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Observations indicate that molecular clouds are strongly magnetized, and that magnetic fields influence the formation of stars. A key observation supporting the conclusion that molecular clouds are significantly magnetized is that the orientation of their internal structure is closely related to that of the magnetic field. At low column densities the structure aligns parallel with the field, whereas at higher column densities, the gas structure is typically oriented perpendicular to magnetic fields, with a transition at visual extinctions $A_V\gtrsim{}3~\rm{}mag$. Here we use far-infrared polarimetric observations from the HAWC+ polarimeter on SOFIA to report the discovery of a further transition in relative orientation, i.e., a return to parallel alignment at $A_V\gtrsim{}21~\rm{}mag$ in parts of the Serpens South cloud. This transition appears to be caused by gas flow and indicates that magnetic supercriticality sets in near $A_V\gtrsim{}21~\rm{}mag$, allowing gravitational collapse and star cluster formation to occur even in the presence of relatively strong magnetic fields., Comment: 16 pages, 4 figures, Published in Nature Astronomy (August 2020). This is the authors' version before final edits. Link to the NA publication: https://www.nature.com/articles/s41550-020-1172-6

Using wide-field $^{13}$CO ($J=1-0$) data taken with the Nobeyama 45-m telescope, we investigate cloud structures of the infrared dark cloud complex in M17 with SCIMES. In total, we identified 118 clouds that contain 11 large clouds with radii larger than 1 pc. The clouds are mainly distributed in the two representative velocity ranges of 10 $-$ 20 km s$^{-1}$ and 30 $-$ 40 km s$^{-1}$. By comparing with the ATLASGAL catalog, we found that the majority of the $^{13}$CO clouds with 10 $-$ 20 km s$^{-1}$ and 30 $-$ 40 km s$^{-1}$ are likely located at distances of 2 kpc (Sagittarius arm) and 3 kpc (Scutum arm), respectively. Analyzing the spatial configuration of the identified clouds and their velocity structures, we attempt to reveal the origin of the cloud structure in this region. Here we discuss three possibilities: (1) overlapping with different velocities, (2) cloud oscillation, and (3) cloud-cloud collision. From the position-velocity diagrams, we found spatially-extended faint emission between $\sim$ 20 km s$^{-1}$ and $\sim$ 35 km s$^{-1}$, which is mainly distributed in the spatially-overlapped areas of the clouds. We also found that in some areas where clouds with different velocities overlapped, the magnetic field orientation changes abruptly. The distribution of the diffuse emission in the position-position-velocity space and the bending magnetic fields appear to favor the cloud-cloud collision scenario compared to other scenarios. In the cloud-cloud collision scenario, we propose that two $\sim$35 km s$^{-1}$ foreground clouds are colliding with clouds at $\sim$20 km s$^{-1}$ with a relative velocity of 15 km s$^{-1}$. These clouds may be substructures of two larger clouds having velocities of $\sim$ 35 km s$^{-1}$ ($\gtrsim 10^3 $ M$_{\odot}$) and $\sim$ 20 km s$^{-1}$ ($\gtrsim 10^4 $ M$_{\odot}$), respectively., Comment: 34 pages, 13 figures, Accepted for publication in PASJ

We present comprehensive spectroscopic and photometric analyses of the intermediate luminosity type lax supernova (SN lax) 2019muj based on multi-band datasets observed through the framework of the OISTER target-of-opportunity program. SN 2019muj exhibits almost identical characteristics to the subluminous SNe lax 2008ha and 2010ae in terms of the observed spectral features and the light curve evolution at the early phase, except for the peak luminosity. The long-term observations unveil late-time flattening light curves as seen in luminous SN lax 2014dt. This can be explained by the existence of an inner dense and optically thick component possibly associated with a bound white dwarf remnant left behind after the explosion. We demonstrate that the weak deflagration model with a wide range of explosion parameters can reproduce the late-phase light curves of other SNe lax. Therefore, we conclude that a common explosion mechanism operates for different subclass SNe lax., DST-JSPS grant [DST/INT/JSPS/P-281/2018]; Optical and Near-infrared Astronomy Inter-University Cooperation Program; JSPS KAKENHI [JP19K23461, 21K13959, JP20H00174, JP20H04737, JP18H04585, JP18H05223, JP17H02864, JP17K14253]; Turkish Scientific and Technical Research Council (TUBITAK-2211C); Turkish Scientific and Technical Research Council (TUBITAK-2214A); [19B-N-CN02]; [19B-N-CT01], We are grateful to the staff at the Seimei and Subaru telescopes for their support. The spectral data using the Seimei telescope were taken under the programs 19B-N-CN02, 19B-N-CT01. The latephase observation was performed with the Subaru telescope under S19B-055. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawaii. We thank the support staff at IAO and CREST who enabled the 2m HCT observations. The IAO is operated by the Indian Institute of Astrophysics, Bangalore, India. D.K.S. and G.C.A. acknowledge partial support through DSTJSPS grant DST/INT/JSPS/P-281/2018. The authors also thank T. J. Moriya and M. Tanaka for insightful comments. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with theNational Aeronautics and Space Administration. The spectral data of comparison SNe were downloaded from the SUSPECT7 (Richardson et al. 2001) and WISeREP8 (Yaron & Gal-Yam 2012) databases. This research has made use of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), a service of the Astrophysics Science Division at NASA/GSFC and of the Smithsonian Astrophysical Observatory's High Energy Astrophysics Division. This work is supported by the Optical and Near-infrared Astronomy Inter-University Cooperation Program. The authors declare that there is no conflict of interests. M.K. acknowledges support by JSPS KAKENHI Grants (JP19K23461, 21K13959). K.M. acknowledges support by JSPS KAKENHI Grants (JP20H00174, JP20H04737, JP18H04585, JP18H05223, JP17H02864). M.Y. is partly supported by JSPS KAKENHI Grant (JP17K14253). U.B. acknowledges the support provided by the Turkish Scientific and Technical Research Council (TUBITAK-2211C and 2214A).

