43 research outputs found
Isotopic Anomalies in Primitive Solar System Matter: Spin-state Dependent Fractionation of Nitrogen and Deuterium in Interstellar Clouds
Organic material found in meteorites and interplanetary dust particles is
enriched in D and 15N. This is consistent with the idea that the functional
groups carrying these isotopic anomalies, nitriles and amines, were formed by
ion-molecule chemistry in the protosolar nebula. Theoretical models of
interstellar fractionation at low temperatures predict large enrichments in
both D and 15N and can account for the largest isotopic enrichments measured in
carbonaceous meteorites. However, more recent measurements have shown that, in
some primitive samples, a large 15N enrichment does not correlate with one in
D, and that some D-enriched primitive material displays little, if any, 15N
enrichment. By considering the spin-state dependence in ion-molecule reactions
involving the ortho and para forms of H2, we show that ammonia and related
molecules can exhibit such a wide range of fractionation for both 15N and D in
dense cloud cores. We also show that while the nitriles, HCN and HNC, contain
the greatest 15N enrichment, this is not expected to correlate with extreme D
enrichment. These calculations therefore support the view that Solar System 15N
and D isotopic anomalies have an interstellar heritage. We also compare our
results to existing astronomical observations and briefly discuss future tests
of this model.Comment: Submitted to ApJ
On the nature of the enigmatic object IRAS 19312+1950: A rare phase of massive star formation?
IRAS 19312+1950 is a peculiar object that has eluded firm characterization
since its discovery, with combined maser properties similar to an evolved star
and a young stellar object (YSO). To help determine its true nature, we
obtained infrared spectra of IRAS 19312+1950 in the range 5-550 m using
the Herschel and Spitzer space observatories. The Herschel PACS maps exhibit a
compact, slightly asymmetric continuum source at 170 m, indicative of a
large, dusty circumstellar envelope. The far-IR CO emission line spectrum
reveals two gas temperature components: of material at
K, and of material at K. The OI 63
m line is detected on-source but no significant emission from atomic ions
was found. The HIFI observations display shocked, high-velocity gas with
outflow speeds up to 90 km s along the line of sight. From Spitzer
spectroscopy, we identify ice absorption bands due to HO at 5.8 m and
CO at 15 m. The spectral energy distribution is consistent with a
massive, luminous () central source surrounded by a
dense, warm circumstellar disk and envelope of total mass
-, with large bipolar outflow cavities. The combination
of distinctive far-IR spectral features suggest that IRAS 19312+1950 should be
classified as an accreting high-mass YSO rather than an evolved star. In light
of this reclassification, IRAS 19312+1950 becomes only the 5th high-mass
protostar known to exhibit SiO maser activity, and demonstrates that 18 cm OH
maser line ratios may not be reliable observational discriminators between
evolved stars and YSOs.Comment: 16 pages. Accepted for publication in Ap
On the accretion process in a high-mass star forming region - A multitransitional THz Herschel-HIFI study of ammonia toward G34.26+0.15
[Abridged] Our aim is to explore the gas dynamics and the accretion process
in the early phase of high-mass star formation. The inward motion of molecular
gas in the massive star forming region G34.26+0.15 is investigated by using
high-resolution profiles of seven transitions of ammonia at THz frequencies
observed with Herschel-HIFI. The shapes and intensities of these lines are
interpreted in terms of radiative transfer models of a spherical, collapsing
molecular envelope. An accelerated Lambda Iteration (ALI) method is used to
compute the models. The seven ammonia lines show mixed absorption and emission
with inverse P-Cygni-type profiles that suggest infall onto the central source.
