Optical observations of close binary systems with a compact component - 10 Outline of a comparative study of disk and halo cataclysmic variables

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Optical observations of close binary systems with a compact component

Augusteijn, T.

Publication date

1994

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Citation for published version (APA):

Augusteijn, T. (1994). Optical observations of close binary systems with a compact

component.

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10 0

Outlinee of a comparative study of disk and halo

cataclysmicc variables

T.. Augusteijn, T. Abbott, R.G.M. Rutten, R. Stehle, J. van Paradijs

Abstract t

Wee have selected samples of dwarf novae, on the basis of their magnitudes in outburstt which represent disk and halo populations. We give a short discussion of ourr selection criteria. Of the 59 sources in our halo sample, only 6 have accurately knownn orbital periods. Our primary aim is to increase the number of Population II dwarff novae with known orbital periods through a search for photometric variability off orbital origin such as eclipses, hot-spot humps and/or ellipsoidal or heating effects inn the secondary. By comparing the orbital period distribution and space densities off our two samples we hope to gain a better understanding of the formation and evolutionn of cataclysmic variables.

10.11 Introduction

AA cataclysmic variable (CV) consists of a Roche-lobe filling, late-type secondary which transfers masss to a white-dwarf primary. The general evolution from the detached binary, or pre-CV, too the semi-detached state of a CV, and the subsequent long-term evolution to shorter orbital periodss is thought to be driven by orbital angular momentum losses caused by gravitational radi-ationn and magnetic braking (see, e.g., King 1988). Although the evolution of the semi-detached CVV phase seems to be fairly well understood, the formation rate and preceding evolutionary pathh of pre-CVs is not well known.

AA standard technique in the study of stellar evolution is to compare groups of stars that are distinguishedd by their spatial distribution (halo/disk), age or kinematics. We want to apply this generall method to CVs, by comparing the period distribution and space density of Population II and Population II CVs. Population II CVs are expected to have relatively uniform high age andd low metallicity (see, however, Stehle and Ritter 1994). By comparing the two groups, we expectt to obtain information on the formation and evolution of (pre-)CVs, and perhaps such otherr aspects of CV evolution as the physics of magnetic braking, the effect of metallicity on accretion,, the outburst mechanism of dwarf novae, the period gap and the minimum period.

Recentt population synthesis of CVs, including CV-birthrates, their secular evolution and selectionn effects, were able to explain the general features of the observed period distribution of CVss (Kolb and Ritter 1992). Within a similar model Stehle et al. (1994) made a prediction for thee period distribution of Population II CVs, but for this population a reliable observed sample iss still lacking.

1400 JO Outline of a comparative study of disk and halo cataclysmic variables

10.22 Halo and disk CVs

Too study and compare the properties of Population I and Population II CVs we have started a projectt to determine the orbital period distribution of comparable samples of "halo" and "disk" dwarff nova type CVs. Our primary goal is to obtain a sample of high galactic latitude CVs (HGL-CVs)) which have a high probability to be different in their evolutionary history from galacticc disk CVs.

Wee selected our halo sample in the following way: we limit our sample to dwarf novae

we use the magnitude in outburst as distance indicator the sources have z > 400 pc

the sources have b > 20°

Forr comparison, we selected a control group of galactic disk CVs with d > 400 pc, but z < 190 pc. .

10.33 Discussion of the selection criteria

D w a r ff novae

Theree are many problems in selecting a well defined sample of CVs for which to determine thee orbital period distribution. There is a large variety of types of CVs distinguished by their eruptivee behavior (novae, dwarf novae), the magnetic field strength of the white dwarf primary (polarss and intermediate polars), or their spectral properties (nova-like variables). Any sample includingg different types will be biased because of the different ways in which the sources are discovered;; e.g., polars and intermediate polars are mainly discovered in X-rays. However, alsoo the photometric behavior in quiescence can play an important role in the resulting period distribution;; e.g., UX UMa type CVs (a subgroup of the nova-like variables) generally do not showw regular photometric variations with their orbital period, and only for those sources that showw eclipses can the orbital period be determined easily. A further problem is posed by the needd of a good distance indicator for the sources.

Selectingg only dwarf novae circumvents many of the problems mentioned above. There is aa fairly large number of known dwarf novae which form a relatively well defined homogeneous groupp of CVs. Furthermore, dwarf novae generally show regular photometric variations with the orbitall period. Dwarf novae in outburst are fairly bright in outburst, and they can be identified outt to large distances. The brightness in outburst can also be used to estimate the distance to thee sources (see below).

