CPD - 71 0 172, a New Binary with a Hot SubdwarfM. V/TON 1, O. BURGARELLA 1, A. CASSATELLA 2* and L. PREVOT3

1 Laboratoire d'Astronomie Spatiale, Marseille, France21UE Observatory, ESA, Madrid, Spain3 Observatoire de Marseille, France

The Very Wide Field Camera r,yWFC)is an experiment developed with, as amajor aim, the detection in a range offaint magnitudes of the stellar objectsshowing a strong ultraviolet excess. Itconsists of an all reflection F/1.9Schmidt camera which refocuses theimage of the sky given by a hyperbolicmirror onto the photocathode of an im­age intensifier. lt provides a useful fieldof about 66° in diameter with a resolu­tion of about 5 arcminutes. Three UVinterference filters centred at 168, 195and 254 nm can be placed alternativelyin the incident beam. The images arerecorded on classical KODAK lIa-Oemulsion. The VWFC was flown aboardSpacelab-1 in December 1983 and 66frames were obtained; because of orbi­tal constraints. only twenty per cent ofthe celestial sphere could be covered bythe deepest survey. Thanks to an ade­quate image data processing particular­Iy weil adapted to correct the imagesfrom straylight background, limitingmagnitudes as faint as m (195 nm) = 9.3were reached.

In the course of identifying starsaround the Small Magellanic Cloud, awell-defined star-like object was found(Fig. 1) with no other counterpart than acool star; according to the data pro-

• AHilialed 10 Ihe Aslrophysies Division, SpaeeSeienee Departmenl, ESA.

vided by the Centre de DonneesStellaires in Strasbourg and to the ESO/SRC Sky Atlas prints, the only possiblecandidate was CPD -71° 172, an FOstar with magnitudes mv- 10.5 andms - 10.9. From these values, the ex­pected 195 nm magnitude was normallyexpected at about 12.3 whereas it wasestimated to be close to 8. A so large UVexcess led us to suspect a hot sub­luminous companion to the cool pri­mary. We confirmed soon the spectraltype of the latter by help of a spectro­gram obtained at ESO La Silla. At thesame time, a first low resolution UVspectrum provided by the IUE satelliteconfirmed the existence of a very hotsubdwarf companion. The binaryassociation of both stars seems evidentafter their angular distance. which islikely less than 1 arcsecond (the pair isoptically still unresolved), and after thedistance inferred from the cool star(700 pe). The absolute magnitude de­duced for the blue star, Mv - 3, is acommon value for this kind of objects.We recall their great interest as theyprovide a direct means of testing theability of models to reproduce the laststages of stellar evolution towards thewhite dwarf degenerates. In this con­nection and from a practical point ofview, the interest of binary systemssimilar to CPD - 71° 172 lies in theopportunity they ofter to determine un-

ambiguously the absolute magnitudesof hot subdwarfs, so long they belong toloose systems in which no significantmass exchange has occurred when theoriginal primary was near the red gianttip.

Visible spectrography was performedon 111 a-J hypersensitized plates usingthe Boiler and Chivens spectrographattached to the ESO 1.52-m telescope.with a dispersion of 39 Amm-'. Weclassified the star as F 2.5 IV with anuncertainty of 1 subclass; no He I or He 11lines were visible.

UV spectra at low resolution havebeen obtained in the range 120 to320 nm with the IUE satellite; they arefully dominated, especially in the shorterwavelength region, by the flux of the hotcomponent, showing a UV gradientsimilar to that of BD 75° 325, a well­studied sdO star.

Johnson, Cousins and Strömgrenphotometries were carried out at ESOand SAAO, with the aim of a photomet­rie deconvolution between the compo­nents. The fitting of intrinsic colours ofboth components to so many colourswas a puzzling task which required aniterative method. We found that a colourexcess E (B-V) = 0.014 was necessaryto reproduce the observed colours. Thedeconvolution implied a fit of the Kurucz(1979) model atmospheres normalizedat V to the flux distribution in the IUE

Figure 1: Negative enlargement of part of a deep frame (195 nm filter, exposure time 304 s) of the VWFC in the region of the Magellanic Clouds:the LMC is at the centre. the SMC to the right (North is roughly up, East to the left). Four other newly identified objects showing UV excess havebeen found in this field.

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October 6 Council in ParisNovember 17 Scientific Technical

CommitteeNovember 19-20 Finance CommitteeNov. 30-Dec. 1 Observing Pro-

grammes CommitteeDecember 7 Committee of CouncilDecember 8 CouncilAll meetings will take place in Garchingunless stated otherwise.

have been identified and spectrographicobservations recently performed at ESOhave revealed seven objects of variousnatures, for which a long-term generalsurvey including multicolour photomet­ry, radial velocity measurements andIUE observations is in progress.

