4. Results

4.1. Near infrared photometry of the unidentified IRAS sources in the sample

In Tables 3 (click here)a to g we show the photometric magnitudes of the 225 IRAS sources measured in the near infrared, together with the estimated associated errors and a number indicating the run in which the source was observed, according to the list given in Table  1 (click here). Letters A and B, following the IRAS name, indicate that two near infrared counterparts were found equidistant to the nominal IRAS position. In addition, in Table  4 (click here), we list the IRAS names of the 42 sources not detected in the K band. Although in many cases they are associated with the faintest sources in our sample showing the lower fluxes at , some of them were detected at a different epoch with a K magnitude well above the detection limit, which may indicate a strong variability. As we can see, there is a high percentage of positive detections, around 80%, which confirms the validity of the method of observation used.

 

 

IRAS name J H K L' M Run Other measurements

00422+6131 - - (9)

00470+6130 - - (9)

02143+5852 - - (9) 1

02395+6244 - - (9)

02528+4350 - - (9) 1

03578+3134 - - (9) 2

04010+5118 - - (9)

04101+3103 - - (2) 2

- - (9)

04185+2022 - - (9) 2

04189+2650 - - (9)

04296+3429 - - (8) 1, 3

04302+4425 - - (8) 1

- - (9)

05113+1347 - (9) 4, 5

05209+2454 - - (9)

05238-0626 - - (9) 6

05284+1945 - - - (8)

05341+0852 - - (8) 1, 6

- - (9)

05355-0117 - - (9)

05471+2351 - - (9) 7

05573+3156 - - (3)

- - (9)

05591+1630 - - (9) 8

06013-1452 - (6)

- - (9)

06464-1644 - - (9) 6

06499+0145 - - (2)

06518-1041 - - (3) 4

- - (9)

06530-0213 - - (2) 9

06549-2330 - - (3)

06556+1623 - - (9) 4, 7

06562-0337 - - (3) 1, 10

- (6)

- - (9)

07027-7934 (4) 11, 12

07227-1320 - (4)

 

IRAS name J H K L' M Run Other measurements

07280-1829 - - (2)

07330-2332 - - (9)

07399-1435 - - (9) 13, 14

07430+1115 - - (9)

07582-4059 - - (6) 9

08057-3417 - (6)

08131-4432 - (4)

08143-4406 - (4)

08189+5314 - - (9) 15

08213-3857 (4)

08229-4051 - - (6)

08242-3828 (4)

08275-6206 - (6)

08281-4850 - - (6)

08351-4634 - - (6)

08355-4027 - - (4)

- - (6)

08425-5116 (6) 6

08470-4321 (4) 16, 17, 18

09024-5019 - - (6)

09119-5150 - - (6)

09362-5413 - - (6)

09425-6040 (4) 12 (6)

10029-5553 - - (4) 9

10115-5640 - - (6)

10178-5958 (4) 19

10197-5750 (4) 14

10215-5916 (4) 20

10256-5628 - (4)

10348-6320 - - (7)

11065-6026 (4) 21

SAO 239162 (4) 12

SAO 251457 (6) 12, 22

11339-6004 - - (7) 9

11387-6113 - (4)

11415-6541 - - (7)

11438-6330 - - (4) 14, 23

- (6)

 

IRAS name J H K L' M Run Other measurements

11444-6150 - - (7)

SAO 223245 (6) 24, 25

12158-6443 - - (7)

SAO 239853 - (4) 12, 20

12358-6323 - - (4)

(6) 9

12360-5740 - - (7)

13110-6629 - (4)

13203-5917 - - (6)

13245-5036 - - (6)

13266-5551 - (4) 12

13356-6249 (4)

13421-6125 - - (4)

- - (6)

13427-6531 - - - - (6)

13428-6233 (4) 26 (6)

13500-6106 - - (7)

14079-6402 - (7) 27

14104-5819 - - (4)

- (7)

14122-5947 (6) 9

14177-5824 (4)

14247-6148 (7)

14331-6435 - - (4)

14562-5637 - - (7)

15066-5532 - (4)

15103-5754 - - (4) (7)

15154-5258 - - (6)

15406-4946 (4)

15408-5413 (4) 23, 28, 29, 30

15514-5323 - - (7) 16

15553-5230 (6)

