5,519 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
Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust
We construct detailed vertical structure models of irradiated accretion disks
around T Tauri stars with interstellar medium dust uniformly mixed with gas.
The dependence of the structure and emission properties on mass accretion rate,
viscosity parameter, and disk radius is explored using these models. The
theoretical spectral energy distributions (SEDs) and images for all
inclinations are compared with observations of the entire population of
Classical T Tauri stars (CTTS) and Class I objects in Taurus. In particular, we
find that the median near-infrared fluxes can be explained within the errors
with the most recent values for the median accretion rates for CTTS. We further
show that the majority of the Class I sources in Taurus cannot be Class II
sources viewed edge-on because they are too luminous and their colors would be
consistent with disks seen only in a narrow range of inclinations. Our models
appear to be too geometrically thick at large radii, as suggested by: (a)
larger far-infrared disk emission than in the typical SEDs of T Tauri stars;
(b) wider dark dust lanes in the model images than in the images of HH30 and HK
Tau/c; and (c) larger predicted number of stars extincted by edge-on disks than
consistent with current surveys. The large thickness of the model is a
consequence of the assumption that dust and gas are well-mixed, suggesting that
some degree of dust settling may be required to explain the observations.Comment: 41 pages, 13 figures, accepted in Ap
Disk evolution in the Ori OB1 association
We analyze multi-band photometry of a subsample of low mass stars in the
associations Ori OB1a and 1b discovered during the CIDA Orion Variability
Survey, which have ages of 7 - 10 Myr and 3 - 5 Myr, respectively. We obtained
UBVRcIc photometry at Mt. Hopkins for 6 Classical T Tauri stars (CTTS) and 26
Weak T Tauri stars (WTTS) in Ori OB1a, and for 21 CTTS and 2 WTTS in Ori OB1b.
We also obtained L band photometry for 14 CTTS at Mt. Hopkins, and 10um and
18um photometry with OSCIR at Gemini for 6 CTTS; of these, all 6 were detected
at 10um while only one was detected at 18um. We estimate mass accretion rates
from the excess luminosity at U, and find that they are consistent with
determinations for a number of other associations, with or without high mass
star formation. The observed decrease of mass accretion rate with age is
qualitatively consistent with predictions of viscous evolution of accretion
disks. We find an overall decrease of disk emission from Taurus to Ori OB1b to
Ori OB1a. This decrease implies that significant grain growth and settling
towards the midplane has taken place in the inner disks of Ori OB1. We compare
the SED of the star detected at both 10um and 18um with disk models for similar
stellar and accretion parameters. We find that the low <= 18 um fluxes of this
Ori OB1b star cannot be due to the smaller disk radius expected from viscous
evolution in the presence of the FUV radiation fields from the OB stars in the
association. Instead, we find that the disk of this star is essentially a flat
disk, with little if any flaring, indicating a a significant degree of dust
settling towards the midplane, as expected from dust evolution in
protoplanetary disks.Comment: 35 pages, 11 figures, to appear in the Astronomical Journal. Full
resolution figures in http://www.cida.ve/~briceno/publications
The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks
We show in this Letter that the spectral details of the FUV radiation fields
have a large impact on the chemistry of protoplanetary disks surrounding T
Tauri stars. We show that the strength of a realistic stellar FUV field is
significantly lower than typically assumed in chemical calculations and that
the radiation field is dominated by strong line emission, most notably Lyman
alpha radiation. The effects of the strong Lyman alpha emission on the chemical
equilibrium in protoplanetary disks has previously been unrecognized. We
discuss the impact of this radiation on molecular observations in the context
of a radiative transfer model that includes both direct attenuation and
scattering. In particular, Lyman alpha radiation will directly dissociate water
vapor and may contribute to the observed enhancements of CN/HCN in disks.Comment: 14 pages, 4 figures, accepted by ApJ Letter
True and apparent scaling: the proximity of the markov- switching multifractal model to long-range dependence
In this paper, we consider daily financial data of a collection of different stock market indices, exchange rates, and interest rates, and we analyze their multi-scaling properties by estimating a simple specification of the Markov-switching multifractal model (MSM). In order to see how well the estimated models capture the temporal dependence of the data, we estimate and compare the scaling exponents H(q) (for q = 1, 2) for both empirical data and simulated data of the estimated MSM models. In most cases the multifractal model appears to generate âapparentâ long memory in agreement with the empirical scaling laws
