adsabs/SciX_UAT_keywords huggingface dataset:
The Paschen Jump as a Diagnostic of the Diffuse Nebular Continuum Emission in Active Galactic Nuclei

Photoionization modeling of active galactic nuclei (AGN) predicts that diffuse continuum (DC) emission from the broad-line region makes a substantial contribution to the total continuum emission from ultraviolet through near-infrared wavelengths. Evidence for this DC component is present in the strong Balmer jump feature in AGN spectra, and possibly from reverberation measurements that find longer lags than expected from disk emission alone. However, the Balmer jump region contains numerous blended emission features, making it difficult to isolate the DC emission strength. In contrast, the Paschen jump region near 8200 Å is relatively uncontaminated by other strong emission features. Here, we examine whether the Paschen jump can aid in constraining the DC contribution, using Hubble Space Telescope Space Telescope Imaging Spectrograph spectra of six nearby Seyfert 1 nuclei. The spectra appear smooth across the Paschen edge, and we find no evidence of a Paschen spectral break or jump in total flux. We fit multicomponent spectral models over the range 6800-9700 Å and find that the spectra can still be compatible with a significant DC contribution if the DC Paschen jump is offset by an opposite spectral break resulting from blended high-order Paschen emission lines. The fits imply DC contributions ranging from ~10% to 50% at 8000 Å, but the fitting results are highly dependent on assumptions made about other model components. These degeneracies can potentially be alleviated by carrying out fits over a broader wavelength range, provided that models can accurately represent the disk continuum shape, Fe II emission, high-order Balmer line emission, and other components. *Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with programs 8136, 8479, 14121, 14744, 15124, and 15413.

Keywords: spectroscopy; active galactic nuclei; seyfert galaxies
Tidal Disruptions of Main-sequence Stars. III. Stellar Mass Dependence of the Character of Partial Disruptions

In this paper, the third in this series, we continue our study of tidal disruption events of main-sequence stars by a nonspinning 106 M⊙ supermassive black hole. Here we focus on the stellar mass dependence of the outcomes of partial disruptions. As the encounter becomes weaker, the debris mass is increasingly concentrated near the outer edges of the energy distribution. As a result, the mass fallback rate can deviate substantially from a t-5/3 power law, becoming more like a single peak with a tail declining as t-p with p ≃ 2-5. Surviving remnants are spun-up in the prograde direction and are hotter than main-sequence stars of the same mass. Their specific orbital energy is ≃10-3× that of the debris, but of either sign with respect to the black hole potential, while their specific angular momentum is close to that of the original star. Even for strong encounters, remnants have speeds at infinity relative to the black hole potential ≲300 km s-1, so they are unable to travel far out into the galactic bulge. The remnants most deeply bound to the black hole go through a second tidal disruption event upon their first return to pericenter; if they have not thermally relaxed, they will be completely disrupted.

Keywords: supermassive black holes; tidal disruption; general relativity; hydrodynamics; hydrodynamical simulations; galactic center; gravitation; stellar dynamics
Characterizing Solar Surface Convection Using Doppler Measurements

The Helioseismic Magnetic Imager on board the Solar Dynamics Observatory records line-of-sight Dopplergram images of convective flows on the surface. These images are used to obtain the multiscale convective spectrum. We design a pipeline to process the raw images to remove large-scale features like differential rotation, meridional circulation, limb shift, and imaging artifacts. The Hierarchical Equal Area Pixelization scheme is used to perform spherical harmonic transforms on the cleaned image. Because we only have access to line-of-sight velocities on half the solar surface, we define a "mixing matrix" to relate the observed and true spectra. This enables the inference of poloidal and toroidal flow spectra in a single step through the inversion of the mixing matrix. Performing inversions on a number of flow profiles, we find that the poloidal flow recovery is most reliable among all the components. We also find that the poloidal spectrum is in qualitative agreement with inferences from Local Correlation Tracking of granules. The fraction of power in vertical motions increases as a function of wavenumber and is at the 8% level for ℓ = 1500. In contrast to seismic results and LCT, the flows show nearly no temporal-frequency dependence. Poloidal flow power peaks in the range of ℓ - ∣m∣ ≈ 150-250, which may potentially hint at a latitudinal preference for convective flows.