16342-3814 (4) 12, 26

16552-3050 - - (5) 9

16594-4656 (4) 26

 

IRAS name J H K L' M Run Other measurements

17009-4154 (4) (7)

17021-3054 - - (4)

17055-3753 - - (4)

17067-3759 - (4)

17074-1845 - - (1)

17088-4221 - (4) 26, 31

SAO 208540 - (4) 26

17106-3046 - (4)

SAO 244567 - - (4)

17130-4029 - - (4)

17149-3053 - - (1)

17150-3224 (4) 26, 32

17153-3814 - (4)

17164-3226 - - (4)

17168-3736 - (4) 31

 17223-2659A - - (1)

 17223-2659B - - (1)

17234-4008 - - (4)

17242-3859 - - (7)

17245-3951 - - (4)

17269-2235 - - (5)

17287-3443 - (4)

17291-2402 - - (5) 4

17311-4924 - (4) 12, 26

17316-3523 - - (4)

17317-3331 (4) 28, 30, 31, 33, 34

17317-2743 - - (1)

17332-2215 - - (5)

17347-3139 - - (1) 26

- - - (7)

17348-2906 - - (5)

17360-2142 - - (3)

17393-2727 - - (5)

17395-0841 - - - (1)

- - (3)

17411-3154 - - (1) 14

17416-2112 - - (5)

17418-3335 - - (7)

 

IRAS name J H K L' M Run Other measurements

17423-1755 - - (3)

17433-1750 - - (1) 9

17441-2411 - - (5) 9, 26

SAO 209306 - (6)

17466-3031 - - (5)

17479-3032 - - (4)

17495-2534 - - (1)

17506-2955 - - (5)

17540-2753 (5) 14

17542-0603 - - (1)

17543-3102 - (4)

 17548-2753A - - (5)

 17548-2753B - - (5)

17550-2800 - - (5)

17550-2120 - - (5)

17560-2027 - - (3)

17579-3121 - - (1)

17580-3111 - - (5) 9

- (6)

17581-2926 - - (5)

17582-2619 - - (5)

17583-3346 (7)

17584-3147 (7)

17597-1442 - - (3)

18011-2057 - - (5)

18019-3121 - - (7)

18025-3906 (4) 9, 12

SAO 85766 - - (3) 35

18075-0924 - - (5)

18087-1440 - - (1)

18095+2704 - - (5) 26, 35, 36, 37

18096-3230 - - (7)

18182-1504 - - (5)

18186-0833 - - (1)

18216-0156 - - - (1)

18229-1127    -       -    (1)

18246-1032 - - (5)

18252-1016 - - (5)

 

IRAS name J H K     L'         M     Run Other measurements

18257-1000 - - (1)

18347-0825 - - (5)

18379-1707 - - (1) 12

18386-1253 - - (1)

18420-0512 - - (3)

18454+0001 - - (5)

18485+0642 - - (3)

18514+0019 - - (1)

18518+0558 - - (5)

18520+0007 - - (5)

18576+0341 - - (1)

18582+0001 - - (1)

19005-0445 - - (3)

19071+0857 - - (5)

19154+0809 - - (3)

19176+1251 - - (5)

19190+1048 - - (5)

19193+1804 - - (5)

19207+2023 - - (5)

19254+1631 - - (5)

19283+1944 - - - (5)

- - - - (8)

19306+1407 - - (5)

19344+2457 - - (1)

19356+0754 - - (5)

19454+2920 - - (5) 35

19475+3119 - - (3) 37

19477+2401 - - (5) 35

19576+2814 - - (8)

19589+4020 - - (5)

19590-1249 - - (1)

20094+3721 - - (3)

20144+4656 - - - (1)

- - (5)

20144+3526    -       -    (1)

20244+3509 - - (3)

20406+2953 - - (5)

20461+3853 - - (9)

 

IRAS name J H K     L'         M     Run Other measurements

20462+3416 - - (9)

20470+4458 - - (9) 1

20490+5934 - - (8) 4

- - (9)

20559+6416 - - (9)

20572+4919 - - (9) 4

20588+5215 - - (9)

21002+4939 - - (9) 38

21289+5815 - - (9)

