Star-forming processes in Cepheus OB2
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h t r o n . Nachr. 310 (1989) 5. 385-388
Star-forming processes in Cepheus OB2
L. G. B A L ~ and M . KUN. Budapest, Hungary
Keceiwd 1988 October 15
The properties of IRAS point sources in the region of the Cepheus bubble were studied. This bubble is a possible very old supernova remnant which includes the association Cepheus OB2. A large number of sources which are probably low- and medium-mass embedded stars are situated around the bubble. I t is concluded that whereas high-mass stars which have formed in the bubble are already on the main sequence. lower-mass stars are still embedded in their parent clouds.
Die Eigenschaften von IRAS-Punktquellen im Gebiet der ,.Cepheus-Blase, einem mtiglicherweise sehr alten Supernovaiiberrest. der auch die Cepheus-OB2-Assodation enthllt, wurden untersucht. Eine groBe Anzahl von Quellen sind i i k r die .,Blase verteilt. bei denen es sich wahrschein- lich urn Sterne kleiner oder mittlerer Masse mii Hullen handelt. Es wird geschlossen, daB Sierne mit g r o k n Massen schon auf der Hauptreihe liegen. wahrend die Sterne kleinerer Masse sich noch in den Wolken, in denen sie entstanden sind. befinden.
Kc>I. words: infrared sources -- infrared stars .- star formation
.4.4 A suhjeci ckussificuiiort : 1 3 I
The interstellar medium is full of shell-like structures which display the various effects of high mass stars on the interstellar matter. These shells. arcs and filaments often have dimensions of several hundred parsecs and are observed at different wavelengths, extending from X-rays to radio. Star formation can be observed in many shells (e.g., HERRST and ASSOUSA 1977). showing that the matter compressed by stellar winds, expanding HI1 regions or supernova explosions or the composite effect of all ofthese became gravitationally unstable. In the present paper weexamine the observable traces ofstar formation in a giant ring of the interstellar matter which can be seen in the IRAS Skyflux maps in the Cepheus constellation. The infrared ring displays a possible genetic link between several optically separated star-forming regions such as S 129, S 131 (IC 1396). NGC 7129 and S 140, each situated at a distance of 900 pc (KuB, BALAZS and T ~ T H 1987). We shall regard these regions as different subgroups of the association Cep OB2 because the high mass star which produced the bubble must have been a member of Cep OB2. The aim of the present paper is to find evidence of star formation around the whole bubble by examining the IRAS point sources of the region.
2. Statistics of IRAS point sources
We have investigated an area between 20h42 and 22h18 in right ascension and 56--68 in decli?ation containing the Ccpheus bubble. The catalogue of IKAS point sources lists 1922 objects in this field. We used the first version (PSCI) in our investigation. Out of the 1922 sources, however. only 88 have high or moderate flux qualities in all of the four bands.
Fig. 1. Two-colour diagram logCf(25)if( 12)) vs. logU(60)if(25)) for all objects with flux qualities higher than one in all three wavelength bands
386 Astron. Nachr. 310 (1989) 5
... . .. .
. .. 561 :. , , . . + . ! +: , . I 22 22 22 04 21 46 21 28 21 10 20 52
s t e ( l a r sources RA
.. . . ..
22 22 22 04 21 46 21 28 21 10 20 52 nonstellar sources RA
60. . . . . -.. . . .. . . .. .. . 58. .
,. ...: : + . ,: 56 22 22 22 04 21 46 21 28 21 10 20 52
nonstellar sources RA
Fig. 2. Distribution of the group of stellar sources among the objects of Fig. 1
Fig. 3. Distribution ofthe group ofnonstellar sources among the objects of Fig. 1
To obtain reliable fluxes and an acceptable number of sources for statistical studies we extracted three subsamples from the originai sample.
