adsabs/SciX_UAT_keywords huggingface dataset:
s-processing in AGB Stars Revisited. III. Neutron Captures from MHD Mixing at Different Metallicities and Observational Constraints

We present post-process neutron-capture computations for Asymptotic Giant Branch (AGB) stars of 1.5-3 M⊙ and metallicities -1.3 ≤ [Fe/H] ≤ 0.1. The reference stellar models are computed with the FRANEC code, using the Schwarzschild's criterion for convection; our motivations for this choice are outlined. We assume that MHD processes induce the penetration of protons below the convective boundary, when the Third Dredge Up occurs. There, the 13C n-source can subsequently operate, merging its effects with those of the 22Ne(α, n)25Mg reaction, activated at the temperature peaks characterizing AGB stages. This work has three main scopes. (i) We provide a grid of abundance yields, as produced through our MHD mixing scheme, which are uniformly sampled in mass and metallicity. From this, we deduce that the solar s-process distribution, as well as the abundances in recent stellar populations, can be accounted for, without the need of the extra primary-like contributions suggested in the past. (ii) We formulate analytic expressions for the mass of the 13C-pockets generated to allow easy verification of our findings. (iii) We compare our results with observations of evolved stars and with isotopic ratios in presolar SiC grains, also noticing how some flux tubes should survive turbulent disruption, carrying C-rich materials into the winds even when the envelope is O-rich. This wind phase is approximated through the G-component of AGB s-processing. We conclude that MHD-induced mixing is adequate to drive slow n-capture phenomena accounting for observations; our prescriptions should permit its inclusion into current stellar evolutionary codes.

Keywords: asymptotic giant branch stars; s-process; stellar nucleosynthesis; meteorite composition; magnetohydrodynamics; barium stars
Optimal Spectral Lines for Measuring Chromospheric Magnetic Fields

This paper identifies spectral lines from X-ray to IR wavelengths which are optimally suited to measuring vector magnetic fields as high as possible in the solar atmosphere. Instrumental and Earth's atmospheric properties, as well as solar abundances, atmospheric properties, and elementary atomic physics are considered without bias toward particular wavelengths or diagnostic techniques. While narrowly focused investigations of individual lines have been reported in detail, no assessment of the comparative merits of all lines has ever been published. Although in the UV, on balance the Mg+ h and k lines near 2800 Å are optimally suited to polarimetry of plasma near the base of the solar corona. This result was unanticipated, given that longer-wavelength lines offer greater sensitivity to the Zeeman effect. While these lines sample optical depths photosphere to the coronal base, we argue that cores of multiple spectral lines provide a far more discriminating probe of magnetic structure as a function of optical depth than the core and inner wings of a strong line. Thus, together with many chromospheric lines of Fe+ between 2585 Å and the h line at 2803 Å, this UV region promises new discoveries concerning how the magnetic fields emerge, heat, and accelerate plasma as they battle to dominate the force and energy balance within the poorly understood chromosphere.

Keywords: solar-terrestrial interactions; active sun; quiet sun; spectropolarimetry
TESS-Gaia Light Curve: A PSF-based TESS FFI Light-curve Product

The Transiting Exoplanet Survey Satellite (TESS) is continuing its second extended mission after 55 sectors of observations. TESS publishes full-frame images (FFIs) at a cadence of 1800, 600, or 200 s, allowing light curves to be extracted for stars beyond a limited number of pre-selected stars. Simulations show that thousands of exoplanets, eclipsing binaries, variable stars, and other astrophysical transients can be found in these FFI light curves. To obtain high-precision light curves, we forward model the FFI with the effective point-spread function (PSF) to remove contamination from nearby stars. We adopt star positions and magnitudes from Gaia DR3 as priors. The resulting light curves, called TESS-Gaia light curves (TGLCs), show a photometric precision closely tracking the prelaunch prediction of the noise level. The TGLCs' photometric precision reaches ≲2% at 16th TESS magnitude even in crowded fields. We publish TGLC aperture and PSF light curves for stars down to 16th TESS magnitude through the Mikulski Archive for Space Telescopes for all available sectors and will continue to deliver future light curves. The open-source package tglc 3 3 Via 10.17909/610m‑9474. is publicly available to enable any user to produce customized light curves.

