[Paper Review] Bright CO ro-vibrational emission lines in the class I source GSS 30 IRS1: Probing the inner disk of a young embedded star

K. M. Pontoppidan, F. L. Schoeier+2 more|ArXiv.org|Jul 18, 2002

Astrophysics and Star Formation Studies14 citations

TL;DR

This study presents the first detection of strong ro-vibrational CO emission in the 4.5–4.8 μm band from an embedded class I young stellar object, GSS 30 IRS1. The emission arises from a single-temperature gas at 515 ± 5 K, best explained by scattering in a bipolar cavity from a dense, dissociative accretion shock in the inner disk (10–50 AU), with a total gas mass of 1–100 M⊕ and a physical extent of 20–100 AU.

ABSTRACT

We present a 4.5-4.85 micron R=5000 spectrum of the low mass class I young stellar object GSS 30 IRS1 (L=25L_sun) in the rho Ophiuchus core, observed with the infrared spectrometer (ISAAC) on the Very Large Telescope (VLT-UT1). Strong line emission from the ro-vibrational transitions of 12CO and 13CO is detected. In total more than 40 distinct lines are seen in the covered region. The line emission is spatially extended and detected up to 2" = 320 AU from the central source but is spectrally unresolved (Delta v < 30 km/s). This is the first time strong emission in the fundamental ro-vibrational band from CO has been observed from an embedded young stellar object. The line fluxes were modeled using a 1-dimensional full radiative transfer code, which shows that the emission is fully consistent with a gas in LTE at a single well constrained temperature (T=515+/-5 K). Furthermore, the ratios between lines from the two detected isotopic species of CO show that the 12CO lines must be optically thick. However, this is inconsistent with the observed spatial extent of the emission, since this implies such low CO column densities that the lines are optically thin. A likely solution to the discrepancy is that the lines are emitted by a smaller more dense region and then scattered in the bipolar cavity present around the central star. This gives a rough estimate of the total molecular gas mass of 1-100 M_earth and a physical extent of ~20-100 AU. We propose that the most likely origin of the line emission is post-shocked gas in a dense dissociative accretion shock from the inner 10-50 AU of a circumstellar disk. The presence of a shock capable of dissociating molecules in the disk will have implications for the chemical evolution of disks around young low mass stars.

Motivation & Objective

• To investigate the origin of strong ro-vibrational CO emission in the 4.5–4.8 μm band from the embedded class I YSO GSS 30 IRS1.
• To determine the physical conditions (temperature, column density, optical depth) of the emitting gas in the inner disk region.
• To resolve the discrepancy between observed line optical depths and spatial extent, which cannot be explained by a single emission region.
• To assess the role of accretion shocks in shaping the chemical and physical evolution of protoplanetary disks around low-mass young stars.

Proposed method

• Acquired a high-resolution (R = 5,000) near-infrared spectrum of GSS 30 IRS1 using the ISAAC spectrometer on the VLT-UT1.
• Identified over 40 distinct ro-vibrational transitions of 12CO and 13CO in the M-band (4.5–4.8 μm).
• Applied a 1D full radiative transfer code to model the line emission, assuming local thermodynamic equilibrium (LTE).
• Used the observed 12CO/13CO line flux ratios to infer optical depth and constrain column density.
• Explored scattering models to reconcile the observed spatial extent (up to 2 arcsec = 320 AU) with high optical depth.
• Evaluated alternative scenarios, including two-temperature gas distributions, and found them inconsistent with observations.

Experimental results

Research questions

• RQ1 What physical conditions (temperature, density, optical depth) are required to reproduce the observed CO ro-vibrational line spectrum in GSS 30 IRS1?
• RQ2 Why is the emission spatially extended (up to 320 AU) yet spectrally unresolved (Δv < 30 km s⁻¹), and how can this be reconciled with high optical depth?
• RQ3 What mechanism can produce a single-temperature gas at ~515 K with high optical depth in a region with low column density?
• RQ4 Is the observed emission consistent with a shock in the inner disk, and if so, what are the implications for disk chemistry and accretion dynamics?

Key findings

• The CO ro-vibrational emission is best modeled by a single-temperature gas at 515 ± 5 K, with a CO column density of 2 ± 0.5 × 10¹⁸ cm⁻².
• The 12CO lines are optically thick, but the observed spatial extent (320 AU) implies low column density, creating a contradiction.
• The discrepancy is resolved by a scattering model: emission originates from a small, dense region and is scattered in a bipolar cavity.
• The observed line fluxes and ratios are inconsistent with a two-temperature gas distribution, ruling out such models.
• The emission is most plausibly explained by a J-shock in a dense accretion flow from the inner 10–50 AU of a circumstellar disk.
• The shock scenario implies significant molecular dissociation and has major implications for chemical evolution in inner disks of low-mass YSOs.