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Spectral changes as U Sco passes through maximum brightness

ATel #15423; U. Munari (INAF Padova), P. Valisa and S. Dallaporta (ANS Collaboration)
on 8 Jun 2022; 15:55 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Subjects: Optical, Nova

Referred to by ATel #: 15435, 15436, 15438, 15465, 15502, 15508, 15513

The current outburst of U Sco has been first detected by M. Moriyama on 2022 June 6.720 (=t0). From the AAVSO light-curve, maximum brightness was reached at V=7.82(±0.02) mag on June 7.125 (±0.05) UT. The first spectrum of U Sco by Siviero and Munari (ATel #15417) actually pre-dates maximum brightness by t-t(max)=–0.15 days, and could be the earliest ever recorded for U Sco in outburst. Taguchi et al. (ATel #15420) reported on a low resolution spectrum they obtained on t-t(max)=+0.76, and Woodward et al. (ATel #15422) on spectra collected at t-t(max)=+0.52 and +1.51 days.

We have recorded a high-resolution, high S/N spectrum of U Sco with the Varese 0.84m + Echelle on June 7.903 UT, corresponding to t-t(max)=+0.78 days. It is shown here in comparison with the t-t(max)=–0.15 days spectrum of Siviero and Munari (ATel #15417). The difference could not be larger: all absorptions lines and P-Cyg components are gone and emission lines now dominate a much flatter continuum. The profile of Balmer lines is squared, with superimposed three narrower peaks, while the profile of all other emission lines is completely different being similar to a simpler, very-broad Gaussian. The FWZI of Halpha is 9850 km/s, which sits exactly of the FWZI=10030 – 270×[t-t(max)] derived by Munari et al. (1999, A&A 347, L39) for the 1999 outburst.

Woodward et al. (ATel #15422) commented how their t-t(max)=+1.51 spectrum is very similar to the early spectrum of U Sco obtained on day 0.831 after optical maximum during the 2010 outburst and shown in Figure 2 of Amupama et al. (2013, A&A, 559, A121). The comparison with the outburst of 1999 may show apparently larger differences though: in this figure we compare our t-t(max)=+0.78 Echelle spectrum with the similar spectra of U Sco for t-t(max)=+0.64 and +1.63 days from Munari et al. (1999, A&A 347, L39). The triple-peaked top for Balmer lines did not initially develop in 1999, but only well into the decline the emission lines evolved to separated triple components. Also the relative intensity of emission lines and the overall excitation degree could be (slightly) different, but this will be better addressed as the outburst progresses.

Photometry of U Sco is being obtained with ANS Collaboration telescopes ID 0311 (SLOAN g'r'i' bands) and ID 0310 (Landolt BVRI bands). The table below lists the data we have collected so far during the current outburst and by comparison we show also the latest measurement predating by less than two days the onset of the 2022 eruption, when U Sco was caught still at flat quiescence.

UT 2022t-t0g'r'i'BVRI
June 04.974–1.74618.43517.92317.773
June 07.872+1.1529.2228.5288.183
June 07.876+1.1569.2949.0308.2977.555
June 07.984+1.2649.4189.1918.4087.687
June 07.990+1.2709.3448.6398.300
June 08.016+1.2969.4519.2478.4187.717