FN Tau is a rare example of very low-mass T Tauri stars that exhibits a spatially resolved nebulosity in near-infrared scattering light .
To directly derive the parameters of a circumstellar disk around FN Tau , observations of dust continuum emission at 340 GHz are carried out with the Submillimeter Array ( SMA ) .
A point-like dust continuum emission was detected with a synthesized beam of \sim 0.7 \arcsec in FWHM .
From the analysis of the visibility plot , the radius of the emission is estimated to be \leq 0.29 \arcsec , corresponding to 41 AU .
This is much smaller than the radius of the nebulosity , 1.85 \arcsec for its brighter part at 1.6 µm .
The 340 GHz continuum emission observed with the SMA and the photometric data at \lambda \leq 70 ~ { } \micron are explained by a power-law disk model whose outer radius and mass are 41 AU and ( 0.24 - 5.9 ) \times 10 ^ { -3 } ~ { } M _ { \sun } , respectively , if the exponent of dust mass opacity ( \beta ) is assumed to be 0 - 2 .
The disk model can not fully reproduce the flux density at 230 GHz obtained with the IRAM 30-meter telescope , suggesting that there is another extended “ halo ” component that is missed in the SMA observations .
By requiring the halo not to be detected with the SMA , the lower limit to the size of the halo is evaluated to be between 174 AU and 574 AU , depending on the assumed \beta value .
This size is comparable to the near-infrared nebulosity , implying that the halo unseen with the SMA corresponds to the origin of the near-infrared nebulosity .
The halo can contain mass comparable to or at most 8 times greater than that of the inner power-law disk , but its surface density should be lower than that at the outer edge of the power-law disk by more than one order of magnitude .
The physical nature of the halo is unclear , but it may be the periphery of a flared circumstellar disk that is not described well in terms of a power-law disk model , or a remnant of a protostellar envelope having flattened structure .