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DO Dra:Recovering to the Base Intermediate Luminosity State

ATel #10982; Ivan L. Andronov (Odessa National Maritime University, Ukraine), Nikolay N. Mishevskiy (Observatory Ananjev (L33), Ukraine), Vitalii V. Breus (Odessa National Maritime University, Ukraine)
on 17 Nov 2017; 23:58 UT
Credential Certification: Ivan L.Andronov (tt_ari@ukr.net)

Subjects: Optical, Binary, Cataclysmic Variable, Variables

DO Dra is a magnetic dwarf nova (or an outbursting intermediate polar) which was previously known as a dwarf nova, so being mostly in a quiet state of V~15.2, which is rarely interrupted by outbursts to V~10-11. The decay time of the outburst is statistically dependent on brightness at the outburst. A new type of TPO (transient periodic oscillations) were detected during 3 nights (2008A&A...486..855A). Further study of hour-scale variability was published in 2017JASS...34...37H . On June 8, 2017, the system went to a much lower state, which seemed to be an unprecedented one, but the analysis of own previous data had shown two more sure detections (2017ATel10477....1B). We have started monitoring of this object, as a part of the international campaign ``Inter-Longitude Astronomy" (ILA) (2017ASPC..511...43A). The low luminosity states are common for intermediate polars (e.g. in V1323 Her (2014ATel.5944....1A), MU Cam (2005JASS...22..197K), FO Aqr (2016ApJ...833...93L), but the third ("low") level in dwarf novae (in an addition to "basic" quiescence level) was not distinguished before. The observations were obtained by N.Mishevskiy using 25-cm (f/4.7) reflector equipped with a CCD camera (totally 42 runs, 2644 observations, filter V, comparison stars "152" and "158" at the http://aavso.org/vsp chart. These observations are published in the aavso.org international database. V.Breus used the 60-cm telescope of the Astronomical Observatory and Planetarium in Hlohovec, Slovakia (totally 19 runs, 1200 observations, filter R, comparison star "135", thanks to Karol Petrik). As no calibration in R was published, we estimated its value as R=13.18 (V-R=0.36). The color indices of the comparison stars follow the statistical relationship (V-Rc)=0.523(B-V). This table is published at uavso.org.ua/data/dodra/dodra_2017_bvv_r.dat. The light curve in filters V and R since the weakening to the low state is shown at uavso.org.ua/data/dodra/DO_Dra-VR.jpg. On JD2457913-69, the object was in a very deep state with a nightly mean of V~16.9, which was interrupted by a two-day outburst to 16.3 on JD2457930-31. Next brightening to such level occurred 40 days after and lasted as a "standstill" for 30 days. Then again a slight weakening and recover to 16.2. Then the observations had shown that DO Dra is gradually returning to a "quiet" luminosity state since JD2458043. Recent points are 15.7 (JD2458055) with a characterstic time scale of 9.2+-0.6 days/mag. On JD2458069, it brightened to usual 15.2-15.4. The continuation of monitoring is suggested. The cease of the mass transfer rate may be caused e.g. by the solar-type activity of the red dwarf (1990AJ.....99.1941B), (1990Ap&SS.169..237A). The recent model explains such lowest states and absence of the dwarf nova outbursts (with an application to another star - FO Aqr) by a disappearance of the accretion disk, so the magnetospheric radius exceeds the circularisation radius (2017A&A...606A...7H). The outburst-like two-day event observed in DO Dra at JD2457930-31 thus may be explained as a temporary mass transfer increase.