Abstract H 2 CO is a small organic molecule widely detected in protoplanetary disks. As a precursor to grain-surface formation of CH 3 OH, H 2 CO is considered an important precursor of O-bearing ...organic molecules that are locked in ices. Still, since gas-phase reactions can also form H 2 CO, there remains an open question on the channels by which organics form in disks, and how much the grain versus the gas pathways impact the overall organic reservoir. We present spectrally and spatially resolved Atacama Large Millimeter/submillimeter Array observations of several ortho- and para-H 2 CO transitions toward the bright protoplanetary disk around the Herbig Ae star HD 163296. We derive column density, excitation temperature, and ortho-to-para ratio (OPR) radial profiles for H 2 CO, as well as disk-averaged values of N T ∼ 4 × 10 12 cm −2 , T ex ∼ 20 K, and OPR ∼ 2.7, respectively. We empirically determine the vertical structure of the emission, finding vertical heights of z / r ∼ 0.1. From the profiles, we find a relatively constant OPR ∼ 2.7 with radius, but still consistent with 3.0 among the uncertainties, a secondary increase of N T in the outer disk, and low T ex values that decrease with disk radius. Our resulting radial, vertical, and OPR constraints suggest an increased UV penetration beyond the dust millimeter edge, consistent with an icy origin but also with cold gas-phase chemistry. This Herbig disk contrasts previous results for the T Tauri disk, TW Hya, which had a larger contribution from cold gas-phase chemistry. More observations of other sources are needed to disentangle the dominant formation pathway of H 2 CO in protoplanetary disks.
We investigate the structure of the core surrounding the recently identified deeply embedded young stellar object Barnard 1c. B1c lies within the Perseus molecular cloud at a distance of 250 pc. It ...is a deeply embedded core of 2.4 M sub( )(Kirk et al.) and a luminosity of 4 c 2 L sub( ). Observations (and resolutions) of super(12)CO J = 1-0 (9".2 x 5".9), super(13)CO J = 1-0, C super(18)O J = 1-0 (14".3 x 6".7), HCO super(+) J = 1-0 (7".6 x 5".8), and N sub(2)H super(+) J = 1-0 (5".9 x 4".6) were obtained with the BIMA array, together with the continuum at 3.3 mm (6".4 x 4".9) and 2.7 mm (9".5 x 6".3). Single-dish measurements of N sub(2)H super(+) J = 1-0 and HCO super(+) J = 1-0 with FCRAO reveal the larger scale emission in these lines with 660 resolution. The super(12)CO and HCO super(+) emission traces the outflow extending over the full field of view (2'.1), which coincides in detail with the S-shaped jet recently found in Spitzer IRAC imaging. The N sub(2)H super(+) emission, which anticorrelates spatially with the C super(18)O emission, originates from a rotating envelope with effective radius 62400 AU and mass 2.1-2.9 M sub( ), as derived from the 3.3 mm continuum emission. N sub(2)H super(+) emission is absent from a 600 AU diameter region around the young star, offset from the continuum peak. The remaining N sub(2)H super(+) emission may lie in a coherent torus of dense material. With its outflow and rotating envelope, B1c closely resembles the previously studied object L483 mm, and we conclude that it is a protostar in an early stage of evolution, i.e., Class 0 or in transition between Class 0 and Class I.
