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J.P. Magué & B. Ménard

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[1]  arXiv:1506.07877 [pdf, other]
Title: Dark Matter Superfluidity and Galactic Dynamics
Comments: 5 pages, 2 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

We propose a unified framework that reconciles the stunning success of MOND on galactic scales with the triumph of the LambdaCDM model on cosmological scales. This is achieved through the physics of superfluidity. Dark matter consists of self-interacting axion-like particles that thermalize and condense to form a superfluid in galaxies, with ~mK critical temperature. The superfluid phonons mediate a MOND acceleration on baryonic matter. Our framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not): dark matter has a higher temperature in clusters, and hence is in a mixture of superfluid and normal phase. The rich and well-studied physics of superfluidity leads to a number of striking observational signatures.

[2]  arXiv:1506.07878 [pdf, other]
Title: KIC 4552982: Outbursts and Asteroseismology from the Longest Pseudo-Continuous Light Curve of a ZZ Ceti
Comments: 13 pages, 15 figures, accepted to ApJ: June 22, 2015
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We present the Kepler light curve of KIC 4552982, the first ZZ Ceti (hydrogen-atmosphere pulsating white dwarf star) discovered in the Kepler field of view. Our data span more than 1.5 years with a 86% duty cycle, making it the longest pseudo-continuous light curve ever recorded for a ZZ Ceti. This extensive data set provides the most complete coverage to-date of amplitude and frequency variations in a cool ZZ Ceti. We detect 20 independent frequencies of variability in the data that we compare with asteroseismic models to demonstrate that this star has a mass M$_*$ > 0.6 M$_{\rm Sun}$. We identify a rotationally split pulsation mode and derive a probable rotation period for this star of 17.47 $\pm$ 0.04 hr. In addition to pulsation signatures, the Kepler light curve exhibits sporadic, energetic outbursts that increase the star's relative flux by 2-17%, last 4-25 hours, and recur on an average timescale of 2.7 days. These are the first detections of a new dynamic white dwarf phenomenon that we believe may be related to the pulsations of this relatively cool (T$_{\rm eff}$ = 10,860 $\pm$ 120 K) ZZ Ceti star near the red edge of the instability strip.

[3]  arXiv:1506.07880 [pdf, ps, other]
Title: Radial Trends in IMF-Sensitive Absorption Features in Two Early-Type Galaxies: Evidence for Abundance-Driven Gradients
Authors: Nicholas J. McConnell (University of Hawaii), Jessica R. Lu (University of Hawaii), Andrew W. Mann (University of Texas)
Comments: 16 page body + 7 page appendix + references. Includes 18 figures. Submitted to ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We have observed two massive early-type galaxies with Keck/LRIS and measured radial gradients in the strengths of stellar absorption features from 4000-5500 \AA$\,$ and 8000-10,000 \AA. We present spatially resolved measurements of the dwarf-sensitive spectral indices NaI (8190 \AA) and Wing-Ford FeH (9915 \AA), as well as indices for species of H, C$_2$, CN, Mg, Ca, TiO, and Fe. Our measurements show a metallicity gradient in both objects, and Mg/Fe consistent with uniform $\alpha$-enhancement, matching widely observed trends for massive early-type galaxies. The NaI index and the CN$_1$ index at 4160 \AA$\,$ exhibit significantly steeper gradients, with a break at $r \sim 0.1 r_{\rm eff}$ ($r \sim 300$ pc). Inside this radius NaI and CN$_1$ increase sharply toward the galaxy center, relative to other indices. We interpret this trend as a rapid central rise in [Na/Fe] and [N/Fe]. In contrast, the FeH index exhibits a marginal decrease toward the galaxy center, relative to Fe. Our investigation is among the first to track FeH as a function of radius, and to demonstrate discrepant behavior between NaI and FeH. We suggest that a shallow gradient in FeH and steep, broken NaI profile reflect unique abundance patterns rather than a gradient in the stellar initial mass function.

[4]  arXiv:1506.07881 [pdf, other]
Title: Radio Pulsars
Comments: Invited review for Space Science Reviews
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Almost 50 years after radio pulsars were discovered in 1967, our understanding of these objects remains incomplete. On the one hand, within a few years it became clear that neutron star rotation gives rise to the extremely stable sequence of radio pulses, that the kinetic energy of rotation provides the reservoir of energy, and that electromagnetic fields are the braking mechanism. On the other hand, no consensus regarding the mechanism of coherent radio emission or the conversion of electromagnetic energy to particle energy yet exists. In this review, we report on three aspects of pulsar structure that have seen recent progress: the self-consistent theory of the magnetosphere of an oblique magnetic rotator; the location, geometry, and optics of radio emission; and evolution of the angle between spin and magnetic axes. These allow us to take the next step in understanding the physical nature of the pulsar activity.

[5]  arXiv:1506.07882 [pdf, other]
Title: The SLUGGS Survey: Globular cluster kinematics in a "double sigma" galaxy - NGC 4473
Comments: 13 pages, 10 figures, accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

NGC 4473 is a so--called double sigma (2$\sigma$) galaxy, i.e. a galaxy with rare, double peaks in its 2D stellar velocity dispersion. Here, we present the globular cluster (GC) kinematics in NGC 4473 out to $\sim10\,R_e$ (effective radii) using data from combined HST/ACS and Subaru/Suprime--Cam imaging and Keck/DEIMOS spectroscopy. We find that the 2$\sigma$ nature of NGC 4473 persists up to 3 $R_e$, though it becomes misaligned to the photometric major axis. We also observe a significant offset between the stellar and GC rotation amplitudes. This offset can be understood as a co--addition of counter--rotating stars producing little net stellar rotation. We identify a sharp radial transition in the GC kinematics at $\sim4\,R_e$ suggesting a well defined kinematically distinct halo. In the inner region ($<4\,R_e$), the blue GCs rotate along the photometric major axis, but in an opposite direction to the galaxy stars and red GCs. In the outer region ($>4\,R_e$), the red GCs rotate in an opposite direction compared to the inner region red GCs, along the photometric major axis, while the blue GCs rotate along an axis intermediate between the major and minor photometric axes. We also find a kinematically distinct population of very red GCs in the inner region with elevated rotation amplitude and velocity dispersion. The multiple kinematic components in NGC 4473 highlight the complex formation and evolutionary history of this 2$\sigma$ galaxy, as well as a distinct transition between the inner and outer components.

