4,930 research outputs found
Accretion Disks Around Young Objects. III. Grain Growth
We present detailed models of irradiated T Tauri disks including dust grain
growth with power-law size distributions. The models assume complete mixing
between dust and gas and solve for the vertical disk structure
self-consistentlyincluding the heating effects of stellar irradiation as well
as local viscous heating. For a given total dust mass, grain growth is found to
decrease the vertical height of the surface where the optical depth to the
stellar radiation becomes unit and thus the local irradiation heating, while
increasing the disk emission at mm and sub-mm wavelengths. The resulting disk
models are less geometrically thick than our previous models assuming
interstellar medium dust, and agree better with observed spectral energy
distributions and images of edge-on disks, like HK Tau/c and HH 30. The
implications of models with grain growth for determining disk masses from
long-wavelength emission are considered.Comment: 29 pages, including 11 figures and 1 table, APJ accepte
Quantitative features of multifractal subtleties in time series
Based on the Multifractal Detrended Fluctuation Analysis (MFDFA) and on the
Wavelet Transform Modulus Maxima (WTMM) methods we investigate the origin of
multifractality in the time series. Series fluctuating according to a qGaussian
distribution, both uncorrelated and correlated in time, are used. For the
uncorrelated series at the border (q=5/3) between the Gaussian and the Levy
basins of attraction asymptotically we find a phase-like transition between
monofractal and bifractal characteristics. This indicates that these may solely
be the specific nonlinear temporal correlations that organize the series into a
genuine multifractal hierarchy. For analyzing various features of
multifractality due to such correlations, we use the model series generated
from the binomial cascade as well as empirical series. Then, within the
temporal ranges of well developed power-law correlations we find a fast
convergence in all multifractal measures. Besides of its practical significance
this fact may reflect another manifestation of a conjectured q-generalized
Central Limit Theorem
Growth and nutritional response of Nemared peach rootstock infected with Pratylenchus vulnus and the mycorrhizal fungus Glomus mosseae
Les effets de l'interaction entre #Pratylenchus vulnus et le champignon mycorrhizien #Glomus mosseae sur les porte-greffe de pêcher "Nemared" ont été étudiés en microparcelles pendant deux saisons de croissance. Le poids frais des pieds, le diamètre de la tige, la longueur des pieds et le poids frais des racines sont significativement plus faibles chez les pieds infestés par le nématode - qu'ils soient ou non colonisés par #G. mosseae à l'aide de mycorrhizes provoquent une diminution de la population finale du nématode et du nombre de nématodes par gramme de racine par rapport aux pieds infestés par #P. vulnus et non traités à l'aide du champignon. La colonisation par les mycorrhizes n'est pas affectée par la présence du nématode. Chez les pieds infestés par le nématode, le Cu est le seul élément déficitaire détecté par analyse foliaire, quoique des taux faibles de fer y aient été observés. Les taux les plus élevés de Ma, Mg, Mn et Zn ont été détectés chez les pieds infestés par #P. vulnus. Les pieds mycorrhizés recèlent les taux les plus élevés de Cu et d'Al. #G. mosseae est bénéfique pour la croissance des pêchers "Nemared" mais ne leur confère aucune protection contre #P. vulnus. (Résumé d'auteur
The spectral energy distribution of self-gravitating protostellar disks
The long wavelength emission of protostellar objects is commonly attributed
to a disk of gas and dust around the central protostar. In the first stages of
disk accretion or in the case of high mass protostars, the disk mass is likely
to be sufficiently large, so that the disk self-gravity may have an impact on
the dynamics and the emission properties of the disk. In this paper we describe
the spectral energy distribution (SED) produced by a simple, non-flaring,
self-gravitating accretion disk model. Self-gravity is included in the
calculation of the rotation curve of the disk and in the energy balance
equation, as a term of effective heating related to Jeans instability. In order
to quantify in detail the requirements on the mass of the disk and on the
accretion rate posed on the models by realistic situations, we compare the SEDs
produced by these models with the observed SEDs of a small sample of
well-studied protostellar objects. We find that relatively modest disks - even
lighter than the central star - can lead to an interesting fit to the infrared
SED of the FU Orionis objects considered, while in the case of T Tauri stars
the required parameters fall outside the range suggested as acceptable by the
general theoretical and observational scenario. On the basis of the present
results, we may conclude that the contribution of a self-gravitating disk is
plausible in several cases (in particular, for FU Orionis objects) and that, in
the standard irradiation dominated disk scenario, it would help softening the
requirements encountered by Keplerian accretion models.Comment: 26 pages, 7 figures, accepted by A&
X-ray emission from dense plasma in CTTSs: Hydrodynamic modeling of the accretion shock
