EVIDENCE FOR AN FU ORIONIS-LIKE OUTBURST FROM A CLASSICAL T TAURI STAR

We present pre- and post-outburst observations of the new FU Orionis-like\nyoung stellar object PTF 10qpf (also known as LkHa 188-G4 and HBC 722). Prior\nto this outburst, LkHa 188-G4 was classified as a classical T Tauri star on the\nbasis of its optical emission-line spectrum superposed on a K8-type\nphotosphere, and its photometric variability. The mid-infrared spectral index\nof LkHa 188-G4 indicates a Class II-type object. LkHa 188-G4 exhibited a steady\nrise by ~1 mag over ~11 months starting in Aug. 2009, before a subsequent more\nabrupt rise of > 3 mag on a time scale of ~2 months. Observations taken during\nthe eruption exhibit the defining characteristics of FU Orionis variables: (i)\nan increase in brightness by > 4 mag, (ii) a bright optical/near-infrared\nreflection nebula appeared, (iii) optical spectra are consistent with a G\nsupergiant and dominated by absorption lines, the only exception being Halpha\nwhich is characterized by a P Cygni profile, (iv) near-infrared spectra\nresemble those of late K--M giants/supergiants with enhanced absorption seen in\nthe molecular bands of CO and H_2O, and (v) outflow signatures in H and He are\nseen in the form of blueshifted absorption profiles. LkHa 188-G4 is the first\nmember of the FU Orionis-like class with a well-sampled optical to mid-infrared\nspectral energy distribution in the pre-outburst phase. The association of the\nPTF 10qpf outburst with the previously identified classical T Tauri star LkHa\n188-G4 (HBC 722) provides strong evidence that FU Orionis-like eruptions\nrepresent periods of enhanced disk accretion and outflow, likely triggered by\ninstabilities in the disk. The early identification of PTF 10qpf as an FU\nOrionis-like variable will enable detailed photometric and spectroscopic\nobservations during its post-outburst evolution for comparison with other known\noutbursting objects.\n