5,951 research outputs found
Strong irradiation of protostellar cores in Corona Australis
The importance of the physical environment in the evolution of newly formed
low-mass stars remains an open question. In particular, radiation from nearby
more massive stars may affect both the physical and chemical structure of these
kinds of young stars. Aims: To constrain the physical characteristics of a
group of embedded low-mass protostars in Corona Australis in the vicinity of
the young luminous Herbig Be star R CrA. Methods: Millimetre wavelength maps of
molecular line and continuum emission towards the low-mass star forming region
IRS7 near R CrA from the SMA and APEX are presented. The maps show the
distribution of 18 lines from 7 species (H2CO, CH3OH, HC3N, c-C3H2, HCN, CN and
SiO) on scales from 3" to 60" (400-8000 AU). Using a set of H2CO lines, we
estimate the temperatures and column densities in the region using LTE and
non-LTE methods. The results are compared with 1-D radiative transfer modelling
of the protostellar cores. These models constrain which properties of the
central source, envelope, and environment can give rise to the observed line
and continuum emission. Results: Most of the H2CO emission from the regions
emerges from two elongated narrow ridges dominating the emission picked up in
both interferometric and single-dish measurements. The temperatures inferred
from the H2CO lines are no less than ~30 K and more likely 50-60 K, and the
line emission peaks are offset by ~2500 AU from the location of the embedded
protostars. The temperatures can not be explained by the heating from the young
stellar objects themselves. Irradiation by the nearby Herbig Be star R CrA
could, however, explain the high temperatures. The elevated temperatures can in
turn impact the physical and chemical characteristics of protostars and lead to
enhanced abundances of typical tracers of photon dominated regions seen in
single-dish line surveys of embedded protostars in the region.Comment: Accepted for publication in A&A; 21 pages, 28 figures; Added footnote
in Section 2.
C18O (3-2) observations of the Cometary Globule CG 12: a cold core and a C18O hot spot
The feasibility of observing the C18O (3-2) spectral line in cold clouds with
the APEX telescope has been tested. As the line at 329.330 GHz lies in the wing
of a strong atmospheric H2O absorption it can be observed only at high altitude
observatories. Using the three lowest rotational levels instead of only two
helps to narrow down the physical properties of dark clouds and globules. The
centres of two C18O maxima in the high latitude low mass star forming region CG
12 were mapped in C18O (3-2) and the data were analyzed together with spectral
line data from the SEST. The T_MB(3-2)/T_MB(2-1) ratio in the northern C18O
maximum, CG 12 N, is 0.8, and in the southern maximum, CG 12 S, ~2. CG 12 N is
modelled as a 120'' diameter (0.4pc) cold core with a mass of 27 Msun. A small
size maximum with a narrow, 0.8 kms-1, C18O (3-2) spectral line with a peak
temperature of T_MB ~11 K was detected in CG 12 S. This maximum is modelled as
a 60'' to 80'' diameter (~0.2pc) hot (80 K < Tex < 200 K) ~1.6 Msun clump. The
source lies on the axis of a highly collimated bipolar molecular outflow near
its driving source. This is the first detection of such a compact, warm object
in a low mass star forming region.Comment: APEX A&A special issue, accepte
Arcsecond resolution images of the chemical structure of the low-mass protostar IRAS 16293-2422
It remains a key challenge to establish the molecular content of different
components of low-mass protostars, like their envelopes and disks, and how this
depends on the evolutionary stage and/or environment of the young stars.
