The extremely truncated circumstellar disc of V410 X-ray 1: a precursor to TRAPPIST-1? [SSA]

Protoplanetary discs around brown dwarfs and very low mass stars offer some of the best prospects for forming Earth-sized planets in their habitable zones. To this end, we study the nature of the disc around the very low mass star V410 X-ray 1, whose SED is indicative of an optically thick and very truncated dust disc, with our modelling suggesting an outer radius of only 0.6 au. We investigate two scenarios that could lead to such a truncation, and find that the observed SED is compatible with both. The first scenario involves the truncation of both the dust and gas in the disc, perhaps due to a previous dynamical interaction or the presence of an undetected companion. The second scenario involves the fact that a radial location of 0.6 au is close to the expected location of the H$_2$O snowline in the disc. As such, a combination of efficient dust growth, radial migration, and subsequent fragmentation within the snowline leads to an optically thick inner dust disc and larger, optically thin outer dust disc. We find that a firm measurement of the CO $J=2$–1 line flux would enable us to distinguish between these two scenarios, by enabling a measurement of the radial extent of gas in the disc. Many models we consider contain at least several Earth-masses of dust interior to 0.6 au, suggesting that V410 X-ray 1 could be a precursor to a system with tightly-packed inner planets, such as TRAPPIST-1.

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D. Boneberg, S. Facchini, C. Clarke, et. al.
Wed, 21 Feb 18

Comments: 10 pages, 8 figures, accepted for publication in MNRAS

Constraints on Sub-GeV Dark Matter-Electron Scattering from the DarkSide-50 Experiment [CEA]

We present new constraints on sub-GeV dark matter particles scattering off electrons in argon based on an analysis of ionization signal data from the DarkSide-50 detector.

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DarkSide. Collaboration, P. Agnes, I. Albuquerque, et. al.
Wed, 21 Feb 18

Comments: N/A

The Cosmic Web Around The Brightest Galaxies During The Epoch Of Reionization [GA]

The most luminous galaxies at high-redshift are generally considered to be hosted in massive dark-matter halos of comparable number density, hence residing at the center of overdensities/protoclusters. We assess the validity of this assumption by investigating the clustering around the brightest galaxies populating the cosmic web at redshift $z\sim8-9$ through a combination of semi-analytic modeling and Monte Carlo simulations of mock Hubble Space Telescope WFC3 observations. The innovative aspect of our approach is the inclusion of a log-normal scatter parameter $\Sigma$ in the galaxy luminosity versus halo mass relation, extending to high-$z$ the conditional luminosity function framework extensively used at low redshift. Our analysis shows that the larger the value of $\Sigma$, the less likely that the brightest source in a given volume is hosted in the most massive halo, and hence the weaker the overdensity of neighbors. We derive a minimum value of $\Sigma$ as a function of redshift by considering stochasticity in the halo assembly times, which affects galaxy ages and star formation rates in our modeling. We show that $\Sigma_{min}(z)\sim0.15-0.3$, with $\Sigma_{min}$ increasing with redshift as a consequence of shorter halo assembly periods at higher redshifts. Current observations ($m_{AB}\sim27$) of the environment of spectroscopically confirmed bright sources at $z>7.5$ do not show strong evidence of clustering and are consistent with our modeling predictions for $\Sigma\geq\Sigma_{min}$. Deeper future observations reaching $m_{AB}\sim28.2-29$ would have the opportunity to clearly quantify the clustering strength, and hence to constrain $\Sigma$, investigating the physical processes that drive star formation in the early Universe.

