Time-Series Photometry of Stars in and around the Lagoon Nebula. I. Rotation Periods of 290 Low-Mass Pre-Main-Sequence Stars in NGC 6530

We have conducted a long-term, wide-field, high-cadence photometric monitoring survey of ~50,000 stars in the Lagoon Nebula \ion{H}{2} region. This first paper presents rotation periods for 290 low-mass stars in NGC 6530, the young cluster illuminating the nebula, and for which we assemble a catalog of infrared and spectroscopic disk indicators, estimated masses and ages, and X-ray luminosities. The distribution of rotation periods we measure is broadly uniform for 0.5 < P < 10 d; the short-period cutoff corresponds to breakup. We observe no obvious bimodality in the period distribution, but we do find that stars with disk signatures rotate more slowly on average. The stars' X-ray luminosities are roughly flat with rotation period, at the saturation level ($\log L_X / L_{\rm bol} \approx -3.3$). However, we find a significant positive correlation between $L_X / L_{\rm bol}$ and co-rotation radius, suggesting that the observed X-ray luminosities are regulated by centrifugal stripping of the stellar coronae. The period-mass relationship in NGC 6530 is broadly similar to that of the Orion Nebula Cluster (ONC), but the slope of the relationship among the slowest rotators differs from that in the ONC and other young clusters. We show that the slope of the period-mass relationship for the slowest rotators can be used as a proxy for the age of a young cluster, and we argue that NGC 6530 may be slightly younger than the ONC, making it a particularly important touchstone for models of angular momentum evolution in young, low-mass stars.Comment: 28 pages, 18 figures, Accepted for publication in ApJ. For a brief video explaining the key results of this paper, see http://www.youtube.com/user/OSUAstronomy#p/u/1/WarGh6GiWu

High-Resolution Spectroscopy during Eclipse of the Young Substellar Eclipsing Binary 2MASS 0535-0546. II. Secondary Spectrum: No Evidence that Spots Cause the Temperature Reversal

We present high-resolution optical spectra of the young brown-dwarf eclipsing binary 2M0535-05, obtained during eclipse of the higher-mass (primary) brown dwarf. Combined with our previous spectrum of the primary alone (Paper I), the new observations yield the spectrum of the secondary alone. We investigate, through a differential analysis of the two binary components, whether cool surface spots are responsible for suppressing the temperature of the primary. In Paper I, we found a significant discrepancy between the empirical surface gravity of the primary and that inferred via fine analysis of its spectrum. Here we find precisely the same discrepancy in surface gravity, both qualitatively and quantitatively. While this may again be ascribed to either cool spots or model opacity errors, it implies that cool spots cannot be responsible for preferentially lowering the temperature of the primary: if they were, spot effects on the primary spectrum should be preferentially larger, and they are not. The Teff we infer for the primary and secondary, from the TiO-epsilon bands alone, show the same reversal, in the same ratio, as is empirically observed, bolstering the validity of our analysis. In turn, this implies that if suppression of convection by magnetic fields on the primary is the fundamental cause of the Teff reversal, then it cannot be a local suppression yielding spots mainly on the primary (though both components may be equally spotted), but a global suppression in the interior of the primary. We briefly discuss current theories of how this might work.Comment: Final ApJ version. Small textual change in summary at the end (Sec 6.2), to include work published after submission of this paper; no changes in our results or conclusion

Near-Infrared Light Curves of the Brown Dwarf Eclipsing Binary 2MASS J05352184-0546085: Can Spots Explain the Temperature Reversal?

We present the JHKs light curves for the double-lined eclipsing binary 2MASS J05352184-0546085, in which both components are brown dwarfs. We analyze these light curves with the published Ic-band light curve and radial velocities to provide refined measurements of the system's physical parameters. The component masses and radii are here determined with an accuracy of ~6.5% and ~1.5%, respectively. We confirm the previous surprising finding that the primary brown dwarf has a cooler effective temperature than its companion. Next, we perform a detailed study of the variations in the out-of-eclipse phases of the light curves to ascertain the properties of any inhomogeneities on the surfaces of the brown dwarfs. Our analysis reveals two low-amplitude periodic signals, one attributable to the rotation of the primary (with a period of 3.293+/-0.001 d) and the other to that of the secondary (14.05+/-0.05 d). Finally, we explore the effects on the derived physical parameters of the system when spots are included in the modeling. The observed low-amplitude rotational modulations are well fit by cool spots covering a small fraction of their surfaces. To mimic the observed ~200 K suppression of the primary's temperature, our model requires that the primary possess a very large spot coverage fraction of ~65%. Altogether, a spot configuration in which the primary is heavily spotted while the secondary is lightly spotted can explain the apparent temperature reversal and can bring the temperatures of the brown dwarfs into agreement with the predictions of theoretical models.Comment: Accepted for publication in The Astrophysical Journal; 13 pages, 8 figures, 6 tables. Updated parameter uncertaintie

The canonical Luminous Blue Variable AG Car and its neighbor Hen 3-519 are much closer than previously assumed