[6]  arXiv:1506.07886 [pdf, ps, other]
Title: The Sloan Digital Sky Survey Reverberation Mapping Project: Ensemble Spectroscopic Variability of Quasar Broad Emission Lines
Comments: 20 pages, 25 figures. ApJ submitted: comments welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We explore the variability of quasars in the MgII and Hbeta broad emission lines and UV/optical continuum emission using the Sloan Digital Sky Survey Reverberation Mapping project (SDSS-RM). This is the largest spectroscopic study of quasar variability to date: our study includes 29 spectroscopic epochs from SDSS-RM over $6$ months, containing 357 quasars with MgII and 41 quasars with Hbeta . On longer timescales, the study is also supplemented with two-epoch data from SDSS-I/II. The SDSS-I/II data include an additional $2854$ quasars with MgII and 572 quasars with Hbeta. The MgII emission line is significantly variable ($\Delta f/f$ 10% on 100-day timescales), indicating that it is feasible to use the broad MgII line for reverberation mapping studies. The data also confirm that continuum variability increases with timescale and decreases with luminosity, and the continuum light curves are consistent with a damped random-walk model on rest-frame timescales of $\gtrsim 5$ days. We compare the emission-line and continuum variability to investigate the structure of the broad-line region. Broad-line variability shows a shallower increase with timescale compared to the continuum emission, demonstrating that the broad-line transfer function is not a $\delta$-function. Hbeta is more variable than MgII (roughly by a factor of $1.5$), suggesting different excitation mechanisms, optical depths and/or geometrical configuration for each emission line. The ensemble spectroscopic variability measurements enabled by the SDSS-RM project have important consequences for future studies of reverberation mapping and black hole mass estimation of $1<z<2$ quasars.

[7]  arXiv:1506.07887 [pdf, ps, other]
Title: Calibrating echelle spectrographs with Fabry-Perot etalons
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

Over the past decades hollow-cathode lamps have been calibration standards for spectroscopic measurements. Advancing to cm/s radial velocity precisions with the next generation of instruments requires more suitable calibration sources with more lines and less dynamic range problems. Fabry-Perot interferometers provide a regular and dense grid of lines and homogeneous amplitudes making them good candidates for next generation calibrators. We investigate the usefulness of Fabry-Perot etalons in wavelength calibration, present an algorithm to incorporate the etalon spectrum in the wavelength solution and examine potential problems. The quasi periodic pattern of Fabry-Perot lines is used along with a hollow-cathode lamp to anchor the numerous spectral features on an absolute scale. We test our method with the HARPS spectrograph and compare our wavelength solution to the one derived from a laser frequency comb. The combined hollow-cathode lamp/etalon calibration overcomes large distortion (50 m/s) in the wavelength solution of the HARPS data reduction software. Direct comparison to the laser frequency comb bears differences of only maximum 10 m/s. Combining hollow-cathode lamps with Fabry-Perot Interferometers can lead to substantial improvements in the wavelength calibration of echelle spectrographs. Etalons can provide economical alternatives to the laser frequency comb, especially for smaller projects.

[8]  arXiv:1506.07891 [pdf, other]
Title: Discovery of low-metallicity stars in the central parsec of the Milky Way
Authors: Tuan Do (1 and 2), Wolfgang Kerzendorf (3 and 4), Nathan Winsor (1 and 5), Morten Støstad (3), Mark R. Morris (2), Jessica R. Lu (6), Andrea M. Ghez (2) ((1) Dunlap Institute, University of Toronto, (2) Physics and Astronomy Department, UCLA, (3) Department of Astronomy, University of Toronto, (4) ESO, (5) Grenfell Campus, Memorial University of Newfoundland, (6) IfA, University of Hawaii)
Comments: 11 pages, 10 figures, ApJ Accepted
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present a metallicity analysis of 83 late-type giants within the central 1 pc of the Milky Way. K-band spectroscopy of these stars were obtained with the medium-spectral resolution integral-field spectrograph NIFS on Gemini North using laser-guide star adaptive optics. Using spectral template fitting with the MARCS synthetic spectral grid, we find that there is large variation in metallicity, with stars ranging from [M/H] $<$ -1.0 to above solar metallicity. About 6\% of the stars have [M/H] $<$ -0.5. This result is in contrast to previous observations, with smaller samples, that show stars at the Galactic center have approximately solar metallicity with only small variations. Our current measurement uncertainties are dominated by systematics in the model, especially at [M/H] $>$ 0, where there are stellar lines not represented in the model. However, the conclusion that there are low metallicity stars, as well as large variations in metallicity is robust. The metallicity may be an indicator of the origin of these stars. The low-metallicity population is consistent with that of globular clusters in the Milky Way, but their small fraction likely means that globular cluster infall is not the dominant mechanism for forming the Milky Way nuclear star cluster. The majority of stars are at or above solar metallicity, which suggests they were formed closer to the Galactic center or from the disk. In addition, our results indicate that it will be important for star formation history analyses using red giants at the Galactic center to consider the effect of varying metallicity.

