KIC 8462852 Hereford Arizona Observatory Photometry Observations #10
Bruce Gary, Last updated: 2020.12.20, 04 UT

I'm taking another break from observing this star.

An abrupt dip began a couple days ago, and appears to have ended. At g' band it was ~ 1.5 % deep (at r' band depth was less, at i' band depth even less). This latest abrupt dip is superimposed upon a shallow dip (~ 0.5 %) that began 11 days ago. These two dips were embedded within a 6-month slow decrease in brightness. The slow decrease fade amount varies with wavelength in a way that is consistent with an obscuring optically thin dust cloud dominated by small particles (greater fade at shorter wavelengths). This is just like dip behavior. So we now know that short fade events, or dips (lasting a couple days), and medium timescale fades (lasting a week or two), and the long-term variations (lasting several months), can be explained using models of optically thin dust clouds dominated by small particles.

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Links on this web page

    g', r' & i' magnitudes vs. date (for last 2 months & last year) 
  
  List of observing sessions (starting 2019 Oct 04)
    Finder image (showing my ref stars) 
    The Big Picture .
    My collaboration policy
    Speculations about physical model
    References    

Links on another web page
 

    HAO precision explained (580 ppm) 
    DASCH comment  

    This is the 10th web page devoted to my observations of Tabby's Star for the date interval 2020.09.27 to present.

  Go back to 9th of 10 web pages  (for dates 2019.01.20 to 2020.01.11) 
  Go back to 8th of 10 web pages  (for dates 2018.10.10 to 2019.01.19)
  Go back to 7th of 10 web pages  (for dates 2018.08.12 to 2018.10.04)
  Go back to 6th of 10 web pages  (for dates 2018.02.25 to 2018.08.01)
  Go back to 5th of 10 web pages  (for dates 2017.11.13 to 2018.01.03)

  Go back to 4th of 10 web pages  (for dates 2017.09.21 to 2017.11.13)
  Go back to 3rd of 10 web pages  (for dates 2017.08.29 to 2017.09.18)
  Go back to 2nd of 10 web pages  (for dates 2017.06.18 to 2017.08.28)
  Go back to 1st of 10 web pages  (for dates 2014.05.02 to 2017.06.17)

    Reference Star Quality Assessment  (the 10 best stars out of 25 evaluated)  
 
g', r' and i' Mag's vs. Date


Figure 1
HAO g', r' and i'-magnitudes for the past month. The horizontal dashed lines are suggestions for OOT levels, set to the brightest magnitudes observed during the past two years (when I began observing in three bands).



Figure 2. HAO g', r' and i'-magnitudes for the past year. The g' trace from JD4 9000 to 9120 are from Sjoed Dufoer's AAVSO-submitted V band magnitudes (shift adjusted to match my g'-mags, and smoothed). The r' and i' traces are departures from the respective OOT levels multiplied by the g'-mag departures from the g' OOT level. The multipliers for r' band and i' band are 0.56  and 0.40. In other words, the r' and i' fade amounts are 56 and 40 % of the g' fade amounts.

Here's my suggestion for understanding the previous two figures:
1) During the past two years there was a 10-day interval (early November, 2019) when all bands were at their maximum brightness. I interpret these to be OOT levels (no dust clouds, just an unobstructed view of the star).
2) There are two components of dust cloud: broad, producing slow fades of brightness (month timescale) and small, producing brief fades (a few days timescale).
3) Both dust cloud components have similar "particle size distributions" (PSDs) that are dominated by small particles (< 0.5 micron radius), so they produce greater fades at shorter wavelengths.
4) The fade ratios for r' to g' and i' to g' would be the same for all dust clouds if they had the same PSDs. Since specific dust cloud PSDs may differ the observed ratios may vary over time.
5) On the assumption that all dust clouds have the same PSD it should be possible to predict r' and i' fade amounts by multiplying the g' fade amount by fixed ratios.
6) So far it appears that the fixed ratios are 0.54 and 0.40. These values should provide a constraint
on PSD functions. (I need help with that.)

