180 research outputs found
Chemical tracers of episodic accretion in low-mass protostars
Aims: Accretion rates in low-mass protostars can be highly variable in time.
Each accretion burst is accompanied by a temporary increase in luminosity,
heating up the circumstellar envelope and altering the chemical composition of
the gas and dust. This paper aims to study such chemical effects and discusses
the feasibility of using molecular spectroscopy as a tracer of episodic
accretion rates and timescales.
Methods: We simulate a strong accretion burst in a diverse sample of 25
spherical envelope models by increasing the luminosity to 100 times the
observed value. Using a comprehensive gas-grain network, we follow the chemical
evolution during the burst and for up to 10^5 yr after the system returns to
quiescence. The resulting abundance profiles are fed into a line radiative
transfer code to simulate rotational spectra of C18O, HCO+, H13CO+, and N2H+ at
a series of time steps. We compare these spectra to observations taken from the
literature and to previously unpublished data of HCO+ and N2H+ 6-5 from the
Herschel Space Observatory.
Results: The bursts are strong enough to evaporate CO throughout the
envelope, which in turn enhances the abundance of HCO+ and reduces that of
N2H+. After the burst, it takes 10^3-10^4 yr for CO to refreeze and for HCO+
and N2H+ to return to normal. The chemical effects of the burst remain visible
in the rotational spectra for as long as 10^5 yr after the burst has ended,
highlighting the importance of considering luminosity variations when analyzing
molecular line observations in protostars. The spherical models are currently
not accurate enough to derive robust timescales from single-dish observations.
As follow-up work, we suggest that the models be calibrated against spatially
resolved observations in order to identify the best tracers to be used for
statistically significant source samples.Comment: Accepted by A&A; 12 pages, 7 figure
Radionuclide Ionization in Protoplanetary Disks: Calculations of Decay Product Radiative Transfer
We present simple analytic solutions for the ionization rate
arising from the decay of short-lived radionuclides (SLRs)
within protoplanetary disks. We solve the radiative transfer problem for the
decay products within the disk, and thereby allow for the loss of radiation at
low disk surface densities; energy loss becomes important outside
for typical disk masses M. Previous studies of
chemistry/physics in these disks have neglected the impact of ionization by
SLRs, and often consider only cosmic rays (CRs), because of the high CR-rate
present in the ISM. However, recent work suggests that the flux of CRs present
in the circumstellar environment could be substantially reduced by relatively
modest stellar winds, resulting in severely modulated CR ionization rates,
, equal to or substantially below that of SLRs
( s). We compute the net ionizing
particle fluxes and corresponding ionization rates as a function of position
within the disk for a variety of disk models. The resulting expressions are
especially simple for the case of vertically gaussian disks (frequently assumed
in the literature). Finally, we provide a power-law fit to the ionization rate
in the midplane as a function of gas disk surface density and time. Depending
on location in the disk, the ionization rates by SLRs are typically in the
range s.Comment: 7 pages, 4 figures, accepted to Ap
Molecule sublimation as a tracer of protostellar accretion: Evidence for accretion bursts from high angular resolution C18O images
The accretion histories of embedded protostars are an integral part of
descriptions of their physical and chemical evolution. In particular, are the
accretion rates smoothly declining from the earlier toward later stages or in
fact characterized by variations such as intermittent bursts? We aim to
characterize the impact of possible accretion variations in a sample of
embedded protostars by measuring the size of the inner regions of their
envelopes where CO is sublimated and relate those to their temperature profiles
dictated by their current luminosities. Using observations from the
Submillimeter Array we measure the extents of the emission from the C18O
isotopologue toward 16 deeply embedded protostars. We compare these
measurements to the predicted extent of the emission given the current
luminosities of the sources through dust and line radiative transfer
calculations. Eight out of sixteen sources show more extended C18O emission
than predicted by the models. The modeling shows that the likely culprit for
these signatures is sublimation due to increases in luminosities of the sources
by about a factor five or more during the recent 10,000 years - the time it
takes for CO to freeze-out again on dust grains. For four of those sources the
increase would have had to have been a factor 10 or more. The compact emission
seen toward the other half of the sample suggests that C18O only sublimates
when the temperature exceeds 30 K - as one would expect if CO is mixed with H2O
in the grain ice-mantles. The small-number statistics from this survey suggest
that protostars undergo significant bursts about once every 20,000 years. This
also illustrates the importance of taking the physical evolutionary histories
into account for descriptions of the chemical structures of embedded
protostars.Comment: Accepted by A&A; 11 pages, 5 figure
Hot water in the inner 100 AU of the Class 0 protostar NGC1333 IRAS2A
Evaporation of water ice above 100 K in the inner few 100 AU of low-mass
embedded protostars (the so-called hot core) should produce quiescent water
vapor abundances of ~10^-4 relative to H2. Observational evidence so far points
at abundances of only a few 10^-6. However, these values are based on spherical
models, which are known from interferometric studies to be inaccurate on the
relevant spatial scales. Are hot cores really that much drier than expected, or
are the low abundances an artifact of the inaccurate physical models? We
present deep velocity-resolved Herschel-HIFI spectra of the 3(12)-3(03) lines
of H2-16O and H2-18O (1097 GHz, Eup/k = 249 K) in the low-mass Class 0
protostar NGC1333 IRAS2A. A spherical radiative transfer model with a power-law
density profile is unable to reproduce both the HIFI data and existing
interferometric data on the H2-18O 3(13)-2(20) line (203 GHz, Eup/k = 204 K).
