CRIRES high-resolution infrared spectroscopy of the long-period Cepheid l Carinae^⋆

¹ Université Côte d’Azur, OCA, CNRS, Lagrange, France
e-mail Nicolas.Nardetto@oca.eu
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
³ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
⁴ European Southern Observatory, Alonso de Córdova 3107, 19001Casilla, Santiago, Chile
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748Garching b. München, Germany
⁶ Observatoire Midi-Pyrénées, Laboratoire d’Astrophysique, UMR 5572, Université Paul Sabatier, Toulouse 3, 14 avenue Edouard Belin, 31400 Toulouse, France
⁷ Departamento de Astronomía, Universidad de Concepción, 160-C Casilla, Concepción, Chile
⁸ Millenium Institute of Astrophysics, Santiago, Chile
⁹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland
¹⁰ LESIA (UMR 8109), Observatoire de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
¹¹ UMI 3386, Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU, France
¹² Departamento de Astronomía, Universidad de Chile, Camino El Observatorio, 1515 Las Condes, Santiago, Chile
¹³ Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
¹⁴ CNRS, UMR5277, Institut de recherche en Astrophysique et Planétologie, 14 Avenue Edouard Belin, 31400 Toulouse, France
¹⁵ Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, M5S 3H4 Toronto, ON, Canada
¹⁶ Leibniz Institute for Astrophysics, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 11 April 2018
Accepted: 30 May 2018

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 the J band, but little data have been secured beyond 1.6 μm. 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 the K-band domain, and its impact on the projection factor and the k-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⁻¹ between 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 the K-band line is also found to be significantly different from that of the optical line.