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Register a new userTemperature and Density Distribution in the Molecular Gas Toward Westerlund 2: Further Evidence for Physical Association
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Furukawa et al. 2009 reported the existence of a large mass of molecular gas associated with the super star cluster Westerlund 2 and the surrounding HII region RCW49, based on a strong morphological correspondence between NANTEN2 12CO(J=2-1) emission and Spitzer IRAC images of the HII region. We here present temperature and density distributions in the associated molecular gas at 3.5 pc resolution, as derived from an LVG analysis of the 12CO(J=2-1), 12CO(J=1-0) and 13CO(J=2-1) transitions. The kinetic temperature is as high as 60-150 K within a projected distance of 5-10 pc from Westerlund 2 and decreases to as low as 10 K away from the cluster. The high temperature provides robust verification that the molecular gas is indeed physically associated with the HII region, supporting Furukawa et al.s conclusion. The derived temperature is also roughly consistent with theoretical calculations of photo dissociation regions (PDRs), while the low spatial resolution of the present study does not warrant a more detailed comparison with PDR models. We suggest that the molecular clouds presented here will serve as an ideal laboratory to test theories on PDRs in future higher resolution studies.
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We present observations of the $^3P_1$-$^3P_0$ fine-structure line of atomic carbon using the ASTE 10 m sub-mm telescope towards RCW38, the youngest super star cluster in the Milky Way. The detected [CI] emission is compared with the CO $J$ = 1-0 image cube presented in Fukui et al. (2016) which has an angular resolution of 40$^{prime prime}$ ($sim$ 0.33 pc). The overall distribution of the [CI] emission in this cluster is similar to that of the $^{13}$CO emission. The optical depth of the [CI] emission was found to be $tau$ = 0.1-0.6, suggesting mostly optically thin emission. An empirical conversion factor from the [CI] integrated intensity to the H$_2$ column density was estimated as $X_{rm [CI]}$ = 6.3 $times$ 10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s (for visual extinction: $A_V$ $le$ 10 mag) and 1.4 $times$ 10$^{21}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s (for $A_V$ of 10-100 mag). The column density ratio of the [CI] to CO ($N_{rm [CI]}/N_{rm CO}$) was derived as $sim$ 0.1 for $A_V$ of 10-100 mag, which is consistent with that of the Orion cloud presented in Ikeda et al. (2002). However, our results cover an $A_V$ regime of up to 100 mag, which is wider than the coverage found in Orion, which reach up to $sim$ 60 mag. Such a high [CI]/CO ratio in a high $A_V$ region is difficult to be explained by the plane-parallel photodissociation region (PDR) model, which predicts that this ratio is close to 0 due to the heavy shielding of the ultraviolet (UV) radiation. Our results suggest that the molecular gas in this cluster is highly clumpy, allowing deep penetration of UV radiation even at averaged $A_V$ values of 100 mag. Recent theoretical works have presented models consistent with such clumped gas distribution with a sub-pc clump size (e.g., Tachihara et al. 2018).
We present ALMA observations of the dwarf starburst galaxy Henize 2-10 in combination with previous SMA CO observations to probe the molecular environments of natal super star clusters. These observations include the HCO$^+$(1-0), HCN(1-0), HNC(1-0), and CCH(1-0) molecular lines, as well as 88 GHz continuum with a spatial resolution of $1.7times 1.6$. After correcting for the contribution from free-free emission to the 88 GHz continuum flux density ($sim$ 60% of the 88 GHz emission), we derive a total gas mass for He~2-10 of $M_{gas} = 4-6times10^8$ M$_{odot}$, roughly 5-20% of the dynamical mass. Based on a principle component analysis, HCO$^+$ is found to be the best general tracer of molecular emission. The line widths and luminosities of the CO emission suggests that the molecular clouds could either be as small as $sim 8$ pc, or alternately have enhanced line widths. The CO emission and 88 GHz continuum are anti-correlated, suggesting that either the dust and molecular gas are not cospatial, which could reflect the 88 GHz continuum is dominated by free-free emission. The CO and CCH emission are also relatively anti-correlated, which is consistent with the CCH being photo-enhanced, and/or the CO being dissociated in the regions near the natal super star clusters. The molecular line ratios of regions containing the natal star clusters are different from the line ratios observed for regions elsewhere in the galaxy. In particular, the regions with thermal radio emission all have CO(2-1)/HCO$^+(1-0) < 16$, and the HCO$^+$/CO ratio appears to be correlated with the evolutionary stage of the clusters.
