Another dip has begun (following the possible D800/TESS dip of Oct 18). It appears to have a depth of 0.6 % and may be increasing. Regarding the predicted transit on Oct 18, I think we observed it but since none of our observing sets registered the ingress or egress structures (because they must have happened between observing sessions), and because I was unable to observe with g', r' and i' on the date of the dip (due to clouds), we can only say that our observations are compatible with Rafik's predicted transit while they are insufficient to confirm the presence of a transit by an optically thick object (in an orbit of 776 days).



Four nights ago was the 2nd of two nights for a predicted window for a 1 % dip lasting < 1 day due to the return of the Kepler D800/TESS dip. On Night 1 Barbara Harris (Florida) and Paul Benni (Boston) observed for 3 or 4 hours; neither found a change in brightness that could be attributed to an ingress (or egress) feature. Yenal Ogmen (Cyprus) didn't observe and Joao Gregorio and I were clouded-out! Joe Garlitz (Oregon) and John Hall (Colorado) are working with their data; Tabby Boyajian has data for each of 7 nights from 3 observing sites (but that data is for intervals that are too brief for detecting ingress or egress structure). On Night 2 I was able to observe, and obtained a brightness level corresponding to "OOT for dips." Paul Benni also got data for Night 2, as did Joao Gregorio (but Paul's data for Night 2 was with a different filter, and may not be usable). Night 3 has become crucial for establishing that a dip of ~ 1 % occurred at the approximate predicted time. By offsetting the Benni data to match Gary data we are able to place the Night 1 and 2 Benni data on the Gary/Harris magnitude scale. This allows for a complete 4-day light curve, and it shows the presence of a 1 % dip that lasted < ~ 1 day (which is consistent with the D800/TESS return prediction).

The goal for having several observers at a range of longitudes observe during Nights 1 and 2 was to try to detect an ingress or egress change in brightness, but the timing of the 1 % fade (and possible transit) have denied us observing coverage during those crucial times. The only chance of using brightness levels to detect the dip was for those of Harris and Gary, since the inter-calibration for these two observers is well-established. We are therefore free to adopt offsets for the other observers that provide a match to Harris and Gary data. In the following graph the data for Benni and Gregorio have been adjusted using offsets that provide agreement with the Harris and Gary observations (the data for Harris and Gary have not been adjusted using offsets). The transit model has been adjusted in two ways: 1) depth was reduced to 1 % (from the TESS value of 1.4 % in 2019), and 2) a time shift of -0.5 day (i.e., earlier) was applied.  Note: The Benni data for both dates was made with a V filter and the same reference stars. An offset was applied to the Benni data that provided a match to the Gary data of Night 3. The fact that the Benni data for Night 1 agrees with the Harris data is reassuring.  


Figure 0.1a. Comparison of 6 data sets for a 4-day interval using offsets for the Benni and Gregorio data that produce agreement with Gary data. The dip model is from a fit for TESS data (2019 Sep 03), shifted to an earlier time by 0.5 day and with a depth reduced from 1.4 % to 1.0 %. (Transit was predicted for 9506.3, or ~ 0.5 days later than shown, but our period of 776.14 days could have been uncertain by this amount.)

I conclude from this comparison of different data sets and the TESS dip model that our data is compatible with the Rafik prediction of P ~ 776 days; however the data do not provide sufficient support for Rafik's prediction that would allow us to claim that we have observational evidence confirming the 776-day periodicity. This lack of support is due to the observing gaps between Nights 0 and 1, and between Nights 1 and 2, where ingress and egress brightness changes may have occurred. Since we were unable to detect the ingress and egress features required by TESS data it is possible that the Oct 18 dip was just another 1 % dip (that have occurred frequently during this observing season). But if this is a repeat of the D800, D1570 and TESS dips, then dip depth is now lower than when TESS observed (i.e., 1.0 % now vs. 1.4 % in 2019, which was actually expected from a trend that was noted and described below, at link).
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This observing season continues a trend of more frequent dips of lower depth than the year before. When Kepler was observing (8 to 12 years ago) dips were rare, but when they occurred they were usually deep. When ground-based measurements first showed dip activity (2017 May), the dips were 1.5 to 3 %, and this was exciting. Every observing season since then as seen lower depth dips, and more of them. During the present observing season we have the most dips per month ever, and they're less deep than ever (~0.5 to 1.0 %). This pattern can be understood as a natural consequence of dips being produced by dust clouds resulting from a cascade of collisions that started with one big collision 10 years ago (in 2011). This "collision cascade model" predicts the following 3 patterns for dips on successive orbits (imagine orbit P = 776 days): 1) a group of dips will be spread out over a longer date interval, 2) there will be more dips per month, and 3) dips will be less deep. This pattern can already be seen. I therefore predict that the next several months will be like the last several months: lots of 0.5 to 1.0 % dips and nothing more dramatic. For more on this explanation see link.

