Alert Notice 488: Observations of AA Tau requested to schedule XMM-Newton

August 1, 2013: Dr. Hans Moritz Guenther (Harvard-Smithsonian Center for Astrophysics) has requested nightly observations of the classical T Tauri star AA Tau in order to schedule x-ray observations with XMM-Newton that have been planned for between 2013 August 15 and September 15.

The purpose of the AAVSO observations is to determine whether AA Tau is at a suitable magnitude for the satellite observations. Taurus is difficult to observe during this time period but that is exactly why AAVSO assistance is needed! AA Tau is a morning object, and also, many of the professional ground-based telescopes are offline because of the US southwest monsoon season. Since it is critical to know the brightness of AA Tau, your observations will be truly essential.

Nightly visual and snapshot (not more than once per night) observations beginning now and continuing through September 20 are needed. Coverage beginning ahead of the XMM window is requested because there is a one- to two-week lead time for the target to be inserted into the telescope schedule. Continuing the nightly observations a few days beyond the end of the XMM window will give better optical context for the x-ray data. AA Tau ranges between ~12.8V and ~16.1V; since December 2011 or earlier it has been at ~14.5V. The most recent observation in the AAVSO International Database shows it at 14.779V on 2013 Feb 5 (JD 2456328.75602) (J. Roe, Bourbon, MO).

Dr. Guenther writes, "AA Tau is a classical T Tauri star in the Taurus-Auriga star forming region. It is surrounded by a thick accretion disk. Material is falling from the accretion disk onto that star. In the case of AA Tau this disk is seen nearly edge-on. For decades the light curve of AA Tau showed regular eclipsing events when the accretion funnel, that connects the star and the disk, rotated through the line of sight. However, earlier this year J. Bouvier and his group found that this behavior changed dramatically: AA Tau now seems to be deeply absorbed all the time (V band 14.5 mag). In collaboration with this group we will perform X-ray observations of AA Tau with the XMM-Newton satellite...

"[Your observations are needed] so we can make sure that is it still in the low state when the XMM-Newton observations happen. Observations in calibrated bands (e.g. V) are preferred, but not required. [High] precision is not required, since even data with large uncertainties should allow us to check if AA Tau is at 14.5 mag or at 12.8 mag...Fainter-than observations of <13.0 or fainter will also be useful to us. Your help with this project will be greatly appreciated."

Coordinates (2000.0): R.A. 04 34 55.42  Dec. +24 28 53.1

Charts for AA Tau may be created using the AAVSO Variable Star Plotter (VSP,

Please report observations as soon as possible to the AAVSO International Database using the name AA TAU.

This campaign is being followed from the AAVSO Observing Campaigns page (

Your observations are crucial to the success of this campaign! Many thanks!

This AAVSO Alert Notice was prepared by Elizabeth O. Waagen.


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