How can we tell that there is an underabundance of OB stars in the Trapezium?

[Ann]

There has been a lot of talk about the putative intermediate mass black hole (IMBH) in the Trapezium Cluster in the Orion Nebula. The major reason to believe in such an IMBH is the stellar dynamics in the Trapezium. But another reason, if I understand it correctly, is the underabundance of OB stars in the Trapezium and in the Orion Nebula.

Ladislav Šubr, Pavel Kroupa, and Holger Baumgardt wrote:

We investigate the dynamical evolution of the Orion Nebula Cluster (ONC) by means of direct N-body integrations. A large fraction of residual gas was probably expelled when the ONC formed, so we assume that the ONC was much more compact when it formed compared with its current size, in agreement with the embedded cluster radius-mass relation from Marks & Kroupa. Hence, we assume that few-body relaxation played an important role during the initial phase of evolution of the ONC. In particular, three-body interactions among OB stars likely led to their ejection from the cluster and, at the same time, to the formation of a massive object via "runaway" physical stellar collisions. The resulting depletion of the high-mass end of the stellar mass function in the cluster is one of the important points where our models fit the observational data.

So the way I understand it, it is generally accepted in the astronomical community that there is a shortage of massive stars of spectral classes O and B in the Orion Nebula and the Trapezium. My question is how they can know that. How can you look at a young cluster and say that there are "too few" OB stars in it?

For that matter, how can you measure the parameters of a young cluster very carefully and say that there are too few OB stars in it?

I know that there is something called the initial mass function (IMF) of a cluster. I know that the IMF says something about how many OB stars you can expect in different kinds of young clusters, and I also know, of course, that high-mass stars are vastly more rare than low-mass stars. But I have never managed to get a real grip of the IMF, because it is way too mathematical for me. I know it says something about how many OB stars you can expect to find in a young cluster whose mass is 100 M_☉, or 1000 M_☉, or 10,000 M_☉. But I have no idea just how many OB stars we should expect to find in a young cluster whose mass is, let's say, 1000 M_☉.

Can you help me here? How many OB stars should there be in a young cluster of 1000 M_☉? How many OB stars should there have been in the Orion Nebula, if the high-mass end of stars there had not been depleted?

Ann

[BDanielMayfield]

Yes Ann, there has been about this. (Sorry if my use of that smilie offended, not my intention.) You have written very intelligently on this question, explaining your reasons for doubting the IBH in the Orion Nebula theory very well I think. I'd like to learn more about the IMF and its predictive merit also, so I hope someone provides more info as you've asked.

Opinions on this question seem to be rather strong. Why so? This is just another mystery to be unraveled by collecting enough data which will show which model is correct.

Personally back when this story was first reported by S&T I was very skeptical too. How could such a massive BH be hiding in that "tiny" space between the Trapezium that I had looked at with my own eyes through my own little telescope? It just didn't seem logical. It would have to have an accretion disk and jets, I thought. But then I learned more about why this wasn't the case. There really wasn't any evidence disproving this theory.

So I thought more about it in terms of galactic and even universal habitability. Which would be better for the planets in the regions around star forming places like Orion, having tens more O and B stars, all of which will go supernova someday, or one IBH that will only mess with things that get really close?

This IBH idea may be growning on astronomers also because their production is needed to help explain SMBHs in galactic cores. The most efficient way for them to grow would be to scarf down lots of IBHs.

So Ann is opposed, while I'm in favor. Personal biases drive opinions. Ann likes blue stars. I like habitability. Data doesn't care.

Bruce

[Ann]

Thanks, Bruce! And your smiley certainly didn't offend me.

But I'm disappointed that no one seems to know anything about the Initial Mass Function, since that might help us understand if there really should be more OB stars in the Trapezium, and if the formation of an intermediate black hole could explain the putative shortage of them. And the supposed shortage of OB stars in the Trapezium is an important factor in the argument that there is an intermediate mass black hole in the Trapezium.

Ann

[Chris Peterson]

So Ann is opposed, while I'm in favor. Personal biases drive opinions. Ann likes blue stars. I like habitability. Data doesn't care.

My bias leans towards avoiding bias. I have no strong opinion one way or the other. There are good arguments against the hypothesis, there are good ones for it. And a shortage of data. What is important is that the hypothesis itself is sound, and we recognize a few observational tests that can support or reject it. That's what science is all about.

I'm satisfied to consider it an interesting subject and wait for more evidence to accumulate. I doubt it will be too long.