tions. The method is very sensitive. By cause a strong variable background, background. Two examples of flare visual inspection, only flares of am­ even when properly subtracted, can are given in Figures 3 and 4. plitudes larger than O~ 6 are found. Here produce systematic exposure effects. The final part of the programme, it seems possible to decrease the limit Another problem appears for almaga­ which is under development, offers the to about O~ 3. The method also yields mated chains in regions of high user the possibility to combine results useful results for plates taken under density. It occasionally happens that from different plates, in order to study non-photometrie conditions; such stars from the two chains merge into the complete available history of all plates, however, are rarely obtained at single images of higher brightness. This stars in the field. For flare stars, this La Silla. can occur at any position in the chain. means better confirmation, especially in The flare candidates are stored in a At last, the discovered flare events cases of low amplitude flares of higher "flare-file". This file contains also infor­ can be inspected in the form of tracings frequency, and thus higher sensitivity of mation about celestial coordinates, or digitized images of the chain. The the photographie flare search. Also, typ­ magnitudes of the images and density images reveal only flare events with suf­ ical time seales for more frequent flares of the sky background. The last value is ficiently large amplitudes, while the trac­ can be determined. Finally, other types especially important for checking the re­ ings clearly show also the fainter flares of variables with longer periods can be liability of a reported flare event, be- and the variations of the sky detected.

Dust in Early-Type M. P. VERON-CETTYand P. VERON, Observatoire de Haute-Provence, France

Introduction fill the gap between E7 and Sa and are galaxies has been made by visual in­ Abrief historical review of the discov­ called SO or SBO (Sandage, 1975). spection of photographie plates rather ery and exploration of elliptical galaxies The SO class has been described for than by measurement of the brightness has been published by de Vaucouleurs the first time in 1961 by Sandage. SO distribution leading to some misclassifi­ (1987). We have made use of it to write galaxies appear to form a transition be­ cation especially when the con­ this introduction. tween the E galaxies and the true spi­ tains dust. The most widely used classification rals; the transition from E to SO is Dust was clearly visible on a photo­ system for galaxies is that proposed by smooth and continuous. The division graph of NGC 1316 taken in 1943 by Hubble in 1926. The sequence of between E and SO is made on the basis Paraskevopoulos; but it was mistaken classification, as originally presented, of the presence or absence of an outer as plate defects by Shapley who could consisted of aseries of elliptical nebulae amorphous envelope or thin fundamen­ not believe that an could ranging from globular (EO) to lenticular tal plane surrounding the nuclear re­ contain dust (Hodge, 1975). Probably (E7) forms, and two parallel series of gions. Some SO galaxies exhibit a struc­ because of the presence of this dust, unwiding spirals, normal (S) and barred tureless envelope similar to that of the NGC 1316 was called Sa pec by San­ (SB). The figure following the symbol E normal SO's, but a sharp, narrow ab­ dage and Tammann (1981); however, is equal to the ellipticity (a-b)/a with the sorption lane is found within the lens; Schweizer (1980) has shown that it is a decimal point omitted, a and b being the lane is in an are concentric with the typical Morgan D-type galaxy with an respectively the major and minor axis of nucleus. These galaxies are called S03' elliptical-like spheroid embedded in an the galaxy. A more quantitative way to separate E extensive envelope. Elliptical galaxies have been de­ from SO has emerged from the study of In the course of a survey of bright scribed by Hubble as highly concen­ the light distribution in galaxies. De Vau­ southern galaxies, a pair of abnormal trated objects with luminosity falling couleurs (1959) has pointed out that objects have been observed by de Vau­ rapidly away from bright, semistellar elliptical galaxies have a single-compo­ couleurs (1953) which shared most of nuclei to undefined boundaries; small nent surface brightness distribution the characteristic peculiarities of the patches of obscuring material are occa­ which follows closely the de Vau­ bright NGC 5128 (Cen­ sionally silhouetted against the lumin­ couleurs relation: taurus A), i. e. radio emissivity and dust; ous background, but otherwise these these galaxies are NGC 1316 (Fornax A) log I (r) cq1/4 nebulae present no structural details. and NGC 1947, to which de Vau­ No elliptical galaxies are known that (r is the distance from the centre of the couleurs added a northern galaxy, are flatter than E7; galaxies which are galaxy) while the surface brightness dis­ NGC 2537; however, at the time, radio flatter invariably show an outer region of tribution for spirals and SO galaxies positions were not accurately mea­ low surface brightness which resembles show two main components: an inner sured; the errors were of the order of a thin fundamental plane. spheroidal component which follows 1°, and only NGC 1316 has been con­ The transition from E7 to Sa appeared the de Vaucouleurs law, and an outer firmed as a radio source; moreover, very abrupt. With accumulating data, exponential component (disk) with: NGC 2537 is an Sc galaxy and of no and especially with the increasing interest here. NGC 1947 is indeed an I (r) = I (0) xe-«r number of good photographs taken with early-type galaxy (S03 pec), but not a the 100-inch Mount Wilson reflector, This result has been confirmed since by radio source (Möllenhoff, 1982). numerous systems have been recog­ a number of more recent works (see for Shobbrook (1963) has discovered a nized as soon as 1936 by Hubble which instance: Freeman, 1970; Kormendy, dust lane in the elliptical galaxy are later (flatter) than E7, but which 1977 a, band cl. NGC 4696 and used it as an argument show no spiral structure. These galaxies However, the classification of most for the identification of this galaxy with

