2. The sample

The sample consists of 126 objects. It includes OH/IR stars with cool envelopes, PPNe that are believed to be oxygen-rich (i.e. they are associated with OH or masers, and/or the central stars are of spectral class M, and/or they have silicon dust features in the infrared spectra), and PNe associated with OH-masers. Five of the objects are more likely to be pre-main sequence objects than evolved objects, but were observed nevertheless. They are included in Table 1 (click here) (labeled "PMS?") and Table 2 (click here), but are not taken into account in the rest of the paper. The sample that is being discussed thus consists of 121 objects.

The objects with cool envelopes were mainly selected from Likkel (1989), Likkel et al. (1991), Omont et al. (1993), and Silva et al. (1993), and OH/IR stars as well as PPNe are included.

More post-AGB candidates were selected from the literature and they can be divided into the following categories:

1. Non-variable OH/IR stars (Herman & Habing 1985; van Langevelde et al. 1990).
2. High-velocity OH or maser sources (te Lintel Hekkert et al. 1988, 1992; te Lintel Hekkert 1991; Likkel et al. 1992). In these objects the velocity extent of the maser emission imply motions with velocities larger than the largest envelope expansion velocities expected for stars on the AGB (about ).
3. Objects where the velocity width of the maser emission exceeds that of the OH emission, indicating that the objects recently have become PPNe (Gomez et al. 1994).
4. Early type stars and/or objects with a double-peaked energy distribution. In some cases there is an association with an OH maser (Hu et al. 1993a, 1994).
5. Low-excitation emission line objects (Hen 1191 and Hen 1379, Le Bertre et al. 1989; M1-92 and M2-56, Bujarrabal et al. 1992).
6. Some sources that are considered to be oxygen-rich post-AGB objects: the Frosty Leo nebula (Forveille et al. 1987), HR 3126 (Chiar et al. 1993; Nyman et al. 1993b), Roberts 22 (Silva et al. 1993), and IRAS 07131-0147 (Scarrott et al. 1990).

 