D i s t a n c ee e s t i m a t e u s i n g t h e m a g n i t u d e in o u t b u r s t

Warnerr (1987) found that the intrinsic visual brightness of dwarf novae in outburst is, in contrast too quiescence, well constrained (the peak magnitudes show a weak, but well-behaved dependence onn orbital period). Szkody and Howell (1992; see also Sect. 10.4) found that some dwarf novae whichh have large amplitude outburst are intrinsically faint in quiescence. Also these sources havee absolute visual magnitudes in outburst which are consistent with Warner's result (see Sect.. 10.5). Furthermore, the magnitude of a dwarf nova in outburst is a much better defined observationall quantity than the magnitude in quiescence.

Ass we do not know the orbital period a priori we have taken the average absolute visual magnitudee for dwarf novae in outburst Mv(out)= 4.7 mag (Vogt 1981; see also Warner 1987).

10.310.3 Discussion of the selection criteria 141 1 Ass we will obtain more information about the sources that are included in our sample (e.g., the orbitall period, the inclination) we will reassess their inclusion in the sample. We also plan to usee (near) infra-red observations to attempt to detect the secondary and derive an independent estimatee for the distance to the sources which can be used to check our selection criteria.

zz > 4 0 0 p c

Allenn (1976) gives the ^-distance for Population II stars as starting at 400 pc. For the CVs in thee thin galactic disk the scale height is H ~ 190 pc (Patterson 1984). The space distribution off Population II stars is spread out to much higher distances above the galactic plane. This halo populationn has a (possibly somewhat flattened) spheroidal shape were the density is a function off the distance to the galactic center (see, e.g., Freeman 1987). For simplicity we will here assumee a Gaussian distribution as function of z with a scale-height equal to that of the thick galacticc disc (intermediate Population), where H11 _{~1500 pc (Freeman 1987).}

Byy taking a Gaussian distribution in the z direction

»U I( 2 )) = n one x p | - ( - ^ n J |

andd a relative population of n^/nó — 0.02 (Freeman 1987) we derive a probability at z = 400 pc forr a CV to be a member of the Halo of P(z=400 pc)= 60%. (Note, however, that this estimate onlyy applies to the intrinsic population and not to the observed population. The observational selectionn effects on the Population I and Population II CVs, which contribute to the population off CVs [at a given height above the galactic plane], could be very different; see Stehle and Augusteijnn 1995).

Afterr deriving the z-distance from observations we plan to make a probability-weighted samplee using the probability

66 > 20

Onee potential problem in selecting sources at large distances is that the sources can be sig-nificantlyy reddened, which results in over-estimating their distances. However, the galactic absorptionn layer is concentrated towards the galactic disk, and the sources in our halo sample aree largely unaffected. Only sources which are at low galactic latitudes are expected to be af-fectedd significantly. We, therefore, have limited our halo sample to sources with b > 20°. At a galacticc latitude of 20° the average extinction is expected to be 0.2 mag (Woltjer 1975), which affectss the selection of our sources only slightly.

T h ee control g r o u p

Onee of the difficulties of studying a sample of halo sources is to take a properly selected control groupp of disk sources. Most of the disk dwarf novae which have been studied in any detail have muchh brighter apparent magnitudes than the sources in our sample of halo, or HGL-C Vs, and thee observational selection effects on the two samples may be very different (see, e.g., Bitter andd Burkert 1986, Dünhuber 1993). We, therefore, selected a second sample of dwarf novae whichh are at a distance of more than 400 pc and is limited by z < 190 pc. Thus, we have not includedd sources between approximately 1 and 2 scale heights to distinguish the two populations ass clearly as possible.

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10.410.4 Our sample 151 1

10.44 Our sample

Fromm the GCVS (Kukarin 1990; see Downes and Shara 1993) we have selected 261 dwarf novae thatt are fainter than ray—12.7 in outburst, i.e., more distant than 400 pc. Our halo sample is limitedd to those sources with z > 400 pc and bu > 20°. Our disk sample is delineated by z < 190

pc.. Thus, we have not included sources with z between 1 and 2 scale heights, to distinguish the twoo populations as clearly as possible. In this way we have selected a total of 59 halo, and 127 diskk dwarf novae.

Thee number of sources in our halo sample is about the same as the number of dwarf novae includedd in the sample of halo CVs by Howell and Szkody (1990; see also Szkody and Howell 1992).. These authors also selected their sources from the GCVS, and their sample was limited too those sources with z > 350 pc and b11 > 40°. They, furthermore, excluded sources with

outburstt amplitude smaller than 1.9 mag. The major differences with the halo sample presented heree is that the distance estimates were based on the magnitudes in quiescence (see Sect. 10.5). Ass a result of the differences in selection criteria, of the 57 dwarf novae as 'halo' CVs in the samplee of Howell and Szkody only 27 are included in our sample.

