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

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3. Data analysis

3.1. Optical data

3.1.1. Wolf diagrams

We have divided the field of the Schmidt plate into 6 subfields (see Fig. 1 and Table 1) in order to study the spatial variations of the extinction.


Table 1. Schmidt plate subfields. Columns are: (1) number of the subfield; (2)-(3) galactic coordinates; (4) average surface brightness at 100 µm from the ISSA image; (5) area of the subfield; (6) number of B, A and F -type stars inside the subfield; (7) comments, e.g. associated objects. See also Fig. 1

Classification of stellar spectra on the objective prism images was made by eye using the set of criteria of the Bonner Spectral Atlas II. (Seitter 1975). We have classified 768 B to K type stars. We estimate the accuracy of the spectral classification for B, A and F stars (13, 240 and 137 stars, respectively) to be [FORMULA] 2 spectral subclasses, which means [FORMULA] [FORMULA] in [FORMULA] and [FORMULA] for A type stars. Our sample is complete down to [FORMULA] for A type stars. We have assigned absolute magnitudes [FORMULA] to all the classified stars (Lang 1992) and derived apparent distance moduli [FORMULA].

Using these results we drew Wolf diagrams (Wolf 1923) in order to estimate the apparent distance moduli at which the presence the of the extinction layers cause a distortion on the curve of the [FORMULA] function i.e. the logarithm of the cumulative star counts as a function of the distance modulus [FORMULA] (see Fig. 4). The cutoff in the [FORMULA] curve for [FORMULA] is due to the limiting magnitude of our observations. On the other side, due to the small number of classified stars for low distance moduli, the proper distance of the extinction features closer than [FORMULA] cannot be identified. The corresponding distance of 150 pc is therefore an upper limit of their distance.

The extinction free curve was created using an extended galaxy model by Wainscoat (1992) (also Balázs 1998, priv. com.). In this way one can estimate the cumulative star counts for any spectral type within a region anywhere in the sky. The effect of an extinction layer is simply a shift towards higher distance moduli, and the measure of this shift is the average extinction caused by the layer itself. Best fit values for the distance and extinction of each subfield are presented in Table 4.

3.1.2. Visual extinction from POSS data

A blue extinction map was created by counting stars on a POSS blue plate (see Sect. 2.1) in the 1[FORMULA] 1o area around Kh 15. The visual extinction [FORMULA] was calculated using the method described by Dickman (1978b). The spatial resolution was 3´[FORMULA]3´ corresponding to the size of the reseau. We derived a value for reference star counts from the "non-cloudy" parts of the POSS plate. Another larger field around Kh 15 was analysed at a lower spatial resolution of 15´[FORMULA]15´ in order to derive an average value of [FORMULA] and compare it with the visual extinction obtained from the Wolf diagrams.

3.2. FIR data

Before using the ISSA and HIRES data all the maps have been calibrated using the COBE maps at appropriate (60 and 100µm) wavelengths. In the first step ISSA images were calibrated with the help of COBE-DIRBE annual average maps (AAM) (Hauser et al. 1998), following the information available at the COBE homepage 6. Determination of the zodiacal background emission on the DIRBE images was done by using the IRSKY 7 software (Ebert 1994). We assumed linear relationships. As a second step, calibrated ISSA maps were used to calibrate the HIRES maps to the COBE scale. Table 2 summarizes the resulting coefficients and levels of correlation.


Table 2. Results of COBE-ISSA-HIRES calibrations. We assumed linear relationships in all cases. The conversion is DIRBE = slope[FORMULA]ISSA + const. and ISSA = slope[FORMULA]HIRES + const.

Using the HIRES and ISSA 60 and 100 µm images, we made 100 µm excess, [FORMULA], maps (Laureijs et al. 1989). We derived the value of [FORMULA] at the outer parts of Kh 15 and obtained [FORMULA].