IRAS name Other name Ra (1950) Dec (1950) OH H₂O SiO Type Reference

01304+6211 01h30m27.0s -55 x x x OH/IR 1, 5

03206+6521 OH 138.0+7.2 03h20m41.6s -37 x x OH/IR 2, 5

03293+6010 OH 141.7+3.5 03h29m23.6s -57 x x x OH/IR 2, 6, 9

04361+2547 04h36m09.4s 6 x x PMS? 1

06319-0501 AFGL 5201 06h31m58.3s -60 x x OH/IR 1

06530-0213 06h53m00.9s 28 x PPN 1, 10, 11

07027-7934 07h02m45.3s -27 x PN 12

07131-0147 07h13m10.9s 0 PPN 13

07180-1314 AFGL 5229 07h18m00.7s 0 x x X OH/IR 2, 9

07399-1435 OH 231.8+4.2 07h39m58.9s 40 x x x PPN 5, 9, 32

07536-2830 HU Pup 07h53m38.6s 0 x x X OH/IR 2, 9

07559-5859 HR 3126 07h55m54.5s 0 PPN 28

08005-2356 08h00m32.5s 0 x PPN 1, 10, 22

08357-1013 OH 235.3+18.1 08h35m44.5s 0 x x X ? 2, 9

09371+1212 Frosty Leo 09h37m11.2s -20 PPN 1, 30, 31

09429-2148 IRC-20197 09h42m58.3s 40 x x x OH/IR 2, 9, 23

10197-5750 Roberts 22 10h19m45.1s 0 x PPN 3

11385-5517 HD 101584 11h38m33.6s 40 x PPN 29

11516-6201 11h51m36.6s 20 x ? 3

12358-6323 12h35m53.0s -3 x ? 10, 11

14086-0730 14h08m38.9s -26 x X OH/IR 2, 9

14122-5947 14h12m14.0s 0 PN 10, 37

14341-6211 14h34m10.7s -23 x PPN 10, 11

15405-4945 15h40m32.1s 50 x PPN 14

16029-3041 OH 345.0+15.7 16h02m59.7s -4 x x ? 2, 9, 41

16086-5255 16h08m37.8s 0 PPN 10

16235-4832 Hen1191 16h23m31.8s -9 PPN 15

16342-3814 16h34m17.1s 50 x PPN 14

16559-2957 16h55m57.3s 56 x PPN 1, 10, 11

16594-4656 16h59m26.7s -26 PPN 16

17103-3702 NGC 6302 17h10m21.3s -40 x PN 4

17106-3046 17h10m39.0s 6 x ? 3

17118-2952 17h11m53.0s -40 x OH/IR 1

17150-3754 OH 349.36-0.20 17h15m04.0s -125 x PN 4

17150-3224 17h15m04.6s 26 x PPN 10, 11, 26

17163-3907 Hen1379 17h16m22.0s 5 PPN 15

17195-2710 17h19m35.5s 0 PPN 10

17207-2856 17h20m44.7s -20 x PN 4

17221-3038 PK 356+2.1 17h22m06.1s 0 x PN 4

17233-2602 OH 0.1+5.1 17h23m23.0s -143 x PPN 10, 11

17292-2727 17h29m15.4s -50 x ? 1

17317-2743 OH 359.8+2.6 17h31m44.9s 45 x ? 1

17347-3139 17h34m45.9s -100 x PN 4

17360-3012 OH 358.16+0.5 17h36m02.3s 2 x x OH/IR 2, 7

17371-2747 17h37m11.8s 115 PN (continuum pos.) 4

17371-2747 17h37m08.4s 115 x PN (OH maser pos.) 4

17375-2759 17h37m29.4s 30 PN (continuum pos.) 4

17375-2759 17h37m26.7s 30 x PN (OH maser pos.) 4

17375-3000 17h37m32.6s 0 x PN 4

17385-3332 17h38m34.4s -236 x PPN 10, 11

17393-2727 OH 0.9+1.3 17h39m24.7s -110 x PN 4

17404-2713 OH 1.2+1.3 17h40m29.5s 0 x PPN 10, 11

17411-3154 17h41m07.4s -20 x X OH/IR 2, 3, 9

17423-1755 He 3-1475 17h42m18.8s 45 x PPN 17, 38

 

 

