So the final version of my paper on the progenitors of Type Ia supernovae is now available through #MNRAS open access! ๐Ÿ˜Ž #AcademicChatter

You can find it here:โ€ฆ

Here's a ๐Ÿงต covering some of the main points: 1/n
Quick primer: Type Ia supernovae (SNe) are the explosions of white dwarfs (WDs), which are the cooling embers of low mass stars, and the future our sun has to look forward to - though it probably won't explode ๐Ÿ˜’ . 2/n
WDs happily sit around and slowly cool, unless they gain a bit too much mass.

If that happens they undergo runaway nuclear fusion reactions and blow themselves up in a Type Ia supernova ๐Ÿ’ฅ 3/n
A big gap in our understanding of these events is that it's not clear how these WDs obtain the extra material that pushes them over the mass threshold and into oblivion.

There are lots of models of what might be possible but it isn't yet clear which of these happen 4/n
In reality it's most likely a combination of several pathways, but their relative rates are not well understood.

In simple terms (๐Ÿคž I don't annoy the moddlers too much) there's two main groups of possibilities. 5/n
Single degenerate: a single WD accretes material from a companion star (gravitationally sucking material directly from it or by capturing material from a stellar wind) in an astronomy equivalent of a vampire ๐Ÿง› 6/n
Double degenerate: two WDs merge or collide and it's the combined mass that sends the resulting object supernova

Perhaps surprisingly these pathways produce supernovae that are very similar. 7/n
This similarity is very helpful if you are using Ia's as bright distance markers, but less so if you are trying to work out what actually exploded and how. 8/n
Unfortunately no Ia has occurred close enough to us for us to observe the system beforehand, leaving us looking at the explosions themselves for subtle hints to decide on the best model. 9/n
Enter Circumstellar Material (CSM): gas and dust near the site of the explosion but isn't a part of the exploding WD(s) 11/n
CSM is helpful as single degenerate models tend to have more 'stuff' surrounding the WD just before it explodes - it's stealing material from a companion so its environment can be thought of as 'dirty'

Double degenerate models tend to have much cleaner environments 12/n
If you see CSM early in the SN's evolution it's a good sign of a single degenerate pathway.

I looked at 2 such signatures: Narrow blueshifted sodium and high velocity calcium absorption features, each probing different distances from the SN and produced differently 13/n
It's important to look at multiple probes to confirm that what you are observing is really linked to the SN and not contamination from other material that happens to sit in between you and the SN. 14/n
Sodium (Na) absorption features are very common in SNe and galaxy spectra, so seeing the feature alone doesn't distinguish between the models but other properties of the feature thankfully can. 15/n
If the Na was from contamination you would expect it to appear redshifted across different objects as often as it appears blueshifted.

But why ...?
If the Na was the result of an unrelated cloud in the host galaxy it would be equally likely to be between Earth and the SN - blueshifted - as behind it - redshifted.

But that's not what we see! 17/n
Across the full sample we see a 24% excess of blueshifted Na features compared to what would be expected through contamination alone and this jumps to an even higher 48% when looking just at spiral galaxies (no excess is observed in ellipticals). 18/n
This shows that overall - its not possible to say on a per object basis - that a large proportion of these features are SN related rather than contamination - which is great and supports previous work with a larger sample.

So what about the Ca? 19/n
High velocity Ca features can be produced when the outermost ejecta from the SN crashes into the surrounding CSM.

With the general premise being the stronger the feature the more material there is surrounding the WD at the time of explosion. 20/n
So strong high velocity (HV) Ca absorption would be another indication of the single degenerate pathway - more material there to crash into.

HV Ca features are very common common in Type Ia SNe around max brightness but their strength varies significantly. 21/n
It's those objects with the strongest features that are the best candidates for single degenerate explosions.

Which leads to a nice 'expected' correlation: Stronger blueshifted Na features = Stronger Ca features

Nice and easy to identify single degenerate explosions! 22/n
Unfortunately the universe has other ideas.

There isn't a correlation between the two in the sample we looked at ๐Ÿ˜•

In the attached plot that would be the blue points in a nice straight line from the bottom left to the top right rather than scattered all over the show. 23/n
That means that either:

One or both of the signatures don't actually track the presence of CSM around WDs as they explode.

Or the more likely scenario; that the presence or absence of CSM at the different distances explored by the pair can't be directly linked 24/n
The paper goes into this in a LOT more detail along with a whole host more analysis of the objects in the sample but I think I've tweeted enough for one day at this point ๐Ÿ˜‚

Thanks to everyone who made this paper happen and to anyone still reading! 25/25
@threadreaderapp save people some time and unroll please ๐Ÿ˜…

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