Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 1B065

Deep O2 observations toward a low-mass protostar with Herschel-HIFI

Yildiz, Umut (Leiden Observatory, Leiden University)
Acharyya, Kinsuk (S.N. Bose National Centre for Basic Sciences)
Goldsmith, Paul (Jet Propulsion Laboratory, California Institute of Technology)
van Dishoeck, Ewine (Leiden Observatory, Leiden University & MPE)
Melnick, Gary (Harvard-Smithsonian Center for Astrophysics)
Snell, Ronald (University of Massachusetts)
Liseau, Rene (Chalmers University of Technology, Onsala Space Observatory)
Chen, Jo-Hsin (Jet Propulsion Laboratory, California Institute of Technology)
Pagani, Laurent (LERMA & Observatoire de Paris)
Bergin, Edwin (University of Michigan)
Caselli, Paola (University of Leeds & INAF)
Herbst, Eric (University of Virginia)
Kristensen, Lars (Harvard-Smithsonian Center for Astrophysics)
Visser, Ruud (University of Michigan)
Lis, Dariusz (California Institute of Technology, Cahill Center for Astronomy and Astrophysics)
Gerin, Maryvonne (LRA/LERMA & Observatoire de Paris & Ecole Normale Superieure)

Oxygen is the third most abundant element in the Universe, after hydrogen and helium. According to traditional gas-phase chemical models, O2 should be abundant in molecular clouds (X(O2)~7e-5 relative to H2), but until recently, attempts to detect interstellar O2 line emission with ground- and space-based observatories have failed. Following the multi-line detection of O2 with low abundances in the Orion and rho OphA molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O2 abundance near a solar-mass protostar. Observations of molecular oxygen, O2, at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333 IRAS 4A, are presented, using the HIFI on the Herschel Space Observatory. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models. The deep HIFI spectrum (rms=1.3 mK) fails to show O2 at the velocity of the dense protostellar envelope, implying one of the deepest abundance upper limits of O2/H2 at <6x10e-9 (3 sigma). The O2/CO abundance ratio is less than 0.005. However, a tentative (4.5 sigma) detection of O2 is seen at the velocity of the surrounding NGC~1333 cloud, shifted by 1 km/s relative to the protostar. Pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (~0.7-1x10e6 years) during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H2O to avoid overproduction of O2 in the dense envelope. The tentative detection of O2 in the surrounding cloud is consistent with a low-density PDR model with a small enhancement of the water-ice photodesorption yield of a factor of two. The low O2 abundance in the collapsing envelope around a low-mass protostar suggests that the gas and ice entering protoplanetary disks is very poor in O2. This research is described in Yildiz et al. (2013, A&A, astro-ph: 1307.8031).

Click here to view poster PDF