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Abstract Studies of the resolved stellar populations of young massive clusters have shown that the slope of the initial mass function (IMF) appears to be the same everywhere, with no dependence on stellar density or metallicity. At the same time, studies of integrated properties of galaxies usually conclude that the IMF does vary and must be top-heavy in starburst regions. In order to investigate this, we have carried out a long-term project to characterize the massive-star content of NGC 3603, the nearest giant Hiiregion, known to have a rich population of massive stars. We used both ground-based and Hubble Space Telescope (HST) imaging to obtain photometry, and we employed Gaia to establish membership. We obtained spectra of 128 stars using the Magellan 6.5 m telescope and HST, and we combine these data to produce a reddening map. After analyzing the data in the same way as we have for 25 other star-forming regions in the Milky Way and the Magellanic Clouds, we find that the IMF slope of NGC 3603 is quite normal compared to other clusters, with Γ = −0.9 ± 0.1. If anything, there are fewer very high mass (>65M_⊙) stars than one would expect by extrapolation from lower masses. This slope is also indistinguishable from what several studies have shown for R136 in the LMC, an even richer region. We speculate that the depreciation of the highest-mass bins in NGC 3603, but not in R136, may indicate that it is harder to form extremely massive stars at the higher metallicity of the Milky Way compared to that of the LMC. 

Abstract The star NGC 3603-A1 has long been known to be a very massive binary, consisting of a pair of O2-3If*/WN5-6 stars which show Wolf–Rayet–like emission due to their luminosities being near the Eddington limit. The system has been poorly characterized until now, due to the difficulties of obtaining reliable radial velocities from broad, blended emission lines and the extreme crowding in the cluster. However, previously unpublished archival Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) spectra revealed that some of the upper Balmer lines (seen in absorption) are well separated at favorable orbital phases, prompting us to obtain our own carefully timed new HST/STIS spectra, which we have analyzed along with the older data. Radial velocities measured from these spectra allow us to obtain an orbit for this 3.77298-day binary. We also used archival STIS imaging of the cluster to obtain a more accurate light curve for this eclipsing system, which we then modeled, yielding the orbital inclination and providing values for the stellar radii and temperatures. Together, these data show that the NGC 3603-A1 system consists of a 93.3 ± 11.0M_⊙O3If*/WN6 primary with an effective temperature of 37,000 K, and a 70.4 ± 9.3M_⊙O3If*/WN5 secondary that is slightly hotter, 42,000 K. Although a more massive binary is known in the LMC, NGC 3603-A1 is as massive as any binary known in our own Galaxy for which a direct measurement of its mass has been made by a fundamental method. The secondary has been spun up by mass accretion from the primary, and we discuss the evolutionary status of this intriguing system. 

Abstract The spatial range for feedback from star formation varies from molecular cloud disruption on parsec scales to supershells and disk blowout on kiloparsec scales. The relative amounts of energy and momentum given to these scales are important for understanding the termination of star formation in any one region and the origin of interstellar turbulence and disk stability in galaxies as a whole. Here, we measure, for 11 THINGS galaxies, the excess kinetic energy, velocity dispersion, and surface density of H i gas associated with regions of excess star formation, where the excess is determined from the difference between the observed local value and the azimuthal average. We find small decreases in the excess kinetic energy and velocity dispersion in regions of excess star formation rate density, suggesting that most of the feedback energy does not go into local H i motion. Most likely, it disrupts molecular clouds and dissipates rapidly at high gas density. Some could also be distributed over larger regions, filling in spaces between the peaks of star formation and contributing to other energy sources from self-gravity and spiral arm shocks.