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Carbon star WZ Cassiopeiae (C9,2 Li) with Star Analyser 100

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#1 mwr

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Posted 01 November 2019 - 10:30 AM

The carbon star WZ Cas is even with the low resolving SA 100 an impressive object. An almost fully saturated Na I line dominates the spectrum:

 

WZ_cas_comb.jpg

 

CN- and C2- (Swan) absorption bands are visible as well. I'm not quite confident about the Li-absorption at 6708 A, which might be an artifact at the red end of the spectrum. 

 

It would be interesting to see other SA 100 spectra of this object.


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#2 descott12

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Posted 01 November 2019 - 10:35 AM

Very cool. I will try to capture tonight if clear.

I noticed you capture in color.  I had learned that monochrome can give better results but you seem to be doing very well in color.



#3 Organic Astrochemist

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Posted 01 November 2019 - 12:13 PM

Nice spectrum and image of the double star.

 

i think that the lithium absorption is close to the limit of the star analyzer. I guess a test would be to look at a similar carbon star that isn’t lithium rich and see the difference.

 

Here’s my attempt:

 

Attached Thumbnails

  • C210B5A7-2E07-439F-8C81-3D7EBC18DA3A.png


#4 mwr

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Posted 01 November 2019 - 01:16 PM

I guess a test would be to look at a similar carbon star that isn’t lithium rich and see the difference.

 

Brilliant idea! Z Piscium would be good test object. It is even directly compared with WZ Cassiopeiae by Richard Walker in his Spectroscopic Atlas. The CN line is very close to the Li line (delta is about 50 A). So a resolution of about 130 should be sufficient. For my optimized system (with an enlarged grating to sensor distance) resolution is around 100. Hence, the putative Li absorption in my WZ Cas spectrum is probably a blend with CN. In your spectrum a shoulder is visible due to the higher resolution of your grab and go system. An improvement of the resolution of my system is probably not feasible as the end of the backfocus has been reached:

 

distance.jpg

 

Anyway, I will do the test with Z Psc and will report on it.


Edited by mwr, 01 November 2019 - 02:03 PM.


#5 Organic Astrochemist

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Posted 07 November 2019 - 12:02 AM

Here's my attempt to compare some spectra 

WZ Cas.png

Z Psc.png

 

and the two overlayed and zoomed in

 

Z Psc vs WZ Cas.png

I think I'm able to resolve the CN and the lithium absorptions.

 

 also found it interesting to compare to this spectrum of Mira which showed TiO absorption nearby at 671.5 nm (apparently there are also lines at 665.1 nm and 668.1 nm that I didn't label and may be blended in my spectrum). TiO is absent in carbon stars like Z Psc and WZ Cas because the excess carbon gobbles up all the oxygen.

Mira gamma stacked corrected.png


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#6 robin_astro

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Posted 07 November 2019 - 08:56 AM

Note though that TiO molecular bands completely dominate the spectrum of M giants so the broad bands seen in the Mira spectrum are produced by hundreds (thousands?) of blended TiO  lines not just the wavelength singled out.  See for example this very high resolution spectrum of a Mira Ceti type star (S Car) from the UVES spectrograph from the UVES POP list

http://www.eso.org/s...rs_uptonow.html

The same is true for Carbon stars though the molecules involved in producing the bands are different. I think it could be difficult to isolate with any certainty the single Li line from all the molecular lines at this low resolution

 

Cheers

Robin

 

S_Car_UVES_POP.png



#7 Organic Astrochemist

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Posted 07 November 2019 - 05:14 PM

Molecules do have fundamentally different spectra than monoatomic ions and atoms because the former can show vibration. As shown in your high resolution spectrum, the lines from the vibrational transitions become so close that they form a blended absorption, deeper than the other lines. This is called the bandhead and, with the other lines, forms a band.The values cited are for bandheads, but they are subject to variations.

Molecular absorptions like TIO and CN occur higher up in the cooler Stellar atmosphere and can overprint absorptions that occur lower in the atmosphere. But metals like sodium and lithium have such low ionization energies that their absorptions also occur high in the atmosphere. WZ Cas is lithium rich and shows lithium with a substantial EW compared to Z Psc. My results are consistent with this fact.
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#8 robin_astro

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Posted 07 November 2019 - 05:48 PM

Hi Jim,

 

I am not disputing that the Li absorption line exists in the WZ Cas spectrum. For example it seen clearly in the medium resolution R~2500 spectrum by Kevin Gurney in the BAA database.  

 

https://britastro.or...php?obs_id=2861

 

Nor am I disputing your detection. I am just asking (with my peer reviewer hat on ;-) ) if the claim of detection in your spectrum is robust. At your R~100 resolving power the narrow Li line will be almost 70A wide FWHM and so will be severely diluted and blended with the TiO features so could potentially disappear into the noise. How does your detection compare with your noise floor and is the observation reproduceable ?

 

Cheers

Robin



#9 robin_astro

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Posted 07 November 2019 - 06:58 PM

At your R~100 resolving power the narrow Li line will be almost 70A wide FWHM and so will be severely diluted and blended with the TiO features so could potentially disappear into the noise.

If we overlay your spectrum on Kevin Gurney's (blue) we see that the pseudo continuum formed by the molecular bands already dips in this region  making it difficult to distinguish between this and the Li line after it is  broadened by the lower resolution

 

Cheers

Robin

Attached Thumbnails

  • JL_KG_Li.png


#10 Organic Astrochemist

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Posted 09 November 2019 - 12:58 AM

Hi Jim,

 

I am not disputing that the Li absorption line exists in the WZ Cas spectrum. For example it seen clearly in the medium resolution R~2500 spectrum by Kevin Gurney in the BAA database.  

