ER UMa Stars Research Articles

BK Lyncis: the oldest old nova and a Bellwether for cataclysmic variable evolution

We summarize the results of a 20-yr campaign to study the light curves of BK Lyn, a nova-like star strangely located below the 2 to 3 h orbital-period gap in the family of cataclysmic variables (CVs). Two apparent superhumps dominate the nightly light curves, with periods 4.6 per cent longer, and 3.0 per cent shorter, than the orbital period. The first appears to be associated with the star's brighter states (V ∼ 14), while the second appears to be present throughout and becomes very dominant in the low state (V ∼ 15.7). It is plausible that these arise, respectively, from a prograde apsidal precession and a retrograde nodal precession of the star's accretion disc. Starting in the year 2005, the star's light curve became indistinguishable from that of a dwarf nova – in particular, that of the ER UMa subclass. No such clear transition has ever been observed in a CV before. Reviewing all the star's oddities, we speculate: (a) BK Lyn is the remnant of the probable nova on 101 December 30, and (b) it has been fading ever since, but it has taken ∼2000 yr for the accretion rate to drop sufficiently to permit dwarf-nova eruptions. If such behaviour is common, it can explain other puzzles of CV evolution. One: why the ER UMa class even exists (because all members can be remnants of recent novae). Two: why ER UMa stars and short-period nova-likes are rare (because their lifetimes, which are essentially cooling times, are short). Three: why short-period novae all decline to luminosity states far above their true quiescence (because they are just getting started in their post-nova cooling). Four: why the orbital periods, accretion rates and white dwarf temperatures of short-period CVs are somewhat too large to arise purely from the effects of gravitational radiation (because the unexpectedly long interval of enhanced post-nova brightness boosts the mean mass-transfer rate). And maybe even five: why very old, post-period-bounce CVs are hard to find (because the higher mass-loss rates have ‘burned them out’). These are substantial rewards in return for one investment of hypothesis: that the second parameter in CV evolution, besides orbital period, is time since the last classical-nova eruption.

IX Draconis, ER UMa-type dwarf nova

AbstractWe present results of our world-wide observing campaign which was dedicated to one of the most active dwarf novae, IX Draconis. We investigated photometric behaviour of the system to derive its basic parameters. This study is important in the context of our understanding of the accretion process of IX Draconis, as well as other active cataclysmic variables, especially those from the still poorly studied type of ER UMa stars.

CURious Variables Experiment (CURVE): CCD photometry of active dwarf nova DI Ursae Majoris

Context. We report an analysis of photometric behaviour of DI UMa, an extremely active dwarf nova. The observational campaign (completed in 2007) covers five superoutbursts and four normal outbursts. Aims. We examined principal parameters of the system to understand peculiarities of DI UMa, and other active cataclysmic variables. Methods. Based on precise photometric measurements, temporal light curve behaviour, O−C analysis, and power spectrum analysis, we investigated physical parameters of the system. Results. We found that the period of the supercycle now equals 31.45 ± 0.3 days. Observations during superoutbursts infer that the period of superhumps equals Psh = 0.055318(11) days (79.66 ± 0.02 min). During quiescence, the light curve reveals a modulation of period Porb = 0.054579(6) days (78.59 ± 0.01 min), which we interpret as the orbital period of the binary system. The values obtained allowed us to determine a fractional period excess of 1.35% ± 0.02%, which is surprisingly small compared to the usual value for dwarf novae (2%–5%). A detailed O−C analysis was performed for two superoutbursts with the most comprehensive coverage. In both cases, we detected an increase in the superhump period with a mean rate of u P/Psh = 4.4(1.0) × 10 −5 . Conclusions. Based on these measurements, we confirm that DI UMa is probably a period bouncer, an old system that reached its period minimum a long time ago, has a secondary that became a degenerate brown dwarf, the entire system evolving now toward longer periods. DI UMa is an extremely interesting object because we know only one more active ER UMa star with similar characteristics (IX Dra).

