unusual “stunted” outbursts in old novae and nova-like cataclysmic variables

THE ASTRONOMICAL JOURNAL, 115 :2527È2538, 1998 June1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

UNUSUAL ““ STUNTED ÏÏ OUTBURSTS IN OLD NOVAE AND NOVA-LIKE CATACLYSMIC VARIABLES

R. K. J. W. AND G. W.HONEYCUTT, ROBERTSON,1 TURNER

Department of Astronomy, Indiana University, Swain Hall West, Bloomington, IN 47405 ; honey=astro.indiana.edu, psjr=atuvm.atu.edu,turner=astro.indiana.edu

Received 1998 January 12 ; revised 1998 February 23

ABSTRACTOutbursts averaging 0.6 mag in amplitude and 10 days in width are described in Ðve old novae and

nova-like cataclysmic variables : UU Aqr, Q Cyg, CP Lac, X Ser, and RW Sex. These stars are thoughtto be high mass transfer rate systems for which the accretion disk is expected to be stable against thethermal instability responsible for dwarf nova outbursts. The widths and spacings of these events aresimilar to those of dwarf nova eruptions, but the amplitudes are signiÐcantly smaller, or ““ stunted. ÏÏ Theoutbursts are sometimes accompanied by dips. These dips have amplitudes that are similar to the out-burstsÏ but have shapes that scatter signiÐcantly more than the shapes of the outbursts. The outburstsand dips sometimes occur as pairs and are sometimes isolated. We are not able at this time to determinea single common mechanism for this behavior, or even to conclude that some mechanisms are preferred.Rather, we characterize these phenomena with regard to outburst shapes and frequency of occurrenceand explore a range of possible causes, including truncated disks, mass transfer modulations, andZ Camelopardalis type behavior. Arguments are assembled for and against such possible mechanisms,and key observations are suggested. It appears unlikely that accretion disk instabilities are the singlecommon cause of these phenomena, and we are left with either a combination of accretion disk andmass transfer events or a situation in which mass transfer events are somehow responsible for all thesevaried behaviors.Key words : novae, cataclysmic variables È

stars : individual (UU Aquarii, Q Cygni, CP Lacertae, X Serpentis, RW Sextantis)

1. INTRODUCTION

Subclasses of cataclysmic variables (CVs) usually denotethe nature of their photometric behavior (Warner 1995a).Dwarf novae (DNs) have outbursts (OBs) of 2È6 mag thatoccur with characteristic repetition rates of weeks to years.These OBs are generally thought to be due to the action of athermal instability in the accretion disk (Cannizzo 1993a ;

& Ritter that operates only in aMeyer-Hofmeister 1993)restricted range of the accretion rate from the donor star,

Nova-like (NL) CVs do not display DN OBs, and theirM0 .relatively steady brightness is thought to be due to their M0exceeding the upper stability limit, for OBs.M0 crit, Cannizzo,Shafter, & Wheeler give an expression for that is(1988) M0 critindependent of the free parameter a, namely,

M0 crit\ 1016r102.6 m1~0.87 g s~1 ,

where is the outer disk radius in units of 1010 cm andr10 m1is the white dwarf mass in solar units. For typical values ofand we Ðnd yr~1.r10 \ 4 m1\ 1, M0 crit ^ 6 ] 10~9 M

_The notion that NLs are above is supported by theM0 critfact that NL CVs have on average, D4 mag brighterMV,

than DNs Observationally, very few post-(Warner 1995a).nova CVs are found to be DNs et(Livio 1987 ; Honeycuttal. Instead, old novae generally have photometric1998a).and spectroscopic characteristics similar to those of NLCVs and may be kept at higher in the decades followingM0the nova by the lingering e†ects of the thermonuclearrunaway responsible for the nova explosion Prial-(Kovetz,nik, & Shara 1988).

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ1 Current address : Department of Physical Sciences, Arkansas Techni-

cal University, Russellville, AR 72801-2222.

The long-term photometric behavior of DNs is quite wellestablished compared with that of old novae and NL CVs.This is because a number of DNs are popular targets forvisual and amateur observers as a result of their brightnessand their frequent large OBs (e.g., SS Cyg ; &CannizzoMattei Bianchini studied the long-term1992). (1990, 1992)variability of old novae in the context of solar-type cycles,Ðnding that they display several kinds of variability ontimescales of weeks to years. Discussions of long-termvariability in old novae and NLs can also be found in

and However, the timeWarner (1995b) Andronov (1995).coverage of these observations is generally insufficient toreveal the kinds of details that have been studied in DNlight curves.

