515 research outputs found
Ionization toward the high-mass star-forming region NGC 6334 I
Context. Ionization plays a central role in the gas-phase chemistry of
molecular clouds. Since ions are coupled with magnetic fields, which can in
turn counteract gravitational collapse, it is of paramount importance to
measure their abundance in star-forming regions. Aims. We use spectral line
observations of the high-mass star-forming region NGC 6334 I to derive the
abundance of two of the most abundant molecular ions, HCO+ and N2H+, and
consequently, the cosmic ray ionization rate. In addition, the line profiles
provide information about the kinematics of this region. Methods. We present
high-resolution spectral line observations conducted with the HIFI instrument
on board the Herschel Space Observatory of the rotational transitions with Jup
> 5 of the molecular species C17O, C18O, HCO+, H13CO+, and N2H+. Results. The
HCO+ and N2H+ line profiles display a redshifted asymmetry consistent with a
region of expanding gas. We identify two emission components in the spectra,
each with a different excitation, associated with the envelope of NGC 6334 I.
The physical parameters obtained for the envelope are in agreement with
previous models of the radial structure of NGC 6334 I based on submillimeter
continuum observations. Based on our new Herschel/HIFI observations, combined
with the predictions from a chemical model, we derive a cosmic ray ionization
rate that is an order of magnitude higher than the canonical value of 10^(-17)
s-1. Conclusions. We find evidence of an expansion of the envelope surrounding
the hot core of NGC 6334 I, which is mainly driven by thermal pressure from the
hot ionized gas in the region. The ionization rate seems to be dominated by
cosmic rays originating from outside the source, although X-ray emission from
the NGC 6334 I core could contribute to the ionization in the inner part of the
envelope.Comment: This paper contains a total of 10 figures and 3 table
Breaking the Redshift Deadlock - I: Constraining the star formation history of galaxies with sub-millimetre photometric redshifts
Future extragalactic sub-millimetre and millimetre surveys have the potential
to provide a sensitive census of the level of obscured star formation in
galaxies at all redshifts. While in general there is good agreement between the
source counts from existing SCUBA (850um) and MAMBO (1.25mm) surveys of
different depths and areas, it remains difficult to determine the redshift
distribution and bolometric luminosities of the sub-millimetre and millimetre
galaxy population. This is principally due to the ambiguity in identifying an
individual sub-millimetre source with its optical, IR or radio counterpart
which, in turn, prevents a confident measurement of the spectroscopic redshift.
Additionally, the lack of data measuring the rest-frame FIR spectral peak of
the sub-millimetre galaxies gives rise to poor constraints on their rest-frame
FIR luminosities and star formation rates. In this paper we describe
Monte-Carlo simulations of ground-based, balloon-borne and satellite
sub-millimetre surveys that demonstrate how the rest-frame FIR-sub-millimetre
spectral energy distributions (250-850um) can be used to derive photometric
redshifts with an r.m.s accuracy of +/- 0.4 over the range 0 < z < 6. This
opportunity to break the redshift deadlock will provide an estimate of the
global star formation history for luminous optically-obscured galaxies [L(FIR)
> 3 x 10^12 Lsun] with an accuracy of 20 per cent.Comment: 14 pages, 22 figures, submitted to MNRAS, replaced with accepted
versio
A spectral line survey of the starless and proto-stellar cores detected by BLAST toward the Vela-D molecular cloud
We present a 3-mm and 1.3-cm spectral line survey conducted with the Mopra
22-m and Parkes 64-m radio telescopes of a sample of 40 cold dust cores,
previously observed with BLAST, including both starless and proto-stellar
sources. 20 objects were also mapped using molecular tracers of dense gas. To
trace the dense gas we used the molecular species NH3, N2H+, HNC, HCO+, H13CO+,
HCN and H13CN, where some of them trace the more quiescent gas, while others
are sensitive to more dynamical processes. The selected cores have a wide
variety of morphological types and also show physical and chemical variations,
which may be associated to different evolutionary phases. We find evidence of
systematic motions in both starless and proto-stellar cores and we detect line
wings in many of the proto-stellar cores. Our observations probe linear
distances in the sources >~0.1pc, and are thus sensitive mainly to molecular
gas in the envelope of the cores. In this region we do find that, for example,
the radial profile of the N2H+(1-0) emission falls off more quickly than that
of C-bearing molecules such as HNC(1-0), HCO+(1-0) and HCN(1-0). We also
analyze the correlation between several physical and chemical parameters and
the dynamics of the cores. Depending on the assumptions made to estimate the
virial mass, we find that many starless cores have masses below the
self-gravitating threshold, whereas most of the proto-stellar cores have masses
which are near or above the self-gravitating critical value. An analysis of the
median properties of the starless and proto-stellar cores suggests that the
transition from the pre- to the proto-stellar phase is relatively fast, leaving
the core envelopes with almost unchanged physical parameters.Comment: Submitted for publication to Astronomy & Astrophysics on January
18th, 201
Reverberation of pulsar wind nebulae (I): Impact of the medium properties and other parameters upon the extent of the compression
The standard approach to the long term evolution of pulsar wind nebulae
(PWNe) is based on one-zone models treating the nebula as a uniform system. In
particular for the late phase of evolved systems, many of the generally used
prescriptions are based on educated guesses for which a proper assessment
lacks. Using an advanced radiative code we evaluate the systematic impact of
various parameters, like the properties of the supernova ejecta, of the inner
pulsar, as well of the ambient medium, upon the extent of the reverberation
phase of PWNe. We investigate how different prescriptions shift the starting
time of the reverberation phase, how this affects the amount of the
compression, and how much of this can be ascribable to the radiation processes.