Infrared Spectroscopy of Short-Period Cataclysmic Variables

We present near-infrared spectra for five dwarf novae with short orbital periods. We performed fittings of the spectral energy distributions (SEDs) by assuming a power-law distribution for the accretion disk and using template spectra of late-type dwarfs for the secondary star. Two are WZ Sge–or WX Cet-type stars, whose spectra show water absorption features, and one of them shows K$\;$ I and Na$\;$ I absorption lines in the $J$-band. We find that the secondary contributions to the overall SED are larger in WZ Sge stars than in ER UMa stars. The SED fitting suggests M9 and L1 type dwarf companions for our WZ Sge systems, and an M2 dwarf for our ER UMa star. We discuss the direct relationship between the high temperatures of secondary stars and high mass-transfer rates in ER UMa stars. SDSS J013701.06–091234.9 is the only object showing a spectrum dominated by the secondary component. The spectrum strongly suggests that the evolutionary path of SDSS J013701.06–091234.9 is different from that of ordinary CVs, and this object is a candidate of EI Psc-like systems.

In-the-gap SU UMa-type dwarf nova, Var73 Dra with a supercycle of about 60 days

An intensive photometric-observation campaign of the recently discovered SU UMa-type dwarf nova, Var73 Dra was conducted from 2002 August to 2003 February. We caught three superoutbursts in 2002 October, December and 2003 February. The recurrence cycle of the superoutburst (supercycle) is indicated to be60 d, the shortest among the values known so far in SU UMa stars and close to those of ER UMa stars. The superhump periods measured during the first two superoutbursts were 0.104885(93) d, and 0.10623(16) d, respectively. A 0.10424(3)-d periodicity was detected in quiescence. The change rate of the superhump period during the second superoutburst was 1:7 10 3 , which is an order of magnitude larger than the largest value ever known. Outburst activity has changed from a phase of frequent normal outbursts and infrequent superoutbursts in 2001 to a phase of infrequent normal outbursts and frequent superoutbursts in 2002. Our observations are negative to an idea that this star is an related object to ER UMa stars in terms of the duty cycle of the superoutburst and the recurrence cycle of the normal outburst. However, to trace the superhump evolution throughout a superoutburst, and from quiescence more eectively, may give a fruitful result on this matter.

Photometric study of new southern SU UMa-type dwarf novae and candidates -- II. Authentication of BF Ara as a normal SU UMa-type dwarf nova with the shortest supercycle

We photometrically observed the 2002 August long outburst of BF Ara. The observation for the first time unambiguously detected superhumps [average period 0.087 97(1) d], qualifying BF Ara as a genuine SU UMa-type dwarf nova. An analysis of the long-term visual light curve yielded a mean supercycle length of 84.3(3) d. The characteristics of outbursts and superhumps more resemble those of usual SU UMa-type dwarf novae rather than those of ER UMa stars. BF Ara is thus confirmed to be a usual SU UMa-type dwarf nova with the shortest known supercycle length. There still remains an unfilled gap of distributions between ER UMa stars and the usual SU UMa-type dwarf novae. We detected a zero period change of the superhumps, which is quite unexpected from our previous knowledge. This discovery implies that a previous interpretation requiring a low would no longer be valid or that a different mechanism is responsible for BF Ara. We propose that the reduced (prograde) apsidal motion of the eccentric disc by pressure forces may be responsible for the unusual period change in BF Ara.

Superoutbursts, superhumps and the tidal-thermal instability model

We include the tidal instability due to the 3:1 resonance in the disc instability model developed by Hameury et al. ([CITE]) and modified by Buat-Ménard et al. ([CITE]). We confirm earlier results by Osaki ([CITE]) that the tidal instability can account for the SU UMa light curves. We show that in ultra-low mass ratio systems such as ER UMa stars and WZ Sge stars, the superoutburst ends while the disc is still eccentric, as proposed by Hellier ([CITE]). However, since the disc shrinks rapidly once a cooling wave has started, the eccentricity should stop shortly after the end of a superoutburst. This result disagrees with the suggestion by Hellier that decoupling the thermal and tidal instability in the TTI model can account for late superhumps and echo outbursts in ultra-low mass ratio systems. We propose instead that ER UMa short supercycles can be explained either by the alternation of narrow and wide outbursts similar to those occurring in SS Cyg, or by the effects of irradiation (Hameury et al. [CITE]). In both cases, we predict that superhumps should be permanent, which is suggested by observations (Gao et al. [CITE]). We can also reproduce light curves similar to those of EG Cnc, varying the mass transfer rate in a TTI model including both irradiation and the presence of an inner hole in the disc.