Because of the relative paucity of long-term NL photom-etry, a program was initiated in 1990 to obtain regularCCD imaging of about D100 CVs, including D65 oldnovae and NL systems. About 6.5 years of data have nowaccumulated on these objects. Details of VY Sculptoristransitions are being revealed Livio, &(Honeycutt,Robertson Cannizzo, & Robertson1993 ; Honeycutt,

Robertson, & Honeycutt as well as1994a ; Hillwig, 1998),occasional 0.5 mag oscillatory light curves in old novae andNLs et al. Robertson, &(Honeycutt 1994b ; Honeycutt,Turner This present paper discusses another kind of1995a).variability among old novae and NLs, namely, repetitiveD0.6 mag OBs, sometimes accompanied by D0.6 mag dips.These OBs and dips occur in D20% of the old novae andNLs that are being monitored. We discuss here the resultsfor Ðve systems : UU Aqr, Q Cyg (Nova Cyg 1876), CP Lac(Nova Lac 1936), X Ser (Nova Ser 1903), and RW Sex.Other systems being monitored also have occasional OBsand dips similar to the ones in these systems (see, e.g., thelight curves of KR Aur and V841 Oph in et al.Honeycutt

2527

1 2

3 AB

4 5 6

CD

E

F

2528 HONEYCUTT, ROBERTSON, & TURNER Vol. 115

FIG. 1.ÈLight curve of UU Aqr from 1990 November to 1997 July.Data points within ^0.0425 phase units of the D1.8 mag deep eclipse areomitted.

but these Ðve are the better examples for the study1994b),of multiple OBs.

2. OBSERVATIONS

The data were acquired by RoboScope, a 0.41 m tele-scope in Indiana equipped for automated di†erential CCDstellar photometry & Turner All observa-(Honeycutt 1992).tory operations are accomplished as fully unattended andunsupervised tasks (including data reduction), making prac-tical the kind of long, homogeneous data streams needed forthis study. Typically, RoboScope obtains one or two 4

FIG. 2.ÈThe UU Aqr data of plotted on an expanded scale forFig. 1three observing seasons 1990È1993. Data points spaced closer than 3.5days are connected by lines. The labels refer to entries in Tables and1 2.

FIG. 3.ÈSame as but for 1994È1997 seasonsFig. 2,

minute exposures per clear night on each of D140 programstars. The data are reduced using the method of incomplete-ensemble photometry and the zero point(Honeycutt 1992),of the di†erential light curve is established using secondarystandards from Henden & Honeycutt (1995, 1997).

Figures show the light curves for each of the Ðve1È17stars discussed here. For clarity, error bars are not plottedin the compressed light curves of Figures and1, 4, 8, 12, 15 ;however, error bars are included on the expanded plots.Photometric errors are typically 0.01 mag at V D 14 and0.06 mag at V D 18 and are generally negligible for thepurposes of this paper. It is apparent in the compressedplots that each star has a relatively well deÐned, slowlyvarying quiescent level from which OBs and dips appear. It

FIG. 4.ÈLight curve of Q Cyg from 1991 July to 1997 July

1

2

A

3 4 5 6 7

8 B

9 10 11

1

A 2

No. 6, 1998 ““ STUNTED ÏÏ OUTBURSTS IN OLD NOVAE 2529

FIG. 5.ÈThe Q Cyg data of plotted on an expanded scale forFig. 4three observing seasons 1991È1994. Data points spaced closer than 3.5days are connected by lines. The labels refer to entries in Tables and1 2.

is also apparent that there are sometimes other interestingvariations in these stars apart from the OBs and dips. Thispaper deals solely with a single common behavior that hasgone largely unexamined, namely, OBs that resemble those


FIG. 7.ÈSame as but for 1997 seasonFig. 5,

FIG. 8.ÈLight curve of CP Lac from 1991 June to 1997 August

of DNs but are of signiÐcantly smaller amplitude (i.e.,““ stunted ÏÏ). These events are of particular interest becauseOBs are not expected to occur in these high-state accretiondisk systems.

3. OUTBURSTS

In these stars, one sees occasional well-deÐned OBslasting from days to weeks superposed on longer timescalerandom variations. Those OBs that are well deÐned arecharacterized in for the Ðve systems. There is someTable 1unavoidably subjective judgement involved in choosing theevents to be included in Table 1 from the background of

FIG. 9.ÈThe CP Lac data of plotted on an expanded scale forFig. 8three observing seasons 1991È1994. Data points spaced closer than 3.5days are connected by lines. The labels refer to entries in Tables and1 2.

3

4

5

6

1 2

2

3



sometimes poorly sampled OBs and random variations. Ingeneral, Table 1 lists only OBs that are sampled wellenough to measure the amplitude and at least one otherparameter, and only OBs that are ““ conspicuous ÏÏ in the

FIG. 11.ÈSame as but for 1997 seasonFig. 9,

FIG. 12.ÈLight curve of X Ser from 1992 July to 1997 July

FIG. 13.ÈThe X Ser data of plotted on an expanded scale forFig. 12three observing seasons 1992È1994. Data points spaced closer than 3.5days are connected by lines. The labels refer to entries in Tables and1 2.

sense that they stand out well from the background. Thereare other less well deÐned OBs in these light curves that arenot included in Table 1 and, perhaps, a few events that arenot really OBs in Table 1. The expectation is that this isnevertheless a representative sample suitable for examiningthe systematics of the phenomenon. The most serious errorthat could occur in this regard is to have chosen fromamong a large number of light curves, each often containingvariations on numerous timescales, just those randomvariations that resemble OBs. We are conÐdent that is notthe case. Most of the 65 old novae and NLs in the programdo not show these OBs at all, and these null detections


1

A

2

B

C


FIG. 15.ÈLight curve of RW Sex from 1990 December to 1997 May.(An unusually faint data point at JD\ 2,449,053.8, V \ 12.06 has beenomitted to allow a favorable scaling on the magnitude axis.)

include systems with random variations both weaker andstronger than the Ðve systems discussed here. The fewsystems that do show OBs usually have numerous OBs,which is inconsistent with these being noise. Finally, theparameters in have, in general, less scatter in ampli-Table 1tude and FWHM among the OBs in a given system thanfrom system to system. All these characteristics stronglysupport the assumption that these are real OBs that arerepeatable in some sense and are not just part of a noisespectrum.

