4,009 research outputs found
Warm water deuterium fractionation in IRAS 16293-2422 - The high-resolution ALMA and SMA view
Measuring the water deuterium fractionation in the inner warm regions of
low-mass protostars has so far been hampered by poor angular resolution
obtainable with single-dish ground- and space-based telescopes. Observations of
water isotopologues using (sub)millimeter wavelength interferometers have the
potential to shed light on this matter. Observations toward IRAS 16293-2422 of
the 5(3,2)-4(4,1) transition of H2-18O at 692.07914 GHz from Atacama Large
Millimeter/submillimeter Array (ALMA) as well as the 3(1,3)-2(2,0) of H2-18O at
203.40752 GHz and the 3(1,2)-2(2,1) transition of HDO at 225.89672 GHz from the
Submillimeter Array (SMA) are presented. The 692 GHz H2-18O line is seen toward
both components of the binary protostar. Toward one of the components, "source
B", the line is seen in absorption toward the continuum, slightly red-shifted
from the systemic velocity, whereas emission is seen off-source at the systemic
velocity. Toward the other component, "source A", the two HDO and H2-18O lines
are detected as well with the SMA. From the H2-18O transitions the excitation
temperature is estimated at 124 +/- 12 K. The calculated HDO/H2O ratio is (9.2
+/- 2.6)*10^(-4) - significantly lower than previous estimates in the warm gas
close to the source. It is also lower by a factor of ~5 than the ratio deduced
in the outer envelope. Our observations reveal the physical and chemical
structure of water vapor close to the protostars on solar-system scales. The
red-shifted absorption detected toward source B is indicative of infall. The
excitation temperature is consistent with the picture of water ice evaporation
close to the protostar. The low HDO/H2O ratio deduced here suggests that the
differences between the inner regions of the protostars and the Earth's oceans
and comets are smaller than previously thought.Comment: Accepted for publication in Astronomy & Astrophysic
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
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
Exploring the Origins of Earth's Nitrogen: Astronomical Observations of Nitrogen-bearing Organics in Protostellar Environments
It is not known whether the original carriers of Earth's nitrogen were
molecular ices or refractory dust. To investigate this question, we have used
data and results of Herschel observations towards two protostellar sources: the
high-mass hot core of Orion KL, and the low-mass protostar IRAS 16293-2422.
Towards Orion KL, our analysis of the molecular inventory of Crockett et al.
(2014) indicates that HCN is the organic molecule that contains by far the most
nitrogen, carrying of nitrogen-in-organics. Following this
evidence, we explore HCN towards IRAS 16293-2422, which we consider a solar
analog. Towards IRAS 16293-2422, we have reduced and analyzed Herschel spectra
of HCN, and fit these observations against "jump" abundance models of IRAS
16293-2422's protostellar envelope. We find an inner-envelope HCN abundance
and an outer-envelope HCN
abundance . We also find the
sublimation temperature of HCN to be ~K; this
measured enables us to predict an HCN binding energy
~K. Based on a comparison of the HCN/H2O ratio
in these protostars to N/H2O ratios in comets, we find that HCN (and, by
extension, other organics) in these protostars is incapable of providing the
total bulk N/H2O in comets. We suggest that refractory dust, not molecular
ices, was the bulk provider of nitrogen to comets. However, interstellar dust
is not known to have 15N enrichment, while high 15N enrichment is seen in both
nitrogen-bearing ices and in cometary nitrogen. This may indicate that these
15N-enriched ices were an important contributor to the nitrogen in
planetesimals and likely to the Earth.Comment: Accepted to ApJ; 21 pages, 4 figure
The effect of a strong external radiation field on protostellar envelopes in Orion
We discuss the effects of an enhanced interstellar radiation field (ISRF) on
the observables of protostellar cores in the Orion cloud region. Dust radiative
transfer is used to constrain the envelope physical structure by reproducing
SCUBA 850 micron emission. Previously reported 13CO, C17O and H2CO line
observations are reproduced through detailed Monte Carlo line radiative
transfer models. It is found that the 13CO line emission is marginally
optically thick and sensitive to the physical conditions in the outer envelope.
An increased temperature in this region is needed in order to reproduce the
13CO line strengths and it is suggested to be caused by a strong heating from
the exterior, corresponding to an ISRF in Orion 10^3 times stronger than the
"standard" ISRF. The typical temperatures in the outer envelope are higher than
the desorption temperature for CO. The C17O emission is less sensitive to this
increased temperature but rather traces the bulk envelope material. The data
are only fit by a model where CO is depleted, except in the inner and outermost
regions where the temperature increases above 30-40 K. The fact that the
temperatures do not drop below approximately 25 K in any of the envelopes
whereas a significant fraction of CO is frozen-out suggest that the
interstellar radiation field has changed through the evolution of the cores.
The H2CO lines are successfully reproduced in the model of an increased ISRF
with constant abundances of 3-5x10^{-10}.Comment: 11 pages, 10 figures. Accepted for publication in A&
High Frame-rate Imaging Based Photometry, Photometric Reduction of Data from Electron-multiplying Charge Coupled Devices (EMCCDs)
The EMCCD is a type of CCD that delivers fast readout times and negligible
readout noise, making it an ideal detector for high frame rate applications
which improve resolution, like lucky imaging or shift-and-add. This improvement
in resolution can potentially improve the photometry of faint stars in
extremely crowded fields significantly by alleviating crowding. Alleviating
crowding is a prerequisite for observing gravitational microlensing in main
sequence stars towards the galactic bulge. However, the photometric stability
of this device has not been assessed. The EMCCD has sources of noise not found
in conventional CCDs, and new methods for handling these must be developed.
