[Paper Review] Viscous Heating and Boundary Layer Accretion in the Disk of Outbursting Star FU Orionis

Aaron Labdon, Stefan Kraus+12 more|arXiv (Cornell University)|Nov 16, 2020

Astrophysics and Star Formation Studies56 references4 citations

TL;DR

This study presents the first J-band interferometric observations of a young stellar object, FU Orionis, using the MIRC-X instrument at the CHARA Array. It reveals a temperature gradient of T ∝ r⁻⁰.⁷⁴±⁰.⁰² in the inner disk, consistent with viscous heating in a steady-state, optically thick accretion disk, and confirms boundary layer accretion down to 0.015 ± 0.007 au with a surface temperature of 5800 ± 700 K.

ABSTRACT

Context. FU Orionis is the archetypal FUor star, a subclass of young stellar object (YSO) that undergo rapid brightening events, often gaining 4-6 magnitudes on timescales of days. This brightening is often associated with a massive increase in accretion; one of the most ubiquitous processes in astrophysics from planets and stars to super-massive black holes. We present multi-band interferometric observations of the FU Ori circumstellar environment, including the first J-band interferometric observations of a YSO. Aims. We investigate the morphology and temperature gradient of the inner-most regions of the accretion disk around FU Orionis. We aim to characterise the heating mechanisms of the disk and comment on potential outburst triggering processes. Methods. Recent upgrades to the MIRC-X instrument at the CHARA array allowed the first dual-band J and H observations of YSOs.Using baselines up to 331 m, we present high angular resolution data of a YSO covering the near-infrared bands J, H, and K. The unprecedented spectral range of the data allows us to apply temperature gradient models to the innermost regions of FU Ori. Results. We spatially resolve the innermost astronomical unit of the disk and determine the exponent of the temperature gradient of the inner disk to $T=r^{-0.74\pm0.02}$. This agrees with theoretical work that predicts $T = r^{-0.75}$ for actively accreting, steady state disks, a value only obtainable through viscous heating within the disk. We find a disk which extends down to the stellar surface at $0.015\pm0.007$ au where the temperature is found to be $5800\pm700$ K indicating boundary layer accretion. We find a disk inclined at $32\pm4^\circ$ with a minor-axis position angle of $34\pm11^\circ$.

Motivation & Objective

• To characterize the temperature structure and morphology of the innermost accretion disk around FU Orionis, a prototypical FU Orionis outbursting star.
• To test theoretical models of viscous heating in steady-state, optically thick accretion disks using high-angular-resolution observations.
• To investigate the presence and impact of boundary layer accretion by probing the disk's inner edge and temperature gradient.
• To constrain the disk inclination and position angle using geometric modeling of interferometric data.
• To search for close companions within 0.5–50 mas that could influence the accretion outburst mechanism.

Proposed method

• Multi-band interferometric observations were conducted using the MIRC-X instrument at the CHARA Array, achieving baselines up to 331 m.
• Simultaneous J and H-band observations enabled high spectral resolution and broad wavelength coverage to model temperature gradients across the disk.
• Geometric modeling of visibility and closure phase data was used to derive disk inclination, position angle, and inner disk radius.
• Temperature gradient models were fitted to the observed flux distribution across J, H, and K bands to infer heating mechanisms.
• A companion search was performed using H-band visibility and closure phase data, setting an upper flux contrast limit of 1.3% for companions between 0.5 and 50 mas.
• Theoretical models of viscously heated, steady-state accretion disks were compared to observed temperature profiles to validate the heating mechanism.

Experimental results

Research questions

• RQ1 What is the radial temperature profile of the innermost disk around FU Orionis, and does it match predictions for viscously heated, steady-state accretion disks?
• RQ2 Does the observed temperature gradient provide evidence for viscous heating as the dominant heating mechanism in the disk's inner region?
• RQ3 What is the extent of the accretion disk down to the stellar surface, and does the temperature at the inner edge confirm boundary layer accretion?
• RQ4 What is the disk's inclination and position angle, and how do they compare to previous estimates and jet/outflow orientation?
• RQ5 Is there evidence for a close companion within 0.5–50 mas that could trigger or influence the FU Orionis outburst?

Key findings

• The temperature gradient in the inner disk is measured as T ∝ r⁻⁰.⁷⁴±⁰.⁰², closely matching the theoretical prediction of T ∝ r⁻⁰.⁷⁵ for viscously heated, steady-state accretion disks.
• The inner disk extends down to 0.015 ± 0.007 au from the star, where the temperature reaches 5800 ± 700 K, consistent with boundary layer accretion.
• The disk is inclined at 32 ± 4° with a minor-axis position angle of 34 ± 11°, providing tighter constraints than previous studies.
• The minor-axis position angle is misaligned by approximately 60° from the direction of the known jet/outflow, suggesting the jet may not be perfectly perpendicular to the disk plane.
• No significant companion is detected within 0.5–50 mas, with an upper flux contrast limit of 1.3% in the H band, ruling out massive companions in this region.
• This study demonstrates the feasibility and power of J-band interferometry for probing the thermal structure of accretion disks in young stellar objects.