AKARI observations of ice absorption bands towards edge-on young stellar objects

¹ Department. of Earth and Planetary SciencesKobe University, 657-8501 Kobe, Japan
e-mail: aikawa@kobe-u.ac.jp
² Department of Astronomy, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
³ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, 181-8588 Tokyo, Japan
⁴ Department of Physics, Scottish Universities Physics Alliance (SUPA), University of Strathclyde, Glasgow G4 ONG, UK
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, 464-8602 Nagoya, Japan
⁷ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, 229-8510 Kanagawa, Japan

Received: 26 October 2010
Accepted: 17 November 2011

Abstract

Context. Circumstellar disks and envelopes of low-mass young stellar objects (YSOs) contain significant amounts of ice. Such icy material will evolve to become volatile components of planetary systems, such as comets in our solar system.

Aims. To investigate the composition and evolution of circumstellar ice around low-mass young stellar objects (YSOs), we observed ice absorption bands in the near infrared (NIR) towards eight YSOs ranging from class 0 to class II, among which seven are associated with edge-on disks.

Methods. We performed slit-less spectroscopic observations using the grism mode of the InfraRed Camera (IRC) on board AKARI, which enables us to obtain full NIR spectra from 2.5 μm to 5 μm, including the CO₂ band and the blue wing of the H₂O band, which are inaccessible from the ground. We developed procedures to carefully process the spectra of targets with nebulosity. The spectra were fitted with polynomial baselines to derive the absorption spectra. The molecular absorption bands were then fitted with the laboratory database of ice absorption bands, considering the instrumental line profile and the spectral resolution of the grism dispersion element.

Results. Towards the class 0-I sources (L1527, IRC-L1041-2, and IRAS  04302), absorption bands of H₂O, CO₂, CO, and XCN are clearly detected. Column density ratios of CO₂ ice and CO ice relative to H₂O ice are 21−28% and 13−46%, respectively. If XCN is OCN⁻, its column density is as high as 2−6% relative to H₂O ice. The HDO ice feature at 4.1 μm is tentatively detected towards the class 0-I sources and HV Tau. Non-detections of the CH-stretching mode features around 3.5 μm provide upper limits to the CH₃OH abundance of 26% (L1527) and 42% (IRAS  04302) relative to H₂O. We tentatively detect OCS ice absorption towards IRC-L1041-2. Towards class 0-I sources, the detected features should mostly originate in the cold envelope, while CO gas and OCN⁻ could originate in the region close to the protostar, where there are warm temperatures and UV radiation. We detect H₂O ice band towards ASR41 and 2MASSJ 1628137-243139, which are edge-on class II disks. We also detect H₂O ice and CO₂ ice towards HV Tau, HK Tau, and UY Aur, and tentatively detect CO gas features towards HK Tau and UY Aur.