231 research outputs found
Spatially resolved H_2 emission from a very low-mass star
Molecular outflows from very low-mass stars (VLMSs) and brown dwarfs have
been studied very little. So far, only a few CO outflows have been observed,
allowing us to map the immediate circumstellar environment. We present the
first spatially resolved H2 emission around IRS54 (YLW52), a ~0.1-0.2 Msun
Class I source. By means of VLT SINFONI K-band observations, we probed the H2
emission down to the first ~50 AU from the source. The molecular emission shows
a complex structure delineating a large outflow cavity and an asymmetric
molecular jet. Thanks to the detection of several H2 transitions, we are able
to estimate average values along the jet-like structure (from source position
to knot D) of Av~28 mag, T~2000-3000 K, and H2 column density N(H2)~1.7x10^17
cm^-2. This allows us to estimate a mass loss rate of ~2x10^-10 Msun/yr for the
warm H2 component . In addition, from the total flux of the Br Gamma line, we
infer an accretion luminosity and mass accretion rate of 0.64 Lsun and ~3x10^-7
Msun/yr, respectively. The outflow structure is similar to those found in
low-mass Class I and CTTS. However, the Lacc/Lbol ratio is very high (~80%),
and the mass accretion rate is about one order of magnitude higher when
compared to objects of roughly the same mass, pointing to the young nature of
the investigated source.Comment: accepted as a Letter in A&
POISSON project - III - Investigating the evolution of the mass accretion rate
As part of the POISSON project (Protostellar Optical-Infrared Spectral Survey
on NTT), we present the results of the analysis of low-resolution NIR spectra
0.9-2.4 um) of two samples of YSOs in Lupus and Serpens (52 and 17 objects),
with masses 0.1-2.0 Msun and ages from 10^5 to a few 10^7 yr. After determining
the accretion parameters of the Lup and Ser targets by analysing their HI
near-IR emission features, we added the results to those from previous regions
(investigated in POISSON with the same methodology). We obtained a final
catalogue (143 objects) of mass accretion rates (Macc) derived in a homogeneous
fashion and analysed how Macc correlates with M* and how it evolves in time. We
derived the accretion luminosity (Lacc) and Macc for Lup and Ser objects from
the Br_gamma line by using relevant empirical relationships from the literature
that connect HI line luminosity and Lacc. To minimise the biases and also for
self-consistency, we re-derived mass and age for each source using the same set
of evolutionary tracks. We observe a correlation MaccM*^2.2, similarly to what
has previously been observed in several star-forming clouds. The time variation
of Macc is roughly consistent with the expected evolution in viscous disks,
with an asymptotic decay that behaves as t^-1.6. However, Macc values are
characterised by a large scatter at similar ages and are on average higher than
the predictions of viscous models. Although part of the scattering may be
related to the employed empirical relationship and to uncertainties on the
single measurements, the general distribution and decay trend of the Macc
points are real. These findings might be indicative of a large variation in the
initial mass of the disks, of fairly different viscous laws among disks, of
varying accretion regimes, and of other mechanisms that add to the dissipation
of the disks, such as photo-evaporation.Comment: 18 pages, 10 figures, accepted by A&
A sub-arcsecond study of the hot molecular core in G023.01-00.41
(Abridged) METHODS: We performed SMA observations at 1.3 mm with both the
most extended and compact array configurations, providing sub-arcsecond and
high sensitivity maps of various molecular lines, including both hot-core and
outflow tracers. We also reconstruct the spectral energy distribution of the
region from millimeter to near infrared wavelengths, using the Herschel/Hi-GAL
maps, as well as archival data. RESULTS: From the spectral energy distribution,
we derive a bolometric luminosity of about 4x10^4 Lsun. Our interferometric
observations reveal that the distribution of dense gas and dust in the HMC is
significantly flattened and extends up to a radius of 8000 AU from the center
of radio continuum and maser emission in the region. The equatorial plane of
this HMC is strictly perpendicular to the elongation of the collimated bipolar
outflow, as imaged on scales of about 0.1-0.5 pc in the main CO isotopomers as
well as in the SiO(5-4) line. In the innermost HMC regions (ca. 1000 AU), the
velocity field traced by the CH3CN(12_K-11_K) line emission shows that
molecular gas is both expanding along the outflow direction following a
Hubble-law, and rotating about the outflow axis, in agreement with the (3-D)
velocity field traced by methanol masers. The velocity field associated with
rotation indicates a dynamical mass of 19 Msun at the center of the core. The
latter is likely to be concentrated in a single O9.5 ZAMS star, consistent with
the estimated bolometric luminosity of G023.01-00.41. The physical properties
of the CO(2-1) outflow emission, such as its momentum rate 6x10^-3 Msun km/s
/yr and its outflow rate 2x10^-4 Msun/yr, support our estimates of the
luminosity (and mass) of the embedded young stellar object.Comment: 24 pages, 11 figures, 6 tables, accepted by Astronomy & Astrophysic
POISSON project - I - Emission lines as accretion tracers in young stellar objects: results from observations of Chamaeleon I and II sources
We present the results of the analysis of LR optical-NIR spectra (0.6-2.4 um)
of a sample 47 YSOs in the ChaI and II star-forming clouds. These data are part
of the POISSON project (Protostellar Optical-Infrared Spectral Survey on NTT).
