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Astron. Astrophys. 337, 757-771 (1998)

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2. RE 1816+541

RE 1816+541 has been discovered as the optical counterpart to a bright EUV source detected during the Wide Field Camera (WFC) All-Sky Survey phase of the ROSAT mission. From the Hubble Space Telescope Guide Star Catalog (Jenkner et al. 1990) the position and magnitude are RA = 18h 16m 16s.8, Dec = [FORMULA] 10' 22" (J2000.0), V=11.7.

Medium and low resolution optical spectroscopy of this object was presented by Jeffries, James & Bromage (1994), who classed it as a single, rapidly-rotating dM1-2e star with a [FORMULA] sini of 61 km s-1. Assuming a main sequence radius, this corresponds to a period, Prot [FORMULA] 12 sini hrs or 0.5 sini d. A high level of activity is indicated by strong emission in H[FORMULA] and Ca II H & K, both commonly used as chromospheric indicators. Based also on its high EUV flux as measured by the WFC, RE 1816+541 is regarded as one of the most active stars in the solar neighbourhood. Arguments for a youthful evolutionary status, based on its rapid rotation and its kinematics, are given by those authors.

Mean magnitudes and colours of RE 1816+541 were given by Schwartz et al. (1995). Robb & Cardinal (1995) obtained differential photometry with the Johnson V filter. From the modulation of the light curve they derived a period of 0.459 d and concluded that the star has large active regions on its surface causing the brightness variations. In order to calculate rotational phases, we have used in this work the ephemeris given by those authors, HJD=2449927.752 + 0.4589E.

The combination of this period with [FORMULA] sini gives a projected radius, [FORMULA]. The absolute visual magnitude, MV, is derived from the photometry published in Schwartz et al. (1995), MV=10.32, assuming a distance of about 20 pc (Jeffries, James & Bromage 1994). Then, using the mass-visual luminosity relation for low-mass stars from Henry & McCarthy (1993), a mass estimate [FORMULA] is obtained. The stellar radius can be determined from calibrated mass-radius relations. According to the empirical mass-radius relation for late M dwarfs produced by Caillault & Patterson (1990) the result is [FORMULA]. This is in good agreement with the value predicted by the mass-radius relation for main-sequence stars given in Demircan & Kahraman (1991), [FORMULA]. The latter relation was obtained using theoretical stellar models for ZAMS stars.

From comparison between the estimates of both R* and R*sini, it follows that the stellar inclination must be close to [FORMULA]. Combining the radius and mass estimates yields a co-rotation radius, [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998