Livestream of Solar Eclipse 2017

As many of you know, there is going to be a total Solar Eclipse today, Aug 21, 2017. Here in Massachusetts we will not experience totality, but because I am running some outreach for the students at Merrimack College, I will be livestreaming through the eyepiece of our solar telescope. Hopefully the stream will be up from 1:30 pm to around 4:00 pm EST. The Youtube link is here:

https://gaming.youtube.com/user/cdustonable/live

To get this image, I am using a solar telescope with an H-alpha filter, which allows us to view things like solar prominances and other structures in the corona. I am viewing directly with my cell phone and a camera app called “Camera VF-5”.

Happy viewing, and also check out the NASA livestream, at https://eclipse2017.nasa.gov/eclipse-live-stream. Of course, they have a bunch more interesting things going on, but we are all at the mercy of the weather!

Primordial Black Holes as Dark Matter?

I was recently asked by a family friend “have you heard about this new idea that primordial black holes could explain dark matter?”

Well I hadn’t, so I did a little investigating and it’s a pretty clever idea. Part of the backstory here is “what can we do with gravitational waves?”, so that’s where I’ll start.

One of the surprising things about the very first direct observations of gravitational waves by LIGO is the masses of the constituent black holes. The first pair was 36 and 29 solar masses, the second was 14 and 8, and the third was 31 and 19. What was immediately understood to be important about these sources is that they are generally more massive than the other stellar-mass black holes we’re found previously (from X-ray studies, usually. Max there is 18 solar masses). Significantly, the larger mass ones should also be *less* likely, from stellar formation scenarios. So while we are only talking about 6 new black holes, we clearly need to know if that will pose a problem for stellar formation models. (there are also some issues in regard to the spins of these black holes, but I won’t go down that particular rabbit hole).

So people started looking at it, and found that it was generally possible to get these kinds of higher-mass black holes, but it did put some constraints on the formation scenarios. Basically, the problem is you need to make giant stars, which generally need to have low metallicity to form. However, the conditions that generate those stars (high star formation rate in the past) generally turn out to produce higher overall metallicity quicker. If you tune the star formation rate a bit so there are actually fewer large-mass stars, you reduce the overall metalicity so you can effectively create massive black holes. So it’s constraining, but not overly so.

But that’s actually not what I want to talk about – what about other formation scenarios for these black holes? Specifically, what about primordial black holes (PBH)? These are black holes that formed as a result of density fluctuations in the early universe. It turns out it’s pretty easy to produce black holes of this mass in this manner (and the spin, which I skipped talking about above, is a little easier to produce as well). So, cool, we have at least two different ways the universe can give us the black holes found by LIGO.

But, are there any other implications of primordial mass black hole production at this rate? Well, without a stellar companion, there would typically not be an accretion disk and we would have no way to observe these black holes. But of course – that’s exactly the condition we need for dark matter!

So, in a recent paper, Juan Garcia-Bellido and his collaborators (who include Sebastien Clesse, Andre Linde, and David Wands) have worked this out in a bit of detail (and apparently there are others working on this as well, such as Alexander Kashlinsky).

The idea that black holes (or other compact objects) could be a model for dark matter is not new, actually. We’ve been looking for microlensing due to compact objects in the galactic halo for years (these objects are called MACHOS), but have essentially found nothing. What’s interesting about their new models is the mass distribution for primordial black holes in the 10-100 range sits right in the region of parameter space which was has not been covered by previous studies:

As you can see in the figure (which comes from the paper), the lower limits on PBH have a gap in between the lower mass MACHO/EROS observations and the higher mass WMAP3/FIRAS observations. It looks to me like that gap peaks around 0.01 of a solar mass and carries up to around 100. Which is broad range for black holes, but look at the range which we are talking about here (25 orders of magnitude!).

So there are lots of other interested details here, but what’s really fascinating about this new paper is that there are apparently a very large set of phenomenological signals we can use to test this hypothesis. It would affect the CMB, star formation in the early universe, X-ray transients, and a whole host of others. One particularly interesting idea is that rather then looking for lensing, we might try to look for the shift of the positions of stars over time. With the new plethora of data on stellar positions (like the GAIA satellite), it also might be the first time someone could actually attempt such a study. So there are a lot of interesting things to check.

As a sidenote, some of these black holes would of course develop an accretion disk through random interactions with stars or gas, and produce point sources that would emit in Gamma or X-ray range. Well, there actually is a large list of unidentified point sources in nearly all the X-ray catalogs. In fact, my undergraduate honors thesis was working on trying to identify unknown point sources in a Chandra X-ray image of the galactic center. The paper suggests that rather than looking at spectral characteristics, one should look for a correlation between the point sources and the expected dark matter distribution.

