skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: MILLIMETER-WAVE SPECTRAL LINE SURVEYS TOWARD THE GALACTIC CIRCUMNUCLEAR DISK AND Sgr A*

Abstract

We have performed unbiased spectral line surveys at the 3 mm band toward the Galactic circumnuclear disk (CND) and Sgr A* using the Nobeyama Radio Observatory 45 m radio telescope. The target positions are two tangential points of the CND and the direction of Sgr A*. We have obtained three wide-band spectra that cover the frequency range from 81.3 GHz to 115.8 GHz, detecting 46 molecular lines from 30 species, including 10 rare isotopomers and 4 hydrogen recombination lines. Each line profile consists of multiple velocity components which arise from the CND, +50 km s{sup –1} and +20 km {sup –1} giant molecular clouds (GMCs), and the foreground spiral arms. We define the specific velocity ranges that represent the CND and the GMCs toward each direction, and classify the detected lines into three categories: the CND, GMC, HBD types, based on the line intensities integrated over the defined velocity ranges. The CND and GMC types are the lines that mainly trace the CND and the GMCs, respectively. The HBD types possesses the both characteristics of the CND and GMC types. We also present lists of line intensities and other parameters, as well as intensity ratios, which must be useful tomore » investigate the difference between the nuclear environments of our Galaxy and others.« less

Authors:
; ; ;  [1];  [2];  [3]
  1. School of Fundamental Science and Technology, Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama, Kanagawa 223-8522 (Japan)
  2. Department of Physics, Institute of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama, Kanagawa 223-8522 (Japan)
  3. Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Publication Date:
OSTI Identifier:
22340176
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal, Supplement Series; Journal Volume: 214; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CLOUDS; GALAXIES; GHZ RANGE; HYDROGEN 4; MOLECULES; RADIO TELESCOPES; RECOMBINATION; SPECTRA

