adsabs/SciX_UAT_keywords huggingface dataset:
The Environmental Dependence of the XCO Conversion Factor

$\mathrm{CO}$ is the most widely used observational tracer of molecular gas. The observable $\mathrm{CO}$ luminosity is translated to ${{\rm{H}}}_{2}$ mass via a conversion factor, ${X}_{\mathrm{CO}}$ , which is a source of uncertainty and bias. Despite variations in ${X}_{\mathrm{CO}}$ , the empirically determined solar neighborhood value is often applied across different galactic environments. To improve understanding of ${X}_{\mathrm{CO}}$ , we employ 3D magnetohydrodynamics simulations of the interstellar medium (ISM) in galactic disks with a large range of gas surface densities, allowing for varying metallicity, far-ultraviolet (FUV) radiation, and cosmic-ray ionization rate (CRIR). With the TIGRESS simulation framework we model the three-phase ISM with self-consistent star formation and feedback, and post-process outputs with chemistry and radiation transfer to generate synthetic CO (1-0) and (2-1) maps. Our models reproduce the observed CO excitation temperatures, line widths, and line ratios in nearby disk galaxies. ${X}_{\mathrm{CO}}$ decreases with increasing metallicity, with a power-law slope of -0.8 for the (1-0) line and -0.5 for the (2-1) line. ${X}_{\mathrm{CO}}$ also decreases at higher CRIR and is insensitive to the FUV radiation. As density increases, ${X}_{\mathrm{CO}}$ first decreases owing to increasing excitation temperature and then increases when the emission is fully saturated. We provide fits between ${X}_{\mathrm{CO}}$ and observable quantities such as the line ratio, peak antenna temperature, and line brightness, which probe local gas conditions. These fits, which allow for varying beam size, may be used in observations to calibrate out systematic biases. We also provide estimates of the CO-dark ${{\rm{H}}}_{2}$ fraction at different gas surface densities, observational sensitivities, and beam sizes.

Keywords: interstellar medium; interstellar molecules; molecular clouds; galaxy disks; magnetohydrodynamical simulations; chemical abundances
A Census of Blue Stragglers in Gaia DR2 Open Clusters as a Test of Population Synthesis and Mass Transfer Physics

We use photometry and proper motions from Gaia DR2 to determine the blue straggler star (BSS) populations of 16 old (1-10 Gyr), nearby (d < 3500 pc) open clusters. We find that the fractional number of BSS compared to red giant branch stars increases with age, starting near zero at 1 Gyr and flattening to ∼0.35 by 4 Gyr. Fitting stellar evolutionary tracks to these BSSs, we find that their mass distribution peaks at a few tenths of a solar mass above the main-sequence turnoff. BSSs more than 0.5 M⊙ above the turnoff make up only ∼25% of the sample, and BSSs more than 1.0 M⊙ above the turnoff are rare. We compare this to Compact Object Synthesis and Monte Carlo Investigation Code population synthesis models of BSSs formed via mass transfer. We find that standard population synthesis assumptions dramatically under-produce the number of BSS in old open clusters. We also find that these models overproduce high-mass BSSs relative to lower-mass BSSs. The expected number of BSSs formed through dynamics do not fully account for this discrepancy. We conclude that in order to explain the observed BSS populations from Roche lobe overflow, mass transfer from giant donors must be more stable than assumed in canonical mass-transfer prescriptions, and including nonconservative mass transfer is important in producing realistic BSS masses. Even with these modifications, it is difficult to achieve the large number of BSSs observed in the oldest open clusters. We discuss some additional physics that may explain the large number of observed blue stragglers among old stellar populations.

