A radio map of the colliding winds in the very massive binary system HD 93129A

¹ Instituto Argentino de Radioastronomía, CCT-La Plata, CONICET, Argentina
e-mail: paula@iar-conicet.gov.ar
² Departament d’Astronomia i Meteorologia, Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès 1, 08028 Barcelona, Spain
³ ASTRON Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁴ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁵ Department of Astrophysics, Geophysics and Oceanography, University of Liège, 17 Allée du 6 Août B5c, 4000 Sart Tilman, Belgium
⁶ NRC Herzberg Astronomy and Astrophysics, DRAO, PO Box 248, Penticton B.C., V0H 1K0, Canada
⁷ Australia Telescope National Facility, CSIRO Astronomy & Space Science PO Box 76, Epping, NSW 1710, Australia

Received: 29 December 2014
Accepted: 25 March 2015

Abstract

Context. Radio observations are an effective tool for discovering particle acceleration regions in colliding-wind binaries through detection of synchrotron radiation. Wind-collision region (WCR) models can reproduce the radio continuum spectra of massive binaries. However, key constraints for models come from high-resolution imaging. Only five WCRs have been resolved to date at radio frequencies on milliarcsec (mas) angular scales. The source HD 93129A, a prototype of the very few known O2 I stars, is a promising target for study. Recently, a second massive, early-type star about 50 mas away was discovered, and a non-thermal radio source was detected in the region. Preliminary long-baseline array data suggest that a significant fraction of the radio emission from the system comes from a putative WCR.

Aims. We seek evidence that HD 93129A is a massive binary system with colliding stellar winds that produce non-thermal radiation through spatially resolved images of the radio emitting regions.

Methods. We completed observations with the Australian Long Baseline Array (LBA) to resolve the system at mas angular resolutions and reduced archival Australia Telescope Compact Array (ATCA) data to derive the total radio emission. We also compiled optical astrometric data of the system in a homogeneous way. We reduced historical Hubble Space Telescope data and obtained absolute and relative astrometry with milliarcsec accuracy.

Results. The astrometric analysis leads us to conclude that the two stars in HD 93129A form a gravitationally bound system. The LBA data reveal an extended arc-shaped non-thermal source between the two stars, which is indicative of a WCR. The wind momentum-rate ratio of the two stellar winds is estimated. The ATCA data show a point source with a change in flux level between 2003-4 and 2008-9, which is modeled with a non-thermal power-law spectrum with spectral indices of −1.03 ± 0.09 and −1.21 ± 0.03, respectively. The mass-loss rates derived from the deduced thermal radio emission and from the characteristics of the WCR are consistent with estimates derived by other authors.