Some critical aspects are the description of the reverse shock evolution, the
efficiency by which at later times material from the ejecta accretes onto the
swept-up shell around the PWN, and finally the density, velocity and pressure
profiles in the surrounding supernova remnant. We have explicitly treated the
cases of the Crab Nebula, and of J1834.9--0846, taken to be representatives of
the more and the less energetic pulsars, respectively. Especially for the
latter object the prediction of large compression factors is confirmed, even
larger in the presence of radiative losses, also confirming our former
prediction of periods of super-efficiency during the reverberation phase of
some PWNe.Comment: 12 pages, 7 figures, accepted for publication in MNRA
A new Classical T Tauri object at the sub-stellar boundary in Chamaeleon II
We have obtained low- and medium-resolution optical spectra of 20 candidate
young low-mass stars and brown dwarfs in the nearby Chamaeleon II dark cloud,
using the Magellan Baade telescope. We analyze these data in conjunction with
near-infrared photometry from the 2-Micron All Sky Survey. We find that one
target, [VCE2001] C41, exhibits broad H(alpha) emission as well as a variety of
forbidden emission lines. These signatures are usually associated with
accretion and outflow in young stars and brown dwarfs. Our spectra of C41 also
reveal LiI in absorption and allow us to derive a spectral type of M5.5 for it.
Therefore, we propose that C41 is a classical T Tauri object near the
sub-stellar boundary. Thirteen other targets in our sample have continuum
spectra without intrinsic absorption or emission features, and are difficult to
characterize. They may be background giants or foreground field stars not
associated with the cloud or embedded protostars, and need further
investigation. The six remaining candidates, with moderate reddening, are
likely to be older field dwarfs, given their spectral types, lack of lithium
and H(alpha).Comment: Astrophysical Journal, accepted June 19, 200
Dynamics of fully coupled rotators with unimodal and bimodal frequency distribution
We analyze the synchronization transition of a globally coupled network of N
phase oscillators with inertia (rotators) whose natural frequencies are
unimodally or bimodally distributed. In the unimodal case, the system exhibits
a discontinuous hysteretic transition from an incoherent to a partially
synchronized (PS) state. For sufficiently large inertia, the system reveals the
coexistence of a PS state and of a standing wave (SW) solution. In the bimodal
case, the hysteretic synchronization transition involves several states.
Namely, the system becomes coherent passing through traveling waves (TWs), SWs
and finally arriving to a PS regime. The transition to the PS state from the SW
occurs always at the same coupling, independently of the system size, while its
value increases linearly with the inertia. On the other hand the critical
coupling required to observe TWs and SWs increases with N suggesting that in
the thermodynamic limit the transition from incoherence to PS will occur
without any intermediate states. Finally a linear stability analysis reveals
that the system is hysteretic not only at the level of macroscopic indicators,
but also microscopically as verified by measuring the maximal Lyapunov
exponent.Comment: 22 pages, 11 figures, contribution for the book: Control of
Self-Organizing Nonlinear Systems, Springer Series in Energetics, eds E.
