Astron. Astrophys. 363, 755-766 (2000)

4. Results

4.1. Extinction and distance

4.1.1. Distances from Wolf diagrams

The Wolf diagrams (Fig. 4) suggest the presence of several extinction layers, which are listed in Table 4. Extinction and distance values were derived by placing a weak extinction feature () into the model Wolf diagram at a given distance modulus. This step was repeated until the difference between model and observed curves became small enough, in our case until , . Our distances differ slightly from the ones using Wolf 's original distance interpretation, since our method derives the distribution of the extinction along the line of sight, and not only determines the distance moduli where breaking points appear on the cumulative starcount curve.

Table 4. Properties of the main extinction layers derived from the Wolf diagrams for six subfields (See Fig. 1). The columns are: (1) Name of the subfield(s);  (2) distance modulus of the extinction layer (non-biased fit);  (3) corresponding distance;  (4) value of visual extinction derived from the Wolf diagram. The error of this extinction value is estimated to be for all the layers.

Distance of Kh 15:

We assign the strongest feature in the Wolf-diagram of Subfield 3 (Table 4 and Fig. 4b) to Kh 15 appearing at (250 pc). According to the FIR map Fig. 1, the other weaker features around Kh 15 probably contain the extinction caused by the wall of GIRL126+10.

Distance of the loop GIRL126+10:

On the Wolf diagram containing the northwestern part of GIRL126+10, (Subfield 1, Fig. 4a) one can see several extinction features, the strongest one being at the distance modulus (200 pc). This feature can also be seen on the Wolf-diagram of Subfield 3 (see above), which also contains a part from GIRL126+10. Furthermore, there are other distance measurements of different parts of GIRL126+10. Obayashi et al. (1998) reported a distance of 180 pc for LDN 1333, which is located at the eastern part of GIRL126+10 (see Fig. 1). The extinction seen on their Fig. 1 is quite extended in space, and therefore the derived 180 pc distance is in a good agreement with our distance derived for this layer. Therefore we adopt 200 pc as GIRL126+10's distance, which gives a diameter of about 25 pc for the sized loop.

Distance of LDN 1308:

Since the subfield containing LDN 1308 (Subfield 5) is rather small, we were not able to construct a Wolf diagram only from the stars counted within this region. In order to increase the number of stars, we added the data of Subfields 3 and 5, and drew a common Wolf diagram (see Fig. 4c and Table 4). It shows a nearer and a more distant layer and . Since at no extinction can be seen in the Wolf diagram of Subfield 3, we identify the closer layer as the extinction of LDN 1308. The further layer is the effect of Kh 15. The distance of LDN 1308 cannot be determined more accurately due to the small star numbers at low distance moduli. Because of the size of GIRL126+10 and the upper limit of the distance of LDN 1308 it does not appear that LDN 1308 is physically connected with GIRL126+10 or Kh 15.

4.1.2. The extinction distribution

The visual extinction resulting from star counts of a POSS image (see Sects. 2.1 and 3.1.2) is shown in Fig. 6 as white overlaying contours. The resulting maximum value of is , derived as . We shall compare with in Sect. 5.1. For the average value of around Kh 15 in the larger, approximately 3.5 deg² sized POSS field, we obtained a value of , which is in a good agreement with the value of , derived as the sum of the extinction values seen at the Wolf diagram of Subfield 3, which contains Kh 15.

4.2. Far-infrared results

IRAS point sources:

Towards Kh 15 we found twenty point sources, which are shown in Fig. 5. The point sources nrs. 1, 2, 3, 9, 11 and 17 (marked by open circles) have good or moderate flux quality in at least two bands, and may be classified as stars according to Walker et al. (1989). Each of them coincides with a star on the POSS image. Point source no. 16 (IRAS 00506+7248) has good (3) flux quality in all bands, and it has been identified as the galaxy MCG+12-01-001. Other sources have poor flux qualities at three of the four wavelengths. Point source no. 12 (IRAS 00449+7208) is located at the very centre of the Kh 15 cloud core and has been detected only at 100 µm by IRAS. Additional FIR observations would be needed to detect the source in other bands, and determine its nature. We assume it to be a small sized cold core and not an embedded (proto)star in Kh 15.