A trend toward absorption at increasingly higher velocities for higher
excitation transitions is clearly seen in the line profiles. The lines show only very weak emission, so these absorption profiles
can be used directly to analyze the inward motion of the gas. This is the first
time a multitransitional study of spectrally resolved rotational ammonia lines
has been used for this purpose. Broad emission is, in addition, mixed with the
absorption in the ortho-NH line, possibly tracing a molecular
outflow from the star forming region. The best-fitting ALI model reproduces the
continuum fluxes and line profiles, but slightly underpredicts the emission and
absorption depth in the ground-state ortho line . The derived
ortho-to-para ratio is approximately 0.5 throughout the infalling cloud core
similar to recent findings for translucent clouds in sight lines toward W31C
and W49N. We find evidence of two gas components moving inwards toward the
central region with constant velocities: 2.7 and 5.3 kms, relative
to the source systemic velocity. The inferred mass accretion rates derived are
sufficient to overcome the expected radiation pressure from G34.26+0.15.Comment: 20 pages, 18 figures, accepted by A&A 3 October 201
Similar levels of deuteration in the pre-stellar core L1544 and the protostellar core HH211
In the centre of pre-stellar cores, deuterium fractionation is enhanced due
to the low temperatures and high densities. Therefore, the chemistry of
deuterated molecules can be used to study the earliest stages of star
formation. We analyse the deuterium fractionation of simple molecules,
comparing the level of deuteration in the envelopes of the pre-stellar core
L1544 in Taurus and the protostellar core HH211 in Perseus. We used single-dish
observations of CCH, HCN, HNC, HCO, and their C-, O- and
D-bearing isotopologues, detected with the Onsala 20m telescope. We derived the
column densities and the deuterium fractions of the molecules. Additionally, we
used radiative transfer simulations and results from chemical modelling to
reproduce the observed molecular lines. We used new collisional rate
coefficients for HNC, HNC, DNC, and DCN that consider the hyperfine
structure of these molecules. We find high levels of deuteration for CCH (10%)
in both sources, consistent with other carbon chains, and moderate levels for
HCN (5-7%) and HNC (8%). The deuterium fraction of HCO is enhanced towards
HH211, most likely caused by isotope-selective photodissociation of CO.
Similar levels of deuteration show that the process is likely equally efficient
towards both cores, suggesting that the protostellar envelope still retains the
chemical composition of the original pre-stellar core. The fact that the two
cores are embedded in different molecular clouds also suggests that
environmental conditions do not have a significant effect on the deuteration
within dense cores. Radiative transfer modelling shows that it is necessary to
include the outer layers of the cores to consider the effects of extended
structures. Besides HCO observations, HCN observations towards L1544 also
require the presence of an outer diffuse layer where the molecules are
relatively abundant.Comment: 27 pages, 17 figures, accepted for publication in A&
A spectral line survey of Orion KL in the bands 486-492 and 541-577 GHz with the Odin satellite I. The observational data
Spectral line surveys are useful since they allow identification of new
molecules and new lines in uniformly calibrated data sets. Nonetheless, large
portions of the sub-millimetre spectral regime remain unexplored due to severe
absorptions by H2O and O2 in the terrestrial atmosphere. The purpose of the
measurements presented here is to cover wavelength regions at and around 0.55
mm -- regions largely unobservable from the ground. Using the Odin
astronomy/aeronomy satellite, we performed the first spectral survey of the
Orion KL molecular cloud core in the bands 486--492 and 541--576 GHz with
rather uniform sensitivity (22--25 mK baseline noise). Odin's 1.1 m size
telescope, equipped with four cryo-cooled tuneable mixers connected to broad
band spectrometers, was used in a satellite position-switching mode. Two mixers
simultaneously observed different 1.1 GHz bands using frequency steps of 0.5
GHz (25 hours each). An on-source integration time of 20 hours was achieved for
most bands. The entire campaign consumed ~1100 orbits, each containing one hour
of serviceable astro-observation. We identified 280 spectral lines from 38
known interstellar molecules (including isotopologues) having intensities in
the range 80 to 0.05 K. An additional 64 weak lines remain unidentified. Apart
from the ground state rotational 1(1,0)--1(0,1) transitions of ortho-H2O, H218O
and H217O, the high energy 6(2,4)--7(1,7) line of para-H2O and the
HDO(2,0,2--1,1,1) line have been observed, as well as the 1,0--0,1 lines from
NH3 and its rare isotopologue 15NH3. We suggest assignments for some
unidentified features, notably the new interstellar molecules ND and SH-.