[9]  arXiv:1506.07900 [pdf, other]
Title: A New Model for Mixing By Double-Diffusive Convection (Semi-Convection). III. Thermal and Compositional Transport Through Non-Layered ODDC
Comments: Submitted to ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

Oscillatory double-diffusive convection (ODDC) (also known as semi- convection) refers to a type of double diffusive instability that occurs in regions of planetary and stellar interiors which have a destabilizing thermal stratification and a stabilizing mean molecular weight stratification. In this series of papers, we use an extensive suite of three-dimensional (3D) numerical simulations to quantify the transport of heat and chemical species by ODDC. Rosenblum et al. (2011) first showed that ODDC can either spontaneously form layers, which significantly enhance the transport of heat and chemical species compared to mi- croscopic transport, or remain in a state dominated by large scale gravity waves, in which there is a more modest enhancement of the turbulent transport rates. Subsequent studies in this series have focused on identifying under what condi- tions layers form (Mirouh et al. 2012), and quantifying transport through layered systems (Wood et al. 2013). Here we proceed to characterize transport through systems that are unstable to the ODDC instability, but do not undergo spon- taneous layer formation. We measure the thermal and compositional fluxes in non-layered ODDC from both 2D and 3D numerical simulations and show that 3D simulations are well approximated by similar simulations in a 2D domain. We find that the turbulent mixing rate in this regime is weak and can, to a first level approximation, be neglected. We conclude by summarizing the findings of papers I through III into a single prescription for transport by ODDC.

[10]  arXiv:1506.07908 [pdf, ps, other]
Title: Core-Collapse Supernova Rate Synthesis Within 11 Mpc
Comments: 11 pages, 5 figures, 6 tables. Accepted by MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

The 11 Mpc H-alpha and Ultraviolet Galaxy (11HUGS) Survey traces the star formation activity of nearby galaxies. In addition within this volume the detection completeness of core-collapse supernovae (CCSNe) is high therefore by comparing these observed stellar births and deaths we can make a sensitive test of our understanding of how stars live and die. In this paper, we use the results of the Binary Population and Spectral Synthesis (BPASS) code to simulate the 11HUGS galaxies H-alpha and far-ultraviolet (FUV) star formation rate indicators (SFRIs) and simultaneously match the core-collapse supernova (CCSN) rate. We find that stellar population including interacting binary stars makes little difference to the total CCSN rate but increases the H-alpha and FUV fluxes for a constant number of stars being formed. In addition they significantly increase the predicted rate of type Ibc supernovae (SNe) relative to type II SNe to the level observed in the 11HUGS galaxies. We also find that instead of assuming a constant star formation history (SFH) for the galaxies our best fitting models have a star formation rate (SFR) that peaked more than 3 Myrs ago.

[11]  arXiv:1506.07914 [pdf, ps, other]
Title: Frozen-in Fractals All Around: Inferring the Large Scale Effects of Small-Scale Magnetic Structure
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The large-scale structure of the magnetic field in the solar corona provides the energy to power large-scale solar eruptive events. Our physical understanding of this structure, and hence our ability to predict these events, is limited by the type of data currently available. It is shown that the multifractal spectrum is a powerful tool to study this structure, by providing a physical connection between the details of photospheric magnetic gradients and current density at all size scales. This uses concepts associated with geometric measure theory and the theory of weakly differentiable functions to compare Amp\`{e}re's law to the wavelet-transform modulus maximum method. The H\"{o}lder exponent provides a direct measure of the rate of change of current density across spatial size scales. As this measure is independent of many features of the data (pixel resolution, data size, data type, presence of quiet-Sun data), it provides a unique approach to studying magnetic-field complexity and hence a potentially powerful tool for a statistical prediction of solar-flare activity. Three specific predictions are provided to test this theory: the multifractal spectra will not be dependent on the data type or quality; quiet-Sun gradients will not persist with time; structures with large current densities at large size scale will be the source of energy storage for solar eruptive events.

[12]  arXiv:1506.07926 [pdf, ps, other]
Title: Dust Cooling in Supernova Remnants in the Large Magellanic Cloud
Comments: 8 pages, 2 figures, 1 table. Accepted for publication in ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA); High Energy Astrophysical Phenomena (astro-ph.HE)

The infrared-to-X-ray (IRX) flux ratio traces the relative importance of dust cooling to gas cooling in astrophysical plasma such as supernova remnants (SNRs). We derive IRX ratios of SNRs in the LMC using Spitzer and Chandra SNR survey data and compare them with those of Galactic SNRs. IRX ratios of all the SNRs in the sample are found to be moderately greater than unity, indicating that dust grains are a more efficient coolant than gas although gas cooling may not be negligible. The IRX ratios of the LMC SNRs are systematically lower than those of the Galactic SNRs. As both dust cooling and gas cooling pertain to the properties of the interstellar medium, the lower IRX ratios of the LMC SNRs may reflect the characteristics of the LMC, and the lower dust-to- gas ratio (a quarter of the Galactic value) is likely to be the most significant factor. The observed IRX ratios are compared with theoretical predictions that yield IRX ratios an order of magnitude larger. This discrepancy may originate from the dearth of dust in the remnants due to either the local variation of the dust abundance in the preshock medium with respect to the canonical abundance or the dust destruction in the postshock medium. The non-equilibrium ionization cooling of hot gas, in particular for young SNRs, may also cause the discrepancy. Finally, we discuss implications for the dominant cooling mechanism of SNRs in low-metallicity galaxies.