The aim is to determine the accretion luminosity (Lacc) and mass accretion rate
(Macc) of the sources through the analysis of the detected emission features.
We also aim at verifying the reliability and consistency of the existing
empirical relationships connecting emission line luminosity and Lacc. We employ
five tracers (OI-6300A, Ha, CaII-8542A, Pab, and Brg) to derive the accretion
luminosity. The tracers provide Lacc values showing different scatters when
plotted as a function of L*. The Brg seems to be the most reliable, because it
gives the minimum Lacc dispersion over the entire range of L*, whereas the
other tracers provide much more scattered Lacc values, which are not expected
for our homogeneous sample. The comparison between Lacc(Brg) and Lacc obtained
from the other tracers also shows systematic differences among the empirical
relationships. These may probably be ascribed to different excitation
mechanisms contributing to the line emission, which may vary between our sample
and those where the relationships were calibrated. Adopting the Lacc derived
from Brg, we find Lacc=0.1L*-1L* for all sources, and Macc of the order of
10^-7-10^-9 Msun/yr. The Macc derived in ChaI are proportional to M*^2, as
found in other low-mass star-forming regions. The discrepancies observed in the
case of Lacc(Brg) and Lacc(Pab) can be related to different intrinsic Pab/Brg,
ratios. The derived ratios show the existence of two different emission
modalities, one that agrees with predictions of both wind and accretion models,
the other suggesting optically thick emission from relatively small regions
(10^21-10^22 cm^-3) with gas at low temperatures (<4000K).Comment: 22 pages, 8 figures, accepted for publication in A&A; institute
affiliations and typos correcte
Exploring the dimming event of RW Aur A through multi-epoch VLT/X-Shooter spectroscopy
RW Aur A is a CTTS that has suddenly undergone three major dimming events
since 2010. We aim to understand the dimming properties, examine accretion
variability, and derive the physical properties of the inner disc traced by the
CO ro-vibrational emission at NIR wavelengths (2.3 mic).
We compared two epochs of X-Shooter observations, during and after the
dimming. We modelled the rarely detected CO bandhead emission in both epochs to
examine whether the inner disc properties had changed. The SED was used to
derive the extinction properties of the dimmed spectrum and compare the
infrared excess between the two epochs. Lines tracing accretion were used to
derive the mass accretion rate in both states. The CO originates from a region
with physical properties of T=3000 K, N=1x10 cm and
vsini=113 km/s. The extinction properties of the dimming layer were derived
with the effective optical depth ranging from teff 2.5-1.5 from the UV to the
NIR. The inferred mass accretion rate Macc is Msun/yr and Msun/yr after and during the dimming respectively. By fitting the
SED, additional emission is observed in the IR during the dimming event from
dust grains with temperatures of 500-700K. The physical conditions traced by
the CO are similar for both epochs, indicating that the inner gaseous disc
properties do not change during the dimming events. The extinction curve is
flatter than that of the ISM, and large grains of a few hundred microns are
thus required. When we correct for the observed extinction, Macc is constant in
the two epochs, suggesting that the accretion is stable and therefore does not
cause the dimming. The additional hot emission in the NIR is located at about
0.5 au from the star. The dimming events could be due to a dust-laden wind, a
severe puffing-up of the inner rim, or a perturbation caused by the recent
star-disc encounter.Comment: Accepted by Astronomy & Astrophysic
Augmented visualization by computer vision and chromatographic fingerprinting on comprehensive two-dimensional gas chromatographic patterns: Unraveling diagnostic signatures in food volatilome
AMBER/VLTI high spectral resolution observations of the Br emitting region in HD 98922. A compact disc wind launched from the inner disc region
We analyse the main physical parameters and the circumstellar environment of
the young Herbig Be star HD 98922. We present AMBER/VLTI high spectral