We have searched 14 dense cores, 11 of which have literature distances of 150–350 pc, for T Tauri stars by using the Wide Field Grism Spectrograph 2 on the University of Hawaii 2.2 m telescope. As a result, we detected 50 Hα emission stars including 24 newly found objects. Twelve out of these 50 stars have a distance of ∼190–370 pc, and 16 of them are within a distance of ∼460 pc. Seven of these 16 objects are newly found Hα emission stars and would serve as new good targets for studying protoplanetary disks and young exoplanets.

Star formation primarily occurs in filaments where magnetic fields are expected to be dynamically important. The largest and densest filaments trace spiral structure within galaxies. Over a dozen of these dense ($\sim$10$^4$\,cm$^{-3}$) and long ($>$10\,pc) filaments have been found within the Milky Way, and they are often referred to as "bones." Until now, none of these bones have had their magnetic field resolved and mapped in their entirety. We introduce the SOFIA legacy project FIELDMAPS which has begun mapping $\sim$10 of these Milky Way bones using the HAWC+ instrument at 214\,$��$m and 18$\farcs$2 resolution. Here we present a first result from this survey on the $\sim$60\,pc long bone G47. Contrary to some studies of dense filaments in the Galactic plane, we find that the magnetic field is often not perpendicular to the spine (i.e., the center-line of the bone). Fields tend to be perpendicular in the densest areas of active star formation and more parallel or random in other areas. The average field is neither parallel or perpendicular to the Galactic plane nor the bone. The magnetic field strengths along the spine typically vary from $\sim$20 to $\sim$100\,$��$G. Magnetic fields tend to be strong enough to suppress collapse along much of the bone, but for areas that are most active in star formation, the fields are notably less able to resist gravitational collapse., Accepted to ApJL; typo in author list metadata corrected

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, {\nlqb trace all gas with column density higher than $3\times 10^{22}$ 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., Comment: We welcome all comments to make the paper better. Please send comments to my email. Thank you

We carried out mapping observations toward three nearby molecular clouds, Orion A, Aquila Rift, and M17, using a new 100 GHz receiver, FOREST, on the Nobeyama 45-m telescope. In the present paper, we describe the details of the data obtained such as intensity calibration, data sensitivity, angular resolution, and velocity resolution. Each target contains at least one high-mass star-forming region. The target molecular lines were $^{12}$CO ($J = 1 - 0$), $^{13}$CO ($J = 1 - 0$), C$^{18}$O ($J = 1 - 0$), N$_2$H$^+$ ($J=1-0$), and CCS ($J_N=8_7-7_6$), with which we covered the density range of 10$^2$ cm$^{-3}$ to 10$^6$ cm$^{-3}$ with an angular resolution of $\sim 20\arcsec$ and a velocity resolution of $\sim$ 0.1 km s$^{-1}$. Assuming the representative distances of 414 pc, 436 pc, and 2.1 kpc, the maps of Orion A, Aquila Rift, and M17 cover most of the densest parts with areas of about 7 pc $\times$ 15 pc, 7 pc $\times$ 7 pc, and 36 pc $\times$ 18 pc, respectively. On the basis of the $^{13}$CO column density distribution, the total molecular masses are derived to be $3.86 \times 10^4 M_\odot$, $2.67 \times 10^4 M_\odot$, and $8.1\times 10^5 M_\odot$ for Orion A, Aquila Rift, and M17, respectively. For all the clouds, the H$_2$ column density exceeds the theoretical threshold for high-mass star formation of $\gtrsim$ 1 g cm$^{-2}$, only toward the regions which contain current high-mass star-forming sites. For other areas, further mass accretion or dynamical compression would be necessary for future high-mass star formation. This is consistent with the current star formation activity. Using the $^{12}$CO data, we demonstrate that our data have enough capability to identify molecular outflows, and for Aquila Rift, we identify 4 new outflow candidates. The scientific results will be discussed in details in separate papers., 50 pages, 39 figures, accepted by PASJ

