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Alert Notice 716: XMM-Newton observing five T Tau stars

August 14, 2020

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Drs. Hans Moritz Guenther (MIT) and Christian Schneider (Universitaet Hamburg, Hamburger Sternwarte) have requested AAVSO observers' assistance in monitoring five T Tau stars in support of several observations with XMM-Newton scheduled for the coming months. Targets, as well as dates for August and September, are given below. The first XMM observation is to take place August 18, and coverage is requested beginning now.

Dr. Guenther writes: "Stars form in large clouds of gas and dust. In the beginning, they are surrounded by the remains of that cloud and can only be seen in X-rays and in the infrared, but as time goes on the gas and dust of the cloud collapses into a disk around the star. At the inner disk, ionized gas falls from the disk into the star, and can cause random or periodic obscuration. Where the infalling plasma hits the stellar surface, a bright spot develops, which also emits X-rays. Further out in the disk, planets form, and thus it's extremely important to study how stars, disks, and stellar flares interact. Stars in this phase are called classical T Tauri stars (CTTS). Over time, the disk material all falls into the star, collapses into planets or is blown out of the system, but the stars remain active with flares and stellar spots because they are young and rotate fast.

"We plan to study the time variability of these stars in X-rays over time scales of several weeks to months (many rotation cycles). Previous observations usually monitored CTTS continuously, but because X-ray observations from space are hard to do, even the longest monitoring only lasted less than a month. We selected a field in the Taurus Molecular cloud, which is one of the closest and oldest of the star forming regions around us. In Taurus, the molecular cloud is already gone, so that the view to the stars themselves is not obscured (except for those where the circumstellar disk is in the way).

"We will have several XMM-Newton observations this fall and winter, while Taurus is visible, and it would be extremely valuable to have optical data for [about five] nights before and after each X-ray observation. That way, we can tell if the X-ray observations happen during a minimum, maximum, or an average part of the optical lightcurve.

"Our XMM-Newton observations will be centered on:

R.A. 04:31:43.5  Dec. +18:13:56.6  (2000.0)

"We are interested in all objects in the field, but the most important ones are the ones that we know to be X-ray bright with optical magnitudes between 10 and 15." They are listed in the table below.

Name Priority RA (2000) Dec (2000) Range Spec Type Var Type VSX link
XZ Tau priority1 04 31 40.09 +18 13 56.6 12.6 - 15.2 V G5V-M3(T)e CTTS/ROT XZ Tau
HL Tau priority 1 04 31 38.47 +18 13 58.1 14.3 - 15.0 V K7:e(T)-M2:e(T) CTTS HL Tau
V710 Tau* priority 2 04 31 57.80 +18 21 36.8 13.3 - 13.8 V M1.7e+M3.3e CTTS V710 Tau
V827 Tau priority 3 04 32 14.57 +18 20 14.8 12.4 - 13.2 V K7/M0Ve WTTS/ROT+UV V827 Tau
V826 Tau priority 3 04 32 15.84 +18 01 38.8 11.7 - 12.2 V K7V TTS/ROT V826 Tau

*Note: V710 Tau is a CTTS binary, but it is faint because its spectral type is M.

The XMM-Newton observations are currently scheduled for the following dates and times (UT):

2020 Aug. 18 13:32:46 - Aug. 19 02:26:05
2020 Aug. 22 13:15:42 - Aug. 23 02:52:22
2020 Aug. 28 19:33:04 - Aug. 29 04:43:04
2020 Sep. 03 22:45:00 - Sep. 04 07:55:00
2020 Sep. 12 06:25:55 - Sep. 12 15:35:55

As with all satellite observations, times are subject to change if urgent target-of-opportunity events occur. Observers are strongly encouraged to subscribe to the forum threads above to learn of any last-minute changes to the schedule.

Beginning now and continuing through September 17, nightly snapshots of the five targets in the table are requested. V, CV, and Rc filters are preferred. If R images are obtained, concurrent V images would be very helpful to be able to obtain V-R, especially for XZ Tau and HL Tau. Dr. Guenther notes that "They can have extinction events, when part of the disk or the accretion column moves through the line of sight. Those extinction events redden the color and it would be great to see that (if it happens)." Visual observations are also welcome.

Charts with comparison stars for the targets may be created using the AAVSO Variable Star Plotter (VSP).

Please report all observations to the AAVSO International Database using the names given in the table.

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

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