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Register a new userAGB dust and gas ejecta in extremely metal-poor environments
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Flavia Dell'Agli Dr.
Publication date
2019
fields
Physics
and research's language is
English
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We present asymptotic giant branch (AGB) models of metallicity $Z=10^{-4}$ and $Z=3times 10^{-4}$, with the aim of understanding how the gas enrichment and the dust production change in very metal-poor environments and to assess the general contribution of AGB stars to the cosmic dust yield. The stellar yields and the dust produced are determined by the change in the surface chemical composition, with a transition occurring at $sim 2.5~M_{odot}$. Stars of mass $M < 2.5~M_{odot}$ reach the carbon stage and produce carbon dust, whereas their higher mass counterparts produce mainly silicates and alumina dust; in both cases the amount of dust manufactured decreases towards lower metallicities. The $Z=10^{-4}$ models show a complex and interesting behaviour on this side, because the efficient destruction of the surface oxygen favours the achievement of the C-star stage, independently of the initial mass. The present results might indicate that the contribution from this class of stars to the overall dust enrichment in metal-poor environments is negligible at redshifts $z>5$.
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We study the evolution of extremely metal-poor AGB stars, with metallicities down to [Fe/H]=-5, to understand the main evolutionary properties, the efficiency of the processes able to alter their surface chemical composition and to determine the gas and dust yields. We calculate two sets of evolutionary sequences of stars in the 1-7.5Msun mass range, evolved from the pre-main sequence to the end of the AGB phase. To explore the extremely metal-poor chemistries we adopted the metallicities Z=3x10^{-5} and Z=3x10^{-7} which correspond, respectively to [Fe/H]=-3 and [Fe/H]=-5. The results from stellar evolution modelling are used to calculate the yields of the individual chemical species. We also modelled dust formation in the wind, to determine the dust produced by these objects. The evolution of AGB stars in the extremely metal-poor domain explored here proves tremendously sensitive to the initial mass of the star. M<2Msun stars experience several third dredge-up events, which favour the gradual surface enrichment of C12 and the formation of significant quantities of carbonaceous dust, of the order of 0.01Msun. The C13 and nitrogen yiel are found to be significantly smaller than in previous explorations of low-mass, metal-poor AGB stars, owing to the weaker proton ingestion episodes experienced during the initial AGB phases. M>5Msun stars experience hot bottom burning and their surface chemistry reflects the equilibria of a very advanced proton-capture nucleosynthesis; little dust production takes place in their wind. Intermediate mass stars experience both third dredge-up and hot bottom burning: they prove efficient producers of nitrogen, which is formed by proton captures on C12 nuclei of primary origin dredged-up from the internal regions.
Chemical modelling of AGB outflows is typically focused on either non-thermodynamic equilibrium chemistry in the inner region or photon-driven chemistry in the outer region. We include, for the first time, a comprehensive dust-gas chemistry in our AGB outflow chemical kinetics model, including both dust-gas interactions and grain-surface chemistry. The dust is assumed to have formed in the inner region, and follows an interstellar-like dust-size distribution. Using radiative transfer modelling, we obtain dust temperature profiles for different dust types in an O-rich and a C-rich outflow. We calculate a grid of models, sampling different outflow densities, drift velocities between the dust and gas, and dust types. Dust-gas chemistry can significantly affect the gas-phase composition, depleting parent and daughter species and increasing the abundance of certain daughter species via grain-surface formation followed by desorption/sputtering. Its influence depends on four factors: outflow density, dust temperature, initial composition, and drift velocity. The largest effects are for higher density outflows with cold dust and O-rich parent species, as these species generally have a larger binding energy. At drift velocities larger than $sim 10$ km s$^{-1}$, ice mantles undergo sputtering; however, they are not fully destroyed. Models with dust-gas chemistry can better reproduce the observed depletion of species in O-rich outflows. When including colder dust in the C-rich outflows and adjusting the binding energy of CS, the depletion in C-rich outflows is also better reproduced. To best interpret high-resolution molecular line observations from AGB outflows, dust-gas interactions are needed in chemical kinetics models.
