157 research outputs found
Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201
We present the detection and characterization of a peculiar low-mass
protostar (IRAS 22129+7000) located ~0.4 pc from Ced 201 Photodissociation
Region (PDR) and ~0.2 pc from the HH450 jet. The cold circumstellar envelope
surrounding the object has been mapped through its 1.2 mm dust continuum
emission with IRAM-30m/MAMBO. The deeply embedded protostar is clearly detected
with Spitzer/MIPS (70 um), IRS (20-35 um) and IRAC (4.5, 5.8, and 8 um) but
also in the K_s band (2.15 um). Given the large "near- and mid-IR excess" in
its spectral energy distribution, but large submillimeter-to-bolometric
luminosity ratio (~2%), IRAS 22129+7000 must be a transition Class 0/I source
and/or a multiple stellar system. Targeted observations of several molecular
lines from CO, 13CO, C18O, HCO+ and DCO+ have been obtained. The presence of a
collimated molecular outflow mapped with the CSO telescope in the CO J=3-2 line
suggests that the protostar/disk system is still accreting material from its
natal envelope. Indeed, optically thick line profiles from high density tracers
such as HCO+ J=1-0 show a red-shifted-absorption asymmetry reminiscent of
inward motions. We construct a preliminary physical model of the circumstellar
envelope (including radial density and temperature gradients, velocity field
and turbulence) that reproduces the observed line profiles and estimates the
ionization fraction. The presence of both mechanical and (non-ionizing)
FUV-radiative input makes the region an interesting case to study triggered
star formation
Tracing the envelopes around embedded low-mass young stellar objects with HCO+ and millimeter-continuum observations
Interferometer observations of millimeter-continuum (OVRO) and single-dish
observations of HCO+ and H13CO+ J=1-0, 3-2, and 4-3 (JCMT, IRAM 30m) are
presented of nine embedded low-mass young stellar objects (YSOs) in Taurus. All
nine objects are detected at 3.4 and 2.7 mm, with fluxes of 4-200 mJy, and
consist of unresolved (<3 arcsec) point sources, plus, toward about half of the
objects, an extended envelope. The point sources likely are circumstellar
disks, showing that these are established early in the embedded phase.
Literature values of 1.1 mm continuum emission are used to trace the envelopes,
carrying 0.001-0.26 M(sol). In HCO+, the 1-0 lines trace the surrounding
clouds, while the 3-2 and 4-3 are concentrated toward the sources with
intensities well correlated with the envelope flux. An HCO+/H2 abundance of
1.2e-8 is derived. The HCO+ line strengths and envelope fluxes can be fit
simultaneously with the simple collapse model of Shu (1977), and related
density power laws with slopes p=1-3. As an indicator of the relative
evolutionary phase of a YSO, the ratio of HCO+ 3-2 line intensity over
bolometric luminosity is proposed, which is roughly proportional to the current
ratio of envelope over stellar mass. It is concluded that HCO+ 3-2 and 4-3 are
excellent tracers of the early embedded phase of star formation.Comment: 45 pages, 10 figures, ApJ/AASLaTeX. To be published in The
Astrophysical Journa
Synthetic Molecular Clouds from Supersonic MHD and Non-LTE Radiative Transfer Calculations
The dynamics of molecular clouds is characterized by supersonic random
motions in the presence of a magnetic field. We study this situation using
numerical solutions of the three-dimensional compressible magneto-hydrodynamic
(MHD) equations in a regime of highly supersonic random motions. The non-LTE
radiative transfer calculations are performed through the complex density and
velocity fields obtained as solutions of the MHD equations, and more than
5x10^5 synthetic molecular spectra are obtained. We use a numerical flow
without gravity or external forcing. The flow is super-Alfvenic and corresponds
to model A of Padoan and Nordlund (1997). Synthetic data consist of sets of
90x90 synthetic spectra with 60 velocity channels, in five molecular
transitions: J=1-0 and J=2-1 for 12CO and 13CO, and J=1-0 for CS. Though we do
not consider the effects of stellar radiation, gravity, or mechanical energy
input from discrete sources, our models do contain the basic physics of
magneto-fluid dynamics and non-LTE radiation transfer and are therefore more
realistic than previous calculations. As a result, these synthetic maps and
spectra bear a remarkable resemblance to the corresponding observations of real
clouds.Comment: 33 pages, 12 figures included, 5 jpeg figures not included (fig1a,
fig1b, fig3, fig4 fig5), submitted to Ap
Molecular line radiative transfer in protoplanetary disks: Monte Carlo simulations versus approximate methods
We analyze the line radiative transfer in protoplanetary disks using several
approximate methods and a well-tested Accelerated Monte Carlo code. A low-mass
