Search

Annular structures (rings and gaps) in disks around pre-main-sequence stars have been detected in abundance towards class II protostellar objects that are approximately 1,000,000 years old1. These structures are often interpreted as evidence of planet formation1–3, with planetary-mass bodies carving rings and gaps in the disk4. This implies that planet formation may already be underway in even younger disks in the class I phase, when the protostar is still embedded in a larger-scale dense envelope of gas and dust5. Only within the past decade have detailed properties of disks in the earliest star-forming phases been observed6,7. Here we report 1.3-millimetre dust emission observations with a resolution of five astronomical units that show four annular substructures in the disk of the young (less than 500,000 years old)8protostar IRS 63. IRS 63 is a single class I source located in the nearby Ophiuchus molecular cloud at a distance of 144 parsecs9, and is one of the brightest class I protostars at millimetre wavelengths. IRS 63 also has a relatively large disk compared to other young disks (greater than 50 astronomical units)10. Multiple annular substructures observed towards disks at young ages can act as an early foothold for dust-grain growth, which is a prerequisite of planet formation. Whether or not planets already exist in the disk of IRS 63, it is clear that the planet-formation process begins in the initial protostellar phases, earlier than predicted by current planet-formation theories11.

In this paper, we present a dust emission map of the starless core TMC-1C taken at 2100 mm. Along with maps at 160, 450, 850, and 1200 mm, we study the dust emissivity spectral index from the (sub)millimeter spectral energy distribution, and find that it is close to the typically assumed value of b = 2. We also map the dust temperature and column density in TMC-1C, and find that at the position of the dust peak (AV [?] 50) the line-of-sight-averaged temperature is [?]7 K. Employing simple Monte Carlo modeling, we show that the data are consistent with a constant value for the emissivity spectral index over the whole map of TMC-1C.

We present the chemistry, temperature, and dynamical state of a sample of 193 dense cores or core candidates in the Perseus Molecular cloud and compare the properties of cores associated with young stars and clusters with those which are not. The combination of our NH3 and CCS observations with previous millimeter, submillimeter, and Spitzer data available for this cloud enables us both to determine core properties precisely and to accurately classify cores as starless or protostellar. The properties of cores in different cluster environments and before-and-after star formation provide important constraints on simulations of star formation, particularly under the paradigm that the essence of star formation is set by the turbulent formation of prestellar cores. We separate the influence of stellar content from that of the cluster environment and find that cores within clusters have (1) higher kinetic temperatures (12.9 K versus 10.8 K) and, (2) lower fractional abundances of CCS (0.6 x 10-9 versus 2.0 x 10-9) and NH3 (1.2 x 10-8 versus 2.9 x 10-8). Cores associated with protostars have (1) slightly higher kinetic temperatures (11.9 K versus 10.6 K), (2) higher NH3 excitation temperatures (7.4 K versus 6.1 K), (3) are at higher column density (1.2 x 1022 cm-2 versus 0.6 x 1022 cm-2), have (4) slightly more nonthermal/turbulent NH3 line widths (0.14 km s-1 versus 0.11 km s-1 FWHM), have (5) higher masses (1.5 M versus 1.0 M ), and have (6) lower fractional abundance of CCS (1.4 x 10-9 versus 2.4 x 10-9). All values are medians. We find that neither cluster environment nor protostellar content makes a significant difference to the dynamical state of cores as estimated by the virial parameter--most cores in each category are gravitationally bound. Only the high precision of our measurements and the size of our sample make such distinctions possible. Overall, cluster environment and protostellar content have a smaller influence on the properties of the cores than is typically assumed, and the variation within categories is larger than the differences between categories.

We describe a model for the thermal and dynamical equilibrium of starless cores that includes the radiative transfer of gas and dust and simple CO chemistry. The model shows that the structure and behavior of the cores is significantly different depending on whether the central density is either above or below about 105 cm[?]3. This density is significant as the critical density for gas cooling by gas-dust collisions and as the critical density for dynamical stability, given the typical properties of the starless cores. Starless cores thus divide into two classes that we refer to as thermally supercritical and thermally subcritical. This two-class distinction allows an improved interpretation of the different observational data of starless cores within a single model.

