318 research outputs found
The flight of Arcadia: spatial CO2/SO2 variations in a cross section above the Nord East crater of Etna volcano
The CO2/SO2 ratio in volcanic plumes of open conduit volcanoes can provide useful information about the magma depth inside a conduit and the possible occurrence of an eruptive event. Moreover, the same CO2 measurement when combined with a SO2 flux measurement, commonly carried out at many volcanoes nowadays, is used to contribute to an improved estimate of global volcanic CO2 budget. Today worldwide at 13 volcanoes automated in-situ instruments (known as Multi-GAS stations) are applied to continuously determine CO2/SO2 ratios and to use this signal as additional parameter for volcanic monitoring. Usually these instruments carry out measurements of half an hour 4 – 6 times/day and thus provide continuous CO2/SO2 values and their variability. The stations are located at crater rims in a position that according to the prevailing winds is invested by the plume. Obviously, although the stations are carefully positioned, it is inevitable that other sources than the plume itself, e.g. soil degassing and surrounding fumaroles, contribute and will be measured as well, covering the ‘real’ values. Between July and September 2014 experiments were carried out on the North East crater (NEC) of Mount Etna, installing a self-made cable car that crossed the crater from one side to the other. The basket, called “Arcadia”, was equipped with an automated standard Multi-GAS station and a GPS, which acquired at high frequency (0.5 Hz) the following parameters : CO2, SO2, H2S, Rh, T, P and geo-coordinates. The choice of NEC of the volcano Etna was based on its accessibility, the relative small diameter (about 230 m) and the presence of a relatively constant and rather concentrated plume. Actually, NEC belongs also to the monitoring network EtnaPlume (managed by the INGV of Palermo). The aim of these experiments was to observe variations of each parameter, in particular the fluctuation of the CO2/SO2 ratio within the plume, moving from the edge to the center of the crater. The gained results give a first possibility to understand if common measurements carried out at the edge of a crater are subject to overor underestimation and about the order of derivations caused by other sources than the plume. A preliminary analysis results in a lower CO2/SO2 ratio in the central part of the crater versus the more peripheral one. The deviation between the average CO2/SO2 ratio and the center of the plume ranges from a minimum of 58% up to a maximum of 74%. An increased CO2/SO2 emission could be caused by the influence of soil and/or fumarolic degassing at the crater rim. This interpretation leads us to the conclusion that measurements by fixed installed stations might overestimate the CO2/SO2 ratio compared to values originating from the “pure” plume. Further on, it means that variations of up to 74%(in our experiment) don’t necessarily correlate with volcanic activity changes
Sequential and Spontaneous Star Formation Around the Mid-Infrared Halo HII Region KR 140
We use 2MASS and MSX infrared observations, along with new molecular line
(CO) observations, to examine the distribution of young stellar objects (YSOs)
in the molecular cloud surrounding the halo HII region KR 140 in order to
determine if the ongoing star-formation activity in this region is dominated by
sequential star formation within the photodissociation region (PDR) surrounding
the HII region. We find that KR 140 has an extensive population of YSOs that
have spontaneously formed due to processes not related to the expansion of the
HII region. Much of the YSO population in the molecular cloud is concentrated
along a dense filamentary molecular structure, traced by C18O, that has not
been erased by the formation of the exciting O star. Some of the previously
observed submillimetre clumps surrounding the HII region are shown to be sites
of recent intermediate and low-mass star formation while other massive starless
clumps clearly associated with the PDR may be the next sites of sequential star
formation.Comment: Accepted for publication in MNRAS, 8 pages, 10 figure
The Molecular Gas Environment around Two Herbig Ae/Be Stars: Resolving the Outflows of LkHa 198 and LkHa 225S
Observations of outflows associated with pre-main-sequence stars reveal
details about morphology, binarity and evolutionary states of young stellar
objects. We present molecular line data from the Berkeley-Illinois-Maryland
Association array and Five Colleges Radio Astronomical Observatory toward the
regions containing the Herbig Ae/Be stars LkHa 198 and LkHa 225S. Single dish
observations of 12CO 1-0, 13CO 1-0, N2H+ 1-0 and CS 2-1 were made over a field
of 4.3' x 4.3' for each species. 12CO data from FCRAO were combined with high
resolution BIMA array data to achieve a naturally-weighted synthesized beam of
6.75'' x 5.5'' toward LkHa 198 and 5.7'' x 3.95'' toward LkHa 225S,
representing resolution improvements of factors of approximately 10 and 5 over
