Re: APOD 2006 May 25 - NGC 1579 - Trifid of the North

[glen youman]

The caption associated with the subject APOD stated that the NGC 1579 is an emission/reflection nebula. I believe that is not correct and is doing a dis-service to amateur astronomers.

I've researched all web available scientific papers on this object and cannot find a single paper that supports the assertion that NGC 1579 is anything other than a reflection nebula. Every scientific paper maintains that NGC 1579 is a cold cloud of dust reflecting the light from the massive star "LKh-alpha 101". LKh-alpha 101 is an Ae/Be star with very strong spectral line emissions in the h-alpha wavelength. NGC 1579 does not contain any significant HII regions. LKh-alpha 101 is not part of the nebula and is encased in a partial cloud of obscuring dust. The only HII region is a small region surrounding LKh-alpha 101. This HII region was confirmed by the 2Mass survey.


The seminal paper is the polarization study conducted by Redman et.al (Cal Tech/JPL). See also Herbig et.al. which notes an absence of OIII emissions among other findings.

Below is the abstract from the polarization study:

"The linear polarization of the reflection nebula NGC 1579 and the CO (1 to 0) emission from the associated molecular gas have been mapped for several minutes of arc around the exciting star LkH-alpha 101. These maps show conclusively that LkH-alpha 101 is the sole significant source of illumination in the region. The dust in the reflection nebula appears to be uniform over the illuminated region and is uniformly illuminated by LkH-alpha 101. Despite the patchy obscuration, the dark cloud which obscures LkH-alpha 101 does not surround the star. LkH-alpha 101 may have formed out of a placental cloud whose remnants now include four molecular cloud fragments, two in front of and two behind the reflection nebula, as well as an H I cloud previously detected in the region. "

It is interesting that most American astrophotographers list NGC 1579 as an emission nebula while almost all Japanese, Finnish, Italian and French astrophotographers list it as a reflection nebula in keeping with the scientific findings.

The subject APOD is now an authority, albeit incorrect, on this object. I believe that APOD should revisit this subject and correct any misconceptions.