Optical and X-ray monitoring, Doppler imaging and space motion of the young star Par 1724 in Orion

R. Neuhäuser, S.J. Wolk, G. Torres, Th. Preibisch, N.M. Stout-Batalha, A.P. Hatzes, S. Frink, R. Wichmann, E. Covino, J.M. Alcala, W. Brandner, F.M. Walter, M.F. Sterzik, and R. Köhler

Astronomy & Astrophysics, 334, 873, (1998)

Abstract

We present a detailed study of the young T Tauri star P1724, located 15' north of the Trapezium cluster in Orion. Our extensive photometric measurements in the V, R and I bands confirm the rotational period to be approximately 5.7 days. Repeated high-resolution spectra show low amplitude variability in the radial velocity of the star. A Doppler-imaging analysis based on high-S/N high-resolution spectra yields an image showing a pronounced dark feature (spot) at relatively low latitude. From its size and temperature we conclude that it is responsible for the observed photometric and spectroscopic variability. Our high-resolution spectra yield a rotational velocity v sin i ~ 71 km/s and a mean radial velocity of ~ +23 km/s. R band imaging reveals no visual companions down to separations of 1 arc second and magnitude differences of up to Delta R = 7 mag. We also collected UBVRIJHK photometry of the star and present its spectral energy distribution, showing that P1724 is a naked T Tauri star.

We estimate the bolometric luminosity to be ~ 51 L_sun, the spectral type to be K0, and the radius to be ~ 9 R_sun. We obtain similar values of the radius from the Boltzmann law and the Barnes-Evans relation. Also, the surface gravity derived from Stromgren photometry, and independently from the location of the star in the H-R diagram are in agreement with our direct measurement from h igh-resolution spectroscopy. This strongly supports a distance for P1724 consistent with ~ 460 pc. Although P1724 appears to have lost all its circumstellar material, its bolometric luminosity places it very close to the stellar birth-line at an age of only ~ 2 10^5 years, with a mass of ~ 3 M_sun. The age from the H-R diagram is consistent with its present location and 3D space motion (~ +20 km/s to the north relative to the Trapezium) under the assumption that it was ejected from the Trapezium ~ 2 10^5 yrs ago.

P1724 thus appears to be a single, very young, naked, weak-line T Tauri star moving north relative to the Trapezium, but sharing the Orion radial velocity. All our observations are consistent with P1724 being a run-away T Tauri star, ejected from its birth place, the Trapezium cluster, ~ 2 10^5 years ago.


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