Citation Formats

Takekawa, Shunya, Oka, Tomoharu, Matsumura, Shinji, Miura, Kodai, Tanaka, Kunihiko, and Sakai, Daisuke, E-mail: shunya@z2.keio.jp. MILLIMETER-WAVE SPECTRAL LINE SURVEYS TOWARD THE GALACTIC CIRCUMNUCLEAR DISK AND Sgr A*. United States: N. p., 2014. Web. doi:10.1088/0067-0049/214/1/2.
Takekawa, Shunya, Oka, Tomoharu, Matsumura, Shinji, Miura, Kodai, Tanaka, Kunihiko, & Sakai, Daisuke, E-mail: shunya@z2.keio.jp. MILLIMETER-WAVE SPECTRAL LINE SURVEYS TOWARD THE GALACTIC CIRCUMNUCLEAR DISK AND Sgr A*. United States. doi:10.1088/0067-0049/214/1/2.
Takekawa, Shunya, Oka, Tomoharu, Matsumura, Shinji, Miura, Kodai, Tanaka, Kunihiko, and Sakai, Daisuke, E-mail: shunya@z2.keio.jp. Mon . "MILLIMETER-WAVE SPECTRAL LINE SURVEYS TOWARD THE GALACTIC CIRCUMNUCLEAR DISK AND Sgr A*". United States. doi:10.1088/0067-0049/214/1/2.
@article{osti_22340176,
title = {MILLIMETER-WAVE SPECTRAL LINE SURVEYS TOWARD THE GALACTIC CIRCUMNUCLEAR DISK AND Sgr A*},
author = {Takekawa, Shunya and Oka, Tomoharu and Matsumura, Shinji and Miura, Kodai and Tanaka, Kunihiko and Sakai, Daisuke, E-mail: shunya@z2.keio.jp},
abstractNote = {We have performed unbiased spectral line surveys at the 3 mm band toward the Galactic circumnuclear disk (CND) and Sgr A* using the Nobeyama Radio Observatory 45 m radio telescope. The target positions are two tangential points of the CND and the direction of Sgr A*. We have obtained three wide-band spectra that cover the frequency range from 81.3 GHz to 115.8 GHz, detecting 46 molecular lines from 30 species, including 10 rare isotopomers and 4 hydrogen recombination lines. Each line profile consists of multiple velocity components which arise from the CND, +50 km s{sup –1} and +20 km {sup –1} giant molecular clouds (GMCs), and the foreground spiral arms. We define the specific velocity ranges that represent the CND and the GMCs toward each direction, and classify the detected lines into three categories: the CND, GMC, HBD types, based on the line intensities integrated over the defined velocity ranges. The CND and GMC types are the lines that mainly trace the CND and the GMCs, respectively. The HBD types possesses the both characteristics of the CND and GMC types. We also present lists of line intensities and other parameters, as well as intensity ratios, which must be useful to investigate the difference between the nuclear environments of our Galaxy and others.},
doi = {10.1088/0067-0049/214/1/2},
journal = {Astrophysical Journal, Supplement Series},
number = 1,
volume = 214,
place = {United States},
year = {Mon Sep 01 00:00:00 EDT 2014},
month = {Mon Sep 01 00:00:00 EDT 2014}
}
  • We have conducted the first comprehensive millimeter-wave molecular emission line surveys of the evolved circumstellar disks orbiting the nearby, roughly solar-mass, pre-main-sequence (T Tauri) stars, TW Hya (D = 54 pc) and V4046 Sgr AB (D = 73 pc). Both disks are known to retain significant residual gaseous components despite the advanced ages of their host stars (∼8 Myr and ∼21 Myr, respectively). Our unbiased broadband radio spectral surveys of the TW Hya and V4046 Sgr disks were performed with the Atacama Pathfinder Experiment 12 m telescope, and are intended to yield a complete census of the bright molecular emissionmore » lines in the range 275-357 GHz (1.1-0.85 mm). We find that lines of {sup 12}CO, {sup 13}CO, HCN, CN, and C{sub 2}H, all of which lie in the higher frequency (>330 GHz) range, constitute the strongest molecular emission from both disks in the spectral region surveyed. The molecule C{sub 2}H is detected here for the first time in both disks, as is CS in the TW Hya disk. The survey results also include the first measurements of the full suite of the hyperfine transitions of CN N = 3 → 2 and C{sub 2}H N = 4 → 3 in both disks. Modeling of these CN and C{sub 2}H hyperfine complexes in the spectrum of TW Hya indicates that the emission from both species is optically thick and may originate from very cold (≲10 K) disk regions. The latter result, if confirmed, would suggest the efficient production of CN and C{sub 2}H in the outer disk and/or near the disk midplane. It furthermore appears that the fractional abundances of CN and C{sub 2}H are significantly enhanced in these evolved protoplanetary disks, relative to the fractional abundances of the same molecules in the environments of deeply embedded protostars. These results, combined with previous determinations of the enhanced abundances of other species (such as HCO{sup +}) in T Tauri star disks, underscore the importance of properly accounting for high-energy (FUV and X-ray) radiation from the central T Tauri star when modeling protoplanetary disk gas chemistry and physical conditions.« less
  • We report the detection of molecular gas which appears in absorption at large negative velocities near the galaxtic center for a number of millimeter-wavelength molecular transitions previously observed only in emisson. We have made millimeter-continuum emission measurements which show that the absorbing line is optically thick in HCO/sup +/ and has an excitation temperature consistent with no collisional excitation form --117 to --83 km s/sup -1/ toward Sgr B2. At these velocities CO is seen in emission at a level of 3 K. We obtain molecular column densities for five species and estimate a molecular hydrogen column density of approx.5more » x 10/sup 23/ cm/sup -2/ to Sgr B2. We associate this absorbing gas with the nuclear disk and esimate that the total mass involved is approx.8 x 10/sup 9/ M/sub sun/. It is shown that spiral arms in the galactic disk also appear as absorption features in HCO/sup +/ and HCN.« less