The strong mass loss of Luminous Blue Variables (LBVs) is thought to play a critical role in massive-star evolution, but their place in the evolutionary sequence remains debated. A key to understanding their peculiar instability is their high observed luminosities, which often depends on uncertain distances. Here we report direct distances and space motions of four canonical Milky Way LBVs---AG~Car, HR~Car, HD~168607, and (candidate) Hen~3-519---from the Gaia first data release. Whereas the distances of HR~Car and HD~168607 are consistent with previous literature estimates within the considerable uncertainties, Hen~3-519 and AG~Car, both at $\sim$2~kpc, are much closer than the 6--8~kpc distances previously assumed. As a result, Hen~3-519 moves far from the locus of LBVs on the HR Diagram, making it a much less luminous object. For AG~Car, considered a defining example of a classical LBV, its lower luminosity would also move it off the S~Dor instability strip. Lower luminosities allow both AG~Car and Hen~3-519 to have passed through a previous red supergiant phase, lower the mass estimates for their shell nebulae, and imply that binary evolution is needed to account for their peculiarities. These results may also impact our understanding of LBVs as potential supernova progenitors and their isolated environments. Improved distances will be provided in the Gaia second data release, which will include additional LBVs. AG~Car and Hen~3-519 hint that this new information may alter our traditional view of LBVs.Comment: 15 pages, 2 figures, 1 table. Accepted by Astronomical Journa

Angular Momentum Evolution of Young Stars: Toward a Synthesis of Observations, Theory, and Modeling

The aim of this AAS Topical Session was to update the community on the current state of knowledge about the angular momentum evolution of young stars. For newcomers to the subject, the session was intended to provide an introduction and general overview and to highlight emerging issues. For experienced workers in this field, the session provided an opportunity for synthesizing recent developments in observations, theory, and modeling of rotation of young stars and for identifying promising new research directions.Comment: 10 pages, conference summary, to appear in April PAS

Magnetic Accretion and Photopolarimetric Variability in Classical T Tauri Stars

We employ a Monte Carlo radiation transfer code to investigate the multi- wavelength photopolarimetric variability arising from a spotted T Tauri star surrounded by a dusty circumstellar disk. Our aim is to assess the ability of the magnetic accretion model to explain the observed photopolarimetric variability of classical T Tauri stars, and to identify potentially useful observational diagnostics of T Tauri star/disk/spot parameters. We model a range of spot sizes, spot latitudes, inner disk truncation radii, and system inclination angles, as well as multiple disk and spot geometries. We find that the amplitude, morphology, and wavelength dependence of the photopolarimetric variability predicted by our models are generally consistent with existing observations; a flared disk geometry is required to reproduce the largest observed polarization levels and variations. Our models can further explain stochastic polarimetric variability if unsteady accretion is invoked, in which case irregular -- but correlated -- photometric variability is predicted, in agreement with observations. Potentially useful observational diagnostics of system parameters of interest are discussed. We also investigate the reliability of modeling spot parameters via analytic fits to multi-band photometric variations.Comment: 25 pages, 8 figures To be published in ApJ, 1999, 51

The Color-Period Diagram and Stellar Rotational Evolution - New Rotation Period Measurements in the Open Cluster M34

We present results from a 5-month photometric survey for stellar rotation periods combined with a 4-year radial-velocity survey for membership and binarity in the 220Myr open cluster M34. We report surface rotation periods for 120 stars, 83 of which are late-type cluster members. A comparison to previous work serves to illustrate the importance of high cadence long baseline photometric observations and membership information. The new M34 periods are less biased against slow rotation and cleaned for non-members. The rotation periods of the cluster members span more than an order of magnitude from 0.5 day up to 11.5 days, and trace two distinct rotational sequences - fast (C) and moderate-to-slow (I) - in the color-period diagram. The sequences represent two different states in the rotational evolution of the late-type cluster members. We use the color-period diagrams for M34 and for younger and older clusters to estimate the timescale for the transition from the C to the I sequence and find ~<150Myr, ~150-300Myr, and ~300-600Myr for G, early-mid K, and late K dwarfs, respectively. The small number of stars in the gap between C and I suggest a quick transition. We estimate a lower limit on the maximum spin-down rate (dP/dt) during this transition to be ~0.06 days/Myr and ~0.08 days/Myr for early and late K dwarfs, respectively. We compare the I sequence rotation periods in M34 and the Hyades for G and K dwarfs and find that K dwarfs spin down slower than the Skumanich rate. We determine a gyrochronology age of 240Myr for M34. We measure the effect of cluster age uncertainties on the gyrochronology age for M34 and find the resulting error to be consistent with the error estimate for the technique. We use the M34 I sequence to redetermine the coefficients in the expression for rotational dependence on color used in gyrochronology (abridged).Comment: 47 pages (12pt, preprint), 14 figures, 2 tables, Accepted for publication in ApJ, format of RA coordinates in Table 2 corrected in latest versio