Shallow extra mixing in solar twins inferred from Be abundances

Abstract

Context. Lithium and beryllium are destroyed at different temperatures in stellar interiors. As such, their relative abundances offer excellent probes of the nature and extent of mixing processes within and below the convection zone. Aims. We determine Be abundances for a sample of eight solar twins for which Li abundances have previously been determined. The analyzed solar twins span a very wide range of age, 0.5-8.2 Gyr, which enables us to study secular evolution of Li and Be depletion.Methods. We gathered high-quality UVES/VLT spectra and obtained Be abundances by spectral synthesis of the Be ii 313 nm doublet. Results. The derived beryllium abundances exhibit no significant variation with age. The more fragile Li, however, exhibits a monotonically decreasing abundance with increasing age. Therefore, relatively shallow extra mixing below the convection zone is necessary to simultaneously account for the observed Li and Be behavior in the Sun and solar twins.

HIP 10725: The first solar twin/analogue field blue straggler

“…Our determined value of v rot sin i = 3.3 km s −1 suggests an age of roughly 1 Gyr, while the Yonsei-Yale isochrones result in an age of 5.2 +1.9 −2.1 Gyr. Our upper limit for the Li abundance (A(Li) ≤ 0.9 dex) is also about 20 times smaller than the Li abundance expected for an age of 1 Gyr (Tucci Maia et al 2015), showing that the star is not young. Thus, the high projected rotational velocity is most likely the consequence of angular momentum transfer during the accretion stage from the former AGB star (Fuhrmann & Bernkopf 1999), which "rejuvenated" HIP 10725, enhancing both rotation and chromospheric activity.…”

“…We took diffusion (including gravitational settling) and rotation-induced mixing into account, as described in our previous works (do Nascimento et al 2009;Castro et al 2011;Tucci Maia et al 2015). We modelled the Be depletion using the diffusion coefficient at the base of the convective zone D bcz , and the half-height width of the tachocline Δ as parameters, which are both free parameters of stellar evolution models.…”

“…Large beryllium depletion is not observed in solar twins, where its abundance is relatively constant with age (Tucci Maia et al 2015) 1 , unlike lithium, which is continuously being depleted as the star evolves Meléndez et al 2010Meléndez et al , 2014aMonroe et al 2013) by extramixing below the convective zone. This suggests that when Be depletion is present in these stars, other mechanisms must be responsible for it.…”

“…We estimated the projected rotational velocity v rot sin i for this star using the same procedures and line list as in Tucci Maia et al (2015). First, we obtained the macroturbulence velocity V macro = 3.6 km s −1 using the relation given in the aforementioned work.…”

The Solar Twin Planet Search

“…Previous studies proposed that rotation can produce extra mixing that is responsible for depleting the light elements Li and Be in their atmospheres (Pinsonneault et al 1989;Charbonnel et al 1994;Tucci Maia et al 2015), which could explain the disconnection between meteoritic and solar abundances of Li (Baumann et al 2010). Moreover, rotation is highly correlated with magnetic activity (e.g., Noyes et al 1984;Soderblom et al 1993;Baliunas et al 1995;Mamajek & Hillenbrand 2008), and this trend is key to understanding how planetary systems and life evolve in face of varying magnetic activity and energy outputs by solar-like stars during the main sequence (Guinan & Engle 2009;Ribas et al 2005;do Nascimento et al 2016).…”