Amongst all the excitement over the first results from Herschel, it’s easy to forget about its comparatively tiny American cousin Spitzer. Launched in 2003 with its 3 instruments IRAC, IRS and MIPS, Spitzer covers the infrared wavelengths from around 3 to 150 microns – a region that from Earth is either totally inaccessible or severely hampered by atmospheric absorption. With its 85-cm diameter primary mirror, it’s easy to dismiss Spitzer as belonging to a former era. But new science is coming out of Spitzer data every day, and vast quantities of data remain unpublished in the archives. The big legacy surveys in particular, such as c2d (Cores to Disks) and the galactic plane surveys GLIMPSE and MIPSGAL, have released a wealth of data into the public domain, throwing light on old problems and unveiling new mysteries to solve.
One interesting phenomenon witnessed on the images from the GLIMPSE survey was a curious population on extended green objects (EGOs). Catalogued by Cyganowski et al in 2008, these “green fuzzies” appear to be associated with regions of massive star formation – many of them lie in or very near to infrared dark clouds, known to harbour the earliest forms of massive star birth, or are associated with methanol masers, strong radio emission caused by excitation of methanol molecules by infrared radiation from dust. Their green colour is in a sense incidental, arising from the way we construct 3-colour images from the Spitzer camera IRAC. IRAC takes images in 4 channels, at 3.6, 4.5, 5.8 and 8 microns, and typically an red-green-blue image uses the 8, 4.5 and 3.6 micron data, respectively. In this picture, “green” indicates that the object has an unusually high flux in the 4.5 micron band.
This characteristic in a spatially extended object instantly raises a flag among star formation aficionados, as this band, stretching from 4 to 5 microns, contains some prominent spectral lines from molecular hydrogen (H2) and carbon monoxide (CO), commonly seen in emission in outflow regions. Young protostars that are growing by accreting material from their surrounding cloud often have strong streams fo outflowing material. Where the outflow collides with the surrounding interstellar medium, the resulting shocks give rise to this strong emission. So this fuzzy greenness could indicate the presence of a young massive star growing deep inside a dense molecular cloud, even though we can’t yet see the actual young star itself.
Even though relatively few massive stars are produced compared with “regular” low mass stars, and their lifetimes are much shorter, their immense output of energy, particularly in heavy elements that they alone can produce and eventually blast into the interstellar medium as supernovae, has galactic-scale influence. But their rapid evolution and long embedded formation stage makes them very elusive objects to study, and astronomers have to rely on indirect signposts of massive stellar birthplaces to get an insight into the process. The prospect of these green fuzzies as an additional telltale sign of young massive protostars is therefore well worth exploring. But IRAC’s broad band imaging alone can’t reveal the true nature of EGOs – for that we need spectroscopy.
This week, De Buizer & Vacca of NASA Ames Research Center posted a paper to astro-ph claiming the first spectroscopic identification of green fuzzies over the IRAC wavelength range of 4-5 microns, allowing them to examine directly the source of the emission in these objects, precisely at the wavelengths that they appear strong in. They used the mid-IR instrument NIRI on the 8-m ground-based Gemini North telescope to observe two of the objects from Cyganowski’s catalogue. And interestingly, the objects appeared to be quite different in nature, suggesting that they’re not a clear-cut signpost of anything.
The first fuzzy, shown on the left above, looks like some sort of object-with-outflow, and the spectrum does suggest that that is the case. The central source’s spectrum is consistent with a deeply embedded massive forming star, with all of its radiation below 3.5 microns or so being absorbed by a thick shell of dust. The green knotty areas surrounding it show only strong emission in molecular hydrogen lines with no underlying continuum emission, probably emanating from shocked gas in an outflow from the central young star.
The second fuzzy, however, has a very different shape – it looks a bit comet-shaped and there’s no clear axis along which you might expect to see an outflow. The spectrum is also very different: it doesn’t show any particular emission features in the IRAC 4.5 micron filter region, although its spectrum does suggest the presence of a deeply embedded young massive star at the brightest location. But there’s no evidence of an outflow. The authors suggest that in this case, the object is not particularly bright at 4.5 microns, it’s unusually faint in the red and blue channels (8 and 3.5 microns, respectively) – and this produces the same appearance. At 3.5 micron, the radiation is likely just being absorbed by dust, while the 8 micron flux is low from a nearby silicate absorption that is strong in embedded young stars.
This is not an groundbreakingly new result. Two objects don’t exactly make “a sample”, so we don’t learn anything conclusive about the nature of the Spitzer green fuzzies. And in a way, it’s not unexpected that there are several mechanisms responsible for the enhanced 4.5 micron flux: the galactic plane is a chaotic cauldron of gas and dust, and these objects we’re seeing in the Spitzer images are all at different distances and depths with different amounts and compositions of intervening material. But the spectra are nice, and it’s a good example of how we’re slowly chipping away at new questions coming out of groundbreaking facilities, even many years after their launch. Hundreds of these green fuzzies have been identified from Spitzer images, and other authors have defined differing methods of identifying them, and all are excellent follow-up fodder for current large ground-based telescopes and new observatories and instruments coming online soon. For a phenomenon as important as the birth formation of massive stars, it’s worth exploring anything that can give us a glimpse into the heart of the formation process.
James M. De Buizer, & William D. Vacca (2010). Direct Spectroscopic Identification of the Origin of ‘Green Fuzzy’ Emission in Star Forming Regions accepted in ApJ arXiv: 1005.2209v1
C. J. Cyganowski, B. A. Whitney, E. Holden, E. Braden, C. L. Brogan, E. Churchwell, R. Indebetouw, D. F. Watson, B. L. Babler, R. Benjamin, M. Gomez, M. R. Meade, M. S. Povich, T. P. Robitaille, & C. Watson (2008). A Catalog of Extended Green Objects (EGOs) in the GLIMPSE Survey: A new
sample of massive young stellar object outflow candidates Astronomical Journal, 136 (6), 2391-2412 arXiv: 0810.0530v1