[en] Using the Gemini infrared camera on the 3 m Shane telescope at Lick Observatory, we have searched for broadband J and K' photometric variability for a sample of 15 L- and T-type brown dwarfs, including 7 suspected spectral binaries. Four of the dwarfs—2MASS J0939–2448, 2MASS J1416+1348A, 2MASS J1711+2232, and 2MASS J2139+0220—exhibit statistically significant variations over timescales ranging from ∼0.5 hr to 6 days. Our detection of variability in 2MASS J2139+0220 confirms that reported by Radigan et al., and periodogram and phase dispersion minimization analysis also confirms a variability period of approximately 7.6 ± 0.2 hr. Remarkably, two of the four variables are known or candidate binary systems, including 2MASS J2139+0220, for which we find only marginal evidence of radial velocity variation over the course of a year. This result suggests that some spectral binary candidates may appear as such due to the blending of cloudy and non-cloudy regions in a single ''patchy'' atmosphere. Our results are consistent with an overall variability fraction of 35% ± 5%, with no clear evidence of greater variability among brown dwarfs at the L dwarf/T dwarf transition.

[en] We present large field H I-line emission maps obtained with the single-dish Green Bank Telescope centered on the dwarf irregular galaxies Sextans A, NGC 2366, and WLM. We do not detect the extended skirts of emission associated with the galaxies that were reported from Effelsberg observations. The ratio of H I at 10¹⁹ atoms cm^–2 to optical extents of these galaxies is instead 2-3, which is normal for this type of galaxy. There is no evidence for a truncation in the H I distribution ≥10¹⁹ atoms cm^–2.

[en] We present a new radial velocity measurement that, together with a trigonometric parallax, proper motion and signs of low gravity from the literature, confirms that SDSS J111010.01+011613.1 is a new T5.5 bona fide member of AB Doradus. Fitting $\mathrm{\lambda <!--\; ??\; -->}/\mathrm{\Delta <!--\; ??\; -->}\mathrm{\lambda <!--\; ??\; -->}$ ≈ 6000 Folded-port InfraRed Echellette spectroscopy in the 1.20–1.33 μm region to BT-Settl atmosphere models yielded a radial velocity of 7.5 ± 3.8 km s⁻¹. At such a young age (110–130 Myr), current evolution models predict a mass of ∼10–12 $M_{\mathrm{J}\mathrm{u}\mathrm{p}}$, thus placing SDSS J1110+0116 well into the planetary-mass regime. We compare the fundamental properties of SDSS J1110+0116 with a sequence of seven recently identified M8–T5 brown dwarf bona fide or high-confidence candidate members of AB Doradus. We also note that its near-infrared J–K color is redder than field T5–T6 brown dwarfs, however its absolute J-band magnitude is similar to them. SDSS J1110+0116 is one of the few age-calibrated T dwarfs known to date, as well as one of the coolest bona fide members of a young moving group.

[en] Empirical calibrations of the stellar age-rotation–activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. We used the Chandra X-ray Observatory to study M37, a 500-Myr-old open cluster that has been extensively surveyed for rotation periods ($P_{\mathrm{r}\mathrm{o}\mathrm{t}}$). M37 was observed almost continuously for five days, for a total of 440.5 ks, to measure stellar X-ray luminosities ($L_{\mathrm{X}}$), a proxy for coronal activity, across a wide range of masses. The cluster’s membership catalog was revisited to calculate updated membership probabilities from photometric data and each star’s distance to the cluster center. The result is a comprehensive sample of 1699 M37 members: 426 with $P_{\mathrm{r}\mathrm{o}\mathrm{t}}$, 278 with X-ray detections, and 76 with both. We calculate Rossby numbers, $R_o=P_{\mathrm{r}\mathrm{o}\mathrm{t}}/\mathrm{\tau <!--\; ??\; -->}$, where τ is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, $L_{\mathrm{X}}/L_{\mathrm{b}\mathrm{o}\mathrm{l}}$, to minimize mass dependencies in our characterization of the rotation-coronal activity relation at 500 Myr. We find that fast rotators, for which $R_o<<!--\; <\; -->0.09\pm <!--\; ??\; -->0.01$, show saturated levels of activity, with log($L_{\mathrm{X}}/L_{\mathrm{b}\mathrm{o}\mathrm{l}}$)$=-<!--\; ???\; -->3.06\pm <!--\; ??\; -->0.04$. For $R_o⩾<!--\; ???\; -->0.09\pm <!--\; ??\; -->0.01$, activity is unsaturated and follows a power law of the form $R_o^{\mathrm{\beta <!--\; ??\; -->}}$, where β = $-<!--\; ???\; -->{2.03}_{-<!--\; ???\; -->0.14}^{+0.17}$. This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4–1.3 $M_{\odot <!--\; ???\; -->}$, $P_{\mathrm{r}\mathrm{o}\mathrm{t}}$ in the range 0.4–12.8 days, and $L_{\mathrm{X}}$ in the range ${10}^{28.4-<!--\; ???\; -->30.5}$ $\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-<!--\; ???\; -->1}$. Our results make M37 a new benchmark open cluster for calibrating the ARAR at ages of $\approx <!--\; ???\; -->500$ Myr.