Instead, the HIFI spectra likely show optically thick emission from a hot core
with a radius of about 100 AU. The mass of the hot core is estimated from the
C18O J=9-8 and 10-9 lines. We derive a lower limit to the hot water abundance
of 2x10^-5, consistent with the theoretical predictions of ~10^-4. The revised
HDO/H2O abundance ratio is 1x10^-3, an order of magnitude lower than previously
estimated.Comment: Accepted by ApJ; 12 pages in emulateapj format; 7 figure
Transient Inactivation of the Medial Prefrontal Cortex Affects Both Anxiety and Decision-Making in Male Wistar Rats
In both humans and rats high levels of anxiety impair decision-making in the Iowa gambling task (IGT) in male subjects. Expression of the immediate early gene c-fos as marker of neural activity in rat studies indicated a role of the medial prefrontal cortex (prelimbic and infralimbic region; mPFC) in mediating the relationship between anxiety and decision-making. To delineate this relationship further and assess the underlying neurobiology in more detail, we inactivated in the present study the mPFC in male rats using a mixture of the GABA-receptor agonists muscimol and baclofen. Rats were exposed to the elevated plus maze (EPM) to measure effects on anxiety and to the rodent version of the IGT (r-IGT). Inactivation led to increased levels of anxiety on the EPM, while not affecting general activity. The effect in the r-IGT (trials 61–120) was dependent on levels of performance prior to inactivation (trial 41–60): inactivation of the mPFC hampered task performance in rats, which already showed a preference for the advantageous option, but not in rats which were still choosing in a random manner. These data suggest that the mPFC becomes more strongly involved as rats have learned task-contingencies, i.e., choose for the best long-term option. Furthermore they suggest, along with the data of our earlier study, that both anxiety and decision-making in rats are mediated through a neural circuitry including at least the mPFC. The data are discussed in relation to recent data of rodent studies on the neural circuitry underlying decision-making
Destruction of Refractory Carbon in Protoplanetary Disks
The Earth and other rocky bodies in the inner solar system contain
significantly less carbon than the primordial materials that seeded their
formation. These carbon-poor objects include the parent bodies of primitive
meteorites, suggesting that at least one process responsible for solid-phase
carbon depletion was active prior to the early stages of planet formation.
Potential mechanisms include the erosion of carbonaceous materials by photons
or atomic oxygen in the surface layers of the protoplanetary disk. Under
photochemically generated favorable conditions, these reactions can deplete the
near-surface abundance of carbon grains and polycyclic aromatic hydrocarbons by
several orders of magnitude on short timescales relative to the lifetime of the
disk out to radii of ~20-100+ au from the central star depending on the form of
refractory carbon present. Due to the reliance of destruction mechanisms on a
high influx of photons, the extent of refractory carbon depletion is quite
sensitive to the disk's internal radiation field. Dust transport within the
disk is required to affect the composition of the midplane. In our current
model of a passive, constant-alpha disk, where alpha = 0.01, carbon grains can
be turbulently lofted into the destructive surface layers and depleted out to
radii of ~3-10 au for 0.1-1 um grains. Smaller grains can be cleared out of the
planet-forming region completely. Destruction may be more effective in an
actively accreting disk or when considering individual grain trajectories in
non-idealized disks.Comment: 15 pages, 9 figures, Accepted for publication in Ap
Outflow forces of low mass embedded objects in Ophiuchus: a quantitative comparison of analysis methods
The outflow force of molecular bipolar outflows is a key parameter in
theories of young stellar feedback on their surroundings. The focus of many
outflow studies is the correlation between the outflow force, bolometric
luminosity and envelope mass. However, it is difficult to combine the results
of different studies in large evolutionary plots over many orders of magnitude
due to the range of data quality, analysis methods and corrections for
observational effects such as opacity and inclination. We aim to determine the
outflow force for a sample of low luminosity embedded sources. We will quantify
the influence of the analysis method and the assumptions entering the
calculation of the outflow force. We use the James Clerk Maxwell Telescope to
map 12CO J=3-2 over 2'x2' regions around 16 Class I sources of a well-defined
sample in Ophiuchus at 15" resolution. The outflow force is then calculated
using seven different methods differing e.g. in the use of intensity-weighted
emission and correction factors for inclination. The results from the analysis
methods differ from each other by up to a factor of 6, whereas observational
properties and choices in the analysis procedure affect the outflow force by up
to a factor of 4. For the sample of Class I objects, bipolar outflows are
detected around 13 sources including 5 new detections, where the three
non-detections are confused by nearby outflows from other sources. When
combining outflow forces from different studies, a scatter by up to a factor of
5 can be expected. Although the true outflow force remains unknown, the
separation method (separate calculation of dynamical time and momentum) is
least affected by the uncertain observational parameters. The correlations
between outflow force, bolometric luminosity and envelope mass are further
confirmed down to low luminosity sources.Comment: 24 pages, 13 figures, Accepted by A&
Photodissociation of interstellar N2
Molecular nitrogen is one of the key species in the chemistry of interstellar
clouds and protoplanetary disks and the partitioning of nitrogen between N and
N2 controls the formation of more complex prebiotic nitrogen-containing
species. The aim of this work is to gain a better understanding of the
interstellar N2 photodissociation processes based on recent detailed
theoretical and experimental work and to provide accurate rates for use in
chemical models.
We simulated the full high-resolution line-by-line absorption + dissociation
spectrum of N2 over the relevant 912-1000 \AA\ wavelength range, by using a
quantum-mechanical model which solves the coupled-channels Schr\"odinger
equation. The simulated N2 spectra were compared with the absorption spectra of
H2, H, CO, and dust to compute photodissociation rates in various radiation
fields and shielding functions. The effects of the new rates in interstellar
cloud models were illustrated for diffuse and translucent clouds, a dense
photon dominated region and a protoplanetary disk.Comment: Online database: http://home.strw.leidenuniv.nl/~ewine/phot
- …