Schoell, S.H.L. Klapp, P. Hoeve
Discovery of weak 6.7-GHz CH3OH masers in a sample of high-mass Hi-GAL sources
Maser lines from different molecular species, including water, hydroxyl, and
methanol, are common observational phenomena associated with massive star
forming regions. In particular, the methanol maser appears as an ideal tool to
study the early phases of massive star formation. However, it is difficult to
establish the exact start of the methanol maser phase, and it would then be
interesting to detect and study low-flux density methanol masers (i.e., < 0.1
Jy or even << 0.1 Jy), in order to determine if they can effectively be used to
mark a specific evolutionary phase in high-mass star formation. Past surveys
have been unable to systematically detect many low-flux density methanol
masers, and thus we do not yet know how many such masers exist in the Galaxy
and what is their physical nature. Out of a sample of 107 observed Hi-GAL
sources we detected a total of 32 methanol masers, with 22 sources being new
and weak (median peak flux density 0.07 Jy) detections, in the Galactic
longitude range [32.0, 59.8]deg. We also detected 12 6.035-GHz OH maser, with 9
objects being new detections. Our survey covers a similar range of source
distances as the "Arecibo Methanol Maser Galactic Plane Survey", but the
methanol masers detected by us are clearly shifted towards lower integrated
flux densities. The newly detected methanol masers are mostly of low-luminosity
and, except for some sources, their weakness is not due to distance effects or
positional offsets. No specific correlation is found with the physical
parameters of the Hi-GAL clumps, except for sources with both CH3OH and OH
masers which tend to have higher mass and luminosity. The intensity of the
methanol masers correlates well with the velocity range of the maser emission,
which suggests that the low brightness of these masers is related to the number
of maser spots in the emitting region and their evolution with time.Comment: This paper contains a total of 14 figures and 7 tables. Submitted for
publication to A&A on November 4th, 201
A Search for H2CO 6cm Emission toward Young Stellar Objects III: VLA Observations
We report the results of our third survey for formaldehyde (H2CO) 6cm maser
emission in the Galaxy. Using the Very Large Array, we detected two new H2CO
maser sources (G23.01-0.41 and G25.83-0.18), thus increasing the sample of
known H2CO maser regions in the Galaxy to seven. We review the characteristics
of the G23.01-0.41 and G25.83-0.18 star forming regions. The H2CO masers in
G23.01-0.41 and G25.83-0.18 share several properties with the other known H2CO
masers, in particular, emission from rich maser environments and close
proximity to very young massive stellar objects.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
SANEPIC: A Map-Making Method for Timestream Data From Large Arrays
We describe a map-making method which we have developed for the Balloon-borne
Large Aperture Submillimeter Telescope (BLAST) experiment, but which should
have general application to data from other submillimeter arrays. Our method
uses a Maximum Likelihood based approach, with several approximations, which
allows images to be constructed using large amounts of data with fairly modest
computer memory and processing requirements. This new approach, Signal And
Noise Estimation Procedure Including Correlations (SANEPIC), builds upon
several previous methods, but focuses specifically on the regime where there is
a large number of detectors sampling the same map of the sky, and explicitly
allowing for the the possibility of strong correlations between the detector
timestreams. We provide real and simulated examples of how well this method
performs compared with more simplistic map-makers based on filtering. We
discuss two separate implementations of SANEPIC: a brute-force approach, in
which the inverse pixel-pixel covariance matrix is computed; and an iterative
approach, which is much more efficient for large maps. SANEPIC has been
successfully used to produce maps using data from the 2005 BLAST flight.Comment: 27 Pages, 15 figures; Submitted to the Astrophysical Journal; related
results available at http://blastexperiment.info/ [the BLAST Webpage
Mass Flows in Cometary UCHII Regions
High spectral and spatial resolution, mid-infrared fine structure line
observations toward two ultracompact HII (UCHII) regions (G29.96 -0.02 and Mon
R2) allow us to study the structure and kinematics of cometary UCHII regions.
In our earlier study of Mon R2, we showed that highly organized mass motions
accounted for most of the velocity structure in that UCHII region. In this
work, we show that the kinematics in both Mon R2 and G29.96 are consistent with
motion along an approximately paraboloidal shell. We model the velocity
structure seen in our mapping data and test the stellar wind bow shock model
for such paraboloidal like flows. The observations and the simulation indicate
that the ram pressures of the stellar wind and ambient interstellar medium
cause the accumulated mass in the bow shock to flow along the surface of the
shock. A relaxation code reproduces the mass flow's velocity structure as
derived by the analytical solution. It further predicts that the pressure
gradient along the flow can accelerate ionized gas to a speed higher than that
of the moving star. In the original bow shock model, the star speed relative to
the ambient medium was considered to be the exit speed of ionized gas in the
shell.Comment: 34 pages, including 14 figures and 1 table, to be published in ApJ,
September 200
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