Solar Abundance Of Heavy Elements Research Articles

Exploring the hypothesis of an inverted Z gradient inside Jupiter

Context. Reconciling models of Jupiter’s interior with measurements of the atmospheric composition still poses a significant challenge. Interior models favour a subsolar or solar abundance of heavy elements, Z, whereas atmospheric measurements suggest a supersolar abundance. One potential solution may be to account for the presence of an inverted Z gradient, namely, an inward decrease of Z, which implies a higher heavy-element abundance in the atmosphere than in the outer envelope. Aims. We investigate two scenarios in which the inverted Z gradient is either located at levels where helium rain occurs (∼Mbar) or at higher levels (∼kbar) where a radiative region could exist. Here, we aim to assess the plausibility of these scenarios. Methods. We calculated interior and evolution models of Jupiter with such an inverted Z gradient and we set constraints on its stability and formation. Results. We find that an inverted Z gradient at the location of helium rain is not feasible, as it would require a late accretion and would involve too much material. We find interior models with an inverted Z gradient at upper levels due to a radiative zone preventing downward mixing, could satisfy the current gravitational field of the planet. However, our evolution models suggest that this second scenario cannot be validated. Conclusions. We find that an inverted Z gradient in Jupiter could indeed be stable, however, its presence either at the Mbar or kbar levels is rather unlikely.

A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b

Close-in giant exoplanets with temperatures greater than 2,000 K (‘ultra-hot Jupiters’) have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble Space Telescope (HST) and Spitzer Space Telescope1–3. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis3–12. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS13 instrument on the JWST. The data span 0.85 to 2.85 μm in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at >6σ confidence) and evidence for optical opacity, possibly attributable to H−, TiO and VO (combined significance of 3.8σ). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy-element abundance (‘metallicity’, {rm{M/H}}=1.0{3}_{-0.51}^{+1.11} times solar) and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the substellar point that decreases steeply and symmetrically with longitude towards the terminators.

Determining the metallicity of the solar envelope using seismic inversion techniques

The solar metallicity issue is a long-lasting problem of astrophysics, impacting multi- ple fields and still subject to debate and uncertainties. While spectroscopy has mostly been used to determine the solar heavy elements abundance, helioseismologists at- tempted providing a seismic determination of the metallicity in the solar convective enveloppe. However, the puzzle remains since two independent groups prodived two radically different values for this crucial astrophysical parameter. We aim at provid- ing an independent seismic measurement of the solar metallicity in the convective enveloppe. Our main goal is to help provide new information to break the current stalemate amongst seismic determinations of the solar heavy element abundance. We start by presenting the kernels, the inversion technique and the target function of the inversion we have developed. We then test our approach in multiple hare-and-hounds exercises to assess its reliability and accuracy. We then apply our technique to solar data using calibrated solar models and determine an interval of seismic measurements for the solar metallicity. We show that our inversion can indeed be used to estimate the solar metallicity thanks to our hare-and-hounds exercises. However, we also show that further dependencies in the physical ingredients of solar models lead to a low accuracy. Nevertheless, using various physical ingredients for our solar models, we determine metallicity values between 0.008 and 0.014.

Helioseismic measurements in the solar envelope using group velocities of surface waves

At intermediate and high degree $l$, solar p- and f modes can be considered as surface waves. Using variational principle, we derive an integral expression for the group velocities of the surface waves in terms of adiabatic eigenfunctions of normal modes, and address the benefits of using group-velocity measurements as a supplementary diagnostic tool in solar seismology. The principal advantage of using group velocities, when compared with direct analysis of the oscillation frequencies, comes from their smaller sensitivity to the uncertainties in the near-photospheric layers. We address some numerical examples where group velocities are used to reveal inconsistencies between the solar models and the seismic data. Further, we implement the group-velocity measurements to the calibration of the specific entropy, helium abundance Y and heavy-element abundance Z in the adiabatically-stratified part of the solar convective envelope, using different recent versions of the equation of state. The results are in close agreement with our earlier measurements based on more sophisticated analysis of the solar oscillation frequencies (Vorontsov et al. 2013, MNRAS 430, 1636). These results bring further support to the downward revision of the solar heavy-element abundances in recent spectroscopic measurements.

JUPITER MODELS WITH IMPROVED AB INITIO HYDROGEN EQUATION OF STATE (H-REOS.2)

The amount and distribution of heavy elements in Jupiter gives indications on the process of its formation and evolution. Core mass and metallicity predictions however depend on the equations of state used, and on model assumptions. We present an improved ab initio hydrogen equation of state, H-REOS.2 and compute the internal structure and thermal evolution of Jupiter within the standard three-layer approach. The advance over our previous Jupiter models with H-REOS.1 by Nettelmann et al.(2008) is that the new models are also consistent with the observed 2 or more times solar heavy element abundances in Jupiter's atmosphere. Such models have a rock core mass Mcore=0-8 ME, total mass of heavy elements MZ=28-32 ME, a deep internal layer boundary at 4 or more Mbar, and a cooling time of 4.4-5.0 Gyrs when assuming homogeneous evolution. We also calculate two-layer models in the manner of Militzer et al.(2008) and find a comparable large core of 16-21 ME, out of which ~11 ME is helium, but a significantly higher envelope metallicity of 4.5 times solar. According to our preferred three-layer models, neither the characteristic frequency (nu0 ~156 microHz) nor the normalized moment of inertia (~0.276) are sensitive to the core mass but accurate measurements could well help to rule out some classes of models.