In the first sample we selected objects with flux qualities higher than one at 12,25 and 60 microns. We obtained 209 sources with this property. These enabled us to construct a logCf(25)/(12)) vs. logCf(60)/(25)) two-colour diagram in order to isolate the ordinary stellar component from those objects which probably are closely related to the recent star formation in the bubble. Figure 1 shows the two colour diagram obtained in this way. There are two significant groups in this diagram and most of the objects belong to one of these. The group at the left bottom part has colour indices which are typical of normal stars (EMERSON, 1986). The other group, however, has colour indices indicating flux densities that increase with increasing wavelength. The fact that they were detected at 12, 25 and 60 microns allows us to suppose that they have stellar cores embedded in dense circumstellar dust clouds (BEICHMAN et al. 1984).
We reprojected both these groups onto the celestial sphere in order to study their relationship to the known star-forming regions which lie along the Cepheus bubble. We have displayed the distribution of the stellar group in the (alpha, delta) plane in Fig. 2 and it shows nothing reminiscent of the bubble. Figure 3 shows the same distribution for the second group. Unlike normal stars these objects nicely reveal the HI1 regions: S 129, IC 1396, S 134 and S 140. The upper part of the bubble is poorly populated and no object is found inside the bubble.
As mentioned above stellar objects just after their birth and embedded in their parent clouds must have infrared fluxes rising steeply from 12 to 100 microns. We can expect a large number of similar embedded sources in a giant star-forming region which are too faint to be detected at 12 microns. In order to study the distribution of the probable fainter or cooler protostellar objects we extracted a second subsample consisting of objects having good flux qualities at 25,60 and 100 microns. Most of them were not detected at 12 microns and are characterized byf(25) < f(60) < f(lO0). They are assumed to have stellar cores. There are 135 sources in the region that meet this criterion. Their surface distribution is shown in Fig. 4. Three of them (21161 + 6141,22109 + 6505 and 22134 + 5834) were included in a CO survey by CASOLI et al. (1986). They found a CO emission peak on all of these sources, and estimated the distance of these CO sources as 0.9- 1 .O kpc, which supports their relationship with Cepheus ring. It is reasonable to believe that most sources with similar infrared energy distribution are molecular condensations around an embedded star. Our third subsample consisted of objects detected only at 60 and 100 microns. They are called warm cirrus sources by BEICHMAN et al. (1984) and are probably small density or
22 22 22 04 21 46 21 28 21 10 20 52 RA embedded sources
Fig. 4. Distribution of the embedded sources Fig. 5. Distribution of the warm cirrus sources
L. G . R~i.li7-s and M. K ~ J N : Star-forming processes in Cepheus OB2 387
temperature enhancements in an extended cloud which will never collapse into stars. We cannot, however. exclude the possibility that they are heated by embedded stars which are too faint to be detected at the shorter wavelengths. Reprojecting these objects onto the celestial sphere one can be convinced that they display beautifully the ring structure observed on the skyflux maps (see Fig. 5). This may be because these sources represent the small-scale inhomogeneities in the matter of the ring and, therefore. they should follow its shape. but it is possible that they indicate that star formation is going on in the cooler and maybe less dense northern part of the bubble.
In the following we make a comparison between the brightness distributions of sources at 60 and 100 microns in different parts of the Cepheus ring. For this purpose we divided the ring into three parts which differ from each other in the proportion of high-mass stars having been formed in them. The first consists of the northern and north-western, faint part. the second is the southern part containing IC 1396; the third part represents the eastern part comprising S 140. Fig. 6 shows the brightness distribution of thc "warm cirrus" sources. Inspecting this figure one has the impression that there is a significant shortage of bright sources in the first part in comparison with the other two parts. We performed two sample Kolmogorov-Smirnov tests pair-wise between the three groups defined. As we expected the distribution of the first group a t 60 microns is different from the other two at a 99'1" and 95% significance level, respectively. These levels are higher than 99'2, at 100 micron. Therefore we concluded that the differences in the brightness distributions are significant.