Discovery of a New ER UMa-Type Dwarf Nova IX Draconis

Abstract We observed a new dwarf nova, IX Dra, for 46 days between 2000 November 10 and 2001 June 17, and revealed that IX Dra is the fifth member of a peculiar group of SU UMa stars, called ER UMa stars. The light curve is almost perfectly identical to that of V1159 Ori: supercycle, 53 d; interval of normal outbursts, 3–4 d; duty cycle, $\sim 30\%$; and the outburst amplitude, 2.5 mag. From a time-series observation during the superoutbursts, we obtained a presumable superhump period of 0.067 d. We note that some clustering may exist in the period distribution of ER UMa stars. It is needed to confirm this clustering by a larger number of examples.

High Spectral Resolution Time‐resolved Optical Spectroscopy of V893 Scorpii

We present high-resolution time-resolved optical spectra of the high-inclination, short orbital period dwarf nova V893 Scorpii. We performed spectral analysis through radial velocity measurements, Doppler mapping, and ratioed Doppler maps. Our results indicate that V893 Sco's accretion disk is dissimilar to WZ Sge's accretion disk and does not fit any of the current accretion disk models. We derive the system parameters M1 and i and present evidence for V893 Sco as a very young cataclysmic variable and an ER UMa star. We advance the hypothesis that all ER UMa stars may be newly formed cataclysmic variables.

CR Boo: the 'helium ER UMa star' with a 46.3-d supercycle

We observed the unique helium cataclysmic variable CR Boo (=PG 1346+082) extensively, using CCD and visual photometry, during the period 1995–1998. The observation revealed the existence of a quasi-periodic alternation of bright and faint states with a period of 46.3 d. During the faint states, the system shows short outbursts with a characteristic recurrence period of 4–8 d. The 1490-s signal, which has been considered as superhumps, almost disappeared during the short outburst on 1998 January 2. The overall patterns of outbursts and the appearance of the superhump signal are remarkably similar to those of hydrogen-rich ER UMa stars. We propose CR Boo as a member of a class of ‘helium’ counterparts of ER UMa stars.

Superhumps in a Peculiar SU Ursae Majoris-Type Dwarf Nova, ER Ursae Majoris.

We report the photometry of a peculiar SU Ursae Majoris-type dwarf nova, ER Ursae Majoris, for 10 nights during 1998 December and 1999 March, covering a complete rise to the supermaximum and a normal outburst cycle. Superhumps have been found during the rise to the superoutburst. A negative superhump appeared in the December 22 light curve, while the superhump on the next night became positive and had a large-amplitude waveform distinct from that of the previous night. In the normal outbursts we captured, superhumps with larger or smaller amplitudes seem to always exist, although it is not necessarily true for every normal outburst. These results show great resemblance to V1159 Ori. It is more likely that superhumps occasionally exist at essentially all phases of the eruption cycles of ER UMa stars, which should be considered in modeling.

WZ Sagittae as a DQ Herculis star

ABSTRA C T We argue that quiescent WZ Sge is a rapidly spinning magnetic rotator in which most of the matter transferred from the secondary is ejected from the system. Assuming that the observed 27.87-s oscillation period results from the spinning white dwarf, we propose that the other observed principal period of 28.96 s is a beat caused by reprocessing of the rotating white dwarf beam on plasma blobs in Keplerian rotation at the outer disc rim. The weaker, transient, 29.69-s period is identified as a beat with the Keplerian period of the magnetosphere. WZ Sge evolves through a cycle of spin-up and spin-down phases. During the spin-down phase it is a DQ Her star; during the spin-up phase it should be a ER UMa star.

