First spectrum on a Be Star
Posted: Mon Oct 29, 2018 6:51 pm
Hello all
Here is my first spectrum taken with the following setup : Alpy600, Newton 200 mm F/5, Atik414Ex (science camera), ASI290mm for guiding.
This is a spectrum of Phi Pers a Be star.
I tried to assess the rotational velocity of the dust disk around the star.
Please tell me if the method I followed is correct or how I can improve the assessment.
Ha et Hb lines in emission are clearly visible on spectrum.
No P Cygni profile is visible, maybe due to the low resolution.
He I absorption line seems to be very weak.
The apparent rotational velocity of the dust disk is related to FWHM of the emission lines.
It can be assessed thanks to these relations from [1] :
v sin i = (FWHM_corr_Ha - 50) / 1,4
v sin i = (FWHM_corr_Hb - 30) / 1,2
FWHM_corr is the FWHM of the line measured and corrected from the instrumental broadening :
FWHM_corr_Ha = (FWHM_Ha² - FWHM_instrum²)^0,5
For the Ha line, the theoretical FWHM_instrum of the Alpy 600 seems to be 6562/600 = 10,9 A.
But I measured the FWHM of 8 isolated and non saturated emission lines of the Ne-Ar lamp in that range of wavelength (6000 - 7000 A) and work out the average.
The true FWHM_instrum seems to be closer to 11,5 A rather than 10,9 A for Ha line.
For the Hb, i could assess the FWHM_instrum with only 1 emission line at 4158 A from the Ne-AR lamp, it was 9,7 A.
The measurement of the FWHM is not really precise and relies on how you choose the limits of the start and the end of the emission lines.
Even by dividing by the continuum it's not easy to know which value to write down.
Few measurements on the same line give me a fluctuation of ~0,1 A ~0,2 A on the FWHM.
How do you measure your FWHM properly ?
So, finally, the results are :
From Ha FWHM : v sin i = 307 km/s
From Hb FWHM : v sin i = 403 km/s
I didn't assess the uncertainties but i would say it is about ~100 km/s.
The literature (2002) gives a rotational velocity of 410 km/s [2].
My results are not that bad !
I'll try now to assess the rotational velocity of the star from the FWHM of the absorption lines...
It should be possible, but dont know how yet !
[1] : http://articles.adsabs.harvard.edu/cgi- ... etype=.pdf :
[2] : http://iopscience.iop.org/article/10.1086/340590/pdf
Here is my first spectrum taken with the following setup : Alpy600, Newton 200 mm F/5, Atik414Ex (science camera), ASI290mm for guiding.
This is a spectrum of Phi Pers a Be star.
I tried to assess the rotational velocity of the dust disk around the star.
Please tell me if the method I followed is correct or how I can improve the assessment.
Ha et Hb lines in emission are clearly visible on spectrum.
No P Cygni profile is visible, maybe due to the low resolution.
He I absorption line seems to be very weak.
The apparent rotational velocity of the dust disk is related to FWHM of the emission lines.
It can be assessed thanks to these relations from [1] :
v sin i = (FWHM_corr_Ha - 50) / 1,4
v sin i = (FWHM_corr_Hb - 30) / 1,2
FWHM_corr is the FWHM of the line measured and corrected from the instrumental broadening :
FWHM_corr_Ha = (FWHM_Ha² - FWHM_instrum²)^0,5
For the Ha line, the theoretical FWHM_instrum of the Alpy 600 seems to be 6562/600 = 10,9 A.
But I measured the FWHM of 8 isolated and non saturated emission lines of the Ne-Ar lamp in that range of wavelength (6000 - 7000 A) and work out the average.
The true FWHM_instrum seems to be closer to 11,5 A rather than 10,9 A for Ha line.
For the Hb, i could assess the FWHM_instrum with only 1 emission line at 4158 A from the Ne-AR lamp, it was 9,7 A.
The measurement of the FWHM is not really precise and relies on how you choose the limits of the start and the end of the emission lines.
Even by dividing by the continuum it's not easy to know which value to write down.
Few measurements on the same line give me a fluctuation of ~0,1 A ~0,2 A on the FWHM.
How do you measure your FWHM properly ?
So, finally, the results are :
From Ha FWHM : v sin i = 307 km/s
From Hb FWHM : v sin i = 403 km/s
I didn't assess the uncertainties but i would say it is about ~100 km/s.
The literature (2002) gives a rotational velocity of 410 km/s [2].
My results are not that bad !
I'll try now to assess the rotational velocity of the star from the FWHM of the absorption lines...
It should be possible, but dont know how yet !
[1] : http://articles.adsabs.harvard.edu/cgi- ... etype=.pdf :
[2] : http://iopscience.iop.org/article/10.1086/340590/pdf