Archive for molecular clouds

Onset of ice formation in the Pipe nebula

Posted in astronomy with tags , , , on December 15, 2017 by Tim Kendall

In a recent paper, Goto et al., (preprint) have used infrared absorption spectroscopy of background stars to probe ice formation on grains in the Pipe nebula (APOD image and credits), detecting water ice absorption in seven out of 21 lines of sight investigated. The peak optical depths of the water ice are about half as large as those on the sources in Taurus with similar visual extinctions. One possible explanation (among others) is that the formation of the ice mantle has just started, that is, the Pipe Nebula is in an earlier phase of ice evolution than Taurus is. Alternatively the interstellar radiation field impinging on the Pipe nebula is larger than that on the Taurus molecular cloud, or the visual extinction through the Pipe nebula may have been overestimated. From the abstract:

Spectroscopic studies of ices in nearby star-forming regions indicate that ice mantles form on dust grains in two distinct steps, starting with polar ice formation (water rich) and switching to apolar ice (CO rich). We test how well the picture applies to more diffuse and quiescent clouds where the formation of the first layers of ice mantles can be witnessed. Medium-resolution near-infrared spectra are obtained toward background field stars behind the Pipe Nebula. The water ice absorption is positively detected at 3.0 micron in seven lines of sight out of 21 sources for which observed spectra are successfully reduced. The peak optical depth of the water ice is significantly lower than those in Taurus with the same visual extinction. The source with the highest water-ice optical depth shows CO ice absorption at 4.7 micron as well. The fractional abundance of CO ice with respect to water ice is 16+7-6 %, and about half as much as the values typically seen in low-mass star-forming regions. A small fractional abundance of CO ice is consistent with some of the existing simulations. Observations of CO2 ice in the early diffuse phase of a cloud play a decisive role in understanding the switching mechanism between polar and apolar ice formation.


T Tauri and Hind’s variable nebula

Posted in astronomy with tags , on December 31, 2016 by Tim Kendall

hindsvariable_goldmanT Tauri stars exist often in association with OB stars, whose short lifetimes mean the lower-mass T Tauri stars must also be young. The history of how this came to be known is recounted in a 2008 paper by Scott J. Kenyon et al., here, and an excerpt is below. Image: Optical image of T Tauri and surroundings (courtesy D. Goldman, APOD). T Tau is the bright yellow star near the centre. Barnard’s nebula is visible as faint nebulosity immediately surrounding T Tau. Hind’s nebula is the bright, arc-shaped cloud that covers some of the lower-right pair of diffraction spikes from the T Tau image. Fainter nebulosity, mostly ionized gas powered by a weak ultraviolet radiation field, covers the rest of the image. Burnham (1894) and Barnard (1895) discuss the relationship between Burnham’s nebula and the more distant Hind’s and Struve’s nebulae.

In October 1852, J. R. Hind ‘noticed a very small nebulous looking object’ roughly 18′′ west of a tenth magnitude star in Taurus. Over the next 15 years, the nebula slowly faded in brightness and in 1868 vanished completely from the view of the largest telescopes. O. Struve then found a new, smaller and fainter, nebulosity roughly 4′ west of Hind’s nebula. While trying to recover these nebulae, Burnham (1890, 1894) discovered a small elliptical nebula surrounding T Tau (above image). In the 1940’s, A. Joy compiled the first lists of ‘T Tauri stars,’ irregular variable stars associated with dark or bright nebulosity, with F5-G5 spectral types and low luminosity (Joy 1945, 1949; Herbig 1962). Intense searches for other T Tauri stars revealed many stars associated with dark clouds and bright nebulae, including a class with A- type spectra (e.g. Herbig 1950a, 1960). Most of these stars were in loose groups, the T associations, or in dense clusters, the O associations (e.g., Herbig 1950b, 1957; Kholopov 1958; Dolidze & Arakelyan 1959). Because O stars have short lifetimes, both types of associations have to be composed of young stars, with ages of 10 Myr or less (Ambartsumian 1957). This realization – now 50 years old – initiated the study of star formation in dark [molecular] clouds.