Keywords: solar photosphere; supergranulation
The Influence of Outflow Feedback in Clumps

We analyzed the influence of outflow feedback from two perspectives: turbulent support and potential disruptive effect, of which 694 clumps and 188 have been identified as outflow candidates. For turbulent support, we find the slopes of E turb - R clump (turbulent energy and radius of the clump) and P turb - R clump (turbulent momentum and radius of the clump) have no difference and are consistent with expected values whether there is outflow feedback in clumps or not. The ratios of the outflow energy and momentum to the turbulence energy and momentum (E flow/E turb, P flow/P turb) show that the majority of clumps have not enough energy and momentum to support turbulence. Meanwhile, there is no correlation between the velocity dispersion and radius. For potential disruptive effects, we conclude that it is impossible for the outflow activities to disrupt entire clumps and as the mass of the clumps increases, the clumps becomes harder to destroy. Finally, we do not see evidence that the virial parameter changes significantly whether the clumps have outflow candidates or not.

Keywords: interstellar clouds; interstellar dynamics; jets; star formation
GRB 160625B: Evidence for a Gaussian-shaped Jet

We present multiwavelength modeling of the afterglow from the long γ-ray burst (GRB) 160625B using Markov Chain Monte Carlo techniques of the afterglowpy Python package. GRB 160625B is an extremely bright burst with a rich set of observations spanning from radio to γ-ray frequencies. These observations range from ∼0.1 days to >1000 days, thus making this event extremely well suited to such modeling. In this work we compare top-hat and Gaussian jet structure types in order to find best-fit values for the GRB jet collimation angle, viewing angle, and other physical parameters. We find that a Gaussian-shaped jet is preferred (2.7σ-5.3σ) over the traditional top-hat model. Our estimate for the opening angle of the burst ranges from 1°26 to 3°90, depending on jet-shape model. We also discuss the implications that assumptions on jet shape, viewing angle, and particularly the participation a fraction of electrons have on the final estimation of GRB intrinsic energy release and the resulting energy budget of the relativistic outflow. Most notably, allowing the participation fraction to vary results in an estimated total relativistic energy of ∼1053 erg. This is two orders of magnitude higher than when the total fraction is assumed to be unity; thus, this parameter has strong relevance for placing constraints on long GRB central engines, details of the circumburst media, and host environment.

Keywords: gamma-ray bursts; relativistic jets
The Contribution of Evolved Stars to Polycyclic Aromatic Hydrocarbon Heating and Implications for Estimating Star Formation Rates

Emission from polycyclic aromatic hydrocarbons (PAHs) is a promising tool for estimating star formation rate (SFR) in galaxies, but the origin of its sources of excitation, which include not only young but possibly also old stars, remains uncertain. We analyze Spitzer mid-infrared mapping-mode spectroscopic observations of the nuclear and extranuclear regions of 33 nearby galaxies to study the contribution of evolved stars to PAH emission. In combination with photometric measurements derived from ultraviolet, Hα, and infrared images, the spatially resolved spectral decomposition enables us to characterize the PAH emission, SFR, and stellar mass of the sample galaxies on subkiloparsec scales. We demonstrate that the traditional empirical correlation between PAH luminosity and SFR has a secondary dependence on specific SFR, or, equivalently, stellar mass. Ultraviolet-faint regions with lower specific SFRs and hence a greater fraction of evolved stars emit stronger PAH emission at fixed SFR than ultraviolet-bright regions. We reformulate the PAH-based SFR estimator by explicitly introducing stellar mass as a second parameter to account for the contribution of evolved stars to PAH excitation. The influence of evolved stars can explain the sublinear correlation between PAH emission and SFR, and it can partly account for the PAH deficit in dwarf galaxies and low-metallicity environments.