Keywords: binary stars; blue straggler stars; open star clusters; roche lobe overflow; multiple star evolution
Inconsistencies Between Local and Global Measures of CME Radial Expansion as Revealed by Spacecraft Conjunctions

The radial expansion of coronal mass ejections (CMEs) is known to occur from remote observations, from the variation of their properties with radial distance, and from local in situ plasma measurements showing a decreasing speed profile throughout the magnetic ejecta (ME). However, little is known on how local measurements compare to global measurements of expansion. Here, we present results from the analysis of 42 CMEs measured in the inner heliosphere by two spacecraft in radial conjunction. The magnetic-field decrease with distance provides a measure of their global expansion. Near 1 au, the decrease in their bulk speed provides a measure of their local expansion. We find that these two measures have little relation with each other. We also investigate the relation between characteristics of CME expansion and CME properties. We find that the expansion depends on the initial magnetic-field strength inside the ME, but not significantly on the magnetic field inside the ME measured near 1 au. This is indirect evidence that CME expansion in the innermost heliosphere is driven by the high magnetic pressure inside the ME, while by the time the MEs reach 1 au, they are expanding due to the decrease in the solar-wind dynamic pressure with distance. We also determine the evolution of the ME tangential and normal magnetic-field components with distance, revealing significant deviations as compared to the expectations from force-free field configurations as well as some evidence that the front half of MEs expand at a faster rate than the back half.

Keywords: solar coronal mass ejections; interplanetary magnetic fields
The Physical Drivers of the Luminosity-weighted Dust Temperatures in High-redshift Galaxies

The underlying distribution of galaxies' dust spectral energy distributions (SEDs) (i.e., their spectra reradiated by dust from rest-frame ∼3 μm to 3 mm) remains relatively unconstrained owing to a dearth of far-IR/(sub)millimeter data for large samples of galaxies. It has been claimed in the literature that a galaxy's dust temperature—observed as the wavelength where the dust SED peaks (λpeak)—is traced most closely by its specific star formation rate (sSFR) or parameterized "distance" to the SFR-M⋆ relation (the galaxy "main sequence"). We present 0"24 resolved 870 μm ALMA dust continuum observations of seven z = 1.4-4.6 dusty star-forming galaxies chosen to have a large range of well-constrained luminosity-weighted dust temperatures. We also draw on similar-resolution dust continuum maps from a sample of ALESS submillimeter galaxies from Hodge et al (2016). We constrain the physical scales over which the dust radiates and compare those measurements to characteristics of the integrated SED. We confirm significant correlations of λpeak with both LIR (or SFR) and ΣIR (∝SFR surface density). We investigate the correlation between log10(λpeak) and log10(ΣIR) and find the relation to hold as would be expected from the Stefan-Boltzmann law, or the effective size of an equivalent blackbody. The correlations of λpeak with sSFR and distance from the SFR-M⋆ relation are less significant than those for ΣIR or LIR; therefore, we conclude that the more fundamental tracer of galaxies' luminosity-weighted integrated dust temperatures are indeed their star formation surface densities in line with local universe results, which relate closely to the underlying geometry of dust in the interstellar medium.

Keywords: galaxy evolution; dust continuum emission; starburst galaxies; high-redshift galaxies; submillimeter astronomy
The Accretion Process in the DQ Tau Binary System

Mass accretion from the circumstellar disk onto the protostar is a fundamental process during star formation. Measuring the mass accretion rate is particularly challenging for stars belonging to binary systems, because it is often difficult to discriminate which component is accreting. DQ Tau is an almost equal-mass spectroscopic binary system where the components orbit each other every 15.8 days. The system is known to display pulsed accretion, i.e., the periodic modulation of the accretion by the components on eccentric orbit. We present multi-epoch ESO/Very Large Telescope X-Shooter observations of DQ Tau, with the aim of determining which component of this system is the main accreting source. We use the absorption lines in the spectra to determine the radial velocity of the two components, and measure the continuum veiling as a function of wavelength and time. We fit the observed spectra with nonaccreting templates to correct for the photospheric and chromospheric contribution. In the corrected spectra, we study in detail the profiles of the emission lines and calculate mass accretion rates for the system as a function of orbital phase. In accordance with previous findings, we detect elevated accretion close to periastron. We measure the accretion rate as varying between 10-8.5 and 10-7.3 M ⊙ yr-1. The emission line profiles suggest that both stars are actively accreting, and the dominant accretor is not always the same component, varying in a few orbits.

