THE RADIAL DISTRIBUTION OF H₂ AND CO IN TW HYA AS REVEALED BY RESOLVED ALMA OBSERVATIONS OF CO ISOTOPOLOGUES

Abstract

CO is widely used as a tracer of molecular gas. However, there is now mounting evidence that gas phase carbon is depleted in the disk around TW Hya. Previous efforts to quantify this depletion have been hampered by uncertainties regarding the radial thermal structure in the disk. Here we present resolved ALMA observations of 13 CO 3-2, C 18 O 3-2, 13 CO 6-5, and C 18 O 6-5 emission in TW Hya, which allow us to derive radial gas temperature and gas surface density profiles, as well as map the CO abundance as a function of radius. These observations provide a measurement of the surface CO snowline at ∼30 AU and show evidence for an outer ring of CO emission centered at 53 AU, a feature previously seen only in less abundant species. Further, the derived CO gas temperature profile constrains the freeze out temperature of CO in the warm molecular layer to <21 K. Combined with the previous detection of HD 1-0, these data constrain the surface density of the warm H 2 gas in the inner ∼30 AU such thatg cm 10 auwarm gas 2.9 3.0 2 1 2 . We find that CO is depleted by two orders of magnitude from -= R 10 60 AU, with the small amount of CO returning to the gas phase inside the surface CO snowline insufficient to explain the overall depletion. Finally, this new data is used in conjunction with previous modeling of the TW Hya disk to constrain the midplane CO snowline to 17-23 AU.

Stellar energetic particle ionization in protoplanetary disks around T Tauri stars

“…There is observational evidence for CO depletion in protoplanetary disks (Dutrey et al 1997;Bruderer et al 2012;Favre et al 2013;Kama et al 2016a;Schwarz et al 2016;McClure et al 2016). The best constraint case is TW Hya.…”

“…However, these results depend on the method or more precisely the molecule used to trace the CO ice line (see Qi et al 2013b;Schwarz et al 2016;Nomura et al 2016 for TW Hya). Further, due to complex chemical processes like the CO sink effect it is possible that the actual location of the CO ice line does not only depend on the CO freeze-out temperature (Aikawa et al 2015, Sect.…”

Far-infrared HD emission as a measure of protoplanetary disk mass

“…The fluxes and upper limits for the observations used can be found in Table B.1 in Kama et al (2016b). The line fluxes of 13 CO 6-5, C 18 O 3-2 and C 18 O 6-5 were calculated from the data presented in Schwarz et al (2016). …”

“…Different studies have shown that the TW Hya disk seems genuinely depleted in volatile carbon and oxygen (e.g., Bergin et al 2010;Bergin et al 2013Bergin et al , 2016Hogerheijde et al 2011;Favre et al 2013;Du et al 2015;Kama et al 2016b). Recently, new observations of the CO isotopologues have become available (see, e.g., Schwarz et al 2016). Therefore, the case of TW Hya is revisited using a combined HD and CO isotopologue analysis.…”

The Gas Disk: Evolution and Chemistry

“…Schwarz et al (2016) uses ALMA observations of several CO transitions (including isotopologues) to derive the radial temperature profile of the TW Hya disk. However, these observations mainly trace the warm molecular layer (≈30-50 K see Sect.…”