PMS OBJECTS IN THE STAR FORMATION REGION Cep OB3. II. YOUNG STELLAR OBJECTS IN THE Ha NEBULA Cep B

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1 Astrophysics, Vol. 56, No. 2, June, 2013 PMS OBJECTS IN THE STAR FORMATION REGION Cep OB3. II. YOUNG STELLAR OBJECTS IN THE Ha NEBULA Cep B E. H. Nikoghosyan Models for the spectral energy distributions of four stellar objects in the bright compact Hα nebula Cep B are constructed. With a high probability, three of them are found to be very young stellar objects of evolutionary class 0/I with ages of years, comparable to the kinematic age of the ionization front of the nebula itself. The IRAS source associated with Cep B is initiated by heated dust. An intermediate-mass (B2-B3) star of evolutionary class III lies at the center of the ionization front. The local density of PMS stars in the immediate neighborhood of the Cep B nebula exceeds that for the cluster as a whole. It is highly probable that this zone is a local source of a new star-formation stage. Keywords: compact star formation region individual: Cep B 1. Introduction Submillimeter wavelength observations ( 12 CO) have revealed an extended molecular cloud containing several hot clumps in the region of Cep OB3 [1,2]. One of these, located a distance of 725 pc from Cep B, is associated with a bright compact Ha nebula [3]. The nebula lies at the edge of the S 155 HII region and has a so-called bubble shape. Cep B is also a source of nonthermal radio emission produced by the collision of ionized gas with dense molecular matter [4]. Given that the expansion velocity of this HII region is on the order of ~2 km/s [5], the relative observed sizes at a distance of 700 pc indicate an estimated age of years [4], while according to observations V. A. Ambartsumyan Byurakan Astrophysical Observatory, Armenia; elena@bao.sci.am Original article submitted November, 2012; accepted for publication March 1, Translated from Astrofizika, Vol. 56, No. 2, pp (May 2013) /13/ Springer Science+Business Media New York 165

2 in the submillimeter range, its mass should be on the order of 0.3 M [6]. The source IRAS lies to the southeast of the ionized gas front. A comparison of its emission intensity in the submillimeter (450 and 850 µm) and infrared (MSX) ranges suggests that IRAS is caused by heated dusty material, rather than by a young stellar source [7]. We may assume that there is a subgroup of young stellar sources in this region that is younger than the age of the stellar population of the Cep OB3b association as a whole [4,5,8]. A compact group of young infrared stars also shows up in the neighborhood of IRAS in data from the 2MASS survey [9]. Studies of compact star-formation groups of this kind, which are deeply embedded in a molecular cloud, are very important because their formation is generally initiated in secondary star-formation stages and depends, to a great extent, on the properties of the parent cluster itself [10,11]. Thus, studies of these groups are of great value for gaining a complete picture of the star-formation process in the cluster as a whole. This paper is a study of young stars in the neighborhood of the bright Hα nebula Cep B. 2. The group of young stars in the neighborhood of the Cep B nebula 2.1. Method. Several sources have been discovered in submillimeter wavelength (450 and 850 µm) images of the neighborhood of IRAS [7,12]. Three have been identified as objects contained in infrared data bases, specifically MSX, WISE, and 2MASS. In addition, data from the Spitzer IRAC telescope have also been used [13]. The photometric parameters of these objects are listed in Table 1. The submillimeter (SM) fluxes and designations in this table correspond to the data shown in Table 2 (the Fundamental Map Data Set) of Ref. 12. The position of the objects is indicated in Fig. 1, which shows images of this region in several wavelength ranges. The position of the source IRAS is denoted by an ellipse. The discrepancy between the coordinates in the submillimeter and infrared ranges is less than 3". The submillimeter sources 4 and 5 of Table 2 of Ref. 12, which are also indicated in Fig. 1, were not identified in the infrared. Table 1 also lists the photometric parameters of one of the radio sources, A-NIR, in this region [4] (see Fig. 1) that is not resolvable in the submillimeter range. The coordinates of this object were derived from images in the 2MASS survey. The V and I stellar magnitudes of this object were also determined. The observational technique is described in part I of this article [14]. The other objects in Table 1 were unresolvable in the optical range. Based on the photometric parameters for each of the stellar objects in Table 1, we have constructed a model of the spectral energy distribution for PMS stars which has been described in detail in Refs. 15 and 16. We have also determined their major parameters, in particular, their age, mass, bolometric luminosity, etc. In constructing the spectral energy distributions, we have assumed (as in Ref. 14) a distance to the cluster of pc [3], but the absorption (A V ) interval has been increased to 20 m. The most probable of the proposed models has been chosen. 166