Keywords: stellar accretion disks; spectroscopic binary stars; young stellar objects; circumstellar disks; stellar accretion; spectroscopy
All Spectral Type LAMOST Spectra Library (ATLAS)

We present an empirical stellar spectra library, ATLAS, with resolution R ~ 1800 and wavelength coverage from 3800-8700 Å. These spectra are homogeneously observed by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and have been released in its DR5. ATLAS is separated into two groups, ATLAS-A and -T. The former contains 5584 spectra covering spectral types from O- to M-type and some special types such as A supergiant, blue horizontal-branch, and carbon stars. All of the spectra have absolutely calibrated fluxes at certain characteristic wavelengths corresponding to the optical passbands with accuracy better than 2.9% by comparing with PanSTARRS1 g-, r-, and i-band photometry. In addition, greater than 1% systematic uncertainty from the fundamental calibration should be considered separately. ATLAS-A contains 5342 spectra with spectral energy distribution effective temperature, surface gravity, and metallicity and 242 spectra with only the effective temperature and surface gravity. These parameters are consistent with the spectroscopic derived parameters of the same stars. Compared to current empirical libraries, ATLAS-A contains more cool giant stars, which may play a critical role in understanding the evolution of galaxies. ATLAS-T, on the other hand, contains 1118 spectral templates averaging over spectra with similar stellar parameters from ATLAS-A. It smooths out some special features in the individual spectra and can be used as a "standard" atlas of stellar spectra. Containing stellar spectra with almost all normal types, ATLAS may not only be a complete training data set for stellar spectra parameter determination but also an ideal legacy for stellar population synthesis.

Keywords: astronomy databases; catalogs; spectrophotometry
Emission Line Intensity Ratios of Fe XXVI/ XXV/ XXIV in Solar Flares Observed by Hinotori

High-resolution spectra observed by the Solar X-ray spectrometer on board the Hinotori mission are revisited. Flat crystals slightly offset to the satellite spin axis produce automatic spectral scans for emission lines emerging from highly charged iron ions in solar flares every half-spin time period. All the downlinked data of the mission are converted to FITS format and major flare spectral data are revived as IDL save files in ISAS/DARTS. Based on these data sets, single-temperature fits are performed for the emission line complex of highly charged iron ions in the wavelength range of 1.75–1.95 Å and compared with theoretical predictions. Synthetic spectra with single electron temperatures estimated from j/w line-intensity ratios fit fairly well for Fe XXIV and Fe XXIII lines in the wavelength range of 1.85–1.88 Å, while intensity ratios of Fe XXV lines (x, y, z) and the inner-shell excitation line of Fe XXIV (q) to the Fe XXV resonance line (w) have systematic excesses. Empirical relations for the observed line ratios are derived. Ion fractions of Fe+25/Fe+24 estimated by intensity ratios of Lyα/w in the temperature range of log T e =7.25–7.45 are consistent with values in ionization equilibrium, and the remaining excesses of the Fe XXV line ratios may suggest problems with the atomic parameters or atomic modeling.

Keywords: solar x-ray emission; atomic spectroscopy; line intensities; atomic data
Correcting Doppler Shifts in He II 30.38 nm Line by Using the EVE and AIA Data from Solar Dynamics Observatory

The Extreme-ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory detects the solar EUV spectra with high temporal cadence and spectral resolution. The wavelength shifts of emission lines provide key information of the dynamics of the Sun. However, some of EVE spectral observations are influenced by the non-uniformly distributed irradiance on the Sun, which may prevent us from correctly understanding the physical processes happening in the solar corona. Here, based on the only published on-orbit calibration data of EVE He II 30.38 nm line on 2011 January 27, we develop a method to correct the He II 30.38 nm line by using AIA 304 imaging data. This correction method is then applied to EVE He II 30.38 nm data from 2010 October 29 to 2011 March 3 to study the Doppler oscillations of the solar He II 30.38 nm line, in which we show that the half-month periodic Doppler oscillation is caused by non-uniformly distributed irradiance mainly due to the presence of active regions. Other EVE coronal lines also present similar Doppler oscillations, suggesting that an appropriate correction must be implemented before interpreting the oscillation phenomena appearing in these lines.

