Can Fluffy Dust Alleviate the Subsolar Interstellar Abundance Problem?

Elements in the interstellar medium (ISM) exist in the form of gas or dust. The interstellar extinction and elemental abundances provide crucial constraints on the composition, size, and quantity of interstellar dust. Most of the extinction modeling efforts have assumed the total (gas and dust) abundances of the dust-forming elements—known as the “interstellar abundances,” “interstellar reference abundances,” or “cosmic abundances”—to be solar and the gas-phase abundances to be environmentally independent. However, it remains unclear whether the solar abundances are an appropriate representation of the interstellar abundances. Meanwhile, the gas-phase abundances are known to exhibit appreciable variations with local environments. Here we explore the viability of the abundances of B stars, the solar and protosolar abundances, and the protosolar abundances augmented by Galactic chemical enrichment (GCE) as an appropriate representation of the interstellar abundances by quantitatively examining the extinction and abundances of 10 interstellar sight lines for which both the extinction curves and the gas-phase abundances of all the major dust-forming elements (i.e., C, O, Mg, Si and Fe) have been observationally determined. Instead of assuming a specific dust model and then fitting the observed extinction curves, for each sight line we apply the model-independent Kramers–Kronig relation, which relates the wavelength-integrated extinction to the total dust volume, to place a lower limit on the dust depletion. This, together with the observationally derived gas-phase abundances, allows us to rule out the B-star, solar, and protosolar abundances as the interstellar reference standard and support the GCE-augmented protosolar abundances as a viable representation of the interstellar abundances.

Based on the heavy element nucleosynthesis theory, with the solar heavy-nuclide abundances and the observed abundances of three elements which are the representatives of the individul neutron-capture processes, a method to determine the relative contributions from the individul neutron-capture processes to the abundances of heavy elements in metal-poor stars is applied. With this method, the abundances of heavy elements in ultra-metal-poor star CS 22892-052 are calculated. It is found that the observed abundances of heavy elements in this star are well matched by our calculations in error limits, except for thorium.

Using the Cerro Tololo Inter-American Observatory 4 m telescope, we have obtained optical spectra of H II regions in five Sculptor group dwarf irregular galaxies. We derive oxygen, nitrogen, and sulfur abundances from the H II region spectra. Oxygen abundances are derived via three different methods (the direct method, the empirical method guided by photoionization modeling of McGaugh [published in 1991], and the purely empirical method of Pilyugin, published in 2000) and are compared. Significant systematic differences are found between the three methods, and we suggest that a recalibration of the empirical abundance scale is required. Until differences between these three methods are better understood, the issue of the degree of uniformity of the interstellar medium abundances in a dwarf galaxy cannot be properly addressed. The N/O ratio for the metal-poor dI ESO 473-G24 of log (N/O) = -1.43 ± 0.03 lies well above the plateau of log (N/O) = -1.60 ± 0.02 found by Izotov & Thuan for a collection of metal-poor, blue compact galaxies. This shows that not all galaxies with 12 + log (O/H) ≤ 7.6 have identical elemental abundance ratios, and this implies that the Izotov & Thuan scenario for low-metallicity galaxies is not universal. Measurements of the H II regions in NGC 625 yield log (N/O) ≈ -1.25. Assuming N production by intermediate-mass stars, this relatively high N/O ratio may be indicative of a long quiescent period prior to the recent active burst of star formation. The oxygen abundances in the Sculptor group dI's are in good agreement with the relationship between metallicity and luminosity observed in the Local Group dI's. Taken together, the observations show a better relationship between metallicity and luminosity than between metallicity and galaxy central surface brightness. The Sculptor group dI's, in general, lie closer to the simple closed-box model evolutionary path than the Local Group dI's. The higher gas contents, lower average star formation rates, and closer resemblance to closed-box evolution could all be indicative of evolution in a relatively low density environment.

The Goddard high-resolution spectrograph aboard the Hubble Space Telescope has been used to produce interstellar abundance measures of gallium, germanium, arsenic, krypton, tin, thallium, and lead, the heaviest elements detected in interstellar gas. These heavy elements arise from stellar nuclear processes (slow- and rapid-process neutron capture) that are different from those that produce zinc and the lighter elements previously observed. These data allow investigators to study how the heavy elements chemically interact with interstellar dust and to compare interstellar heavy element abundances in the current galactic epoch to those present at the time of the formation of the solar system. For example, the data indicate that the abundance of atoms in interstellar dust cannot be explained by simple condensation models alone and must be heavily influenced by chemistry in the interstellar medium. Also, the data for some elements suggest that their true galactic cosmic abundances may be different from the "fossil" abundances incorporated into the solar system 4.6 billion years ago.

