Abstracts of Interest
Selected by:
Jarryd Day
Abstract: 1808.07896
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Title: Cooldown Strategies and Transient Thermal Simulations for the Simons Observatory
(Submitted on 23 Aug 2018)
Abstract: The Simons Observatory (SO) will provide precision polarimetry of the cosmic microwave background (CMB) using a series of telescopes which will cover angular scales from arc-minutes to tens of degrees, contain over 60,000 detectors, and observe in frequency bands between 27 GHz and 270 GHz. SO will consist of a six-meter-aperture telescope initially coupled to ~35,000 detectors along with an array of 0.5m aperture refractive cameras, coupled to an additional 30,000+ detectors. The large aperture telescope receiver (LATR) is coupled to a six-meter crossed Dragone telescope and will be 2.4m in diameter, weigh over 3 tons, and have five cryogenic stages (80 K, 40 K, 4 K, 1 K and 100 mK). The LATR is coupled to the telescope via 13 independent optics tubes containing cryogenic optical elements and detectors. The cryostat will be cooled by by two Cryomech PT90 (80 K) and three Cryomech PT420 (40 K and 4 K) pulse tube cryocoolers, with cooling of the 1 K and 100 mK stages by a commercial dilution refrigerator system. The second component, the small aperture telescope (SAT), is a single optics tube refractive cameras of 42cm diameter. Cooling of the SAT stages will be provided by two Cryomech PT420, one of which is dedicated to the dilution refrigeration system which will cool the focal plane to 100 mK. SO will deploy a total of three SATs. In order to estimate the cool down time of the camera systems given their size and complexity, a finite difference code based on an implicit solver has been written to simulate the transient thermal behavior of both cryostats. The result from the simulations presented here predict a 35 day cool down for the LATR. The simulations suggest additional heat switches between stages would be effective in distribution cool down power and reducing the time it takes for the LATR to cool. The SAT is predicted to cool down in one week, which meets the SO design goals.
Abstract: 1808.08059
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Full Text: [ PostScript, PDF]
Title: Capture into first-order resonances and long-term stability of pairs of equal-mass planets
(Submitted on 24 Aug 2018)
Abstract: Massive planets form within the lifetime of protoplanetary disks and undergo orbital migration due to planet-disk interactions. When the first planet reaches the inner edge of the disk its migration stops and the second planet is locked in resonance. We detail the resonant trapping comparing semi-analytical formulae and numerical simulations in the case of two equal-mass coplanar planets trapped in first order resonances. We describe the family of resonant stable equilibrium points (zero-amplitude libration orbits) using expanded and non-expanded Hamiltonians. We show that during convergent migration the planets evolve along these families of equilibria. Eccentricity damping from the disk leads to a final equilibrium configuration that we predict analytically. The fact that observed multi-exoplanetary systems are rarely seen in resonances suggests that the resonant configurations achieved by migration can become unstable after the removal of the disk. We probe the stability of the resonances as a function of planetary mass. To do this, we fictitiously increase the masses of the planets, adiabatically maintaining the low-amplitude libration regime until instability. We discuss two hypotheses for instability: low-order secondary resonance of the libration frequency with a fast synodic frequency, and minimal approach distance between planets. We show that secondary resonances do not seem to impact resonant systems at low-amplitude of libration. Resonant systems are more stable than non-resonant ones for a given minimal distance at close encounters, but we show that the latter still play the decisive role in the destabilisation of resonant pairs. We show that, as the planetary mass increases and the minimal distance between planets gets smaller in terms of mutual Hill radius, the region of stability around the resonance center shrinks, until the equilibrium point itself becomes unstable.
