Nova TCP J20210770+2914093 turns into an FeII-type as it keeps rising in brightness
ATel #14816; U. Munari (INAF Padova), S. Moretti, A. Maitan, V. Andreoli (ANS Collaboration)
on 29 Jul 2021; 20:47 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
The transient TCP J20210770+2914093 was discovered by K. Itagaki on
2021 Jul 16.492 UT at unfiltered magnitude 12.0, and classified as a nova by
Munari et al. (ATel #14793) based on a spectrum taken on July 17.102 UT.
This initial spectrum resembled novae of the He/N type, with Balmer,
HeI, and NII lines present in emission. However, Munari et al. (ATel
#14793) warned that with the nova possibly still progressing toward maximum
light, the spectral type could evolve toward the FeII type. This has been
indeed what happened and here we report about the evolution of the nova during
the last two weeks.
We have been monitoring the nova at daily cadence in BVRI bands with
various ANS Collaboration telescopes, primarily ID 1205, 2202, 2203, and
3600. After an initial decline of 0.7mag to a minimum brightness reached
around July 18.8/19.0 UT (two and a half days past discovery), the nova has
since then steadily rised in brightness, being now two mag brighter than the
initial minimum. The following are excerpts at representative dates from our
monitoring:
July 2021 (UT) | B | V | R | I |
---|
16.877 | 13.087 | 12.395 | 11.706 | 11.126 |
18.897 | 13.752 | 13.111 | 12.410 | 11.925 |
22.929 | 13.220 | 12.447 | 11.786 | 11.162 |
28.939 | 11.828 | 11.088 | 10.514 | 9.956 |
Spectra of TCP J20210770+2914093 have been collected at various dates
with various instruments, in particular on July 18.944 and 28.942 UT with
the Echelle spectrograph mounted on the Asiago 1.82m telescope (range
3650-7350 Ang, resolving power 20,000).
The Echelle spectrum for Jul 18.944 UT coincides with the initial
photometric minimum; compared with the spectrum for July 17.102 UT from ATel
#14793, it still shows the HeI and NII emission lines although at a much
reduced intensity, while the FWHM of Balmer lines has sharpened from 1700 to
1400 km/s and the velocity of Balmer P-Cyg absorption reduced from -810 to
-380 km/s (for Hbeta).
In the Echelle spectrum for July 28.942 UT, all HeI and NII features are
gone, to be replaced by a forest of strong FeII lines (in particular
multiples 27, 28, 37, 38, 42, 48, 49, 55, and 74) all flanked by deep P-Cyg
absorptions as for the Balmer lines. This spectrum is a textbook example
for a nova of the FeII-type. The typical radial velocity of P-Cyg
absorptions is -230 km/s, and the FWHM of the emissions is around 240
km/s. To be resolved, such low velocities requires the high-resolution provided by an
Echelle spectrograph.
It is not yet clear what is the reddening affecting the nova. The
profile of interstellar CaII 3934, 3968 and NaI 5890, 5896, at heliocentric
velocity -7.5 km/s, is the blend of at least two unresolved components.
These atomic lines are saturated and cannot be used to derive the reddening
except to place a lower limit of ~0.5 to E(B-V). The equivalent width of
DIB 6614 is 0.23 Ang; combined with the calibration by Munari (2014, ASPC
490, 183) it returns E(B-V)=1.0 which agrees with value inferred from the
IPHAS 3D map of Galactic extinction (Sales et al. 2014, MNRAS 443, 2907).
On the other hand, comparing the intrinsic colors of novae at maximum from
van den Bergh and Younger (1987 A&AS, 70, 125) with photometry in the table above, a
value E(B-V)=0.5 is obtained, which is consistent with the prediction from the 3D
maps of Galactic extinction by Lallement et al (2014, A&A, 561) and Green et
al. (2019, ApJ 887, 93).