So, we’ve got LIGO finding a new class of black holes, which could be created in the early universe, and a new model for dark matter. Given how much trouble the particle model for dark matter is having (sorry LHC!), we should be taking these new ideas seriously. And what’s great about this is there are *bunch* of great ways to look for this primordial black hole signal. Of course, maybe that means it won’t last long as an explanation for dark matter, but it’s something new to look at that doesn’t require any exotic new physics.

And, not to belabor the point, but all of this wouldn’t have been possible with LIGO. Thanks LIGO!

 

Merging Stars, Star Formation, and Planetary Nebulae

I was reading the latest issue of Sky and Telescope this week and came across an article by Monica Young talking about the formation of massive stars (here a link to the highlights, you’ll need an account to actually read it). The gist of the article is that forming massive stars is difficult – as mass accumulates and nuclear reactions begin, the radiation pressure from the young (not yet massive) star will tend to blow material away, halting the growth. This happens around 10 solar masses, so it’s a bit mysterious how we end up with more massive stars then that (and we do – although they are rare, Type O stars are over 15 solar masses, and the most massive stars are over 25). The article covers a few modern approaches, mostly which involve particular dynamics by which material is accumulated in a different physical location then the photon flux from the new star. But, it was also mentioned that some massive stars are simply caused by merging younger stars, which was the topic of my master’s thesis! Since I’ve never written about it here (and it’s only been published at the academic library), I thought I would give a quick overview on the cute idea and nice results we worked out (“we” being myself and my adviser at the time, Robin Ciardullo).

The problem we were tackling had to do with the Planetary Nebula Luminosity Function (PNLF – there is even a Wikipedia page about this now!). As medium-sized and smaller (under 10 solar masses or so) stars reach the end of their life, they turn into really pretty objects called Planetary Nebula (PNe, and here are some cool Hubble pics). Massive stars a) evolve faster and b) make brighter PNe then their less massive siblings, so over time less and less bright PNe should be produced by any given population of stars. Further, the luminosity from a PNe is primarily due to excitation from the central white dwarf, which also dims over time. Therefore, PNe in a single population of stars should be generally getting less luminous over time. Problem is, that is not observed, at all!

The Brightest PNes in a population are the same luminosity, regardless of the age of the population.

The Brightest PNes in a population are the same luminosity, regardless of the age of the population.

The figure above comes from Ciardullo (2006), and demonstrates the problem – all the brightest PNe have the same absolute magnitude, regardless of the age of the stellar population (which goes old to young from top to bottom). This allows you to use PNe as a secondary method to find astronomical distances, but it also shows that there is something fundamentally incorrect with the nice picture of stellar evolution I’ve presented above. The idea explored in my thesis was that as the population aged, stellar mergers produced a ready supply of massive blue stars (called “Blue Stragglers”) which would form the brightest PNe. The advantage of a model like this is that it does not require a significant amount of detailed physics, such as the effects of stellar rotation, wind, or other micro-astrophysics. It is simply a population synthesis approach – we essentially created stellar populations, used standard stellar evolutionary models, but included a small fraction of stars (around 10%) which merged to form more massive stars.

First, let’s take a look at the “standard picture”, with no Blue Stragglers:

Simulated PNLFs with no merging stars.

Simulated PNLFs with no merging stars.

The ages of the stellar populations are shown in the upper lefthand corner (1-10 Gyr). It clearly displays the effect I talked about – the brightest PNe fade over time as the population ages.

Now let’s take a look at our basic model, including 10% blue stragglers into a population of several different ages:

The PNLF single burst models with 10% blue straggler fraction.

The PNLF single burst models with 10% blue straggler fraction.

As we expected, the brightest PNe held pretty constant for a variety of stellar population ages (1-10 Gyr, shown in the upper corner, with the 1 Gyr being a bit of an outlier). The absolute magnitude ended up being a little high, and the initial shape was more shallow then the observations, but it was clear that the blue stragglers were able to keep the maximum luminosity of the PNLF relatively constant over a wide range in population ages.

It’s worth noting that the two populations of blue stragglers which we are discussing here are actually disjoint. Since PNe form from stars under 10 solar masses, the usual formation scenarios have no trouble making them. It’s only for the stars over 10 solar masses that the merging scenario is invoked for a creation mechanism. On the other hand, both of these merger scenarios are based on stars which form in binary systems, and then merge at a later time. So although the end masses are different the formation mechanism from a blue straggler point of view is the same. It would be interesting to see if one could reproduce the required blue straggler fraction by using the initial binary population. Using both the PNLF and mass star formation considerations, one might be able to check this over the entire mass range of the initial mass function of binaries. Not something I can see spending time on at the moment, but an interesting question which even might make a nice undergraduate project!

If you are interesting in reading the whole thesis, you can check it out here. What I’ve talked about above the only half the story – there is also the “dip” found in some PNLFs (but not M31, for instance), which the model tried to address as well.