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

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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).

Galactic H2CO Densitometry I: Pilot survey of Ultracompact HII regions and methodology

[ ADS ] [ Full Version ]

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We examined 24 UCHII regions using the GBT to observe the 2-2 line of ortho-Formaldehyde. We measured the local gas density and compared to GMCs and other galaxies. We found densities in GMCs that are 1-2 orders of magnitude higher than the mean density in GMCs, implying that extreme overdensities are common even in "quiescent" (non-star-forming) GMCs. This work was almost entirely my own, but was seeded by Jeremy Darling Some highlight figures from the paper:

A sample spectrum. On the top, black is Formaldehyde 1-1, red is Formaldehyde 2-2. On the bottom, blue is 13CO, red is H77α, black is H110α.

Histograms of GMC mean densities and the densities measured directly from our survey. Note that even for "serendipitious" line of sight GMCs, the densities are MUCH higher than typical GMC densities.

Comparison of the measured column densities of formaldehyde and inferred number densities of H₂.

The optically bright post-AGB population of the LMC

[ ADS ] [ arXiv ]
E. van Aarle, H. Van Winckel, T. Lloyd Evans, T. Ueta, P. R. Wood, and A. G. Ginsburg

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A catalog of post-AGB stars in the LMC, useful in particular because they are at a common distance. Post-AGB stars are quite luminous (> 1000 L_O usually), so easily detected in the LMC.

My contribution to this work was years ago; I worked on the SAGE project for a few months at the University of Denver with Toshiya Ueta. I generated a catalog of post-AGB objects and an online catalog with automatic SED plotter. It was a pretty neat project, but I left before I was able to convince others that my catalog was definitive; nonetheless it was eventually used in this publication. As an aside, that was my first foray into data languages, and I ended up using the Perl Data Language long before I learned of python and before I got a free IDL license.

Characterizing Precursors to Stellar Clusters with Herschel

[ ADS ] [ arXiv ]
C. Battersby, J. Bally, A. Ginsburg, J.-P. Bernard, C. Brunt, G.A. Fuller, P. Martin, S. Molinari, J. Mottram, N. Peretto, L. Testi, M.A. Thompson 2011

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The paper includes a careful characterization of the properties of both dense and diffuse regions within the two Hi-Gal Science Demonstration Phase fields (l=30 and l=59). It demonstrated that star formation tracers are more common at higher temperatures, and that IRDCs exist on both the near and far side of the galaxy, where the difference between near and far is around 5-7 kpc.

I helped develop the iterative background subtraction method and contributed to the discussion and conclusions; most of the work was Cara's but my experience with iterative flux estimation from the BGPS pipeline proved useful.

An Infrared Through Radio Study of the Properties and Evolution of IRDC Clumps

ADS
arXiv

I assisted in many of the technical aspects of the paper including Bolocam and VLA data reduction.

We identify a potential evolutionary sequence among millimeter "clumps". HCO+ and N2H+ maps are used to measure the physical properties of some of these clumps.

Star-forming Region Sh 2-233IR. I. Deep Near-infrared Observations toward the Embedded Stellar Clusters

ADS
arXiv

My contributions to this paper were minor. Analysis of luminosity functions in the IRAS 05358+3543 region have identified different aged groups. The paper contributes to the picture of sequential (triggered?) star formation in the S233-235 region in Auriga.

Herschel observations of the W43 "mini-starburst"

ADS
arXiv

A minor work as part of the Herschel Hi-Gal early science special issue.

The Bolocam Galactic Plane Survey IV: λ = 1.1 and 0.35 mm Dust Continuum Emission in the Galactic Center Region

ADS
arXiv

As with other Bolocam papers, I reduced the data.

We identify a number of interesting mm sources in the Galactic Center and characterize some of their properties using Bolocam and SHARC data.

The Bolocam Galactic Plane Survey. III. Characterizing Physical Properties of Massive Star-forming Regions in the Gemini OB1 Molecular Cloud

ADS
arXiv

As with the other Bolocam papers, I reduced the data. All of the analysis work was done by Miranda.

Because of Gem OB1's known distance (~2 kpc), it was possible to derive the physical properties of all of the 34 BGPS sources. NH₃ temperatures and line widths were combined to measure total dust mass, density, and virial parameter. This paper is the best characterization of "typical" BGPS sources, at least for the outer galaxy.

The Bolocam Galactic Plane Survey. II. Catalog of the Image Data

ADS
arXiv

My primary contribution to this work was reducing the data from which catalogs were derived. As part of this process, I also made the noise maps that were part of the Bolocat process and did some of the by-eye verification work.

Bolocat extracted >8000 sources from the BGPS using a watershed decomposition method. Most of these sources have now been observed with the HHT in HCO+ and N2H+ to determine their velocity and kinematic distance.

The Bolocam Galactic Plane Survey: Survey Description and Data Reduction

ADS
arXiv

I wrote about half of this paper and did the majority of the work described, including writing the data pipeline and reducing all of the data. However, there was a good deal of work done on the project before I showed up on the scene.

The paper describes the methods by which we reduced the Bolocam observations of the Northern Galactic Plane and the tests we ran to determine how well we recovered the "true" sky structure.

Outflows and Massive Stars in the protocluster IRAS 05358+3543

ADS
arXiv

A mm and near-IR study of the IRAS 05358+3543 system. Recently (2011), it has become clear that "protocluster" is unlikely to be the right label for IRAS 05358+3543. "proto-association" might be more accurate.

Perhaps the most interesting result of this paper is the discovery that the central source is likely to be a 400-AU binary with two massive (>8 msun) stars at different evolutionary stages. It is just barely in ALMA's range...

A spatially resolved study of photoelectric heating and [C II] cooling in the LMC. Comparison with dust emission as seen by SAGE

ADS
arXiv

My contribution to this paper was quite minor. I was briefly a member of the SAGE team and this is the only SAGE paper I'm included on.

Kinematics and H2 morphology of the multipolar post-AGB star IRAS 16594-4656

ADS
arXiv

An interesting examination of the morphology of the circumstellar material around IRAS 16594-4656. I did this work with Toshiya Ueta at the University of Denver. My contribution was azimuthally interpolating the 7 high-resolution long-slit spectra onto a 3-dimensional grid, creating a full 3D position-velocity cube. I used the Perl Data Language, which I have since abandoned.