Keywords: polycyclic aromatic hydrocarbons; star formation; interstellar emissions; interstellar medium; evolved stars
The Hubble Space Telescope PanCET Program: An Optical to Infrared Transmission Spectrum of HAT-P-32Ab

We present a 0.3-5 μm transmission spectrum of the hot Jupiter HAT-P-32Ab observed with the Space Telescope Imaging Spectrograph and Wide Field Camera 3 instruments mounted on the Hubble Space Telescope, combined with Spitzer Infrared Array Camera photometry. The spectrum is composed of 51 spectrophotometric bins with widths ranging between 150 and 400 Å, measured to a median precision of 215 ppm. Comparisons of the observed transmission spectrum to a grid of 1D radiative-convective equilibrium models indicate the presence of clouds/hazes, consistent with previous transit observations and secondary eclipse measurements. To provide more robust constraints on the planet's atmospheric properties, we perform the first full optical to infrared retrieval analysis for this planet. The retrieved spectrum is consistent with a limb temperature of ${1248}_{-92}^{+92}$ K, a thick cloud deck, enhanced Rayleigh scattering, and ∼10× solar H2O abundance. We find log(Z/Z⊙) = ${2.41}_{-0.07}^{+0.06}$ , and compare this measurement with the mass-metallicity relation derived for the solar system.

Keywords: exoplanet atmospheric composition; exoplanet atmospheres; exoplanets; hot jupiters
Detrending Exoplanetary Transit Light Curves with Long Short-term Memory Networks

The precise derivation of transit depths from transit light curves is a key component for measuring exoplanet transit spectra, and henceforth for the study of exoplanet atmospheres. However, it is still deeply affected by various kinds of systematic errors and noise. In this paper we propose a new detrending method by reconstructing the stellar flux baseline during transit time. We train a probabilistic long short-term memory (LSTM) network to predict the next data point of the light curve during the out-of-transit, and use this model to reconstruct a transit-free light curve—i.e., including only the systematics—during the in-transit. By making no assumption about the instrument, and using only the transit ephemeris, this provides a general way to correct the systematics and perform a subsequent transit fit. The name of the proposed model is TLCD-LSTM, standing for transit light-curve detrending-LSTM. Here we present the first results on data from six transit observations of HD 189733b with the IRAC camera on board the Spitzer Space Telescope, and discuss some of its possible further applications.

Keywords: exoplanet atmospheres; photometric systems; astronomy data analysis; extrasolar gaseous giant planets; neural networks
Search for the Gravitational-wave Background from Cosmic Strings with the Parkes Pulsar Timing Array Second Data Release

We perform a direct search for an isotropic stochastic gravitational-wave background (SGWB) produced by cosmic strings in the Parkes Pulsar Timing Array (PPTA) Data Release 2 (DR2). We find no evidence for such an SGWB, and therefore place a 95% confidence level upper limit on the cosmic string tension, G μ, as a function of the reconnection probability, p, which can be less than 1 in the string-theory-inspired models or pure Yang–Mills theory. The upper bound on the cosmic string tension is G μ ≲ 5.1 × 10‑10 for p = 1, which is about five orders of magnitude tighter than the bound derived from the null search of individual gravitational-wave bursts from cosmic string cusps in the PPTA DR2, and comparable to previous bounds derived from the null search of the SGWB from cosmic strings.

Keywords: millisecond pulsars
Distinguishing between Wet and Dry Atmospheres of TRAPPIST-1 e and f