Since early 2021 the "long timescale" level has been ~ 1 % below the "complete clearing" level (defined as no debris of any timescale).  The only "complete clearing" occurred in late 2019.

This observing season will provide a test of two lperiodicity predictions made a few years ago: 1601 days by Rafik Bourne and 1574 days by Gary Sacco. The following table may be useful in evaluating whether either of these two periodicities have support by this year's observations:

Table 1. Predictions for 2021 Observing Season Based on Competing Periodicities (1574 vs 1601 days)

Kepler Name	Depth	JD4	2 x 1574	2x1601	Date	Date
D800	16 %	5626	8774	8828	2019 Oct 18	2019 Dec 11
D1520	21 %	6353	9501	9555	2021 Oct 14	2021 Dec 07
D1540	3 %	6373	9521	9575	2021 Nov 03	2021 Dec 27
D1570	8 %	6401	9549	9603	2021 Dec 01	2022 Jan 24

In an article submitted to MNRAS on 2017 Nov by Bourne, Gary and Plakhov (link) we made the following predictions:



Prediction 1 is partially confirmed. An abrupt 2 % rise in brightness began 4 months after this window of dates (on 2018 Apr 7). Prediction 2 will be evaluated later this year. Prediction 3 is a failure (a U-shaped fade of ~ 3 %, lasting up to 2 years, did not commence this year). Prediction 4 will be evaluated later this year. By the way, in this article we neglected to predict a fade corresponding to the return of the Kepler D800 dip (listed in above table as JD4 = 8828); there was indeed a significant dip centered on this date (as shown in Fig 1c, below). The reduction of D800 depth and increase in duration could be easily explained using an orbit shear argument (see Fig. 6.11 in "Speculations about physical model", at link below, for description).

Recently, Rafik Bourne has noticed very interesting coincidences that involve a periodicity of 776.14 days. The first deep Kepler dip, referred to as D800 (JD4 = 1527), and the last deep Kepler dip, referred to as D1570 (JD4 = 6402) and a TESS dip at JD4 = 8730, are all separated by multiples of 776.14 days. In addition, their depths form a pattern of decreasing by half every 776-day hypothetical orbit.  In addition, according to Rafik, each transit can be modeled by a transit velocity of ~ 26 km/s and a similar impact parameter (that specifies star latitude for transit crossing path). A 776 day orbit corresponds to a orbit with radius ~ 1.9 AU (if circular), where orbit velocity ~ 26 km/s. A final coincidence is that all three transits can be explained by similar geometries - a circular object with size slightly larger than Neptune (implying mass ~ 0.7 times Jupiter) that have dust disks that decrease versus time in physical extent and density. These matters are discussed below, at link. Based on this 776-day pattern Rafik is predicting a return dip on JD4 = 9506.30 (2021 Oct 18.80). This prediction is summarized in the following table: 
 
Table 2. Prediction for Return of Kepler D800 Dip During 2021 Observing Season Based on Periodicity of 776.14 Days

	5th orbit return of D800	Predicted Depth
Ingress	JD4 = 9505.90, 2021 Oct 18, 10 UT
Mid-transit	JD4 = 9506.30, 2021 Oct 18, 19 UT	~ 1.2 %
Egress	JD4 = 9506.70, 2021 Oct 19, 05 UT

Prediction for 2021 Oct 18/19 dip, a return of the Kepler D800 dip after 5 orbits (with P = 776.14 days).

"Tabby's Star" continues to puzzle everyone! We don't even know what's creating the dust clouds that produce brightness variations. The best guess is collision cascades model, but we really don't even know that for sure. We'll be handicapped in understanding this star system until a periodicity is established. About the only thing we can be sure of is that variability exists on a range of timescales. There are slow changes, with timescales of many months to many years, and faster changes, with timescales of less than a day to several days (referred to as "dips"). The dips have smaller depths at longer wavelengths, and this is surely evidence for the presence of a small size component of dust (< 0.5 micron radius).  This is still true for the 2021 observing season dips. We have some evidence for the long timescale variations to exhibit the same wavelength dependence. This is just a tentative result because we haven't yet established the "complete clearing" level. I continually hope to observe such a clearing again, to verify the earlier tentative result for a "complete clearing" level in late 2019.