 

https://britastro.or...php?obs_id=2861

 

Nor am I disputing your detection. I am just asking (with my peer reviewer hat on ;-) ) if the claim of detection in your spectrum is robust. At your R~100 resolving power the narrow Li line will be almost 70A wide FWHM and so will be severely diluted and blended with the TiO features so could potentially disappear into the noise. How does your detection compare with your noise floor and is the observation reproduceable ?

 

Cheers

Robin

Hi Robin,

I certainly appreciate all your feedback.

 

I'm not sure I'm sophisticated enough to respond to your questions. I have measured S/N before with ISIS, but I'm not really sure what it meant. I did look at WZ Cas a couple times that night and every sub seemed to show a dip around 671 nm. I honestly don't completely understand reproduceability in spectroscopy. How many times does the radiologist have to reproduce an x-ray to say a bone is broken?

 

Thanks for the spectra by Kevin Gurney. Could you easily calculate the EW and FWHM for the sodium and lithium lines? I tried to estimate this roughly (difficult to see the continuum). 

WZ Cas Na.png

In my spectrum the Na I was roughly 55-60  angstroms FWHM which seemed close to what is seen in Kevin's spectrum.

 

WZ Cas Li.png

In my spectrum the Li I was roughly 14 angstroms FWHM compared to Kevin's spectrum for which I estimated the FWHM to be about 8 angstroms.

 

I'm not really out to prove anything, but it would feel nice if my data were at least consistent with other data. Of course, I may be asking more of my data than it can reliably provide.


Edited by Organic Astrochemist, 09 November 2019 - 01:00 AM.


#11 mwr

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Posted 10 November 2019 - 03:28 AM

If we overlay your spectrum on Kevin Gurney's (blue) we see that the pseudo continuum formed by the molecular bands already dips in this region  making it difficult to distinguish between this and the Li line after it is  broadened by the lower resolution

 

Cheers

Robin

I have downloaded Kevin Gurney's spectrum and tried to open it with VSpec. The plan was to apply a Gaussian filter to verify how the Li line is broadened and how this compares to the SA-100 spectrum. However, without success - neither VSpec nor RSpec does open the fit' - file:

 

WZ_Cas.jpg

 

 

Another test with a BAA-spectrum from R Aqr did work. Any suggestions how to open Kevin Gurney's spectrum for modelling?


Edited by mwr, 10 November 2019 - 03:33 AM.


#12 robin_astro

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Posted 10 November 2019 - 01:14 PM

 

 

Another test with a BAA-spectrum from R Aqr did work. Any suggestions how to open Kevin Gurney's spectrum for modelling?

That's strange. It works for me.  The individual downloaded fits files from the database have inverted comas round the file name for some reason, which have to be removed. It then loads in VSpec (and ISIS) as a profile. 

 

Robin


Edited by robin_astro, 10 November 2019 - 01:16 PM.


#13 mwr

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Posted 10 November 2019 - 03:05 PM

That's strange. It works for me.  The individual downloaded fits files from the database have inverted comas round the file name for some reason, which have to be removed. It then loads in VSpec (and ISIS) as a profile. 

 

Robin

Ok! Now it works. I've not recognized  the inverted comas.

I have now broadened the high resolution spectrum (orange curve) by Gaussian filtering and blended in my low resolution SA-100 spectrum (upper violet curve) (https://www.cloudyni...-100/?p=9741930)

 

WZ_Cas.jpg

 

At least the "Lithium-dip" is at the correct position. 


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#14 robin_astro

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Posted 10 November 2019 - 06:36 PM

You two are wearing down my resistance. I am almost ready to agree that you can detect the Li line :-) Great stuff !

 

Robin


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#15 Organic Astrochemist

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Posted Yesterday, 11:50 PM

Finally a night with a few clear patches! I don't know why I'm surprised, the hurricane season really isn't very good for astronomy!
And I didn't forget everything about the Alpy and ISIS!
I present WZ Cas:
WZ Cas.png
The FWHM for the Li I absorption is 11 Angstroms compared to 14 that I measured with the Star Analyzer
WZ Cas FWHM 11.png

The FWHM for the Na I absorption is 44 Angstroms compared to 55-60 with the Star Analyzer
WZ Cas Na FWHM 44.png

Edited by Organic Astrochemist, Today, 12:33 AM.

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#16 robin_astro

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Posted Today, 07:16 AM

Wow that Na D line feature is very wide.  The native Na D doublet from the star will be much narrower. That has to be a blend with molecular bands (CN?) with perhaps doppler shifted interstellar contributions.

 

I have just been reading up about carbon star spectral classification in Gray and Corbally.  I did not realise the diversity. It is as complex as all the other stars put together !  I normally think of carbon stars as all being very cool but there are carbon star equivalents from G-M.  On top of that Keenan (1993) developed a classification system based on 5 different sub types depending on chemical signatures (R,N,H,J,Hd).  WZ Cas is apparently J,  for stars with unusually low carbon 12/13 isotope ratio (~13  compared with 92 in our solar system)

 

Cheers

Robin


Edited by robin_astro, Today, 07:19 AM.


#17 robin_astro

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Posted Today, 07:32 AM

On top of that Keenan (1993) developed a classification system based on 5 different sub types depending on chemical signatures (R,N,H,J,Hd). 

https://ui.adsabs.ha.....419B/abstract

 

if you have not come across it already. you can also download the spectra from Vizier as dat files 


Edited by robin_astro, Today, 07:40 AM.


#18 robin_astro

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Posted Today, 08:35 AM

In this system WZ Cas is C-N7 III: C2 2 Li 10

https://iopscience.i.../10.1086/133252




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