Keywords: solar active regions; solar extreme ultraviolet emission; solar rotation; solar spectral irradiance; doppler shift
The Origin of Star-forming Rings in S0 Galaxies

Spatially resolved integral field spectroscopic maps in a sample of 532 S0 galaxies from the MaNGA survey have unveiled the existence of inner rings (<R> ~ 1 R e) betraying ongoing star formation in a number of these objects. Activity gradients averaged over bins of galactocentric radius up to ~1.5 R e have been measured in the subspace defined by the first two principal components of the optical spectra of these galaxies. We find that the sign of the gradients is closely related to the presence of such rings in the spectral maps, which are especially conspicuous in the equivalent width of the Hα emission line, EW(Hα), with a fractional abundance-21%-34%-notably larger than that inferred from optical images. While the numbers of S0s with positive, negative, and flat activity gradients are comparable, star-forming rings are largely found in objects for which quenching proceeds from the inside out, in good agreement with predictions from cosmological simulations studying S0 buildup. Assessment of these ringed structures indicates that their frequency increases with the mass of their hosts, that they have shorter lifetimes in galaxies with ongoing star formation, that they may feed on gas from the disks, and that the local environment does not play a relevant role in their formation. We conclude that the presence of inner rings in EW(Hα) is a common phenomenon in fully formed S0s, possibly associated with annular disk resonances driven by weakly disruptive mergers preferentially involving a relatively massive primary galaxy and a tiny satellite strongly bound to the former.

Keywords: lenticular galaxies; galaxy spectroscopy; galaxy structure; galaxy evolution
A Statistical Analysis on the Morphology and Color of Galaxies Hosting Radio-loud Active Galactic Nuclei

The properties of galaxies hosting radio-loud active galactic nuclei (AGN) have been investigated in order to find out the relation between morphology and color of galaxies and radio loudness. The concentration index is used to map the morphology of galaxies as early or late type. 4000 Å break strength and star-formation rate are used to quantify the state of galaxies as blue star forming or red quenched. Due to the environmental dependence of these properties, galaxies with the same environment have been considered. The results show that the prevalence of radio-loud AGN in early-type galaxies is independent of environment. This is confirmed by comparing a sample of galaxies hosting radio-loud AGN with a sample of galaxies hosting radio-quiet AGN or with a randomly selected galaxy sample from the total population of galaxies used in this study. In addition, it is shown that galaxies hosting radio-loud AGN are bluer indicating the role of cold gas in triggering AGN activity. Finally, the optically-selected radio-loud AGN show to have higher [O III] luminosities compared with their radio-quiet counterparts. We argue the relation between radio size and luminosity with the [O III] luminosity.

Keywords: active galactic nuclei; galaxy interactions; radio continuum emission; radio active galactic nuclei
Physical Conditions in the LMC's Quiescent Molecular Ridge: Fitting Non-LTE Models to CO Emission

The Molecular Ridge in the LMC extends several kiloparsecs south from 30 Doradus, and it contains ~30% of the molecular gas in the entire galaxy. However, the southern end of the Molecular Ridge is quiescent-it contains almost no massive star formation, which is a dramatic decrease from the very active massive-star-forming regions 30 Doradus, N159, and N160. We present new Atacama Large Millimeter/submillimeter Array and Atacama Pathfinder Experiment observations of the Molecular Ridge at a resolution as high as ~16″ (~3.9 pc) with molecular lines 12CO(1-0), 13CO(1-0), 12CO(2-1), 13CO(2-1), and CS(2-1). We analyze these emission lines with our new multiline non-LTE fitting tool to produce maps of Tkin, ${n}_{{{\rm{H}}}_{2}}$ , and NCO across the region based on models from RADEX. Using simulated data for a range of parameter space for each of these variables, we evaluate how well our fitting method can recover these physical parameters for the given set of molecular lines. We then compare the results of this fitting with LTE and XCO methods of obtaining mass estimates and how line ratios correspond with physical conditions. We find that this fitting tool allows us to more directly probe the physical conditions of the gas and estimate values of Tkin, ${n}_{{{\rm{H}}}_{2}}$ , and NCO that are less subject to the effects of optical depth and line-of-sight projection than previous methods. The fitted ${n}_{{{\rm{H}}}_{2}}$ values show a strong correlation with the presence of young stellar objects (YSOs), and with the total and average mass of the associated YSOs. Typical star formation diagnostics, such as mean density, dense gas fraction, and virial parameter do not show a strong correlation with YSO properties.

