WD 1856+534 Transit Light Curve Photometry
Bruce L. Gary, 2020 Oct 23, 05 UT

WD1856 exhibits transits that are 57 % deep every 1.41 days. The original goal for the photometry observations reported here was to provide an accurate transit light curve that could be used to improve the TESS transit depth (5 %) and shape, in order to establish a case for obtaining expensive observing time on professional telescopes (the 10.4-meter Gran Telescopio Canarias, the world's largest optical telescope, and the Spitzer Space Telescope). Those measurements allowed modelers to establish that the occulting body is a planet the size of Jupiter, with a mass less than 7 times Jupiter's. Currently, the goal for the observations on this web page is to search for transit timing variations (TTV), and possible depth changes, both of which could be produced by a massive planet in an outer orbit of the same system. 

Why WD 1856+054 is Important

For decades astronomers have wondered if planets could survive the transition that most stars undergo after their normal lifetime of converting hydrogen to helium in their cores. When the hydrogen is exhausted the star swells to thousands of times its original size, becoming a red giant, and in the process consuming inner solar system planets. Eventually the red giant shrinks to an even smaller size than when it was a normal star, becoming a white dwarf that is only slightly larger than the Earth. Our sun (and solar system) is 4.5 billion years old; it has another 4 or 5 billion years before it undergoes the red giant/white dwarf transition. Afterward, our sun will remain a white dwarf forever – for 100 billion years, or however long the universe lasts. Stars like our sun will therefore spend most of their total lifetime as a white dwarf. Astronomers have had a long-standing question: “Will any of our solar system planets survive and orbit our diminutive sun during its white dwarf eternity?” With the discovery of a planet orbiting white dwarf WD 1856+534 we can now say that this is possible; survival of some of our outer planets may continue to orbit our white dwarf sun forever. The planet orbiting WD1856 is the size of Jupiter, which is 7 times larger than the white dwarf. Its mass could be comparable to Jupiter's, and it orbits the star every 1.4 days. If our sun has a similar fate it might be said that for the majority of our solar system’s lifetime (of 100 billion or more years) it will be a white dwarf orbited by only Jupiter, for example.


Introduction

TESS observations were used to identify WD1856 as exhibiting transits with a depth of ~ 5 %. TESS spatial resolution is poor, and in fact for this star the TESS photometry aperture included two other stars. This led to a large under-estimate of transit depth. On 2019.10.03 Andrew Vanderburg asked for ground-based follow-up observations by amateurs Bruce Gary and Tom Kaye with 3 "backyard" observatories (16", 16" and 32"). A week later we were able to observe a transit, showing that depth was much deeper, 57 %. With our improved transit depth and shape there was justification for requesting observations with the 10.4-meter Gran Telescopio Canarias, the world's largest optical telescope, and the Spitzer Space Telescope. The Spitzer observations of a transit at 4.5 microns ruled-out the scenario of the transiting object being a brown dwarf star (because transit depth at 4.5 micron was the same as at optical wavelengths). Modelers have concluded that the transiting object is a planet the size of Jupiter with a mass probably < 7 times Jupiter's. Since 2019 I have continued to observe WD1856 in order to search for transit-timing-variations. Such TTV could reveal the presence of a massive planet in an outer orbit that long ago (after the star's transition to white dwarf) caused the transiting planet's orbit to become highly eccentric, bringing it closer to the WD star, where it eventually circularized to its present small size with a 1.4-day orbit.

Links on this web page
    Overall results to dates
    Individual observing sessions listing
    Finder image
    References    
    External links of related material  
 
Basic Info for WD1856

RA/DE = 18:57:39.3 +53:30:33. V-mag = 16.88, Teff  ~ = 4710 K, Rwd = 1.429 x R_earth, Rpl = 10.4 x R_earth, Transit ephemeris: BJD = ~ 2458779.3750828 + E x 1.4079405. Transit depth = 57 %, length = 7.356 minutes (ingress to egress). Observing season is centered on July 7. These values are listed in the discovery paper (Vanderburg, 2020). I recommend a new P = 1.4079389 (2) days.

Overall Results to Date

Here's a TTV plot for 2019 October to now. It shows "observed mid-transit time minus predicted mid-transit time" as well as dip depth. 


Figure 1. Transit Timing Variation (TTV) and dip depth versus date.

The TTV slope is merely a slight refinement of the adopted period (a 1.5-sigma departure from the published period).
According to an analysis by Dr. Saul Rappaport there is no statistically significant quadratic curvature (i.e., due to orbit decay) or sinusoidal variation (due to an outer orbit massive planet). This is a long-term project, and we shouldn't expect to see any departure from a straight line for another 2 or 3 years. 

For my TTV analysis I have adopted a dip shape corresponding to "ingress to egress interval" L = 0.1226 hrs, Fp (fraction of transit that's partial) = 0.620, F2 (ratio of depths at contact 2 to OOT), fade depth at mid-transit
= 906 mmag (depth = 56.6 %), and a "shape" parameter = 0.37 (too complex to explain here). This choice of parameters is based on my model's fit to the very accurate GTC LC. After obtaining a TTV reading (mid-transit time difference with respect to a reference ephemeris) the depth parameter is allowed to vary and a solved-for value is recorded for later use in searching for depth changes versus date.


