21 research outputs found
Modeling and Analysis Generic Interface for eXternal numerical codes (MAGIX)
The modeling and analysis generic interface for external numerical codes
(MAGIX) is a model optimizer developed under the framework of the coherent set
of astrophysical tools for spectroscopy (CATS) project. The MAGIX package
provides a framework of an easy interface between existing codes and an
iterating engine that attempts to minimize deviations of the model results from
available observational data, constraining the values of the model parameters
and providing corresponding error estimates. Many models (and, in principle,
not only astrophysical models) can be plugged into MAGIX to explore their
parameter space and find the set of parameter values that best fits
observational/experimental data. MAGIX complies with the data structures and
reduction tools of ALMA (Atacama Large Millimeter Array), but can be used with
other astronomical and with non-astronomical data.Comment: 12 pages, 15 figures, 2 tables, paper is also available at
http://www.aanda.org/articles/aa/pdf/forth/aa20063-12.pd
Multitechnique testing of the viscous decretion disk model I. The stable and tenuous disk of the late-type Be star CMi
The viscous decretion disk (VDD) model is able to explain most of the
currently observable properties of the circumstellar disks of Be stars.
However, more stringent tests, focusing on reproducing multitechnique
observations of individual targets via physical modeling, are needed to study
the predictions of the VDD model under specific circumstances. In the case of
nearby, bright Be star CMi, these circumstances are a very stable
low-density disk and a late-type (B8Ve) central star. The aim is to test the
VDD model thoroughly, exploiting the full diagnostic potential of individual
types of observations, in particular, to constrain the poorly known structure
of the outer disk if possible, and to test truncation effects caused by a
possible binary companion using radio observations. We use the Monte Carlo
radiative transfer code HDUST to produce model observables, which we compare
with a very large set of multitechnique and multiwavelength observations that
include ultraviolet and optical spectra, photometry covering the interval
between optical and radio wavelengths, optical polarimetry, and optical and
near-IR (spectro)interferometry. Due to the absence of large scale variability,
data from different epochs can be combined into a single dataset. A parametric
VDD model with radial density exponent of = 3.5, which is the canonical
value for isothermal flaring disks, is found to explain observables typically
formed in the inner disk, while observables originating in the more extended
parts favor a shallower, = 3.0, density falloff. Modeling of radio
observations allowed for the first determination of the physical extent of a Be
disk (35 stellar radii), which might be caused by a binary
companion. Finally, polarization data allowed for an indirect measurement of
the rotation rate of the star, which was found to be , i.e.,
very close to critical.Comment: 19 pages (35 including online material), 17 figures, 2 online
figures, 2 online tables with dat
Short-term variability and mass loss in Be stars III. BRITE and SMEI satellite photometry of 28 Cygni
The BRITE Constellation of nanosatellites obtained mmag photometry of 28
Cygni for 11 months in 2014-2016. Observations with the Solar Mass Ejection
Imager in 2003-2010 and 118 H line profiles were added.
For decades, 28 Cyg has exhibited four large-amplitude frequencies: two
closely spaced frequencies of spectroscopically confirmed modes near 1.5
c/d, one slightly lower exophotospheric (Stefl) frequency, and at 0.05 c/d the
difference frequency between the two g modes. This top-level framework is
indistinguishable from eta Cen (Paper I), which is also very similar in
spectral type, rotation rate, and viewing angle. The Stefl frequency is the
only one that does not seem to be affected by the difference frequency. The
amplitude of the latter undergoes large variations; around maximum the amount
of near-circumstellar matter is increased, and the amplitude of the Stefl
frequency grows by some factor. During such brightenings dozens of transient
spikes appear in the frequency spectrum, concentrated in three groups. Only
eleven frequencies were common to all years of BRITE observations.