We aim to investigate how the normal photometric reduction steps from
conventional CCDs should be adjusted to be applicable to EMCCD data. One
complication is that a bias frame cannot be obtained conventionally, as the
output from an EMCCD is not normally distributed. Also, the readout process
generates spurious charges in any CCD, but in EMCCD data, these charges are
visible as opposed to the conventional CCD. Furthermore we aim to eliminate the
photon waste associated with lucky imaging by combining this method with
shift-and-add.
A simple probabilistic model for the dark output of an EMCCD is developed.
Fitting this model with the expectation-maximization algorithm allows us to
estimate the bias, readout noise, amplification, and spurious charge rate per
pixel and thus correct for these phenomena. To investigate the stability of the
photometry, corrected frames of a crowded field are reduced with a PSF fitting
photometry package, where a lucky image is used as a reference.
We find that it is possible to develop an algorithm that elegantly reduces
EMCCD data and produces stable photometry at the 1% level in an extremely
crowded field.Comment: Submitted to Astronomy and Astrophysic
Electron transport in single wall carbon nanotube weak links in the Fabry-Perot regime
We fabricated reproducible high transparency superconducting contacts
consisting of superconducting Ti/Al/Ti trilayers to gated single-walled carbon
nanotubes (SWCNTs). The reported semiconducting SWCNT have normal state
differential conductance up to and exhibit clear Fabry-Perot
interference patterns in the bias spectroscopy plot. We observed subharmonic
gap structure in the differential conductance and a distinct peak in the
conductance at zero bias which is interpreted as a manifestation of a
supercurrent. The gate dependence of this supercurrent as well as the excess
current are examined and compared to a coherent theory of superconducting point
contacts with good agreement.Comment: 10 pages, 4 figure
The Co-evolution of Disk and Star in Embedded Stages: The Case of the Very Low-mass Protostar
We have observed the CCH (N=3-2, J=7/2-5/2, F=4-3 and 3-2) and SO (6_7-5_6)
emission at a 0"2 angular resolution toward the low-mass Class 0 protostellar
source IRAS 15398-3359 with ALMA. The CCH emission traces the
infalling-rotating envelope near the protostar with the outflow cavity extended
along the northeast-southwest axis. On the other hand, the SO emission has a
compact distribution around the protostar. The CCH emission is relatively weak
at the continuum peak position, while the SO emission has a sharp peak there.
Although the maximum velocity shift of the CCH emission is about 1 km s^-1 from
the systemic velocity, a velocity shift higher than 2 km s^{-1} is seen for the
SO emission. This high velocity component is most likely associated with the
Keplerian rotation around the protostar. The protostellar mass is estimated to
be 0.007^{+0.004}_{-0.003} from the velocity profile of the SO emission. With
this protostellar mass, the velocity structure of the CCH emission can be
explained by the model of the infalling-rotating envelope, where the radius of
the centrifugal barrier is estimated to be 40 au from the comparison with the
model. The disk mass evaluated from the dust continuum emission by assuming the
dust temperature of 20 K-100 K is 0.1-0.9 times the stellar mass, resulting in
the Toomre Q parameter of 0.4-5. Hence, the disk structure may be partly
unstable. All these results suggest that a rotationally-supported disk can be
formed in the earliest stages of the protostellar evolution
Subarcsecond resolution observations of warm water towards three deeply embedded low-mass protostars
Water is present during all stages of star formation: as ice in the cold
outer parts of protostellar envelopes and dense inner regions of circumstellar
disks, and as gas in the envelopes close to the protostars, in the upper layers
of circumstellar disks and in regions of powerful outflows and shocks. In this
paper we probe the mechanism regulating the warm gas-phase water abundance in
the innermost hundred AU of deeply embedded (Class~0) low-mass protostars, and
investigate its chemical relationship to other molecular species during these
stages. Millimeter wavelength thermal emission from the para-H2-18O
3(1,3)-2(2,0) (Eu=203.7 K) line is imaged at high angular resolution (0.75";
190 AU) with the IRAM Plateau de Bure Interferometer towards the deeply
embedded low-mass protostars NGC 1333-IRAS2A and NGC 1333-IRAS4A. Compact
H2-18O emission is detected towards IRAS2A and one of the components in the
IRAS4A binary; in addition CH3OCH3, C2H5CN, and SO2 are detected. Extended
water emission is seen towards IRAS2A, possibly associated with the outflow.
The detections in all systems suggests that the presence of water on <100 AU
scales is a common phenomenon in embedded protostars. We present a scenario in
which the origin of the emission from warm water is in a flattened disk-like
structure dominated by inward motions rather than rotation. The gas-phase water
abundance varies between the sources, but is generally much lower than a
canonical abundance of 10^-4, suggesting that most water (>96 %) is frozen out
on dust grains at these scales. The derived abundances of CH3OCH3 and SO2
relative to H2-18O are comparable for all sources pointing towards similar
chemical processes at work. In contrast, the C2H5CN abundance relative to
H2-18O is significantly lower in IRAS2A, which could be due to different
chemistry in the sources.Comment: 12 pages, 9 figure
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