SCUBA sub-millimeter observations of gamma-ray bursters - II. GRB 991208, 991216, 000301C, 000630, 000911, 000926

We discuss our ongoing program of Target of Opportunity sub-millimeter obse rvations of gamma-ray bursts (GRBs) using the Sub-millimetre Common-User Bo lometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). Sub-mill imeter observations of the early afterglows are of interest because this is where the emission peaks in some bursts in the days to weeks following the burst. Of increasing interest is to look for underlying quiescent sub-mill imeter sources that may be dusty star-forming host galaxies. In this paper, we present observations of GRB 991208, 991216, 000301C, 000630, 000911, an d 000926. For all these bursts, any sub-millimeter emission is consistent w ith coming from the afterglow. This means that we did not conclusively dete ct quiescent sub-millimeter counterparts to any of the bursts that were stu died from 1997 through 2000. The inferred star formation rates (M greater t han or equal to5 M-circle dot) are typically less than or similar to 300 M- circle dot yr(-1). If GRBs are due to the explosions of high-mass stars, th is may indicate that the relatively small population of extremely luminous dusty galaxies does not dominate the total star formation in the universe a t early epochs. Instead, the GRBs may be predominantly tracing slightly low er luminosity galaxies. The optical faintness of some host galaxies is unli kely to be explained as due to dust absorption in the host.