H{sub 2}CO AND N{sub 2}H{sup +} IN PROTOPLANETARY DISKS: EVIDENCE FOR A CO-ICE REGULATED CHEMISTRY

Journal Article · Fri Mar 01 04:00:00 EST 2013 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/765/1/34· OSTI ID:22167651

Qi, Chunhua; Wilner, David J. ^[1]; Oeberg, Karin I. ^[2]

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1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
2. Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904 (United States)

We present Submillimeter Array (SMA) observations of H{sub 2}CO and N{sub 2}H{sup +} emission in the disks around the T Tauri star TW Hya and the Herbig Ae star HD 163296 at 2''-6'' resolution and discuss the distribution of these species with respect to CO freezeout. The H{sub 2}CO and N{sub 2}H{sup +} emission toward HD 163296 does not peak at the continuum emission center that marks the stellar position but is instead significantly offset. Using a previously developed model for the physical structure of this disk, we show that the H{sub 2}CO observations are reproduced if H{sub 2}CO is present predominantly in the cold outer disk regions. A model where H{sub 2}CO is present only beyond the CO snow line (estimated at a radius of 160 AU) matches the observations well. We also show that the average H{sub 2}CO excitation temperature, calculated from two transitions of H{sub 2}CO observed in these two disks and a larger sample of disks around T Tauri stars in the DISCS (the Disk Imaging Survey of Chemistry with SMA) program, is consistent with the CO freezeout temperature of {approx}20 K. In addition, we show that N{sub 2}H{sup +} and H{sub 2}CO line fluxes in disks are strongly correlated, indicative of co-formation of these species across the sample. Taken together, these results imply that H{sub 2}CO and N{sub 2}H{sup +} are generally present in disks only at low temperatures where CO depletes onto grains, consistent with fast destruction of N{sub 2}H{sup +} by gas-phase CO, and in situ formation of H{sub 2}CO through hydrogenation of CO ice. In this scenario H{sub 2}CO, CH{sub 3}OH, and N{sub 2}H{sup +} emission in disks should appear as rings with the inner edge at the CO midplane snow line. This prediction can be tested directly using observations from ALMA with higher resolution and better sensitivity.

• OSTI ID: 22167651
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 765; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ASTRONOMY
ASTROPHYSICS
CARBON MONOXIDE
COSMOCHEMISTRY
EXCITATION
HERBIG-HARO OBJECTS
HYDROGENATION
ICE
ION EMISSION
METHANOL
MOLECULES
PHOTON EMISSION
PROTOPLANETS
RESOLUTION
SNOW
STAR EVOLUTION
T TAURI STARS