Photometric and spectroscopic observations of the symbiotic star CM Aql in outburst
ATel #13647; U. Munari (INAF Padova), P. Ochner (Univ. Padova), S. Dallaporta, A. Maitan, and P. Valisa (ANS Collaboration)
on 18 Apr 2020; 15:55 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
Subjects: Optical, Cataclysmic Variable, Transient, Variables
CM Aql is a poorly studied symbiotic star harboring an M2 giant. On 2020
Feb 28, it was caught in outburst by Gaia as alert Gaia20bcf (=AT 2020dsz).
We begun photometric BVRI observations on March 7 as soon as the object rose
above 2.5 airmass on the pre-dawn sky. The table summarizes our photometry
collected with ANS Collaboration telescope ID 310 (total error budget -
including transformation to the Landolt equatorial standards - less than
0.013 mag in all colors and all dates).
| UT | B | V | B-V | V-R | V-I |
|---|
| Mar 07.146 | 13.578 | 12.412 | 1.166 | 0.910 | 1.934 |
| Mar 08.184 | 13.594 | 12.425 | 1.169 | 0.917 | 1.945 |
| Mar 16.179 | 13.628 | 12.439 | 1.189 | | 1.964 |
| Apr 05.116 | 13.741 | 12.537 | 1.204 | 0.968 | 2.007 |
| Apr 16.080 | 13.777 | 12.542 | 1.235 | 0.972 | 2.014 |
| Apr 18.060 | 13.781 | 12.544 | 1.237 | 0.973 | 2.025 |
Our last observations obtained just before the Solar conjunction in late 2019
with ANS Collaboration telescope ID 2202 reported CM Aql averaging to B=16.23
B-V=+1.46, V-R=+1.76, V-I=+3.24. The outburst amplitude has therefore been at
least 2.7 mag in the B band, with a marked blueing of all colors all the way
through the I-band. The decline from maximum is so far smooth and characterized
by a 0.005 mag/day rate in B band.
A low resolution, absolutely fluxed spectrum on CM Aql has been obtained
on April 18.074 UT with the Asiago 1.22m telescope (3300-7900 Ang, 2.31
Ang/pix). With an identical set-up we observed CM Aql for the last time
prior to the current outburst on 2019 Sept 20.820 UT. A strong blue and
smooth continuum has now veiled most of the molecular bands from the M2
giant. The current outburst is of the "cool" type, with a marked drop in
the ionization stage. The HeII 4686 and [FeVII] 6087 emission lines that
were outstanding in quiescence are now gone, and the integrated absolute
flux of HeI lines has declined to half the value in quiescence. Halpha,
Hbeta and Hgamma Balmer lines are visible in emission, with an increase of
20 percent in integrated flux compared to quiescence, while higher order
Balmer lines appear in absorption.
A high resolution Echelle spectrum of CM Aql has been obtained on Apr
16.125 with the Varese 0.84m telescope (range 4200-8560, res.pow. 15000).
It nicely confirms the disappearance of the high ionization emission lines,
and shows OI 8446 in strong emission while OI 7772 is absent, suggesting
fluorescence pumping by absorption of hydrogen Lyman-beta photons in a high
density medium. The emission lines are rather narrow (FWHM about 70 km/s
after correction for instrumental PSF). A narrow absorption component
is visible on the blue wing of the Halpha profile, at -148 km/s
from its emission peak. No similar absorption components are visible in our
quiescence Echelle spectra for 2015 and 2017, but one is present in the
Halpha profile for 1989 Sept 9 shown in the atlas by van Winckel et al.
(1993, A&AS 102, 401).
