Near-infrared echelle spectroscopy of Class I protostars: molecular hydrogen emission-line (MHEL) regions revealed

Abstract

Infrared echelle spectra are used to trace dynamic activity in the immediate vicinity of Class I outflow sources. The H 2 and Brγ observations presented here trace different components of these emission‐line regions; indeed, they are thought to trace the orthogonal processes of outflow and infall respectively. High‐velocity H 2 emission is detected in the extended lobes of nine outflows. In addition, complex H 2 line emission is observed within a few hundred au of nine of the outflow sources. We refer to these H 2 emission regions as ‘molecular hydrogen emission‐line’ regions, or MHELs, and compare their properties to those of forbidden emission‐line regions (FELs) observed in classical T Tauri and some Herbig AeBe stars. Like the FELs, both low‐ and high‐velocity components (LVCs and HVCs) are observed in H 2, with blueshifted velocities of the order of 5–20 and 50–150 km s−1 respectively. LVCs are more common than HVCs in MHEL regions, and like their FEL counterparts, the latter are spatially further offset from the exciting source in each case. The MHEL regions – which are in all cases preferentially blueshifted – are assumed to be associated with the base of each outflow. Brγ profiles are detected towards four of the Class I sources observed (SVS 13, IRAS 04239+2436, HH 34‐IRS and GGD 27(1)) as well as towards the T Tauri star AS 353A. These lines are all broad and symmetric, the line peaks being blueshifted by ∼30 km s−1. The profiles are typical of the permitted hydrogen line profiles observed in many T Tauri stars, and probably derive from magnetospheric accretion flows. We do not observe redshifted absorption features (inverse P‐Cygni profiles) in any of the sources, however. Nor do we detect a dependence on linewidth with inclination angle of the system to the line of sight, as is predicted by such accretion models. No Brγ is detected in the extended flow lobes. Instead, the emission is confined to the source and is spatially unresolved along each flow axis.

IR diagnostics of embedded jets: kinematics and physical characteristics of the HH46-47 jet

“…The [Fe ii] and H 2 emission structure at the base of HH46-47 is very similar to the one observed in several Class I jets (e.g., Davis et al 2001bDavis et al , 2003Takami et al 2006). As in the case of CTTS jets, the forbidden-emission line (FEL) regions of Class I jets are characterised by the presence of two velocity components at the jet base at high and low velocity.…”

“…Both show a broad line emission near the source which consists of a collimated and large scale jet at high velocity (the so-called HVC) and a slower velocity component (the so-called LVC) that is only detected around the central source. Noticeably, this kind of velocity structure is often observed in both the atomic and molecular components of the jets, the latter probed through the H 2 emission and called molecular hydrogen emission line regions (MHEL; Davis et al 2001b). Concerning the physical properties, Class I jets show higher densities and mass loss rates than CTTS jets, as expected for less evolved objects.…”

Spatially resolved H2emission from a very low-mass star

“…In general, MHEL properties are consistent with shockheated gas from the inner regions of Herbig-Haro objects or from spatially extended wide-angle winds (Davis et al 2001;Caratti o Garatti et al 2006;Beck et al 2008;Davis et al 2011). The reported gas temperatures and excitation properties of IRS54 are consistent with shock-heated material, as well.…”

“…The mass-loss rate carried by the warm molecular component (Ṁ H 2 ) can be computed from the derived N(H 2 ) values using the expressionṀ H 2 = 2 μ m H N(H 2 ) A dv t /dl t (see, e.g., Davis et al 2001Davis et al , 2011. Here, A is the area of the emitting region, μ is the mean atomic weight, and dl t and dv t are the projected length and the tangential velocity.…”

“…The presence of MHEL regions are typical of the spectrum of low-mass Class I sources (Davis et al 2001;Garcia Lopez et al 2008;Davis et al 2011), and they are usually associated with an FEL region. Thus, one should also expect MHEL regions around young BDs and VLMSs.…”