The pre-eruption state of T CrB as observed with ALMA in 2024

TL;DR

This study presents pre-eruption ALMA observations of T CrB across Bands 1–8 to characterize the circumstellar environment before the imminent nova eruption. The 2024 quiescent spectrum is faint and well described by a power law with $α=0.56\pm0.11$, consistent with free-free emission from a partially ionized red-giant wind and significantly softer than the 2016/17 high state, which was well fit by a fully ionized wind with a mass-loss rate of order $10^{-8}\,M_\odot\,\mathrm{yr}^{-1}$. The data indicate a turnover frequency well above 350 GHz in 2024, incompatible with the simple fully ionized-wind model that explained the 2016/17 state, implying a markedly different wind ionization state in quiescence. The absence of extended circum-binary emission further constrains the environment, and the authors advocate broader simultaneous frequency coverage (including Band 9) to tighten constraints on $ν_t$ and wind properties as the system approaches eruption around 2025–2026.

Abstract

T CrB is a nearby symbiotic binary and a recurrent nova with a period of ca. 80 years. The next eruption is expected to take place in 2025 or 2026. We present our pre-eruption observations made in ALMA frequency Bands 1, 3, 4, 6, 7, and 8 in August to November 2024 and constrain the properties of the environment into which the imminent next nova will erupt. We find that in the second half of 2024, the quiescent T CrB was a faint mm source with a spectral energy distribution well described by a powerlaw with index $α=$0.56$\pm$0.11 and a flux density of ca. 0.1 mJy at 44 GHz and 0.4 mJy at 400 GHz. There is no significant line emission. This is in agreement with expectations for free-free emission from the partially ionized wind of the red giant donor star and, in extrapolation to 35 GHz, a factor 5 fainter than the emission observed in 2016/17 during the latest high state. Comparing the spectra from that high-state between 13.5 GHz and 35 GHz with our spectrum from 2024, our spectrum is softer. The spectral index is on average lower by 0.34$\pm$0.11 . Our per-band and aggregate bandwidth images of T CrB show an unresolved point source with no evidence for extended structure. A simple model of a free-free emitting, fully-ionized stellar wind seems to describe well the 2016/17 high state of T CrB but not our 2024 ALMA measurements with their low flux and high turnover frequency suggesting that in 2024, the wind was far from fully ionized. (See the unabridged version of the abstract in the paper.)

Figures (7)

• Figure 1: Our 2024 ALMA T CrB flux density measurements from Table \ref{['table:fluxes']} plotted vs. centroid frequency and coloured by orbital phase (see legend). The error bars include the systematic errors. The solid line shows a power law fit to the points for orbital phase 0.43 while the dashed line shows a power law fit to all points.
• Figure 2: Wide-band image of T CrB obtained from a joint MTMFS deconvolution of all our ALMA 12 m array data on the target (bands 1 - 8) from 2024 as listed in Table \ref{['table:obs']}. The size of the synthesized beam (angular resolution) is given by the ellipse plotted in the lower left corner. The parameters (major axis, minor axis, position angle) are 1.62 arcsec, 1.16 arcsec, 19.8$^\circ$. The image resembles that of the point spread function. The object is detected with 15 $\sigma$ significance but there are no resolved spatial features. ALMA was tracking the object at all times taking into account its proper motion. The spatial coordinates shown are those for 10 August 2024, the start of the observations.
• Figure 3: Continuum image obtained from combining only our band 7 and 8 observations of T CrB where we achieve the best angular resolution. The two magenta circles represent the boundaries of an annulus around the position of T CrB with inner radius 0.8 arcsec (twice the angular resolution of the observation) and outer radius 1.6 arcsec, inside which we derive an upper limit on diffuse emission in the circum-binary medium.
• Figure 4: Image from the peak channel of spectral window 4 in our complete ALMA band 7 observations of the region around T CrB in 2024 (see Tab. \ref{['table:speclimits']}), where we achieve the best angular resolution and sensitivity for an individual line. This spectral window is centered (in LSRK) on the rest frequency of HCN v=0 J=4-3 line. The radio velocity shown in the upper right is relative to this line. The image is consistent with pure noise. The signal, for which this image shows the peak channel, is extracted from the annulus given by the two magenta circles around the image center, which are the same as in Fig. \ref{['fig:trc-b78-cont']}. The beam major and minor axes are 0.48 arcsec and 0.30 arcsec. We show this image in gray-scale to draw attention the fact that this is a narrow-band image of a single spectral channel.
• Figure 5: Comparison of our ALMA spectrum of T CrB for orbital phase 0.43 from 10 Aug 2024 (blue points) with a representative spectrum of the same object observed on 14 July 2016 with the VLA by 2019ApJ...884....8L (green circles). The fit parameters given in the caption are those of a power law fit to our points as also shown in Fig. \ref{['fig:alma-spec']}.