3,657 research outputs found
VLBA determination of the distance to nearby star-forming regions I. The distance to T Tauri with 0.4% accuracy
In this article, we present the results of a series of twelve 3.6-cm radio
continuum observations of T Tau Sb, one of the companions of the famous young
stellar object T Tauri. The data were collected roughly every two months
between September 2003 and July 2005 with the Very Long Baseline Array (VLBA).
Thanks to the remarkably accurate astrometry delivered by the VLBA, the
absolute position of T Tau Sb could be measured with a precision typically
better than about 100 micro-arcseconds at each of the twelve observed epochs.
The trajectory of T Tau Sb on the plane of the sky could, therefore, be traced
very precisely, and modeled as the superposition of the trigonometric parallax
of the source and an accelerated proper motion. The best fit yields a distance
to T Tau Sb of 147.6 +/- 0.6 pc. The observed positions of T Tau Sb are in good
agreement with recent infrared measurements, but seem to favor a somewhat
longer orbital period than that recently reported by Duchene et al. (2006) for
the T Tau Sa/T Tau Sb system.Comment: 24 pages, 3 pages, AASTEX format, accepted for publication in Ap
Sub-au imaging of water vapour clouds around four Asymptotic Giant Branch stars
We present MERLIN maps of the 22-GHz H2O masers around four low-mass
late-type stars (IK Tau U Ori, RT Vir and U Her), made with an angular
resolution of ~ 15 milliarcsec and a velocity resolution of 0.1 km s-1. The H2O
masers are found in thick expanding shells with inner radii ~ 6 to 16 au and
outer radii four times larger. The expansion velocity increases radially
through the H2O maser regions, with logarithmic velocity gradients of 0.5--0.9.
IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset
between the near and far sides of the shell, which suggests that the masers are
distributed in oblate spheroids inclined to the line of sight. U Ori and U Her
have elongated poorly-filled shells with indications that the masers at the
inner edge have been compressed by shocks; these stars also show OH maser
flares. MERLIN resolves individual maser clouds, which have diameters of 2 -- 4
au and filling factors of only ~ 0.01 with respect to the whole H2O maser
shells. The CSE velocity structure gives additional evidence the maser clouds
are density bounded. Masing clouds can be identified over a similar timescale
to their sound crossing time (~2 yr) but not longer. The sizes and observed
lifetimes of these clouds are an order of magnitude smaller than those around
red supergiants, similar to the ratio of low-mass:high-mass stellar masses and
sizes. This suggests that cloud size is determined by stellar properties, not
local physical phenomena in the wind.Comment: 21 pages, including 14 figures and 8 tables. Accepted for publication
in MNRA
Semi-inclusive charged-current neutrino-nucleus cross sections in the relativistic plane wave impulse approximation
Neutrino-nucleus quasielastic scattering is studied in the plane wave impulse
approximation for three nuclear models: the relativistic Fermi gas (RFG), the
independent-particle shell model (IPSM) and the natural orbitals (NO) model
with Lorentzian dependence of the excitation energy. A complete study of the
kinematics of the semi-inclusive process and the associated cross sections are
presented and discussed for 40 Ar and 12 C. Inclusive cross sections are also
obtained by integrating the semi-inclusive expressions over the outgoing
hadron. Results are consistent with previous studies restricted to the
inclusive channel. In particular, a comparison with the analytical results for
the RFG model is performed. Explicit expressions for the hadronic tensor and
the 10 semi-inclusive nuclear responses are given. Theoretical predictions are
compared with semi-inclusive experimental data from T2K experiment.Comment: 55 pages, 22 figures, submitted to Physical Review
Superscaling in the resonance region for neutrino-nucleus scattering: The SuSAv2-DCC model
In this work the SuSAv2 and dynamical coupled-channels (DCC) models have been
combined and tested in the inelastic regime for electron and neutrino reactions
on nuclei. The DCC model, an approach to study baryon resonances through
electron and neutrino induced meson production reactions, has been implemented
