  We measure the transmission spectrum of WASP-19b from 3 transits using
low-resolution optical spectroscopy from the HST Space Telescope Imaging
Spectrograph (STIS). The STIS spectra cover a wavelength range of 0.29-1.03
microns with resolving power R=500. The optical data are combined with archival
near-IR data from the HST Wide Field Camera 3 (WFC3) G141 grism, covering the
wavelength range 1.087-1.687 micron, with resolving power R=130. We obtain the
transmission spectrum from 0.53-1.687 microns with S/N levels between 3000 and
11,000 in 0.1 micron bins. WASP-19 is a very active star, with optical stellar
flux varying by a few per cent over time. We correct the transit light curves
for the effects of stellar activity using ground-based activity monitoring with
the Cerro Tololo Inter-American Observatory (CTIO). While we were not able to
construct a transmission spectrum using the blue optical data due to the
presence of large occulted star spots, we were able to use the spot crossings
to help constrain the mean stellar spot temperature. To search for predicted
features in the hot-Jupiter atmosphere, we also define spectral indices for
differential radius measurements to specifically search for the presence of TiO
and alkali line features. Our measurements rule out TiO features predicted for
a planet of WASP-19b's equilibrium temperature (2050 K) in the transmission
spectrum at the 2.7-2.9 sigma confidence level, depending on atmospheric model
formalism. The WFC3 transmission spectrum shows strong absorption features due
to the presence of H2O, which is detected at the 4 sigma confidence level
between 1.1 and 1.4 microns. The results indicate that WASP-19b is a planet
with no or low levels of TiO and without a high C/O ratio. The lack of
observable TiO features are possibly due to rainout, breakdown from stellar
activity or the presence of other absorbers in the optical.