Abstract: 1808.08232
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Full Text: [ PostScript, PDF]
Title: DUNE as the Next-Generation Solar Neutrino Experiment
(Submitted on 24 Aug 2018)
Abstract: We show that the Deep Underground Neutrino Experiment (DUNE) has the potential to deliver world-leading results in solar neutrinos. Significant but realistic new efforts would be required. With an exposure of 100 kton-year, DUNE could detect $\gtrsim 10^5$ signal events above 5 MeV. Separate precision measurements of neutrino-mixing parameters and the $^8$B flux could be made using two detection channels ($\nu_e + \, ^{40}$Ar and $\nu_{e,\mu,\tau} + e^-$) and the day-night effect ($> 10 \sigma$). New particle physics may be revealed through the comparison of solar neutrinos (with matter effects) and reactor neutrinos (without), which is discrepant by $\sim 2 \sigma$ (and could become $5.6 \sigma$). New astrophysics may be revealed through the most precise measurement of the $^8$B flux (to 2.5%) and the first detection of the hep flux (to 11%). DUNE is required: No other experiment, even proposed, has been shown capable of fully realizing these discovery opportunities.
Abstract: 1808.08543
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Full Text: [ PostScript, PDF]
Title: Predicting a solar cycle before its onset using a flux transport dynamo model
(Submitted on 26 Aug 2018)
Abstract: We begin with a review of the predictions for cycle~24 before its onset. After summarizing the basics of the flux transport dynamo model, we discuss how this model had been used to make a successful prediction of cycle~24, on the assumption that the irregularities of the solar cycle arise due to the fluctuations in the Babcock--Leighton mechanism. We point out that fluctuations in the meridional circulation can be another cause of irregularities in the cycle.
Abstract: 1808.08757
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Full Text: [ PostScript, PDF]
Title: Escape of High Energy Particles from Bow-Shock Pulsar Wind Nebulae
(Submitted on 27 Aug 2018)
Abstract: The detection of bright X-ray features and large TeV halos around old pulsars that have escaped their parent Supernova Remnants and are interacting directly with the ISM, suggest that high energy particles, more likely high energy pairs, can escape from these systems, and that this escape if far more complex than a simple diffusive model can predict. Here we present for the first time a detailed analysis of how high energy particles escape from the head of the bow shock. In particular we focus our attention on the role of the magnetic field geometry, and the inclination of the pulsar spin axis with respect to the direction of the pulsar kick velocity. We show that asymmetries in the escape pattern of charged particles are common, and they are strongly energy dependent. More interestingly we show that the flow of particles from bow-shock pulsar wind nebulae is likely to be charge separated, which might have profound consequences on the way such flow interacts with the ISM magnetic field, driving local turbulence.
Abstract: 1808.08761
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Full Text: [ PostScript, PDF]
Title: Analysis of vertex-contained high energy neutrino events for the KM3NeT/ARCA detector
(Submitted on 27 Aug 2018)
Abstract: KM3NeT is a research infrastructure housing the next generation neutrino detectors in the depths of the Mediterranean Sea. The ARCA detector, which is currently under construction, is optimized for searches for neutrinos from astrophysical sources as well as measurements of the diffuse astrophysical flux. The unambiguous detection of neutrinos of extraterrestrial origin by IceCube has led to the first measurement of a high energy astrophysical neutrino flux. The properties of sea water allow for a measurement of the neutrino direction with an excellent angular resolution for both track and cascade events. Here a method to differentiate track and shower events and a method to reject the atmospheric muon background from starting track-like events are combined in one analysis. The analysis on the discovery potential of KM3NeT/ARCA for a diffuse astrophysical neutrino flux using events with the reconstructed vertex inside the detector volume will be presented.
Abstract: 1808.08870
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Full Text: [ PostScript, PDF]
Title: Role of gaseous giants in the dynamical evolution of terrestrial planets and water delivery in the habitable zone
(Submitted on 27 Aug 2018)
Abstract: In the present research, we study the effects of a single giant planet in the dynamical evolution of water-rich embryos and planetesimals, located beyond the snow line of systems around Sun-like stars, in order to determine what kind of terrestrial-like planets could be formed in the habitable zone (hereafter HZ) of these systems. To do this, we carry out N-body simulations of planetary accretion, considering that the gas has been already dissipated from the disk and a single giant planet has been formed beyond the snow line of the system, at 3 au. We find that a giant planet with a value of mass between Saturn-mass and Jupiter-mass, represents a limit from which the amount of water-rich embryos that moves inward from beyond the snow line starts to decrease. From this, our research suggests that giant planets more massive than one Jupiter-mass become efficient dynamical barriers to inward-migrating water-rich embryos. Moreover, we infer that the number of these embryos that survive in the HZ significantly decreases for systems that host a giant planet more massive than one Jupiter-mass. This result has important consequences concerning the formation of terrestrial-like planets in the HZ with very high water contents and could provide a selection criteria in the search of potentially habitable exoplanets in systems that host a gaseous giant around solar-type stars.