Observational Constraints on Evolution of Dwarf Novae in and Below ThePeriod Gap

There is a gap in the distribution of the orbital periods of cataclysmic variables (CVs) between 2 and 3 hours. The period gap is explained at present by cessation of the mass transfer making CVs fainter and preventing them from being discovered. After restarting the mass transfer, CVs have been believed to evolve with the orbital periods becoming shorter, as angular momentum is released by gravitational wave radiation. In this view, the mass transfer rate depends almost only on the orbital period.However, reconsideration of these views is urged by a series of recent discoveries of ER UMa stars (a subclass of SU UMa stars having enormous mass-transfer rates), dwarf novae bridging “classical” SU UMa stars and ER UMa stars, and the first in-the-gap dwarf nova, PG 1510+234. These objects suggest two possibilities; 1) (a part of) SU UMa stars undergo large secular changes of the mass-transfer rate, and 2) there exist parameters overlooked but strongly influential in the evolution of CVs.

The 1995 Superoutburst of the WZ Sge–type Dwarf Nova AL Comae Berenices

We report time-resolved photometric observations of the first superoutburst confirmed since 1975 in a most enigmatic cataclysmic variable, AL Comae Berenices, during 1995 April-June. We detected double-peaked modulations in the early phase of the outburst, the period of which is slightly shorter than the superhump period. Three possibilities for the mechanism causing these oscillations are discussed. We also refer to similar modulations observed in ER UMa stars.

The link between soft X-ray transients and ER Ursae Majoris stars

There are now at least five sources classified as Soft X-ray Transients (SXTs) which show light curve modulations during their active states that are reminiscent of superhumps in SU UMa stars. We compare the morphology of the humps in SXTs with those in SU UMa stars and find that they are more analogous to the small subclass of ER UMa systems than to the bulk of the SU UMa's. The ER UMa stars apparently differ from other SU UMa systems only in having a much higher mass-transfer rate, one that is close to the stability limit of the thermal instability in the disc. In general SU UMa stars have very low-mass Roche-lobe filling components because a low mass ratio is required to excite the resonant instability, that probably drives the superhumps, in the outer edge of their accretion discs. If the degenerate components of SXTs are black holes rather than white dwarfs the system and hence the Roche-lobe filling component can be much more massive and still excite superhumps. Because X-ray irradiation of the disc by a central accreting neutron star or black hole stabilizes the thermal instability at lower mass-transfer rates, it is difficult to get transient behaviour in black-hole systems. Thus, mass-transfer rates in SXTs must naturally be close to the instability limit, and consequently these systems are similar to ER UMa stars in this sense. We test the determination of the mass, or at least a lower limit on it, of the compact primary of SXTs by means of the superhump period.

THE 1994 SUPEROUTBURST OF THE NEW SU UMA-TYPE DWARF NOVA, SX LEONIS MINORIS

ABSTRACT We present observations of SX LMi obtained during the 1994 long outburst at Ouda Station. Superhumps, with a period of 0.06950 (± 0.00002) d, are detected. This confirms SX LMi as a new member of the SU UMa-type dwarf novae. We measured the recurrence cycle of normal outbursts and superoutbursts to be about 35 d and 250 d, repsectively, from outburst records by RoboScope (Indiana University) and amateur observers. These recurrence cycles are typical values for normal SU UMa stars. However, the outburst amplitude (∼3.8 mag) of SX LMi is small for an SU UMa star and close to those of ER UMa stars. This small outburst amplitude might be understood if the viscosity at quiescence and the mass-transfer rate are slightly high in SX LMi.

