S. Yoshiike

MOLECULAR CLOUDS IN THE TRIFID NEBULA M20: POSSIBLE EVIDENCE FOR A CLOUD-CLOUD COLLISION IN TRIGGERING THE FORMATION OF THE FIRST GENERATION STARS

A large-scale study of the molecular clouds toward the Trifid Nebula, M20, has been made in the J = 2-1 and J = 1-0 transitions of 12CO and 13CO. M20 is ionized predominantly by an O7.5 star HD164492. The study has revealed that there are two molecular components at separate velocities peaked toward the center of M20 and that their temperatures—30-50 K as derived by a large velocity gradient analysis—are significantly higher than the 10 K of their surroundings. We identify the two clouds as the parent clouds of the first generation stars in M20. The mass of each cloud is estimated to be ~103 M ☉ and their separation velocity is ~8 km s–1 over ~1-2 pc. We find that the total mass of stars and molecular gas in M20 is less than ~3.2 × 103 M ☉, which is too small by an order of magnitude to gravitationally bind the system. We argue that the formation of the first generation stars, including the main ionizing O7.5 star, was triggered by the collision between the two clouds in a short timescale of ~1 Myr, a second example alongside Westerlund 2, where a super-star cluster may have been formed due to cloud-cloud collision triggering.

INTERSTELLAR PROTONS IN THE TeV γ-RAY SNR HESS J1731-347: POSSIBLE EVIDENCE FOR THE COEXISTENCE OF HADRONIC AND LEPTONIC γ-RAYS

HESS J1731-347 (G353.6-0.7) is one of the TeV γ-ray SNRs which shows the shell-like morphology. We have made a new analysis of the interstellar protons toward the SNR by using both the CO(J=1–0) and Hi datasets. The results indicate that the TeV γ-ray shell shows significant spatial correlation with the interstellar protons at a velocity range from −90 km s to −75 km s, and the distance corresponding to the velocity range is ∼ 5.2 kpc, a factor of 2 larger than the previous figure 3 kpc. The total mass of the interstellar protons is estimated to be 6.4 × 10M⊙ , 25% of which is atomic gas. We have identified the cold Hi gas observed as self-absorption which shows significant correspondence with the northeastern γ-ray peak. While the good correspondence between the interstellar protons and TeV γ-rays in the north of the SNR lends support to the hadronic scenario for the TeV γ-rays, the southern part of the shell shows a break in the correspondence; in particular, the southwestern rim of the SNR shell shows a significant decrease of the interstellar protons by a factor of 2. We argue that this discrepancy can be explained as due to leptonic γ-rays, because this region well coincides with the bright shell which emit non-thermal radio continuum emission and non-thermal X-rays, suggesting that the γ-rays of HESS J1713-347 consist of both the hadronic and leptonic components. The leptonic contribution then corresponds to ∼20% of the total γ-rays. The total energy of cosmic-ray protons is estimated to be 10 erg for the γ-ray energy range of 1 GeV-100 TeV by assuming that 80% of the total γ-ray is of the hadronic origin. Subject headings: cosmic rays γ-rays: ISM Hii regions ISM:coluds ISM: individual objects(HESS J1731-347)

Recombining Plasma and Hard X-Ray Filament in the Mixed-Morphology Supernova Remnant W 44

We report on new features of the typical mixed-morphology supernova remnant W 44. In X-ray spectra obtained with Suzaku, radiative recombination continua of highly ionized atoms were detected for the first time. The spectra are well reproduced by a thermal plasma in a recombining phase. The best-fit parameters suggest that the electron temperature of the shock-heated matter rapidly cooled down from � 1 keV to � 0.5 keV, possibly due to adiabatic expansion (rarefaction), occurred � 20000 yr ago. We also discovered hard X-ray emission, which shows an arclike structure spatially correlating with a radio continuum filament. The surface-brightness distribution has a clear anticorrelation with 12 CO (J = 2–1) emission from a molecular cloud observed with NANTEN2. While the hard X-ray is most likely due to a synchrotron enhancement in the vicinity of the cloud, no current model can quantitatively predict the observed flux.

