2,738 research outputs found
Chemistry and kinematics of the pre-stellar core L1544: Constraints from H2D+
This paper explores the sensitivity of line profiles of H2D+, HCO+ and N2H+,
observed towards the center of L1544, to various kinematic and chemical
parameters. The total width of the H2D+ line can be matched by a static model
and by models invoking ambipolar diffusion and gravitational collapse. The
derived turbulent line width is b=0.15 km/s for the static case and <~ 0.05
km/s for the collapse case. However, line profiles of HC18O+ and N2H+ rule out
the static solution. The double-peaked H2D+ line shape requires either infall
speeds in the center that are much higher than predicted by ambipolar diffusion
models, or a shell-type distribution of H2D+, as is the case for HCO+ and N2H+.
At an offset of ~20 arcsec from the dust peak, the H2D+ abundance drops by a
factor of ~5.Comment: four pages, two colour figures; to appear in The Dense Interstellar
Medium in Galaxies, proceedings of the fourth Cologne-Bonn-Zermatt Symposium,
Sept 22-26, 200
Highly deuterated pre-stellar cores in a high-mass star formation region
We have observed the deuterated gas in the high-mass star formation region
IRAS 05345+3157 at high-angular resolution, in order to determine the
morphology and the nature of such gas. We have mapped the N2H+ (1-0) line with
the Plateau de Bure Interferometer, and the N2D+ (3-2) and N2H+ (3-2) lines
with the Submillimeter Array. The N2D+ (3-2) integrated emission is
concentrated in two condensations, with masses of 2-3 and 9 M_sun and diameters
of 0.05 and 0.09 pc, respectively. The high deuterium fractionation (0.1) and
the line parameters in the N2D+ condensations indicate that they are likely
low- to intermediate-mass pre-stellar cores, even though other scenarios are
possible.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and
Astrophysic
Chemical evolution in the environment of intermediate mass young stellar objects: NGC7129--FIRS2 and LkH234
We have carried out a molecular survey of the Class 0 IM protostar NGC 7129
-- FIRS 2 (hereafter FIRS 2) and the Herbig Be star LkH 234 with the
aim of studying the chemical evolution of the envelopes of intermediate-mass
(IM) young stellar objects (YSOs). Both objects have similar luminosities (~500
Lsun) and are located in the same molecular cloud which minimizes the chemical
differences due to different stellar masses or initial cloud conditions.
Moreover, since they are located at the same distance, we have the same spatial
resolution in both objects. A total of 17 molecular species (including rarer
isotopes) have been observed in both objects and the structure of their
envelopes and outflows is determined with unprecedent detail.
Our results show that the protostellar envelopes are dispersed and warmed up
during the evolution to become a pre-main sequence star. In fact, the envelope
mass decreases by a factor >5 from FIRS 2 to LkH234, while the kinetic
temperature increases from ~13K to 28K. On the other hand, there is no
molecular outflow associated with LkH234. The molecular outflow seems
to stop before the star becomes visible. These physical changes strongly affect
the chemistry of their envelopes.
Based on our results in FIRS2 and LkH 234, we propose some abundance
ratios that can be used as chemical clocks for the envelopes of IM YSOs. The
SiO/CS, CN/N2H+, HCN/N2H+, DCO+/HCO+ and D2CO/DCO+ ratios are good diagnostics
of the protostellar evolutionary stage.Comment: 24 pages, 17 figure
H ortho-to-para conversion on grains: A route to fast deuterium fractionation in dense cloud cores?
Deuterium fractionation, i.e. the enhancement of deuterated species with
respect to the non-deuterated ones, is considered to be a reliable chemical
clock of star-forming regions. This process is strongly affected by the
ortho-to-para (o-p) H ratio. In this letter we explore the effect of the
o-p H conversion on grains on the deuteration timescale in fully depleted
dense cores, including the most relevant uncertainties that affect this complex
process. We show that (i) the o-p H conversion on grains is not strongly
influenced by the uncertainties on the conversion time and the sticking
coefficient and (ii) that the process is controlled by the temperature and the
residence time of ortho-H on the surface, i.e. by the binding energy. We
find that for binding energies in between 330-550 K, depending on the
temperature, the o-p H conversion on grains can shorten the deuterium
fractionation timescale by orders of magnitude, opening a new route to explain
the large observed deuteration fraction in dense molecular
cloud cores. Our results suggest that the star formation timescale, when
estimated through the timescale to reach the observed deuteration fractions,
might be shorter than previously proposed. However, more accurate measurements
of the binding energy are needed to better assess the overall role of this
process.Comment: Accepted for publication in ApJ Letter
Mobile robot navigation in enclosed large-scale space
Journal ArticleIn a large-scale s ace, navigation may occur among very dispersed landmarks, further apart than the range of sensing of an autonomous vehicle. In this work we investigate the feasibility of construction of a landmark-based cognitive map, whose elements are the obstacles perceived by a robotic vehicle during exploration of an unknown, large-scale environment. This cognitive map can then be used as an aid for goal-oriented navigation in such a challenging environment. A map construction algorithm is described suitable for a mobile robot with the ability of temporarily marking a single location in an enclosed environment containing polygonal objects. The algorithm is being verifies with a LEGO-Technic-based autonomous vehicle, equipped with a 2 d.0.f. arm and relying on inaccurate odometric and short-range proximity sensing . In spite of its limited and inaccurate internal and external sensing abilities basic skills experimentally demonstrated by the robot include pick-and-place of a portable marker, obstacle detection, as well as characterization and recognition of polygonal objects. These skills, in conjunction with the approximate odometric measurements collected by the vehicle, also represent the repertoire of behaviors exploited in map-assisted navigation