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Links on this web page

    Basic info for KIC846  
    Long timescale variations (based on AAVSO and other sources)
    g', r' & i' magnitudes vs. date
    List of observing sessions (for the 2021 observing season)
    Finder image (showing my ref stars) 
    Miscellaneous LC presentations .
    Data exchange files for download   
    My collaboration policy
    Speculations about physical model ..  
    References    

Links on other web pages
 
    HAO precision explained (580 ppm) 
    DASCH comment  

    This is the 11th web page devoted to my observations of Tabby's Star for the date interval 2021.04.25 to 2021.10.22.
  The next set of observations, starting with 2021.10.29, are at http://www.brucegary.net/ts12/.

  Go back to 10th of 11 web pages  (for dates 2020.09.27 to 2020.12.20) 
  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)
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  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)  

Basic Info for KIC846

RA/DE = 20:06:15.44 +44:27:24.9
V-mag = 11.85, g'-mag =12.046, B-V = +0.51 (APASS)
Spectral type: F3V
T_eff = 6750 K
R = 1.58 R_sun (1.10e+6 km)
M = 1.43 M_sun (2.84e+30 Kg)
Observing season centered on Jul 24

Long Timescale Variations




Figure 1a. 4-month LC for V and g' measurements.


Figure 1b. A 1-year LC of ground-based observations that are in the "public domain" (and are good quality), normalized to the V magnitude scale of observer HBB.


Figure 1c. A 2-year LC of ground-based observations that are in the "public domain" (such as from AASVO), normalized to the V magnitude scale of observer HBB.


Figure 1d. A 5-year light curve for V- and g'-band measurements (including magnitude offsets to achieve internal consistency).
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Figure 1e. 15 years of space-based and ground-based observations that are in the "public domain" (such as from AASVO), normalized to the V magnitude scale of observer HBB. (The Kepler data are from Montet and Simon, 2016, which omits dips). 

The next paragraphs are a highly speculative attempt to explain the above brightness variations.

Imagine that Tabby's Star is orbited by a Jupiter-class planet that has recently been perturbed into an eccentric orbit that brought its periapsis into the region of an asteroid belt. Occasional asteroid/planet collisions produce a ring of debris centered on the planet orbit. Due to Keplerian orbit shear the debris is approximately uniform along the entire planet orbit. Imagine that the planet orbit, and its debris belt, has a period of > 15 years. This debris ring produces long timescale brightness variations (e.g., months to years).

Now imagine that because this Jupiter-class planet enters the asteroid belt during its periapsis some asteroid orbits are perturbed, producing occasional asteroid/asteroid collisions. The debris produced by these collisions will orbit with a period similar to those of the asteroids (e.g., ~ 2 years). Due to radiation pressure from the star these dust clouds won't last long. Because of their shorter lifetimes (e.g., years) they won't have accumulated much Keplerian orbit shear, so during their lifetime they will be small clumps of dust. They orbit faster than the planet, and due to this, plus their smaller size, they transit the star on short timescales (e.g., a day to a week).

Both components of fading will be superimposed upon each other. On rare occasions the outer belt of dust that produces long timescale fades will have clearings, allowing us to determine the true brightness of Tabby's Star. I think this happened in late 2019. This unobstructed view corresponds to a brightness level shown in the my graphs by the green dotted line labeled "OOT (no debris)." 


Figure 1f. Illustration of two components of fading: outer orbit belt (long timescale) and inner orbit dips (short timescale).

 
HAO g', r' and i' Mag's vs. Date


The last two r'-band measurements are solid. I don't understand why they appears to be low.


Figure 2a. A 4-month LC of HAO g' and r' magnitudes. The dotted traces are an attempt to represent the component of slow variations (timescale of months). The fast variations, or dip structure, is superimposed upon the slow variations.   


Figure 2b. HAO g', r' and i'-magnitudes for the past 12 months. The horizontal dashed lines are suggestions for OOT levels, set to the brightest magnitudes observed during the past 3 years (when I began observing in three bands).


Figure 2c. HAO g', r' and i'-magnitudes for the past two years. The r' and i' model 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 0.60  and 0.35. In other words, the r' and i' fade amounts are 60 and 35 % of the g' fade amounts. The slow variations are more noticeable in this graph.

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!