Element abundance ratios in the quiet Sun transition region [SSA]


Element abundance ratios of magnesium to neon (Mg/Ne) and neon to oxygen (Ne/O) in the transition region of the quiet Sun have been derived by re-assessing previously published data from the Coronal Diagnostic Spectrometer on board the Solar and Heliospheric Observatory in the light of new atomic data. The quiet Sun Mg/Ne ratio is important for assessing the effect of magnetic activity on the mechanism of the first ionization potential (FIP) effect, while the Ne/O ratio can be used to infer the solar photospheric abundance of neon, which can not be measured directly. The average Mg/Ne ratio is found to be $0.52\pm 0.11$, which applies over the temperature region 0.2–0.7~MK, and is consistent with the earlier study. The Ne/O ratio is, however, about 40\%\ larger, taking the value $0.24\pm 0.05$ that applies to the temperature range 0.08–0.40~MK. The increase is mostly due to changes in ionization and recombination rates that affect the equilibrium ionization balance. If the Ne/O ratio is interpreted as reflecting the photospheric ratio, then the photospheric neon abundance is $8.08\pm 0.09$ or $8.15\pm 0.10$ (on a logarithmic scale for which hydrogen is 12), according to whether the oxygen abundances of M.~Asplund et al.\ or E.~Caffau et al.\ are used. The updated photospheric neon abundance implies a Mg/Ne FIP bias for the quiet Sun of $1.6\pm 0.6$.

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P. Young
Fri, 19 Jan 18

Comments: Submitted to ApJ, 10 pages, 3 figures

$τ$ Ori and $τ$ Lib: Two new massive heartbeat binaries [SSA]


We report the discovery of two massive eccentric systems with BRITE data, $\tau$ Ori and $\tau$ Lib, showing heartbeat effects close to the periastron passage. $\tau$ Lib exhibits shallow eclipses that will soon vanish due to the apsidal motion in the system. In neither system, tidally excited oscillations were detected.

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A. Pigulski, M. Kaminska, K. Kaminski, et. al.
Fri, 19 Jan 18

Comments: 3 pages, 1 figure, to appear in the Proceedings of the 3rd BRITE Science Conference

Magnetic braids in eruptions of a spiral structure in the solar atmosphere [SSA]


We report on high-resolution imaging and spectral observations of eruptions of a spiral structure in the transition region, which were taken with the Interface Region Imaging Spectrometer (IRIS), the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI). The eruption coincided with the appearance of two series of jets, with velocities comparable to the Alfv\’en speeds in their footpoints. Several pieces of evidence of magnetic braiding in the eruption are revealed, including localized bright knots, multiple well-separated jet threads, transition region explosive events and the fact that all these three are falling into the same locations within the eruptive structures. Through analysis of the extrapolated three-dimensional magnetic field in the region, we found that the eruptive spiral structure corresponded well to locations of twisted magnetic flux tubes with varying curl values along their lengths. The eruption occurred where strong parallel currents, high squashing factors, and large twist numbers were obtained. The electron number density of the eruptive structure is found to be $\sim3\times10^{12}$ cm$^{-3}$, indicating that significant amount of mass could be pumped into the corona by the jets. Following the eruption, the extrapolations revealed a set of seemingly relaxed loops, which were visible in the AIA 94 \AA\ channel indicating temperatures of around 6.3 MK. With these observations, we suggest that magnetic braiding could be part of the mechanisms explaining the formation of solar eruption and the mass and energy supplement to the corona.

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Z. Huang, L. Xia, C. Nelson, et. al.
Fri, 19 Jan 18

Comments: 11 figs, accepted for publication in ApJ

The Of?p stars of the Magellanic Clouds: Are they strongly magnetic? [SSA]


All known Galactic Of?p stars have been shown to host strong, organized, magnetic fields. Recently, five Of?p stars have been discovered in the Magellanic Clouds. They posses photometric \citep{Naze} and spectroscopic \citep{Walborn} variability compatible with the Oblique Rotator Model (ORM). However, their magnetic fields have yet to be directly detected. We have developed an algorithm allowing for the synthesis of photometric observables based on the Analytic Dynamical Magnetosphere (ADM) model of \citet{Owocki}. We apply our model to OGLE photometry in order to constrain their magnetic geometries and surface dipole strengths. We predict that the field strengths for some of these candidate extra-Galactic magnetic stars may be within the detection limits of the FORS2 instrument.

