Silicate features in the circumstellar envelopes of the Class I binary driving source of HH 250

Abstract

Context. The silicate feature near 10 μm is one of the main tools available to study the mineralogy of circumstellar disks and envelopes, providing information on the thermal processing, growth, location, and circulation of dust grains. Aims. We investigate the silicate feature of the two Class I components of HH 250-IRS, a resolved binary system with a separation of 0″53 driving a Herbig-Haro flow. Each component has its own circumstellar envelope, and the system is surrounded by a circumbinary disk. Methods. We carried out low resolution spectroscopy in the 8–13 μm range using VISIR, the thermal infrared imager and spectrograph at ESO’s Very Large Telescope. Results. The silicate features of both sources are clearly different. The northwest (NW) component has a broad, smooth absorption profile lacking structure. We attribute most of it to foreground interstellar dust absorption, but estimate that additional absorption by amorphous silicates takes place in the circumstellar envelope of the young stellar object. The southeast (SE) component shows the silicate feature in emission, with structure longward of 9.5 μm indicating the presence of crystalline dust in the dominant form of forsterite. The apparent lack of an absorption feature caused by foreground dust is probably due to the filling of the band with emission by amorphous silicates in the envelope of the object. Conclusions. Despite their virtually certain coevality, the differences in the components of the HH 250-IRS binary are most likely due to markedly different circumstellar environments. The NW component displays an unevolved envelope, whereas dust growth and crystallization has taken place in the SE component. The weak or absent signatures of enstatite in the latter are fairly unusual among envelopes with crystalline dust, and we tentatively relate it to a possible wide gap or an inner truncation of the disk already hinted at in previous observations by a drop in the L′-band flux, which might indicate that the SE component could actually be a very close binary. We speculate that the clear differences between the silicate feature spectra of both components of HH 250-IRS may be due either to disk evolution sped up by multiplicity, or by accretion variability leading to episodes of crystal formation. Different inclinations with respect to the line of sight may play a role as well, although it is very unlikely that they are the sole element for the differences between both objects.