523 research outputs found
Exploring molecular complexity in the Galactic Center with ALMA
The search for complex organic molecules (COMs) in the ISM has revealed
chemical species of ever greater complexity. This search relies heavily on the
progress made in the laboratory to characterize the rotational spectra of these
molecules. Observationally, the advent of ALMA with its high angular resolution
and sensitivity has allowed to reduce the spectral confusion and detect
low-abundance molecules that could not be probed before. We present results of
the EMoCA survey conducted with ALMA toward the star-forming region Sgr B2(N).
This spectral line survey aims at deciphering the molecular content of Sgr
B2(N) in order to test the predictions of astrochemical models and gain insight
into the chemical processes at work in the ISM. We report on the tentative
detection of N-methylformamide, on deuterated COMs, and on the detection of a
branched alkyl molecule. Prospects for probing molecular complexity in the ISM
even further are discussed at the end.Comment: Invited contribution to appear in "Astrochemistry VII -- Through the
Cosmos from Galaxies to Planets", proceedings of the IAU Symposium No. 332,
2017, Puerto Varas, Chile. M. Cunningham, T. Millar and Y. Aikawa, eds. (12
pages, 8 figures
Observation of rotation in star forming regions: clouds, cores, disks, and jets
Angular momentum plays a crucial role in the formation of stars and planets.
It has long been noticed that parcels of gas in molecular clouds need to reduce
their specific angular momentum by 6 to 7 orders of magnitude to participate in
the building of a typical star like the Sun. Several physical processes on
different scales and at different stages of evolution can contribute to this
loss of angular momentum. In order to set constraints on these processes and
better understand this transfer of angular momentum, a detailed observational
census and characterization of rotation at all stages of evolution and over all
scales of star forming regions is necessary. This review presents the main
results obtained in low-mass star forming regions over the past four decades in
this field of research. It addresses the search and characterization of
rotation in molecular clouds, prestellar and protostellar cores, circumstellar
disks, and jets. Perspectives offered by ALMA are briefly discussed.Comment: 43 pages, 8 figures. To appear in the Proceedings of the Evry
Schatzman School 2012 of PNPS and CNRS/INSU on the "Role and mechanisms of
angular momentum transport during the formation and early evolution of
stars", Eds. P.Hennebelle and C.Charbonne
Strongly triggered collapse model confront observations
Detailed modelling of individual protostellar condensations, is important to
test the various theories. Here we present comparisons between strongly induced
collapse models with one young class-0, IRAS4A in the Perseus cloud and one
prestellar cloud observed in the Coalsack molecular cloud.Comment: IAU 237, Triggering of star formation in turbulent molecular clouds,
eds B. Elmegreen and J. Palou
Molecular line study of the very young protostar IRAM 04191 in Taurus: Infall, rotation, and outflow
We present a detailed millimeter line study of the circumstellar environment
of the low-luminosity Class 0 protostar IRAM 04191+1522 in the Taurus molecular
cloud. New line observations demonstrate that the ~14000 AU radius protostellar
envelope is undergoing both extended infall and fast, differential rotation.
