Abstracts of Interest
Selected by:
Jarryd Day
Abstract: 1902.08618
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Discrepancy in the Upper Bound Mass of Neutron Stars
(Submitted on 22 Feb 2019)
Abstract: Observations have indicated that we do not see neutron stars (NS) of mass near the theoretical upper limit as predicted. Here we invoke the role of dark matter (DM) particles in star formation, and their role in lowering the mass of remnants eventually formed from these stars. Massive stars can capture DM particles more effectively than the lower mass stars, thus further softening the equation of state of neutron star. We also look at the capture of DM particles by the NS, which could further soften the upper mass limit of NS. The admixture of DM particles would be higher at earlier epochs (high z).
Abstract: 1902.08840
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Oxygen fractionation in dense molecular clouds
(Submitted on 23 Feb 2019)
Abstract: We have developed the first gas-grain chemical model for oxygen fractionation (also including sulphur fractionation) in dense molecular clouds, demonstrating that gas-phase chemistry generates variable oxygen fractionation levels, with a particularly strong effect for NO, SO, O2, and SO2. This large effect is due to the efficiency of the neutral 18O + NO, 18O + SO, and 18O + O2 exchange reactions. The modeling results were compared to new and existing observed isotopic ratios in a selection of cold cores. The good agreement between model and observations requires that the gas-phase abundance of neutral oxygen atoms is large in the observed regions. The S16O/S18O ratio is predicted to vary substantially over time showing that it can be used as a sensitive chemical proxy for matter evolution in dense molecular clouds.
Abstract: 1902.09361
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Differentially rotating strange star in general relativity
(Submitted on 25 Feb 2019)
Abstract: Rapidly and differentially rotating compact stars are believed to be formed in binary neutron star merger events, according to both numerical simulations and the multi-messenger observation of GW170817. The lifetime and evolution of such a differentially rotating star, is tightly related to the observations in the post-merger phase. Various studies on the maximum mass of differentially rotating neutron stars have been done in the past, most of which assume the so-called $j$-const law as the rotation profile inside the star and consider only neutron star equations of state. In this paper, we extend the studies to strange star models, as well as to a new rotation profile model. Significant differences are found between differentially rotating strange stars and neutron stars, with both differential rotation laws. A moderate differential rotation rate for neutron stars is found to be too large for strange stars, resulting in a rapid drop in the maximum mass as the differential rotation degree is increased further from $\hat{A}\sim2.0$, where $\hat{A}$ is a parameter characterizing the differential rotation rate for $j$-const law. As a result the maximum mass of a differentially rotating self-bound star drops below the uniformly rotating mass shedding limit for a reasonable degree of differential rotation. The continuous transition to the toroidal sequence is also found to happen at a much smaller differential rotation rate and angular momentum than for neutron stars. In spite of those differences, $\hat{A}$-insensitive relation between the maximum mass for a given angular momentum is still found to hold, even for the new differential rotation law. Astrophysical consequences of these differences and how to distinguish between strange star and neutron star models with future observations are also discussed.
Abstract: 1902.09547
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Evolution of supernovae-driven superbubbles with conduction and cooling
(Submitted on 25 Feb 2019)
Abstract: We use spherically symmetric hydrodynamic simulations to study the dynamical evolution and internal structure of superbubbles (SBs) driven by clustered supernovae (SNe), focusing on the effects of thermal conduction and cooling in the interface between the hot bubble interior and cooled shell. Our simulations employ an effective diffusivity to account for turbulent mixing from nonlinear instabilities that are not captured in 1D. The conductive heat flux into the shell is balanced by a combination of cooling in the interface and evaporation of shell gas into the bubble interior. This evaporation increases the density, and decreases the temperature, of the SB interior by more than an order of magnitude relative to simulations without conduction. However, most of the energy conducted into the interface is immediately lost to cooling, reducing the evaporative mass flux required to balance conduction. As a result, the evaporation rate is typically a factor of $\sim$3-30 lower than predicted by the classical similarity solution of Weaver et al., which neglects cooling. Blast waves from the first $\sim$30 SNe remain supersonic until colliding with the shell because reduced evaporation from the interface lowers the mass they sweep up in the hot interior. Updating the Weaver solution to include cooling, we construct a new analytic model to predict the cooling rate, evaporation rate, and temporal evolution of SBs. The cooling rate, and hence the hot gas mass and momentum delivered by SBs, is set by the ambient ISM density and the efficiency of nonlinear mixing at the bubble/shell interface.
