Final Sirius Spectrum

[johnb]

So Sirius for the last time (I think)

Below is the Raw, Instrument Corrected and Normalised Spectrum's, taken with an SA100 and processed in RSpec

The only thing I could not identify are the lines at 6863 and 7169 ?

Thanks for looking

John B

Raw Spectrum

Calibrated and Instrument Response Corrected

and Normalised

[Moonshane]

do these spectra isolate the main component or could e.g. the spectrum of Sirius A be affected by the make-up of Sirius B?

[robin_astro]

do these spectra isolate the main component or could e.g. the spectrum of Sirius A be affected by the make-up of Sirius B?

This is a slitless spectrum so there will be contamination from Sirius B but since it is ~10mag (10000x)  fainter in the visible wavelengths it will be completely buried in the noise.

Isolating the spectrum of Sirius B using a slit spectrograph would be an interesting (though very tough) challenge. The spectrum of the white dwarf is actually quite similarin appearence  to Sirius A with prominent Balmer lines (somewhat hotter though and with much broader lines due to the very high surface gravity)

eg here is a Sirius B spectrum from the HST

http://www.einstein-online.info/spotlights/redshift_white_dwarfs#section-3

Cheers

Robin

[Moonshane]

cheers Robin, interesting stuff. I suppose my question at least shows that I almost understand how it works.

[robin_astro]

The only thing I could not identify are the lines at 6863 and 7169 ?

Hi John,

These are also Telluric bands from O2 and H2O.   Christian buils annotated medium resolution spectrum of Vega is a good source of information on these

http://www.astrosurf.com/buil/us/vatlas/vatlas.htm

It might  even be possible to download is dat file of the spectrum, import it into RSpec and overlay it on your spectrum.

You sirius spectrum should look very similar to the Vega spectrum. There is something amiss with the instrument response correction. In a hot star like this the spectrum should continue rising to the blue end to around 4000A, as in Christian Buil's Vega spectrum.  On possible cause could be atmospheric extinction which will be significant for a low altitude target like Sirius and will reduce the intensity at the blue end.  To correct this, your reference star that you use to calculate the instrument response needs t obe taken at a similar (low) altitude 

There are still some artifacts in your normalised spectrum seen as "emission" overshoots either side of the Balmer absorption lines. This is most probably caused by not completely removing the lines when calculating the smoothed continuum.  The remnants of the lines then appear as "emission" artifacts after the division. You need to crop out the lines carefully before smoothing the result. Not exactly how this is done  in Rspec though as I rarely use it. 

Cheers

Robin

[robin_astro]

There are still some artifacts in your normalised spectrum seen as "emission" overshoots either side of the Balmer absorption lines. This is most probably caused by not completely removing the lines when calculating the smoothed continuum.  The remnants of the lines then appear as "emission" artifacts after the division. You need to crop out the lines carefully before smoothing the result. Not exactly how this is done  in Rspec though as I rarely use it. 

Cheers

Robin

John,

Here is an example of normalising a spectrum to the continuum (HD214994, an A1v spectral type, like Sirius, taken with my ALPY 600 spectrograph)

The blue line is the instrument response corrected spectrum, the orange line is the continuum (after removing the lines and smoothing) and the green line is the normalised result after dividing the blue by the orange. This was done in Visual spec but of course the same thing would be possible in RSpec

Cheers

Robin

[nytecam]

Thanks Robin for the Hubble link to Sirius B spectrum :-)

[thiago_]

I beg your pardon for my ignorance, but if Sirius A´s effective surface temperature is around 10.000 K, shouldn´t the maximum of the emission spectrum be around 290 nm ? Thanks a lot in advance!

[robin_astro]

I beg your pardon for my ignorance, but if Sirius A´s effective surface temperature is around 10.000 K, shouldn´t the maximum of the emission spectrum be around 290 nm ? Thanks a lot in advance!

