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More Be Shell Star Spectroscopy, Alpy 600

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#1 old_frankland

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Posted 12 August 2017 - 09:56 PM

I continue to compare Be type stars that are designated ‘shell stars.’  A shell stars’ circumstellar decretion disk is seen edge-on so that line of sight looks through disk material that variously obscures our view of the photosphere.  Spectra of ‘shell stars’ are typically a combination of H-balmer lines which occur in both the photosphere and the decretion disk, either in absorption or emission.  Plus, He I absorption lines, which are primarily photospheric in nature, and, metal lines, Fe II, Ca II and Mg II, referred to as ‘shell lines’ that develop in the cooler decretion disk. 

 

I’ve put together a web page displaying spectra of ‘shell stars’ I have recorded in recent months.  The two figures at the top of the page are a comparison of 6 stars that show presence of shell lines, Fe II, in roughly descending intensity.  59 Cyg’s H-alpha and H-beta lines are in strong emission, and likely all of its H-Balmer lines would show some degree of emission with a higher resolution spectrograph.  Fe II lines are also in emission in 59 Cyg’s spectrum.  The next 4 stars also show H-alpha in emission, with shell absorption line intensities tapering off.  The last star, 4 Her, is a well studied ‘shell star’ that presently shows little evidence of a decretion disk, lacking both shell lines and Balmer line emission. 

What is fascinating is that all six of these stars have been observed to go through one or more periods of intense change over the last 100 years.  Presenting, at some point, spectra of ordinary B type stars (like present day 4 Her), Be type spectra with Balmer emission lines (88 Her), and Be spectra containing weak to strong ‘shell lines.’

 

All of these stars are fast rotators – 200-450 km/s.  Depending upon stellar diameter, many of these stars are rotating at 70% to 80% of critical rotation…..bordering on breaking up, though, no evidence has yet been presented that a star has actually spun itself out of existence.

 

Five of the six stars are 2-4x the size of the sun with 59 Cyg a whopping 7x solar diameter.  All six have suspected or confirmed faint binary companions.  59 Cyg’s is a sub-dwarf O type star, the remaining companions are not classified in the papers I have been reading.

 

Farther on down the web page you can see individual spectra of these and half a dozen addition stars.  The web page is still a work in progress, lacking annotations and commentary which I am presently working on.  Be type stars are a fascinating study, even at very low spectral resolution. Spectroscopy rocks!

 

http://www.lafterhal...ectroscopy.html

 

Be_shell_series6_2017-jun-jul_002.png

 

be-shell_h-alpha_series6_jferreira_2017-


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#2 555aaa

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Posted 13 August 2017 - 12:59 AM

These are awesome!  What sort of exposure (integration) time are you using? How long does it typically take to post-process the data?



#3 old_frankland

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Posted 13 August 2017 - 11:18 AM

These are awesome!  What sort of exposure (integration) time are you using? How long does it typically take to post-process the data?

All spectra were recorded using a 23 micron slit.  4th and 5th magnitude stars, 20-80 seconds.  7th and 8th magnitude stars typically 600 seconds, occasionally more depending on sky transparency.  I typically stack 4 to 8 images.  I use BASS software to process the spectra.  If I go the full route of flats, darks, comparison star for instrument response and neon calibration, I'll spend a couple hours calibrating and processing.  Most of these stars only saw spectral calibration using the H-balmer and telluric lines, then rectifying of profiles.  For my low resolution comparative purposes, that's really all the processing they need -- very qualitative.

 

Early on in this project, I spent hours digging through on-line research papers learning which lines should be present, then labeling the spectral plots.  Now, I dare say, I have a pretty good grasp of what to look for in the Be star spectra so processing typically takes less than an hour......usually.




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