Radiative transfer modeling of multitransition CS and C34S maps indicate an
infall velocity v_inf ~ 0.15 km/s at r ~ 1500 AU and v_inf ~ 0.1 km/s up to r ~
11000 AU, as well as a rotational angular velocity Omega ~ 3.9 x 10^{-13}
rad/s, strongly decreasing with radius beyond 3500 AU down to a value Omega ~
1.5-3 x 10^{-14} rad/s at ~ 11000 AU. Two distinct regions, which differ in
both their infall and their rotation properties, therefore seem to stand out:
the inner part of the envelope (r ~< 2000-4000 AU) is rapidly collapsing and
rotating, while the outer part undergoes only moderate infall/contraction and
slower rotation. These contrasted features suggest that angular momentum is
conserved in the collapsing inner region but efficiently dissipated due to
magnetic braking in the slowly contracting outer region. We propose that the
inner envelope is in the process of decoupling from the ambient cloud and
corresponds to the effective mass reservoir (~0.5 M_sun) from which the central
star is being built. Comparison with the rotational properties of other objects
in Taurus suggests that IRAM 04191 is at a pivotal stage between a prestellar
regime of constant angular velocity enforced by magnetic braking and a
dynamical, protostellar regime of nearly conserved angular momentum. The
rotation velocity profile we derive for the inner IRAM 04191 envelope should
thus set some constraints on the distribution of angular momentum on the scale
of the outer Solar system at the onset of protostar/disk formation.Comment: 23 pages, 16 figures, 1 table, Accepted by Astronomy & Astrophysic
Widening of Protostellar Outflows: an Infrared Outflow Survey in Low Luminosity Objects
We present an outflow survey toward 20 Low Luminosity Objects (LLOs), namely
protostars with an internal luminosity lower than 0.2 Lsun. Although a number
of studies have reported the properties of individual LLOs, the reasons for
their low luminosity remain uncertain. To answer this question, we need to know
the evolutionary status of LLOs. Protostellar outflows are found to widen as
their parent cores evolve, and therefore, the outflow opening angle could be
used as an evolutionary indicator. The infrared scattered light escapes out
through the outflow cavity and highlights the cavity wall, giving us the
opportunity to measure the outflow opening angle. Using the
Canada-France-Hawaii Telescope, we detected outflows toward eight LLOs out of
20 at Ks band, and based on archival Spitzer IRAC1 images, we added four
outflow-driving sources from the remaining 12 sources. By fitting these images
with radiative transfer models, we derive the outflow opening angles and
inclination angles. To study the widening of outflow cavities, we compare our
sample with the young stellar objects from Arce & Sargent 2006 and Velusamy et
al. 2014 in the plot of opening angle versus bolometric temperature taken as an
evolutionary indicator.Our LLO targets match well the trend of increasing
opening angle with bolometric temperature reported by Arce & Sargent and are
broadly consistent with that reported by Velusamy et al., suggesting that the
opening angle could be a good evolutionary indicator for LLOs. Accordingly, we
conclude that at least 40% of the outflow-driving LLOs in our sample are young
Class 0 objects.Comment: Accepted for publication in AJ, 13 pages, 9 figure
APEX telescope observations of new molecular ions
Hydrides are key ingredients of interstellar chemistry since they are the
initial products of chemical networks that lead to the formation of more
complex molecules. The fundamental rotational transitions of light hydrides
fall into the submillimeter wavelength range. Using the APEX telescope, we
observed the long sought hydrides SH+ and OH+ in absorption against the strong
continuum source Sagittarius B2(M). Both, absorption from Galactic center gas
as well as from diffuse clouds in intervening spiral arms over a large velocity
range is observed. The detected absorption of a continuous velocity range on
the line of sight shows these hydrides to be an abundant component of diffuse
clouds. In addition, we used the strongest submillimeter dust continuum sources
in the inner Galaxy to serve as background candles for a systematic census of
these hydrides in diffuse clouds and massive star forming regions of our Galaxy
and initial results of this survey are presented.Comment: To appear in Spectroscopy of Molecular Ions in the Laboratory and in
Space (SMILES 2010), AIP Conference Proceedings, in pres
Search for interstellar methoxyacetonitrile and cyanoethanol: insights into coupling of cyano- to methanol and ammonia chemistry
As part of an effort to study gas-grain chemical models in star-forming regions as they relate to molecules
containing cyanide (–C≡N) groups, we present here a search for the molecules 2-cyanoethanol (OHCH_2CH_2CN) and methoxyacetonitrile (CH_3OCH_2CN) in the galactic center region SgrB2. These species are structural isomers
of each other and are targeted to investigate the cross-coupling of pathways emanating from the photolysis products
of methanol and ammonia with pathways involving cyano-containing molecules. Methanol and ammonia ices are two of the main repositories of the elements C, O, and N in cold clouds and understanding their link to cyanide chemistry could give important insights into prebiotic molecular evolution. Neither species was positively detected, but the upper limits we determined allow comparison to the general patterns gleaned from chemical models. Our results indicate the need for an expansion of the model networks to better deal with cyanochemistry, in particular with respect to pathways including products of methanol photolysis. In addition to these results, the two main observational routes for detecting new interstellar molecules are discussed. One route is by decreasing detection limits at millimeter wavelength through spatial filtering with interferometric studies at the Atacama Large Millimeter Array (ALMA), and the second is by searching for intense torsional states at THz frequencies using the Herschel Space Observatory. 2-cyanoethanol and methoxyacetonitrile would both be good test beds for exploring the capabilities of ALMA and Herschel in the study of complex interstellar chemistry
Interstellar HOCN in the Galactic center region
Aims. Our aim is to confirm the interstellar detection of cyanic acid, HOCN,
in the Galactic center clouds. It has previously been tentatively detected only
in Sgr B2(OH).
