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Be shell star zeta Tauri with SA-200

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#1 mwr

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Posted 02 January 2020 - 06:23 AM

The Be shell star zeta Tauri is a bright star with an extensive observational history, especially in the ProAm community. Attracted by this huge interest I have tried to record a spectrum of this famous star with my low budget, low resolution set up (50 spectra were stacked and processed with VSpec and RSpec). 

I have compared the spectrum of zeta Tauri with the spectrum of beta Psc, which is an "ordinary" Be star:

 

zeta_tauri3.jpg

 

 

The He I absorption lines (generated in the circumstellar disk ?) are well visible and distinguish this shell star from the Be star beta Psc.

 

To check, if the determination of the equivalent width would deliver meaningful data, I have compared the H alpha line with a high resolution line profile that was aquired at Dec. 20th by Bryssinck from the BeSS database (http://basebe.obspm.fr/basebe/):

 

zeta_tauri4.jpg

 

In theory, the measured equivalent width should be independent from spectrometer dispersion but this is obviously not the case here. It would be interesting to see other low resolution spectra of zeta Tauri. Perhaps it is possible to follow the long term variability of the H alpha EW at least semi-quantitatively??

 

 

 


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#2 happylimpet

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Posted 02 January 2020 - 08:33 AM

The Be shell star zeta Tauri is a bright star with an extensive observational history, especially in the ProAm community. Attracted by this huge interest I have tried to record a spectrum of this famous star with my low budget, low resolution set up (50 spectra were stacked and processed with VSpec and RSpec). 

I have compared the spectrum of zeta Tauri with the spectrum of beta Psc, which is an "ordinary" Be star:

 

attachicon.gifzeta_tauri3.jpg

 

 

The He I absorption lines (generated in the circumstellar disk ?) are well visible and distinguish this shell star from the Be star beta Psc.

 

To check, if the determination of the equivalent width would deliver meaningful data, I have compared the H alpha line with a high resolution line profile that was aquired at Dec. 20th by Bryssinck from the BeSS database (http://basebe.obspm.fr/basebe/):

 

attachicon.gifzeta_tauri4.jpg

 

In theory, the measured equivalent width should be independent from spectrometer dispersion but this is obviously not the case here. It would be interesting to see other low resolution spectra of zeta Tauri. Perhaps it is possible to follow the long term variability of the H alpha EW at least semi-quantitatively??

Nice work.

 

Cant say whether the two measurements give the same EW as there's no scale, and I suspect the scale is different between them (as they both fill the y axis). EW generally constantly changes over time for most/all Be stars, so i wouldn't be at all surprised if you get a different result. What are the numbers?

 

As you say, dispersion and resolution shouldnt affect measured EW, provided you can detect the line. As long as youve got the zero point of the flux scale correct (I expect you have) then your results should be good, the line is very clearly seen....

 

HeI can be in absorption for Be stars, so this doesnt identify it as a shell star, if thats what you meant.


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#3 mwr

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Posted 02 January 2020 - 09:59 AM

 

Cant say whether the two measurements give the same EW as there's no scale, and I suspect the scale is different between them (as they both fill the y axis). EW generally constantly changes over time for most/all Be stars, so i wouldn't be at all surprised if you get a different result. What are the numbers?

 

As you say, dispersion and resolution shouldnt affect measured EW, provided you can detect the line. As long as youve got the zero point of the flux scale correct (I expect you have) then your results should be good, the line is very clearly seen....

 

HeI can be in absorption for Be stars, so this doesnt identify it as a shell star, if thats what you meant.

Thanks for the very helpful comments!

I have compared the spectra in RSpec and obviously I had forgoten to de-check the box "use 2nd y-axis" and was immediately discouraged by optical inspection of the emission lines (still learning.....). Now it looks much better:

 

zeta_tauri5.jpg

 

Both spectra are rectified (intensity of continuum = 1) and I have roughly determined the EW's: ProAm high resolution spectrum approx, 15 Angström vs. my low resolution and low signal to noise spectrum: approx. 10 Angström:

 

zeta_tauri6.jpg

zeta_tauri7.jpg

 

Concerning the He I absorption line you are of course correct! I have been imprecise.

