Abstract
Context. The dynamical structure of the atmosphere of Cepheids has been well studied in the optical. Several authors have found very interesting spectral features in theJband, but little data have been secured beyond 1.6μm. However, such observations can probe different radial velocities and line asymmetry regimes, and are able to provide crucial insights into stellar physics.Aims. Our goal was to investigate the infrared line-forming region in theK-band domain, and its impact on the projection factor and thek-term of Cepheids.Methods. We secured CRIRES observations for the long-period Cepheid l Car, with a focus on the unblended spectral line NaI 2208.969 nm. We measured the corresponding radial velocities (by using the first moment method) and the line asymmetries (by using the bi-Gaussian method). These quantities are compared to the HARPS visible spectra we previously obtained on l Car.Results. The optical and infrared radial velocity curves show the same amplitude (only about 3% of difference), with a slight radial velocity shift of about 0.5 ± 0.3 km s−1between the two curves. Around the minimum radius (phase ≃ 0.9) the visible radial velocity curve is found in advance compared to the infrared one (phase lag), which is consistent with an infrared line forming higher in the atmosphere (compared to the visible line) and with a compression wave moving from the bottom to the top of the atmosphere during maximum outward velocity. The asymmetry of theK-band line is also found to be significantly different from that of the optical line.
Highlights
For almost a century, the Baade–Wesselink (BW) method is used to derive the distance of Cepheids (Lindemann 1918; Baade 1926; Wesselink 1946)
Around the minimum radius the visible radial velocity curve is found in advance compared to the infrared one, which is consistent with an infrared line forming higher in the atmosphere and with a compression wave moving from the bottom to the top of the atmosphere during maximum outward velocity
The amplitude of the RVcc−g is slightly larger, as already discussed in Nardetto et al (2017). Another striking feature of the infrared radial velocity curve is the delay shift with respect to its visible counterpart that can be clearly seen in the descending branch
Summary
The Baade–Wesselink (BW) method is used to derive the distance of Cepheids (Lindemann 1918; Baade 1926; Wesselink 1946). Cepheids are usually observed at infrared wavelengths, whereas at optical wavelengths the pulsation has been resolved for only three stars, β Dor, η Aql (Jacob 2008), and the long-period l Car (Davis et al 2009) For the latter, the angular diameter curves derived respectively from the infrared surface brightness relation and infrared interferometry are consistent (Kervella et al 2004a), while it is not the case for the short-period Cepheid δ Cep (Ngeow et al 2012). We compared each observed spectrum with its corresponding atmospheric model generated with Molecfit (including telluric lines) and we derived a mean offset of 10.7 ± 0.3 km s−1 This value was used to calibrate the wavelength of all spectra.
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