The overall conclusion from these observations is that both the short-timescale dipping and long-timescale variations are caused by dust clouds dominated by small particles!


Figure 3. Comparison of Sjoed Dufoer's AAVSO-submitted V band magnitudes and my g' band magnitudes (shifted for approximate "agreement").

ASASS SN B band measurements support the above fade variation (as Rafik Bourne has determined).

List of Observations (for all earlier observations, before 2020.09.27 go to link)

2020.12.20  
2020.12.09  
2020.11.19  
2020.11.18  
2020.10.30  
2020.10.29  
2020.10.28  
2020.10.27  
2020.10.25  
2020.10.24  
2020.10.23  

2020.10.22  
2020.10.21  
2020.10.20  
2020.10.19  
2020.10.18  
2020.10.17  
2020.10.15  
2020.10.12  
2020.10.10  
2020.10.05  
2020.10.03
2020.09.28  
2020.09.27  


Daily Observing Session Information (most recent at top)




2020.12.20  



2020.12.09  



2020.11.19  





2020.11.18  





2020.10.30  







2020.10.29  







2020.10.28  







2020.10.27  







2020.10.25  







2020.10.24  







2020.10.23  







2020.10.22  




2020.10.21  





I have the image sets for i' in case they need to be processed.

2020.10.20  



I have the image sets for r' & i' in case they need to be processed.

2020.10.19  







2020.10.18  







2020.10.17  



2020.10.15  







2020.10.12  







2020.10.10  







2020.10.05  







2020.10.03  







2020.09.28  






 

2020.09.27  




 


Finder Image


Figure 5.1. Finder image showing the 17 reference stars that I use. KIC846 is in the blue square. FOV = 15.6 x 10.5 'arc, NE at upper-left.



The Big Picture

What is the overall character of KIC846 brightness variations?

I like to distinguish between short-term and long-term variations. The short-term variations are referred to as "dips." The dips last a few days typically. By long-term I refer to whatever is left over after removing the dip data. The long-term data can have variations with timescales of months to years. The next plot covers a 14 year interval and includes both Kepler and ground-based data, and it shows long-term variations (red model traces).


Figure 6.1. 14 years of Kepler and ground-based measurements. The black dots are Kepler data with dips removed; these data show the long-term variation during the 4 years of Kepler observations. Starting in 2017 (with only ground-based data) the dip and long-term data are shown with different symbols. None of Tabby's LCO data are shown (because a digital version of this data is not in the public domain) and none of the AAVSO data are shown (because most of it is noisy and adding the less noisy data would make the plot too "busy").  


Figure 6.2. Ground-based HAO g' measurements during the past 3 years (plus TESS).


Figure 6.3. Ground-based HAO g', r' & i' measurements during previous (2018/19) observing season.

Now let's return to the Kepler data that has long-term variations removed, allowing us to see just the short-term ("dip") activity.


Figure 6.5a. Kepler short-term version of data for the entire 4-year of Kepler observations.


Figure 6.5b. Same Kepler data but with an expanded normalized flux scale.


Figure 6.5c. Last 3 months of Kepler data showing the one set of dips with a complex and sometimes deep dipping structure.

As an aside, allow me to show what TESS observed recently:

Let's do the same removal of long-term variations for recent ground-based data.



Figure 6.7. Ground-based (HAO) data, plus TESS data, with long-term variations removed (showing only dip activity) for the 3 years preceding this observing season.

Here's an expanded version of the TESS data in the above plot.


Figure 6.8. TESS measurements of two short-timescale dips.


Figure 6.9. Ground-based (HAO) data with long-term variations removed (showing only dip activity) for the last 2 months of last year's observing season.

Other ground-based data exists but some of it is not in the public domain in digital form (LCO data) and I apologize to the AAVSO observers with data that is not included above. I'll try to add some AAVSO data if I get time for processing and selecting it.

