Dust around HII regions - III. IRAS evidence for an old SNR in the W 75N region ?

IRAS Calibrated Raw Detector Data (CRDD) are presented for the W75 region. Activity in W75, W75N, DR21 and several far-infrared sources is interpreted in terms of a star-forming loop in the interstellar medium (ISM). Two possible causes of this loop are explored - a wind-blown bubble around an OB association, and an SNR. The former is excluded on the grounds of the number and age of the OB stars required, and the data are found to be consistent with a previously unknown, old SNR which has swept up an arc of dust in an expanding shell-like shock wave and may have triggered star formation in these regions. The age of the SNR is estimated to be 100,000 yr and its diameter is 35 pc. Color temperature analysis indicates dust grains at around 40-50 K, as well as a population of transiently heated very small grains or PAHs, and the total mass of swept-up gas and dust is calculated to be 45,000 solar masses.