Starship Asterisk*

Just to be complete - I mentioned before that the Lynds and O'Neil distance estimate of 1680 parsecs = 5500 light years is based on an estimate of the distance to HD 164514. This is the supergiant that illuminates the blue reflection nebula north of the main Trifid nebula.

This is also not quite correct. Although the Spitzer paper does indeed cite this paper on the Trifid reflection nebula, the 1680 parsecs estimate is actually derived in a 1985 Lynds paper here:

http://adsabs.harvard.edu/abs/1985ApJ...288..164L

which also examines HD 164492, the central star system of NGC 6514.

The Lynds paper looks only at the A component of HD 164492, and derives a slightly different E(B-V) figure of 0.255. The Kohoutek, et.al. paper determines this to be 0.32. The main issue, however, is that Kohoutek, et.al. found quite a bit more dust around the other components of HD 164492 and so derives a larger distance estimate.

The Lynds paper explicitly points out that the region around HD 164492 is dustier than is typical for interstellar calculations and takes that into account to a certain extent. The Kohoutek, et.al. paper mentions the Lynds conclusion and says that their distance estimate would be slightly lower (2500 parsecs) if this was taken into account.

So this does not change my main conclusion - the Kharchenko et.al. results show that the rest of NGC 6514 is much less dusty than the region around the core star system HD 164492, and so finds that NGC 6514 is much closer than originally estimated by the previous studies.

My apology for this rather long winded analysis, but the bottom line seems to be that the Trifid nebula has a very dusty central star system and is much closer (816 parsecs = 2600 light years) than anyone had previously realised.

I've followed up on my question on the distance to the Trifid nebula and looked at the differences between the Kohoutek et..al. and Kharchenko et.al. estimates to the distance of NGC 6514, the Trifid star cluster.

First, I was incorrect about the Kohoutek et..al. study - it is not based on full cluster photometry but on an analysis of the distance to seven different components of the O6 class multiple star HD 164492. This is the star system at the heart of NGC 6514, but obviously is not the entire cluster.

I'm not an expert on this, but it appears that the key difference between the Kohoutek et.al. and Kharchenko et.al. distance estimates is their results on interstellar reddening. The Kohoutek et.al. study found E(B-V) to range between 0.32 and 0.44 depending upon the component of HD 164492.

On the other hand, the Kharchenko et.al. results, which looked at NGC 6514 as a whole, found E(B-V) to be 0.19.

In other words, Kohoutek et.al. looking at the central star system of NGC 6514, found a lot more interstellar dust than Kharchenko et.al., which looked at the cluster as a whole.

Now, as I said, I am not an expert on this, but it seems to me that one possible way to reconcile the two papers is to conclude that the distance to NGC 6514 (and hence the Trifid nebula) is indeed 816 pc = 2600 light years, but that the central star system is surrounded by a lot of local dust which makes it look much further away than it actually is.

The Spitzer paper mostly discusses the nebula itself, but there is a paragraph on the region to the north (which is the area you mention - as you say, the APOD has rotated the original image):

Just north of the Trifid Nebula is a region containing a surprising number of protostars (R.A. 18h02m42.28s and Dec. −22◦48′44.8′′, J2000). Both the diffuse mid-infrared emission and the distribution of young stellar objects in the 8μm and 24μm maps suggest that this is a small H II region with active star formation ... The ionization rate is consistent with a young B0/O9.5 star or an evolved B0.5 (giant or supergiant). Hence, we suggest a massive star is hidden in the area, which implies that the region north of the Trifid Nebula proper is a separate H II region, which we named “Trifid Junior.”

Yes, this is an amazing image that seems to reveal quite a few new objects.

I'm curious about the distance estimate that was used in the caption. The caption says that Trifid nebula "lies only 5,500 light-years away". This seems to be (quite reasonably) taken from the 2006 Spitzer paper that released the image.

When I read the Spitzer paper, I saw that the estimate (1600 parsecs) was taken from the Lynds and O'Neil paper on the Trifid, which was published in 1986, more than 20 years ago. More recent estimates are available

Presumably the best estimates would be based on photometry for NGC 6514, the star cluster associated with the Trifid. The Lynds and O'Neil estimate was not based on cluster photometry, but rather on a distance estimate to a single star HD 164514, which could never be as reliable.

A photometry study of NGC 6514 done by Kohoutek, Mayer and Lorenz in 1999 found a distance of 2500 to 2800 parsecs (8000 to 9000 light years).

On the other hand, the latest version of the Diaz, et.al. star cluster catalog, here:

http://www.astro.iag.usp.br/~wilton/

gives the distance to NGC 6514 as half the Lynds distance, or 816 parsecs.

This would be 2600 light years.

This figure is taken from WEBDA and is the same as the distance estimate in the 2005 Kharchenko cluster catalog:

http://vizier.u-strasbg.fr/viz-bin/Cat?J/A%2bA/438/1163

The distance estimates from this cluster catalog are also based on cluster photometry.

It's a bit surprising that astronomers can't seem to nail down the distance to the Trifid a bit more accurately and that there are widely varying estimates that seem to be based on observations of the same star cluster. In any case, I do think that there are more recent estimates for the Trifid based on real cluster photometric data and those are more likely to be reliable than the one from the Lynds and O'Neil paper.

http://antwrp.gsfc.nasa.gov/apod/ap070707.html

Is that a bubble or a bow shock wave that I can see? :

[img]http://www.andywade.co.uk/space/trifid.jpg[/img]

You can see it better in the larger picture, which is rotated from the APOD version:

http://antwrp.gsfc.nasa.gov/apod/image/0707/trifid_spitzer_f.jpg