IRAS name Other name Ra (1950) Dec (1950) OH H₂O SiO Type Reference

17433-1750 17h43m20.6s 120 x PPN 1, 10, 11

17443-2519 17h44m22.2s 29 x OH/IR 2

17443-2949 17h44m23.8s -10 x PN 4

17459-3057 17h45m56.5s 6 x X OH/IR 2, 9

17484-1511 17h48m28.2s 94 x ? 1

17505-3143 17h50m31.5s 4 x X OH/IR 2, 9

17516-2525 17h51m37.8s -15 x PPN 2, 24

17534+2603 89 Her 17h53m24.0s -5 PPN 27, 33

17565-2035 17h56m35.7s 204 x ? 2

17579-3121 17h57m59.1s 10 x PPN 3

17580-3111 17h58m08.0s 20 x PN 4

18016-2743 18h01m35.0s 74 x PPN 10, 11

18025-3906 18h02m35.0s -116 x PPN 3

18044-1947 18h04m29.1s 0 x OH/IR 2

18055-1833 AX Sgr 18h05m31.4s 0 x OH/IR 2, 40

18071-1727 OH 12.8+0.9 18h07m11.1s 24 x PPN 2, 24

18081-0338 18h08m11.1s 7 x OH/IR 18

18095+2704 18h09m30.9s 0 x PPN 2, 25

18105-1935 18h10m34.7s 17 x PPN 10, 11

18107-0710 18h10m47.6s 18 x X OH/IR 1, 9

18135-1456 OH 15.7+0.8 18h13m34.5s -1 x x x PPN 5, 9, 19

18161-1713 18h16m07.4s 0 ? 2

18167-1209 OH 18.5+1.4 18h16m47.4s 175 x ? 19

18176-1848 OH 12.8-1.9 18h17m40.0s 13 x x x OH/IR 2, 8, 9

18257-1000 OH 21.5+0.5 18h25m43.1s 116 x x x OH/IR 2, 5, 9

18276-1431 OH 17.7-2.0 18h27m39.8s 61 x x PPN 15

18281+2149 AC Her 18h28m08.8s 0 PPN 2

18298-2111 AFGL 5501 18h29m51.2s 98 x OH/IR 1

18303-0519 18h30m20.5s 0 Variable star 2

18310-2834 18h31m03.8s -115 x OH/IR 1

18314-2759 18h31m26.2s 22 x ? 1

18340-0839 18h34m03.5s 0 x X OH/IR 2, 9

18348-0526 OH 26.5+0.6 18h34m52.5s 27 x x x OH/IR 2, 5, 7, 9

18437-0643 OH 26.4-1.9 18h43m45.3s 25 x x x OH/IR 2, 5, 9

18450-0148 W 43A 18h45m05.2s 34 x x PPN 20

18454+0250 18h45m29.9s 14 x PMS? 1

18460-0254 OH 30.1-0.7 18h46m04.9s 99 x x x OH/IR 2, 9

18479-2514 18h47m56.4s -30 x OH/IR 1

18491-0207 18h49m10.7s 80 x PPN 17

18526+0140 18h52m39.6s -115 x ? 2

18550+0130 18h55m02.5s 0 X Variable star 2, 9

18560+0638 OH 39.7+1.5 18h56m03.9s 20 x x x OH/IR 2, 6

18596+0315 OH 37.1-0.8 18h59m36.4s 84 x x ? PPN 5, 21

19010+0526 19h01m05.9s 0 x OH/IR 2

19067+0811 OH 42.3-0.1 19h06m43.9s 64 x x x PPN 5, 9, 21

19075+0921 19h07m34.0s 0 x OH/IR 2, 39

19114+0002 AFGL 2343 19h11m25.0s 100 x PPN 1

19127+1717 19h12m45.5s 16 x PN 1, 27

19134+2131 19h13m26.6s -67 x PPN 20

19161+2343 CRL 2362 19h16m08.6s 29 x x OH/IR 2

19178-2620 CRL 2370 19h17m51.4s 5 x X OH/IR 2, 9

19190+1102 19h19m03.8s -23 x x PMS? 1

19192+0922 OH 44.8-2.3 19h19m13.1s -72 x x x OH/IR 2, 8, 9

 