FIG. 16.ÈThe RW Sex data of plotted on an expanded scale forFig. 15three observing seasons 1991È1994. Data points spaced closer than 3.5days are connected by lines. The labels refer to entries in Tables and1 2.


In the Ðrst column is a label for the OB, keyed toTable 1,the labels on the expanded light curves. The second columnlists the time of the OB, taken as the Julian Date of thebrightest data point. The uncertainty of these times is typi-

TABLE 1

SELECTED OUTBURSTS IN OLD NOVAE AND NOVA-LIKE CVS

JD Amplitude FWHM qrise qfallOutburst (2,448,000]) (mag) (days) (days) (days)(1) (2) (3) (4) (5) (6)

UU Aqr :1 . . . . . . . 882 0.6 4 6 72 . . . . . . . 891 0.5 3 7 73 . . . . . . . 1229 [0.9 7 13 \74 . . . . . . . 1591 [0.6 [6 29 : . . .5 . . . . . . . 1631 0.6 6 25 86 . . . . . . . 1683 [0.6 4 3 8

Q Cyg :1 . . . . . . . 551 0.4 5È8 35 . . .2 . . . . . . . 1215 [0.6 9 10 63 . . . . . . . 1504 0.5 13 24 . . .4 . . . . . . . 1544 0.7 12 21 115 . . . . . . . 1611 [0.7 23 60 \106 . . . . . . . 1654 0.5 \14 \15 217 . . . . . . . 1688 [0.7 \16 19 . . .8 . . . . . . . 1948 0.6 20 \17 209 . . . . . . . 2312 0.4 9 21 \1410 . . . . . . 2338 0.4 10 9 4511 . . . . . . 2388 [0.5 10 \18 17

CP Lac :1 . . . . . . . 606 [0.6 6È9 6 : . . .2 . . . . . . . 1372 0.7 6 5 6 :3 . . . . . . . 1664 [0.5 8 \8 \44 . . . . . . . 1991 0.5 4 \3 35 . . . . . . . 2391 [0.6 \6 4 \86 . . . . . . . 2668 0.4 7 : 3 : . . .

X Ser :1 . . . . . . . 840 [1.5 60 25 302 . . . . . . . 1075 0.9 10È20 \9 . . .3 . . . . . . . 1447 0.7 12È18 \12 28

RW Sex :1 . . . . . . . 684 [0.6 \30 . . . . . .2 . . . . . . . 1443 0.3 11 30 60

2532 HONEYCUTT, ROBERTSON, & TURNER

cally 2È4 days because of the sampling. Column (3) lists theamplitude measured from the local quiescent level. If thequiescent level was changing, the amplitude was measuredfrom this sloping baseline. These amplitudes are formallynearly always lower limits because the sampling of 1È4 daysmeans the peak is often not actually resolved. etHoneycuttal. measured OBs in V446 Her that were of similar(1998b)amplitude and width to the OBs measured here, and whichwere obtained with the same mean data spacing. Becausethose V446 Her OBs are so numerous, some were in factwell resolved, and it was shown that the mean lower limitswere only D20% smaller than the mean of the resolvedpeaks. We will assume that the same is true for this data setand will therefore treat amplitude lower limits as trueamplitudes, with little error. Column (4) is the FWHM indays, with a typical error of 3 days. Columns (5) and (6) arethe e-folding times for the rise and fall, respectively, of theOB, with typical errors of 25%.

The amplitudes of the OBs are relatively uniform, at 0.5È0.7 mag. When the OBs are frequent, as in the top panel of

for UU Aqr and in the top and bottom panels ofFigure 3for Q Cyg, the spacing is seen to be D25È50 days.Figure 6

The widths can vary appreciably but are commonly D10days. Except for the small amplitude, these OB parametersfall within the range of such parameters for DN OBs. Thissuggests that perhaps these OBs are DN events whoseamplitude has been attenuated by the presence of additionalsystem light.

The issue of additional system light is confused by therecent history of the OBs in V446 Her (\Nova Her 1960).Those OBs were originally interpreted as being due to masstransfer events Robertson, & Turner(Honeycutt, 1995b),partly because their mean amplitude of D1.5 mag fallsbelow the range of most observed DN OBs. However, it wassubsequently determined that these are really D2.5 magOBs contaminated by the light of two nearby optical com-panions et al. These V446 Her OBs were(Honeycutt 1998a).also shown to have shapes and spacings very similar tothose of DNs, and the conclusion was that V446 Her is oneof a very few old novae that now have DN OBs.

plots the amplitude versus FWHM for the OBsFigure 18in Note that, in general, the scatter in OB param-Table 1.eters for a given star is smaller than the scatter from star tostar. This suggests that the OBs in each star have a pre-ferred strength and shape, a situation that might eventuallyhelp determine the origin of these OBs.

also includes for comparison the mean ofFigure 18similar measures for OBs in V446 Her from et al.Honeycutt

We see that the widths of these stunted OBs are(1998a).similar to those in V446 Her while the amplitude in V446Her is larger (but still smaller than most DNs). Might thestunted OBs reported here be due to the same e†ect ofblended companions as is the case in V446 Her? We do notthink so. shows expanded views of direct imagesFigure 19of these Ðve stars. Some of these images were obtained atthe WIYN and others at the US NavalObservatory2Observatory Flagsta† Station as part of a program forestablishing secondary standards (Henden & Honeycutt

With the exception of V446 Her (which is1995, 1997).

