J0537.5+0959 | J053745.9+095759 | J053745.9+095759 | -11.35 | 2.64 | 0.02 | 1.47 | -0.89 | -5.39 | 0.38 | 2.49
J0539.2$-$6333 | J054002.9-633216 | J054002.9-633216 | -11.79 | 2.09 | -0.03 | 1.04 | -1.04 | -4.03 | 1.19 | 2.55
J0544.8+5209 | J054424.5+521513 | J054424.5+521513 | -11.6 | 2.64 | -0.7 | 0.94 | -0.69 | -5.19 | 1.12 | 2.56
J0610.8$-$4911 | J061100.0-491034 | J061100.0-491034 | -11.98 | 1.93 | -0.65 | 1.21 | 0.31 | -4.77 | 0.31 | 2.15
J0610.8$-$4911 | J061031.8-491222 | J061031.8-491222 | -11.98 | 1.93 | -0.65 | 1.21 | -0.75 | -1.92 | -0.02 | 0.01
J0620.7$-$5034 | J062045.7-503350 | J062045.7-503350 | -11.98 | 1.88 | 0.16 | 1.32 | -0.28 | -4.43 | 0.45 | 2.06
J0633.9+5840 | J063400.1+584036 | J063400.1+584036 | -11.9 | 2.1 | -0.6 | 0.92 | -1.03 | -5.33 | 0.53 | 2.11
J0650.6+2055 | J065035.4+205557 | J065035.4+205557 | -11.25 | 1.79 | 0.3 | 0.98 | 0.05 | -4.01 | 0.64 | 1.66
J0704.3$-$4829 | J070421.8-482645 | J070421.8-482648 | -11.61 | 2.07 | 0.08 | 1.24 | -0.39 | -4.64 | 0.64 | 1.73
J0738.6+1311 | J073843.4+131330 | J073843.4+131330 | -11.57 | 2.6 | -0.36 | 1.23 | -0.97 | -2.22 | -0.03 | 0.08
J0800.1$-$5531 | J080013.1-553408 | J080013.1-553408 | -11.64 | 2.47 | -0.49 | 1.02 | -0.89 | -4.74 | 0.2 | 1.21
J0800.1$-$5531 | J075949.3-553254 | J075949.3-553254 | -11.64 | 2.47 | -0.49 | 1.02 | -0.94 | -4.15 | 1.12 | 2.36
J0800.9+0733 | J080056.5+073235 | J080056.5+073235 | -11.94 | 1.83 | 0.55 | 0.61 | -0.34 | -3.92 | 0.75 | 2.19
J0827.0$-$4111 | J082705.4-411159 | J082705.4-411159 | -11.54 | 2.1 | -0.14 | 0.99 | -0.4 | -5.37 | 0.63 | 1.74
J0838.5+4013 | J083902.8+401548 | J083903.0+401546 | -11.99 | 1.69 | -1.68 | 0.5 | -0.4 | -4.6 | 0.39 | 1.54
J0903.5+4057 | J090342.8+405503 | J090342.8+405503 | -12.02 | 1.96 | 0.11 | 1.03 | -0.89 | -4.14 | 1.02 | 2.76
J0906.1$-$1011 | J090616.2-101430 | J090616.1-101426 | -11.76 | 2.07 | -0.3 | 0.48 | -0.69 | -4.73 | 1.12 | 2.56
J0910.1$-$1816 | J091003.9-181613 | J091003.9-181613 | -11.8 | 1.95 | 0.36 | 1.1 | -0.21 | -4.77 | 0.35 | 2.2
J0914.5+6845 | J091430.0+684509 | J091429.7+684509 | -11.9 | 1.88 | 0.24 | 1.02 | -0.31 | -4.56 | 0.47 | 2.27
J0928.4$-$5256 | J092818.7-525701 | J092818.7-525701 | -11.38 | 2.0 | -0.17 | 0.89 | -0.21 | -5.13 | 0.4 | 2.41
J0930.9$-$3030 | J093058.0-303118 | J093057.9-303118 | -11.67 | 1.96 | -0.36 | 0.74 | -0.67 | -4.75 | 0.38 | 1.91
J0934.5+7223 | J093334.0+722101 | J093333.7+722101 | -11.64 | 2.91 | -4.0 | 1.03 | -0.74 | -4.46 | 0.73 | 3.05
J0938.8+5155 | J093835.0+515455 | J093834.8+515453 | -11.87 | 2.06 | 0.01 | 1.08 | -0.67 | -4.69 | 0.88 | 2.83
J1008.2$-$1000 | J100749.3-094910 | J100749.4-094912 | -11.39 | 3.08 | -0.03 | 1.05 | -1.11 | -4.87 | 1.05 | 2.67
J1008.2$-$1000 | J100802.5-095918 | J100802.5-095918 | -11.39 | 3.08 | -0.03 | 1.05 | -0.39 | -4.77 | 1.15 | 3.31
J1008.2$-$1000 | J100848.6-095450 | J100848.6-095450 | -11.39 | 3.08 | -0.03 | 1.05 | 0.61 | -3.12 | 0.68 | 2.21
J1011.1$-$4420 | J101132.0-442255 | J101132.0-442255 | -11.81 | 2.02 | -0.16 | 0.96 | 0.86 | -3.91 | 0.4 | 2.13
J1016.1$-$4247 | J101620.8-424723 | J101620.7-424723 | -11.42 | 1.87 | 0.43 | 1.04 | -0.56 | -4.45 | 0.68 | 1.86
J1016.2$-$5729 | J101625.7-572807 | J101625.7-572807 | -10.9 | 2.54 | 0.07 | 1.08 | -0.34 | -4.9 | 0.49 | 0.68
J1018.1$-$2705 | J101750.2-270550 | J101750.2-270550 | -11.47 | 2.54 | 0.23 | 1.04 | -0.29 | -4.49 | 0.33 | 1.8
J1018.1$-$4051 | J101807.6-404408 | J101807.6-404408 | -11.57 | 2.52 | 0.27 | 1.09 | -1.09 | -5.28 | 0.92 | 2.64
J1018.1$-$4051 | J101801.5-405520 | J101801.4-405519 | -11.57 | 2.52 | 0.27 | 1.09 | -1.03 | -4.63 | 1.08 | 1.83
J1024.5$-$4543 | J102432.6-454428 | J102432.5-454428 | -11.6 | 1.92 | -0.03 | 0.89 | 0.1 | -4.94 | 0.32 | 1.75
J1034.7$-$4645 | J103438.7-464405 | J103438.7-464405 | -11.7 | 2.17 | -0.13 | 1.13 | 0.21 | -5.27 | 0.54 | 2.23
J1048.4$-$5030 | J104824.2-502941 | J104824.2-502941 | -11.74 | 1.69 | 0.31 | 0.94 | -0.78 | -4.27 | 0.68 | 2.23
J1049.8+2741 | J104938.8+274217 | J104938.8+274213 | -11.79 | 2.13 | -0.41 | 1.09 | -0.68 | -4.11 | 0.29 | 1.14
J1106.7+3623 | J110636.7+362648 | J110636.5+362650 | -11.5 | 2.51 | -0.07 | 1.86 | -1.0 | -4.66 | 1.08 | 2.6
J1111.4+0137 | J111114.2+013431 | J111114.2+013431 | -11.77 | 2.42 | -0.48 | 0.7 | 0.11 | -4.88 | 0.48 | 1.47
J1119.9$-$1007 | J111948.2-100704 | J111948.4-100707 | -11.69 | 2.15 | 0.29 | 1.38 | -0.69 | -4.56 | 0.81 | 2.52
J1122.0$-$0231 | J112213.8-022916 | J112213.7-022914 | -11.71 | 2.37 | 0.16 | 0.92 | -0.29 | -4.22 | 0.51 | 2.18
J1146.0$-$0638 | J114600.8-063851 | J114600.8-063851 | -11.46 | 1.75 | 0.27 | 0.82 | -0.55 | -4.86 | 0.34 | 1.98
J1155.2$-$1111 | J115514.7-111125 | J115514.7-111125 | -11.74 | 1.93 | 0.28 | 1.05 | -0.55 | -5.16 | 0.19 | 2.73
J1220.1$-$2458 | J122014.5-245949 | J122014.5-245949 | -11.56 | 2.03 | -0.34 | 1.05 | 0.01 | -4.83 | 0.51 | 2.07
J1243.7+1727 | J124351.6+172643 | J124351.8+172645 | -11.95 | 2.01 | -0.17 | 0.69 | -1.0 | -4.38 | 0.51 | 2.58
J1256.8+5329 | J125630.4+533203 | J125630.5+533205 | -11.4 | 2.64 | 0.07 | 1.02 | -1.15 | -4.71 | 1.16 | 2.53
