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Register a new userDisentangling the excitation conditions of the dense gas in M17 SW
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Juan-Pablo Perez-Beaupuits JP
Publication date
2015
fields
Physics
and research's language is
English
Authors
J.P. Perez-Beaupuits
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We probe the chemical and energetic conditions in dense gas created by radiative feedback through observations of multiple CO, HCN and HCO$^+$ transitions toward the dense core of M17 SW. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain maps of the $J=16-15$, $J=12-11$, and $J=11-10$ transitions of $^{12}$CO. We compare these maps with corresponding APEX and IRAM 30m telescope data for low- and mid-$J$ CO, HCN and HCO$^+$ emission lines, including maps of the HCN $J=8-7$ and HCO$^+$ $J=9-8$ transitions. The excitation conditions of $^{12}$CO, HCO$^+$ and HCN are estimated with a two-phase non-LTE radiative transfer model of the line spectral energy distributions (LSEDs) at four selected positions. The energy balance at these positions is also studied. We obtained extensive LSEDs for the CO, HCN and HCO$^+$ molecules toward M17 SW. The LSED shape, particularly the high-$J$ tail of the CO lines observed with SOFIA/GREAT, is distinctive for the underlying excitation conditions. The critical magnetic field criterion implies that the cold cloudlets at two positions are partially controlled by processes that create and dissipate internal motions. Supersonic but sub-Alfvenic velocities in the cold component at most selected positions indicates that internal motions are likely MHD waves. Magnetic pressure dominates thermal pressure in both gas components at all selected positions, assuming random orientation of the magnetic field. The magnetic pressure of a constant magnetic field throughout all the gas phases can support the total internal pressure of the cold components, but it cannot support the internal pressure of the warm components. If the magnetic field scales as $B propto n^{2/3}$, then the evolution of the cold cloudlets at two selected positions, and the warm cloudlets at all selected positions, will be determined by ambipolar diffusion.
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Since the main cooling lines of the gas phase are important tracers of the interstellar medium in Galactic and extragalactic sources, proper and detailed understanding of their emission, and the ambient conditions of the emitting gas, is necessary for a robust interpretation of the observations. With high resolution (7-9) maps (~3x3 pc^2) of mid-J molecular lines we aim to probe the physical conditions and spatial distribution of the warm (50 to few hundred K) and dense gas (n(H_2)>10^5 cm^-3) across the interface region of M17 SW nebula. We have used the dual color multiple pixel receiver CHAMP+ on APEX telescope to obtain a 5.3x4.7 map of the J=6-5 and J=7-6 transitions of 12CO, the 13CO J=6-5 line, and the {^3P_2}-{^3P_1} 370 um fine-structure transition of [C I] in M17 SW. LTE and non-LTE radiative transfer models are used to constrain the ambient conditions. The warm gas extends up to a distance of ~2.2 pc from the M17 SW ridge. The 13CO J=6-5 and [C I] 370 um lines have a narrower spatial extent of about 1.3 pc along a strip line at P.A=63 deg. The structure and distribution of the [C I] {^3P_2}-{^3P_1} 370 um map indicate that its emission arises from the interclump medium with densities of the order of 10^3 cm^-3. The warmest gas is located along the ridge of the cloud, close to the ionization front. An LTE approximation indicates that the excitation temperature of the embedded clumps goes up to ~120 K. The non-LTE model suggests that the kinetic temperature at four selected positions cannot exceed 230 K in clumps of density n(H_2)~5x10^5 cm^-3, and that the warm T_k>100 K and dense (n(H_2)>10^4 cm^-3) gas traced by the mid-J 12CO lines represent just about 2% of the bulk of the molecular gas. The clump volume filling factor ranges between 0.04 and 0.11 at these positions.
The non-uniform distribution of gas and protostars in molecular clouds is caused by combinations of various physical processes that are difficult to separate. We explore this non-uniform distribution in the M17 molecular cloud complex that hosts massive star formation activity using the $^{12}$CO ($J=1-0$) and $^{13}$CO ($J=1-0$) emission lines obtained with the Nobeyama 45m telescope. Differences in clump properties such as mass, size, and gravitational boundedness reflect the different evolutionary stages of the M17-H{scriptsize II} and M17-IRDC clouds. Clumps in the M17-H{scriptsize II} cloud are denser, more compact, and more gravitationally bound than those in M17-IRDC. While M17-H{scriptsize II} hosts a large fraction of very dense gas (27%) that has column density larger than the threshold of $sim$ 1 g cm$^{-2}$ theoretically predicted for massive star formation, this very dense gas is deficient in M17-IRDC (0.46%). Our HCO$^+$ ($J=1-0$) and HCN ($J=1-0$) observations with the TRAO 14m telescope, { lqb trace all gas with column density higher than $3times 10^{22}$ cm$^{-2}$}, confirm the deficiency of high density ($gtrsim 10^5$ cm$^{-3}$) gas in M17-IRDC. Although M17-IRDC is massive enough to potentially form massive stars, its deficiency of very dense gas and gravitationally bound clumps can explain the current lack of massive star formation.
