HD 172555: detection of 63μm [OI] emission in a debris disc

P Riviere-Marichalar,F Ménard,W R F Dent,I Mendigutía,J Lebreton,W F Thi,C Eiroa,G Meeus,G Duchêne,J.-C Augereau,A Roberge,D Barrado,N Huélamo,C Howard,C Pinte,B Montesinos,G Sandell

doi:10.1051/0004-6361/201219745

Abstract

Context. HD 172555 is a young A7 star belonging to the Beta Pictoris Moving Group that harbours a debris disc. The Spitzer IRS spectrum of the source showed mid-IR features such as silicates and glassy silica species, indicating the presence of a warm dust component with small grains, which places HD 172555 among the small group of debris discs with such properties. The IRS spectrum also shows a possible emission of SiO gas. Aims. We aim to study the dust distribution in the circumstellar disc of HD 172555 and to asses the presence of gas in the debris disc. Methods. As part of the GASPS Open Time Key Programme, we obtained Herschel-PACS photometric and spectroscopic observations of the source. We analysed PACS observations of HD 172555 and modelled the Spectral Energy Distribution (SED) with a modified blackbody and the gas emission with a two-level population model with no collisional de-excitation. Results. We report for the first time the detection of OI atomic gas emission at 63.18 microns in the HD 172555 circumstellar disc. We detect excesses due to circumstellar dust toward HD 172555 in the three photometric bands of PACS (70, 100, and 160 microns). We derive a large dust particle mass of 4.8e-4 Earth masses and an atomic oxygen mass of 2.5e-2*R^2 Earth masses, where R in AU is the separation between the star and the inner disc. Thus, most of the detected mass of the disc is in the gaseous phase.

Highlights

We aim to study the dust distribution in the circumstellar disc of HD 172555 and to asses the presence of gas in the debris disc
We report for the first time the detection of [OI] atomic gas emission at 63.18 μm in the HD 172555 circumstellar disc
Circumstellar discs play a key role in understanding how planets form and evolve, since it is thought that they originate in gas-rich circumstellar discs around young stars, the so-called protoplanetary discs

Summary

Introduction

Circumstellar discs play a key role in understanding how planets form and evolve, since it is thought that they originate in gas-rich circumstellar discs around young stars, the so-called protoplanetary discs. With ages greater than about 5 to 10 Myr, another type of disc is seen: dusty, optically thin debris discs that are produced by destructive collisions between planetesimals formed in the protoplanetary disc phase. The younger debris discs (∼5−100 Myr) are the likely sites of ongoing terrestrial planet formation, while the older ones (0.1–1 Gyr) correspond to the “clearing out” phase, when most planetesimals left over from planet formation are removed from the system. There are a few debris discs that show strong solid-state midIR spectral features, indicating the presence of small, warm grains.

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Discussion

Conclusion