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

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 $\mu$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$\alpha$ 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.