flaring disk model with uniform as well as stratified molecular abundances is
adopted. Radiative transfer in low and high rotational lines of CO, C18O, HCO+,
DCO+, HCN, CS, and H2CO is simulated. The corresponding excitation
temperatures, synthetic spectra, and channel maps are derived and compared to
the results of the Monte Carlo calculations. A simple scheme that describes the
conditions of the line excitation for a chosen molecular transition is
elaborated. We find that the simple LTE approach can safely be applied for the
low molecular transitions only, while it significantly overestimates the
intensities of the upper lines. In contrast, the Full Escape Probability (FEP)
approximation can safely be used for the upper transitions (J_{\rm up} \ga 3)
but it is not appropriate for the lowest transitions because of the maser
effect. In general, the molecular lines in protoplanetary disks are partly
subthermally excited and require more sophisticated approximate line radiative
transfer methods. We analyze a number of approximate methods, namely, LVG, VEP
(Vertical Escape Probability) and VOR (Vertical One Ray) and discuss their
algorithms in detail. In addition, two modifications to the canonical Monte
Carlo algorithm that allow a significant speed up of the line radiative
transfer modeling in rotating configurations by a factor of 10--50 are
described.Comment: 47 pages, 12 figures, accepted for publication in Ap
Numerical methods for non-LTE line radiative transfer: Performance and convergence characteristics
Comparison is made between a number of independent computer programs for
radiative transfer in molecular rotational lines. The test models are
spherically symmetric circumstellar envelopes with a given density and
temperature profile. The first two test models have a simple power law density
distribution, constant temperature and a fictive 2-level molecule, while the
other two test models consist of an inside-out collapsing envelope observed in
rotational transitions of HCO+. For the 2-level molecule test problems all
codes agree well to within 0.2%, comparable to the accuracy of the individual
codes, for low optical depth and up to 2% for high optical depths (tau=4800).
The problem of the collapsing cloud in HCO+ has a larger spread in results,
ranging up to 12% for the J=4 population. The spread is largest at the radius
where the transition from collisional to radiative excitation occurs. The
resulting line profiles for the HCO+ J=4-3 transition agree to within 10%,
i.e., within the calibration accuracy of most current telescopes. The
comparison project and the results described in this paper provide a benchmark
for future code development, and give an indication of the typical accuracy of
present day calculations of molecular line transfer.Comment: Accepted for publication in A&
Campus Alberta: A Collaborative, Multi-University Counsellor Training Initiative
Permission to include this article granted by NATCONThe need for trained counselors/counseling psychologists, and for counselor education, training, and accreditation, are topics of continued discussion within professional organizations, accrediting bodies, universities, and other training institutions. Following an exploratory meeting in 1998 to discuss the need for additional graduate counselor education in Alberta, an advisory committee was formed with representatives from Alberta universities and major stakeholder groups. This paper describes an innovative approach to inter-university collaboration in the delivery of graduate programming emerging from that initiative. The initiative works on the premise that the current model of graduate education must be changed to reduce the barriers to continuing professional development. The new graduate program in counseling has a two-stage process. The first stage focuses on the fundamentals of counseling theory and practice. In the second stage, students select relevant assessment and intervention modules and develop an area of counseling specialization. The program is a learner-driven delivery system within the context of adult education and professional development. Through this collaboration, the needs of students can be better served while maintaining high standards of academic excellence and professional practice. (JDM
Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
Ultraviolet absorption from interstellar 12CO and 13CO was detected toward
rho Oph A and chi Oph. The measurements were obtained at medium resolution with
the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column
density ratios, N(12CO)/N(13CO), of 125 \pm 23 and 117 \pm 35 were derived for
the sight lines toward rho Oph A and chi Oph, respectively. A value of 1100 \pm
600 for the ratio N(12C16O)/N(12C18O) toward rho Oph A was also obtained.
Absorption from vibrationally excited H_2 (v" = 3) was clearly seen toward this
star as well.
