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Register a new userThe Photon Dominated Region in the IC 348 molecular cloud
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In this paper we discuss the physical conditions of clumpy nature in the IC 348 molecular cloud. We combine new observations of fully sampled maps in [C I] at 492 GHz and 12CO 4--3, taken with the KOSMA 3 m telescope at about 1 resolution, with FCRAO data of 12CO 1--0, 13CO 1--0 and far-infrared continuum data observed by HIRES/IRAS. To derive the physical parameters of the region we analyze the three different line ratios. A first rough estimate of abundance is obtained from an LTE analysis. To understand the [C I] and CO emission from the PDRs in IC 348, we use a clumpy PDR model. With an ensemble of identical clumps, we constrain the total mass from the observed absolute intensities. Then we apply a more realistic clump distribution model with a power law index of 1.8 for clump-mass spectrum and a power law index of 2.3 for mass-size relation. We provide detailed fits to observations at seven representative positions in the cloud, revealing clump densities between 4 10$^{4}$ cm$^{-3}$ and 4 10$^{5}$ cm$^{-3}$ and C/CO column density ratios between 0.02 and 0.26. The derived FUV flux from the model fit is consistent with the field calculated from FIR continuum data, varying between 2 and 100 Draine units across the cloud. We find that both an ensemble of identical clumps and an ensemble with a power law clump mass distribution produce line intensities which are in good agreement (within a factor ~ 2) with the observed intensities. The models confirm the anti-correlation between the C/CO abundance ratio and the hydrogen column density found in many regions.
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The IC 348 young star cluster contains more than 300 confirmed members. It is embedded in the Perseus molecular cloud, making any clustering analysis subject to an extinction bias. In this work, we derive the extinction map of the cloud and revisit the content of IC 348 through a statistical approach that uses the 2MASS data. Our goal was to address the question of the completeness of IC 348 and of young clusters in general. We performed a combined analysis of the star color and density in this region, in order to establish the surface density map of the cluster. We reached the conclusion that IC 348 has structures up to 25 from the cluster center, and we estimate that about 40 members brighter than Ks=13 mag are still unidentified. Although we cannot use our statistical method to identify these new members individually, the surface density map gives a strong indication of their actual location. They are distributed in the outer regions of the cluster, where very few dedicated observations have been made so far, which is probably why they escaped previous identification. In addition, we propose the existence of a new embedded cluster associated to the infrared source MSX6C G160.2784-18.4216, about 38 south of IC 348.
IC 348 is a young (t$sim$3Myr) and nearby (d$sim$340pc) star forming region in the Perseus molecular cloud. We performed a deep imaging survey using the MEGACAM (z-band) and WIRCAM (JHK and narrowband CH${_4}$ on/off) wide-field cameras on the Canada-France-Hawaii Telescope. From the analysis of the narrowband CH${_4}$ on/off deep images, we report 4 T-dwarf candidates, of which 3 clearly lie within the limits of the IC 348 cluster. An upper limit on the extinction was estimated for each candidate from colour-magnitude diagrams, and found consistent with extinction maps of the cloud. Initial comparisons with T-dwarf spectral models suggest these candidates have a spectral type between T3 and T5, and perhaps later, potentially making these among the lowest mass isolated objects detected in a young star forming region so far.
In this paper, we address two issues related to primordial disk evolution in three clusters (NGC 1333, IC 348, and Orion A) observed by the INfrared Spectra of Young Nebulous Clusters (IN-SYNC) project. First, in each cluster, averaged over the spread of age, we investigate how disk lifetime is dependent on stellar mass. The general relation in IC 348 and Orion A is that primordial disks around intermediate mass stars (2--5$M_{odot}$) evolve faster than those around loss mass stars (0.1--1$M_{odot}$), which is consistent with previous results. However, considering only low mass stars, we do not find a significant dependence of disk frequency on stellar mass. These results can help to better constrain theories on gas giant planet formation timescales. Secondly, in the Orion A molecular cloud, in the mass range of 0.35--0.7$M_{odot}$, we provide the most robust evidence to date for disk evolution within a single cluster exhibiting modest age spread. By using surface gravity as an age indicator and employing 4.5 $mu m$ excess as a primordial disk diagnostic, we observe a trend of decreasing disk frequency for older stars. The detection of intra-cluster disk evolution in NGC 1333 and IC 348 is tentative, since the slight decrease of disk frequency for older stars is a less than 1-$sigma$ effect.
We present [Ci] and [Cii] observations of a linear edge region in the Taurus molecular cloud, and model this region as a cylindrically symmetric PDR exposed to a low-intensity UV radiation field. The sharp, long profile of the linear edge makes it an ideal case to test PDR models and determine cloud parameters. We compare observations of the [C i], 3P1 -> 3P0 (492 GHz), [C i] 3P2 -> 3P1 (809 GHz), and [Cii] 2P3/2 -> 2P1/2 (1900 GHz) transitions, as well as the lowest rotational transitions of 12CO and 13CO, with line intensities produced by the RATRAN radiative transfer code from the results of the Meudon PDR code. We constrain the density structure of the cloud by fitting a cylindrical density function to visual extinction data. We study the effects of variation of the FUV field, 12C/13C isotopic abundance ratio, sulfur depletion, cosmic ray ionization rate, and inclination of the filament relative to the sky-plane on the chemical network of the PDR model and resulting line emission. We also consider the role of suprathermal chemistry and density inhomogeneities. We find good agreement between the model and observations, and that the integrated line intensities can be explained by a PDR model with an external FUV field of 0.05 G0, a low ratio of 12C to 13C ~ 43, a highly depleted sulfur abundance (by a factor of at least 50), a cosmic ray ionization rate (3 - 6) x 10-17 s^-1, and without significant effects from inclination, clumping or suprathermal chemistry.
We consider the excitation of water in the Photon Dominated Region (PDR). With the use of a three-dimensional escape probability method we compute the level populations of ortho- and para-H_2O up to 350 K (i.e., 8 levels), as well as line intensities for various transitions. Homogeneous and inhomogeneous models are presented with densities of 10^4-10^5 cm^{-3} and the differences between the resulting intensities are displayed. Density, temperature, and abundance distributions inside the cloud are computed with the use of a self-consistent physi-chemical (in)homogeneous model in order to reproduce the line intensities observed with SWAS, and to make predictions for various lines that HIFI will probe in the future. Line intensities vary from 10^{-13} erg cm^{-2} s^{-1} sr^{-1} to a few times 10^{-6} erg cm^{-2} s^{-1} sr^{-1}. We can reproduce the intensity for the 1_{10}-1_{01} line observed by the SWAS satellite. It is found that the 2_{12}-1_{01} line is the strongest, whereas the 3_{12}-2_{21} line is the weakest, in all the models. It is found that the 1_{10}-1_{01} line probes the total column, while higher excitation lines probe the higher density gas (e.g., clumps).
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