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ2 The WIYN Observatory is jointly operated by the University of

Wisconsin, Indiana University, Yale University, and the National OpticalAstronomy Observatories.

FIG. 18.ÈParameters of the outbursts in Ðve systems, as listed in TableEntries in Table 1 having only limits for FWHM are omitted from the1.

plot. However, entries with only limits for amplitude are included sincethese limits di†er little from the true values. The plotted values of ampli-tude and FWHM for two of the data points have been adjusted in minorways from the values in Table 1 to avoid confusion of overlapping points.The point for outburst 1 of X Ser is o† the plot. The point for V446 Her isthe average of seven measures of FWHM from the ““ weak ÏÏ outbursts listedin et al. the error bar is the standard deviation of theHoneycutt (1998a) ;mean. The mean amplitude of the outbursts in V446 Her is D1.5 magwhen the light of the close optical companions is included and is D2.5 magwhen corrected for the light of the companions.

shown in for comparison), the few nearby stars inFig. 19these images are faint and could only decrease the OBamplitude by at most a few tenths of a magnitude if inadver-tently included in the photometry. Furthermore, we do notdetect any image elongation indicative of unresolved com-panions. Therefore we Ðnd no evidence to support thishypothesis other than the (misleading, we think) example ofV446 Her.

4. DIPS

In addition to the OBs in these Ðve stars, there are dipsvisible in the light curves that bear a strong superÐcialresemblance to inverted OBs. For example, if the light curveof RW Sex in is inverted, the apparent characterFigure 15of the variations is hardly changed! However, this result issomewhat misleading. More detailed examination revealsthat the dips in these Ðve stars are less numerous and showconsiderably more diverse behavior than the OBs. Thesedips resemble somewhat the dips seen superposed on theslower VY Sculptoris type transition in DW UMa

et al.(Honeycutt 1993).tabulates the properties of some of these dips.Table 2

The table entries are analogous to those for We seeTable 1.that while the mean dip amplitude of D0.5 mag is quitesimilar to the amplitudes of the OBs, the durations andshapes of the dips vary rather widely. Because the dips arefewer in number than the OBs and because of their greaterdiversity, they are more difficult to distinguish unam-biguously from random variations. Therefore, the caveats

Q Cyg CP Lac

UU Aqr Her V446

X SerRW SexFIG. 19.ÈExpanded V -band images of the Ðve program stars, showing no evidence for close companions that might attenuate outburst amplitudes. The

aperture for the long-term photometry was D4A. These images are 30A on a side, north at the top and east to the left. Images for Q Cyg, CP Lac, UU Aqr, andV446 Her were obtained with the WIYN telescope ; they have a pixel size of and have FWHM in the range Images for RW Sex and X Ser were0A.2 0A.6È0A.7.obtained with the US Naval Observatory Flagsta† Station 1 m reÑector ; they have a pixel size of and have FWHM in the range V446 Her is0A.7 1A.5È2A.0.shown as an example of a system for which close companions do attenuate the apparent outburst amplitude in the long-term photometry.


TABLE 2

SELECTED DIPS IN OLD NOVAE AND NOVA-LIKE CVS

JD Amplitude FWHM qfall qriseDip (2,448,000]) (mag) (days) (days) (days) Typea

UU Aqr :A . . . . . . 1239 0.5 [0.7 11 : . . . fB . . . . . . 1296 [0.5 . . . 8 17 : fC . . . . . . 1934 0.4 \4 \5 18 i :D . . . . . . 1955 [0.2 \3 \2 \1 iE . . . . . . 1966 [0.3 2 8 \2 iF . . . . . . 2357 [0.3 4 \1 8 : f :

Q Cyg :A . . . . . . 1227 0.4 6 11 9 fB . . . . . . 1991 0.5 50 11 180 f :

CP Lac :A . . . . . . 1268 0.6 40 115 14 i

RW Sex :A . . . . . . 1027 [0.7 3 6 6 iB . . . . . . 1463 0.4 10 11 30 fC . . . . . . 2058 0.3 7 17 19 . . .

a Here f denotes that the dip immediately follows an outburst ; i is an isolated dip.

described earlier in connection with the selection of OBsthat were included in apply even more strongly toTable 1the dips in We have included in Table 2 only dipsTable 2.for which at least two parameters could be measured. Thereare undoubtedly additional real dips that are not listed inTable 2, but the expectation is that Table 2 is a representa-tive (albeit small) sample of the behavior.