Note, as Rafik Bourne pointed-out to me, TESS is sensitive to just long wavelengths (Rc/Ic/z') which does not include g'-band, and since dip depth is consistently less at longer wavelengths TESS dip depths will always be less than g'-band depths. For example, in the above figure the TESS dip showing depth = 1.2 % would probably have been observed with a g' filter to have a depth of 2.0 or 2.5 %.

We can now ask the question: Are the long-term and short-term (dip) activities for the past 3 years similar or different from what Kepler observed during 4 years?

Long-term Variation Differences

Referring back to an earlier figure, repeated here, the long term variation during the past 3 years has been considerably greater than during Kepler's 4 years of measurements.


Repeat of Figure 6.1. The Kepler data with dip activity removed (black dots) exhibit just one large change (2.2 %) following a slow fade (1 %). The ground-based data, starting in 2017, exhibit several changes, or variations, each about 1 % but adding up to ~ 3.5 % during 3 years.      

Short-Term (Dip Activity) Differences

Again, there are significant differences between the Kepler 4-year record of dip activity and the 3-year record of ground-based dip activity. Consider the following figure showing the two "short-term only data" using the same scale for normalized flux but with the ground-based data shifted in time.
 

Figure 6.10. Comparing dip activity of Kepler and ground-based (HAO) data (i.e., long-term variations removed).

It is apparent in this comparison plot of dip activity that the past 4 years have exhibited more short-term ("dip") activity than a comparable interval of Kepler data. Another difference is that during the Kepler dates when dips were present they could be much deeper! This suggests to me that before Kepler KIC846 underwent an event that produced a few dense dust clouds with slightly different periods, and lately the dust clouds have dissipated (lower optical depth) and are spread out (more frequent dipping).

Physical Model Speculations

A possible explanation for this dip activity pattern (in the above figure) is that the Kepler observations were closer in time to an event that created a well-defined cluster of dust-producing fragments within an orbit, and during the course of the last 11 years, at least, the fragments have dispersed along the orbit owing to their orbital periods not being exactly the same. The total amount of light blocking dust may have not changed much, but since fragment-based dust clouds spread apart over time they produce more dips with lower depth. 

An important fraction of exoplanetary systems show evidence for having been disturbed by orbit migration. The most cited mechanism is Kozai-Lidov. According to this "high eccentricity migration" scenario, or HEM, a massive outer planet causes an inner planet's inclination to change in a way that increases eccentricity (an exchange of momentum). This might be one of the explanations for the presence of so many "hot Jupiter" exoplanets. It is also a suggested mechanism for bringing asteroids and planets into close-in orbits to white dwarfs (with evidence that 1/3 of WDs have close-in asteroids or planets). If the highly eccentric orbit causes the planet's closest approach to the star (periastron) to be close to the Roche radius of the star, then tidal forces can cause shape distortion, tidal heating, and in addition can cause surface material to "drift away" from the planet surface and orbit the star with a slightly different period than the planet. The fragments of material in their own orbits can collide with each other to produce dust clouds. Their orbits may slowly circularize with periods much smaller than for the mother planet. Material could therefore reside in a wide range of orbits spanning the planet's highly eccentric orbit, with long periods, to nearly circular orbits with much shorter periods. In theory, this range of periodicities could account for the presence of fade timescales ranging from many months to less than a day.

I want to call attention to three possibly distinct variation timescales: 1) long timescale variation, lasting many months to a few years, 2) medium timescale, lasting one or two weeks, and 3) short timescale, also called "dips," lasting from 1/2 day to 2 or 3 days. The following figures show examples of these three variations.


Figure 7.1. Slow variation, that my last 2 years when it is over.


Figure 7.2.
Medium timescale variation, lasting 9 days.

Finally, the brief dip in the above figure at JD4 = 9148 is an example of a short timescale variation, or "dip." However, the next figure is a better illustration of a dip having a similar length and depth (occurring 418 days earlier).


Figure 7.3. Detail of short timescale variation, also called a "dip," measured by TESS on 2019 Sep 03. It lasted 18 hours.

