“Of course, this identification is simply wrong.” That one remark caused a subdued but audible commotion in the audience, and a handful of shocked responses to be posted on twitter. The remark in question had been made by Jacek Krełowski, one of the most distinguished researchers in the particular field being discussed in the talk which had just finished. Hearing such a direct challenge to someone’s work at an international conference is, in my experience, rare enough. When the challenger is reknowned enough to have some scientific phenomenon named after them, it’s virtually unheard of. I’ll admit, it’s not like I hadn’t thought along similar lines during that particular talk (and indeed since shortly after hearing about this piece of research), but as Krełowski went on to explain the reasons behind his challenge I suddenly felt quite sorry for the junior researcher still standing at the front of the lecture room.
In some fields, one quickly learns to be careful what to say and what not to say, and the study of the diffuse interstellar bands is very definitely one of those fields. And with good reason. A scientific puzzle spanning over 100 years is most certainly not something to be taken lightly. Over 400 cryptic absorption lines originating in the icy depths of interstellar space, with not a single accepted identification to date of anything that might be causing them. Discovered originally late in the 19th century and realised to be interstellar in 1934, entire careers and, indeed, entire lives have been devoted to the underlying cause of the diffuse interstellar bands. A set of spectroscopic absorption features blocking out specific frequencies of light in the optical and near-infrared parts of the electromagnetic spectrum, the cause of which is expected by many to be due to any number of interstellar molecules. So elusive are the specific molecules, however, that I’ve heard it said there’s no quicker way for a scientist to ruin their reputation than to assert an identification of one of the diffuse interstellar bands.
Nonetheless, that’s precisely what a group of researchers, let by John Maier, did last year. Maier’s group put out a paper, boldly entitled “Identification of H2CCC as a Diffuse Interstellar Band Carrier”. This act triggered quite a response from a number of leading interstellar medium researchers. This paper, by Liszt et al, would seem to be the final nail in the coffin of this particular idea.
There are lots of problems with H2CCC, also known as l-H2C3, with its mickey mouse ears, being a diffuse band carrier, and the biggest one is that there simply isn’t enough of it. There would need to be three times as much l-H2C3 as this paper finds in order to qualify this molecule as truly being a diffuse band carrier. As nice as it would be to finally have a confirmed detection of a diffuse band carrier, this almost certainly isn’t one. Interestingly though, these authors note that Krełowski actually proposed a different molecule as a diffuse band carrier – H-C4H-H+ – in 2010. A suggestion which received criticism by Maier. It seems the backstory here goes a bit farther back than I’d realised.
Aside from definitively condemning these particular small molecules as diffuse band carriers, Liszt et al find a few other interesting snippets in this paper, regarding interstellar molecules, such as the overwhelming abundance of C2H, and the disappointingly low abundance of interstellar C4H–. It’s disappointing for me, anyway, given my penchant for interstellar anions. But I should stop before I digress.
As the door closes with finality on this particular hypothesis, it seems the most interesting thing to note in this case isn’t necessarily the science itself, but the method behind the science. The most skeptical person in any scientific research should always be the one conducting the research. If you can pick holes in your own work but choose not to, someone else will undoubtedly do so for you. It’s safe to say that it’s best not to let it come to that. Personally, I like to spend hours metaphorically hitting my own work with a hammer and seeing if it survives, before allowing myself to make any assertion. And speaking of assertions, we should all be wary of making brash claims – something I’ve seen on several occasions, and not once did that move end well!
Image from Rauw et al (2004)
Harvey Liszt, Paule Sonnentrucker, Martin Cordiner, & Maryvonne Gerin (2012). The abundance of C3H2 and other small hydrocarbons in the diffuse interstellar medium ApJL (Accepted) arXiv: 1206.0342v1