21542+5558 - - (5)

21546+4721 - - (5)

- - (9)

22023+5249 - - (5)

22036+5306 - - (5)

- - (8)

- (9)

22223+4327 - - (2)

- - (5) 5

SAO 34504 - - (1) 1, 26, 37

22331+5809 - - (5)

23198-0230 - - (9) 39

23304+6147 - - (9) 1, 3, 4

23312+6028 - - (5)

23436+6306 - - (9)

1. Paper I 14. Lepine et al. (1995) 27. Persi et al. (1987)

2. Kenyon et al. (1990) 15. Miroshnichenko et al. (1996) 28 Le Bertre (1993)

3. Hrivnak & Kwok (1991) 16. Gaylard & Whitelock (1988) 29. Epchtein et al. (1987)

4. Paper II 17. Persson & Campbell (1988) 30. Nyman et al. (1993)

5. Kwok et al. (1995) 18. Liseau et al. (1992) 31. Epchtein & Nguyen-Q-Rieu (1982)

6. Blommaert et al. (1993) 19. Allen & Glass (1975) 32. Hu et al. (1993b)

7. Allen (1974) 20. Hrivnak et al. (1989) 33. Persi et al. (1990)

8. Allen (1973) 21. Hu et al. (1990) 34. Jones et al. (1982)

9. Hu et al. (1993a) 22. Hu et al. (1989) 35. Lawrence et al. (1990)

10. Campbell et al. (1989) 23. Gaylard et al. (1989) 36. Hrivnak et al. (1988)

11. Zijlstra et al. (1991) 24. Elias (1978) 37. Kastner & Weintraub (1995)

12. Fouque et al. (1992) 25. Lloyd-Evans (1985) 38. Cohen (1974)

13. Kastner et al. (1992) 26. van der Veen et al. (1989) 39. Whitelock et al. (1995).

 

 

IRAS name Run IRAS name Run

01475-0740 (9) 18011-1847 (3)

04117+6402 (9) 18083-2155 (3)

04172+4411 (9) 18355-0712 (5)

 05284+1945¹ (9) 18385+1350 (5)

05318+2749 (9) 18524+0544 (5)

06055-0653 (9) 18533+0523 (5)

 06499+0145² (9) 19024+0044 (5)

08574-5011 (4) 19182+1806 (5)

 09024-5019³ (4) 19219+1533 (5)

09032-3953 (4) 19480+2504 (5)

09370-4826 (6) 20042+3259 (8)

10194-5625 (4,7) 20103+3419 (5)

11472-7834 (6) 20272+3535 (5)

11544-6408 (4) 20404+4527 (5)

 14562-5637⁴ (4) 21206+5145 (5)

16040-4708 (7) 21388+5622 (5)

16114-4504 (4) 21480+5640 (5)

17021-3109 (4) 21537+6435 (9)

17086-2403 (1) 21554+6204 (9)

17448-2131 (1) 22568+6141 (5,9)

17521-2938 (5) 23125+5921 (5) ¹ Detected in run (8). ² The object detected in run (2) is 40'' away from the       IRAS position. ³ Detected in run (6). ⁴ Detected in run (7).

All the IRAS sources included in Table  3 (click here) satisfy the selection criteria described in Paper I with the exceptions of IRAS 19344+2457, identified as a new OH/IR star, and for which no previous near infrared photometry was available, and IRAS 19590-1249, a recently discovered hot post-AGB star at high galactic latitude with nebular emission lines (McCausland et al. 1992). Both show far infrared colours very similar to those required to be included in the sample but do not strictly satisfy all the selection criteria.

We have plotted in Fig. 3 (click here) the position of the observed infrared sources in the near infrared two-colour diagram J-H vs. H-K. Again, as in Fig. 1 (click here), we have divided the diagram into Regions (from I to V) for our analysis.

The distribution observed in Fig. 3 (click here) looks quite different to that shown by the sample of well identified objects in Fig. 1 (click here), specially the fact that only three sources are found in Region V, where we expect to find well evolved PNe. The three of them have recently been identified as new PNe through optical spectroscopy (García-Lario et al. 1997a), which confirms the validity of this method to detect new PNe.

