Near-infrared spectroscopic monitoring of exor variables: First results

Abstract

We present low-resolution (R ∼ 250) spectroscopy in the near-IR (0.8-2.5 μm) of the EXor variables. These are the initial results (obtained during the period 2007-2008) from a long-Term photometric and spectroscopic program aimed to study the variability in the accretion processes of pre-main-sequence stars, by correlating the continuum fluctuations with the spectroscopic properties. Eight sources have been observed in different epochs, for a total of 25 acquired spectra. EXor spectra show a wide variety of emission features dominated by Hi recombination (Paschen and Brackett series). We have investigated whether line and continuum variability could be due to a variable extinction, but such a hypothesis is applicable only to the peculiar source PV Cep. By comparing the observed spectra with a wind model, mass loss rates in the range (2-10) × 10-8M⊙ yr-1 are derived, along with other wind parameters. Consistent results are also obtained by assuming that Hi lines are due to accretion. A CO overtone is also detected in the majority of the sources both in absorption and in emission. It appears to come from regions more compact than winds, likely the stellar photosphere (when in absorption) and the circumstellar disk (when in emission). Na i and Ca i IR lines behave as the CO does, thus they are thought to arise in the same locations. For some targets multiple spectra correspond to different activity stages of the source. Those exhibiting the largest continuum variation at 2 μm (ΔK ≳ 1 mag) present a significant line flux fading during the continuum declining phases. In particular, CO absorption (emission) appears associated with inactive (active) stages, respectively.