J1320.3$-$6410 | J132016.9-641353 | J132015.9-641349 | -11.33 | 2.63 | 0.56 | 1.15 | 2.32 | -4.29 | 0.01 | -0.17
J1326.0+3507 | J132544.4+350450 | J132544.4+350450 | -11.67 | 2.15 | 0.24 | 1.75 | -0.84 | -4.68 | 0.66 | 1.83
J1326.0+3507 | J132622.3+350628 | J132622.2+350625 | -11.67 | 2.15 | 0.24 | 1.75 | -1.19 | -4.57 | 1.06 | 2.41
J1415.9$-$1504 | J141546.1-150229 | J141546.1-150229 | -11.68 | 2.18 | 0.19 | 0.98 | -0.55 | -4.56 | 0.67 | 1.61
J1429.8$-$0739 | J142949.7-073302 | J142949.5-073305 | -11.9 | 2.34 | 0.59 | 0.96 | -0.24 | -3.57 | 0.5 | 3.25
J1513.0$-$3118 | J151244.8-311647 | J151244.8-311647 | -11.42 | 2.43 | 0.42 | 1.0 | -0.46 | -3.02 | -0.02 | 0.11
J1514.8+4448 | J151451.0+444957 | J151451.0+444957 | -11.18 | 2.36 | 0.62 | 1.85 | -1.66 | -4.95 | 0.72 | 2.7
J1528.4+2004 | J152836.0+200424 | J152835.8+200421 | -11.91 | 2.14 | -0.36 | 0.98 | -0.17 | -4.83 | 0.36 | 2.36
J1545.0$-$6642 | J154459.0-664148 | J154458.9-664147 | -11.42 | 1.71 | -0.1 | 0.81 | 0.41 | -4.68 | 0.6 | 1.51
J1557.2+3822 | J155711.9+382032 | J155711.9+382032 | -12.14 | 1.98 | 0.09 | 0.98 | -0.4 | -4.88 | 0.35 | 1.89
J1623.7$-$2315 | J162334.1-231750 | J162334.1-231750 | -11.18 | 2.62 | -0.07 | 0.85 | -0.55 | -4.69 | 0.93 | 2.94
J1631.8+4144 | J163146.7+414634 | J163146.7+414633 | -11.93 | 1.8 | 0.01 | 1.16 | -0.44 | -4.23 | 0.65 | 2.39
J1637.5+3005 | J163739.3+301015 | J163739.2+301013 | -11.68 | 2.82 | -0.3 | 0.91 | -0.23 | -5.12 | 1.01 | 3.13
J1637.5+3005 | J163728.2+300958 | J163728.1+300953 | -11.68 | 2.82 | -0.3 | 0.91 | -1.27 | -4.94 | 1.2 | 2.53
J1644.8+1850 | J164457.3+185149 | J164457.2+185150 | -11.61 | 2.06 | 0.38 | 0.97 | -0.89 | -4.56 | 0.81 | 2.84
J1645.0+1654 | J164500.0+165510 | J164459.8+165513 | -11.73 | 2.26 | -0.11 | 0.85 | -0.41 | -4.54 | 0.41 | 1.44
J1650.9$-$4420 | J165124.2-442142 | J165124.2-442142 | -10.57 | 2.64 | 0.64 | 0.88 | 3.18 | -5.96 | 0.18 | 0.2
J1651.7$-$7241 | J165151.5-724310 | J165151.5-724310 | -11.98 | 1.99 | -1.11 | 0.49 | 0.0 | -4.71 | 0.35 | 1.57
J1720.6$-$5144 | J172032.7-514413 | J172032.7-514413 | -11.57 | 2.06 | 0.29 | 0.87 | 0.44 | -4.75 | 0.37 | 1.72
J1818.5+2533 | J181830.9+253707 | J181831.2+253707 | -11.4 | 2.6 | 0.38 | 1.17 | -1.23 | -3.84 | 0.2 | 1.67
J1820.4$-$1609 | J182029.6-161044 | J182029.6-161044 | -10.19 | 2.67 | -0.48 | 1.08 | -0.89 | -4.91 | 0.54 |
J1846.9$-$0227 | J184650.7-022904 | J184650.7-022904 | -10.47 | 2.5 | 0.73 | 0.98 | 1.5 | -3.56 | -0.09 | -0.16
J1910.8+2856 | J191059.4+285635 | J191059.4+285635 | -11.45 | 1.81 | 0.32 | 1.34 | -0.96 | -4.84 | 0.18 | 2.56
J1910.8+2856 | J191052.2+285624 | J191052.2+285624 | -11.45 | 1.81 | 0.32 | 1.34 | -0.02 | -5.23 | 0.63 | 2.3
J1918.0+0331 | J191803.6+033030 | J191803.6+033030 | -11.44 | 1.77 | 0.31 | 1.25 | -0.47 | -5.44 | 0.5 | 1.26
J1927.5+0154 | J192731.3+015357 | J192731.3+015357 | -11.56 | 1.79 | 0.29 | 1.16 | -0.3 | -4.79 | 0.52 | 2.34
J1955.3$-$5032 | J195512.5-503012 | J195512.5-503012 | -11.62 | 2.43 | 0.22 | 1.21 | -0.36 | -4.95 | 0.42 | 2.08
J2008.4+1619 | J200827.6+161844 | J200827.6+161844 | -11.65 | 2.12 | 0.31 | 1.12 | -0.19 | -4.99 | 0.55 | 2.16
J2041.1$-$6138 | J204112.0-613952 | J204112.0-613949 | -11.59 | 2.18 | -0.0 | 0.63 | -0.75 | -4.44 | 0.54 | 2.09
J2046.9$-$5409 | J204700.5-541246 | J204700.7-541245 | -11.8 | 1.9 | 0.33 | 0.95 | -0.58 | -4.67 | 0.34 | 2.17
J2109.6+3954 | J210936.4+395513 | J210936.4+395513 | -11.48 | 1.69 | 0.47 | 1.11 | -0.38 | -5.43 | 0.68 | 2.37
J2114.9$-$3326 | J211452.0-332532 | J211452.1-332533 | -11.44 | 2.07 | -0.52 | 1.07 | -0.25 | -4.72 | 0.55 | 1.8
J2159.6$-$4620 | J215935.9-461954 | J215936.1-461953 | -11.55 | 1.72 | 0.04 | 1.15 | -0.79 | -4.42 | 0.52 | 2.6
J2207.1+2222 | J220704.3+222234 | J220704.1+222232 | -11.84 | 1.85 | -0.12 | 1.15 | -0.6 | -4.58 | 0.48 | 3.06
J2222.9+1507 | J222253.9+151052 | J222253.9+151055 | -11.88 | 2.11 | -0.07 | 0.87 | -0.69 | -4.11 | 0.77 | 3.15
J2225.8$-$0804 | J222552.9-080416 | J222552.9-080416 | -11.65 | 1.97 | 0.12 | 1.08 | -0.77 | -5.01 | 0.25 | 2.95
J2237.2$-$6726 | J223709.3-672614 | J223709.3-672614 | -11.76 | 2.01 | -0.12 | 1.25 | -0.26 | -4.81 | 0.4 | 1.8
J2240.3$-$5241 | J224017.5-524117 | J224017.7-524113 | -11.2 | 2.16 | 0.04 | 1.27 | -1.51 | -4.66 | 0.58 | 2.08
J2247.7$-$5857 | J224745.0-585501 | J224745.0-585501 | -11.65 | 2.55 | -0.5 | 1.12 | -0.66 | -5.03 | 0.34 | 2.69
J2303.9+5554 | J230351.7+555618 | J230351.7+555618 | -11.63 | 1.83 | 0.11 | 0.89 | 0.12 | -5.12 | 0.42 | 1.57
J2311.6$-$4427 | J231145.6-443221 | J231145.6-443221 | -11.84 | 2.27 | 0.05 | 0.98 | -0.89 | -4.34 | 0.79 | 2.53
J2317.7+2839 | J231740.0+283954 | J231740.0+283954 | -11.51 | 2.0 | -0.47 | 1.43 | -1.43 | -4.71 | 0.48 | 2.53
J2326.9$-$4130 | J232653.2-412713 | J232653.1-412711 | -11.47 | 2.78 | -1.19 | 1.34 | -0.97 | -3.87 | 1.1 | 2.8
J2336.9$-$8427 | J233627.1-842648 | J233627.1-842648 | -11.72 | 2.15 | -0.5 | 0.95 | -0.39 | -4.75 | 0.51 | 2.26
J2337.7$-$2903 | J233730.2-290241 | J233730.2-290241 | -11.87 | 2.16 | 0.24 | 0.61 | -0.19 | -4.56 | 0.66 | 2.41
J2351.4$-$2818 | J235136.5-282154 | J235136.5-282154 | -11.79 | 2.35 | -0.01 | 0.65 | -0.73 | -3.31 | -0.04 | 1.26