The nearby TRAPPIST-1 planetary system is an exciting target for characterizing the atmospheres of terrestrial planets. The planets e, f, and g lie in the circumstellar habitable zone and could sustain liquid water on their surfaces. During the extended pre-main-sequence phase of TRAPPIST-1, however, the planets may have experienced extreme water loss, leading to a desiccated mantle. The presence or absence of an ocean is challenging to determine with current and next-generation telescopes. Therefore, we investigate whether indirect evidence of an ocean and/or a biosphere can be inferred from observations of the planetary atmosphere. We introduce a newly developed photochemical model for planetary atmospheres, coupled to a radiative-convective model, and validate it against modern Earth, Venus, and Mars. The coupled model is applied to the TRAPPIST-1 planets e and f, assuming different surface conditions and varying amounts of CO2 in the atmosphere. As input for the model we use a constructed spectrum of TRAPPIST-1, based on near-simultaneous data from X-ray to optical wavelengths. We compute cloud-free transmission spectra of the planetary atmospheres and determine the detectability of molecular features using the Extremely Large Telescope (ELT) and the James Webb Space Telescope (JWST). We find that under certain conditions the existence or nonexistence of a biosphere and/or an ocean can be inferred by combining 30 transit observations with ELT and JWST within the K band. A nondetection of CO could suggest the existence of an ocean, whereas significant CH4 hints at the presence of a biosphere.

Keywords: exoplanet atmospheres; exoplanet atmospheric composition; planetary atmospheres
Faculae Cancel out on the Surfaces of Active Suns

Surfaces of the Sun and other cool stars are filled with magnetic fields, which are either seen as dark compact spots or more diffuse bright structures like faculae. Both hamper detection and characterization of exoplanets, affecting stellar brightness and spectra, as well as transmission spectra. However, the expected facular and spot signals in stellar data are quite different, for instance, they have distinct temporal and spectral profiles. Consequently, corrections of stellar data for magnetic activity can greatly benefit from the insight on whether the stellar signal is dominated by spots or faculae. Here, we utilize a surface flux transport model to show that more effective cancellation of diffuse magnetic flux associated with faculae leads to spot area coverages increasing faster with stellar magnetic activity than that by faculae. Our calculations explain the observed dependence between solar spot and facular area coverages and allow its extension to stars that are more active than the Sun. This extension enables anticipating the properties of stellar signal and its more reliable mitigation, leading to a more accurate characterization of exoplanets and their atmospheres.

Keywords: stellar astronomy; solar physics
A New Insight into the Linear Theory of Magnetoacoustic Waves in the Homogeneous Flux Tubes with an Abrupt Boundary

An important and interesting issue is brought to notice in the standard linear theory of the magnetoacoustic waves in the homogeneous magnetic flux tubes with an abrupt magnetic boundary, which are representative of the coronal, chromospheric, and photospheric loops of the solar atmosphere. It is shown that the dispersion relation given for the magnetoacoustic waves of such flux tubes by the linear standard theory is generally inapplicable because it does not take into account the electrodynamic boundary condition and the applicability is only restricted to the case β = 0, where β is the usual plasma parameter given by the ratio of the thermodynamic gas pressure to the magnetic pressure. This issue is then addressed, which gives us new insight into the standard linear theory of such modes, and a new dispersion relation is obtained, which is generally applicable i.e., for all β and would reduce to the earlier expression as a special case for β = 0. Finally a comparison is drawn between the dispersion curves described by the newly derived dispersion relation and its previous counterpart.

Keywords: solar corona
The Not So Simple Stellar System ω Cen. II. Evidence in Support of a Merging Scenario

We present multiband photometry covering ∼5° × 5° across ω Cen collected with the Dark Energy Camera on the 4 m Blanco telescope, combined with Hubble Space Telescope and Wide Field Imager data for the central regions. The unprecedented photometric accuracy and field coverage allows us to confirm the different spatial distribution of blue and red main-sequence stars, and of red giant branch (RGB) stars with different metallicities. The ratio of the number of blue to red main-sequence stars shows that the blue main-sequence stellar subpopulation has a more extended spatial distribution compared to the red main-sequence one, with the frequency of blue main-sequence stars increasing at a distance of ∼20' from ω Cen's center. Similarly, the more metal-rich RGB stars show a more extended spatial distribution compared to the more metal-poor ones in the outskirts of the cluster. Moreover, the centers of the distributions of metal-rich and metal-poor RGB stars are shifted in different directions with respect to the geometrical center of ω Cen. We constructed stellar density profiles for the blue and red main-sequence stars; they show that the blue main-sequence stellar subpopulation has a more extended spatial distribution compared to the red main-sequence one in the outskirts of ω Cen, confirming the results based on the number ratio. We also computed the ellipticity profile of ω Cen, which has a maximum value of 0.16 at a distance of ∼8' from the center, and a minimum of 0.05 at ∼30'; the average ellipticity is ∼0.10. The circumstantial evidence presented in this work suggests a merging scenario for the formation of the peculiar stellar system ω Cen.