Some of the dips in these light curves are temporallyassociated with the OBs. Good examples can be seen in thebottom panel of for UU Aqr, the bottom panel ofFigure 2

for Q Cyg, and the bottom panel of forFigure 5 Figure 16RW Sex. The last column of notes whether the dipTable 2is isolated or immediately follows an OB. There is a ten-dency for the dips that are paired with OBs to have shapesand durations similar to those of the preceding OB, and theisolated dips are systematically the most diverse dips.Despite the sometimes fragmentary data and the uncertaincharacter of some of the entries in the last column of Table2, there are nevertheless two rather Ðrm conclusions : (1)pairings of OBs and dips occur much more frequently thana random distribution of each type of event would predict,and (2) dips nevertheless deÐnitely sometimes occur wellisolated from any OB.

5. DISCUSSION OF CANDIDATE MECHANISMS

In this section we list a number of mechanisms that couldaccount for the photometric behavior just described, sug-gesting further observational tests of each. We cannot ruleout the possibility that more than one mechanism is atwork for this sample, even in the same star.

5.1. Mass Transfer ModulationsCV disks in the high state are nearly steady state, with

large viscosity that allows the disk brightness to followchanges in from the secondary star. We know that suchM0

occur in at least some CVs. Highly magneticM0 -changesCVs (polars) often have large brightness variations thatcannot be attributed to disk instabilities in these disklesssystems. VY Sculptoris type CVs with disks also have largebrightness variations, as the accretion from the secondarystar sometimes turns o† completely. Possible mechanismsfor mass transfer modulations include solar-type cycles onthe secondary star (Warner 1988 ; Bianchini 1990 ; Richman,

Applegate, & Patterson the migration of starspots1994),under the inner Lagrangian point & Pringle(Livio 1994),and cyclic induced by irradiation of the secondary starM0by the disk Wickramasinghe, & Warner Early(Wu, 1995).work aimed at explaining DN OBs as triggered by a peri-odic instability in the atmosphere of the donor star (Bath

is seldom considered viable nowadays in the original1975)context, but may nevertheless be a candidate for thesestunted OBs. (Note, however, criticisms of this mass transferinstability by various authors, summarized in Warner

pp. 168È169.) points out that a varia-1995a, Warner (1995b)tion of 0.7 mag in a steady state disk requires nearly a factorof 10 range of through the disk. Such variations in areM0 M0not seen in DNs (they would show up as variations in thebrightness of the hot spot), and Warner therefore concludesthat modulations seen on timescales of tens of days in NLsdo not originate in from the secondary star. We noteM0here that if mass transfer modulations are invoked as themechanism for these stunted OBs, then the similarity inspacing and duration to OBs in DNs must be taken ascoincidental.

There are several possible tests for mass transfer modula-tions. The hot spot produced where the accretion streamintersects the disk will be enhanced in brightness during amass transfer burst, producing observable e†ects in systemsof suitably high inclination. Also, & VerbuntLivio (1988)and others (see have shown that there is anWarner 1995a)expected characteristic change in disk radius in responseto a temporary enhancement of mass transfer. However,both these kinds of tests are observationally demanding,requiring coordination of orbit-resolved photometry andspectroscopy with the appearance of stunted outbursts.

5.2. Dwarf NovaÈType OutburstsThe durations and spacings of these stunted OBs are

consistent with those of DNs. However, the amplitudes falloutside the range of observed DN amplitudes et(Pattersonal. being too small by 1.5È5.5 mag. Inappropriate1996),inclusion of extra light in the measurement from unsus-pected optical companion stars could lower the observedamplitude, but we do not think this is the case, for reasonscited in Furthermore, it would be a rather impressive° 3.coincidence if unsuspected companions always contributed


just the brightness necessary to produce consistent D0.6mag OB amplitudes for this sample of Ðve stars.

If for these stars has fallen to the instability regime,M0then the disk should be cooler than for NLs,high-M0resulting in a lower excitation spectrum. In weFigure 20,update a plot originally presented in et al.Honeycutt

showing the location of old novae and NLs, Z Cam-(1995b)elopardalis systems, and DNs in excitation space, as mea-sured by the equivalent widths of spectral features havingdi†erent excitation energies. (The data point for Q Cyg isimproperly labeled in et al. the properHoneycutt 1995b ;location of Q Cyg is shown in The data are takenFig. 20.)from the literature compilation assembled by Echevarr•� a

from measures made on the CV spectra in(1988),Kaitchuck, & Schlegel and using unpub-Honeycutt, (1987),

lished CV spectra from WIYN and KPNO. Scatter is intro-duced into this plot because of the e†ects of inclination,both long-term and orbital variations in line-strengthratios, and contributions to the lines from winds or accre-tion columns. Nevertheless, there is the expected progres-sion in this plot from DN, to Z Cam system, to nova andNL, showing that the excitation level increases with M0 .Four of these Ðve program stars are also plotted, using forthe most part unpublished spectra from WIYN. These fourare seen to lie mostly in the regime of the novae and NLCVs. However, they are scattered widely enough in the plotthat conclusions about their collective would be riskyM0from this plot alone.