We conducted near-infrared (JHKs) imaging polarimetry toward the infrared dark cloud (IRDC) M17 SWex, including almost all of the IRDC filaments as well as its outskirts, with the polarimeter SIRPOL on the IRSF 1.4 m telescope. We revealed the magnetic fields of M17 SWex with our polarization-detected sources that were selected by some criteria based on their near-IR colors and the column densities toward them, which were derived from the Herschel data. The selected sources indicate not only that the ordered magnetic field is perpendicular to the cloud elongation as a whole, but also that at both ends of the elongated cloud the magnetic field appears to bent toward its central part, i.e., large-scale hourglass-shaped magnetic field perpendicular to the cloud elongation. In addition to this general trend, the elongations of the filamentary subregions within the dense parts of the cloud appear to be mostly perpendicular to their local magnetic fields, while the magnetic fields of the outskirts appear to follow the thin filaments that protrude from the dense parts. The magnetic strengths were estimated to be ~70-300 microG in the subregions, of which lengths and average number densities are ~3-9 pc and ~2-7x10^3 cm^{-3}, respectively, by the Davis-Chandrasekhar-Fermi method with the angular dispersion of our polarization data and the velocity dispersion derived from the C^{18}O (J=1-0) data obtained by the Nobeyama 45 m telescope. These field configurations and our magnetic stability analysis of the subregions imply that the magnetic field have controlled the formation/evolution of the M17 SWex cloud., Comment: 16 pages, 13 figures, accepted in PASJ

A survey of molecular cores covering the infrared dark cloud known as the M17 southwest extension (M17 SWex) has been carried out with the 45 m Nobeyama Radio Telescope. Based on the N2H+ (J= 1-0) data obtained, we have identified 46 individual cores whose masses are in the range 43 to 3026 Mo. We examined the relationship between the physical parameters of the cores and those of young stellar objects (YSOs) associated with the cores found in the literature. The comparison of the virial mass and the core mass indicates that most of the cores can be gravitationally stable if we assume a large external pressure. Among the 46 cores, we found four massive cores with YSOs. They have large mass of >~ 1000Mo and line width of >~ 2.5 km s^-1 which are similar to those of clumps forming high mass stars. However, previous studies have shown that there is no active massive star formation in this region. Recent measurements of near-infrared polarization infer that the magnetic field around M17 SWex is likely to be strong enough to support the cores against self-gravity. We therefore suggest that the magnetic field may prevent the cores from collapsing, causing the low-level of massive star formation in M17 SWex., 12 figures, accepted for publication in PASJ (Publications of the Astronomical Society of Japan)

We made near-infrared polarimetric observations toward Serpens South. This region contains three dense filaments that are roughly parallel to one another. Using the histogram of relative orientations, the three filaments are found to be roughly perpendicular to the global magnetic field. The morphology of the plane-of-sky (POS) magnetic field and molecular gas suggests that the magnetic field plays an important role in the filament formation and evolution. Applying the Davis–Chandrasekhar–Fermi method, the POS magnetic field strengths are estimated to be 10–80$\, \mu$G. The evaluated mass-to-flux ratios indicate that the center filament is magnetically supercritical, while the others are approximately magnetically critical. We speculate that the filaments are formed by fragmentation of a sheet-like cloud that was created through the gravitational contraction of a magnetized, turbulent cloud.

