125 research outputs found
Nitrogen Fractionation in External Galaxies
In star forming regions in our own Galaxy, the 14N/15N ratio is found to vary
from 100 in meteorites, comets and protoplanetary disks up to
1000 in pre-stellar and star forming cores, while in external galaxies the very
few single-dish large scale measurements of this ratio lead to values of
100-450. The extent of the contribution of isotopic fractionation to these
variations is, to date, unknown. In this paper we present a theoretical
chemical study of nitrogen fractionation in external galaxies in order to
determine the physical conditions that may lead to a spread of the 14N/15N
ratio from the solar value of 440 and hence evaluate the contribution of
chemical reactions in the ISM to nitrogen fractionation. We find that the main
cause of ISM enrichment of nitrogen fractionation is high gas densities, aided
by high fluxes of cosmic rays.Comment: Accepted by MNRA
The dynamical properties of dense filaments in the infrared dark cloud G035.39-00.33
Infrared Dark Clouds (IRDCs) are unique laboratories to study the initial
conditions of high-mass star and star cluster formation. We present
high-sensitivity and high-angular resolution IRAM PdBI observations of N2H+
(1-0) towards IRDC G035.39-00.33. It is found that G035.39-00.33 is a highly
complex environment, consisting of several mildly supersonic filaments
(sigma_NT/c_s ~1.5), separated in velocity by <1 km s^-1 . Where multiple
spectral components are evident, moment analysis overestimates the non-thermal
contribution to the line-width by a factor ~2. Large-scale velocity gradients
evident in previous single-dish maps may be explained by the presence of
substructure now evident in the interferometric maps. Whilst global velocity
gradients are small (<0.7 km s^-1 pc^-1), there is evidence for dynamic
processes on local scales (~1.5-2.5 km s^-1 pc^-1 ). Systematic trends in
velocity gradient are observed towards several continuum peaks. This suggests
that the kinematics are influenced by dense (and in some cases, starless)
cores. These trends are interpreted as either infalling material, with
accretion rates ~(7 \pm 4)x10^-5 M_sun yr^-1 , or expanding shells with
momentum ~24 \pm 12 M_sun km s^-1 . These observations highlight the importance
of high-sensitivity and high-spectral resolution data in disentangling the
complex kinematic and physical structure of massive star forming regions.Comment: 25 pages, 23 figures, accepted for publication in MNRA
Zooming in to Massive Star Birth
We present high resolution (0.2", 1000 AU) 1.3 mm ALMA observations of
massive infrared dark cloud clump, G028.37+00.07-C1, thought to harbor the
early stages of massive star formation. Using (3-2) we resolve the
previously identified C1-S core, separating the bulk of its emission from two
nearby protostellar sources. C1-S is thus identified as a massive
(), compact (pc diameter) starless core, e.g., with
no signs of outflow activity. Being highly deuterated, this is a promising
candidate for a pre-stellar core on the verge of collapse. An analysis of its
dynamical state indicates a sub-virial velocity dispersion compared to a
trans-Alfv\'enic turbulent core model. However, virial equilibrium could be
achieved with sub-Alfv\'enic conditions involving mG magnetic field
strengths.Comment: 19 pages, 15 figures, 4 tables, accepted by Ap
A high resolution study of complex organic molecules in hot cores
We present the results of a line identification analysis using data from the
IRAM Plateau de Bure Inferferometer, focusing on six massive star-forming hot
cores: G31.41+0.31, G29.96-0.02, G19.61-0.23, G10.62-0.38, G24.78+0.08A1 and
G24.78+0.08A2. We identify several transitions of vibrationally excited methyl
formate (HCOOCH) for the first time in these objects as well as transitions
of other complex molecules, including ethyl cyanide (CHCN), and
isocyanic acid (HNCO). We also postulate a detection of one transition of
glycolaldehyde (CH(OH)CHO) in two new hot cores. We find G29.96-0.02,
G19.61-0.23, G24.78+0.08A1 and 24.78+0.08A2 to be chemically very similar.
G31.41+0.31, however, is chemically different: it manifests a larger chemical
inventory and has significantly larger column densities. We suggest that it may
represent a different evolutionary stage to the other hot cores in the sample,
or it may surround a star with a higher mass. We derive column densities for
methyl formate in G31.41+0.31, using the rotation diagram method, of
10 cm and a T of 170 K. For G29.96-0.02,
G24.78+0.08A1 and G24.78+0.08A2, glycolaldehyde, methyl formate and methyl
cyanide all seem to trace the same material and peak at roughly the same
position towards the dust emission peak. For G31.41+0.31, however,
glycolaldehyde shows a different distribution to methyl formate and methyl
cyanide and seems to trace the densest, most compact inner part of hot cores.Comment: Accepted to MNRA
Search for massive protostar candidates in the southern hemisphere: II. Dust continuum emission
In an ongoing effort to identify and study high-mass protostellar candidates
we have observed in various tracers a sample of 235 sources selected from the
IRAS Point Source Catalog, mostly with dec < -30 deg, with the SEST antenna at
millimeter wavelengths. The sample contains 142 Low sources and 93 High, which
are believed to be in different evolutionary stages. Both sub-samples have been
studied in detail by comparing their physical properties and morphologies.