Perhaps more than almost any other subject of astrophysical study, the interstellar medium can be studied at any wavelength—from gamma rays to the radio—with each waveband providing unique and significant information. However, there are substantial limitations on the available data. One such limitation is the fact that very few lines of sight have been adequately observed in multiple wavebands. This in turn restricts our understanding of topics as fundamental as interstellar abundances and dust composition. I will review the reasons that such data limitations exist and specifically describe two accepted observing programs that will, in the near future, utilize a multiwavelength approach in studying interstellar dust. The first is a Chandra/HST program that will provide unique insight into dust composition by disentangling the gas‐phase and dust‐phase abundances of oxygen and iron. The second is a Spitzer/HST program that will examine the extinction curves of possible “translucent clouds” at wavelengths fr...

Some form of hydrogenated amorphous carbon (HAC) is widely regarded as a likely component of dust in the diffuse interstellar medium of the Galaxy. The most direct observational evidence is the ∼2950 cm −1 C-H stretch feature seen in absorption along heavily attenuated lines of sight. We have used the measured properties of one well-characterized HAC sample, together with well-established correlations between the structural, optical, and infrared (IR) characteristics of HAC materials in general, to estimate the amount of carbon relative to hydrogen that must be bound in this particular solid in interstellar space. The solution to this problem remains uncertain because of the strong dependence of the ∼2950 cm −1 mass-absorption coefficient upon the H/C ratio in HAC materials. We estimate that about 80 ppM carbon relative to hydrogen is locked up in interstellar HAC grains with H/C ≃ 0.5. A H/C ratio lower than this value would require higher carbon depletions and result in a ∼ 2950 cm −1 absorption profile inconsistent with observations. Substantially higher H/C ratios, which would imply carbon depletions as low as 20 ppM, would lead to materials exhibiting efficient blue-green photoluminescence, which is not observed. Combined with observations of the gas-phase interstellar abundance of carbon and the revised interstellar reference abundances of refractory elements, this result implies that the bulk of solid carbon in interstellar space is in the form of HAC. In addition to a careful analysis of the IR properties of HAC, we present a summary of the UV/visible optical properties, including the complex index of refraction, measured for one particularly well-characterized HAC sample that is shown to have an IR absorption spectrum which is quantitatively consistent with the latest IR observations of the diffuse interstellar medium. Subject headings: dust, extinction — infrared: ISM: lines and bands — ISM: abundances

Context.A number of He-rich hot subdwarf stars present high abundances for trans-iron elements, such as Sr, Y, Zr, and Pb. Diffusion processes are important in hot subdwarf stars and it is generally believed that the high abundances of heavy elements in these peculiar stars are due to the action of radiative levitation. However, during the formation of He-rich hot subdwarf stars, hydrogen can be ingested into the convective zone driven by the He-core flash. It is known that episodes of protons being ingested into He-burning convective zones can lead to neutron-capture processes and the formation of heavy elements.Aims.In this work, we aim to explore, for the first time, whether neutron-capture processes can occur in late He-core flashes taking place in the cores of the progenitors of He-rich hot subdwarfs. We aim to explore the possibility of a self-synthesized origin for the heavy elements observed in some He-rich hot subdwarf stars.Methods.We computed a detailed evolutionary model for a stripped red-giant star using a stellar evolution code with a nuclear network comprising 32 isotopes. Then we post-processed the stellar models in the phase of helium and hydrogen burning using a post-processing nucleosynthesis code with a nuclear network of 1190 species, which allowed us to follow the neutron-capture processes in detail.Results.We find the occurrence of neutron-capture processes in our model, with neutron densities reaching a value of ∼5 × 1012 cm−3. We determined that the trans-iron elements are enhanced in the surface by 1 to 2 dex, as compared to initial compositions. Moreover, the relative abundance pattern [Xi/Fe] produced by neutron-capture processes closely resembles those observed in some He-rich hot subdwarf stars, hinting at a possible self-synthesized origin for the heavy elements in these stars.Conclusions.We conclude that intermediate neutron-capture processes can occur during a proton ingestion event in the He-core flash of stripped red-giant stars. This mechanism offers a natural channel for the production of the heavy elements observed in certain He-rich hot subdwarf stars.

