[Paper Review] Gas in Protoplanetary Discs (GASPS) 1. First results

G. S. Mathews, W. R. F. Dent+51 more|arXiv (Cornell University)|May 21, 2010

Astrophysics and Star Formation Studies17 citations

TL;DR

This study uses Herschel's PACS instrument to conduct far-infrared spectroscopy and photometry of four protoplanetary discs, detecting [OI] 63 µm emission in three sources (HD 169142, TW Hydrae, RECX 15) but not [CII] 158 or [OI] 145. The results reveal a wide range in [OI] 63 line-to-continuum ratios, suggesting extreme gas-to-dust ratios in some discs, particularly RECX 15, which is undetected at 850 µm, indicating a rare, gas-rich, dust-poor disc regime previously unexplored.

ABSTRACT

Context - Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel space observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution. Aims - We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase. Methods - We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at lambda ~60-190 micron using the Photodetector Array Camera and Spectrometer instrument on the Herschel space observatory. Results - We detect [OI] 63 micron emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 micron. Extensive models are presented in associated papers.

Motivation & Objective

• To detect and characterize far-infrared gas emission from protoplanetary discs using Herschel’s high-sensitivity spectroscopy and photometry.
• To investigate the gas content and excitation conditions in circumstellar discs during the early stages of planetary system formation.
• To explore the prevalence of gas-rich, dust-poor discs by measuring line-to-continuum flux ratios and comparing with models.
• To assess the limitations of previous studies focused on either hot inner-disc emission or cold outer-disc mm lines.
• To determine whether [CII] 158 emission is detectable in discs with known large radii, such as HD 169142.

Proposed method

• Conducted medium-resolution spectroscopy at 60–190 µm using the Photodetector Array Camera and Spectrometer (PACS) on the Herschel space observatory.
• Performed high-sensitivity photometry at 70 and 160 µm to constrain dust distribution and continuum emission.
• Analyzed line flux ratios of [OI] 63 µm, [CII] 158 µm, and [OI] 145 µm against model predictions to assess emission origin.
• Used 3σ upper limits and lower limits to interpret non-detections, particularly for [CII] 158 and [OI] 145.
• Compared observed line-to-continuum flux ratios with theoretical models to infer gas-to-dust ratios and disc structure.
• Evaluated consistency with disc emission versus outflow/jet sources by comparing line ratios with known outflowing systems.

Experimental results

Research questions

• RQ1 Can far-infrared [OI] 63 µm emission be detected in protoplanetary discs using Herschel’s sensitivity, and what does it reveal about gas excitation?
• RQ2 Why is [CII] 158 µm emission not detected in any of the four targets despite expectations from disc models?
• RQ3 What explains the exceptionally high [OI] 63 line-to-continuum flux ratio in RECX 15 compared to other discs?
• RQ4 To what extent do the observed line flux ratios support disc emission rather than outflow or jet activity?
• RQ5 How common are gas-rich, dust-poor discs, and what does the non-detection of RECX 15 at 850 µm imply about its gas-to-dust ratio?

Key findings

• [OI] 63 µm emission was detected in HD 169142, TW Hydrae, and RECX 15, with fluxes ranging from 4 to 90 × 10⁻¹⁸ W/m² at 140 pc.
• No significant detections were found for [CII] 158 µm or [OI] 145 µm in any of the four targets, despite model predictions suggesting detectability with longer integration.
• The [OI] 63 line-to-continuum flux ratio reached 17 in RECX 15, significantly higher than 0.3–1 in HD 169142 and TW Hydrae, indicating a possible enhanced gas-to-dust ratio.
• RECX 15 is undetected at 850 µm, consistent with a high gas-to-dust ratio, similar to the 49 Ceti system, suggesting a rare, gas-rich, dust-poor disc.
• The line flux ratios for all three detected sources are consistent with disc emission rather than outflow, based on comparison with jet sources.
• Model predictions suggest that with longer integration times, [CII] 158 may be detectable in TW Hydrae and RECX 15, and [OI] 145 in HD 169142.