4,437 research outputs found
Protostellar half-life: new methodology and estimates
(Abridged) Protostellar systems evolve from prestellar cores, through the
deeply embedded stage and then disk-dominated stage, before they end up on the
main sequence. Knowing how much time a system spends in each stage is crucial
for understanding how stars and associated planetary systems form, because a
key constraint is the time available to form such systems. Equally important is
understanding what the spread in these time scales is. The most commonly used
method for inferring protostellar ages is to assume the lifetime of one
evolutionary stage, and then scale this to the relative number of protostars in
the other stages, i.e., assuming steady state. This method does not account for
the underlying age distribution and apparent stochasticity of star formation,
nor that relative populations are not in steady state. To overcome this, we
propose a new scheme where the lifetime of each protostellar stage follows a
distribution based on the formalism of sequential nuclear decay. The main
assumptions are: Class 0 sources follow a straight path to Class III sources,
the age distribution follows a binomial distribution, and the star-formation
rate is constant. The results are that the half-life of Class 0, Class I, and
Flat sources are (2.4+/-0.2)%, (4.4+/-0.3)%, and (4.3+/-0.4)% of the Class II
half-life, respectively, which translates to 47+/-4, 88+/-7, and 87+/-8 kyr,
respectively, for a Class II half-life of 2 Myr for protostars in the Gould
Belt clouds with more than 100 protostars. The mean age of these clouds is
1.2+/-0.1 Myr, and the star formation rate is (8.3+/-0.5)x10^-4 Msun/yr. The
critical parameters in arriving at these numbers are the assumed half-life of
the Class II stage, and the assumption that the star-formation rate and
half-lives are constant. This method presents a first step in moving from
steady-state to non-steady-state solutions of protostellar populations.Comment: Accepted for publication in A&
Origin of the wide-angle hot H2 in DG Tauri: New insight from SINFONI spectro-imaging
We wish to test the origins proposed for the extended hot H2 at 2000K around
the atomic jet from the T Tauri star DGTau, in order to constrain the
wide-angle wind structure and the possible presence of an MHD disk wind. We
present flux calibrated IFS observations in H2 1-0 S(1) obtained with
SINFONI/VLT. Thanks to spatial deconvolution by the PSF and to accurate
correction for uneven slit illumination, we performed a thorough analysis and
modeled the morphology, kinematics, and surface brightness. We also compared
our results with studies in [FeII], [OI], and FUV-pumped H2. The
limb-brightened H2 emission in the blue lobe is strikingly similar to
FUV-pumped H2 imaged 6yr later, confirming that they trace the same hot gas and
setting an upper limit of 12km/s on any expansion proper motion. The wide-angle
H2 rims are at lower blueshifts than probed by narrow long-slit spectra. We
confirm that they extend to larger angle and to lower speed the onion-like
velocity structure observed in optical atomic lines. The latter is shown to be
steady over more/equal than 4yr but undetected in [FeII] by SINFONI, probably
due to strong iron depletion. The H2 rim thickness less/equal than 14AU rules
out excitation by C-shocks, and J-shock speeds are constrained to 10km/s. We
find that explaining the H2 wide-angle emission with a shocked layer requires
either a recent outburst (15yr) into a pre-existing ambient outflow or an
excessive wind mass flux. A slow photoevaporative wind from the dense
irradiated disk surface and an MHD disk wind heated by ambipolar diffusion seem
to be more promising and need to be modeled in more detail
Water emission tracing active star formation from the Milky Way to high- galaxies
(Abridged) The question of how most stars in the Universe form remains open.
While star formation predominantly occurs in young massive clusters, the
current framework focuses on isolated star formation. One way to access the
bulk of protostellar activity within star-forming clusters is to trace
signposts of active star formation with emission from molecular outflows. These
outflows are bright in water emission, providing a direct observational link
between nearby and distant galaxies. We propose to utilize the knowledge of
local star formation as seen with molecular tracers to explore the nature of
star formation in the Universe. We present a large-scale statistical galactic
model of emission from galactic active star-forming regions. Our model is built
on observations of well-resolved nearby clusters. By simulating emission from
molecular outflows, which is known to scale with mass, we create a proxy that
can be used to predict the emission from clustered star formation at galactic
scales. We evaluated the impact of the most important global-star formation
parameters (i.e., initial stellar mass function (IMF), molecular cloud mass
distribution, star formation efficiency (SFE), and free-fall time efficiency)
on simulation results. We observe that for emission from the para-H2O 202 - 111
line, the IMF and molecular cloud mass distribution have a negligible impact on
the emission, both locally and globally, whereas the opposite holds for the SFE
and free-fall time efficiency. Moreover, this water transition proves to be a
low-contrast tracer of star formation. The fine-tuning of the model and
adaptation to morphologies of distant galaxies should result in realistic
predictions of observed molecular emission and make the galaxy-in-a-box model a
tool to analyze and better understand star formation throughout cosmological
times.Comment: Accepted for publication in A&A. 16 pages, 13 figure
Far infrared CO and HO emission in intermediate-mass protostars
Intermediate-mass young stellar objects (YSOs) provide a link to understand
how feedback from shocks and UV radiation scales from low to high-mass star
forming regions. Aims: Our aim is to analyze excitation of CO and HO in
deeply-embedded intermediate-mass YSOs and compare with low-mass and high-mass
YSOs. Methods: Herschel/PACS spectral maps are analyzed for 6 YSOs with
