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Register a new userChemical evidence for planetary ingestion in a quarter of Sun-like stars
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Stellar members of binary systems are formed from the same material, therefore they should be chemically identical. However, recent high-precision studies have unveiled chemical differences between the two members of binary pairs composed by Sun-like stars. The very existence of these chemically inhomogeneous binaries represents one of the most contradictory examples in stellar astrophysics and source of tension between theory and observations. It is still unclear whether the abundance variations are the result of chemical inhomogeneities in the protostellar gas clouds or instead if they are due to planet engulfment events occurred after the stellar formation. While the former scenario would undermine the belief that the chemical makeup of a star provides the fossil information of the environment where it formed, a key assumption made by several studies of our Galaxy, the second scenario would shed light on the possible evolutionary paths of planetary systems. Here, we perform a statistical study on 107 binary systems composed by Sun-like stars to provide - for the first time - unambiguous evidence in favour of the planet engulfment scenario. We also establish that planet engulfment events occur in stars similar to our own Sun with a probability ranging between 20 and 35$%$. This implies that a significant fraction of planetary systems undergo very dynamical evolutionary paths that can critically modify their architectures, unlike our Solar System which has preserved its planets on nearly circular orbits. This study also opens to the possibility of using chemical abundances of stars to identify which ones are the most likely to host analogues of the calm Solar System.
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We present a summary of the splinter session Sun-like stars unlike the Sun that was held on 09 June 2016 as part of the Cool Stars 19 conference (Uppsala, Sweden). We discussed the main limitations (in the theory and observations) in the derivation of very precise stellar parameters and chemical abundances of Sun-like stars. We outlined and discussed the most important and most debated processes that can produce chemical peculiarities in solar-type stars. Finally, in an open discussion between all the participants we tried to identify new pathways and prospects towards future solutions of the currently open questions.
The X-ray and extreme-ultraviolet (EUV) emissions from the low-mass stars significantly affect the evolution of the planetary atmosphere. However, it is, observationally difficult to constrain the stellar high-energy emission because of the strong interstellar extinction of EUV photons. In this study, we simulate the XUV (X-ray+EUV) emission from the Sun-like stars by extending the solar coronal heating model that self-consistently solves, with sufficiently high resolution, the surface-to-coronal energy transport, turbulent coronal heating, and coronal thermal response by conduction and radiation. The simulations are performed with a range of loop lengths and magnetic filling factors at the stellar surface. With the solar parameters, the model reproduces the observed solar XUV spectrum below the Lyman edge, thus validating its capability of predicting the XUV spectra of other Sun-like stars. The model also reproduces the observed nearly-linear relation between the unsigned magnetic flux and the X-ray luminosity. From the simulation runs with various loop lengths and filling factors, we also find a scaling relation, namely $log L_{rm EUV} = 9.93 + 0.67 log L_{rm X}$, where $L_{rm EUV}$ and $L_{rm X}$ are the luminosity in the EUV and X-ray range, respectively, in cgs. By assuming a power-law relation between the Rossby number and the magnetic filling factor, we reproduce the renowned relation between the Rossby number and the X-ray luminosity. We also propose an analytical description of the energy injected into the corona, which, in combination with the conventional Rosner-Tucker-Vaiana scaling law, semi-analytically explains the simulation results. This study refines the concepts of solar and stellar coronal heating and derives a theoretical relation for estimating the hidden stellar EUV luminosity from X-ray observations.
Recently published, precise stellar photometry of 72 Sun-like stars obtained at the Fairborn Observatory between 1993 and 2017 is used to set limits on the solar forcing of Earths atmosphere of $pm$ 4.5 W m$^{-2}$ since 1750. This compares with the +2.2 $pm$ 1.1 W m$^{-2}$ IPCC estimate for anthropogenic forcing. Three critical assumptions are made. In decreasing order of importance they are: (a) most of the brightness variations occur within the average time-series length of $approx$17 years; (b) the Sun seen from the ecliptic behaves as an ensemble of middle-aged solar-like stars; and (c) narrow-band photometry in the Stromgren $b$ and $y$ bands are linearly proportional to the total solar irradiance. Assumption (a) can best be relaxed and tested by obtaining more photometric data of Sun-like stars, especially those already observed. Eight stars with near-solar parameters have been observed from 1999, and two since 1993. Our work reveals the importance of continuing and expanding ground-based photometry, to complement expensive solar irradiance measurements from space.
Reinhold et al. (Science, 1 May 2020, p. 518) provided two possible interpretations of measurements showing that the Sun is less active than other solar-like stars. We argue that one of those interpretations anticipates the observed differences between the properties of their two stellar samples. This suggests that solar-like stars become permanently less variable beyond a specific evolutionary phase.
The chemical composition of the Sun is a fundamental yardstick in astronomy, relative to which essentially all cosmic objects are referenced. We reassess the solar abundances of all 83 long-lived elements, using highly realistic solar modelling and state-of-the-art spectroscopic analysis techniques coupled with the best available atomic data and observations. Our new improved analysis confirms the relatively low solar abundances of C, N, and O obtained in our previous 3D-based studies: $logepsilon_{text{C}}=8.46pm0.04$, $logepsilon_{text{N}}=7.83pm0.07$, and $logepsilon_{text{O}}=8.69pm0.04$. The revised solar abundances for the other elements also typically agree well with our previously recommended values with just Li, F, Ne, Mg, Cl, Kr, Rb, Rh, Ba, W, Ir, and Pb differing by more than $0.05$ dex. The here advocated present-day photospheric metal mass fraction is only slightly higher than our previous value, mainly due to the revised Ne abundance from Genesis solar wind measurements: $X_{rm surface}=0.7438pm0.0054$, $Y_{rm surface}=0.2423pm 0.0054$, $Z_{rm surface}=0.0139pm 0.0006$, and $Z_{rm surface}/X_{rm surface}=0.0187pm 0.0009$. Overall the solar abundances agree well with those of CI chondritic meteorites but we identify a correlation with condensation temperature such that moderately volatile elements are enhanced by $approx 0.04$ dex in the CI chondrites and refractory elements possibly depleted by $approx 0.02$ dex, conflicting with conventional wisdom of the past half-century. Instead the solar chemical composition resembles more closely that of the fine-grained matrix of CM chondrites. The so-called solar modelling problem remains intact with our revised solar abundances, suggesting shortcomings with the computed opacities and/or treatment of mixing below the convection zone in existing standard solar models.
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