Keywords: globular star clusters; stellar populations; galaxy stellar content; dwarf galaxies; stellar evolution; hertzsprung russell diagram
Statistical Analysis of the Dearth of Super-eccentric Jupiters in the Kepler Sample

Hot Jupiters may have formed in situ, or been delivered to their observed short periods through one of two categories of migration mechanisms: disk migration or high-eccentricity migration. If hot Jupiters were delivered by high-eccentricity migration, we would expect to observe some "super-eccentric" Jupiters in the process of migrating. We update a prediction for the number of super-eccentric Jupiters we would expect to observe in the Kepler sample if all hot Jupiters migrated through high-eccentricity migration and estimate the true number observed by Kepler. We find that the observations fail to match the prediction from high-eccentricity migration with 94.3% confidence and show that high-eccentricity migration can account for at most ~62% of the hot Jupiters discovered by Kepler.

Keywords: transit photometry; exoplanet migration; exoplanets; star-planet interactions; exoplanet dynamics; hot jupiters
Effects of Active Galactic Nucleus Feedback on Cold Gas Depletion and Quenching of Central Galaxies

We investigate the influence of active galactic nucleus (AGN) feedback on the galaxy cold gas content and its connection to galaxy quenching in three hydrodynamical simulations of Illustris, IllustrisTNG, and SIMBA. By comparing to the observed atomic and molecular neutral hydrogen measurements for central galaxies, we find that Illustris overpredicts the cold gas masses in star-forming galaxies and significantly underpredicts them for quenched galaxies. IllustrisTNG performs better in this comparison than Illustris, but quenched galaxies retain too much cold gas compared with observations. SIMBA shows good agreement with observations, by depleting the global cold gas reservoir for quenched galaxies. We find that the discrepancies in IllustrisTNG are caused by its weak kinetic AGN feedback that only redistributes the cold gas from the inner disks to the outer regions and reduces the inner cold gas densities. It agrees with observations much better when only the cold gas within the stellar disk is considered to infer the star formation rates. From dependences of the cold gas reservoir on the black hole mass and Eddington ratio, we find that the cumulative energy release during the black hole growth is the dominant reason for the cold gas depletion and thus the galaxy quenching. We further measure the central stellar surface density within 1 kpc (Σ1) for the high-resolution run of IllustrisTNG and find a tight correlation between Σ1 and black hole mass. It suggests that the observed decreasing trend of cold gas mass with Σ1 is also a reflection of the black hole growth.

Keywords: agn host galaxies; galaxy quenching; h i line emission; interstellar atomic gas; star formation; hydrodynamical simulations
Recovering Density Fields inside Quasar Proximity Zones at z 6

The matter density field at z ~ 6 is very challenging to probe. One of the traditional probes of the low density intergalactic medium that works successfully at lower redshifts is the Lyα forest in quasar spectra. However, at the end of reionization, the residual neutral hydrogen usually creates saturated absorption, thus much of the information about the gas density is lost. Luckily, in a quasar proximity zone, the ionizing radiation is exceptionally intense, thus creating a large region with nonzero transmitted flux. In this study we use the synthetic spectra from simulations to investigate how to recover the density fluctuations inside the quasar proximity zones. We show that, under ideal conditions, the density can be recovered accurately with a small scatter. We also discuss how systematics such as the quasar continuum fitting and reionization models affect the results. This study shows that by analyzing the absorption features inside quasar proximity zones we can potentially constrain quasar properties and the environments they reside in.

Keywords: reionization; quasars; intergalactic medium; large-scale structure of the universe; cosmology