Note. — All the parameters are described in Table 1.

Table 3: The association results based on the neural network probabilities for all the 4FGL unassociated sources  4FGL Name | UVOT | P_fsrq | SIMBAD/SDSS | redshift
---|---|---|---|---
|  |  | FSRQ candidates from NN results |
J1008.2$-$1000 | J100802.5$-$095918 | 0.993 | QSO | 1.66(Krogager et al., 2018)
J1008.2$-$1000 | J100749.4$-$094912 | 0.993 |  $\cdots$ |  $\cdots$
J1637.5+3005 | J163728.1+300953 | 0.992 | QSO | 1.2; Fig 10a
J1637.5+3005 | J163739.2+301013 | 0.991 | Seyfert I | 0.155; Fig 9f
|  |  | BL Lac candidates from NN results |
J0026.1$-$0732 | J002611.6$-$073115 | 0.007 | blazar candidate | 0.5phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0031.5$-$5648 | J003135.1$-$564640 | 0.008 |  $\cdots$ |  $\cdots$
J0057.9+6326 | J005758.1+632642 | 0.006 | blazar candidate | 0.18phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0156.3$-$2420 | J015624.6$-$242003 | 0.007 | blazar candidate | Chang et al. (2019)
J0159.0+3313 | J015905.0+331255 | 0.008 |  $\cdots$ |  $\cdots$
J0231.0+3505 | J023112.2+350445 | 0.006 |  $\cdots$ |  $\cdots$
J0301.6$-$5617 | J030115.1$-$561644 | 0.008 |  $\cdots$ |  $\cdots$
J0302.5+3354 | J030226.7+335448 | 0.007 |  $\cdots$ |  $\cdots$
J0327.6+2620 | J032737.2+262008 | 0.007 |  $\cdots$ |  $\cdots$
J0409.2+2542 | J040921.6+254440 | 0.008 |  $\cdots$ |  $\cdots$
J0610.8$-$4911 | J061031.8$-$491222 | 0.007 |  $\cdots$ |  $\cdots$
J0610.8$-$4911 | J061100.0$-$491034 | 0.007 | QSO | Warwick et al. (2012)
J0620.7$-$5034 | J062045.7$-$503350 | 0.007 | BL Lac | 0.25phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0633.9+5840 | J063400.1+584036 | 0.009 | blazar candidate | 0.29phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0650.6+2055 | J065035.4+205557 | 0.006 | BL Lac | 0.3phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0704.3$-$4829 | J070421.8$-$482648 | 0.009 |  $\cdots$ |  $\cdots$
J0800.9+0733 | J080056.5+073235 | 0.007 | blazar candidate | 0.44phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0827.0$-$4111 | J082705.4$-$411159 | 0.009 |  $\cdots$ |  $\cdots$
J0838.5+4013 | J083903.0+401546 | 0.006 | BL Lac | 0.19; Fig 8b
J0903.5+4057 | J090342.8+405503 | 0.009 | QSO | 0.89; Fig 8c
J0910.1$-$1816 | J091003.9$-$181613 | 0.007 | blazar candidate | 0.45phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0914.5+6845 | J091429.7+684509 | 0.007 | blazar candidate | 0.45phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0928.4$-$5256 | J092818.7$-$525701 | 0.007 |  $\cdots$ |  $\cdots$
J0930.9$-$3030 | J093057.9$-$303118 | 0.007 |  $\cdots$ |  $\cdots$
J1011.1$-$4420 | J101132.0$-$442255 | 0.007 |  $\cdots$ |  $\cdots$
J1016.1$-$4247 | J101620.7$-$424723 | 0.007 |  $\cdots$ |  $\cdots$
J1024.5$-$4543 | J102432.5$-$454428 | 0.007 | BL Lac | 0.37phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1048.4$-$5030 | J104824.2$-$502941 | 0.006 |  $\cdots$ |  $\cdots$
J1146.0$-$0638 | J114600.8$-$063851 | 0.006 | BL Lac | 0.64phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1155.2$-$1111 | J115514.7$-$111125 | 0.007 | BL Lac | 0.47phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1220.1$-$2458 | J122014.5$-$245949 | 0.007 | blazar candidate | 0.48phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1243.7+1727 | J124351.8+172645 | 0.007 | QSO | 0.94** The redshift values provided by the SDSS$-$DR16 and SIMBAD catalogs are different; 0.94 and 2.14 respectively. In addition, the redshift estimate has varied from 0.5 to 2.14 in different data releases for SDSS. The value provided in this table corresponds to the latest data release, DR16.; Fig 9b
J1545.0$-$6642 | J154458.9$-$664147 | 0.006 | BL Lac | 0.23phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1557.2+3822 | J155711.9+382032 | 0.008 |  $\cdots$ |  $\cdots$
J1631.8+4144 | J163146.7+414633 | 0.006 | galaxy | 0.71; Fig 9e
J1651.7$-$7241 | J165151.5$-$724310 | 0.007 |  $\cdots$ |  $\cdots$
J1720.6$-$5144 | J172032.7$-$514413 | 0.008 |  $\cdots$ |  $\cdots$
J1910.8+2856 | J191052.2+285624 | 0.007 | BL Lac | 0.3phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1910.8+2856 | J191059.4+285635 | 0.006 | CV | Pourbaix et al. (2004)
J1918.0+0331 | J191803.6+033030 | 0.006 | blazar candidate | 0.23phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1927.5+0154 | J192731.3+015357 | 0.006 |  $\cdots$ |  $\cdots$
J2046.9$-$5409 | J204700.7$-$541245 | 0.007 |  $\cdots$ |  $\cdots$
J2109.6+3954 | J210936.4+395513 | 0.007 |  $\cdots$ |  $\cdots$
J2114.9$-$3326 | J211452.1$-$332533 | 0.007 |  $\cdots$ |  $\cdots$
J2159.6$-$4620 | J215936.1$-$461953 | 0.006 | blazar candidate | 0.52phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J2207.1+2222 | J220704.1+222232 | 0.007 | blazar candidate | 0.55; Fig 10b
J2225.8$-$0804 | J222552.9$-$080416 | 0.008 |  $\cdots$ |  $\cdots$
J2237.2$-$6726 | J223709.3$-$672614 | 0.008 |  $\cdots$ |  $\cdots$
J2303.9+5554 | J230351.7+555618 | 0.006 |  $\cdots$ |  $\cdots$
J2317.7+2839 | J231740.0+283954 | 0.007 | galaxy | 0.57 ; Fig 10c
|  |  | ambiguous candidates from NN results |
J0004.4$-$4001 | J000434.2$-$400035 | 0.018 | BL Lac | 0.10phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J0025.4$-$4838 | J002536.9$-$483810 | 0.014 |  $\cdots$ |  $\cdots$
J0037.2$-$2653 | J003729.5$-$265045 | 0.597 |  $\cdots$ |  $\cdots$
J0058.3$-$4603 | J005806.3$-$460419 | 0.011 |  $\cdots$ |  $\cdots$
J0118.3$-$6008 | J011824.0$-$600753 | 0.035 |  $\cdots$ |  $\cdots$
J0120.2$-$7944 | J011914.7$-$794510 | 0.947 |  $\cdots$ |  $\cdots$
J0125.9$-$6303 | J012548.1$-$630245 | 0.022 |  $\cdots$ |  $\cdots$
J0209.8+2626 | J020946.5+262528 | 0.101 | blazar candidate | 0.68; Fig. 8a
J0240.2$-$0248 | J024004.6$-$024505 | 0.406 |  $\cdots$ |  $\cdots$
J0259.0+0552 | J025857.6+055244 | 0.01 |  $\cdots$ |  $\cdots$
J0406.2+0639 | J040607.7+063919 | 0.244 |  $\cdots$ |  $\cdots$
J0427.8$-$6704binbin this source is classified as binary source in the most recent third data release 4FGL catalog (4FGL$-$DR3) (Abdollahi et al., 2022) | J042749.6$-$670435 | 0.141 |  $\cdots$ |  $\cdots$
J0537.5+0959 | J053745.9+095759 | 0.938 |  $\cdots$ |  $\cdots$
J0539.2$-$6333 | J054002.9$-$633216 | 0.015 |  $\cdots$ |  $\cdots$
J0544.8+5209 | J054424.5+521513 | 0.959 |  $\cdots$ |  $\cdots$
J0738.6+1311 | J073843.4+131330 | 0.695 |  $\cdots$ |  $\cdots$
J0800.1$-$5531 | J075949.3$-$553254 | 0.52 |  $\cdots$ |  $\cdots$
J0800.1$-$5531 | J080013.1$-$553408 | 0.29 |  $\cdots$ |  $\cdots$
J0906.1$-$1011 | J090616.1$-$101426 | 0.015 |  $\cdots$ |  $\cdots$
J0934.5+7223 | J093333.7+722101 | 0.3 |  $\cdots$ |  $\cdots$
J0938.8+5155 | J093834.8+515453 | 0.014 | QSO | 0.41; Fig 8d
J1008.2$-$1000 | J100848.6$-$095450 | 0.972 | Seyfert I | 0.05(Jones et al., 2009)
J1016.2$-$5729 | J101625.7$-$572807 | 0.461 |  $\cdots$ |  $\cdots$
J1018.1$-$2705 | J101750.2$-$270550 | 0.647 |  $\cdots$ |  $\cdots$
J1018.1$-$4051 | J101801.4$-$405519 | 0.951 |  $\cdots$ |  $\cdots$
J1018.1$-$4051 | J101807.6$-$404408 | 0.966 |  $\cdots$ |  $\cdots$
J1034.7$-$4645 | J103438.7$-$464405 | 0.017 | blazar candidate | 0.33phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1049.8+2741 | J104938.8+274213 | 0.012 | radio galaxy | 0.14; Fig 8e
J1106.7+3623 | J110636.5+362650 | 0.963 | QSO | 1.55; Fig 8f
J1111.4+0137 | J111114.2+013431 | 0.125 | radio galaxy | 0.26; Fig 9a
J1119.9$-$1007 | J111948.4$-$100707 | 0.029 |  $\cdots$ |  $\cdots$
J1122.0$-$0231 | J112213.7$-$022914 | 0.191 | BL Lac |  $\cdots$
J1256.8+5329 | J125630.5+533205 | 0.984 | QSO | 0.99; Fig 9c
J1320.3$-$6410 | J132015.9$-$641349 | 0.71 |  $\cdots$ |  $\cdots$
J1326.0+3507 | J132622.2+350625 | 0.055 | QSO | 1.04; Fig 9d
J1326.0+3507 | J132544.4+350450 | 0.028 |  $\cdots$ |  $\cdots$
J1415.9$-$1504 | J141546.1$-$150229 | 0.015 |  $\cdots$ |  $\cdots$
J1429.8$-$0739 | J142949.5$-$073305 | 0.36 |  $\cdots$ |  $\cdots$
J1513.0$-$3118 | J151244.8$-$311647 | 0.496 | Rot Variable Star | Kiraga (2012)
J1514.8+4448 | J151451.0+444957 | 0.831 |  $\cdots$ |  $\cdots$
J1528.4+2004 | J152835.8+200421 | 0.013 | blazar candidate | 0.52phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J1623.7$-$2315 | J162334.1$-$231750 | 0.958 |  $\cdots$ |  $\cdots$
J1644.8+1850 | J164457.2+185150 | 0.015 |  $\cdots$ |  $\cdots$
J1645.0+1654 | J164459.8+165513 | 0.031 |  $\cdots$ |  $\cdots$
J1650.9$-$4420 | J165124.2$-$442142 | 0.692 |  $\cdots$ |  $\cdots$
J1818.5+2533 | J181831.2+253707 | 0.721 |  $\cdots$ |  $\cdots$
J1846.9$-$0227 | J184650.7$-$022904 | 0.469 |  $\cdots$ |  $\cdots$
J1955.3$-$5032 | J195512.5$-$503012 | 0.487 |  $\cdots$ |  $\cdots$
J2008.4+1619 | J200827.6+161844 | 0.014 |  $\cdots$ |  $\cdots$
J2041.1$-$6138 | J204112.0$-$613949 | 0.012 |  $\cdots$ |  $\cdots$
J2222.9+1507 | J222253.9+151055 | 0.013 |  $\cdots$ |  $\cdots$
J2240.3$-$5241 | J224017.7$-$524113 | 0.013 | blazar candidate | 0.25phph photometric redshift from the High$-$Synchrotron Peaked blazars catalog by Chang et al. (2019)
J2247.7$-$5857 | J224745.0$-$585501 | 0.552 |  $\cdots$ |  $\cdots$
J2311.6$-$4427 | J231145.6$-$443221 | 0.073 |  $\cdots$ |  $\cdots$
J2326.9$-$4130 | J232653.1$-$412711 | 0.938 |  $\cdots$ |  $\cdots$
J2336.9$-$8427 | J233627.1$-$842648 | 0.01 |  $\cdots$ |  $\cdots$
J2337.7$-$2903 | J233730.2$-$290241 | 0.017 |  $\cdots$ |  $\cdots$
J2351.4$-$2818 | J235136.5$-$282154 | 0.159 | Brightest Galaxy in the Cluster | Lauer et al. (2014)


