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Post Info TOPIC: UltraCool White Dwarf


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Posts: 131433
Date:
TVLM 513-46546.
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Tiny, Ultracool Star is Super Stormy

The research team targeted a well-known red dwarf star located about 35 light-years from Earth in the constellation Boîtes. The object is so small and cool that its right on the dividing line between stars (which fuse hydrogen) and brown dwarfs (which dont). One of the things that makes this small star remarkable is that it spins rapidly, completing a full rotation about every 2 hours. Compare that with our Sun, which takes nearly a month to spin once on its axis.
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Posts: 131433
Date:
TVLM 513-46546 and 2MASS J0036+1821104
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Title: 325 MHz VLA Observations of Ultracool Dwarfs TVLM 513-46546 and 2MASS J0036+1821104
Authors: T. R. Jaeger, R. A. Osten, T. J. Lazio, N. Kassim, R. L. Mutel

We present 325 MHz (90 cm wavelength) radio observations of ultracool dwarfs TVLM 513-46546 and 2MASS J0036+1821104 using the Very Large Array (VLA) in June 2007. Ultracool dwarfs are expected to be undetectable at radio frequencies, yet observations at 8.5 GHz (3.5 cm) and 4.9 GHz (6 cm) of have revealed sources with > 100 {\mu}Jy quiescent radio flux and > 1 mJy pulses coincident with stellar rotation. The anomalous emission is likely a combination of gyrosynchrotron and cyclotron maser processes in a long-duration, large-scale magnetic field. Since the characteristic frequency for each process scales directly with the magnetic field magnitude, emission at lower frequencies may be detectable from regions with weaker field strength. We detect no significant radio emission at 325 MHz from TVLM 513-46546 or 2MASS J0036+1821104 over multiple stellar rotations, establishing 2.5{\sigma} total flux limits of 795 {\mu}Jy and 942 {\mu}Jy respectively. Analysis of an archival VLA 1.4 GHz observation of 2MASS J0036+1821104 from January 2005 also yields a non-detection at the level of < 130 {\mu}Jy . The combined radio observation history (0.3 GHz to 8.5 GHz) for these sources suggests a continuum emission spectrum for ultracool dwarfs which is either flat or inverted below 2-3 GHz. Further, if the cyclotron maser instability is responsible for the pulsed radio emission observed on some ultracool dwarfs, our low-frequency non-detections suggest that the active region responsible for the high-frequency bursts is confined within 2 stellar radii and driven by electron beams with energies less than 5 keV.

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Posts: 131433
Date:
Ultracool subdwarfs
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Ultracool stars take 'wild rides' around, outside the Milky Way
Astronomers announced today that stars of a recently discovered type, dubbed ultracool subdwarfs, take some pretty wild rides as they orbit around the Milky Way, following paths that are very different from those of typical stars. One of them may actually be a visitor that originated in another galaxy.
Adam Burgasser and John Bochanski of the Massachusetts Institute of Technology presented the findings on Tuesday, June 9, in a press conference at the American Astronomical Society's semi-annual meeting in Pasadena, California. The result clarifies the origins of these peculiar, faint stars, and may provide new details on the types of stars the Milky Way has acquired from other galaxies.

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Posts: 131433
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RE: UltraCool White Dwarf
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Title: The UKIDSS-2MASS Proper Motion Survey I: Ultracool dwarfs from UKIDSS DR4
Authors: N.R. Deacon (1), N.C. Hambly (2), R.R. King (3), M.J. McCaughrean (3) ((1) Department of Astrophysics, Radboud University Nijmegen, (2) SUPA Institute for Astronomy, University of Edinburgh (3) Astrophysics Group, School of Physics, University of Exeter)

The UKIRT Infrared Deep Sky Survey (UKIDSS) is the first of a new generation of infrared surveys. Here we combine the data from two UKIDSS components, the Large Area Survey (LAS) and the Galactic Cluster Survey (GCS), with 2MASS data to produce an infrared proper motion survey for low mass stars and brown dwarfs. In total we detect 267 low mass stars and brown dwarfs with significant proper motions. We recover all ten known single L dwarfs and the one known T dwarf above the 2MASS detection limit in our LAS survey area and identify eight additional new candidate L dwarfs. We also find one new candidate L dwarf in our GCS sample. Our sample also contains objects from eleven potential common proper motion binaries. Finally we test our proper motions and find that while the LAS objects have proper motions consistent with absolute proper motions, the GCS stars may have proper motions which are significantly under-estimated. This is due possibly to the bulk motion of some of the local astrometric reference stars used in the proper motion determination.

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Posts: 131433
Date:
TVLM 513-46546
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Title: Optical variability of the ultracool dwarf TVLM 513-46546: evidence for inhomogeneous dust clouds
Authors: S.P. Littlefair, V.S. Dhillon, T.R. Marsh, T. Shahbaz, E.L. Martin, C. Copperwheat

We present multi-colour photometry of the M8.5V ultracool dwarf "pulsar" TVLM 513-46546 (hereafter TVLM 513) obtained with the triple-beam photometer ULTRACAM. Data were obtained simultaneously in the Sloan-g' and Sloan-i' bands. The previously reported sinusoidal variability, with a period of 2-hrs, is recovered here. However, the Sloan-g' and Sloan-i' lightcurves are anti-correlated, a fact which is incompatible with the currently proposed starspot explanation for the optical variability. The anti-correlated nature and relative amplitudes of the optical lightcurves are consistent with the effects of persistent dust clouds rotating on the surface of the star. In the absence of other plausible explanations for the optical variability of TVLM 513, it seems likely that dust cloud coverage combined with the rapid rotation of TVLM 513 is responsible for the optical variability in this object. However, crude modelling of a photosphere with partial dust cloud coverage shows that the anti-correlation can only be reproduced using cooler models than the literature temperature of TVLM 513. We suggest this discrepancy can be removed if more dust is present within the photosphere of TVLM 513 than theoretical model atmospheres predict, though a definitive statement on this matter will require the development of self-consistent models of partially dusty atmospheres.

