Young and Intermediate-age Distance Indicators [SSA]

Distance measurements beyond geometrical and semi-geometrical methods, rely mainly on standard candles. As the name suggests, these objects have known luminosities by virtue of their intrinsic proprieties and play a major role in our understanding of modern cosmology. The main caveats associated with standard candles are their absolute calibration, contamination of the sample from other sources and systematic uncertainties. The absolute calibration mainly depends on their chemical composition and age. To understand the impact of these effects on the distance scale, it is essential to develop methods based on different sample of standard candles. Here we review the fundamental properties of young and intermediate-age distance indicators such as Cepheids, Mira variables and Red Clump stars and the recent developments in their application as distance indicators.

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S. Subramanian, M. Marengo, A. Bhardwaj, et. al.
Fri, 22 Sep 17

Comments: Review article, 63 pages (28 figures), Accepted for publication in Space Science Reviews (Chapter 3 of a special collection resulting from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space Age)

Nanoflare Heating: Observations and Theory [SSA]

This is a review of the observational and theoretical evidence for nanoflare heating of the magnetically-closed corona.

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J. Klimchuk
Fri, 22 Sep 17

Comments: This is a section for a mega-paper entitled “Achievements of Hinode in the First Ten Years.” The invited speakers of the Hinode 10 Conference in Nagoya were asked to each write a section. The contribution should be cited as Klimchuk, J. A. and Hinode Review Team, “Achievements of Hinode in the First Ten Years,” 2017, PASJ, submitted

White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8 [SSA]

We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere white dwarfs (DAVs, a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The space-based photometry reveals pulsation properties previously inaccessible to ground-based observations. We observe a sharp dichotomy in oscillation mode linewidths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode linewidths, more reminiscent of a damped harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also permits extensive mode identification: We identify the spherical degree of 61 out of 154 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4m-class telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low- and intermediate-mass stars. We find that 0.51-to-0.73-solar-mass white dwarfs, which evolved from 1.7-to-3.0-solar-mass ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Finally, we announce an online repository for our Kepler data and follow-up spectroscopy, which we collect at this http URL

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J. Hermes, B. Gaensicke, S. Kawaler, et. al.
Fri, 22 Sep 17

Comments: 33 pages, 31 figures, 5 tables; accepted for publication in ApJS. All raw and reduced data are collected at this http URL

Rayleigh scattering in dense fluid helium [SSA]

Iglesias et al. (2002) showed that the Rayleigh scattering from helium atoms decreases by collective effects in the atmospheres of cool white dwarf stars. Their study is here extended to consider an accurate evaluation of the atomic polarizability and the density effects involved in the Rayleigh cross section over a wide density-temperature region. The dynamic dipole polarizability of helium atoms in the ground state is determinated with the oscillator-strength distribution approach. The spectral density of oscillator strength considered includes most significant single and doubly excited transitions to discrete and continuum energies. Static and dynamic polarizability results are confronted with experiments and other theoretical evaluations shown a very good agreement. In addition, the refractive index of helium is evaluated with the Lorentz-Lorenz equation and shows a satisfactory agreement with the most recent experiments. The effect of spatial correlation of atoms on the Rayleigh scattering is calculated with Monte Carlo simulations and effective energy potentials that represent the particle interactions, covering fluid densities between 0.005 and a few g/cm$^3$ and temperatures between $1000$ K and $15000$ K. We provide analytical fits from which the Rayleigh cross section of fluid helium can be easily calculated at wavelength $\lambda>505.35$ \AA. Collision-induced light scattering was estimated to be the dominant scattering process at densities greater than 1-2 g/cm$^3$ depending on the temperature.

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R. Rohrmann
Fri, 22 Sep 17

Comments: Accepted for publication in MNRAS; 14 pages, 14 figures

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

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

Comments: 10 pages, 8 figures

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

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

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]

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

Comments: Astronomy and Astrophysics, accepted