The MAGIC project – II. Discovery of two new Galactic lithium-rich Cepheids

Abstract

We report the discovery of two new lithium-rich Cepheids in the Milky Way (A(Li) ≈ 2.9 dex) among the targets of the MAGIC spectroscopic survey with the Southern African Large Telescope spectrographs. The 6707 Å Li feature is usually not detectable in the atmosphere of Cepheids. Only three Cepheids (two in the Milky Way and one in the Large Magellanic Cloud) had been reported so far with high lithium abundances. We examine the possibility that the Li-rich Cepheids are evolving towards the red giant branch, crossing the instability strip for the first time, as well as other possible scenarios. The two stars are mixed-mode Cepheids pulsating simultaneously in the first and in the second overtone modes. We briefly discuss the possible connection between their unusual chemical composition and their pulsational properties.

V363 Cassiopeiae: a new lithium-rich Galactic Cepheid

“…This would then explain the low number of Li-rich DCEPs. As noted by Kovtyukh et al (2019), about 80% of the DCEPs that are expected to be at their first crossing (about 5% of the total) are Li-depleted. Therefore it can be hypothesised that the progenitors of the Li-rich and Li-depleted DCEPs (B stars) were slow and fast rotating stars, respectively, when they were on the MS.…”

“…In this Letter we report the discovery of a new rare lithiumrich DCEP. Only four DCEPs showing an enhanced lithium abundance (through detection of the Li i 6707.766 Å) have been discovered in the Galaxy so far (Luck & Lambert 2011;Kovtyukh et al 2016Kovtyukh et al , 2019, and an additional DCEP was detected in the Large Magellanic Cloud (LMC, Luck & Lambert 1992). All these objects show a lithium abundance A(Li) ∼ 3.0 dex, while the majority of the Galactic DCEPs show A(Li) < 1.2 dex (Luck & Lambert 2011).…”

“…envelopes do not show the signature of nuclear processes that occurred during the main-sequence (MS) phase. According to Kovtyukh et al (2019), at least three of out of the four Milky Way (MW) Li-rich DCEPs also show abundances of the CNO species that are consistent with the solar values, that is, they are not processed by the CN-cycle. However, the 1DU is not the only phenomenon capable of depleting lithium.…”

“…Of the four MW Li-rich DCEPs, ASAS J075842-2536.1 and ASAS J131714-6605.0 both pulsate in the first and second overtone (DCEP_1O2O), V371 Per pulsates in the fundamental and first overtone (DCEP_F1O), and V1033 Cyg is only a fundamental mode (DCEP_F) pulsator. According to Kovtyukh et al (2019), multi-mode DCEPs have a less efficient mixing in their envelope than DCEP_F, which means that they would preferentially tend to retain their Li.…”

“…Even if other more complex processes can address the presence of lithium in DCEPs (see Kovtyukh et al 2019, for a detailed discussion), the basic mechanism to explain Li-rich DCEPs is their passage through the IS at the first crossing. This occurrence can be verified by measuring the rate of period change that is due to evolution along the Hertzsprung-Russell diagram (HRD), as the period is expected to increase at the first and third crossing but to decrease at the second crossing (see e.g.…”