907 research outputs found
Submillimeter continuum observations of Sagittarius B2 at subarcsecond spatial resolution
We report the first high spatial resolution submillimeter continuum
observations of the Sagittarius B2 cloud complex using the Submillimeter Array
(SMA). With the subarcsecond resolution provided by the SMA, the two massive
star-forming clumps Sgr B2(N) and Sgr B2(M) are resolved into multiple compact
sources. In total, twelve submillimeter cores are identified in the Sgr B2(M)
region, while only two components are observed in the Sgr B2(N) clump. The gas
mass and column density are estimated from the dust continuum emission. We find
that most of the cores have gas masses in excess of 100 M and column
densities above 10 cm. The very fragmented appearance of Sgr
B2(M), in contrast to the monolithic structure of Sgr B2 (N), suggests that the
former is more evolved. The density profile of the Sgr B2(N)-SMA1 core is well
fitted by a Plummer density distribution. This would lead one to believe that
in the evolutionary sequence of the Sgr B2 cloud complex, a massive star forms
first in an homogeneous core, and the rest of the cluster forms subsequently in
the then fragmenting structure.Comment: 4 pages, 2 figures, accepted by A&A letter
High Velocity Outflow in CO J=7-6 from the Orion Hot Core
Using the Caltech Submillimeter Observatory 10.4-meter telescope, we
performed sensitive mapping observations of 12CO J=7-6 emission at 807 GHz
towards Orion IRc2. The image has an angular resolution of 10", which is the
highest angular resolution data toward the Orion Hot Core published for this
transition. In addition, thanks to the on-the-fly mapping technique, the
fidelity of the new image is rather high, particularly in comparison to
previous images. We have succeeded in mapping the northwest-southeast
high-velocity molecular outflow, whose terminal velocity is shifted by ~70-85
km/s with respect to the systemic velocity of the cloud. This yields an
extremely short dynamical time scale of ~900 years. The estimated outflow mass
loss rate shows an extraordinarily high value, on the order of 10^{-3} Msun/yr.
Assuming that the outflow is driven by Orion IRc2, our result agrees with the
picture so far obtained for a 20 Msun (proto)star in the process of formation.Comment: accepted by ApJ main journal, 13 pages 5 color figure
Searches for HCl and HF in comets 103P/Hartley 2 and C/2009 P1 (Garradd) with the Herschel space observatory
HCl and HF are expected to be the main reservoirs of fluorine and chlorine
wherever hydrogen is predominantly molecular. They are found to be strongly
depleted in dense molecular clouds, suggesting freeze-out onto grains in such
cold environments. We can then expect that HCl and HF were also the major
carriers of Cl and F in the gas and icy phases of the outer solar nebula, and
were incorporated into comets. We aimed to measure the HCl and HF abundances in
cometary ices as they can provide insights on the halogen chemistry in the
early solar nebula. We searched for the J(1-0) lines of HCl and HF at 626 and
1232 GHz, respectively, using the HIFI instrument on board the Herschel Space
Observatory. HCl was searched for in comets 103P/Hartley 2 and C/2009 P1
(Garradd), whereas observations of HF were conducted in comet C/2009 P1. In
addition, observations of HO and HO lines were performed in C/2009
P1 to measure the HO production rate. Three lines of CHOH were
serendipitously observed in the HCl receiver setting. HCl is not detected,
whereas a marginal (3.6-) detection of HF is obtained. The upper limits
for the HCl abundance relative to water are 0.011% and 0.022%, for 103P and
C/2009 P1, respectively, showing that HCl is depleted with respect to the solar
Cl/O abundance by a factor more than 6 in 103P, where the error is
related to the uncertainty in the chlorine solar abundance. The marginal HF
detection obtained in C/2009 P1 corresponds to an HF abundance relative to
water of (1.80.5) 10, which is approximately consistent
with a solar photospheric F/O abundance. The observed depletion of HCl suggests
that HCl was not the main reservoir of chlorine in the regions of the solar
nebula where these comets formed. HF was possibly the main fluorine compound in
the gas phase of the outer solar nebula.Comment: Accepted for publication in Astronomy & Astrophysic
Dynamics and thermalization of the nuclear spin bath in the single-molecule magnet Mn12-ac: test for the theory of spin tunneling
The description of the tunneling of a macroscopic variable in the presence of
a bath of localized spins is a subject of great fundamental and practical
interest, and is relevant for many solid-state qubit designs. Instead of
focusing on the the "central spin" (as is most often done), here we present a
detailed study of the dynamics of the nuclear spin bath in the Mn12-ac
single-molecule magnet, probed by NMR experiments down to very low temperatures
(T = 20 mK). We find that the longitudinal relaxation rate of the 55Mn nuclei
in Mn12-ac becomes roughly T-independent below T = 0.8 K, and can be strongly
suppressed with a longitudinal magnetic field. This is consistent with the
nuclear relaxation being caused by quantum tunneling of the molecular spin, and
we attribute the tunneling fluctuations to the minority of fast-relaxing
molecules present in the sample. The transverse nuclear relaxation is also
T-independent for T < 0.8 K, and can be explained qualitatively and
quantitatively by the dipolar coupling between like nuclei in neighboring
molecules. We also show that the isotopic substitution of 1H by 2H leads to a
slower nuclear longitudinal relaxation, consistent with the decreased tunneling
probability of the molecular spin. Finally, we demonstrate that, even at the
lowest temperatures, the nuclear spins remain in thermal equilibrium with the
lattice phonons, and we investigate the timescale for their thermal
