bluuebunny/crossref_metadata_2025_split huggingface dataset:
Distant Comet C/2017 K2 and the Cohesion Bottleneck
Authors: Jewitt David , Agarwal Jessica , Hui Man-To , Li Jing , Mutchler Max , Weaver Harold 
Published: January 2019

Abstract
The distant long-period comet C/2017 K2 (PANSTARRS) has been outside the planetary region of the solar system for ∼3 Myr, negating the possibility that heat retained from the previous perihelion could be responsible for its activity. This inbound comet is also too cold for water ice to sublimate and too cold for amorphous water ice, if present, to crystallize. C/2017 K2 thus presents an ideal target in which to investigate the mechanisms responsible for activity in distant comets. We have used the Hubble Space Telescope to study the comet in the pre-perihelion heliocentric distance range 13.8 ≤ r

H
≤ 15.9 au. In this range, the coma maintains a logarithmic surface brightness gradient m = −1.010 ± 0.004, consistent with mass loss proceeding in steady state. The absence of a radiation pressure swept tail indicates that the effective particle size is large (radius ≳0.1 mm) and the mass-loss rate is ∼200 kg s−1, remarkable for a comet still beyond the orbit of Saturn. Extrapolation of the photometry indicates that activity began in 2012.1 ± 0.5, at r

H
= 25.9 ± 0.9 au, where the isothermal blackbody temperature is only T

BB
= 55 K. This large distance and low temperature suggest that cometary activity is driven by the sublimation of a super-volatile ice (e.g., CO), presumably preserved by K2's long-term residence in the Oort cloud. The mass-loss rate can be sustained by CO sublimation from an area ≲2 km2, if located near the hot subsolar point on the nucleus. However, while the drag force from sublimated CO is sufficient to lift millimeter-sized particles against the gravity of the cometary nucleus, it is 102–103 times too small to eject these particles against interparticle cohesion. Our observations thus require either a new understanding of the physics of interparticle cohesion or the introduction of another mechanism to drive distant cometary mass loss. We suggest thermal fracture and electrostatic supercharging in this context.
Atmospheric Dynamics and the Variable Transit of KELT-9 b*
Authors: Cauley P. Wilson , Shkolnik Evgenya L. , Ilyin Ilya , Strassmeier Klaus G. , Redfield Seth , Jensen Adam 
Published: January 2019

Abstract
We present a spectrally and temporally resolved detection of the optical Mg i triplet at 7.8σ in the extended atmosphere of the ultra-hot Jupiter KELT-9 b, adding to the list of detected metal species in the hottest gas giant currently known. Constraints are placed on the density and radial extent of the excited hydrogen envelope using simultaneous observations of Hα and Hβ under the assumption of a spherically symmetric atmosphere. We find that planetary rotational broadening of

km s−1 is necessary to reproduce the Balmer line transmission profile shapes, where the model including rotation is strongly preferred over the non-rotating model using a Bayesian information criterion comparison. The time series of both metal line and hydrogen absorption show remarkable structure, suggesting that the atmosphere observed during this transit is dynamic rather than static. We detect a relative emission feature near the end of the transit which exhibits a P-Cygni-like shape, evidence of material moving at ≈50–100 km s−1 away from the planet. We hypothesize that the in-transit variability and subsequent P-Cygni-like profiles are due to a flaring event that caused the atmosphere to expand, resulting in unbound material being accelerated to high speeds by stellar radiation pressure. Further spectroscopic transit observations will help establish the frequency of such events.
Optical Detection of Star Formation in a Cold Dust Cloud in the Counterjet Direction of Centaurus A
Authors: Keel William C. , Banfield Julie K. , Medling Anne M. , Neff Susan G. 
Published: January 2019

