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Post Info TOPIC: TW Hydrae Association
Blobrana


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RE: TW Hydrae Association
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Title: Identification of the TW Hya Association member 2M1235$-$39: a tertiary component of the HR 4796 system
Authors: Joel H. Kastner (Laboratoire d'Astrophysique de Grenoble and Rochester Institute of Technology), B. Zuckerman (University of California, Los Angeles), M. Bessell (Research School of Astronomy and Astrophysics, The Australian National University)

The association of the late-type star 2MASS J12354893-3950245 (2M1235-39) with a bright X-ray source detected serendipitously by ROSAT and XMM-Newton, combined with its proximity to the well-studied (A+M binary) system HR 4796, suggests this star is a member of the TW Hya Association (TWA). To test this hypothesis, we used optical spectroscopy to establish the Li and H-alpha line strengths and spectral type of 2M1235-39, and determined its proper motion via optical imaging. The Li absorption and H-alpha emission line strengths of 2M1235-39, its near-IR and X-ray fluxes, and its proper motion all indicate that 2M1235-39 is a TWA member. Most likely this star is a wide (13,500 AU) separation, low-mass (M4.5), tertiary component of the HR 4796 system.

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Blobrana


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Posts: 131433
Date:
RE: TW Hydrae
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Title: TW Hydrae: evidence of stellar spots instead of a Hot Jupiter
Authors: N. Huelamo, P. Figueira, X. Bonfils, N.C. Santos, F. Pepe, M. Guillon, R. Azevedo, T. Barman, M. Fernandez, E. di Folco, E.W. Guenther, C. Lovis, C.H.F. Melo, D. Queloz, S. Udry

TW Hydrae shows significant radial-velocity variations in the optical regime. They have been attributed to a 10 Jupiter Mass planet orbiting the star at 0.04 AU. In this work, we have tested whether the observed RV variations can be caused by stellar spots. We have also analysed new optical and infrared data to confirm the signal of the planet companion. We fitted the RV variations of TW Hya using a cool spot model. Our model shows that a cold spot covering 7% of the stellar surface and located at a latitude of 54 deg can reproduce the reported RV variations. The model also predicts a bisector semi-amplitude variation <10 m/s, which is less than the errors of the RV measurements discussed in an earlier publication. The analysis of new optical RV data, with typical errors of 10 m/s, shows a larger RV amplitude that varies depending on the correlation mask used. A slight correlation between the RV variation and the bisector is also observed, although not at a very significant level. The infrared H-band RV curve is almost flat, showing a small variation (<35 m/s) that is not consistent with the optical orbit. All these results support the spot scenario rather than the presence of a hot Jupiter around TW Hya.

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Blobrana


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Title: Resolving the chemistry in the disk of TW Hydrae I. Deuterated species
Authors: C. Qi, D.J. Wilner, Y. Aikawa, G.A. Blake, M.R. Hogerheijde

We present Submillimeter Array (SMA) observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN J=3-2 and DCO+ J=3-2 lines, and upper limits to the HDO 3(1,2)-2(2,1), ortho-H2D+ 1(1,0)-1(1,1) and para-D2H+ 1(1,0)-1(0,1) transitions. We also present observations of the HCN J=3-2, HCO+ J=3-2 and H13CO+ J=4-3 lines for comparison with their deuterated isotopologues. We constrain the radial and vertical distributions of various species in the disk by fitting the data using a model where the molecular emission from an irradiated accretion disk is sampled with a 2D Monte Carlo radiative transfer code. We find that the distribution of DCO+ differs markedly from that of HCO+. The D/H ratios inferred change by at least one order of magnitude (0.01 to 0.1) for radii <30 AU to >70 AU and there is a rapid falloff of the abundance of DCO+ at radii larger than 90 AU. Using a simple analytical chemical model, we constrain the degree of ionisation, x(e-)=n(e-)/n(H2), to be ~10^-7 in the disk layer(s) where these molecules are present. Provided the distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined to be 1.7±0.5 \times 10^-2; however, this ratio is very sensitive to the poorly constrained vertical distribution of HCN. The resolved radial distribution of DCO+ indicates that {\it in situ} deuterium fractionation remains active within the TW Hydrae disk and must be considered in the molecular evolution of circumstellar accretion disks.

