Submillimeter Molecular Line Research Articles

Recent Updates to the Gas-phase Chemical Reactions and Molecular Lines in CLOUDY: Their Effects on Millimeter and Submillimeter Molecular Line Predictions

Here, we present our current updates to the gas-phase chemical reaction rates and molecular lines in the spectral synthesis code cloudy, and its implications in spectroscopic modeling of various astrophysical environments. We include energy levels, and radiative and collisional rates for HF, CF+, HC3N, ArH+, HCl, HCN, CN, CH, and CH2. Simultaneously, we expand our molecular network involving these molecules. For this purpose, we have added 561 new reactions and have updated the existing 165 molecular reaction rates involving these molecules. As a result, cloudy now predicts all the lines arising from these nine molecules. In addition, we also update H2–H2 collisional data up to rotational levels J = 31 for v = 0. We demonstrate spectroscopic simulations of these molecules for a few astrophysical environments. Our existing model for globules in the Crab Nebula successfully predicts the observed column density of ArH+. Our model predicts a detectable amount of HeH+, OH+, and CH+ for the Crab Nebula. We also model the interstellar medium toward HD185418, W31C, and NGC 253, and our predictions match with most of the observed column densities within the observed error bars. Very often molecular lines trace various physical conditions. Hence, this update will be very supportive for spectroscopic modeling of various astrophysical environments, particularly involving submillimeter and mid-infrared observations using the Atacama Large Millimeter/submillimeter Array and the James Webb Space Telescope, respectively.

Velocity-inverted Three-dimensional Distribution of the Gas Clouds in the Type 2 AGN NGC 1068

Abstract Spatially resolved velocity maps at high resolutions of 1–10 pc are becoming available for many nearby active galactic nuclei (AGNs) in both optical/infrared atomic emission lines and submillimeter molecular lines. For the former, it is known that a linear relationship appears to exist between the velocity of the ionized gas clouds and the distance from the nucleus in the inner ∼100 pc region, where these clouds are outflowing. Here we demonstrate that, in such a case, we can actually derive the three-dimensional (3D) geometrical distribution of the clouds directly from the velocity map. Revisiting such a velocity map taken by the Hubble Space Telescope for the prototypical Type 2 AGN NGC 1068, we implement the visualization of the 3D distribution derived from the map, and show that this inner narrow-line region has indeed a hollow-cone structure, consistent with previous modeling results. Quite possibly, this is the outer extended part of the polar elongated dusty material seen in the recent mid-infrared interferometry at parsec scale. Conversely, the latter small-scale geometry is inferred to have a hollow-cone outflowing structure as the inward extension of the derived 3D distribution above. The AGN obscuring “torus” is argued to be the inner optically thick part of this hollow-cone outflow, and its shadowed side would probably be associated with the molecular outflow seen in certain submillimeter lines. We discuss the nature of the linear velocity field, which could be from an episodic acceleration that occurred ∼105 yr ago.

Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations

Abstract Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be underabundant and/or subthermally excited. We performed one of the most sensitive (down to τ rms CO ∼ 0.002 and τ rms HCO + ∼ 0.0008 ) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to A v < 0.32 mag and HCO+ was detected down to A v < 0.2 mag , where atomic gas and dark molecular gas start to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures T ex of 4.1 and 2.7 K, for CO and for HCO+, respectively, which are close to the cosmic microwave background and explain their being undercounted in emission surveys. A stronger linear correlation was found between N HCO + and N H 2 (Pearson correlation coefficient P ∼ 0.93) than that of N CO and N H 2 (P ∼ 0.33), suggesting HCO+ is a better tracer of H2 than CO in diffuse gas. The derived CO-to-H2 conversion factor (the CO X-factor) of (14 ± 3) × 1020 cm−2 (K km s − 1 )−1 is approximately six times larger than the average value found in the Milky Way.