41 a radio source now known as therefore two ideas emerged: that ellipti­ We have made a study of a complete PKS 1245-41; this identification has cal galaxies could contain dust and that volume limited sampie of 78 early-type since been confirmed. dusty ellipticals are more often radio galaxies, looking for the presence of Westerlund and Smith (1966) have sources than galaxies without dust. dust in order to check if there is any identified the early-type galaxy Dust is now known to be frequent in E correlation between the presence of NGC 612 with the radiosource and SO galaxies (see for instance dust and that of radio or far-infrared PKS 0131-36 and noted the presence Hawarden et al., 1981, and Ebneter et emission. of a dust lane. The identification has al. , 1988); moreover, a number of early­ been confirmed by Bolton et al. (1965). type galaxies have been detected at The Observations In the meantime, it was shown that 100 IJ.m by the Infra-Red Astronomical powerful extragalactic radio sources Satellite (I RAS) (Neugebauer et al., The revised Shapley-Ames Cata­ were always associated with elliptical 1984), and this far-infrared emission is logue (Sandage and Tammann, 1981) galaxies rather than with spiral or SO attributed to thermal emission by cold contains 47 ellipticals and 29 SO galaxies (Matthews et al. , 1964), and dust (Jura, 1986). galaxies at declination ö < + 20°, with NGC 524 NGC 4696 p ..·r..-s.... ••··•••• -e- '/." ... , •••• :..;'. ''Z -. 1'")01. '-$(1...1:""': , 1 -..~",~ :. • r,• ...... •,...... eJ... .~, "'. c:·•• IL...... ",,~ ~... ' " '. ,.,je • ,r:-~ ~ ' ••' • .... 1.7 . ~. -"~' '. ',' •i;,,"" .... • ... I• J.-. ".;. ~..' •._.1 ''''' • .. :..'~"... ," ••.. ..: .1'" ..,..,~ ;~:o\a'.,...... f . --"-.'••••.. 1 • r • • • '.1' '• .- ~', .. ' '_I . "'...... , 't-Jo:r' : ,~., ",_,: Ä~ ••• '. .. .. • ," f· ., ~ Ir. ," .... • .••~,.• . '",. _~' ., J"Ia: l .... .r .i .-.' •• ' ,.J~ t:'-r .~' ":~ I ~ ..;.~;.;'••••• ~ , ~":' ,.~ "l •• :' '.' •, ',' .:.1 ..' •• i .," ,e I ... • Ja.' I.... ., ...... 1.lJ.... I, , ";Il. _. ,,-::,.,' ;"'~..."" '.~ ,:'\." '. . :(, .;~.~ l'~_"r: .~ , c ••. :-', .~.... lI1' .,.Jt:.~- ",'...... ' :; . ".:;"~ • t' "T": ~ ;,,~ '~'" , "" 1 ry--t$ .:t",;. e ',> ,'.:.' . r ,. • , ~'.:..I. ' ,- ,,rJ,f"'-1 '.' " '.)f " ,.. .. r • '~ I...' .' . ~._ .. :~ .. '.'l\4}:~.',. .,....,·~r ~ .~ " ". ~ ~"'S: .,j'1' • • .' . ..L~ --:...... --- • I:·.... ' . • • . " ...... • ..• .1 . '- "..... ,; '- " -.' - .- . ""'~.. ~. '.' .~ .. . .' ...... , : •..., ."'. ••­ .'.' . . r.. '\ '.

