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The abstract of the arXiv preprint SN 2023ixf in Messier 101: A Variable Red Supergiant as the Progenitor Candidate to a Type II Supernova (itself recently "discovered" in the observatory) includes:

Fitting to blackbody and red supergiant (RSG) spectral-energy distributions (SEDs), we find that the source is anomalously cool with a significant mid-IR excess.

From Figure 3 I can see that the 1640 K (orange) curve seems to be consistent with all the red(measured) and pink (upper limit) dots, and I think that can support anomalously cool" as a singular explanation, or it not being anomalously cool but there being an additional added significant mid-IR excess.

I'm sure it's in there, but it's hard for me to read; how do they support that both are happening? Is it just something like "we model a normal temperature star surrounded by a dust cloud that absorbs and re-radiates in such a way that it looks like 1640 K"? Or are they actually ruling out a single 1640 K source as an explanation for the observed data?


"Figure 3. SED of the pre-explosion counterpart to SN 2023ixf..." from https://arxiv.org/abs/2306.04722

Figure 3. SED of the pre-explosion counterpart to SN 2023ixf, with red circles denoting detections and pink circles denoting upper limits (described in Section 3.2). We fit the HST, Spitzer, and ground-based photometry photometry with a 1640 K blackbody (orange line), which describes the data but is much cooler than typical effective temperatures for the RSG progenitor stars of SN II (e.g., in Smartt 2015). We also show a RSG SED for a reddened RSG supergiant with a Teff = 3780 K photosphere inside of a 880 K dust shell exhibiting mid-infrared excess (green; from Kilpatrick & Foley 2018). The individual components of the overall reddened RSG SED (star and dust shell) are shown in blue and red, respectively

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The phrasing is slightly vague; I think what they're implicitly saying is that both of those things are true compared to red supergiants. In other words, the measurements imply that there is material that is cooler than the surface temperatures of red supergiants and is creating infrared emission in excess of what we see from red supergiants (or our models of them).

To go into more detail: The peak of the single-black body fit implies a temperature of $T_{\mathrm{eff}}=1640\;\mathrm{K}$. This is much lower than the temperatures of the coolest red supergiants, which tend to be around $T_{\mathrm{eff}}\simeq3400\;\mathrm{K}$; this is where they get "anomalously cool". The measured spectrum, being shifted to lower temperatures, has more long-wavelength emission -- hence "infrared excess".

The fit of a black body with $T_{\mathrm{eff}}=3780\;\mathrm{K}$ surrounded by circumstellar dust with $T=880\;\mathrm{K}$ lets you keep the progenitor star at a realistic red supergiant temperature while still explaining the observed spectrum. It also addresses the fact that applying the Stefan-Boltzmann law to the luminosity and the temperature of the single-black body fit would require a star with a radius rather larger than expected from most supergiants, something that immediately points to circumstellar material.

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