Good spot !  If  Sirius  were a perfect black body the flux in the spectrum should indeed continue to rise to a maximum in the UV well beyond where we are measuring. There are however various processes going on the the star's atmosphere (the photosphere), absorbing radiation at certain wavelengths and re-emitting it at others which redistributes the energy spectrum. The most significant one in the visible spectrum for stars like Sirius is the "Balmer Jump"  

As we go to shorter wavelengths the Balmer absorption lines (produced by absorption of photons by electrons in the hydrogen atom 2nd energy level jumping to higher levels)  get closer and closer together until they eventually merge into a continuous absorption, when  the electrons gain at least enough energy to  escape completely from  the atom (ionisation) This puts a big hole in the black body curve at the UV end. This happens completely above 3646A though we see the effect earlier in low resolution spectra as the lines merge together. Attached is a spectrum (including the UV end) of a hot A star with the theoretical black body curve overlaid.

Robin

[thiago_]

Good spot !  If  Sirius  were a perfect black body the flux in the spectrum should indeed continue to rise to a maximum in the UV well beyond where we are measuring. There are however various processes going on the the star's atmosphere (the photosphere), absorbing radiation at certain wavelengths and re-emitting it at others which redistributes the energy spectrum. The most significant one in the visible spectrum for stars like Sirius is the "Balmer Jump"  

As we go to shorter wavelengths the Balmer absorption lines (produced by absorption of photons by electrons in the hydrogen atom 2nd energy level jumping to higher levels)  get closer and closer together until they eventually merge into a continuous absorption, when  the electrons gain at least enough energy to  escape completely from  the atom (ionisation) This puts a big hole in the black body curve at the UV end. This happens completely above 3646A though we see the effect earlier in low resolution spectra as the lines merge together. Attached is a spectrum (including the UV end) of a hot A star with the theoretical black body curve overlaid.

Robin

Thanks a lot for that great answer, Robin!

Before reading your reply, I was guessing that would be because the spectrum was recorded using a terrestrial telescope, and since the Earth´s atmosfere absorbs in the UV region, that might play a role. The same issue would arise if the telescope´s optics weren´t transparent to UV light. Now, if I understand correctly, even if one uses a spatial telescope to record a hot A star´s spectrum, one would get such "void" at the UV end shortward from 364.6 nm due to hydrogen ionisation (the Balmer Jump as you pointed out, i.e., the continuum of transitions from n=2 up to the ionisation threshold at 364.6 nm). Now, that raises a new question: how does one actually determine the effective temperature of a star? Just by fitting a theoretical black body curve to the visible portion of the spectrum?

[robin_astro]

Well the atmosphere does affect the spectrum significantly even in the visible region (both by adding (telluric) lines from for example H2O and O2 and by changing the general overall shape of the spectrum with increasing absorption towards the blue end so that has to be corrected for, as has been done here by measuring a standard star with a known spectrum under the same conditions and with the same equipment. The correction curve obtained by dividing the measured spectrum by the catalogued spectrum for the standard star can then be applied to our unknown star. This gets rid of both instrument and atmospheric affects. 

The interstellar medium however also absorbs some light and changes the shape of the spectrum so in general fitting a black body curve is not a very reliable measure of a star's temperature.

Fortunately there are better ways. Exactly  which lines appear and their relative strength  in the spectrum depends on the temperature so by examining the spectrum and classifying it, we can determine the temperature more accurately. ( Spectral classification (OBAFGKM) was the key to  identifying the relationship that led to the Hertzprung Russel diagram for example)  

The temperature (among other factors) also affects the shape of the lines (only seen at high resolution) The shape (profile) of a line can be modelled using temperature, surface gravity, rotation, microturbulence etc as variables) and compared with the measured line profile to determine the exact physical conditions in the star. This is why spectroscopy is such a powerful tool for astronomers.

Robin

[robin_astro]

 The correction curve obtained by dividing the measured spectrum by the catalogued spectrum for the standard star can then be applied to our unknown star. 

But where does the cataogue spectrum for the standard star come from I hear you ask ;-)

Well these have been determined by people much cleverer than me !  For example this reference gives an idea of what is involved

http://ukads.nottingham.ac.uk/abs/1985IAUS..111..225H

Robin