# On the Relationship between CH3OD Abundance and Temperature in the Orion KL
Nebula

Olivia H. Wilkins  Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, CA 91125 USA<EMAIL_ADDRESS>Geoffrey A. Blake  Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, CA 91125 USA

###### Abstract

The relative abundances of singly-deuterated methanol isotopologues,
[CH2DOH]/[CH3OD], in star-forming regions deviate from the statistically
expected ratio of 3. In Orion KL, the nearest high-mass star-forming region to
Earth, the singly-deuterated methanol ratio is about 1, and the cause for this
observation has been explored through theory for nearly three decades. We
present high-angular resolution observations of Orion KL using the Atacama
Large Millimeter/submillimeter Array to map small-scale changes in CH3OD
column density across the nebula, which provide a new avenue to examine the
deuterium chemistry during star and planet formation. By considering how CH3OD
column densities vary with temperature, we find evidence of chemical processes
that can significantly alter the observed column densities. The astronomical
data are compared with existing theoretical work and support D-H exchange
between CH3OH and heavy water (i.e., HDO and D2O) at methanol’s hydroxyl site
in the icy mantles of dust grains. The enhanced CH3OD column densities are
localized to the Hot Core-SW region, a pattern that may be linked to the
coupled evolution of ice mantel chemistry and star formation in giant
molecular clouds. This work provides new perspectives on deuterated methanol
chemistry in Orion KL and informs considerations that may guide future
theoretical, experimental, and observational work.

Current address: NASA Postdoctoral Program Fellow, NASA Goddard Space Flight
Center, Greenbelt, MD, 20771 USA Division of Geological and Planetary
Sciences, California Institute of Technology, Pasadena, CA 91125 USA

##  1 Introduction

The relative abundances of site-specific stable isotopologues, particularly
those involving deuterated compounds, are powerful tools that can be used to
trace chemical evolution in the interstellar medium, and during star and
planet formation. For example, the relative abundances of heavy water (i.e.,
HDO and D2O) in comets and meteorites can provide insights into the processing
of water between the primordial molecular cloud and present-day Earth.1, 2
Interstellar compounds such as N2H+ and CH3OH have D/H ratios that are higher
than the cosmic value of ${\sim}10^{-5}$,3 and that ratio is a function of
temperature, with higher D/H ratios signalling formation in colder, typically
denser, environments.4

In many high-mass and low-mass protostars alike, the deuterium chemistry of
methanol—namely, the relative abundances of the singly-deuterated isotopomers
CH2DOH and CH3OD—has presented itself as a mystery. Methanol is one of the
simplest complex (having $\geq$6 atoms) organic molecules, and it is found at
every stage of star formation, from cold cloud cores and hot cores/corinos to
outflows and circumstellar disks. 5, 6 As such, it is commonly used as a
tracer of other complex organics.

In prestellar and protostellar cores, methanol forms primarily via successive
hydrogenation of frozen CO on grain mantles (eq 1).7, 8

$\ch{CO->[H]HCO->[H]H2CO->[H]H3CO->[H]CH3OH}$ (1)

Statistically, we would expect the [CH2DOH]/[CH3OD] ratio to be 3 since there
are three methyl hydrogen sites compared to a single hydroxyl site. This
statistical ratio has been observed in the massive star-forming region NGC
7538-IRS1 but not toward many other star-forming regions.9 Low-mass cores
generally exhibit ratios ${>}3$ and as much as $\utilde{>}$10,10, 11 while
high-mass protostars tend to have [CH2DOH]/[CH3OD] ratios of ${<}3$.12, 13, 14
Astrochemical models have predicted that the [CH2DOH]/[CH3OD] ratio should be
$\geq$10 in prestellar cores and that CH3OD is only efficiently formed on icy
grains at later evolutionary stages when the ices are warmed due to the
presence of young (proto)stars.15 Deviations from the statistical ratio in
high-mass star-forming regions have also been attributed to grain surface
chemistry,16, 17 but investigations into the intricacies of such processes—and
the potential role of gas processing—are ongoing.

