- ID:
- ivo://CDS.VizieR/J/A+A/494/403
- Title:
- Low temperature Rosseland opacities
- Short Name:
- J/A+A/494/403
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We provide low temperature opacity data that incorporate varied abundances of the elements carbon and nitrogen. In the temperature range that we focus at, molecules are the dominant opacity source. Our dataset spans a large metallicity range and shall deliver the necessary input data for stellar evolution models as well as other applications. We conduct chemical equilibrium calculations in order to evaluate the partial pressures of neutral atoms, ions and molecules. Based on a large dataset containing atomic line and continuum data, and, most importantly, a plethora of molecular lines, we subsequently calculate Rosseland mean opacity coefficients. This is done not only for a number of different metallicities, but also for varied abundances of the isotopes ^12^C and ^14^N at each metallicity. The molecular data comprise the main opacity sources at either an oxygen-rich or carbon-rich chemistry. We tabulate the opacity coefficients as a function of temperature and, basically, density. Already within a certain chemistry regime an alteration in the carbon abundance causes, due to the special role of the CO molecule, considerable changes in the Rosseland opacity. The transition from a scaled solar (i.e. oxygen-rich) mixture to the carbon-rich regime results in opacities that can, at low temperatures, be orders of magnitude different compared to the initial situation. The reason is that different molecular absorbers make up the mean opacity in either case. A varying abundance of nitrogen has less pronounced effects but, nevertheless, cannot be neglected.
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- ID:
- ivo://CDS.VizieR/J/A+A/550/A106
- Title:
- Low-velocity shocks models
- Short Name:
- J/A+A/550/A106
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Large-scale motions in galaxies (supernovae explosions, galaxy collisions, galactic shear etc.) generate turbulence, which allows a fraction of the available kinetic energy to cascade down to small scales before it is dissipated. We establish and quantify the diagnostics of turbulence dissipation in mildly irradiated diffuse gas in the specific context of shock structures. We incorporated the basic physics of photon-dominated regions into a state-of-the-art steady-state shock code. We examined the chemical and emission properties of mildly irradiated (G_0_=1) magnetised shocks in diffuse media (n_H_=10^2^ to 10^4^cm^-3^) at low- to moderate velocities (from 3 to 40km/s). The formation of some molecules relies on endoergic reactions. Their abundances in J shocks are enhanced by several orders of magnitude for shock velocities as low as 7km/s. Otherwise most chemical properties of J-type shocks vary over less than an order of magnitude between velocities from about 7 to about 30km/s, where H_2_ dissociation sets in. C-type shocks display a more gradual molecular enhancement with increasing shock velocity. We quantified the energy flux budget (fluxes of kinetic, radiated and magnetic energies) with emphasis on the main cooling lines of the cold interstellar medium. Their sensitivity to shock velocity is such that it allows observations to constrain statistical distributions of shock velocities. We fitted various probability distribution functions (PDFs) of shock velocities to spectroscopic observations of the galaxy-wide shock in Stephan's Quintet and of a Galactic line of sight which samples diffuse molecular gas in Chamaeleon. In both cases, low velocities bear the greatest statistical weight and the PDF is consistent with a bimodal distribution. In the very low velocity shocks (below 5km/s), dissipation is due to ion-neutral friction and it powers H_2_ low-energy transitions and atomic lines. In moderate velocity shocks (20km/s and above), the dissipation is due to viscous heating and accounts for most of the molecular emission. In our interpretation a significant fraction of the gas in the line of sight is shocked (from 4% to 66%). For example, C^+^ emission may trace shocks in UV irradiated gas where C^+^ is the dominant carbon species.} Low- and moderate velocity shocks are important in shaping the chemical composition and excitation state of the interstellar gas. This allows one to probe the statistical distribution of shock velocities in interstellar turbulence.
- ID:
- ivo://CDS.VizieR/J/A+AS/110/329
- Title:
- LTE model atmospheres coeff.
- Short Name:
- J/A+AS/110/329
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Linear, quadratic and square root limb-darkening coefficients have been computed by means of least squares fits to model atmospheres by Kurucz (1991) with the main purpose of facilitating the analysis of light curves of eclipsing binaries. Results are compared with previously published values using the same of different models and various fitting procedures.