Large-scale structure:

We have studied the large-scale structure of the interstellar dust towards Kh 15 using the IRAS maps. We present the ISSA 100 µm image of the region around Kh 15 in Fig. 1. The r.m.s. noise in this 100 µm image is 0.62 MJysr^-1.

Fig. 1 shows that Kh 15 may be associated with the diameter loop GIRL126+10 (Tóth et al. 1996), and with a string running NE-SW through LDN 1308 and Kh 15.

We analysed the 12, 25, 60 and 100 high resolution IRAS images (HIRES), and found that the cloud was not detected at 12 and 25 µm (the mean value of the excess intensity is about the same as the standard deviation). At 100 µm Kh 15 is well-resolved and shows a clumpy structure. The 100 µm resolution was about  75".

Dust properties: We constructed an map ( = 1.67(I₁₀₀ - I₆₀/0.21), see Boulanger et al. 1998), using the 60 and 100 µm COBE calibrated HIRES images, presented in Fig. 7 as white overlaying contours. The scaling factor corrects for the fraction of the 100 µm emission which is lost in the subtraction. Therefore we used in the following instead of the 100 µm excess to calculate the parameters of the dust, especially the dust mass. is free of the radiation coming from smaller sized grains (the so called cirrus component), therefore one can assume that its total emission can be described by one grain family, the big grains (see e.g. Désert et al. 1990), and only by one dust temperature. Since much of the 60 µm emission originates from smaller grain emission at higher temperatures, it cannot be used to derive the dust temperature.

Fig. 7. Grayscale with black contours: integrated intensity map (OSO-20m). Contours are from 1.5 Kkms-1 with 0.25 Kkms-1 steps, the velocity interval is 1 kms 5 kms-1. The center of the map is l=, b=. The beam size is indicated in the bottom-left corner. White contours: map of the central part of Kh 15, made from the 60 and 100 µm HIRES images. Contours are from 7.0 MJysr-1 with 2.0 MJysr-1 steps. The center of the map is the same as above. The spatial resolution was estimated to be 75". The 5.0 MJysr-1 contour, over which the mass estimation was done, approximately encircles the region presented here.

Lagache et al. (1998) found, that the FIR emission of the interstellar cold dust can be described by two components. One of these is associated with the galactic cirrus, shows good correlation at the 60 and 100 µm IRAS bands and its radiation can be described by a 17.51.5 K temperature, assuming a grey body spectral energy distribution (SED), with emissivity law. The other component is in particular present in the direction of molecular regions and its dust temperature is around 15 K, assuming the same SED as before. Using I, we separated this component from the FIR emission. Since we cannot determine the dust temperature of this component for Kh 15, we applied a uniform value of 15 K (the mean value according to Lagache et al., 1998), where ( 3 level over the background). Using these, we could estimate the 100 µm optical depth, the dust column density and mass (Hildebrand 1983). Because our cloud is small and has a relatively low density, we also calculated the 100 µm optical depth assuming an upper limit for the dust temperature of 17 K (see Fig. 7. in Lagache et al. 1998). The resulted cold dust masses are = 0.35 and = 0.12 using a uniform dust temperature of 15 K and 17 K, respectively, over the Kh 15 core region. The contours in Fig. 7 show three small cores with strong emission (denoted on the figure by C1, C2 and C3). The estimated masses of the C1, C2 and C3 cores are 8, 9 and 7 percents of the total dust mass of this area, respectively.