IRAS name Other name Ra (1950) Dec (1950) OH H₂O SiO Type Reference

19219+0947 VY2-2 19h21m59.3s -60 x PN 4

19244+1115 IRC+10420 19h24m26.8s 78 x Supergiant 34

19283+1944 OH 55.0+2.7 19h28m18.1s 27 x X OH/IR 1, 9

19288+2923 19h28m51.4s -40 x x x OH/IR 2, 8

19297+2300 19h29m43.5s 11 x PMS? 1

19343+2926 M1 92 19h34m17.6s 0 x PPN 35

19352+2030 OH 56.4-3.0 19h35m12.7s 17 x OH/IR 1

19511+3935 19h51m06.9s 23 x ? 1

19566+3423 19h56m38.1s -43 x x ? 1

19576+2814 OH 65.7+0.7 19h57m36.3s -59 x OH/IR 1

20043+2653 OH 65.3-2.7 20h04m18.2s -5 x x OH/IR 1, 5, 9

20272+3535 OH 75.3-1.8 20h27m12.5s -4 x OH/IR 1

20406+2953 20h40m41.5s 15 x ? 1, 10, 11

20491+4236 OH 83.4-0.9 20h49m09.5s -39 x x x OH/IR 2, 6, 9

21206+5145 21h20m38.7s -100 x x PMS? 1

21554+6204 21h55m29.6s -20 x ? 2

22036+5306 22h03m40.0s -40 x PPN 17

22177+5936 OH 104.9+2.4 22h17m43.1s -25 x x x OH/IR 2, 5

22480+6002 22h48m00.9s 0 X Supergiant 2, 9

23321+6545 23h32m06.3s -15 x PPN 1, 10, 11

23541+7031 M2 56 23h54m06.6s 0 PPN 36

References:
1. Likkel (1989), 2. Omont et al. (1993), 3. Silva et al. (1993), 4. Zijlstra et al. (1989), 5. Jewell et al. (1991), 6. Nyman et al. (1986), 7. Nyman et al. (1993), 8. Gomez et al. (1990), 9. This work, 10. Hu et al. (1993a), 11. Hu et al. (1994), 12. Zijlstra et al. (1991), 13. Scarrot et al. (1990), 14. te Lintel Hekkert et al. (1988), 15. Le Bertre et al. (1989), 16. Loup et al. (1990), 17. te Lintel Hekkert (1991), 18. David et al. (1993), 19. van Langevelde et al. (1990), 20. Likkel et al. (1992), 21. Gomez et al. (1994), 22. Slijkhuis et al. (1991), 23. Haikala (1990), 24. van der Veen et al. (1989a), 25. Hrivnak et al. (1988), 26. Hu et al. (1993b), 27. Likkel et al. (1991), 28. Chiar et al. (1993), and Nyman et al. (1993b), 29. te Lintel Hekkert et al. (1992), 30. Forveille et al. (1987), 31. Scarrott & Scarrott (1994), 32. Morris et al. (1982), 33. Waters et al. (1993), 34. Oudmaijer et al. (1994), 35. Bowers & Knapp (1989), 36. Bujarrabal et al. (1992), 37. van de Steene & Pottasch (1993), 38. Bobrowsky et al. (1995), 39. Le Squeren et al. (1992), 40. van der Veen et al. (1989b), 41. Haikala et al. (1994).

The PNe were selected from Zijlstra et al. (1989, 1991). For two PNe there were significant differences between the radio continuum and OH maser positions, and therefore both positions were observed.

Table 1 (click here) lists the sources, observed positions, center velocities of the spectra, presence of maser emission (the detections made in this work are included, and our new detections are marked with a capital "X"), the evolutionary status of the object, and references to papers from which the sources were selected and to SiO maser data. The evolutionary status of the objects given in Table 1 (click here) have been classified in the following ways:

1. OH/IR: objects with double-peaked OH spectra that have been found to have variable OH, and/or optical, and/or infrared emission. If no special monitor observations have been reported in the literature, we regard an object as variable if the IRAS variability index is > 90. None of the characteristics given below for PPNe have been reported for these objects.
2. PPN: objects that have one or more of the following characteristics: bipolar appearance, an early type central star, a double peaked energy distribution, high velocity masers, and/or the velocity width of the emission exceeds that of the OH emission.
3. PN: Objects that have been detected in radio continuum, indicating the presence of ionized gas.
4. Two objects where OH maser emission has not been detected, but with variable central stars are labeled as "variable star".
5. Two objects are labeled "supergiant": IRAS 22480+6002 that has been classified as a supergiant (Winfrey et al. 1994), and IRC+10420 which may be a post-AGB object or a supergiant (e.g. Oudmaijer et al. 1994).

These definitions cover 86% of the sample. The rest of the objects do not fit into the above criteria because of lack of relevant observations. We have not made any attempt to separate them into possible OH/IR objects or PPNe, although most of them probably are OH/IR objects. They are labeled "?" in Table 1 (click here).

The OH/IR stars most likely contain a mixture of AGB stars, supergiants, and perhaps also a few very young PPNe that have just started to evolve off the AGB. The objects classified as PPNe most likely consist of PPNe at different ages as well as a objects that may not be true PPNe, e.g. some of the binary systems that produce bipolar nebulae.

The sample roughly consists of 37% variable stars (mostly OH/IR objects), 36% PPNe, 13% PNe, and 14% unclassified objects.