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phys. 118, 217.Iben, I.Jr., Tutukov, AV. 1984, Ap. J Suppl.

54,335.Koester, 0., Schulz, H.,. Weidemann, V.

1979, Astron. Astrophys. 76, 262.Liebert, J., Probst, R. G. 1987, Ann. Rev. As­

tron. Ap. 25, 473.

Matteucci, F., Greggio, L. 1986, Astron. As­trophys. 154,279.

McCook, G. P., Sion, E. M. 1984, A Catalogueof Spectroscopically Identified WhiteDwarfs, Villanova University Press.

Nomoto, K., Iben, I.Jr. 1985, Ap. J, 297, 531.Pier, J. R. 1983, Ap. J Suppl. 53, 791.Probst, R. G., O'Connell, R. W. 1982, Ap. J

Lett. 252, L69.Renzini, A, Fusi Pecci, F. 1988, Ann. Rev.

Astron. Ap. in press; ESO Preprint No. 565.Robinson, E.L., Shafter, AW. 1987, Ap. J

322,296.Saffer, R.A., Liebert, J. W., Olszewski, E.

1987, Bull. A.A.S.19, 1041.Saio, H., Nomoto, K. 1985, Astron. Astro­

phys. 150, L21.

A Search tor Flare Stars With the GPO AstrographR. ANIOL, H. W. OUERBECK, W.' C. SEITTER, Astronomisches Institut der Universität Münster,

F. R. GermanyM. K. TSVETKOV, Department ofAstronomy, Academy ofSciences, Sofia, Bulgaria

Figure 1: Number of plates per year taken with the GPO telescope since the beginning of itsoperation on La Silla.

The elose eorrespondenee betweenflare stars in aggregates and in the solarvieinity, as weil as some analogy be­tween the flares in these stars and thesolar flares suggest theoretieal modelsfor the underlying physieal proeess(es).However, a general pieture whieh in­eludes all aspects of the problem ofeorrelating the properties of flare stars,UV Ceti stars and older main sequeneestars, has not yet been developed.

2. The GPO Astrograph

The GPO astrograph was one of thefirst teleseopes on La Silla. At its previ­ous loeation Zeekoegat in South Afriea(1961 -1966) and during its first yearson La Silla, it was mainly used to obtainobjeetive prism speetra of stars in thedireetion towards the MagellanieClouds. One of the rare speetra of Sk- 69° 202, the star that later beeameSN 1987 A, was taken with this instru­ment.

1985

3. The Suitability ot the GPO tor aFlare Star Search

Until now, patrol observations of flarestar fields were predominantly earriedout with Sehmidt teleseopes in thenorthern hemisphere. The Sehmidt tele­seopes in the southern hemispherewere to a large extent oeeupied by atlas

A new ehapter for the GPO wasopened with inereasing interest in direetplates. Most of them are used for as­trometrie purposes: for asteroid obser­vations, proper motion studies, and forthe input eatalogue of the HIPPARCOSastrometrie satellite mission. Reeenthighlights of the southern sky, Halley'sComet and the Supernova 1987 A,tempted more observers to "try" theGPO. We will report on one of our twophotometrie surveys, the seareh for flarestars with the GPO. Figure 1 displaysthe observational aetivity of the last twodeeades.

19751970

1200

'-coQ)

>-- 800Cf)Q)....co

Q.

400

1. Flare Stars in Young StellarAggregates

The first systematic investigation offlare stars by the Mexican astronomerGuillermo Haro marks the beginning oflong-term, multi-site studies of theseobjects in young stellar aggregates. Hisfirst series of observations with theSchmidt telescope of the TonantzintlaObservatory already showed the eloserelation between stars in the T Tauristage and eool dwarf stars with en­haneed UV aetivity (= flare stars), bothtypes of objeets being still in their earlystages of evolution.

After 30 years of subsequent investi­gations, some of the eonclusions byHaro are substantiated on the basis ofextensive observational material, whilethe new results also gave a elearer pie­ture of the nature of flare stars. Some ofthe main aehievements and results to besubstantiated by further studies are:- More than 1,000 flare stars in the

aggregates of Orion, the Pleiades,NGC 7000 (North Ameriea nebula),Praesepe, Coma, and others werediseovered.

- The physieal eorrespondents of flarestars in aggregates are the UV Cetistars in the solar neighbourhood.

- By observing multiple flares in givenstars, the lower limit of the number offlare stars in various aggregateseould be estimated: Orion - 1,500;Pleiades - 1,000, NGC 7000 - 400,Praesepe - 300, Coma - 100 stars.

- It was found that, for a given aggre­gate, the speetral type of the bright­est flare star and the maximum flareamplitude both eorrelate with the ageof the aggregate.

- In stellar evolution, the T Tauri stageis followed by the flare star stage.

