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Alert Notice 654: XZ Tau and HL Tau monitoring needed for Chandra observations

Note: The November 15 observation with Chandra has been rescheduled to 2018 Nov. 11 17:37 - Nov. 12 00:01 UT and 2018 Nov. 12 08:01 - Nov. 12 14:27 UT. Please see AAVSO Alert Notice 656 for details. - Elizabeth O. Waagen

October 29, 2018: Dr. David Principe (MIT) has requested AAVSO observers' assistance in monitoring the classical T Tauri star binary XZ Tau in support of Chandra observations scheduled between tonight and November 15. Coverage of the very nearby HL Tau, a classical T Tauri star, is also requested but at a lower priority.

Dr. Principe writes: "XZ Tau (binary separation ~ 0.3") is classified as an EXor type object, one that undergoes random mass-accretion outbursts on timescales of days-months.  During an outburst, the combined brightness of the binary can increase several magnitudes in the optical. The brightness of the combined (unresolved) binary is typically between Vmag 13-14.5 but has reached as high as Vmag ~10.5 during an extreme outburst (Dodin et al. 2016, Astronomy Letters, Volume 42, Issue 1; Krist et al. 2008, AJ 136, 5) and can vary by 1mag in V on timescales of weeks.  

"XZ Tau is scheduled to be observed with the Chandra X-ray observatory multiple times over several days in the coming months with the next sequence of observations scheduled to start on October 30, 2018 (see table below). We request photometric observations in any optical band (preferably V, B, and/or R in that order) of the XZ Tau binary.

"If possible, it would be ideal to observe [XZ Tau] during any/all of the below Chandra sequences (or within a few days of the Chandra observations). Its outburst state is not expected to change much on timescales of days and thus one or two observations per night would be useful in determining its accretion state (whether it is in outburst or not). The purpose of the Chandra observations are to identify signatures of accretion shocks on the surface of the star and thus determining its outburst state with optical photometry is helpful.

"The classical T Tauri star HL Tau (Vmag~15.1) is ~30" away from XZ Tau and is also a target of these Chandra observations. We request to include this object in the field of view of any XZ Tau photometric measurements if possible, however, this target is not a priority as it is not expected to display optical variability of the same type as XZ Tau. Near-IR J, H, K photometric measurements of XZ Tau and HL Tau could also be helpful but are not necessary.

Schedule of Chandra observations of XZ Tau and HL Tau:
2018 Oct. 30 17:36 - 21:00 UT
2018 Oct. 31 13:15 - 23:00
2018 Nov. 02 10:10 - 13:00
2018 Nov. 03 04:54 - 15:00
2018 Nov. 15 00:00 - 13:00

Coordinates (2000, from VSX), Range, Type:
XZ Tau   R.A. 04 31 40.08   Dec. +18 13 57.1   10.4 - 16.62 B   CTTS/ROT
HL Tau   R.A. 04 31 38.47   Dec. +18 13 58.1   15.1 - 17.2: B   INT

Charts with comparison star sequences for XZ Tau and HL Tau may be created using the AAVSO Variable Star Plotter (VSP).

Please submit observations to the AAVSO International Database using the names XZ TAU and HL TAU.

AAVSO Forums: This campaign is the topic of the AAVSO Campaigns and Observing Reports forum thread https://www.aavso.org/xz-tau-hl-tau-chandra-campaign

and the AAVSO YSO forum thread https://www.aavso.org/xz-tau-hl-tau-chandra-campaign-0

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material provided by Dr. David Principe.

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