We probe the column densities and masses traced by the ionized and neutral atomic carbon with spectrally resolved maps, and compare them to the diffuse and dense molecular gas traced by [C I] and low-$J$ CO lines toward the star-forming region M17SW. We mapped a 4.1pc x 4.7pc region in the [C I] 609 m$mu$ line using the APEX telescope, as well as the CO isotopologues with the IRAM 30m telescope. We analyze the data based on velocity channel maps that are 1 km/s wide. We correlate their spatial distribution with that of the [C II] map obtained with SOFIA/GREAT. Optically thin approximations were used to estimate the column densities of [C I] and [C II] in each velocity channel. The spatial distribution of the [C I] and all CO isotopologues emission was found to be associated with that of [C II] in about 20%-80% of the mapped region, with the high correlation found in the central (15-23 km/s ) velocity channels. The excitation temperature of [C I] ranges between 40 K and 100 K in the inner molecular region of M17 SW. Column densities in 1 km/s channels between ~10$^{15}$ and ~10$^{17}$ cm$^{-2}$ were found for [C I]. Just ~20% of the velocity range (~40 km/s) that the [C II] line spans is associated with the star-forming material traced by [C I] and CO. The total gas mass estimated from the [C II] emission gives a lower limit of ~4.4x10$^3$ $M_{odot}$. At least 64% of this mass is not associated with the star-forming material in M17SW. We also found that about 36%, 17%, and 47% of the [C II] emission is associated with the HII, HI, and H_2 regimes, respectively. Comparisons with the H41$alpha$ line shows an ionization region mixed with the neutral and part of the molecular gas, in agreement with the clumped structure and dynamical processes at play in M17SW. These results are also relevant to extra-galactic studies in which [C II] is often used as a tracer of star-forming material.
With new THz maps that cover an area of ~3.3x2.1 pc^2 we probe the spatial distribution and association of the ionized, neutral and molecular gas components in the M17 SW nebula. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain a 5.7x3.7 map of the 12CO J=13-12 transition and the [C II] 158 um fine-structure line in M17 SW and compare the spectroscopically resolved maps with corresponding ground-based data for low- and mid-J CO and [C I] emission. For the first time SOFIA/GREAT allow us to compare velocity-resolved [C II] emission maps with molecular tracers. We see a large part of the [C II] emission, both spatially and in velocity, that is completely non-associated with the other tracers of photon-dominated regions (PDR). Only particular narrow channel maps of the velocity-resolved [C II] spectra show a correlation between the different gas components, which is not seen at all in the integrated intensity maps. These show different morphology in all lines but give hardly any information on the origin of the emission. The [C II] 158 um emission extends for more than 2 pc into the M17 SW molecular cloud and its line profile covers a broader velocity range than the 12CO J=13-12 and [C I] emissions, which we interpret as several clumps and layers of ionized carbon gas within the telescope beam. The high-J CO emission emerges from a dense region between the ionized and neutral carbon emissions, indicating the presence of high-density clumps that allow the fast formation of hot CO in the irradiated complex structure of M17 SW. The [C II] observations in the southern PDR cannot be explained with stratified nor clumpy PDR models.
We present high-resolution observations (0.2-1.5) of multiple dense gas tracers, HCN and HCO$^+$ ($J$ = 1-0, 3-2, and 4-3), HNC ($J$ = 1-0), and CS ($J$ = 7-6) lines, toward the nearby luminous infrared galaxy VV 114 with the Atacama Large Millimeter/submillimeter Array. All lines are robustly detected at the central gaseous filamentary structure including the eastern nucleus and the Overlap region, the collision interface of the progenitors. We found that there is no correlation between star formation efficiency and dense gas fraction, indicating that the amount of dense gas does not simply control star formation in VV 114. We predict the presence of more turbulent and diffuse molecular gas clouds around the Overlap region compared to those at the nuclear region assuming a turbulence-regulated star formation model. The intracloud turbulence at the Overlap region might be excited by galaxy-merger-induced shocks, which also explains the enhancement of gas-phase CH$_3$OH abundance previously found there. We also present spatially resolved spectral line energy distributions of HCN and HCO$^+$ for the first time, and derive excitation parameters by assuming optically-thin and local thermodynamic equilibrium (LTE) conditions. The LTE model revealed that warmer, HCO$^+$-poorer molecular gas medium is dominated around the eastern nucleus, harboring an AGN. The HCN abundance is remarkably flat ($sim$3.5 $times$ 10$^{-9}$) independently of the various environments within the filament of VV 114 (i.e., AGN, star formation, and shock).
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