[13]  arXiv:1506.07927 [pdf, ps, other]
Title: Investigation of Power8 processors for astronomical adaptive optics real-time control
Authors: Alastair Basden
Comments: Accepted for publication in MNRAS
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

The forthcoming Extremely Large Telescopes all require adaptive optics systems for their successful operation. The real-time control for these systems becomes computationally challenging, in part limited by the memory bandwidths required for wavefront reconstruction. We investigate new POWER8 processor technologies applied to the problem of real-time control for adaptive optics. These processors have a large memory bandwidth, and we show that they are suitable for operation of first-light ELT instrumentation, and propose some potential real-time control system designs. A CPU-based real-time control system significantly reduces complexity, improves maintainability, and leads to increased longevity for the real-time control system.

[14]  arXiv:1506.07929 [pdf, ps, other]
Title: Analysis of EMCCD and sCMOS readout noise models for Shack-Hartmann wavefront sensor accuracy
Authors: Alastair Basden
Comments: Accepted for publication in JATIS
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

In recent years, detectors with sub-electron readout noise have been used very effectively in astronomical adaptive optics systems. Here, we compare readout noise models for the two key faint flux level detector technologies that are commonly used: EMCCD and scientific CMOS (sCMOS) detectors. We find that in almost all situations, EMCCD technology is advantageous, and that the commonly used simplified model for EMCCD readout is appropriate. We also find that the commonly used simple models for sCMOS readout noise are optimistic, and recommend that a proper treatment of the sCMOS rms readout noise probability distribution should be considered during instrument performance modelling and development.

[15]  arXiv:1506.07934 [pdf, ps, other]
Title: Spherical Collapse in Modified Gravity Theories
Comments: 15 pages, 6 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)

We study the spherical collapse in the Parametrized Post-Friedmannian (PPF) scheme. We use a general form of the PPF parameter related to the Poisson equation and found the equations to solve that includes a non-trivial fifth force coming from the convolution of the modified gravity term in the k-space. In order to compute a concrete model, we use the parametrization proposed by Bertschinger and Zukin. The equations of the spherical collapse are solved assuming a Gaussian density profile and we show there is no shell crossing before reaching the turn around point. We show that the fifth force does not satisfy the Birkhoff's theorem and introduces different behaviors for the density threshold $\delta_{c}$, which in this case depends on the size and shape of the initial density profile, and therefore one expects a different statistic of the collapsed objects in the universe.

[16]  arXiv:1506.07948 [pdf, ps, other]
Title: Pulsar Observations of Extreme Scattering Events
Comments: Accepted by ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); High Energy Astrophysical Phenomena (astro-ph.HE)

Extreme scattering events (ESEs) in the interstellar medium (ISM) were first observed in regular flux measurements of compact extragalactic sources. They are characterized by a flux variation over a period of weeks, suggesting the passage of a "diverging plasma lens" across the line of sight. Modeling the refraction of such a lens indicates that the structure size must be of order AU and the electron density of order 10s of cm^{-3}. Similar structures have been observed in measurements of pulsar intensity scintillation and group delay. Here we report observations of two ESEs showing increases in both intensity scintillation and dispersion made with the Parkes Pulsar Timing Array (PPTA). These allow us to make more complete models of the ESE, including an estimate of the "outer-scale" of the turbulence in the plasma lens. These observations show clearly that the ESE structure is fully turbulent on an AU scale. They provide some support for the idea that the structures are extended along the line of sight, such as would be the case for a scattering shell. The dispersion measurements also show a variety of AU scale structures which would not be called ESEs, yet involve electron density variations typical of ESEs and likely have the same origin.

[17]  arXiv:1506.07981 [pdf, other]
Title: Characterization of the Atmospheric Muon Flux in IceCube
Comments: 33 pages, 36 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Muons produced in atmospheric cosmic ray showers account for the by far dominant part of the event yield in large-volume underground particle detectors. The IceCube detector, with an instrumented volume of about a cubic kilometer, has the potential to conduct unique investigations on atmospheric muons by exploiting the large collection area and the possibility to track particles over a long distance. Through detailed reconstruction of energy deposition along the tracks, the characteristics of muon bundles can be quantified, and individual particles of exceptionally high energy identified. The data can then be used to constrain the cosmic ray primary flux and the contribution to atmospheric lepton fluxes from prompt decays of short-lived hadrons.
In this paper, techniques for the extraction of physical measurements from atmospheric muon events are described and first results are presented. The multiplicity spectrum of TeV muons in cosmic ray air showers for primaries in the energy range from the knee to the ankle is derived and found to be consistent with recent results from surface detectors. The single-muon energy spectrum is determined up to PeV energies and shows a clear indication for the emergence of a distinct spectral component from prompt decays of short-lived hadrons. The magnitude of the prompt flux, which should include a substantial contribution from light vector meson di-muon decays, is consistent with current theoretical predictions.
The variety of measurements and high event statistics can also be exploited for the evaluation of systematic effects. In the course of this study, internal inconsistencies were found which indicate the presence of an unexplained effect outside the range of detector systematics. The underlying cause could be related to the hadronic interaction models used to describe muon production in air showers.