Abstract
Measuring the gas mass of protoplanetary disks, the reservoir available for giant planet formation, has proven to be difficult. We currently lack a far-infrared observatory capable of ...observing HD, and the most common gas mass tracer, CO, suffers from a poorly constrained CO-to-H
2
ratio. Expanding on previous work, we investigate if N
2
H
+
, a chemical tracer of CO-poor gas, can be used to observationally measure the CO-to-H
2
ratio and correct CO-based gas masses. Using disk structures obtained from the literature, we set up thermochemical models for three disks, TW Hya, DM Tau and GM Aur, to examine how well the CO-to-H
2
ratio and gas mass can be measured from N
2
H
+
and C
18
O line fluxes. Furthermore, we compare these gas masses to gas masses independently measured from archival HD observations. The N
2
H
+
(3 − 2)/C
18
O(2 − 1) line ratio scales with the disk CO-to-H
2
ratio. Using these two lines, we measure 4.6 × 10
−3
M
⊙
≤
M
disk
≤ 1.1 × 10
−1
M
⊙
for TW Hya, 1.5 × 10
−2
M
⊙
≤
M
disk
≤ 9.6 × 10
−2
M
⊙
for GM Aur and 3.1 × 10
−2
M
⊙
≤
M
disk
≤ 9.6 × 10
−2
M
⊙
for DM Tau. These gas masses agree with values obtained from HD within their respective uncertainties. The uncertainty on the N
2
H
+
+ C
18
O gas mass can be reduced by observationally constraining the cosmic-ray ionization rate in disks. These results demonstrate the potential of using the combination of N
2
H
+
and C
18
O to measure gas masses of protoplanetary disks.
Aperture-synthesis and single-dish (sub-) millimeter molecular-line and continuum observations reveal in great detail the envelope structure of deeply embedded young stellar objects (SMM 1 = FIRS 1, ...SMM 2, SMM 3, SMM 4) in the densely star-forming Serpens Molecular Cloud. SMM 1, 3, and 4 show partially resolved (>2" = 800 AU) continuum emission in the beam of the Owens Valley Millimeter Array at lambda = 3.4-1.4 mm. The continuum visibilities accurately constrain the density structure in the envelopes, which can be described by a radial power law with slope -2.0 +/- 0.5 on scales of 300 to 8000 AU. Inferred envelope masses within a radius of 8000 AU are 8.7, 3.0, and 5.3 Msolar for SMM 1, 3, and 4, respectively. A point source with 20%-30% of the total flux at 1.1 mm is required to fit the observations on long baselines, corresponding to warm envelope material within approximately 100 AU or a circumstellar disk. No continuum emission is detected interferometrically toward SMM 2, corresponding to an upper limit of 0.2 Msolar assuming Td = 24 K. The lack of any compact dust emission suggests that the SMM 2 core does not contain a central protostar. Aperture-synthesis observations of the 13CO, C18O, HCO+, H13CO+, HCN, H13CN, N2H+ 1-0, SiO 2-1, and SO 2(2)-1(1) transitions reveal compact emission toward SMM 1, 3, and 4. SMM 2 shows only a number of clumps scattered throughout the primary field of view, supporting the conclusion that this core does not contain a central star. The compact molecular emission around SMM 1, 3, and 4 traces 5"-10" (2000-4000 AU) diameter cores that correspond to the densest regions of the envelopes, as well as material directly associated with the molecular outflow. Especially prominent are the optically thick HCN and HCO+ lines that show up brightly along the walls of the outflow cavities. SO and SiO trace shocked material, where their abundances may be enhanced by 1-2 orders of magnitude over dark-cloud values. A total of 31 molecular transitions have been observed with the James Clerk Maxwell and Caltech Submillimeter telescopes in the 230, 345, 490, and 690 GHz atmospheric windows toward all four sources, containing, among others, lines of CO, HCO+, HCN, H2CO, SiO, SO, and their isotopomers. These lines show 20-30 km s-1 wide line wings, deep and narrow (1-2 km s-1) self-absorption, and 2-3 km s-1 FWHM line cores. The presence of highly excited lines like 12CO 4-3 and 6-5, 13CO 6-5, and several H2CO transitions indicates the presence of material with temperatures > or approximately 100 K. Monte Carlo calculations of the molecular excitation and line transfer show that the envelope model derived from the dust emission can successfully reproduce the observed line intensities. The depletion of CO in the cold gas is modest compared to values inferred in objects like NGC 1333 IRAS 4, suggesting that the phase of large depletions through the entire envelope is short lived and may be influenced by the local star formation density. Emission in high-excitation lines of CO and H2CO requires the presence of a small amount of approximately 100 K material, comprising less than 1% of the total envelope mass and probably associated with the outflow or the innermost region of the envelope. The derived molecular abundances in the warm (Tkin > 20 K) envelope are similar to those found toward other class 0 YSOs like IRAS 16293-2422, though some species appear enhanced toward SMM 1. Taken together, the presented observations and analysis provide the first comprehensive view of the physical and chemical structure of the envelopes of deeply embedded young stellar objects in a clustered environment on scales between 1000 and 10,000 AU.
