Accretion rates and accretion tracers of Herbig Ae/Be stars

¹ Centro de Astrobiología, Departamento de Astrofísica (CSIC-INTA), ESAC Campus, PO Box 78, 28691 Villanueva de la Cañada, Madrid Spain
e-mail: ignacio.mendigutia@cab.inta-csic.es
² Departamento de Física Teórica, Módulo 15, Facultad de Ciencias, Universidad Autónoma de Madrid, PO Box 28049, Cantoblanco, Madrid, Spain
³ Department of Astronomy, University of Michigan, 830 Dennison Building, 500 Church Street, Ann Arbor, MI 48109, USA
⁴ GAIA Science Operations Centre, ESA, European Space Astronomy Centre, PO Box 78, 28691, Villanueva de la Cañada, Madrid, Spain
⁵ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD, 21218, USA
⁶ School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
⁷ Herschel Science Centre, ESA, European Space Astronomy Centre, PO Box 78, 28691, Villanueva de la Cañada, Madrid, Spain

Received: 9 June 2011
Accepted: 2 September 2011

Abstract

Context. The scarcity of accretion rate estimates and accretion tracers available for Herbig Ae/Be (HAeBe) stars contrasts with the extensive studies for lower mass objects.

Aims. This work aims to derive accretion rates from the UV Balmer excess for a sample of 38 HAeBe stars. We look for possible empirical correlations with the strength of the Hα, [O i]6300, and Brγ emission lines.

Methods. Shock modelling within the context of magnetospheric accretion (MA) was applied to each star. We obtained the accretion rates from the excess in the Balmer discontinuity, derived from mean values of multi-epoch Johnson’s UB photometry. The accretion rates were related to both mean Hα luminosities, Hα 10% widths, and [O i]6300 luminosities from simultaneous spectra, and to Brγ luminosities from the literature.

Results. The typical -median- mass accretion rate is 2  ×  10^-7 M_⊙ yr^-1 in our sample, 36% of the stars showing values  ≤10^-7 M_⊙ yr^-1, 35% between 10^-7 and 10^-6, and 29%  >  10^-6 M_⊙ yr^-1. The model fails to reproduce the large Balmer excesses shown by the four hottest stars (T_∗ > 12   000 K). When accretion is related to the stellar masses and luminosities (1 ≤ M_∗/M_⊙ ≤ 6; 2 ≤ L_∗/L_⊙ ≤ 10³), we derive Ṁ_acc ∝ M_∗⁵ and L_acc ∝ L_∗^1.2, with scatter. Empirical calibrations relating the accretion and the Hα, [O i]6300, and Brγ luminosities are provided. The slopes in our expressions are slightly shallower than those for lower mass stars, but the difference is within the uncertainties, except for the [O i]6300 line. The Hα 10% width is uncorrelated with Ṁ_acc, unlike for the lower mass regime. The mean Hα width shows higher values as the projected rotational velocities of HAe stars increase, which agrees with MA. The accretion rate variations in the sample are typically lower than 0.5 dex on timescales of days to months. Our data suggest that the changes in the Balmer excess are uncorrelated to the simultaneous changes of the line luminosities.

Conclusions. The Balmer excesses and Hα line widths of HAe stars can be interpreted within the context of MA, which is not the case for several HBes. The steep trend relating Ṁ_acc and M_∗ can be explained from the mass-age distribution characterizing HAeBe stars. The line luminosities used for low-mass objects are also valid to estimate typical accretion rates for the intermediate-mass regime under similar empirical expressions. However, we suggest that several of these calibrations are driven by the stellar luminosity.