Investigating Cepheid ℓ Carinae's cycle-to-cycle variations via contemporaneous velocimetry and interferometry

Abstract

Baade-Wesselink-type (BW) techniques enable geometric distance measurements\nof Cepheid variable stars in the Galaxy and the Magellanic clouds. The leading\nuncertainties involved concern projection factors required to translate\nobserved radial velocities (RVs) to pulsational velocities and recently\ndiscovered modulated variability. We carried out an unprecedented observational\ncampaign involving long-baseline interferometry (VLTI/PIONIER) and spectroscopy\n(Euler/Coralie) to search for modulated variability in the long-period (P\n$\\sim$ 35.5 d) Cepheid Carinae. We determine highly precise angular diameters\nfrom squared visibilities and investigate possible differences between two\nconsecutive maximal diameters, $\\Delta_{\\rm{max}} \\Theta$. We characterize the\nmodulated variability along the line-of-sight using 360 high-precision RVs.\nHere we report tentative evidence for modulated angular variability and confirm\ncycle-to-cycle differences of $\\ell$ Carinae's RV variability. Two successive\nmaxima yield $\\Delta_{\\rm{max}} \\Theta$ = 13.1 $\\pm$ 0.7 (stat.) {\\mu}as for\nuniform disk models and 22.5 $\\pm$ 1.4 (stat.) {\\mu}as (4% of the total angular\nvariation) for limb-darkened models. By comparing new RVs with 2014 RVs we show\nmodulation to vary in strength. Barring confirmation, our results suggest the\noptical continuum (traced by interferometry) to be differently affected by\nmodulation than gas motions (traced by spectroscopy). This implies a previously\nunknown time-dependence of projection factors, which can vary by 5% between\nconsecutive cycles of expansion and contraction. Additional interferometric\ndata are required to confirm modulated angular diameter variations. By\nunderstanding the origin of modulated variability and monitoring its long-term\nbehavior, we aim to improve the accuracy of BW distances and further the\nunderstanding of stellar pulsations.\n