Searching for spatial variations of alpha^2/mu in the Milky Way
  (Abridged) A procedure is suggested to explore the value of F = alpha^2/mu,
where mu = m_e/m_p is the electron-to-proton mass ratio, and alpha is the
fine-structure constant. The fundamental physical constants, which are measured
in different physical environments of high (terrestrial) and low (interstellar)
densities of baryonic matter are supposed to vary in chameleon-like scalar
field models, which predict that both masses and coupling constant may depend
on the local matter density. The parameter Delta F/F = (F_obs - F_lab)/F_lab
can be estimated from the radial velocity offset, Delta V = V_rot-V_fs, between
the low-laying rotational transitions in carbon monoxide 13CO and the
fine-structure transitions in atomic carbon [CI]. A model-dependent constraint
on Delta alpha/alpha can be obtained from Delta F/F using Delta mu/mu
independently measured from the ammonia method. Currently available radio
astronomical datasets provide an upper limit on |Delta V| < 110 m/s (1sigma).
When interpreted in terms of the spatial variation of F, this gives |Delta F/F|
< 3.7*10^-{7}. An order of magnitude improvement of this limit will allow us to
test independently a non-zero value of Delta mu/mu = (2.2 +/- 0.4_stat +/-
0.3_sys)*10^{-8} recently found with the ammonia method. Taking into account
that the ammonia method restricts the spatial variation of mu at the level of
|Delta mu/mu| <= 3*10^{-8} and assuming that Delta F/F is the same in the
entire interstellar medium, one obtains that the spatial variation of alpha
does not exceed the value |Delta alpha/alpha| < 2*10^{-7}. Since extragalactic
gas clouds have densities similar to those in the interstellar medium, the
bound on Delta alpha/alpha is also expected to be less than 2*10^{-7} at high
redshift if no significant temporal dependence of alpha is present.