Keywords: star formation; large magellanic cloud; molecular clouds; millimeter astronomy; interstellar medium
Diffuse Ionized Gas in Simulations of Multiphase, Star-forming Galactic Disks

It has been hypothesized that photons from young, massive star clusters are responsible for maintaining the ionization of diffuse warm ionized gas seen in both the Milky Way and other disk galaxies. For a theoretical investigation of the warm ionized medium (WIM), it is crucial to solve radiation-transfer equations where the interstellar medium (ISM) and clusters are modeled self-consistently. To this end, we employ a solar neighborhood model of Three-phase Interstellar Medium in Galaxies Resolving Evolution with Star Formation and Supernova Feedback (TIGRESS), a magnetohydrodynamic simulation of the multiphase, star-forming ISM, and post-process the simulation with an adaptive ray tracing method to transfer UV radiation from star clusters. We find that the WIM volume filling factor is highly variable, and sensitive to the rate of ionizing photon production and ISM structure. The mean WIM volume filling factor rises to ∼0.15 at $| z| \sim 1\,\,\mathrm{kpc}$ . Approximately half of ionizing photons are absorbed by gas and half by dust; the cumulative ionizing photon escape fraction is 1.1%. Our time-averaged synthetic ${\rm{H}}\alpha $ line profile matches Wisconsin ${\rm{H}}\alpha $ Mapper observations on the redshifted (outflowing) side, but has insufficient intensity on the blueshifted side. Our simulation matches the Dickey-Lockman neutral density profile well, but only a small fraction of snapshots have high-altitude WIM density consistent with Reynolds Layer estimates. We compute a clumping correction factor ${{ \mathcal C }}_{{n}_{{\rm{e}}}}\equiv \langle {n}_{{\rm{e}}}\rangle /\langle {n}_{{\rm{e}}}^{2}{\rangle }^{1/2}\sim 0.2$ that is remarkably constant with distance from the midplane and time; this can be used to improve estimates of ionized gas mass and mean electron density from observed ${\rm{H}}\alpha $ surface brightness profiles in edge-on galaxies.

Keywords: radiative transfer; warm ionized medium; interstellar medium; galaxy structure
Mass-Velocity Dispersion Relation in MaNGA Brightest Cluster Galaxies

We investigate a kinematic scaling relation between the baryonic mass and the flat velocity dispersion, i.e., mass-velocity dispersion relation (MVDR), from the brightest cluster galaxies (BCGs) to the galaxy clusters. In our studies, the baryonic mass of BCGs is mainly estimated by photometry. The velocity dispersion profiles are explored with the integrated field unit by Mapping Nearby Galaxies at Apache Point Observatory (MaNGA). For the first time, we reveal two significant results with 54 MaNGA BCGs: (1) the flat velocity dispersion profiles; (2) a tight empirical relation on the BCG-cluster scale together with cluster samples, i.e., MVDR, $\mathrm{log}({M}_{\mathrm{bar}}/{M}_{\odot })={4.1}_{-0.1}^{+0.1}$ $\mathrm{log}({\sigma }_{\mathrm{los}}/\mathrm{km}\,{{\rm{s}}}^{-1})\,+{1.6}_{-0.3}^{+0.3}$ , with a tiny lognormal intrinsic scatter of ${10}_{-1}^{+2} \%$ . This slope is identical to the acceleration relation in galaxy clusters, which is reminiscent of the spiral galaxies, albeit at a larger characteristic acceleration scale. The residuals of the MVDR represent a Gaussian distribution, displaying no correlations with four properties: baryonic mass, scale length, surface density, and redshift. Notably, the MVDR on the BCG-cluster scale provides a strict test, which disfavors the general prediction of the slope of three in the dark matter model.

Keywords: galaxy kinematics; brightest cluster galaxies; giant elliptical galaxies; galaxy clusters; dark matter