23 Nov 2019

Dust in the interstellar medium follows the normal extinction lawR=A/E(B-V)=3.1; however, dust in the immediate locality of an HIIregion may not. The environment around the HII region Sh2-121 isparticularly dusty; Sh2-121 seems to be enveloped by a dust cloudof its own. Three stars are seen ionizing the nebula. The distanceto Sh2-121 is known independent of the stellar information below,at 4.2 kpc from the Sum.

a. What the the average total extinction A toward the start in theHII region?

b. The foreground reddening E(B-V) to the nebula can be estimatedfrom the total hydrogen column density in the ISM towards thenebula, which I have measured for you N(HI+H2)=7.2X10^21 cm^-2.Using the mean ratio of total neutral hydrogen to colour excess inBohlin, Savage & Drake (1978, The Astrophysical Journal, no.224, p. 132) what is the foreground reddening to Sh2-121?


Star 1, V(mag)=14.91, B-V(mag)=1.86, Sp Type & LC=B0II

Star 2, V(mag)=15.44, B-V(mag)=1.76, Sp Type & LC=O4V

Star 3, V(mag)=15.48, B-V(mag)=1.76, Sp Type & LC=B2II

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