The fate of the progenitors of luminous red novae: Infrared detection of LRNe years after the outburst

TL;DR

This study investigates the fate of luminous red nova progenitors by assembling late-time optical, near-infrared, and mid-infrared data for nine extragalactic LRNe. The authors show that while optical emission fades below progenitor levels, remnants persist in the NIR/MIR for years, often displaying red giant or red supergiant-like properties. SED analyses reveal cool, expanded remnants and significant dust formation, with dust masses of order 10^-4 to a few 10^-3 solar masses and IR echoes tracing evolving dusty shells. The work demonstrates LRNe are non-terminal outcomes of binary mergers, highlighting dust production and long-lived IR emission as key signatures, with JWST providing crucial late-time constraints. Overall, the results unify the late-time behavior of LRNe and reinforce their interpretation as merger-driven transients with dusty circumbinary environments.

Abstract

We present late-time optical and infrared (IR) observations of a sample of nine extragalactic luminous red novae (LRNe) discovered in the past three decades. In all of these cases, the LRN survivors fade below the pre-outburst luminosity of the progenitors in the optical region. However, they remain visible in the near-IR (NIR) and bright in the mid-IR (MIR) domains for years. We recover AT 1997bs in Spitzer images from 2004, and a residual source is visible in HST and JWST NIR images 27 years after the outburst. The spectral energy distribution (SED) of AT 1997bs is consistent with that of an orange giant star with a photospheric temperature of 3750-4250 K and a radius of 120-150 $R_{\odot}$, without a significant circumstellar dust attenuation. Similarly, the SED of AT 2019zhd after three years is compatible with a red supergiant star with $T_{ph}=3100\pm100$ K and $R=350\pm50$ $R_{\odot}$. Another LRN, AT 2011kp, is detected by JWST 12.5 years after the outburst. Its SED, with two excesses at 1.8 and 7.7 $μ$m, can be explained by a cold ($T\sim450$ K) dusty shell composed of amorphous carbon surrounding a cold expanded source, plus emission from the Pa$α$ line. We constructed the [3.6]-[4.5] colour curves extending up to more than 7 years for six LRNe, which show a similar evolution: The MIR colour is $\sim-0.5$ mag before the optical maximum light, it becomes bluer after around one year, and then it gradually turns to redder colours before reaching $\sim+1.0$ mag after 7 years. We also estimated the masses and the temperatures of newly formed dust years after the LRN onset. We find that LRNe produce dust masses of the order of (1-5)x$10^{-4}$ (and up to 2x$10^{-3}$) $M_{\odot}$ between 7 and 13 years after the outbursts. Finally, we find that the remnants of LRNe detected years or decades after the merger are expanded and cool objects, similar to red supergiant stars.

Figures (20)

• Figure 1: Very late time detections of AT 1997bs. Top-left: Detection of AT 1997bs in an SST $Ch2$ filter image obtained in 2004 (after the subtraction of a template image). Top-right: Same, but for the $Ch4$ filter (using a JWST/MIRI $F770W$ image as a template). The green circles are centred at the transient position and has a radius of $5"$. Bottom-left: Detection of AT 1997bs in $F814W$ filter in HST/ACS images obtained in 2023. Bottom-right: Detection of AT 1997bs in $F150W$-filter JWST/NIRCam images obtained in 2023. The green circle are centred at the position of the transient and have a radius of $0.15"$.
• Figure 2: Detection of AT 2007sv in 2010 images obtained by SST in the $Ch1$ (left) and $Ch2$ (right) filters. The green circles are centred at the transient position and have a radius of $3"$.
• Figure 3: Images from HST of the field of AT 2011kp. Left: $F814W$ filter image obtained on 2013 October 30. Centre: $F814W$ filter image taken on 2014 January 12. Right: $F657N$ HST image obtained on 2014 October 23. The green circles are centred at the coordinates of the transient and have a radius of $0.5"$.
• Figure 4: Mid-infrared light curve of AT 2011kp spanning 12.5 years of evolution. The 2024 JWST/NIRCam $F444W$ filter measurement is also shown as its bandpass is similar to that of the $Ch2$ filter. Upper limits are marked with downwards arrows. For reference, the $r$-band light curve published by Pastorello2019review is also shown with green squares.
• Figure 5: JWST MIRI and NIRCam images of AT 2011kp. Panels are as follow: ($a$): $F770W$, ($b$): $F560W$, ($c$): $F444W$, ($d$): $F335M$, ($e$): $F187N$, and ($f$): $F115W$. In the MIR, a bright source is clearly visible 12.5 years after the first peak. In the top row stamps, the green circles are centred at the position of the transient and have a radius of $0.5"$. In the $F335M$ stamp, the circle has a radius of $0.3"$, while in the $F187N$ and $F115W$ stamps it has a radius of $0.1"$. The latter two stamps are more zoomed-in to better identify the counterpart.