NGC 4526 NGC 2911

.....,...,...,'. L Ir ..... ~.",. -.r.19' Figure 1, This figure shows the ratio of the B to the z images for four of the galaxies with obvious dust clouds, Red, i. e. dust, shows up in white, In NGC 524, the dust appears to form an almost face-on disk; in NGC 4526, the dust is a very regular disk or torus seen almost edge-on; in NGC 4696, there is a thin filament of dust first discovered by Shobbrook in 1963; while in NGC 2911, the dust is in a very irregular cloud, 42 a gaiactocentric ever, as we have seen above, galaxies about 10-6; this is a strong indication Va< 3,000 km S-I and a blue total ab­ may have been misclassified and, more that the assumption that dust and in­ solute magnitude MB < -21. If the RSA specifically, E galaxies with dust may frared emission are independent is not

catalogue was complete to mpg = 12.9, have been calied SO, just because for a valid and that, therefore, the infrared this would constitute a volume limited long time it was believed that there is no emission of early-type galaxies is indeed sam pie; however, according to Sandage dust in ellipticals. For instance, Sandage due to thermal emission by cold dust.

and Tammann, at mpg = 12.9, the com­ and Tammann have classified One of the brightest galaxies at pleteness of the RSA catalogue is only NGC 6868 as an E3/S02 and NGC 7196 100 ~m is NGC 7213; however, in this 28%. Two other galaxies, NGC 1316 as an E3/S03, suggesting that they are galaxy, the presence of dust has been and 7213, classified as Sa by Sandage elliptical with dust. suspected (Phillips, 1979) but has still to and Tammann, have been added to the A careful study of NGC 5266, be confirmed. It is a Seyfert 1 (Filippen­ sampie; as we have seen above, classified as an S03 pec by Sandage ko and Halpern, 1984) and, in this case, NGC 1316 is a Morgan D-type galaxy and Tammann, has shown that it is an the infrared emission could be related to while, according to Phillips (1979), E4 galaxy, without trace of disk (Varnas the activity of the nucleus rather than NGC 7213 is a pole-on SO. et al. , 1987). Similarly, van den Bergh due to cold dust. Jura et al. (1987) have In the course of four observing runs at (1976) has concluded that NGC 5128 is detected NGC 4486 at 12, 60 and La Silla (October 1984, February and an EO galaxy. NGC 1316 is an Sa pec 100 ~m; this infrared spectrum is rather June 1985 and September 1987), we according to Sandage and Tammann "hot", and unlikely to be due to cold observed 69 of the galaxies with the (1981), however, the brightness profile dust; it could rather be associated with 2.2-m telescope. For each galaxy, we established beyond doubt that it is the optical jet discovered by Curtis in have obtained two ten-minute direct im­ essentially an elliptical (Schweizer, 1918. A good photograph of this jet has ages through a Johnson Band a Gunn z 1980). been published by Nieto and Lelievre filter (Schneider et al., 1983) with a thin, It is therefore necessary to measure (1982). back-i1luminated RCA CCD having the brightness profile of all the dusty ~m galaxies to determine without ambiguity 512 x 320 30 pixels. The scale of Conclusions these images is 0."36/pixel. if they have a disk or not and find out the In addition, on January 24, 1988, one fraction of ellipticals and SOs which con­ The two main conclusions we have of the sampie galaxies, NGC 2911, was tain dust. reached are that: observed with EFOSC (Dekker et al., 1. There is no clear evidence that 1988) at the Cassegrain focus of the radio galaxies more often contain dust Dust and Radio Sources 3.6-m telescope. The detector was a than radio quiet galaxies. thin back-illuminated RCA 15 x 15 ~m Out of the 78 galaxies studied, 17 2. Galaxies detected at 100 ~m by pixel CCD used in the 2 x 2 binned have a 5000 MHz flux density larger IRAS are those which show clear evi­ mode; the effective scale corresponds than 0.1 Jy. At the level of detection, dence for dust on direct images, indicat­ to 0."675 per 30~m pixel. The exposure 31 % (6/19) of the dusty galaxies are ing that the infrared emission is most time was 6 minutes with each of the B radio sources, while 19 % (11/59) of the probably due to thermal emission by and z filters. non dusty galaxies are radio sources. this dust. However, galaxies with an ac­ In the near-infrared, fringes on the There is therefore no strong evidence in tive nucleus and showing no evidence RCA CCD chip are very prominent; to our data for early-type radio galaxies to for dust may have a detectable far-in­ remove them, we have obtained flat­ preferentially contain dust. frared flux, in which cases this infrared fields during each observing run, by tak­ emission would be associated with the ing three sky images in a field almost activity of the nucleus. Dust and Infrared Emission void of stars, the telescope being moved by a few arcseconds after each Jura (1986) has found that at least a References exposure; comparison of the three ex­ third of all early-type galaxies are de­ van den Bergh, S., 1976, Astrophys. J. 208, POsures (with the routine FCOMPARE of tected in the far-infrared by IRAS. The 673. ~m IHAP) allows the removal of the stars infrared emission is strongest at 100 Bolton, J. G., Clarke, M. E. and Ekers, R.O., and results in a flat-field wh ich removes and quite often the far-infrared luminosi­ 1965, Australian J. Phys.18, 627. 8 almost perfectly the fringes. ty is in excess of 10 .; the infrared flux Curtis, H.O., 1918, Publ. Liek Obs. 13. The sky level, estimated from the cor­ most probably results from dust repro­ Oekker, H., O'Odorico, S. and Arsenault, R., ners of each frame, was removed from cessing of starlight (Jura, 1982). 1988, Asuon. Asuophys. J. 189,353. each image and then, for each galaxy, Of the 78 galaxies in our sampie, one Ebneter, K., Ojorgovski, S. and Oavis, M., we have divided the B by the z frame. has no IRAS coverage (NGC 3078) and 1988, Astron. J. 95, Astron. 422. The presence of dust was detected by 21 have been detected in the infrared Filippenko, A V. and Halpern, J. P., 1984, As­ trophys. J. 285, 458. visual inspection of these images in 16 (Lonsdale et al., 1985). Among the 21 Freeman, K. C., 1970, Astrophys. J. 160, 811. galaxies (four examples are given in galaxies detected by IRAS, 14 contain Hawarden, T. G., Elson, R. A. W., Longmore, Fig. 1) to which one can add NGC 1316, dust visible on the direct images, while AJ., Tritton, S.B. and Corwin, H.G., 1981, 5018 and 5128 for which we have no of the 19 galaxies with dust, 14 have Monthly Notiees Roy. Astr. Soe. 196,747. observations, but which are weil known been detected by IRAS, all at 100 ~m. Hodge, P. W., 1975, Sky and Te/eseope 49, to contain dust. In our sampie of 78 galaxies, 21 have 354. been detected in the infrared. Let us Hubble, E., 1926, Astrophys. J. 64, 321. Hubble, E., 1936, The realm of Nebulae, Yale Elliptical and SO Galaxies assume that there is no correlation be­ tween the presence of dust as seen on University Press. Of the 19 galaxies with dust, 6 are direct images and the detectability at Jura, M., 1982, Astrophys. J. 254, 70. Jura, M., 1986, Astrophys. J. 306, 483. ellipticals, 12 SO and one Sa (according 100 ~m. We have found 19 galaxies Jura, M., Kim, O. W., Knapp, G. R. and to Sandage and Tammann, 1981), sug­ with dust; we expect five of them to be Gahathakurta, P., 1987, Astrophys. J. 312, gesting that dust is more frequent in SO detected at 100 ~m while there are 14. L11. than in E galaxies (there are 47 E's and The probability to find 14 or more galax­ Kormendy, J., 1977a, Astrophys. J. 214, 359. 29 SO's in the complete sampie). How- ies with dust in the IRAS subsampie is Kormendy, J., 1977b, Astrophys. J. 217, 406.

