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16 January 2004

The January Sky

by Paul Curtin

Gemini, Canis Major and Orion are the most striking figures in the Winter Milky Way for Northern observers. Faint Monoceros and Puppis contain a number of interesting deep sky objects. The Southern Hemisphere features Orion, Carina, Southern Puppis and a fine view of the Large Magellanic Cloud.

Orion, apart from being one of the most distinct constellations, also boasts one of the brightest and most interesting gas nebulae-M42, the Orion Nebula. Anything from binoculars to huge telescopes can be used to explore M42. Begin by scanning the area of Orion's sword. Try drawing the details of the nebula-the dark lanes and fine features of the luminous gas become more apparent upon repeated observation. In addition to the nebulosity observers should note the multiple star system Theta-one Orionis. These stars form the Trapezium situated at the very heart of the nebula. Use the highest power possible to draw these and neighboring stars.

The planets this month

Mercury can be found in the Eastern sky just before sunrise after January 17. Mars, in Pisces can be seen for a large part of the night although surface features can only be resolved with large telescopes. Jupiter is in Leo rising just after dark and is a fine object throughout the night. Saturn, which was at opposition December 31, is still very bright and should be watched carefully while the disk and rings appear their largest. Now is the best time to look for details in the atmosphere and divisions in the rings.

Herbig-Haro Objects: Mystery formations in protostellar neighborhoods.

Orion is surely one of the most popular destinations for amateur astronomers. M42 and environs hold the promise of rich rewards to both casual observers and serious theoreticians. It is within the roiling layers of these dark, reflective and emission nebulae that the secrets of star birth and evolution are hidden. One feature of such regions, associated with very young stars, is the Herbig-Haro object. These objects were described independently in the 1950's by George Herbig and Guillamaro Haro, both of whom were investigating the neighborhoods of T-Tauri stars.

Herbig-Haro objects, or HH objects, appear as non-homogeneous gas clouds at the leading edge of bipolar jets streaming away from extremely young stars. They have a V or U shape characteristic of bow shock. Such structures in molecular clouds known for star formation activity that show strongly in [SII], H₂ and [Fe] are catalogued as HH objects. O'Dell et al., using the Hubble Space Telescope, identified two regions within the Orion Nebula that meet these criteria. See Reipurth's online catalogue for the most complete, current list of known HH objects.

Although we have been able to identify these structures for nearly half a century the physical nature of HH objects eluded researchers until quite recently. Proposals ranged from treating HH objects as newly 'ignited'* stars, H₂O masers ('interstellar bullets'*), stellar winds racing through nebulae and the jet-induced shock model in which young stars send out material that collides with the interstellar medium. This last theory was long considered the most credible but, although it successfully accounted for the observed emissions, it also predicted that HH objects should emit x-rays-one of the very few wavelengths not associated with them.

Assuming that previous null results were attributable to technological limitations, Pravdo et al. observed one of the brightest HH objects (HH2, located just south of the Trapezium) employing the advanced CCD device on the Chandra x-ray observatory. In addition to detecting numerous known and hitherto unknown x-ray sources in the area, they detected x-ray emissions from the HH object as well. The source appeared to be too large and too cool to be a stellar object. This indicates that HH objects are formed when gas, ejected from young stars in the form of high velocity jets, collides with interstellar material. Further work remains to more accurately describe the dynamics of the bow shock and to check the other catalogued objects for x-ray emissions.

Bibliography

Burnham, Robert Jr. Burnham's Celestial Handbook. New York: Dover, 1978.

Cohen, M. Are We Beginning to Understand T-Tauri Stars? Sky and Telescope, October 1981.

Gupta, R. (ed.) Observers Handbook 2004. Toronto: University of Toronto Press, 2003.

Kaler, J. B. Extreme Stars. Cambridge: Cambridge University Press, 2001.

Kepple, G. R., Sanner, G. W. The Night Sky Observers Guide. Richmond, VA: Willman-Bell, 1999.

Pravado, S. H. et al. Discovery of X-rays from the protostellar outflow object HH2. Nature vol.413, 707-711 (Oct. 2001).

Reipurth, Bo. A General Catalogue of Herbig-Haro Objects. Electronic Version, 1994-1. (http://casa.colorado.edu/hhcat)

Webb, T. W. Celestial Objects for Common Telescopes. New York: Dover, 1962.

Wu, J., et al. A study of the energy sources of Herbig-Haro objects. Chin. J. Astron. Astrophysics. Vol. 2 (2002), No. 1, 33-42.