Fig. 6. Brightness distributions of the "warm cirrus" sources for ( I ) the northern and north-western part, (2) the southern part. and (3) theeastcm part of thecepheus ring
3. Physical nature of point sources
In order to clarify the nature of the point sources forming the ring structure we looked for optical identifications with young stellar objects. The largest number of such identifications can be found among the stellar sources. About one-third of the early type members of Cepheus OB2 classified by SIMONSON (1968) have counterparts in the PSC. They are mostly Be stars. Some B-type stars, however, coincide with IRAS sources having embedded energy distribution, which suggests that these stars are surrounded by circumstellar clouds. A few T Tauri and faint H-alpha emission stars of Cep OB2 also have IKAS counterparts either with.f(l2) > j(25) or.f(l2) < f(25). "Cirrus" sources which were detected only at 60 and 100 microns have no optical identifications and their relationship with star formation is uncertain. Embedded sources are probably the most reliable young stellar objects. A lower limit can be given for their luminosities with the assumption that they radiate most of their energy in the far infrared region covered by the IRAS bands (CASOIJ et al. 1986). Then, with the assumption that the sources are heated by a central object for which log(L/L ) = 3.61 log(M/Md (AI.I.ES 1973), the mass of this central star can be estimated. Because luminosity is a strongly increasing function of mass, stars with slightly different masses produce very large differences in luminosity. All the objects which we regard as embedded stars probably have masscs of 1.5-7M0. We shall examine the three parts of the region separately. They contain 14, 76 and 40 sources with the above spectral characteristics, respectively. Their average 25-, 60- and 100-micron fluxes and flux ratios are given in Table 1. Although the average brightnesses differ significantly for the three regions, the flux ratios characterizing the dust temperature are remarkably similar. Table I lists the average luminosities and masses of the central objects as well.
part . / I 2 9 ,/(60) /'(100) J(25)1./(60) .f(60)/./(100) LiL, MIM, . . ____ .
. - .- -
I 0.35 1.38 7.76 0.24 0. I9 6.5 1.7 2 0.82 3.72 18.20 0.22 0.20 15 2.14 3 1.25 4.27 21.38 0.29 0.20 18 2.24
- . . . - .... . ._ - .. . . .
388 Astron. Sachr . 310 (1989) 5
The giant infrared ring in Cepheus OB2contains at least 135 far infrared sources which show an energy distribution character- istic of young stellar objects shrouded by dusty circumstellar envelopes. A crude estimation of their luminosities and masses shows that they represent low- or medium-mass stars. It is reasonable to assume that there are somc contracting protostars among the fainter IRAS sources detected only at 60 and 100 microns. With the assumption that their energy distribution is similar to those observed at 12 and 25 microns their luminosities and masses can be estimated and we find that these sources are probably heated by stars whose mass is 1 - 1.5 solar masses. There are no sources corresponding to high-luminosity 0- or B-type stars. If the star formation in this region began 3 million years ago which is the estimated age of the bubble. the largest clumps have already evolved onto the main sequence because of their shorter contraction times.
ALLEN. W. C.: 1973, Astrophysical Quantities, 3rd ed.. The Athlone Press. Univ. of London. B1:lCHMAN. C. A., EMERSON. J . P., JENNINGS, R. E., HARRIS, S., BAUD. B.. and YOUNG. E. T. : 1984, in : Nearby Molecular Clouds (ed. G. SERRA),
CASOIJ. F., DLPRAZ. C.. GERIN. M., C'OMBES, F.. and ~ U L A N G E R , F.: 1986, Astron. Astrophys. 169, 281. EMERSON. J. P . : 1986. in: IAU Symp. No. 115, Star Forming Regions (eds. M. PEIMBERT and J. JUGAKU), Reidel, Dordrecht, p. 19. HERBST, W. and ASSOLSA, G. A.: 1977. Astrophys. J. 217. 473. KUN, M.. BALAZS. L. G.. and T ~ T H , 1.: 1987, Astrophys. Sp. Sci. 134,211. SIMONSON, S. C.: 1968. Astrophys. J . 154,923.
Address of the authors: L. G. BAI.AZS. M . KUN Konkoly Observatory Box 67 14-1 525 Budapest Hungary
Springer. Berlin, p. 95.