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Register a new userHigh Atomic Carbon Abundance in Molecular Clouds in the Galactic Center Region
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This letter presents a Nyquist-sampled, high-resolution [CI] 3P1-3P0 map of the -0.2 deg < l < 1.2 deg x -0.1 deg < b < 0 deg region in the Central Molecular Zone (CMZ) taken with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. We have found that molecular clouds in the CMZ can be classified into two groups according to their [CI]/13CO intensity ratios: a bulk component consisting with clouds with a low, uniform [CI]/13CO ratio (0.45) and another component consisting of clouds with high [CI]/13CO ratios (> 0.8). The [CI]-enhanced regions appear in M-0.02-0.07, the circumnuclear disk, the 180-pc ring and the high velocity compact cloud CO+0.02-0.02. We have carried out a large velocity gradient (LVG) analysis and have derived the C^0/CO column density ratio for M-0.02-0.07 as 0.47, which is approximately twice that of the bulk component of the CMZ (0.26). We propose several hypotheses on the origin of high C^0 abundance in M-0.02-0.07, including cosmic-ray/X-ray dissociation and mechanical dissociation of CO in the pre-existing molecular clouds. We also suggest the possibility that M-0.02-0.07 is a cloud at an early stage of chemical evolution from diffuse gas, which was possibly formed by the bar-induced mass inflow in the Galactic Center region.
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Aims: To reveal the morphology, chemical composition, kinematics and to establish the main processes prevalent in the gas at the foot points of the giant molecular loops (GMLs) in the Galactic center region Methods: Using the 22-m Mopra telescope, we mapped the M$-3.8+0.9$ molecular cloud, placed at the foot points of a giant molecular loop, in 3-mm range molecular lines. To derive the molecular hydrogen column density, we also observed the $^{13}$CO $(2-1)$ line at 1 mm using the 12-m APEX telescope. From the 3 mm observations 12 molecular species were detected, namely HCO$^+$, HCN, H$^{13}$CN, HNC, SiO, CS, CH$_3$OH, N$_2$H$^+$, SO, HNCO, OCS, and HC$_3$N. Results: Maps revealing the morphology and kinematics of the M$-3.8+0.9$ molecular cloud in different molecules are presented. We identified six main molecular complexes. We derive fractional abundances in 11 selected positions of the different molecules assuming local thermodynamical equilibrium. Conclusions: Most of the fractional abundances derived for the M$-3.8+0.9$ molecular cloud are very similar over the whole cloud. However, the fractional abundances of some molecules show significant difference with respect to those measured in the central molecular zone (CMZ). The abundances of the shock tracer SiO are very similar between the GMLs and the CMZ. The methanol emission is the most abundant specie in the GMLs. This indicates that the gas is likely affected by moderate $sim $ 30 km s$^{-1}$ or even high velocity (50 km s$^{-1}$) shocks, consistent with the line profile observed toward one of the studied position. The origin of the shocks is likely related to the flow of the gas throughout the GMLs towards the foot points.
C-fractionation has been studied from a theoretical point of view with different models of time-dependent chemistry, including both isotope-selective photodissociation and low-temperature isotopic exchange reactions. Recent chemical models predict that the latter may lead to a depletion of $^{13}$C in nitrile-bearing species, with $^{12}$C/$^{13}$C ratios two times higher than the elemental abundance ratio of 68 in the local ISM. Since the carbon isotopic ratio is commonly used to evaluate the $^{14}$N/$^{15}$N ratios with the double-isotope method, it is important to study C-fractionation in detail to avoid incorrect assumptions. In this work we implemented a gas-grain chemical model with new isotopic exchange reactions and investigated their introduction in the context of dense and cold molecular gas. In particular, we investigated the $^{12}$C/$^{13}$C ratios of HNC, HCN, and CN using a grid of models, with temperatures and densities ranging from 10 to 50 K and 2$times$10$^{3}$ to 2$times$10$^{7}$ cm$^{-3}$, respectively. We suggest a possible $^{13}$C exchange through the $^{13}$C + C$_{3}$ $rightarrow$ $^{12}$C +$^{13}$CC$_{2}$ reaction, which does not result in dilution, but rather in $^{13}$C enhancement, for molecules formed starting from atomic carbon. This effect is efficient in a range of time between the formation of CO and its freeze-out on grains. Furthermore, we show that the $^{12}$C/$^{13}$C ratios of nitriles are predicted to be a factor 0.8-1.9 different from the local value of 68 for massive star-forming regions. This result also affects the $^{14}$N/$^{15}$N ratio: a value of 330 obtained with the double-isotope method is predicted to be 260-1150, depending on the physical conditions. Finally, we studied the $^{12}$C/$^{13}$C ratios by varying the cosmic-ray ionization rate: the ratios increase with it because of secondary photons and cosmic-ray reactions.
We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free-free absorption along the line of sight. We focus on one cloud, G0.13--0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (alpha^{74MHz}_{327MHz}=1.3pm0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l~0.2^0 and is a strong source of 74 MHz continuum, SiO (2-1) and FeI Kalpha 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13--0.13 molecular cloud and that low energy cosmic ray electrons are responsible for the FeI Kalpha line emission. The high cosmic ray ionization rate ~10-13 s-1 H-1 is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high velocity dispersion. LVG modeling of multi-transition SiO observations of this cloud implies H2 densities ~104-5 cm-3 and high temperatures. The lower limit to the temperature of G0.13-0.13 is ~100K, whereas the upper limit is as high as 1000K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N2H+/HCO+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.
We demonstrate that the accretion disk model for the Galactic Center region by Linden et al (1993a) is applicable for at least one order of magnitude in radius from the Galactic Center (10 ... 100 pc). The viscosity $ u$ is shown to be weakly dependent on the radius $s$: $ u sim s^{0.4}$. Finally, we discuss the influence of the inner boundary on the structure of the inner disk regions.
Ever since their discovery, Infrared dark clouds (IRDCs) are generally considered to be the sites just at the onset of high-mass (HM) star formation. In recent years, it has been realized that not all IRDCs harbour HM Young Stellar Objects (YSOs). Only those IRDCs satisfying a certain mass-size criterion, or equivalently above a certain threshold density, are found to contain HMYSOs. In all cases, IRDCs provide ideal conditions for the formation of stellar clusters. In this paper, we study the massive stellar content of IRDCs to re-address the relation between IRDCs and HM star formation. For this purpose, we have identified all IRDCs associated to a sample of 12 Galactic molecular clouds (MCs). The selected MCs have been the target of a systematic search for YSOs in an earlier study. The catalogued positions of YSOs have been used to search all YSOs embedded in each identified IRDC. In total, we have found 834 YSOs in 128 IRDCs. The sample of IRDCs have mean surface densities of 319 Mo/pc2, mean mass of 1062 Mo, and a mass function power-law slope -1.8, which are similar to the corresponding properties for the full sample of IRDCs and resulting physical properties in previous studies. We find that all those IRDCs containing at least one intermediate to high-mass young star satisfy the often-used mass-size criterion for forming HM stars. However, not all IRDCs satisfying the mass-size criterion contain HM stars. We find that the often used mass-size criterion corresponds to 35% probability of an IRDC forming a massive star. Twenty five (20%) of the IRDCs are potential sites of stellar clusters of mass more than 100 Mo.
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