Photometry and spectroscopy of the 2021 outburst of the symbiotic star YY Her
ATel #14464; U. Munari (INAF Padova), P. Ochner (Univ. Padova), S. Dallaporta, G. L. Righetti, M. Graziani, F. Castellani, A. Maitan, S. Moretti, (ANS Collaboration), and N. Masetti (INAF Bologna).
on 16 Mar 2021; 18:04 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
Subjects: Optical, Cataclysmic Variable
Referred to by ATel #: 14469
The symbiotic star YY Her has just been announced in outburst (ATel #14458),
based on unfiltered telephoto lens images taken on March 15.006 UT during the
New Milky Way survey for transients.
As part of our continuous monitoring of symbiotic stars, we have been
following the event with ANS Collaboration telescopes in UBVRI bands with
their observations transformed to the Landolt system of equatorial
standards. Some representative results on YY Her from our monitoring
are:
| 2021 UT | U | B | V | R | I | status |
|---|
| Feb 16.155 | | 14.159 | 13.081 | 12.030 | 10.722 | quiescence |
| Feb 25.134 | | 13.882 | 12.956 | 11.878 | 10.692 | on the rise |
| Mar 13.081 | | 13.135 | 12.379 | 11.424 | 10.447 | current |
| Mar 16.157 | 12.421 | 13.146 | 12.328 | 11.468 | 10.465 | current |
showing the star still at quiescence on Feb 16, and already on the rise by
Feb 25. For comparison, the average values of the YY Her epoch photometry
listed by Henden and Munari (2008, BaltA 17, 293) in their photometric atlas
of symbiotic stars are: U=14.322, B=14.300, V=12.925, R=11.842, and I=10.574.
We have obtained on March 16.08 UT a deep low-resolution spectrum of YY Her with
the Asiago 1.22m telescope (3300-8000 Ang, 2.3 Ang/pix). The major difference with similar
spectra for the preceding quiescence are a Balmer
continuum in much stronger emission, a more pronounced veiling of the M giant spectrum
by a blue continuum, and a significant increase in the integrated flux of
emission lines. In the following table we compare the integrated fluxes of
some representative emission lines measured on the March 16.08 UT spectrum
and on a similar spectrum taken on Aug 11, 2019 at exactly the same orbital
phase, so to avoid introducing spurious differences due to varying orbital aspect.
The two spectra have been taken 583 days apart, the orbital period of YY Her
being 590 days, with the last transit of the M giant at inferior conjunction
occurring in May 2020.
| line | lambda | flux (10-13erg cm-2s-1) |
|---|
| | 2019/08/11 | 2021/03/16 |
|---|
| OIII | 3341 | 3.3 | 1.8 |
| [NeIII] | 3869 | 0.2 | 1.6 |
| Hgamma | 4340 | 5.8 | 11.0 |
| [OIII] | 4363 | 0.6 | 2.1 |
| HeII | 4686 | 3.6 | 12.4 |
| Hbeta | 4861 | 14.8 | 26.8 |
| [OIII] | 5007 | 0.2 | 2.3 |
| HeII | 5412 | | 1.0 |
| Halpha | 6563 | 70.2 | 110.0 |
| HeI | 6678 | 5.2 | 6.5 |
| HeI | 7065 | 3.1 | 4.1 |
The current outburst seems to be of the "hot type" with the larger increase
in flux affecting the higher ionization emission lines, as opposed to
outbursts of the "cool type" characterized by the near disappearance of
higher ionization lines. The latter is the most frequent type of outburst among symbiotic stars, with some objects showing both types, most notably AG Dra.