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K. Ren, M. Trenti and S. Mutch
Wed, 21 Feb 18

Comments: 12 pages, 7 figures, 1 table; accepted for publication in ApJ

Fundamental Frequencies in the Schwarzschild Spacetime [HEAP]

We consider the Keplerian, radial and vertical fundamental frequencies in the Schwarzschild spacetime to study the so-called kilohertz quasi-periodic oscillations from low-mass X-ray binary systems. We show that, within the Relativistic Precession Model, the interpretation of observed kilohertz quasi-periodic oscillations in terms of the fundamental frequencies of test particles in the Schwarzschild spacetime, allows one to infer the total mass $M$ of the central object, the internal $R_{in}$ and external $R_{ex}$ radii of accretion disks, and innermost stable circular orbits $r_{ISCO}$ for test particles in a low-mass X-ray binary system. By constructing the relation between the upper and lower frequencies and exploiting the quasi-periodic oscillation data of the Z and Atoll sources we perform the non-linear model fit analysis and estimate the mass of the central object. Knowing the value of the mass we calculate the internal $R_{in}$ and external $R_{ex}$ radii of accretion disks and show that they are larger than $r_{ISCO}$, what was expected.

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K. Boshkayev, M. Muccino, J. Rueda, et. al.
Wed, 21 Feb 18

Comments: 7 pages, 6 figures, 1 table

Implosive collapse about magnetic null points: A quantitative comparison between 2D and 3D nulls [SSA]

Null collapse is an implosive process whereby MHD waves focus their energy in the vicinity of a null point, forming a current sheet and initiating magnetic reconnection. We consider, for the first time, the case of collapsing 3D magnetic null points in nonlinear, resistive MHD using numerical simulation, exploring key physical aspects of the system as well as performing a detailed parameter study. We find that within a particular plane containing the 3D null, the plasma and current density enhancements resulting from the collapse are quantitatively and qualitatively as per the 2D case in both the linear and nonlinear collapse regimes. However, the scaling with resistivity of the 3D reconnection rate – which is a global quantity – is found to be less favourable when the magnetic null point is more rotationally symmetric, due to the action of increased magnetic back-pressure. Furthermore, we find that with increasing ambient plasma pressure the collapse can be throttled, as is the case for 2D nulls. We discuss this pressure-limiting in the context of fast reconnection in the solar atmosphere and suggest mechanisms by which it may be overcome. We also discuss the implications of the results in the context of null collapse as a trigger mechanism of Oscillatory Reconnection, a time-dependent reconnection mechanism, and also within the wider subject of wave-null point interactions. We conclude that, in general, increasingly rotationally-asymmetric nulls will be more favourable in terms of magnetic energy release via null collapse than their more symmetric counterparts.

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J. Thurgood, D. Pontin and J. McLaughlin
Wed, 21 Feb 18

Comments: Accepted in ApJ, will be published gold open access, refer to main journal

The Planetary Nebula Luminosity Function (PNLF): Contamination from Supernova Remnants [GA]

The planetary nebula luminosity function (PNLF) has been used as an extragalactic distance indicator since the 1980’s, but there are still unsolved problems associated with its use. One of the most serious involves PNLF distances beyond ~ 10 Mpc, which tend to be slightly smaller than those of other methods. We consider the implications of previous spectroscopic investigations that found that several of the brightest planetary nebula (PN) candidates in M74 are actually compact supernova remnants (SNRs). Using narrow-band imaging data from the KPNO 4-m telescope, we measure the [O III] $\lambda$5007 and H$\alpha$ fluxes of all the known SNRs in M31 and M33, and test whether those objects could be misidentified as bright PNe at distances beyond ~ 10 Mpc. Our results suggest that compact SNRs are not an important source of contamination in photometric surveys for extragalactic PNe.

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B. Davis, R. Ciardullo, J. Feldmeier, et. al.
Wed, 21 Feb 18

Comments: 2 pages, 1 figure

Comets in UV [EPA]

Comets are important “eyewitnesses” of Solar System formation and evolution. Important tests to determine the chemical composition and to study the physical processes in cometary nuclei and coma need data in the UV range of the electromagnetic spectrum. Comprehensive and complete studies require for additional ground-based observations and in-situ experiments. We briefly review observations of comets in the ultraviolet (UV) and discuss the prospects of UV observations of comets and exocomets with space-born instruments. A special refer is made to the World Space Observatory-Ultraviolet (WSO-UV) project.

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B. Shustov, M. Sachkov, A. Castro, et. al.
Wed, 21 Feb 18

Comments: International symposium “Ultraviolet Sky Surveys: the need and the means” 10 – 14 July 2017, Tel Aviv University, Israel