The size, luminosity, and motion of the extreme carbon star IRC+10216 (CW Leonis)

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: kmenten@mpifr-bonn.mpg.de; kaminski@mpifr-bonn.mpg.de
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street/MS42, Cambridge MA 02138, USA
e-mail: reid@cfa.harvard.edu
³ National Radio Astronomy Observatory, Array Operations Center, PO Box O, Socorro, NM 87801, USA
e-mail: mclausse@nrao.edu

Received: 16 April 2012
Accepted: 30 May 2012

Abstract

Very Large Array observations of the extreme carbon star IRC+10216 at 7 mm wavelength with 40 milli-arcsec resolution resolve the object’s radio emission, which forms an almost round uniform disk of 83 milli-arcsec diameter, corresponding to 11 AU (for an assumed distance of 130 pc). We find a brightness temperature of 1630 K for the radio photosphere. Since the emission is optically thick, we can directly estimate IRC+10216’s average luminosity, which is 8600 L_⊙. This value is in excellent agreement with what is predicted from the period-luminosity relation for carbon-rich Miras. Assuming an effective temperature of 2750 K for IRC+10216, it implies an optical photospheric diameter of 3.8 AU. Our precise determination of IRC+10216’s proper motion fits the picture presented by far-ultraviolet and far-infrared wavelength observations of its interaction region with the interstellar medium (its “astrosphere”): the star moves roughly in the direction expected from the morphology of the termination shock and its astrotail structures. Calculation of its three dimensional velocity and an analysis of the kinematics of its surrounding interstellar medium (ISM) suggest an appreciable relative velocity of 42 km s^-1, which is about half the value discussed in recent studies. This suggests a lower (time-averaged) mass loss rate and/or a higher ISM density than previously assumed.