Abstract
We have identified a set of optical emission-line features 700″ (12 kpc) to the southwest of the nucleus of Centaurus A, roughly opposite to the radio jet and well-known optical emission filaments associated with the northern radio structure. This location is roughly along the axis of the southwestern radio lobes, although there is no coherent jet at this radius. We use integral-field optical spectroscopy to trace the ratios of strong emission lines, showing changes in excitation across the region, and significant local reddening. The emission regions are spatially associated with far-infrared emission peaks in one of two cold dust clouds identified using Herschel and Spitzer data, and there may be a mismatch between the low temperature of the dust and the expected heating effect of young stars. The strong emission lines have ratios consistent with photoionization in normal H ii regions, requiring only modest numbers of OB stars; these stars and their cooler accompanying populations must be largely obscured along our line of sight. These data fit with a picture of fairly ordinary formation of clusters in a large giant molecular cloud, or network of such clouds. The location, projected near the radio-source axis and within the radius where a starburst wind has been inferred on the other side of the galaxy, raises the question of whether this star-forming episode was enhanced or indeed triggered by an outflow from the central parts of Centaurus A. However, the level of star formation is normal for the associated cold-gas mass and column density, and optical emission-line ratios and line widths limit the role of shocks on the gas, so any interaction with an outflow, associated either with the radio source or star formation in the gas-rich disk of Centaurus A, can at most have compressed the gas weakly. We speculate that the presence of similar star-forming regions on both sides of the galaxy, contrasted with the difference in the character of the emission-line clouds, reflects the presence of a collimated radio jet to the northeast and perhaps anisotropic escape of ionizing radiation from the AGN as well. In this view, the star formation on the southwestern side of Cen A could be enhanced indirectly via compression by a broad outflow (whether originated by a starburst or AGN), distinct from the radio jet and lobes.
The Impact of the Environment on the Early Stages of Radio Source Evolution
Authors: Sobolewska Małgosia , Siemiginowska Aneta , Guainazzi Matteo , Hardcastle Martin , Migliori Giulia , Ostorero Luisa , Stawarz Łukasz 
Published: January 2019

Abstract
Compact symmetric objects (CSOs) show radio features such as jets, lobes, and hot spots that are contained within the central 1 kpc region of their host galaxy. Thus, they are thought to be among the progenitors of large-scale radio galaxies. A debate regarding whether the CSOs are compact primarily because they are young or because they are surrounded by a dense medium impacting their expansion is ongoing. Until now, attempts to discriminate between the environmental and genuine youthfulness scenarios have been inconclusive. We present a study of three CSOs selected on the basis of their puzzling X-ray absorbing properties in prior Beppo-SAX and/or Chandra X-ray Observatory data. Our new XMM-Newton observations unambiguously confirm the nature of their X-ray absorbers. Furthermore, for the first time, our X-ray data reveal the existence of a population of CSOs with intrinsic hydrogen column density N
H > 1023 cm−2 that is different from the population of X-ray unabsorbed CSOs. The two groups appear to be separated in the linear size versus radio power plane. This finding suggests that a dense medium in X-ray obscured CSOs may be able to confine the radio jets. Alternatively, X-ray obscured CSOs could be seen as radio brighter than their unobscured counterparts either because they reside in a dense environment or because they have larger jet powers. Our results help constrain the origin of the X-ray emission and the location and size of the X-ray obscurer in CSOs, and indicate that the environment may play a key role during the initial expansion of a radio source.
An Adaptive Optics Survey of Stellar Variability at the Galactic Center
Authors: Gautam Abhimat Krishna , Do Tuan , Ghez Andrea M. , Morris Mark R. , Martinez Gregory D. , Hosek Matthew W. , Lu Jessica R. , Sakai Shoko , Witzel Gunther , Jia Siyao , Becklin Eric E. , Matthews Keith 
Published: January 2019

Abstract
We present an ≈11.5 yr adaptive optics (AO) study of stellar variability and search for eclipsing binaries in the central ∼0.4 pc (∼10″) of the Milky Way nuclear star cluster. We measure the photometry of 563 stars using the Keck II NIRC2 imager (K′-band, λ
0 = 2.124 μm). We achieve a photometric uncertainty floor of Δm