3.3. CO data

Fig. 2 shows the profiles of the 12CO and 13CO (J=1-0) lines taken at (l, b) = ([FORMULA], [FORMULA]), the position where the 13CO integrated intensity has a maximum on the Nagoya map (see Fig. 6). In the Nagoya 12CO spectrum, one can see two emission peaks at [FORMULA] [FORMULA] 3 and -1.8 kms-1. The former, more intense and much broader peak, has a prominent 13CO counterpart, and it seems likely that it is emitted by the molecular gas in the Kh 15 cloud. The weaker velocity component is telluric line, also seen in the 12CO (3-2) line observed with the KOSMA telescope. Table 3 lists the parameters of the three CO isotopic lines at [FORMULA] [FORMULA] 3 kms-1 obtained by fitting single Gaussians. A spectrum showing the strongest line of the KOSMA-3m observation of LDN 1308 is presented in Fig. 3.


Table 3. Overview of the CO measurements and characteristic line parameters.

[FIGURE] Fig. 1. IRAS 100 [FORMULA] image of the region around Kh 15 (ISSA data). The 5o diameter field of the Schmidt plate is marked with a black circle. An IRAS loop GIRL126+10 (Tóth et al. 1996) is marked with an ellipse drawn with a thick white line. Contours are drawn from 6 MJysr-1 with steps of 3 MJysr-1. We have indicated Kh 15 in association with the [FORMULA] diameter loop GIRL126+10 and with a string of cloudlets running through the dark cloud LDN 1308 (Lynds 1962). S1-S6 mark the six subfields of the Schmidt-plate (see Table 1).

[FIGURE] Fig. 2a-f. Sample spectra of Kh 15 at (l, b) = ([FORMULA], [FORMULA])
a and b : Nagoya-4m telescope (HPBW = [FORMULA]), 12CO (1-0) and 13CO (1-0)
c and d : Onsala-20m telescope (HPBW = [FORMULA]), 13CO (1-0) and C18O(1-0)
e and f : KOSMA-3m telescope, 12CO (2-1) and 12CO (3-2) spectrum at (l, b) = ([FORMULA], [FORMULA]), HPBW = [FORMULA] and [FORMULA], respectively.

[FIGURE] Fig. 3. KOSMA-3m (HPBW=[FORMULA]) spectrum of LDN 1308 towards l=[FORMULA], b=[FORMULA], of the [FORMULA] line. HPBW=[FORMULA].

[FIGURE] Fig. 4a-d. Wolf diagram (left) and distribution of visual extinction along the line of sight (right). In the Wolf diagrams the continuous curves present the observed star counts. Dashed and dotted curves present the extinction-free and -modeled star count curves (see Table 4 and Sect. 4.1). The dashed horizontal line in the extinction distribution figures represent the experimental detection limit ([FORMULA] 3[FORMULA]).
a for Subfield 1; b for Subfield 3 (Kh 15); c for Subfield 3 and 5 (Kh 15 and LDN 1308); d for the reference fields 2, 4 and 6;

[FIGURE] Fig. 5. Location of IRAS point sources near Kh 15 on the POSS (Palomar Observatory Sky Survey) image. Contours show the 100 [FORMULA] emission from 11.3 MJysr-1, with 4 MJysr-1 steps (ISSA image without galactic background subtraction). Point sources marked by circles have star-like IRAS colours; point sources marked by diamonds, boxes and triangles have good flux quality (3) only at 100, 60 and 25[FORMULA]m, respectively. Point source no. 16 (signed by a cross) is identified as the galaxy MCG+12-02-001 (see the text for details).

[FIGURE] Fig. 6. Grayscale with black contours: 13CO (1-0) integrated intensity map of Kh 15 (Nagoya-4m), centered at (l, b) = ([FORMULA], [FORMULA]), the velocity interval is: 1 kms-1 [FORMULA] [FORMULA] [FORMULA] 5 kms-1. Contours are from [FORMULA] with [FORMULA] steps and the beam size is indicated in the bottom-left corner. The 8´[FORMULA]10´ sized rectangle indicates the area mapped with OSO-20m (see Fig. 7).
White contours: blue extinction map of Kh 15. Contours are from [FORMULA] with [FORMULA] steps. The center of the field is the same as above. The square in the bottom-right corner indicates the size of the reseau.

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Online publication: December 11, 2000