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Results in Journal The Astrophysical Journal: 123,142

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, , Timothy Shimwell, Mateusz Olech, , Harish K. Vedantham, Glenn J. White, Marco Iacobelli, Alexander Drabent
The Astrophysical Journal, Volume 919; https://doi.org/10.3847/1538-4357/ac0fda

Abstract:
We present observations of planetary nebulae with the LOw Frequency ARray (LOFAR) between 120 and 168 MHz. The images show thermal free–free emission from the nebular shells. We have determined the electron temperatures for spatially resolved, optically thick nebulae. These temperatures are 20%–60% lower than those estimated from collisionally excited optical emission lines. This strongly supports the existence of a cold plasma component, which co-exists with hot plasma in planetary nebulae. This cold plasma does not contribute to the collisionally excited lines, but does contribute to recombination lines and radio flux. Neither of the plasma components are spatially resolved in our images, although we infer that the cold plasma extends to the outer radii of planetary nebulae. However, more cold plasma appears to exist at smaller radii. The presence of cold plasma should be taken into account in modeling of radio emission of planetary nebulae. Modelling of radio emission usually uses electron temperatures calculated from collisionally excited optical and/or infrared lines. This may lead to an underestimate of the ionized mass and an overestimate of the extinction correction from planetary nebulae when derived from the radio flux alone. The correction improves the consistency of extinction derived from the radio fluxes when compared to estimates from the Balmer decrement flux ratios.
, Fabien R. Baron, , Claudia Paladini, Matthew D. Anderson, , Gail H. Schaefer, Xiao Che, , Michael S. Connelley, et al.
The Astrophysical Journal, Volume 919; https://doi.org/10.3847/1538-4357/ac0c7e

Abstract:
We present H-band interferometric observations of the red supergiant (RSG) AZ Cyg that were made with the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy (CHARA) Array. The observations span 5 yr (2011–2016), which offers insight into the short and long-term evolution of surface features on RSGs. Using a spectrum of AZ Cyg obtained with SpeX on the NASA InfraRed Telescope Facility (IRTF) and synthetic spectra calculated from spherical MARCS, spherical PHOENIX, and SAtlas model atmospheres, we derive Teff is between 3972 K and 4000 K and between −0.50 and 0.00, depending on the stellar model used. Using fits to the squared visibility and GAIA parallaxes, we measure its average radius . Reconstructions of the stellar surface using our model-independent imaging codes SQUEEZE and OITOOLS.jl show a complex surface with small bright features that appear to vary on a timescale of less than one year and larger features that persist for more than one year. The 1D power spectra of these images suggest a characteristic size of 0.52–0.69 R⋆ for the larger, long lived features. This is close to the values of 0.51–0.53 R⋆ that are derived from 3D RHD models of stellar surfaces. We conclude that interferometric imaging of this star is in line with predictions of 3D RHD models but that short-term imaging is needed to more stringently test predictions of convection in RSGs.
The Astrophysical Journal, Volume 919; https://doi.org/10.3847/1538-4357/ac110a

Abstract:
We present detailed radio observations of the tidal disruption event (TDE) AT2019dsg, obtained with the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), and spanning 55–560 days post disruption. We find that the peak brightness of the radio emission increases until ∼200 days and subsequently begins to decrease steadily. Using a standard equipartition analysis, including the effects of synchrotron cooling as determined by the joint VLA–ALMA spectral energy distributions, we find that the outflow powering the radio emission is in roughly free expansion with a velocity of ≈0.07 c, while its kinetic energy increases by a factor of about 5 from 55 to 200 days and plateaus at ≈4.4 × 1048 erg thereafter. The ambient density traced by the outflow declines as radius ≈R−1.7 on a scale of ≈(1–4) × 1016 cm (≈6300–25,000 Rs), followed by a steeper decline to ≈7 × 1016 cm (≈44,000 Rs). Allowing for a collimated geometry, we find that to reach even mildly relativistic velocities (Γ = 2) the outflow requires an opening angle of θj ≈ 2°, which is narrow even by the standards of gamma-ray burst jets; a truly relativistic outflow requires an unphysically narrow jet. The outflow velocity and kinetic energy in AT2019dsg are typical of previous non-relativistic TDEs, and comparable to those from type Ib/c supernovae, raising doubts about the claimed association with a high-energy neutrino event.
Soojeong Jang, , , , , , Yeon-Han Kim
The Astrophysical Journal, Volume 920; https://doi.org/10.3847/2041-8213/ac2a46

Abstract:
We present, for the first time, a deep learning model that returns the three-dimensional (3D) coronal electron density from coronagraphic images. The intensity of coronagraphic observations arises from the Thomson scattering of photospheric light by the coronal electrons. We use MHD numerical simulations to obtain realistic 3D electron density and construct error-free training sets consisting of input (observation) and target (electron density) images. In the training sets, the input images are directly synthesized from the target 3D electron density by applying the Thomson scattering theory. The input and target images are in the form of latitude–longitude maps given at a radius, often referred to as synoptic maps. Using synoptic maps reduces a tomographic method to an image translation problem. We use pix2pixHD, one of the well-established supervised image translation methods and develop models for six selected heights: 2.0, 2.2, 2.5, 4.0, 6.0, and 12.0 solar radii. All six models have similar performance and the mean absolute percent error of the generated density images is less than 7% with respect to the ground-truth simulated data sets.
A. Albert, S. Alves, M. André, M. Anghinolfi, G. Anton, M. Ardid, J.-J. Aubert, J. Aublin, B. Baret, S. Basa, et al.
The Astrophysical Journal, Volume 920; https://doi.org/10.3847/1538-4357/ac16d6

Jia-Ming Chen, Zhao-Yang Peng, Tan-Tan Du, Yue Yin, Hui Wu
The Astrophysical Journal, Volume 920; https://doi.org/10.3847/1538-4357/ac14b8

, S. L. Xiong, , L. M. Song, F. J. Lu, Y. Huang, C. Cai, Q. B. Yi, X. Y. Song, W. Chen, et al.
The Astrophysical Journal, Volume 920; https://doi.org/10.3847/1538-4357/ac1420

Sara Frederick, Suvi Gezari, Matthew J. Graham, , , , , Charlotte Ward, Erica Hammerstein, Tiara Hung, et al.
The Astrophysical Journal, Volume 920; https://doi.org/10.3847/1538-4357/ac110f

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