4.3. Molecular cloud

Nagoya-4m:

Fig. 6 shows the Nagoya-4m integrated intensity map of the ¹³CO (1-0) emission. The ¹³CO cloud has a size of 20´15´, and is elongated in the NE-SW direction. The velocities at the ¹³CO line peaks vary systematically in the SW-NE dierction from 2.8 to 3.2 in Kh 15. The central velocity of the line at the cloud centre is 3.2 kms^-1. We define the boundary of the molecular cloud at the level in Fig. 6, i.e. approximately at the level. The physical parameters of the ¹³CO cloud core are summarized in Table 5. Assuming local thermodynamic equilibrium, we estimated kinetic temperatures and ¹³CO column densities following Nozawa et al. (1991). We assumed a uniform excitation temperature () throughout the core using the value derived at the cloud centre. It is difficult to choose the appropriate ratio of the H₂ column density and the ¹³CO column density N(¹³CO). Since Kh 15 is relatively small and isolated, it is expected, that the relative abundance of the CO isotopes is lower than in larger and more complex star forming regions (see e.g. Tóth et al. 1995and Harjunpää & Mattila 1996). Therefore we used to calculate the molecular hydrogen column densities, which is a median value from the papers sited above. It should be noticed that the densities and masses estimated below are highly dependent on the abundance and might be overestimated by a factor of 2. This ratio can also show spatial variations inside the cloud.

Table 5. Derived parameters of the ¹³CO cloud core in Kh 15 from the Nagoya-4m and Onsala-20m observations (see Fig. 6 and Fig. 7). The optical depth and column densities are average values over the core. Size, mass and number density are based on the distance of 250 pc.

We derived an average H₂ density, (H₂), in the cloud core by dividing the peak value of the H₂ column density by the geometrical mean, , of the major and minor diameters. The total mass in the cloud core was calculated for an uniform sphere with diameter and average density , taking into account the mass of helium of 0.4 times the total hydrogen mass.
The excitation temperature at the centre of the cloud, 7.1 K, is lower than the temperatures derived in other dark clouds in the Cepheus-Cassiopeia region (Sato et al. 1994, Kun et al. 1994). On the other hand, the ¹³CO optical depth is higher than in most of them; only the C¹⁸O core `E' in LDN 1251 containing a compact molecular outflow source has a similar value of the ¹³CO optical depth (Sato et al. 1994). It should be noticed, that the ¹²CO spectra (Fig. 2) apparently show self-absorption, which may account for the low excitation temperature. On the other hand, the excitation temperatures derived from the Onsala-20m ¹³CO and C¹⁸O spectra present very similar values (see next paragraph).

We have defined the cloud core in the map where , which has a size of 12´ 8´. The total gas mass in the cloud core is 16 (taking into account the helium mass), which is listed in the last column of Table 5. The total mass of the cloud inside the region with cm^-2 ( detection limit) was estimated to be . Thus, the ¹³CO core contains at most 47% of the total mass of the cloud.

Onsala-20m:

We mapped the central region of Kh 15, which remained unresolved by the Nagoya-4m beam. The intergated intensity map (Fig. 7) shows three main condensations (cores), which we call C1, C2 and C3. The three cores shows the same geometrical structure as the corresponding cores (see also Fig. 7), although they appear at slightly different sky positions. Spatial differences may be due to the large ( 5´) size of the IRAS 100 µm detector pixels, and the upcoming HIRES processing. Therefore, we identify the CO-cores as the counterparts of the cores called the same. Because of the discrepancy of the CO and FIR map, we do not compare the distribution of molecular gas and dust in this finer scale. The derived properties of the Onsala C1, C2, C3 cores and the whole mapped region is summarized in Table 5. We calculated the ¹³CO optical depth and the ¹³CO column density N(¹³CO) as described in the previous subsection. The C¹⁸O optical depth and column density N(C¹⁸O) were calculated following Nozawa et al. (1991). The derivation of the molecular hydrogen number density was done as described in the previous subsection, but in the positions where ¹³CO was too thick optically () we used the C¹⁸O data to derived the molecular hydrogen number density as . This value was also derived as an average of the values listed in Harjunpää & Mattila (1996), as in the previous paragraph. The derivation of the number density of Onsala cores was done by removing the background molecular hydrogen column density value ( 2.010²¹ cm^-2), and dividing the remaining by the effective diameter, , of the core. Masses have been calculated using the sum of the background removed values of the column density over the core. The central line velocities at the column density peaks of all the three cores are 3.20.1 kms^-1, and the noticed velocity shift in the Nagoya ¹³CO data might be due the (relatively) coarse velocity resolution of those spectra, including the errors of the fits of the Gaussians.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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