We report the discovery of a thin stellar stream found in Pan-STARRS1 photometry near the Galactic bulge in the constellation of Ophiuchus. It appears as a coherent structure in the colour-selected stellar density maps produced to search for tidal debris around nearby globular clusters. The stream is exceptionally short and narrow; it is about 2.5 • long and 6 ′ wide in projection. The colour-magnitude diagram of this object, which harbours a blue horizontalbranch, is consistent with an old and relatively metal-poor population ([Fe/H] ∼ −1.3) located 9.5 ± 0.9 kpc away at (l, b) ∼ (5 • , +32 • ), and 5.0 ± 1.0 kpc from the Galactic centre. These properties argue for a globular cluster as progenitor. The finding of such a prominent, nearby stream suggests that many streams could await discovery in the more densely populated regions of our Galaxy.

We present the detection of 6,371 RR Lyrae (RRL) stars distributed across ∼14,000 deg 2 of the sky from the combined data of the Sloan Digital Sky Survey (SDSS), the Panoramic Survey Telescope and Rapid Response System 1 (PS1), and the second photometric catalogue from the Catalina Survey (CSDR2), out of these, ∼2,021 RRL stars (∼572 RRab and 1,449 RRc) are new discoveries. The RRL stars have heliocentric distances in the 4-28 kpc distance range. RRL-like color cuts from the SDSS and variability cuts from the PS1 are used to cull our candidate list. We then use the CSDR2 multi-epoch data to refine our sample. Periods were measured using the Analysis of Variance technique while the classification process is performed with the Template Fitting Method in addition to the visual inspection of the light curves. A cross-match of our RRL star discoveries with previous published catalogs of RRL stars yield completeness levels of ∼50% for both RRab and RRc stars, and an efficiency of ∼99% and ∼87% for RRab and RRc stars, respectively. We show that our method for selecting RRL stars allows us to recover halo structures. The full lists of all the RRL stars are made publicly available.

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Variable stars play a crucial role as standard candles and provide valuable insights into stellar physics. They can be modeled either through fully fledged hydrodynamical simulations or analytically as systems of coupled differential equations describing the evolution of relevant physical quantities. Typically, such equations are arrived at by simplified physical assumptions concerning the conservation laws governing stellar interiors. Here we apply a data-driven technique—sparse identification of nonlinear dynamics (SINDy)—to automatically learn governing equations from observed light curves. We apply SINDy to 3100 light curves of three different variable types from the Catalina Sky Survey. The success rate depends systematically on variable type, with possible implications for variable star classification; however, it does not obviously depend on amplitude or period. Successful models can be reduced to the generalized Lienard equation x ̈ + ( a + bx + c x ̇ ) x ̇ + x = 0 . Members of the Lienard class of ordinary differential equations, such as the well-studied van der Pol oscillator, already saw some application to variable star modeling. For a, b = 0 the equation can be solved exactly, and it admits both periodic and nonperiodic solutions. We find a condition on the coefficients of the general equation for the presence of a limit cycle, which is also observed numerically in several instances.