Synoptic IR Observations of T Crb

ATel #16120; C. E. Woodward (U. Minnesota), D. P.K. Banerjee (PRL, India), A. Evans (Keele U., UK)
on 7 Jul 2023; 19:16 UT
Credential Certification: C.E. Woodward (chickw024@gmail.com)

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The recurrent nova T CrB, which had its last eruption in 1946, has been showing a super-active accretion phase since late 2014. This peaked in April 2016 and appears to have culminated in mid-2023 (Munari 2023, arXiv230700255M). The end of this active phase has been suggested to be the precursor to an imminent and new outburst, which has been predicted to occur in 2024.4 +/- 0.3 (Schaefer et al. 2023, AAVSO announcement, https://www.aavso.org/news/t-crb-pre-eruption-dip). In order to monitor the pre-outburst evolution, assuming the outburst occurs as predicted, we conducted synoptic observations in the near-infrared covering four epochs during the active phase, 2017 Feb., 2020 June, 2021 July, and most recently 2023 July.

Here, we report the latest spectrum, covering 0.7 to 4.2 micron, obtained on 2023 July 06.23 UT on the 3.2-m IRTF telescope using SpeX in cross-dispersed mode with an 0.5 arcsecond slit (R=1200) under photometric conditions and moderate seeing (0.9 arcsec at K-band). The 2017, 2020 and 2021 spectra had also been taken at the same resolution with the same instrument. The spectra at all epochs are similar and essentially dominated by the SED of the cool giant secondary, displaying the standard atomic and molecular features expected for such stars. Among the molecular absorption features are the CN bands (Delta(v) = 0) at approximately 1.09 micron, strong first overtone CO bands at 2.29 microns and beyond, and SiO first overtone bands (2-0, 3-1, 4-2, 5-3) near 4 microns. Superposed on the spectrum are narrow and very weak emission lines e.g., HeI 1.083 (flux of order 1.4e-12 erg/s/cm2; FWHM = 4.1e-4 micron, with a narrow P-Cygni feature), OI 1.129, Pa beta 1.282, Br gamma 2.166 and Brackett alpha 4.05 micron (upper flux limit 7.6e-14 erg/s/cm2). The emission lines have been fading monotonically since 2017 and many have faded below detection limits in the latest 2023 spectrum. A similar decline in line strength has also been reported in the optical region (Munari, 2023).

We see no evidence yet for freshly formed dust that might account for the pre-eruption dip that commenced in June-July 2023 (Schaefer et al. 2023). Our 2023 SpeX spectrum (taken during the pre-eruption dip) is virtually identical to the 2021 spectrum (used as a reference template). If any significant amount of dust had recently formed, an upturn at longer wavelengths would be expected, but no IR excess is seen out to 4.2 micron. Had any hot dust formed (e.g., at T = 1700 K), then it would have affected the SED in the H band, but no major changes are seen in that region. Preliminary simulations show that if dust had recently formed causing the pre-eruption dip, over a reasonable temperature range of 900-1700 K and composed of either amorphous carbon or silicate grains, then the dust mass is small and less than 1.0e-10 solar masses. This is much smaller than that typically produced in dust forming novae.

However, synoptic near-IR spectrophotometric monitoring is continuing, and observers are encouraged to obtain broadband photometric JHK observations and beyond. We thank the IRTF staff and telescope operator for assisting with scheduling and observations conducted under program 2023A015.