Molecular tracers of radiative feedback in Orion (OMC-1) Widespread CH+ (J = 1-0), CO (10-9), HCN (6-5), and HCO+ (6-5) emission.

Young massive stars regulate the physical conditions, ionization, and fate of their natal molecular cloud and surroundings. It is important to find tracers that help quantifying the stellar feedback processes that take place at different spatial scales. We present ~85 arcmin2 (~1.3 pc2) velocity-resolved maps of several submillimeter molecular lines, taken with Herschel/HIFI, toward the closest high-mass star-forming region, the Orion molecular cloud 1 core (OMC-1). The observed rotational lines include probes of warm and dense molecular gas that are difficult, if not impossible, to detect from ground-based telescopes: CH+ (J = 1-0), CO (J = 10-9), HCO+ (J = 6-5) and HCN (J = 6-5), and CH (N, J =1, 3/2-1, 1/2). These lines trace an extended but thin layer (A V ≃3-6 mag or ~1016 cm) of molecular gas at high thermal pressure, P th = n H · T k ≈ 107 - 109 cm-3 K, associated with the far ultraviolet (FUV) irradiated surface of OMC-1. The intense FUV radiation field, emerging from massive stars in the Trapezium cluster, heats, compresses and photoevaporates the cloud edge. It also triggers the formation of specific reactive molecules such as CH+. We find that the CH+ (J = 1-0) emission spatially correlates with the flux of FUV photons impinging the cloud: G 0 from ~103 to ~105. This correlation is supported by constant-pressure photodissociation region (PDR) models in the parameter space P th/G 0 ≈ [5 · 103 - 8 · 104] cm-3 K where many observed PDRs seem to lie. The CH+ (J = 1-0) emission spatially correlates with the extended infrared emission from vibrationally excited H2 (v ≥ 1), and with that of [C ii] 158 μm and CO J = 10-9, all emerging from FUV-irradiated gas. These correlations link the presence of CH+ to the availability of C+ ions and of FUV-pumped H2 (v ≥ 1) molecules. We conclude that the parsec-scale CH+ emission and narrow-line (Δv ≃ 3 km s-1) mid-J CO emission arises from extended PDR gas and not from fast shocks. PDR line tracers are the smoking gun of the stellar feedback from young massive stars. The PDR cloud surface component in OMC-1, with a mass density of 120-240 M ⊙ pc-2, represents ~5% to ~10% of the total gas mass, however, it dominates the emitted line luminosity; the average CO J = 10-9 surface luminosity in the mapped region being ~35 times brighter than that of CO J = 2-1. These results provide insights into the source of submillimeter CH+ and mid-J CO emission from distant star-forming galaxies.

THE REMARKABLE MOLECULAR CONTENT OF THE RED SPIDER NEBULA (NGC 6537)

Millimeter and sub-millimeter molecular-line observations of planetary nebula (PN) NGC 6537 (Red Spider) have been carried out using the Sub-Millimeter Telescope and the 12 m antenna of the Arizona Radio Observatory in the frequency range 86-692 GHz. CN, HCN, HNC, CCH, CS, SO, H2CO, HCO+ and N2H+, along with the J = 3 → 2 and 6 → 5 lines of CO and those of several isotopologues, were detected toward the Red Spider, estimated to be ~1600 yr old. This extremely high excitation PN evidently fosters a rich molecular environment. The presence of CS and SO suggest that sulfur may be sequestered in molecular form in such nebulae. A radiative transfer analysis of the CO and CS spectra indicate a kinetic temperature of T K ~ 60-80 K and gas densities of n(H2) ~ 1-8 × 105 cm–3 in NGC 6537. Column densities of the molecules in the nebula and their fractional abundances relative to H2 ranged from N tot ~ 1016 cm–2 and f ~ 10–4 for CO, to ~7 × 1011 cm–2 and f ~ 8 × 10–9 for the least abundant species, N2H+. For SO and CS, N tot ~ 2 × 1012 cm–2 and 1013 cm–2, respectively, with f ~ 10–7 and 2 × 10–8. It was also found that HCN/HNC 2. A low 12C/13C ratio of ~4 was measured, indicative of hot-bottom burning. These results, coupled with past observations, suggest that molecular abundances in PNe are governed principally by the physical and chemical properties of the individual object and its progenitor star, rather than nebular age.

