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Molecular spectra of metal oxides in pure S stars - R Gem

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#1 mwr

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Posted 23 February 2021 - 06:13 AM

R Gem is a Mira type variable star and can be observed near its maximum light right now (differential photometry: 6.8 mag at 2021-02-20 using the green channel of a modified Canon EOS 450 Da):

 

Folie1.JPG

 

The low resolution spectrum (resolution approx. 12 Angström at 4861 Angström)  was recorded using the SA-200 grism in the converging beam setup. The rectified spectrum was compared with a ProAm spectrum from the BAA database and showed a good agreement:

 

Folie2.JPG

 

The spectrum shows the main characteristics of a pure S-type spectrum, namely, the dominance of ZrO bands (triplet system), and the absence of strong TiO bands. Other s-process elements like La, Ce and Y were detectable as well in form of their oxides.The band heads of the ZrO triplet system could be assigned with the help of an atlas of the spectra of pure S stars http://adsabs.harvar...MNRAS.184..127W:

 

Folie3.JPG

 

Vanadium oxide with a dissociation energy less than 7 eV is relatively fragile and could not be observed in this maximum light spectrum. Maybe someone can post here a minimum light spectrum of R Gem for direct comparison (the BAA database contains no minimum light spectrum of R Gem and my setup is not sensitive enough to record a spectrum of a 13 mag star).


Edited by mwr, 23 February 2021 - 11:33 AM.

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#2 Organic Astrochemist

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Posted 23 February 2021 - 05:33 PM

Great work as always.

You didn’t have confidence to assign any CN bands?

If I understood Wyckoff, I think these should increase near maximum light relative to ZrO.

In comparing all these diatomic molecules, I’m still struggling about what we can infer, if anything, from this vibronic spectroscopy about their relative bond orders, vibration constants etc. that might reveal something about the star or even approximately from what depth or temperature these features originate.

Do these spectra reflect a Boltzmann distribution near one average temperature or do they show a weighted average over a range of optical and geometric depths and temperatures?

#3 mwr

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Posted 24 February 2021 - 12:04 PM

Great work as always.

You didn’t have confidence to assign any CN bands?

If I understood Wyckoff, I think these should increase near maximum light relative to ZrO.

 

Hi Jim, thanks for your feedback! The CN bands around 7850 Angström were quite weak and partially overlapping with the LaO bands, so I didn't assign them.

 

 


In comparing all these diatomic molecules, I’m still struggling about what we can infer, if anything, from this vibronic spectroscopy about their relative bond orders, vibration constants etc. that might reveal something about the star or even approximately from what depth or temperature these features originate.

Do these spectra reflect a Boltzmann distribution near one average temperature or do they show a weighted average over a range of optical and geometric depths and temperatures?

Some research groups have presented results of rotational contour calculations for SiC2 in  carbon stars assuming local thermodynamic equilibrium (LTE). The results were consistent with SiC2 lying in the upper atmosphere of the star with a rotational temperature substantially lower than that of the photosphere: https://academic.oup...9/1/103/1074946. So I think the in-depth analysis of rovibronic spectra of molecules in stellar atmospheres can indeed deliver something but the theory behind these calculations is beyond my basic knowledge. Maybe you can recommend a good textbook or a good review on this topic?



#4 mwr

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Posted 25 February 2021 - 09:26 AM

In comparing all these diatomic molecules, I’m still struggling about what we can infer, if anything, from this vibronic spectroscopy about their relative bond orders, vibration constants etc. that might reveal something about the star or even approximately from what depth or temperature these features originate.

As a chemist I would like to add a nice quote here with a wink of the eye:

 

"Eddington lamented,

after the discovery of the first diatomic molecules, that
‘‘atoms are physics but molecules are chemistry.’’ As a
physicist, he regretted the loss of innocence when simple
physical formulas have to give way to the complex chemical
solutions of a molecular universe. However, the multitude of
rotational, vibrational, and electronic transitions of molecules
provide unique opportunities to probe the Universe in much
greater detail than ever before and we have just started to
explore this."

 

Tielens in "The molecular universe"

https://journals.aps...pkEsmOlLDGMjj2Q



#5 Organic Astrochemist

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Posted 25 February 2021 - 06:21 PM

Hi Jim, thanks for your feedback! The CN bands around 7850 Angström were quite weak and partially overlapping with the LaO bands, so I didn't assign them.

 

You did such a great job with W Orionis , I thought that you could use the bands you were confident about in W Ori to help identify bands in R Gem. Sort of a cross-correlation technique.

 

R Gem has a pretty high C/O. I bet there is CN somewhere in R Gem, at some point. This article suggests C2 and CN are some of the few molecules "which are solely formed in the

deep, warm layers of the photosphere". No doubt the high bond dissociation energy helps keep CN around so that we can observe it.

 

Speaking of bond dissociation energies, that of MgO seems pretty low. Although the paper by Wyckoff you cited identifies MgO, this paper boldly proclaims "Other metal oxides and hydroxides such as CaO, CaOH, FeOH, MgO, and MgOH have been searched in molecular clouds and stars without success." Astronomers often seem to have so little regard for chemistry that they refer to everything besides H as "metals". I think most chemists would understand that MgO was much further on a continuum towards an ionic bond compared to the metal oxides that you have identified. 

 

I am afraid that, at best, my understanding of diatomic spectroscopy is at the level of an undergraduate. For any of the big molecules I've ever worked with, a Jablonski diagram was good enough. So thank you for making me review all this fundamental molecular spectroscopy. I have found that even a review of atomic spectroscopy has been helpful. Why is there a sodium doublet? I do find it fascinating the interplay between the very small (subatomic structure) and the very large (stellar and galactic structure). Learning about one has informed my understanding of the other.


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