Debris Disks around M Dwarfs
Submm Excesses and Optical Imaging

After primordial disks around forming stars dissipate, coalescing into larger planetesimals and planets, collisions between these more massive bodies can produce what is known as a "debris" disk. As material is collisionally reduced again to dust grains, the star once again exhibits an infrared excess. Hence, observation of these excesses is typically associated with the presence of a debris disk.

Figure 1: SEDs of GL 803 and GJ 182 including the SCUBA 850 micron detections, and the 450 micron marginal detection and upper limit for the two stars, respectively. Optical data are from SIMBAD and the Hipparcos catalog; the IR data to 100 microns are from the 2MASS catalog and the IRAS color-corrected Faint Source Catalog and SCANPI photometry. In the case of GL 803, a modified blackbody of 40 K and beta 0.8 is fit to the SED longward of 60 micron. At this temperature, blackbody grains would not exist inward of a radius of 17 AU from the star. This inner gap could indicate the presence of planets within this radius. GL 803 is only 12 Myr old, so any planets present should be detectable by thermal emission.

As part of a SCUBA survey of nearby stars selected based on their ages and membership in the beta Pictoris and Local Association moving groups, we report submillimetre excesses around two M dwarfs. These are the first detections of disks such low mass stars. One of these, GL 803 (AU Microscopium) was known to have an IR excess based on IRAS 60 micron data, but for the second, GJ 182, this is the first indication of excess emission associated with disks. Figure 1 shows the SEDs of these two stars including our detections at 850 micron with SCUBA. Neither has been signficantly detected at 450 micron, but for GL 803, we have a marginal 2 sigma detection consistent with the fit of a modified blackbody of 40 K with beta of 0.8. These results are presented in Liu, Matthews, Williams & Kalas (2004, ApJ, in press), which will appear shortly on astro-ph.

Figure 2: Coronographic imaging of GL 803 reveals a large, nearly edge-on disk in scattered optical (R-band) light. The coronograph covers the inner 50 AU radius of the disk, but it is detected out to a radius of 210 AU. SCUBA imaging has thus far failed to resolve the disk.

Most importantly, of the three M dwarfs we observed, excesses indicative of disks were detected around two (the third, GL 799, was not detected). If many low mass stars harbour debris disks, this could be taken as evidence that planet formation is possible around a large fraction of low mass stars. To date, only the M dwarf GJ 876 is known to have planets, detected at approximately 0.2 AU by a radial velocity survey (Marcy et al. 2001). The 17 AU inner hole inferred for GL 803 suggests that planets may form at much larger separations, and do so within 10 Myr. Based on the SCUBA data, we estimate the masses of these disks to be between 0.01 and 0.03 Earth masses.

GL 803 (M1V) has been an object of interest due to its IR excess for some time; it is included on Spitzer and HST ACS GTO programs. However, we have already followed up our JCMT detection with optical imaging of the disk (see Figure 2) using the UH 88" coronograph (as reported in Kalas, Liu & Matthews, 2004, Science). This is only the fourth optically detected debris disk; the first was beta Pictoris (A5V). These two stars are members of the same moving group. Together, these stars offer an opportunity to understand planet formation around stars of different mass formed from the same natal cloud.