A Hot Subdwarf in NGC 1851

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Session 50 -- Globular Clusters
Display presentation, Wednesday, 9:20-6:30, Pauley Room

[50.05] A Hot Subdwarf in NGC 1851

W.B.Landsman (HSTX), R.Saffer (STScI), P.Hintzen (NASA/GSFC), M.Hamuy, M.M.Phillips (CTIO)

Images of the globular cluster NGC 1851 obtained with the Ultraviolet Imaging Telescope (UIT) reveal the presence of a hot (T$_{eff}$ $\sim$ 30,000 K) optically faint (V $\sim$ 18.2) star about $3'$ from the cluster center. We have obtained a CTIO 4-m spectrum of this star, which is \# 43 in the UIT color-magnitude study of Parise et al.\ (1992, submitted to ApJ). The CTIO spectrum shows only weak Balmer absorption lines. The measured radial velocity of 330 km s$^{-1}$ confirms the membership of UIT-43 in NGC 1851. By simultaneously fitting the Balmer lines, we derive T$_{eff} = 29700 \pm 1630$ K and $\log g= 5.39 \pm 0.27$. These values place UIT-43 somewhat above the zero-age extended horizontal branch (HB).


The other HB stars in NGC 1851 detected in the Parise study, and in the ground-based study of Walker (1992, PASP, 104, 1063), are all cooler than 12,000 K. If UIT-43 is a product of single star evolution, then a mechanism must be found to explain its large separation in effective temperature from the remaining HB stars. Alternatively, UIT-43 could be a merger product of a white dwarf pair, as outlined by Bailyn and Iben (1989, ApJL, L21).

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