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
Probing the embedded YSOs of the R CrA region through VLT-ISAAC spectroscopy
Near IR spectra obtained with ISAAC at VLT, have been used to pose
constraints on the evolutionary state and accretion properties of a sample of
five embedded YSOs located in the R CrA core. This sample includes three Class
I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6
and IRS3). Absorption lines have been detected in the medium resolution spectra
of all the observed targets, together with emission lines likely originating in
the disk-star-wind connected regions. We derived spectral types, veiling and
stellar luminosity of the five observed sources, which in turn have been used
to infer their mass and age adopting pre-main sequence evolutionary tracks. We
find that in HH100 IR and IRS2 most of the bolometric luminosity is due to
accretion, while the other three investigated sources, including the Class I
object IRS5a, present a low accretion activity (L_{acc}/L_{bol} < 0.2). We
observe a general correlation between the accretion luminosity, the IR veiling
and the emission line activity of the sources. A correlation between the
accretion activity and the spectral energy distribution slope is recognizable
but with the notable exception of IRS5a. Our analysis therefore shows how the
definition of the evolutionary stage of deeply embedded YSOs by means of IR
colors needs to be more carefully refined.Comment: 17 pages, 13 figures, accepted on A&
Probing the Dust and Gas in the Transitional Disk of CS Cha with Spitzer
Here we present the Spitzer IRS spectrum of CS Cha, a member of the ~2 Myr
old Chamaeleon star-forming region, which reveals an optically thick
circumstellar disk truncated at ~43 AU, the largest hole modeled in a
transitional disk to date. Within this inner hole, ~5x10^-5 lunar masses of
dust are located in a small optically thin inner region which extends from 0.1
to 1 AU. In addition, the disk of CS Cha has bigger grain sizes and more
settling than the previously modeled transitional disks DM Tau, GM Aur, and
CoKu Tau/4, suggesting that CS Cha is in a more advanced state of dust
evolution. The Spitzer IRS spectrum also shows [Ne II] 12.81 micron
fine-structure emission with a luminosity of 1.3x10^29 ergs s^-1, indicating
that optically thin gas is present in this ~43 AU hole, in agreement with
H_alpha measurements and a UV excess which indicate that CS Cha is still
accreting 1.2x10^-8 M_sun yr^-1. We do not find a correlation of the [Ne II]
flux with L_X, however, there is a possible correlation with mass accretion
rate, which if confirmed would suggest that EUV fluxes due to accretion are the
main agent for formation of the [Ne II] line.Comment: accepted to ApJ Letter
Spitzer observations of the Orion OB1 association: disk census in the low mass stars
We present new Spitzer Space Telescope observations of two fields in the
Orion OB1 association. We report here IRAC/MIPS observations for 115 confirmed
members and 41 photometric candidates of the ~10 Myr 25 Orionis aggregate in
the OB1a subassociation, and 106 confirmed members and 65 photometric
candidates of the 5 Myr region located in the OB1b subassociation. The 25
Orionis aggregate shows a disk frequency of 6% while the field in the OB1b
subassociation shows a disk frequency of 13%. Combining IRAC, MIPS and 2MASS
photometry we place stars bearing disks in several classes: stars with
optically thick disks (class II systems), stars with an inner transitional
disks (transitional disk candidates) and stars with "evolved disks"; the last
exhibit smaller IRAC/MIPS excesses than class II systems. In all, we identify 1
transitional disk candidate in the 25 Orionis aggregate and 3 in the OB1b
field; this represents ~10% of the disk bearing stars, indicating that the
transitional disk phase can be relatively fast. We find that the frequency of
disks is a function of the stellar mass, suggesting a maximum around stars with
spectral type M0. Comparing the infrared excess in the IRAC bands among several
stellar groups we find that inner disk emission decays with stellar age,
showing a correlation with the respective disk frequencies. The disk emission
at the IRAC and MIPS bands in several stellar groups indicates that disk
dissipation takes place faster in the inner region of the disks. Comparison
with models of irradiated accretion disks, computed with several degrees of
settling, suggests that the decrease in the overall accretion rate observed in
young stellar groups is not sufficient to explain the weak disk emission
observed in the IRAC bands for disk bearing stars with ages 5 Myr or older.Comment: Accepted in the Astrophysical Journa
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