[18]  arXiv:1506.07982 [pdf, other]
Title: Mastering the effects of peculiar velocities in cosmic voids
Comments: 12 pages, 12 figures and 2 tables, prepared for submission to MNRAS. Comments welcome ( )
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

How do peculiar velocities affect observed voids? To answer this question we use the VIDE toolkit to identify voids in mock galaxy populations embedded within an N-body simulation both with and without peculiar velocities included. We compare the resulting void populations to assess the impact on void properties. We find that void abundances and spherically-averaged radial density profiles are mildly affected by peculiar velocities. However, peculiar velocities can distort by up to 10% the shapes for a particular subset of voids depending on the void size and density contrast, which can lead to increased variance in Alcock-Paczy\'nski test. We offer guidelines for performing optimal cuts on the void catalogue to reduce this variance by removing the most severely affected voids while preserving the unaffected ones. In addition, since this shape distortion is largely limited to the line of sight, we show that the void radii are only affected at the $\sim$ 10% level and the macrocenter positions at the $\sim$ 20% (even before performing cuts), meaning that cosmological probes based on the Integrated Sachs-Wolfe and gravitational lensing are not severely impacted by peculiar velocities.

[19]  arXiv:1506.07986 [pdf]
Title: CCD polarimetry of distant comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at the 6-m telescope of the SAO RAS
Comments: 28 pages, 13 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

We present first measurements of the degree of linear polarization of distant comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at heliocentric distances r= 5.9 - 7.0 AU. Observations were carried out with the SCORPIO-2 focal reducer at the 6-m telescope of the SAO RAS. Both comets showed considerable level of activity beyond a zone where water ice sublimation is negligible (up to 5 AU). Significant spatial variations both in the intensity and polarization are found in both comets. The slope of radial profiles of intensity changes gradually with the distance from the photocenter: from - 0.7 near the nucleus up to about - 1.3 for larger distances (up to 100000 km). The variation in polarization profiles indicates the non uniformity in the polarization distribution over the coma. The polarization degree over the coma gradually increases (in absolute value) with increasing the photocentric distance from of about - 1.9% up to - 3% for comet C/2010 S1 (LINEAR), and from of about - 2.5% up to - 3.5% for comet C/2010 R1 (LINEAR). These polarization values are significantly higher than typical value of the whole coma polarization (-1.5%) for comets at heliocentric distances less than 5 AU. The obtained photometric and polarimetric data are compared with those derived early for other comets at smaller heliocentric distances. Numerical modeling of light scattering characteristics was performed for media composed of particles with different refractive index, shape, and size. The computations were made by using the superposition T-matrix method. We obtained that for comet C/2010 S1 (LINEAR), the dust in the form of aggregates of overall radius R ~ 1.3 {\mu}m composed of N = 1000 spherical monomers with radius a = 0.1 {\mu}m, refractive index m = 1.65 + i 0.05, allows to obtain a satisfactory agreement between the results of polarimetric observations of comet C/2010 S1 and computations.

[20]  arXiv:1506.07994 [pdf, other]
Title: Mass ratio of the 2 pc binary brown dwarf LUH16 and limits on planetary companions from astrometry
Comments: 5 pages, 7 figures. Submitted to MNRAS letters
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

We analyse FORS2/VLT I-band imaging data to monitor the motions of both components in the most nearby known binary brown dwarf WISE J104915.57-531906.1AB (LUH16) over one year. The astrometry is dominated by parallax and proper motion, but with a precision of $\sim$0.2 milli-arcsecond per epoch we accurately measure the relative position change caused by the orbital motion of the pair. This allows us to directly determine a mass ratio of $q=0.78\pm0.10$ for this system. We also search for the signature of a planetary-mass companion around either of the A and B component and exclude at 3-$\sigma$ the presence of planets with masses larger than $2\,M_\mathrm{Jup}$ and orbital periods of 20-300 d. We update the parallax of LUH16 to $500.51\pm0.11$ mas, i.e. just within 2 pc. This study yields the first direct constraint on the mass ratio of LUH16 and shows that the system does not harbour any close-in giant planets.

[21]  arXiv:1506.08034 [pdf, ps, other]
Title: CO Core Candidates in the Gemini Molecular Cloud
Comments: Accepted for Publication in AJ, 23 Pages, 15 figures. arXiv admin note: text overlap with arXiv:astro-ph/0604427 by other authors
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present observations of a 4 squared degree area toward the Gemini cloud obtained using J = 1-0 transitions of $^{12}$CO, $^{13}$CO and C$^{18}$O. No C$^{18}$O emission was detected. This region is composed of 36 core candidates of $^{13}$CO. These core candidates have a characteristic diameter of 0.25 pc, excitation temperatures of 7.9 K, line width of 0.54 km s$^{-1}$ and a mean mass of 1.4 M$_{\sun}$. They are likely to be starless core candidates, or transient structures, which probably disperse after $\sim$10$^6$ yr.

[22]  arXiv:1506.08035 [pdf, other]
Title: Triggering an eruptive flare by emerging flux in a solar active-region complex
Comments: Accepted for publication in Topical Issue of Solar Physics: Solar and Stellar Flares. 25 pages, 12 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

A flare and fast coronal mass ejection originated between solar active regions NOAA 11514 and 11515 on July 1, 2012 in response to flux emergence in front of the leading sunspot of the trailing region 11515. Analyzing the evolution of the photospheric magnetic flux and the coronal structure, we find that the flux emergence triggered the eruption by interaction with overlying flux in a non-standard way. The new flux neither had the opposite orientation nor a location near the polarity inversion line, which are favorable for strong reconnection with the arcade flux under which it emerged. Moreover, its flux content remained significantly smaller than that of the arcade (approximately 40 %). However, a loop system rooted in the trailing active region ran in part under the arcade between the active regions, passing over the site of flux emergence. The reconnection with the emerging flux, leading to a series of jet emissions into the loop system, caused a strong but confined rise of the loop system. This lifted the arcade between the two active regions, weakening its downward tension force and thus destabilizing the considerably sheared flux under the arcade. The complex event was also associated with supporting precursor activity in an enhanced network near the active regions, acting on the large-scale overlying flux, and with two simultaneous confined flares within the active regions.

