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provided by St Andrews Research Repository MNRAS 453, 976–1001 (2015) doi:10.1093/mnras/stv1576
A spectroscopic survey of Herbig Ae/Be stars with X-shooter – I. Stellar parameters and accretion rates
J. R. Fairlamb,1† R. D. Oudmaijer,1 I. Mendigut´ıa,1 J. D. Ilee2 and M. E. van den Ancker3 1School of Physics and Astronomy, EC Stoner Building, University of Leeds, Leeds LS2 9JT, UK 2 SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK 3European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany
Accepted 2015 July 13. Received 2015 July 13; in original form 2015 March 3 Downloaded from
ABSTRACT Herbig Ae/Be stars (HAeBes) span a key mass range that links low- and high-mass stars, and thus provide an ideal window from which to explore their formation. This paper presents Very
Large Telescope/X-shooter spectra of 91 HAeBes, the largest spectroscopic study of HAeBe http://mnras.oxfordjournals.org/ accretion to date. A homogeneous approach to determining stellar parameters is undertaken for the majority of the sample. Measurements of the ultraviolet are modelled within the context of magnetospheric accretion, allowing a direct determination of mass accretion rates. Multiple correlations are observed across the sample between accretion and stellar properties: the youngest and often most massive stars are the strongest accretors, and there is an almost 1:1 relationship between the accretion luminosity and stellar luminosity. Despite these overall trends of increased accretion rates in HAeBes when compared to classical T Tauri stars, we
also find noticeable differences in correlations when considering the Herbig Ae and Herbig at University of St Andrews on November 2, 2015 Be subsets. This, combined with the difficulty in applying a magnetospheric accretion model to some of the Herbig Be stars, could suggest that another form of accretion may be occurring within Herbig Be mass range. Key words: accretion, accretion discs – techniques: spectroscopic – stars: early-type – stars: formation – stars: pre-main-sequence – stars: variables: T Tauri, Herbig Ae/Be.
been relaxed since then in order to find more potential targets (see 1 INTRODUCTION Finkenzeller & Mundt 1984;The,´ de Winter & Perez 1994; Vieira Herbig Ae/Be stars (HAeBes) are pre-main-sequence, PMS, stars et al. 2003). In these surveys, more attention has been drawn to- that bridge the key mass range of 2–10 M , between low- and wards the colours of the objects, particularly in the infrared, IR. high-mass stars. Their importance lies in linking the reasonably This is because the IR often displays an excess of emission, com- well understood formation of low-mass stars, like the numerous pared to main-sequence, MS, stars of the same spectral type, and is classical T Tauri stars, CTTs (which have M < 2M and will go on the product of the circumstellar disc around HAeBes. This has been to form stars like our own Sun), to the rarer, more deeply embedded confirmed in numerous studies (van den Ancker et al. 2000; Meeus high-mass stars (or massive young stellar objects, MYSOs). The et al. 2001), and by direct observations in the optical (McCaughrean greater number of HAeBes compared to forming high-mass stars, &O’dell1996; Grady et al. 2001), sub-mm (Mannings & Sargent along with them being closer and optically visible, makes them a 1997), and of scattered, polarized light (Vink et al. 2002, 2005). It powerful link and important tool in furthering our understanding of is these indicators which ultimately point to the stars being young star formation up to the high-mass star formation regime. and in the PMS phase. This young nature, suspected by Herbig, HAeBes were originally identified by Herbig (1960)inanat- was confirmed by Strom et al. (1972) who observed the HAeBes tempt to push the mass boundaries in understanding of CTTs, and to have lower surface gravities than MS stars. With their PMS na- had to meet the criteria of: ‘spectral type A or earlier with emission ture established, the next obvious questions are: what happens with lines, lies in an obscured region, the star illuminates fairly bright the disc–star interaction; how do they both evolve; and what phys- nebulosity in its immediate vicinity’. The latter two criteria have ical mechanisms are present in these interactions? And of course, how are these aspects related to each other between the CTTs and MYSOs? Based on observations using the ESO Very Large Telescope, at Cerro With regard to the disc–star interaction, an ultraviolet, UV,excess Paranal , under the observing programme 084.C-0952A. in the CTTs (Garrison 1978; Gullbring et al. 1998) has been shown † E-mail: [email protected] to match well with the theory of disc-to-star accretion within the