 

Anyway, I was surprised to learn by the study of several ProAm publications ( e.g. https://arxiv.org/pdf/0801.4530.pdf), that H alpha observations of zeta Tauri were analysed using the EW of this line, which might not be the best way of analysing this emission line (https://www.cloudyni...work/?p=9837788).

 

Am I completely wrong here? To my understanding the EW is the continuum related energy flux and the peak intensity of an emission line may fluctuate independently from it.



#4 robin_astro

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Posted 02 January 2020 - 01:01 PM

 

Am I completely wrong here? To my understanding the EW is the continuum related energy flux and the peak intensity of an emission line may fluctuate independently from it.

You are correct. To follow trends in emission line strength the possibility of the continuum level varying has to be taken into account. Pollmann has done this in later similar studies by correcting EW using photometric V mag values but even this is not ideal as variations in the continuum flux at H alpha  may not follow those at the V passband. The V/R ratio measurements however are valid of course.

 

(Note that emission line EW are negative by convention)


Edited by robin_astro, 02 January 2020 - 01:03 PM.

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#5 mwr

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Posted 03 January 2020 - 01:24 AM

You are correct. To follow trends in emission line strength the possibility of the continuum level varying has to be taken into account. Pollmann has done this in later similar studies by correcting EW using photometric V mag values but even this is not ideal as variations in the continuum flux at H alpha  may not follow those at the V passband. The V/R ratio measurements however are valid of course.

 

(Note that emission line EW are negative by convention)

Thanks Robin for your feedback which I always highly appreciate! 

As far as the usage of the EW is concerned: it seems that even professional astronomers are quite "nonchalant" about this issue. In a paper about the long-term observation of zeta Tauri the H alpha emission line strength is reported to be positive in all curves: (https://www.aanda.or.../aa10526-08.pdf). "Astronomy & Astrophysics" is actually a peer reviewed journal with a good impact factor. May be this point has been too unimportant to the reviewers and editors ;-)

 

Anyway, I have started to dive a little bit deeper into the theory and practice of observational astrophysics and I did follow your advice to consult the book of Trypsteen and Walker on "Spectroscopy for Amateur Astronomers". A good textbook to start with (as I find) is: "To measure the Sky - An Introduction to Observational Astronomy" by Frderick Chromey.


Edited by mwr, 03 January 2020 - 01:34 AM.


#6 happylimpet

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Posted 03 January 2020 - 04:28 AM

You are correct. To follow trends in emission line strength the possibility of the continuum level varying has to be taken into account. Pollmann has done this in later similar studies by correcting EW using photometric V mag values but even this is not ideal as variations in the continuum flux at H alpha  may not follow those at the V passband. The V/R ratio measurements however are valid of course.

 

(Note that emission line EW are negative by convention)

To a first approximation its not too bad a thing to do as the continuum excess correlates with the Ha EW fairly well for Be stars, so as the EW increases (in magnitude! negative...) the underlying continuum increases which will slightly reduce the EW, but in a fairly linear manner so it still provides a good metric of the emission, but yes its important to note that an EW of -20nm is more than 10x the emission flux of an EW of -2nm, (ignoring the underlying absorption profile).

 

 

As far as the usage of the EW is concerned: it seems that even professional astronomers are quite "nonchalant" about this issue. In a paper about the long-term observation of zeta Tauri the H alpha emission line strength is reported to be positive in all curves: (https://www.aanda.or.../aa10526-08.pdf). "Astronomy & Astrophysics" is actually a peer reviewed journal with a good impact factor. May be this point has been too unimportant to the reviewers and editors ;-)

 

Yeah thats sloppy. Interestingly they do note that 'on this occasion emission is indicated by positive EW' but its very bad practice. I'm surprised that got through the peer-review stage.

