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Register a new userA new study of an old sink of sulfur in hot molecular cores: the sulfur residue
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Sulfur appears to be depleted by an order of magnitude or more from its elemental abundance in star-forming regions. In the last few years, numerous observations and experiments have been performed in order to to understand the reasons behind this depletion without providing a satisfactory explanation of the sulfur chemistry towards high-mass star-forming cores. Several sulfur-bearing molecules have been observed in these regions, and yet none are abundant enough to make up the gas-phase deficit. Where, then, does this hidden sulfur reside? This paper represents a step forward in our understanding of the interactions among the various S-bearing species. We have incorporated recent experimental and theoretical data into a chemical model of a hot molecular core in order to see whether they give any indication of the identity of the sulfur sink in these dense regions. Despite our model producing reasonable agreement with both solid-phase and gas-phase abundances of many sulfur-bearing species, we find that the sulfur residue detected in recent experiments takes up only ~6 per cent of the available sulfur in our simulations, rather than dominating the sulfur budget.
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The abundances of chemical elements and their depletion factors are essential parameters for understanding the composition of the gas and dust that are ultimately incorporated into stars and planets. Sulfur is an abundant but peculiar element in the sense that, despite being less volatile than other elements (e.g., carbon), it is not a major constituent of dust grains in diffuse interstellar clouds. Here, we determine the gas-phase carbon-to-sulfur abundance ratio, [C]/[S], and the sulfur abundance [S] in a dense star-forming cloud from new radio recombination lines (RRLs) detected with the Yebes 40m telescope - at relatively high frequencies (~40 GHz ~7 mm) and angular resolutions (down to 36) - in the Orion Bar, a rim of the Orion Molecular Cloud (OMC). We detect nine Cnalpha RRLs (with n=51 to 59) as well as nine narrow line features separated from the Cnalpha lines by delta v=-8.4+/-0.3 km s^-1. Based on this velocity separation, we assign these features to sulfur RRLs, with little contribution of RRLs from the more condensable elements Mg, Si, or Fe. Sulfur RRLs lines trace the photodissociation region (PDR) of the OMC. In these predominantly neutral gas layers, up to A_V~4, the ions C+ and S+ lock in most of the C and S gas-phase reservoir. We determine a relative abundance of [C]_Ori/[S]_Ori=10.4+/-0.6 and, adopting the same [C]_Ori measured in the translucent gas toward star theta^1 Ori B, an absolute abundance of [S]_Ori=(1.4+/-0.4)x10^-5. This value is consistent with emission models of the observed sulfur RRLs if N(S+)~7x10^17 cm^-2 (beam-averaged). The [S]_Ori is the initial sulfur abundance in the OMC, before an undetermined fraction of the [S]_Ori goes into molecules and ice mantles in the cloud interior. The inferred abundance [S]_Ori matches the solar abundance, thus implying that there is little depletion of sulfur onto rocky dust grains, with D(S)=0.0+/-0.2 dex.
Recently, an experimental study developed an efficient way to obtain sulfur-doped gamma-graphdiyne. This study has shown that this new material could have promising applications in lithium-ion batteries, but the complete understanding of how the sulfur atoms are incorporated into the graphdiyne network is still missing. In this work, we have investigated the sulfur doping process through molecular dynamics and density functional theory simulations. Our results suggest that the doped induced distortions of the gamma-graphdiyne pores prevent the incorporation of more than two sulfur atoms. The most common configuration is the incorporation of just one sulfur atom per the graphdiyne pore.
We re-examine a 50+ year-old problem of deep central reversals predicted for strong solar spectral lines, in contrast to the smaller reversals seen in observations. We examine data and calculations for the resonance lines of H I, Mg II and Ca II, the self-reversed cores of which form in the upper chromosphere. Based on 3D simulations as well as data for the Mg II lines from IRIS, we argue that the resolution lies not in velocity fields on scales in either of the micro- or macro-turbulent limits. Macro-turbulence is ruled out using observations of optically thin lines formed in the upper chromosphere, and by showing that it would need to have unreasonably special properties to account for critical observations of the Mg II resonance lines from the IRIS mission. The power in turbulence in the upper chromosphere may therefore be substantially lower than earlier analyses have inferred. Instead, in 3D calculations horizontal radiative transfer produces smoother source functions, smoothing out intensity gradients in wavelength and in space. These effects increase in stronger lines. Our work will have consequences for understanding the onset of the transition region, the energy in motions available for heating the corona, and for the interpretation of polarization data in terms of the Hanle effect applied to resonance line profiles.
We present an observational study of the sulfur (S)-bearing species towards Orion KL at 1.3 mm by combining ALMA and IRAM-30,m single-dish data. At a linear resolution of $sim$800 au and a velocity resolution of 1 $mathrm{km, s^{-1}, }$, we have identified 79 molecular lines from 6 S-bearing species. In these S-bearing species, we found a clear dichotomy between carbon-sulfur compounds and carbon-free S-bearing species in various characteristics, e.g., line profiles, spatial morphology, and molecular abundances with respect to $rm H_2$. Lines from the carbon-sulfur compounds (i.e., OCS, $^{13}$CS, H$_2$CS) exhibit spatial distributions concentrated around the continuum peaks and extended to the south ridge. The full width at half maximum (FWHM) linewidth of these molecular lines is in the range of 2 $sim$ 11 $mathrm{km, s^{-1}, }$. The molecular abundances of OCS and H$_2$CS decrease slightly from the cold ($sim$68 K) to the hot ($sim$176 K) regions. In contrast, lines from the carbon-free S-bearing species (i.e., SO$_2$, $^{34}$SO, H$_2$S) are spatially more extended to the northeast of mm4, exhibiting broader FWHM linewidths (15 $sim$ 26 $mathrm{km, s^{-1}, }$). The molecular abundances of carbon-free S-bearing species increase by over an order of magnitude as the temperature increase from 50 K to 100 K. In particular, $mathrm{^{34}SO/^{34}SO_2}$ and $mathrm{OCS/SO_2}$ are enhanced from the warmer regions ($>$100 K) to the colder regions ($sim$50 K). Such enhancements are consistent with the transformation of SO$_2$ at warmer regions and the influence of shocks.
Sulfur abundances are derived for a sample of ten B main-sequence star members of the Orion association. The analysis is based on LTE plane-parallel model atmospheres and non-LTE line formation theory by means of a self-consistent spectrum synthesis analysis of lines from two ionization states of sulfur, SII and SIII. The observations are high-resolution spectra obtained with the ARCES spectrograph at the Apache Point Observatory. The abundance distribution obtained for the Orion targets is homogeneous within the expected errors in the analysis: A(S)=7.15+/-0.05. This average abundance result is in agreement with the recommended solar value (both from modelling of the photospheres in 1-D and 3-D, and meteorites) and indicates that little, if any, chemical evolution of sulfur has taken place in the last ~4.5 billion years. The sulfur abundances of the young stars in Orion are found to agree well with results for the Orion nebulae, and place strong constraints on the amount of sulfur depletion onto grains as being very modest or nonexistent. The sulfur abundances for Orion are consistent with other measurements at a similar galactocentric radius: combined with previous results for other OB-type stars produce a relatively shallow sulfur abundance gradient with a slope of -0.037+/-0.012 dex/kpc.
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