5,908 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 pĂȘcher "Nemared" ont Ă©tĂ© Ă©tudiĂ©s en microparcelles pendant deux saisons de croissance. Le poids frais des pieds, le diamĂštre de la tige, la longueur des pieds et le poids frais des racines sont significativement plus faibles chez les pieds infestĂ©s par le nĂ©matode - qu'ils soient ou non colonisĂ©s par #G. mosseae Ă l'aide de mycorrhizes provoquent une diminution de la population finale du nĂ©matode et du nombre de nĂ©matodes par gramme de racine par rapport aux pieds infestĂ©s par #P. vulnus et non traitĂ©s Ă l'aide du champignon. La colonisation par les mycorrhizes n'est pas affectĂ©e par la prĂ©sence du nĂ©matode. Chez les pieds infestĂ©s par le nĂ©matode, le Cu est le seul Ă©lĂ©ment dĂ©ficitaire dĂ©tectĂ© par analyse foliaire, quoique des taux faibles de fer y aient Ă©tĂ© observĂ©s. Les taux les plus Ă©levĂ©s de Ma, Mg, Mn et Zn ont Ă©tĂ© dĂ©tectĂ©s chez les pieds infestĂ©s par #P. vulnus. Les pieds mycorrhizĂ©s recĂšlent les taux les plus Ă©levĂ©s de Cu et d'Al. #G. mosseae est bĂ©nĂ©fique pour la croissance des pĂȘchers "Nemared" mais ne leur confĂšre aucune protection contre #P. vulnus. (RĂ©sumĂ© d'auteur
Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk
Using the Hubble Space Telescope, the 4 m Blanco telescope at the Cerro
Tololo Inter-American Observatory, and the Spitzer Space Telescope, we have
performed deep imaging from 0.8 to 8 um of the southern subcluster in the
Chamaeleon I star-forming region. In these data, we have discovered an object,
Cha 110913-773444, whose colors and magnitudes are indicative of a very
low-mass brown dwarf with a circumstellar disk. In a near-infrared spectrum of
this source obtained with the Gemini Near-Infrared Spectrograph, the presence
of strong steam absorption confirms its late-type nature (>=M9.5) while the
shapes of the H- and K-band continua and the strengths of the Na I and K I
lines demonstrate that it is a young, pre-main-sequence object rather than a
field dwarf. A comparison of the bolometric luminosity of Cha 110913-773444 to
the luminosities predicted by the evolutionary models of Chabrier and Baraffe
and Burrows and coworkers indicates a mass of 8+7/-3 M_Jup, placing it fully
within the mass range observed for extrasolar planetary companions (M<=15
M_Jup). The spectral energy distribution of this object exhibits mid-infrared
excess emission at >5 um, which we have successfully modeled in terms of an
irradiated viscous accretion disk with M'<=10e-12 M_sun/year. Cha 110913-773444
is now the least massive brown dwarf observed to have a circumstellar disk, and
indeed is one of the least massive free-floating objects found to date. These
results demonstrate that the raw materials for planet formation exist around
free-floating planetary-mass bodies.Comment: 5 pages, accepted to Astrophysical Journal Letter
Short Gas Dissipation Timescales: Diskless Stars in Taurus and Chamaeleon I
We present an Advanced Camera for Surveys/ Solar Blind Channel
far-ultraviolet (FUV) study of \h2 gas in 12 weak T Tauri stars in nearby
star-forming regions. The sample consists of sources which have no evidence of
inner disk dust. Our new FUV spectra show that in addition to the dust, the gas
is depleted from the inner disk. This sample is combined with a larger FUV
sample of accretors and non-accretors with ages between 1 and 100 Myr, showing
that as early as 1--3 Myr, systems both with and without gas are found.
Possible mechanisms for depleting gas quickly include viscous evolution, planet
formation and photoevaporation by stellar radiation fields. Since these
mechanisms alone cannot account for the lack of gas at 1--3 Myr, it is likely
that the initial conditions (e.g. initial disk mass or core angular momentum)
contribute to the variety of disks observed at any age. We estimate the angular
momentum of a cloud needed for most of the mass to fall very close to the
central object and compare this to models of the expected distribution of
angular momenta. Up to 20% of cloud cores have low enough angular momenta to
form disks with the mass close to the star, which would then accrete quickly;
this percentage is similar to the fraction of diskless stars in the youngest
star forming regions. With our sample, we characterize the chromospheric
contribution to the FUV luminosity and find that saturates at
.Comment: 5 pages, 4 figures, Accepted to ApJ
A simplified protocol for detecting two systemic bait markers (Rhodamine B and iophenoxic acid) in small mammals
We developed a method of quantifying levels of fluorescence in the whiskers of wild stoats (Mustela erminea) using fluorescence microscopy and Axiovision 3.0.6.1 software. The method allows for discrimination between natural fluorescence present in or on a whisker, and the fluorescence resulting from the ingestion of the systemic marker Rhodamine B (RB), although some visual judgement is still required. We also developed a new high performance liquid chromatography (HPLC) protocol for detecting the systemic marker iophenoxic acid (IPA) in the blood of laboratory rats (Rattus norvegicus) and wild stoats. With this method, the blood of an animal that has consumed IPA can be tested for the presence of the foreign IPA compound itself. This is a more reliable test than the previous method, which measured the raised level of natural blood protein-bound iodine correlated with IPA absorption. The quantity of blood required from animal subjects is very small (10 ÎŒl), so the testing is less intrusive and the method can be extended to smaller species. The extraction technique uses methanol, rather than acids and heavy metal salts, thereby simplifying the procedure. Recovery of IPA is quantitative, giving a highly reliable reading. In experiments on captive rats the IPA method proved successful. Of 12 positively marked carcasses, two that had not been frozen for the 24 h before blood samples were taken showed relatively lower IPA levels. The same IPA detection method, as well as the whisker analysis for RB, was applied successfully to a population of wild stoats to which both Rhodamine B and IPA were made available at bait stations. The presence of both bait markers was detectable in rats for at least 21 days and in stoats for at least 27 days
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&
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