- ID:
- ivo://CDS.VizieR/J/A+A/524/A45
- Title:
- Overproduction factors of s-nuclei in massive stars
- Short Name:
- J/A+A/524/A45
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using the "post-processing" technique, we explore the role of the convective overshooting on the production of s-nuclei in stellar models of different initial mass and metallicity (M_ZAMS_ in the range 15-25M_{sun}_ Z in the range 10^-4^-0.02), considering a range of values for the parameter f, which determines the overall efficiency of convective overshooting. We find enhancements in the production of s-nuclei until a factor ~6 (measured as the average overproduction factor of the 6 s-only nuclear species with 60<A<90) in all our models of different initial mass and metallicity having f in the range 0.01-0.035 (i.e. models with overshooting), compared to the production obtained with "no-overshooting" models (i.e. models having the same initial mass and metallicity but f=10^-5^). Moreover the results indicate that the link between the overshooting parameter f and the s-process efficiency is essentially monotonic in all our models of different initial mass and metallicity. Also evident is the higher s-process efficiency when, for a given f value, we progressively increase both the mass of the models from 15 to 25M_{sun}_ and the Z value from 10^-4^ to 0.02.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/566/A146
- Title:
- Pair-instability supernovae models
- Short Name:
- J/A+A/566/A146
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Both recent observations and stellar evolution models suggest that pair-instability supernovae (PISNe) could occur in the local Universe, at metallicities below Z_{sun}_/3. Previous PISN models were mostly produced at very low metallicities in the context of the early Universe. We present new PISNe models at a metallicity of Z=0.001, which are relevant for the local Universe. We take the self-consistent stellar evolutionary models of pair-instability progenitors with initial masses of 150 and 250 solar masses at metallicity of Z=0.001 by Langer et al. (2007A&A...475L..19L) and follow the evolution of these models through the supernova explosions, using a hydrodynamics stellar evolution code with an extensive nuclear network including 200 isotopes. Both models explode as PISNe without leaving a compact stellar remnant. Our models produce a nucleosynthetic pattern that is generally similar to that of Population III PISN models, which is mainly characterized by the production of large amounts of elements and a strong deficiency of the odd-charged elements. However, the odd-even effect in our models is significantly weaker than that found in Population III models. The comparison with the nucleosynthetic yields from core-collapse supernovae at a similar metallicity (Z=0.002) indicates that PISNe could have strongly influenced the chemical evolution below Z=0.002, assuming a standard initial mass function. The odd-even effect is predicted to be most prominent for the intermediate mass elements between silicon and calcium. With future observations of chemical abundances in Population II stars, our result can be used to constrain the number of PISNe that occurred during the past evolution of our Galaxy.
- ID:
- ivo://CDS.VizieR/J/MNRAS/399/699
- Title:
- Parameters of atmospheres of white dwarfs
- Short Name:
- J/MNRAS/399/699
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- It is possible to reliably identify white dwarfs (WDs) without recourse to spectra, instead using photometric and astrometric measurements to distinguish them from main-sequence stars and quasars. WDs' colours can also be used to infer their intrinsic properties (effective temperature, surface gravity, etc.), but the results obtained must be interpreted with care. The difficulties stem from the existence of a solid angle degeneracy, as revealed by a full exploration of the likelihood, although this can be masked if a simple best-fitting approach is used. Conversely, this degeneracy can be broken if a Bayesian approach is adopted, as it is then possible to utilize the prior information on the surface gravities of WDs implied by spectroscopic fitting. The benefits of such an approach are particularly strong when applied to outliers, such as the candidate halo and ultracool WDs identified by Vidrih et al. A reanalysis of these samples confirms their results for the latter sample, but suggests that most of the halo candidates are thick-disc WDs in the tails of the photometric noise distribution.
- ID:
- ivo://CDS.VizieR/J/ApJ/788/59
- Title:
- Parametric model for circumstellar disks gas mass
- Short Name:
- J/ApJ/788/59
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The disks that surround young stars are mostly composed of molecular gas, which is harder to detect and interpret than the accompanying dust. Disk mass measurements have therefore relied on large and uncertain extrapolations from the dust to the gas. We have developed a grid of models to study the dependencies of isotopologue CO line strengths on disk structure and temperature parameters and find that a combination of ^13^CO and C^18^O observations provides a robust measure of the gas mass. We apply this technique to Submillimeter Array observations of nine circumstellar disks and published measurements of six well studied disks. We find evidence for selective photodissociation of C^18^O and determine masses to within a factor of about three. The inferred masses for the nine disks in our survey range from 0.7 to 6 M_Jup_, and all are well below the extrapolation from the interstellar medium gas-to-dust ratio of 100. This is consistent with the low masses of planets found around such stars, and may be due to accretion or photoevaporation of a dust-poor upper atmosphere. However, the masses may be underestimated if there are more efficient CO depletion pathways than those known in molecular clouds and cold cores.