We present observations of HCN J = 1-0 and CH3OH J(Ka , Kc ) = 3(1, 3)-4(0, 4) A+ emission from comet C/2002 T7 (LINEAR) obtained simultaneously with the Owens Valley Radio Observatory (OVRO) and ...Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometers. We combined the data from both arrays to increase the (u, v) sampling and signal to noise of the detected line emission. We also report the detection of CH3OH J(Ka , Kc ) = 8(0, 8)-7(1, 7) A+ with OVRO data alone. Using a molecular excitation code that includes the effects of collisions with water and electrons, as well as pumping by the Solar infrared photons (for HCN alone), we find a production rate of HCN of 2.9 X 1026 s-1 and for CH3OH of 2.2 X 1027 s-1. Compared to the adopted water production rate of 3 X 1029 s-1, this corresponds to an HCN/H2O ratio of 0.1% and a CH3OH/H2O ratio of 0.7%. We critically assess the uncertainty of these values due to the noise (~10%), the uncertainties in the adopted comet model (~50%), and the uncertainties in the adopted collisional excitation rates (up to a factor of 2). Pumping by Solar infrared photons is found to be a minor effect for HCN, because our 15'' synthesized beam is dominated by the region in the coma where collisions dominate. Since the uncertainties in the derived production rates are at least as large as one-third of the differences found between comets, we conclude that reliable collision rates and an accurate comet model are essential. Because the collisionally dominated region critically depends on the water production rate, using the same approximate method for different comets may introduce biases in the derived production rates. Multiline observations that directly constrain the molecular excitation provide much more reliable production rates.
We report the first science observations and results obtained with the 'extended' SMA (eSMA), which is composed of the SMA (Submillimeter Array), JCMT (James Clerk Maxwell Telescope), and CSO ...(Caltech Submillimeter Observatory). Redshifted absorptions at z = 0.886 of C I (3 P 1-3 P 0) were observed with the eSMA with an angular resolution of 055 X 022 at 1.1 mm toward the southwestern image of the remarkable lensed quasar PKS 1830 - 211, but not toward the northeastern component at a separation of ~1''. Additionally, SMA observations of CO, 13CO, and C18O (all J = 4-3) were obtained toward this object: CO was also detected toward the southwest component, but none of the isotopologues were. This is the first time C I has been detected in this object, allowing the first direct determination of relative abundances of neutral atomic carbon to CO in the molecular clouds of a spiral galaxy at z > 0.1. The C I and CO profiles can be decomposed into two and three velocity components, respectively. We derive C/CO column density ratios ranging from 0.5 (representative of dense cores) to ~2.5 (close to translucent cloud values). This could indicate that we are seeing environments with different physical conditions or that we are witnessing the chemical evolution of regions where C has not completely been converted into CO.
