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Astron. Astrophys. 334, 969-975 (1998)

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1. Introduction

The pre-main-sequence object Z Canis Majoris (Z CMa, [FORMULA] (1950) = [FORMULA] [FORMULA] 22:s52, [FORMULA] (1950) = [FORMULA] [FORMULA] [FORMULA]) is a member of the CMa OB1 association for which distance estimates range from [FORMULA] pc (Ibragimov & Shevchenko 1990) to 1150 pc (Claria 1974). It was classified as an Herbig Ae/Be object (Herbig 1960, Thé et al. 1994) as well as a FU Orionis variable (Hartmann et al. 1989) whose luminosity is dominated by the contribution of an accretion disk during the early phase.

At large spatial scales, narrow-band imaging and long-slit spectroscopy by Poetzel et al. (1989) led to the detection of Herbig-Haro objects and a jet emanating from Z CMa. The Herbig-Haro objects can be traced out to a distance of [FORMULA] on the blue-shifted side (SW) and to [FORMULA] on the red-shifted side (NE) at the position angle [FORMULA]. The blue-shifted jet has three velocity components. The fast one (-620 km/s) can be found up to a distance of [FORMULA] from the source while the other two (-400 km/s, -100 km/s) show up only within [FORMULA] which may be an indication of a multiple shell structure.

An accompanying molecular outflow was detected by mapping of the CO line emission (Evans et al. 1994, Liljeström & Olofsson 1997) whose extent is roughly [FORMULA] at the FWHM level. The positional offset between the blue and the red wings amounts to about [FORMULA].

In a field of [FORMULA] Nakajima & Golimowski (1995) obtained R- and I-band coronographic images of the vicinity of Z CMa around the [FORMULA] occulting disk of the coronograph. They found a warped, disk-like reflexion nebula (of `one-sided hourglass geometry') with an extension of [FORMULA] at a position angle of about [FORMULA] which is nearly perpendicular to the outflow direction. The brightest part of the nebula lies [FORMULA] north of the central object.

Bieging et al. (1984) have surveyed the region around Z CMa with the VLA at 6 cm. Their maps show an elongated, double peaked radio structure. The main peak is shifted eastward by [FORMULA] from the star. The secondary peak lies at the position angle of about [FORMULA] at a distance of [FORMULA] close to the outflow direction. Bieging et al. (1984) concluded that the detected 6 cm continuum emission is connected with the mechanism which causes a direct mass loss (which was not observed up to this time).

Using speckle observations, Z CMa is found to be double in the IR (Koresko et al. 1991, Haas et al. 1993, Malbet et al. 1993, Tessier et al. 1994) and in the optical (Barth et al. 1994, Thiébaut et al. 1995) with a separation of [FORMULA] at a position angle of about [FORMULA]. Whereas at optical wavelengths the southeastern component is more luminous, the northwestern component is the brighter one from 2.2 µm to longer wavelengths. Therefore, we call the southeastern component in the following the primary.

Besides the mapping observations mentioned above, aperture measurements were performed. Weintraub et al. (1991) reported millimeter and submillimeter continuum observations of Z CMa. They modeled their submillimeter data best as thermal flux from a circumstellar disk. For the Z CMa system they deduced a very large disk mass of 0.2 [FORMULA] which is reasonable for extremely young objects or massive central stars.

Koresko et al. (1991) combined their speckle flux ratios in the J, H, K, [FORMULA], M, N-bands with photometric measurements of Z CMa to estimate the spectral energy distribution of the Z CMa system. They proposed a three component model consisting of an PMS object of FU Orionis type ([FORMULA]) for the southeastern component, a deeply (optically thick) embedded PMS object ([FORMULA]) for the northwestern component and around both a second (circumbinary) disk which may be the reservoir for accretion onto the FU Orionis disk.

Zinnecker & Preibisch (1994) unambiguously detected Z CMa clearly as an X-ray source. They explained the X-ray emission by a mechanism which is related to the stellar wind interaction with remnant circumstellar material.

Covino et al. (1984) found a close linking between the photometric activity and the presence of emission lines. They measured the strongest variation in the spectrum of Z CMa in the H [FORMULA] profile. As a possible explanation for the triple structure of the H [FORMULA] absorption component they suggested an envelope of three different expanding shells.

Results of aperture polarimetry using Johnson U, B, V, R, I and H [FORMULA] filters and different aperture sizes were published during the last 30 years (Hall 1958, Serkowski 1970, Breger 1974, Vrba 1975, Garrison & Anderson 1978, Vrba et al. 1987, Jain et al. 1990, Miroshnichenko et al. 1993, Jain & Bhatt 1995, see Fig. 3). The polarization degrees reach up to [FORMULA]. Only the measurement by Hall (1958) at B has given a significant larger value: 7.6%, [FORMULA]. Measurements from different epochs show that Z CMa is polarimetrically variable like most Herbig Ae/Be objects (Jain & Bhatt 1995).

Whitney et al. (1993) investigated Z CMa by optical spectropolarimetry and found a larger polarization (up to 6%) in the emission lines than in the continuum (1-2%). They argued that the primary (northwestern component) is the emission line source (perhaps a Herbig Ae/Be star) of the Z CMa binary which is hidden from view but observable by scattered, and in this way, polarized light. In their model (for their observations) the optical primary (southeastern component) does not contribute to the flux in the lines.

In recent years, evidence has been accumulated that most stars form as binaries (e.g. Leinert et al. 1997, Testi et al. 1997, Pirzkal et al. 1997). While there are theoretical studies which suggest that the binary formation is the immediate result of the fragmentation of the protostellar cores (Burkert et al. 1997), other scenarios for binary formation are possible (capture, e.g. Turner et al. 1995). By the study of the morphology of dusty circumstellar disks/envelopes around young stars, conclusions about the formation mechanism of binary stars can be drawn. This is based on the fact that the circumstellar dust (agent and remnant of star formation) becomes observable by scattered and thereby polarized light. Particularly, the observed polarization pattern contains information on the orientation of the disk (Fischer et al. 1996). Thus, polarimetry offers a valuable method for the study of young binary stars.

If both stars were formed by fission of a fragment, we would expect either aligned individual disks or a circumbinary disk. In both cases, the net polarization angle of both components should be similar (projection effects have to be taken into account). Otherwise, if binaries were formed by capture, we would in general expect different polarization angles. This anticipated difference in the polarization behaviour led us to initiate high-resolution imaging polarimetry of young binary stars. In the following, we present results for one object of our sample - Z CMa.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998