Severe blends have been detected in the line wings of the H218O, H217O and 13CO
lines changing the true linewidths of the outflow emission.Comment: 21 pages, 10 figures, 7 tables, accepeted for publication in
Astronomy and Astrophysics 30 August 200
Upper limits to interstellar NH^+ and para-NH_2^− abundances. Herschel-HIFI observations towards Sgr B2 (M) and G10.6−0.4 (W31C)
The understanding of interstellar nitrogen chemistry has improved significantly with recent results from the Herschel Space Observatory. To set even better constraints, we report here on deep searches for the NH^+ ground state rotational transition J = 1.5−0.5 of the ^2Π_(1/2) lower spin ladder, with fine-structure transitions at 1013 and 1019 GHz, and the para-NH_2^− 1_(1,1)−0_(0,0) rotational transition at 934 GHz towards Sgr B2 (M) and G10.6−0.4 (W31C) using the Herschel Heterodyne Instrument for the Far-Infrared (HIFI). No clear detections of NH^+ are made and the derived upper limits relative to the total number of hydrogen nuclei are ≲2 × 10^(-12) and ≲7 × 10^(-13) in the Sgr B2 (M) molecular envelope and in the G10.6−0.4 molecular cloud, respectively. The searches are, however, complicated by the fact that the 1 013 GHz transition lies only −2.5 km s^(-1) from a CH_2NH line, which is seen in absorption in Sgr B2 (M), and that the hyperfine structure components in the 1019 GHz transition are spread over 134 km s^(-1). Searches for the so far undetected NH_2^− anion turned out to be unfruitful towards G10.6−0.4, while the para-NH_2^− 1_(1,1)−0_(0,0) transition was tentatively detected towards Sgr B2 (M) at a velocity of 19 km s^(-1). Assuming that the absorption occurs at the nominal source velocity of +64 km s^(-1), the rest frequency would be 933.996 GHz, offset by 141 MHz from our estimated value. Using this feature as an upper limit, we found N(p-NH_2^−) ≲4 × 10^(11) cm^(-2), which implies an abundance of ≲8 × 10^(-13) in the Sgr B2 (M) molecular envelope. The upper limits for both species in the diffuse line-of-sight gas are less than 0.1 to 2% of the values found for NH, NH_2, and NH_3 towards both sources, and the abundance limits are ≲2−4 × 10^(-11). An updated pseudo time-dependent chemical model with constant physical conditions, including both gas-phase and surface chemistry, predicts an NH^+ abundance a few times lower than our present upper limits in diffuse gas and under typical Sgr B2 (M) envelope conditions. The NH_2^− abundance is predicted to be several orders of magnitudes lower than our observed limits, hence not supporting our tentative detection. Thus, while NH_2^− may be very difficult to detect in interstellar space, it could, on the other hand, be possible to detect NH^+ in regions where the ionisation rates of H_2 and N are greatly enhanced
Ground-state ammonia and water in absorption towards Sgr B2
We have used the Odin submillimetre-wave satellite telescope to observe the
ground state transitions of ortho-ammonia and ortho-water, including their 15N,
18O, and 17O isotopologues, towards Sgr B2. The extensive simultaneous velocity
coverage of the observations, >500 km/s, ensures that we can probe the
conditions of both the warm, dense gas of the molecular cloud Sgr B2 near the
Galactic centre, and the more diffuse gas in the Galactic disk clouds along the
line-of-sight. We present ground-state NH3 absorption in seven distinct
velocity features along the line-of-sight towards Sgr B2. We find a nearly
linear correlation between the column densities of NH3 and CS, and a
square-root relation to N2H+. The ammonia abundance in these diffuse Galactic
disk clouds is estimated to be about (0.5-1)e-8, similar to that observed for
diffuse clouds in the outer Galaxy. On the basis of the detection of H218O
absorption in the 3 kpc arm, and the absence of such a feature in the H217O
spectrum, we conclude that the water abundance is around 1e-7, compared to
~1e-8 for NH3. The Sgr B2 molecular cloud itself is seen in absorption in NH3,
15NH3, H2O, H218O, and H217O, with emission superimposed on the absorption in
the main isotopologues. The non-LTE excitation of NH3 in the environment of Sgr
B2 can be explained without invoking an unusually hot (500 K) molecular layer.
A hot layer is similarly not required to explain the line profiles of the
1_{1,0}-1_{0,1} transition from H2O and its isotopologues. The relatively weak
15NH3 absorption in the Sgr B2 molecular cloud indicates a high [14N/15N]
isotopic ratio >600. The abundance ratio of H218O and H217O is found to be
relatively low, 2.5--3. These results together indicate that the dominant
nucleosynthesis process in the Galactic centre is CNO hydrogen burning.Comment: 10 pages, 5 figure
The composition of the protosolar disk and the formation conditions for comets
Conditions in the protosolar nebula have left their mark in the composition
of cometary volatiles, thought to be some of the most pristine material in the
solar system. Cometary compositions represent the end point of processing that
began in the parent molecular cloud core and continued through the collapse of
that core to form the protosun and the solar nebula, and finally during the
evolution of the solar nebula itself as the cometary bodies were accreting.
Disentangling the effects of the various epochs on the final composition of a
comet is complicated. But comets are not the only source of information about
the solar nebula. Protostellar disks around young stars similar to the protosun
provide a way of investigating the evolution of disks similar to the solar
nebula while they are in the process of evolving to form their own solar
systems. In this way we can learn about the physical and chemical conditions
under which comets formed, and about the types of dynamical processing that
shaped the solar system we see today.
This paper summarizes some recent contributions to our understanding of both
cometary volatiles and the composition, structure and evolution of protostellar
disks.Comment: To appear in Space Science Reviews. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11214-015-0167-