Surveying the 217.5 nm interstellar extinction feature with a small satellite

The interstellar 217.5 nm extinction feature in the ultraviolet (UV) was identified by Stecher (1965). A broader investigation was conducted with the International Ultraviolet Explorer (IUE) during its 20 years of operation starting in 1979. However, this feature is still not well understood, and did not benefit from a dedicated mission. The Russian-Spanish WSO/UV mission planned to launch in 2019 is promising, but UV extinction is not one of its primary science goals. We propose here a small and inexpensive satellite that will enable a deeper study of the 217.5 nm extinction, and provide physical insights into its nature. Results obtained until now show that the central wavelength of the feature is stable at 217.4 � 1.7 nm, however, it is ubiquitously broad with a FWHM ∼50 nm (varying between 36 to 60 nm), and its profile of the extinction vs. 1/λ can be approximated by a Drude profile for different total A_v to selective A_B - A_v extinction ratios. Our preliminary feasibility study shows that the absorbed energy around the centre of the feature represents well the total absorbed energy of the entire feature. Hence, establishing the existence of the feature, as well as studying its strength, can be done using a UV telescope with two broad-band filters. One filter centred on 217.5 nm, together with a continuum level measurement at longer wavelengths, will enable the evaluation of the absorbed energy of the feature. The continuum level will be defined by two filters, one covering the range 270 to 320 nm and another for the range 330 and 360 nm. This last filter can be either space or ground based. Assuming one year of space operation, we will be able to measure tens of thousands of stars, starting from a complete Gould Belt survey, which will include hundreds of O and B stars. A periodic calibration of the instrument will be conducted using UV standard stars. This mission can be performed for a reasonable cost with a small 6U student-built cubesat, equipped with a UV camera and two filters. The telescope aperture will be ∼ 10 cm. In order to minimize power consumption, as well as for simplicity of design, we propose to use CMOS detectors. In this paper, we present details of the feasibility study, as well as the proposed layout of the satellite.