Resolved Hubble Space spectroscopy of ultracool binary systems
Using the low-resolution mode of the Space Telescope Imaging Spectrograph (STIS) aboard the \emph{Hubble Space Telescope} (HST), we have obtained spatially resolved spectra of 20 ultracool dwarfs. 18 of them belong to 9 known very low-mass binary systems with angular separations in the range 0.37-0.098 arcseconds. We have derived spectral types in the range dM7.5 to dL6 from the PC3 index, and by comparing our STIS spectra with ground-based spectra of similar spectral resolution from Mart{\'ı}n et al. (1999). We have searched for H$_α$ emission in each object but it was clearly detected in only 2 of them. We find that the distribution of H$_α$ emission in our sample is statistically different from that of single field dwarfs, suggesting an intriguing anticorrelation between chromospheric activity and binarity for M7--M9.5 dwarfs. We provide measuments of the strength of the main photospheric features and the PC3 index, and we derive calibrations of spectral subclasses versus F814W and K-band absolute magnitudes for a subset of 10 dwarfs in 5 binaries that have known trigonometric parallaxes.