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  • ELSEVIER Chinese Astronomy and Astrophysics 38 (2014) 265293

    CHINESEASTRONOMYAND ASTROPHYSICS

    12CO J=2-1 and J=3-2 Line Observations ofMolecular Clouds toward the Directions of 59

    EGOs in the Northern Sky

    LI Zhi-guang1,2,3 HE Jin-hua1,21Yunnan Astronomical Observatory, Chinese Academy of Sciences, Kunming 650011

    2Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy ofSciences, Kunming 650011

    3University of Chinese Academy of Sciences, Beijing 100049

    Abstract In order to investigate the differences between the molecular cloudswhich are associated with the massive star forming regions and those which arenot, we have performed the single-dish simultaneous observations of 12CO J=2-1and J=3-2 lines toward a sample of 59 Spitzer Extended Green Objects (EGOs)as the massive star forming regions in the northern sky. Combining our resultswith the data of the 12CO J=1-0 observations toward the same sample EGOs inthe literature, we have made the statistical comparisons on the intensities andlinewidths of multiple 12CO lines between the molecular clouds associated withEGOs (EGO molecular clouds, in brief) and other non-EGO molecular clouds.On this basis, we have discussed the effects of the gas temperature, density, andvelocity field distributions on the statistical characteristics of the two kinds ofmolecular clouds. It is found that both the EGO molecular clouds and non-EGO molecular clouds have similar mass ranges, hence we conclude that forthe formation of massive stars, the key-important factor is probably not thetotal mass of a giant molecular cloud (GMC), but the volume filling factor of themolecular clumps in the GMC(or the compression extent of the molecular gasin the cloud).

    Key words: stars: formationISM: cloudsISM: spectral lines and bandsISM: molecules submillimeter: ISM

    Supported by National Natural Science Foundation (11173056)Received 20130402; revised version 20130429

    A translation of Acta Astron. Sin. Vol. 55, No. 1, pp. 4065, 2014 lzhg04wj@ynao.ac.cn

    0275-1062/01/$-see front matter c 2014 Elsevier Science B. V. All rights reserved.PII:

    0275-1062/14/$-see front matter 2014 Elsevier B.V. All rights reserved.doi:10.1016/j.chinastron.2014.07.005

    http://crossmark.crossref.org/dialog/?doi=10.1016/j.chinastron.2014.07.005&domain=pdf

  • 266 Li Zhi-guang and He Jin-hua / Chinese Astronomy and Astrophysics 38 (2014) 265293

    1. INTRODUCTION

    The study of large-mass star formation is a very active research field at present. By meansof stellar wind, jet, HII region collision, and supernova explosion, the massive stars releasea large amount of heavy elements and ultraviolet radiation to the cosmic space, which affectthe formation processes of other stars and planets, and further the chemical and physicalproperties, as well as the morphologic structure of the whole galaxy via the interactions withthe parent molecular cloud.

    However, the human knowledge on the large-mass star formation is much less than thatof small-mass star formation. In which the main reason is the difficulty of observation. Thelarge-mass star forming regions are generally very distant, and most of them are situatedon the Galactic plane, hence heavy extinction exists at the optical waveband, in addition tothe short evolutionary timescale of large-mass stars, the number of forming large-mass starswhich can be observed and studied is very small.

    According to the review made by Zinnecker et al.[1], now in observation the processof large-mass star formation is roughly divided into four stages: (1) IR dark cloud; (2) hotmolecular core; (3) hypercompact and ultracompact HII regions; and (4) compact and clas-sical HII regions. Similarly in theory the process of large-mass star formation can also bedivided into four stages: (1) the formation of cool and dense molecular cloud core or fila-mentary structure; (2) the gravitational collapse of molecular cloud core; (3) the accretionof mass; and (4) the disruption of parent molecular cloud. But these theoretically dividedevolutionary stages can hardly correspond one by one strictly to the stages divided in ob-servation. These are only the rough divisions of the process of large-mass star formation,in fact, about the formation of large-mass stars there are still many problems remained tosolve, for example, the formation of a large-mass star is caused by whether the accretionof a single star or the merge of small molecular clouds? how about the cloud structure inthe young massive star formation region? what are the conditions for the formation of alarge-mass star? what is the relation between the mass of the finally formed star and theproperty of its parent molecular cloud? and so on.

    From the Galactic infrared survey observation of the Spitzer infrared space telescope,Cyganowski et al.[2] found a group (about 300) of new objectsExtended Green Objects (EGOs).By the statistical study on the correlations of the EGOs with the infrared dark clouds andtype-II methanol masers, as well as their positions on the infrared color-color diagram, theyverified the reasonableness that the EGOs may be the candidates of large-mass star forma-tion regions. This increases about one fold of observational sample for the exploration oflarge-mass star formation, and provides us with new opportunities to study the process oflarge-mass star formation by observations.

