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Astron. Astrophys. 329, 827-839 (1998)

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1. Introduction

The Cloverleaf is the gravitationally lensed image of the distant quasar H1413+117 (14 [FORMULA] 15 [FORMULA] 46 [FORMULA].23; [FORMULA] J2000.0) at [FORMULA] showing four spots with angular separations from [FORMULA] to [FORMULA]. Since its discovery (Magain et al. 1988), the Cloverleaf has been observed spectroscopically and imaged with ground based telescopes in various bands from B to I (Kayser et al 1990, hereafter K90, Angonin et al 1990, Arnould et al 1993) as well as at 3.6 cm with the VLA (K90). Models of the gravitational lens, involving one or two galaxies, were derived from these data sets, with some emphasis put on the VLA radio map (K90). Recently, Keeton, Kochanek and Seljak (1997) (hereafter KKS) in a theoretically motivated paper studying "shear" in gravitational lens quads system, presented Singular Isothermal Sphere (SIS) plus external shear lens models for most quads and in particular for the Cloverleaf. The main difficulty with these models lies in the fact that they predict, for a z=1.44 lens, a mass of [FORMULA] 2.5 [FORMULA] [FORMULA] M [FORMULA] within [FORMULA] (6 [FORMULA] kpc) radius, which would correspond to a relatively bright normal galaxy: so far, searches in the K band of the predicted `bright' lensing galaxy have been unsuccessful (Lawrence 1996) and this fact remains a mystery.

HST observations of the Cloverleaf have been done in different modes: Falco (1993) presented a short report on the pre-COSTAR HST/WFPC image (used for the lens model derived by Yun et al (1997)), Turnshek (1995) discussed FOS observations, and a recent paper by Turnshek et al (1997) described PC observations (pre- and post-COSTAR). However, the post-COSTAR HST/WFPC2 (PC and WF) data have not been fully exploited yet, though they may bring new constraints on the gravitational lens modeling.

New information also comes from the detection of the Cloverleaf in the molecular CO transitions (Barvainis et al 1994, Wilner et al 1995, Barvainis et al 1997, Yun et al 1997, Alloin et al 1997). Both the physical conditions of the molecular gas in the quasar can be studied in detail (Barvainis et al 1997) and the gravitational lens model could be improved. Indeed, unveiling the gravitational lens nature of the Cloverleaf in CO (Alloin et al 1997) provides the true intensity ratios of the four spots, unaffected by absorption from intervening material and unaffected by microlensing effects as the CO source in the quasar is not point-like (a gravitational shear is measured on the 3 brightest CO spots A, B & C). However, the different parts of the CO source suffering a different amplification, the CO intensity ratios cannot be compared directly to the optical ones: this explains the corresponding observed differences reported by Alloin et al (1997). An improved CO(7-6) map, presented in this paper, has been obtained with the IRAM interferometer, giving further insight into the geometry of the quasar CO source.

In addition, CCD images of the Cloverleaf over a 5' field of view (CFHT/FOCAM archives) allow precise astrometry of the field and a better registration of the maps obtained through various wavebands, with a particular interest in the optical and the millimeter ones.

In the current paper, we take into account all these new constraints in order to derive an improved model of the gravitational lens and of the quasar molecular source. We present in Sect. 2 the new CO(7-6) data set and discuss in Sect. 3 the post-COSTAR HST/WFPC2 data, from the point of view of the Cloverleaf itself and of its environment. We provide in Sect. 4 the results of the astrometry performed from archived CFHT/FOCAM images and, using the CO(7-6) high resolution map obtained with the IRAM interferometer, we position very accurately the optical and millimeter data sets. A new gravitational lens model is computed and presented in Sect. 5, while the CO source in the quasar is discussed in Sect. 6. A final discussion and prospective analysis is given in Sect. 7. Throughout the paper, we use H0 = 50 [FORMULA] km/s/Mpc, [FORMULA] =1 and [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997