SIMBAD references

Spectroscopic and photometric observations of the variable carbon-rich post-AGB star HD 56126 are analysed in order to improve the knowledge of its pulsation and of its fundamental parameters. New high resolution spectra (R=42 000), taken as regularly as possible (every 7-9 days) from October 1995 to April 1996 at the Haute-Provence Observatory, are presented. Important profile variations of the Hα line are confirmed, and variations of Hβ are also shown for the first time. These variations can be significant over time intervals as short as ∼7 days. Photospheric radial velocities were derived and merged with other data from different sources to form a set of 89 values spanning ∼8 years. We also considered a completely independent set of 87 V-band Geneva photometric measurements made over 7 years and covering approximatively the same epoch. The results of the Fourier analysis of the radial velocity and photometric series agree remarkably well: a strong main period of P=36.8±0.2days is found in both data sets. This period supersedes the value of 27 days previously found by us with much fewer data points. Weaker secondary oscillations, probably related to the irregularity of the pulsation, are also detected. There is no prominent Fourier component at ∼2P and the pulsation of HD 56126 is certainly quite different from that of RV Tauri variables. Linear nonadiabatic analysis has been applied. It is found that the only linear radial mode that can fit the main period is the first overtone. This, together with the requirement of some driving of the pulsation and with the core mass-luminosity relation, favours the following range of fundamental parameters: logg=0.38-0.49, M=0.57-0.63M_☉, T_eff=5700-6100K and L=5250-8160L_☉. Owing to its higher temperature, the star seems more evolved than post-AGB RV Tauri's. Hydrodynamical models are also discussed. The spectroscopic and photometric variations of the star appear to be consistent with a radial pulsation yielding shockwaves which propagate through the atmosphere and probably generate a complex, asynchronous motion of the outer hydrogen layers.