Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~tghosh/feb09/arceibo_gal_chem_v1.pdf
Дата изменения: Mon Jan 26 04:16:01 2009
Дата индексирования: Sun Apr 10 22:21:07 2016
Кодировка: IBM-866

Поисковые слова: arp 220
Introduction The study of complex molecules in the interstellar medium (ISM) is crucial for understanding the physical and chemical processes in the ISM as well as the origins of life. To date, over 140 molecules have been identified in space. Some of these are quite complex (containing as many as 8 to 13 atoms) and are considered "pre-biotic" molecules ннmolecules thought to be important to life, as they are needed to synthesize amino acids. It has been suggested that chemical processes in the interstellar medium provided the necessary material that allowed the emergence of life on Earth (Maeda & Ohno 2006). A thorough chemical inventory is therefore needed to establish if there is any connection between life and the interstellar medium. Moreover, the detection and identification of complex molecules in different environments (i.e., warm star forming regions, PDRs, cicumstellar envelopes of evolved stars, etc.) are essential for better understanding the possible formation (and destruction) pathways of these molecules, as well as for constraining chemical models (Ikeda et al. 2001). Unbiased spectral line surveys provide essential information needed for characterizing the physical and chemical conditions of a region, and the relatively unexplored spectral range between 1 and 10 GHz has the potential to provide new and exciting discoveries. The majority of interstellar molecules have been detected at mm wavelengths, as molecules with small moments of inertia are the most abundant cosmically, with their rotational lines occurring at mm wavelengths or shorter. However, although less abundant, many complex molecules have spectral lines in the radio regime for l > 3 cm, where "line confusion" does not set a limit to their delectability. Many transitions of small polycyclic aromatic hydrocarbons (PAHs), of pre-biotic molecules, and even a number of transitions of the simplest amino acid, glycine, fall between 1 and 10 GHz. In addition, observations at this frequency range are complementary to (sub)-mm spectral line surveys as they probe colder and lower density gas. The few existent cm-wave spectral line surveys exhibit very interesting results and show the need for further studies for understanding the physical and chemical conditions in different ISM environments. Recently, members of our team detected a number of pre-biotic molecules in the distant ultra luminous infra-red galaxy, Arp 220, in spectral made at Arecibo using (and commissioning) the dual-board mode of the WAPP spectrometer (Salter et al. 2008). Methanine, a molecule important for the formation of glycine (e.g., Dickerson 1978), was one of the species discovered in Arp220. Since then this molecule (and other complex species) have been detected in Arp220-like starbust galaxies as part of an on-going cm-wave study Arecibo and the GBT (Minchin et al. 2009). In addition, the Arecibo observations by Kalenskii et al. (2004) and Araya et al. (2003) of the TMC1 cold dark cloud and the asymptotic giant branch carbon star IRC +10ђ216, respectively, produced the first detections of lines of several complex and simple molecules. The authors also used their multi-molecular line observations to investigate the chemical richness of the regions, estimate the temperature of the gas, and study the formation mechanism of molecules. Update Inspired by the success of the Salter et al. (2008) study, on October 2007 we proposed to carry out a spectral survey of Galactic sources, using the Arecibo Observatory with the (1 GHz wide) Mock spectrometers (in single-pixel mode). The project was given an A-grade. Unfortunately, the Mock's were not ready in single-pixel mode in time for the October-November 2008 observations, so we had to use the WAPPs (which give a maximum of bandwidth of about 340 MHz) and were forced to substantially cut back on our integration time per band. We therefore did not achieve the required sensitivity to detect complex pre-biotic molecules (see below for required integration times). Regardless of the set backs we were able to make some exciting discoveries....