Tentative Time-tableof Council Sessionsand Committee Meetingsin 1987

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Figure 2: Absolute flux distribution of CPD - 71°172AB from Lyman alpha to the Johnson Kband (2.2 {im) expressed in erg (cm 2 s Ar' versus the wavelength in Aunits. Strömgren uvby,Cousins Re le and Johnson JHK fluxes are displayed by large filled dots. Small dots in the UVrange represent the observations with IUE.

Star Mv Tot! BC Log (U4J) R/R(!) log g

CPD -71° 172A 1.80 6,700 -0.10 1.15 2.80 3.7CPD -71° 172B 2.86 55,000 -4.80 2.70 0.24 5.4

range, following the method of Heber etal. (1984) and Heber (1986) though pres­ently only LTE models were available. Itprovided a low estimate of the effectivetemperature Tell - 50,000° K for thehot star, from which were derived itsintrinsic optical colours; some iterationswere necessary to adjust the V mag­nitudes and the reddening so that allcolours of the primary would be those ofan early F star. The adopted solutionwas the following:

Star CPD CPD-71° 172A -71° 172B

Spectral type F 3-41V SdOBY or.V 10.99 12.05b-y 0.265 - 0.155m, 0.161 0.060C, 0.643 - 0.210U-B 0.019 - 1.210B-V 0.403 - 0.327V-Re 0.240 - 0.16V-Ie 0.481 - 0.32V-J 0.77 - 0.74V-H 0.96 - 0.91V-K 1.01 - 0.97

The visual absolute magnitudes ofboth stars were derived from theadopted spectral type and colours ofthe F component, using the Tables ofCrawford (1975) and of FitzGerald(1970) to estimate its degree of evolu­tion; the inferred intrinsic parameterswere checked by means of the Barnes­Evans (1978) relation. We determined inthat way:

Figure 2 shows the fit of the sum ofthe Kurucz models adopted for eachcomponent to the IUE and visible ob­served composite colours. As it can beseen, the overall agreement betweenmodels and observations is quite satis­factory. A subsequent check using the Rindex of Schonberner and Drilling (1984)yielded Tefl - 50,000 to 60,000° K.

We have compared CPD - 71 ° 172 Bto two well-known subdwarfs:

(1) BD 75° 325 is a field sdO with Teff- 50,000° K, log 9 - 5.3, helium richafter Kudritzki et al. (1980). The shortwavelength IUE spectra of both starshave revealed comparable ionizationand excitation temperatures; the He I1line at 164 nm is much stronger in thespectrum of BD 75° 325, suggestingthat helium is moderately to fairly de­pleted in CPD -71 ° 172 B. The abun­dance of nitrogen is roughly normalwhile silicon and carbon are depletedlike helium.

(2) The ultraviolet spectrum ofLS I1 + 18° 9, which is a helium normalstar with Teff - 60,000° K, confirmsboth the normal abundance of nitrogenand the slight depletion of helium in thephotosphere of CPD -71 ° 172 B. Incase where these results would be con­firmed by the analysis of IUE high reso­lution observations, this newly discov­ered subdwarf would be one of the firsthelium poor sdO's with Teff >40,0000K.

So far, 14 other potential candidates

ReferencesBarnes, T.G., Evans, D.S. and Moffet, T.J.:

1978, Mon. Not. R. Astron. Soc. 183,285.Crawford, D. L.: 1975, Astron. J. 80, 955.FitzGerald, P. M.: 1970, Astron. Astrophys. 4,

234.Heber, U., Hunger, K., Jonas, G. and Kudritz­

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Heber, U.: 1986, Astron. Astrophys. 155,33.Kudritzki, R. P., Hunger, K., Gruschinske, J.

and Simon, K. P.: 1980, Proc. of 2ndEurop. IUE Cont., ESA SP-157, 307.

Kurucz, R. L.: 1979, Astrophys. J. Suppl. 40,1.

Schonberner, D. and Drilling, J. S.: 1984, As­~ophys. J. 278, 702.

List of ESO Preprints506. V. Castellani and M. L. Quarta: The

Oosterhoff Dichotomy Revisited. I. TheRanking of RR Lyrae Periods VersusMetallicity. Astronomy and Astrophy­sics. June 1987.

507. L. B. Lucy and M. Perinotto: Models forthe Wind of the Central Star of NGC6543. Astronomy and Astrophysics.June 1987.

508. A. F. M. Moorwood: IRSPEC: Design,Performance and First Scientific Re­sults. Invited paper presented at theWorkshop on "Ground-Based As­tronomical Observations with InfraredArray Detectors", University of Hawaiiat Hilo, March 1987. June 1987.

509. A. Renzini: Some Embarrassments inCurrent Treatments of ConvectiveOvershooting. Astronomy and As­trophysics. June 1987.

510. P. Crane, A. Stockton and W. C. Sas­law: The Optical Spectral Index in theSouth Radio Lobe of 3C 33. Astronomyand Astrophysics. June 1987.

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