Masquerade! Dwarf Nova Faces on Parade


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Title: Classical Novae Masquerading as Dwarf Novae? Outburst Properties of Cataclysmic Variables with ASAS-SN
Authors: A. Kawash et al.
First Author’s Institution: Michigan State University
Status: Accepted to ApJ

Who Is Who

My favorite star is a cataclysmic variable star, or Gillian Anderson, depending on the context of the question. This type of variable star is my favorite, because it’s actually a binary star system, instead of just a single star. In this system, a white dwarf accretes matter from a donor star, usually (but not always) one on the main sequence. In most cases, an accretion disk will also form around the white dwarf. See the cover image above for an illustrated example. Sometimes explosions will occur within the binary, and they’re called “novae.” A “classical nova” (CN; CNe plural) happens on the surface of the white dwarf and is caused by thermonuclear runaway. A “dwarf nova” (DN; DNe plural) happens in the accretion disk and is thought to be caused by thermal instabilities. It’s important to remember that although they might sound similar, novae are very different from supernovae and should not be confused.

Using observations of galaxies like ours (Andromeda, for example) and theoretical models, we can predict how often we should expect to see a CN. Unexpectedly, the observed detection rate is significantly below the theoretical detection rate. The authors of today’s paper hypothesize that maybe this isn’t because we aren’t detecting them; instead, maybe we do see CNe but just misclassify them. DNe are one of the most common types of galactic transient and come from the same star type as CNe, so maybe we’re just confusing the two.

Stop and Stare at the Sea of Smiles Around You

Before immediately testing the sample of all known DNe, the authors wanted to create a baseline for what they expected to find. Two of the most important characteristics of a nova — dwarf or classical — are the time it takes to become fainter by two magnitudes from peak brightness (called “t2” in this paper) and the magnitude difference between peak and quiescent brightness (called the “outburst amplitude”). There are 9,333 (more now!) DNe in the VSX catalog, one of the largest variable star catalogs. The authors compared this to the ASAS-SN catalog of variable star light curves and selected the 2,688 that were observed during outburst. The ASAS-SN telescopes are only sensitive to luminosities brighter than 18 magnitudes, and so to get robust quiescent magnitudes, the authors further trimmed the sample to the 1,617 DNe that were also detected in the (more sensitive) Pan-STARRS catalog. To create a sample of 132 CNe, 40 were selected using the method above to be combined with a 92 CNe sample from Strope et al. (2010).

Let the Spectacle Astound You

Like any reasonable scientist, after the authors collected all their data, they plotted it! Visually, you can see a separation between the two samples (CNe in red & DNe in blue) in Figure 1. On average, CNe had an outburst amplitude of 11.43 ± 0.25 magnitudes, while DNe had an outburst amplitude of 5.13 ± 0.04 magnitudes. Furthermore, the authors found a 15% overlap in the outburst amplitude. The results for t2 were a little more complicated. The CNe had a t2 value of 18.7 ± 1.9 days, but the DNe sample needed to be split into a “fast” group (~12% of the sample) and a “slow” group (~88%). The average DNe t2 values were 2.4 ± 0.2 days and 10.5 ± 0.2 days, respectively. The authors were able to find fits to both samples in the form of log(t2) = B*(Amp – <Amp>) + a. The fit to the CNe sample was not very significant (~3σ) and had a negative slope (B = –0.083), while the fit to the DNe sample was very significant (~10σ) and had a positive slope (B = 0.061).

Basically the two samples are distinct, but there’s enough overlap that maybe we’re misclassifying CNe as DNe. Colloquially, they’re saying there’s a chance.

Plot describing properties of novae.

Figure 1: Comparison of the outburst amplitude to t2 (the time it takes to reduce by 2 magnitudes from peak brightness). CNe are shown in red, while DNe are shown in blue. [Kawash et al. 2021]

Hide Your Face So The World Will Never Find You

Once the authors knew what to look for, they critically analyzed the 2,688 ASAS-SN DNe sample. From analysis of the CNe luminosity function from Shafter 2017, the authors determined that a transient must have an absolute magnitude brighter than –4.2. Using apparent magnitudes from ASAS-SN, distance constraints, and dust extinction estimates, the authors were able to eliminate all but 201 novae in the sample from being (possible) CNe. They further reduced this sample to 94 after eliminating those that were quickly recurrent and those with outburst amplitudes less than an apparent magnitude of 5. These cuts were made since no classical nova is known to recur on timescales less than a decade and 5 was the lowest apparent magnitude limit on the CN outburst amplitude (from Figure 1). Finally, all but 27 of these 94 are spectroscopically confirmed DNe. To analyze the remaining 27, the authors used “quiescent multi-band photometry.” If a source is pretty blue, it’s likely to be close by and therefore likely to have a lower luminosity during outburst (hence, likely to be a DN), and if it’s red, it’s probably further away and is more likely to have a higher luminosity during outburst (hence, likely to be a CN). Basically, blue sources are DNe, and red sources are CNe. Using this method, the authors found that 19 novae are consistent with DNe, 0 are consistent with CNe, and 8 are ambiguous. So, at most 8 out of 2,688 — or 0.29% — of ASAS-SN classified DNe could be CNe.

To quote the authors, “the transient community appears to be doing an effective job classifying CV (cataclysmic variable star) outbursts.” Sadly this means that there is no masquerade and another explanation (maybe dust extinction?) is needed to explain the missing CNe.

Original astrobite edited by Gloria Fonseca Alvarez.
A French translation of this article is available on Astrobites, written by Celeste Hay.

About the author, Huei Sears:

Huei Sears is a third-year graduate student at Northwestern University studying astrophysics! Her research is focused on gamma-ray burst host galaxies. In addition to research, she cares a lot about science communication, and is always looking for ways to make science more accessible. In her free time, she enjoys walking along the lake, listening to Taylor Swift, & watching the X-Files.