The accretion of material from protoplanetary disks onto their central stars is a fundamental process in the evolution of these systems and a key diagnostic in constraining the disk lifetime. We ...analyze the relationship between the stellar accretion rate and the disk mass in 32 intermediate-mass Herbig Ae/Be systems and compare them to their lower-mass counterparts, T Tauri stars. We find that the $\dot{M}$ – M _disk relationship for Herbig Ae/Be stars is largely flat at ∼10 ^−7 M _☉ yr ^−1 over 3 orders of magnitude in dust mass. While most of the sample follows the T Tauri trend, a subset of objects with high accretion rates and low dust masses are identified. These outliers (12 out of 32 sources) have an inferred disk lifetime of less than 0.01 Myr and are dominated by objects with low infrared excess. This outlier sample is likely identified in part by the bias in classifying Herbig Ae/Be stars, which requires evidence of accretion that can only be reliably measured above a rate of ∼10 ^−9 M _☉ yr ^−1 for these spectral types. If the disk masses are not underestimated and the accretion rates are not overestimated, this implies that these disks may be on the verge of dispersal, which may be due to efficient radial drift of material or outer disk depletion by photoevaporation and/or truncation by companions. This outlier sample likely represents a small subset of the larger young, intermediate-mass stellar population, the majority of which would have already stopped accreting and cleared their disks.
Comets are some of the most primitive bodies left over from the Solar System's early history. They may preserve both interstellar material and material from the proto-solar nebula, and so studies of ...their volatile components can provide clues about the evolution of gases and ices, as a collapsing molecular cloud transforms into a mature planetary system,. Previous observations of emission from rotational transitions in molecules have averaged over large areas of the inner coma, and therefore include both molecules that sublimed from the nucleus and those that result from subsequent chemical processes in the coma. Here we present high-resolution observations of emission from the molecules HNC, DCN and HDO associated with comet Hale-Bopp. Our data reveal arc-like structures-icy jets-offset from (but close to) the nucleus. The measured abundance ratios on 1-3″ scales are substantially different from those on larger scales, and cannot be accounted for by models of chemical processes in the coma,,; they are, however, similar to the values observed in the cores of dense interstellar clouds and young stellar objects. We therefore propose that sublimation from millimetre-sized icy grains ejected from the nucleus provides access to relatively unaltered volatiles. The D/H ratios inferred from our data suggest that, by mass, Hale-Bopp (and by inference the outer regions of the early solar nebula) consists of 15-40% of largely unprocessed interstellar material.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present high-quality data of Saturn and its ring system from the Berkeley-Illinois-Maryland Association and Owens Valley Radio Observatory arrays. We observed at wavelengths of 1.3 and 3.0 mm in ...2002 October and 2003 March, respectively. At that time Saturn's rings were near their maximum ring opening angle (|B| ~ 26°), which allowed us to map the brightness distribution on Saturn's disk, as well as from its rings, despite the moderate resolution achieved (~25-4''). Clear latitudinal structure is seen on Saturn's disk, indicative of an overall upwelling of gas on the southern hemisphere and subsidence of dry air in the north. The rings are brighter at 1.3 mm than at 3.0 mm, as expected from the increase in ring particles' thermal emission at shorter wavelengths. We use our radiative transfer code to model the relative contributions of the scattered and thermal radiation emanating from the rings. Unlike previous (centimetric) observations, the present (millimetric) data serve as an adequate test of the model's ability to account for the thermal contribution from the rings. A comparison between the model and data shows that Saturn's light transmitted and scattered through the B ring, as well as the radiation reflected from the A ring behind and just off the planet's south pole, is much brighter than predicted by our model. Hence, the ring particles appear to scatter more efficiently both in the forward and backward directions than implied by an isotropic phase function.
Chemical Evolution of Protostars Hogerheijde, Michiel R
Astrophysics and space science,
1/2005, Letnik:
295, Številka:
1-2
Journal Article
Recenzirano
Odprti dostop
This review discusses the evolution of the chemical composition of the molecular gas throughout the process of star formation. It covers the initial stages of cloud-core formation, gravitational ...collapse and the formation of the star, the effects of stellar radiation and outflows on the surrounding cloud, and the composition of the circumstellar disk. A brief overview of relevant chemical processes sets the stage. The ensuing evolution of the chemistry is governed by several competing timescales and by energy input by the young star(s). Special attention is given to the role of molecular freeze-out on dust grains and to deuterium fractionation in cold gas.
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