EXOPLANET ALBEDO SPECTRA AND COLORS AS A FUNCTION OF PLANET PHASE, SEPARATION, AND METALLICITY

First generation optical coronagraphic telescopes will obtain images of cool gas and ice giant exoplanets around nearby stars. The albedo spectra of exoplanets at planet-star separations larger than about 1 AU are dominated by reflected light to beyond 1 {\mu}m and are punctuated by molecular absorption features. We consider how exoplanet albedo spectra and colors vary as a function of planet-star separation, metallicity, mass, and observed phase for Jupiter and Neptune analogs from 0.35 to 1 {\mu}m. We model Jupiter analogs with 1x and 3x the solar abundance of heavy elements, and Neptune analogs with 10x and 30x. Our model planets orbit a solar analog parent star at separations of 0.8 AU, 2 AU, 5 AU, and 10 AU. We use a radiative-convective model to compute temperature-pressure profiles. The giant exoplanets are cloud-free at 0.8 AU, have H2O clouds at 2 AU, and have both NH3 and H2O clouds at 5 AU and 10 AU. For each model planet we compute moderate resolution spectra as a function of phase. The presence and structure of clouds strongly influence the spectra. Since the planet images will be unresolved, their phase may not be obvious, and multiple observations will be needed to discriminate between the effects of planet-star separation, metallicity, and phase. We consider the range of these combined effects on spectra and colors. For example, we find that the spectral influence of clouds depends more on planet-star separation and hence temperature than metallicity, and it is easier to discriminate between cloudy 1x and 3x Jupiters than between 10x and 30x Neptunes. In addition to alkalis and methane, our Jupiter models show H2O absorption features near 0.94 {\mu}m. We also predict that giant exoplanets receiving greater insolation than Jupiter will exhibit higher equator to pole temperature gradients than are found on Jupiter and thus may have differing atmospheric dynamics.

METAL-ABSORPTION COLUMN DENSITIES IN FAST RADIATIVE SHOCKS

In this paper we present computations of the integrated metal-ion column densities produced in the post-shock cooling layers behind fast, radiative shock-waves. For this purpose, we have constructed a new shock code that calculates the non-equilibrium ionization and cooling; follows the radiative transfer of the shock self-radiation through the post-shock cooling layers; takes into account the resulting photoionization and heating rates; follows the dynamics of the cooling gas; and self-consistently computes the initial photoionization state of the precursor gas. We discuss the shock structure and emitted radiation, and study the dependence on the shock velocity, magnetic field, and gas metallicity. We present a complete set of integrated post-shock and precursor metal-ion column densities of all ionization stages of the elements H, He, C, N, O, Ne, Mg, Si, S, and Fe, for shocks with velocities of 600 and ~2000 km/s, corresponding to initial post-shock temperatures of 5e6 and 5e7 K, cooling down to 1000 K. We consider shocks in which the magnetic field is negligible (B=0) so that the cooling occurs at approximately constant pressure ("isobaric"), and shocks in which the magnetic pressure dominates everywhere such that the cooling occurs at constant density (isochoric). We present results for gas metallicities Z ranging from 1e-3 to twice the solar abundance of heavy elements, and we study how the observational signatures of fast radiative shocks depend on Z. We present our numerical results in convenient online figures and tables, available at http://wise-obs.tau.ac.il/~orlyg/shocks/

Single and binary evolution of Population III stars and their supernova explosions

We present stellar evolution calculations for Population III stars for both single- and binary- star evolutions. Our models include 10- and 16.5-Msingle stars and a 10-Mmodel star that undergoes an episode of accretion resulting in a final mass of 16.1 M� . For comparison, we present the evolution of a solar heavy element abundance model. We use the structure from late-stage evolution models to calculate simulated supernova light curves. Light curve comparisons are made between accretion and non-accretion progenitor models, and models for single-star evolution of comparable masses. Where possible, we make comparisons to previous works. Similar investigations have been carried out, but primarily for solar or near-solar heavy metal abundance stars and not including both the evolution and the supernova explosions in one work.