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Posts: 131433
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RE: UltraCool White Dwarf
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Title: The Future of Ultracool Dwarf Science with JWST
Authors: Mark S. Marley (NASA ARC), S.K. Leggett (Gemini Observatory)

Ultracool dwarfs exhibit a remarkably varied set of characteristics which hint at the complex physical processes acting in their atmospheres and interiors. Spectra of these objects not only depend upon their mass and effective temperature, but also their atmospheric chemistry, weather, and dynamics. As a consequence divining their mass, metallicity and age solely from their spectra has been a challenge. JWST, by illuminating spectral blind spots and observing objects with constrained masses and ages should finally unearth a sufficient number of ultracool dwarf Rosetta Stones to allow us to decipher the processes underlying the complex brown dwarf cooling sequence. In addition the spectra of objects invisible from the ground, including very low mass objects in clusters and nearby cold dwarfs from the disk population, will be seen for the first time. In combination with other ground- and space-based assets and programs, JWST will usher in a new golden era of brown dwarf science and discovery.

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Google earth file: COMBO-17 J114356.08-0144032.kmz (2kb, kmz)



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Title: A New Ultracool White Dwarf Discovered in the SuperCOSMOS Sky Survey
Authors: N. Rowell, M. Kilic, N. C. Hambly

We present photographic B, R and I photometry, and optical and near-infrared spectroscopy, of a new ultracool white dwarf (UCWD) discovered in the SuperCOSMOS Sky Survey. The spectrum of SSSJ1556-0806 shows strong flux suppression due to the presence of collisionally induced absorption by molecular hydrogen (H2CIA), a feature characteristic of the cool, high density environments found in the atmospheres of ultracool white dwarfs. SSSJ1556-0806 therefore joins a list of <10 ultracool white dwarfs displaying extreme flux suppression. Synthetic model fitting suggests an effective temperature <3000K, which if true would make this one of the coolest white dwarfs currently known. We also exploit the similarity between the SEDs of SSSJ1556-0806 and the well-studied UCWD LHS 3250 to aid in the determination of the atmospheric parameters in a regime where models consistently fail to reproduce observations. SSSJ1556-0806 is relatively bright (R ~ 17.8), making it particularly amenable to follow up observations to obtain trigonometric parallax and IR photometry.

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Title: New UltraCool and Halo White Dwarf Candidates in SDSS Stripe 82
Authors: S. Vidrih (1,2), D. M. Bramich (1,3), P. C. Hewett (1), N. W. Evans (1), G. Gilmore (1), S. Hodgkin (1), M. Smith (1), L. Wyrzykowski (1), V. Belokurov (1), M. Fellhauer (1), M. J. Irwin (1), R. G. McMahon (1), D. Zucker (1), J. A. Munn (4), H. Lin (5), G. Miknaitis (5), H. C. Harris (4), R. H. Lupton (6), D. P. Schneider (7) ((1) Cambridge, (2) ARI, (3) Isaac Newton Group of Telescopes, (4) US Naval Observatory, (5) Fermi National Accelerator Laboratory, (6) Princeton, (7) Pennsylvania)

A 2.5 x 100 degree region along the celestial equator (Stripe 82) has been imaged repeatedly from 1998 to 2005 by the Sloan Digital Sky Survey. A new catalogue of ~4 million light-motion curves, together with over 200 derived statistical quantities, for objects in Stripe 82 brighter than r~21.5 has been constructed by combining these data by Bramich et al. (2007). This catalogue is at present the deepest catalogue of its kind. Extracting the ~130000 objects with highest signal-to-noise ratio proper motions, we build a reduced proper motion diagram to illustrate the scientific promise of the catalogue. In this diagram disk and halo subdwarfs are well-separated from the cool white dwarf sequence. Our sample of 1049 cool white dwarf candidates includes at least 8 and possibly 21 new ultracool DA type white dwarfs (T_eff < 4000K) and one new ultracool DB type white dwarf candidate identified from their SDSS optical and UKIDSS infrared photometry. At least 10 new halo white dwarfs are also identified from their kinematics.

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Title: Rotational Modulation of M/L Dwarfs due to Magnetic Spots
Authors: C. Lane, G. Hallinan, R. T. Zavala, R. F. Butler, R. P. Boyle, S. Bourke, A. Antonova, J. G. Doyle, F. J. Vrba, A. Golden

We find periodic I-band variability in two ultracool dwarfs, TVLM 513-46546 and 2MASS J00361617+1821104, on either side of the M/L dwarf boundary. Both of these targets are short-period radio transients, with the detected I-band periods matching those found at radio wavelengths (P=1.96 hr for TVLM 513-46546, and P=3 hr for 2MASS J00361617+1821104). We attribute the detected I-band periodicities to the periods of rotation of the dwarfs, supported by radius estimates and measured v sin i values for the objects. Based on the detected period of rotation of TVLM 513-46546 (M9) in the I-band, along with confirmation of strong magnetic fields from recent radio observations, we argue for magnetically induced spots as the cause of this periodic variability. The I-band rotational modulation of L3.5 dwarf 2MASS J00361617+1821104 appeared to vary in amplitude with time. We conclude that the most likely cause of the I-band variability for this object is magnetic spots, possibly coupled with time-evolving features such as dust clouds.

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