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Type EB or EW?

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Nicole_Crumpler
Type EB or EW?

This variable star candidate has a period suggestive of a type EB variable, but a shape suggestive of a type EW. I'm looking for suggestions on which type is more likely, and any other general comments or advice. I have attached two gnuplots for this object. The SuperWASP data had larger error bars, so one plot has the more complete SuperWASP data while the other only has measurements with a lower error.

libmar96
This looks to be an EW

This looks to be an EW (searched for VSX, there are still thousands of stars with period between 0.8-1.2d), but so small amplitude is only because it's a triple (Gaia DR2 source, see attachment). Did you deblend?

The separation is too small to find out which target (12.6 or 13.0) is responsible for variability (that 15+ mag star is too faint). Sebastian, do you know if Gaia DR1 and Gaia DR2 G_mag differences (or e_mag values) can be used to find out which star could be variable?

Generally we deblend and give range for the real variable source, but in this case I don't know if they should be calculated for brighter target (range will be OK for an EW) or both (~0.16 mag), because we are not able determine which one is the variable. More answers are needed...

Regards,

Gabriel Murawski

File upload: 
Sebastian Otero
Sebastian Otero's picture
EW or EB

Hello Nicole and Gabriel,

I am not entirely sure the companion exists, it is not the first time that both Gaia DR2 and Pan-STARRS1 list two entries in the case of bright stars (Pan-STARRS1 may list several). The companion's position in each catalogue is different and that is suspicious.
So we can't apply any correction here. The 15.7 Vmag. companion 3" away is too faint to be the variable and will only shrink the amplitude by 0.01 mag. (with the actual magnitudes being fainter by 0.05).
None of the datasets you used has standard magnitudes, nowadays ASAS-SN magnitudes are recommended because they give you a V zero point. For 12th mag. stars ASAS-SN is perfect. You can simply add 0.05 mag. to the ASAS-SN zero point in this case because the amplitude change is meaningless.
So do not even mention the close companion. Do mention the star 3" away.

Regarding the variability type, this is an example of how arbitrary the classification can be. Both EB or EW could fit here. The limit of 1.0 d. for EWs is arbitrary. The different minima depth is also something that might be confusing because long period systems may show similar minima and short period objects may show different minima.
EW are usually low mass systems of late spectral types and short periods.
EW is okay since the colors of this system are not blue even when its period is rather long.

Cheers,
Sebastian

Nicole_Crumpler
Added ASAS-SN data

Thank you for your advice. It was really helpful. This updated plot now contains ASAS-SN data.

File upload: 
PVEA
PVEA's picture
Type EB or EW?

Hi Nicole,

To classify the eclipsing binary stars as EW or EB is not so important and this plays only preliminary role for the catalogues. As this classification is based only on the phenomenological shape of the stars light curves, as you see from the discussion above (previous posts) it is not possible to judge clearly where the truth is. The use of EB type is a suspicious. The eclipse depths of eclipsing binary systems depend on the mass ratio, degree of contact, orbital inclination and the temperatures of the components. Of course in the EW case the components have nearly equal temperatures. As a result the eclipse depths are practically independent of the effective temperatures but depend strongly on the mass ratio q, orbital inclination i and surface potential (Rucinski 2001).

In general the EA (Algol) and EW (W UMa) types cover everything that we need to phenomenologically classify eclipsing binary stars for the catalogues.

The important scientific data are the real physical properties of the eclipsing binary stars which allows us to classify the binary star with big confidence. The modern morphological classification of the eclipsing binary stars is based on the equipotential surfaces of the total gravitational field of the pair. These are surfaces where the sum of the rotational (centrifugal force) and gravitational energy per unit of mass is constant. The equipotential surface of the total gravitational field of the pair passing through the first Lagrangian point (L1) is called the Roche limit. For each component in a binary system, the space covered by the Roche limit is defined as Roche lobe. The co-rotational matter inside the Roche limit remains gravitationally associated with the corresponding component. Depending on the equipotential surfaces, the eclipsing binary systems are divided into three types:

  • Detached eclipsing binary systems (D) in which the two components are enclosed within individual equipotential surfaces and do not fill up their Roche lobes. Their evolution is more or less independent of each other. It could be associated with EA (Algol) type.
  • Semi-detached binary systems (SD) in which one of the star fills up the equipotential surface of its Roche lobe and through the first Lagrangian point (L1) transfers mass to the second component. With many, many constrains one could associated SD with EB type that I WOULD NOT recommend.
  • Contact eclipsing binary systems (C), where both components fill up their Roche lobes and continuously exchange masses through the L1 point. The mass exchange between the two components evolves their evolution. It is possible the binary system to create so-called common convective envelope (CCE) of the material embracing the stars (in cases like this, some authors use the term (still under lots of discussion), over-contact eclipsing binary systems). The contact eclipsing binary systems could be associated with EW (W UMa) type.

By modeling the photometric data one can find the mass ratio, individual masses, radiuses, luminosities, and the distance between the components. It is possible to judge whether the components develop a common envelope or not, the global distant to the star etc. That physical properties allows to classify the eclipsing binary stars as detached (D), semi-detached (SD) or contact (C) ones. The modeling could be done using all modification of the Wilson–Devinney (WD) code (Wilson & Devinney 1971; Wilson 1979, 1993) or other software as the PHOEBE code (Prsa & Zwitter 2005; Prsa et al. 2011, 2016) which is based on the Wilson–Devinney (WD) code.

To do this it is necessary to have relevant photometric data. The photometric errors (scatter) of the ASAS-SN, ROTSE and SWASP data are huge. The time resolving capability of these sequences should be taken into account with a great deal of uncertainty. So you need observational data with a good S/N ratio that has to cover entire light curve including both primary and secondary minima and the maxima in quadratures as well. The time series should give you good time resolving power to measure the time of minima (ToM). Based on the ToM we can improve the ephemeris of the eclipsing binary stars. In the case when the eclipsing binary do not undergo total eclipse you will need spectral data to get radial velocities of the components and to calculate the mass ratio of the binary system with confidence.

Once modeling the eclipsing binary star and getting the physical properties of the system one could classify the star according the modern concepts.

I apologize for the long post but I would like to point out that the observations of the eclipsing binary systems plays significant role of understanding the stars life and that the binary systems are the real laboratory for investigations.

Regards,

Velimir

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