Cygnus OB2 no. 12: A Most Peculiar Star

But for the obscuring effect of interstellar dust, the 10th magnitude star Cygnus OB2 no. 12 would be one of the brightest stars in the night time sky. For more than 60 years, since the earliest studies of this region, investigators have been puzzled by the surprising fact that one of the most luminous stars in our Galaxy happens by chance to lie behind a local concentration of interstellar matter that adds several magnitudes to the visual extinction compared with its siblings in this young star cluster. In a new study published today in Astrophysical Journal, Doug Whittet reassesses the nature of the dust in the line of sight. In particular, this research seeks to test the oft-held assumption that the dust toward Cygnus OB2 no. 12 is prototypical of the “diffuse” interstellar medium where H is predominantly atomic rather than molecular and conditions inhibit the growth of icy mantles on the dust grains.

It is shown that the excess relative to other cluster members occurs in translucent clumps within an extensive network of clouds in the region. Attenuation of the ambient radiation field is sufficient in the cores of the clumps to support the presence of gas-phase molecules, but not to sustain detectable ice formation. In general, the optical properties of dust in the clumps are, indeed, closely similar to those observed in typical diffuse interstellar material, with the notable exception of an unusually low value for the wavelength of maximum polarization. The implied enhancement of polarization by small grains is attributed to increased alignment efficiency in an enhanced magnetic field. This caveat apart, the results of the current work provide reassurance that Cygnus OB2 no. 12 is, indeed, an appropriate choice for studies that target diffuse and translucent phases of the ISM.

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