Chemical analysis of early-type stars with planets

Abstract

Aims. Our goal is to explore the chemical pattern of early-type stars with planets, searching for a possible signature of planet formation. In particular, we study a likely relation between the λ Boötis chemical pattern and the presence of giant planets. Methods. We performed a detailed abundance determination in a sample of early-type stars with and without planets via spectral synthesis. Fundamental parameters were initially estimated using Strömgren photometry or literature values and then refined by requiring excitation and ionization balances of Fe lines. We derived chemical abundances for 23 different species by fitting observed spectra with an iterative process. Synthetic spectra were calculated using the program SYNTHE together with local thermodynamic equilibrium ATLAS12 model atmospheres. We used specific opacities calculated for each star, depending on the individual composition and microturbulence velocity vmicro through the opacity sampling method. The complete chemical pattern of the stars were then compared to those of λ Boötis stars and other chemically peculiar stars. Results. We compared the chemical pattern of the stars in our sample (13 stars with planets and 24 stars without detected planets) with those of λ Boötis and other chemically peculiar stars. We have found four λ Boötis stars in our sample, two of which present planets and circumstellar disks (HR 8799 and HD 169142) and one without planets detected (HD 110058). We have also identified the first λ Boötis star orbited by a brown dwarf (ζ Del). This interesting pair, the λ Boötis star and brown dwarf, could help to test stellar formation scenarios. We found no unique chemical pattern for the group of early-type stars bearing giant planets. However, our results support, in principle, a suggested scenario in which giant planets orbiting pre-main-sequence stars possibly block the dust of the disk and result in a λ Boötis-like pattern. On the other hand, we do not find a λ Boötis pattern in different hot-Jupiter planet host stars, which does not support the idea of possible accretion from the winds of hot-Jupiters, recently proposed in the literature. As a result, other mechanisms should account for the presence of the λ Boötis pattern between main-sequence stars. Finally, we suggest that the formation of planets around λ Boötis stars, such as HR 8799 and HD 169142, is also possible through the core accretion process and not only gravitational instability.

Testing the accretion scenario of lambda Boo stars

“…The stellar parameters T eff and log g were estimated iteratively, similar to previous works (Saffe et al 2021). They were first estimated by using the Strömgren uvbyβ mean photometry of Hauck & Mermilliod (1998) or considering previously published results.…”

“…These initial values were refined (when necessary and/or possible) by imposing excitation and ionization balances of the iron lines. A similar strategy was previously applied in the literature (e.g., Saffe & Levato 2014;Saffe et al 2021). The values derived in this way are listed in the Table 2, with an average dispersion of ∼115 K and ∼0.13 dex for T eff and log g, respectively.…”

“…With the new abundance values, we derived a new model atmosphere and started the process again. In each step, opacities were calculated for an arbitrary composition and v micro using the opacity sampling (OS) method, similar to previous works (Saffe et al 2020(Saffe et al , 2021. Possible differences originated from the use of opacities with solar-scaled composition instead of an arbitrary composition, were recently estimated for solar-type stars (Saffe et al 2018(Saffe et al , 2019.…”

“…If necessary, T eff and log g were refined to achieve the balance of Fe I and Fe II lines. In this way, abundances and parameters are consistently derived until reach the same input and output abundance values (for more details, see Saffe et al 2021).…”

“…In this case, the planet acts as the source of gas poor in refractory species. However, Saffe et al (2021) have recently shown that eight earlytype stars hosting hot-Jupiter planets do not display the λ Boo peculiarity. This would let the interaction of the star with a diffuse cloud, as the more plausible scenario to explain the λ Boo phenomena in main-sequence stars.…”