In contrast, the majority of sources in Fig. 3 (click here) are located in Regions I and II of the diagram where basically all kind of objects can be present, as we can see in Table 2 (click here), complicating the identification process.

The lack of well evolved PNe among the unidentified objects in our sample can simply be explained as a selection effect. Bright PNe are easily recognized in the optical range through the detection of the many nebular emission lines covering their optical spectra. Those not yet discovered probably belong to the group of very oung and dusty-PNe and, thus, if present in our sample, will probably show unusual near infrared colours, as we will confirm later.

4.2. Classification of the unidentified IRAS sources

4.2.1. PNe

As we have already shown, only PNe displaying the characteristic excess observed in the J band can be unambiguously recognized as such, based on near infrared data alone. Unfortunately, only 3 objects have been found in Region V of the near infrared two-colour diagram among the unidentified IRAS sources in our sample. These are SAO 244567, IRAS 18186-0833 and IRAS 17074-1845, and the three of them have been identified as new PNe through optical spectroscopy.

For the detection of the rest of PNe showing peculiar near infrared colours we need to use additional information obtained in other spectral ranges. The far infrared IRAS colours are not useful in this case, since PNe are known to be widely distributed throughout the whole selected region in the IRAS two-colour diagram. Fortunately, the ionized gas in their envelopes can easily be detected in the radio continuum and this has been used in a few cases in Table 6 to classify previously unidentified objects as new PNe, once confirmed its nature via optical spectroscopy (van de Steene et al. 1996a,b). In addition, optical spectra were also taken for many of the objects located in the IRAS two-colour diagram in the region where no overlap exists with young stellar objects, active galactic nuclei or variable OH/IR stars, whenever an optical counterpart in the Palomar or ESO prints was found (García-Lario et al. 1997a). They were the best candidates for being new PNe and, in fact, we identified in this way a few additional PNe which are also included in Table 6 as new detections. However, our optical spectroscopy revealed that most of the unidentified objects in this region of the diagram were not PNe, but transition objects in the post-AGB stage, some of them showing very bright optical counterparts, as we will see below.

Table 5 (click here) shows the distribution of the new PNe in the near infrared two-colour diagram. As we can see, most of them are associated with colours which are only unusually observed in well known PNe (see Table 2 (click here) for comparison). Some are found in Regions III and IV of the diagram, sometimes extremely reddened, such as IRAS 07027-7934. As we have previously shown, PNe in these regions of the diagram are expected to be very young and dusty. On the other hand, others are also observed in Regions I and II and may be PNe in binary systems, where the emission detected in the near infrared is coming from the photosphere of the companion star. This is true, at least, for IRAS 17395-0841, with a star located at only 2'', which is the main responsible for the near infrared emission observed. However, we have found that many of these new PNe show a faint nebular emission of a very low excitation class and bright central stars with a low effective temperature ( 25000 K) which are completely dominating the emission observed in the near infrared. They are also probably very young PNe, very similar to M1-26 and CRL 618, although with not such a strong circumstellar reddening.

Finally, we should also mention that some of the objects not detected in the near infrared above our detection limit turned out to be new PNe when observed in the optical. They are faint PNe but they do not show any indication of being young. This supports the idea that only the brightest PNe and those very young and dusty are easily detectable in the near infrared and can explain why we did not detect a large number of evolved PNe among the unidentified objects in our sample. The selection effect is clear and must be taken into account if we want to derive statistical conclusions.

 

 

Class Region I Region II Region III Region IV Region V Not Detected Detected (Total)

Planetary Nebulae 8 16 4 9 3 6 40 (46)

Post-AGB 58 28 4 20 0 15 110 (125)

Non-variable OH/IR 3 25 3 1 0 4 32 (36)

Variable OH/IR 3 24 1 0 0 3 28 (31)

Young Stellar Objects 11 12 18 26 0 15 67 (82)

Galaxies 0 0 0 0 0 2 0 (2)

 

4.2.2. Late AGB/Post-AGB stars

As we can see in Table 5 (click here), two thirds of the previously unidentified objects in our sample have been classified either as late-AGB or post-AGB stars. Compared to the small percentage found among the previously known ones, it seems clear that there also exists a strong selection effect which favours the detection of this kind of objects. This is probably due to the fact that many of these stars can only be identified as such in the far infrared, so that only with the advent of IRAS data it has been possible to recognize stars in this short-lived transition phase which precedes the formation of a PN.

