THE EVIDENCE FOR CLUMPY ACCRETION IN THE HERBIG AE STAR HR-5999

Analysis of IUE high- and low-dispersion spectra of the young Herbig A e star HR 5999 (HD 144668) covering 1978-1992 has revealed dramatic ch anges in the Mg II h and k (2795.5, 2802.7 angstrom) emission profiles , changes in the column density and distribution in radial velocity of accreting gas, and flux in the Lyalpha, O I and CIV emission lines, w hich are correlated with the UV excess luminosity. We also observe var iability in the spectral type inferred from the UV spectral energy dis tribution, ranging from A5 IV-III in high state to A7 III in the low s tate. The trend of earlier inferred spectral type with decreasing wave length and with increasing UV continuum flux has previously been noted as a signature of accretion disks in lower mass pre-main sequence sta rs (PMS) and in systems undergoing FU Orionis-type outbursts. Our data represent the first detection of similar phenomena in an intermediate mass (M greater-than-or-equal-to 2 M.) PMS star. Recent IUE spectra s how gas accreting toward the star with velocities as high as +300 km s -1, much as is seen toward beta Pic, and suggest that we also view thi s system through the debris disk. The absence of UV lines with the rot ational broadening expected given the optical data (A7 IV, v sin i = 1 80 +/- 20 km s-1) for this system also suggests that most of the UV li ght originates in the disk, even in the low continuum state. The drama tic variability in the column density of accreting gas, consistent wit h clumpy accretion, such as has been observed toward beta Pic, is a ha llmark of accretion onto young stars, and is not restricted to the cle aring phase, since detectable amounts of accretion are present for sta rs with 0.5 < t(age) < 2.8 Myr. The implications for models of beta Pi c and similar systems are briefly discussed.