  The radio spectra of main-sequence stars remain largely unconstrained due to
the lack of observational data to inform stellar atmosphere models. As such,
the dominant emission mechanisms at long wavelengths, how they vary with
spectral type, and how much they contribute to the expected brightness at a
given radio wavelength are still relatively unknown for most spectral types. We
present radio continuum observations of Altair, a rapidly rotating A-type star.
We observed Altair with NOEMA in 2018 and 2019 at 1.34 mm, 2.09 mm, and 3.22 mm
and with the VLA in 2019 at 6.7 mm and 9.1 mm. In the radio spectra, we see a
brightness temperature minimum at millimeter wavelengths followed by a steep
rise to temperatures larger than the optical photosphere, behavior that is
unexpected for A-type stars. We use these data to produce the first
sub-millimeter to centimeter spectrum of a rapidly rotating A-type star
informed by observations. We generated both PHOENIX and KINICH-PAKAL model
atmospheres and determine the KINICH-PAKAL model better reproduces Altair's
radio spectrum. The synthetic spectrum shows a millimeter brightness
temperature minimum followed by significant emission over that of the
photosphere at centimeter wavelengths. Together, these data and models show how
the radio spectrum of an A-type star can reveal the presence of a chromosphere,
likely induced by rapid rotation, and that a Rayleigh Jean's extrapolation of
the stellar photosphere is not an adequate representation of a star's radio
spectrum.
