Alert Notice 731: Monitoring needed for TW Hya for HST and TESS observations

February 24, 2021

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Dr. Fred Walter (Stony Brook University) requests AAVSO assistance in providing multicolor CCD photometry of the T Tau star TW Hya in support of upcoming HST and TESS observations.

Dr. Walter writes: "Part II of the ODYSSEUS pre-main sequence star program starts next month. ODYSSEUS is an international collaborative campaign to understand the processes involved in the formation of low mass stars, their circumstellar disks, and their planets, via the time-variability of these systems.

"The HST will monitor the 10 million year old K7 pre-main sequence star TW Hya intensively, obtaining 12 sets of ultra-violet spectra spread over about 13 days, 7-19 March 2021, simultaneously with TESS observations.

"TESS will observe this part of the sky nearly continuously for 27 days, from 7 March through 2 April, with a 10 minute cadence. TESS data, both in terms of length of coverage and data quality, cannot be matched from the ground, but TESS is a single channel photometer operating in the red. We need ground-based optical photometry to place the TESS magnitudes in context.

  - Are stellar variations due to enhanced accretion events or long-lived flares, which case the star to become bluer, or
  - are they due to variable circumstellar extinction, in which case the star will become redder, or
  - are they due to obscuration of the photosphere by optically-thick irregularities in the circumstellar disk, in which case the color should not change?

"Changes in extinction in particular are important for interpreting the UV spectra, which are very sensitive to the absorption.

"TW Hya is fairly bright, with a mean V ~ 11, and a historical range from 10.6 to 11.3 (from VSX). The typical day-to-day range of variability is +/- 0.1 mag. I seek to extract colors accurate to better than 2% (0.02 mag), which requires CCD photometry accurate to better the 0.01 mag.

Observations are requested from 1 March through 5 April. Dr. Walter continues:
- "Highest priority will be during the HST observations, but those exact times  are not known yet. [They will be announced in the forum threads given above.]
- Next highest priority are nightly observations 6 through 19 March inclusive, spanning the time of the HST observations. No more than one observation is needed per night.
- Lower priority would be from 20 March through 2 April, when only TESS is on the target."
- The observations before the HST window and after the TESS window are to provide a baseline for each observer's data.

"Please observe in 4 bands (B,V and either Cousins RI (preferred) or Sloan ri).

"When I learn exactly when the HST observations will be scheduled, I will provide that information. But contemporaneous photometry more than suffices for our purposes.

"Please upload any data you get to the AAVSO database. Thanks in advance for your contributions to this campaign."

Coordinates (2000.0): 11 01 51.91 -34 42 17.0 (from VSX page for TW Hya)

Finder charts with comparison stars may be created using the AAVSO Variable Star Plotter (VSP).

Please submit observations to the AAVSO International Database using the name TW HYA.

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


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