Astronomy & Astrophysics

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A&A 416, 941-953 (2004)
DOI: 10.1051/0004-6361:20031743

Cepheid distances from infrared long-baseline interferometry

I. VINCI/VLTI observations of seven Galactic Cepheids

P. Kervella¹, N. Nardetto², D. Bersier³, D. Mourard² and V. Coudé du Foresto⁴

¹ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
² Département Fresnel, UMR CNRS 6528, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁴ LESIA, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France

(Received 19 June 2003 / Accepted 26 November 2003)

Abstract
We report the angular diameter measurements of seven classical Cepheids, X Sgr, $\eta$ Aql, W Sgr, $\zeta$ Gem, $\beta$ Dor, Y Oph and $\ell$ Car that we have obtained with the VINCI instrument, installed at ESO's VLT Interferometer (VLTI). We also present reprocessed archive data obtained with the FLUOR/IOTA instrument on $\zeta$ Gem, in order to improve the phase coverage of our observations. We obtain average limb darkened angular diameter values of $\overline{\theta_{\rm LD}}[{\rm X\,Sgr}] = 1.471 \pm 0.033$ mas, $\overline{\theta_{\rm LD}}[\eta\,{\rm Aql}] = 1.839 \pm 0.028$ mas, $\overline{\theta_{\rm LD}}[{\rm W\,Sgr}] = 1.312 \pm 0.029$ mas, $\overline{\theta_{\rm LD}}[\beta\,{\rm Dor}] = 1.891 \pm 0.024$ mas, $\overline{\theta_{\rm LD}}[\zeta\,{\rm Gem}] =1.747 \pm 0.061$ mas, $\overline{\theta_{\rm LD}}[{\rm Y\,Oph}] = 1.437 \pm 0.040$ mas, and $\overline{\theta_{\rm LD}}[\ell\,{\rm Car}] = 2.988 \pm 0.012$ mas. For four of these stars, $\eta$ Aql, W Sgr, $\beta$ Dor, and $\ell$ Car, we detect the pulsational variation of their angular diameter. This enables us to compute directly their distances, using a modified version of the Baade-Wesselink method: $d[\eta\,{\rm Aql}] = 276^_$ pc, $d[{\rm W\,Sgr}] = 379^_$ pc, $d[\beta {\rm Dor}] = 345^_$ pc, $d[\ell\,{\rm Car}] = 603^_$ pc. The stated error bars are statistical in nature. Applying a hybrid method, that makes use of the Gieren et al. (1998) Period-Radius relation to estimate the linear diameters, we obtain the following distances (statistical and systematic error bars are mentioned): $d[{\rm X\,Sgr}] = 324 \pm 7 \pm 17$ pc, $d[\eta\,{\rm Aql}] = 264 \pm 4 \pm 14$ pc, $d[{\rm W\,Sgr}] = 386 \pm 9 \pm 21$ pc, $d[\beta {\rm Dor}] = 326 \pm 4 \pm 19$ pc, $d[\zeta\,{\rm Gem}] = 360 \pm 13 \pm 22$ pc, $d[{\rm Y\,Oph}] = 648 \pm 17 \pm 47$ pc, $d[\ell\,{\rm Car}] = 542 \pm 2 \pm 49$ pc.

Key words: techniques: interferometric -- stars: variables: Cepheids -- stars: oscillations

Offprint request: P. Kervella, pkervell@eso.org

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