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1. Localizing Sources of Variability in Crowded TESS Photometry

   https://doi.org/10.3847/1538-3881/acb20c  

   Higgins, Michael E. ; Bell, Keaton J. ( March 2023 , The Astronomical Journal)

   The Transiting Exoplanet Survey Satellite (TESS) has an exceptionally large plate scale of 21″ px⁻¹, causing most TESS light curves to record the blended light of multiple stars. This creates a danger of misattributing variability observed by TESS to the wrong source, which would invalidate any analysis. We developed a method that can localize the origin of variability on the sky to better than one fifth of a pixel. Given measured frequencies of variability (e.g., from periodogram analysis), we show that the best-fit sinusoid amplitudes to raw light curves extracted from each pixel are distributed in the same way as light from the variable source. The primary assumption of this method is that other nearby stars are not variable at the same frequencies. Essentially, we are using the high frequency resolution of TESS to overcome limitations from its low spatial resolution. We have implemented our method in an open-source Python package,TESS_localize(github.com/Higgins00/TESS-Localize), that determines the location of a variable source on the sky and the most likely Gaia source given TESS pixel data and a set of observed frequencies of variability. Our method utilizes models of the TESS pixel response function, and we characterize systematics in the residuals of fitting these models to data. We find that even stars more than three pixels outside a photometric aperture can produce significant contaminant signals in the extracted light curves. Given the ubiquity of source blending in TESS light curves, verifying the source of observed variability should be a standard step in TESS analyses.

    

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2. Discovery of dipolar chromospheres in two white dwarfs

   https://doi.org/10.1093/mnras/stad2184  

   Farihi, J. ; Hermes, J. J. ; Littlefair, S. P. ; Howarth, I. D. ; Walters, N. ; Parsons, S. G. ( July 2023 , Monthly Notices of the Royal Astronomical Society)

   This paper reports the ULTRACAM discovery of dipolar surface spots in two cool magnetic white dwarfs with Balmer emission lines, while a third system exhibits a single spot, similar to the prototype GD 356. The light curves are modelled with simple, circular, isothermal dark spots, yielding relatively large regions with minimum angular radii of 20°. For those stars with two light-curve minima, the dual spots are likely observed at high inclination (or colatitude); however, identical and antipodal spots cannot simultaneously reproduce both the distinct minima depths and the phases of the light-curve maxima. The amplitudes of the multiband photometric variability reported here are all several times larger than that observed in the prototype GD 356; nevertheless, all DAHe stars with available data appear to have light-curve amplitudes that increase towards the blue in correlated ratios. This behaviour is consistent with cool spots that produce higher contrasts at shorter wavelengths, with remarkably similar spectral properties given the diversity of magnetic field strengths and rotation rates. These findings support the interpretation that some magnetic white dwarfs generate intrinsic chromospheres as they cool, and that no external source is responsible for the observed temperature inversion. Spectroscopic time-series data for DAHe stars is paramount for further characterization, where it is important to obtain well-sampled data, and consider wavelength shifts, equivalent widths, and spectropolarimetry.

    

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3. The Directly Imaged Exoplanet Host Star 51 Eridani is a Gamma Doradus Pulsator

   https://doi.org/10.3847/1538-4357/ac9229  

   Sepulveda, Aldo G. ; Huber, Daniel ; Zhang, Zhoujian ; Li, Gang ; Liu, Michael C. ; Bedding, Timothy R. ( October 2022 , The Astrophysical Journal)