3 TABLE 1. Photometric Parameters of the SM and IR Sources Object SM1 SM2 SM3 A-NIR RA (2000) Dec (2000) (Jy) 0.31 (R = 25".3) 0.98 (R = ".3) 7.31 (R = 49".3) (Jy) 6.64 (R = 25".3) (R = ".3) (R = 49".3) - MSX A (Jy) MSX C (Jy) MSX D (Jy) MSX E (Jy) WISE WISE WISE WISE IRAC IRAC IRAC IRAC MASS J MASS H MASS K I V Results. Spectral energy distributions were constructed for all the objects listed in Table 1. The results are shown in Fig. 2 (left). The major parameters for the most probable models are listed to the right of the graphs. We consider each object separately. The evolutionary classes were determined based on the models for the spectral energy distribution discussed in Refs. 15 and 17. SM1. A young stellar object with a mass essentially equal to that of the sun. The spectral energy distribution corresponds to an object in evolutionary class I with a massive shell and disk. SM2. A young stellar object of intermediate mass in evolutionary class I with a massive shell and disk. This is consistent with the conclusions (B-NIR [4]) of a study based on data regarding nonthermal emission from this object in the millimeter range. This object is also an x-ray source [18]. SM3. The spectral energy distribution has been constructed at only five points. However, the rather wide range of the photometric parameters and the fairly high level of agreement ( χ 2 = 126 ) nevertheless suggest that this, the brightest of the objects in the submillimeter range, is also a young stellar object of intermediate mass, with a ratio 167

4 Dec (2000) 38:00 62:37:00 36:00 35:00 39:00 38:00 37:00 36: :57: :57:00 56:50 Dec (2000) 39:00 62:38:00 37:00 39:00 62:38:00 37:00 36:00 36: :57:00 56:50 RA (2000) :57:00 56:50 RA (2000) Fig. 1. Images of the region surrounding the IRAS source in different wavelength bands. of the photospheric, disk, and shell emission corresponding to evolutionary class 0/I. A-NIR. This is a star with an intermediate mass that is considerably older than the others. It has a negligible infrared excess and is most likely in evolutionary class III. Its considerable reddening is explained by high interstellar absorption in this region. This is consistent with a photometric study in the near infrared [4]. This star is also an x- ray source [18]. It should be noted that this star lies essentially at the center of the bubble-shaped Ha nebula mentioned above and an ionization front region shows up clearly around it in infrared images of this region. Its intensity increases toward longer wavelengths (see Fig. 1). According to our data, this object is in spectral class B2-B3 with a bolometric luminosity on the order of 100 M. These characteristics are in quite good agreement with the estimates of Ref. 4 and this again confirms our assumption that the emission from this B-star causes heating of the material in a molecular cloud and the formation of an ionization front. The interstellar absorption for three of the above stars exceeds the average for the cluster as a whole [14]. 168

5 IRAS An attempt was made to construct the spectral energy distribution for this source as well. To do this we have used photometric data from the submillimeter range [7] and the IRAS and MSX data bases. However, even for the most probable model, χ 2 > This confirms the assumption that this IRAS source is not driven by a stellar source, but by heated dust. Note that its coordinates coincide with the brightest part of the ionization front (see Fig. 1). λf λ (erg/cm 2 /s) SM1 2 χ = 114 Inclination= 18 o A v = 4 m.8 Distance = 0.71 kpc 4 Age = years Mass of star in M = Temperature of star (K) = 3974 Bolometric luminosity in L = Mass of disk in M = Mass of shell in M = Accretion rate in = M λf λ (erg/cm 2 /s) SM λ( µm) χ 2 = 691 Inclination = 18 o A v = 16 m.81 Distance = 0.74 kpc 3 Age = years Mass of star in M = 5. 2 Temperature of star (K) = 4219 Bolometric luminosity in L = Mass of disk in M = Mass of shell in M = Accretion rate in = M λ( µm) λf λ (erg/cm 2 /s) SM λ( µm) χ 2 = 126 Inclination = 41 o A v = 15 m.65 Distance = 0.78kpc 4 Age = years Mass of star in M = Temperature of star (K) = 4566 Bolometric luminosity in L = Mass of disk in M = Mass of shell in M = Accretion rate in = M 2 Fig. 2. Spectral energy distributions and major parameters of the young stellar objects. 169

6 λf λ (erg/cm 2 /s) A-NIR λ( µm) χ 2 = 189 Inclination = 76 o A v = 14 m.06 Distance = 0.71 kpc Age = years Mass of star in M = Temperature of star (K) = Bolometric luminosity in L = Mass of disk in M = Mass of shell in M = Accretion rate in = M 3 Fig. 2. (Conclusion) 3. Local density distribution Figure 3 shows the distribution of the local density of all the Ha emission stars [14]. The local density was determined for each object within an area with a radius equal to the distance to the n-th nearest emission star. Overall, the local density was determined for 149 stars with n = 10. To a first approximation the density distribution of the emission stars is relatively uniform. No groups were observed for which the density exceeds the average over all the objects by more than 3 σ. However, in the region immediately adjacent to the source IRAS , the emission Y (arcsec) X (arcsec) Fig. 3. Distribution of the local density of Ha emission stars. The position (0,0) corresponds to the coordinates of the source IRAS (22 h 57 m 04 s.93; +62 o 37'49".57). 170

