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)
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 2022 | t-t0 | g' | r' | i' | B | V | R | I |
June 04.974 | –1.746 | 18.435 | 17.923 | 17.773 | | | | |
June 07.872 | +1.152 | 9.222 | 8.528 | 8.183 | | | | |
June 07.876 | +1.156 | | | | 9.294 | 9.030 | 8.297 | 7.555 |
June 07.984 | +1.264 | | | | 9.418 | 9.191 | 8.408 | 7.687 |
June 07.990 | +1.270 | 9.344 | 8.639 | 8.300 | | | | |
June 08.016 | +1.296 | | | | 9.451 | 9.247 | 8.418 | 7.717 |