   51 Eri is well known for hosting a directly imaged giant planet and for its membership to theβPictoris moving group. Using 2 minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS), we detect multiperiodic variability in 51 Eri that is consistent with pulsations of Gamma Doradus (γDor) stars. We identify the most significant pulsation modes (with frequencies between ∼0.5 and 3.9 cycles day⁻¹and amplitudes ranging between ∼1 and 2 mmag) as dipole and quadrupole gravity modes, as well as Rossby modes, as previously observed in KeplerγDor stars. Our results demonstrate that previously reported variability attributed to stellar rotation is instead likely due toγDor pulsations. Using the mean frequency of theℓ= 1 gravity modes, together with empirical trends of the KeplerγDor population, we estimate a plausible stellar core rotation period of${0.9}_{-0.1}^{+0.3}$days for 51 Eri. We find no significant evidence for transiting companions around 51 Eri in the residual light curve. The detection ofγDor pulsations presented here, together with follow-up observations and modeling, may enable the determination of an asteroseismic age for this benchmark system. Future TESS observations would allow a constraint on the stellar core rotation rate, which in turn traces the surface rotation rate, and thus would help clarify whether or not the stellar equatorial plane and orbit of 51 Eri b are coplanar.

    

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4. Recovery of TESS Stellar Rotation Periods Using Deep Learning

   https://doi.org/10.3847/1538-4357/ac498f  

   Claytor, Zachary R. ; van Saders, Jennifer L. ; Llama, Joe ; Sadowski, Peter ; Quach, Brandon ; Avallone, Ellis A. ( March 2022 , The Astrophysical Journal)

   We used a convolutional neural network to infer stellar rotation periods from a set of synthetic light curves simulated with realistic spot-evolution patterns. We convolved these simulated light curves with real TESS light curves containing minimal intrinsic astrophysical variability to allow the network to learn TESS systematics and estimate rotation periods despite them. In addition to periods, we predict uncertainties via heteroskedastic regression to estimate the credibility of the period predictions. In the most credible half of the test data, we recover 10% accurate periods for 46% of the targets, and 20% accurate periods for 69% of the targets. Using our trained network, we successfully recover periods of real stars with literature rotation measurements, even past the 13.7 day limit generally encountered by TESS rotation searches using conventional period-finding techniques. Our method also demonstrates resistance to half-period aliases. We present the neural network and simulated training data, and introduce the softwarebutterpyused to synthesize the light curves using realistic starspot evolution.

    

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5. Photometry of Outer Solar System Objects from the Dark Energy Survey. I. Photometric Methods, Light-curve Distributions, and Trans-Neptunian Binaries

   https://doi.org/10.3847/1538-4365/acf6bf  

   Bernardinelli, Pedro H. ; Bernstein, Gary M. ; Jindal, Nicholas ; Abbott, T. M. C. ; Aguena, M. ; Alves, O. ; Andrade-Oliveira, F. ; Annis, J. ; Bacon, D. ; Bertin, E. ; et al ( October 2023 , The Astrophysical Journal Supplement Series)

   We report the methods of and initial scientific inferences from the extraction of precision photometric information for the >800 trans-Neptunian objects (TNOs) discovered in the images of the Dark Energy Survey (DES). Scene-modeling photometry is used to obtain shot-noise-limited flux measures for each exposure of each TNO, with background sources subtracted. Comparison of double-source fits to the pixel data with single-source fits are used to identify and characterize two binary TNO systems. A Markov Chain Monte Carlo method samples the joint likelihood of the intrinsic colors of each source as well as the amplitude of its flux variation, given the time series of multiband flux measurements and their uncertainties. A catalog of these colors and light-curve amplitudesAis included with this publication. We show how to assign a likelihood to the distributionq(A) of light-curve amplitudes in any subpopulation. Using this method, we find decisive evidence (i.e., evidence ratio <0.01) that cold classical (CC) TNOs with absolute magnitude 6 <H_r< 8.2 are more variable than the hot classical (HC) population of the sameH_r, reinforcing theories that the former form in situ and the latter arise from a different physical population. Resonant and scattering TNOs in thisH_rrange have variability consistent with either the HCs or CCs. DES TNOs withH_r< 6 are seen to be decisively less variable than higher-H_rmembers of any dynamical group, as expected. More surprising is that detached TNOs are decisively less variable than scattering TNOs, which requires them to have distinct source regions or some subsequent differential processing.

    

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