EDP Sciences

Vol. 602
In section 6. Interstellar and circumstellar matter

SPHERE/ZIMPOL observations of the symbiotic system R Aquarii. I. Imaging of the stellar binary and the innermost jet clouds

by H.M. Schmid, A. Bazzon, J. Milli, et al. A&A 602, A53

The symbiotic binary system R Aqr consists of a Mira variable, a hot companion with a spectacular jet outflow, and an extended emission line nebula. The authors observed R Aqr in the Halpha and other filters with the visual camera subsystem ZIMPOL, which is part of the new extreme adaptive optics (AO) instrument SPHERE at the VLT. They analyzed the high resolution ≈ 25 mas images from SPHERE-ZIMPOL and determined the position, size, geometric structure, and line fluxes of the jet source and the clouds in the innermost region 2′′ ( 400 AU) f R Aqr. They resolved for the first time the R Aqr binary and found that the jet source is 46 ± 1 mas west (position angle −85.5◦ ± 1.0◦) of the Mira star. The central jet source is the strongest Hα component. Within 0.5′′ (100 AU) in the SW of the central source, they observed a string of bright clouds arranged in a zig-zag pattern, and further out, at 1′′ − 2′′, fainter and more extended bubbles. In the N and NE they found a bright, very elongated filamentary structure between 0.2′′ − 0.7′′ and faint perpendicular “wisps” further out. Cloud densities are very high, implying a short recombination timescale of a year or less.

Vol. 602
In section 7. Stellar structure and evolution

Kepler sheds new and unprecedented light on the variability of a blue supergiant: Gravity waves in the O9.5Iab star HD188209

by C. Aerts, S. Símon-Díaz, S. Bloemen, et al. A&A 602, A32

Space missions such as CoRoT and Kepler have had a tremendous impact on asteroseismological studies of low mass stars. Higher-mass stars (> 8 M_sun) and their evolved descendants have not been as amenable to similar scrutiny primarily because their oscillation periods are longer, leading to multi-periodic beating patterns with periods of several years, thus rendering observations unfeasible. For massive O and B supergiants, the situation is further complicated by the presence of other phenomena such as strong stellar winds. Although the saturation limit for the Kepler CCDs is ~11.5 mag., Aerts et al. were able to observe a 5.5 mag. O supergiant, HD188209, using a strategic placement of masks, thereby enabling a study of the target in scattered light over a time period of about four years. Extensive ground-based spectroscopy was also acquired. By combining these unique datasets, the authors were able to infer the presence of convectively driven internal gravity waves, making HD188209 the first case of a massive supergiant for which such variability has been observed.

Vol. 601
In section 1. Letters

The HI content of isolated ultra-diffuse galaxies: A sign of multiple formation mechanisms?

by E Papastergis, E.A.K. Adams, and A.J. Romanowsky A&A 601, L10


Ultra-diffuse galaxies (UDG) are distinctive because of their extremely low surface density of stars, with respect to the mass and size of their dark matter halo. The latter are derived from their relatively large rotational velocity, and large size. UDG are very abundant in galaxy clusters, and one of the best scenarios is that they have been stripped of their gas by the cluster environment, which stopped their star formation. In isolation, they could be quenched by starburst-driven outflows. The present article reports of 21cm observations of the HI component in four isolated UDG, leading to three upper limits and one detection. Combining with the literature, this means six field UDG have been observed, with three upper limits. The observed dichotomy in the gaseous properties of field UDGs is interpreted as a sign of the existence of multiple mechanisms for their formation, with the formation of the quiescent gas-poor UDGs remaining a mystery.

Vol. 601
In section 5. Galactic structure, stellar clusters and populations

The Gaia-ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations

byA. Rojas-Arriagada, A. Recio-Blanco, P. de Laverny, et al. A&A 601, A140

This paper uses the fourth internal data release of the Gaia-ESO spectroscopic survey to characterize the properties of 2500 individual red clump stars in the Galactic bulge, sampling the range of longitude from -10deg to +8deg and latitude from -10deg to -4deg. This is the first time that such a large and homogeneous sample, over such a wide area of the bulge, has been analyzed looking at [Fe/H] and [Mg/Fe] with such high resolution and high S/N spectra. The authors present the spatial distribution of these measurements, and the correlation of these properties with kinematics. The metalicity distribution function of the bulge is confirmed to be bimodal across the whole sampled area, with metal-poor stars dominating at higher latitudes. The metal-poor stars have isotropic hot kinematics, not consistent with an X-shaped bulge, that could be the product of an early prompt dissipative collapse dominated by massive stars. The population shows a knee in the [Mg/Fe] measurements at higher [Fe/H] than in the thick disk, which suggests a higher star-formation rate. In contrast, the metal-rich stars exhibit bar-like kinematics and appear tightly associated with an X-shape bulge. These metal-rich stars also overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. The bar boxy-peanut bulge thus seems likely to have formed from the secular evolution of the early thin disk. The authors present a chemical evolution model for the entire bulge that requires a fast (< 1 Gyr) intense burst of stellar formation at early epochs.

Vol. 601
In section 9. The Sun

Simulating Ellerman bomb-like events

by S. Danilovic A&A 601, A122


Discovered exactly a century ago, Ellerman bombs are prominent brightenings in seen in H-alpha observations of newly emerging active regions (sunspots) on the Sun. These events play a crucial role in our understanding of the magnetic reconnection on the Sun, and cartoon scenarios have been drawn for decades. This this paper shows in a 3D MHD simulation how these events form in a self-consistent, way and highlights that solar convection pushes together magnetic field of opposite magnetic polarity and through this initiates reconnection in the photosphere of the Sun. The H-alpha emission derived from the simulations shows the same characteristic flame-like appearance as that found in observations.

Vol. 601
In section 10. Planets and planetary systems

Evidence for depletion of heavy silicon isotopes at comet 67P/Churyumov-Gerasimenko

by M. Rubin, K. Altwegg, H. Balsiger, et al. A&A 601, A123


The heterogeneity of comets as carriers of primordial abundances is highlighted by the identification of the silicon isotopic ratio variations between comets - especially from this detailed, in situ study. Most importantly, the isotopic ratios point to different origins, espcially among the AGB stars and supernovae. While this has been known for some time from meteorites, this study is a first for the in situ sampling of a comet (thought to be the most primitive material in the solar system). It is important that the silicon is a far less chemistry-dependent tracer of the source(s) than the CNO isotopes.