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Blobrana


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Posts: 131433
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RE: TW Hydrae Association
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Title: New Brown Dwarf Disks in the TW Hydrae Association
Authors: B. Riaz, J. E. Gizis

In our analysis of Spitzer/IRS archival data on the stellar and sub-stellar members of the TW Hydrae Association (TWA), we have discovered two new brown dwarf disks: a flat optically thick disk around SSSPM J1102-3431 (SSSPM 1102), and a transition disk around 2MASS J1139511-315921 (2M1139). The disk structure for SSSPM 1102 is found to be very similar to the known brown dwarf disk 2MASSW J1207334-393254 (2M1207), with excess emission observed at wavelengths as short as 5 \micron. 2M1139 shows no excess emission shortward of  ~20 \micron, but flares up at longer wavelengths, and is the first transition disk detected among the sub-stellar members of TWA. We also report on the  Spitzer/70 \micron observations, and the presence of a weak  absorption 10 \micron silicate feature for 2M1207. The absorption can be attributed to a close to edge-on disk at a 75degrees inclination. The 10 \micron spectrum for 2M1207 shows crystalline forsterite features, with a peak in absorption near 11.3 \micron. No silicate absorption/emission is observed towards SSSPM 1102. While only 6 out of 25 stellar members show excess emission at these mid-infrared wavelengths, {\it all} of the TWA brown dwarfs that have been observed so far with  Spitzer show signs of disks around them, resulting in a disk fraction of at least 60%. This is a considerable fraction at a relatively older age of ~10 Myr. A comparison with younger clusters indicates that by the age of the TWA  (~10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of ~2 for the higher mass stars. This suggests longer disk decay time scales for brown dwarfs compared to higher mass stars.

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Blobrana


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Title: Spitzer IRS Observations of Disks around Brown Dwarfs in the TW Hydra Association
Authors: A. L. Morrow, K. L. Luhman, C. Espaillat, P. D'Alessio, L. Adame, N. Calvet, W. J. Forrest, B. Sargent, L. Hartmann, D. M. Watson, C. J. Bohac

Using SpeX at the NASA Infrared Telescope Facility and the Spitzer Infrared Spectrograph, we have obtained infrared spectra from 0.7 to 30um for three young brown dwarfs in the TW Hydra Association (~10 Myr), 2M J1207-3932, 2M J1139-3159, and SS J1102-3431. The spectral energy distribution for 2M J1139-3159 is consistent with a stellar photosphere for the entire wavelength range of our data while the other two objects exhibit significant excess emission at >5um. We are able to reproduce the excess emission from each brown dwarf using our models of irradiated accretion disks. According to our model fits, both disks have experienced a high degree of dust settling. We also find that silicate emission at 10 and 20um is absent from the spectra of these disks, indicating that grains in the upper disk layers have grown to sizes larger than ~5um. Both of these characteristics are consistent with previous observations of decreasing silicate emission with lower stellar masses and older ages. These trends suggest that either 1) the growth of dust grains, and perhaps planetesimal formation, occurs faster in disks around brown dwarfs than in disks around stars, or 2) the radii of the mid-IR-emitting regions of disks are smaller for brown dwarfs than for stars, and grains grow faster at smaller disk radii. Finally, we note the possible detection of an unexplained emission feature near 14um in the spectra of both of the disk-bearing brown dwarfs.

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Blobrana


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RE: TW Hydrae
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German astronomers have discovered a "giant baby" planet in deep space which they say has a mass 10 times as dense as Jupiter but is a scant 10 million years old - a newborn by cosmic standards.
The scientists at the Max Planck Institute for Astronomy in Heidelberg say it is the youngest known planet outside of our solar system.

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Blobrana


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German  astronomers have discovered a "giant baby" planet in deep space which they say has a mass 10 times as dense as Jupiter but is a scant 10 million years old - a newborn by cosmic standards. The scientists at the Max Planck Institute for Astronomy in Heidelberg say it is the youngest known planet outside of our solar system.
Its host star is still surrounded by the disk of gas and dust from which it was only recently born. It orbits its mother star every 3.65 days.

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Blobrana


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Posts: 131433
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TW Hydrae b
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Astronomers have discovered a newborn planet in a solar system that is still in the process of forming - the first example of this ever found.
Planets are believed to develop within swirling discs of dust and gas around nascent stars.
So studying very young examples could tell astronomers much about the birth and evolution of planetary systems - including our own Solar System.
Details of the discovery, by a team in Germany, appear in the journal Nature.

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Blobrana


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Posts: 131433
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RE: TW Hydrae Association
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Variation der Radialgeschwindigkeit des Sterns TW Hydrae, gemessen im Frühjahr 2007

This diagram shows the variations of the radial velocity of TW Hydrae as observed in early 2007. The data can best be described as an oscillation with a period of 3.6 days, caused by a giant planet orbiting around the star.
Picture: Max Planck Institute for Astronomy

-- Edited by Blobrana at 20:02, 2008-01-03

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Blobrana


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A planet has been found within a disc of dust and gas around a young star for the first time, a new study reports. The research confirms predictions that planets can coalesce within 10 million years, but it does not settle the debate over just how that formation proceeds.

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