We use data gathered by the COMPLETE survey of star-forming regions to find new calibrations of the "X-factor" and 13CO abundance within the Perseus molecular cloud. We divide Perseus into six subregions, using groupings in a dust temperature vs. LSR velocity plot. The standard X-factor, X [?] N(H2)/W(12CO) , is derived both for the whole Perseus complex and for each of the six subregions with values consistent with previous estimates. However, the X-factor is heavily affected by the saturation of the emission above AV [?] 4 mag, and variations are also found between regions. Linear fits to relate W(12CO) and AV using only points below 4 mag of extinction yield a better estimate of the AV than the X-factor. Linear relations of W(13CO) , N(13CO) , and W(C18O) with AV are derived. The extinction thresholds above which 13CO(1-0) and C18O(1-0) are detected are about 1 mag larger than previous estimates, so that a more efficient shielding is needed for the formation of CO than previously thought. The 12CO and 13CO lines saturate above 4 and 5 mag, respectively, whereas C18O(1-0) never saturates in the whole AV range probed by our study (up to 10 mag). Approximately 60% of the positions with 12CO(1-0) emission have subthermally excited lines, and almost all positions have excitation temperatures below the dust temperature. PDR models, using the Meudon code, can explain the 12CO(1-0) and 13CO(1-0) emission with densities ranging between 103 and 104 cm[?]3. In general, local variations in the volume density and nonthermal motions (linked to different star formation activity) can explain the observations. Higher densities are needed to reproduce CO data toward active star-forming sites, such as NGC 1333, where the larger internal motions driven by the young protostars allow more photons from the embedded high-density cores to escape the cloud. In the most quiescent region, B5, the 12CO and 13CO emission appears to arise from an almost uniform thin layer of molecular material at densities around 104 cm[?]3, and in this region the integrated intensities of the two CO isotopologues are the lowest in the whole complex.

The gravitational collapse of a spherical cloud core is investigated by numerical calculations. The initial conditions of the core lie close to the critical Bonnor-Ebert sphere with a central density of ~104 cm-3 in one model (a = 1.1), while gravity overwhelms pressure in the other (a = 4.0), where a is the internal gravity-to-pressure ratio. The a = 1.1 model shows reasonable agreement with the observed velocity field in prestellar cores. Molecular distributions in cores are calculated by solving a chemical reaction network that includes both gas-phase and grain-surface reactions. When the central density of the core reaches 105 cm-3, carbon-bearing species are significantly depleted in the central region of the a = 1.1 model, while the depletion is only marginal in the other model. The two different approaches encompass the observed variations of molecular distributions in different prestellar cores, suggesting that molecular distributions can be probes of contraction or accumulation timescales of cores. The central enhancement of the NH3/N2H+ ratio, which is observed in some prestellar cores, can be reproduced under certain conditions by adopting recently measured branching fractions for N2H+ recombination. Various molecular species, such as CH3OH and CO2, are produced by grain-surface reactions. The ice composition depends sensitively on the assumed temperature. Multideuterated species are included in our most recent gas-grain chemical network. The deuterated isotopomers of Himg1.gif are useful as probes of the central regions of evolved cores, in which gas-phase species with heavy elements are strongly depleted. At 10 K, our model can reproduce the observed abundance ratio of ND3/NH3 but underestimates the isotopic ratios of deuterated to normal methanol.