The Orion Kleinmann-Low (Orion KL) nebula is a high-mass star-forming region
notable for its peculiar methanol deuteration. At a distance of $\sim$388
pc,18 Orion KL is uniquely situated to explore the relationship between
relative deuterated methanol abundances and environmental conditions because
sub-environments within the nebula can be resolved, even with modest imaging
capabilities. The two most well-studied regions within Orion KL are the Hot
Core and Compact Ridge. The Hot Core region contains denser and warmer gas
($n_{\rm H_{2}}\sim 10^{7}$ cm-3, $T_{\rm kin}\sim 200$ K) whereas the Compact
Ridge, to the southwest,***In astronomical maps, north is up, east is left,
and west is right. is cooler and less dense ($n_{\rm H_{2}}\utilde{<}10^{6}$
cm-3, $T_{\rm kin}\sim 100{-}150$ K).19, 20 These regions are also the
prominent sites of nitrogen-bearing and oxygen-bearing compounds,
respectively.19, 21 Extending southwest from the Hot Core toward the Compact
Ridge is the Hot Core-SW, which is physically and chemically heterogeneous,
with possible sources of internal heating22 and both oxygen- and nitrogen-
bearing compounds.23, 24 Flanking these regions are compact sources, such as
Source I—an edge-on disk thought to be internally heated25, 26—to the west
within the Hot Core region and the (sub)millimeter sources SMA1 and C22—a
protostar and possible hot core, respectively27, 21—to the southwest of Source
I and along the northwestern edge of the Hot Core-SW.

Jacq et al. 28 reported the first definitive detection of CH2DOH toward Orion
KL on angular scales between 12′′ and 26′′ (centered on the Hot Core and
Source I region). They combined their measurements with past CH3OD
measurements to report a [CH2DOH]/[CH3OD] ratio in the range of 1.1-1.5. Neill
et al. 29 similarly reported a ratio of $1.2\pm 0.3$ based on local
thermodynamic equilibrium models of ${\sim}30{-}44^{\prime\prime}$
observations of the Hot Core and the Compact Ridge, while Peng et al. 30
reported an even lower ratio of $0.7\pm 0.3$ toward Orion KL, using an angular
resolution of $3.\\!\\!^{\prime\prime}6\times 2.\\!\\!^{\prime\prime}3$.

There has been extensive debate about whether the apparent CH3OD enhancements
are the result of grain-surface or gas-phase processes. Early on, Jacq et al.
28 concluded that their observed ratio was evidence of grain-surface
processing followed by injection into the gas phase, perhaps by thermal
desorption. Shortly after, chemical models of gas-phase exchange rejected the
grain-surface hypothesis on the premise that such chemistry would require
unrealistically high [HDO]/[H2O] ratios.31 Rodgers and Charnley 32 criticized
the assumed statistical [CH2DOH]/[CH3OD] ratio of 3 since D and H react with
species other than CO and H2CO, which could affect the relative abundances of
the singly-deuterated methanol isotopologues. Osamura et al. 33 used models to
suggest that ion-molecule reactions in the gas phase lead to the loss of
CH3OD, which they conclude accounts for high [CH2DOH]/[CH3OD] ratios in low-
mass star-forming regions, provided methanol is efficiently regenerated in the
dissociative recombination of protonated methanol with electrons. This work
also finds that D-H exchange on the methyl site is inefficient.

Nevertheless, D-H exchange at the hydroxyl group of methanol on icy grain
mantles has emerged as a favored explanation for the [CH2DOH]/[CH3OD] ratios
observed in massive star-forming regions. 34, 17, 30 In this mechanism,
deuterated water in the ice reacts with CH3OH to produce CH3OD:

$\ch{CH3OH+HDO<=>CH3OD+H2O}$ (2) $\ch{CH3OH+D2O<=>CH3OD+HDO}$ (3)

However, the intricacies of this exchange are still being investigated.

This work provides a new observational perspective on the possibility of D-H
exchange at the methanol hydroxyl site by mapping gas-phase CH3OD abundances
in Orion KL at sub-arcsecond ($\sim$0$.\\!\\!^{\prime\prime}$7) angular
resolution using the Atacama Large Millimeter/submillimeter Array (ALMA),
which corresponds to linear scales of $\sim$270 au at the nebula’s distance.
This allows us to plot gas-phase CH3OD as a function of the local line-of-site
temperature across relatively small scales within the nebula and explore
temperature-dependent chemical processes that may affect the observed CH3OD
chemistry.

##  2 Methods

###  2.1 Observations

Observations of Orion KL were taken in ALMA Band 4 during Cycle 5 (project
code: ADS/JAO.ALMA#2017.1.01149, PI: Wilkins) on 2017 December 14, completely
on the main 12-m array. The pointing center was set to
$\alpha_{\mbox{\scriptsize J2000}}=05^{\mbox{\scriptsize
h}}35^{\mbox{\scriptsize m}}14.\\!\\!^{\mbox{\scriptsize s}}50$,
$\delta_{\mbox{\scriptsize
J2000}}=-05^{\circ}22^{\prime}30.\\!\\!^{\prime\prime}9$. These observations
employed 49 antennas during one execution block. All spectra were obtained in
a single local oscillator set-up consisting of 10 spectral windows; as such,
the uncertainties for quantities derived from these spectra are dominated by
thermal noise and mostly unaffected by calibration uncertainty. Of these
spectral windows, the targeted CH3OD lines were contained in three spectral
windows with a spectral resolution of 244 kHz ($\sim$0.5 km s-1) covering
143.51-143.97, 153.16-153.40, and 154.84-155.07 GHz. Projected baselines were
between 15.1 m and 3.3 km (7.6 and 1650 k$\lambda$, where $\lambda\sim 2$ mm
is the wavelength), and the primary beam was 39.1′′. The on-source integration
time was 2062 s. Precipitable water vapor was 3.7 mm, and typical system
temperatures were around 75-125 K.

Calibration was completed using standard CASA (version 5.1.1-5) calibration
pipeline scripts. The source J0423$-$0120 was used as a calibrator for
amplitude, atmosphere, bandpass, pointing, and WVR (Water Vapor Radiometer)
variations, and J0541$-$0211 was used as the phase and WVR calibrator.

The CH3OD data introduced here were prepared in the same way as the ^13CH3OH
images presented by Wilkins et al. 22 In brief, the data cubes were created
from measurement sets split to include only baselines of $\leq$500 m,
resulting in a synthesized beam of $0.\\!\\!^{\prime\prime}74\times
0.\\!\\!^{\prime\prime}63$. Cubes were reduced with continuum emission
estimated from line-free channels subtracted using the uvcontsub function
followed by imaging using the tclean algorithm with robust weighting, a Briggs
parameter of 1.5 (i.e., semi-natural weighting) for deconvolution, and the
‘auto-multithresh’ masking algorithm 35 in conjunction with interactive
tclean. The images have a noise-level of $\sigma_{\mbox{\scriptsize RMS}}\sim
1.3$ mJy beam-1.

###  2.2 Deriving CH3OD Parameters

The CH3OD column density as a function of position was derived via pixel-by-
pixel fits of the CH3OD transitions shown listed in Table 1 assuming optically
thin lines (see Table S1 of the Supporting Information) in local thermodynamic
equilibrium (LTE).†††Python script available at
https://github.com/oliviaharperwilkins/LTE-fit. Integrated intensity maps of
each transition are providing in the Supporting Information (Figure S2). For
each coordinate-space pixel in the data cubes, a spectrum within a single
synthesized beam centered on that pixel was extracted. The CH3OD rotational
temperature ($T_{\rm rot}$) profile was assumed to be the same as that
previously derived22 from ^13CH3OH since the transitions of both isotopologues
have similar upper energy states $E_{u}$. Column density, line width
(${\sim}0.8{-}3.5$ km s-1), and local standard of rest (LSR) velocity
(${\sim}7{-}9$ km s-1) were determined by simultaneous fits using LMFIT, a
least-squares fitting software package.‡‡‡https://doi.org/10.5281/zenodo.11813
Because all lines used in the fit were observed simultaneously, the
uncertainties in excitation, which are derived from relative fluxes, should be
dominated by thermal noise rather than by multiple sources of calibration
uncertainty.

