A Comparison of the Near-Infrared and Visible Spectra of Three Bright Cepheids

Recent studies have suggested that significant differences may exist between the IR and the visible radial velocity curves and line profiles for Cepheid variable stars. Such wavelength-dependent phenomena would affect the relationship between pulsational and radial velocities and the derived surface brightness (Baade-Wesselink) distances for Cepheids. In a further investigation of this problem, radial velocity curves and line profile asymmetries have been obtained from 100 absorption lines in the near-IR (8000 Å) for the bright Cepheids δ Cep (5 day period), η Aql (7 day), and X Cyg (16 day). A fiducial wavelength scale has been provided by the embedded telluric absorption lines. The relative radial velocity errors of individual stellar absorption lines have been reduced to a few hundred meters per second. The shape and amplitude of velocity curves from lines in the near-IR are very similar to those from lines of the same atomic species and similar excitation observed in the visible region. For the two shorter period Cepheids in this study, the near-IR and visible velocity curves differ by less than 1 km s-1. The longer period Cepheid X Cyg shows velocity differences of as much as 3 km s-1. The near-IR line profile asymmetries are very similar to those observed in the visible. As in the case of the visible lines, the high excitation potential (high-EP) lines in the near-IR appear to have a greater asymmetry amplitude than the low-EP lines. Radial velocity curves and line profiles for Hα and Hβ have also been obtained. The Cepheids in this sample show a clear trend with increasing pulsation period. The Hα and Hβ line profiles of the shortest period star (δ Cep) remain relatively symmetric throughout the entire pulsation cycle, and their respective radial velocity curves remain continuous at maximum infall velocity. The Hβ radial velocity curve shows only small deviations from that of the standard (Fe I) velocity curve. The Hβ line of the longer period star, η Aql, remains relatively symmetric, but Hα becomes extremely asymmetric near the phase of maximum infall. The Hα radial velocity amplitude is 57% larger than the standard Fe I velocity curve. Both the Hα and Hβ velocity curves are very sharply peaked (almost discontinuous) at maximum infall velocity, and they lag the standard velocity curve (derived from strong 3-6 eV Fe I lines in the 5000-6300 Å region) by 0.06 in phase. Both Hα and Hβ show extreme asymmetries. For the longest period Cepheid in the sample, X Cyg, the Hα profile splits into two components for half the pulsation cycle. The Hα radial velocity curve is completely out of phase with the standard velocity curve, while the Hβ velocity curve remains relatively well behaved, lagging the Fe I velocity curve by 0.12 in phase.

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