Consider the fact that some dips last only ~ 0.75 day, from start to finish (e.g., the TESS dip on 2019.09.03, shown above). How fast must this dust cloud be moving to cross the star's diameter in that time? The answer is 35 km/s, which corresponds to a circular orbit with a radius of 1.12 AU and an orbital period of exactly a year.
An alternative interpretation would be for a smaller orbit that is eccentric so that the orbital motion vector is not perpendicular to our viewing direction. Regardless of orbital motion geometry that we consider, we can say that if the dust cloud was opaque (with abrupt edges) it would have to be as large as 10 % the diameter of the star. We know it wasn't opaque because of the dip's shape, so we require a larger size dust cloud that is semi-transparent (i.e., "optically thin").

The Roche radius for the KIC846 star is located approximately at the star's surface. Therefore, any planet whose eccentricity rises so high that it comes close to the star's Roche radius would either be enveloped by the star or heated to temperatures that would evaporate it (after a few passes). We therefore conclude that tidal disruption of a migrating planet has not played a role in producing dust clouds. Instead, collisions remain the next candidate to consider for producing dust clouds.



We should keep in mind the possibility that the long-term variations in brightness that seem to have increased during the past 11years (cf. Fig. 6.1) could be caused by:1) reflection of starlight when the dust cloud is on the far side of the star, or 2) forward scattering when the dust cloud is on the near side of the star (close to our line-of-sight). With a more spread-out configuration of dust clouds there is less chance of one cloud blocking the reflection, or forward scattering, of another cloud.

Note: these are just speculations by an amateur; actual modeling of these and other ideas are needed by more-qualified people. 

My Collaboration Policy

At my age of 80 I'm entitled to have fun and avoid work. Photometric observing and figuring things out are fun. Writing papers is work. So if anyone wants to use any of my observations for a publication you're welcome to do so. But please don't invite me for co-authorship!

My light curve observations are "in the public domain." This means anyone can and may download my LC observations, and use (or misuse) any of that data for whatever purpose. If my data is essential to any publication just mention this in the acknowledgement section.
 