Table 1: Transitions of CH3OD used for line fits.⋆

Transition | $\nu$ | $E_{u}$ | $S_{ij}\mu^{2}$ | $g_{u}$
---|---|---|---|---
(GHz) | (K) | (Debye2)
$5_{(1,4)}-5_{(0,5)}$ A | 143.7417 | 39.48 | 11.2 | 11
$7_{(1,6)}-7_{(0,7)}$ A | 153.3240 | 68.05 | 14.7 | 15
$3_{(-1,2)}-2_{(0,1)}$ E | 154.9628 | 17.71 | 2.2 | 7

⋆From Anderson et al. 36 Column (2): rest frequency $\nu$ of the transitions;
(3): upper state energy $E_{u}$; (4): product of the transition line strength
and the square of the electric dipole moment $S_{ij}\mu^{2}$; (5): upper-state
degeneracy $g_{u}$

##  3 Results and Discussion

###  3.1 CH3OD Column Density

Figure 1: Derived CH3OD column density and percent propagated uncertainty
shown by the color maps in the upper and lower panels, respectively. The 2 mm
($\sim$150 GHz) continuum emission is shown by the grey contours at
$2\sigma_{RMS},4\sigma_{RMS},8\sigma_{RMS},16\sigma_{RMS},32\sigma_{RMS},64\sigma_{RMS}$.
The Hot Core (HC), Source I (I), IRc7, and IRc4 are shown in red; SMA1 and C22
are shown by the teal diamonds; the methyl formate emission peak (MF1)24 is
labeled in yellow, and the Compact Ridge (CR) is labeled in blue. The
0$.\\!\\!^{\prime\prime}$7 synthesized beam is shown by the black ellipse in
the bottom left corner.

The column density profile derived from a pixel-by-pixel fit of the ALMA data
image cubes was used to show small-scale variations in CH3OD column densities
across Orion KL. As shown in Figure 1, the derived CH3OD column density
($N_{\rm tot}$) is generally on the order of $10^{17}$ cm-2 and peaks south of
SMA1 and C22. In general, the uncertainties (standard errors calculated using
LMFIT) for these values are $<$10% throughout the region southwest of the Hot
Core (Hot Core-SW), which is the region of interest for most of the discussion
in this work; higher uncertainties, up to $\sim$25%, characterize IRc4 and the
western (left) edge of the Compact Ridge. The relationship between the CH3OD
and ^13CH3OH column densities (Figure S3 in the Supporting Information) and
rotational temperature ($T_{\rm rot}$, Figure S4 in the Supporting
Information) is illustrated by Figures 2a and 2b. In general,
[CH3OD]/[^13CH3OH] falls between 1.5 and 5.3. Assuming a local interstellar
^12C/^13C ratio of $68\pm 15$,37 this suggests [CH3OD]/[CH3OH] $\approx
0.015$-$0.104$, which encompasses the ratio of 0.01-0.06 in Orion KL reported
by Mauersberger et al. 38 on much larger angular scales of 15′′-23′′. The
discrepancy on the higher end of these ranges may be the result of CH3OD
abundance enhancements on smaller spatial scales being diluted in the single-
dish data, but otherwise, the agreement suggests that the flux recovered in
our observations is representative of that collected by single-dish
observations. A comparison of [CH3OD]/[^12CH3OH] ratios derived from different
assumed ^12C/^13C values is presented in Table S2 of the Supporting
Information.

(a)

(b)

Figure 2: (a) Column densities $N_{\rm tot}$ of ^13CH3OH (horizontal axis,
from Wilkins et al. 22) and CH3OD (vertical axis, this work) with each point
representing a single pixel in Figure 1. Each pixel is colored by its
rotational temperature. The grey lines are labeled by the [CH3OD]/[^13CH3OH]
ratios (1 to 5) they represent. (b) Two-dimensional histogram (50 points per
bin) showing the methanol [CH3OD]/[^13CH3OH] ratios plotted as a function of
temperature.

(a)

(b)

Figure 3: (a) Two-dimensional histogram (50 points per bin) showing the
derived CH3OD column densities $N_{\rm tot}$ against rotational temperature
$T_{\rm rot}$ of ^13CH3OH. The blue solid line shows the power-law fit to the
data if there were no D-H exchange (eq 4). The black dashed line shows the
modeled D-H exchange (eq 5) followed by desorption based on ice experiments by
Souda et al. 39 and assumptions by Faure et al. 40 The blue dotted curve is
the power-law fit after the CH3OD enhanced by D-H exchange thermally desorbs
off the grains. (b) The solid curve shows the density profile with the
underlying power-law (blue curve in a, eq 4) subtracted. The dotted curve
shows the sputtered CH3OD2+ profile reported by Souda et al. 39 but scaled for
comparison to the CH3OD column density profile in this work.

Figure 3a, in which each point represents 50 binned pixels for which a column
density and rotational temperature pair were derived, shows that the CH3OD
column density increases with rotational temperature, which is characteristic
of thermal desorption in which material is sublimed from the grains as the
environment warms. 41 However, the profile also contains a “shark-tooth”
feature where the column density starts to rise more steeply at $\sim$110 K
before peaking close to 185 K. At temperatures higher than 185 K, there is a
sharp decrease in the CH3OD column density, and the relationship between
column density and rotational temperature returns to the underlying trend.

Power-law distributions are commonly used to characterize the temperature and
density profiles of star-forming regions and young stellar objects (YSOs).42,
43 In Figure 3a, the fitted underlying power-law relationship between $T_{\rm
rot}$ and $N_{\rm tot}$, described by eq 4, is shown by the solid blue line.

$N_{\rm tot}=1.5\times 10^{14}T_{\rm rot}^{1.4148}$ (4)

The solid black line in Figure 3b shows the “shark-tooth” from Figure 3a with
the underlying power-law between $T_{\rm rot}$ and $N_{\rm tot}$ subtracted.

###  3.2 Grain-Surface Processes

The rapid rise in gas-phase CH3OD column density between $\sim$110 K and
$\sim$120 K is consistent with D-H exchange between methanol and heavy water
(HDO, D2O) on the ices at $\sim$100 K. Souda et al. 39 experimentally
investigated hydrogen bonding between water and methanol in low-temperature
ices warmed from 15 K to 200 K under ultrahigh vacuum conditions. They
observed that when CH3OH was adsorbed onto D2O ice, secondary CH3OD2+
ions—evidence of D-H exchange at the hydroxyl site—sputtered off the ice
analogue surfaces predominantly between 140 and 175 K. Follow-up analyses by
Kawanowa et al. 44 describes this as a “rapid and almost complete H/D
exchange” to yield the sputtered CH3OD2+ species. The fact that we see a
similar sudden increase in CH3OD column densities at similar temperatures
(Figure 3b, dotted line), with discrepancies owing to the differences in
pressure between ultrahigh vacuum and even the densest regions of interstellar
medium, supports a similar rapid exchange in Orion KL.

Models of D-H exchange between water and methanol in ices by Faure et al. 40
successfully reproduced gas-phase CH3OH deuterium fractionation in Orion KL
using initial ice abundances of $n_{S}(\ch{CH3OH})=2.0\times
10^{-6}n_{\ch{H}}$, $n_{S}(\ch{HDO})=3.0\times 10^{-7}n_{\ch{H}}$, and
$n_{S}(\ch{CH3OD})=6.0\times 10^{-9}n_{\ch{H}}$. Taking these initial ice
abundances, we modeled the change in gas-phase CH3OD column density following
rapid D-H exchange on the ices and subsequent desorption. Specifically, we
assumed an initial ice column density of $N_{S}(\ch{CH3OD})=6.0\times
10^{-9}N_{\ch{H}}=6.0\times 10^{14}\mbox{ cm}^{-2}$, since $N_{\ch{H}}\sim
10^{23}$ cm-2 across Orion KL (including the Hot Core, Compact Ridge, and
Extended Ridge),45, 46 and an initial gas-phase column density of
$N(\ch{CH3OD})=1\times 10^{16}\mbox{ cm}^{-2}$, based on the column densities
measured at 100 K in this work after subtracting the underlying power-law in
eq 4.