K′ ∼ 0.03 (≈3%), comparable to the highest precision achieved in other AO studies. Approximately half of our sample (50% ± 2%) shows variability: 52% ± 5% of known early-type young stars and 43% ± 4% of known late-type giants are variable. These variability fractions are higher than those of other young, massive star populations or late-type giants in globular clusters, and can be largely explained by two factors. First, our experiment time baseline is sensitive to long-term intrinsic stellar variability. Second, the proper motion of stars behind spatial inhomogeneities in the foreground extinction screen can lead to variability. We recover the two known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We constrain the Galactic center eclipsing binary fraction of known early-type stars to be at least 2.4% ± 1.7%. We find no evidence of an eclipsing binary among the young S-stars nor among the young stellar disk members. These results are consistent with the local OB eclipsing binary fraction. We identify a new periodic variable, S2-36, with a 39.43 days period. Further observations are necessary to determine the nature of this source.
The JCMT Transient Survey: An Extraordinary Submillimeter Flare in the T Tauri Binary System JW 566
Authors: Mairs Steve , Lalchand Bhavana , Bower Geoffrey C. , Forbrich Jan , Bell Graham S. , Herczeg Gregory J. , Johnstone Doug , Chen Wen-Ping , Lee Jeong-Eun , Hacar Alvaro 
Published: January 2019

Abstract
The binary T Tauri system JW 566 in the Orion Molecular Cloud underwent an energetic, short-lived flare observed at submillimeter wavelengths by the SCUBA-2 instrument on 2016 November 26 (UT). The emission faded by nearly 50% during the 31 minute integration. The simultaneous source fluxes averaged over the observation are

at 450 μm and

at 850 μm. The 850 μm flux corresponds to a radio luminosity of

, approximately one order of magnitude brighter (in terms of

) than that of a flare of the young star GMR-A, detected in Orion in 2003 at 3mm. The event may be the most luminous known flare associated with a young stellar object and is also the first coronal flare discovered at submillimeter wavelengths. The spectral index between 450 and 850 μm of α = 0.11 is broadly consistent with nonthermal emission. The brightness temperature was in excess of

. We interpret this event to be a magnetic reconnection that energized charged particles to emit gyrosynchrotron/synchrotron radiation.
Quantifying the CMB Degeneracy between the Matter Density and Hubble Constant in Current Experiments
Authors: Kable Joshua A. , Addison Graeme E. , Bennett Charles L. 
Published: January 2019

Abstract
We revisit the degeneracy between the Hubble constant, H
0, and matter density, Ω
m
, for current cosmic microwave background (CMB) observations within the standard ΛCDM model. We show that Planck, Wilkinson Microwave Anisotropy Probe (WMAP), South Pole Telescope (SPT), and Atacama Cosmology Telescope Polarimeter (ACTPol) temperature power spectra produce different values of the exponent x from minimizing the variance of the product

. The distribution of x from the different data sets does not follow the Markov Chain Monte Carlo (MCMC) best-fit values for H
0 or Ω
m
. Particularly striking is the difference between Planck multipoles ℓ ≤ 800 (x = 2.81), and WMAP (x = 2.94), despite very similar best-fit cosmologies. We use a Fisher matrix analysis to show that, in fact, this range in exponent values is exactly as expected in ΛCDM given the multipole coverage and power spectrum uncertainties for each experiment. We show that the difference in x from the Planck ℓ ≤ 800 and WMAP data is explained by a turning point in the relationship between x and the maximum effective multipole, at around ℓ = 700. The value of x is determined by several physical effects, and we highlight the significant impact of gravitational lensing for the high-multipole measurements. Despite the spread of H
0 values from different CMB experiments, the experiments are consistent with their sampling of the Ω
m
− H
0 degeneracy and do not show evidence for the need for new physics or for the presence of significant underestimated systematics according to these tests. The Fisher calculations can be used to predict the Ω
m
− H
0 degeneracy of future experiments.
Searching a Thousand Radio Pulsars for Gamma-Ray Emission
Authors: Smith D. A. , Bruel P. , Cognard I. , Cameron A. D. , Camilo F. , Dai S. , Guillemot L. , Johnson T. J. , Johnston S. , Keith M. J. , Kerr M. , Kramer M. , Lyne A. G. , Manchester R. N. , Shannon R. , Sobey C. , Stappers B. W. , Weltevrede P. 
Published: January 2019