Observations at submillimeter wavelengths provide a direct possibility to study
the chemical composition of low-mass protostars through transitions probing
temperatures up to a few hundred K in the gas surrounding these sources. This
paper presents a large molecular line survey of the deeply embedded
protostellar binary IRAS 16293-2422 from the Submillimeter Array (SMA) -
including images of individual lines down to approximately 1.5-3" (190-380 AU)
resolution. More than 500 individual transitions are identified related to 54
molecular species (including isotopologues) probing temperatures up to about
550 K. Strong chemical differences are found between the two components in the
protostellar system with a separation between, in particular, the sulfur- and
nitrogen-bearing species and oxygen-bearing complex organics. The action of
protostellar outflow on the ambient envelope material is seen in images of CO
and SiO and appear to influence a number of other species, including
(deuterated) water, HDO. The effects of cold gas-phase chemistry is directly
imaged through maps of CO, N2D+ and DCO+, showing enhancements of first DCO+
and subsequently N2D+ in the outer envelope where CO freezes-out on dust
grains.Comment: Accepted for publication in A&A, 30 pages, 22 figure
The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars
(Abridged) The water deuterium fractionation (HDO/HO abundance ratio) has
traditionally been used to infer the amount of water brought to Earth by
comets. Measuring this ratio in deeply-embedded low-mass protostars makes it
possible to probe the critical stage when water is transported from clouds to
disks in which icy bodies are formed. We present sub-arcsecond resolution
observations of HDO in combination with HO from the PdBI toward the
three low-mass protostars NGC 1333-IRAS 2A, IRAS 4A-NW, and IRAS 4B. The
resulting HDO/HO ratio is for IRAS 2A,
for IRAS 4A-NW, and for IRAS
4B. Derived ratios agree with radiative transfer models within a factor of 2-4
depending on the source. Our HDO/HO ratios for the inner regions (where
K) of four young protostars are only a factor of 2 higher than those
found for pristine, solar system comets. These small differences suggest that
little processing of water occurs between the deeply embedded stage and the
formation of planetesimals and comets.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Atomic jet from SMM1 (FIRS1) in Serpens uncovers non-coeval binary companion
We report on the detection of an atomic jet associated with the protostellar
source SMM1 (FIRS1) in Serpens. The jet is revealed in [FeII] and [NeII] line
maps observed with Spitzer/IRS, and further confirmed in HiRes IRAC and MIPS
images. It is traced very close to SMM1 and peaks at ~5 arcsec" from the source
at a position angle of $\sim 125 degrees. In contrast, molecular hydrogen
emission becomes prominent at distances > 5" from the protostar and extends at
a position angle of 160 degrees. The morphological differences suggest that the
atomic emission arises from a companion source, lying in the foreground of the
envelope surrounding the embedded protostar SMM1. In addition the molecular and
atomic Spitzer maps disentangle the large scale CO (3-2) emission observed in
the region into two distinct bipolar outflows, giving further support to a
proto-binary source setup. Analysis at the peaks of the [FeII] jet show that
emission arises from warm and dense gas (T ~1000 K, n(electron) 10^5 - 10^6
cm^-3). The mass flux of the jet derived independently for the [FeII] and
[NeII] lines is 10^7 M(sun)/yr, pointing to a more evolved Class~I/II protostar
as the driving source. All existing evidence converge to the conclusion that
SMM1 is a non-coeval proto-binary source.Comment: 10 pages, 7 figures, 1 table. Accepted for publication in Astronomy
\& Astrophysic
Single wall carbon nanotube double quantum dot
We report on two top-gate defined, coupled quantum dots in a semiconducting
single wall carbon nanotube, constituting a tunable double quantum dot system.
The single wall carbon nanotubes are contacted by titanium electrodes, and
gated by three narrow top-gate electrodes as well as a back-gate. We show that
a bias spectroscopy plot on just one of the two quantum dots can be used to
extract the addition energy of both quantum dots. Furthermore, honeycomb charge
stability diagrams are analyzed by an electrostatic capacitor model that
includes cross capacitances, and we extract the coupling energy of the double
quantum dot.Comment: Published in Applied Physics Letters 4 December 2006.
http://link.aip.org/link/?APL/89/23211
On the origin of H_2CO abundance enhancements in low-mass protostars
High angular resolution H_2CO 218 GHz line observations have been carried out toward the low-mass protostars IRAS 16293-2422 and L1448-C using the Owens Valley Millimeter Array at ~2" resolution. Simultaneous 1.37 mm continuum data reveal extended emission which is compared with that predicted by model envelopes constrained from single-dish data. For L1448-C the model density structure works well down to the 400 AU scale to which the interferometer is sensitive. For IRAS 16293-2422 , a known proto-binary object, the interferometer observations indicate that the binary has cleared much of the material in the inner part of the envelope, out to the binary separation of ~800 AU. For both sources there is excess unresolved compact emission centered on the sources, most likely due to accretion disks ≾200 AU in size with masses of ≳0.02 M_☉ (L1448-C) and ≳0.1 M_☉ (IRAS 16293-2422). The H_2CO data for both sources are dominated by emission from gas close to the positions of the continuum peaks. The morphology and velocity structure of the H_2CO array data have been used to investigate whether the abundance enhancements inferred from single-dish modelling are due to thermal evaporation of ices or due to liberation of the ice mantles by shocks in the inner envelope. For IRAS 16293-2422 the H_2CO interferometer observations indicate the presence of rotation roughly perpendicular to the large scale CO outflow. The H_2CO distribution differs from that of C^(18)O, with C^(18)O emission peaking near MM1 and H_2CO stronger near MM2. For L1448-C, the region of enhanced H_2CO emission extends over a much larger scale >1" than