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M. Munoz, G. Wade, Y. Naze, et. al.
Fri, 19 Jan 18

Comments: 5 pages, 2 figures

Where can a Trappist-1 planetary system be produced? [EPA]


We study the evolution of protoplanetary discs that would have been precursors of a Trappist-1 like system under the action of accretion and external photoevaporation in different radiation environments. Dust grains swiftly grow above the critical size below which they are entrained in the photoevaporative wind, so although gas is continually depleted, dust is resilient to photoevaporation after only a short time. This means that the ratio of the mass in solids (dust plus planetary) to the mass in gas rises steadily over time. Dust is still stripped early on, and the initial disc mass required to produce the observed $4\,M_{\oplus}$ of Trappist-1 planets is high. For example, assuming a Fatuzzo & Adams (2008) distribution of UV fields, typical initial disc masses have to be $>30\,$per cent the stellar (which are still Toomre $Q$ stable) for the majority of similar mass M dwarfs to be viable hosts of the Trappist-1 planets. Even in the case of the lowest UV environments observed, there is a strong loss of dust due to photoevaporation at early times from the weakly bound outer regions of the disc. This minimum level of dust loss is a factor two higher than that which would be lost by accretion onto the star during 10 Myr of evolution. Consequently even in these least irradiated environments, discs that are viable Trappist-1 precursors need to be initially massive ($>10\,$per cent of the stellar mass).

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T. Haworth, S. Facchini, C. Clarke, et. al.
Fri, 19 Jan 18

Comments: 15 pages. Accepted for publication in MNRAS

Spectroscopic and photometric confirmation of chromospheric activity in four stars [SSA]


We present analysis of medium resolution optical spectra and long term V band photometry of four cool stars, BD+13 5000, BD+11 3024, TYC 3557-919-1 and TYC 5163-1764-1. Our spectroscopic analysis reveals that the stars are giant or sub-giant from K0 or K1 spectral type, and all of them exhibit emission features in their Ca ii H& K lines. These features appear to be modulated with the rotation of the stars. Except BD+11 3024, we observe that the radial velocities of the target stars are not stable, which suggests that each of them might be a member of a binary system. Global analysis of photometric data indicates clear cyclic variation for BD+13 5000 and TYC 5163-1764-1 with a period of 8.0$\pm$0.3 and 5.04$\pm$0.04 year, respectively. Besides that, we observe a dramatic increase ($\sim$0\fm7) in the mean brightness of BD+11 3024, accompanied with a 2.87$\pm$0.12 cyclic variation, embedded into the global brightening trend, which indicates possible multiple cycles on this star.

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O. Ozdarcan and H. Dal
Fri, 19 Jan 18

Comments: 10 pages, 6 figures, 5 tables, accepted for publication in AN

Sun-to-Earth MHD Simulation of the 14 July 2000 "Bastille Day" Eruption [SSA]


Solar eruptions are the main driver of space-weather disturbances at the Earth. Extreme events are of particular interest, not only because of the scientific challenges they pose, but also because of their possible societal consequences. Here we present a magnetohydrodynamic (MHD) simulation of the 14 July 2000 Bastille Day eruption, which produced a very strong geomagnetic storm. After constructing a thermodynamic MHD model of the corona and solar wind, we insert a magnetically stable flux rope along the polarity inversion line of the eruption’s source region and initiate the eruption by boundary flows. More than 10^33 ergs of magnetic energy are released in the eruption within a few minutes, driving a flare, an EUV wave, and a coronal mass ejection (CME) that travels in the outer corona at about 1500 km/s, close to the observed speed. We then propagate the CME to Earth, using a heliospheric MHD code. Our simulation thus provides the opportunity to test how well in situ observations of extreme events are matched if the eruption is initiated from a stable magnetic-equilibrium state. We find that the flux-rope center is very similar in character to the observed magnetic cloud, but arrives about 8.5 hours later and about 15 degrees too far to the North, with field strengths that are too weak by a factor of about 1.6. The front of the flux rope is highly distorted, exhibiting localized magnetic-field concentrations as it passes 1 AU. We discuss these properties with regard to the development of space-weather predictions based on MHD simulations of solar eruptions.

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T. Torok, C. Downs, J. Linker, et. al.
Fri, 19 Jan 18

Comments: 27 pages, 13 figures, under revision for publication in the Astrophysical Journal