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Peremennye Zvezdy (Variable Stars) 38, No. 2, 2018 Received 12 April; accepted 17 April.
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Article in PDF |
New Eclipsing Cataclysmic Variable Candidate in the Period Gap
D. Panov1, D.V. Denisenko2
- Erudit Lyceum, Rubtsovsk, Russia
- Sternberg Astronomical Institute, Moscow State University, Universitetskij pr. 13, Moscow 119992, Russia; d.v.denisenko@gmail.com
We report the discovery of a blue eclipsing variable
Dan V1 in Sagittarius with the orbital period
(2 2626). The object is also an UV source, indicating the
presence of a white dwarf in the binary system. We consider two
possibilities: a cataclysmic (nova-like) variable and an eclipsing
hot subdwarf of the HW Vir type. Our analysis favors the first
(CV) scenario. If confirmed, Dan V1 is a very rare eclipsing
cataclysmic variable in the period gap.
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1. Discovery
The new variable in Sagittarius was discovered by a 10th grade
student Danil Panov during the Moscow University LANAT summer camp
in August, 2017. D. Panov was blinking images of the field
centered at NSV 11918, obtained with the 0.51-m T31 instrument of
the iTelescope network in Siding Spring, Australia. The star
indicated in Fig. 1 was found to be in eclipse on the images from
11:01 to 11:08 UT, Aug. 15, and reappearing on the 11:09 UT image.
The position of the variable (J2000.0) was taken from the PPMXL
catalog (Roeser et al. 2010): 19
21
27
38,
-31
13
49
5. Checking the AAVSO VSX website (Watson et
al. 2007) has shown no known variable star at these coordinates as
of 2017 August 15. The new object was designated Dan V1 by the
discoverer.
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Fig. 1.
The finding chart of Dan V1 from the DSS
Red plate. The FOV is |
', North is up and East, to
the left.
2. Period Determination
In addition to twelve 2017 Aug. 15 images, we had 57 additional
60-second exposures from 2016 Aug. 08, covering 12 of
observations. Another eclipsing variable, DDE 47, was discovered
on those images (Denisenko 2016). We obtained additional
observations of the field with T31 on 2017 Aug. 17 (11:18-11:32
UT), Aug. 19 (11:01-11:15 UT), and with the 0.43-m T32 instrument
on Aug. 20 (11:13-11:30 UT). Unfortunately, no eclipses were
detected in these data. However, they allowed us to exclude
several possible periods we used for computing trial eclipse
ephemerides.
The area of Dan V1 was observed by the NEAT survey (Teegarden et
al. 2003) on 10 nights from August 1998 to August 2002, with a
total of 31 images. FITS crops with 
FOV were
downloaded from the SkyMorph website at
https://skys.gsfc.nasa.gov/ skymorph/obs.html, and the visual
inspection revealed three additional eclipses in NEAT data. The
magnitudes of the new variable were measured for all images using
USNO-B1.0 0587-0835084 (
) as the reference star.
The observing times were converted from JD to Barycentric Julian
Dates using the online period search service
http://scan.sai.msu.ru/lk/ by Kirill Sokolovsky. The best period,
0
094277(2), was obtained using the Lafler-Kinman method.
With this period and the Aug. 15 eclipse as the initial epoch, we
predicted the next eclipse to occur around 11:18 UT on 2017 Aug.
28. The observations with T31 on 11:08-11:33 UT confirmed the
prediction. The eclipse with epoch number 138 was successfully
observed. The phased light curve using all data is presented in
Fig. 2.
The light elements of Dan V1 from combined iTelescope and NEAT observations are:
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Fig. 2.
The light curve of Dan V1
from the T31 and NEAT data folded with the best orbital period,
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, and the initial epoch 
HJD2457980.967.
3. Discussion
The new variable Dan V1 has the following identifications and magnitudes in astronomical catalogues:
GALEX J192127.3-311349 (
, 
);
GALEX J192127.4-311350 (
, 
);
USNO-A2.0 0525-40560552 (
, 
);
USNO-B1.0 0587-0835138 (
, 
, 
,
, 
).
The star was not detected in the 2MASS and WISE infrared all-sky surveys. Its color indices clearly indicate the presence of a hot white dwarf in the binary system. There are two possibilities for short-period eclipsing binaries: a cataclysmic variable (CV) or a hot subdwarf system of the HW Vir type. In our case, the light curve shape is in favor of a cataclysmic variable. Namely, there is no brightness rise towards the 0.5 phase that should be expected in an HW Vir system due to reflection effect on the secondary from the subdwarf. Also, the light curve of Dan V1 folded with the orbital period does not reproduce itself exactly from one epoch to another, which is an indication of interaction between the two components. Thus, we conclude that Dan V1 is an eclipsing CV in the period gap. According to the AAVSO Variable Star Register, VSX (Watson et al. 2007), there are about 5000 CVs among 500 thousand variable stars known as of February 2018. 54 CVs fall in the period gap from 2.2 to 2.8 hours, and only 15 of them are eclipsing. Figure 3 shows the position of Dan V1 in the period distribution of cataclysmic variables.
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Fig. 3.
Position of Dan V1 (the
vertical dashed line) in the period gap of cataclysmic variables.
The bin size along the period axis is 0 |
2. Cataclysmic
variables with periods longer than 10 hours are not shown.
4. Conclusion
A new candidate cataclysmic variable in Sagittarius, in the period
gap, has been discovered. We would like to encourage multi-color
photometry and time-resolved spectroscopy of Dan V1, needed to
confirm the nature of this object. Note that the new variable is
located in
from another eclipsing cataclysmic
variable, DDE 47 (Denisenko 2016), which is a dwarf nova with
unknown period.
Acknowledgments: Danil Panov's participation in the LANAT
summer camp was supported by the Andrey Melnichenko Foundation and
Siberian Generating Company. The authors are grateful to Dr.
S.V. Antipin for useful discussion.
References:
Denisenko, D., 2016, Astronomer's Telegram, No. 9342
Roeser, S., Demleitner, M., Schilbach, E., 2010, Astron. J., 139, 2440
Teegarden, B.J., Pravdo, S.H., Hicks, M., et al., 2003, Astrophys. J., 589, L51
Watson, C.L., Henden, A.A., Price A., 2007, Journal of the AAVSO, 35, 414