Alert Notice 809: Monitoring of AB Aur requested for HST support

Note: HST observations of AB Aur have been scheduled for 2023 December 31 - 2024 January 1 UT. Please see Alert Notice 844 for more information.  -  Elizabeth O. Waagen, December 20, 2023

January 17, 2023

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Drs. Lauren Biddle, Brendan Bowler, and Yifan Zhou (University of Texas) request AAVSO observers' assistance in monitoring the bright (6.9-8.4V) young, accreting A0 Herbig Ae star, AB Aur, in support of HST observations planned for 2023.

One set of HST observations has been completed, and the final set of HST observations will take place in February, but observers are asked to observe AB Aur beginning immediately and continuing through the end of the observing season. Building the light curve in the weeks leading up to the HST observations is essential so that, Dr. Biddle writes: "we have a complete and accurate understanding of the star’s accretion activity and the behavior of its variability." Observers will be notified about the exact HST dates as soon as the information becomes available.

Dr. Biddle continues: "Recently, a pointlike source has been detected with direct imaging of the circumstellar disk of AB Aur, indicating the potential existence of an accreting protoplanet at an orbital distance of ~93 au (Currie et al. 2022; Zhou et al. 2022). However, unresolved features of protoplanetary disks can mimic planets by scattering light from the central star, causing false-positive signals — a major challenge in validating candidate protoplanets. Additional evidence is required to validate the existence of the protoplanet candidate, AB Aur b.

"Our team has an approved HST program that will confirm or refute the existence of the candidate protoplanet with accretion light echoes. We seek to answer the question: Is emission from this pointlike feature a bona fide accreting protoplanet, or is it an unresolved compact disk structure seen in scattered light? If flux from the companion source varies synchronously with the host star, that would point to the disk scattering scenario, whereas uncorrelated behavior would support the planet interpretation.

"The nature of this experiment relies heavily on variability of the central star, and so it is essential that we understand the nature of its variability to the fullest extent during the months leading up to, and overlapping with, the HST observations. This monitoring campaign will provide critical support for the success of our HST program by enabling us to recover the characteristics of AB Aur’s variability. New science resulting from this campaign will be published in the Astrophysical Journal alongside the HST results as a multi-part series."

CCD or photoelectric (PEP) photometry is requested. Rc is preferred, then B and V. Unfiltered photometry (CV or CR) and DSLR observations are acceptable but will receive lower priority in analysis than filtered CCD/PEP observations. CCD observers are reminded to be very careful not to saturate their exposures of this bright target.

Observers with Halpha filters are asked to prioritize this filter over all others, as observations made with it will best support the science goals of the campaign. Please use the Rc magnitude of the comp star for your Ha photometry reference and report to the AAVSO database as an Ha magnitude.

Comparison Stars
All observers are requested to use GSC 2387-0879 (the 116 in the AAVSO AB Aur sequence) as their single comp star and GSC 2387-0935 (the 120 in the sequence) as their check star. Single comp star photometry is preferred for this campaign.

Dr. Biddle writes: "The science goals of this campaign would be best supported by a time-series with observational cadence that covers a range of timescales: high-cadence (hourly), medium cadence (weekly), and low-cadence (monthly). An ideal observing plan that satisfies the desired cadences would look something like: 3 – 10 observations over the course of the night, with each night separated by a few days to a week, repeated over several months."

Since the comp and check stars for this campaign are about 5 magnitudes fainter than AB Aur, observers are asked to take multiple exposures in each filter used and stack the exposures to attain comp star S/N of 100 or more while not overexposing AB Aur.

Dr. Biddle also notes that observers who provide significant contributions to the monitoring campaign will be considered for co-authorship on publications resulting from the reduced data they provide.

When the HST observations are scheduled, observers will be notified via the forum threads above and likely via a follow-up AAVSO Alert Notice.

Coordinates (2000.0):  R.A. 04 55 45.84  Dec. +30 33 04.3  (from VSX page for AB Aur)

Charts with comparison stars for AB Aur may be created using the AAVSO Variable Star Plotter (VSP). It is recommended that observers use the 'E' scale when creating charts for AB Aur. Be sure to create new charts for this campaign - do not use ones you may have from earlier observing, as there have been changes to the comparison stars. Observers should be aware that the 116 comp star appeared in earlier versions of the AB Aur sequence as 117. If you are using AUIDs to identify the comp and check stars when you submit your observations, note that the AUID of the 116 is not the same as that star had when it was 117; be sure to use the AUID from the current chart's photometry table.

Please submit observations to the AAVSO International Database using the name AB AUR.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using information provided by Dr. Biddle.


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