We present results of an extensive mapping survey of N2H+ (1-0) in about 60 low-mass cloud cores already mapped in the NH3 (1, 1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution of 54'', about 1.5 times finer than the previous ammonia observations made at the Haystack telescope. The comparison between N2H+ and NH3 maps shows strong similarities in the size and morphology of the two molecular species, indicating that they are tracing the same material, especially in starless cores. Cores with stars typically have map sizes about a factor of 2 smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. The mean aspect ratio is ~2. Significant correlations are found between NH3 and N2H+ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Starless cores are less massive (img1.gif [?] 3 M) than cores with stars (img1.gif [?] 9 M). Velocity gradients range between 0.5 and 6 km s-1 pc-1, similar to what has been found with NH3 data, and the ratio b of rotational kinetic energy to gravitational energy has magnitudes between ~10-4 and 0.07, indicating that rotation is not energetically dominant in the support of the cores. "Local" velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complex motions not interpretable as simple solid-body rotation. Integrated intensity profiles of starless cores present a "central flattening" and are consistent with a spherically symmetric density law n [?] r-a, where a = 1.2 for r < rbreak and a = 2 for r > rbreak, with rbreak ~ 0.03 pc. Cores with stars are better modeled with single density power laws with a [?] 2, in agreement with observations of submillimeter continuum emission. Line widths change across the core, but we did not find a general trend: there are cores with significant positive as well as negative linear correlations between Dv and the impact parameter b. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~10 coherence lengths. The corresponding value of the coherence length, ~0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased "coherence" of line widths on small scales. Despite finer angular resolution, the majority of N2H+ and NH3 maps show a similar "simple" structure, with single peaks and no elongation.

We report Swedish-ESO Submillimetre Telescope observations of seven bipolar outflows thought to be associated with Class 0 objects in the u = 0J = 3 - 2 and J = 2 - 1 transitions of SiO and Jk = 3k - 2k and Jk = 2k - 1k transitions of CH3OH. Methanol and silicon monoxide emission from outflowing gas were detected toward the lobes of four objects (NGC 2264G, IRAS 16293-2422, Serpens S68N, and Serpens SMM 4). The SiO line profiles are characteristics of C-type bow shocks, showing a peak at a radial velocity close to but displaced from the ambient cloud velocity and a gradual decrease in intensity from the peak toward higher flow velocities. There is a significant correlation between the column density of SiO and the terminal SiO flow velocity, which suggests a velocity-selective enhancement in the production of SiO molecules. We find that the SiO abundance in the lobes is enhanced with respect to that of the ambient cloud by a factor of at least 330 in IRAS 16293-2422 and SMM 4, 170 in NGC 2264G, and 80 in S68N. The CH3OH abundance is enhanced by a factor of 500 in IRAS 16293-2422, 330 in SMM 4, 80 in S68N, and 23 in NGC 2264G. In addition, we find that the dependence of the SiO/CO and CH3OH/CO abundance ratios with radial flow velocity shows a steep increase in the range from ~0 to ~4-5 km s-1 and a gradual decline toward higher flow velocities. In the remaining three sources (CG 30, IRAS 13036-7644, and VLA 1623-243), emission in methanol was detected from a narrow line at the velocity of the ambient cloud, and no emission was detected in silicon monoxide. Weak methanol emission from a low-velocity outflow component was detected toward CG 30.

We have used the 3 mm receiver and upgraded antenna of Haystack Observatory to make high spatial and spectral resolution observations of lines of N2H+and C3H2in 60 dense cores. Both species are detected in most of the cores, and the velocities, and line widths are well correlated. This suggests that ions and neutrals are well coupled. We found vD,max? 0.03 km s-1, which is the first observational constraint on the relative speed of ions and neutrals in star-forming dense cores. Twenty of the cores were also observed in the JN= 43-32line of CCS. From our N2H+observations, the fractional abundance of the molecular nitrogen N2is found to be 7 × 10-5, consistent with all the nitrogen being in molecular form and not depleted onto dust grains.

We have used the 3 mm receiver and upgraded antenna of Haystack Observatory to make high spatial and spectral resolution observations of lines of N2H+ and C3H2 in 60 dense cores. Both species are detected in most of the cores, and the velocities, and line widths are well correlated. This suggests that ions and neutrals are well coupled. We found vD,max [?] 0.03 km s-1, which is the first observational constraint on the relative speed of ions and neutrals in star-forming dense cores. Twenty of the cores were also observed in the JN = 43-32 line of CCS. From our N2H+ observations, the fractional abundance of the molecular nitrogen N2 is found to be [?]7 x 10-5, consistent with all the nitrogen being in molecular form and not depleted onto dust grains.

