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Question about an object's amplitude

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GDonnelly
Question about an object's amplitude

Hello everyone, 

I'm working on an EW currently, and I'm almost ready to submit. The amplitude I'm currently getting for this object is 0.070 for magnitude varying between 11.23-11.30 (V), and that comes from a phasing of ROTSE1:Orphans, ROTSE1:NSVS, SuperWASP, and ASAS-SN. The problem is, this is too small of an amplitude for a typical EW. 

I have run into this problem before, and in that case, the object was a visual double being substantially blended with its companion. Therefore, a remark saying so is necessary in the submission. However, I know of no other object that could this object could be blending with to cause the amplitude to be incorrect. WDS doesn't show a visual companion for this object, and I cannot find any catalog that does.

The nearest object is about 6" away. However, this object's magnitude is too faint to be able to blend significantly with my variable. I have attached an image showing this, the variable is the "Big Star," the faint object is the "Small Star." The table below it shows some magnitudes for each. A search in APASS for each object's coordinates yields the same data for each object. The coordinates of the "Big Star" are J00217.031+284749.90, and the "Small Star" are J00216.586+284748.52.

In this situation, is it possible to fix the magnitude ranges and amplitude? If not, what should I include as a remark in the submission?

Many thanks in advance,

Grant

File Upload: 
libmar96
Hi Grant,

Hi Grant,

At first I apologize for this short message which was much longer before. Before sending I decided to click last time on your attached file, but after going back, everything cleared... So here's in short:

Are you sure the fainter one is variable? Based on 2MASS J-K value and G_mag from Gaia DR2, I calculated V magnitude 16.65 mag for companion (from the spreadsheet). While J-K values look real, Gaia DR2 with 16.3 mag really surprised me - I expected something around 15 mag from DSS image. But it really looks that faint on PANSTARRS DR1 images, so I think we should trust this value.

If we give 16.65 mag for mean magnitude of companion, it will never cause 0.07 mag variations. Having combined values, increase of 0.035 mag (half amplitude) will require to reach 14.75 mag, but decrease of 0.035 mag from mean value - actually never. It's just too faint. A disappearance of 16.65 mag object decreases combined magnitude by 0.008 mag only. With 0.07 mag amplitude, the companion needed to have 12-14 mag to cause real EW changes.

What would I do? If the object has clear EW shape (no phase plot attached here), then brighter object could be a trinary with a very close EW companion (1+2). If no, I would rethink about ELL type (information about period is useful too). I looked at SWASP quickly and I think the first one is likely the solution. I would put in Remarks note that the amplitude is small and the possible solution is a close companion (and good to inform that this 16.5 V star is too faint to cause such magnitude changes).

Regards,

Gabriel Murawski

GDonnelly
Hi Gabriel,

Hi Gabriel,

Thank you very much for the thoughtful response, I really appreciate the insight. 

I may not have been clear, I think the "Big Star" (the brighter one) is the variable, and I've attached a phase plot to this reply if you want to see that. The plot has the period as well.

When you say this could be a trinary, do you mean a system of three stars that are gravitationally in the same system? Like an EW of two stars, orbiting with a third, separated star? If you don't mind, I would love to understand more how this kind of thing effects the amplitude. 

Or alternatively, do you mean it could be a trinary in the visual sense, with a thrid star not gravitationally interacting with the EW, just messing with its amplitude by coincidence?

Again, thank you very much for your time.

Grant

File upload: 
libmar96
Hi Grant,

Hi Grant,

We would need high resolution observation (like what Keck does for transiting exoplanets), so maybe the companion could be revealed. We don't know what brightness could it have, as Gaia (the best source) couldn't catch it. If magnitude of stable star was 12.00 V, then the EW range would be 11.94-12.08 V. If 11.50 V, we have 12.81-13.16 V. So, for now you can only give combined magnitude with some words in Remarks section. Both reasons might be real, but a system with three stars is more likely. This is because coincidence of such bright 11-14 mag stars in almost the same position (even for this far from Milky Way area) is quite rare.

Regards,

Gabriel Murawski

bpablo
Third Light

Hey Grant,

What Gabriel is saying about the object being a trinay is essentially alluding to the same thing you noticed before, and  which in the binary community is referred to as third light.  Third Light is the idea that their is contribution to the total light measured which is not coming from the binary itself. Before when you had blending what happened was the eclipses appeared dimmer because you had a third source which was close on the sky to the object you cared about. Since the total light was greater, the effect you see is diluted. However, in that case you knew that there was another star in your psf that was unrelated to the object you were interested. In the case of a trinary consider this. You have two stars in a nice short period orbit, and then a third star which is orbiting the short period pair with a period of  several hundred years. Depending on how far the system is away, all three of those stars could be unresolved in your psf. You have the light of the binary plus the light of a 3rd component that is so close that  you don't even know it is there. Since you are measuring the light of all three stars, it would strongly dilute the eclipses in the short period pair. This light unrelated to the binary itself is called third light regardless of whether it's actually in the system or just a background star that happens to have a similar position.  

Actually knowing if there is a 3rd component in the system is difficult and typically requires spectroscopy. In your case I am unsure if this is likely or not without digging a little more into spectral types (if they exist) and making some educated guesses. Ellipsoidal variation is definitely possible as well and the phased light curve you showed definitely doesn't preclude it. Unfortunately, the data isn't quite good enough to tell simply from that curve. 

Hope this helps. If you have any other questions, please ask.

Thanks,

Bert Pablo
Staff Astronomer
AAVSO

GDonnelly
Thank you!

I understand what I need to do now. Thank you everyone so much for the very helpful responses!

Grant

Sebastian Otero
Sebastian Otero's picture
An EW ready to submit

Hi Grant,
this was a nice example of pre-submission collaboration!.
The star is an EW object because of the light curve shape (shallow maxima, sharp minima) and short period so Gabriel's suggestion is possible.
If the system is far from an inclination of 90º compared to our line of sight, amplitudes can be smaller too (although such small value looks suspicious). I wouldn't add any remark here if we don't have evidence. It is likely to be a triple system but we do not know.
One thing to keep in mind, the potential third star shrinking the amplitude is not the 16.6 mag. companion, which is an unrelated object (Gaia DR2 parallaxes are very different) not contributing to the combined light (a mag. difference of >5 mag. means contribution will be <0.01 mag.). Just to avoid confusions in that regard. Thus there is no need to mention the companion in the remarks either.

I see your plot and you can submit it as it is.

Cheers,
Sebastian

GDonnelly
Hi Mr. Otero,

Hi Mr. Otero,

Okay, I'll double check everything and submit. Thanks for the input!

Grant

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