It is important to note that under the group of late-AGB stars in our sample we expect to find only heavily obscured variable OH/IR stars, most of them with no optical counterpart in the Palomar or ESO prints. Those with bluer colours or optically bright Mira variables with OH maser emission, which can also be considered late-AGB stars, are located outside the region of the IRAS two-colour diagram under study, as we can see in Fig. 2 (click here). On the other hand, non-variable OH/IR stars, although belonging to the group of post-AGB stars, will be considered as a separate group in the following. As we also see in Table 5 (click here), they show near infrared properties which are quite similar to those observed in variable OH/IR stars, probably because they have just very recently left the AGB stage and are still heavily obscured by their circumstellar envelopes.

OH/IR stars are easily recognized because of the presence of the characteristic double-peaked OH maser emission at 1612 MHz and the strong silicate absorption feature at 9.8 m in their LRS, when available. However, it is more difficult to determine whether individual objects under this class should be classified either as variable or as non-variable stars. Although, as we will see below their distribution in the IRAS two-colour diagram is not exactly the same, only with a detailed monitoring of the emission observed in the near infrared or in the OH maser we can unambiguously determine to which of these groups the source belongs. In the absence of data, the IRAS variability index has been used in Table 6 to classify OH/IR stars into one of these two classes. To avoid this problem, whenever it has been possible, the sources showing OH maser emission in association with a low IRAS variability index were reobserved in the near infrared. In this way, a few objects initially classified as non-variable turned out to be strongly variable. This was the case, for instance, of IRAS 11438-6330, for which we found extraordinary large variations in the near infrared, when observed at two different epochs separated about two years, with a colour index H-K close to 8 magnitudes and an amplitude of more than 3.5 magnitudes in the K band.

With such a red colour, the most heavily obscured variable OH/IR stars might not be detectable in the K band, specially if they are observed close to a minimum in their light curves. As we can see in Table 5 (click here), some objects classified as OH/IR stars were not detected in the near infrared in a first visit while, when reobserved at a different epoch, they were succesfuly measured well above the detection limit.

In the near infrared two-colour diagram both variable and non-variable OH/IR stars are found strongly concentrated in Region II, as we can see in Table 5 (click here). This region corresponds to objects showing an extremely reddened stellar emission, as expected. In some cases, the near infrared colours are so reddened that the objects fall close to or outside the limits of our plots, as is the case of IRAS 05284+1945, IRAS 11438-6330, IRAS 14247-6148, IRAS 16437-3140, IRAS 17583-3346 and IRAS 20043+2653, all them identified as strongly obscured OH/IR stars.

In the far infrared IRAS two-colour diagram, however, variable and non-variable OH/IR stars are observed following a different distribution. While objects identified as variable OH/IR stars only appear in Region b) of this diagram, as expected, a significant fraction of the IRAS sources classified as non-variable are found well outside the limits of this region, showing a similar distribution to that observed in more evolved post-AGB stars.

Among the IRAS sources classified as post-AGB stars in Table 6 we find not only objects with optically bright counterparts, but also heavily obscured stars still occulted behind their expanding circumstellar shells, showing a low IRAS variability index. Like non-variable OH/IR stars, they are also probably in a very early post-AGB stage but, in this case, the OH maser emission is not detected any more. Most of them are easy to recognize because they are located in the far infrared IRAS two-colour diagram in the region where the only existing overlap is with PNe. Sometimes, however, they are found in regions where a strong overlap exists with other heavily obscured objects, such as compact HII regions or Herbig-Haro objects. In this case, previously unidentified objects have been classified as post-AGB stars and not as young stellar objects only when, after a visual inspection of the Palomar or ESO prints, we found that the IRAS source was not located in the direction of any dark nebula or molecular cloud and, of course, not in association with any known star forming region.

The detection of broad CO molecular emission lines in some of these post-AGB stars suggests that they may be surrounded by C-rich neutral envelopes. This could be the reason why they were not detected in OH. Some of them must be strongly obscured since they are very bright in the far infrared, with an LRS showing a featureless and very red continuum, but they have not been detected in the K band above our detection limit. This is the case of IRAS 09032-3953, IRAS 19480+2504 or IRAS 20028+3910.

