X-shooter spectroscopy of young stellar objects

II. Impact of chromospheric emission on accretion rate estimates^⋆,⋆⋆

¹ European Southern Observatory, Karl Schwarzschild Str. 2, 85748 Garching, Germany
e-mail: cmanara@eso.org
² INAF − Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Department of Planetary Science, Lunar and Planetary Lab, University of Arizona, 1629, E. University Blvd, Tucson AZ 85719, USA
⁴ INAF − Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
⁵ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁶ INAF − Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁷ INAF − Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate ( LC), Italy
⁸ INAF − Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁹ INAF − Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone ( RM), Italy

Received: 14 December 2012
Accepted: 10 January 2013

Abstract

Context. The lack of knowledge of photospheric parameters and the level of chromospheric activity in young low-mass pre-main sequence stars introduces uncertainties when measuring mass accretion rates in accreting (Class II) young stellar objects. A detailed investigation of the effect of chromospheric emission on the estimates of mass accretion rate in young low-mass stars is still missing. This can be undertaken using samples of young diskless (Class III) K and M-type stars.

Aims. Our goal is to measure the chromospheric activity of Class III pre main sequence stars to determine its effect on the estimates of the accretion luminosity (L_acc) and mass accretion rate (Ṁ_acc) in young stellar objects with disks.

Methods. Using VLT/X-shooter spectra, we analyzed a sample of 24 nonaccreting young stellar objects of spectral type between K5 and M9.5. We identified the main emission lines normally used as tracers of accretion in Class II objects, and we determined their fluxes in order to estimate the contribution of the chromospheric activity to the line luminosity.

Results. We have used the relationships between line luminosity and accretion luminosity derived in the literature for Class II objects to evaluate the impact of chromospheric activity on the accretion rate measurements. We find that the typical chromospheric activity would bias the derived accretion luminosity by L_acc,noise< 10^-3  L_⊙, with a strong dependence on the T_eff of the objects. The noise on Ṁ_acc depends on stellar mass and age, and the typical values of log(Ṁ_acc,noise) range between ~−9.2 to −11.6  M_⊙/yr.

Conclusions. Values of L_acc ≲10^-3  L_⊙ obtained in accreting low-mass pre main sequence stars through line luminosity should be treated with caution because the line emission may be dominated by the contribution of chromospheric activity.