Abstract: 1902.09629
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:The Dark Matter Distributions in Low-Mass Disk Galaxies. I. H$α$ Observations Using the Palomar Cosmic Web Imager
(Submitted on 25 Feb 2019)
Abstract: Dark-matter-only simulations predict that dark matter halos have cusp-like inner density profiles, while observations of low-mass galaxies have found a range of inner slopes that are typically much shallower. It is still not well established whether this discrepancy can be explained by baryonic feedback or if it may require modified dark matter models. To better understand the diversity of dark matter profiles in dwarf galaxies, we undertook a survey of 26 low-mass galaxies ($\log M_*/\textrm{M}_\odot = 8.4-9.8$, $v_{\rm max} = 50-140$ km s$^{-1}$) within 30 Mpc using the Palomar Cosmic Web Imager, which is among the largest integral field spectroscopic surveys of its type. In this paper, we derive H$\alpha$ velocity fields for the full sample with a typical spatial resolution of $\sim$160 pc. We extract rotation curves and verify their robustness to several choices in the analysis. We present a method for improving the velocity precision obtained from image slicing spectrographs using narrowband H$\alpha$ images. For 11 galaxies, we compare the H$\alpha$ velocity fields to CO kinematics measured using CARMA, finding the maps to be in good agreement. The standard deviation of the difference is typically $\sim$7 km s$^{-1}$, comparable to the level of turbulence in the interstellar medium, showing that the two tracers have substantially the same bulk kinematics. In a companion paper, we will use the rotation curves produced here to construct mass models of the galaxies and determine their dark matter density profiles.
Abstract: 1902.09650
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Constraining the non-gravitational scattering of baryons and dark matter with early cosmic structure formation
(Submitted on 25 Feb 2019)
Abstract: We derive new constraints on the non-gravitational baryon-dark-matter scattering (BDMS) by evaluating the mass thresholds of dark matter (DM) haloes in which primordial gas can cool efficiently to form Population III (Pop~III) stars, based on the timing of the observed 21-cm absorption signal. We focus on the BDMS model with interaction cross-section $\sigma=\sigma_{1}[v/(1\ \mathrm{km\ s^{-1}})]^{-4}$, where $v$ is the relative velocity of the encounter. Our results rule out the region in parameter space with $\sigma_{1}\gtrsim 10^{-19}\ \mathrm{cm^{2}}$ and DM particle mass $m_{\chi}c^{2}\lesssim 3\times 10^{-2}$~GeV, where the cosmic number density of Pop~III hosts at redshift $z\sim 20$ is at least three orders of magnitude smaller than in the standard Lambda cold DM ($\Lambda$CDM) case. In these BDMS models, the formation of Pop~III stars is significantly suppressed for $z\gtrsim 20$, inconsistent with the timing of the observed global 21-cm absorption signal. For the fiducial BDMS model with $m_{\chi}c^{2}=0.3$~GeV and $\sigma_{1}=8\times 10^{-20}\ \mathrm{cm^{2}}$, capable of accommodating the measured absorption depth, the number density of Pop~III hosts is reduced by a factor of $3-10$ at $z\sim 15-20$, when the 21-cm signal is imprinted, compared with the $\Lambda$CDM model. The confluence of future detailed cosmological simulations with improved 21-cm observations promises to probe the particle-physics nature of DM at the small-scale frontier of early structure formation.
Abstract: 1902.09654
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Interpreting the relation between the gamma-ray and infrared luminosities of star-forming galaxies
(Submitted on 25 Feb 2019)
Abstract: It has been found that there is a quasi-linear scaling relationship between the gamma-ray luminosity in GeV energies and the total infrared luminosity of star-forming galaxies, i.e. $L_{\gamma}\propto L_{\rm IR}^{\alpha}$ with $\alpha\simeq 1$. However, the origin of this linear slope is not well understood. Although extreme starburst galaxies can be regarded as calorimeters for hadronic cosmic ray interaction and thus a quasi-linear scaling may hold, it may not be the case for low star-formation-rate (SFR) galaxies, as the majority of cosmic rays in these galaxies are expected to escape. We calculate the gamma-ray production efficiency in star-forming galaxies by considering realistic galaxy properties, such as the gas density and galactic wind velocity in star-forming galaxies. We find that the slope for the relation between gamma-ray luminosity and the infrared luminosity gets steeper for low infrared luminosity galaxies, i.e. $\alpha\rightarrow 1.6$, due to increasingly lower efficiency for the production of gamma-ray emission. We further find that the measured data of the gamma-ray luminosity is compatible with such a steepening. The steepening in the slope suggests that cosmic-ray escape is very important in low-SFR galaxies.