Abstract: 1808.08912
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Full Text: [ PostScript, PDF]
Title: The superluminal motion of the jet launched in GW170817, the Hubble constant, and critical tests of gamma ray bursts theory
(Submitted on 27 Aug 2018 (v1), last revised 28 Aug 2018 (this version, v2))
Abstract: The direction of the axis of the orbital motion of the merging binary neutron stars in the GW170817 event coincided with that of the apparent superluminal jet, which produced the short hard gamma ray burst (SHB) 170817A, if the local value of the Hubble constant is that provided by standard candle Type Ia supernovae, H_0=73.24 +/- 1.74 km/s Mpc. This value differs by 3 sigma from the cosmic value H_0=67.74 +/- 0.46 km/s Mpc obtained from the cosmic microwave background radiation by Planck assuming the standard Lambda-CDM cosmology. The measured superluminal motion of the jet also allows critical tests of the assumed production mechanism of SHBs in general and of SHB170817A in particular.
Abstract: 1808.08237
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Full Text: [ PostScript, PDF]
Title: To $B$ or not to $B$: Primordial magnetic fields from Weyl anomaly
(Submitted on 24 Aug 2018)
Abstract: The quantum effective action for the electromagnetic field in an expanding universe has an anomalous dependence on the scale factor of the metric arising from virtual charged particles in the loops. It has been argued that this Weyl anomaly of quantum electrodynamics sources cosmological magnetic fields in the early universe. We examine this long-standing claim by using the effective action beyond the weak gravitational field limit which has recently been determined. We introduce a general criteria for assessing the quantumness of field fluctuations, and show that the Weyl anomaly is not able to convert vacuum fluctuations of the gauge field into classical fluctuations. We conclude that there is no production of coherent magnetic fields in the universe from the Weyl anomaly of quantum electrodynamics, irrespective of the number of massless charged particles in the theory.
Abstract: 1808.09082
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Full Text: [ PostScript, PDF]
Title: Revised Distances to 21 Supernova Remnants
(Submitted on 28 Aug 2018)
Abstract: We carry out a comprehensive study of HI 21 cm line observations and $^{13}$CO line observations of 21 supernova remnants (SNRs). The aim of the study is to search for HI absorption features to obtain kinematic distances in a consistent manner. The 21 SNRs are in the region of sky covered by the Very Large Array Galactic Plane Survey (HI 21 cm observations) and Galactic Ring Survey ($^{13}$CO line observations). We obtain revised distances for 10 SNRs based on new evidence in the HI and $^{13}$CO observations. We revise distances for the other 11 SNRs based on an updated rotation curve and new error analysis. The mean change in distance for the 21 SNRs is $\simeq25\%$, i.e. change of 1.5 kpc compared to a mean distance for the sample of 6.4 kpc. This has a significant impact on interpretation of the physical state of these SNRs. For example, using a Sedov model, age and explosion energy scale as the square of distance, and inferred ISM density scales as distance.
Abstract: 1808.09098
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Full Text: [ PostScript, PDF]
Title: Search for Low-Mass Dark Matter with CDMSlite Using a Profile Likelihood Fit
(Submitted on 28 Aug 2018)
Abstract: The Cryogenic Dark Matter Search low ionization threshold experiment (CDMSlite) searches for interactions between dark matter particles and germanium nuclei in cryogenic detectors. The experiment has achieved a low energy threshold with improved sensitivity to low-mass (<10 GeV/c$^2$) dark matter particles. We present an analysis of the final CDMSlite data set, taken with a different detector than was used for the two previous CDMSlite data sets. This analysis includes a data "salting" method to protect against bias, improved noise discrimination, background modeling, and the use of profile likelihood methods to search for a dark matter signal in the presence of backgrounds. We achieve an energy threshold of 70 eV and significantly improve the sensitivity for dark matter particles with masses between 2.5 and 10 GeV/c$^2$ compared to previous analyses. We set an upper limit on the dark matter-nucleon scattering cross section in germanium of 5.4$\times$10$^{-42}$ cm$^2$ at 5 GeV/c$^2$, a factor of $\sim$2.5 improvement over the previous CDMSlite result.
