161 research outputs found
MHD simulations of radiative jets from young stellar objects: Halpha emission
We study the H emission from jets using two-dimensional
axisymmetrical simulations. We compare the emission obtained from hydrodynamic
(HD) simulations with that obtained from magnetohydrodynamics (MHD)
simulations. The magnetic field is supposed to be present in the jet only, and
with a toroidal configuration. The simulations have time-dependent ejection
velocities and different intensities for the initial magnetic field. The
results show an increase in the H emission along the jet for the
magnetized cases with respect to the HD case. The increase in the emission is
due to a better collimation of the jet in the MHD case, and to a small increase
in the shock velocity. These results could have important implications for the
interpretation of the observations of jets from young stellar objects.Comment: 7 pages, 4 figures; accepted for publication by A&
Atmospheric mass loss by stellar wind from planets around main sequence M stars
We present an analytic model for the interaction between planetary
atmospheres and stellar winds from main sequence M stars, with the purpose of
obtaining a quick test-model that estimates the timescale for total atmospheric
mass loss due to this interaction. Planets in the habitable zone of M dwarfs
may be tidally locked and may have weak magnetic fields, because of this we
consider the extreme case of planets with no magnetic field. The model gives
the planetary atmosphere mass loss rate as a function of the stellar wind and
planetary properties (mass, atmospheric pressure and orbital distance) and an
entrainment efficiency coefficient . We use a mixing layer model to
explore two different cases: a time-independent stellar mass loss and a stellar
mass loss rate that decreases with time. For both cases we consider planetary
masses within the range of M and atmospheric pressures with
values of 1, 5 and 10 atm. For the time dependent case, planets without
magnetic field in the habitable zone of M dwarfs with initial stellar mass
losses of M yr, may retain their
atmospheres for at least 1 Gyr. This case may be applied to early spectral type
M dwarfs (earlier than M5). Studies have shown that late type M dwarfs (later
than M5) may be active for long periods of time (Gyr), and because of
that our model with constant stellar mass loss rate may be more accurate. For
these stars most planets may have lost their atmospheres in 1 Gyr or less
because most of the late type M dwarfs are expected to be active. We emphasize
that our model only considers planets without magnetic fields. Clearly we must
expect a higher resistance to atmospheric erosion if we include the presence of
a magnetic field.Comment: Icarus, submitted. 18 pages, 6 figure
The dynamics of internal working surfaces in MHD jets
The dynamical effects of magnetic fields in models of radiative, Herbig-Haro
(HH) jets have been studied in a number of papers. For example, magnetized,
radiative jets from variable sources have been studied with axisymmetric and 3D
numerical simulations. In this paper, we present an analytic model describing
the effect of a toroidal magnetic field on the internal working surfaces that
result from a variability in the ejection velocity. We find that for parameters
appropriate for HH jets the forces associated with the magnetic field dominate
over the gas pressure force within the working surfaces. Depending on the ram
pressure radial cross section of the jet, the magnetic field can produce a
strong axial pinch, or, alternatively, a broadening of the internal working
surfaces. We check the validity of the analytic model with axisymmetric
numerical simulations of variable, magnetized jets.Comment: 14 pages, 4 figures. ApJ in pres
Relation between source and temperature fluctuations in photoionized nebulae
The magnitude of the temperature fluctuations (t^2) required to explain the
observed inconsistencies between metallicities inferred from recombination
lines and from forbidden lines cannot be attained by steady-state equilibrium
photoionization models. If on the other hand the nebular ionizing source was
variable, the temperature fluctuations t^2 would be significantly larger. We
investigate the time-dependent response of the nebular ionization and
temperature structure when photoionized by a periodically varying source. We
study how the asymptotic mean value, , behaves as a function of the period
or amplitude of the source variability. We find that the temperature
fluctuations occur only in the outer section of the nebula, close to the
ionization front, within a zone corresponding to 8-20% of the ionized layer's
thickness. We conclude that the amplitude of the exciting star variations
required to achieve a = 0.025 (as in the Orion nebula) is unacceptably
large. Source variability is therefore not a viable mechanism to explain the
observed values of t^2. We reach a similar conclusion from studies of the
temporal variability resulting from intermittent shadows behind opaque
condensations. We find that photoionized nebulae are on average less massive
but somewhat hotter in the case of cyclicly variable ionizing sources.Comment: 15 pages, 6 figures, submitted to Revista Mexicana de Astronomia y
Astrofisica, revised versio
HAWC response to atmospheric electricity activity
The HAWC Gamma Ray observatory consists of 300 water Cherenkov detectors
(WCD) instrumented with four photo multipliers tubes (PMT) per WCD. HAWC is
located between two of the highest mountains in Mexico. The high altitude (4100
m asl), the relatively short distance to the Gulf of Mexico (~100 km), the
large detecting area (22 000 m) and its high sensitivity, make HAWC a good
instrument to explore the acceleration of particles due to the electric fields
existing inside storm clouds. In particular, the scaler system of HAWC records
the output of each one of the 1200 PMTs as well as the 2, 3, and 4-fold
multiplicities (logic AND in a time window of 30 ns) of each WCD with a
sampling rate of 40 Hz. Using the scaler data, we have identified 20
enhancements of the observed rate during periods when storm clouds were over
HAWC but without cloud-earth discharges. These enhancements can be produced by
electrons with energy of tens of MeV, accelerated by the electric fields of
tens of kV/m measured at the site during the storm periods. In this work, we
present the recorded data, the method of analysis and our preliminary
conclusions on the electron acceleration by the electric fields inside the
clouds.Comment: Presented at the 35th International Cosmic Ray Conference (ICRC2017),
Bexco, Busan, Korea. See arXiv:1708.02572 for all HAWC contribution
Kinematics of Herbig-Haro Objects and Jets in the Orion Nebula
We have surveyed the inner 5' of the Orion Nebula by means of Halpha and
[NII] Fabry-Perot imaging spectroscopy to present a kinematical study of the
Herbig-Haro objects in the nebula. The objects studied in this work are HH 202,
203, 204, 529, 269 and other associated features. For HH 202 we find new
features that, because of their high velocities (up to 100 km/seg) indicate the
presence of an outflow that probably is a HH flow not catalogued previously. HH
202 could be only a part of this larger outflow. Large internal motions are
found in the fainter regions of HH 203-204, as well as evidence of transverse
density gradients that could account for the asymmetry in the brightness
distribution of HH 204. We report for the first time a high blueshifted
velocity (-118 km/seg) associated with HH 204, and show that the apex of HH 204
is indeed the zone of maximum velocity, in agreement with bow shock models. We
also studied the radial velocity field of HH 269 finding features associated
with the HH object. From our studies, we find kinematic evidence that suggests
that HH 203-204 and HH 202 are part of a big (aprox. 0.55 pc) bipolar HH
outflow.Comment: 24 pages, 10 figures (in three jpg files). To be Published in
Astronomical Journa
- …