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GI2TREGAIN spectro-interferometry with a new infrared beam combiner

a r X i v :a s t r o -p h /0004184v 1 13 A p r 2000

GI2T/REGAIN spectro-interferometry with a new infrared

beam combiner

G.Weigelt a ,D.Mourard b ,L.Abe b ,U.Beckmann a ,

O.Chesneau a ,C.Hillemanns a ,K.-H.Hofmann a ,S.Ragland b ,D.Schertl a ,M.Scholz c ,P.Stee b ,N.Thureau b ,F.Vakili b

a MPI

f¨u r Radioastronomie,Auf dem Huegel 69,53121Bonn,Germany

b Departement Fresnel,CNRS/UMR 6528,Observatoire de la Cote d’Azur,2130,route de l’Observatoire,Caussols,06460St Vallier de Thiey,France

c Institut f¨u r Theoretische Astrophysik der Universitaet Heidelberg,Tiergartenstrasse 15,

69121Heidelberg,Germany

ABSTRACT

We have built an infrared beam combiner for the GI2T/REGAIN interferometer of the Observatoire de la Cote d’Azur.The beam combiner allows us to record spectrally dispersed Michelson interference fringes in the near-infrared J-,H-or K-bands.The beam combiner has the advantage that Michelson interferograms can simultaneously be recorded in about 128di?erent spectral channels.The tilt of the spectrally dispersed fringes is a measure of the instantaneous optical path di?erence.We present the optical design of the beam combiner and GI2T/REGAIN observations of the Mira star R Cas with this beam combiner in the spectral range of 2.00μm –2.18μm (observations on 22and 25August 1999;variability phase 0.08;V-magnitude approximately 6;seven baselines between 12m and 24m;reference stars Vega and βPeg).The spectrograph of the beam combiner consists of an anamorphotic cylindrical lens system,an image plane slit,and a grism.The detector is a 256x256pixel Rockwell PICNIC array camera.The shortest possible exposure time is 10ms.A system of digital signal processors calculates the ensemble average power spectrum of the spectrally dispersed Michelson interferograms,the instantaneous optical path di?erence error,and several other useful parameters in real time.From the observed R Cas visibilities at baselines 12.0m,13.8m and 13.9m,a 2.1μm uniform-disk diameter of 25.3mas ±3.3mas was derived.The unusually high visibility values at baselines ≥16m show that the stellar surface of R Cas is more complex than previously assumed.The visibility values at baselines ≥16m can be explained by high-contrast surface structure on the stellar surface of R Cas or other types of unexpected center-to-limb variations.The R Cas observations were compared with theoretical Mira star models 1,2.We obtained the following results for R Cas at variablity phase 0.08:Angular Rosseland radius R a :12.1mas ±1.7mas

Linear Rosseland radius R :276R ⊙±66R ⊙(using the HIPPARCOS parallax of 9.36mas ±1.10mas)

E?ective temperature:2685K ±238K (derived from the angular Rosseland radius and the JHKLM-photometry).Keywords:interferometry -infrared -spectroscopy -evolved stars -Mira stars

1.INTRODUCTION

The GI2T/REGAIN interferometer 3,4is an optical interferometer of two 1.5m telescopes.For this interferometer we have built an infrared beam combiner and used it to record spectrally dispersed infrared Michelson interferograms of the Mira star R Cas.In this paper we present the design of our new infrared beam combiner and R Cas observations obtained in August 1999.This beam combiner allows us to simultaneously record Michelson fringes in about 128di?erent spectral channels from 2.00to 2.18μm.The tilt of the spectrally dispersed fringes is a measure of the instantaneous optical path di?erence.The R Cas observations reported in this paper are the ?rst NIR interferometry observations with large 1.5m telescopes.The aim of our Mira star project is to resolve the stellar disk of Mira stars,to reveal photospheric asymmetries and surface structures,and to study the wavelength and phase dependence of the diameter.Previous speckle or long-baseline interferometry observations of Mira stars were,for example,reported in Refs.5-16.Theoretical studies 1,2,17?19show that interferometric diameter measurements can considerably improve our understanding of cool stellar atmospheres.

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