Abstract
Identifying as many gamma-ray pulsars as possible in the Fermi Large Area Telescope (LAT) data helps test pulsar emission models by comparing predicted and observed properties for a large, varied sample with as little selection bias as possible. It also improves extrapolations from the observed population to estimate the contribution of unresolved pulsars to the diffuse gamma-ray emission. We use a recently developed method to determine the probability that a given gamma-ray photon comes from a known position in the sky, convolving the photon’s energy with the LAT’s energy-dependent point-spread function, without the need for an accurate spatial and spectral model of the gamma-ray sky around the pulsar. The method is simple and fast and, importantly, provides probabilities, or weights, for gamma-rays from pulsars too faint for phase-integrated detection. We applied the method to over a thousand pulsars for which we obtained rotation ephemerides from radio observations, and discovered gamma-ray pulsations from 16 pulsars, 12 young and 4 recycled. PSR J2208+4056 has spindown power

erg s−1, about three times lower than the previous observed gamma-ray emission “deathline.” PSRs J2208+4056 and J1816−0755 have radio interpulses, constraining their geometry and perhaps enhancing their gamma-ray luminosity. We discuss whether the deathline is an artifact of selection bias due to the pulsar distance.
Identifying the 3FHL Catalog. I. Archival Swift Observations and Source Classification
Authors: Kaur A. , Ajello M. , Marchesi S. , Omodei N. 
Published: January 2019

Abstract
We present the results of an identification campaign of unassociated sources from the Fermi Large Area Telescope 3FHL catalog. Out of 200 unidentified sources, we selected 110 sources for which archival Swift-XRT observations were available, 52 of which were found to have exactly one X-ray counterpart within the 3FHL 95% positional uncertainty. In this work, we report the X-ray, optical, IR, and radio properties of these 52 sources using positional associations with objects in various catalogs. The Wide-field Infrared Survey Explorer color–color plot for sources suggests that most of these belong to the blazar class family. The redshift measurements for these objects range from z = 0.277 to z = 2.1. Additionally, under the assumption that the majority of these sources are blazars, three machine-learning algorithms are employed to classify the sample into flat spectrum radio quasars or BL Lacertae objects. These suggest that the majority of the previously unassociated sources are BL Lac objects, in agreement with the fact the BL Lac objects represent by far the most numerous population detected above 10 GeV in 3FHL.
Implications of the Warm Corona and Relativistic Reflection Models for the Soft Excess in Mrk 509
Authors: García Javier A. , Kara Erin , Walton Dominic , Beuchert Tobias , Dauser Thomas , Gatuzz Efrain , Balokovic Mislav , Steiner James F. , Tombesi Francesco , Connors Riley M. T. , Kallman Timothy R. , Harrison Fiona A. , Fabian Andrew , Wilms Jörn , Stern Daniel , Lanz Lauranne , Ricci Claudio , Ballantyne David R. 
Published: January 2019

Abstract
We present the analysis of the first Nuclear Spectroscopic Telescope Array observations (∼220 ks), simultaneous with the last Suzaku observations (∼50 ks), of the active galactic nucleus of the bright Seyfert 1 galaxy Mrk 509. The time-averaged spectrum in the 1–79 keV X-ray band is dominated by a power-law continuum (Γ ∼ 1.8–1.9), a strong soft excess around 1 keV, and signatures of X-ray reflection in the form of Fe K emission (∼6.4 keV), an Fe K absorption edge (∼7.1 keV), and a Compton hump due to electron scattering (∼20–30 keV). We show that these data can be described by two very different prescriptions for the soft excess: a warm (kT ∼ 0.5–1 keV) and optically thick (τ ∼ 10–20) Comptonizing corona or a relativistically blurred ionized reflection spectrum from the inner regions of the accretion disk. While these two scenarios cannot be distinguished based on their fit statistics, we argue that the parameters required by the warm corona model are physically incompatible with the conditions of standard coronae. Detailed photoionization calculations show that even in the most favorable conditions, the warm corona should produce strong absorption in the observed spectrum. On the other hand, while the relativistic reflection model provides a satisfactory description of the data, it also requires extreme parameters, such as maximum black hole spin, a very low and compact hot corona, and a very high density for the inner accretion disk. Deeper observations of this source are thus necessary to confirm the presence of relativistic reflection and further understand the nature of its soft excess.
Sources and processes that control the submicron organic aerosol composition in an urban Mediterranean environment (Athens): a high temporal-resolution chemical composition measurement study
Authors: Stavroulas Iasonas , Bougiatioti Aikaterini , Grivas Georgios , Paraskevopoulou Despina , Tsagkaraki Maria , Zarmpas Pavlos , Liakakou Eleni , Gerasopoulos Evangelos , Mihalopoulos Nikolaos 
Published: January 2019

