11,889 research outputs found
Solving the excitation and chemical abundances in shocks: the case of HH1
We present deep spectroscopic (3600 - 24700 A) X-shooter observations of the
bright Herbig-Haro object HH1, one of the best laboratories to study the
chemical and physical modifications caused by protostellar shocks on the natal
cloud. We observe atomic fine structure lines, HI, and He, recombination lines
and H_2, ro-vibrational lines (more than 500 detections in total). Line
emission was analyzed by means of Non Local Thermal Equilibiurm codes to derive
the electron temperature and density, and, for the first time, we are able to
accurately probe different physical regimes behind a dissociative shock. We
find a temperature stratification in the range 4000 - 80000 K, and a
significant correlation between temperature and ionization energy. Two density
regimes are identified for the ionized gas, a more tenuous, spatially broad
component (density about 10^3 cm^-3), and a more compact component (density >
10^5 cm^-3) likely associated with the hottest gas. A further neutral component
is also evidenced, having temperature lass than 10000 K and density > 10^4
cm^-3. The gas fractional ionization was estimated solving the ionization
equilibrium equations of atoms detected in different ionization stages. We find
that neutral and fully ionized regions co-exist inside the shock. Also,
indications in favor of at least partially dissociative shock as the main
mechanism for molecular excitation are derived. Chemical abundances are
estimated for the majority of the detected species. On average, abundances of
non-refractory/refractory elements are lower than solar of about 0.15/0.5 dex.
This testifies the presence of dust inside the medium, with a depletion factor
of Iron of about 40%.Comment: Accepted by The Astrophysical Journa
Towards a better classification of unclear eruptive variables: the cases of V2492 Cyg, V350 Cep, and ASASSN-15qi
Eruptive variables are young stars that show episodic variations of
brightness: EXors/FUors variations are commonly associated with enhanced
accretion outbursts occurring at intermittent cadence of months/years (EXors)
and decades/centuries (FUors). Variations that can be ascribed to a variable
extinction along their line of sight are instead classified as UXors. We aim at
investigating the long-term photometric behaviour of three sources classified
as eruptive variables. We present data from the archival plates of the Asiago
Observatory relative to the fields where the targets are located. For the sake
of completeness we have also analysed the Harvard plates of the same regions
that cover a much longer historical period, albeit at a lower sensitivity,
however we are only able to provide upper limits. A total of 273 Asiago plates
were investigated, providing a total of more than 200 magnitudes for the three
stars, which cover a period of about 34 yr between 1958 and 1991. We have
compared our data with more recently collected literature data. Our plates
analysis of V2492 Cyg provides historical upper limits that seem not to be
compatible with the level of the activity monitored during the last decade.
Therefore, recently observed accretion phenomena could be associated with the
outbursting episodes, more than repetitive obscuration. While a pure extinction
does not seem the only mechanism responsible for the ASASSN-15qi fluctuations,
it can account quite reasonably for the recent V350 Cep variations.Comment: 12 pages, accepted by A&
On the rapidity dependence of the average transverse momentum in hadronic collisions
The energy and rapidity dependence of the average transverse momentum
in and collisions at RHIC and LHC energies are
estimated using the Colour Glass Condensate (CGC) formalism. We update previous
predictions for the - spectra using the hybrid formalism of the CGC
approach and two phenomenological models for the dipole - target scattering
amplitude. We demonstrate that these models are able to describe the RHIC and
LHC data for the hadron production in , and collisions at GeV. Moreover, we present our predictions for and
demonstrate that the ratio decreases with the rapidity and has a behaviour similar to that
predicted by hydrodynamical calculations.Comment: 11 pages, 7 figures; revised version: new results for the average
transverse momentum at partonic level added in fig. 4; Results and Discussion
section has been improved and enlarge
On the binarity of the classical Cepheid X Sgr from interferometric observations
Optical-infrared interferometry can provide direct geometrical measurements
of the radii of Cepheids and/or reveal unknown binary companions of these
stars. Such information is of great importance for a proper calibration of
Period-Luminosity relations and for determining binary fraction among Cepheids.
We observed the Cepheid X Sgr with VLTI/AMBER in order to confirm or disprove
the presence of the hypothesized binary companion and to directly measure the
mean stellar radius, possibly detecting its variation along the pulsation
cycle. From AMBER observations in MR mode we performed a binary model fitting
on the closure phase and a limb-darkened model fitting on the visibility. Our
analysis indicates the presence of a point-like companion at a separation of
10.7 mas and 5.6 magK fainter than the primary, whose flux and position are
sharply constrained by the data. The radius pulsation is not detected, whereas
the average limb-darkened diameter results to be 1.48+/-0.08 mas, corresponding
to 53+/-3 R_sun at a distance of 333.3 pc.Comment: 5 pages, 3 figures, research not
Testing the running coupling -factorization formula for the inclusive gluon production
The inclusive gluon production at midrapidities is described in the Color
Glass Condensate formalism using the - factorization formula, which was
derived at fixed coupling constant considering the scattering of a dilute
system of partons with a dense one. Recent analysis demonstrated that this
approach provides a satisfactory description of the experimental data for the
inclusive hadron production in collisions. However, these studies
are based on the fixed coupling - factorization formula, which does not
take into account the running coupling corrections, which are important to set
the scales present in the cross section. In this paper we consider the running
coupling corrected - factorization formula conjectured some years ago and
investigate the impact of the running coupling corrections on the observables.
In particular, the pseudorapidity distributions and charged hadrons
multiplicity are calculated considering , and
collisions at RHIC and LHC energies. We compare the corrected running coupling
predictions with those obtained using the original - factorization
assuming a fixed coupling or a prescription for the inclusion of the running of
the coupling. Considering the Kharzeev - Levin - Nardi unintegrated gluon
distribution and a simplified model for the nuclear geometry, we demonstrate
that the distinct predictions are similar for the pseudorapidity distributions
in collisions and for the charged hadrons multiplicity in
collisions. On the other hand, the running coupling corrected -
factorization formula predicts a smoother energy dependence for in
collisions.Comment: 9 pages and 4 figure
The 2016-2017 peak luminosity of the pre-main sequence variable V2492 Cyg
V2492 Cyg is a young pre-main sequence star presenting repetitive brightness
variations of significant amplitude (Delta R > 5 mag) whose physical origin has
been ascribed to both extinction (UXor-type) and accretion (EXor-type)
variability, although their mutual proportion has not been clarified yet.
Recently, V2492 Cyg has reached a level of brightness ever registered in the
period of its documented activity. Optical and near-infrared photometry and
spectroscopy have been obtained in October 2016 and between March and July
2017. The source has remained bright until the end of May 2017, then it started
to rapidly fade since the beginning of June at a rate of about 0.08 mag/day. On
mid-July 2017 the source has reached the same low-brightness level as two years
before. Extinction and mass accretion rate were derived by means of the
luminosity of the brightest lines, in particular Halpha and Hbeta. A couple of
optical high-resolution spectra are also presented to derive information on the
gas kinematics. Visual extinction variations do not exceed a few magnitudes,
while the mass accretion rate is estimated to vary from less than 10^-8 up to a
few 10^-7 M_sun/yr. This latter is comparable to that estimated on the previous
high-state in 2010, likely occurred under more severe extinction conditions.
The combined analysis of the optical and near-infrared (NIR) observations
extends to the present event the original suggestion that the V2492 Cyg
variability is a combination of changing extinction and accretion.Comment: Accepted by A&
The effective tax rates in the EU Commission Study on corporate taxation: methodological aspects, main results and policy implications
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