The Peculiar Balmer Decrement of SN 2009ip: Constraints on Circumstellar Geometry

 [ arXiv ]

Emily M. Levesque, Guy S. Stringfellow, Adam G. Ginsburg, John Bally, Brian A. Keeney

---

SN2009ip is a particularly interesting "supernova" since it left a bright remnant behind, indicating that it was instead a supernova impostor.  The 2012B event, a brightening in late September 2012, looked like it might be a "genuine supernova", but maybe not.  Have a look at the other papers on this object: http://arxiv.org/abs/1210.3568, http://arxiv.org/abs/1210.3347, http://arxiv.org/abs/1209.6320.

A few folks at CU, primarily Guy, had acquired some spectra of SN2009ip right around the 2012B event.  We had a quick look at the data and decided there was something really interesting in it that other groups (who probably had more complete data) had overlooked.

The most interesting single point we noted was a peculiar "Balmer Decrement" (which is the ratio of H-alpha to H-beta).  The normal Balmer decrement is ~3 (2.87 at n~10³ and 10000K), but we observed a decrement of ~1.4 (and so did others, see e.g. http://users.northnet.com.au/~bohlsen/Nova/sn2009ip.htm).

This Balmer decrement is weird, because all normal effects will increase rather than decrease the Balmer decrement.

• Interstellar reddening - affects the blue more than the red, therefore should decrease H-beta relative to H-alpha.  Of course, the reddening towards SN2009ip has been measured to be quite low.
• Line Splitting due to optical depth in the H-beta line (photons become "trapped" in the n=4 state and "escape" via Paschen-alpha and H-alpha) - but this only serves to decrease H-beta and increase H-alpha.  This is known as "Case C recombination" (see Xu et al 1992, Figure 1)

So what can increase the Balmer decrement?  Two possibilities: 

1. Hydrogen reaches local densities above 10¹³ cm^-3: above these densities, it reaches collisional equilibrium with the gas and adopts the gas temperature.  At 10000K, H-beta will be a few times brighter than H-alpha [to-do: put exactly how much brighter...]
2. H-alpha becomes optically thick, while H-beta remains optically thin.  This is essentially a geometric argument, and is explored a little in the text.  If H-alpha becomes optically thick, more hydrogen won't increase the H-alpha brightness, but if H-beta remains optically thin, more hydrogen will increase its brightness.  Simple argument, and it hasn't been fully explored yet (does the radiative transfer work, or does the "Case-C" situation kick in too hard first?), but it is a plausible alternative.

If you want to see my calculations in action, check out the ipython notebook performing the calculations.

In case you're interested in Case C recombination, here's a first step: a hydrogen level diagram with levels connected by the (sum of the) Einstein A values between the relevant levels (from http://physics.nist.gov/cgi-bin/ASD/lines1.pl, generated with https://github.com/keflavich/energyleveldiagrams).

Short timescale variability in Orion? Nope.

I attempted to use TripleSpec finder / guider images to search for variability  on short (~5s) timescales in the Orion Nebula region.  This project was an offshoot of the related http://adamginsburg.blogspot.com/2012/10/using-guider-images-to-achieve.html.  I successfully matched the images using a particular catalog & the ever-cool astrometry.net with a great deal of assistance from Dustin Lang and some from David Hogg.  I'll post details of that process later; it proved challenging but will DEFINITELY be useful again in the future.  Short story: I had to build my own indices, then fit twice - once for the first match, once after distortion was applied to re-extract the sources in the right location.

Conveniently, one of the products output by the astrometry.net code is ra,dec locations and fluxes (background subtracted) for all of the identified sources.  So, I used a simple script to match all sources within 2" of a known source from my input catalog and generate timestream data.  It took about an hour of processing (nested for loops - awful, but the only obvious choice at first.  Half of the time was probably file IO anyway).

I now had a few hundred timestreams of Orion stars.  At this point, I hadn't done any kind of quality control, which was obviously a problem because the Trapezium and neighbors were clearly saturated.  Also, there are known hot pixels that will drive some stars into weird behavior.  But I ignored all of that.  All I did for quality control was reject stars with more than about 5% missing data.

I elected to go straight for PCA analysis.  I assumed the primary component (the most correlated component) would be from the atmosphere, which is almost certainly true.  Subtracting that off (shown in the notebook linked below) left some clearly correlated components left over - some of these are instrumental, others are correlated backgrounds, others are correlated saturation levels.  I don't honestly know what all of the components are, but I DO know that correlated components are very unlikely to be intrinsic phenomena - stellar brightness isn't correlated.

So I tried to cut out ALL correlated components.  However, I think in the process I probably removed any useful information - even un-correlated signal should probably have a high amplitude (if it's interesting...) that will get removed through the process I used.  Nonetheless, I probably have some upper limits on the 5s scale variability of these stars in K-band.  Not enough to do anything like asteroseismology, but it was worth a day's effort.

Here's the ipython notebook (gist & nbviewer) for those interested:
https://gist.github.com/4109329
http://nbviewer.ipython.org/4109329/