SPIRE FTS Spectroscopy

The SPIRE instrument onboard the Herschel Space Observatory has a 3-band imaging bolometer array and a Fourier Transform Spectrometer (FTS). The FTS covers an unprecedented frequency range simultaneously (450-1550 GHz, 194-670 microns), with a resolving power that ranges from 1200 at the highest frequencies to 350 at the lowest frequencies. Astronomers at UCL have been participating in a number of programmes that exploit the sensitivity and large wavelength coverage of the SPIRE FTS.

Among the objects we are studying are stars which have run out of hydrogen fuel and are nearing the ends of their lives. Such stars shed much of their mass during their final stages of evolution, in the form of a slow molecular wind. This mass loss profoundly affects the evolution of the star and eventually returns material enriched in the products of stellar nuclear burning to the interstellar medium.

Figure 1. Hubble Space Telescope images of AFGL 618 (l), AFGL 2688 (c) and NGC 7027 (r), showing the gases which are being shed as the stars near the ends of their lives.

Using the SPIRE spectrograph on board Herschel, we have carried out a study of the three evolved objects shown in Figure 1 above: AFGL 2688, AFGL 618 and NGC 7027. These three are all carbon-rich objects, at differing stages of their post-main sequence evolution. With the unprecedented spectral coverage and resolution afforded by Herschel, we have measured 150-200 emission lines in the spectrum of each object.  We have detected numerous molecules, some of which have not previously been detected in these objects. The strongest lines in the spectra are those due to carbon monoxide. Example spectra are shown in Figures 2 and 3 below.

CO Lines
Figure 2. Herschel-SPIRE spectrum of the planetary nebula NGC 7027, showing the lines of carbon monoxide which are very strong in this wavelength range.

Among the most puzzling observations is the detection of water in all three objects, as shown in the spectrum presented in Figure 3. In a carbon-rich environment, one would not expect oxygen-bearing molecules like water to be formed. Possible origins include the evaporation of cometary bodies in the outflow from the star, and chemical reactions following the dissociation of carbon monoxide molecules by interstellar ultraviolet light. Herschel data may permit us to determine which mechanism is behind the formation of water in these objects.

CRL618 Spectrum
Figure 3.  Herschel-SPIRE spectrum of the protoplanetary nebula AFGL 618.  The species responsible for the stronger emission lines are indicated. Click on the image to see the full spectrum.

Reference: Wesson et al, 2010, A&A Herschel special issue

Page last modified on 16 jul 10 14:55 by Fabrizio Sidoli