Changing environments of pre-main-sequence stars and their effect on the 3 μm ice feature

We present 3 μm spectra of three pre-main-sequence stellar objects. The heavily embedded infrared source HH100-IR, and two optically visible stars that apparently power the Herbig-Haro objects HH52, 53, 54, and HH57, are likely to represent different stages during early stellar evolution. Our medium-resolution spectra of HH100-IR confirm the existence of a very strong ice band absorption (optical depth 0.8-1.2 at 3.07 μm) discovered by Whittet & Blades [MNRAS, 191, 309 (1980)] with lower resolution. Each of the visible stars shows a weaker (optical depth 0.2-0.4) water-ice feature but with a relatively prominent absorption dip at 2.97 μm which we believe is due to ammonia ice. A detailed inspection of the line profiles suggests that the environment around the optically visible stars is significantly warmer (∼100 K) than that around HH100-IR (∼10-20 K) and that processing of remnant circumstellar dust is already underway.