Accretion in Ophiuchus brown dwarfs: infrared hydrogen line ratios

Astronomy & Astrophysics, Dec 2006

Context.Mass accretion rate determinations are fundamental for an understanding of the evolution of pre-main sequence star circumstellar disks.Aims.Magnetospheric accretion models are used to derive values of the mass accretion rates in objects of very different properties, from brown dwarfs to intermediate-mass stars; we test the validity of these models in the brown dwarf regime, where the stellar mass and luminosity, as well as the mass accretion rate, are much lower than in T Tauri stars.Methods.We have measured nearly simultaneously two infrared hydrogen lines, Paβ and Br, in a sample of 16 objects in the star-forming region ρ -Oph. The sample includes 7 very low mass objects and brown dwarfs and 9 T Tauri stars.Results.Brown dwarfs where both lines are detected have a ratio Paβ/Brof ~2. Larger values, 3.5, are only found among the T Tauri stars. The low line ratios in brown dwarfs indicate that the lines cannot originate in the column of gas accreting from the disk onto the star along the magnetic field lines, and we suggest that they form instead in the shocked photosphere, heated to temperatures of ~3500 K. If so, in analogy to veiling estimates in T Tauri stars, the hydrogen infrared line fluxes may provide a reliable measure of the accretion rate in brown dwarfs.

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Accretion in Ophiuchus brown dwarfs: infrared hydrogen line ratios

A&A Accretion in ρ Ophiuchus brown dwarfs: infrared hydrogen line ratios , T. Gatti 1 2 L. Testi 2 A. Natta 2 S. Randich 2 J. Muzerolle 0 0 Steward Observatory, University of Arizona , USA 1 Università di Firenze , Dipartimento di Astronomia, Largo E. Fermi 5, 50125 Firenze , Italy 2 Osservatorio Astrofisico di Arcetri, INAF , Largo E. Fermi 5, 50125 Firenze , Italy Context. Mass accretion rate determinations are fundamental for an understanding of the evolution of pre-main sequence star circumstellar disks. Aims. Magnetospheric accretion models are used to derive values of the mass accretion rates in objects of very different properties, from brown dwarfs to intermediate-mass stars; we test the validity of these models in the brown dwarf regime, where the stellar mass and luminosity, as well as the mass accretion rate, are much lower than in T Tauri stars. Methods. We have measured nearly simultaneously two infrared hydrogen lines, Paβ and Brγ, in a sample of 16 objects in the starforming region ρ-Oph. The sample includes 7 very low mass objects and brown dwarfs and 9 T Tauri stars. Results. Brown dwarfs where both lines are detected have a ratio Paβ/Brγof ∼2. Larger values, >∼3.5, are only found among the T Tauri stars. The low line ratios in brown dwarfs indicate that the lines cannot originate in the column of gas accreting from the disk onto the star along the magnetic field lines, and we suggest that they form instead in the shocked photosphere, heated to temperatures of ∼3500 K. If so, in analogy to veiling estimates in T Tauri stars, the hydrogen infrared line fluxes may provide a reliable measure of the accretion rate in brown dwarfs. accretion; accretion disks - stars; formation - stars; low-mass; brown dwarfs 1. Introduction In pre-main sequence stars, circumstellar disks feed matter onto their central stars over a period of a few million years. This accretion process does not significantly alter the properties of the star during most of this time; however, it can have a significant influence on the lifetime and evolution of the disks and of the planetary systems that may form. The physical conditions of the accretion process, in particular the mass accretion rate, can be investigated by studying the line and continuum emission produced by the accreting material. The model which has been more successful in explaining the accretion phenomena in T Tauri stars (TTS) is magnetospheric accretion, where the disk is truncated by the effect of a stellar magnetic field near or within the corotation radius. The disk material, which is slowly drifting radially toward the star, reaches the truncation radius and is then lifted above the disk midplane and accretes onto the star along magnetic field lines, impacting on the star at approximately the escape velocity. A shock forms on the stellar surface, creating a hot spot which emits the excess UV continuum and lines observed in TTS. Optical and IR line emission is expected to come from the accreting columns of gas, where the temperature has to be of the order of 10 000 K. Magnetospheric accretion models have been developed in detail under the assumption of dipole magnetic field by, e.g., Hartmann et al. (1994), Calvet & Gullbring (1998) , Muzerolle et al. (1998a, 2001) , Lamzin (1995, 1998), to predict the expected amount of veiling and line profiles and intensities. They have been used to derive accretion rates for TTS, brown dwarfs (BDs), some intermediate-mass TTS and Herbig Ae stars (Gullbring et al. 1998; Muzerolle et al. 2001, 2003, 2004, 2005; Natta et al. 2004, 2006; Calvet et al. 2004; Garcia Lopez et al. 2006) . Testing magnetospheric accretion models is therefore very important and extensive checks have been carried out in several of the above papers. They have been mostly focussed on TTS, where it is possible to observe the accretion-driven emission over a large range of wavelengths, both in lines and continuum. Although it is likely that the topology of the stellar magnetic field is much more complex than the dipole field for which most models have been developed, the models seem able to account satisfactorily for TTS activity (see Bouvier et al. 2006, and references therein). The situation is different for BDs, where at present the accreting matter is usually detected through optical and near-IR line emission. In fact, the vast majority of the existing estimates of M˙ acc in BDs are derived by fitting the observed Hα profiles with emission in magnetospheric accreting columns (Muzerolle et al. 2003, 2005; Natta et al. 2004) or by means of secondary indicators, such as the luminosity of the hydrogen near-IR lines or of the Ca IR triplet, which have been calibrated using the Hα-measured accretion rates (Natta et al. 2004; Mohanty et al. 2005). This paper reports the result of a project aimed at testing the capability of current magnetospheric models to describe quantitatively the accretion in very low mass objects and BDs by compari (...truncated)


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T. Gatti, L. Testi, A. Natta, S. Randich, J. Muzerolle. Accretion in Ophiuchus brown dwarfs: infrared hydrogen line ratios, Astronomy & Astrophysics, 2006, pp. 547-553, Volume 460, Issue 2, DOI: 10.1051/0004-6361:20066095