existing data. By using uniform weighting, we achieved another factor of two
improvement. The outflow around LkHa 198 resolves into at least four outflows,
none of which are centered on LkHa 198-IR, but even at our resolution, we
cannot exclude the possibility of an outflow associated with this source. In
the LkHa 225S region, we find evidence for two outflows associated with LkHa
225S itself and a third outflow is likely driven by this source. Identification
of the driving sources is still resolution-limited and is also complicated by
the presence of three clouds along the line of sight toward the Cygnus
molecular cloud. 13CO is present in the environments of both stars along with
cold, dense gas as traced by CS and (in LkHa 225S) N2H+. No 2.6 mm continuum is
detected in either region in relatively shallow maps compared to existing
continuum observations.Comment: 14 pages, 10 figures (5 color), accepted for publication in Ap
A GLIMPSE into the Nature of Galactic Mid-IR Excesses
We investigate the nature of the mid-IR excess for 31 intermediate-mass stars
that exhibit an 8 micron excess in either the Galactic Legacy Infrared
Mid-Plane Survey Extraordinaire or the Mid-Course Space Experiment using high
resolution optical spectra to identify stars surrounded by warm circumstellar
dust. From these data we determine projected stellar rotational velocities and
estimate stellar effective temperatures for the sample. We estimate stellar
ages from these temperatures, parallactic distances, and evolutionary models.
Using MIPS [24] measurements and stellar parameters we determine the nature of
the infrared excess for 19 GLIMPSE stars. We find that 15 stars exhibit Halpha
emission and four exhibit Halpha absorption. Assuming that the mid-IR excesses
arise in circumstellar disks, we use the Halpha fluxes to model and estimate
the relative contributions of dust and free-free emission. Six stars exhibit
Halpha fluxes that imply free-free emission can plausibly explain the infrared
excess at [24]. These stars are candidate classical Be stars. Nine stars
exhibit Halpha emission, but their Halpha fluxes are insufficient to explain
the infrared excesses at [24], suggesting the presence of a circumstellar dust
component. After the removal of the free-free component in these sources, we
determine probable disk dust temperatures of Tdisk~300-800 K and fractional
infrared luminosities of L(IR)/L(*)~10^-3. These nine stars may be
pre-main-sequence stars with transitional disks undergoing disk clearing. Three
of the four sources showing Halpha absorption exhibit circumstellar disk
temperatures ~300-400 K, L(IR)/L(*)~10^-3, IR colors K-[24]< 3.3, and are warm
debris disk candidates. One of the four Halpha absorption sources has K-[24]>
3.3 implying an optically thick outer disk and is a transition disk candidate.Comment: 17 figures. Accepted for publication in Ap
Evolution of emission line activity in intermediate mass young stars
We present optical spectra of 45 intermediate mass Herbig Ae/Be stars.
Together with the multi-epoch spectroscopic and photometric data compiled for a
large sample of these stars and ages estimated for individual stars by using
pre-main sequence evolutionary tracks, we have studied the evolution of
emission line activity in them. We find that, on average, the H_alpha emission
line strength decreases with increasing stellar age in HAeBe stars, indicating
that the accretion activity gradually declines during the PMS phase. This would
hint at a relatively long-lived (a few Myr) process being responsible for the
cessation of accretion in Herbig Ae/Be stars. We also find that the accretion
activity in these stars drops substantially by ~ 3 Myr. This is comparable to
the timescale in which most intermediate mass stars are thought to lose their
inner disks, suggesting that inner disks in intermediate mass stars are
dissipated rapidly after the accretion activity has fallen below a certain
level. We, further find a relatively tight correlation between strength of the
emission line and near-infrared excess due to inner disks in HAeBe stars,
indicating that the disks around Herbig Ae/Be stars cannot be entirely passive.
We suggest that this correlation can be understood within the frame work of the
puffed-up inner rim disk models if the radiation from the accretion shock is
also responsible for the disk heating.Comment: 39 pages, accepted for publication in Ap
Optical spectroscopic variability of Herbig Ae/Be stars
We analysed 337 multi-epoch optical spectra of 38 Herbig Ae/Be (HAeBe) stars
to gain insights into the variability behaviour of the circumstellar (CS)
atomic gas. Equivalent widths (EWs) and line fluxes of the Halpha, [OI]6300,
HeI5876 and NaID lines were obtained for each spectrum; the Halpha line width
at 10% of peak intensity (W10) and profile shapes were also measured and
classified. The mean line strengths and relative variabilities were quantified
for each star. Simultaneous optical photometry was used to estimate the line
fluxes.