The local Galactic HI gas was found to contain cold neutral medium (CNM) filaments that are aligned with polarized dust emission. These filaments appear to be dominated by the magnetic field and in this case turbulence is expected to show distinct anisotropies. We use the Galactic Effelsberg--Bonn HI Survey (EBHIS) to derive 2D turbulence spectra for the HI distribution in direction to 3C196 and two more comparison fields. Prior to Fourier transform we apply a rotational symmetric 50% Tukey window to apodize the data. We derive average as well as position angle dependent power spectra. Anisotropies in the power distribution are defined as the ratio of the spectral power in orthogonal directions. We find strong anisotropies. For a narrow range in position angle, in direction perpendicular to the filaments and the magnetic field, the spectral power is on average more than an order of magnitude larger than parallel. In the most extreme case the anisotropy reaches locally a factor of 130. Anisotropies increase on average with spatial frequency as predicted by Goldreich and Sridhar, at the same time the Kolmogorov spectral index remains almost unchanged. The strongest anisotropies are observable for a narrow range in velocity and decay with a power law index close to --8/3, almost identical to the average isotropic spectral index of $-2.9 < gamma < -2.6$. HI filaments, associated with linear polarization structures in LOFAR observations in direction to 3C196, show turbulence spectra with marked anisotropies. Decaying anisotropies appear to indicate that we witness an ongoing shock passing the HI and affecting the observed Faraday depth.
We have compared the TeV gamma-rays with the new 12CO J=2-1 data toward HESS J1745-303 in the Galactic center and confirmed that the molecular gas MG358.9-0.5 toward (l, b)=(358.9, -0.5 at VLSR=-100-0 km s-1 shows a reasonable positional agreement with the primary peak (northern part) of the gamma-ray source. For the southern part of HESS J1745-303, we see no CO counterpart, whereas the HI gas in the Parkes 21 cm HI dataset shows a possible counterpart to the gamma-ray source. This HI gas may be optically thick as supported by the HI line shape similar to the optically thick 12CO. We estimate the total mass of interstellar protons including both the molecular and atomic gas to be 2x10^6 Mo and the cosmic-ray proton energy to be 6x10^{48} ergs in the hadronic scenario. We discuss possible origins of the cosmic-ray protons including the nearby SNR G359.1-0.5. The SNR may be able to explain the northern gamma-ray source but the southern source seems to be too far to be energized by the SNR. As an alternative, we argue that the second-order Fermi acceleration in the inter-clump space surrounded by randomly moving high-velocity clumps may offer a possible mechanism to accelerate protons. The large turbulent motion with velocity dispersion of ~15 km s-1 has energy density two orders of magnitude higher than in the solar vicinity and is viable as the energy source.
We have discovered remarkable jet- and arc-like molecular features toward the rich and young stellar cluster Westerlund2. The jet has a length of ~100 pc and a width of ~10 pc, while the arc shows a crescent shape with a radius of ~30 pc. These molecular features each have masses of ~10000 solar mass and show spatial correlations with the surrounding lower density HI gas. The jet also shows an intriguing positional alignment with the core of the TeV gamma ray source HESS J1023-575 and with the MeV/GeV gamma-ray source recently reported by the Fermi collaboration. We argue that the jet and arc are caused by an energetic event in Westerlund 2, presumably due to an anisotropic supernova explosion of one of the most massive member stars. While the origin of the TeV and GeV gamma-ray sources is uncertain, one may speculate that they are related to the same event via relativistic particle acceleration by strong shock waves produced at the explosion or by remnant objects such as a pulsar wind nebula or microquasar.
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