The Extreme Occulation of a Sun-like Star

Jonathan Marshall^1*, Carlos del Burgo², Steve Ertel³, Peter Scicluna⁴, Francisca Kemper^5,1, Sascha Zeegers¹, Oscar Morata⁵

¹Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
²INAOE, Puebla, Mexico
³UoA, Tucson, USA
⁴ESO, Santiago, Chile
⁵ESO, Garching, Germany

* Presenter:Jonathan Marshall, email:jmarshall@asiaa.sinica.edu.tw

A drastic drop in the brightness of Gaia EDR3 5539970601632026752 (ASASSN-21qj) was recently reported, falling from 13.5 mag to > 16 mag in the g' band over the space of several days after previously exhibiting no large-scale variability. The host star is Sun-like (R_star = 1.004±0.015 R_Sun, L_star = 1.059±0.020 L_Sun), inferred from Gaia data and stellar evolution models, and lies at a distance of 556~pc. We believe that this object is a newly identified "little dipper" star, of which very few are known and about which we know very little. These unusual objects exhibit deep, aperiodic occultation events indicative of the presence of substantial amounts of circumstellar dust, but without corroborating evidence through the presence of infrared excess emission. In the case of this newly identified potential member of the cohort, the occultation depth is 90 % of the typical stellar flux; four times deeper than KIC 8462852's event, that was previously the most extreme, and prototypical, member of this class of objects.

In the last few years, an enigmatic class of irregularly variable main-sequence stars has emerged, often referred to as the "little dipper" stars. The first object of this class to be identified was KIC 8462852 ("Boyajian's star") and around a dozen ``little dipper'' stars have been identified to date. Short events blocking more than 20 % of the starlight have been observed, as well as prolonged asymmetric obscurations of a few %, with durations of 10 to 20 days. Frequency analysis of the dimming is inconclusive on the periodicity of KIC 8462852, with estimates ranging up to several years, long term monitoring is refining this parameter. Several possible causes for the peculiar optical light curves of so-called "little dippers" have been explored, and a stream of exocomets orbiting the host star remains the mostly plausible explanation. This idea has received support from calculations estimating the disruption of a Ceres-like planetesimal into smaller fragments, or the start of a late heavy bombardment type event, could explain the observations. Further evidence for the exocomet hypothesis comes from multi-wavelength observations of KIC 8462852, exhibiting a wavelength dependent transit depth consistent with an optically thin clump of sub-micron-sized dust grains occulting the star.

In this presentation I will provide an overview of the ''little dipper'' phenomenon and a summary of previous observations. I will then focus on the detection and evolution of a recent extreme occultation event of ASASSN-21qj. This event has been monitored in optical/near-infrared photometry using the LCO global network of telescopes. I then present our analysis of the event, constraining the size distribution of the dust grains from the time-dependent colour of the occultation, and the total mass of the dust clump from its shape and duration. Finally, I will give context to this event by comparison with the sample of known ''little dippers'', and speculate on its possible origins.

Keywords: Planetary systems, Exocomets, Dust, Circumstellar matter, Stars