equilibration. After a review of the theory of macroscopic spin tunneling in
the presence of a spin bath, we argue that most of our experimental results are
consistent with that theory, but the thermalization of the nuclear spins is
not.Comment: 24 pages, 18 figures. Experimental study of the spin bath dynamics in
quantum nanomagnets, plus an extensive review and application of the theor
Ammonia and other parent molecules in comet 10P/Tempel 2 from Herschel/HIFI and ground-based radio observations
The Jupiter-family comet 10P/Tempel 2 was observed during its 2010 return
with the Herschel Space Observatory. We present here the observation of the (J,
K) = (1, 0)-(0, 0) transition of ammonia at 572 GHz in this comet with the
Heterodyne Instrument for the Far Infrared (HIFI) of Herschel. We also report
on radio observations of other molecules (HCN, CH3OH, H2S and CS) obtained
during the 1999 return of the comet with the CSO telescope and the JCMT, and
during its 2010 return with the IRAM 30-m telescope. Molecular abundances
relative to water are 0.09%, 1.8%, 0.4%, and 0.08% for HCN, CH3OH, H2S, and CS,
respectively. An abundance of 0.5% for NH3 is obtained, which is similar to the
values measured in other comets. The hyperfine structure of the ammonia line is
resolved for the first time in an astronomical source. Strong anisotropy in the
outgassing is present in all observations from 1999 to 2010 and is modelled to
derive the production rates.Comment: 6 pages and 8 figures. Accepted for publication in Astronomy &
Astrophysic
Li abundances in F stars: planets, rotation and galactic evolution
We find that hot jupiter host stars within the T range
5900-6300K show lower Li abundances, by 0.14 dex, than stars without detected
planets. This offset has a significance at the level 7, pointing to a
stronger effect of planet formation on Li abundances when the planets are more
massive and migrate close to the star. However, we also find that the average v
\textit{sin}i of (a fraction of) stars with hot jupiters is higher on average
than for single stars in the same T region, suggesting that
rotationally-induced mixing (and not the presence of planets) might be the
cause for a greater depletion of Li. We confirm that the mass-metallicity
dependence of the Li dip is extended towards [Fe/H] 0.4 dex (beginning
at [Fe/H] -0.4 dex for our stars) and that probably reflects the
mass-metallicity correlation of stars of the same T on the Main
Sequence. We find that for the youngest stars ( 1.5 Gyr) around the Li dip,
the depletion of Li increases with v \textit{sin}i values, as proposed by
rotationally-induced depletion models. This suggests that the Li dip consists
of fast rotators at young ages whereas the most Li-depleted old stars show
lower rotation rates (probably caused by the spin-down during their long
lifes). We have also explored the Li evolution with [Fe/H] taking advantage of
the metal-rich stars included in our sample. We find that Li abundance reaches
its maximum around solar metallicity but decreases in the most metal-rich
stars, as predicted by some models of Li Galactic production.Comment: 10 pages, accepted to A&
Submillimeter absorption from SH+, a new widespread interstellar radical, 13CH+ and HCl
We have used the Atacama Pathfinder Experiment 12 m telescope (APEX) to carry
out an absorption study of submillimeter wavelength rotational ground-state
lines of H35Cl, H37Cl, 13CH+, and, for the first time, of the SH+ radical
(sulfoniumylidene or sulfanylium). We detected the quartet of ground-state
hyperfine structure lines of SH+ near 683 GHz with the CHAMP+ array receiver
against the strong continuum source Sagittarius B2, which is located close to
the center of our Galaxy. In addition to absorption from various kinematic
components of Galactic center gas, we also see absorption at the radial
velocities belonging to intervening spiral arms. This demonstrates that SH+ is
a ubiquitous component of the diffuse interstellar medium. We do not find clear
evidence for other SH+ lines we searched for, which is partially due to
blending with lines from other molecules. In addition to SH+, we observed
absorption from H35Cl, H37Cl, and 13CH+. The observed submillimeter absorption
is compared in detail with absorption in 3 mm transitions of H13CO+ and c-C3H2
and the CO J = 1-0 and 3-2 transitions.Comment: 17 pages, 9 figure
Warm H2 in the Galactic center region
We present ISO observations of several H2 pure-rotational lines (from S(0) to
S(5)) towards a sample of 16 molecular clouds distributed along the central ~
500 pc of the Galaxy. We also present C18O and 13CO J=1->0 and J=2->1
observations of these sources made with the IRAM-30m telescope. With the CO
data we derive H2 densities of 10e(3.5-4.0) cm-3 and H2 column densities of a
few 10e22 cm-2. We have corrected the H2 data for ~ 30 magnitudes of visual
extinction using a self-consistent method. In every source, we find that the H2
emission exhibits a large temperature gradient. The S(0) and S(1) lines trace
temperatures (T) of ~150 K while the S(4) and S(5) lines indicate temperatures
of ~ 600K. The warm H2 column density is typically ~1-2 x 10e22 cm-2, and is
predominantly gas with T=150 K. This is the first direct estimate of the total
column density of the warm molecular gas in the Galactic center region. These
warm H2 column densities represent a fraction of ~ 30 % of the gas traced by
the CO isotopes emission. The cooling by H2 in the warm component is comparable
to that by CO. Comparing our H2 and CO data with available ammonia NH3
observations from literature one obtains relatively high NH3 abundances of a
few 10e(-7) in both the warm and the cold gas. A single shock or
Photo-Dissociation Region (PDR) cannot explain all the observed H2 lines.
Alternatives for the heating mechanisms are discussed.Comment: 14 pages including figures, to be published in A&
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