TV Cas

Ask your questions, show your results
Post Reply
David Cejudo
Posts: 45
Joined: Mon Apr 23, 2018 9:01 pm

TV Cas

Post by David Cejudo »

Hello.

TV Cas is an Algol type eclipsing binary composed of a bright B9V star and a F7IV companion. The bright star, according to the article by K.Walter(1979), makes the F7IV very difficult to distinguish. It has a 1.81 days' period.
Threre was a predicted minimum at the beggining of the night of September, the 29th. Althought i have many years experience in photometry, i only started doing spectroscopy about a year ago. I have obtained many spectrums of different types so far and i wanted to try my luck in detecting variability in a binary.
So i started obtaining Mile's HD3360 B2V star spectrum to be used as instrumental response and then continued obtaining TV Cas 180 seg spectrums and then left the observatory unattended for the rest of the night. Autoguiding went well and i got 150 spectrums. I stacked then in groups of fifteen using Demetra and finnally got 10 spectrums. I processed them twice. Once without using instrumental response and another using it.
I also obtained many images using the finder to measure TV Cas photometry. Next figure shows the resulting curve, just as predicted for the minimum.
TV Cas photometry.jpg
TV Cas photometry.jpg (61.27 KiB) Viewed 3466 times
airmass.jpg
airmass.jpg (38.51 KiB) Viewed 3466 times
According to Walker's Spectroscopy Atlas, F type stars have as charasteristics a decreasing Balmer lines intensity, but increasing Mg triplet (5168-83) and CH band(4300).
So i measured, using the spectrums without the instrumental response, EW at Hgamma and Mg. To achieve it, i cropped the spectrums to the area around the absotion line, eliminated the continuum and normalized all of them so that the intensity value was one in the flat part out of the absortion line. Then i measured EW in each spectrum carefully trying to use the same limits at the figure. For this analysis i used BASS software.
I obtained this curve for H gamma.
H gamma.jpg
H gamma.jpg (42.23 KiB) Viewed 3466 times
As expected, as the bright star brightness is less and the F star spectrum may show up, Hgamma intensity is minimum coincidental with photometry minimum. Then again, i consider that measuring EW correctly is difficult and my result could be just a coincidence. Maybe the air mass is to blame?
I would love to hear comments on this.
On the other hand, i also measured the Mg triplet EW and obtained just the opposite curve, that is, maximum EW at the minimum in the photometry curve, also as expected.

Also, i had a look at the spectrums in which i used the instrumental response. The star used for this response was acquired only once, at the beggining of the session. It was close to TV Cas.
Untitled-2.jpg
Untitled-2.jpg (40.33 KiB) Viewed 3466 times
In it it is clearly shown the CH band at 4300 Angstron in the spectrum at minimum brightness and its ausence in the spectrum at maximum brightness.

I also added up in BASS the two B and F type synthetic spectrums to check the result, and it seems very similar to what i obtained at minimum brightness.
Reference.jpg
Reference.jpg (64.66 KiB) Viewed 3466 times
As a summary, i think i could observe that mimimum in the spectrums. But i am just starting in this spectroscopy world and i am not sure if the procedures i used are correct.
Specially in the EW measurement and the use of a star as instrumental response only once.

Any advice would be very welcome.

Equipment used for the spectrums:
-Meade 12"LX200 with an Optec reducer, making it f/5.5
-Atik 460 EX camera
-LISA spectroscope
Equipment used for photometry:
-EVOGuide 50 telescope
-Atik Titan camera
David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: TV Cas

Post by Robin Leadbeater »

Hi David,

I would say that is a good result. The changes in the features match what would be expected from an increase in the contribution of the F star.

The EW of absorption lines are unaffected by air mass or instrument response as it is measured relative to the local continuum but as a check for best accuracy I try to take at least reference spectra at the start and end of long runs as SCT used with focal reducers suffer from some chromatism which can change the shape of the continuum depending in focus (and atmospheric refraction can have a similar affect if not running at the parallactic angle)

Cheers
Robin
Last edited by Robin Leadbeater on Sat Nov 20, 2021 3:08 pm, edited 1 time in total.
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: TV Cas

Post by Robin Leadbeater »

An alternative approach which can show changes more clearly is to produce a template from the outside eclipse spectra and subtract it from the other spectra. (I would expect to see a general increase in the contribution from many other metal lines pop out too). This can also make it easier to measure changes in EW as any unchanged features in spectrum surrounding the line being measured cancel out making it easier to get good reference points in the continuum.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Post Reply