7 stars are positioned more compactly and form a small group with comet-shaped density contours. The density of this group exceeds the average over all the emission objects by a factor of two. On the charts of the 2MASS survey this same object also contains a compact group of young stellar sources [9]. The density contours for the infrared group have a similar shape. To some extent this shape of the density contours, i.e., a significant reduction in the density of Hα emission stars to the east of IRAS , may be explained by increased interstellar absorption in this direction [3,14]. 4. Discussion and conclusion Photometric data in the submillimeter, infrared, and optical ranges have been used to construct models for the spectral energy distribution of four stellar sources located in the neighborhood of the bright compact Hα nebula Cep B. These results show that three of the four objects are highly likely to be very young stellar objects of evolutionary class 0/I with a ages of years. Their age is comparable to the kinematic age of the ionization front of the nebula itself, which is estimated to be on the order of 10 4 years. The ionization region along the edges of the Hα nebula shows up clearly in the infrared images. The relative brightness of the ionization region increases at longer wavelengths. The IRAS source associated with Cep B has the same coordinates as the brightest part of the ionization front. An attempt has also been made to construct the spectral energy distribution of this source using data from the data bases of the IRAS and MSX surveys, as well as observations in the submillimeter range. However, even for the most probable model χ 2 > All of this confirms the earlier assumption that the IRAS source itself is not initiated by a young star, but by heated dust. The other submillimeter sources (4 and 5 in Ref. 12), which were not identified in the infrared, may simply be bunches of dust or very young cold protostars. A star of intermediate mass and spectral class B2-B3 lies at the center of the ionization front. The most probable model derived from the photometric data for this star corresponds to evolutionary class III and an age of years. It is probably the source of the radiation that has led to heating of the interstellar matter in a molecular cloud and to the formation of the ionization front. In the immediate neighborhood of the Cep B nebula the local density of PMS stars is almost twice the density of young stellar objects in the cluster as a whole. These results again confirm the earlier assertions that a group of young stellar sources is deeply embedded in a molecular cloud in the region of Cep B. This zone is most likely the local source of a new, third stage of star formation in the Cep OB3b association. REFERENCES 1. A. L. Sargent, Astrophys. J. 218, 736 (1977). 2. A. L. Sargent, Astrophys. J. 233, 163 (1979). 3. M. A. Moreno-Corral, C. Chavarria-K, E. de Lara, and S. Wagner, Astron. Astrophys. 273, 619 (1993). 171

8 4. L. Testi, L. Olmi, L. Hunt, et al., Astron. Astrophys. 3, 881 (1995). 5. N. Panagia and C. Thum, Astron. Astrophys. 98, 295 (1981). 6. M. A. Thompson, J. Hatchell, A. J. Walsh, G. H. Macdonald, and T. J. Millar, Astron. Astrophys. 453, 1003 (2006). 7. S. J. Williams, G. A. Fuller, and T. K. Sridharan, Astron. Astrophys. 417, 115 (2004). 8. M. Felli, G. Tofani, R. H. Harten, and N. Panagia, Astron. Astrophys. 69, 199 (1978). 9. M. S. N. Kumar, E. Keto, and E. Clerkin, Astron. Astrophys. 449, 1033 (2006). 10. B. G. Elmegreen, Y. Efremov, R. Pudritz, and H. Zinnecker, in: eds. F. A. P. Mannings, S. S. Boss, and S. S. Russell, Protostars & Planets IV (2000), p C. J. Lada and E. A. Lada, Ann. Rev. Astron. Astrophys. 41, 57 (2003). 12. J. Di Francesco, D. Johnstone, H. Kirk, T. MacKenzie, and E. Ledwosinska, Astrophys J. Suppl. Ser. 175, 277 (2008). 13. K. V. Getman, E. D. Feigelson, K. L. Luhman, et al., Astron. J. 699, 1454 (2009). 14. E. H. Nikoghosyan, Astrofizika 56, 33 (2013). 15. T. P. Robitaille, B. A. Whitney, R. Indebetouw, K. Wood, and P. Denzmore, Astrophys. J. Suppl. 167, 256 (2006). 16. T. P. Robitaille, B. A. Whitney, R. Indebetouw, and K. Wood, Astrophys. J. Suppl. 169, 328 (2007). 17. B. A. Whitney, K. Wood, J. E. Bjorkman, and M. Cohen, Astrophys. J. 598, 1079 (2003). 18. K. V. Getman, E. D. Feigelson, L. Townsley, et al., Astron. J. Suppl. 163, 6 (2006). 172