Precise reaction cross sections (oR) for 24_38M g on C targets at energies around 240 M eV /nucleon have been measured at the Radioactive Isotope Beam Factory at RIKEN. The oR for 36-38 Mg have been measured for the first time. An enhancement o f oR compared to the systematics for spherical stable nuclei has been observed, especially in the neutron-rich region, which reflects the deformation of those isotopes. In the vicinity of the drip line the aR for 37Mg is especially large. It is shown by analysis using a recently developed theoretical method that this prominent enhancement of oR for 37Mg should come from the p-orbital halo formation breaking the N = 28 shell gap.Since the early years of the study of atomic nuclei, the nuclear shell model has been the basic framework for understanding nuclear structure. The high stability of nuclei with certain numbers of neutrons (or protons) observed in stable nuclei indicates the existence of the shells filled at certain so-called "magic numbers." Studies in the last few decades have revealed that those magic numbers are sometimes broken or changed in unstable nuclei [1], The breakdown of the N = 20 shell gap between the sd and f p shells has been extensively studied since the irregularities in binding energies and 2+ excitation energies were observed in neutron-rich nuclei around N = 20 [2-6]. The term "island of inversion" was applied to this region [6] and deformed nuclear structures related to the changing of shell structures have been reported in this region [7]. The vanishing of the N = 28 shell closure has been also extensively studied, starting from neutronrich S-Ar isotopes [8][9][10][11][12][13][14]. The development of deformation observed in those nuclei could be interpreted as degeneracy of the f p shell, which induces strong quadrupole deformation [9][10][11][13][14][15][16][17][18]. Such deformation has been reported also for Si isotopes [19,20], and studies have recently indicated that this * takechi @ np.gs .niigata-u. ac .jp PACS number(s): 21.10.Gv, 25.60.Dz phenomenon could be seen even in a very neutron-rich Mg region [21].The purpose of our present study is to elucidate the changes of nuclear structures, such as a development of deformation, a breakdown of the magic numbers and possible halo formation in Mg isotopes, from the stability line to the vicinity of the neutron drip line. For this purpose, precise measurements of reaction cross sections for 24_38Mg have been performed at the Radioactive Isotope Beam Factory (RIBF) at RIKEN. The reaction cross section aR or interaction cross section ay reflects the nuclear size, and has been a powerful probe in searching for halo formation since the first study by Tanihata et al. [22], Recently, measurements of o, for Ne isotopes performed at RIBF [23] have successfully revealed the halo structure of 3lNe in which the sd-pf shell inversion associated with nuclear deformation causes the formation of a halo [23][24][25]. Moreover, theoretical studies on those data have shown that a precise data set on crR is v...

Based on optical and infrared observations, we study the albedo and the temperature of the dust grains associated with the spectacular 2007 outburst of Jupiter-family comet 17P/Holmes. We found that the albedo at the solar phase angle ∼16 • was 0.03-0.12. While the color temperature around 3-4 μm was 360 ± 40 K, the color temperature at 12.4 μm and 24.5 μm was ∼200 K, which is consistent with that of a blackbody. We studied the equilibrium temperature of the dust grains at 2.44 AU and found that the big discrepancy in the temperature was caused by the heterogeneity in particle size, that is, hotter components consist of submicron absorbing grains whereas colder components consist of large ( 1 μm) grains. The contemporaneous optical and mid-infrared observations suggest that the albedo and the temperature could decrease within ∼ 3 days after the outburst and stabilized at typical values of the other comets. We estimated the total mass injected into the coma by the outburst on the basis of the derived albedo and the optical magnitude for the entire dust cloud, and found that at least 4 × 10 10 kg (equivalent to a few meter surface layer) was removed by the initial outburst event. The derived mass suggests that the outburst is explainable by neither the exogenetic asteroidal impact nor water ice sublimation driven by solar irradiation, but by an endogenic energy source. We conclude that the outburst was triggered by the energy sources several meters or more below the nuclear surface.

Aims. We examined the physical properties of molecular clouds (morphology, column density, number density, mass) to investigate the mechanism of triggered star formation by UV radiation from a massive star.Methods. We made extensive, high-resolution maps of molecular clouds associated and interacting with the W5-East H ii region using the 45-m telescope at the Nobeyama Radio Observatory (HPBW = 15. 6) in 13 CO (J = 1−0) and C 18 O (J = 1−0) to reveal details in the high-density regions of the molecular clouds. In addition, to investigate the spatial distributions of young stellar objects (YSOs) in the W5-East H ii region, we mapped the spatial distributions of Class I and II candidates.Results. We identified eight 13 CO molecular clouds (three of them are known bright-rimmed clouds) and nine C 18 O clumps. The masses of the clouds and clumps range from 460 to 36 000 M and from 55 to 740 M , respectively. The peak 13 CO column densities of the clouds facing the H ii region are twice as large as the others. They have steep density gradients toward the H ii region, indicating interactions with the H ii region. We selected 55 Class I candidates and 778 Class II candidates associated with the W5-East H ii region from the previous Spitzer IRAC/MIPS survey. Most Class I candidates are located around integrated intensity peaks of 13 CO, whereas most Class II candidates are distributed along the front sides of the BRC arcs close to the exciting star. Conclusions. The alignments of the YSO candidates and the molecular clouds in order of age indicate that triggered star formation occurs in the W5-East H ii region as a result of gas compression by strong UV radiation. Based on the column densities of 13 CO and the spatial distribution of YSO candidates, we identified a new bright-rimmed cloud candidate on the west side of the W5-East H ii region.

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We have made a new survey of emission-line stars in the W 5 E H II region to investigate the population of PMS stars near the OB stars by using the Wide Field Grism Spectrograph 2 (WFGS2). A total of 139 H˛emission stars were detected and their g 0 i 0-photometry was performed. Their spatial distribution shows three aggregates, i.e., two aggregates near the bright-rimmed clouds at the edge of the W 5 E H II region (BRC 13 and BRC 14) and one near the exciting O7 V star. The age and mass of each H˛star were estimated from an extinction-corrected color-magnitude diagram and theoretical evolutionary tracks. We found, for the first time in this region, that the young stars near the exciting star are systematically older (4 Myr) than those near the edge of the H II region (1 Myr). This result supports that the formation of stars proceeds sequentially from the center of H II region to the eastern bright rim. We further suggest a possibility that the birth of low-mass stars near the exciting star of the H II region precedes the production of massive OB stars in the pre-existing molecular cloud.