High spectral resolution X-ray observations of CTTSs demonstrate the presence
of plasma at T~2-3X10^6 K and n_e~10^11-10^13 cm^-3, unobserved in
non-accreting stars. Stationary models suggest that this emission is due to
shock-heated accreting material, but they do not allow to analyze the stability
of such material and its position in the stellar atmosphere. We investigate the
dynamics and the stability of shock-heated accreting material in CTTSs and the
role of the stellar chromosphere in determining the position and the thickness
of the shocked region. We perform 1-D HD simulations of the impact of the
accretion flow onto chromosphere of a CTTS, including the effects of gravity,
radiative losses from optically thin plasma, thermal conduction and a well
tested detailed model of the stellar chromosphere. Here we present the results
of a simulation based on the parameters of the CTTS MP Mus. We find that the
accretion shock generates an hot slab of material above the chromosphere with a
maximum thickness of 1.8X10^9 cm, density n_e~10^11-10^2 cm^-3, temperature
T~3X10^6 K and uniform pressure equal to the ram pressure of the accretion flow
(~450 dyn cm^-2). The base of the shocked region penetrates the chromosphere
and stays where the ram pressure is equal to the thermal pressure. The system
evolves with quasi-periodic instabilities of the material in the slab leading
to cyclic disappearance and re-formation of the slab. For an accretion rate of
~10^-10 M_sun yr^-1, the shocked region emits a time-averaged X-ray luminosity
L_X~7X10^29 erg s^-1, which is comparable to the X-ray luminosity observed in
CTTSs of the same mass. Furthermore, the X-ray spectrum synthesized from the
simulation matches in detail all the main features of the O VIII and O VII
lines of the star MP Mus.Comment: Accepted for publication as a Letter in Astronomy & Astrophysic
A Deep Chandra X-ray Spectrum of the Accreting Young Star TW Hydrae
We present X-ray spectral analysis of the accreting young star TW Hydrae from
a 489 ks observation using the Chandra High Energy Transmission Grating. The
spectrum provides a rich set of diagnostics for electron temperature T_e,
electron density N_e, hydrogen column density N_H, relative elemental
abundances and velocities and reveals its source in 3 distinct regions of the
stellar atmosphere: the stellar corona, the accretion shock, and a very large
extended volume of warm postshock plasma. The presence of Mg XII, Si XIII, and
Si XIV emission lines in the spectrum requires coronal structures at ~10 MK.
Lower temperature lines (e.g., from O VIII, Ne IX, and Mg XI) formed at 2.5 MK
appear more consistent with emission from an accretion shock. He-like Ne IX
line ratio diagnostics indicate that T_e = 2.50 +/- 0.25 MK and N_e = 3.0 +/-
0.2 x 10^(12) cm^(-3) in the shock. These values agree well with standard
magnetic accretion models. However, the Chandra observations significantly
diverge from current model predictions for the postshock plasma. This gas is
expected to cool radiatively, producing O VII as it flows into an increasingly
dense stellar atmosphere. Surprisingly, O VII indicates N_e = 5.7
^(+4.4}_(-1.2) x 10^(11) cm^(-3), five times lower than N_e in the accretion
shock itself, and ~7 times lower than the model prediction. We estimate that
the postshock region producing O VII has roughly 300 times larger volume, and
30 times more emitting mass than the shock itself. Apparently, the shocked
plasma heats the surrounding stellar atmosphere to soft X-ray emitting
temperatures and supplies this material to nearby large magnetic structures --
which may be closed magnetic loops or open magnetic field leading to mass
outflow. (Abridged)Comment: 13 pages (emulateapj style), 10 figures, ApJ, in pres
Spectral Energy Distributions of T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison with ISO LWS Observations
We improve upon the radiative, hydrostatic equilibrium models of passive
circumstellar disks constructed by Chiang & Goldreich (1997). New features
include (1) account for a range of particle sizes, (2) employment of
laboratory-based optical constants of representative grain materials, and (3)
numerical solution of the equations of radiative and hydrostatic equilibrium
within the original 2-layer (disk surface + disk interior) approximation. We
explore how the spectral energy distribution (SED) of a face-on disk depends on
grain size distributions, disk geometries and surface densities, and stellar
photospheric temperatures. Observed SEDs of 3 Herbig Ae and 2 T Tauri stars,
including spectra from the Long Wavelength Spectrometer (LWS) aboard the
Infrared Space Observatory (ISO), are fitted with our models. Silicate emission
bands from optically thin, superheated disk surface layers appear in nearly all
systems. Water ice emission bands appear in LWS spectra of 2 of the coolest
stars. Infrared excesses in several sources are consistent with vertical
settling of photospheric grains. While this work furnishes further evidence
that passive reprocessing of starlight by flared disks adequately explains the
origin of infrared-to-millimeter wavelength excesses of young stars, we
emphasize how the SED alone does not provide sufficient information to
constrain particle sizes and disk masses uniquely.Comment: Accepted to ApJ, 35 pages inc. 14 figures, AAS preprin
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