S U S I

1. The Pulsation of the Cepheid l Car Measured with SUSI J. Davis, M.J. Ireland, A. Jacob, J.R. North, S.M. Owens, J.G. Robertson, W.J. Tango, P.G. Tuthill S U S I Abstract SUSI’s newly commissioned beam-combination system, working at the red end of the visual spectrum, has enabled the pulsation of the Cepheid l Car to be measured. SUSI limb-darkened angular diameters of l Car measured at 700 nm as a function of pulsation phase Observations and Results Observations were made on 20 nights during Feb-May 2004, primarily with a 40 m baseline, interleaving the observations of l Car with observations of b Car, i Car and S Car as calibrators. Each night provided a value of visibility squared (V2) and, with the assumption that the value of V2 for zero baseline is unity, a uniform disk angular diameter was obtained. The assumption was justified by separate short baseline observations that were consistent with a value of unity at zero baseline. Limb-darkening corrections were obtained from the tabulation of Davis, Tango & Booth (MNRAS, 318,387,2000) using effective temperature and log g values as a function of pulsation phase from Taylor (PhD thesis, University of Sydney). The resulting limb-darkened angular diameters are listed against pulsation phase in the table and plotted in the diagram. The observed amplitude of the pulsation is approximately 18% of the mean angular diameter compared with an average uncertainty of ± 1.7% in each measured angular diameter. Comparison with ESO’s VLTI Results l Car has been observed with ESO’s VLTI at 2.2 mm (Kervella et al., ApJ, 604, L113, 2004) and the VLTI limb-darkened angular diameters are plotted in the diagram below with the SUSI 700 nm values. The Distance to l Car The distance to a Cepheid can be determined by combining angular diameter measurements with measurements made by other techniques. Preliminary estimates of the distance to l Car have been made by two approaches: • By the ratio of the linear radial displacement (DD) to angular displacement (DqLD): • d = DD/Dq • The linear radial displacement from Taylor et al (MNRAS, 292, 662, 1997) combined with the angular displacement measured with SUSI gives the distance as 560 ± 60 pc. • By the ratio of the mean linear diameter (Dm) to the mean angular diameter (qLDm): • d = Dm/qLDm • The mean linear diameter from Laney & Stobie (MNRAS, 274, 337, 1995), determined by the Baade-Wesselink method, combined with the mean angular diameter determined with SUSI gives the distance as 591 ± 15 pc. The VLTI and SUSI results are in good agreement with the SUSI values appearing, on average, to be less than the VLTI values when the diameter is increasing. Unfortunately values around the epoch of maximum angular diameter were not obtained with SUSI this year due to bad weather for one maximum and a major technical snafu (now fixed) for another. It it emphasised that these are preliminary results but they are nevertheless in good agreement with recent published values (e.g. 603 +24/-19 pc, Kervella et al, A&A, 416, 941, 2004). Sydney University Stellar Interferometer