Search for Cold Debris Disks around M-dwarfs. II
  Although 70 % of the stars in the Galaxy are M-dwarfs, thermal emission
searches for cold debris disks have been conducted mostly for A-type and
solar-type stars. We report on new lambda=1.2 mm continuum observations of
thirty M-dwarfs, using the MAMBO-2 bolometer array camera at the IRAM 30m
telescope. For a statistical analysis, we combine these data with our prior
SCUBA and MAMBO-2 observations of 20 other M-dwarfs. Our total sample divides
in M-dwarfs in moving groups, with relatively young ages, and in nearby
M-dwarfs with unknown ages. Only one cold debris disk (GJ842.2) was detected
significantly. We compare the implied disk abundance constraints with those
found in two comparable submillimeter surveys of 10 to 190 Myr old A- and
FGK-type stars. For the 19 youngest (ages less than 200 Myr) M-dwarfs in our
sample, we derive a cold disk fraction of 5.3^{+10.5}_{-5.0} %, compared to 15
+/-11.5 % for FGK-stars and 22^{+33}_{-20} % for A-stars. Hence, for this age
group, there is an apparent trend of fewer cold disks for later stellar types,
i.e., lower star masses. Although its statistical significance is marginal,
this trend is strengthened by the deeper sensitivity of observations in the
M-dwarf sample. We derive a cold disk fraction of < 10 % for the older (likely
a few Gyr) M-dwarfs in our sample. Finally, although inconclusively related to
a debris disk, we present the complex millimeter structure found around the
position of the M1.5 dwarf GJ526 in our sample.