    Recently, people have made a series of succeeding multi-wavelength observations on thisgroup of EGOs, including the millimeter wave single-point spectral observations of H13CO+

    J=1-0, 12CO J=1-0, 13CO J=1-0, and C18O J=1-0 lines, that were made by Chen et al.[3]

    toward a group of EGOs in the northern sky with the 13.7m millimeter radio telescope ofPurple Mountain Observatory at Delingha. They obtained the conclusion that the infallmotion of matter exists in these EGOs. Dr. Cyganowski, the discoverer of EGOs, madehimself the millimeter wave single-dish spectral observations and the interferometric imagingobservations on some selected EGOs[45] in detail, and made comparison with the result of

  • Li Zhi-guang and He Jin-hua / Chinese Astronomy and Astrophysics 38 (2014) 265293 267

    methanol maser observations, he confirmed that the phenomena of jets really exist in theseobjects, and discussed the mechanism for the coexistence of different kinds of methanolmasers in the EGOs. He et al.[6] made the single-dish spectral observations at the molecularlines H13CO+ J=3-2, SiO J=6-5 and the SO, CH3OH lines to trace the dense gas of a groupof EGOs in the northern sky, and made a joint statistical analysis in combination with theobserved data of the CO isotopic lines 12CO J=1-0, 13CO J=1-0, and C18O J=1-0, theyrevealed that most EGO molecular clouds are associated with dense gas, many EGOs exhibitjets and shocks, and that the different EGO molecular clouds exhibit universal similaritiesin density and temperature structures.

    In this work, using the KOSMA 3m sub-millimeter wave telescope we make the spec-tral observations at the rotational transition lines 12CO J=2-1 and J=3-2 on a group ofEGOs in the northern sky, and in combination with the data observed previously at the12CO J=1-0 line with the Delingha 13.7m telescope[3], we statistically analyze the differ-ences between the molecular clouds associated with EGOs (EGO molecular clouds) and theother molecular clouds without large-mass star formation in the same directions (non-EGOmolecular clouds), and hereby to reveal the environmental characteristics of the molecularclouds associated with large-mass star formation regions.

    2. OBSERVATION AND DATA PROCESSING

    On 5th-13th Nov. 2009, with the German 3m diameter KOSMA sub-millimeter wave tele-scope on the Gornergrat mountain in the Swiss Alps before its moving to Yangbajing inTibet, we made the single-point spectral observations at the 12CO J=2-1 (230.538GHz) andJ=3-2 (345.796GHz) molecular lines toward 59 EGOs in the northern sky. The positions ofthese objects were taken from Reference [2]. The KOSMAs double-sideband receiver wasused to observe simultaneously the two spectral lines, this would help to reduce the fluxcalibration error between the two spectral lines. The acousto-optic spectrometers (AOSs) ofhigh resolution (HRS) and of low resolution (LRS), and a digital Fourier transform spec-trometer (DFT) were simultaneously used to analyze the spectral signal of the 12CO J=2-1line, and an AOS of variable resolution (VRS) was used to analyze the spectral signal of the12CO J=3-2 line. In this work, we adopted mainly the data of LRS (1.419GHz bandwidthfor 1048 channels) and VRS (0.695GHz bandwidth for 2048 channels). Because of the oc-casional failure of the LRS, for the G35.20-0.74, G39.39-0.14, G44.01-0.03, G57.61+0.02,and G59.79+0.63 five objects, we adopted the 12CO J=2-1 line data recorded by the DFT(1GHz bandwidth for 16 384 channels). At the 12CO J=2-1 line, the KOSMAs main-beamwidth was 130, the main-beam efficiency was 0.68; at the 12CO J=3-2 line, they were 82

    and 0.70, respectively. During the observations, the mode of position switching was adoptedto reduce the effect of atmospheric noise. Since all the directions of our observations are veryclose to the Galactic plane, for most observations the reference position (off) of the positionswitching had been checked, the position without significant CO radiation or with only apiece of narrow spectral feature was adopted as the off-position. Only in a few observations,their off-positions had not been checked, the influences of the possible 12CO emission atthe off-position on the observed spectra are unknown, this may cause some effects on thespectra of a small part of molecular clouds.

  • 268 Li Zhi-guang and He Jin-hua / Chinese Astronomy and Astrophysics 38 (2014) 265293

    In order to make co