[23]  arXiv:1506.08039 [pdf, other]
Title: Rotation periods of late-type dwarf stars from time-series high-resolution spectroscopy of chromospheric indicators
Comments: 13 pages, 13 figures, 5 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

We determine rotation periods of a sample of 48 late F-type to mid-M dwarf stars using time-series high-resolution spectroscopy of the Ca II H&K and H-alpha chromospheric activity indicators. We find good agreement between the rotation periods obtained from each of these two indicators. An empirical relationship between the level of chromospheric emission measured by log (R'HK) and the spectroscopic rotation periods is reported. This relation is largely independent of the spectral type and the metallicity of the stars and can be used to make a reliable prediction of rotation periods for late K to mid-M dwarfs with low levels of activity. For some stars in the sample, the measured spectroscopic rotation periods coincide, or are very close, to the orbital periods of postulated planets. In such cases, further studies are needed to clarify whether the associated periodic radial velocity signals reveal the existence of planets or are due to magnetic activity.

[24]  arXiv:1506.08043 [pdf, ps, other]
Title: Looking for the least luminous BL Lac objects
Authors: Alessandro Capetti (1) Claudia M. Raiteri (1) ((1) INAF - Osservatorio Astronomico di Torino, Italy)
Comments: Accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Among active galactic nuclei, BL Lac objects show extreme properties that have been interpreted as the effect of relativistic beaming on the emission from a plasma jet oriented close to the line of sight. The Doppler amplification of the jet emission makes them ideal targets for studying jet physics. In particular, low-power BL Lacs (LPBL) are very interesting because they probe the jet formation and emission processes at the lowest levels of accretion. However, they are difficult to identify since their emission is swamped by the radiation from the host galaxy in most observing bands. In this paper we propose a new LPBL selection method based on the mid-infrared emission, in addition to the traditional optical indices. We considered the radio-selected sample of Best & Heckman (2012, MNRAS, 421, 1569) and cross-matched it with the WISE all-sky survey. In a new diagnostic plane including the W2-W3 color and the Dn(4000) index, LPBL are located in a region scarcely populated by other sources. By filtering objects with small emission line equivalent width, we isolated 36 LPBL candidates up to redshift 0.15. Their radio luminosity at 1.4 GHz spans the range log L_r = 39.2-41.5 [erg/s]. Considering the completeness of our sample, we analyzed the BL Lac luminosity function (RLF), finding a dramatic paucity of LPBL with respect to the extrapolation of the RLF toward low power. This requires a break in the RLF located at log L_r~40.6 [erg/s]. The consequent peak in the BL Lacs number density is possibly the manifestation of a minimum power required to launch a relativistic jet.

[25]  arXiv:1506.08048 [pdf, other]
Title: Early optical spectra of nova V1369 Cen show presence of Lithium
Comments: 6 pages, 5 figures, accepted for publication in ApJLetters
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We present early high resolution spectroscopic observations of the nova V1369 Cen. We have detected an absorption feature at 6695.6 \AA\, that we have identified as blue--shifted $^7$Li I $\lambda$6708 \AA. The absorption line, moving at -550 km/s, was observed in five high-resolution spectra of the nova obtained at different epochs. On the basis of the intensity of this absorption line we infer that a single nova outburst can inject in the Galaxy $M_{Li} =$ 0.3 - 4.8 $\times 10^{-10}$ M$_{\odot}$. Given the current estimates of Galactic nova rate, this amount is sufficient to explain the puzzling origin of the overabundance of Lithium observed in young star populations.

[26]  arXiv:1506.08081 [pdf, other]
Title: The nature of the KFR08 stellar stream
Comments: 9 pages, 6 figures, accepted for publication in A&A
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The origin of a new kinematically identified metal-poor stellar stream, the KFR08 stream, has not been established. We present stellar parameters, stellar ages, and detailed elemental abundances for Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Ni, Zn, Sr, Y, Zr, Ba, La, and Eu for 16 KFR08 stream members based on analysis of high resolution spectra. Based on the abundance ratios of 14 elements, we use the chemical tagging method to identify the stars which have the same chemical composition, and thus, might have a common birthplace, such as a cluster. Although three stars were tagged with similar elemental abundances ratios, we find that, statistically, it is not certain that they originate from a dissolved star cluster. This conclusion is consistent with the large dispersion of [Fe/H] ($\sigma_{\rm{[Fe/H]}} = 0.29$) among the 16 stream members. We find that our stars are $\alpha$ enhanced and that the abundance patterns of the stream members are well matched to the thick disk. In addition, most of the stream stars have estimated stellar ages larger than 11 Gyr. These results, together with the hot kinematics of the stream stars, suggest that the KFR08 stream is originated from the thick disk population which was perturbed by a massive merger in the early universe.

[27]  arXiv:1506.08089 [pdf, other]
Title: Realistic uncertainties on Hapke model parameters from photometric measurement
Comments: 36 pages, 18 figures, submitted in Icarus
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Hapke proposed a convenient and widely used analytical model to describe the spectro-photometry of granular materials. Using a compilation of the published data, Hapke (2012, Icarus, 221, 1079-1083) recently studied the relationship of b and c for natural examples and proposed the hockey stick relation (excluding b>0.5 and c>0.5). For the moment, there is no theoretical explanation for this relationship. One goal of this article is to study a possible bias due to the retrieval method. We expand here an innovative Bayesian inversion method in order to study into detail the uncertainties of retrieved parameters. On Emission Phase Function (EPF) data, we demonstrate that the uncertainties of the retrieved parameters follow the same hockey stick relation, suggesting that this relation is due to the fact that b and c are coupled parameters in the Hapke model instead of a natural phenomena. Nevertheless, the data used in the Hapke (2012) compilation generally are full Bidirectional Reflectance Diffusion Function (BRDF) that are shown not to be subject to this artifact. Moreover, the Bayesian method is a good tool to test if the sampling geometry is sufficient to constrain the parameters (single scattering albedo, surface roughness, b, c, opposition effect). We performed sensitivity tests by mimicking various surface scattering properties and various single image-like/disk resolved image, EPF-like and BRDF-like geometric sampling conditions. The second goal of this article is to estimate the favorable geometric conditions for an accurate estimation of photometric parameters in order to provide new constraints for future observation campaigns and instrumentations.