Abstract: 1902.09663
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Are starburst galaxies a common source of high energy neutrinos and cosmic rays?
(Submitted on 25 Feb 2019)
Abstract: A recent analysis of cosmic ray air showers observed at the Pierre Auger Observatory indicates that nearby starburst galaxies (SBGs) might be the cause of ~ 10% of the Ultra-High-Energy Cosmic Ray flux at energies E > 39 EeV. Since high energy neutrinos are a direct product of cosmic ray interactions, we investigate SBGs as a possible source of some of the 0.1-1 PeV neutrinos observed at IceCube. A statistical analysis is performed to establish the degree of positional correlation between the observed neutrinos and a set of nearby, radio- and infrared-bright SBGs. Our results are consistent with no causal correlation. However, a scenario where ~ 10% of the neutrino data are coming from the candidate SBGs is not excluded. The same conclusion is reached for two different IceCube data sets (and their subsets, including shower-like and track-like events), as well as two different subsets of SBGs motivated by the Pierre Auger Observatory analysis.
Abstract: 1902.09677
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Calibration of gamma-ray bursts luminosity correlations using gravitational waves as standard sirens
(Submitted on 26 Feb 2019)
Abstract: Gamma-ray bursts (GRBs) are a potential tool to probe high-redshift universe. However, the circularity problem enforces people to find model-independent methods to study the luminosity correlations of GRBs. Here, we present a new method which uses gravitational waves as standard sirens to calibrate GRB luminosity correlations. For the third-generation ground-based GW detectors (i.e., Einstein Telescope), the redshifts of gravitational wave (GW) events accompanied electromagnetic counterparts can reach out to $\sim 4$, which is more distant than type Ia supernovae ($z\lesssim 2$). The Amati relation and Ghirlanda relation are calibrated using mock GW catalogue from Einstein Telescope. We find that the $1\sigma$ uncertainty of intercepts and slopes of these correlations can be constrained to less than 0.2\% and 8\% respectively. Using calibrated correlations, the evolution of dark energy equation of state can be tightly measured, which is important for discriminating dark energy models.
Abstract: 1902.09719
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Kepler-411: a four-planet system with an active host star
(Submitted on 26 Feb 2019)
Abstract: We present a detailed characterization of the Kepler-411 system (KOI 1781). This system was previously known to host two transiting planets: one with a period of 3 days ($R=2.4R_\oplus$; Kepler-411b) and one with a period of 7.8 days ($R=4.4R_\oplus$; Kepler-411c), as well as a transiting planetary candidate with a 58-day period ($R=3.3R_\oplus$; KOI 1781.03) from Kepler photometry. Here, we combine Kepler photometry data and new transit timing variation (TTV) measurements from all the Kepler quarters with previous adaptive-optics imaging results, and dynamical simulations, in order to constrain the properties of the Kepler-411 system. From our analysis, we obtain masses of 25.6$\pm$2.6$M_\oplus$ for Kepler-411b and 26.4$\pm$5.9 $M_\oplus$ for Kepler-411c, and we confirm the planetary nature of KOI 1781.03 with a mass of 15.2$\pm$5.1$M_\oplus$, hence the name Kepler-411d. Furthermore, by assuming near-coplanarity of the system (mutual inclination below $30^\circ$), we discover a nontransiting planet, Kepler-411e, with a mass of 10.8$\pm$1.1$M_\oplus$ on a 31.5-day orbit, which has a strong dynamical interaction with Kepler-411d. With densities of $1.71\pm0.39$~g\,cm$^{-3}$ and $2.32\pm0.83$~g\,cm$^{-3}$, both Kepler-411c and Kepler-411d belong to the group of planets with a massive core and a significant fraction of volatiles. Although Kepler-411b has a sub-Neptune size, it belongs to the group of rocky planets.
Abstract: 1902.09838
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Neutron-induced cross sections -- from raw data to astrophysical rates
(Submitted on 26 Feb 2019)
Abstract: Neutron capture cross sections are one of the most important nuclear inputs to models of stellar nucleosynthesis of the elements heavier than iron. The activation technique and the time-of-flight method are mostly used to determine the required data experimentally. Recent developments of experimental techniques allow for new experiments on radioactive isotopes. Monte-Carlo based analysis methods give new insights into the systematic uncertainties of previous measurements. We present an overview over the state-of-the-art experimental techniques, a detailed new evaluation of the $^{197}$Au(n,$\gamma$) cross section in the keV-regime and the corresponding re-evaluation of 63 more isotopes, which have been measured in the past relative to the gold cross section.