- There are two major types of flares,fast and slow ones. It is eoneludedthat all flares have the same physiealorigin, sinee there are also rare flaresof intermediate speed.

39

of two chains with equal coordinates indeclination) are not analysed at thisstage. The resulting mean distances be­tween images are determined within0.5" to 1.0".

It is now possible to look for allchains. The amalgamated chains cangenerally be separated without difficultyon account of their larger number ofmembers. Shorter chains and evensingle images can be found using thecoordinate list of objects found on themaster plate. Before storage, the chainsare marked with identifiers, which givethe total number of images. Incompletechains or single images are stored sepa­rately, because they have a higher prob­ability of resulting from a fJare star.

The third part of the programme isdevoted to the search for flare events. Astandard light curve (variation of mag­nitude across a representative chain)must be known in order to determine thepresence of a flare by comparison with agiven chain. Because of the scatter inthe magnitudes of individual chains, amean light curve must be used. In orderto avoid systematic effects, mean lightcurves are obtained separately for givenmagnitude intervals.

A chain showing at least one signifi­cant deviation from the mean Iightcurvemarks a flare candidate, though, as arule, a true flare produces several devia-

Figure 4: Intensity tracing of a fast flare ob­served in a newly discovered flare star inOrion.

The programme package developed byone of us (R. A.) includes:1. plate preprocessing2. chain recognition3. search for f1are event candidates4. history of variability.

In part one, the data are brought intothe form needed for the subsequentprocesses. Image segmentation andrestauration is performed with a pro­gramme written by H. Horstmann for agalaxy survey on ESO/SRC atlas plates.The microdensitometer coordinates aretransformed into celestial coordinates,using an astrometric programme writtenby H.J. Tucholke. The first step is thetransformation of the coordinates for asingle long exposure master plate. Thisis necessary because the deep plateregisters stars which may be visible onthe multiple exposure plate only duringa stellar brightening. The accuracy isbetter than 1", when stars from the SAOcatalogue are used. Next, coordinatetransformations are made between themaster plate and the multiple exposureplates. The coordinates are assigned tothe eastern image of each chain of ex­posures.

Finally, the machine magnitudes ofthe images are calculated using a stan­dard characteristic curve.

The data are now ready for the sec­ond part of the programme, whose pur­pose it is to determine the mean distan­ces between the images in the chainsand their standard errors. Chains with asmaller number of images (weak starsdrowned in a variable sky background)or chains with more images than ex­posures (caused by the almagamation

4. Methods of Reduction

Figure 2: Seetion of a multiple exposure plate of the Orion nebula region, showing the rise of aslow flare of KO Ori (emulsion /la-O, exposure time 6x 10 minutes).

work. It thus seemed worthwhile to trythe GPO telescope for a survey ofsouthern fields.

While an astrograph has weak trans­mission for ultraviolet light, where mostof the radiation of the flare is emitted, ithas a more suitable image growth whichmakes small magnitude differences bet­ter measurable. In order to calibrate thedetection power of the GPO, we chosethe weil studied region of the Orionnebula (M42/M43), the centre of therichest field of UV Ceti and T Tauri stars.

The visual inspection of the test plates(by M. K. T.) showed that the UV defi­ciency is indeed compensated by theenhanced image growth and that atleast as many flare events per unit areaand unit time are discovered on GPOplates as on Schmidt plates (Tsvetkovet al., Comm. Konkoly Obs. No. 86,Budapest 1986, p. 429.)

New fields of the survey are the youngclusters 0 Vel (IC 2391), e Car (JC 2603),and the dark cloud regions Cha T 1,Sco-Oph, Q Oph, and Sco-Sgr.

Generally, five to seven exposures often minutes duration each, separated by30", are made per plate. An example isshown in Figure 2, which portraits one ofthe rare slow flares, here of the star KOOri.

For the beginning of the flare starproject, we intended to reduce the mul­tiple-exposure plates fully automatically.All plates are scanned with thePOS 2020GM plus microdensitometerat the Astronomical Institute Münster.

40

tions. The method is very sensitive. Byvisual inspection, only flares of am­plitudes larger than O~ 6 are found. Hereit seems possible to decrease the limitto about O~ 3. The method also yieldsuseful results for plates taken undernon-photometrie conditions; suchplates, however, are rarely obtained atLa Silla.

The flare candidates are stored in a"flare-file". This file contains also infor­mation about celestial coordinates,magnitudes of the images and densityof the sky background. The last value isespecially important for checking the re­liability of a reported flare event, be-

cause a strong variable background,even when properly subtracted, canproduce systematic exposure effects.Another problem appears for almaga­mated chains in regions of high stardensity. It occasionally happens thatstars from the two chains merge intosingle images of higher brightness. Thiscan occur at any position in the chain.

