[Paper Review] Tracing jet emission at the base of a high-mass YSO. First AMBER/VLTI observations of the Brγemission in IRAS 13481-6124

A. Caratti o Garatti, B. Stecklum+10 more|Figshare|Mar 22, 2016

Stellar, planetary, and galactic studies34 references12 citations

TL;DR

This study presents the first AMBER/VLTI medium-resolution interferometric observations of the Brγ emission line in the high-mass young stellar object IRAS 13481-6124. It reveals that the Brγ emission originates primarily from a collimated, ionized jet with high-velocity outflow (up to 500 km s⁻¹), while low-velocity components trace gas within 2 au of the central source, likely the jet footpoint or disc wind region.

ABSTRACT

To probe the circumstellar environment of IRAS 13481-6124, a 20 M_sun high-mass young stellar object (HMYSO) with a parsec-scale jet and accretion disc, we investigate the origin of its Brγ-emission line through NIR interferometry. We present the first AMBER/VLTI observations of the Brγ-emitting region in an HMYSO at R~1500. Our AMBER/VLTI observations reveal a spatially and spectrally resolved Brγ-line in emission with a strong P Cygni profile, indicating outflowing matter with a terminal velocity of ~500 km/s. Visibilities, differential phases, and closure phases are detected in our observations within the spectral line and in the adjacent continuum. Both total visibilities (continuum plus line emitting region) and pure-line visibilities indicate that the Brγ-emitting region is more compact (2-4 mas in diameter or ~6-13 au at 3.2 kpc) than the continuum-emitting region (~5.4 mas or ~17 au). The absorption feature is also spatially resolved at the longest baselines (81 and 85 m) and has a visibility that is slightly smaller than the continuum-emitting region. The differential phases at the four longest baselines display an \u2018S\u2019-shaped structure across the line, peaking in the blue- and red-shifted high-velocity components. The calibrated photocentre shifts are aligned with the known jet axis, i.e they are probably tracing an ionised jet. The high-velocity components (v_r~100-500 km/s) are located far from the source, whereas the low-velocity components (0-100 km/s) are observed to be closer, indicating a strong acceleration of the gas flow in the inner 10 au. Finally, a non-zero closure phase along the continuum is detected. By comparing our observations with the synthetic images of the continuum around 2.16 um, we confirm that this feature originates from the asymmetric brightness distribution of the continuum owing to the inclination of the inner disc.

Motivation & Objective

• To investigate the origin of the Brγ emission line in the high-mass YSO IRAS 13481-6124 using high-angular-resolution interferometry.
• To determine whether the Brγ emission arises from accretion, outflow, or jet structures in the innermost circumstellar environment.
• To resolve spatial and spectral structure of the Brγ line to distinguish between competing physical models of emission origin.
• To constrain the geometry and kinematics of the ionized outflow and its connection to the central accretion disc and parsec-scale jet.

Proposed method

• Performed medium-resolution (R ≈ 1500) near-infrared interferometry with the AMBER instrument on the VLTI to observe the Brγ emission line in IRAS 13481-6124.
• Measured total visibilities, differential phases, and closure phases across the Brγ spectral line and adjacent continuum to probe spatial structure and asymmetries.
• Used calibrated photocentre shifts derived from differential phases to trace the direction and extent of moving gas components.
• Compared observed interferometric observables with synthetic images of the continuum (2.16 µm) to interpret asymmetries as due to disc inclination.
• Analyzed the P Cygni profile in the Brγ line to infer outflow velocity and mass-loss characteristics.
• Distinguished between high-velocity (100–500 km s⁻¹) and low-velocity (0–100 km s⁻¹) components based on visibility and phase behavior.

Experimental results

Research questions

• RQ1 What is the spatial origin of the Brγ emission line in IRAS 13481-6124, and which physical component (accretion, disc wind, jet) dominates it?
• RQ2 How do the kinematics and geometry of the Brγ-emitting gas relate to the known parsec-scale collimated jet and inner disc?
• RQ3 Is the Brγ emission spatially resolved, and what does its visibility profile reveal about the size and structure of the emitting region?
• RQ4 What causes the observed photocentre shifts, and how do they align with the jet axis and velocity components?
• RQ5 Can the interferometric data distinguish between ionized jet emission and other potential sources such as infall or accretion flows?

Key findings

• The Brγ emission line exhibits a strong P Cygni profile with a terminal velocity of ~500 km s⁻¹, indicating a fast ionized outflow.
• The Brγ-emitting region is more compact (2–4 mas, or ~6–13 au at 3.2 kpc) than the continuum-emitting region (~5.4 mas, or ~17 au).
• High-velocity components (100–500 km s⁻¹) are spatially resolved at longer baselines and show photocentre shifts aligned with the known jet axis, indicating collimated outflow.
• Low-velocity components (0–100 km s⁻¹) are more compact (≤2 au) and likely originate from the jet footpoint or disc wind region near the central source.
• The differential phase exhibits an 'S'-shaped structure peaking in the blue- and red-shifted high-velocity wings, confirming asymmetric, outflowing motion.
• A non-zero closure phase in the continuum is attributed to the asymmetric brightness distribution of the inclined inner disc, not intrinsic asymmetry in the jet.