An additional indication of whether these Ðve systemsmay have such low that DN OBs are possible is whetherM0they fall close to the DN regime in an versus orbitalM

vperiod diagram. In Figure 9.8 of only twoWarner (1995a),of these systems are plotted, and neither CP Lac nor RWSex is systematically near the DNs. (There is evidence,

FIG. 20.ÈDependence of CV line excitation on variability type. Datafor novae, NLs, Z Cam systems, and DNs are taken from the literature andfrom other unpublished measures. The boldface letters show new data forfour of the systems that have stunted outbursts ; X \ X Ser, Q \ Q Cyg,U \ UU Aqr, and C \ CP Lac. The point for CP Lac is an upper limit forboth equivalent width ratios.

however, that UU Aqr has a low near the DN instabilityM0regime, as discussed in Among the systems for which° 6.1.)

et al. derived from IR colors, neitherWeight (1994) M0Q Cyg nor CP Lac has low for an old nova or NL.M0

DN disk instabilities are nevertheless an attractive expla-nation for at least some of these OBs and dips if an extrasource of light can be invoked. This would be particularlytrue if random variations in this extra light were to beuncoupled from the from the secondary star and if theM0extra light were somehow associated with an increased exci-tation level.

5.3. Dwarf Nova Outbursts in a Truncated DiskA truncated disk is one in which the inner portion of the

disk is missing yet the remaining material retains many ofthe usual characteristics of accretion disks. The inner partmay be missing because of heating by magnetic torques

& Pringle by a coronal siphon &(Livio 1992), (MeyerMeyer-Hofmeister or by a white dwarf magnetic Ðeld1994),that is strong enough to directly disrupt the disk. The latterscenario has received signiÐcant attention &(AngeliniVerbunt Abbott, & Shafter1989 ; Wood, 1992 ; Lasota,Hameury, & Hure� Livio, & Tout1995 ; Warner, 1996)because a variety of otherwise difficult to explain propertiesof disks in particular systems can be addressed by invokinga truncated disk.

Though the extant models are not yet very general, itappears that in a truncated disk inside-out OBs are lessfrequent, somewhat more narrow, and somewhat less bright

& Verbunt et al.(Angelini 1989 ; Cannizzo 1993b ; Lasotaet al. If the inner disk radius is large1995 ; Warner 1996).

enough to exclude the temperature needed to initiate aninside-out OB, then OBs can be suppressed altogether. Thiscondition is more likely to occur in than inlow-M0 high-M0systems such as NLs. Truncated disks are of interest withrespect to these stunted OBs because of the potential toreduce the apparent amplitude of the OB. This is a possi-bility because not only are the modeled OBs up to 1 magfainter, but also only a portion of the available gravitationalpotential energy has been released in a truncated disk. If theremaining energy available from the inner disk edge downto the boundary layer can be released at least partially atoptical wavelengths, then the DN OBs in the truncated diskcould appear at signiÐcantly lower amplitude.

The observational signatures of a truncated disk mightinclude modiÐed emission-line proÐles due to the missinghigher disk velocities, modiÐed proÐles of strong UVabsorption lines Diaz, & Hubeny or a modi-(Wade, 1996),Ðed energy distribution due to the missing hotter part of thedisk Abbott, & Shafter van Paradijs,(Wood, 1992 ; Rutten,& Tinbergen If the truncated disk is due to a magne-1992).tized white dwarf, then the well-studied photometric signa-tures of intermediate polars might be(Patterson 1994)apparent, i.e., the presence of period(s) in addition to theorbital period.

5.4. Z CamÈlike OutburstsA recent study of the long-term light curves of Z Camelo-

pardalis systems et al. discussed two(Honeycutt 1998b)kinds of behavior that may be related to these stunted OBs.First, the amplitudes of Z Cam oscillations are seen tobehave like a damped oscillation as standstill is entered.The timescale of the damping is approximately equal to themean OB recurrence interval, and the oscillations can often


be followed to amplitudes as small as a few tenths of amagnitude as standstill is entered. Second, it is not unusualto see an exit from standstill that persists for just one fulloscillation before returning to standstill, much as we seeoccurring in the paired OBs and dips (see examples forZ Cam and AH Her in et al. However,Honeycutt 1988b).these OB-dip pairs in Z Cam stars have full amplitudes ofD2.5 mag, much larger than the amplitudes of these stuntedevents. In Z Cam stars, the exit from standstill can be eitherinto an outburst or into a dip. All of the OB-dip pairs inthese stunted events appear to begin with an OB, but thestatistics are small. The analogy of some of these stuntedOBs to Z Cam behavior may be strengthened by recog-nizing that Z Cam systems presumably enter standstillbecause they have very near the boundary between NLM0and DN CVs, a regime these systems with stunted OBs maybe near.

A difficulty with this idea as a universal mechanism forstunted OBs is that Z Cam OBs always have a very highduty cycle with a nearly sinusoidal variation. This is theexpected behavior for DNs near (see, e.g., Fig. 10 inM0 critand the models of & OsakiCannizzo 1993a Ichikawa 1994).The models of Tuchman, & WheelerMineshige, (1990)show signiÐcantly reduced outburst amplitude for irradi-ated disk annuli that might be appropriate for the environ-ment of some accretion disks, but the duty cycle remainsnearly 100%, at variance with some of these stunted OBs.

