Mass, radius, and age measurements of young (≲100 Myr) planets have the power to shape our understanding of planet formation. However, young stars tend to be extremely variable in both photometry and radial velocity (RV) measurements, which makes constraining these properties challenging. The V1298 Tau system of four ∼0.5 R J planets transiting a pre-main-sequence star presents an important, if stress-inducing, opportunity to observe and measure directly the properties of infant planets. Suárez Mascareño et al. published radial-velocity-derived masses for two of the V1298 Tau planets using a state-of-the-art Gaussian process regression framework. The planetary densities computed from these masses were surprisingly high, implying extremely rapid contraction after formation in tension with most existing planet-formation theories. In an effort to constrain further the masses of the V1298 Tau planets, we obtained 36 RVs using Keck/HIRES, and analyzed them in concert with published RVs and photometry. Through performing a suite of cross-validation tests, we found evidence that the preferred model of Suárez Mascareño et al. suffers from overfitting, defined as the inability to predict unseen data, rendering the masses unreliable. We detail several potential causes of this overfitting, many of which may be important for other RV analyses of other active stars, and recommend that additional time and resources be allocated to understanding and mitigating activity in active young stars such as V1298 Tau.

Line-of-sight extinction estimates of well-studied young T Tauri and Herbig Ae/Be stars are based on many different measurements and analysis methods. This has resulted in wide scatter among the published A V values for the same star. In this work, we discuss the challenges in measuring extinction to actively accreting and especially outbursting young stellar objects (YSOs). We then explore a method not previously applied to young stars utilizing diffuse interstellar bands (DIBs). In early-type stars, narrow correlations exist between the DIB equivalent widths and the column density of interstellar material, and therefore the line-of-sight extinction. Here, we measure the equivalent widths of the 5780 and 6614 Å DIB features in a sample of actively accreting YSOs, and apply a DIB-reddening calibration to estimate reddening and subsequently extinction. Our calibration is newly derived from a composite of available literature data and fully accounts for the scatter in these measurements. We also compare the DIBs-inferred optical line-of-sight extinction values with previous extinction estimates for our sample stars.

Molecular hydrogen is the most abundant molecule in the galaxy and plays important roles in planets, their circumstellar environments, and many of their host stars. We have confirmed the presence of molecular hydrogen in the AU Mic system using high-resolution FUV spectra from HST-STIS during both quiescence and a flare. AU Mic is a ∼23 Myr M dwarf that hosts a debris disk and at least two planets. We estimate the temperature of the gas at 1000–2000 K, consistent with previous detections. Based on the radial velocities and widths of the H2 line profiles and the response of the H2 lines to a stellar flare, the H2 line emission is likely produced in the star, rather than in the disk or the planet. However, the temperature of this gas is significantly below the temperature of the photosphere (∼3650 K) and the predicted temperature of its starspots (≳2650 K). We discuss the possibility of colder starspots or a cold layer in the photosphere of a pre-main-sequence M dwarf.

Line-of-sight extinction estimates to well-studied young T Tauri and Herbig Ae/Be stars are based on many different measurements and analysis methods. This has resulted in wide scatter among the published A V values for the same star. In this work, we discuss the challenges in measuring extinction to actively accreting and especially outbursting young stellar objects (YSOs), and explore a method not previously applied to young stars, utilizing diffuse interstellar bands (DIBs). In early-type stars, narrow correlations exist between DIB equivalent widths and the column density of interstellar material, and therefore the line-of-sight extinction.Here, we measure equivalent widths of the 5780 Å and 6614 Å DIB features in a sample of actively accreting YSOs, and apply established DIB-reddening calibrations to derive reddening, and subsequently extinction. We also compare the DIBs-inferred optical line-of-sight extinction values with previous extinction estimates for our sample stars.