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Classical T Tauri Stars (CTTSs) are highly variable stars that possess gas- and dust-rich disks from which planets form. Much of their variability is driven by mass accretion from the surrounding disk, a process that is still not entirely understood. A multi-epoch optical spectral monitoring campaign of four CTTSs (TW Hya, RU Lup, BP Tau, and GM Aur) was conducted along with contemporaneous HST UV spectra and ground-based photometry in an effort to determine accretion characteristics and gauge variability in this sample. Using an accretion flow model, we find that the magnetospheric truncation radius varies between 2.5-5 R* across all of our observations. There is also significant variability in all emission lines studied, particularly Halpha, Hbeta, and Hgamma. Using previously established relationships between line luminosity and accretion, we find that, on average, most lines reproduce accretion rates consistent with accretion shock modeling of HST spectra to within 0.5 dex. Looking at individual contemporaneous observations, however, these relationships are less accurate, suggesting that variability trends differ from the trends of the population and that these empirical relationships should be used with caution in studies of variability., Comment: 39 pages, 10 figures, 12 tables

Classical T Tauri Stars (CTTSs) are young, low-mass stars which accrete material from their surrounding protoplanetary disk. To better understand accretion variability, we conducted a multi-epoch, multi-wavelength photometric monitoring campaign of four CTTSs: TW Hya, RU Lup, BP Tau, and GM Aur, in 2021 and 2022, contemporaneous with HST UV and optical spectra. We find that all four targets display significant variability in their light curves, generally on days-long timescales (but in some cases year-to-year) often due to periodicity associated with stellar rotation and to stochastic accretion variability. Their is a strong connection between mass accretion and photometric variability in all bands, but the relationship varies per target and epoch. Thus, photometry should be used with caution as a direct measure of accretion in CTTSs., Comment: 28 pages, 12 figures

The Classical T Tauri Star (CTTS) stage is a critical phase of the star and planet formation process. In an effort to better understand the mass accretion process, which can dictate further stellar evolution and planet formation, a multi-epoch, multi-wavelength photometric and spectroscopic monitoring campaign of four CTTSs (TW Hya, RU Lup, BP Tau, and GM Aur) was carried out in 2021 and 2022/2023 as part of the Outflows and Disks Around Young Stars: Synergies for the Exploration of ULYSSES Spectra (ODYSSEUS) program. Here we focus on the HST UV spectra obtained by the HST Director's Discretionary Time UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. Using accretion shock modeling, we find that all targets exhibit accretion variability, varying from short increases in accretion rate by up to a factor of 3 within 48 hours, to longer decreases in accretion rate by a factor of 2.5 over the course of 1 year. This is despite the generally consistent accretion morphology within each target. Additionally, we test empirical relationships between accretion rate and UV luminosity and find stark differences, showing that these relationships should not be used to estimate the accretion rate for individual target. Our work reinforces that future multi-epoch and simultaneous multi-wavelength studies are critical in our understanding of the accretion process in low-mass star formation., Comment: 37 pages, 14 figures

We review the current knowledge of the population of `older' stars in the Orion OB1 association, specifically those in subgroups 1a and 1b. We briefly outline the history of the subject and then continue with a summary of the present state of knowledge of the early-type stars in Orion OB1. New results from the Hipparcos parallaxes and proper motions will be presented. The main result is that subgroup 1a is located at about 330 pc from the Sun, much closer than the previously determined distance, and about 100 pc distant from the other subgroups of the association and the Orion molecular clouds. Unfortunately, due to the unfavorable kinematics of the association with respect to the Galactic background, Hipparcos proper motions do not allow a clear kinematic separation of the association from the field. For this purpose accurate and homogeneous radial velocities are needed. Traditionally, the massive O and B stars have received most of the attention in the studies of OB associations. However, we will present results showing that significant numbers of low-mass stars are associated with Orion OB1. Unbiased, optically complete, spectroscopic and photometric surveys of areas within subgroups 1a and 1b have the potential to determine the complete low-mass stellar population, down to the brown dwarf limit. This will provide much insight into the overall initial mass function and studies of the kinematics of the low-mass stars will yield insights into the dispersal of the association., Comment: To appear in The Orion Complex Revisited, eds. M. J. McCaughrean & A. Burkert (San Francisco, ASP), gzipped tar-file, 22 pages 7 EPS-figures, LaTeX using paspconf.sty and psfig.tex. Wrongly quoted errors on the average parallaxes of the Orion OB1 subgroups were corrected (Section 4)

This grant supported the analysis of two RXTE observations of low mass pre-main sequence stars. The data were obtained without problem, and both sources were clearly detected. I analyzed the data at GSFC in July 1999 (it was necessary to defer the analysis because the sources are weak, and the background model was not adequate until recently). The data are good and, although there is still some tweaking of the background subtraction and model fitting remaining to be done, we expect to work up the data for publication this spring.

This grant supported the analysis of a ROSAT observation of a field in the Orion OB1A star forming region. The data were obtained without problem, and a number of sources were detected. The data have been analyzed. We have obtained the optical and near-IR photometry, and some of the optical spectroscopy (the latter in November 1999) we need to fully analyzed and understand the data.