 

And great result with your EW measurement.....


Edited by happylimpet, 03 January 2020 - 04:30 AM.

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#7 robin_astro

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Posted 03 January 2020 - 10:24 AM

To a first approximation its not too bad a thing to do as the continuum excess correlates with the Ha EW fairly well for Be stars, so as the EW increases (in magnitude! negative...) the underlying continuum increases which will slightly reduce the EW, but in a fairly linear manner so it still provides a good metric of the emission, 

Many Be stars are also variables.eg

https://arxiv.org/abs/1609.08449

Is there evidence that this variability is solely due to variations in flux from the disc and not the photosphere? 


Edited by robin_astro, 03 January 2020 - 10:25 AM.


#8 robin_astro

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Posted 03 January 2020 - 10:50 AM

I am still trying to track down the example where Pollmann corrected H alpha EW using V mag but this paper is also of interest where it discusses how variations  in H apha EW can show either positive or negative correlations with V mag depending on the geometry of the disc.  

https://arxiv.org/pdf/1301.3721.pdf


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#9 mwr

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Posted 04 January 2020 - 05:58 AM

I am still trying to track down the example where Pollmann corrected H alpha EW using V mag

Finally I have found a paper by Pollmann and others dealing with the analysis of the H alpha line of zeta Tauri by using its central absorption depth: http://www.arasbeam....ort_may2017.pdf (Figure 1 on page 6).

 

"While the Hα emission line samples

the disk as a whole, the region probed by the shell lines (CA) is restricted to the line of sight.
The diagnostics they provide should not be neglected, as their properties (absorption depth)
reflect the structure and dynamics of the disk in the observer’s direction (Escolano et al.
2015)."



#10 happylimpet

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Posted 06 January 2020 - 05:20 AM

Many Be stars are also variables.eg

https://arxiv.org/abs/1609.08449

Is there evidence that this variability is solely due to variations in flux from the disc and not the photosphere? 

You're quite right, its an assumption I'm making. I'm not aware that early OB type stars like these tend to exhibit significant variability from their photospheres; I was always of the impression (and I have read quite a lot of literature, I did my PhD on BeXRBs) that at the very least the vast majority of the variability comes from the effect of the disk. However, in retrospect, I realise that this wasn't something I checked or looked into. It certainly isnt widely discussed in the BeXRB literature. And as you point out, a shell type Be star might be expected to allow for negative correlations with Ha EW if the disk is thick and blocking a significant portion of the visible photosphere, for example. The main object of my study, A0535+262 or HDE245770, has an inclination of something like 45degrees so this wasn't a significant factor.



#11 robin_astro

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Posted 06 January 2020 - 09:01 AM

 I did my PhD on BeXRBs

Fascinating systems. I am currently keeping an eye on X Per for Paul Roche.

https://britastro.org/node/15540

 

It amazes me how the B star survives the supernova explosion. 

 

Robin



#12 happylimpet

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Posted 06 January 2020 - 10:35 AM

Fascinating systems. I am currently keeping an eye on X Per for Paul Roche.

https://britastro.org/node/15540

 

It amazes me how the B star survives the supernova explosion. 

 

Robin

Ah cool, yes I know Paul a bit, we had the same PhD supervisor and he slept on my lounge floor once.....

 

And yeah, I dont know of any quantified modelling of the effects of the SN on the Be star, but it must be quite something, if short-lived. Would be interested if you know of any.


Edited by happylimpet, 06 January 2020 - 10:35 AM.


#13 descott12

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Posted 07 January 2020 - 10:52 AM

... with my low budget, low resolution set up (50 spectra were stacked and processed with VSpec and RSpec). 

 

 

 

Hello,

What are you using to stack the spectra? I have struggled with this. I have tried Autostackkert with various settings but it tends to just blur the spectrum into a featureless hump. I did get a great result once but since then it has failed.