Keywords: light curves; astronomy software; astronomy databases; exoplanet astronomy; variable stars; eclipsing binary stars
Assessing Environmental Factors of Alluvial Fan Formation on Titan

Present-day environmental conditions on Titan, the largest moon of Saturn, may do active geomorphic work on its surface. On Earth, the hydrologic water cycle erodes and weathers its continents. Deluges over elevated terrain create debris flows and sheetfloods that spread into alluvial fans as the topographic slope decreases. Mars also shows evidence of past fluvial erosion, but fluvial activity cannot be ongoing in the present. On Titan, however, fluvial erosion is likely ongoing. In this study, we focus on understanding the environmental controls on the spatial distributions of alluvial fans, a type of fluvial depositional feature observed globally on the surface of Titan. To do this, we utilize probabilistic models to determine the strength of spatial correlations between spatial distributions of alluvial fans and present-day environmental factors. We find that the spatial distribution of alluvial fans on Titan correlates well with several present-day environmental conditions, including average precipitation, precipitation variability, and elevation. Based on our model, we also provide predictions of the likelihood of alluvial fan occurrences for areas of Titan not mapped with Cassini, which may be of interest for future missions to Titan.

Keywords: titan; saturnian satellites; land-atmosphere interactions; planetary atmospheres; planetary surfaces
The Chandra High-resolution X-Ray Spectrum of Quiescent Emission from Sgr A*

In quiescence, Sgr A* is surprisingly dim, shining 100,000 times less than expected for its environment. This problem has motivated a host of theoretical models to explain radiatively inefficient accretion flows. The Chandra Galactic Center X-ray Visionary Program obtained approximately 3 Ms (1 month) of Chandra high-energy transmission grating (HETG) data, offering the only opportunity to examine the quiescent X-ray emission of Sgr A* with high-resolution spectroscopy. Utilizing custom background regions and filters for removing overlapping point sources, this work provides the first-ever look at stacked HETG spectra of Sgr A*. We model the background data sets with a cubic spline and fit the unbinned Sgr A* spectra with a simple parametric model of a power law plus Gaussian lines under the effects of interstellar extinction. We detect a strong 6.7 keV iron emission line in the HEG spectra and a 3.1 keV emission line in the MEG spectra. In all cases, the line centroids and equivalent widths are consistent with those measured from low-resolution CCD spectra. An examination of the unbinned, stacked HEG ± 1 spectrum reveals fine structure in the iron line complex. In addition to resolving the resonant and forbidden lines from He-like iron, there are apparent emission features arising with higher statistical significance at lower energy, potentially associated with Fe XX-XXIV ions in a ∼1 keV plasma arising near the Bondi radius of Sgr A*. With this work, we release the cleaned and stacked Sgr A* and background HETG spectra to the public as a special legacy data set.

Keywords: galactic center; accretion; supermassive black holes; high resolution spectroscopy
The Persistence of Pancakes and the Revival of Self-gravity in Tidal Disruption Events

The destruction of a star by the tides of a supermassive black hole (SMBH) powers a bright accretion flare, and the theoretical modeling of such tidal disruption events (TDEs) can provide a direct means of inferring SMBH properties from observations. Previously it has been shown that TDEs with β = rt/rp = 1, where rt is the tidal disruption radius and rp is the pericenter distance of the star, form an in-plane caustic, or "pancake," where the tidally disrupted debris is compressed into a one-dimensional line within the orbital plane of the star. Here we show that this result applies generally to all TDEs for which the star is fully disrupted, i.e., that satisfy β ≳ 1. We show that the location of this caustic is always outside of the tidal disruption radius of the star and the compression of the gas near the caustic is at most mildly supersonic, which results in an adiabatic increase in the gas density above the tidal density of the black hole. As such, this in-plane pancake revitalizes the influence of self-gravity even for large β, in agreement with recent simulations. This finding suggests that for all TDEs in which the star is fully disrupted, self-gravity is revived post-pericenter, keeps the stream of debris narrowly confined in its transverse directions, and renders the debris prone to gravitational instability.