Figure 2. Merging of data for the best 11 observing sessions of this year. 


Figure 3.  The first 6 dates subset of above data.
Model is a fit to this data subset.


Figure 4. The last 5 dates subset of data in Fig. 2. Model is a fit to this data subset.

This last two figure can be used to answer the question "Has the dip changed depth, width or shape since 2019 Oct 22?" We have three epochs to compare: 2019 Oct 22 (GCT), 2020.06.11 to 2020.08.09 (HAO), and 2020.0819 to 2020.10.10 (HAO). Width hasn't changed (7.36 min, 7.56 min, 7.44 min), nor has depth (56.8 %, 57.3 % and 57.3 %). SEs will eventually be needed.
 

Individual Observing Sessions

2020.10.20 - BG16    
2020.10.10 - BG16 
2020.10.03 - BG16  
2020.09.26 - BG16  
2020.09.16 - BG16  
2020.09.02 - BG16  
2020.08.26 - BG16  
2020.08.19 - BG16  
2020.08.09 - BG16  
2020.07.26 - BG16   
2020.06.25 - BG16    
2020.06.18 - BG16  
2020.06.15 - BG16  
2020.06.11 - BG16
2020.05.18 - BG16  
2019.10.17 - BG16  
2019.10.17 - TK16  
2019.10.17 - JBO32  
2019.10.10 - BG16  
2019.10.10 - TK16  
2019.10.10 - TK32  


2020.10.20 - BG16    





2020.10.10 - BG16 





2020.10.03 - BG16  





2020.09.26 - BG16  





2020.09.16 - BG16  





2020.09.02 - BG16  





2020.08.26 - BG16  





2020.08.19 - BG16  





2020.08.09 - BG16 





2020.07.26 - BG16  





2020.06.25 - BG16 





2020.06.18 - BG16  





2020.06.15 - BG16  





2020.06.11 - BG16  





2020.05.18 - BG16  





2019.10.17 - BG16





2019.10.17 - TK16





2019.10.17 - JBO





2019.10.10 - BG16  





2019.10.10 - TK16  





2019.10.10 - TK32  





Finder Chart


FOV = 15.7 x 10.5 'arc, NE upper-left.

References

Maldonado, R. F., E. Vilaver, A. J. Mustill, M. Chavez, E. Bertone (2020), "Do Instabilities in High-Multiplicity Systems Explain the Existence of Close-in White Dwarf Planets?",  arXiv:2010.11403v1

Stephan, Alexander O., Smadar Naoz and B. Scott Gaudi (2020), "Giant Planets, Tiny Stars: Producing Short-Period Planets around White Dwarfs with Eccentric Kozai-Lidov Mechanism," submitted to Earth and Planetary Astrophysics, arXiv

Lagos, F., M. R. Schreiber, M. Zorotovic, B. T. Gansicke, M. P. Ronco and Adrian S. Hamers (2020), " WD 1856 b: a Close Giant Planet around a White Dwarf that Cold have Survived a Common-Envelope Phase," submitted to Earth and Planetary Astrophysics, arXiv 

Munoz, Diego J. and Cristobal Petrovich, "Kozai Migration Naturally Explains the White Dwarf Planet WD1856b," submitted to Earth and Planetary Astrophysics, arXiv  

Kaltenegger, Lisa., Ryan J. MacDonald, Thea Kozakis, Nikole K. Lewis, Eric E. Mamajek, Jonathan C. McDowell and Andrew Vanderburg, 2020, "The White Dwarf Opportunity: Robust Detections of Molecules in Earth-like Exoplanet Atmospheres with the James Webb Space Telescope," Astrophysical Journal Letters, 901, 1, link & arXiv 
 
Vanderburg, A., S. A. Rappaport, S. Xu, I. Crossfield, J. C. Becker, B. Gary, (and 58 others), "A Giant Planet Candidate Transiting a White Dwarf," (2020), Nature, 585, 363-367, 2020 September 17, digital version, Abstract (must pay for complete article), link. Or get a free copy at arXiv 


External Links of Related Material

    arXiv of Vanderburg et al. paper, link

    Nature article abstract (20.09.17, pay to see entire article), link

    Nature commentary (News and Views) (20.09.17), link

    Univ. Wisconsin press release (2020.09.16), link

    JPL press release (2020.09.16), link

    Goddard Space Flight Center press release (2020.09.16), link 

    CNN article, link 

    Space.com article, link 

    Physics World (20.09.16), link

    Science Alert (20.09.17), link  

    Sci News article (20.09.16), link  

    Astronomy magazine article (20.09.16), link  

    Tom Kaye YouTube description of amateur's role, link 

    B.L.Gary resume 

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 This site opened: 2019.10.18