for the first time in the SuSAv2-inelastic model to analyze the resonance
region. Within this framework, we also present a novel description about other
inelasticities in the resonance region (SoftDIS). The outcomes of these
approaches are firstly benchmarked against (e,e') data on 12C. The description
is thus extended to the study of neutrino-nucleus inclusive cross sections on
12C and 40Ar and compared with data from the T2K, MicroBooNE, ArgoNEUT and
MINERvA experiments, thus covering a wide kinematical range.Comment: Submitted to Physical Review
Thermal Emission from HII Galaxies: Discovering the Youngest Systems
We studied the radio properties of very young massive regions of star
formation in HII galaxies, with the aim of detecting episodes of recent star
formation in an early phase of evolution where the first supernovae start to
appear. Our sample consists of 31 HII galaxies, characterized by strong
Hydrogen emission lines, for which low resolution VLA 3.5cm and 6cm
observations were obtained. The radio spectral energy distribution has a range
of behaviours; 1) there are galaxies where the SED is characterized by a
synchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies with
possible free-free absorption at long wavelengths. The latter SEDs were found
in a few galaxies and represent a signature of heavily embedded massive star
clusters closely related to the early stages of massive star formation. Based
on the comparison of the star formation rates determined from the recombination
lines and those determined from the radio emission we find that SFR(Ha) is on
average five times higher than SFR(1.4GHz). We confirm this tendency by
comparing the ratio between the observed flux at 20 cm and the expected one,
calculated based on the Ha star formation rates, both for the galaxies in our
sample and for normal ones. This analysis shows that this ratio is a factor of
2 smaller in our galaxies than in normal ones, indicating that they fall below
the FIR/radio correlation. These results suggest that the emission of these
galaxies is dominated by a recent and massive star formation event in which the
first supernovae (SN) just started to explode. We conclude that the systematic
lack of synchrotron emission in those systems with the largest equivalent width
of Hb can only be explained if those are young starbursts of less than 3.5Myr
of age.Comment: Accepted for publication in Ap
Differences among origins on nutritional and quality parameters of soybean meal
Soybean meal (SBM) is the main protein source in livestock feeds. United States (USA), Brazil (BRA), and Argentine (ARG) are the major SBM exporter countries. The nutritive value of SBM varies because genetics, environment, farming conditions, and processing of the beans influence strongly the content and availability of major nutrients. The present research was conducted to determine the influence of origin (USA, BRA and ARG) on nutritive value and protein quality of SBM
The LMT Galaxies' 3 mm Spectroscopic Survey: First Results
The molecular phase of the interstellar medium (ISM) in galaxies offers
fundamental insight for understanding star-formation processes and how stellar
feedback affects the nuclear activity of certain galaxies. We present here
Large Millimeter Telescope spectra obtained with the Redshift Search Receiver,
a spectrograph that cover simultaneously the 3 mm band from 74 to 111 GHz with
a spectral resolution of around 100 km/s. The observed galaxies that have been
detected previously in HCN, have different degrees of nuclear activity, one
normal galaxy (NGC 6946), the starburst prototype (M 82) and two ultraluminous
infrared galaxies (ULIRGs, IRAS 17208-0014 and Mrk 231). We plotted our data in
the HCO+/HCN vs. HCN/13CO diagnostic diagram finding that NGC 6946 and M 82 are
located close to other normal galaxies; and that both IRAS 17208-0014 and Mrk
231 are close to the position of the well known ULIRG Arp 220 reported by Snell
et al. (2011). We found that in Mrk 231 -- a galaxy with a well known active
galactic nucleus -- the HCO+/HCN ratio is similar to the ratio observed in
other normal galaxies.Comment: Proceedings to appear in "Massive Young Star Clusters Near and Far:
From the Milky Way to Reionization", 2013 Guillermo Haro Conference. Eds. Y.
D. Mayya, D. Rosa-Gonzalez, & E. Terlevich, INAOE and AMC. 5 pages, 1 figur
- …