Our current understanding of the chemical evolution of the Universe is that a first generation of stars was formed out of primordial material, completely devoid of metals (Pop III stars). This first population of stars comprised massive stars that exploded as supernovae disseminating the metals they synthesised in the interstellar medium. These massive stars are long dead and cannot be observed in the local Universe. Among very metal poor stars (metallicity below -2.0) we expect to find the direct descendants of these pristine metal factories. The chemical composition of these stars provides us indirect information on the nature of the Pop III stars, their masses, luminosities and mode of explosion. The constraints are stronger if the chemical inventory is more complete, more chemical elements and isotopic ratios are measured for each star. Unfortunately the lower the metallicity of the star, the weaker the lines. Access to the space UV spectral range gives us crucial supplementary information. To start with, it allows access to some very strong Fe lines that may allow to measure the abundance of this element in stars for which this was not possible from the ground-accessible UV spectra. The number of such stars is steadily increasing. Next the UV range allows us to measure elements that cannot be measured from ground-based spectra like P, Ge, As, Se, Cd, Te, Lu, Os, Ir, Pt, Au. In addition it is fundamental for measuring other elements that can be accessed from earth, but with great difficulty, like C, S, Cu, Zn, Pb. The Hubble space telescope, with its limited collecting power made this possible only for very few stars. Old metal poor stars are cool, of spectral types F,G,K, and their UV flux is low. The availability of a UV high resolution spectrograph fed by a large area space telescope will open an unprecedented window on the early evolution of our Galaxy.
We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky. In particular, we outline our photometric selection procedures and describe the low-resolution ($R$ $approx$ 3000) spectroscopic follow-up observations that are used to provide estimates of effective temperature, surface gravity and metallicity ([Fe/H]) for the candidates. The selection process is very efficient: of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to -2.0, 41% have [Fe/H] $leq$ -2.75 and only $sim$7% have [Fe/H] $>$ -2.0 dex. The most metal-poor candidate in the sample has [Fe/H] $<$ -4.75 and is notably carbon-rich. Except at the lowest metallicities ([Fe/H] $<$ -4), the stars observed spectroscopically are dominated by a `carbon-normal population with [C/Fe]$_{1D,LTE}$ $leq$ +1 dex. Consideration of the A(C)$_{1D, LTE}$ versus [Fe/H]$_{1D, LTE}$ diagram suggests that the current selection process is strongly biased against stars with A(C)$_{1D, LTE}$ $>$ 7.3 (predominantly CEMP-$s$) while any bias against stars with A(C)$_{1D, LTE}$ $<$ 7.3 and [C/Fe]$_{LTE}$ $>$ +1 (predominantly CEMP-no) is not readily quantifiable given the uncertainty in the SkyMapper $v$-band DR1.1 photometry. We find that the metallicity distribution function of the observed sample has a power-law slope of $Delta$(Log N)/$Delta$[Fe/H] = 1.5 $pm$ 0.1 dex per dex for -4.0 $leq$ [Fe/H] $leq$ -2.75, but appears to drop abruptly at [Fe/H] $approx$ -4.2, in line with previous studies.
Sulfur is important: the site of its formation is uncertain, and at very low metallicity the trend of [S/Fe] against [Fe/H] is controversial. Below [Fe/H]=-2.0, [S/Fe] remains constant or it decreases with [Fe/H], depending on the author and the multiplet used in the analysis. Moreover, although sulfur is not significantly bound in dust grains in the ISM, it seems to behave differently in DLAs and in old metal-poor stars. We aim to determine precise S abundance in a sample of extremely metal-poor stars taking into account NLTE and 3D effects. NLTE profiles of the lines of the multiplet 1 of SI have been computed using a new model atom for S. We find sulfur in EMP stars to behave like the other alpha-elements, with [S/Fe] remaining approximately constant for [Fe/H]<-3. However, [S/Mg] seems to decrease slightly as a function of [Mg/H]. The overall abundance patterns of O, Na, Mg, Al, S, and K are best matched by the SN model yields by Heger & Woosley. The [S/Zn] ratio in EMP stars is solar, as found also in DLAs. We obtain an upper limit on the abundance of sulfur, [S/Fe] < +0.5, for the ultra metal-poor star CS 22949-037. This, along with a previous reported measurement of zinc, argues against the conjecture that the light-element abundances pattern in this star, and, by analogy, the hyper metal-poor stars HE 0107-5240 and HE 1327-2326, are due to dust depletion.
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