LDN 1780: A Translucent High-Galactic Latitude Interstellar Cloud in a UV-rich, Anisotropic Radiation Field

We combined high-resolution optical imaging observations in 12 intermediate-band (BATC) filter and deep mid- and far-IR Spitzer maps of LDN 1780 to characterize the external radiation field illuminating this high-latitude (l = 359 deg; b = 36.5 deg; distance 100 pc) translucent cloud and the infrared emission of dust within LDN 1780 in response to this external illumination.

The overall energy density of the incident radiation field is approximately equal to that of the ISRF near the Sun, resulting in a large dust grain equilibrium temperature ranging from 14.5 K -16.8 K. However, the incident radiation field is highly anisotropic, with the southern portions of LDN 1780 being most strongly illuminated, especially at shorter wavelengths. This anisotropy is a result of the cloud's proximity to the Sco OB2 association (est. center: l = 322 deg; b = 10 deg).

The southwestern portion and the optically-thin eastern tail of LDN 1780 exhibit strong intensity excesses at 24 micron (Spitzer MIPS) and at 8 micron (Spitzer IRAC Ch. #4) compared to dust in the diffuse ISM of the Milky Way Galaxy. We interpret these excesses as enhanced emission from stochastically-heated very small grains (VSG) and from PAH ions, respectively. These excesses, however, are not necessarily the result a greater relative abundance of these two small-particle components but rather reflect the increased frequency of photon-grain interactions (e.g. heating, excitation, or ionization) within a UV-rich radiation field.

This research has been supported by grants from NASA and the NSF to the University of Toledo as well as by contributions from corporate sponsors AstroDon, RC Optical Systems, Santa Barbara Instrument Group, Software Bisque, and New Mexico Skies, for which we are grateful. We especially acknowledge the NSF-REU program at the University of Toledo, which supported the analysis of these data.