We present SCUBA-2 850 \mu m observations of 7 very low mass stars ( VLMS ) and brown dwarfs ( BDs ) .
3 are in Taurus and 4 in the TW Hydrae Association ( TWA ) , and all are classical T Tauri ( cTT ) analogs .
We detect 2 of the 3 Taurus disks ( one only marginally ) , but none of the TWA ones .
For standard grains in cTT disks , our 3 \sigma limits correspond to a dust mass of 1.2 M _ { \earth } in Taurus and a mere 0.2 M _ { \earth } in the TWA ( 3–10 \times deeper than previous work ) .
We combine our data with other sub-mm/mm surveys of Taurus , \rho Oph and the TWA to investigate the trends in disk mass and grain growth during the cTT phase .
Assuming a gas-to-dust mass ratio of 100:1 and fiducial surface density and temperature profiles guided by current data , we find the following . ( 1 ) The minimum disk outer radius required to explain the upper envelope of sub-mm/mm fluxes is \sim 100 AU for intermediate-mass stars , solar-types and VLMS , and \sim 20 AU for BDs . ( 2 ) While the upper envelope of apparent disk masses increases with M _ { \ast } from BDs to VLMS to solar-type stars , no such increase is observed from solar-type to intermediate-mass stars .
We propose this is due to enhanced photoevaporation around intermediate stellar masses . ( 3 ) Many of the disks around Taurus and \rho Oph intermediate-mass and solar-type stars evince an opacity index of \beta \sim 0–1 , indicating significant grain growth .
Of the only four VLMS/BDs in these regions with multi-wavelength measurements , three are consistent with considerable grain growth , though optically thick disks are not ruled out . ( 4 ) For the TWA VLMS ( TWA 30A and B ) , combining our 850 \mu m fluxes with the known accretion rates and ages suggests substantial grain growth by 10 Myr , comparable to that in the previously studied TWA cTTs Hen 3-600A and TW Hya .
The degree of grain growth in the TWA BDs ( 2M1207A , SSPM1102 ) remains largely unknown . ( 5 ) A Bayesian analysis shows that the apparent disk-to-stellar mass ratio has a roughly constant mean of log _ { 10 } [ M _ { disk } / M _ { \ast } ] \approx - 2.4 all the way from intermediate-mass stars to VLMS/BDs , supporting previous qualitative suggestions that the ratio is \sim 1 % throughout the stellar/BD domain . ( 6 ) Similar analysis shows that the disk mass in close solar-type Taurus binaries ( sep < 100 AU ) is significantly lower than in singles ( by a factor of 10 ) , while that in wide solar-type Taurus binaries ( \geq 100 AU ) is closer to that in singles ( lower by a factor of 3 ) . ( 7 ) We discuss the implications of these results for planet formation around VLMS/BDs , and for the observed dependence of accretion rate on stellar mass .