- ID:
- ivo://CDS.VizieR/J/A+A/374/265
- Title:
- LTE spectrum synthesis in magnetic atmospheres
- Short Name:
- J/A+A/374/265
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Files table4.dat contain calculated Stokes IQUV local line profiles of Fe II 4923.9 shown in Fig. 4 of this paper. Profile sets are provided for 0.1, 5 and 20kG magnetic fields, with vector orientation {psi}=40{deg}, {phi}=0{deg}, disc centre and eps_fe_=4.6.
- ID:
- ivo://CDS.VizieR/J/ApJ/746/169
- Title:
- Luminosity function of broad-line quasars
- Short Name:
- J/ApJ/746/169
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We jointly constrain the luminosity function (LF) and black hole mass function (BHMF) of broad-line quasars with forward Bayesian modeling in the quasar mass-luminosity plane, based on a homogeneous sample of ~58000 Sloan Digital Sky Survey (SDSS) Data Release 7 quasars at z~0.3-5. We take into account the selection effect of the sample flux limit; more importantly, we deal with the statistical scatter between true BH masses and FWHM-based single-epoch virial mass estimates, as well as potential luminosity-dependent biases of these mass estimates. The LF is tightly constrained in the regime sampled by SDSS and makes reasonable predictions when extrapolated to ~3 mag fainter. Downsizing is seen in the model LF. On the other hand, we find it difficult to constrain the BHMF to within a factor of a few at z >~ 0.7 (with Mg II and C IV-based virial BH masses). This is mainly driven by the unknown luminosity-dependent bias of these mass estimators and its degeneracy with other model parameters, and secondly driven by the fact that SDSS quasars only sample the tip of the active BH population at high redshift. Nevertheless, the most likely models favor a positive luminosity-dependent bias for Mg II and possibly for C IV, such that at fixed true BH mass, objects with higher-than-average luminosities have overestimated FWHM-based virial masses. There is tentative evidence that downsizing also manifests itself in the active BHMF, and the BH mass density in broad-line quasars contributes an insignificant amount to the total BH mass density at all times. Within our model uncertainties, we do not find a strong BH mass dependence of the mean Eddington ratio, but there is evidence that the mean Eddington ratio (at fixed BH mass) increases with redshift.
- ID:
- ivo://CDS.VizieR/J/ApJ/764/45
- Title:
- Luminosity function of broad-line quasars. II.
- Short Name:
- J/ApJ/764/45
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We employ a flexible Bayesian technique to estimate the black hole (BH) mass and Eddington ratio functions for Type 1 (i.e., broad line) quasars from a uniformly selected data set of ~58000 quasars from the Sloan Digital Sky Survey (SDSS) DR7. We find that the SDSS becomes significantly incomplete at M_BH_<~3x10^8^M_{sun}_ or L/L_Edd_<~0.07, and that the number densities of Type 1 quasars continue to increase down to these limits. Both the mass and Eddington ratio functions show evidence of downsizing, with the most massive and highest Eddington ratio BHs experiencing Type 1 quasar phases first, although the Eddington ratio number densities are flat at z<2. We estimate the maximum Eddington ratio of Type 1 quasars in the observable universe to be L/L_Edd_~3. Consistent with our results in Shen & Kelly (Paper I, Cat. J/ApJ/746/169), we do not find statistical evidence for a so-called sub-Eddington boundary in the mass-luminosity plane of broad-line quasars, and demonstrate that such an apparent boundary in the observed distribution can be caused by selection effect and errors in virial BH mass estimates. Based on the typical Eddington ratio in a given mass bin, we estimate growth times for the BHs in Type 1 quasars and find that they are comparable to or longer than the age of the universe, implying an earlier phase of accelerated (i.e., with higher Eddington ratios) and possibly obscured growth. The large masses probed by our sample imply that most of our BHs reside in what are locally early-type galaxies, and we interpret our results within the context of models of self-regulated BH growth.
