Alert Notice 493: Request to monitor BP Tau in support of Chandra X-ray observations

December 18, 2013: Dr. Hans Moritz Guenther (Harvard-Smithsonian Center for Astrophysics) has requested nightly observations of the classical T Tauri star BP Tau in support of upcoming Chandra X-ray observations in his study of the accretion processes underway in this star.

Dr. Guenther writes: "... BP Tau is a classical T Tauri star in the Taurus-Aurigae star forming region. It is surrounded by a thick accretion disk. Material is falling from the accretion disk onto that star and we want to study this accretion shock and its interaction with the other layers of the star. The accretion shock is very hot (2-4 Mio Kelvin) and can be best seen in X-rays, but support from optical observers is needed. Specifically, we want to find out (1) if the X-ray spot is seen at the same time as bright or dark spots on the stellar surface, and (2) if the optical brightness changes, when the X-ray flux suddenly goes up or down due to changes of the rate of accretion or stellar flares. To do all this we need optical light curves over several weeks, so that we can see the rotational modulation and the average fluctuation in the light curve. For this we ask for the help of the AAVSO.

"BP Tau was observed with the XMM-Newton satellite a few years ago and it is (after TW Hya) the second X-ray brightest classical T Tauri star in the sky. Now, we will observe BP Tau with Chandra for 9 days. This will be the longest X-ray exposure ever obtained for a young star. Unfortunately, the Chandra satellite is ageing and very restricted on where it can point on the sky, so the observation has to be split over several months.

"The first half of the observation is scheduled to happen between 23-Dec-2013 and 06-Jan-2014 in many small chunks.

"It would be a great help if [the AAVSO] observers could support us again."

Nightly visual and snapshot (not more than once or twice per night) observations from now through the end of the observing season are requested. Multicolor observations are welcome; if choosing one band V is preferred. BP Tau ranges between ~11.3V and ~13.0V (~13.4B). The most recent observations in the AAVSO International Database show it at 12.129V on 2013 December 10.8995 UT (2456637.3995, D. Barrett, St. Leger Bridereix, France) and at 11.598V on December 18.2368 (2456644.73676, J. Roe, Bourbon, MO).

Coordinates (2000.0): R.A. 04 19 15.83  Dec. +29 06 26.8

Charts for BP 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 BP 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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