In the near infrared two-colour diagram, heavily obscured post-AGB stars are usually detected in Region II, with an identical distribution to that observed in variable and non-variable OH/IR stars, which seems reasonable considering their evolutionary connection. As for the OH/IR stars, the near infrared colours observed can be interpreted as stellar emission combined with a moderate circumstellar reddening. With the dilution of the envelope, as a consequence of the expansion of the circumstellar shell, we expect the emission coming from the central star to become dominant in the near infrared. This is the case of the optically bright post-AGB stars found in Region I of the diagram.

The direct connection between non-variable OH/IR stars and optically bright post-AGB stars is confirmed by the detection of objects, like IRAS 16559-2957, showing both the characteristic double-peaked OH maser emission at 1612 MHz and, at the same time, a bright optical counterpart of intermediate spectral type. In this particular case, the detection of a faint H emission over an F5 I stellar continuum confirms the right identification of the optical counterpart.

A direct evolutionary connection between heavily obscured post-AGB stars and PNe is also possible, as observed in the case of IRAS 19016-2330. Not detected by us in June 1986, it was observed by van der Veen et al. (1989) in June 1987 with a K magnitude close to our detection limit and very red colours corresponding to our Region III in the near infrared two-colour diagram. Recently, we have detected the faint optical counterpart of this star and we have found that it already shows nebular emission of a very low excitation class over a very red continuum, while the source still show a deep silicate absorption band in the mid-infrared (van der Veen et al. 1989).

The detection of optically bright post-AGB stars with intermediate spectral types is expected in the case of low mass progenitors, since they evolve slow enough to become observable again in the optical when the effective temperature of the central is still relatively low. In contrast, high mass progenitor stars would evolve so fast that the central star could reach an effective temperature hot enough to produce the onset of the ionization of the envelope when the circumstellar shell is still optically thick.

Supporting this possibility, a few heavily obscured OH/IR stars have been found to show both OH maser emission and radio continuum emission (Zijlstra et al. 1989). It is tempting to speculate that they may be the result of the rapid evolution of high mass progenitors. However, some of these so-called "OHPN" stars are known to be peculiar. Sometimes, as observed in IRAS 17347-3139, our photometric data indicate that the central star is still strongly variable. In addition, the OH maser emission observed usually show multiple peaks which have been interpreted as an indication of bipolarity. On the other hand, it is well known that bipolar structures are usually found among type I PNe, which are considered to be the result of the evolution of high mass progenitors. Some of the most heavily obscured new PNe found, like IRAS 07027-7934 or IRAS 17423-1755, have also been detected in OH, confirming that OH maser emission and ionization may coexist in some peculiar PNe. At least in the case of IRAS 17423-1755 we know that the source is strongly bipolar and it shows a very high velocity outflow (Riera et al. 1995).

A considerable number of post-AGB stars with optically bright counterparts are located in Region IV of the diagram. This position cannot be explained only in terms of interstellar or circumstellar reddening and requires the presence of hot dust surrounding the central star. Hot dust is only expected if the mass loss has not completely stopped after the end of the AGB phase. It is well known that some post-AGB stars show sporadic mass loss episodes which can accelerate the transition towards the PN stage. Supporting this interpretation, we have found that most of the post-AGB stars in Region IV of the diagram show H emission in their optical spectra (García-Lario et al. 1997a).

A similar situation is observed for many objects in Region I of the diagram identified as post-AGB stars. Some of them are located to the right of the main-sequence and giant stars and, again, this position cannot be explained in terms of interstellar or circumstellar extinction only. The near infrared excess observed is also atributed to hot dust formed as a consequence of recent post-AGB mass loss and this has also been confirmed in many cases through the detection of H emission.

4.2.3. Young stellar objects

Among the previously unidentified IRAS sources in our sample we have also found a considerable fraction of young stellar objects (25%), as expected, most of them concentrated in known star-forming regions, such as the Taurus-Auriga complex and Orion. This fraction is similar to that observed in the group of well identified IRAS sources. Their identification, however, is not possible based on near infrared data alone, and additional criteria or observations in other spectral ranges have been used.

