Optical observations of the fast nova V2491 Cyg

ATel #1475; T. Tomov, M. Mikolajewski, E. Ragan, E. Swierczynski, P. Wychudzki (Uniwersytet Mikolaja Kopernika, Torun, Poland)
on 14 Apr 2008; 22:27 UT
Credential Certification: M. Mikolajewski (mamiko@astri.uni.torun.pl)

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We report on optical spectral observations and UBVRI brightness estimations obtained with 60/90 cm Schmidt and 60 cm Cassegrain telescopes of the Nicolaus Copernicus University Observatory (Torun, Poland). The nova V2491 Cyg was discovered on Apr. 10.728 UT with about 7.7 mag on unfiltered CCD frames (IAUC# 8934 ). Additionally, the X-ray emission was detected for the prenova several months ago (ATel#1473). The magnitudes estimated on Apr. 13.957 UT were U=9.02, B=9.21, V=9.03, R=8.04, I=7.72 using HD 331150 as the comparison star and assuming 10.30, 10.254, 10.25, 10.26 and 10.27 for its UBVRI magnitudes, respectively. The accuracy of our photometry is about 0.01 magnitude. A comparison with the data from IAUC# 8934 shows that our photometry was obtained about 2.2-2.3 days after the maximum when the star reached about 7.1-7.4 magnitude in the V light. Prismatic optical spectra (4000-9500 AA), with a resolution ~18 A around H-alpha and ~6 A around H-beta, were obtained on Apr. 11.99 UT and Apr. 13.95 UT. In both spectra the Balmer lines from H-alpha to H-delta are seen in emission. The FWHM of H-alpha is ~4800 km/s on Apr. 11.99 and ~4600 km/s on Apr. 13.95. The H-beta FWHM is ~4860 km/s and ~4220 km/s, respectively. The equivalent width of H-alpha increased twice, from ~380 A on Apr. 11.99 to ~760 A on Apr. 13.95. In the same time the H-beta equivalent width changed from ~140 A on Apr. 11.99 to ~180 A on Apr. 13.95. The FWHM and equivalent width of H-alpha observed on Apr. 11.99 are very close to those observed near the maximum (CBET#1334). P Cyg absorptions with a velocity about -3900 km/s on Apr. 11.99 and about -4000 km/s on Apr. 13.95 are well visible in H-beta and H-gamma profiles. It is possible that a second P Cyg absorption component, with velocity of about -6400 km/s, appeared in H-beta on Apr. 13.95 UT. An additional emission component with a velocity about +2300 km/s presents in the H-beta and H-gamma profiles on both dates. A strong asymmetry of the red H-alpha wing is most probably caused by a similar emission component. In addition to the Balmer lines in the spectrum of V2491 Cyg, relatively strong lines of Fe II multiplets 37, 38, 42 and 49 and OI 8446 A are present as well. Both, the expansion velocities and rapid fading of the brightness, indicate that V2491 Cyg belongs to the group of the extremely fast novae.