Observations of C18O, H13CO+, and DCO+ toward 23 low-mass cores are used to constrain the fractional ionization (electron abundance) within them. Chemical models have been run over a wide range of densities, cosmic-ray ionization rates, and elemental depletions, and we find that we can fit 20 of the 23 cores for densities of nH2 $_{{&rm; H}2}$ =(1-3)x10 4 cm-3, moderate C and O abundance variations, and a cosmic-ray ionization rate of zH2 $_{{&rm; H}2}$ =5x10 [?]17 s-1. The derived ionization fractions lie within the range 10-7.5 to 10-6.5, with a median value of xe,m = 9 x 10-8 and typical errors for each individual core equal to a factor of 3. These values imply that the cores are weakly coupled to the magnetic field and that MHD waves can propagate within them. The ambipolar diffusion timescale is about an order of magnitude greater than the free-fall time, and the cores can be considered to be in quasi-static equilibrium. There is no significant difference between the ionization fraction for cores with and without embedded stars, which suggests that the molecular ionization in cores is primarily governed by cosmic rays alone.

The rate equation approach to the chemistry occurring on grain surfaces in interstellar clouds has been criticized for not taking the discrete nature of grains into account. Indeed, investigations of simple models show that results obtained from rate equations can be significantly different from results obtained by a Monte Carlo procedure. Some modifications of the rate equations have been proposed that have the effect of eliminating most of the differences with the Monte Carlo procedure for simplified models of interstellar clouds at temperatures of 10 K and slightly above. In this study we investigate the use of the modified rate equations in more realistic chemical models of dark interstellar clouds with complex gas-grain interactions. Our results show some discrepancies between the results of models with unmodified and modified rate equations; these discrepancies are highly dependent, however, on the initial form of hydrogen chosen. If the initial form is mainly molecular, at early stages of cloud evolution there are some significant differences in calculated molecular abundances on grains, but at late times the two sets of results tend to converge for the main components of the grain mantles. If the initial form is atomic hydrogen, there are essentially no differences in results between models based on the unmodified rate equations and those based on the modified rate equations, except for the abundances on grains of some minor complex molecules. Thus, the major results of previous gas-grain models of cold, dark interstellar clouds remain at least partially intact.

Observations of C18O, H13CO+, and DCO+toward 23 low-mass cores are used to constrain the fractional ionization (electron abundance) within them. Chemical models have been run over a wide range of densities, cosmic-ray ionization rates, and elemental depletions, and we find that we can fit 20 of the 23 cores for densities of nH2=(1-3)×104cm-3, moderate C and O abundance variations, and a cosmic-ray ionization rate of ?H2=5×10?17s-1. The derived ionization fractions lie within the range 10-7.5to 10-6.5, with a median value of xe,m= 9 × 10-8and typical errors for each individual core equal to a factor of 3. These values imply that the cores are weakly coupled to the magnetic field and that MHD waves can propagate within them. The ambipolar diffusion timescale is about an order of magnitude greater than the free-fall time, and the cores can be considered to be in quasi-static equilibrium. There is no significant difference between the ionization fraction for cores with and without embedded stars, which suggests that the molecular ionization in cores is primarily governed by cosmic rays alone.

The rate equation approach to the chemistry occurring on grain surfaces in interstellar clouds has been criticized for not taking the discrete nature of grains into account. Indeed, investigations of simple models show that results obtained from rate equations can be significantly different from results obtained by a Monte Carlo procedure. Some modifications of the rate equations have been proposed that have the effect of eliminating most of the differences with the Monte Carlo procedure for simplified models of interstellar clouds at temperatures of 10 K and slightly above. In this study we investigate the use of the modified rate equations in more realistic chemical models of dark interstellar clouds with complex gas-grain interactions. Our results show some discrepancies between the results of models with unmodified and modified rate equations; these discrepancies are highly dependent, however, on the initial form of hydrogen chosen. If the initial form is mainly molecular, at early stages of cloud evolution there are some significant differences in calculated molecular abundances on grains, but at late times the two sets of results tend to converge for the main components of the grain mantles. If the initial form is atomic hydrogen, there are essentially no differences in results between models based on the unmodified rate equations and those based on the modified rate equations, except for the abundances on grains of some minor complex molecules. Thus, the major results of previous gas-grain models of cold, dark interstellar clouds remain at least partially intact.