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    CO2 Ice Toward Low-Luminosity Embedded Protostars: Evidence For Episodic Mass Accretion Via Chemical History

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    Date
    2012-10
    Author
    Kim, Hyo Jeeong
    Evans, Neal J.
    Dunham, Michael M.
    Lee, Jeong-Eun
    Pontoppidan, Klaus M.
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    Abstract
    We present Spitzer IRS spectroscopy of CO2 ice bending mode spectra at 15.2 mu m toward 19 young stellar objects (YSOs) with luminosity lower than 1L(circle dot) (3 with luminosity lower than 0.1 L-circle dot). Ice on dust grain surfaces can encode the history of heating because pure CO2 ice forms only at elevated temperature, T > 20 K, and thus around protostars of higher luminosity. Current internal luminosities of YSOs with L < 1L(circle dot) do not provide the conditions needed to produce pure CO2 ice at radii where typical envelopes begin. The presence of detectable amounts of pure CO2 ice would signify a higher past luminosity. Many of the spectra require a contribution from a pure, crystalline CO2 component, traced by the presence of a characteristic band splitting in the 15.2 mu m bending mode. About half of the sources (9 out of 19) in the low-luminosity sample have evidence for pure CO2 ice, and 6 of these have significant double-peaked features, which are very strong evidence of pure CO2 ice. The presence of the pure CO2 ice component indicates that the dust temperature, and hence luminosity of the central star/accretion disk system, must have been higher in the past. An episodic accretion scenario, in which mixed CO-CO2 ice is converted to pure CO2 ice during each high-luminosity phase, explains the presence of pure CO2 ice, the total amount of CO2 ice, and the observed residual (CO)-O-18 gas.
    Department
    Astronomy
    Subject
    astrochemistry
    ism: molecules
    stars: formation
    stars: low-mass
    stars: protostars
    spitzer c2d survey
    pre-protostellar cores
    molecular cloud cores
    star-formation
    infrared-spectroscopy
    interstellar ices
    surface-reactions
    space-telescope
    evolutionary signatures
    carbon-dioxide
    astronomy & astrophysics
    URI
    http://hdl.handle.net/2152/35132
    xmlui.dri2xhtml.METS-1.0.item-citation
    Kim, Hyo Jeong, Neal J. Evans II, Michael M. Dunham, Jeong-Eun Lee, and Klaus M. Pontoppidan. "CO2 Ice Toward Low-luminosity Embedded Protostars: Evidence for Episodic Mass Accretion via Chemical History." The Astrophysical Journal, Vol. 758, No. 1 (Oct., 2012): 38.
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