- ID:
- ivo://CDS.VizieR/J/ApJ/808/194
- Title:
- Performance of exoplanet search space missions
- Short Name:
- J/ApJ/808/194
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an analytic model to estimate the capabilities of space missions dedicated to the search for biosignatures in the atmosphere of rocky planets located in the habitable zone of nearby stars. Relations between performance and mission parameters, such as mirror diameter, distance to targets, and radius of planets, are obtained. Two types of instruments are considered: coronagraphs observing in the visible, and nulling interferometers in the thermal infrared. Missions considered are: single-pupil coronagraphs with a 2.4m primary mirror, and formation-flying interferometers with 4x0.75m collecting mirrors. The numbers of accessible planets are calculated as a function of {eta}_Earth_. When Kepler gives its final estimation for {eta}_Earth_, the model will permit a precise assessment of the potential of each instrument. Based on current estimations, {eta}_Earth_=10% around FGK stars and 50% around M stars, the coronagraph could study in spectroscopy only ~1.5 relevant planets, and the interferometer ~14.0. These numbers are obtained under the major hypothesis that the exozodiacal light around the target stars is low enough for each instrument. In both cases, a prior detection of planets is assumed and a target list established. For the long-term future, building both types of spectroscopic instruments, and using them on the same targets, will be the optimal solution because they provide complementary information. But as a first affordable space mission, the interferometer looks the more promising in terms of biosignature harvest.
- ID:
- ivo://CDS.VizieR/J/A+A/574/A17
- Title:
- Period spacings in gamma Dor stars
- Short Name:
- J/A+A/574/A17
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The analysis of stellar oscillations is one of the most reliable ways to probe stellar interiors. Recent space missions such as Kepler have provided us with an opportunity to study these oscillations with unprecedented detail. For many multi-periodic pulsators such as gamma Doradus stars, this led to the detection of dozens to hundreds of oscillation frequencies that could not be found from ground-based observations. We aim to detect non-uniform period spacings in the Fourier spectra of a sample of gamma Doradus stars observed by Kepler. Such detection is complicated by both the large number of significant frequencies in the space photometry and by overlapping non-equidistant rotationally split multiplets. Guided by theoretical properties of gravity-mode oscillation of gamma Doradus stars, we developed a period-spacing detection method and applied it to Kepler observations of a few stars, after having tested the performance from simulations. The application of the technique resulted in the clear detection of non-uniform period spacing series for three out of the five treated Kepler targets. Disadvantages of the technique are also discussed, and include the disability to distinguish between different values of the spherical degree and azimuthal order of the oscillation modes, without additional theoretical modelling. Despite the shortcomings, the method is shown to enable solid detections of period spacings for gamma Doradus stars, which will allow future asteroseismic analyses of these stars.
- ID:
- ivo://CDS.VizieR/J/ApJ/762/123
- Title:
- PHAT. IV. Initial Mass Function
- Short Name:
- J/ApJ/762/123
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a probabilistic approach for inferring the parameters of the present-day power-law stellar mass function (MF) of a resolved young star cluster. This technique (1) fully exploits the information content of a given data set; (2) can account for observational uncertainties in a straightforward way; (3) assigns meaningful uncertainties to the inferred parameters; (4) avoids the pitfalls associated with binning data; and (5) can be applied to virtually any resolved young cluster, laying the groundwork for a systematic study of the high-mass stellar MF (M>~1M_{sun}_). Using simulated clusters and Markov Chain Monte Carlo sampling of the probability distribution functions, we show that estimates of the MF slope, {alpha}, are unbiased and that the uncertainty, {Delta}{alpha}, depends primarily on the number of observed stars and on the range of stellar masses they span, assuming that the uncertainties on individual masses and the completeness are both well characterized. Using idealized mock data, we compute the theoretical precision, i.e., lower limits, on {alpha}, and provide an analytic approximation for {Delta}{alpha} as a function of the observed number of stars and mass range. Comparison with literature studies shows that ~3/4 of quoted uncertainties are smaller than the theoretical lower limit. By correcting these uncertainties to the theoretical lower limits, we find that the literature studies yield <{alpha}>=2.46, with a 1{sigma} dispersion of 0.35dex. The precision on MF slope recovery in this paper are lower limits, as we do not explicitly consider all possible sources of uncertainty, including dynamical effects (e.g., mass segregation), unresolved binaries, and non-coeval populations.