ORBITAL PERIODS FOR THE UNUSUAL DWARF NOVAE ER URSAE MAJORIS AND V1159 ORIONIS

ABSTRACT ER UMa (PG 0943+521), V1159 Ori, and RZ LMi constitute a small recently- recognized group of dwarf novae, called the RZ LMi stars or the ER UMa stars. They share many features of the SU UMa-type, but have shorter outburst cycles than had been known previously. Here we establish orbital periods, based on emission-line radial velocities, for ER UMa (0.06366 ± 0.00003 d = 91.67 ± 0.04 min) and V1159 Ori (0.06217801 ± 1.3 × 10-7 d = 89.5363 ± 0.0002 min). The precise orbital period for V1159 Ori depends on an extrapolation of cycle count from 1991 to 1994, which is slightly uncertain, but the daily cycle count is reasonably secure. These orbital periods are similar to those of the more common, less frequently outbursting SU UMa stars. As is generally the case, the orbital periods derived here are slightly shorter than the periods of photometric superhumps which appear in superoutburst. The superhump period excesses found here are compatible with, but marginally larger than, those seen in other SU UMa stars of similar period. While the empirical link between ER UMa stars and SU UMa stars is generally strengthened by these results, the trend toward slightly larger period excesses (if true) can be interepreted as a tendency for systems with larger mass ratios to develop larger mass-transfer rates at a given orbital period.

Discovery of a New SU UMa Star, CI Ursae Majoris

We detected superhumps with a period of 0.0625 (±0.0007) d in CI UMa during a bright outburst in 1995 January. From this, CI UMa was first proven to be a SU UMa-type dwarf nova. We found the interval of the superoutburst to be about 140 d from historical records of the outburst. If this represents the cycle length of the superoutburst (supercycle), CI UMa has the third-shortest supercycle among usual SU UMa stars, except for ER UMa stars. We regard CI UMa as being an intermediate object between ER UMa stars and the usual SU UMa stars based on the short orbital period estimated from the superhump period and the short supercycle. It may be crucial to clarify whether the supercycle has changed by closely monitoring CI UMa in order to solve any problem concerning the evolutionary path of ER UMa stars.

DI Ursae Majoris: Discovery of an Extreme ER UMa-Type Dwarf Nova with the Shortest Orbital Period

The long-term, time-resolved CCD photometry of a dwarf nova, DI UMa, has revealed this object to be a peculiar SU UMa-type dwarf nova having both the shortest ever observed superhump period (0.0555 d) and a very short (25 d) supercycle length. These features make DI UMa the fourth established member of a recently discovered small group of dwarf novae, called ER UMa stars or RZ LMi stars, which are now believed to be SU UMa-type dwarf novae with exceptionally high mass-transfer rates. DI UMa in all respects resembles a peculiar dwarf nova RZ LMi, but is more extreme in its short superhump period. The high-incident discovery of objects with such an extreme combination of parameters, which can not be expected from the standard evolution of cataclysmic variables (CVs), should provide a unique tool in probing the long-debated problem concerning the origin of mass-transfer in CVs and a secular alteration between CV subclasses.

DWARF-NOVA OUTBURSTS

ABSTRACT Outburst mechanisms of dwarf novae are discussed. There is a rich variety in outburst behaviors of non-magnetic cataclysmic variable stars, starting from non-outbursting nova-like stars to various sub-classes of dwarf novae (i.e., U Gem-type, Z Cam-type, and SU UMa-type). A unification model for dwarf nova outbursts is proposed within the basic framework of the disk instability model in which two different intrinsic instabilities (i.e., the thermal instability and the tidal instability) within accretion disks play an essential role. Non-magnetic cataclysmic variables are classified into four regions in the orbital-period versus mass-transfer-rate diagram, showing different combinations of stability behavior for the two intrinsic instabilities in accretion disks, and their different outburst behaviors are basically understood in this diagram. We discuss several problems concerning the thermal limit-cycle instability model for dwarf novae above the period gap. We then discuss the thermal-tidal instability model for SU UMa stars, dwarf novae below the period gap, in which the coupling of the two intrinsic instabilities in the accretion disk plays a unique role. In particular, a rich variety of outburst behaviors of cataclysmic variables below the period gap (i.e., starting from "permanent superhumpers", to Z Cam-like SU UMa stars, "ER UMa stars", to ordinary SU UMa stars, and finally to WZ Sge stars) is understood by the thermal-tidal instability model.