Skewed Distributions and Opposite Velocity Gradients of Submillimeter Molecular Lines in Low-Mass Protostellar Envelopes

Abstract We have made mapping observations of L 1551 IRS 5, L 1551 NE, L 723, and L 43 and single-point observations of IRAS 16293$-$2422 in the submillimeter CS ($J$$=$ 7–6) and HCN ($J$$=$ 4–3) lines with Atacama Submillimeter Telescope Experiment (ASTE). Using the present data and our previous ASTE observations of L 483 and B 335 together, we found a definite correlation between source bolometric luminosities and total integrated intensities of the submillimeter lines ($I_{\rm CS}$$\propto$$L_{\rm bol}^{0.92}$). The combined ASTE $+$ Submillimeter Array CS (7–6) image of L 1551 IRS 5 exhibits an extended ($\sim$ 2000 AU) component tracing the associated reflection nebula at west and southwest, as well as a compact ($\lesssim$ 500 AU) component centered on the protostellar position. The peaks of the CS and HCN emissions in L 1551 NE are not located at the protostellar position but offset ($\sim$ 1400 AU) toward the associated reflection nebula at west. By statistical analyses, we confirmed the velocity gradients of the CS (7–6) emission that are opposite to those of the millimeter lines along the outflow direction, which we reported in our early paper. The magnitudes of the submillimeter velocity gradients are estimated to be (9.7$\ \pm\ $1.7) $\times$ 10$^{-3}\ $km s$^{-1}\ $arcsec$^{-1}\ $ in L 1551 IRS 5 and (7.6$\ \pm\ $2.4) $\times$ 10$^{-3}\ $km s$^{-1}\ $arcsec$^{-1}\ $ in L 483. We suggest that the “skewed” submillimeter molecular emissions toward the associated reflection nebulae at a few thousand AU scale trace the warm ($\gtrsim$ 40 K) walls of the envelope cavities, excavated by the associated outflows and directly irradiated by the central protostars. The opposite velocity gradients along the outflow direction likely reflect the dispersing gas motion at the wall of the cavity in envelopes perpendicular to the outflow.

A NEW LOOK AT THE GALACTIC CIRCUMNUCLEAR DISK

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) × 105 M ☉ 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, H13CN, HCO+, and HNC, but it is barely traced by N2H+, SiO, CCS, and HC3N 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 k 63 K and H2 density of n(H2) 104.1 cm–3. The bulk of the CND seems to have an overall, systematic infall motion, with a velocity of V infall 50 km s–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 2 pc. It would trigger a burst of star formation, and subsequent processes could enhance the mass accretion rate to Sgr A*.

Discovery of an Extremely High Velocity, Massive, and Compact Molecular Outflow in Norma

G331.5–0.1 in the Norma spiral arm is one of the most luminous and extended cores of a giant molecular cloud (GMC), containing at least six massive and dense dust condensations. Here we report the discovery, from observations of several submillimeter molecular lines that were made using the Atacama Submillimeter Telescope (ASTE) and the Atacama Pathfinder Experiment Telescope (APEX), of an unresolved, extremely high velocity molecular outflow toward the brightest and most massive dust condensation. The outflow is massive and energetic (flow mass of ~55 M☉; momentum of ~2.4 × 103 M☉ km s−1; kinetic energy of ~1.4 × 1048 ergs). These values are characteristic of flows driven by young massive stellar objects with Lbol ~ 1 × 105 L☉. We also report the detection, using the Australia Telescope Compact Array (ATCA), of a compact radio continuum source that is located at the center of the outflow and therefore likely to be its driving energy source. It has an spectral index between 4.8 and 8.6 GHz of 1.1 ± 0.2, suggesting that it might correspond to a collimated jet.