Methods. We used a complete line survey of the hot cores Sgr B2(N) and (M) in
the 3 mm range, complemented by additional observations carried out with the
IRAM 30 m telescope at selected frequencies in the 2 mm band and towards four
additional positions in the Sgr B2 cloud complex in the 2 and 3 mm bands. The
spectral survey was analysed in the local thermodynamical equilibrium
approximation (LTE) by modeling the emission of all identified molecules
simultaneously. This allowed us to distinguish weak features of HOCN from the
rich line spectrum observed in Sgr B2(N) and (M). Lines of the more stable (by
1.1 eV) isomer isocyanic acid, HNCO, in these sources, as well as those of HOCN
and HNCO towards the other positions, were analysed in the LTE approximation as
well.
Results. Four transitions of HOCN were detected in a quiescent molecular
cloud in the Galactic center at a position offset in (R.A., decl.) by
(20'',100'') from the hot core source Sgr B2(M), confirming its previous
tentative interstellar detection. Up to four transitions were detected toward
five other positions in the Sgr B2 complex, including the hot cores Sgr B2(M),
(S), and (N). A fairly constant abundance ratio of ~ 0.3 - 0.8 % for HOCN
relative to HNCO was derived for the extended gas components, suggesting a
common formation process of these isomers
The dynamical state of the First Hydrostatic Core Candidate Cha-MMS1
Observations of First Hydrostatic Core candidates, a theoretically predicted
evolutionary link between the prestellar and protostellar phases, are vital for
probing the earliest phases of star formation. We aim to determine the
dynamical state of the First Hydrostatic Core candidate Cha-MMS1. We observed
Cha-MMS1 in various transitions with the APEX and Mopra telescopes. The
molecular emission was modeled with a radiative transfer code to derive
constraints on the envelope kinematics. We derive an internal luminosity of
0.08 - 0.18 Lsol. An average velocity gradient of 3.1(0.1) km/s/pc over 0.08 pc
is found perpendicular to the filament in which Cha-MMS1 is embedded. The
gradient is flatter in the outer parts and at the innermost 2000 - 4000 AU.
These features suggest solid-body rotation beyond 4000 AU and slower,
differential rotation beyond 8000 AU. The origin of the flatter gradient in the
innermost parts is unclear. The classical infall signature is detected in HCO+
3-2 and CS 2-1. The radiative transfer modeling indicates a uniform infall
velocity in the outer parts of the envelope. An infall velocity field scaling
with r^(-0.5) is consistent with the data for r < 9000 AU. The infall
velocities are 0.1 - 0.2 km/s at r > 3300 AU and 0.04 - 0.6 km/s at r < 3300
AU. Both the internal luminosity of Cha-MMS1 and the infall velocity field in
its envelope are consistent with predictions of MHD simulations for the first
core phase. There is no evidence for a fast, large-scale outflow stemming from
Cha-MMS1 but excess emission from the high-density tracers CS 5-4, CO 6-5, and
CO 7-6 suggests the presence of higher-velocity material at the inner core. Its
internal luminosity excludes Cha-MMS1 being a prestellar core. The kinematical
properties of its envelope are consistent with Cha-MMS1 being a first core
candidate or a very young Class 0 protostar.(abridged).Comment: Accepted for publication in A&A. 27 pages, 22 figures, 13 tables. A
version with high-resolution figures is available on request to the first
autho
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