  If quark stars exist, they may be enveloped in thin electron layers (electron
seas), which uniformly surround the entire star. These layers will be affected
by the magnetic fields of quark stars in such a way that the electron seas
would transmit hydromagnetic cyclotron waves, as studied in this paper.
Particular attention is devoted to vortex hydrodynamical oscillations of the
electron sea. The frequency spectrum of these oscillations is derived in
analytic form. If the thermal X-ray spectra of quark stars are modulated by
vortex hydrodynamical vibrations, the thermal spectra of compact stars,
foremost central compact objects (CCOs) and X-ray dim isolated neutron stars
(XDINSs), could be used to verify the existence of these vibrational modes
observationally. The central compact object 1E 1207.4-5209 appears particularly
interesting in this context, since its absorption features at 0.7 keV and 1.4
keV can be comfortably explained in the framework of the hydro-cyclotron
oscillation model.