Star formation In Bright-Rimmed Clouds: a comparison of wind-driven triggering with millimeter observations

Bright-rimmed clouds (BRCs) are logical laboratories in which to study triggered star formation, however it is difficult in any single cloud to definitively show that star formation was triggered. In this study we compare the hydrodynamic models produced by Vanhala & Cameron (1998) that treat the problems of star-formation triggered by wind-driven implosion to millimeter and submillimeter molecular line observations of BRCs with embedded IRAS sources. These latter sources are derived from a catalog by Sugitani, Fukui, & Ogura (1991) In order to make an accurate comparison we implement a radiative transfer model based on the Sobolev or LVG approximation, and generate molecular line maps which can be directly compared to our observations. We observed several millimeter and submillimeter transitions of CO, C 18O, HCO+, and H13CO+ using the FCRAO, SMT, CSO, and SMA observatories (De Vries, Narayanan, & Snell 2002). We compare these observations with 3 hydrodynamic models of wind-driven shock fronts interacting with pre-existing, but unbound cloud cores. In two cases these model cores are triggered to collapse under the influence of the external wind.

Star Formation in Bright‐rimmed Clouds. I. Millimeter and Submillimeter Molecular Line Surveys

We present the results of the first detailed millimeter and submillimeter molecular line survey of bright-rimmed clouds, observed at FCRAO in the CO (J = 1 → 0), C18O (J = 1 → 0), HCO+ (J = 1 → 0), H13CO+ (J = 1 → 0), and N2H+ (J = 1 → 0) transitions, and at the Heinrich Hertz Telescope in the CO (J = 2 → 1), HCO+ (J = 3 → 2), HCO+ (J = 4 → 3), H13CO+ (J = 3 → 2), and H13CO+ (J = 4 → 3) molecular line transitions. The source list is composed of a selection of bright-rimmed clouds from the catalog of such objects compiled by Sugitani et al. We also present observations of three Bok globules done for comparison with the bright-rimmed clouds. We find that the appearance of the millimeter CO and HCO+ emission is dominated by the morphology of the shock front in the bright-rimmed clouds. The HCO+ (J = 1 → 0) emission tends to trace the swept-up gas ridge and overdense regions, which may be triggered to collapse as a result of sequential star formation. Five of the seven bright-rimmed clouds we observe seem to have an outflow; however, only one shows the spectral line blue-asymmetric signature that is indicative of infall in the optically thick HCO+ emission. We also present evidence that in bright-rimmed clouds the nearby shock front may heat the core from outside-in, thereby washing out the normally observed line infall signatures seen in isolated star-forming regions. We find that the derived core masses of these bright-rimmed clouds are similar to other low- and intermediate-mass star-forming regions.

Molecular Gas in NGC 7129

This paper reports new submillimeter molecular line observations of NGC 7129, a reflection nebula and star-forming region. Maps of 12CO and 13CO 2-1 and 3-2 emission show a cavity surrounded by bright ridges of denser molecular gas. Known molecular outflows in the region issue from deeply embedded sources in these ridges. Only the red lobe of the most prominent outflow is seen clearly, but there is some evidence to suggest that a blue counterjet is escaping through the front of the cavity. We suggest a picture of the region's evolution in which the oldest star, BD +65°1638, created the cavity by sweeping the surrounding interstellar gas into the surrounding ridges. This led to further star formation, as evidenced by the outflow sources found in the ridges. One of these sources, FIRS 2, thought to be an intermediate-mass analog of a Class 0 young stellar object, is coincident with a pointlike source seen in a near-IR (K') image.

Submillimeter molecular line observations of IRC +10216: Searches for MgH, SiH2, and HCO(+), and detection of hot HCN

Submillimeter molecular line observations of IRC +10216: Searches for MgH, SiH2, and HCO(+), and detection of hot HCN