Inn summary:

"Halo":: z > 400 pc ; 6 > 20° 59 sources "Disk":: d > 400 pc ; z < 190 pc 127 sources

Inn Tables 10.1 and 10.2 we list the sources in our halo and disk sample, respectively. In thesee tables we list for each source the position, the magnitudes in outburst and quiescence, thee classification, the galactic latitude, the period (if known), and some comments including the observedd photometric variations and some relevant references; in the last column we indicate if thee source has already been observed in our programme.

Inn the future we hope to extend our samples as more sources, particularly at high galactic latitudee (see, e.g., Drissen et al. 1994), are detected. An additional benefit of our project will be thatt we extend the number of all CVs with known orbital period to fainter magnitudes, which iss expected to reduce the bias of the presently known sample towards high mass white dwarf CVss (Ritter and Buckert 1986, Dünhuber 1993).

10.55 High Galactic Latitude CVs revisited

Howelll and Szkody (1990; see also Szkody and Howell 1992) were the first to attempt a compre-hensivee study of HGL-CVs. They noted that a different CV population might be observed at highh galactic latitudes and that studying HGL-CV might give us new insight into the birth rates andd into the secular evolution of CVs. The orbital period distribution of their sample was found too be weighted towards short periods and the outbursts of dwarf novae have on average larger amplitudess than 'disk' dwarf nova. Also, for several of these 'halo' dwarf novae, the intrinsic brightnesss in quiescence was found to be substantially lower than that of 'disk' dwarf novae.

Thee dwarf novae included in the sample of HGL-CVs of Howell and Szkody (1990) were selectedd on the basis of their magnitudes in quiescence. From a study of dwarf novae with knownn distances Warner (1987) found that the absolute visual magnitude of dwarf novae in quiescencee is a, roughly linear, function of orbital period. As the orbital periods were not known theyy assumed a priori an average absolute visual magnitudee in quiescence of M<jUi = 7.5 mag, and

152 2 1010 Outline of a comparative study of disk and halo cataclysmic variables O O

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F i g u r ee 1 0 . 1 . The expected absolute magnitude in quiescence, M,„i(z=350 pc), if the dwarff novae taken from the sample of Howell and Szkody were placed at a distance of 3500 pc above the galactic plane, as a function of orbital period. The drawn line gives the relationn between the absolute magnitude in quiescence and the orbital period derived by Warnerr (1987). The dashed line shows the same relation shifted upward by 0.7 mag, which reflectss the rms value of the individual points around this relation. See the text for the meaningg of the different symbols

i n c l u d e dd those sources with distances z above t h e galactic plane greater t h a n 350 p c . Here we r e - e x a m i n ee t h e inclusion of those dwarf novae in t h e sample of H G L - C V of Howell a n d Szkody (1990)) for which t h e o r b i t a l p e r i o d has b e e n measured. In Fig. 10.1 we show the expected a b s o l u t ee m a g n i t u d e in quiescence, M? m( z = 3 5 0 p c ) , if the dwarf novae taken from the sample of

Howelll and Szkody were placed at a distance of 350 pc above t h e galactic plane; i.e. t h e lower limitt for these sources t o be included in t h e sample. In this figure sources below M9 U !( z = 3 5 0

p c ) == 7.5 are include in t h e sample of Howell and Szkody. I nn Fig. 10.1 we also indicate:

•• those sources which are included in our sample (squares)

•• t h e relation between the absolute m a g n i t u d e in quiescence and t h e orbital period derived byy Warner (1987; d r a w n line). Sources below this line are expected to be at a distance g r e a t e rr t h a n 350 pc above t h e galactic plane

•• t h e s a m e relation shifted upward by 0.7 m a g (dashed line), corresponding t o t h e r m s deviationn of t h e individual points a r o u n d t h e relation derived by Warner

•• those sources t h a t show eclipses in their light curve (indicated with a plus sign). As one observess t h e accretion disk under a large angle these sources appear substantially fainter

10.610.6 Large-amplitude dwarf novae 153 3

thann expected from a source with the average absolute brightness, and their distance will bee over-estimated. This difference is a strong function of inclination, and can be as large ass ~ 3 mag (see, e.g., Warner 1987)

•• sources which have large, A >5.7 mag, outbursts (filled in symbols). For some of these sourcess Szkody and Howell (1992) have found that they are exceptionally faint in quiescence (seee Sect. 10.6).

