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arXiv 0802.1732astro-ph
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Spitzer IRS对TW水螅星协中褐矮星周围恒星的观测

Spitzer IRS Observations of Disks around Brown Dwarfs in the TW Hydra Association

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

摘要:

利用美国宇航局红外望远镜设备上的SpeX和斯皮策红外光谱仪,我们获得了TW Hydra星协中三颗年轻棕矮星(约1000万年),2 M J1207-3932,2 M J1139-3159和SS J1102-3431的0.7 - 30 μ m的红外光谱。2 M J1139-3159的光谱能量分布与我们数据的整个波长范围内的恒星光球一致,而其他两个物体在> 5 μ m处表现出显着的过量发射。我们能够重现多余的排放从每个棕矮星使用我们的模型照射吸积盘。根据我们的模型拟合,两个磁盘都经历了高度的灰尘沉降。我们还发现,在10和20 μ m的硅酸盐发射是缺席的光谱这些磁盘,表明在磁盘上层的晶粒已经长大到大于~ 5 μ m的尺寸。这两个特征都与以前的观测结果相一致,即硅酸盐发射量随着恒星质量的降低和年龄的增大而减少。这些趋势表明,1)尘埃颗粒的生长,也许是星子的形成,在棕矮星周围的圆盘中比在恒星周围的圆盘中发生得更快,或者2)棕矮星圆盘的中红外辐射区域的半径比恒星小,并且颗粒在较小的圆盘半径处生长得更快。最后,我们注意到可能检测到的一个无法解释的发射功能在14微米附近的光谱的两个盘轴承棕矮星。

英文摘要:

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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