The enhancement of gas-phase CH3OD column density from D-H exchange with water
was then modeled. In the absence of directly analogous temperature-programmed
desorption measurements of CH3OH and CH3OD themselves, we fit the CH3OD2+
curve of Souda et al. 39 (Figure 3b, dotted line) between 110 K and 145 K. The
relative intensity amplitude was normalized to the column densities observed
for CH3OD in our Orion KL observations, resulting in a relationship of the
form

$N_{S}^{\prime}(\ch{CH3OD})=6.2\times 10^{14}T-6.0\times 10^{16}$ (5)

where $N_{S}^{\prime}$ is the additional (solid) CH3OD available [cm-2] for
desorption at a given temperature $T$. The desorption rate coefficient is
expressed as

$k_{des}=\nu_{des}e^{(-E_{d}/T)}$ (6)

where the pre-exponential factor $\nu_{des}$ and the binding energy $E_{d}$
are taken to be approximately the values for annealed amorphous solid
water—$2.0\times 10^{12}$ s-1 and 5200 K, respectively—under the assumption
that the methanol desorbs with water, which is in excess.47, 48, 49, 50, 40,
51

It follows that the change in gas-phase $N(\ch{CH3OD})$ is approximated by
multiplying the rate coefficient (eq 6) by the total solid CH3OD column
density, which includes the additional solid CH3OD available following D-H
exchange (eq 5) at temperature $T$. Thus, the rate at which $N(\ch{CH3OD})$
changes in the gas phase between 100 and 150 K is approximated by

$\frac{d}{dt}N(\ch{CH3OD})=k_{des}\left[N_{S}(\ch{CH3OD})+N_{S}^{\prime}(\ch{CH3OD})\right]$
(7)

using temperature steps of 1 K, the corresponding time steps for which were
determined by

$\Delta t=\left(\frac{T-T_{0}}{T_{max}-T_{0}}\right)^{1/n}t_{h}$ (8)

where $\Delta t$ is the time elapsed since $t=0$; $T_{0}$ and $T_{max}$ are
the initial (10 K) and maximum (300 K) temperatures, respectively; $t_{h}$ is
the heating timescale; and $n$ is the order of heating, which is assumed to be
2 following the previous work.52 The initial gas and solid CH3OD column
densities were assumed, respectively, to be $N(\ch{CH3OD})=9.0\times 10^{15}$
cm-2 (approximated from the CH3OD density profile with the underlying power-
law subtracted) and $N_{S}(\ch{CH3OD})=6.0\times 10^{-9}N_{\ch{H}}$ cm-2
(based on assumptions used by Faure et al. 40 in their D-H exchange models).

The resulting desorption model from eq 7 with $t_{h}=10^{3}$ yr is shown by
the black dashed line in Figure 3a. Longer timescales (i.e., $t_{h}\geq
10^{4}$ yr) characteristic of massive YSOs do not follow the increasing CH3OD
profile as closely. Although potential internal heating sources have been
suggested within the Hot Core-SW,22 Li et al. 53 conclude that this region,
part of the “elongated ridge” comprising the Hot Core and Source I, is
predominantly heated externally by shocks induced by the Orion KL explosion,
which took place about 500 years ago. Furthermore, the models by Faure et al.
40 suggest the D-H exchange in Orion KL reaches steady-state in ${<}10^{3}$
yr. After D-H exchange, the power-law relationship from eq 4 applies, but with
a coefficient of $3.2\times 10^{14}$ (dotted blue curve in Fig. 3a).

The hypothesis that the CH3OD column density profile in Figure 3 is the result
of rapid D-H exchange on the grains relies on two assumptions regarding
temperature. First, we assume that the rotational temperature $T_{\rm rot}$ is
an appropriate estimate for the kinetic temperature $T_{kin}$. This assumption
is based on the fact that the Compact Ridge, a spatial component toward the
southwestern region of Orion KL that is characterized as being rich in oxygen-
bearing molecules, has a fairly high density of ${\sim}10^{6}$ cm-3, 19, 20
implying that LTE is a reasonable assumption. Second, we assume that the dust
and gas are thermally coupled. Li et al. 54 found that other quiescent regions
(no infrared sources, no evident outflows) in the Orion Molecular Cloud are
thermally coupled. Models by Bruderer et al. 55 also support coupling between
dust and gas temperature at the relevant densities. This gas-grain thermal
coupling was also demonstrated in models of several massive star-forming
regions, including the Orion KL Compact Ridge, by Garrod and Herbst.56 As
such, it is reasonable to assume that the gas temperatures shown in Figure 3
are also representative of the temperatures of the dust, on which methanol
forms, and that any decoupling between the dust and gas is negligible for the
purposes of this discussion.

###  3.3 Enhanced Deuteration in the Hot Core-SW

Above $\sim$180 K, the profile in Figure 3a drops sharply. We attribute this
to the enhanced deuteration being localized to the Hot Core-SW and that there
simply is little gas above 180 K in this region. Figure 4a shows the
distribution of CH3OD column densities minus the underlying power-law
relationship (eq 4). Figure 4b maps [CH3OD]/[^13CH3OH], confirming that the
profile in Figure 4a is indeed the result of excess CH3OD in the Hot Core-SW
and not enhanced abundances of methanol in general.

(a)

(b)

Figure 4: (a)Map of gas-phase CH3OD excess column density across Orion KL
after subtracting the underlying power-law relationship (eq 4) between
rotational temperature and column density. Darker shading indicates a larger
deviation from the underlying power-law. (b) Map of [CH3OD]/[^13CH3OH].

We considered two chemical explanations for this trend, neither of which
adequately explain the observed patterns. One avenue that has been proposed
for CH3OD depletion is gas-phase D-H exchange via protonation of the hydroxyl
group (eqs 9 and 10) followed by dissociative recombination (eqs. 11 and
12).33.

$\ch{CH3OD+H3O+->CH3ODH++H2O}$ (9)
${\color[rgb]{0,0,0}\definecolor[named]{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@gray@stroke{0}\pgfsys@color@gray@fill{0}\ch{CH2DOH+H3O+->CH2DOH2++H2O}}$
(10) $\ch{CH3ODH++e-->CH3OH+D},\ch{CH3OD+H}$ (11) $\ch{CH2DOH2++e-->CH2DOH+H}$
(12)

The methyl H/D site is not exchangeable; therefore, in this model, only CH3OD
can be depleted while CH2DOH cannot, which has been suggested as an
explanation for the low relative CH3OD abundances in low-mass star-forming
regions.33 However, this is unlikely to account for the decrease in gas-phase
CH3OD at warmer temperatures in the Orion KL Hot Core-SW following enrichment
on grain surfaces because dissociative recombination reactions tend to destroy
gas-phase methanol (and its isotopologues), with methanol production
comprising the smallest branching ratio (3%) listed in KIDA57. A drop in the
^13CH3OH column density at temperatures above 180 K is not seen in Orion KL
(see Figure S5 in the Supporting Information), suggesting that the observed
decrease of the [CH3OD]/[^13CH3OH] ratio (Figure 2b) and of the CH3OD column
density (Figure 3a) above 180 K cannot simply be explained by protonation of
the methanol (e.g., by H3O+) followed by dissociative recombination.

Another mechanism considered was the neutral-radical reaction with the
hydroxyl radical (OH); however, this reaction is too slow at $\sim$200 K to
account for the observed patterns (see Section S4 of the Supporting
Information).

Instead, we propose that the drop in enhanced CH3OD column density is the
result of environment rather than chemistry. That is, the enhanced CH3OD
column density profile is limited to warm gas in the Hot Core-SW with
temperatures that max out around 200 K. The enhanced deuteration in this
region, brought about by temperature-dependent surface D-H exchange, may be
the result of evolutionary state. However, this region is not directly
associated with any known YSOs, including SMA1 (a young, high-mass protostar)
and C22 (a possible hot core).27, 21 Although there is no known self-
illuminated source (e.g., embedded protostar) in the Hot Core-SW to drive
grain warming and associated D-H exchange in ices, it has been suggested that
there is a potential (hidden) source of internal heating there.22 And, the
complex history and star formation patterns in Orion KL will lead to quite
varied thermal histories of various sub-regions of the giant molecular cloud
complex. Thus, dedicated work to elucidate the nature of the Hot Core-SW
region is needed to better understand how this environment could affect the
observed chemistry.

###  3.4 Methyl Group Chemistry

If the hump observed in the CH3OD column density versus temperature profile is
indeed evidence of surface D-H exchange at the hydroxyl site, then we would
expect a smooth profile (i.e., without a similar hump) in the profile of
CH2DOH. Unfortunately, we do not have sufficient CH2DOH transitions in our
data to test this hypothesis directly. However, Carroll 58 mapped the physical
parameters of CH2DCN toward Orion KL using data from ALMA (project:
ADS/JAO.ALMA#2013.1.01034, PI: Crockett). Figure 5 shows a histogram of the
CH2DCN column density and rotational temperature using these data where they
overlap with CH3OD emission in the current observations. In this plot, we see
a consistent power-law relationship between temperature and column density,
which supports the conclusion by Osamura et al. 33 that the methyl groups of
complex organics would not undergo grain-surface or gas-phase D-H exchange. In
other words, the hump visible in Figure 3a is indeed likely the result of
chemistry specific to the hydroxyl site of methanol. Future dedicated
observations of CH2DOH lines with ALMA in this region are necessary to confirm
this hypothesis.

Figure 5: Two-dimensional histogram with 20 points per bin showing the derived
CH2DCN column density against rotational temperature, using data presented by
Carroll 58.