- ID:
- ivo://CDS.VizieR/J/A+A/608/A139
- Title:
- 237 Lyman-alpha spectra of MUSE-Wide survey
- Short Name:
- J/A+A/608/A139
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We compare 237 Lyman-{alpha} (Ly{alpha}) spectra of the "MUSE-Wide survey" (Herenz et al., 2017, Cat. J/A+A/606/A12) to a suite of radiative transfer simulations consisting of a central luminous source within a concentric, moving shell of neutral gas, and dust. This six parameter shell-model has been used numerously in previous studies, however, on significantly smaller data-sets. We find that the shell-model can reproduce the observed spectral shape very well - better than the also common 'Gaussian-minus-Gaussian' model which we also fitted to the dataset. Specifically, we find that ~94% of the fits possess a goodness-of-fit value of p(chi^2^)>0.1. The large number of spectra allows us to robustly characterize the shell-model parameter range, and consequently, the spectral shapes typical for realistic spectra. We find that the vast majority of the Ly{alpha} spectral shapes require an outflow and only ~5% are well-fitted through an inflowing shell. In addition, we find ~46% of the spectra to be consistent with a neutral hydrogen column density <10^17^cm^-2^ -- suggestive of a non-negligible fraction of continuum leakers in the MUSE-Wide sample. Furthermore, we correlate the spectral against the Ly{alpha} halo properties against each other but do not find any strong correlation.
- ID:
- ivo://CDS.VizieR/J/ApJ/811/30
- Title:
- Machine learning metallicity predictions using SDSS
- Short Name:
- J/ApJ/811/30
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Extremely metal-poor (EMP) stars ([Fe/H]<=-3.0dex) provide a unique window into understanding the first generation of stars and early chemical enrichment of the universe. EMP stars are exceptionally rare, however, and the relatively small number of confirmed discoveries limits our ability to exploit these near-field probes of the first ~500Myr after the Big Bang. Here, a new method to photometrically estimate [Fe/H] from only broadband photometric colors is presented. I show that the method, which utilizes machine-learning algorithms and a training set of ~170000 stars with spectroscopically measured [Fe/H], produces a typical scatter of ~0.29dex. This performance is similar to what is achievable via low-resolution spectroscopy, and outperforms other photometric techniques, while also being more general. I further show that a slight alteration to the model, wherein synthetic EMP stars are added to the training set, yields the robust identification of EMP candidates. In particular, this synthetic-oversampling method recovers ~20% of the EMP stars in the training set, at a precision of ~0.05. Furthermore, ~65% of the false positives from the model are very metal-poor stars ([Fe/H]<=-2.0dex). The synthetic-oversampling method is biased toward the discovery of warm (~F-type) stars, a consequence of the targeting bias from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding survey. This EMP selection method represents a significant improvement over alternative broadband optical selection techniques. The models are applied to >12 million stars, with an expected yield of ~600 new EMP stars, which promises to open new avenues for exploring the early universe.
- ID:
- ivo://CDS.VizieR/J/A+A/506/1341
- Title:
- Magnetic fields in white dwarfs
- Short Name:
- J/A+A/506/1341
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A large number of magnetic white dwarfs discovered in the SDSS have so far only been analyzed by visual comparison of the observations with relatively simple models of the radiation transport in a magnetised stellar atmosphere. We model the structure of the surface magnetic fields of the hydrogen-rich white dwarfs in the SDSS.
- ID:
- ivo://CDS.VizieR/J/ApJ/854/78
- Title:
- Magnetohydrodynamic (MHD) simulations. II.
- Short Name:
- J/ApJ/854/78
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- During the lifetime of Sun-like or low-mass stars a significant amount of angular momentum is removed through magnetized stellar winds. This process is often assumed to be governed by the dipolar component of the magnetic field. However, observed magnetic fields can host strong quadrupolar and/or octupolar components, which may influence the resulting spin-down torque on the star. In Paper I (Finley & Matt 2017ApJ...845...46F), we used the magnetohydrodynamic (MHD) code PLUTO Mignone+ 2007ApJS..170..228M ; Mignone 2009MSAIS..13...67M) to compute steady-state solutions for stellar winds containing a mixture of dipole and quadrupole geometries. We showed the combined winds to be more complex than a simple sum of winds with these individual components. This work follows the same method as Paper I, including the octupole geometry, which not only increases the field complexity but also, more fundamentally, looks for the first time at combining the same symmetry family of fields, with the field polarity of the dipole and octupole geometries reversing over the equator (unlike the symmetric quadrupole). We show, as in Paper I, that the lowest-order component typically dominates the spin-down torque. Specifically, the dipole component is the most significant in governing the spin-down torque for mixed geometries and under most conditions for real stars. We present a general torque formulation that includes the effects of complex, mixed fields, which predicts the torque for all the simulations to within 20% precision, and the majority to within ~5%. This can be used as an input for rotational evolution calculations in cases where the individual magnetic components are known.
