YSO Bulletin
- December 2025 -

Young Binaries studied

Recent results suggest that the first steps towards planet formation may be already taking place in protoplanetary discs during the first 100 000 years after stars form. It is therefore crucial to unravel the physical and chemical structures of such discs in their earliest stages while they are still embedded in their natal envelopes and compare them with more evolved systems.
Oph-IRS 67 is one such system studied in a Danish paper. As a Class I protobinary source, the team used the Submillimeter Array (SMA) on 1–2 arcsec scales (representing 150–300 AU at distance). SMA's wide bandwidth provided detections of a range of molecular transitions tracing different physics, such as CO regions, sulphur-bearing species, and carbon-chain molecules. Different groups of species reveal the various processes at work. For example, the CO and sulphur-bearing species show a rotational profile and trace the larger-scale circumbinary disc structure, while the carbon-based molecules peak at the southern edge of the disc at blue-shifted velocities. The detected molecular transitions can be grouped into three main components: cold regions far from the system, the circumbinary disc, and a UV-irradiated region likely associated with the surface layers of the disc that are reached by the UV radiation from the host.
IRS 67 is, therefore, highlighting the intermediate chemistry between deeply embedded sources and T Tauri discs

Still with Discography

A study of the well-known Upper Scorpius OB association reveals how the presence of stellar companions affects disc evolution. Of the 50 G0-M3 members with discs in the sample, only seven host a stellar companion within 2” and brighter than K = 15, compared to 35 of 75 members without discs. This matches a trend seen in the much younger 1-2 Myr old Taurus region (predominantly less massive stars are forming here), where systems with a stellar companion within 40 AU have a lower fraction of infrared-identified discs than those without such companions, indicating shorter disc lifetimes in close multiple systems.
However, the fractions of disc systems with a stellar companion within 40 AU match in Upper Sco and Taurus. Additionally, they saw no difference in the millimeter brightnesses of discs in Upper Sco systems with and without companions, in contrast to Taurus where systems with a companion within 300 AU are significantly fainter than wider and single systems. These results suggest that the effects of stellar companions on disc lifetimes occur within the first 1-2 Myr of disc evolution, after which companions play little further role. By contrast, discs around single stars lose the millimeter-sized dust grains in their outer regions between ages of 1-2 Myr and 5-11 Myr (roughly the age of the Upper Sco association). The end result of small dust disc sizes and faint millimeter luminosities is the same, whether the disc has been truncated by a companion or has evolved through internal processes.

...and still!

The silicate feature near 10μm 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. A team which includes our very own Bo Reipurth in 2019 investigated the silicate feature of the two Class I components of HH 250-IRS, a resolved binary system in the dark nebula LDN 643 with a separation of half an arcsec driving a Herbig-Haro flow HH 250. Each component has its own circumstellar envelope, and the system is surrounded by a circumbinary disk. As revealed by the thermal infrared imager and spectrograph at ESOs Very Large Telescope, the silicate features of both sources are clearly different. The NW component has a broad, smooth absorption profile lacking structure, which they mainly attribute to foreground interstellar dust absorption near the Aquila Rift where the object is located, but estimate that additional absorption by amorphous silicates takes place in the circumstellar envelope of the YSO. The SE component however shows the silicate feature in emission, with structure longwards of 9.5μm indicating the presence of crystalline dust in the dominant form of forsterite (found in some meteors and cometary dust).
Despite their virtually certain coevality, the differences between the components of the 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 (found in the Earth's mantle rocks) in the latter are fairly unusual among envelopes with crystalline dust, and they tentatively relate it to a possible wide gap or an inner truncation of the disk, which might indicate that the SE component could actually be a very close binary, and they further speculate that the clear differences between the silicate feature spectra of both components of the system may be due either to disk evolution sped up by multiplicity, or by accretion variability leading to episodes of crystal formation.