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Astron. Astrophys. 334, 873-894 (1998)

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1. Introduction

Parenago (1954) listed object number 1724, now called Par 1724 or simply P1724, as a relatively bright ([FORMULA] mag) star located at [FORMULA] and [FORMULA], i.e. only [FORMULA] north of the Trapezium cluster in Orion. It has recently received the variable star designation V1321 Ori (Kazarovets & Samus 1997). Other designations include GSC 4774 0910, Brun 490 (Brun 1934), JW 238 (Jones & Walker 1988), and Strand 51 (Strand 1958); throughout this paper we use P1724, the most common designation for the star. P1724 is listed in the Herbig & Bell (1988) catalog (HBC 452) as a T Tauri star (TTS) with spectral type G8, displaying [FORMULA] in emission and strong lithium 6708Å absorption.

P1724 appears to be one of the most active TTS known. It has shown one of the most powerful X-ray flares seen on a pre-main sequence star (Gagné et al. 1995; Preibisch, Neuhäuser, & Alcalá 1995).

Tagliaferri et al. (1994) studied the star in considerable detail. From their high-resolution spectra they found no indications of (spectroscopic) multiplicity. They estimated the projected rotational velocity to be [FORMULA], and determined a lithium abundance of [FORMULA] (Li) [FORMULA], suggesting a very young age. Tagliaferri et al. (1994) classified the star as a classical TTS (cTTS).

Preibisch et al. (1995) recently studied P1724 optically also. They estimated the spectral type to be about K3 and presented a low-resolution spectrum showing [FORMULA] Å and [FORMULA] (Li) [FORMULA] Å . Hence, P1724 is more properly classified as a weak-line TTS (wTTS).

The X-ray variability of P1724 was studied by Preibisch et al. (1995) and Neuhäuser et al. (1995b). Assuming that it is located in Orion at a distance of [FORMULA], Preibisch et al. (1995) observed the largest flare (in terms of total radiated energy) ever reported on a star (surpassed since then; cf. Grosso et al. 1997). If P1724 is actually foreground to Orion, the flare would still be exceptional (cf. Montmerle et al. 1983, Preibisch et al. 1993, Gagné et al. 1995, Kürster & Schmitt 1996).

TTS tend to cluster in dark clouds and form T associations. There are star forming regions (SFR) where it seems that only low-mass stars form (e.g. Taurus-Auriga), as well as others with both low- and high-mass stars such as the Orion OB association, at a distance of [FORMULA] (Genzel & Stutski 1989, Brown et al. 1994). Most stars in a T association share the same kinematics, i.e. radial velocity and proper motion. There have been several studies of the proper motion of P1724, and the probability of membership to the Orion association as estimated by different authors ranges from [FORMULA] to [FORMULA] (cf. Sect. 9).

One of our goals in this paper is to understand why P1724 shows the extreme behavior in X-rays mentioned above. Due to its possibly peculiar space motion, it may also provide supporting evidence for the hypothesis that low-mass pre-main sequence (PMS) stars can escape from multiple protostellar systems (or dense clusters) and become run-away T Tauri stars. One such star, possibly ejected from the Trapezium, has been presented by Marschall & Mathieu (1988): P1540, a double-lined spectroscopic binary, shows radial velocity consistent with membership to Orion, but its proper motion is different and points away from the Trapezium cluster, from which it might have been ejected [FORMULA] years ago. P1724 may be similar to this star in many respects.

Our paper is organized as follows: In Sect. 2, we discuss the photometry to check the rotational period of P1724. In Sect. 3, we present high-resolution spectra used to monitor the radial velocity and determine the physical parameters of the star. Then, in Sect. 4, we investigate the long-term photometric variations over [FORMULA] years and compare it to our recent data on photometric and radial velocity variations, presumably related to spot activity. The spectral energy distribution is presented in Sect. 5, leading to estimates of the mass, radius, and luminosity. Additional high-resolution spectroscopy is described in Sect. 6, and used for reconstruction of the surface temperature distribution by Doppler imaging techniques. We discuss some activity and/or youth indicators in P1724 (H [FORMULA], Ca II H & K, and lithium) in Sect. 7, as well as X-ray observations in Sect. 8. The space motion of P1724 is studied in Sect. 9. In Sect. 10, we present R - and K -band imaging and discuss the multiplicity of the star. Finally, we summarize our findings and conclude in Sect. 11.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998