6. COMMENTS ON INDIVIDUAL SYSTEMS

6.1. UU AqrUU Aqr is a well-studied & Steiner eclipsing(Diaz 1991)

CV with an orbital period of 3.9 hr. Steiner, &Baptista,Cieslinski and Steiner, & Horne con-(1994) Baptista, (1996)cluded from analysis of eclipse photometry that UU Aqr isa NL system with steady mass accretion throughhigh-M0the disk. However, the outer disk temperature is only 6000K, which helped (along with leadM

VD 5.5) Warner (1997)

to conclude that this system is barely above and thatM0 critsmall variations in could lead to DN OBs. SupportingM0this notion is the fact that UU Aqr is one of the few NLs tohave a rather conspicuous ““ S wave ÏÏ component in thespectral lines et al. a feature generally(Kaitchuck 1994),associated only with DNs in which the nonÈsteady statedisk is faint, leaving a prominent hot spot. Shu-Volkov,garov, & Seregina reported UU Aqr having varied(1986)from 14.0 to 9.6 mag, suggesting that DN OBs haveoccurred in the past. The stunted OBs we report here aredistinct from any possible excursion to that extreme bright-ness, and these stunted OBs are also distinct from the 0.3mag variations that et al. attributed toBaptista (1996) M0 -

These latter 0.3 mag variations can be seen invariations.as the slow mean trend over 7 years.Figure 1

Both the OBs and dips reported here in UU Aqr arenarrow, averaging less than 5 days FWHM. Dips A and Bare paired with OBs, and OBs 5 and 6 may also be followedby dips. However, there are deÐnitely some isolated dipsand some isolated OBs.

There are times when the OBs in UU Aqr have nearly100% duty cycle (near OBs 1 and 2), with a characteristicrecurrence interval of D10 days, while at other times (nearOBs 4, 5, and 6) the OBs have low duty cycle and a recur-rence interval of D45 days. If this change in duty cycle is

due to changes in from the secondary, then the meanM0system brightness should be larger when the OBs are morefrequent. However, the mean system brightness (includingOBs) near and during the widely spaced OBs is at least asbright as near the closely spaced OBs.

Because of the evidence that UU Aqr may be near M0 crit,attributing these stunted OBs to a DN disk instability is anattractive option if a mechanism can be identiÐed to makethe amplitudes appear small. The 1.8 mag deep eclipse inUU Aqr, the presence of an emission-line hot spot, and therelatively bright apparent magnitude combine to make UUAqr perhaps the most promising candidate for additionalwork in identifying the mechanism for these stunted OBs.We hope that orbital-resolved photometry and/or spectros-copy can eventually be coordinated with the appearance ofstunted OBs. If the emission-line or photometric hot spot isenhanced during an OB, then mass transfer events are indi-cated.

6.2. Q CygQ Cyg was a fast nova in 1876, whose photometric

behavior on timescales of weeks to years has receivedoccasional attention Potter, & Shara(Shara, 1989 ;

et al. Of particular interestBianchini 1990 ; Goransky 1997).are the reports by Shugarov of 0.6È1.0 mag(1982, 1983)OBs having 7È10 day duration and occurring about every60 days. These events must surely be the same phenomenonreported here, and they indicate that this activity has per-sisted for at least several decades.

During times when Q Cyg OBs are frequent, the recur-rence interval in these data varies between 25 and 50 days ;there are also quite long intervals during which OBs do notappear. Few of the Q Cyg OBs have associated dips, but theOB-dip pair near JD 2,449,220 is one of the better examplesof this phenomenon. The OBs are mostly narrow comparedwith the spacing, leading to a relatively small duty cycle ;that is, there generally is a quiescent level between the OBs.

If stunted OBs are DN-type disk instabilities, then theywould be expected to occur systematically during thoseintervals when a given system is faint. However, we see nocorrelation of OB frequency with the level of superposedrandom variations in these Ðve systems. In fact, the OBs inQ Cyg appear to occur mostly when the system is brightest.This suggests that either these Q Cyg OBs are not due tothermal instabilities in the disk or that the random bright-ness variations are not faithfully recording variations in M0 .Having OBs when the system is brightest, coupled with thelow duty cycle of the Q Cyg OBs, would appear to rule outa model in which Q Cyg is undergoing DN-type OBs byvirtue of having dipped just belowM0 M0 crit.

6.3. CP L acCP Lac was a nova in 1936 and has received relatively

little attention. The multiyear light curve of showsFigure 8a slow 1.2 mag decline starting in 1995 October and lastingfor about 450 days, with a temporary partial recovery nearthe middle. This behavior is similar to that of a VY Sculp-toris star and must surely reÑect a diminished from theM0secondary star. There does not appear to be any correlationof the stunted OBs with this change in quiescent level. Thestunted OBs are narrow, like those in UU Aqr, and thesingle dip tabulated in may also be a VY SculptorisTable 1type event because of the slow ingress.


6.4. X SerX Ser was a nova in 1903 and has received relatively little

attention. The multiyear light curve of showsFigure 12slow, large-amplitude variations of up to 2 mag in thequiescent level. The OBs are measured from this varyingbackground ; no dips are visible. The OBs in X Ser are oflarger amplitude and larger width than for most of the otherevents tabulated in Tables and raising the possibility1 2,that they are simply part of a spectrum of random varia-tions. Although we cannot rule out this possibility com-pletely, we think that similarity of the OBs to one anotherand the fact that they stand out rather well from the varyingbackground in Figures and argues that they should be13 14considered part of the phenomenon of stunted OBs.

