  Millimeter observations of CO gas in planetesimal belts show a high detection
rate around A stars, but few detections for later type stars. We present the
first CO detection in a planetesimal belt around an M star, TWA 7. The
optically thin CO (J=3-2) emission is co-located with previously identified
dust emission from the belt, and the emission velocity structure is consistent
with Keplerian rotation around the central star. The detected CO is not well
shielded against photodissociation, and must thus be continuously replenished
by gas release from exocomets within the belt. We analyze in detail the process
of exocometary gas release and destruction around young M dwarfs and how this
process compares to earlier type stars. Taking these differences into account,
we find that CO generation through exocometary gas release naturally explains
the increasing CO detection rates with stellar luminosity, mostly because the
CO production rate from the collisional cascade is directly proportional to
stellar luminosity. More luminous stars will therefore on average host more
massive (and hence more easily detectable) exocometary CO disks, leading to the
higher detection rates observed. The current CO detection rates are consistent
with a ubiquitous release of exocometary gas in planetesimal belts, independent
of spectral type.
