3. Absorption features and continuum
The 7.35-7.85 µm spectrum of W 33A shows a wealth of broad and narrow absorption features (Fig. 1). The spectrum short-ward of 7.44 µm drops because of an absorption band at 7.39 µm. It has been detected before in ground based observations (Lacy et al. 1991) and will be discussed by Schutte et al. (in preparation). Another broad absorption band is present at 7.60 µm. It is blended with a narrower band at 7.67 µm. The latter can be attributed to absorption by solid CH4 in a matrix of polar molecules (Boogert et al. 1996). The peak position and width of the 7.60 µm band indicate that it is not associated with interstellar CH4. This feature was previously detected in a KAO HIFOGS spectrum at low signal-to-noise, and was tentatively identified with absorption due to solid in a -rich ice (Boogert et al. 1997). However, the present high quality ISO-SWS spectrum shows clear evidence for an extent of this broad band up to 7.76 µm. This is inconsistent with absorption due to , and the origin of this feature remains unclear. A number of narrow lines, with peak depths of 1-4% of the continuum, can be discerned as well. The prominent line at 7.66 µm coincides with the wavelength of the Q-branch of gaseous CH4 (all gas phase CH4 wavelengths cited in this paper were taken from the HITRAN database; Rothman et al. 1992). It is blended with the CH4 ice band at 7.67 µm and we used a laboratory ice spectrum to define the continuum for the Q-branch (; Boogert et al. 1996). For the other narrow absorption lines, local continuum points were determined by hand and interpolated with a cubic spline. A deep line is present at 7.537 µm, which corresponds to the wavelength of the R(3) line of gaseous CH4 (Table 1). Weaker lines at 7.452, 7.478, 7.505, 7.568, 7.596, 7.621, 7.729, 7.762, and 7.793 µm coincide with the wavelengths of the R(6), R(5), R(4), R(2), R(1), R(0), P(2), P(3), and P(4) lines respectively, taken into account that our wavelength resolution is 0.009 µm (Table 1). A tentative detection of the R(0) line was made by Lacy et al. (1991), with an equivalent width of . At the resolution of our observation, this corresponds to a central depth of %. This is a factor 2 larger than the line detected in our ISO-SWS observation (Fig. 1; Table 1). We note that the line detected toward W 33A by Lacy et al. (1991) is heavily blended with telluric CH4 lines.
Table 1. Equivalent widths of observed gaseous CH4 P and R branch lines. The standard deviation is given in parentheses. The wavelengths () were taken from the HITRAN database (Rothman et al. 1992). The wavelengths of the observed interstellar lines are in excellent agreement with these values.
Toward C 7538 : IRS9, the CH4 ice band dominates the spectrum (Fig. 1). There is also a hint for the presence of a 7.39 µm band toward this source. Contrary to W 33A, no broad 7.60 µm band is apparent. At 1-2% of the continuum, and 2-7 significance, we identify the Q-branch, R(3), R(2), R(1), P(2), and P(3) lines (Table 1). Weak evidence is found for the P(4) line as well. Lacy et al. (1991) claim ground based detections of the R(0) and R(2) lines towards this source. Due to the large radial velocity of C 7538 : IRS9 with respect to the earth, these lines are not blended with telluric CH4. They report equivalent widths of and respectively. Our detection of the R(2) line is in reasonable agreement with this value (Table 1), while the depth of Lacy's R(0) line (0.7% of the continuum at ) is at the noise level of the ISO-SWS spectrum (Fig. 1).
© European Southern Observatory (ESO) 1998
Online publication: July 7, 1998