2,885 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
On the stability of nonisothermal Bonnor-Ebert spheres. III. The role of chemistry in core stabilization
Aims. We investigate the effect of chemistry on the stability of starless
cores against gravitational collapse.
Methods. We combine chemical and radiative transfer simulations in the
context of a modified Bonnor-Ebert sphere to model the effect of chemistry on
the gas temperature, and study the effect of temperature changes on core
stability.
Results. We find that chemistry has in general very little effect on the
nondimensional radius which parametrizes the core stability.
Cores that are initially stable or unstable tend to stay near their initial
states, in terms of stability (i.e., constant), as the
chemistry develops. This result is independent of the initial conditions. We
can however find solutions where decreases at late times () which correspond to increased stabilization caused by
the chemistry. Even though the core stability is unchanged by the chemistry in
most of the models considered here, we cannot rule out the possibility that a
core can evolve from an unstable to a stable state owing to chemical evolution.
The reverse case, where an initially stable core becomes ultimately unstable,
seems highly unlikely.
Conclusions. Our results indicate that chemistry should be properly accounted
for in studies of star-forming regions, and that further investigations of core
stability especially with hydrodynamical models are warranted.Comment: 8 pages, 10 figures; accepted for publication in A&
Chemical tracers in proto-brown dwarfs: CO, ortho-HCO, para-HCO, HCO, CS observations
We present a study of the CO isotopologues and the high-density tracers
HCO, HCO, and CS in Class 0/I proto-brown dwarfs (proto-BDs). We
have used the IRAM 30m telescope to observe the CO (2-1), CO
(2-1), CO (2-1), CO (2-1), HCO (3-2), HCO (3-2), and
CS (5-4) lines in 7 proto-BDs. The hydrogen column density for the proto-BDs
derived from the CO gas emission is 2-15 times lower than that derived
from the dust continuum emission, indicating CO depletion from the gas-phase.
The mean HCO ortho-to-para ratio is 3 for the proto-BDs and
indicates gas-phase formation for HCO. We have investigated the
correlations in the molecular abundances between the proto-BDs and protostars.
Proto-BDs on average show a factor of 2 higher ortho-to-para HCO
ratio than the protostars. Possible explanations include a difference in the
HCO formation mechanism, spin-selective photo-dissociation,
self-shielding effects, or different emitting regions for the ortho and para
species. There is a tentative trend of a decline in the HCO and HCO
abundances with decreasing bolometric luminosity, while the CS and CO
abundances show no particular difference between the proto-BDs and protostars.
These trends reflect the scaled-down physical structures for the proto-BDs
compared to protostars and differences in the peak emitting regions for these
species. The CO isotopologue is detected in all of the proto-BDs as well
as the more evolved Class Flat/Class II BDs in our sample, and can probe the
quiescent gas at both early and late evolutionary stages.Comment: Accepted in MNRAS. arXiv admin note: text overlap with
arXiv:1809.1016
Chemical tracers in proto-brown dwarfs: CN, HCN, and HNC observations
We present results from a study of nitrogen chemistry in Class 0/I
proto-brown dwarfs (proto-BDs). We have used the IRAM 30 m telescope to observe
the CN (2-1), HCN (3-2), and HNC (3-2) lines in 7 proto-BDs. All proto-BDs show
a large CN/HCN abundance ratio of >20, and a HNC/HCN abundance ratio close to
or larger than unity. The enhanced CN/HCN ratios can be explained by high UV
flux originating from an active accretion zone in the proto-BDs. The larger
than unity HNC/HCN ratio for the proto-BDs is likely caused by a combination of
low temperature and high density. Both CN and HNC show a flat distribution with
CO, indicating that these species can survive in regions where CO is depleted.
We have investigated the correlations in the molecular abundances of these
species for the proto-BDs with Class 0/I protostars. We find tentative trends
of CN (HCN) abundances being about an order of magnitude higher (lower) in the
proto-BDs compared to protostars. HNC for the proto-BDs shows a nearly constant
abundance unlike the large spread of ~2 orders of magnitude seen for the
protostars. Also notable is a rise in the HNC/HCN abundance ratio for the
lowest luminosity objects, suggesting that this ratio is higher under
low-temperature environments. None of the relatively evolved Class Flat/Class
II brown dwarfs in our sample show emission in HNC. The HNC molecule can be
considered as an efficient tracer to search and identify early stage
sub-stellar mass objects.Comment: Accepted in MNRA
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
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
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
H_2D^+ in the High-mass Star-forming Region Cygnus X
H_2D^+ is a primary ion that dominates the gas-phase chemistry of cold dense gas. Therefore, it is hailed as a unique tool in probing the earliest, prestellar phase of star formation. Observationally, its abundance and distribution is, however, just beginning to be understood in low-mass prestellar and cluster-forming cores. In high-mass star-forming regions, H_2D^+ has been detected only in two cores, and its spatial distribution remains unknown. Here, we present the first map of the ortho-H_2D^+J_(k^+,k^-) = 1_(1,0) → 1_(1,1) and N_2H^+ 4-3 transition in the DR21 filament of Cygnus X with the James Clerk Maxwell Telescope, and N_2D^+ 3-2 and dust continuum with the Submillimeter Array. We have discovered five very extended (≤34, 000 AU diameter) weak structures in H2D+ in the vicinity of, but distinctly offset from, embedded protostars. More surprisingly, the H_2D^+ peak is not associated with either a dust continuum or N_2D^+ peak. We have therefore uncovered extended massive cold dense gas that was undetected with previous molecular line and dust continuum surveys of the region. This work also shows that our picture of the structure of cores is too simplistic for cluster-forming cores and needs to be refined: neither dust continuum with existing capabilities nor emission in tracers like N_2D^+ can provide a complete census of the total prestellar gas in such regions. Sensitive H_2D^+ mapping of the entire DR21 filament is likely to discover more of such cold quiescent gas reservoirs in an otherwise active high-mass star-forming region
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