Spectroscopic monitoring of long secondary period variables. The case of J-type carbon stars

Abstract

Long-secondary-period (LSP) variables are a subclass of long-period variables whose origin is a matter of debate. The current leading hypotheses involve binarity or non-radial pulsations. We aim to investigate the spectroscopic signatures associated with the LSP variability by targeting two J-type carbon stars exhibiting an LSP - VZ Per and UV Cam - and further constrain its origin. Decade-long spectroscopic monitoring was used to obtain a long-term radial-velocity (RV) curve for the two stars. From a frequency analysis, we isolated an LSP signal. The retrieved periodicity was then compared with changes in spectral profiles and available light-curve variability to further constrain the origin of the modulation. For VZ Per, the LSP derived from the RV curve (∼,960 days) is correlated with the light-curve dimming event and a variation in the Balmer absorption line profile. These results translate into changes in the gas and dust column densities with the phase of the LSP, with the maximum gas column density occurring ahead of the dust. For UV Cam, the RV monitoring yields a period of ∼,1178 d, with an amplitude of about 1,km/s. When interpreting the RV modulation as a Keplerian signal for both stars, a low-mass ($<0.1, ̊m M_⊙$) close companion is found. In this scenario, variations in gas and dust column densities with (orbital) phase trace the material in the expanding comet-like tail trailing the substellar companion. These observations provide time-dependent variations of the gas and dust in LSP variables that are consistent with the current binary scenario. Furthermore, the detection of LSP among J-type carbon stars anchors their current evolutionary status in the dust-producing region of the thermally pulsing AGB phase.