2,513 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
Detection of N15NH+ in L1544
Excess levels of 15N isotopes which have been detected in primitive solar
system materials are explained as a remnant of interstellar chemistry which
took place in regions of the protosolar nebula. Chemical models of nitrogen
fractionation in cold clouds predict an enhancement in the gas-phase abundance
of 15N-bearing molecules, thus we have searched for 15N variants of the N2H+
ion in L1544, which is one of the best candidate sources for detection owing to
its low central core temperature and high CO depletion. With the IRAM 30m
telescope we have obtained deep integrations of the N2H+(1-0) line at 91.2 GHz.
The N2H+(1-0) line has been detected toward the dust emission peak of L1544.
The 14N/15N abundance ratio in N2H+ resulted 446+/-71, very close to the
protosolar value of ~450, higher than the terrestrial ratio of ~270, and
significantly lower than the lower limit in L1544 found by Gerin et al. (2009,
ApJ, 570, L101) in the same object using ammonia isotopologues.Comment: Accepted for publication in Astronomy and Astrophysic
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
First measurements of 15N fractionation in N2H+ toward high-mass star forming cores
We report on the first measurements of the isotopic ratio 14N/15N in N2H+
toward a statistically significant sample of high-mass star forming cores. The
sources belong to the three main evolutionary categories of the high-mass star
formation process: high-mass starless cores, high-mass protostellar objects,
and ultracompact HII regions. Simultaneous measurements of 14N/15N in CN have
been made. The 14N/15N ratios derived from N2H+ show a large spread (from ~180
up to ~1300), while those derived from CN are in between the value measured in
the terrestrial atmosphere (~270) and that of the proto-Solar nebula (~440) for
the large majority of the sources within the errors. However, this different
spread might be due to the fact that the sources detected in the N2H+
isotopologues are more than those detected in the CN ones. The 14N/15N ratio
does not change significantly with the source evolutionary stage, which
indicates that time seems to be irrelevant for the fractionation of nitrogen.
We also find a possible anticorrelation between the 14N/15N (as derived from
N2H+) and the H/D isotopic ratios. This suggests that 15N enrichment could not
be linked to the parameters that cause D enrichment, in agreement with the
prediction by recent chemical models. These models, however, are not able to
reproduce the observed large spread in 14N/15N, pointing out that some
important routes of nitrogen fractionation could be still missing in the
models.Comment: 2 Figures, accepted for publication in ApJ
The observed chemical structure of L1544
Prior to star formation, pre-stellar cores accumulate matter towards the
centre. As a consequence, their central density increases while the temperature
decreases. Understanding the evolution of the chemistry and physics in this
early phase is crucial to study the processes governing the formation of a
star. We aim at studying the chemical differentiation of a prototypical
pre-stellar core, L1544, by detailed molecular maps. In contrast with single
pointing observations, we performed a deep study on the dependencies of
chemistry on physical and external conditions. We present the emission maps of
39 different molecular transitions belonging to 22 different molecules in the
central 6.25 arcmin of L1544. We classified our sample in five families,
depending on the location of their emission peaks within the core. Furthermore,
to systematically study the correlations among different molecules, we have
performed the principal component analysis (PCA) on the integrated emission
maps. The PCA allows us to reduce the amount of variables in our dataset.
Finally, we compare the maps of the first three principal components with the
H column density map, and the T map of the core. The results of
our qualitative analysis is the classification of the molecules in our dataset
in the following groups: (i) the -CH family (carbon chain
molecules), (ii) the dust peak family (nitrogen-bearing species), (iii) the
methanol peak family (oxygen-bearing molecules), (iv) the HNCO peak family
(HNCO, propyne and its deuterated isotopologues). Only HCO and
CS do not belong to any of the above mentioned groups. The principal
component maps allow us to confirm the (anti-)correlations among different
families that were described in a first qualitative analysis, but also points
out the correlation that could not be inferred before.Comment: 29 pages, 19 figures, 2 appendices, accepted for publication in A&A,
arXiv abstract has been slightly modifie
O2 signature in thin and thick O2-H2O ices
Aims. In this paper we investigate the detectability of the molecular oxygen
in icy dust grain mantles towards astronomical objects. Methods. We present a
systematic set of experiments with O2-H2O ice mixtures designed to disentangle
how the molecular ratio affects the O2 signature in the mid- and near-infrared
spectral regions. All the experiments were conducted in a closed-cycle helium
cryostat coupled to a Fourier transform infrared spectrometer. The ice mixtures
comprise varying thicknesses from 8 10 to 3 m. The
absorption spectra of the O2-H2O mixtures are also compared to the one of pure
water. In addition, the possibility to detect the O2 in icy bodies and in the
interstellar medium is discussed. Results. We are able to see the O2 feature at
1551 cm even for the most diluted mixture of H2O : O2 = 9 : 1,
comparable to a ratio of O2/H2O = 10 % which has already been detected in situ
in the coma of the comet 67P/Churyumov-Gerasimenko. We provide an estimate for
the detection of O2 with the future mission of the James Webb Space Telescope
(JWST).Comment: 11 pages, 10 figures, article in press, to appear in A&A 201
Protonated CO2 in massive star-forming clumps
Interstellar CO2 is an important reservoir of carbon and oxygen, and one of
the major constituents of the icy mantles of dust grains, but it is not
observable directly in the cold gas because has no permanent dipole moment. Its
protonated form, HOCO+, is believed to be a good proxy for gaseous CO2.