The composition of a planet’s atmosphere is intricately linked to the chemical makeup of the protoplanetary disc in which it formed. Determining the elemental abundances from key volatiles within discs is therefore essential for establishing connections between the composition of discs and planets. The disc around the Herbig Ae star HD 169142 is a compelling target for such a study due to its molecule-rich nature and the presence of a newly forming planet between two prominent dust rings. In this work, we probe the chemistry of the HD 169142 disc at small spatial scales, drawing links between the composition of the disc and the planet-accreted gas. Using thermochemical models and archival data, we constrain the elemental abundances of volatile carbon, oxygen, and sulfur. Carbon and oxygen are only moderately depleted from the gas phase relative to their interstellar abundances, with the inner ~60 au appearing enriched in volatile oxygen. The carbon-to-oxygen ratio is approximately solar within the inner disc (~0.5) and rises above this in the outer disc (>0.5), as expected across the H$_2$O snowline. The gas-phase sulfur abundance is depleted by a factor of ~1000, consistent with a number of other protoplanetary discs. Interestingly, the observed SiS emission near the HD 169142 b protoplanet vastly exceeds chemical model predictions, supporting previous hypotheses suggesting its origin in shocked gas or a localized outflow. We contextualize our findings in terms of the potential atmospheric composition of the embedded planet, and highlight the utility of sulfur-bearing molecules as probes of protoplanetary disc chemistry.

Context. Recently, a revision of the solar abundances of C, N, and O to substantially lower values has led to a controversy on solar opacities in the solar standard model and to the suggestion to revise the solar abundance of neon upward by as much as a factor of 1.6 leading to enhanced solar neon/oxygen abundance ratios by a factor of 3. Neon and oxygen are neighboring elements with easily defined charged states in the solar wind, and they have been well identified and measured for over two decades in the solar wind under many circumstances and with several instruments. The solar wind Ne/O ratio is 0.14 with a conservative error estimate of ±0.03, consistent with the coronal value derived from solar energetic particle measurements. Aims. We investigate, whether solar wind observations are consistent with the newly proposed elemental solar abundances of neon and oxygen. Methods. The solar helium abundance has been derived from helioseismological observations. Helium and neon abundances in the solar wind have been well determined with the Apollo Foil experiments and, more recently, confirmed with the Genesis sample return mission. With these observations and the neon/oxygen solar wind abundance ratio determined by in-situ mass-spectrometry and using a simple theoretical model of Coulomb-drag fractionation for the solar wind, we estimate solar abundances for neon and oxygen. Results. From the variability of the helium/oxygen and the helium/neon ratio in the solar wind and from theoretical considerations, we conclude that the helium/neon ratio in the outer solar convective zone is 900 ± 110. Our best estimates of the solar neon and oxygen abundances in logarithmic dex-units are [Ne] = 7.96 ± 0.13, and [O] = 8.87 ± 0.11. Conclusions. Our solar neon/oxygen abundance ratio is consistent with the ratio derived from EUV-spectra from SOHO/CDS. However, our absolute abundance for oxygen is only marginally compatible with the newly derived value for oxygen, and our neon value is clearly incompatible with the recently proposed enhancement of the solar neon abundance.

view Abstract Citations (83) References (94) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Abundances in Dense, Moderately Reddened Lines of Sight. I. Observational Evidence for Density-dependent Depletion Joseph, Charles L. ; Snow, Theodore P., Jr. ; Seab, C. Gregory ; Crutcher, Richard M. Abstract The nature of dust-gas interactions, which are capable of modifying the size distribution of interstellar grains and thus causing changes in the selective extinction curve, are investigated through depletion studies. The gaseous abundances of 15 elements have been determined for several lines of sight toward moderately reddened stars, each having an anomalous extinction curve and a large abundance of cyanogen (CN). The basic result of this study is that certain elements appear to deplete preferentially in interstellar clouds having a large abundance of CN. The distinctly different signature of the element-to-element depletion pattern provides an important new diagnostic tool for determining the relative strengths of competing depletion processes in these denser clouds. Publication: The Astrophysical Journal Pub Date: October 1986 DOI: 10.1086/164647 Bibcode: 1986ApJ...309..771J Keywords: Abundance; Interstellar Chemistry; Interstellar Extinction; Interstellar Matter; Iue; Line Of Sight; Particle Size Distribution; Spectral Line Width; Astrophysics; INTERSTELLAR: ABUNDANCES; INTERSTELLAR: GRAINS; INTERSTELLAR: MOLECULES full text sources ADS | data products SIMBAD (5) MAST (1)

