What drives the M*-SFR relation turning over at high masses? The role of
  bulges
  It is unclear whether bulge growth is responsible for the flattening of the
star formation main sequence (MS) at the high mass end. To investigate the role
of bulges in shaping the MS, we compare the NUV$-r$ color between the central
($r<R_{50}$) and outer regions for a sample of 6401 local star-forming
galaxies. The NUV$-r$ color is a good specific star formation rate indicator.
We find that at $M_{\ast}<10^{10.2}M_{\sun}$, the central NUV$-r$ is on average
only $\sim$ 0.25 mag redder than the outer NUV$-r$. Above
$M_{\ast}=10^{10.2}M_{\sun}$, the central NUV$-r$ becomes systematically much
redder than the outer NUV$-r$ for more massive galaxies, indicating that the
central bulge is more evolved at the massive end. When dividing the galaxies
according to their S\'ersic index $n$, we find that galaxies with $n$>2.0 tend
to be redder in the central NUV$-r$ color than those with $n$<2.0, even at
fixed B/T and $M_{\ast}$. This suggests that star formation in bulges is more
strongly dependent on $n$ (or central mass density) than on B/T. Finally, we
find that the fraction of galaxies with $n$>2.0 rapidly increases with
$M_{\ast}$ at $M_{\ast}>10^{10.2}M_{\sun}$, which is consistent with the
turning over of the MS at the same transition mass. We conclude that the
increasing fraction of low-sSFR dense bulges in $M_{\ast}>10^{10.2}M_{\sun}$
galaxies, rather than increasing B/T, is responsible for the flattened slope of
the $M_{\ast}$$-$SFR relation at high masses.