The Arecibo Galactic Chemistry Survey


Goals and Methodology Here we propose to carry out the first set of observations of the Arecibo Galactic Chemistry Survey. The proposed spectral survey will: 1) produce a molecular line inventory of the observed regions; 2) study the physical and chemical conditions of different sources; and 3) search for new interstellar molecules and for previously undetected transitions. The success of the survey depends on using the wideband Mock spectrometers in single-pixel mode (which are scheduled to be operational in a few months). These spectrometers are capable of providing 20.7 kHz-wide channels over a 1 GHz bandwidth. Thus, they make highly sensitive, complete 1-10 GHz line searches of Galactic sources feasible at the Arecibo Observatory, despite requiring high-velocity resolution (<1 km s-1). The unmatched sensitivity and angular resolution of the world's largest radio telescope, along with its new wide-band spectrometers, makes the AO the perfect instrument to carry out our proposed project. In this "precursor" run, where we intend to test our methodology, we aim to study two chemically active hot cores with strong continuum emission. Hot cores are compact (d < 0.1pc), dense (n >106 cm3) and hot (T~200K) regions that harbor massive protostars (Kurtz et al. 2000). In these regions, the high temperature evaporates the icy dust mantles, which is made of many different molecules. The molecules in the gas phase are then able to combine with each other and form more complex species (Charnley et al. 1992). We propose to concentrate our observations first on W51 IRS1 and W49A, two of the strongest continuum sources in the sky associated with massive star forming regions (REF). These sources have been the targets of successful mm-wave searches for large molecules (e.g., Remijan et al. 2003; 2004). The W51 and W49 regions are relatively far away (about 7 and 12 kpc, respectively) and thus they can also be used to study foreground clouds through absorption lines along their lines of sight, against their bright continuum emission (e.g., Neufeld et al. 2002). This is one reason why these two regions are important targets in the Herschel Space Telescope key project PRISMAS: PRobing InterStellar Molecules with Absorption line Studies. Our cm-wave line survey ннwhich will probe the low-energy molecular transitionsнн will complement and help in the interpretation of the Herschel submm study. Strong continuum sources are also favorable for our study as they can slightly amplify lowenergy transition lines of complex molecules through "weak masing" (e.g., Chengalur & Kenekar 2003). In addition to providing complementary information to the mm and sub-mm line surveys, our cm study has the potential to provide new and exciting discoveries in this relatively unexplored spectral window. We note that W51 IRS1 was included in our original (A-grade) proposal, but the TAC did not allow us to observe because it is in the Galactic center quadrant ннa region committed to PALFA and I-GALFA. The latter of these two projects will be done by this summer, so time will become available for us to be able to study both of our proposed sources. We will use our data to build a molecular line catalog of each source, and will search for previously undetected complex (and pre-biotic) ISM molecules, such as glycine and small PAHs, that are hard to detect in the mm regime due to line confusion or because they yet lack a measured mm wavelength transition (e.g., Thorwirth et al. 2007). Rotational diagrams using several line transitions of the same molecules will be used to obtain their column density (and relative abundance) and rotational temperature (e.g. Goldsmith & Langer 1999). By comparing our results to chemical models we will also be able to study the formation mechanism of complex molecules in our regions of interest (e.g., Charnley et al. 1992; Ikeda et al. 2001; Pardo et al. 2007). We expect to expand our source list in the future to build a statistically sound sample of observed chemically active galactic sources. Technical Details The molecule HCOOH is a complex species that is commonly found in hot cores that have been searched for complex molecules, with a typical column density of 1015 н 1016 cm-2 (other complex


molecules have similar abundances in chemically active regions, see, e.g., Remijan et al. 2004; Requena-Torres et al. 2006). Assuming a temperature of ~100K, the expected line strength of the HCOOH (211-212) at 4.9 GHz would be about 0.03K. Using HCOOH as an example, we would then need to achieve an rms of about 3mK to detect complex (pre-biotic) molecules. Given that the both W51 and W49 are strong continuum emitters, we will employ a modified Double Position Switching (DPS) technique, as optimized for our Arp 220 observations (A2234). For this, each 5-min ON/OFF position-switched observation on a target is preceded (or followed) by a 1-min ON/OFF on a nearby strong ( 1 Jy) continuum source, which acts both as a band-pass calibrator, and provides an independent spectral check for RFI and system artifacts such as band-pass ripples, and trapped modes. This strategy has been vital for validating our detections in Arp 220. Each DPS cycle will require about 15 min (including the 1 min wait between ON and OFF phases and ~1 min for the calibration cycle.)

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