LBT/LUCIFER near-infrared spectroscopy of PV Cephei

An outbursting young stellar object with an asymmetric jet^⋆,⋆⋆

¹ Max–Planck–Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: acaratti;rgarcia;weigelt@mpifr-bonn.mpg.de
² Pulkovo Astronomical Observatory of the Russian Academy of Sciences, Pulkovskoe shosse 65, 196140 St. Petersburg, Russia
e-mail: lvtamb@mail.ru; grinin@gao.spb.ru
³ The V.V. Sobolev Astronomical Institute of the St. Petersburg University, Petrodvorets, 198904 St. Petersburg, Russia
⁴ School of Physics and Astronomy, EC Stoner Building, University of Leeds, Leeds, LS2 9JT, UK
e-mail: pyjdi@leeds.ac.uk
⁵ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
e-mail: jdi3@st-andrews.ac.uk

Received: 6 March 2013
Accepted: 16 April 2013

Abstract

Context. Young stellar objects (YSOs) occasionally experience enhanced accretion events, the nature of which is still poorly understood. The discovery of various embedded outbursting YSOs has recently questioned the classical definition of EXors and FUors.

Aims. We present a detailed spectroscopic investigation of the young eruptive star PV Cep, to improve our understanding of its nature and characterise its circumstellar environment after its last outburst in 2004.

Methods. The analysis of our medium-resolution spectroscopy in the near-infrared (NIR, 0.9–2.35 μm), collected in 2012 at the Large Binocular Telescope with the IR spectrograph LUCIFER, allows us to infer the main stellar parameters (visual extinction, accretion luminosity, mass accretion and ejection rates), and model the inner disc, jet, and wind.

Results. The NIR spectrum displays several strong emission lines associated with accretion/ejection activity and circumstellar environment. Our analysis shows that the brightness of PV Cep is fading, as well as the mass accretion rate (2 × 10^-7 M_⊙ yr^-1 in 2012 vs.  ~5 × 10^-6 M_⊙ yr^-1 in 2004), which is more than one order of magnitude lower than in the outburst phase.

Among the several emission lines, only the [Fe ii] intensity increased after the outburst. The observed [Fe ii] emission delineates blue- and red-shifted lobes, both with high- and low-velocity components, which trace an asymmetric jet and wind, respectively. The observed emission in the jet has a dynamical age of 7–8 years, indicating that it was produced during the last outburst. The visual extinction decreases moving from the red-shifted (A_V(red) = 10.1 ± 0.7 mag) to the blue-shifted lobe (A_V(blue) = 6.5 ± 0.4 mag). We measure an average electron temperature of 17 500 K and electron densities of 30 000 cm^-3 and 15 000 cm^-3 for the blue and the red lobe, respectively. The mass ejection rate in both lobes is  ~1.5 × 10^-7 M_⊙ yr^-1, approximately matching the high accretion rate observed during and immediately after the outburst (Ṁ_out/Ṁ_acc ~ 0.05–0.1). The observed jet/outflow asymmetries are consistent with an inhomogeneous medium.

Our modelling of the CO emission hints at a small-scale gaseous disc ring, extending from  ~0.2–0.4 AU to  ~3 AU from the source, with an inner temperature of  ~3000 K. Our H i lines modelling indicates that most of the observed emission comes from an expanding disc wind at T_e = 10 000 K. The line profiles are strongly affected by scattering, disc screening, and outflow self-absorption.

Conclusions. According to the classical definition, PV Cep is not an EXor object, because it is more massive and younger than typical EXors. Nevertheless, its spectrum shows the signature of an “EXor-like” outburst, suggesting a common origin.