6.5. RW SexRW Sex is a bright NL system with an orbital period of

5.8 hr. Stasiewski, & Schwope foundBeuermann, (1992)that the inner hemisphere of the secondary star producesnarrow Balmer emission. By all indications the disk is hot(C III/N III 4650 is strong) and bright et al.(Beuermann 1992derive and therefore not near the DN regime.M

VD 4.8)

The OBs in RW Sex are few and not particularly welldeÐned except for the OB-dip pairing near JD 2,449,450.Dip A, near JD 2,449,030, appears to be followed by a seriesof dips of decreasing amplitude, but the sampling is not asgood as desired in this region. There is little variation in thequiescent level of RW Sex over this 6.5 yr interval, so it isdifficult to blame the varying OB-dip behavior on M0 -variations.

7. DISCUSSION AND CONCLUSIONS

UU Aqr and Q Cyg are probably the most valuablesystems for trying to understand the stunted OBs and dips,because the events are frequent and the stars are relativelywell studied. The other three systems serve mostly todemonstrate that the phenomena are not particularly rareamong NLs and old novae.

The traits that favor an accretion disk instability originfor the stunted OBs are that (1) the spacings and durationsare consistent with this hypothesis and (2) the paired OBsand dips Ðnd a ready explanation as a momentary lapseinto instability. However, this explanation requires amechanism to attenuate the amplitude. Adding additionalsystem light is an attractive mechanism to achieve thisattenuation because there is not good evidence that inM

Vthese systems has fallen to the disk instability regime for asimple accretion disk. Therefore, the conjectured extra lightcan serve two useful purposes : attenuating the OBs andkeeping the systemÏs appearance bright.

The traits that argue against an accretion disk instabilityorigin for the stunted OBs are several. First, if the etVolkoval. 4.4 mag excursion to V ^ 9.6 in UU Aqr was a(1986)DN OB, then the 0.6 mag stunted OBs in UU Aqr must besomething di†erent. Second, the character of the OBs donot change in the expected manner with system brightness.Q Cyg OBs sometimes appear when the system brightens,and the duty cycle of the UU Aqr OBs does not increasewith increased system brightness. This problem could beconveniently addressed by assuming that the additionallight is responsible for the erratic slow brightness variationsand that these variations are uncoupled from the diskbrightness. A third problem for an accretion disk originmight be the isolated dips. A disk (or truncated disk) just

above the upper instability boundary for might wellM0enter the instability regime by initiating a cooling front anddipping below the quiescence level. However, it wouldappear to be difficult to obtain an isolated dip this waybecause disk instability just below would produceM0 critoscillations above and below the standstill brightness. Iso-lated OBs, on the other hand, might result from a disk thatis well below A related problem is that OB-dip pairsM0 crit.occur in systems that also sometimes have low duty cycleOBs, without any systematic change in the backgroundbrightness. Extra light whose contribution is uncoupledfrom the mean onto the disk could address this difficulty,M0but (in UU Aqr at least) the extra light would have to beunrealistically controlled to keep the mean brightness con-stant as the outburst behavior changes.

The SW Sextantis stars et al.(Thorstensen 1991 ; Murray& Chiang have single-peaked emission lines despite1996)their being high-inclination disk systems, constituting evi-dence for at least some nondisk contribution to the totalaccretion luminosity. Furthermore, et al. andRutten (1992)others have found that the disks in many SW Sextantis starshave a Ñatter radial temperature dependence than predictedfor a equilibrium disk, suggesting a disk whose innerportion is truncated. Despite this possible truncation of thebrightest part of the disk, these SW Sextantis stars manageto maintain their bright compared with DNs, againM

Vsuggesting that accretion structures other than the disk(such as a wind or accretion column) contribute signiÐ-cantly to the total light. Although it is far from clear howthese various e†ects are related to stunted OBs, it must beadmitted that the accretion structures in NL CVs are notwell constrained observationally and that an extra lightcontribution is not an unreasonable assumption.

It is worth noting that because of the D4 mag brighterof NLs compared with DNs, the amplitude of theseM

vstunted OBs in intensity units is well within the range ofDN OBs. That is, a 0.6 mag OB from a level 4 mag aboveDN quiescence is equivalent in energy to a 3.6 mag OBfrom DN quiescence. (If one were to consider these stuntedOBs to be conventional 3.6 mag DN OBs seen against extralight, then of course the e-folding times in would beTable 1smaller.)

There are good reasons to seriously consider accretiondisk instabilities as candidates for some or all of theseevents, but it seems clear that taking this path armed onlywith models already in the literature (which were computedfor other purposes) quickly leads to the requirement foradditional hypotheses and perhaps the need for a com-bination of disk instabilities and mass transfer events. If oneseeks a single mechanism for these events, then mass trans-fer modulations would appear to be the route to explore.However, we again note that concludes thatWarner (1995b)light variations on timescales of tens of days in NLs are notexpected to originate in from the secondary star, byM0analogy with the absence of similar hot-spot variations inquiescent DNs. For the moment it appears that the natureof these stunted OBs remains frustratingly elusive.

We thank Constantine Deliyannis for exposing some ofthe WIYN images in Figure 19. We gratefully acknowledgethe e†orts of Brice Adams, Bill Kopp, and Richard LeBeauin helping keep the automated telescope running pro-ductively. This work was supported by the National ScienceFoundation through a grant to Indiana University.

2538 HONEYCUTT, ROBERTSON, & TURNER

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unusual “stunted” outbursts in old novae and nova-like cataclysmic variables

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