The supernova remnant W44: Confirmations and challenges for cosmic-ray acceleration

The middle-aged supernova remnant (SNR) W44 has recently attracted attention because of its relevance regarding the origin of Galactic cosmic-rays. For the first time for a SNR, the gamma-ray missions AGILE and Fermi have established the spectral continuum below 200 MeV, which can be attributed to a neutral pion emission. Confirming the hadronic origin of the gamma-ray emission near 100 MeV is then of the greatest importance. Our paper is focused on a global re-assessment of all available data and models of particle acceleration in W44 with the goal of determining the hadronic and leptonic contributions to the overall spectrum on a firm ground. We also present new gamma-ray and CO NANTEN2 data on W44 and compare them to recently published AGILE and Fermi data. Our analysis strengthens previous studies and observations of the W44 complex environment and provides new information for more detailed modeling. In particular, we determine that the average gas density of the regions emitting 100 MeV‐10 GeV gammarays is relatively high (n 250‐300 cm 3 ). The hadronic interpretation of the gamma-ray spectrum of W44 is viable and supported by strong evidence. It implies a relatively large value for the average magnetic field (B 10 2 G) in the SNR surroundings,which is a sign of field amplification by shock-driven turbulence. Our new analysis establishes that the spectral index of the proton energy distribution function is p1 = 2:2 0:1 at low energies and p2 = 3:2 0:1 at high energies. We critically discuss hadronic versus leptonic-only models of emission taking radio and gamma-ray data into account simultaneously. We find that the leptonic models are disfavored by the combination of radio and gamma-ray data. Having determined the hadronic nature of the gamma-ray emission on firm ground, a number of theoretical challenges remain to be addressed.

NON-THERMAL X-RAYS AND INTERSTELLAR GAS TOWARD THE γ-RAY SUPERNOVA REMNANT RX J1713.7–3946: EVIDENCE FOR X-RAY ENHANCEMENT AROUND CO AND H I CLUMPS

RX J1713.7–3946 is the most remarkable very high energy γ-ray supernova remnant that emits synchrotron X-rays without thermal features. We made a comparative study of CO, H I, and X-rays in order to better understand the relationship between the X-rays, and the molecular and atomic gas. The results indicate that the X-rays are enhanced around the CO and H I clumps on a pc scale, but are decreased inside the clumps on a 0.1 pc scale. Magnetohydrodynamic numerical simulations of the shock interaction with molecular and atomic gas indicate that the interaction between the shock waves and the clumps excite turbulence, which amplifies the magnetic field around the clumps. We suggest that the amplified magnetic field around the CO and H I clumps enhances the synchrotron X-rays and possibly the acceleration of cosmic-ray electrons.

DISCOVERY OF POSSIBLE MOLECULAR COUNTERPARTS TO THE INFRARED DOUBLE HELIX NEBULA IN THE GALACTIC CENTER

We have discovered two molecular features at radial velocities of ?35?km?s?1 and 0?km?s?1 toward the infrared Double Helix Nebula (DHN) in the Galactic center with NANTEN2. The two features show good spatial correspondence with the DHN. We have also found two elongated molecular ridges at these two velocities distributed vertically to the Galactic plane over 0.?8. The two ridges are linked by broad features in velocity and are likely connected physically with each other. The ratio between the 12CO J = 2-1 and J = 1-0 transitions is 0.8 in the ridges which is larger than the average value 0.5 in the foreground gas, suggesting the two ridges are in the Galactic center. An examination of the K band extinction reveals a good coincidence with the CO 0?km?s?1 ridge and is consistent with a distance of 8 ? 2?kpc. We discuss the possibility that the DHN was created by a magnetic phenomenon incorporating torsional Alfv?n waves launched from the circum-nuclear disk and present a first estimate of the mass and energy involved in the DHN.

Interstellar gas towards CTB 37A and the TeV gamma-ray source HESS J1714−385

Observations of dense molecular gas towards the supernova remnants CTB 37A (G348.5+0.1) and G348.5-0.0 were carried out using the Mopra and Nanten2 radio telescopes. We present CO(2-1) and CS(1-0) emission maps of a region encompassing the CTB 37A TeV gamma-ray emission, HESS J1714-385, revealing regions of dense gas within associated molecular clouds. Some gas displays good overlap with gamma-ray emission, consistent with hadronic gamma-ray emission scenarios. Masses of gas towards the HESS J1714-385 TeV gamma-ray emission region were estimated, and were of the order of 10^3-10^4 solar masses. In the case of a purely hadronic origin for the gamma-ray emission, the cosmic ray flux enhancement is ~80-1100 times the local solar value. This enhancement factor and other considerations allow a discussion of the age of CTB 37A, which is consistent with ~10^4 yr.

A Detailed Study of the Interstellar Protons toward the TeV γ-Ray SNR RX J0852.0–4622 (G266.2–1.2, Vela Jr.): The Third Case of the γ-Ray and ISM Spatial Correspondence

We present a new analysis of the interstellar protons toward the TeV

Dense gas towards the RX J1713.7-3946 supernova remnant

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RCW 36 in the Vela Molecular Ridge: Evidence for high-mass star-cluster formation triggered by cloud–cloud collision

-ray SNR RX J0852.0