Radio Continuum Emission from the Central Stars of M20, and the Detection of a New Supernova Remnant near M20

The Trifid nebula (M20) is a well-known prominent optical HII regiontrisected by obscuring dust lanes. Radio continuum VLA observations of thisNebula show free-free emission at lambda 3.6 and 6cm from three stellar sourceslying close to the O7V star at the center of the nebula. We argue that neutralmaterial associated with these stars are photoionized externally by the UVradiation from the hot central star. We also report the discovery of abarrel-shaped SNR G7.06--0.12 at the northwest rim of the nebula and twoshell-like features G6.67--0.42 and G6.83--0.21 adjacent to W28 and M20. Wediscuss the nature of these features and their possible relationship to thepulsar PSR 1801-2306 and W28 OH (1720 MHz) masers.Comment: 11 pp incl 1 fig and 1 table plus 8 jpegs, LaTeX, uses aaspp4.sty; ApJ in pres