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Kontruksi gerak brown
ABSTRAK
Gerak Brown dengan mean p dan variansi a2 adalah suatu proses stokastik ( Xt t 0 ), dengan Xt.f. - Xt merupakan penambahan - penambahan selaina interval waktu (t,t+s] untuk
s > 0. Karena itu, Xtfs Xt adalah Gaussian dengan mean
p(s) dan variansi az(s), dan penambahan Xt+s bebas
dengan ( Xu ; 0 u 5 t ).
Dengan pengulangan dan interpolasi linier pada interval waktu berhingga, yaitu [0,1], diperoleh model dari suatu ukuran peluang P pada (O, Y) := ( ER 0 CO [ie CO ) ),
sedemikian sehingga B = ( Bt,'tB0 5 t < 03 ) adalah gerak Brown dimensi satu standar
UVES spectra of young brown dwarfs in Cha I: radial and rotational velocities
Based on high-resolution UVES spectra we found that the radial velocity (RV)
dispersion of nine of twelve known young bona fide and candidate brown dwarfs
in the Cha I dark cloud is 2.0 km/s, i.e. significantly smaller than the RV
dispersion of T Tauri stars in Cha I (3.6 km/s) and only slightly larger than
the dispersion of the surrounding molecular gas (1.2 km/s) (Mizuno et al.
1999). This result indicates that the majority of these brown dwarfs are not
ejected with high velocity out of a dense region as proposed by some formation
scenarios for brown dwarfs. The mean RV values are consistent with the objects
being kinematic members of Cha I. The RV dispersion of the T Tauri stars
confined to the Cha I region is based on a compilation of T Tauri stars with
known RVs from the literature plus three T Tauri stars observed with UVES and
unpublished RVs for nine T Tauri stars. Time-resolved spectroscopy revealed RV
variations for five out of nine of the bona fide and candidate brown dwarfs in
Cha I, which could be due to orbiting planets or surface features. Furthermore
we derived rotational velocities vsin(i) and the Lithium 6708 \AA equivalent
width.Comment: A&A Letter, in pres
Detectability of dirty dust grains in brown dwarf atmospheres
Dust clouds influence the atmospheric structure of brown dwarfs, and they
affect the heat transfer and change the gas-phase chemistry. However, the
physics of their formation and evolution is not well understood. In this
letter, we predict dust signatures and propose a potential observational test
of the physics of dust formation in brown dwarf atmosphere based on the
spectral features of the different solid components predicted by dust formation
theory. A momentum method for the formation of dirty dust grains (nucleation,
growth, evaporation, drift) is used in application to a static brown dwarf
atmosphere structure to compute the dust grain properties, in particular the
heterogeneous grain composition and the grain size. Effective medium and Mie
theory are used to compute the extinction of these spherical grains. Dust
formation results in grains whose composition differs from that of grains
formed at equilibrium. Our kinetic model predicts that solid amorphous SiO2[s]
(silica) is one of the most abundant solid component followed by amorphous
MgSiO4[s] and MgSiO3[s], while SiO2[s] is absent in equilibrium models
because it is a metastable solid. Solid amorphous SiO2[s] possesses a strong
broad absorption feature centered at 8.7mum, while amorphous
Mg2SiO4[s]/MgSiO3[s] absorb at 9.7mum beside other absorption features at
longer wavelength. Those features at lambda < 15mum are detectable in
absorption if grains are small (radius < 0.2mum) in the upper atmosphere as
suggested by our model. We suggest that the detection of a feature at 8.7mum in
deep infrared spectra could provide evidence for non-equilibrium dust formation
that yields grains composed of metastable solids in brown dwarf atmospheres.
This feature will shift towards 10mum and broaden if silicates (e.g. fosterite)
are much more abundant.Comment: A&A Letter, accepte
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