At last, the discovered flare eventscan be inspected in the form of tracingsor digitized images of the chain. Theimages reveal only flare events with suf­ficiently large amplitudes, while the trac­ings clearly show also the fainter flaresand the variations of the sky

background. Two examples of flarestars are given in Figures 3 and 4.

The final part of the programme,which is under development, offers theuser the possibility to combine resultsfrom different plates, in order to studythe complete available history of allstars in the field. For flare stars, thismeans better confirmation, especially incases of low amplitude flares of higherfrequency, and thus higher sensitivity ofthe photographie flare search. Also, typ­ical time seales for more frequent flarescan be determined. Finally, other typesof variables with longer periods can bedetected.

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

Introduction

Abrief historical review of the discov­ery and exploration of elliptical galaxieshas been published by de Vaucouleurs(1987). We have made use of it to writethis introduction.

The most widely used classificationsystem for galaxies is that proposed byHubble in 1926. The sequence ofclassification, as originally presented,consisted of aseries of elliptical nebulaeranging from globular (EO) to lenticular(E7) forms, and two parallel series ofunwiding spirals, normal (S) and barred(SB). The figure following the symbol Eis equal to the ellipticity (a-b)/a with thedecimal point omitted, a and b beingrespectively the major and minor axis ofthe galaxy.

Elliptical galaxies have been de­scribed by Hubble as highly concen­trated objects with luminosity fallingrapidly away from bright, semistellarnuclei to undefined boundaries; smallpatches of obscuring material are occa­sionally silhouetted against the lumin­ous background, but otherwise thesenebulae present no structural details.

No elliptical galaxies are known thatare flatter than E7; galaxies which areflatter invariably show an outer region oflow surface brightness which resemblesa thin fundamental plane.

The transition from E7 to Sa appearedvery abrupt. With accumulating data,and especially with the increasingnumber of good photographs taken withthe 100-inch Mount Wilson reflector,numerous systems have been recog­nized as soon as 1936 by Hubble whichare later (flatter) than E7, but wh ichshow no spiral structure. These galaxies

fill the gap between E7 and Sa and arecalled SO or SBO (Sandage, 1975).

The SO class has been described forthe first time in 1961 by Sandage. SOgalaxies appear to form a transition be­tween the E galaxies and the true spi­rals; the transition from E to SO issmooth and continuous. The divisionbetween E and SO is made on the basisof the presence or absence of an outeramorphous envelope or thin fundamen­tal plane surrounding the nuclear re­gions. Some SO galaxies exhibit a struc­tureless envelope similar to that of thenormal SO's, but a sharp, narrow ab­sorption lane is found within the lens;the lane is in an are concentric with thenucleus. These galaxies are called S03'

A more quantitative way to separate Efrom SO has emerged from the study ofthe light distribution in galaxies. De Vau­couleurs (1959) has pointed out thatelliptical galaxies have a single-compo­nent surface brightness distributionwhich follows closely the de Vau­couleurs relation:

log I (r) cq1/4

(r is the distance from the centre of thegalaxy) while the surface brightness dis­tribution for spirals and SO galaxiesshow two main components: an innerspheroidal component which followsthe de Vaucouleurs law, and an outerexponential component (disk) with:

I (r) = I (0) xe-«r

This result has been confirmed since bya number of more recent works (see forinstance: Freeman, 1970; Kormendy,1977 a, band cl.

However, the classification of most

galaxies has been made by visual in­spection of photographie plates ratherthan by measurement of the brightnessdistribution leading to some misclassifi­cation especially when the galaxy con­tains dust.

Dust was clearly visible on a photo­graph of NGC 1316 taken in 1943 byParaskevopoulos; but it was mistakenas plate defects by Shapley who couldnot believe that an elliptical galaxy couldcontain dust (Hodge, 1975). Probablybecause of the presence of this dust,NGC 1316 was called Sa pec by San­dage and Tammann (1981); however,Schweizer (1980) has shown that it is atypical Morgan D-type galaxy with anelliptical-like spheroid embedded in anextensive envelope.

In the course of a survey of brightsouthern galaxies, a pair of abnormalobjects have been observed by de Vau­couleurs (1953) which shared most ofthe characteristic peculiarities of thebright radio galaxy NGC 5128 (Cen­taurus A), i. e. radio emissivity and dust;these galaxies are NGC 1316 (Fornax A)and NGC 1947, to which de Vau­couleurs added a northern galaxy,NGC 2537; however, at the time, radiopositions were not accurately mea­sured; the errors were of the order of1°, and only NGC 1316 has been con­firmed as a radio source; moreover,NGC 2537 is an Sc galaxy and of nointerest here. NGC 1947 is indeed anearly-type galaxy (S03 pec), but not aradio source (Möllenhoff, 1982).

Shobbrook (1963) has discovered adust lane in the elliptical galaxyNGC 4696 and used it as an argumentfor the identification of this galaxy with

41