resolution (R =12000) interferometric observations across the Br line,
accompanied by UVES high-resolution spectroscopy and SINFONI-AO assisted
near-infrared integral field spectroscopic data. To interpret our observations,
we develop a magneto-centrifugally driven disc-wind model. Our analysis of the
UVES spectrum shows that HD 98922 is a young (~5x10^5 yr) Herbig Be star
(SpT=B9V), located at a distance of 440(+60-50) pc, with a mass accretion rate
of ~9+/-3x10^(-7) M_sun yr^(-1). SINFONI K-band AO-assisted imaging shows a
spatially resolved circumstellar disc-like region (~140 AU in diameter) with
asymmetric brightness distribution. Our AMBER/VLTI UT observations indicate
that the Br emitting region (radius ~0.31+/-0.04 AU) is smaller than
the continuum emitting region (inner dust radius ~0.7+/-0.2 AU), showing
significant non-zero V-shaped differential phases (i.e. non S-shaped, as
expected for a rotating disc). The value of the continuum-corrected pure
Br line visibility at the longest baseline (89 m) is ~0.8+/-0.1, i.e.
the Br emitting region is partially resolved. Our modelling suggests
that the observed Br line-emitting region mainly originates from a disc
wind with a half opening angle of 30deg, and with a mass-loss rate of ~2x10(-7)
M_sun yr^(-1). The observed V-shaped differential phases are reliably
reproduced by combining a simple asymmetric continuum disc model with our
Br disc-wind model. The Br emission of HD 98922 can be modelled
with a disc wind that is able to approximately reproduce all interferometric
observations if we assume that the intensity distribution of the dust continuum
disc is asymmetric.Comment: Accepted for publication on Astronomy \& Astrophysics. High
resolution figures published on the main journal (see Astronomy &
Astrophysics: Forthcoming) or at
www.researchgate.net/profile/Alessio_Caratti_o_Garatti/publication
A highly-collimated SiO jet in the HH212 protostellar outflow
We mapped the HH212 Class 0 outflow in SiO(2--1, 5--4) and continuum using
the PdBI in its extended configurations. The unprecedented angular resolution
(down to 0.34") allows accurate comparison with a new, deep H2 image obtained
at the VLT. The SiO emission is confined to a highly-collimated bipolar jet
(width 0.35") along the outflow axis. The jet can be traced down to within 500
AU of the protostar, in a region that is heavily obscured in H2 images. Where
both species are detected, SiO shows the same overall kinematics and structure
as H2, indicating that both molecules are tracing the same material. We find
that the high-velocity SiO gas near the protostar is not tracing a wide-angle
wind but is already confined to a flow inside a narrow cone of half-opening
angle < 6 deg.Comment: Astronomy and Astrophysics Letter, in pres
Relative yield of wheat in coexistence with concurrent plants as indicator of competitiveness.
One of the factors that limit the yield of wheat is the weed competition, being the competitive potential of plants affected by their morphophysiological characteristics. The objective of this study was to determine the competitiveness of wheat when in coexistence with Italian ryegrass (Lolium multiflorum) and radish (Raphanus sativus) competitive plants or with wheat as simulator competition plant. There were realized four experiments in the greenhouse of the Agricultural Department of Environmental Sciences of the UFSM, Campus of Frederico Westphalen ? RS, during July to September, 2012. The treatments were arranged in a replacement series, in the proportions of 100:00 (wheat monoculture), 75:25, 50:50, 25:75 and 100:00 (competitor plant monoculture) for wheat plants (cv. Fundacep Cristalino) and for Italian ryegrass, radish and wheat (cv. BRS Guamirim) as competitor plants. The wheat is more competitive than Italian ryegrass in initial coexistence, even in the smallest proportion of plants. The radish when intercropped with wheat demonstrates similar competitive potential for environmental resources to the crop. There is mutual beneficial effect when wheat cultivars Fundacep Cristalino and BRS Guamirim coexisted, and each cultivar seems to have mechanism to avoid competition
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