Be stars seem to be controlled by several coupled clocks, most of which are
not very regular on timescales of weeks to months but function for decades. The
combination of g modes to the low difference frequency and/or the atmospheric
response to it appears significantly nonlinear. Like in eta Cen, the
difference-frequency variability seems the main responsible for the modulation
of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of difference
frequencies may reach the longest timescales known of the Be phenomenon.Comment: 17 pages, 21 figures, submitted to Astronomy & Astrophysic
Herschel/PACS observations of young sources in Taurus: the far-infrared counterpart of optical jets
Observations of the atomic and molecular line emission associated with jets
and outflows emitted by young stellar objects can be used to trace the various
evolutionary stages they pass through as they evolve to become main sequence
stars.
To understand the relevance of atomic and molecular cooling in shocks, and
how accretion and ejection efficiency evolves with the source evolutionary
state, we will study the far-infrared counterparts of bright optical jets
associated with Class I and II sources in Taurus (T Tau, DG Tau A, DG Tau B, FS
Tau A+B, and RW Aur).
We have analysed Herschel/PACS observations of a number of atomic ([OI]63um,
145um, [CII]158um) and molecular (high-J CO, H2O, OH) lines, collected within
the OTKP GASPS. To constrain the origin of the detected lines we have compared
the FIR emission maps with the emission from optical-jets and
millimetre-outflows, and the line fluxes and ratios with predictions from shock
and disk models.
All of the targets are associated with extended emission in the atomic lines
correlated with the direction of the optical jet/mm-outflow. The atomic lines
can be excited in fast dissociative J-shocks. The molecular emission, on the
contrary, originates from a compact region, that is spatially and spectrally
unresolved. Slow C- or J- shocks with high pre-shock densities reproduce the
observed H2O and high-J CO lines; however, the disk and/or UV-heated outflow
cavities may contribute to the emission.
While the cooling is dominated by CO and H2O lines in Class 0 sources, [OI]
becomes an important coolant as the source evolves and the environment is
cleared. The cooling and mass loss rates estimated for Class II and I sources
are one to four orders of magnitude lower than for Class 0 sources. This
provides strong evidence to indicate that the outflow activity decreases as the
source evolves.Comment: 18 pages, 9 figures, accepted by A&
Molecule survival in magnetized protostellar disk winds. I. Chemical model and first results
Molecular counterparts to atomic jets have been detected within 1000 AU of
young stars. Reproducing them is a challenge for proposed ejection models. We
explore whether molecules may survive in an MHD disk wind invoked to reproduce
the kinematics and tentative rotation signatures of atomic jets in T Tauri
stars. The coupled ionization, chemical and thermal evolution along dusty flow
streamlines is computed for a prescribed MHD disk wind solution, using a method
developed for magnetized shocks in the interstellar medium. Irradiation by
wind-attenuated coronal X-rays and FUV photons from accretion hot spots is
included, with self-shielding of H2 and CO. Disk accretion rates of 5e-6, 1e-6
and 1e-7 solar masses per year are considered, representative of low-mass young
protostars (Class 0), evolved protostars (Class I) and very active T Tauri
stars (Class II). The disk wind has an onion-like thermo-chemical structure,
with streamlines launched from larger radii having lower temperature and
ionisation, and higher H2 abundance. The coupling between charged and neutral
fluids is sufficient to eject molecules from the disk out to 9 AU. The launch
radius beyond which most H2 survives moves outward with evolutionary stage. CO
survives in the Class 0 but is significantly photodissociated in the Class
I/II. Balance between ambipolar heating and molecular cooling establishes an
asymptotic temperature 700-3000 K, with cooler jets at earlier protostellar
stages. Endothermic formation of H2O is efficient with abundances up to 1e-4,
while CH+ and SH+ can exceed 1e-6 in the Class I/II winds. A centrifugal MHD
disk wind launched from beyond 0.2-1 AU can produce molecular jets/winds up to
speeds 100 km/s in young low-mass stars. The model predicts a high ratio H2/CO
and an increase of molecular launch radius, temperature, and flow width as the
source evolves, in agreement with current observed trends.Comment: 21 pages, 13 figures; Astronomy & Astrophysics, 23 Nov 201