[28]  arXiv:1506.08093 [pdf, ps, other]
Title: Observational Hints of a Pre--Inflationary Scale?
Comments: 7 pages, LaTeX, 2 eps figures. Essay Written for the 2015 Gravity Research Foundation Awards for Essays on Gravitation. Selected for Honorable Mention
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

We argue that the lack of power exhibited by cosmic microwave background (CMB) anisotropies at large angular scales might be linked to the onset of inflation. We highlight observational features and theoretical hints that support this view, and present a preliminary estimate of the physical scale that would underlie the phenomenon.

[29]  arXiv:1506.08101 [pdf, ps, other]
Title: Prediction and typicality in multiverse cosmology
Authors: Feraz Azhar
Comments: 13 pages, 1 figure. arXiv admin note: text overlap with arXiv:hep-th/0508006 by other authors
Journal-ref: Classical and Quantum Gravity, 31(3), 035005, (2014)
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); History and Philosophy of Physics (physics.hist-ph)

In the absence of a fundamental theory that precisely predicts values for observable parameters, anthropic reasoning attempts to constrain probability distributions over those parameters in order to facilitate the extraction of testable predictions. The utility of this approach has been vigorously debated of late, particularly in light of theories that claim we live in a multiverse, where parameters may take differing values in regions lying outside our observable horizon. Within this cosmological framework, we investigate the efficacy of top-down anthropic reasoning based on the weak anthropic principle. We argue contrary to recent claims that it is not clear one can either dispense with notions of typicality altogether or presume typicality, in comparing resulting probability distributions with observations. We show in a concrete, top-down setting related to dark matter, that assumptions about typicality can dramatically affect predictions, thereby providing a guide to how errors in reasoning regarding typicality translate to errors in the assessment of predictive power. We conjecture that this dependence on typicality is an integral feature of anthropic reasoning in broader cosmological contexts, and argue in favour of the explicit inclusion of measures of typicality in schemes invoking anthropic reasoning, with a view to extracting predictions from multiverse scenarios.

[30]  arXiv:1506.08102 [pdf]
Title: Intensity Conserving Spline Interpolation (ICSI): A New Tool for Spectroscopic Analysis
Comments: 9 pages, 7 figures, 1 table; submitted to Astrophysical Journal
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The detailed shapes of spectral line profiles provide valuable information about the emitting plasma, especially when the plasma contains an unresolved mixture of velocities, temperatures, and densities. As a result of finite spectral resolution, the intensity measured by a spectrometer is the average intensity across a wavelength bin of non-zero size. It is assigned to the wavelength position at the center of the bin. However, the actual intensity at that discrete position will be different if the profile is curved, as it invariably is. Standard fitting routines (spline, Gaussian, etc.) do not account for this difference, and this can result in significant errors when making sensitive measurements. Detection of asymmetries in solar coronal emission lines is one example. Removal of line blends is another. We have developed an iterative procedure called Intensity Conserving Spline Interpolation (ICSI) that corrects for this effect. As its name implies, it conserves the observed intensity within each wavelength bin, which ordinary fits do not. Given the rapid convergence, speed of computation, and ease of use, we suggest that ICSI be made a standard component of the processing pipeline for spectroscopic data.

[31]  arXiv:1506.08115 [pdf, ps, other]
Title: Optimal Electron Energies for Driving Chromospheric Evaporation in Solar Flares
Comments: Accepted for publication to ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

In the standard model of solar flares, energy deposition by a beam of electrons drives strong chromospheric evaporation leading to a significantly denser corona and much brighter emission across the spectrum. Chromospheric evaporation was examined in great detail by Fisher, Canfield, & McClymont (1985a,b,c), who described a distinction between two different regimes, termed explosive and gentle evaporation. In this work, we examine the importance of electron energy and stopping depths on the two regimes and on the atmospheric response. We find that with explosive evaporation, the atmospheric response does not depend strongly on electron energy. In the case of gentle evaporation, lower energy electrons are significantly more efficient at heating the atmosphere and driving up-flows sooner than higher energy electrons. We also find that the threshold between explosive and gentle evaporation is not fixed at a given beam energy flux, but also depends strongly on the electron energy and duration of heating. Further, at low electron energies, a much weaker beam flux is required to drive explosive evaporation.

[32]  arXiv:1506.08121 [pdf, ps, other]
Title: On the Implications of Recent Observations of the Inner Knot in the Crab Nebula
Authors: Yajie Yuan, Roger Blandford (KIPAC, Stanford University)
Comments: 17 pages, 13 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Recent observations of the Crab Nebula (Rudy et al 2015) have maintained its reputation for high energy astrophysical enlightenment and its use as a testbed for theories of the behaviour of magnetized, relativistic plasma. In particular, new observations of the inner knot located 0.65'' SE from the pulsar confirm that it is compact, elongated transversely to the symmetry axis and curved concave towards the pulsar. 60 percent polarization has been measured along the symmetry axis (Moran et al 2013). The knot does not appear to be involved in the gamma ray flares. The new observations both reinforce the interpretation of the knot as dissipation of the pulsar wind at a strong shock and challenge the details of existing models of this process. In particular, it is argued that the compactness, high polarization and curvature are difficult to reconcile with simple relativistic shock models. Alternative possibilities include deflection of the outflow ahead of the shock and spatial variation in which the knot is interpreted as a caustic. Some future observations are proposed and new theoretical investigations are suggested.