Abstract: 1902.10626
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Identifying Galaxy Mergers in Observations and Simulations with Deep Learning
(Submitted on 27 Feb 2019 (v1), last revised 1 Mar 2019 (this version, v2))
Abstract: Mergers are an important aspect of galaxy formation and evolution. We aim to test whether deep learning techniques can be used to reproduce visual classification of observations, physical classification of simulations and highlight any differences between these two classifications. With one of the main difficulties of merger studies being the lack of a truth sample, we can use our method to test biases in visually identified merger catalogues. A convolutional neural network architecture was developed and trained in two ways: one with observations from SDSS and one with simulated galaxies from EAGLE, processed to mimic the SDSS observations. The SDSS images were also classified by the simulation trained network and the EAGLE images classified by the observation trained network. The observationally trained network achieves an accuracy of 91.5% while the simulation trained network achieves 74.4% on the visually classified SDSS and physically classified EAGLE images respectively. Classifying the SDSS images with the simulation trained network was less successful, only achieving an accuracy of 64.3%, while classifying the EAGLE images with the observation network was very poor, achieving an accuracy of only 49.7% with preferential assignment to the non-merger classification. This suggests that most of the simulated mergers do not have conspicuous merger features and visually identified merger catalogues from observations are incomplete and biased towards certain merger types. The networks trained and tested with the same data perform the best, with observations performing better than simulations, a result of the observational sample being biased towards conspicuous mergers. Classifying SDSS observations with the simulation trained network has proven to work, providing tantalizing prospects for using simulation trained networks for galaxy identification in large surveys.
Abstract: 1902.10719
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:A Dynamical Model for Clustered Star Formation in the Galactic Disk
(Submitted on 27 Feb 2019)
Abstract: The clustered nature of star formation should produce a high degree of structure in the combined phase and chemical space in the Galactic disk. To date, observed structure of this kind has been mostly limited to bound clusters and moving groups. In this paper we present a new dynamical model of the Galactic disk that takes into account the clustered nature of star formation. This model predicts that the combined phase and chemical space is rich in substructure, and that this structure is sensitive to both the precise nature of clustered star formation and the large-scale properties of the Galaxy. The model self-consistently evolves 4 billion stars over the last 5 Gyr in a realistic potential that includes an axisymmetric component, a bar, spiral arms, and giant molecular clouds (GMCs). All stars are born in clusters with an observationally-motivated range of initial conditions. As direct \textit{N}-body calculations for billions of stars is computationally infeasible, we have developed a method of initializing star cluster particles to mimic the effects of direct \textit{N}-body effects, while the actual orbit integrations are treated as test particles within the analytic potential. We demonstrate that the combination of chemical and phase space information is much more effective at identifying truly co-natal populations than either chemical or phase space alone. Furthermore, we show that co-moving pairs of stars are very likely to be co-natal if their velocity separation is $< 2$ km s$^{-1}$ and their metallicity separation is $< 0.05$ dex. The results presented here bode well for harnessing the synergies between \textit{Gaia} and spectroscopic surveys to reveal the assembly history of the Galactic disk.
Abstract: 1902.11271
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Particle Physics at Ultrahigh Energies
(Submitted on 28 Feb 2019)
Abstract: We explore particle physics beyond accelerator energies, motivated by questions exposed in astroparticle physics observations: 1) Are there reasonable modifications to the standard extrapolations of LHC-tuned hadronic interaction models, so that ultrahigh energy cosmic ray (UHECR) showers are well-described with a purely protonic primary composition rather than requiring a tuned, energy-dependent composition mixture as needed in conventional models? 2) What modifications to standard models can solve the deficiency in the predicted ground signal found in hybrid UHECR observations? We find that a pure proton composition provides an excellent fit to shower observations, if the QCD inelastic cross section increases more rapidly above $E_{\rm cm} \approx 60$ TeV than in conventional models, and speculate as to possible reasons this may happen; the "muon deficiency" can be cured by relatively minor modifications to particle ratios in unexplored kinematic regimes below and above LHC energies.
This page created: Tue Mar 5 10:56:33 ACDT 2019 by Jarryd Day
For a printable title listing click here
For details on generating this page see the instructions. If there are problems with this page (and I expect there will be from time to time) contact Jose.
For previous lists of abstracts of interest click Previous abstracts of interest