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Photometry and spectroscopy of FeII nova ASASSN-17hx, finally passing through maximum

ATel #10572; U. Munari (INAF Padova), F.-J. Hambsch, A. Frigo, F. Castellani (ANS Collaboration),G. La Mura (Univ. Padova), G. Traven (Univ. Ljubljana), M. Ozbey Arabaci, T. Saguner (Ataturk Univ.)
on 13 Jul 2017; 10:08 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Subjects: Optical, Nova

Referred to by ATel #: 10613, 10641, 10725

More than two weeks past its discovery, nova ASASSN-17hx has finally reached what appears to be its peak optical brightness. Announced as a candidate nova on June 23.7 UT (ATel #10523), we begun daily photometric monitoring of ASASSN-17hx on June 24.07 UT when we measured V=12.39, B-V=+0.72, V-I=+1.19. After a monotonic - although structured - rise toward maximum, peak brightness has been reached at V=10.90, B-V=+0.85 on June 10, followed by a slow decline.

We are collecting high and low resolution spectroscopic observations of ASASSN-17hx with several telescopes over the 3200-9100 Ang range: Asiago 1.82m and 1.22m, Varese 0.6m, and Tubitak 1.5m. Spectra recorded around peak brightness shows a textbook example of a FeII nova, with prominent emission from Balmer and Paschen series, FeII (strongest multiplets 27, 38, 42, 48, 49, 74), Si II, OI (7772, 8446) and CaII far-red triplet (8498, 8542, 8662 Ang). All lines are flanked by structured P-Cyg absorptions of a width varying with the ion and multiplet. They appear sharpest for FeII multiplet 42, for which on July 11.87 UT we measured the strongest absorption components at -451, -359 and -285 km/s heliocentric velocity, with the emission component at +11 km/s and FWHM=265 km/s. From the equivalent width of the diffuse interstellar band at 6614 Ang we derive a reddening E(B-V)=0.68 following the calibration by Munari (2014, ASPC 490, 183), well matched by the E(B-V)=0.62 implied by the B-V color at maximum when compared with the mean intrinsic value for novae by van den Bergh and Younger (1987, A&AS 70, 125). Interstellar lines from NaI appear as a saturated blend of several individual and unresolved components. Other interstellar absorptions present in our spectra include CaI, CaII, CH+, KI and several DIBs.

Soon after discovery, ASASSN-17hx was spectroscopically classified as an He/N type by Kurtenkov et al. (ATel #10527) from low resolution spectra obtained on June 24.0 UT. At that time the nova was still on the fireball rise toward maximum, about ~1.5 mag away, and thus much hotter than at peak brightness. As it happened for Nova Oph 2015 (Atel #7367), there seems to be no anomalous change in the spectral type of ASASSN-17hx. It is a FeII type, and while cooling toward maximum it passed through temperatures characteristic of He/N novae, as it would be observed for all FeII novae if spectra of them were secured well before maximum. Our spectra of ASASSN-17hx, taken at different epochs during the rise to maximum, show the gradual weakening of Helium lines and the parallel emergence and reinforcement of FeII (see also ATel #10558), mirroring the cooling of the B-V color.