  • We report the results of millimeter and submillimeter molecular line mapping observations of the Galactic circumnuclear disk (CND). The CND appears as a large, asymmetric disk of warm molecular gas with a high CO J = 3-2/CO J = 1-0 intensity ratio exceeding 1.5. It has a mass of (2-5) x 10{sup 5} M{sub sun} and a diameter of about 10 pc, including a well-known 2-pc radius ring of dense molecular gas around the minispiral. The CND can be clearly traced by the J = 1-0 lines of HCN, H{sup 13}CN, HCO{sup +}, and HNC, but it is barely tracedmore » by N{sub 2}H{sup +}, SiO, CCS, and HC{sub 3}N lines. These data confirm the entity of the CND, and the 2-pc ring is just a part of it. Line ratios suggest that the CND is chemically immature, having higher density and higher temperature than the ambient gas. A one-zone large-velocity-gradient analysis finds that molecular gas in the CND has a typical kinetic temperature of T{sub k} {approx_equal} 63 K and H{sub 2} density of n(H{sub 2}) {approx_equal} 10{sup 4.1} cm{sup -3}. The bulk of the CND seems to have an overall, systematic infall motion, with a velocity of V{sub infall} {approx_equal} 50 km s{sup -1}. Our results are consistent with the scenario that the CND has been formed by tidal capture and disruption of a giant molecular cloud (GMC). The progenitor GMC may have been formed recently inside the 120-pc ring, possibly by the encounter with the small inner bar of the Galaxy. Toomre's Q parameter indicates that the CND is gravitationally stable now, but it will become unstable and fragment as gas accumulates at r {approx_equal} 2 pc. It would trigger a burst of star formation, and subsequent processes could enhance the mass accretion rate to Sgr A*.« less
  • We present new observations of HCN and HCO{sup +} in the circumnuclear disk (CND) of the Galaxy, which we obtained with the Atacama Pathfinder Experiment telescope. We mapped emission in rotational lines of HCN J = 3-2, 4-3, and 8-7, as well as of HCO{sup +} J = 3-2, 4-3, and 9-8. We also present spectra of H{sup 13}CN J = 3-2 and 4-3 as well as H{sup 13}CO{sup +} J = 3-2 and 4-3 toward four positions in the CND. Using the intensities of all of these lines, we present an excitation analysis for each molecule using the non-LTEmore » radiative transfer code RADEX. The HCN line intensities toward the northern emission peak of the CND yield log densities (cm{sup –3}) of 5.6{sub −0.6}{sup +0.6}, consistent with those measured with HCO{sup +} as well as with densities recently reported for this region from an excitation analysis of highly excited lines of CO. These densities are too low for the gas to be tidally stable. The HCN line intensities toward the CND's southern emission peak yield log densities of 6.5{sub −0.7}{sup +0.5}, higher than densities determined for this part of the CND with CO (although the densities measured with HCO{sup +}, log [n] = 5.6{sub −0.2}{sup +0.2}, are more consistent with the CO-derived densities). We investigate whether the higher densities we infer from HCN are affected by midinfrared radiative excitation of this molecule through its 14 μm rovibrational transitions. We find that radiative excitation is important for at least one clump in the CND, where we additionally detect the J = 4-3, v {sub 2} = 1 vibrationally excited transition of HCN, which is excited by dust temperatures of ≳125-150 K. If this hot dust is present elsewhere in the CND, it could lower our inferred densities, potentially bringing the HCN-derived densities for the southern part of the CND into agreement with those measured using HCO{sup +} and CO. Additional sensitive, high-resolution submillimeter observations, as well as midinfrared observations, would be useful to assess the importance of the radiative excitation of HCN in this environment.« less
  • This paper reports the discovery of evidence for physical contact between the Galactic circumnuclear disk (CND) and an exterior giant molecular cloud. The central 10 pc of our Galaxy has been imaged in the HCN J  = 1–0, HCO{sup +} J  = 1–0, CS J  = 2–1, H{sup 13}CN J  = 1–0, SiO J  = 2–1, SO N{sub J}  = 2{sub 3}–1{sub 2}, and HC{sub 3}N J  = 11–10 lines using the Nobeyama Radio Observatory 45 m radio telescope. Based on our examination of the position–velocity maps of several high-density probe lines, we have found that an emission “bridge” may be connecting the +20 km s{sup −1} cloudmore » (M–0.13–0.08) and the negative-longitude extension of the CND. Analyses of line intensity ratios imply that the chemical property of the bridge is located between the +20 km s{sup −1} cloud and the CND. We introduce a new interpretation that a part of the CND may be colliding with the 20 km s{sup −1} cloud and the collision may be responsible for the formation of the bridge. Such collisional events could promote mass accretion onto the CND or into the inner ionized cavity, which may be further tested by proper motion studies.« less