An opening criterion for dust gaps in protoplanetary discs [SSA]

We aim to understand under which conditions a low mass planet can open a gap in viscous dusty protoplanetary discs. For this purpose, we extend the theory of dust radial drift to include the contribution from the tides of an embedded planet and from the gas viscous forces. From this formalism, we derive i) a grain size-dependent criterion for dust gap opening in discs, ii) an estimate of the location of the outer edge of the dust gap and iii) an estimate of the minimum Stokes number above which low-mass planets are able to carve gaps which appear only in the dust disc. These analytical estimates are particularly helpful to appraise the minimum mass of an hypothetical planet carving gaps in discs observed at long wavelengths and high resolution. We validate the theory against 3D SPH simulations of planet-disc interaction in a broad range of dusty protoplanetary discs. We find a remarkable agreement between the theoretical model and the numerical experiments.

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G. Dipierro and G. Laibe
Tue, 25 Apr 17

Comments: 17 pages, 13 figures. Accepted for publication in MNRAS

Siberian Radioheliograph: First Results [SSA]

Regular observations of active processes in the solar atmosphere have been started using the first stage of the multiwave Siberian Radioheliograph (SRH), a T-shaped 48-antenna array with a 4-8 GHz operating frequency range and a 10 MHz instantaneous receiving band. Antennas are mounted on the central antenna posts of the Siberian Solar Radio Telescope. The maximum baseline is 107.4 m, and the angular resolution is up to 70″. We present examples of observations of the solar disk at different frequencies, “negative” bursts, and solar flares. The sensitivity to compact sources reaches 0.01 solar flux units ($\approx 10^{-4}$ of the total solar flux) with an accumulation time of about 0.3 s. The high sensitivity of SRH enables monitoring of solar activity and allows studying active processes from characteristics of their microwave emission, including faint events, which could not be detected previously.

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S. Lesovoi, A. Altyntsev, A. Kochanov, et. al.
Tue, 25 Apr 17

Comments: 15 pages, 12 figures

The VLT-FLAMES Tarantula Survey XXVII. Physical parameters of B-type main-sequence binary systems in the Tarantula nebula [SSA]

A spectroscopic analysis has been undertaken for the B-type multiple systems (excluding those with supergiant primaries) in the VLT-FLAMES Tarantula Survey (VFTS). Projected rotational velocities, $v$sin$i$, for the primaries have been estimated using a Fourier Transform technique and confirmed by fitting rotationally broadened profiles. A subset of 33 systems with $v$sin$i$$\leq$ 80 km/s have been analysed using a TLUSTY grid of model atmospheres to estimate stellar parameters and surface abundances for the primaries. The effects of a potential flux contribution from an unseen secondary have also been considered. For 20 targets it was possible to reliably estimate their effective temperatures (T${eff}$) but for the other 13 objects it was only possible to provide a constraint of 20,000$\leq$T${eff}$$\leq$26,000 K — the other parameters estimated for these targets will be consequently less reliable. The estimated stellar properties are compared with evolutionary models and are generally consistent with their membership of 30 Doradus, while the nature of the secondaries of 3 SB2 system is discussed. A comparison with a sample of single stars with $v$sin$i$$\leq$ 80 km/s obtained from the VFTS and analysed with the same techniques implies that the atmospheric parameters and nitrogen abundances of the two samples are similar. However, the binary sample may have a lack of primaries with significant nitrogen enhancements, which would be consistent with them having low rotational velocities and having effectively evolved as single stars without significant rotational mixing. This result, which may be actually a consequence of the limitations of the pathfinder investigation presented in this paper, should be considered as a motivation for spectroscopic abundance analysis of large samples of binary stars, with high quality observational data.

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R. Garland, P. Dufton, C. Evans, et. al.
Tue, 25 Apr 17

Comments: 14 pages, 5 figures

The nova V1369 Cen — a short review [SSA]

We briefly present the spectroscopic evolution of the recent outburst of the classical nova V1369 Cen, and the presence of a narrow absorption line identified as due to the resonance of neutral lithium at 6708 \AA. We also discuss the consequences for the chemical evolution of lithium in the Galaxy.