Massive dust clumps have been detected in all but 8 regions, with usually more
than one clump per region. The dust emission shows a variety of complex
morphologies, sometimes with multiple clumps forming filaments or clusters. The
mean clump has a linear size of ~0.5 pc, a mass of ~320 Msolar for a dust
temperature Td=30 K, an H_2 density of 9.5E5 cm-3, and a surface density of 0.4
g cm-2. The median values are 0.4 pc, 102 Msolar, 4E4 cm-3, and 0.14 g cm-2,
respectively. The mean value of the luminosity-to-mass ratio, L/M ~99
Lsolar/Msolar, suggests that the sources are in a young, pre-ultracompact HII
phase. We have compared the millimeter continuum maps with images of the mid-IR
MSX emission, and have discovered 95 massive millimeter clumps non-MSX
emitters, either diffuse or point-like, that are potential prestellar or
precluster cores. The physical properties of these clumps are similar to those
of the others, apart from the mass that is ~3 times lower than for clumps with
MSX counterpart. Such a difference could be due to the potential prestellar
clumps having a lower dust temperature. The mass spectrum of the clumps with
masses above M ~100 Msolar is best fitted with a power-law dN/dM proportional
to M-alpha with alpha=2.1, consistent with the Salpeter (1955) stellar IMF,
with alpha=2.35.Comment: 83 pages, 10 figures, 3 tables. Accepted for publication by A&A. The
full paper, including Fig.2 with the maps of all the individual regions,
complete Tables 1 and 2 can be found at
http://www.arcetri.astro.it/~starform/publ2005.ht
First measurement of the 14N/15N ratio in the analogue of the Sun progenitor OMC-2 FIR4
We present a complete census of the 14N/15N isotopic ratio in the most
abundant N-bearing molecules towards the cold envelope of the protocluster
OMC-2 FIR4, the best known Sun progenitor. To this scope, we analysed the
unbiased spectral survey obtained with the IRAM-30m telescope at 3mm, 2mm and
1mm. We detected several lines of CN, HCN, HNC, HC3N, N2H+, and their
respective 13C and 15N isotopologues. The lines relative fluxes are compatible
with LTE conditions and moderate line opacities have been corrected via a
Population Diagram method or theoretical relative intensity ratios of the
hyperfine structures. The five species lead to very similar 14N/15N isotopic
ratios, without any systematic difference between amine and nitrile bearing
species as previously found in other protostellar sources. The weighted average
of the 14N/15N isotopic ratio is 270 +/- 30. This 14N/15N value is remarkably
consistent with the [250-350] range measured for the local galactic ratio but
significantly differs from the ratio measured in comets (around 140).
High-angular resolution observations are needed to examine whether this
discrepancy is maintained at smaller scales. In addition, using the CN, HCN and
HC3N lines, we derived a 12C/13C isotopic ratio of 50 +/- 5.Comment: Accepted for publication in ApJ ; 19 pages, 5 tables, 12 figure
A Virialized Filamentary Infrared Dark Cloud
The initial conditions of massive star and star cluster formation are
expected to be cold, dense and high column density regions of the interstellar
medium, which can reveal themselves via near, mid and even far-infrared
absorption as Infrared Dark Clouds (IRDCs). Elucidating the dynamical state of
IRDCs thus constrains theoretical models of these complex processes. In
particular, it is important to assess whether IRDCs have reached virial
equilibrium, where the internal pressure balances that due to the
self-gravitating weight of the cloud plus the pressure of the external
environmental. We study this question for the filamentary IRDC G035.39-00.33 by
deriving mass from combined NIR & MIR extinction maps and velocity dispersion
from C18O (1-0) & (2-1) line emission. In contrast to our previous moderately
super-virial results based on 13CO emission and MIR-only extinction mapping,
with improved mass measurements we now find that the filament is consistent
with being in virial equilibrium, at least in its central parsec-wide region
where ~1000 M_Sun snakes along several parsecs. This equilibrium state does not
require large-scale net support or confinement by magnetic fields.Comment: 4 pages, 2 figures, Accepted to ApJ
Massive Star Formation
The enormous radiative and mechanical luminosities of massive stars impact a
vast range of scales and processes, from the reionization of the universe, to
the evolution of galaxies, to the regulation of the interstellar medium, to the
formation of star clusters, and even to the formation of planets around stars
in such clusters. Two main classes of massive star formation theory are under
active study, Core Accretion and Competitive Accretion. In Core Accretion, the
initial conditions are self-gravitating, centrally concentrated cores that
condense with a range of masses from the surrounding, fragmenting clump
environment. They then undergo relatively ordered collapse via a central disk
to form a single star or a small-N multiple. In this case, the pre-stellar core
mass function has a similar form to the stellar initial mass function. In
Competitive Accretion, the material that forms a massive star is drawn more
chaotically from a wider region of the clump without passing through a phase of
being in a massive, coherent core. In this case, massive star formation must
proceed hand in hand with star cluster formation. If stellar densities become
very high near the cluster center, then collisions between stars may also help
to form the most massive stars. We review recent theoretical and observational
progress towards understanding massive star formation, considering physical and
chemical processes, comparisons with low and intermediate-mass stars, and
connections to star cluster formation.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C.
Dullemond, Th. Hennin
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