IUE observations of Large Magellanic Cloud members and detection of the 2,200 Å feature

Abstract

THE dominant feature in the UV spectra of reddened stars in our Galaxy is the broad absorption feature of interstellar origin, centred near 2,200 Å and having a halfwidth of 350 Å (ref. 1). The wavelength position of this feature and the correlation of its strength with other interstellar extinction parameters, for example, the visual colour excess E(B –V) are remarkably uniform throughout the Galaxy within 2 kpc from the Sun². The 2,200 Å feature is attributed to small graphite spheres with radii between 100 and 200 Å (ref. 3), but a molecular origin cannot be ruled out⁴. It is, therefore, the most important feature to search for in the interstellar extinction laws of other galaxies. The Magellanic Clouds are the nearest extragalactic systems and the Large Magellanic Cloud (LMC), in particular the region near 30 Doradus, contains a significant amount of dust⁵. In the past few years considerable doubt has been expressed about the existence of the 2,200 Å feature in the LMC extinction law as derived from surface brightness measurements of the region around 30 Doradus⁶, the results being rather inconclusive due to the difficulties arising from uncertainties in the luminosity function for the region concerned⁷. The interstellar extinction law can be reliably determined only from studies of the individual spectra of reddened and unreddened stars of similar spectral types. We present here the UV observations of a moderately reddened and a slightly reddened early type supergiant in the LMC obtained with the IUE. These observations answer the question of whether or not the agents responsible for the 2,200 Å feature are present in interstellar dust in the LMC.