The ratios are larger than the isotopic ratios for carbon and oxygen
appropriate for ambient interstellar material. Since for both carbon and oxygen
the more abundant isotopomer is enhanced, selective isotopic photodissociation
plays the key role in the fractionation process for these directions. The
enhancement arises because the more abundant isotopomer has lines that are more
optically thick, resulting in more self shielding from dissociating radiation.
A simple argument involving the amount of self shielding [from N(12CO)] and the
strength of the ultraviolet radiation field premeating the gas (from the amount
of vibrationally excited H_2) shows that selective isotopic photodissociation
controls the fractionation seen in these two sight lines, as well as the sight
line to zeta Oph.Comment: 40 pages, 8 figures, to appear in 10 July 2003 issue of Ap
Primordial helium recombination. I. Feedback, line transfer, and continuum opacity
Precision measurements of the cosmic microwave background temperature anisotropy on scales â>500 will be available in the near future. Successful interpretation of these data is dependent on a detailed understanding of the damping tail and cosmological recombination of both hydrogen and helium. This paper and two companion papers are devoted to a precise calculation of helium recombination. We discuss several aspects of the standard recombination picture, and then include feedback, radiative transfer in He i lines with partial redistribution, and continuum opacity from H i photoionization. In agreement with past calculations, we find that He ii recombination proceeds in Saha equilibrium, whereas He i recombination is delayed relative to Saha due to the low rates connecting excited states of He i to the ground state. However, we find that at z<2200 the continuum absorption by the rapidly increasing H i population becomes effective at destroying photons in the He i 21Po-11S line, causing He i recombination to finish around zâ1800, much earlier than previously estimated
Systematic Molecular Differentiation in Starless Cores
(Abridged) We present evidence that low-mass starless cores, the simplest
units of star formation, are systematically differentiated in their chemical
composition. Molecules including CO and CS almost vanish near the core centers,
where the abundance decreases by one or two orders of magnitude. At the same
time, N2H+ has a constant abundance, and the fraction of NH3 increases toward
the core center. Our conclusions are based on a study of 5 mostly-round
starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have
mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and
(2,2), and the 1.2 mm continuum. For each core we have built a model that fits
simultaneously the radial profile of all observed emission and the central
spectrum for the molecular lines. The observed abundance drops of CO and CS are
naturally explained by the depletion of these molecules onto dust grains at
densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to
densities of several 10^5, while the NH3 abundance may be enhanced by reactions
triggered by the disappearance of CO from the gas phase. With the help of our
models, we show that chemical differentiation automatically explains the
discrepancy between the sizes of CS and NH3 maps, a problem which has remained
unexplained for more than a decade. Our models, in addition, show that a
combination of radiative transfer effects can give rise to the previously
observed discrepancy in the linewidth of these two tracers. Although this
discrepancy has been traditionally interpreted as resulting from a systematic
increase of the turbulent linewidth with radius, our models show that it can
arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap
Multi-Resolution Radiative Transfer for Line Emission
We present describe a new computer code that solves the radiative transfer
problem on multi-resolution grids. If the cloud model is from an MHD simulation
on a regular cartesian grid, criteria based for example on local density or
velocity gradients are used to refine the grid by dividing selected cells into
sub-cells. Division can be repeated hierarchically. Alternatively, if the cloud
model is from an MHD simulations with adaptive mesh refinement, the same
multi-resolution grid used for the MHD simulation is adopted in the radiative
transfer calculations. High discretization is often needed only in a small
fraction of the total volume. This makes it possible to simulate spectral line
maps with good accuracy, also minimizing the total number of cells and the
computational cost (time and memory). Multi-resolution models are compared with
models on regular grids. In the case of moderate optical depths (tau ~ a few)
an accuracy of 10% can be reached with multi-resolution models where only 10%
of the cells of the full grid are used. For optically thick species (tau~100),
the same accuracy is achieved using 15% of the cells. The relation between
accuracy and number of cells is not found to be significantly different in the
two MHD models we have studied. The new code is used to study differences
between LTE and non-LTE spectra and between isothermal and non-isothermal cloud
models. We find significant differences in line ratios and individual spectral
line profiles of the isothermal and LTE models relative to the more realistic
non-isothermal case. The slope of the power spectrum of integrated intensity is
instead very similar in all models.Comment: Submitted to ApJ, 34 pages, 14 figure
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