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Register a new userStudy of Interstellar Molecular Clouds using Formaldehyde Absorption toward Extragalactic Radio Sources
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Added by
Esteban Araya Ph.D.
Publication date
2014
fields
Physics
and research's language is
English
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We present new Very Large Array 6cm H2CO observations toward four extragalactic radio continuum sources (B0212+735, 3C111, NRAO150, BL Lac) to explore the structure of foreground Galactic clouds as revealed by absorption variability. This project adds a new epoch in the monitoring observations of the sources reported by Marscher and collaborators in the mid 1990s. Our new observations confirm the monotonic increase in H$_2$CO absorption strength toward NRAO150. We do not detect significant variability of our 2009 spectra with respect to the 1994 spectra of 3C111, B0212+735 and BL Lac; however we find significant variability of the 3C111 2009 spectrum with respect to archive observations conducted in 1991 and 1992. Our analysis supports that changes in absorption lines could be caused by chemical and/or geometrical gradients in the foreground clouds, and not necessarily by small scale (~10 AU) high density molecular clumps within the clouds.
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We report on Atacama Large Millimeter/submillimeter Array (ALMA) detections of molecular absorption lines in Bands 3, 6 and 7 toward four radio-loud quasars, which were observed as the bandpass and complex gain calibrators. The absorption systems, three of which are newly detected, are found to be Galactic origin. Moreover, HCO absorption lines toward two objects are detected, which almost doubles the number of HCO absorption samples in the Galactic diffuse medium. In addition, high HCO to H13CO+ column density ratios are found, suggesting that the interstellar media (ISM) observed toward the two calibrators are in photodissociation regions, which observationally illustrates the chemistry of diffuse ISM driven by ultraviolet (UV) radiation. These results demonstrate that calibrators in the ALMA Archive are potential sources for the quest for new absorption systems and for detailed investigation of the nature of the ISM.
Galactic and extra-galactic sources produce X-rays that are often absorbed by molecules and atoms in giant molecular clouds (GMCs), which provides valuable information about their composition and physical state. We mimic this phenomenon with a laboratory Z-pinch X-ray source, which is impinged on neutral molecular gas. The novel technique produces a soft X-ray pseudo continuum using a pulsed-current generator. The absorbing gas is injected from a 1 cm long planar gas-puff without any window or vessel along the line of sight. An X-ray spectrometer with a resolving power of $lambda/Deltalambdasim$420, comparable to that of astrophysical space instruments, records the absorbed spectra. This resolution clearly resolves the molecular lines from the atomic lines; therefore, motivating the search of molecular signature in astrophysical X-ray spectra. The experimental setup enables different gas compositions and column densities. K-shell spectra of CO$_2$, N$_2$ and O$_2$ reveal a plethora of absorption lines and photo-electric edges measured at molecular column densities between $sim$10$^{16}$ cm$^{-2}$ -- 10$^{18}$ cm$^{-2}$ typical of GMCs. We find that the population of excited-states, contributing to the edge, increases with gas density.
We present a new catalogue of ALMA observations of 3,364 bright, compact radio sources, mostly blazars, used as calibrators. These sources were observed between May 2011 and July 2018, for a total of 47,115 pointings in different bands and epochs. We have exploited the ALMA data to validate the photometry given in the new Planck Multi-frequency Catalogue of Non-thermal sources (PCNT), for which an external validation was not possible so far. We have also assessed the positional accuracy of Planck catalogues and the PCNT completeness limits, finding them to be consistent with those of the Second Planck Catalogue of Compact Sources. The ALMA continuum spectra have allowed us to extrapolate the observed radio source counts at 100 GHz to the effective frequencies of ALMA bands 4, 6, 7, 8 and 9 (145, 233, 285, 467 and 673 GHz, respectively), where direct measurements are scanty, especially at the 3 highest frequencies. The results agree with the predictions of the Tucci et al. model C2Ex, while the model C2Co is disfavoured.
We perform ideal MHD high resolution AMR simulations with driven turbulence and self-gravity and find that long filamentary molecular clouds are formed at the converging locations of large-scale turbulence flows and the filaments are bounded by gravity. The magnetic field helps shape and reinforce the long filamentary structures. The main filamentary cloud has a length of ~4.4 pc. Instead of a monolithic cylindrical structure, the main cloud is shown to be a collection of fiber/web-like sub-structures similar to filamentary clouds such as L1495. Unless the line-of-sight is close to the mean field direction, the large-scale magnetic field and striations in the simulation are found roughly perpendicular to the long axis of the main cloud, similar to 1495. This provides strong support for a large-scale moderately strong magnetic field surrounding L1495. We find that the projection effect from observations can lead to incorrect interpretations of the true three-dimensional physical shape, size, and velocity structure of the clouds. Helical magnetic field structures found around filamentary clouds that are interpreted from Zeeman observations can be explained by a simple bending of the magnetic field that pierces through the cloud. We demonstrate that two dark clouds form a T-shape configuration which are strikingly similar to the Infrared dark cloud SDC13 leading to the interpretation that SDC13 results from a collision of two long filamentary clouds. We show that a moderately strong magnetic field (M_A ~ 1) is crucial for maintaining a long and slender filamentary cloud for a long period of time ~0.5 million years.
We present follow-up observations to those of Geballe & Oka (2010), who found high column densities of H3+ ~100 pc off of the Galactic center (GC) on the lines of sight to 2MASS J17432173-2951430 (J1743) and 2MASS J17470898-2829561 (J1747). The wavelength coverages on these sightlines have been extended in order to observe two key transitions of H3+, R(3,3)l and R(2,2)l, that constrain the temperatures and densities of the environments. The profiles of the H3+ R(3,3)l line, which is due only to gas in the GC, closely matches the differences between the H3+ R(1,1)l and CO line profiles, just as it does for previously studied sightlines in the GC. Absorption in the R(2,2)l line of H3+ is present in J1747 at velocities between -60 and +100 km/s. This is the second clear detection of this line in the interstellar medium after GCIRS 3 in the Central Cluster. The temperature of the absorbing gas in this velocity range is 350 K, significantly warmer than in the diffuse clouds in other parts of the Central Molecular Zone. This indicates that the absorbing gas is local to Sgr B molecular cloud complex. The warm and diffuse gas revealed by Oka et al. (2005) apparently extends to ~100 pc, but there is a hint that its temperature is somewhat lower in the line of sight to J1743 than elsewhere in the GC. The observation of H3+ toward J1747 is compared with the recent Herschel observation of H2O+ toward Sgr B2 and their chemical relationship and remarkably similar velocity profiles are discussed.
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