A Comprehensive Multi-wavelength Study of the Accretion Variability of GM AurPre-main sequence low-mass accreting stars known as classical T Tauri Stars (CTTS) have been observed to be highly variable. Short cadence light curves from limited K2 and ground-based studies show variability on timescales down to a few minutes that remains relatively uncharacterized. To study this short-timescale variability, we propose to obtain 6 near-ultraviolet (NUV) to near-infrared (NIR) STIS spectra of GM Aur during simultaneous approved high-cadence TESS observations. Using HST to benchmark TESS, these observations will have a lasting significant impact on accretion variability studies of young stars. Additional approved ground-based observations with ALMA (mm), CHIRON (medium-resolution optical spectra), Very Large Array (VLA, cm), and the Discovery Channel Telescope (DCT, optical photometry) are scheduled to overlap with this visibility window. Adding HST will strengthen this multi-wavelength multi-epoch campaign, the first such study of a CTTS.We will obtain 6 consecutive daily STIS spectra of GM Aur, an accreting young star with a disk in the Taurus star forming region. These observations will be taken simultaneously with approved 2-minute cadence broadband photometric observations from TESS. Because the science goals of this proposal rely on simultaneity with TESS observations, this does introduce some scheduling constraints on the particular days during which these observations must be made, described in more detail below.
These observations will follow a similar pattern to the previous 8 successful STIS observations of GM Aur. The observations consist of low-resolution spectra with continuous wavelength coverage from the NUV to the NIR. We need STIS/NUV-MAMA spectra taken with the G230L grating to break the degeneracy between accretion column density and surface coverage. STIS/CCD observations with the G430L and G750L gratings are necessary for accurate estimates of the contribution to the total emission from the undisturbed photosphere and aid in the process of fitting the accretion shock models to the observations and are relatively quick to obtain compared to the G230L observations. We require contemporaneous fringe-flats to correct the G750L spectra beyond 7000 where severe fringing occurs. We will employ the TIME-TAG mode in the STIS/NUV-MAMA observations because it contains information on shorter timescales than the full integration length which are of particular interest because of the 2-minute cadence TESS data.
The only overlap between the TESS observations and the HST visibility window for this target is between Dec 3rd, 2019 and Dec 10th, 2019. As such, our observations must occur between these dates. Each visit has a timing requirement such that it must occur between 0.75 and 1.25 days after the previous visit to give us a roughly daily cadence. Each daily visit to GM Aur can be completed within a single orbit.
We have obtained SNR estimates of these objects using the previous STIS observations of GM Aur as the expected emission. These simulated observations show that we can expect to obtain an SNR of at least 15 over most of the NUV continuum, and an SNR > 15 per spectral element in the optical/NIR continuum. This is more than sufficient to accomplish the scientific goals presented in this proposal, even with the possibility of catching GM Aur in a low accretion state. When using the TIME-TAG data to explore variability on minute-long timescales, we plan to bin multiple spectral channels to increase the SNR, as the lower resolution is still sufficient to characterize the accretion behavior.
The details for each of the observing modes are described as follows. For the NUV observations, we use the buffer time suggested by the ETC. For the optical and NIR observations, we used the CR-SPLIT = 2 option to mitigate the effects of cosmic rays. We chose the 52x2 slit width to maximize the SNR observations because data quality is more important for our scientific objectives than resolution. The exception to this is the slit used for the fringe flats, which was chosen to be 0.3x0.9 based on the recommendation in the STIS user manual. Each of the visits has been optimized to maximize SNR in the NUV observations and remove any unused orbital visibility. Other than the scheduling constraint of needing to occur after the previous visit, each visit is identical. The acquisition will be done with the standard F28X50LP aperture using the ACQ mode. The exposure time was set to be 0.5s, which gives an expected SNR of 56, which is well above the required SNR of 40. Because the expected time until the exposure saturates is about 143 seconds, there is no risk of overexposure. While the reduced gyro mode should not directly affect these observations, future multi-observatory observations will become more difficult to schedule with reduced visibility window.C. Robinson has previously worked with STIS data of accreting young stars.
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