Abstract: 1808.09153
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Full Text: [ PostScript, PDF]
Title: A Semi-Analytical Computation of the Theoretical Uncertainties of the Solar Neutrino Flux
(Submitted on 28 Aug 2018)
Abstract: We present a comparison between Monte Carlo simulations and a semi-analytical approach that reproduces the theoretical probability distribution functions of the solar neutrino fluxes, stemming from the $pp$, $pep$, $hep$, $^7\mathrm{Be}$, $^8\mathrm{B}$, $^{13}\mathrm{N}$, $^{15}\mathrm{O}$, and $^{17}\mathrm{F}$ source reactions. We obtain good agreement between the two approaches. Thus, the semi-analytical method yields confidence intervals that closely match those found, based on Monte Carlo simulations, and points towards the same general symmetries of the investigated probability distribution functions. Furthermore, the negligible computational cost of this method is a clear advantage over Monte Carlo simulations, making it trivial to take new observational constraints on the input parameters into account.
Abstract: 1808.09228
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Full Text: [ PostScript, PDF]
Title: Infrared molecular hydrogen lines in GRB host galaxies
(Submitted on 28 Aug 2018)
Abstract: Molecular species, most frequently H_2, are present in a small, but growing, number of gamma-ray burst (GRB) afterglow spectra at redshifts z~2-3, detected through their rest-frame UV absorption lines. In rare cases, lines of vibrationally excited states of H_2 can be detected in the same spectra. The connection between afterglow line-of-sight absorption properties of molecular (and atomic) gas, and the observed behaviour in emission of similar sources at low redshift, is an important test of the suitability of GRB afterglows as general probes of conditions in star formation regions at high redshift. Recently, emission lines of carbon monoxide have been detected in a small sample of GRB host galaxies, at sub-mm wavelengths, but no searches for H_2 in emission have been reported yet. In this paper we perform an exploratory search for rest-frame K band rotation-vibrational transitions of H_2 in emission, observable only in the lowest redshift GRB hosts (z<0.22). Searching the data of four host galaxies, we detect a single significant rotation-vibrational H_2 line candidate, in the host of GRB 031203. Re-analysis of Spitzer mid-infrared spectra of the same GRB host gives a single low significance rotational line candidate. The (limits on) line flux ratios are consistent with those of blue compact dwarf galaxies in the literature. New instrumentation, in particular on the JWST and the ELT, can facilitate a major increase in our understanding of the H_2 properties of nearby GRB hosts, and the relation to H_2 absorption in GRBs at higher redshift.
Abstract: 1808.09231
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Full Text: [ PostScript, PDF]
Title: Massive Star Evolution: Binaries as Two Single Stars
(Submitted on 28 Aug 2018)
Abstract: Binary stars are of course more than two stars, but they are also at least two stars. In this chapter we will review some aspects of the physics governing the evolution of single massive stars. We will also review the uncertainties of key physical ingredients: mass loss, rotation and convection.
