular expansion of these knots and derived an age of 850(-150)(+280) yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton deep observations of A30. The X-ray emission from A30 can be separated into two components: a point source at the central star and diffuse emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters and wind properties of the central star of A30 using a non-LTE model fit to its optical and UV spectra. The spatial distribution and spectral properties of the diffuse X-ray emission are highly suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. This emission can be attributed to shock-heated plasma, as the hydrogen-poor knots are ablated by the stellar winds, under which circumstances the efficient mass loading of the present fast stellar wind raises its density and damps its velocity to produce the observed diffuse soft X-rays. Charge transfer reactions between the ions of the stellar winds and material of the born-again ejecta have also been considered as a possible mechanism for the production of diffuse X-ray emission, and upper limits on the expected X-ray production by this mechanism have been derived. The origin of the X-ray emission from the central star of A30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible. KW - planetary nebulae: general KW - planetary nebulae: individual (A30) KW - stars: winds, outflows KW - X-rays: ISM Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/755/2/129 SN - 0004-637X VL - 755 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER -