Gravitational waves from single neutron stars: an advanced detector era survey [HEAP]

http://arxiv.org/abs/1709.07049


With the doors beginning to swing open on the new gravitational wave astronomy, this review provides an up-to-date survey of the most important physical mechanisms that could lead to emission of potentially detectable gravitational radiation from isolated and accreting neutron stars. In particular we discuss the gravitational wave-driven instability and asteroseismology formalism of the f- and r-modes, the different ways that a neutron star could form and sustain a non-axisymmetric quadrupolar “mountain” deformation, the excitation of oscillations during magnetar flares and the possible gravitational wave signature of pulsar glitches. We focus on progress made in the recent years in each topic, make a fresh assessment of the gravitational wave detectability of each mechanism and, finally, highlight key problems and desiderata for future work.

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K. Glampedakis and L. Gualtieri
Fri, 22 Sep 17
38/75

Comments: 39 pages, 12 figures, 2 tables. Chapter of the book “Physics and Astrophysics of Neutron Stars”, NewCompStar COST Action 1304

The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit [SSA]

http://arxiv.org/abs/1709.07327


The CO(J=3-2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. The estimated average mass-loss rate of W~Aql agrees with previous results. The size of the emitting region is consistent with photodissociation models. The CO(J=3-2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of 10″ and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yrs) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.

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S. Ramstedt, S. Mohamed, W. Vlemmings, et. al.
Fri, 22 Sep 17
44/75

Comments: 10 pages, 8 figures

Observations et modélisations spectro-interférométriques longue base des étoiles et de leur environnement proche [SSA]

http://arxiv.org/abs/1709.07334


With the construction of the VLTI (Very Large Telescope Interferometer of the European Observatory ESO for the southern hemisphere) it is now possible to make observations with resolutions of the order of milli-arc-seconds, especially in IR with AMBER instrument (Astronomical Multi Beam Recombine). These new capabilities allow us to better constrain the stellar structures such as polar jets, equatorial disks and flattened photospheres of rotating stars. Thus the estimation of stellar fundamental parameters allows to explore in detail the mechanisms of mass loss, pulsation and magnetism governing the variability and the evolution of the stars.
This thesis presents the results of fast rotating stars observations carried out on the AMBER spectro-interferometer VLTI in its high \& medium spectral resolutions modes. The observations were highly degraded by the optical defects of AMBER and their analysis required the development of specific digital reduction tools to reach the necessary precision for the interferometric measurements interpretation. In order to interpret those measures I developed a chromatic semi-analytical model of rapidly rotating star that allowed me to estimate, from the differential phases; the degree of flattening, the equatorial radius, the rotation velocity, the angle of inclination, the position angle of the star rotation axis in the sky, the local distribution of the effective temperature and the surface gravity of the star within the von Zeipel theorem. The results for four massive stars of spectral types B, A and F have allowed me to characterize the mechanisms discussed above and thus open some prospect for more systematic studies of similar objects, with extending later these studies to the relationship photosphere – circumstellar envelope.

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M. Hadjara
Fri, 22 Sep 17
47/75

Comments: PhD Thesis 2015, Universite Nice-Sophia Antipoles, in French

Normal and counter Evershed flows in the photospheric penumbra of a sunspot. SPINOR 2D inversions of Hinode-SOT/ SP observations [SSA]

http://arxiv.org/abs/1709.07386


Context. The Evershed effect, a nearly horizontal outflow of material seen in the penumbrae of sunspots in the photospheric layers, is a common characteristic of well-developed penumbrae, but is still not well understood. Even less is known about photospheric horizontal inflows in the penumbra, also known as counter Evershed flows. Aims. Here we present a rare feature observed in the penumbra of the main sunspot of AR NOAA 10930. This spot displays the normal Evershed outflow in most of the penumbra, but harbors a fast photospheric inflow of material over a large sector of the disk-center penumbra. We investigate the driving forces of both, the normal and the counter Evershed flows. Methods. We invert the spectropolarimetric data from Hinode SOT/SP using the spatially coupled version of the SPINOR inversion code, which allows us to derive height-dependent maps of the relevant physical parameters in the sunspot. These maps show considerable fine structure. Similarities and differences between the normal Evershed outflow and the counter Evershed flow are investigated. Results. In both the normal and the counter Evershed flows, the material flows from regions with field strengths of the order of 1.5-2 kG to regions with stronger fields. The sources and sinks of both penumbral flows display opposite field polarities, with the sinks (tails of filaments) harboring local enhancements in temperature, which are nonetheless colder than their sources (heads of filaments). Conclusions. The anti-correlation of the gradients in the temperature and magnetic pressure between the endpoints of the filaments from the two distinct penumbral regions is compatible with both the convective driver and the siphon flow scenarios. A geometrical scale of the parameters is necessary to determine which is the dominant force driving the flows.

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A. Siu-Tapia, A. Lagg, S. Solanki, et. al.
Fri, 22 Sep 17
48/75

Comments: Astronomy and Astrophysics, accepted

The arXiv of the future will not look like the arXiv [CL]

http://arxiv.org/abs/1709.07020


The arXiv is the most popular preprint repository in the world. Since its inception in 1991, the arXiv has allowed researchers to freely share publication-ready articles prior to formal peer review. The growth and the popularity of the arXiv emerged as a result of new technologies that made document creation and dissemination easy, and cultural practices where collaboration and data sharing were dominant. The arXiv represents a unique place in the history of research communication and the Web itself, however it has arguably changed very little since its creation. Here we look at the strengths and weaknesses of arXiv in an effort to identify what possible improvements can be made based on new technologies not previously available. Based on this, we argue that a modern arXiv might in fact not look at all like the arXiv of today.

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A. Pepe, M. Cantiello and J. Nicholson
Fri, 22 Sep 17
71/75

Comments: The authors of this document welcome public comments and ideas from its readers, at the online version of this article (this https URL)

The arXiv of the future will not look like the arXiv [CL]

http://arxiv.org/abs/1709.07020


The arXiv is the most popular preprint repository in the world. Since its inception in 1991, the arXiv has allowed researchers to freely share publication-ready articles prior to formal peer review. The growth and the popularity of the arXiv emerged as a result of new technologies that made document creation and dissemination easy, and cultural practices where collaboration and data sharing were dominant. The arXiv represents a unique place in the history of research communication and the Web itself, however it has arguably changed very little since its creation. Here we look at the strengths and weaknesses of arXiv in an effort to identify what possible improvements can be made based on new technologies not previously available. Based on this, we argue that a modern arXiv might in fact not look at all like the arXiv of today.

Read this paper on arXiv…

A. Pepe, M. Cantiello and J. Nicholson
Fri, 22 Sep 17
71/75

Comments: The authors of this document welcome public comments and ideas from its readers, at the online version of this article (this https URL)

Electron Heating and Saturation of Self-regulating Magnetorotational Instability in Protoplanetary Disks [EPA]

http://arxiv.org/abs/1709.07026


Magnetorotational instability (MRI) has a potential to generate the vigorous turbulence in protoplanetary disks, although its turbulence strength and accretion stress remains debatable because of the uncertainty of MRI with low ionization fraction. We focus on the heating of electrons by strong electric fields which amplifies nonideal magnetohydrodynamic effects. The heated electrons frequently collide with and stick to dust grains, which in turn decreases the ionization fraction and is expected to weaken the turbulent motion driven by MRI. In order to quantitatively investigate the nonlinear evolution of MRI including the electron heating, we perform magnetohydrodynamical simulation with the unstratified shearing box. We introduce a simple analytic resistivity model depending on the current density by mimicking resistivity given by the calculation of ionization. Our simulation confirms that the electron heating suppresses magnetic turbulence when the electron heating occurs with low current density. We find a clear correlation between magnetic stress and its current density, which means that the magnetic stress is proportional to the squared current density. When the turbulent motion is completely suppressed, laminar accretion flow is caused by ordered magnetic field. We give an analytical description of the laminar state by using a solution of linear perturbation equations with resistivity. We also propose a formula that successfully predicts the accretion stress in the presence of the electron heating.

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S. Mori, T. Muranushi, S. Okuzumi, et. al.
Fri, 22 Sep 17
73/75

Comments: 12 pages, 12 figures, accepted for publication in ApJ