Seismic Study of the Chemical Composition of the Solar Convection Zone

Recent downward revision of solar heavy-element abundances using three-dimensional atmospheric model has introduced serious discrepancies between standard solar models and helioseismic inferences about solar structure. In this paper, we investigate the possibility of determining the heavy-element abundances using helioseismic inversion techniques with the hope of providing an independent estimate. We use the adiabatic index, Gamma_1 (logarithmic partial derivative of pressure with respect to density at constant entropy) as a probe to examine the effects of the total heavy-element abundance, as well as the effects due to the abundance of individual elements. Our inversion results show that the new, lower, abundance increases the discrepancy between the Sun and the solar models.

Importance of Compton scattering for radiation spectra of isolated neutron stars

Model atmospheres of isolated neutron stars with low magnetic field are calculated with Compton scattering taking into account. Radiation spectra computed with Compton scattering are softer than computed with Thomson scattering at high energies (E>5 keV) for hot (T eff>106 K) atmospheres with hydrogen-helium composition. Compton scattering is more significant to models with low surface gravity. Compton scattering is less important to models with solar abundance of heavy elements.

Importance of Compton scattering for radiation spectra of isolated neutron stars with weak magnetic fields

Aims. Emergent model spectra of neutron-star atmospheres are widely used to fit the observed soft X-ray spectra of different types of isolated neutron stars. We investigate the effect of Compton scattering on the emergent spectra of hot (T eff > 10 6 K) isolated neutron stars with weak magnetic fields. Methods. In order to compute model atmospheres in hydrostatic and radiative equilibrium we solve the radiation transfer equation with the Kompaneets operator. We calculate a set of models with effective temperatures in the range 1-5 x 10 6 K, with two values of surface gravity (log g = 13.9 and 14.3) and different chemical compositions. Results. Radiation spectra computed with Compton scattering are softer than those computed without Compton scattering at high energies (E > 5 keV) for light-element (H or He) model atmospheres. The Compton effect is more significant in H model atmospheres and models with low surface gravity. The emergent spectra of the hottest (T eff > 3 x 10 6 K) model atmospheres can be described by diluted blackbody spectra with hardness factors ∼1.6-1.9. Compton scattering is less important in models with solar abundance of heavy elements.

The Solar Heavy‐Element Abundances. I. Constraints from Stellar Interiors

The latest solar atmosphere models including non-LTE corrections and three-dimensional hydrodynamic convection simulations predict a significant reduction in the solar metal abundance. This leads to a serious conflict between helioseismic data and the predictions of solar interiors models. We demonstrate that the helioseismic constraints on the surface convection zone depth and helium abundance combined with stellar interiors models can be used to constrain chemical composition. A detailed examination of the errors in the theoretical models disfavors strongly (disagreeing at the 15 σ level with the seismic data) the proposed new low abundance, while the models constructed with the older and higher solar abundances are consistent (within 2 σ). We then use the sensitivity of the seismic properties to abundance changes to invert the problem and infer a seismic solar heavy-element abundance mix with two components: meteoritic abundances and the light metals CNONe. Seismic degeneracies between the best solutions for the elements arise for changes in the relative CNONe abundances and their effects are quantified. We obtain Fe/H = 7.50 ± 0.045 ± 0.003(CNNe) and O/H = 8.86 ± 0.041 ± 0.025(CNNe) on the logarithmic scale, where H = 12 for the relative CNNe mixtures in the Grevese & Sauval mixture; the second error term reflects the uncertainty in the overall abundance scale from errors in the C, N, and Ne abundances relative to oxygen. These are consistent within the errors with the previous standard solar mixture but in strong conflict with the low oxygen abundance inferred from the three-dimensional hydro models. Changes in the Ne abundance can mimic changes in oxygen for the purposes of scalar constraints. However, models constructed with low oxygen and high neon are inconsistent with the solar sound speed profile. Implications for the solar abundance scale are discussed.

Determining Solar Abundances Using Helioseismology

The recent downward revision of solar photospheric abundances of oxygen and other heavy elements has resulted in serious discrepancies between solar models and solar structure as determined through helioseismology. In this work we investigate the possibility of determining the solar heavy-element abundance without reference to spectroscopy by using helioseismic data. Using the dimensionless sound-speed derivative in the solar convection zone, we find that the heavy-element abundance Z = 0.0172 ± 0.002, which is closer to the older, higher value of the abundances.

Recent (1981–82) evolution of galactic haloes

Recent (1981–82) evolution of galactic haloes

A PHOTOMETRIC STUDY OF SW ANDROMEDAE.

ABSTRACT Photometric (uvbyfl) observations of the strong-line RR Lyrae variable SW And are described. The photometry suggests that the star is reddened by E(b - y) . Intrinsic (b - y) and c1 values are used to derive the variations in and log g. The mean values are 6680 K and log g 2.9. Wesselink's method is used to derive a radius of R/R 4.45. The radius, , and log g values lead to M = and 9)' = 0.6 9) , Some peculiarities in the behavior of the m1 index are discussed. The mean m1 value at light minimum indicates that SW And has a near solar abundance of heavy elements. Key words: RR Lyrae variable-photometry-physical elements-Wesselink's method