We present a homogeneous spectroscopic database of HAeBe stars. The lines are
variable in practically all stars and timescales, although 30 % of the objects
show a constant EW in [OI]6300, which is also the only line that shows no
variability on timescales of hours. The HeI5876 and NaID EW relative
variabilities are typically the largest, followed by those in [OI]6300 and
Halpha. The EW changes can be larger than one order of magnitude for the
HeI5876 line, and up to a factor 4 for Halpha. The [OI]6300 and Halpha EW
relative variabilities are correlated for most stars in the sample. The Halpha
mean EW and W10 are uncorrelated, as are their relative variabilities. The
Halpha profile changes in 70 % of the objects. The massive stars in the sample
usually show more stable Halpha profiles with blueshifted self-absorptions and
less variable 10% widths.
Our data suggest multiple causes for the different line variations, but the
[OI]6300 and Halpha variability must share a similar origin in many objects.
The physical mechanism responsible for the Halpha line broadening does not
depend on the amount of emission; unlike in lower-mass stars, physical
properties based on the Halpha luminosity and W10 would significantly differ.
Our results provide additional support to previous works that reported
different physical mechanisms in Herbig Ae and Herbig Be stars.Comment: 10 pages, 5 figures, 2 appendixe
Temporal Modulation of the Control Parameter in Electroconvection in the Nematic Liquid Crystal I52
I report on the effects of a periodic modulation of the control parameter on
electroconvection in the nematic liquid crystal I52. Without modulation, the
primary bifurcation from the uniform state is a direct transition to a state of
spatiotemporal chaos. This state is the result of the interaction of four,
degenerate traveling modes: right and left zig and zag rolls. Periodic
modulations of the driving voltage at approximately twice the traveling
frequency are used. For a large enough modulation amplitude, standing waves
that consist of only zig or zag rolls are stabilized. The standing waves
exhibit regular behavior in space and time. Therefore, modulation of the
control parameter represents a method of eliminating spatiotemporal chaos. As
the modulation frequency is varied away from twice the traveling frequency,
standing waves that are a superposition of zig and zag rolls, i.e. standing
rectangles, are observed. These results are compared with existing predictions
based on coupled complex Ginzburg-Landau equations
Direct observation of twist mode in electroconvection in I52
I report on the direct observation of a uniform twist mode of the director
field in electroconvection in I52. Recent theoretical work suggests that such a
uniform twist mode of the director field is responsible for a number of
secondary bifurcations in both electroconvection and thermal convection in
nematics. I show here evidence that the proposed mechanisms are consistent with
being the source of the previously reported SO2 state of electroconvection in
I52. The same mechanisms also contribute to a tertiary Hopf bifurcation that I
observe in electroconvection in I52. There are quantitative differences between
the experiment and calculations that only include the twist mode. These
differences suggest that a complete description must include effects described
by the weak-electrolyte model of electroconvection
Modeling the Halpha line emission around classical T Tauri stars using magnetospheric accretion and disk wind models
Spectral observations of classical T Tauri stars show a wide range of line
profiles, many of which reveal signs of matter inflow and outflow. Halpha is
the most commonly observed line profile due to its intensity, and it is highly
dependent on the characteristics of the surrounding environment of these stars.
Our aim is to analyze how the Halpha line profile is affected by the various
parameters of our model which contains both the magnetospheric and disk wind
contributions to the Halpha flux. We used a dipolar axisymmetric stellar
magnetic field to model the stellar magnetosphere and a modified Blandford &
Payne model was used in our disk wind region. A three-level atom with continuum
was used to calculate the required Hydrogen level populations. We use the
Sobolev approximation and a ray-by-ray method to calculate the integrated line
profile. Through an extensive study of the model parameter space, we have
investigated the contribution of many of the model parameters on the calculated
line profiles. Our results show that the Halpha line is strongly dependent on
the densities and temperatures inside the magnetosphere and the disk wind
region. The bulk of the flux comes, most of the time, from the magnetospheric
component for standard classical T Tauri stars parameters, but the disk wind
contribution becomes more important as the mass accretion rate, the
temperatures and densities inside the disk wind increase. We have also found
that most of the disk wind contribution to the Halpha line is emitted at the
innermost region of the disk wind. Models that take into consideration both
inflow and outflow of matter are a necessity to fully understand and describe
classical T Tauri stars.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysics. Revised version with English correction
- …