Megahertz emission of massive early-type stars in the Cygnus region
  Massive, early type stars have been detected as radio sources for many
decades. Their thermal winds radiate free-free continuum and in binary systems
hosting a colliding-wind region, non-thermal emission has also been detected.
To date, the most abundant data have been collected from frequencies higher
than 1 GHz. We present here the results obtained from observations at 325 and
610 MHz, carried out with the Giant Metrewave Radio Telescope, of all known
Wolf-Rayet and O-type stars encompassed in area of ~15 sq degrees centred on
the Cygnus region. We report on the detection of 11 massive stars, including
both Wolf-Rayet and O-type systems. The measured flux densities at decimeter
wavelengths allowed us to study the radio spectrum of the binary systems and to
propose a consistent interpretation in terms of physical processes affecting
the wide-band radio emission from these objects. WR 140 was detected at 610
MHz, but not at 325 MHz, very likely because of the strong impact of free-free
absorption. We also report - for the first time - on the detection of a
colliding-wind binary system down to 150 MHz, pertaining to the system of WR
146, making use of complementary information extracted from the TIFR GMRT Sky
Survey. Its spectral energy distribution clearly shows the turnover at a
frequency of about 600 MHz, that we interpret to be due to free-free
absorption. Finally, we report on the identification of two additional
particle-accelerating colliding-wind binaries, namely Cyg OB2 12 and ALS 15108
AB.