Keywords: black hole physics; active galactic nuclei; analytical mathematics; astrophysical fluid dynamics; tidal disruption
A Study of Six Extreme Low Mass Ratio Contact Binary Systems

Multi-band (B, V and R) photometric and spectroscopic observations of six poorly studied contact binaries carried out at the Western Sydney University and Las Cumbres Observatory were analyzed using a recent version of the Wilson-Devenney code. All six were found to be of extreme low mass ratio ranging from 0.073 to 0.149. All are of F spectral class with the mass of the primary component ranging from 1.05M ☉ to 1.48M ☉. None show light curve features of enhanced choromospheric activity (O'Connel Effect) however five of the six do have significant ultraviolet excess indicating the presence of increased magnetic and chromospheric activity. Period analysis based on available survey data suggests two systems have a slowly increasing period suggesting mass transfer from the secondary to the primary, two have a slow declining period with likely mass transfer from primary to the secondary, while one shows a steady period, and one undergoing transition from a declining to increasing period suggesting possible mass transfer reversal. We also compare light curve solutions against theoretical markers of orbital stability and show that three of the six systems have mass ratios within the theoretical instability limit and may be regarded as potential merger candidates.

Keywords: contact binary stars; low mass stars; stellar mergers; photometry
Radial and Local Density Dependence of Star Formation Properties in Galaxy Clusters from the Hyper Suprime-Cam Survey

This study examines the impact of cluster environments on galaxy properties using data from the Hyper Suprime-Cam Subaru Strategic Program and an optically selected CAMIRA cluster sample. Specifically, the study analyzes the fractions of quiescent and green valley galaxies with stellar masses above 108.6 M ⊙ at z ~ 0.2 and 109.8 M ⊙ at z ~ 1.1, investigating their trends in radius and density. The results indicate that a slow quenching mechanism is at work, as evidenced by a radially independent specific star formation rate reduction of 0.1 dex for star-forming galaxies in a cluster environment. The study also finds that slow quenching dominates fast quenching only for low-mass galaxies (<109.2 M ⊙) near the cluster edge, based on their contributions to the quiescent fraction. After controlling for M *, z, and local overdensity, the study still finds a significant radial gradient in the quiescent fraction, indicating active ram pressure stripping in dense environments. That said, analyzing the density trend of the quiescent fraction with other fixed parameters suggests that radial and density-related quenching processes are equally crucial for low-mass cluster galaxies. The study concludes that ram pressure stripping is the primary environmental quenching mechanism for high stellar mass galaxies in clusters. By contrast, ram pressure stripping and density-related quenching processes act comparably for low-mass cluster galaxies around the center. Near the cluster boundary, starvation and harassment become the leading quenching processes for low stellar mass galaxies.

Keywords: galaxy clusters; galactic and extragalactic astronomy; galaxy evolution; extragalactic astronomy; galaxy environments; large-scale structure of the universe
A Jet-bases Emission Model of the EHT2017 Image of M87*

We carry out general relativistic ray-tracing radiative-transfer calculations to study whether a localized emission from plasma rings created at the stagnation surface in the jet funnel, to which we refer as stagnation rings, can explain the ring image of M87* observed by the Event Horizon Telescope (EHT) 2017. The resultant images consist of the direct image of the stagnation rings and the ring images formed via the strong deflection by the black hole (BH) gravity, to which we refer as quasi photon-ring. For the model with the BH spin a* = 0.99, the direct image of the counter-jet and quasi photon-ring are almost coincident to the photon ring with diameter ∼ 40 μas, while the approaching jet shows the small ring image inside them. The synthetic observation image assumes that the EHT2017 array is consistent with that observed in M87*, because the array is a bit sparse to detect the inner ring image. This indicates that the ring image in M87* may contain the important feature of the jet bases in addition to the photon ring. We find that forthcoming EHT observations can resolve the stagnation ring image and may enable us to explore the plasma-injection mechanism in the jet funnel.

