Proposal #195

Optical Photometry in support of HST/Odysseus observations. II. Monitoring TW Hya

The first phase of the Odysseus campaign was a resounding success. HST obtained UV spectra of 13 low mass pre-main sequence stars in the Orion OB1b star formation region simultaneous with TESS observations. Through program P159, I obtained high quality BVRI and BRri data which will be used to study the color variations and seek correlations with the brightness changes seen in the TESS light curves (the TESS data just became public last week). Data are reported at
http://www.astro.sunysb.edu/fwalter/SMARTS/Odysseus/odysseus.html

Part 2 of the program starts next month. The HST will monitor the 10 million year old K7 pre-main sequence star TW Hya intensively, with 12 observations spread over about 13 days, again simultaneously with TESS observations. TESS observes nearly continuously for 27 days with a 10 minute cadence. The 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 put 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?
Changes in extinction in particular are important for interpreting the UV spectra, which are very sensitive to the absorption.

TW Hya is at 11:01:51.9 -34:42:17.0. It is fairly bright, with a mean V ~ 11, and a historical range from 10.6 to 11.3 (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 photometry accurate to better the 0.01 mag.

It is a southern target, which limits the number of AAVSO sites.
I request one set of observation per night from each AAVSOnet telescope that can reach TW Hya, for 14 nights, from 6-19 March, inclusive.
One observation per night per telescope will provide some range in longitude, and in observing times. Possible telescopes are the BSM sites in
Australia and Hawaii, and the OC61 in New Zealand.
(We also plan photometric observations from Las Cumbres observatory and from South Africa.)

Each target should be observed in 4 bands (B,V and either Cousins RI or Sloan ri.)
Three successive frames should be observed in each band for cosmic-ray and satellite-trail filtering.
Integration times of 1 minute per frame in V,R and 2 minutes in B,I should suffice, based on our experience with the first targets, so each observation will require about 18 minutes plus overhead each night.
The limiting consideration is not the brightness of the target, but the brightness of the APASS stars in the field, which I use to calibrate the photometry. I need to have good S/N for stars at least 3 mag fainter than the target, or to about V~14.

When I learn exactly when the HST observations will be scheduled, I will provide that information if it is possible to schedule time simultaneous with the HST
observations. But contemporaneous photometry more than suffices for our purposes.