Fromm Fig. 10.1 we conclude that the dwarf novae in the sample of Howell and Szkody (1990) aree likely to be heavily contaminated with disk CVs.

Wee also identify several other potential problems with this study: i) The selection of dwarf novaee as members of the halo was based on distance estimates assuming an average absolute magnitudee in quiescence. As Howell and Szkody found themselves, this assumption is not valid (seee also Van Paradijs 1983, Warner 1987). ii) Each source was initially observed for 2-4 hrs, andd re-observed only if it showed any indication of a possible periodic variation. This results in aa strong bias towards shorter periods, iii) The control group consisted of of disk CVs which are muchh brighter than the 'halo' sample, and the observational selection effects on the two groups mayy be very different (see Bitter and Buckert 1986, Dünhuber 1993).

10.66 Large-amplitude dwarf novae

Inn Table 10.3 we give for some dwarf novae which have large-amplitude outbursts the absolute magnitudee in quiescence, as estimated by Szkody and Howell (1992). In this table we also show thee amplitude of the outburst and the resulting absolute magnitude in outburst. In the next columnn we give the absolute magnitude in outburst calculated from the relation by Warner (1987) betweenn the absolute magnitude in outburst and the orbital period. For the two sources which doo not have a measured orbital period we list the range in absolute magnitude expected from thiss relation for periods in the range 80 min to 6 hrs. In the last two columns we give estimates of thee distance above the galactic plane as derived from the observed apparent magnitudes using thee average magnitude in quiescence (as used by Howell and Szkody 1990) and the average magnitudee in outburst (My- 4.7; Vogt 1981), respectively.

Tablee 10.3 Dwarf novae with large amplitude outburst Source e

B C U M a a WWW Cet VZZ Aqr AKK Cnc

Mv(qui)) Ampl. Mv(out) M^8t(out) (mag)) (mag) (mag) (mag) 11.0-13.55 7.4 3.6-6.1 5.25 <9.4-12.00 5.7 <3.4-6.0 4.55 10.5-12.00 5.9 4.6-6.1 4.1-5.3 10.4-11.99 5.9 4.5-6.0 4.1-5.3 zz estimate in pc Mv(qui)== 7.5 Mv( o u t ) = 4.7 13100 157 3511 79 5122 123 10100 242

Forr all sources the derived absolute magnitudes in outburst are consistent with the relation byy Warner (1987) and the average magnitude in outburst derived by Vogt (1981). We further notee that the distance estimates using the magnitude in outburst are systematically smaller than usingg the magnitude in quiescence, which indicates that the former method to select halo CVs iss more conservative independent of the validity of either assumption.

154 4 _{References References}

Acknowledgements Acknowledgements

Thee authors gratefully acknowledge Michiel Berger, Orly Barziv, Frank van der Hooft, Jeroen dee Jong, Martin Marinus, Coen Schrijvers and Hanno Spreeuw, without whose help in obtaining observationss and reducing data this project would not be possible.

References s

Allen,, C.W. 1976, Astrophysical Quantities, Athlone, London Downes,, R.A., Shara, M.M. 1993, PASP, 105, 127

Drissen,, L., Shara, M.M., Dopita, M., Wickramasinghe, D.T. 1994, AJ, in press Dünhuber,, J.D. 1993, Ph.D. Thesis, Universitat München

Freeman,, K.C. 1987, ARA&A, 25, 603

Howell,, S.B. 1993, in: Cataclysmic variables and Related Physics, 2n d Technion Haifa Confer-ence,, Eds. O. Regev and G. Shaviv, p. 67

Howell,, S.B., Szkody, P. 1990, ApJ ,356, 623 King,, A. 1988, Qua. Journ. RAS, 29,1 Kolb,, U., Ritter, H. 1992, A&A, 254, 213 Patterson,, J. 1984, ApJ Suppl., 54, 443 Ritter,, H., Burkert, A. 1986, A&A, 158, 161

Stehle,, R., Kolb, U., Ritter, H. 1994, in Interacting Binary Stars, Ed. A. Shafter, ASP Conf. Series,, San Francisco, in press

Stehle,, R., Ritter, H. 1994, proceedings of the the Abano-Padova conference on Cataclysmic variables,, Italy, June 20-25 1994

Stehle,, R., Augusteijn, T. 1995, in preparation Szkody,, P., Howell, S.B. 1992, ApJ Suppl., 78, 537 Vann Paradijs, J. 1983, A&A, 125, L16

Vogt,, N. 1981, Mitt. Astron. Gesell., 57, 79 Warner,, B. 1987, MNRAS, 227, 23