2:15 PM - 2:30 PM
[PCG20-03] First detection of HC18O+ in a protoplanetary disk: exploring oxygen isotope fractionation of CO
Keywords:Protoplaneatry disk, Oxygen isotope fractionation
First detection of HC18O+ in a protoplanetary disk: exploring oxygen isotope fractionation of CO
The element oxygen has three stable isotopes: 16O, 17O, and 18O. It has been well established that the solar system materials, including chondrules, Ca-Al-rich inclusions (CAIs), and the Earth's ocean, are enriched in 17O and 18O compared to the Sun, and their oxygen isotope compositions show mass-independent variations. The oxygen isotope fractionation scenario, which has been developed to explain the oxygen isotope anomaly in the solar system materials, predicts that CO gas is depleted in 18O and 17O in protoplanetary disks, while H2O is enriched in 18O and 17O (e.g., Yurimoto & Kuramoto 2004; Lyons & Young 2005). One way to test the scenario is to measure the oxygen isotopic composition of CO gas in protoplanetary disks. However, it is not straightforward, because C16O is often optically thick in disks.
Based on ALMA observations, we report the first detection of HC18O+ in a Class II protoplanetary disk (TW Hya). This detection allows us to explore the oxygen isotope fractionation of CO in the TW Hya disk from optically thin HCO+ isotopologues as a proxy of optically thicker CO isotopologues. Using the H13CO+ data previously obtained with SMA, we derived the H13CO+/HC18O+ ratio in the central ~100 au regions of the disk. We construct a chemical model of the TW Hya disk with carbon and oxygen isotope fractionation chemistry, and estimate the conversion factor from H13CO+/HC18O+ to 13CO/C18O. With the conversion factor (=0.8), the 13CO/C18O ratio is estimated to be 8.3 +- 2.6, which is consistent with the elemental abundance ratio in the local ISM (8.1 +- 0.8) within error margin. Then there is no clear evidence of 18O depletion in CO gas in the central ~100 au regions of the disk, although we could not draw any robust conclusion due to uncertainties. In conclusion, optically thin lines of HCO+ isotopologues are useful tracers of CO isotopic ratios, which are hardly constrained directly from optically thick lines of CO isotopologues. Future higher sensitivity observations of H13CO+ and HC18O+ would be able to allow us to better constrain the oxygen fractionation in the disk.
The element oxygen has three stable isotopes: 16O, 17O, and 18O. It has been well established that the solar system materials, including chondrules, Ca-Al-rich inclusions (CAIs), and the Earth's ocean, are enriched in 17O and 18O compared to the Sun, and their oxygen isotope compositions show mass-independent variations. The oxygen isotope fractionation scenario, which has been developed to explain the oxygen isotope anomaly in the solar system materials, predicts that CO gas is depleted in 18O and 17O in protoplanetary disks, while H2O is enriched in 18O and 17O (e.g., Yurimoto & Kuramoto 2004; Lyons & Young 2005). One way to test the scenario is to measure the oxygen isotopic composition of CO gas in protoplanetary disks. However, it is not straightforward, because C16O is often optically thick in disks.
Based on ALMA observations, we report the first detection of HC18O+ in a Class II protoplanetary disk (TW Hya). This detection allows us to explore the oxygen isotope fractionation of CO in the TW Hya disk from optically thin HCO+ isotopologues as a proxy of optically thicker CO isotopologues. Using the H13CO+ data previously obtained with SMA, we derived the H13CO+/HC18O+ ratio in the central ~100 au regions of the disk. We construct a chemical model of the TW Hya disk with carbon and oxygen isotope fractionation chemistry, and estimate the conversion factor from H13CO+/HC18O+ to 13CO/C18O. With the conversion factor (=0.8), the 13CO/C18O ratio is estimated to be 8.3 +- 2.6, which is consistent with the elemental abundance ratio in the local ISM (8.1 +- 0.8) within error margin. Then there is no clear evidence of 18O depletion in CO gas in the central ~100 au regions of the disk, although we could not draw any robust conclusion due to uncertainties. In conclusion, optically thin lines of HCO+ isotopologues are useful tracers of CO isotopic ratios, which are hardly constrained directly from optically thick lines of CO isotopologues. Future higher sensitivity observations of H13CO+ and HC18O+ would be able to allow us to better constrain the oxygen fractionation in the disk.