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Astron. Astrophys. 331, 742-748 (1998)

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3. Results

In order to compare the emission detected by ISOPHOT with the emission in the IRAS 12 µm band (7.5 to 15.0 µm) we have summed up our values at 7.3, 10, 11.3, 12.8, and 16 µm (multiplied with appropriate weights). The in-band fluxes thus determined for the 7.5 - 15 µm wavelength band are given in Table 2 together with the IRAS in-band values for the 12, 25, 60, and 100 µm bands. As the comparison shows, similar values are obtained for the 7.5 - 15 µm emission both by ISOPHOT and IRAS. This provides a further confirmation for our flux calibration.


[TABLE]

Table 2. The integrated emission 7.5 - 15.0 µm as observed with ISOPHOT and the IRAS in-band fluxes at 12, 25, 60, and 100 µm. All values are relative to a zero point determined at the reference positions. Error estimates are given in the last line. The last column gives the ratio of the 12 and 100 µm IRAS in-band fluxes.


The observed in-band power of the cirrus emission at the three ON positions is given in Table 3. For the three shortest wavelengths only upper limits could be derived. They correspond to 2 [FORMULA] deviations from a fitted first or second order polynomial through all (ON and REF) data points of the sparse maps as shown in Fig. 3. In order to estimate the contributions of the UIR bands we have assumed a flat continuum, [FORMULA] = const, between 6 - 10 µm, i.e we have used the observed continuum at 10 µm for the 7.3 and 7.7 µm filter bands. For the 11.3 and 12.8 µm filter bands we have interpolated the continuum linearly between the 10 and 16 µm observations. For the 3.29 µm filter the same upper limit as in Table 3 is given. The resulting in-band emission of the UIR features in the bands 3.29, 7.3, 7.7, 11.3, and 12.8 µm are given in columns (2) - (6) of Table 4. In column (7) we give the total emission power (bands + continuum) between 6 - 16.5 µm. The contribution by UIR bands is 38 to 52% of the total emission in the 6 - 16.5 µm band (see Table 4). In column (8) we list the ratio of the UIR band contribution to the total emission power and in columns (9), (10), and (11) we give the UIR band ratios P(11.3)/P(7.7), [P(11.3) + P(12.8)]/P(7.3), and P(3.29)/P(11.3), respectively.


[TABLE]

Table 3. The in-band emission power of the cirrus cloud (bands + continuum) as observed with ISOPHOT in the different filter bands. The values are relative to a zero level determined at the reference positions. The error estimates given in parentheses include (external) statistical errors and a 10% calibration error. The unit is 10-12 W cm-2 sr-1.



[TABLE]

Table 4. The in-band emission power of UIR bands as observed with ISOPHOT in the different filter bands (columns 2 - 6). The values are relative to a continuum level as explained in the text. The error estimates given in parentheses include (external) statistical errors and a 10% calibration error. The unit is 10-12 W cm-2 sr-1. Column (7) gives the total emission power (bands + continuum) in the 6 - 16.5 µm region. Columns (8) - (11) give bands-to-total and three different band ratios. In the lower part of the table values from literature are given for comparison: diffuse emission of the galactic disk (DGBE), Mattila et al. (1996), Onaka et al. (1996), Tanaka et al. (1996); [FORMULA] Oph, Boulanger et al. (1996); RN, PN, HII regions, see Cohen et al. (1989), Roelfsma et al. (1996), Cesarsky et al. (1996), Verstraete et al. (1996).


Based on the spectral energy distributions for the three positions in G 300.2-16.8 (see Fig. 4 and Tables 3 and 4) we infer the following results:
(1) The excess emission in the 7.7 and 11.3 µm narrow filter bands indicates the presence of these UIR bands. The band ratio P(11.3)/P(7.7) = 0.33 - 0.43 is within the range found for the high-ISRF objects.
(2) The broad-band 7.3 µm filter indicates an integrated UIR band emission between 6 - 9 µm by factor [FORMULA] 3 larger than the power in the 7.7 µm band. Thus, in addition to the 7.7 µm band, contributions by the UIR bands at 6.2 and 8.6 µm and/or the broad plateau emission between 6 and 9 µm are suggested.
(3) A contribution of the 12.7 µm band is suggested by the enhanced level of the 12.8 µm filter.
(4) An upper limit to the 3.3 µm UIR band is obtained. For position O1 the band ratio P(3.3)/P(11.3) is [FORMULA] 0.14 which is marginally compatible with those of high-ISRF objects.
(5) Continuum emission is detected at 10 and 16 µm in all three spectra. At 3.6 and 4.85 µm only an upper limit to the continuum can be determined. Over the wavelength region 6 - 16 µm the contribution of the continuum emission is roughly equal to the emission power in the bands. There are distinct differences in the slopes of the 10 - 16 µm continuum between the three positions.
(6) The absolute level of the average UIR band emission in the 7.7 µm filter is [FORMULA] 3 10-12 W cm-2 µm-1 sr-1 which is by a factor [FORMULA] fainter than in typical RNs or HII regions (e.g. NGC 2023, Orion Bar).
(7) The ratio of the integrated emission powers (in W cm-2 sr-1) of the two UIR band groups at 11 - 14 and 6 - 9 µm is measured between 0.23 and 0.31 and is within the range found for the high ISRF objects.

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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