title: Change in the chemical composition of infalling gas forming a disk around a protostar.
abstract: IRAS 04368+2557 is a solar-type (low-mass) protostar embedded in a protostellar core (L1527) in the Taurus molecular cloud, which is only 140 parsecs away from Earth, making it the closest large star-forming region. The protostellar envelope has a flattened shape with a diameter of a thousand astronomical units (1 AU is the distance from Earth to the Sun), and is infalling and rotating. It also has a protostellar disk with a radius of 90 AU (ref. 6), from which a planetary system is expected to form. The interstellar gas, mainly consisting of hydrogen molecules, undergoes a change in density of about three orders of magnitude as it collapses from the envelope into the disk, while being heated from 10 kelvin to over 100 kelvin in the mid-plane, but it has hitherto not been possible to explore changes in chemical composition associated with this collapse. Here we report that the unsaturated hydrocarbon molecule cyclic-C3H2 resides in the infalling rotating envelope, whereas sulphur monoxide (SO) is enhanced in the transition zone at the radius of the centrifugal barrier (100 ± 20 AU), which is the radius at which the kinetic energy of the infalling gas is converted to rotational energy. Such a drastic change in chemistry at the centrifugal barrier was not anticipated, but is probably caused by the discontinuous infalling motion at the centrifugal barrier and local heating processes there.
authors: Department of Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors: Nami
authors: N
authors: Sakai
authors: Department of Communication Engineering and Informatics, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan.
authors: Takeshi
authors: T
authors: Sakai
authors: National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan.
authors: Tomoya
authors: T
authors: Hirota
authors: Department of Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors: Yoshimasa
authors: Y
authors: Watanabe
authors: Institut de Planétologie et d'Astrophysique de Grenoble, BP 53, 38041 Grenoble Cedex 9, France.
authors: Cecilia
authors: C
authors: Ceccarelli
authors: Institut de Planétologie et d'Astrophysique de Grenoble, BP 53, 38041 Grenoble Cedex 9, France.
authors: Claudine
authors: C
authors: Kahane
authors: 1] Université de Toulouse, Université Paul Sabatier, Observatoire Midi-Pyrénées (UPS-OMP), Institut de Recherche en Astrophysique et Planétologie (IRAP), Toulouse, France [2] Centre National de la Recherche Scientifique (CNRS), IRAP, 9 Avenue Colonel Roche, BP 44346, Toulouse 31028 Cedex 4, France.
authors: Sandrine
authors: S
authors: Bottinelli
authors: 1] Université de Toulouse, Université Paul Sabatier, Observatoire Midi-Pyrénées (UPS-OMP), Institut de Recherche en Astrophysique et Planétologie (IRAP), Toulouse, France [2] Centre National de la Recherche Scientifique (CNRS), IRAP, 9 Avenue Colonel Roche, BP 44346, Toulouse 31028 Cedex 4, France.
authors: Emmanuel
authors: E
authors: Caux
authors: 1] Université de Toulouse, Université Paul Sabatier, Observatoire Midi-Pyrénées (UPS-OMP), Institut de Recherche en Astrophysique et Planétologie (IRAP), Toulouse, France [2] Centre National de la Recherche Scientifique (CNRS), IRAP, 9 Avenue Colonel Roche, BP 44346, Toulouse 31028 Cedex 4, France.
authors: Karine
authors: K
authors: Demyk
authors: 1] Université de Toulouse, Université Paul Sabatier, Observatoire Midi-Pyrénées (UPS-OMP), Institut de Recherche en Astrophysique et Planétologie (IRAP), Toulouse, France [2] Centre National de la Recherche Scientifique (CNRS), IRAP, 9 Avenue Colonel Roche, BP 44346, Toulouse 31028 Cedex 4, France.
authors: Charlotte
authors: C
authors: Vastel
authors: 1] Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, Copenhagen 2100 Østerbro, Denmark [2] Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, Copenhagen 1350 K, Denmark.
authors: Audrey
authors: A
authors: Coutens
authors: National Aeronautics and Space Administration (NASA), Goddard Space Flight Center (GSFC), Astrochemistry Laboratory, Mail Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
authors: Vianney
authors: V
authors: Taquet
authors: 1] Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, Taipei 10617, Taiwan [2] Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, Hawaii 96720, USA.
authors: Nagayoshi
authors: N
authors: Ohashi
authors: Institute of Astrophysics, National Taiwan University, Taipei 10617, Taiwan.
authors: Shigehisa
authors: S
authors: Takakuwa
authors: 1] Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, Taipei 10617, Taiwan [2] Institute of Astrophysics, National Taiwan University, Taipei 10617, Taiwan.
authors: Hsi-Wei
authors: HW
authors: Yen
authors: Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan.
authors: Yuri
authors: Y
authors: Aikawa
authors: Department of Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors: Satoshi
authors: S
authors: Yamamoto