Alert Notice 842: AB Aur monitoring requested for HST observations

November 15, 2023
Note: A webinar on AB Aur and this campaign will take place on November 19, 2023. See below under "What to Expect" for details and the free registration link.

AAVSO Forum threads (scroll to the bottom of a thread for latest posts):
- Campaigns & Observation Reports:
- Young Stellar Objects:

Please subscribe to these threads if you are observing this star so you can be updated by the astronomers and by HQ. Join in the discussion or ask questions there!

Drs. Lauren Biddle, Brendan Bowler (University of Texas), Yifan Zhou (University of Virginia), and Mr. Walt Cooney (Madrona Peak Observatory) 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 December 2023 - January 2024. They provide the following information:

Two epochs of HST observations have been completed, and the final 2 sets of HST observations will take place in late December and January, but observers are asked to observe AB Aur beginning immediately and continuing through February. Building the light curve in the weeks leading up to the HST observations [and afterwards] 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, including bright massive accretion events and rare dimming events". Observers will be notified about the exact HST dates as soon as the information becomes available.

Dr. Biddle continues: "Recently, a point-like 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 ~100 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 point-like 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 the variability signature of the central star, and so it is essential that we understand the nature of its variability to the fullest extent during the weeks 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 the final 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 02387-00489 (the 104 in the AAVSO AB Aur sequence, Rc=10.071) as their single comp star and GSC 2387-0935 (the 120 in the sequence, RC=11.483) as their check star. Single comp star photometry is strongly 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: medium-cadence (daily), and low-cadence (weekly and monthly). An ideal observing plan that satisfies the desired cadences would look something like: 3 – 5 observations over the course of the night, with each night separated by a few days to a week, repeated throughout the months of November 2023 through February 2024."

Because the comp and check stars for this campaign are 3 and 5 magnitudes fainter than AB Aur, observers are asked to take multiple exposures in each filter and stack the exposures to attain a comp star S/N of 100 or more while not overexposing AB Aur.

What to Expect:
This alert is the continuation of a year-long effort of AAVSO observers (Alert Notices 809, 811, 814), and Lauren Biddle and Walt Cooney are hosting a webinar on Sunday, November 19 at 1:00PM CST (19:00 UT) to present their initial findings of AB Aur's variability from the first observing season, discuss what observers should expect from this star, and answer questions that observers may have about relevant observing procedures and/or science goals. We encourage observers to attend this webinar to ensure that contributed observations are effective in achieving the goals of the campaign and that observers are getting the most out of their experience as contributors to this ambitious project. Register for the webinar here (free). Please note that in order to register, you need to have a Zoom account.

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 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, but is not the recommended comp star to use for these observations.

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 Lauren Biddle and Walt Cooney.



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