For this purpose, as already shown, IRAS data can be efficiently used, since we know that T-Tauri and Herbig Ae/Be stars are located in a well defined region of the IRAS two-colour diagram. Unfortunately, as we know, in this region we can also find PNe, extremely reddened OH/IR stars and galaxies. On the other hand, compact H II regions and other heavily obscured young stellar objects are also observed only in a very specific region of the IRAS two-colour diagram but, again, some overlap exists, in this case usually with PNe. For the objects located in one of these two overlapping regions, and in the absence of data taken in other spectral ranges, the visual inspection of the Palomar or ESO prints and the search for possible associations with known star forming regions is very useful. In this way, for instance, we have identified IRAS 23312+6028 as the central star of an extended H II region, clearly visible on the Palomar print.

Of course, the absence of OH maser emission, the association with a low IRAS variability index or a very low galactic latitude are all criteria which can also be used as additional indicators of a young stellar nature. An abnormal concentration of IRAS sources in a small region of the sky has also been used to identify new star forming regions, specially if the IRAS colours for all them are consistent with a young stellar nature. In addition, sometimes a narrow CO emission line is observed towards sources embedded in molecular clouds, in contrast with the broad CO emission line observed in the expanding shells of C-rich post-main sequence stars. Moreover, in high density regions, H₂O maser emission (sometimes also NH₃), is frequently detected, usually associated to the presence of Herbig-Haro objects.

As we can see in Table 5 (click here), most of the previously unidentified IRAS sources now classified as young stellar objects appear concentrated in Regions III and IV of the near infrared two-colour diagram. As expected, the most heavily obscured ones, usually identified as compact H II regions or Herbig-Haro objects, tend to concentrate in Region III of the diagram, although a few of them are also found not too far in Region II. Embedded in their parent molecular clouds, some of them do no show any optical counterpart and the near infrared colours observed are consistent with hot dust emission at temperatures betwen 800 and 1500 K.

In contrast, those located in Region IV of the diagram usually show bright optical counterparts and are identified as T-Tauri or Herbig Ae/Be stars. Many of these stars are known to be surrounded by circumstellar disks which are probably responsible for the strong near infrared excess observed. The few objects identified as T-Tauri or Herbig Ae/Be stars found in Region I of the diagram with no or very little near infrared excess may be the result of the observation of these circumstellar disks pole-on.

4.2.4. Galaxies

As we know, only the brightest active galactic nuclei are expected to fulfill our selection criteria, and these are usually well known objects in the literature. Thus, it is not surprising that only two objects have been considered as possible new active galactic nuclei among the unidentified IRAS sources in our sample.

One of them, IRAS 01475-0740, has already been confirmed as a new Seyfert galaxy through optical spectroscopy (Pérez et al. 1990). The other one, IRAS 04117+6402, has tentatively been classified as a possible galaxy because it shows the characteristic colours both in the far infrared and the near infrared and it is located at a relatively high galactic latitude without being associated to any known star forming region. However, a spectroscopic confirmation is still pending.

Although active galactic nuclei appear concentrated in well defined regions of both the far infrared and the near infrared two-colour diagrams, their detection in the near infrared is very difficult, since even the brightest sources are expected to be very faint. In fact, both IRAS 01475-0740 and IRAS 04117+6402 were not detected in the K band above our detection limit, suggesting that a few others may be hidden among the rest of sources not detected in the K band listed in Table 4 (click here).

The final classification, including all objects observed in the near infrared so far, is shown in Table 6. In this Table we give the sequence number of each source in our sample (Col. 1), the IRAS name (Col. 2) together with any other name usually associated to the IRAS source (Col. 3), the IRAS flux at 12 m in Janskys as quoted in Version 2 of the Point Source Catalogue (IRAS Science Team 1988) (Col. 4), the non-colour corrected IRAS colours [12]-[25] (Col. 5) and [25]-[60] (Col. 6) as defined in Sect. 4, the LRS class when available (Col. 7), the IRAS variability index (Col. 8), a code from I to V indicating the position of the source in the near infrared two-colour diagram or the letters ND if the object was not detected (Col. 9), information about the molecular emission observations carried out in CO (Cols. 10 and 11) and OH (Cols. 12 and 13), and our tentative classification (Col. 14).