The Mass Accretion Rates of Intermediate-Mass T Tauri Stars

Abstract

We present Hubble Space Telescope ultraviolet spectra and supporting ground-based data for a sample of nine intermediate-mass T Tauri stars (IMTTSs; 1.5–4 M⊙). The targets belong to three star-forming regions: T Tau, SU Aur, and RY Tau in the Taurus clouds; EZ Ori, P2441, and V1044 Ori in the Ori OB1c association surrounding the Orion Nebula cluster; and CO Ori, GW Ori, and GX Ori in the ring around λ Ori. The supporting ground-based observations include nearly simultaneous UBV(R I)C photometry, 6 A resolution spectra covering the range 3900–7000 A, optical echelle observations in the range 5800–8600 A, and K-band near-infrared spectra. We use these data to determine improved spectral types and reddening corrections and to obtain physical parameters of the targets. We find that an extinction law with a weak 2175 A feature but high values of AUV/AV is required to explain the simultaneous optical-UV data; the reddening laws for two B-type stars located behind the Taurus clouds, HD 29647 and HD 283809, meet these properties. We argue that reddening laws with these characteristics may well be representative of cold, dense molecular clouds. Spectral energy distributions and emission-line profiles of the IMTTSs are consistent with expectations from magnetospheric accretion models. We compare our simultaneous optical-UV data with predictions from accretion shock models to get accretion luminosities and mass accretion rates () for the targets. We find that the average mass accretion rate for IMTTSs is ~3 × 10-8 M⊙ yr-1, a factor of ~5 higher than that for their low-mass counterparts. The new data extend the correlation between and stellar mass to the intermediate-mass range. Since the IMTTSs are evolutionary descendants of the Herbig Ae/Be stars, our results put limits to the mass accretion rates of their disks. We present luminosities of the UV lines of highly ionized metals and show that they are well above the saturation limit for magnetically active cool stars but correlate strongly with accretion luminosity, indicating that they are powered by accretion, in agreement with previous claims but using a sample in which reddening and accretion luminosities have been determined self-consistently. Finally, we find that the relation between accretion luminosity and Brγ luminosity found for low-mass T Tauri stars extends to the intermediate-mass regime.