43 Kormendy, J., 1977 c, Astrophys. J. 218, 333. Sandage, A., 1961, Hubble Atlas of galaxies, Shobbrook, R. R., 1963, The ObservatolY, 83, lonsdale, C. J., Helou, G., Good, J. C. and Carnegie Institution of Washington Publi­ 36. Rice, W. 1985, "Catalogued galaxies and cation 618. Varnas, S. R., Bertola, F., Galletta, G., Free­ quasars observed in the IRAS Survey", JPl Sandage, A., 1975, in Galaxies and the Uni­ man, K. C. and Carter, D., 1987, Astrophys. publication D. 1932. verse, Sandage, A., Sandage, M. and Kris­ J. 313, 69. Matthews, T.A., Morgan, W. M. and Schmidt, tian, J. eds. University of Chicago Press; de Vaucouleurs, G., 1953, The Observa­ M., 1964, Astrophys. J. 140,35. p.1. tory,73, 252. Möllenhoff, C., 1982, Astron. Astrophys. 108, Sandage, A. and Tammann, G.A., 1981, A de Vaucouleurs, G., 1959, Handbuch der 130. revised Shapley-Ames cata/og of bright Physik 53, 311. Neugebauer, G. et al., 1984, Astrophys. J. galaxies, Carnegie Institution of Washing­ de Vaucouleurs, G., 1987, lAU Symp. 127, 278, l1. ton Publication 635. p.3. Nieto, J.-L. and lelievre, G., 1982, Astron. Schneider, D. P., Gunn, J. E. and Hoessel, Westerlund, B. E. and Smith, L. F., 1964, Aus­ Astrophys. 109,95. J. G., 1983, Astrophys. J. 264, 337. tra/ian J. Phys.. 17, 340. Phillips, M. M., 1979, Astrophys. J. 227, Schweizer, F., 1980, Astrophys. J. 237, 303. l121.

Long-Term Walraven Photometry of Cataclysmic Variables s. VAN AMERONGEN and J. VAN PARAOIJS, Astronomicallnstitute "Anton Pannekoek", University ofAmsterdam, the Netherlands

Ouring the last seven years we have lerian orbit around the white dwarf. It is in a spasmodic way. Long time intervalls been observing cataclysmic variables generally believed that viscous pro­ of the order of months of a low accretion (CV's) with the Walraven photometer cesses give rise to exchange of angular rate (and therefore luminosity) are in­ behind the gO-cm Outch telescope at La momentum, by wh ich an initially formed terrupted by outbursts, lasting for a few Silla. An advantage of this photometrie ring spreads and forms a flat disk-like days to about two weeks, during which system for strongly variable blue sour­ configuration, the , the accretion rate increases by a large ces is that it allows simultaneous detec­ around the white dwarf. Matter trans­ factor (see Fig. 2). tion in five passbands, extending from ferred from the secundary gradually spi­ the optical (5400 to the ultraviolet rals inward along quasi-Keplerian orbits. A) Intermediate Polars (3200 A). We have observed these CV's Ouring this diffusion of the particles, the by monitoring them for several hours, other half is radiated away. In these cataclysmic variables the and reduced the measurements diffe­ This radiation from the accretion disk white dwarf has a magnetic field that is rentially with respect to nearby compari­ generally dominates the optical and UV strong enough to dominate the motion son stars. luminosity of cataclysmic variables. The of the inflowing matter within a certain Results obtained so far include mea­ kinetic energy of the inflowing material is distance from its centre. Within this surements of changes in the rotation eventually dissipated close to the white "magnetospheric radius" the matter is periods of accreting white dwarfs in dwarf surface, e. g. in a transition region channeled onto regions near the CV's, quite strong upper limits to secu­ from the accretion disk to the white magnetic poles of the white dwarf. In­ lar changes in optical brightness of dwarf surface. side this magnetosphere an accuration dwarf novae (ON) between outbursts, Based on observational characteris­ disk cannot exist; what fraction of the and the recognition that accretion in­ tics, cataclysmic variables have been accretion disk remains depends on the stabilities similar to ON outbursts also divided into several groups. Ouring our size of the magnetosphere, relative to occur in systems not classified as such. observing project, we have paid most of that of the Roche lobe around the white our attention to the following types of dwarf. cataclysmic variables: Introduction Brightness Variations Cataclysmic variables are close bi­ DwarfNovae nary stars consisting of a white dwarf Most cataclysmic variables with an and a low-mass « 1 MG) near-main­ In the dwarf-nova systems the accre­ accretion disk show brightness varia­ sequence companion. This "secondary" tion rate onto the white dwarf changes tions with the orbital period, wh ich are star fills its so-called Roche lobe, i. e. the critical equipotential surface, outside which matter is no longer bound to the star (see Fig. 1). At the point on the line through the two stellar centres, where the Roche lobes of the secondary and the white dwarf touch each other, mat­ ter from the secondary can easily fall into the gravitational potential weil of the white dwarf. Such matter, flowing from the secon­ dary star, has angular momentum with respect to the white dwarf, due to the orbital revolution of the binary. It will Figure 1: A sketch of the structure ofa cataclysmic binary. Matter from the secondary falls into therefore settle into a more or less Kep- the potential weil of the white dwarf and hits the accretion disk at the bright 01' "hot" spot. 44