We present new measurements of the interstellar gas-phase oxygen abundance along the sight lines toward 19 early-type Galactic stars at an average distance of 2.6 kpc. We derive O i column densities from Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) observations of the weak 1355 Ainter- system transition. We derive total hydrogen column densities (NðH i Þþ 2NðH2Þ) using HST/STIS observa- tions of Lyand Far Ultraviolet Spectroscopic Explorer (FUSE) observations of molecular hydrogen. The molecular hydrogen content of these sight lines ranges from f ðH2 Þ¼ 2NðH2Þ=½NðH i Þþ 2NðH2Þ� ¼ 0:03 to 0.47. The average hHtot=EBV i of 6:3 � 10 21 cm � 2 mag � 1 with a standard deviation of 15% is consistent with previous surveys. The mean oxygen abundance along these sight lines, which probe a wide range of Galactic environments in the distant interstellar medium, is 10 6 (O/H)gas¼ 408 � 13 (1 � in the mean). We see no evi- dence for decreasing gas-phase oxygen abundance with increasing molecular hydrogen fraction, and the rela- tive constancy of (O/H)gas suggests that the component of dust containing the oxygen is not readily destroyed. We estimate that, if 60% of the dust grains are resilient against destruction by shocks, the distant interstellar total oxygen abundance can be reconciliated with the solar value derived from the most recent measurements of 10 6 (O/H)gas� ¼ 517 � 58 (1 � ). We note that the smaller oxygen abundances derived for the interstellar gas within 500 pc or from nearby B star surveys are consistent with a local elemental deficit. Subject headings: dust, extinction — ISM: abundances — ultraviolet: ISM

Geochemistry and genesis of the angrites

Aims. We report the results of a novel determination of the solar oxygen abundance using spatially resolved observations and inversions. We seek to derive the photospheric solar oxygen abundance with a method that is robust against uncertainties in the model atmosphere. Methods. We use observations with spatial resolution obtained at the Vacuum Tower Telescope to derive the oxygen abundance at 40 different spatial positions in granules and intergranular lanes. We first obtain a model for each location by inverting the Fe I lines with the NICOLE inversion code. These models are then integrated into a hierarchical Bayesian model that is used to infer the most probable value for the oxygen abundance that is compatible with all the observations. The abundance is derived from the [O I] forbidden line at 6300 Å taking into consideration all possible nuisance parameters that can affect the abundance. Results. Our results show good agreement in the inferred oxygen abundance for all the pixels analyzed, demonstrating the robustness of the analysis against possible systematic errors in the model. We find a slightly higher oxygen abundance in granules than in intergranular lanes when treated separately (log(ϵO) = 8.83 ± 0.02 vs. log(ϵO) = 8.76 ± 0.02), which is a difference of approximately 2-σ. This tension suggests that some systematic errors in the model or the radiative transfer still exist but are small. When taking all pixels together, we obtain an oxygen abundance of log(ϵO) = 8.80 ± 0.03, which is compatible with both granules and lanes within 1-σ. The spread of results is due to both systematic and random errors.

The solar photospheric oxygen abundance is still widely debated. Adopting the solar chemical composition based on the "low" oxygen abundance, as determined with the use of three-dimensional (3D) hydrodynamical model atmospheres, results in a well-known mismatch between theoretical solar models and helioseismic measurements that is so far unresolved. We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the OI IR triplet lines at 777 nm in different sets of spectra with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTETD. The idea is to simultaneously derive the oxygen abundance,A(O), and the scaling factor SH that describes the cross-sections for inelastic collisions with neutral hydrogen relative the classical Drawin formula. The best fit of the center-to-limb variation of the triplet lines achieved with the CO5BOLD 3D solar model is clearly of superior quality compared to the line profile fits obtained with standard 1D model atmospheres. Our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 +/- 0.02, with the scaling factor SH in the range between 1.2 and 1.8. All 1D non-LTE models give much lower oxygen abundances, by up to -0.15 dex. This is mainly a consequence of the assumption of a $\mu$-independent microturbulence.

High-resolution far-UV spectra of the O-type subdwarf BD +28°4211 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to measure the interstellar deuterium, nitrogen, and oxygen abundances in this direction. The interstellar D I transitions are analyzed down to Lyι at 920.7 A. The star was observed several times at different target offsets in the direction of spectral dispersion. The aligned and co-added spectra have high signal-to-noise ratios (S/N = 50-100). D I, N I, and O I transitions were analyzed with curve-of-growth and profile fitting techniques. A model of interstellar molecular hydrogen on the line of sight was derived from H2 lines in the FUSE spectra and used to help analyze some features where blending with H2 was significant. The H I column density was determined from high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph spectra of Lyα to be log N(H ) = 19.846 ± 0.035 (2 σ), which is higher than is typical for sight lines in the local interstellar medium (ISM) studied for D/H. We found that D/H = (1.39 ± 0.21) × 10-5 (2 σ) and O/H = (2.37 ± 0.55) × 10-4 (2 σ). O/H toward BD +28°4211 appears to be significantly below the mean O/H ratio for the ISM and the Local Bubble.