First ALMA maps of HCO, an important precursor of complex organic molecules, towards IRAS 16293-2422
The formyl radical HCO has been proposed as the basic precursor of many
complex organic molecules such as methanol (CHOH) or glycolaldehyde
(CHOHCHO). Using ALMA, we have mapped, for the first time at high angular
resolution (1, 140 au), HCO towards the Solar-type
protostellar binary IRAS 162932422, where numerous complex organic molecules
have been previously detected. We also detected several lines of the chemically
related species HCO, CHOH and CHOHCHO. The observations revealed
compact HCO emission arising from the two protostars. The line profiles also
show redshifted absorption produced by foreground material of the circumbinary
envelope that is infalling towards the protostars. Additionally, IRAM 30m
single-dish data revealed a more extended HCO component arising from the common
circumbinary envelope. The comparison between the observed molecular abundances
and our chemical model suggests that whereas the extended HCO from the envelope
can be formed via gas-phase reactions during the cold collapse of the natal
core, the HCO in the hot corinos surrounding the protostars is predominantly
formed by the hydrogenation of CO on the surface of dust grains and subsequent
thermal desorption during the protostellar phase. The derived abundance of HCO
in the dust grains is high enough to produce efficiently more complex species
such as HCO, CHOH, and CHOHCHO by surface chemistry. We found that
the main formation route of CHOHCHO is the reaction between HCO and
CHOH.Comment: Accepted in Monthly Notices of the Royal Astronomical Society; 19
pages, 12 figures, 7 table
On the chemical ladder of esters. Detection and formation of ethyl formate in the W51 e2 hot molecular core
The detection of organic molecules with increasing complexity and potential
biological relevance is opening the possibility to understand the formation of
the building blocks of life in the interstellar medium. One of the families of
molecules with astrobiological interest are the esters, whose simplest member,
methyl formate, is rather abundant in star-forming regions. The next step in
the chemical complexity of esters is ethyl formate, CHOCHO. Only two
detections of this species have been reported so far, which strongly limits our
understanding of how complex molecules are formed in the interstellar medium.
We have searched for ethyl formate towards the W51 e2 hot molecular core, one
of the most chemically rich sources in the Galaxy and one of the most promising
regions to study prebiotic chemistry, especially after the recent discovery of
the PO bond, key in the formation of DNA. We have analyzed a spectral line
survey towards the W51 e2 hot molecular core, which covers 44 GHz in the 1, 2
and 3 mm bands, carried out with the IRAM 30m telescope. We report the
detection of the trans and gauche conformers of ethyl formate. A Local
Thermodynamic Equilibrium analysis indicates that the excitation temperature is
7810 K and that the two conformers have similar source-averaged column
densities of (2.00.3)10 cm and an abundance of
10. We compare the observed molecular abundances of ethyl formate
with different competing chemical models based on grain surface and gas-phase
chemistry. We propose that grain-surface chemistry may have a dominant role in
the formation of ethyl formate (and other complex organic molecules) in hot
molecular cores, rather than reactions in the gas phase.Comment: Accepted in A&A; 11 pages, 6 figures, 7 Table
Detection of a hot core in the intermediate-mass (IM) Class 0 protostar NGC 7129--FIRS2
We report high angular resolution (~0.6''x0.5'' at 1.3mm) observations of the
Class 0 intermediate-mass (IM) protostar NGC 7129--FIRS 2 using the PdBI. Our
observations show the existence of a hot core at the position of the Class 0
object. This is, up to our knowledge, the first IM hot core detected so far.
Interferometric maps of the region in the the continuum at 3mm and 1.3mm and in
the CH3CN 5_k-->4_k, CH3OH 5_kk'-->4_kk', and D2CO 4_04-->3_03 lines are
presented in this paper. Enhanced CH3CN and CH3OH abundances are measured
towards the hot core (X(CH3CN)~7.010^{-9}, X(CH3OH)~3 10^{-8}-- a few 10^{-7}).
While intense D2CO emission is detected towards the hot core, the N2D+ line has
not been detected in our interferometric observations. The different behaviors
of D2CO and N2D+ emissions suggest different formation mechanisms for the two
species and different deuteration processes for H2CO and N2H+ (surface and
gas-phase chemistry, respectively). Finally, the spectrum of the large
bandwidth correlator show a forest of lines at the hot core position revealing
that this object is extraordinarily rich in complex molecules.Comment: 14 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
Identifying communicative functions in discourse with content types
Texts are not monolithic entities but rather coherent collections of micro illocutionary acts which help to convey a unitary message of content and purpose. Identifying such text segments is challenging because they require a fine-grained level of analysis even within a single sentence. At the same time, accessing them facilitates the analysis of the communicative functions of a text as well as the identification of relevant information. We propose an empirical framework for modelling micro illocutionary acts at clause level, that we call content types, grounded on linguistic theories of text types, in particular on the framework proposed by Werlich in 1976. We make available a newly annotated corpus of 279 documents (for a total of more than 180,000 tokens) belonging to different genres and temporal periods, based on a dedicated annotation scheme. We obtain an average Cohen’s kappa of 0.89 at token level. We achieve an average F1 score of 74.99% on the automatic classification of content types using a bi-LSTM model. Similar results are obtained on contemporary and historical documents, while performances on genres are more varied. This work promotes a discourse-oriented approach to information extraction and cross-fertilisation across disciplines through a computationally-aided linguistic analysis
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