# The two channels of Type Ia SNe and their use as standard candles

As we know, Type Ia supernovae are used as standard candles since their absolute luminosities are expected to be roughly constant. The consensus model, as seen from the Wikipedia page, suggests this is because Type Ia SNe come from a binary star system with a white dwarf and a red giant, where the mass of the red giant slowly accretes onto the white dwarf, until it reaches the Chandrasekhar limit, where runaway thermonuclear fusion begins, causing the supernova.

However, a paper linked in the Wikipedia page suggests that the above "single degenerate model" only accounts for $$\leq 20$$% of Type Ia supernova. As far as I understand, the main alternative to the single degenerate scenario is a double degenerate scenario, i.e. the merger of two white dwarfs. Of course, the double degenerate scenario is not constrained by the Chandrasekhar limit, and consequently does not have a fixed luminosity.

Question. If only <20% of Type Ia SNe are single-degenerate and have a fixed luminosity, then how can we use them as standard candles at all?

I get the impression from the above paper that it is in general very difficult to understand whether a particular Type Ia supernova fits into the single or double degenerate scenario. In this case, how can we have any confidence in distance measurements with Type Ia SNe?

• I believe this has been asked before, though I don't recall an answer really being given. I think this is the Q&A I'm thinking of. It's slightly different, so I'm thinking not a duplicate, but ProfRob's answer does bring up the basis of your question (variation in the origins of Ia supernovae). Jan 25 at 8:01
• @zibadawatimmy Thank you! I don't think that answers my question exactly, but it is very helpful nonetheless. Jan 25 at 9:21
• Ok maybe I’m missing something here, but I read through the paper and I’m not seeing any justifications or citing for this 20% number; it seems they spend the entirety of the paper trying to decide specifically what happened to SN 1006 and then just at the end say “combined with previous results” and then that 20% number, with no citations or further explanation. Again, maybe I’m missing something, but that just seems a bit unfounded given their evidence, maybe some other literature can supply additional light. Jan 25 at 16:20

The fraction of type Ia explosions caused by single degenerate (SD, mass accretion from a "normal" companion) and double degenerate (DD, white dwarf mergers) is highly uncertain. That is because there are no unambiguous observational distinguishing characteristics in the majority of cases.

It is quite feasible, or even likely, for a DD merger to produce a supernova that looks similar to that produced in an SD scenario. That is because in both cases, the thing that explodes is a near-Chandrasekhar mass white dwarf. In the SD case, mass is accumulated from a higher mass, non-degenerate secondary until it approaches the Chandrasekhar limiting mass and ignites. In the DD case, the mass is accumulated from the tidally disrupted lower mass white dwarf in the merging system; it is then (only) the primary WD that explodes, again at just below the Chandrasekhar mass.

There are I suppose two (main) questions that are important for using Type Ia supernovae as "standard candles". Can the observational characteristics of a given supernova be used to "standardise" the candle and hence estimate its absolute luminosity; and is the standardisation calibration universal, in the sense of being the same at all cosmic epochs and hence distances?

The answer to the first question appears to be yes. The majority of local type Ia supernovae (with distance established by other means) appear to follow a reasonably tight relationship between light curve width and peak absolute magnitude. This is known as the Phillips relationship. There are outliers - maybe at the 20% level - but most of these do show other observational peculiarities and could be filtered out given sufficiently detailed data for more distant supernovae. An example is shown below.

The relationship between absolute magnitude and light curve decay width/rate for local type Ia supernovae (from Taubenberger 2017). The majority of SNe follow the tight Phillips relationship that facilitates their calibration as standard candles, but there are a significant fraction of outliers.

The answer to the second question is more uncertain, since one might expect changes in the relative proportions of SD and DD progenitors at different cosmic epochs and metallicities. DD supernovae are likely to have shorter delays between star formation and explosion and so could be more predominant at higher redshift? The balance of progenitor types could be different in different metallicity environments? Whatever dispersion exists in the Phillips relation is likely different for SD and DD supernovae and so this could bias results.

There are no firm answers to these questions as yet, but the premise of your query isn't quite correct. Although many, if not most, SN Ia explosions may have DD progenitors, the majority of (local) SN Ia follow a tightly empirically calibrated absolute magnitude vs light curve duration relation.

• Thanks for the insight! What is the reason why the DD scenario still has tightly constrained luminosities? I would have expected that the DD case could reach higher masses than the SD case (because of much more rapid mass accumulation), exceeding the Chandrasekhar mass and causing higher luminosities. Why is this incorrect? Jan 26 at 9:25
• Because the higher mass WD in the pair accretes from the tidally disrupted secondary WD until it reaches the Chandrasekhar limit. @YiFan Jan 26 at 9:27
• @YiFan statements in wikipedia should be backed up with references to the original literature - which should be cited if the specific information is used. If they are not, then beware! In this case the relevant paragraph only contains citations to some junior-level lecture notes and a New Scientist article from 2007 both discussing one unusually bright supernova that is an outlier. Jan 26 at 9:33
• There are outliers in the plot - the one discussed in the New Scientist article is in the top-left group of 3. But none of these outliers are conclusively explained. Since most people believe DDs form a large fraction of type Ia SNe then a corollary is that most DD supernovae must look very similar to SD supernovae. @zibadawatimmy I believe a possible problem with your scenario is that it could undergo accretion induced collapse to a neutron star, rather than a thermonuclear type Ia SN - but "super Chandrasekhar" candidates are labelled on the plot - there are 6 of them. Jan 26 at 9:48
• For what it's worth, here's a 2016 paper about super Chandrasekhar mergers of white dwarfs. The ones they modeled don't end in supernovas, but end up collapsing into a neutron star or possibly other forms of dwarf remnant. Basic idea seems to be that off-center burning initiates before the core ignites and does not trigger explosive run-away fusion (then there's a sequence of core burning to off-center burning etc.). Neat. Jan 26 at 10:24