Photometric History for the Past Century of ASASSN-V J060000.76-310027.83 (J060000)

ATel #13450; Bradley E. Schaefer (Louisiana State Univ.)
on 5 Feb 2020; 17:37 UT
Credential Certification: Bradley E. Schaefer (schaefer@lsu.edu)

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The ASAS-SN Team (ATel #13346) reported their discovery of an ordinary early-K star (ATel #13349, #13361), constant in magnitude from 2013 Oct 24 until 2019 Oct 24 (ATel #13346), that displayed an exponentially increasing fade into a 0.8 mag dip in brightness with a minimum on 2019 Dec 11 (ATel #13357).

I examined roughly a hundred archival photographic plates at Harvard College Observatory, with the significant detections spanning from 1922-1948. Except for three plates from late 1937, all the plates have an average magnitude of B=14.94, with an RMS scatter of 0.12 mag, consistent with the usual photometric accuracy of measurement. From 1937, I measured B=15.3 for Sep 13, B=15.3 for Nov 5, and B>15.3 for Nov 24. With three solid measures that the star was significantly faint by around 0.4 mag for over a month, I conclude that J060000 had a dip in brightness in late 1937, much like the 2019/2020 dip.

The original Palomar Sky Survey 103a-O plate is closely in the B system, when calibrated with measured image radii from 16 nearby stars of similar B magnitude. I measure B=14.90 ± 0.28 for 1956 Mar 9.

Three all-sky surveys, with public on-line light curves, are showing J060000 to be constant to within the measurement uncertainty as shown by nearby comparison stars. ASAS has 437 magnitudes from 2001 to 2009, with V=13.65. SuperWASP has 11292 magnitudes from 2006 to 2008, with a mean magnitude of 13.96 for a bandpass from 400 to 700 nm. ASAS-SN has 1159 magnitudes from 2013-2018 with V=13.64, plus 1359 magnitudes from 2017 to 2019 Sep with g=14.21.

The 314 individual magnitudes for the APASS survey used 7 filters on 17 nights from 2011-2014. The RMS scatter of the magnitudes for J060000 are consistent with the stated photometric uncertainties and with the RMS scatters of nearby comparison stars, so J060000 appears constant. I calculate B=14.88, V=13.63, g'=14.23, r'=13.09, i'=12.65, and Z=12.44.

After AAVSO Alert Notice 691, many observers contributed 3484 B, V, R, and I magnitudes from 2019 Dec 20 to 2020 Feb 1. The light curve shows two local minima, with these second and third dips being of larger amplitude in blue light and smaller amplitude in red light. The minimum of 2020 Jan 2 has B=15.40, V=14.05, and I=12.48, while the minimum of 2020 Jan 27 has B=15.55, V=14.11, R=13.18, and I=12.51. For the maximum between these dips (around 2020 Jan 9), B=15.13, V=13.83, R=13.03, and I=12.35.

For a scenario with the dips being caused by some occulting dust cloud, we can calculate R = A_V/E(B-V): (1) From the normal un-dimmed state to the bottom of the third dip, A_V=14.11-13.63=0.48, while the B-V changed from 14.88-13.63=1.25 to 15.55-14.11=1.44 for E(B-V)=0.19. This gives R=2.5. (2) From the baseline to the maximum between the dips, R=4.0. (3) For the dust adding extinction from the maximum between dips to the bottom of the third dip, R=2.0.

I conclude: (1) J060000 had a dip in late 1937 that was much like the current episode of dips. (2) Other than the two dip episodes, J060000 is constant (to within typically 0.04 mag or less), with no trends, flares, or oscillations for the last century. (3) The 2019/2020 episode is chromatic (the dips are much deeper in blue than red light), with brightnesses varying as expected for occultation by dust clouds. (4) The R value for the dust varies from 2.0 to 4.0, implying some segregation of dust by size. (5) The only known class of star displaying rare, irregular, and chromatic dips on ordinary main sequence stars, with no infrared excess, is the 'dipper' class, with the Boyajian Star (KIC 8462852) as the prototype.