the radius of 50-100 K (0."6-0".15) where thermal evaporation can occur. The red-blue asymmetry of the emission is consistent with the outflow; however the velocities are significantly lower. The H_2CO 3_(22)-2_(21)/3_(03)-2_(02) flux ratio derived from the interferometer data is significantly higher than that found from single-dish observations for both objects, suggesting that the compact emission arises from warmer gas. Detailed radiative transfer modeling shows, however, that the ratio is affected by abundance gradients and optical depth in the 3_(03)-2_(02) line. It is concluded that a constant H_2CO abundance throughout the envelope cannot fit the interferometer data of the two H_2CO lines simultaneously on the longest and shortest baselines. A scenario in which the H_2CO abundance drops in the cold dense part of the envelope where CO is frozen out but is undepleted in the outermost region provides good fits to the single-dish and interferometer data on short baselines for both sources. Emission on the longer baselines is best reproduced if the H_2CO abundance is increased by about an order of magnitude from ~ 10^(-10) to ~ 10^(-9) in the inner parts of the envelope due to thermal evaporation when the temperature exceeds ~50 K. The presence of additional H_2CO abundance jumps in the innermost hot core region or in the disk cannot be firmly established, however, with the present sensitivity and resolution. Other scenarios, including weak outflow-envelope interactions and photon heating of the envelope, are discussed and predictions for future generation interferometers are presented, illustrating their potential in distinguishing these competing scenarios
Classifying the embedded young stellar population in Perseus and Taurus & the LOMASS database
Context. The classification of young stellar objects (YSOs) is typically done
using the infrared spectral slope or bolometric temperature, but either can
result in contamination of samples. More accurate methods to determine the
evolutionary stage of YSOs will improve the reliability of statistics for the
embedded YSO population and provide more robust stage lifetimes. Aims. We aim
to separate the truly embedded YSOs from more evolved sources. Methods. Maps of
HCO+ J=4-3 and C18O J=3-2 were observed with HARP on the James Clerk Maxwell
Telescope (JCMT) for a sample of 56 candidate YSOs in Perseus and Taurus in
order to characterize emission from high (column) density gas. These are
supplemented with archival dust continuum maps observed with SCUBA on the JCMT
and Herschel PACS to compare the morphology of the gas and dust in the
protostellar envelopes. The spatial concentration of HCO+ J=4-3 and 850 micron
dust emission are used to classify the embedded nature of YSOs. Results.
Approximately 30% of Class 0+I sources in Perseus and Taurus are not Stage I,
but are likely to be more evolved Stage II pre-main sequence (PMS) stars with
disks. An additional 16% are confused sources with an uncertain evolutionary
stage. Conclusions. Separating classifications by cloud reveals that a high
percentage of the Class 0+I sources in the Perseus star forming region are
truly embedded Stage I sources (71%), while the Taurus cloud hosts a majority
of evolved PMS stars with disks (68%). The concentration factor method is
useful to correct misidentified embedded YSOs, yielding higher accuracy for YSO
population statistics and Stage timescales. Current estimates (0.54 Myr) may
overpredict the Stage I lifetime on the order of 30%, resulting in timescales
of 0.38 Myr for the embedded phase.Comment: 33 pages, 21 figures, 6 tables, Accepted to be published in A&
Magnetic-Field Dependence of Tunnel Couplings in Carbon Nanotube Quantum Dots
By means of sequential and cotunneling spectroscopy, we study the tunnel
couplings between metallic leads and individual levels in a carbon nanotube
quantum dot. The levels are ordered in shells consisting of two doublets with
strong- and weak-tunnel couplings, leading to gate-dependent level
renormalization. By comparison to a one- and two-shell model, this is shown to
be a consequence of disorder-induced valley mixing in the nanotube. Moreover, a
parallel magnetic field is shown to reduce this mixing and thus suppress the
effects of tunnel renormalization.Comment: 5 pages, 3 figures; revised version as publishe
The organic chemistry in the innermost, infalling envelope of the Class 0 protostar L483
Context: The protostellar envelopes, outflow and large-scale chemistry of
Class~0 and Class~I objects have been well-studied, but while previous works
have hinted at or found a few Keplerian disks at the Class~0 stage, it remains
to be seen if their presence in this early stage is the norm. Likewise, while
complex organics have been detected toward some Class~0 objects, their
distribution is unknown as they could reside in the hottest parts of the
envelope, in the emerging disk itself or in other components of the
protostellar system, such as shocked regions related to outflows.
Aims: In this work, we aim to address two related issues regarding
protostars: when rotationally supported disks form around deeply embedded
protostars and where complex organic molecules reside in such objects.
Methods: We observed the deeply embedded protostar, L483, using Atacama Large
Millimeter/submillimeter Array (ALMA) Band~7 data from Cycles~1 and 3 with a
high angular resolution down to ~0.1 (20~au) scales.
Results: We find that the kinematics of CS~-- and
HCN~-- are best fitted by the velocity profile from infall under
conservation of angular momentum and not by a Keplerian profile. The spatial
extents of the observed complex organics are consistent with an estimated ice
sublimation radius of the envelope at ~50~au, suggesting that the complex
organics exist in the hot corino of L483.
Conclusions: We find that L483 does not harbor a Keplerian disk down to at
least ~au in radius. Instead, the innermost regions of L483 are undergoing
a rotating collapse. This result highlights that some Class~0 objects contain
only very small disks, or none at all, with the complex organic chemistry
taking place on scales inside the hot corino of the envelope, in a region
larger than the emerging disk.Comment: 19 pages, 11 figure
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