Forum der Vereinigung der Sternfreunde

Firstly I would like to thank all on this forum who have given me help to get to where I am now able to capture and process spectra with my MRes spectrograph with the minimum intervention. Without the help given here I would not have got IRAF to be able to process my data automatically.

Giving my Python program which controlls the mount and cameras (3 in all) a list of targets I can now collect the data without intervention other than powering up the equipment and setting the temperature controller on the spectrograph. IRAF does the reduction once given the target names and files.

I have decided to study the 3 yellow hypergiants in Cassiopeia as while rho Cas has been well studied the remaining two do not seem to have been.

Here is some data on two of them, Rho Cas and HR8752, from 16th Nov 2017.

Regards Andrew

.....and you are sure, the Na lines are from the star?

berthold

I think so Berthold. For Rho Cas they match the results of Lobel et al. The theory is that the star is surrounded by a shell of gas that is emitting and these emissions fill in the absorption lines from the star. You can see similar effects in other lines ( not shown above). Both Rho Cas and HR8752 match archive spectra form Elodie.

In addition I did not get any Na lines in an A2V star which I imaged on the same night and at a similar airmass.

Do you have any specific reason to doubt them?

Regards Andrew

Dear Andrew,
The NaD lines most probably do have a contribution from the interstellar matter along the line of sight (not terrestrial). The strength of these interstellar lines is correlated with the distance of the stars frpm the sun and galactic latitude. Therefore you will not see them in the A2V star (which is likely much closer) with similar strength. Strong interstellar NaD lines are common in supergiants of all spectral classes, since these stars tend o be at large distances.

In the yellow supergiants, a stellar contribution is also likely, however. The relative contributions are not easy to disentangle. The best you can do is to observe an early-type star close in the sky and at a similar distance from the sun.

Best regards,
Otmar

P.S. The Elodie spectrum seem to show deeper absorptions. Can you overplot the spectra? The difference may indicate that your removal of scattered light is not perfect. Saturated interstellar absorptions are a good test for scattered light.

Thank you for your advice Otmar, I will see if I can find an early star at a similar distance to test for an interstellar contribution to Na D.

On the comparison with the Elodie spepctra I would make two points. Firstly Rho Cas is variable and the ratio of absorption to emission varies considerably through the cycle and secondly they are at different resolutions (mine being lower) so you would expect some difference on both counts.

I will check again for scattered light but it did seem to be low.

Could you elaborate on "Saturated interstellar absorptions " as I am not sure I understand you?

Thanks for taking an interest in my work.

Regards Andrew

Dear Andrew,
Yes, the difference in resolution is certainly important. However, if the absorption is mostly interstellar, the residual intensity will not change much. In addition, the absorption is quite broad, so the difference may be significant. How did you check for scattered light? Very deep absorption lines are, I think, the most convincing check. Interstellar lines can reach almost zero residual intensity, which is normally not observed in stellar lines. Therefore they are well suited to check for small amounts of scattered light. I have a long-standing interest in very luminous supergiants (mostly blue supergiants, but also yellow supergiants), so I was immediately interested in your results. It may be worthwhile to follow these stars. Low cadence observations are enough, since these stars are varying very slowly.

Best regards,
Otmar

Thanks again Otmar.

I only did a simple check for scattered light by looking at the inter-order level which was low. I will try to use you proposal.

I choose these stars as they are bright and only need low cadence as I am located in the North West of the UK and under what seems like perma-cloud! Thus I can analyse the many interesting lines in the spectra when clouded out

I have been googling interstellar lines and along with your comments I think I understand better now.

Unfortunately the only early stars with a similar parallax to Rho Cas are rather too faint for my system so I will seek out professional spectra of some close to Rho Cas.

Are there any particular blue hypergiants you would recommend I monitor?

If they will be of interest to you I will be putting my spectra in the BAA spectral database R ~10000.

Regards Andrew

Hi Andrew,
Yes, excellent choice for this kind of weather

These lines have been known for almost 100 years now: http://adsabs.harvard.edu/abs/1919PASP...31..304H
V373 Cas is relatively close on the sky. It is but probably not as far away as rho Cas, but maybe worthwhile to try. In addition, your second target (HD217476) does show very similar double Sodium lines. I think this is also an argument in favor of the interstellar nature of these lines.
Blue hypergiants are varying faster than yellow hypergiants, but beta Ori, alpha Cygni and 55 Cygni could be worthwhile.
This is a good idea. I am certainly not the only interested person.

Best regards,
Otmar

Thanks Otmar,

Not sure why Simbad did not find V373 Cas according to Hippacros 2007 it is over twice as far away as Rho Cas so it will be interesting to see what I get. Thanks for the tip.

Thanks for the Blue hypergiants.

Regards Andrew

Hi Andrew,
I think, considering the errors, both parallaxes measured by Hipparcos are not significant, i.e., compatible with zero. V373 Cas very likely is less distant than rho Cas.
You are welcome.

Best regards,
Otmar

Hi Otmar,

On interstellar lines you said "These lines have been known for almost 100 years now: http://adsabs.harvard.edu/abs/1919PASP...31..304H" I intended to add there is a huge gap between what is known and what I know.

Regards Andrew

Thanks for the hint from Berthold and the education from Otmar I now realise the Na D absorption is from the ISM. To confirm this I followed Otmar's suggestion and took a spectra of V373 Cas spectral class B0.5 II-III and fairly close to Rho Cas in RA & Dec.

It show the same strong absorption.

Regards Andrew

The exposure is 12 x 600s not 600