Infrared observations of circumstellar disks of Beta Pictoris analogs

We present infrared spectrophotometry and intermediate resolution ($\lambda/\Delta\lambda\approx50)$ spectroscopy of $\beta$ Pictoris and other early-type main sequence stars with infrared excesses. Spectrophotometric observations were conducted in the narrowband ($\rm\Delta\lambda\approx1\mu m)$ "silicate" filters with the MSFC bolometer array camera and the IRTF 2-30 $\mu$m Facility bolometer system. Spectroscopic observations were conducted with the 3-14 $\mu$m Aerospace Corp. Infrared Spectrograph. These observations were motivated by the previously reported detection of silicate emission in $\beta$ Pic. We detected 10 $\mu$m silicate emission from the B9.5 Ve star 51 Oph. We show that the silicate features of $\beta$ Pic and 51 Oph are broader and exhibit more structure than the features from amorphous and/or hydrated silicates, such as found in the interstellar medium or in most circumstellar shells of M stars. The 10 $\mu$m silicate feature of $\beta$ Pic and 51 Oph is similar to that in several Solar System comets. The presence of high temperature phases of silicates such as olivines and pyroxenes is evident in the $\beta$ Pic and 51 Oph spectra, as in the case of comets and interplanetary dust particles.

In a subsequent photometric survey of ten $\beta$ Pic analogs, we obtained the most precise determinations of their mid-infrared excesses. None of these ten sources showed the prominent silicate feature of $\beta$ Pic and 51 Oph. The 10 $\mu$m excess fluxes of $\zeta$ Lep and $\beta$ UMa suggest the presence of weak silicate emission. Intermediate-resolution spectra of these two sources are similar to the weak and broad emission features seen in dust-poor comets.

We searched for gaseous spectral features around 2.3 $\mu$m from CO overtone transitions ($\nu$ = 0-2) in $\beta$ Pic and analogous systems. We utilized the high-resolution near-infrared Echelle spectrograph (CSHELL) at the IRTF. Our inferred upper limits to circumstellar CO column density in $\beta$ Pic possibly indicate an enhancement of CO abundance such as observed in cometary environments.