Infrared emission from HII regions
Gareth Wynn-Williams, University of Hawaii
Here are two pieces of recent work which indicate how one can use a combination of
infrared and radio observations to determine what is happening inside star-forming region
that are totally obscured at visible wavelengths.
A 1.2 - 2.2 µm study of the HII region W51
Graduate
student Jeffrey Goldader and I used the NICMOS camera on the University of Hawaii 2.2
meter telescope to map the 1.2 - 2.2 µm infrared emission from the the luminous H II
regions W51 IRS1 and IRS2, which lies about 7.5 kpc from the Sun. The appearances of the H
II regions at 2.2 µm correspond closely to their appearances at radio wavelengths.
Comparing a narrow band Brackett- gamma filter image to the broadband 2.2 µm image
reveals that the extended emission visible throughout our images is consistent with the
thermal emission expected from the ionized hydrogen and helium in the HII regions. By
comparing the surface brightnesses of this diffuse emission at radio and near-infrared
wavelengths, we derive an extinction to the HII regions of Av= 24 ± 3 mag,
with localized areas of higher obscuration. More than 300 stars are visible in our images.
Almost all the stars seen at 1.2 µm have the blue colors of slightly reddened foreground
stars; they are distributed uniformly across our images. Red stars visible only at 1.6 µm
and 2.2 µm are concentrated atop the HII regions. Their colors are redder than can be
accounted for by simple foreground extinction, suggesting the presence of emission from
hot dust close to the photospheres of these young stars. We believe we have identified the
mid-infrared peak of IRS 2 in our images; broadband photometry and narrowband
spectrophotometry indicate it may be a highly embedded massive star or stars. We have
discovered a bright, red object, which we call IRS3, that also has the spectral
characteristics of a dust-embedded massive star.
The picture show a 90 arcsec (~ 3 pc) region around the center of W51 with the 1.2 µm,
1.6 µm, and 2.2 µm, data represented as blue, green and red respectively
For more information see:
Goldader, J. D. & Wynn-Williams, C. G. Astrophys. J. 433:164-178. (1994)
A submillimeter-wave study of the HII region W3
My colleagues and I used the James Clerk Maxwell telescope to map the 450-800 µm
emission from the W3(Main) star forming region. We found three major peaks. which we call
SMS1, SMS2, SMS3.. One of the submillimeter sources is identified with W3-IRS5, a
well-known candidate protostar. However, to our surprise, we found that none of the
submillimeter peaks coincides with any of the prominent compact HII regions in the area,
as seen by the 6-cm radio map. We estimate that the three submillimeter sources together
contribute 35-50% of the total bolometric luminosity of the region and speculate that the
contribution of luminous radio-quiet sources to the total luminosity of HII
region/molecular cloud complexes may be larger than is often assumed.
For more information see:
Ladd, E. F., Deane, J. R., Sanders, D. B., & Wynn-Williams, C. G. Astrophys. J.
419:186-189.(1993)
Gareth Wynn-Williams (wynnwill@ifa.hawaii.edu)