S R ] 1 5 O ct 2 01 8 Massive and Evolved Stars with the ngVLA

The Next Generation Very Large Array will have excellent sensitivity for detecting the thermal emission from massive stars and from red giants. This will allow direct imaging of the winds for a large number of hot massive stars. It will also allow using the radio emission for the massive stars as a way to detect stars to allow high resolution measurements can be made, even with large extinction. A few examples of the utility of the high resolution measurements are given: dynamics of globular clusters with red giants, detection of intermediate mass stripped stars in binaries, and measurement of masses of stars in massive binaries. 1. Description of the problem Winds of hot massive stars are strong, spatially extended sources of free-free emission in the radio band. The acceleration of stellar winds is often probed via optical and ultraviolet emission lines, but is rarely directly resolved. Stellar winds from many classes of stars allow us to have bright radio sources that can be used as dynamical tracers, both for measuring the masses of massive stars in binaries and for measuring the gravitational potential fields in globular clusters, and should also provide a check on orbits around the Galactic Center. Estimates of the masses of the most massive stars have been extremely difficult in the past, with luminosities and single star evolutionary tracks and/or the Eddington limit used for mass estimates, rather than dynamical measurements typically used as a technique. By providing a tool for obtaining precise astrometry and high dynamic range, even in highly obscurred parts of the sky, radio measurements of stellar masses will provide crucial information for understanding massive stars. With globular clusters, it is quite controversial about whether the clusters contain intermediate mass black holes. Radial velocity measurements and proper motions of stars in the optical band are a valuable tool for probing the inner dynamics of star clusters, but are complicated by hard-to-constrain crowding effects. Using radio tracers of stellar dynamics will allow a complementary test of whether there are intermediate mass black holes in clusters, and will also allow measurements of the proper motions of the most extincted clusters.