An independent distance estimate to CW Leonis ^⋆,⋆⋆

¹ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
e-mail: marting@oma.be
² Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
³ Instituut voor Sterrenkunde, Universiteit van Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Astrophysics Dept, CAB (INTA-CSIC), Crta Ajalvir km 4, 28805 Torrejon de Ardoz, Madrid, Spain
⁵ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 Amsterdam, The Netherlands
⁶ School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff, Wales CF24 3YB, UK
⁷ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Wien, Austria
⁸ Dept. of Physics and Astronomy, University of Denver, Mail Stop 6900, Denver, CO 80208, USA
⁹ Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
¹⁰ Dept of Astronomy, Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, 10691 Stockholm, Sweden
¹¹ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

Received: 15 May 2012
Accepted: 22 June 2012

Abstract

CW Leo has been observed six times between October 2009 and June 2012 with the SPIRE instrument on board the Herschel satellite. Variability has been detected in the flux emitted by the central star with a period of 639  ±  4 days, in good agreement with determinations in the literature. Variability is also detected in the bow shock around CW Leo that had previously been detected in the ultraviolet and Herschel PACS/SPIRE data. Although difficult to prove directly, our working hypothesis is that this variability is directly related to that of the central star. In this case, fitting a sine curve with the period fixed to 639 days results in a time-lag in the variability between bow shock and the central star of 402  ±  37 days. The orientation of the bow shock relative to the plane of the sky is unknown (but see below). For an inclination angle of zero degrees, the observed time-lag translates into a distance to CW Leo of 130  ±  13 pc, and for non-zero inclination angles the distance is smaller. Fitting the shape of the bow shock with an analytical model (Wilkin 1996, ApJ, 459, L31), the effect of the inclination angle on the distance may be estimated. Making the additional assumption that the relative peculiar velocity between the interstellar medium (ISM) and CW Leo is determined entirely by the star space velocity with respect to the local standard of rest (i.e. a stationary ISM), the inclination angle is found to be (−33.3  ±  0.8)° based on the observed proper motion and radial velocity. Using the Wilkin model, our current best estimate of the distance to CW Leo is 123  ±  14 pc. For a distance of 123 pc, we derive a mean luminosity of 7790  ±  150 L_⊙ (internal error).