EDP Sciences

Vol. 620
In section 6. Interstellar and circumstellar matter

An imaging spectroscopic survey of the planetary nebula NGC 7009 with MUSE

by J. R. Walsh, A. Monreal-Ibero, M. J. Barlow, et al., A&A 620, A169


This paper presents a comprehensive and remarkable application of integral field spectroscopic methods to the bright, O-rich iconic planetary nebula, NGC 7009 during the science verification observation with Multi Unit Spectroscopic Explorer (MUSE) ion VLT. The wide-field mode, with an FOV of about 1 arcmin^2, is an ideal match for this target. The authors present density, ionization, and abundance maps at a spatial resolution of about 0.7 arcsec (from seeing), although kinematic information cannot be obtained because the lines were unresolved in velocity (resolution of about 100 km/s). The spectral coverage was 4750-9300A, covering lines from three ionization stages of oxygen, two of sulfur (forbidden lines of S II and III), He I and II, Balmer and Paschen recombination lines), and a host of other forbidden lines, for instance [Mn V], [Cl III], [Ar II], and [N II], and even relatively weak recombination lines, for example, C II and N II. The authors present electron temperature and density maps using line ratio diagnostics and individual lines with different sensitivities; the Paschen jump is also used to obtain continuum measures of the plasma conditions (the appendix will be especially useful to future observers). Neutral helium line optical depth maps are also shown. Significant fine structure appears from comparisons among the different temperature and density maps. The authors show that the He abundance is constant to within about 2\% and that N/O varies considerably, although these still require ionization correction factors. They also compare integrated spectra with photoionization models. This pathfinder study demonstrates the unique and extensive capabilities of IFS observations to unravel complex regions and, extended to targets with larger velocity gradients, even to obtain three dimensional structures.

Vol. 620
In section 1. Letters to the Editor

Direct evidence of a full dipole flip during the magnetic cycle of a sun-like star

by S. Boro Saikia, T. Lueftinger, S. V. Jeffers, C. P. Folsom, V. See, P. Petit, S. C. Marsden, A. A. Vidotto, J. Morin, A. Reiners, M. Guedel, and the BCool collaboration A&A 620, L11


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Using spectropolarimetric observations, the authors have investigated the temporal evolution of the dipolar field of a sun-like star, the K dwarf 61 Cyg A. They have provided unprecedented sampling of the field over a single magnetic cycle of a star other than the Sun, and found that the behavior of the field during the cycle resembles the solar magnetic cycle. They show that 61 Cyg A has a dominant dipolar geometry except at chromospheric activity maximum. The dipole axis migrates from the southern to the northern hemisphere during the cycle. It is located at higher latitudes at chromospheric activity cycle minimum and mid-latitudes during cycle maximum. The dipole is the strongest at activity cycle minimum and much weaker at activity cycle maximum.

Vol. 620
In section 7. Stellar structure and evolution

Physical properties of AM CVn stars: New insights from Gaia DR2

by G. Ramsay, M. Green, T. R. Marsh, et al. A&A 620, A141


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AM CVn stars are important gravitational wave sources. The authors have determined absolute magnitudes and mass-transfer rates of AM CVn stars as function of orbital periods, which are crucial to our understanding of evolutionary channels and their space density.

Vol. 620
In section 7. Stellar structure and evolution

Gaia DR2 study of Herbig Ae/Be stars

by M. Vioque, R. D. Oudmaijer, D. Baines, et al.A&A 620, A128


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The authors use Gaia Data Release 2 (DR2) to place 252 Herbig Ae/Be stars in the HR diagram, and investigate their characteristics and properties. They provide masses, ages, luminosities, distances, photometric variabilities, and infrared excesses homogeneously derived for the most complete sample of Herbig Ae/Be stars to date. The authors find that high-mass stars (B-type above 7M⊙) have a much smaller infrared excess as well as a much lower optical variability compared to lower mass stars, and that ∼25% of all Herbig Ae/Be stars are strongly variable. They note that the strongly variable objects display doubly peaked Hα line profiles, indicating an edge-on disk. This supports the hypothesis that the variability is in most cases due to asymmetric dusty disk structures seen edge-on, as also observed in A-type UX Ori stars. While the break in dust properties occuring at 7M⊙ hints at different disk-dispersal mechanisms in A and B-type stars, various properties in the UV and optical spectral ranges differ at about 3M⊙ instead. This suggests that different accretion mechanisms are at work in low and high mass stars.

Vol. 620
In section 4. Extragalactic astronomy

Kinematics of the outer halo of M87 as mapped by planetary nebulae

by A. Longobardi, M. Arnaboldi, O. Gerhard, C. Pulsoni, and I. Soldner-Rembold A&A 620, A111


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Planetary nebulae (PN) are easy to detect through their ionized gas envelope, and because the Doppler effect on their ionized gas lines gives an overview of the kinematics of the stellar haloes of galaxies. The present work focuses on a sample of nearly 300 PN around the brightest central galaxy of the Virgo cluster, M87. The velocity dispersion and its radial distribution, reveal subcomponents of the halo, which can be identified according to the specific angular momentum of their stars. Close to M87 (inside 10kpc), the dispersion decreases from 400 to 270km/s, and then rises again to 300km/s until a radius of 60kpc, and then falls again to 100km/s at 135kpc. The specific angular momentum remains low, which confirms that M87 is a slow rotator. However the dispersion is quasi isotropic inside the M87 halo (up to 70-90kpc), while radial anisotropy is strong in the outer parts. In the intermediate radii, there is a shell in the sea of stars, also known as a "crown" component, which confirms previous observations from the same group. This crown could be made of the stars from a recently accreted satellite. Outside of the crown, the PN traces a medium which has the dynamics of the Virgo cluster, and more specifically the Virgo A sub-cluster. The various components seen in the outer M87 halo are a warning to interpret the kinematics in terms of masses in the outer parts.

Vol. 620
In section 9. The Sun

Ultrahigh-resolution model of a breakout CME embedded in the solar wind

by S. Hosteaux, E. Chané, B. Decraemer, et al. A&A 620, A57


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An ejection of mass and magnetic field from the Sun, in other words, a coronal mass ejection (CME), is a global disturbance of the heliosphere that can also affect the Earth's magnetosphere. The actual driver of the eruption, and therefore the predictability of such events, is still poorly understood. Traditionally, reconnection in the current sheet in the center of the flare that is associated with the event has been thought to play the main role. The new simulations at ultrahigh resolution presented here show that reconnection at the edge of the ejection is the main driver that launches the CME.