###  3.5 Comparison to Past Studies

The evidence presented here adds to the growing list of observational and
theoretical evidence in favor of a grain-surface mechanism for CH3OD
enrichment in massive star-forming regions. A key difference between this work
and that of past observations is that, here, we map CH3OD column density
across much of Orion KL, including the Compact Ridge, whereas past work
derives one value for the Compact Ridge as a whole. Furthermore, resolving the
small-scale structure of CH3OD column densities (and temperature) allows us to
look at how column density is related to the line-of-site temperature,
something that has not yet been extensively investigated through observations
but is now possible with the sensitivity and spatial grasp of ALMA.

Computational models to assess D-H exchange generally have investigated
temporal variations in relative CH3OD abundances at a single temperature or
have looked at singly-deuterated methanol chemistry across a range of
temperatures but less than 140 K.33, 40, 14 Previous models of grain-surface
D-H exchange in Orion KL have assumed methanol is completely sublimated at
temperatures $>$110 K, whereas our observations suggest D-H exchange on the
ice may be important up to 125 K. Thus, the observations presented here probe
a temperature regime beyond that of existing astrochemical models and call for
revised models to investigate D/H chemistry at higher temperatures.
Specifically, temperature-dependent models of grain-surface D-H exchange in
which methanol sublimates completely at higher temperatures (e.g., 125 K) are
needed to more robustly explain the patterns observed in this work.

Perhaps the leading criticism of proposed grain-surface chemistry prompting
the enhancement of CH3OD abundances relative to CH2DOH is that such processes
would require a large initial [HDO]/[H2O] ratio. For example, models by
Charnley et al. 31 suggest that the initial [HDO]/[H2O] ratio in the ice
mantles would need to be $\sim$0.1,31 which is significantly larger than the
ratio of $\sim$0.003 reported by Neill et al. 45 for compact regions of Orion
KL. However, Thi et al. 59 suggest that the [HDO]/[H2O] ratio can exceed 0.01
in dense (${\geq}10^{6}$ cm-3), warm ($T>100$ K) regions (such as those
observed here) via neutral-neutral reactions—such as the formation of HDO from
OH + HD, OD + H2, and OD + OH—which may be promising for the hypothesis of a
grain-surface CH3OD enhancement. Even more promising is a model presented by
Faure et al. 40, who reproduced observed [CH2DOH]/[CH3OD] ratios in the
Compact Ridge assuming a primitive [HDO]/[H2O] fractionation of 0.006, only a
factor of 2 larger than the observed ratio reported by Neill et al. 45. That
is, the key initial condition is that of the deuteration state of methanol and
water in the icy grain mantles, which are exceedingly difficult to measure
directly with previous observational capabilities. JWST will offer greatly
improved capabilities to attempt such measurements, going forward.

A remaining question, then, is what makes methanol deuteration in low-mass
star-forming regions so different from that in high-mass star-forming regions?
Ratajczak et al. 12 suggest observational biases, namely that, since high-mass
objects tend to be further away than those low-mass objects where deuterium
chemistry has been studied, measurements of the [CH2DOH]/[CH3OD] ratio may be
affected, particularly if the spatial distributions of the two isotopomers are
different. High angular resolution mapping of high-mass star-forming regions,
like that presented in Figure 1, would address this by comparing the CH2DOH
and CH3OD column densities only where their emission overlapped, as was done
for different spectral components of the high-mass star-forming region NGC
6334I by Bøgelund et al. 14 As stated previously, the observations presented
here do not have sufficient CH2DOH lines available to test the spatial
correlation of site-specific deuterated isotopologues, and dedicated high
angular resolution observations targeting low-energy CH2DOH lines are
necessary to further address this issue.

Another conjecture for the different deuterium fractionation patterns in
massive YSOs compared to low-mass star-forming regions is that there is simply
less deuteration in massive protostars because of the warmer environments.14
Faure et al. 40 reproduced relative singly-deuterated methanol abundances for
Orion KL (a high-mass source) and IRAS 16293-2422 (a low-mass object) using
kinetic models that were identical except for the initial deuterium
fractionation ratios. They reported that the Orion KL Compact Ridge’s gas-
phase deuterium chemistry could be modeled assuming similar primitive
deuteration of water and methanol ices ($\sim$0.2-0.3%), whereas IRAS 16293’s
gas-phase deuterated methanol chemistry required a significantly higher
deuterium fractionation in methanol (12%) than water (1%). Their model shows
complete methanol desorption by $\sim$110 K. Such conditions of extreme
deuteration only occur in very cold, dense environments where extensive
molecular depletion occurs, including that of CO and N2. Under such
conditions, D${}_{3}^{+}$ becomes the dominant molecular ion, whose
dissociative recombination results in the arrival of hydrogen atoms onto grain
mantles with a D/H ratio of $>$1.

As seen in Figure 3a, the desorption model based on work by Faure et al. 40
(dashed line) matches nicely the CH3OD column density rise between 100 and 110
K; however, the CH3OD in our data increases at temperatures up to $\sim$125 K.
This slight discrepancy might be addressed by temperature-programmed
desorption experiments, for example, studying the release of CH3OD directly
rather than via sputtered CH3OD2+ detected by Souda et al. 39 Furthermore,
thermal desorption strongly depends on the composition of the underlying
surface. Such questions require more robust chemical networks for deuterium
chemistry as well as a better understanding of the initial chemical conditions
of both high-mass and low-mass star-forming regions.

##  4 Conclusion

We provide observational evidence in support of rapid D-H exchange in
methanol-containing ices, specifically at the hydroxyl site, between $\sim$100
and 125 K in Orion KL, using high angular resolution ALMA Band 4 observations
of CH3OD to map the small-scale variations in CH3OD column density for the
first time, and to compare the observed column densities to the line-of-site
rotational temperatures mapped at the same angular resolution (and derived
previously from ^13CH3OH).22

We fit power-law relationships and toy models of D-H exchange at methanol’s
hydroxyl (-OH) site followed by CH3OD thermal desorption to the observed CH3OD
column density profile in Orion KL. These analyses suggest that D-H exchange
and rapid CH3OD desorption increase the gas-phase CH3OD column density between
100 and 125 K. Enhanced CH3OD column densities are limited to the Hot Core-SW,
which has been previously suggested to harbor a potential source of internal
heating, which could explain the enhanced CH3OD column densities between 125
and 185 K. In this interpretation, there is simply little gas in this region
at higher temperatures that would display CH3OD enhancements.

Future investigations—through observations, experiments, and computational
models—are needed to further constrain the peculiar D-methanol chemistry in
Orion KL and other star-forming regions. The work presented here would be
aided by dedicated high-resolution observations of CH2DOH, spectroscopic
experiments measuring the kinetics of CH3OD formation via D-H exchange in
heavy water ice (HDO and D2O) and subsequent desorption, and temperature-
dependent astrochemical models of possible CH3OD loss at higher temperatures
(185-225 K).

The Supporting Information includes an explanation of optical depth
approximations (Section S1); supplemental maps (Section S2), specifically
CH3OD integrated intensity (Figure S2), ^13CH3OH column density (Figure S3),
and ^13CH3OH rotational temperature (Figure S4) maps; a comparison of derived
[CH3OD]/[^12CH3OH] ratios (Section S3); and loss calculations of CH3OD by gas-
phase reactions with OH (Section S4).

## Acknowledgements

This work makes use of the following ALMA data: ADS/JAO.ALMA#2017.1.01149 and
ADS/JAO.ALMA#2013.1.01034.ALMA is a partnership of ESO (representing its
member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST
and ASIAA (Tawain), and KASI (Republic of Korea), in cooperation with the
Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO,
and NAOJ. The National Radio Astronomy Observatory (NRAO) is a facility of the
National Science Foundation (NSF) operated under Associated Universities, Inc.
(AUI). This research made use of APLpy, an open-source plotting package for
Python,60 and the KInetic Database for Astrochemistry (KIDA), an online
database of kinetic data.57

This work has been supported by the NSF Graduate Research Fellowship Program
under grant No. DGE-1144469 and NRAO Student Observing Support under Award No.
SOSPA6-014. OHW was additionally supported by an ARCS Los Angeles Founder
Chapter scholarship. GAB gratefully acknowledges support from the NSF AAG
(AST-1514918) and NASA Astrobiology (NNX15AT33A) and Exoplanet Research (XRP,
NNX16AB48G) programs. This work benefited from discussions with Brandon
Carroll, Dana Anderson, Aida Behmard, Cam Buzard, Steve Charnley, and
Catherine Walsh. Many thanks to the anonymous referees for their thoughtful
comments and help in improving the manuscript. OHW thanks Erica Keller, Sarah
Wood, and the NRAO North American ALMA Science Center (NAASC) for their
assistance with the data reduction.

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