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THE SPECTRA OF COMETS


Intro - Linear 2000WM1 - Neat 2001 Q4 - Machholz 2004 Q2



Spectrum of comet 2004 Q2 (Machholz)


Observations of December 13th 2004

In the picture 1 is shown the spectrum of comet 2002 Q2 taken on Dec 13th 2004. Its distance from sun was 1.37 AU while the distance from earth was 0.48 AU. Even if this comet was a 5th mag object, the spectrum is faint because only very few light of the comet can enter the narrow slit of the spectrograph (2x480 arc sec).
If compared to other cometary spectra, this one shows a very strong solar reflection within the central 10 arc sec (about 5000 Km at the distance of the comet), while the contribution from radicals is poor, perhaps due to the still quite large distance from the sun.
This fact can be quantitatively shown with a principal component decomposition of each spectrum that forms on the CCD along the slit. I fitted each row of the CCD with a sum of solar reflected spectrum (dust), C2 spectrum, CN spectrum and light pollution spectrum. Light pollution spectrum is used as a "flat reference" as it should be constant along the slit. An example of such fitting is shown in the lower part of the picture 1. Results are displayed in the picture 2.

Picture 1: Spectrum of comet C/2004 Q2 (Machholz) recorded on 13.95 UT Dec 2004 with 10 min exposure for each one of two spectral fields necessary to cover the full 3700-6000 ┼ range with 2.6 ┼/pixel dispersion. Spectral resolution is about 2 pixels.
The telescope we used is a Cassegrain 0.6 m reflector at F/15 and the unvignetted portion of slit spans 240x2 arc seconds with the long size aligned with R.A.
The spectrum is not corrected for instrumental and atmospheric spectral response. A sensitivity curve is available here .
In the lower part of the image a spectral decomposition is shown. This procedure is carried out for each raw of the image and produces the intensities profiles of picture 2 for each one of the main components that produces the light of the comet.

Spectral decomposition is more precise and selective than simple images with filters but can be done only along a selected profile or, if more profiles are acquired, a huge amount of exposure time is required.
I hope to be able to record a better spectrum of this comet in early January and then compare the profiles in picture 2 with the new ones.

Picture 2: Intensity profiles for reflected solar light (dust), C2 and CN radicals along a profile of the comet across the nucleus.
Distribution of dust peaks sharply around the nucleus, while C2 distribution is broader. Distribution of CN and C2 seem different but the low signal level make this result not sure.


Observations of January 4th 2005

In the picture 3-5 the spectrum of the coma of comet Machholz is shown from 3800 to 7000 ┼.
Due to the good brightness of the comet (around 4th magnitude) and very good sky conditions, this is, up to date, my best cometary spectrum and I tried to identify in it as much bands as I can find on books, papers, and web links. The lines that are marked with an asterisk arise from light pollution or earth atmosfphere absorption.
It appeared that this spectrum is absolutely normal for a comet coma with no strange bands or anomalous intensities that can arise when a comet approaches the sun closer than about 0.5 AU.
The comet on Jan 4.8 UT was at the closest distance from the earth (0.347 AU) but still quite distant from the sun (1.245 AU), 20 days before perihelion (24.9 Jan)

Picture 3: Spectrum of comet C/2004 Q2 (Machholz) from 3700 to 5000 ┼ recorded on 4.8 UT Jan 2005 with 15 min exposure with 2.6 ┼/pixel dispersion. Spectral resolution is about 2 pixels.
The telescope we used is a Cassegrain 0.6 m reflector at F/15 and the unvignetted portion of slit spans 420x3 arc seconds with the long size aligned with R.A. The detector is a 512x290 MX5 CCD.
The spectrum is not corrected for instrumental and atmospheric spectral response. A sensitivity curve is available here .

Picture 4: Spectrum of comet C/2004 Q2 (Machholz) recorded on 4.8 UT Jan 2005 with 15 min exposure with 2.6 ┼/pixel dispersion. Spectral resolution is about 2 pixels.
Instrumental details are the same as above.

Picture 5: Red and Near IR spectrum of comet C/2004 Q2 (Machholz) recorded on 4.8 UT Jan 2005 with 15 min exposure with 2.6 ┼/pixel dispersion. Spectral resolution is about 2 pixels.
Instrumental details are the same as above.

Picture 6: Spatial distribution along the slit of the principal spectral components building up the total spectrum of the comet in pictures 3-5. The fitting method is the same as indicated in picture 1. The good S/N ratio allowed to add also CN and C3. It is remarkable that maximum of C2 emission is not at the same point as maximum of dust reflection but it is clearly shifted to the right (West, thus toward the sun).


Go to introduction or other comets spectra:
Introduction - 2000 WM - Neat 2001 Q4



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20/12/2004