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L. Izzo, M. Valle, F. Matteucci, et. al.
Tue, 25 Apr 17

Comments: 8 pages, 5 figures, 2 tables. To appear in the proceedings of the conference “The Golden Age of Cataclysmic Variables and Related Objects – III” held in Palermo, Italy, 7-12 September 2015

Be discs in coplanar circular binaries: Phase-locked spectroscopic variations [SSA]

The first results of radiative transfer calculations on decretion discs of binary Be stars are presented. A smoothed particle hydrodynamics code computes the structure of Be discs in coplanar circular binary systems for a range of orbital and disc parameters. The resulting disc configuration is fed into a Monte Carlo code, which calculates the radiative transfer along the line of sight for various observational coordinates. Making use of the property of steady disc structure in coplanar circular binaries, various observables are computed as functions of the orbital phase. We demonstrate that binary tidal effects give rise to phase-locked variability of the violet-to-red (V/R) ratio of hydrogen emission lines. The amplitude of V/R variations is maximal for stars seen edge-on, low-viscosity discs, large orbital separations, high secondary-to-primary mass ratios. This study opens a way in constraining the parameters of binary systems that exhibit phase-locked variations.

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D. Panoglou, A. Carciofi, D. Faes, et. al.
Tue, 25 Apr 17

Comments: 5 pages, 14 figures; submitted to MNRAS

Stellar Cosmic Rays in a Habitable Zone [SSA]

According to recent observations relative number of flare stars does not change very much from cool dwarfs to hot A stars. Flare energies are strongly correlated with stellar luminosity and radius. Whence it follows that the typical magnetic field associated with a flare is several tens gauss and the typical flare loop length-scales are parts of the stellar radius. Flares on O–B stars were not observed, but they are possible, since strong magnetic fields are detected on O–B stars. Therefore stars of O–M spectral classes are potential sources of cosmic rays. Energy estimates of a magnetic field strength in a tube in photospheres of O–M stars are performed. Basing on their values possible flare energies and numbers of accelerated protons are estimated. The values obtained for the Sun correspond to observations by order of magnitude that justify estimates for other stars. Values of magnetic field strength in a tube differ less than five times for O and M flares (700 and 3500~G), but corresponding flare energies and numbers of accelerated protons for O stars are greater by five orders. Contrary fluencies of stellar protons appear to be five orders less.

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A. Struminsky and A. Sadovski
Tue, 25 Apr 17

Comments: N/A

On the Sunspot Group Number Reconstruction: The Backbone Method [SSA]

We discuss two recent papers very critical of our Group Sunspot Number Series (Svalgaard & Schatten [2016]). Unfortunately, we cannot support any of the concerns they raise. We first show that almost always there is simple proportionality between the counts of different observers and that taking the small, occasional non-linearities into account makes very little difference. Among other examples: we verify that the RGO group count was drifting the first twenty years of observations. We then show that our group count matches the diurnal variation of the geomagnetic field with high fidelity, and that the heliospheric magnetic field derived from geomagnetic data is consistent with our group number series. We evaluate the ‘correction matrix’ approach [Usoskin et al. 2016] and show that it fails to reproduce the observational data. We clarify the notion of daisy-chaining and point out that our group number series has no daisy-chaining for the period 1794-1996 and therefore no accumulation of errors over that span. We compare with the cosmic ray record for the last 400+ years and find good agreement. We note that the Active Day Fraction method (of Usoskin et al.) has the problem that at sunspot maximum, every day is an ‘active day’ so ADF is nearly always unity and thus does not carry information about the statistics of high solar activity. This ‘information shadow’ occurs for even moderate group numbers and thus need to be extrapolated to higher activity. We compare a new application of the correction matrix method utilizing good RGO data after 1900 and find remarkable linear agreement (after correcting for an unphysical offset) with our group series. We conclude that the criticism of Svalgaard & Schatten [2016] is invalid and detrimental to progress in the important field of long-term variation of solar activity, although we would welcome well-reasoned re-examinations of our findings.

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L. Svalgaard and K. Schatten
Tue, 25 Apr 17

Comments: 36 pages, 45 figures