Proposal #207

Southern Low Mass Pre-main Sequence Stars in Orion Simultaneous with HST and TESS
Part III of the ODYSSEUS program
F.M.Walter for the Odysseus Team

The Hubble Space Telescope (HST) HST ULLYSES project (Ultraviolet Legacy Library of Young Stars as Essential Standards -- https://www.stsci.edu/stsci-research/research-topics-and-programs/ullyses); is designed to obtain a statistically complete catalog of ultraviolet observations of a set of galactic stars while the HST can still obtain UV spectra. Our interest is in a sample of T Tauri stars. This proposal will support a larger program called ODYSSEUS (Outflows and Disks around Young Stars: Synergies for the Exploration of Ullyses Spectra), which includes ground-based optical and near-IR photometry and spectroscopy. The ODYSSEUS campaign is described at https://sites.bu.edu/odysseus/

The first part of this project, targeting 13 stars in the Orion OB1 and sigma Ori star forming regions in November/December 2020, was highly successful. AAVSOnet proposal 159 played an important part in this. One early result was in helping us understand why V505 Ori was undetected by the HST - ground based photometric monitoring showed that the star faded by 2 mag just prior to the HST observation, probably due to obscuration by the accretion disk. This let us successfully replan the observation in February of this year.
Part II involved monitoring of TW Hya, again with HST and TESS. It was planned for 12 days, but HST went into safe mode just before the program was to start, delaying the start by 2 weeks, and ended on April 9. This was supported by AAVSOnet proposal 195. The data are still being ingested. I thank the AAVSO for their generous time allocations to this program. AAVSOnet data from those campaigns can be seen at http://www.astro.sunysb.edu/fwalter/SMARTS/Odysseus/ .

Part III of the program is HST snapshots of a sample of southern pre-main sequence stars in the Chamaeleon and Lupus star forming regions. I am requesting time on the AAVSOnet southern telescopes to observe 16 targets brighter than V=16. Eleven of these are currently planned by HST to be observed April 29 though June 6; the other 5 will probably be planned for observations during June. The targets are in the TESS field of view either April 28-May 25 or May 26-June 24. Precise HST scheduling will be known 1-2 weeks in advance, and will most likely occur within the plan windows, which are typically 12 hours long.

These targets will be observed with the HST while they are in the TESS field of view. TESS gets broadband (600-1000nm) optical photometry on a 10 minute cadence for typically 27 days. The HST will obtain ultraviolet spectroscopy. The goal of this observing campaign is to supplement the TESS photometry with filter photometry. It is hard to beat the time coverage of the S/N of the TESS data, or the continuous 27 day cadence, but AAVSOnet data can supply the color information which helps distinguish the possible causes of brightenings and fadings in the light curve, which may include magnetic flares, enhanced accretion rates, starspots, and obscuration by a warped dust disk.

The science goals are to understand the disk accretion processes that build up stars and drive their outbursts. The T Tauri stars vary by up to 2 magnitudes. Some are completely irregular; others have characteristic periods of typically 4-10 days, consistent with either the stellar rotation or the inner edge of the accretion disk.

I request sets of BVRI observations on the 3 nights bracketing the time of the HST observation. If possible, one should be simultaneous with the HST, but this is generally not possible. In addition, I request occasional sets of BVRI observations during the TESS window in order to "calibrate" the TESS photometry, by seeing how predictably the colors change with brightness.

This program is challenging for all because of the dearth of observing facilities, both amateur and professional, in the south. The ULYSSES program has some photometric time on LCO for this program, but our other professional partners are all located in the north. AAVSOnet data, supplemented by data from southern AAVSO observers, will be instrumental in guaranteeing the success of this program.

Target and dates of observations are listed below:
Target RA (2000) DEC <V> HST plan TESS
CHX18N 11 11 46.32 -76 20 09.2 11.9 April 29 14:00-01:00 4/28 - 6/24
HM Lup 15 47 50.63 -35 28 35.4 14.8 May 3 13:00-01:30 4/28 - 5/25
GQ Lup 15 49 12.10 -35 39 05.1 11.4 May 4 11:30-23:00 4/28 - 5/25
GW Lup 15 46 44.73 -34 30 35.5 13.5 May 5 11:00-23:30 4/28 - 5/25
Sz77 15 51 46.95 -35 56 44.1 13.3 May 7 10:30-22:30 4/28 - 5/25

Sz76 15 49 30.74 -35 49 51.4 15.2 May 10 0900-20:00 4/28 - 5/25
Sz66 15 39 28.28 -34 46 18.0 15.0 May 24 03:30-16:00 4/28 - 5/25
Sz45 11 17 37.00 -77 04 38.1 13.5 May 25 03:00-14:00 4/28 - 6/24
XX Cha 11 11 39.65 -76 20 15.2 15.3 May 29/30 0:00-11:30 4/28 - 6/24
GX Cha (HN 5) 11 06 41.79 -76 35 48.9 15.6 May 31 0:00-09:30 5/26 - 6/24
IN Cha 11 12 09.84 -76 34 36.6 16.5 June 5/6 21:00-07:00 5/26 - 6/24
SZ Cha
TW Cha
SST J1600-4221
Sz130
Sz111