Keywords: black hole physics; astrophysical black holes; event horizons; relativistic jets; photon sphere; radiative transfer; radiative processes; very long baseline interferometers; rotating black holes
Testing Photoevaporation and MHD Disk Wind Models through Future High-angular Resolution Radio Observations: The Case of TW Hydrae

We present theoretical predictions for the free-free emission at centimeter wavelengths obtained from photoevaporation and magnetohydrodynamic (MHD) wind disk models adjusted to the case of the TW Hydrae young stellar object. For this system, disk photoevaporation with heating due to the high-energy photons from the star has been proposed as a possible mechanism to open the gap observed in the dust emission with the Atacama Large Millimeter/submillimeter Array. We show that the photoevaporation disk model predicts a radial profile for the free-free emission that is made of two main spatial components, one originated from the bound disk atmosphere at 0.5-1 au from the star, and another more extended component from the photoevaporative wind at larger disk radii. We also show that the stellar X-ray luminosity has a significant impact on both these components. The predicted radio emission from the MHD wind model has a smoother radial distribution which extends to closer distances to the star than the photoevaporation case. We also show that a future radio telescope such as the Next Generation Very Large Array would have enough sensitivity and angular resolution to spatially resolve the main structures predicted by these models.

Keywords: protoplanetary disks
Sunspot Catalog (1921-1935) and Area Series (1886-1940) from the Stonyhurst College Observatory

A sunspot observation program was started at the end of the 19th century at the Stonyhurst College Observatory (hereafter SCO) by Father Perry, director of the observatory at that time. A digitization of the daily sunspot area series recorded in this observatory from 1886 to 1940 (with a gap between 1889 and 1897) is provided in this work. This depicts one of the oldest sunspot area series available. A comparison of this series with contemporary area series made in other observatories shows that SCO generally recorded larger areas than those in some of the observatories of that time such as, for example, the Royal Greenwich Observatory (RGO). Furthermore, SCO published a sunspot group catalog for the period 1921-1935. We provide a machine-readable version of this catalog. We compared the SCO group number series with other sunspot data obtained from other observatories. In this case, for example, the RGO systematically recorded more groups than the SCO. We compared SCO and RGO area distribution functions obtaining the calibration constant between both data sets. We also obtained the butterfly diagram from the group latitudes recorded by SCO and compared the percentages of group types computed from the SCO catalog with those from Valencia Observatory (following the Cortie morphological classification of sunspot groups), identifying their similarities and differences.

Keywords: astronomy databases; catalogs; sunspot cycle; sunspot groups
The Equilibrium Temperature of Planets on Eccentric Orbits: Time Scales and Averages

From estimates of the near-surface heat capacity of planets it is shown that the thermal timescale is larger than the orbital period in the presence of a global ocean that is well-mixed to a depth of 100 m, or of an atmosphere with a pressure of several tens of bars. As a consequence, the temperature fluctuations of such planets on eccentric orbits are damped. The average temperature should be calculated by taking the temporal mean of the irradiation over an orbit, which increases with $1/\sqrt{1-{e}^{2}}$ . This conclusion is independent of the orbital distance and valid for Sun-like stars; the damping is even stronger for low-mass main sequence hosts.

Keywords: exoplanet atmospheric variability; exoplanet dynamics; planetary climates
Climate Change of over 20 °C Induced by Continental Movement on a Synchronously Rotating Exoplanet

It is generally believed that the addition of continents cools the climate of an aquaplanet with a similar orbit to Earth; this is because continents have a higher surface albedo than oceans. A similar effect has been shown in climate simulations for exoplanets. Here we demonstrate that the influence of a continent on ocean circulation could have a dominative effect on the climate of a synchronously rotating exoplanet compared with the effect of the surface albedo, especially when the rotation of the exoplanet is relatively slow (e.g., the rotational period is 40 Earth days). The global mean surface temperature could vary by more than 26° C, simply by moving a small continent to a different location. The ocean circulation on a synchronously rotating exoplanet is characterized by a strong westerly jet along the equator and one large gyre in each hemisphere. The surface temperature decreases when the equatorial westerly or the western branch of either of the gyres is blocked by a continent or an island arc chain. However, if the continent blocks the eastern branch of the gyre, the equatorial westerly is strengthened and the climate warms. A large number of potentially habitable exoplanets have been found orbiting around M-dwarfs in a tidally locked manner; our results indicate that their climates, as well as their atmospheric chemistry, may deviate from previous estimates if a small continent, or even an island arc chain, is present.

