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Faherty J.K.,American Museum of Natural History | Faherty J.K.,State University of New York at Stony Brook | Faherty J.K.,Universities for Research in Astronomy | Burgasser A.J.,University of California at San Diego | And 5 more authors.
Astronomical Journal | Year: 2011

We report our discovery of NLTT 20346 as an M5+M6 companion system to the tight binary (or triple) L dwarf 2MASS J0850359+105716. This nearby (31pc), widely separated (7700AU) quadruple system was identified through a cross-match of proper motion catalogs. Follow-up imaging and spectroscopy of NLTT 20346 revealed it to be a magnetically active M5+M6 binary with components separated by 2″ (50-80AU). Optical spectroscopy of the components shows only moderate Hα emission corresponding to a statistical age of 5-7Gyr for both M dwarfs. However, NLTT 20346 is associated with the XMM-Newton source J085018.9+105644, and based on X-ray activity the age of NLTT 20346 is between 250 and 450Myr. Strong Li absorption in the optical spectrum of 2MASS J0850+1057 indicates an upper age limit of 0.8-1.5Gyr, favoring the younger age for the primary. Using evolutionary models in combination with an adopted system age of 0.25-1.5Gyr indicates a total mass for 2MASS J0850+1057 of 0.07 0.02 M ⊙, if it is a binary. NLTT 20346/2MASS J0850+1057 joins a growing list of hierarchical systems containing brown dwarf binaries and is among the lowest binding energy associations found in the field. Formation simulations via gravitational fragmentation of massive extended disks have successfully produced a specific analog to this system. © 2011. The American Astronomical Society. All rights reserved.

Neugent K.F.,Lowell Observatory | Massey P.,Lowell Observatory | Massey P.,Universities for Research in Astronomy
Astrophysical Journal | Year: 2011

Wolf-Rayet (WR) stars are evolved massive stars, and the relative number of WC-type and WN-type WRs should vary with metallicity, providing a sensitive test of stellar evolutionary theory. The observed WC/WN ratio is much higher than that predicted by theory in some galaxies but this could be due to observational incompleteness for WN types, which have weaker lines. Previous studies of M33's WR content show a galactocentric gradient in the relative numbers of WCs and WNs, but only small regions have been surveyed with sufficient sensitivity to detect all of the WNs. Here, we present a sensitive survey for WRs covering all of M33, finding 55 new WRs, mostly of WN type. Our spectroscopy also improves the spectral types of many previously known WRs, establishing in one case that the star is actually a background quasar. The total number of spectroscopically confirmed WRs in M33 is 206, a number we argue is complete to ∼ 5%, with most WRs residing in OB associations, although ∼2% are truly isolated. The WC/WN ratio in the central regions (<2kpc) of M33 is much higher than that predicted by the current Geneva evolutionary models, while the WC/WN ratios in the outer regions are in good accord, as are the values in the Small Magellanic Cloud and Large Magellanic Cloud. The WC/WN ratio and the WC subtype distribution both argue that the oxygen abundance gradient in M33 is significantly larger than that found by some recent studies, but are consistent with the two-component model proposed by Magrini et al. © 2011. The American Astronomical Society. All rights reserved.

Meibom S.,Harvard - Smithsonian Center for Astrophysics | Mathieu R.D.,University of Wisconsin - Madison | Stassun K.G.,Vanderbilt University | Stassun K.G.,Fisk University | And 3 more authors.
Astrophysical Journal | Year: 2011

We present the results of a 5 month photometric time-series 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 kinematic and photometric 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 over more than an order of magnitude from 0.5 days up to 11.5days, and trace two distinct rotational sequences - fast (C) and moderate-to-slow (I) - in the color-period diagram. The sequences represent two different states (fast and slow) 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 suggests a quick transition. We estimate a lower limit on the maximum spin-down rate (dP/dt) during this transition to be 0.06daysMyr-1 and 0.08daysMyr-1 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 √t rate. We determine a gyrochronology age of 240Myr for M34. The gyro-age has a small formal uncertainty of 2% which reflects the tight I sequence in the M34 color-period diagram. We measure the effect of cluster age uncertainties on the gyrochronology age for M34 and find the resulting error on the gyro-age to be consistent with the 15% error estimate for the technique in general. We use the M34 I sequence to redetermine the coefficients in the expression for rotational dependence on color used in gyrochronology. Finally, we propose that stability in the phase, shape, and amplitude of the photometric variability for the 120 rotators over the 5 month duration of our survey is due to spot generation at active stellar longitudes. © 2011. The American Astronomical Society. All rights reserved.

De Kleer K.,University of California at Berkeley | de Pater I.,University of California at Berkeley | de Pater I.,Technical University of Delft | de Pater I.,SRON Netherlands Institute for Space Research | And 3 more authors.
Icarus | Year: 2013

We present the first high-resolution near-infrared (1.18-2.38. μm) spectrum of the rings of Uranus, as observed with adaptive optics on the W.M. Keck II telescope in August 2010. We derive ring equivalent widths, as well as ring and particle reflectivities for the {small element of}; ring and ringlet groups based on H- and K-band data. We find the rings to be gray, indicating that they are dominated by large particles rather than dust, and we find no evidence for water ice. We present a reflectivity spectrum for the {small element of}; ring alone, which we also find to be consistent with a flat spectrum. We derive H-band ring particle reflectivities of 0.022. ±. 0.010, 0.051. ±. 0.009 0.042. ±. 0.012, and 0.043. ±. 0.001 and K-band ring particle reflectivities of 0.016. ±. 0.010, 0.034. ±. 0.012, 0.047. ±. 0.008 and 0.041. ±. 0.002 for the 456, αβ, ηγδ, and {small element of}; ring groups. Previous observations have found ring particle reflectivities in the 0.033-0.044 range (de Pater, I., Gibbard, S., Macintosh, B.A., Roe, H.G. [2002]. Icarus 160, 359-374; Gibbard, S.G., de Pater, I., Hammel, H.B. [2005]. Icarus 174, 253-262), and are generally consistent with our results. © 2013 Elsevier Inc.

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