Anomalous Extinction toward NGC 1938

Abstract

Abstract Intrigued by the extended red giant clump (RC) stretching across the color–magnitude diagram of the stars in a 50 × 50 pc2 region of the Large Magellanic Cloud (LMC) containing the clusters NGC 1938 and NGC 1939, we have studied the stellar populations to learn about the properties of the interstellar medium (ISM) in this area. The extended RC is caused by a large and uneven amount of extinction across the field. Its slope reveals anomalous extinction properties, with A V /E(B − V) ≃ 4.3, indicating the presence of an additional gray component in the optical contributing about 30% of the total extinction in the field and requiring big grains to be about twice as abundant as in the diffuse ISM. This appears to be consistent with the number of big grains injected into the surrounding ISM by the about 70 SN II explosions estimated to have occurred during the lifetime of the ∼120 Myr old NGC 1938. Although this cluster appears relatively small today and would be hard to detect beyond the distance of M31, with an estimated initial mass of ∼4800 M ⊙, NGC 1938 appears to have seriously altered the extinction properties in a wide area. This has important implications for the interpretation of luminosities and masses of star-forming galaxies both nearby and in the early universe.