Archive for molecular clouds

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.