142 research outputs found
Saturn Forms by Core Accretion in 3.4 Myr
We present two new in situ core accretion simulations of Saturn with planet
formation timescales of 3.37 Myr (model S0) and 3.48 Myr (model S1), consistent
with observed protostellar disk lifetimes. In model S0, we assume rapid grain
settling reduces opacity due to grains from full interstellar values (Podolak
2003). In model S1, we do not invoke grain settling, instead assigning full
interstellar opacities to grains in the envelope. Surprisingly, the two models
produce nearly identical formation timescales and core/atmosphere mass ratios.
We therefore observe a new manifestation of core accretion theory: at large
heliocentric distances, the solid core growth rate (limited by Keplerian
orbital velocity) controls the planet formation timescale. We argue that this
paradigm should apply to Uranus and Neptune as well.Comment: 4 pages, including 1 figure, submitted to ApJ Letter
Detection of Formaldehyde Towards the Extreme Carbon Star IRC+10216
We report the detection of H2CO (formaldehyde) around the carbon-rich AGB
star, IRC+10216. We find a fractional abundance with respect to molecular
hydrogen of x(H2CO)= (1.3 {+1.5}{-0.8}) x 10^{-8}. This corresponds to a
formaldehyde abundance with respect to water vapor of x(H2CO)/x(H2O)=(1.1 +/-
0.2) x 10^{-2}, in line with the formaldehyde abundances found in Solar System
comets, and indicates that the putative extrasolar cometary system around
IRC+10216 may have a similar chemical composition to Solar System comets.
However, we also failed to detect CH3OH (methanol) around IRC+10216 and our
upper limit of x(CH3OH)/x(H2O) < 7.7 x 10^{-4}, (3 sigma), indicates that
methanol is substantially underabundant in IRC+10216, compared to Solar System
comets. We also conclude, based on offset observations, that formaldehyde has
an extended source in the envelope of IRC+10216 and may be produced by the
photodissociation of a parent molecule, similar to the production mechanism for
formaldehyde in Solar System comet comae. Preliminary mapping observations also
indicate a possible asymmetry in the spatial distribution of formaldehyde
around IRC+10216, but higher signal-to-noise observations are required to
confirm this finding. This study is based on observations carried out with the
IRAM 30m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and
IGN (Spain). (abridged)Comment: accepted to ApJ, 45 pages, 11 figure
Warm Molecular Layers in Protoplanetary Disks
We have investigated molecular distributions in protoplanetary disks,
adopting a disk model with a temperature gradient in the vertical direction.
The model produces sufficiently high abundances of gaseous CO and HCO+ to
account for line observations of T Tauri stars using a sticking probability of
unity and without assuming any non-thermal desorption. In regions of radius R >
10 AU, with which we are concerned, the temperature increases with increasing
height from the midplane. In a warm intermediate layer, there are significant
amounts of gaseous molecules owing to thermal desorption and efficient
shielding of ultraviolet radiation by the flared disk. The column densities of
HCN, CN, CS, H2CO, HNC and HCO+ obtained from our model are in good agreement
with the observations of DM Tau, but are smaller than those of LkCa15.
Molecular line profiles from our disk models are calculated using a
2-dimensional non-local-thermal-equilibrium (NLTE) molecular-line radiative
transfer code for a direct comparison with observations. Deuterated species are
included in our chemical model. The molecular D/H ratios in the model are in
reasonable agreement with those observed in protoplanetary disks.Comment: 11 pages, Latex (aa.cls), to be published in Astronomy and
Astrophysic
Dark cloud cores and gravitational decoupling from turbulent flows
We test the hypothesis that the starless cores may be gravitationally bound
clouds supported largely by thermal pressure by comparing observed molecular
line spectra to theoretical spectra produced by a simulation that includes
hydrodynamics, radiative cooling, variable molecular abundance, and radiative
transfer in a simple one-dimensional model. The results suggest that the
starless cores can be divided into two categories: stable starless cores that
are in approximate equilibrium and will not evolve to form protostars, and
unstable pre-stellar cores that are proceeding toward gravitational collapse
and the formation of protostars. The starless cores might be formed from the
interstellar medium as objects at the lower end of the inertial cascade of
interstellar turbulence. Additionally, we identify a thermal instability in the
starless cores. Under par ticular conditions of density and mass, a core may be
unstable to expansion if the density is just above the critical density for the
collisional coupling of the gas and dust so that as the core expands the
gas-dust coupling that cools the gas is reduced and the gas warms, further
driving the expansion.Comment: Submitted to Ap
Ice Lines, Planetesimal Composition and Solid Surface Density in the Solar Nebula
To date, there is no core accretion simulation that can successfully account
for the formation of Uranus or Neptune within the observed 2-3 Myr lifetimes of
protoplanetary disks. Since solid accretion rate is directly proportional to
the available planetesimal surface density, one way to speed up planet
formation is to take a full accounting of all the planetesimal-forming solids
present in the solar nebula. By combining a viscously evolving protostellar
disk with a kinetic model of ice formation, we calculate the solid surface
density in the solar nebula as a function of heliocentric distance and time. We
find three effects that strongly favor giant planet formation: (1) a decretion
flow that brings mass from the inner solar nebula to the giant planet-forming
region, (2) recent lab results (Collings et al. 2004) showing that the ammonia
and water ice lines should coincide, and (3) the presence of a substantial
amount of methane ice in the trans-Saturnian region. Our results show higher
solid surface densities than assumed in the core accretion models of Pollack et
al. (1996) by a factor of 3 to 4 throughout the trans-Saturnian region. We also
discuss the location of ice lines and their movement through the solar nebula,
and provide new constraints on the possible initial disk configurations from
gravitational stability arguments.Comment: Version 2: reflects lead author's name and affiliation change,
contains minor changes to text from version 1. 12 figures, 7 tables, accepted
for publication in Icaru
High Resolution 4.7 um Keck/NIRSPEC Spectra of Protostars. I: Ices and Infalling Gas in the Disk of L1489 IRS
We explore the infrared M band (4.7 um) spectrum of the class I protostar
L1489 IRS in the Taurus Molecular Cloud. This is the highest resolution wide
coverage spectrum at this wavelength of a low mass protostar observed to date
(R=25,000; Dv=12 km/s). Many narrow absorption lines of gas phase 12CO, 13CO,
and C18O are detected, as well as a prominent band of solid 12CO. The gas phase
12CO lines have red shifted absorption wings (up to 100 km/s), likely
originating from warm disk material falling toward the central object. The
isotopes and the 12CO line wings are successfully fitted with a contracting
disk model of this evolutionary transitional object (Hogerheijde 2001). This
shows that the inward motions seen in millimeter wave emission lines continue
to within ~0.1 AU from the star. The colder parts of the disk are traced by the
prominent CO ice band. The band profile results from CO in 'polar' ices (CO
mixed with H2O), and CO in 'apolar' ices. At the high spectral resolution, the
'apolar' component is, for the first time, resolved into two distinct
components, likely due to pure CO and CO mixed with CO2, O2 and/or N2. The ices
have probably experienced thermal processing in the upper disk layer traced by
our pencil absorption beam: much of the volatile 'apolar' ices has evaporated
and the depletion factor of CO onto grains is remarkably low (~7%). This study
shows that high spectral resolution 4.7 um observations provide important and
unique information on the dynamics and structure of protostellar disks and the
evolution of ices in these disks.Comment: 11 pages, 6 figures Scheduled to appear in ApJ 568 n2, 1 April 200
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