680 research outputs found
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
Forming Planetesimals by Gravitational Instability: II. How Dust Settles to its Marginally Stable State
Dust at the midplane of a circumstellar disk can become gravitationally
unstable and fragment into planetesimals if the local dust-to-gas density ratio
mu is sufficiently high. We simulate how dust settles in passive disks and ask
how high mu can become. We settle the dust using a 1D code and test for
dynamical stability using a 3D shearing box code. This scheme allows us to
explore the behavior of small particles having short but non-zero stopping
times in gas: 0 < t_stop << the orbital period. The streaming instability is
thereby filtered out. Dust settles until shearing instabilities in the edges of
the dust layer threaten to overturn the entire layer. In this state of marginal
stability, mu=2.9 for a disk whose bulk (height-integrated) metallicity is
solar. For a disk whose bulk metallicity is 4x solar, mu reaches 26.4. These
maximum values of mu, which depend on the background radial pressure gradient,
are so large that gravitational instability of small particles is viable in
disks whose bulk metallicities are just a few (<4) times solar. Earlier studies
assumed that dust settles until the Richardson number Ri is spatially constant.
Our simulations are free of this assumption but provide support for it within
the dust layer's edges, with the proviso that Ri increases with bulk
metallicity in the same way that we found in Paper I. Only modest enhancements
in bulk metallicity are needed to spawn planetesimals directly from small
particles.Comment: Accepted to Ap
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
The complete catalogue of gamma-ray bursts observed by the Wide Field Cameras on board BeppoSAX
We present the complete on-line catalogue of gamma-ray bursts observed by the
two Wide Field Cameras on board \sax in the period 1996-2002. Our aim is to
provide the community with the largest published data set of GRB's prompt
emission X-ray light curves and other useful data. This catalogue
(BS-GRBWFCcat) contains data on 77 bursts and a collection of the X-ray light
curves of 56 GRB discovered or noticed shortly after the event and of other
additional bursts detected in subsequent searches. Light curves are given in
the three X-ray energy bands (2-5, 5-10, 10-26 keV). The catalogue can be
accessed from the home web page of the ASI Science Data Center-ASDC
(http://www.asdc.asi.it)Comment: 4 pages, 3 figure
Impact of grain evolution on the chemical structure of protoplanetary disks
We study the impact of dust evolution in a protoplanetary disk around a T
Tauri star on the disk chemical composition. For the first time we utilize a
comprehensive model of dust evolution which includes growth, fragmentation and
sedimentation. Specific attention is paid to the influence of grain evolution
on the penetration of the UV field in the disk. A chemical model that includes
a comprehensive set of gas phase and grain surface chemical reactions is used
to simulate the chemical structure of the disk. The main effect of the grain
evolution on the disk chemical composition comes from sedimentation, and, to a
lesser degree, from the reduction of the total grain surface area. The net
effect of grain growth is suppressed by the fragmentation process which
maintains a population of small grains, dominating the total grain surface
area. We consider three models of dust properties. In model GS both growth and
sedimentation are taken into account. In models A5 and A4 all grains are
assumed to have the same size (10(-5) cm and 10(-4) cm, respectively) with
constant gas-to-dust mass ratio of 100. Like in previous studies, the
"three-layer" pattern (midplane, molecular layer, hot atmosphere) in the disk
chemical structure is preserved in all models, but shifted closer to the
midplane in models with increased grain size (GS and A4). Unlike other similar
studies, we find that in models GS and A4 column densities of most gas-phase
species are enhanced by 1-3 orders of magnitude relative to those in a model
with pristine dust (A5), while column densities of their surface counterparts
are decreased. We show that column densities of certain species, like C2H,
HC(2n+1)N (n=0-3), H2O and some other molecules, as well as the C2H2/HCN
abundance ratio which are accessible with Herschel and ALMA can be used as
observational tracers of early stages of the grain evolution process in
protoplanetary disks.Comment: 50 pages, 4 tables, 11 figures, accepted to the Ap
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
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
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