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Silicon dicarbide in carbon stars: Merrill-Sanford absorption bands with SA-100

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#1 mwr

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Posted 17 November 2019 - 06:18 AM

Merill-Sanford (MS) absorption bands are molecular absorption bands of SiC2 in the blue-green spectra of carbon stars. I have tried to detect these rather weak but famous bands using the SA-100 (R=100 in my setup) and VX And as object (red curve; 38 spectra at 180 sec. each were stacked). As reference object I used a spectrum of RY Dra from the BAA-database (R=430; blue curve) (https://britastro.or...ment=&plot=Plot).

 

Folie1.JPG

 

The most intense MS-band at 4985 A is already visible in the original SA-100 spectrum. Two more MS-bands could be detected:

 

Folie2.JPG

 

VX And is classified as C-J4.5 J 5 MS 5

RY Dra is classified as C-N3 III: C2 6.5 J 3.5 MS 4.5

 

(MK classification: https://iopscience.i...1086/133252/pdf)

 

The structure of SiC2 is quite interesting showing a T-shaped form (see Fig. 1 as inset; taken from http://theop11.chem....7_1195_SiC2.pdf) with a C2v-symmetry (vetrical symmetry plane with a 180 degrees rotational symmetry axis). The broad shape of the bands is due to overlapping electronic and vibrational energy levels (see inset showing the assignments; taken from https://iopscience.i...1086/310336/pdf).

 

A comparison of VX And with another SA-100 carbon star spectrum of Z Psc (C-N5 C24) shows the absence of these MS-bands and demonstrates that carbon stars can, indeed, be classified using a low resolution setup. 

 

VX_And_Z_Pisc2.jpg

 

VX_And_Z_Pisc.jpg

 

SiC2 bands are possibly generated in cooler material high above in the photosphere. Real cool chemistry in cool stars!


Edited by mwr, 17 November 2019 - 06:29 AM.

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#2 John Tucker

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Posted 17 November 2019 - 08:43 AM

That is one funky chemical structure to be sure.  At one time there was some controversy as to whether the all-carbon analog was capable of existence, or would spontaneously rearrange to a non-cyclic form.  Dunno if it ever got resolved, but putting a triple bond (prefers to be linear) in a 3 membered ring is quite a "stretch". 

 

   https://pubs.acs.org...021/ja00529a057


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#3 mwr

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Posted 17 November 2019 - 08:50 AM

VT photometry using a Canon 450 Da and the green channel (see Kloppenborg et al. JAAVSO 40 (2012) 815) with Tycho 2 VT data yielded a magnitude of 8.5 for VX And (UTC 20:40 2019-11-16):

 

Folie3.JPG

 

 

 

 



#4 Organic Astrochemist

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Posted 17 November 2019 - 04:04 PM

Great job!
I guess the relative intensities of the lines are also important.

My ALPY spectrum of WZ Cas has many small absorbances but the relative intensities didn’t match at all.

I understand there is an inverse correlation of MS strength with temperature. Why doesn’t WZ show MS bands?

#5 mwr

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Posted 18 November 2019 - 02:24 PM

I understand there is an inverse correlation of MS strength with temperature. Why doesn’t WZ show MS bands?

Typically, the SiC2 bands are either invisible or quite strong. Plots of densities of the SiC2 molecule against temperature show a steep rise to a narrow maximum followed by a steep descent. It is suspected that these bands will appear in the spectrum of any carbon star lying within the rather narrow range of effective temperature and having a sufficient excess of carbon in the atmosphere:

 

http://articles.adsa...000421.000.html

 

However, there might be more recent research on this topic which I'm not aware of. 




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