However, it has been detected in only a few star-forming regions so far, so
that its interstellar chemistry is not well understood. We present new
detections of HOCO+ lines in 11 high-mass star-forming clumps. Our observations
increase by more than three times the number of detections in star-forming
regions so far. We have derived beam-averaged abundances relative to H2 in
between 0.3 and 3.8 x 10^{-11}. We have compared these values with the
abundances of H13CO+, a possible gas-phase precursor of HOCO+, and CH3OH, a
product of surface chemistry. We have found a positive correlation with H13CO+,
while with CH3OH there is no correlation. We suggest that the gas-phase
formation route starting from HCO+ plays an important role in the formation of
HOCO+, perhaps more relevant than protonation of CO2 (upon evaporation of this
latter from icy dust mantles).Comment: 5 pages, 4 figures, 1 table, accepted for publication in MNRA
The Initial Conditions of Clustered Star Formation III. The Deuterium Fractionation of the Ophiuchus B2 Core
We present N2D+ 3-2 (IRAM) and H2D+ 1_11 - 1_10 and N2H+ 4-3 (JCMT) maps of
the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular
cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+
data reveal the deuterium fractionation in the high density gas across Oph B2.
The average deuterium fractionation R_D = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2,
with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is
consistent with previous results in isolated starless and protostellar cores.
The column density distributions of both H2D+ and N2D+ show no correlation with
total H2 column density. We find, however, an anticorrelation in deuterium
fractionation with proximity to the embedded protostars in Oph B2 to distances
>= 0.04 pc. Destruction mechanisms for deuterated molecules require gas
temperatures greater than those previously determined through NH3 observations
of Oph B2 to proceed. We present temperatures calculated for the dense core gas
through the equating of non-thermal line widths for molecules (i.e., N2D+ and
H2D+) expected to trace the same core regions, but the observed complex line
structures in B2 preclude finding a reasonable result in many locations. This
method may, however, work well in isolated cores with less complicated velocity
structures. Finally, we use R_D and the H2D+ column density across Oph B2 to
set a lower limit on the ionization fraction across the core, finding a mean
x_e, lim >= few x 10^{-8}. Our results show that care must be taken when using
deuterated species as a probe of the physical conditions of dense gas in
star-forming regions.Comment: ApJ accepte
N2H+(1-0) survey of massive molecular cloud cores
We present the results of N2H+(1-0) observations of 35 dense molecular cloud
cores from the northern and southern hemispheres where massive stars and star
clusters are formed. Line emission has been detected in 33 sources, for 28
sources detailed maps have been obtained. The optical depth of (23-12)
component toward peak intensity positions of 10 sources is ~ 0.2-1. In total,
47 clumps have been revealed in 26 sources. Integrated intensity maps with
aspect ratios < 2 have been fitted with a power-law radial distribution
convolved with the telescope beam. Mean power-law index is close to
unity corresponding to the density profile provided N2H+
excitation conditions do not vary inside these regions. Line widths of the
cores either decrease or stay constant with distance from the center. The ratio
of rotational to gravitational energy is too low for rotation to play a
significant role in the dynamics of the cores. A correlation between mean line
widths and sizes of clumps has been found.Comment: 17 pages, Late
Kinematics of dense gas in the L1495 filament
We study the kinematics of the dense gas of starless and protostellar cores
traced by the N2D+(2-1), N2H+(1-0), DCO+(2-1), and H13CO+(1-0) transitions
along the L1495 filament and the kinematic links between the cores and the
surrounding molecular cloud.
We measure velocity dispersions, local and total velocity gradients and
estimate the specific angular momenta of 13 dense cores in the four transitions
using the on-the-fly observations with the IRAM 30 m antenna. To study a
possible connection to the filament gas, we use the fit results of the
C18O(1-0) survey performed by Hacar et al. (2013).
All cores show similar properties along the 10 pc-long filament. N2D+(2-1)
shows the most centrally concentrated structure, followed by N2H+(1-0) and
DCO+(2-1), which show similar spatial extent, and H13CO+(1-0). The non-thermal
contribution to the velocity dispersion increases from higher to lower density
tracers. The change of magnitude and direction of the total velocity gradients
depending on the tracer used indicates that internal motions change at
different depths within the cloud. N2D+ and N2H+ show smaller gradients than
the lower density tracers DCO+ and H13CO+, implying a loss of specific angular
momentum at small scales. At the level of cloud-core transition, the core's
external envelope traced by DCO+ and H13CO+ is spinning up, consistent with
conservation of angular momentum during core contraction. C18O traces the more
extended cloud material whose kinematics is not affected by the presence of
dense cores. The decrease in specific angular momentum towards the centres of
the cores shows the importance of local magnetic fields to the small scale
dynamics of the cores. The random distributions of angles between the total
velocity gradient and large scale magnetic field suggests that the magnetic
fields may become important only in the high density gas within dense cores.Comment: Accepted for publication in A&A. The abstract is shortene
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