2000ApJ...540..842Y

The Trifid Nebula (M20) is a well-known prominent optical H II region trisected by obscuring dust lanes. Radio continuum VLA observations of this nebula show free-free emission at λ=3.6 and 6 cm from three stellar sources lying close to the O7 V star at the center of the nebula. We argue that neutral material associated with these stars is photoionized externally by the UV radiation from the hot central star. We also report the discovery of a barrel-shaped supernova remnant, SNR G7.06-0.12, at the northwest rim of the nebula, and two shell-like features, G6.67-0.42 and G6.83-0.21, adjacent to W28 and M20. We discuss the nature of these features and their possible relationship to the pulsar PSR 1801-2306 and W28 OH (1720 MHz) masers.