- ID:
- ivo://CDS.VizieR/J/ApJ/745/77
- Title:
- Photochemical model for planet WASP-12b
- Short Name:
- J/ApJ/745/77
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The hot-Jupiter WASP-12b is a heavily irradiated exoplanet in a short-period orbit around a G0-star with twice the metallicity of the Sun. A recent thermochemical equilibrium analysis based on Spitzer and ground-based infrared observations suggests that the presence of CH_4_ in its atmosphere and the lack of H_2_O features can only be explained if the carbon-to-oxygen ratio in the planet's atmosphere is much greater than the solar ratio ([C]/[O]=0.54). Here, we use a one-dimensional photochemical model to study the effect of disequilibrium chemistry on the observed abundances of H_2_O, CO, CO_2_, and CH_4_ in the WASP-12b atmosphere. We consider two cases: one with solar [C]/[O] and another with [C]/[O]=1.08. The solar case predicts that H_2_ O and CO are more abundant than CO_2_ and CH_4_, as expected, whereas the high [C]/[O] model shows that CO, C_2_H_2_, and HCN are more abundant. This indicates that the extra carbon from the high [C]/[O] model is in hydrocarbon species. H_2_ O photolysis is the dominant disequilibrium mechanism that alters the chemistry at higher altitudes in the solar [C]/[O] case, whereas photodissociation of C_2_H_2_ and HCN is significant in the super-solar case. Furthermore, our analysis indicates that C_2_H_2_ is the major absorber in the atmosphere of WASP-12b and the absorption features detected near 1.6 and 8{mu}m may be arising from C_2_H_2_ rather than CH_4_. The Hubble Space Telescope's WFC3 can resolve this discrepancy, as C_2_H_2_ has absorption between 1.51 and 1.54{mu}m, while CH_4_ does not.
- ID:
- ivo://CDS.VizieR/J/A+A/574/A127
- Title:
- Photodissociation with mechanical heating
- Short Name:
- J/A+A/574/A127
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- CO observations in active galactic nuclei and starbursts reveal high kinetic temperatures. Those environments are thought to be very turbulent due to dynamic phenomena, such as outflows and high supernova rates. We investigate the effect of mechanical heating on atomic fine-structure and molecular lines and on their ratios. We try to use those ratios as a diagnostic to constrain the amount of mechanical heating in an object and also study its significance on estimating the H_2_ mass. Equilibrium photodissociation models (PDRs hereafter) were used to compute the thermal and chemical balance for the clouds. The equilibria were solved for numerically using the optimized version of the Leiden PDR-XDR code. Large velocity-gradient calculations were done as post-processing on the output of the PDR models using RADEX. High-J CO line ratios are very sensitive to mechanical heating ({GAMMA}mech hereafter). Emission becomes at least one order of magnitude brighter in clouds with n~10^5^cm^-3^ and a star formation rate of 1M_{sun}/yr (corresponding to {GAMMA}mech=2x10^-19^erg/cm^3^/s). The Emission of low-J CO lines is not as sensitive to {GAMMA}mech, but they do become brighter in response to {GAMMA}mech. Generally, for all of the lines we considered, {GAMMA}mech increases excitation temperatures and decreases the optical depth at the line centre. Hence line ratios are also effected, strongly in some cases. Ratios involving HCN are a good diagnostic for {GAMMA}mech , where the HCN(1-0)/CO(1-0) increases from 0.06 to 0.25, and the HCN(1-0)/HCO^+^ (1-0) increase from 0.15 to 0.5 for amounts of {GAMMA}mech that are equivalent to 5% of the surface heating rate. Both ratios increase to more than 1 for higher {GAMMA}mech , as opposed to being much less than unity in pure PDRs. The first major conclusion is that low-J to high-J intensity ratios will yield a good estimate of the mechanical heating rate (as opposed to only low-J ratios). The second one is that the mechanical heating rate should be taken into account when determining AV or, equivalently, NH, and consequently the cloud mass. Ignoring {GAMMA}mech will also lead to large errors in density and radiation field estimates.
- ID:
- ivo://CDS.VizieR/J/A+A/654/A172
- Title:
- Photoionization of the CH radical
- Short Name:
- J/A+A/654/A172
- Date:
- 07 Mar 2022 07:12:57
- Publisher:
- CDS
- Description:
- The primary motivation of this paper is to provide accurate data for the photoionization of the CH radical, including absolute total photoionization cross section, partial cross sections and photo electron angular distribution. In addition,the near threshold features in the photoionization curve (which are absent in previous studies) are produced with high precision. A multichannel wavefunction based on R-matrix approach, which uses the configuration interaction (CI) method to describe electronic correlation, is carried out in the present calculations. A set of B-spline orbitals is employed to represent the accurate continuum. The distinctive feature of the present calculations allows us to generate a more accurate description of the bound and continuum states than those employed before. Total photoionization cross sections from the ground state of CH radicals and partial cross sections corresponding to 1{pi}, 3{sigma}, and 2{sigma} states of CH+ ions are presented for photon energies ranging from threshold to 80eV. Extensive resonance structures, which are absent in previous studies, near the ionization threshold, are observed for the first time. The cross-section dataset obtained from the present calculations is expected to be sufficiently accurate and comprehensive for most current modeling applications involving the photon and CH radicals scattering system.
