Proposal #159

Low Mass Pre-main Sequence Stars in Orion Simultaneous with HST and TESS
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 first part of this project involves obtaining UV spectra of 13 stars in the Orion OB1 and sigma Ori star forming regions in November/December 2020.

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.

The 13 low mass PMS stars in Orion will be observed with TESS 11/19 - 12/17/2020, and with the HST for a few hours during that window. The targets are listed,
in order of decreasing mean brightness, in the table. The dates in the table are the current windows for the HST observations and are subject to change. The orbits column gives the number of 96 minute orbits that will be scheduled for each target. The exact times of the HST observations will be known about 2 weeks in advance.

Target RA (J2000) Dec mean V orbits dates (2020)
TX Ori 5h 38m 33.69s -2d 44m 14.1s 12.1 4 29 Nov - 2 Dec
CVSO-165 5h 39m 02.57s -1d 20m 32.3s 13.7 5 13 - 15 Dec
CVSO-109 5h 32m 32.66s -1d 13m 46.1s 14.0 3 26 - 29 Nov %V462 Ori
CVSO-146 5h 35m 46.01s -0d 57m 52.2s 14.0 4 8 - 11 Dec %V499 Ori
V505 Ori 5h 38m 27.26s -2d 45m 09.7s 14.2 4 29 Nov - 1 Dec
CVSO-104 5h 32m 06.49s -1d 11m 00.8s 14.2 4 25 - 28 Nov %NSV 2091
V510 Ori 5h 39m 39.83s -2d 31m 21.9s 14.3 5 6 - 9 Dec
CVSO-90 5h 31m 20.63s -0d 49m 19.8s 14.6 3 15 - 16 Dec
CVSO-107 5h 32m 25.79s -0d 36m 53.4s 14.8 5 3 - 6 Dec
CVSO-58 5h 29m 23.26s -1d 25m 15.5s 14.9 6 30 Nov - 2 Dec
CVSO-176 5h 40m 24.15s -0d 31m 21.3s 15.6 5 28 Nov - 1 Dec %V609 Ori
CVSO-36 5h 25m 50.37s 1d 49m 37.3s 16.1 4 3 - 5 Dec
CVSO-17 5h 23m 04.72s 1d 37m 15.3s 16.2 7 4 - 7 Dec

Observation Details
We seek one set of BVRI CCD filter photometry of each of the targets:
A. simultaneous with each HST orbit visible from the AAVSOnet telescopes;
B. the nights before and after the HST observations;
C. on 3 other nights during the TESS window.
In sum there will be between 5 and 8 observations of each target.
The R and I filters should be Cousins filters.

A: Simultaneity with the HST data will give the colors of the targets while the UV spectra are being obtained. In the most fortuitous case, a target will be scheduled during 4 consecutive orbits observable from the ground at night. In this case, we ask for a minimum of one set of BVRI observations during one of the ~50 minute long observations, and a maximum of one set of BVRI observations during each 96 minutes HST orbit, if possible.
TESS will get the short-timescale variability.
I will supply the HST observing windows, which will be available about 2 weeks prior to the observations.

B: These observations will be used to establish color trends, especially if the HST observation occurs during local day.

C: The purpose of these observations is to obtain a color calibration of TESS. Basically, by hitting these stars at random times, plus the times in parts A and B, we sample random brightnesses. We want to know if the star is bluer or redder when bright.

These observation should be scheduled on the SRO, NMSU, or MJUO telescopes.
As the SRO and NMSU facilities are at similar longitudes; both should not be scheduled on the same night.
The targets are too faint for the BSM telescopes.
These observations will complement a planned AAVSO campaign, and other photometry planned from Catania, Konkoly, and HOYS

AAVSOnet observations will complement these other data and assist our goal of achieving dense photometric coverage on this large number of targets.