I capture .ser files (typically about 100 frames) over about 5 seconds in SharpCap, then attempt to stack in AS3.  Is there something else I could try?  One of my issues is figuring out where to place align points in AS3.
Thanks in advance.



#14 mwr

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Posted 07 January 2020 - 12:23 PM

Hello,

What are you using to stack the spectra? I have struggled with this. I have tried Autostackkert with various settings but it tends to just blur the spectrum into a featureless hump. I did get a great result once but since then it has failed.

I capture .ser files (typically about 100 frames) over about 5 seconds in SharpCap, then attempt to stack in AS3.  Is there something else I could try?  One of my issues is figuring out where to place align points in AS3.
Thanks in advance.

Hello Dave,

 

I'm using Fitswork to stack individual raw files from my Canon 450 Da DSLR camera: https://www.fitswork...re/softw_en.php

 

Fitswork can read AVI files but I'm not sure if it works with your SER files. I have tried to stack video files with EOS Camera Movie Record and AutoStakkert as well to try "lucky spectroscopy" but with very limited success. This programm has been developed for planetray processing and is obviously not a good choice for lucky spectroscopy (maybe someone can chime in here and prove me wrong ...).

Principally, lucky spectroscopy is possible: https://www.semantic...3fd966ec9e8d884

The conclusion of the authors was that "with lucky spectroscopy that is sky background limited the sensitivity should be improved by a factor of approx. 25 (more than 3.5 magnitudes). This broadens considerably the attraction of lucky image selection techniques for carrying out wide field spectroscopic surveys." They used an objective transmission grating (1000 frame run at 12 Hz frame rate) but they didn't report on their data reduction work flow.



#15 happylimpet

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Posted 07 January 2020 - 12:36 PM

Hello,

What are you using to stack the spectra? I have struggled with this. I have tried Autostackkert with various settings but it tends to just blur the spectrum into a featureless hump. I did get a great result once but since then it has failed.

I capture .ser files (typically about 100 frames) over about 5 seconds in SharpCap, then attempt to stack in AS3.  Is there something else I could try?  One of my issues is figuring out where to place align points in AS3.
Thanks in advance.

I would be very happy to give it a go. Im quite proficient at AS!3 as Ive been doing planetary imaging for years and Ive used it for all sorts of things - I even used it for stacking security camera images of a car which knocked my friends front wall down!

 

Can you put it on wetransfer or dropbox or something?


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#16 descott12

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Posted 07 January 2020 - 01:27 PM

Hello Dave,

 

I'm using Fitswork to stack individual raw files from my Canon 450 Da DSLR camera: https://www.fitswork...re/softw_en.php

 

Fitswork can read AVI files but I'm not sure if it works with your SER files. I have tried to stack video files with EOS Camera Movie Record and AutoStakkert as well to try "lucky spectroscopy" but with very limited success. This programm has been developed for planetray processing and is obviously not a good choice for lucky spectroscopy (maybe someone can chime in here and prove me wrong ...).

Principally, lucky spectroscopy is possible: https://www.semantic...3fd966ec9e8d884

The conclusion of the authors was that "with lucky spectroscopy that is sky background limited the sensitivity should be improved by a factor of approx. 25 (more than 3.5 magnitudes). This broadens considerably the attraction of lucky image selection techniques for carrying out wide field spectroscopic surveys." They used an objective transmission grating (1000 frame run at 12 Hz frame rate) but they didn't report on their data reduction work flow.

Yes, I think I read that you an others used Fitswork but I think you are correct about the ser files. I will see if I can capture avi files instead.



#17 descott12

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Posted 07 January 2020 - 01:28 PM

I would be very happy to give it a go. Im quite proficient at AS!3 as Ive been doing planetary imaging for years and Ive used it for all sorts of things - I even used it for stacking security camera images of a car which knocked my friends front wall down!

 

Can you put it on wetransfer or dropbox or something?

Definitely. Thanks for offering to try it out. I will put something together right away. My files are actually pretty small as the high of each from is only 300 pixels.


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