A Direct Measurement of the Total Gas Column Density in Orion KL

The large number of high-J lines of C^(18)O available via the Herschel Space Observatory provide an unprecedented ability to model the total CO column density in hot cores. Using the emission from all the observed lines (up to J = 15-14), we sum the column densities in each individual level to obtain the total column after correcting for the population in the unobserved states. With additional knowledge of source size, V_(LSR), and line width, and both local thermodynamic equilibrium (LTE) and non-LTE modeling, we have determined the total C^(18)O column densities in the Extended Ridge, Outflow/Plateau, Compact Ridge, and Hot Core components of Orion KL to be 1.4 × 10^(16) cm^(–2), 3.5 × 10^(16) cm^(–2), 2.2 × 10^(16) cm^(–2), and 6.2 × 10^(16) cm^(–2), respectively. We also find that the C^(18)O/C^(17)O abundance ratio varies from 1.7 in the Outflow/Plateau, 2.3 in the Extended Ridge, 3.0 in the Hot Core, and to 4.1 in the Compact Ridge. This is in agreement with models in which regions with higher ultraviolet radiation fields selectively dissociate C^(17)O, although care must be taken when interpreting these numbers due to the size of the uncertainties in the C^(18)O/C^(17)O abundance ratio

Molecular line survey of the high-mass star-forming region NGC 6334I with Herschel/HIFI and the SMA

We aim at deriving the molecular abundances and temperatures of the hot molecular cores in the high-mass star-forming region NGC 6334I and consequently deriving their physical and astrochemical conditions. In the framework of the Herschel guaranteed time key program CHESS, NGC 6334I is investigated by using HIFI aboard the Herschel Space Observatory. A spectral line survey is carried out in the frequency range 480-1907 GHz, and auxiliary interferometric data from the SMA in the 230 GHz band provide spatial information for disentangling the different physical components contributing to the HIFI spectrum. The spectral lines are identified with the aid of former surveys and spectral line catalogs. The observed spectrum is then compared to a simulated synthetic spectrum with XCLASS, assuming local thermal equilibrium, and best fit parameters are derived using the model optimization package MAGIX. A total of 46 molecules are identified, with 31 isotopologues, resulting in about 4300 emission and absorption lines. High- energy levels of the dominant emitter methanol and vibrationally excited HCN are detected. The number of unidentified lines remains low with 75, or less than 2 percent of the lines detected. The modeling suggests that several spectral features need two or more components to be fitted properly. Other components could be assigned to cold foreground clouds or to outflows, most visible in the SiO emission. A chemical variation between the two embedded hot cores is found, with more N-bearing molecules identified in SMA1 and O-bearing molecules in SMA2. Spectral line surveys give powerful insights into the study of the interstellar medium. Different molecules trace different physical conditions like the inner hot core, the envelope, the outflows or the cold foreground clouds. The derived molecular abundances provide further constraints for astrochemical models.Comment: 30 pages including appendix, 49 figures, accepted for publication in Astronomy and Astrophysic

Molecular line survey of the high-mass star-forming region NGC 6334I with Herschel/HIFI and the Submillimeter Array

Aims. We aim at deriving the molecular abundances and temperatures of the hot molecular cores in the high-mass star-forming region NGC 6334I and consequently deriving their physical and astrochemical conditions. Methods. In the framework of the Herschel guaranteed time key program CHESS (Chemical HErschel Surveys of Star forming regions), NGC 6334I is investigated by using the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory. A spectral line survey is carried out in the frequency range 480–1907 GHz, and further auxiliary interferometric data from the Submillimeter Array (SMA) in the 230 GHz band provide spatial information for disentangling the different physical components contributing to the HIFI spectrum. The spectral lines in the processed Herschel data are identified with the aid of former surveys and spectral line catalogs. The observed spectrum is then compared to a simulated synthetic spectrum, assuming local thermal equilibrium, and best fit parameters are derived using a model optimization package. Results. A total of 46 molecules are identified, with 31 isotopologues, resulting in about 4300 emission and absorption lines. High-energy levels (E_u > 1000 K) of the dominant emitter methanol and vibrationally excited HCN (ν_2 = 1) are detected. The number of unidentified lines remains low with 75, or <2% of the lines detected. The modeling suggests that several spectral features need two or more components to be fitted properly. Other components could be assigned to cold foreground clouds or to outflows, most visible in the SiO and H_(2)O emission. A chemical variation between the two embedded hot cores is found, with more N-bearing molecules identified in SMA1 and O-bearing molecules in SMA2. Conclusions. Spectral line surveys give powerful insights into the study of the interstellar medium. Different molecules trace different physical conditions like the inner hot core, the envelope, the outflows or the cold foreground clouds. The derived molecular abundances provide further constraints for astrochemical models

Systematic Molecular Differentiation in Starless Cores

(Abridged) We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Molecules including CO and CS almost vanish near the core centers, where the abundance decreases by one or two orders of magnitude. At the same time, N2H+ has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a study of 5 mostly-round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and (2,2), and the 1.2 mm continuum. For each core we have built a model that fits simultaneously the radial profile of all observed emission and the central spectrum for the molecular lines. The observed abundance drops of CO and CS are naturally explained by the depletion of these molecules onto dust grains at densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to densities of several 10^5, while the NH3 abundance may be enhanced by reactions triggered by the disappearance of CO from the gas phase. With the help of our models, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem which has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the linewidth of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent linewidth with radius, our models show that it can arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap

Continuum radiative transfer Modeling of Sagittarius B2

We present results from radiative transfer modeling of the continuum emission towards Sagittarius B2 (hereafter Sgr B2). We have developed a radiative transfer framework – Pandora – that employs RADMC-3D (Dullemond 2012) for a self-consistent determination of the dust temperature. With this pipeline, we have set-up a single model that consistently reproduces the thermal dust and free-free continuum emission of Sgr B2 spanning four orders of magnitude in spatial scales (0.02–45 pc) and two orders of magnitude in frequency (20–4000 GHz)

Magnetic field regulated infall on the disc around the massive protostar Cepheus A HW2

We present polarization observations of the 6.7-GHz methanol masers around the massive protostar Cepheus A HW2 and its associated disc. The data were taken with the Multi-Element Radio Linked Interferometer Network. The maser polarization is used to determine the full three-dimensional magnetic field structure around Cepheus A HW2. The observations suggest that the masers probe the large scale magnetic field and not isolated pockets of a compressed field. We find that the magnetic field is predominantly aligned along the protostellar outflow and perpendicular to the molecular and dust disc. From the three-dimensional magnetic field orientation and measurements of the magnetic field strength along the line of sight, we are able to determine that the high density material, in which the masers occurs, is threaded by a large scale magnetic field of ~23 mG. This indicates that the protostellar environment at ~1000 AU from Cepheus A HW2 is slightly supercritical (lambda approximately 1.7) and the relation between density and magnetic field is consistent with collapse along the magnetic field lines. Thus, the observations indicate that the magnetic field likely regulates accretion onto the disc. The magnetic field dominates the turbulent energies by approximately a factor of three and is sufficiently strong to be the crucial component stabilizing the massive accretion disc and sustaining the high accretion rates needed during massive star-formation.Comment: 10 pages, 6 figures; accepted for publication in MNRAS. High resolution version can be found at http://www.astro.uni-bonn.de/~wouter/papers/papers.shtm