References

    Gonzalez, M. J. Martinez and 15 others, 2108, "High-Resolution Spectroscopy of Boyajian's Star During Optical Dimming Evetnts," arXiv:1812.06837
    Wright, Jason T., "A Reassessment of Families of Solutions to the Puzzle of Boyajian's Star," arXiv  (a 1.1-page paper)
    Schaefer, Bradely E., Rory O. Bentley, Tabetha S. Boyajian and 19 others, 2018, "The KIC 8462852 Light Curve From 2015.75 to 2018.18 Shows a Variable Secular Decline," submitted to MNRAS, arXiv 
    Bodman, Eva, Jason Wright, Tabetha Boyajian, Tyler Ellis, 2018, "The Variable Wavelength Dependence of the Dipping event of KIC 8462852," submitted to AJ, arXiv.
    Bodman, Eva, 2018, "The Transiting Dust of Boyajian's Star," AAS presentation, link 
    Yin, Yao and Alejandro Wilcox, 2018, "Multiband Lightcurve of Tabby's Star: Observations & Modeling," AAS presentation, link (navigate down, etc)
    Sacco, Gary, Linh D. Ngo and Julien Modolo, 2018, "A 1574-Day Periodicity of Transits Orbiting KIC 8462552," JAAVSO, #3327, link
    Boyajian, Tabetha S. and 198 others, 2018, "The First Post-Kepler Brightness Dips of KIC 8462852," arXiv 
    Deeg, H. J., R. Alonso, D. Nespral & Tabetha Boyajian, 2018, "Non-grey dimming events of KIC 8462852 from GTC spectrophotometry" arXiv 
    Bourne, R., B. L. Gary and A. Plakhov, 2017, "Recent Photometric Monitoring of KIC 8462852, the Detection of a Potential Repeat of the Kepler Day 1540 Dip and a Plausible Model," arXiv:1711.10612     
    Bourne, Rafik and Bruce Gary, 2017, "KIC 8462852: Potential repeat of the Kepler day 1540 dip in August 2017," submitted to AAS Research Notes, preprint: arXiv:1711.07472
    Xu, S., S. Rappaport, R. van Lieshout & 35 others, 2017, "A dearth of small particles in the transiting material around the white dwarf WD 1145+017," approved for publication by MNRAS link, preprint arXiv: 1711.06960 
    Gary, Bruce and Rafik Bourne, 2017, "KIC 8462852 Brightness Pattern Repeating Every 1600 Days," published by Research Notes of the AAS at link; preprint at arXiv:1711.04205
    Gary, B. L., S. Rappaport, T. G. Kaye, R. Alonso, J.-F. Hambsch, 2017, "WD 1145+017 Photometric Observations During Eight Months of High Activity", MNRAS, 2017, 465, 3267-3280; arXiv
    Neslusan, L. and J. & Budaj, 2016, "Mysterious Eclipses in the Light Curve of KIC8462852: a Possible Explanation, arXiv: 1612.06121v2  (a "tour de force"; I highly recommend this publication)
    Neslusan & Budaj web site with animation of their way of explaining Kepler D1540 dip:  http://www.astro.sk/~budaj/kic8462.html
    Wyatt, W. C., R. van Lieshout, G. M. Kennedy, T. S. Boyajian, 2017, "Modeling the KIC8462852 light curves: compatibility of the dips and secular dimming with an exocomet interpretation," submitted to MNRAS, arXiv  
    Grindlay interview about Schaefer's assertion that KIC846 exhibited a century long fade using DASCH data: link
    Hippke, Michael and Daniel Angerhausen, 2017, "The year-long flux variations in Boyajian's star are asymmetric or aperiodic," submitted to ApJL, arXiv 
    Sacco, G., L. Ngo and J. Modolo, 2017, "A 1574-day Periodicity of Transits Orbiting KIC 8462852," arXiv
    Rappaport, S., B. L. Gary, A. Vanerdurg, S. Xu, D. Pooley and K. Mukai, 2017, "WD 1145+017: Optical Activity During 2016-2017 and Limits on the X-Ray Flux," arXiv, Mon. Not. Royal Astron. Soc.
    Steele, I. A. & 4 others, 2017, "Optical Polarimetry of KIC 8462852 in May-August 2017,"MNRAS (accepted), arXiv.
    Simon, Joshua D., Benjamen J. Shappee and 6 others, "Where is the Flux Going? The Long-Term Photometric Variability of Boyajian's Star," arXiv:1708.07822 
    Meng, Huan Y. A., G. Rieke and 12 others (including Boyajian), "Extinction and the Dimming of KIC 8462852," arXiv: 1708.07556  
    Sucerquita, M., Alvarado-Montes, J.A. and two others, "Anomalous Lightcurves of Young Tilted Exorings," arXiv: 1708.04600   Also: New Scientist link and Universe Today link.
    Rappaport, S., A. Vanderburg and 9 others, "Likely Transiting Exocomets Detected by Kepler," arXiv: 1708.06069 
    Montet, Benjamin T. and Joshua D. Simon, 2016, arXiv 
    Boyajian et al, 2015, MNRAS, "Planet Hunters X. KIC 8462852 - Where's the flux?" link
    Ballesteros, F. J., P. Arnalte-Mur, A. Fernandez-Soto and V. J. Martinez, 2017, "KIC8462852: Will the Trojans Return in 2011?", arXiv
    Washington Post article, 2015.10.15: link
    AAVSO Campaign Notice requesting KIC646 observations
    AAVSO LC Generator https://www.aavso.org/data/lcg (enter KIC 8462852)
    Web page tutorial: Tips for amateurs observating faint asteroids (useful for any photometry observing)
    Book: Exoplanet Observing for Amateurs, Gary (2014): link (useful for any photometry observing) 
    wikipedia description of Tabby's Star  
    My web pages master list, resume


    B L G a r y at u m i c h dot e d u    Hereford Arizona Observatory    resume 
 
This site opened: 
2020.10.05. 
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