Photometric and Spectroscopic Observations of a Diffuse Interstellar Filament in the Foreground of the Magellanic Clouds

Abstract

We report the results of a photometric and spectroscopic study of stars that appear toward an infrared bright filament in the region between the LMC and SMC in the constellation Mensa. The filament was chosen for study because of its large angular size, intense IR emission, and proximity to an X-ray source observed using ROSAT. We have observed 38 stars in the vicinity of the filament with combined uvby-Hβ photometry, and nine stars with a high-resolution spectrograph (λ/δλ > 70,000) in the wavelength of the Na I D lines. For two stars, we present high-dispersion Ca II K line absorption spectra from the filament. We have derived a distance estimate of 230 ± 30 pc for the filament, and a mean color excess E(B-V) = 0.17 ± 0.05 mag. The resulting extinction toward the cloud is AV = 0.53 mag, assuming RV = 3.1. The Na I D line profiles are strongly saturated for stars at distances of d ≥ 200 pc, and fits to the profiles trace multiple components with vLSR = 0.0 km s-1 to vLSR = 10 km s-1 and b-values ranging from 1.0 km s-1 < b < 3.5 km s-1. We observe strikingly different Ca II absorption line profiles in the spectra of two of the background stars. The sight line toward HD 22252 has at least seven separate components of Ca II absorption, while the sight line toward HD 26109 appears to have only a single component. The spectroscopic results suggest that the warm gas component of the filament traced by Ca II has highly variable kinematics from one end to the other, perhaps as a result of shock excitation. Examination of IRAS colors for the cloud reveals values of R(12, 100) and R(60, 100) enhanced by more than a factor of 2 at the high galactic latitude end of the cloud near the HD 22252 sight line and near the center of the cloud, which could arise from shock-heated dust grains. By combining our distance estimate with the ROSAT X-ray intensity, we can derive an estimate of the hot gas density in the local bubble of ne = 3.47 × 10-3 cm-3. The IR filament appears to offer an excellent laboratory for probing a wide range of interstellar conditions within a single diffuse interstellar cloud. At the estimated distance of 230 pc and with its Galactic coordinates of (290, -43), it is also possible that the filament could have been formed from the Geminga supernova; further observations are needed to confirm this possibility.