[33]  arXiv:1506.08122 [pdf, other]
Title: The star RR Lyr and the Cepheid variables in the era of the space photometry revolution
Comments: Conference Proceedings - CoRoT3-KASC7 The Space Photometry Revolution, Toulouse, France, July 6-11 2014. Paper on CoRoT Cepheids submitted to MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The long-term behaviours of the pulsation and Blazhko periods of RR Lyr are investigated by means of Kepler and ground-based observations. The difficulties in detecting additional modes in the Cepheids monitored with CoRoT are discussed.

[34]  arXiv:1506.08160 [pdf, ps, other]
Title: 870 microns continuum observations of the bubble-shaped nebula Gum 31
Comments: 15 pages, 10 figures. Accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

We are presenting here a study of the cold dust in the ring nebula Gum 31. We aim at deriving the physical properties of the molecular gas and dust associated with the nebula, and investigating its correlation with the star formation in the region, that was probably triggered by the expansion of the ionization front. We use 870 microns data obtained with LABOCA to map the dust emission. The obtained LABOCA image was compared to archival IR,radio continuum, and optical images. The 870 microns emission follows the 8 microns (Spitzer), 250 microns, and 500 microns (Herschel) emission distributions showing the classical morphology of a spherical shell. We use the 870 microns and 250 microns images to identify 60 dust clumps in the collected layers of molecular gas using the Gaussclumps algorithm. The clumps have effective deconvolved radii between 0.16 pc and 1.35 pc, masses between 70 Mo and 2800 Mo, and volume densities between 1.1x10^3 cm^-3 and 2.04x10^5 cm^-3. The total mass of the clumps is 37600 Mo. The dust temperature of the clumps is in the range from 21 K to 32 K, while inside the HII region reaches ~ 40 K. The clump mass distribution is well-fitted by a power law dN/dlog(M/Mo) proportional to M^(-alpha), with alpha=0.93+/-0.28. The slope differs from those obtained for the stellar IMF in the solar neighborhood, suggesting that the clumps are not direct progenitors of single stars/protostars. The mass-radius relationship for the 41 clumps detected in the 870 microns emission shows that only 37% of them lie in or above the high-mass star formation threshold, most of them having candidate YSOs projected inside. A comparison of the dynamical age of the HII region with the fragmentation time, allowed us to conclude that the collect and collapse mechanism may be important for the star formation at the edge of Gum 31, although other processes may also be acting.

[35]  arXiv:1506.08173 [pdf, other]
Title: Pluto's atmosphere from stellar occultations in 2012 and 2013
Comments: 41 pages, 13 figures, 4 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

We analyze two multi-chord stellar occultations by Pluto observed on July 18th, 2012 and May 4th, 2013, and monitored respectively from five and six sites. They provide a total of fifteen light-curves, twelve of them being used for a simultaneous fit that uses a unique temperature profile, assuming a clear (no-haze) and pure N_2 atmosphere, but allowing for a possible pressure variation between the two dates. We find a solution that fits satisfactorily (i.e. within the noise level) all the twelve light-curves, providing atmospheric constraints between ~1,190 km (pressure ~ 11 \mubar) and ~ 1,450 km (pressure ~0.1 \mubar) from Pluto's center. Our main results are: (1) the best-fitting temperature profile shows a stratosphere with strong positive gradient between 1,190 km (at 36 K, 11 \mubar) and r = 1,215 km (6.0 \mubar), where a temperature maximum of 110 K is reached; above it is a mesosphere with negative thermal gradient of -0.2 K/km up to ~ 1,390 km (0.25 \mubar), where, the mesosphere connects itself to a more isothermal upper branch around 81 K; (2) the pressure shows a small (6 %) but significant increase (6-\sigma level) between the two dates; (3) without troposphere, Pluto's radius is found to be R_P = 1,190 +/- 5km. Allowing for a troposphere, R_P is constrained to lie between 1,168 and 1,195 km; (4) the currently measured CO abundance is too small to explain the mesospheric negative thermal gradient. Cooling by HCN is possible, but only if this species is largely saturated; Alternative explanations like zonal winds or vertical compositional variations of the atmosphere are unable to explain the observed mesospheric trend.

[36]  arXiv:1506.08177 [pdf, other]
Title: Intergalactic Magnetogenesis at Cosmic Dawn by Photoionization
Comments: Submitted to MNRAS
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We present a detailed analysis of an astrophysical mechanism that generates cosmological magnetic fields during the Epoch of Reionization. It is based on the photoionization of the Intergalactic Medium by the first sources formed in the Universe. First the induction equation is derived, then the characteristic length and time scales of the mechanism are identified, and finally numerical applications are carried out for first stars, primordial galaxies and distant powerful quasars. In these simple examples, the strength of the generated magnetic fields varies between the order of $10^{-23}$ G on hundreds of kiloparsecs to $10^{-19}$ G on hundreds of parsecs in the neutral Intergalactic Medium between the Str\"omgren spheres of the sources. Thus this mechanism contributes to the premagnetization of the whole Universe before large scale structures are in place. It operates with any ionizing source, at any time during the Epoch of Reionization. Finally, the generated fields possess a characteristic spatial configuration which may help discriminate these seeds from those produced by different mechanisms.