Abstract: 1808.09373
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Full Text: [ PostScript, PDF]
Title: An ultra-fast inflow in the luminous Seyfert PG1211+143
(Submitted on 28 Aug 2018)
Abstract: Blueshifted absorption lines in the X-ray spectra of AGN show that ultra-fast outflows with typical velocities $v \sim 0.1c$ are a common feature of these luminous objects. Such powerful AGN winds offer an explanation of the observed M-$\sigma$ relation linking the mass of the supermassive black hole and the velocity dispersion in the galaxy's stellar bulge. An extended XMM-Newton study of the luminous Seyfert galaxy PG1211+143 recently revealed a variable multi-velocity wind. Here we report the detection of a short-lived, ultra-fast inflow during the same observation. Previous reports of inflows used single absorption lines with uncertain identifications, but this new result identifies an array of resonance absorption lines of highly ionised Fe, Ca, Ar, S and Si, sharing a common redshift when compared with a grid of realistic photoionization spectra. The redshifted absorption arises in a column of highly ionized matter close to the black hole, with a line-of-sight velocity, $v \sim 0.3c$, inconsistent with the standard picture of a plane circular accretion disc. This may represent the first direct evidence for chaotic accretion in AGN, where accretion discs are generally misaligned to the black hole spin. For sufficient inclinations, the Lense-Thirring effect can break the discs into discrete rings, which then precess, collide and shock, causing near free-fall of gas towards the black hole. The observed accretion rate for the reported infall is comparable to the hard X-ray luminosity in PG1211+143, suggesting that direct infall may be a significant contributor to inner disc accretion.
Abstract: 1808.09460
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Full Text: [ PostScript, PDF]
Title: Constraining the Time Interval for the Origin of Life on Earth
(Submitted on 28 Aug 2018)
Abstract: Estimates of the time at which life arose on Earth make use of two types of evidence. First, astrophysical and geophysical studies provide a timescale for the formation of Earth and the Moon, for large impact events on early Earth, and for the cooling of the early magma ocean. From this evidence, we can deduce a habitability boundary, which is the earliest point at which Earth became habitable. Second, biosignatures in geological samples, including microfossils, stromatolites, and chemical isotope ratios, provide evidence for when life was actually present. From these observations we can deduce a biosignature boundary, which is the earliest point at which there is clear evidence that life existed. Studies with molecular phylogenetics and records of the changing level of oxygen in the atmosphere give additional information that helps to determine the biosignature boundary. Here, we review the data from a wide range of disciplines to summarize current information on the timings of these two boundaries. The habitability boundary could be as early as 4.5 Ga, the earliest possible estimate of the time at which Earth had a stable crust and hydrosphere, or as late as 3.9 Ga, the end of the period of heavy meteorite bombardment. The lack of consensus on whether there was a late heavy meteorite bombardment that was significant enough to prevent life is the largest uncertainty in estimating the time of the habitability boundary. The biosignature boundary is more closely constrained. Evidence from carbon isotope ratios and stromatolite fossils both point to a time close to 3.7 Ga. Life must have emerged in the interval between these two boundaries. The time taken for life to appear could, therefore, be within 200 Myr or as long as 800 Myr.
Abstract: 1808.09578
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Full Text: [ PostScript, PDF]
Title: Identifying AGNs in low-mass galaxies via long-term optical variability
(Submitted on 28 Aug 2018)
Abstract: We present an analysis of the nuclear variability of $\sim28,000$ nearby ($z<0.15$) galaxies with Sloan Digital Sky Survey (SDSS) spectroscopy in Stripe 82. We construct light curves using difference imaging of SDSS g-band images, which allows us to detect subtle variations in the central light output. We select variable AGN by assessing whether detected variability is well-described by a damped random walk model. We find 135 galaxies with AGN-like nuclear variability. While most of the variability-selected AGNs have narrow emission lines consistent with the presence of an AGN, a small fraction have narrow emission lines dominated by star formation. The star-forming systems with nuclear AGN-like variability tend to be low-mass ($M_{\ast}<10^{10}~M_{\odot}$), and may be AGNs missed by other selection techniques due to star formation dilution or low-metallicities. We explore the AGN fraction as a function of stellar mass, and find that the fraction of variable AGN increases with stellar mass, even after taking into account the fact that lower mass systems are fainter. There are several possible explanations for an observed decline in the fraction of variable AGN with decreasing stellar mass, including a drop in the supermassive black hole occupation fraction, a decrease in the ratio of black hole mass to galaxy stellar mass, or a change in the variability properties of lower-mass AGNs. We demonstrate that optical photometric variability is a promising avenue for detecting AGNs in low-mass, star formation-dominated galaxies, which has implications for the upcoming Large Synoptic Space Telescope.
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