Abstract. Submicron aerosol chemical composition was studied during a year-long
period (26 July 2016–31 July 2017) and two wintertime intensive campaigns
(18 December 2013–21 February 2014 and 23 December 2015–17 February 2016),
at a central site in Athens, Greece, using an Aerosol Chemical Speciation
Monitor (ACSM). Concurrent measurements included a particle-into-liquid
sampler (PILS-IC), a scanning mobility particle sizer (SMPS), an AE-33
Aethalometer, and ion chromatography analysis on 24 or 12 h filter samples.
The aim of the study was to characterize the seasonal variability of the main
submicron aerosol constituents and decipher the sources of organic aerosol
(OA). Organics were found to contribute almost half of the submicron mass,
with 30 min resolution concentrations during wintertime reaching up to
200 µg m−3. During winter (all three campaigns combined),
primary sources contributed about 33 % of the organic fraction, and comprised
biomass burning (10 %), fossil fuel combustion (13 %), and cooking
(10 %), while the remaining 67 % was attributed to secondary aerosol.
The semi-volatile component of the oxidized organic aerosol (SV-OOA;
22 %) was found to be clearly linked to combustion sources, in
particular biomass burning; part of the very oxidized,
low-volatility component (LV-OOA; 44 %) could also be attributed to the
oxidation of emissions from these primary combustion sources. These results, based on the combined contribution of biomass burning organic
aerosol (BBOA) and SV-OOA, indicate the importance of increased biomass
burning in the urban environment of Athens as a result of the economic recession.
During summer, when concentrations of fine aerosols are considerably lower,
more than 80 % of the organic fraction is attributed to secondary aerosol
(SV-OOA 31 % and LV-OOA 53 %). In contrast to winter, SV-OOA appears
to result from a well-mixed type of aerosol that is linked to fast photochemical
processes and the oxidation of primary traffic and biogenic emissions.
Finally, LV-OOA presents a more regional character in summer, owing to the
oxidation of OA over the period of a few days.
What Is a Good Mediator?
Authors: Garby Thierry 
Published: January 2019

What Is a Good Mediator?
When the time comes to select a mediator, the judge, the lawyers, the parties or the mediation centre will want to find a good one. This raises two questions: what is a good mediator and how to find one?The answer to the first question seems rather simple: a good mediator is one that the parties are happy with. This raises another set of questions: if the parties were happy with a mediator in one case, would they be happy with the same person in another case? Would other parties be happy with this mediator as well? If the answers are not a yes without reservation to both of those questions, then the question becomes: who would be a good mediator for this case between these parties?
Channels
Authors: Connor Steven 
Published: January 2019

What follows is a meditation on a metaphor that continues to provide a way of conceiving our relation to the space of media. It may seem puzzling that I spend as long as I am going to thinking about the prehistory of the idea of the channel in non-communicative contexts. My offer is to try to measure the continuing force of the literality, whatever that may be, of the idea of a channel. Towards the end, I will offer some reflections on the dissolution of the idea of the channel, or of the spatial disposition that has previously constrained what a channel is or can do. I will be saying that the channel depends not upon broadband expression, but on its low bandwidth, on the fact that data must be squeezed through a narrow pipe. My proposition is that decompression of data, the removal of this constraint means a net loss rather than a net gain of information. With no resistance, you can have very high voltage with no amperage. Those of you who want to get quickly to the payload of this piece should scroll through to the last 15 minutes or so. But you can’t if you are istening to this, because this talk is itself a narrow pipe. And, if you could, you would get less information, not more, because there would be less noise for it to have overcome.