bolometric luminosities of . The maps
cover spatial scales of AU in several CO and HO lines located
in the m range. Results: Rotational diagrams of CO show two
temperature components at K and
K, comparable to low- and high-mass protostars
probed at similar spatial scales. The diagrams for HO show a single
component at K, as seen in low-mass protostars, and
about K lower than in high-mass protostars. Since the uncertainties in
are of the same order as the difference between the
intermediate and high-mass protostars, we cannot conclude whether the change in
rotational temperature occurs at a specific luminosity, or whether the change
is more gradual from low- to high-mass YSOs. Conclusions: Molecular excitation
in intermediate-mass protostars is comparable to the central AU of
low-mass protostars and consistent within the uncertainties with the high-mass
protostars probed at AU scales, suggesting similar shock
conditions in all those sources.Comment: Accepted to Astronomy & Astrophysics. 4 pages, 5 figures, 3 table
ALMA CO J=6-5 observations of IRAS16293-2422: Shocks and entrainment
Observations of higher-excited transitions of abundant molecules such as CO
are important for determining where energy in the form of shocks is fed back
into the parental envelope of forming stars. The nearby prototypical and
protobinary low-mass hot core, IRAS16293-2422 (I16293) is ideal for such a
study. The source was targeted with ALMA for science verification purposes in
band 9, which includes CO J=6-5 (E_up/k_B ~ 116 K), at an unprecedented spatial
resolution (~0.2", 25 AU). I16293 itself is composed of two sources, A and B,
with a projected distance of 5". CO J=6-5 emission is detected throughout the
region, particularly in small, arcsecond-sized hotspots, where the outflow
interacts with the envelope. The observations only recover a fraction of the
emission in the line wings when compared to data from single-dish telescopes,
with a higher fraction of emission recovered at higher velocities. The very
high angular resolution of these new data reveal that a bow shock from source A
coincides, in the plane of the sky, with the position of source B. Source B, on
the other hand, does not show current outflow activity. In this region, outflow
entrainment takes place over large spatial scales, >~ 100 AU, and in small
discrete knots. This unique dataset shows that the combination of a
high-temperature tracer (e.g., CO J=6-5) and very high angular resolution
observations is crucial for interpreting the structure of the warm inner
environment of low-mass protostars.Comment: Accepted for publication in A&A Letter
Star-formation-rate estimates from water emission
(Abridged) The star-formation rate (SFR) quantitatively describes the
star-formation process in galaxies. Current ways to calibrate this rate do not
usually employ observational methods accounting for the low-mass end of stellar
populations as their signatures are too weak. Accessing the bulk of
protostellar activity within galactic star-forming regions can be achieved by
tracing signposts of ongoing star formation. One such signpost is molecular
outflows, which are bright in molecular emission. We propose to utilize the
protostellar outflow emission as a tracer of the SFR. In this work, we
introduce a novel version of the galaxy-in-a-box model, which can be used to
relate molecular emission from star formation in galaxies with the SFR. We
measured the predicted para-H2O emission at 988 GHz and corresponding SFRs for
galaxies with LFIR = - L in a distance-independent
manner, and compared them with expectations from observations. We evaluated the
derived results by varying the star formation efficiency, the free-fall time
scaling factor, and the initial mass function. For the chosen H2O transition,
relying on the current Galactic observations and star formation properties, we
are underestimating the total galactic emission, while overestimating the SFRs,
particularly for more starburst-like configurations. The current version of the
galaxy-in-a-box model accounts for a limited number of processes and
configurations, that is, it focuses on ongoing star formation in massive young
clusters in a spiral galaxy. Therefore, the inferred results, which
underestimate the emission and overestimate the SFR, are not surprising: known
sources of emission are not included in the model. To improve the results, the
next version of the model needs to include a more detailed treatment of the
entire galactic ecosystem and other processes that would contribute to the
emission.Comment: Accepted for publication in A&A. 11 pages, 6 figure
Catch Crops in Organic Farming Systems without Livestock Husbandry - Model Simulations
During the last years, an increasing number of stockless farms in Europe converted to organic farming practice without re-establishing a livestock. Due to the lack of animal manure as a nutrient input, the relocation and the external input of nutrients is limited in those organic cropping systems. The introduction of a one-year green manure fallow in a 4-year crop rotation, including clover-grass mixtures as a green manure crop is the classical strategy to solve at least some of the problems related to the missing livestock. The development of new crop rotations, including an extended use of catch crops and annual green manure (legumes) may be another possibility avoiding the economical loss during the fallow year.
Modelling of the C and N turnover in the soil-plant-atmosphere system using the soil-plant-atmosphere model DAISY is one of the tools used for the development of new organic crop rotations. In this paper, we will present simulations based on a field experiment with incorporation of different catch crops.
An important factor for the development of new crop rotations for stockless organic farming systems is the expected N mineralisation and immobilisation after incorporation of the plant materials. Therefore, special emphasise will be put on the simulation of N-mineralisation/-immobilisation and of soil microbial biomass N. Furthermore, particulate organic matter C and N as an indicator of remaining plant material under decomposition will be investigated
- …