Keywords: exoplanet dynamics; exoplanets; extrasolar rocky planets; habitable planets; exoplanet atmospheres; planetary climates; ocean planets
A Deep Investigation of Two Poorly Studied Open Clusters Haffner 22 and Melotte 71 in the Gaia era

This paper presents a deep investigation of two open clusters, Haffner 22 and Melotte 71, using astrometric and photometric data from Gaia EDR3. We identified 382 and 597 most probable cluster members with membership probability higher than 50%. Mean proper motions in R.A. and decl. are estimated as (-1.631 ± 0.009, 2.889 ± 0.008) and (-2.398 ± 0.004, 4.210 ± 0.005) mas yr-1 for Haffner 22 and Melotte 71, respectively. A comparison of observed CMDs with the theoretical isochrones leads to an age of 2.25 ± 0.25 and 1.27 ± 0.14 Gyr for these clusters. The distances 2.88 ± 0.10 and 2.28 ± 0.15 kpc based on the parallax are comparable with the values derived by the isochrone fitting method. Five and four blue straggler stars (BSS) are identified as cluster members in Haffner 22 and Melotte 71. Based on the relative number of high-velocity (binary) and single stars, we inferred binary fractions for both clusters in the range of ~10% ≤ f bin ≤ 14%, for both core and o?-core regions. We found binary content is larger in the core region. Mass function slope is in good agreement with the Salpeter's value for Melotte 71 (x = 1.23 ± 0.38 within mass range 1-3.4 M ⊙) while it is quite a flat slope for Haffner 22 (x = 0.63 ± 0.30 within mass range 1-2.3 M ⊙). Evidence for the existence of mass-segregation effect is observed in both clusters. Using the Galactic potential model, Galactic orbits are derived, indicating that both clusters follow a circular path around the Galactic center, evolving slowly.

Keywords: open star clusters; binary stars; initial mass function; stellar dynamics; blue straggler stars
Reconstruction of Photospheric Velocity Fields from Highly Corrupted Data

The analysis of the photospheric velocity field is essential for understanding plasma turbulence in the solar surface, which may be responsible for driving processes such as magnetic reconnection, flares, wave propagation, particle acceleration, and coronal heating. Currently, the only available methods to estimate velocities at the solar photosphere transverse to an observer's line of sight infer flows from differences in image structure in successive observations. Due to data noise, algorithms such as local correlation tracking may lead to a vector field with wide gaps where no velocity vectors are provided. In this paper, a novel method for image inpainting of highly corrupted data is proposed and applied to the restoration of horizontal velocity fields in the solar photosphere. The restored velocity field preserves all the vector field components present in the original field. The method shows robustness when applied to both simulated and observational data.

Keywords: solar photosphere; astrophysical fluid dynamics; space plasmas; magnetohydrodynamics; computational methods
A Radio Molecular Line Search for Evidence of an O-rich Environment Around Binary in Silicate Carbon Stars

We report observations of oxygen-rich (O-rich) radio molecular line tracers, including H2CO, H2O, and HCO+ toward 28 silicate carbon stars (SCSs). SCSs are special evolved star systems that consist of a carbon star associated with peculiar O-rich circumstellar material. Absorption in the 6 cm ${{\rm{H}}}_{2}\mathrm{CO}$ transition was detected from seven SCSs using the Shanghai 65 m Tianma radio telescope. In order to further investigate their O-rich material, several 3 mm molecular lines including HCO+ and CO were also observed with the Purple Mountain Observatory 13.7 m telescope. Double-peak profiles in some of these lines suggest the possible presence of a companion in the SCS system. A negative association between H2O masers and H2CO absorption was found in our sample of SCSs. The ${{\rm{H}}}_{2}{\rm{O}}$ maser sources may correspond to an earlier evolutionary stage when the circumstellar envelope (CSE) material is O-rich and limited to a compact and denser region. As the star evolves the CSE gradually expands and carbon abundance is enhanced, resulting in the ${{\rm{H}}}_{2}{\rm{O}}$ masers disappearing and H2CO absorption arising.

Keywords: evolved stars; silicon stars; circumstellar envelopes; circumstellar matter; binary stars; asymptotic giant branch stars; post-asymptotic giant branch stars; carbon stars; chemically peculiar stars