We present the results of mapping observations toward an outflow-shocked region, OMC-2 FIR 4, using the Nobeyama 45 m telescope. We observed the area in 13CO (J = 1–0), C18O (J = 1–0), N2H+ (J = 1–0), CCS (JN = 87–76), HCO+ (J = 1–0), H13CO+ (J = 1–0), HN13C (J = 1–0), H13CN (J = 1–0), DNC (J = 1–0), N2D+ (J = 1–0), and DC3N (J = 9–8). We detected a dense molecular clump that contains FIR 4/5. We also detected, in the 13CO line, blueshifted and redshifted components driven presumably by protostellar outflows in this region. The axes of the FIR 3 and VLA 13 outflows, projected on the plane of the sky, appear to point to the FIR 4 clump, suggesting that it may be compressed by protostellar outflows from Class I sources, FIR 3 and VLA 13. Applying a hyperfine fit of N2H+ lines, we estimated the excitation temperature to be ∼20 K. The high excitation temperature is consistent with the fact that the clump contains protostars. CCS emission was detected in this region for the first time. Its abundance is estimated to be a few × 10−12, indicating that the region is chemically evolved at ∼105 yr, which is comparable to the typical lifetime of Class I protostars. This timescale is consistent with the scenario that star formation in FIR 4 is triggered by dynamical compression of the protostellar outflows. The [HNC]/[HCN] ratio was evaluated to be ∼0.5 in the dense clump and the outflow lobes, whereas it is somewhat larger in the envelope of the dense clump. The small [HNC]/[HCN] ratio indicates that HNC formation was prevented due to high temperatures. Such high temperatures seem to be consistent with the scenario that either protostellar radiation, or outflow compression, or both affected the thermal properties of this region.

The RCW 106 molecular cloud complex is an active massive star-forming region where a ministarburst is taking place. We examined its magnetic structure by near-IR polarimetric observations with the imaging polarimeter SIRPOL on the IRSF 1.4 m telescope. The global magnetic field is nearly parallel to the direction of the Galactic plane and the cloud elongation. We derived the magnetic field strength of $\sim100$-$1600~\mu$G for 71 clumps with the Davis-Chandrasekhar-Fermi method. We also evaluated the magnetic stability of these clumps and found massive star-forming clumps tend to be magnetically unstable and gravitationally unstable. Therefore, we propose a new criterion to search for massive star-forming clumps. These details suggest that the process enhancing the clump density without an increase of the magnetic flux is essential for the formation of massive stars and the necessity for accreting mass along the magnetic field lines., Comment: 14 pages, 4 figures, accepted to ApJL

We present results of the mapping observations covering a large area of 1 square degree around W40 and Serpens South carried out in the 12CO (J=1-0), 13CO (J=1-0), C18O (J=1-0), CCS (J_{N}=8_{7}-7_{6}), and N_2H+ (J=1-0) emission lines with the 45 m Nobeyama Radio Telescope. W40 is a blistered Hii region, and Serpens South is an infrared dark cloud accompanied by a young cluster. The relationship between these two regions which are separated by ~20' on the sky has not been clear so far. We found that the C18O emission is distributed smoothly throughout the W40 and Serpens South regions, and it seems that the two regions are physically connected. We divided the C18O emission into four groups in terms of the spatial distributions around the HII region which we call 5, 6, 7, and 8 km s^{-1} components according to their typical LSR velocities, and propose a three-dimensional model of the W40 and Serpens South complex. We found two elliptical structures in position-velocity diagrams, which can be explained as a part of two expanding shells. One of the shells is the small inner shell just around the HII region, and the other is the large outer shell corresponding to the boundary of the HII region. Dense gas associated with the young cluster of Serpens South is likely to be located at the surface of the outer shell, indicating that the natal clump of the young cluster is interacting with the outer shell being compressed by the expansion of the shell. We suggest that the expansion of the shell induced the formation of the young cluster., 18 figures, accepted for publication in PASJ (the special issue of the Nobeyama FOREST receiver, "Nobeyama 45m Telescope: Legacy Projects and Receiver FOREST")

We present a method for determining the age of early-M type pre-main sequence (PMS) stars based on estimations of the surface gravity. The surface gravity was measured using high-resolution near-infrared K-band spectroscopy. The age of the PMS stars can be determined from the surface gravity, which correlates with the photospheric contraction. To estimate the surface gravity while avoiding veiling contamination, we developed a surface gravity indicator using equivalent width ratios (EWRs) of nearby absorption lines. We derived a relationship between the ratios of the Sc (22057.8 u A and 22071.3 u A) and Na (22062.4 u A and 22089.7 u A) absorption lines and the surface gravity by observing giants and main-sequence stars. The surface gravities of early-M type stars were determined with an accuracy of 0.1 in logg. The ages of target PMS stars were estimated within a factor of 1.5 by comparing the surface gravity with the evolution model of Baraffe et al. (1998, A&A, 337, 403). The ages of 4 PMS stars were estimated to be older than indicated by previous age determinations made using the photometric method. The EWR method allows estimating the age of PMS stars without contaminating the uncertainty of the distance, extinction, and veiling.

We present a large-scale view of the magnetic field in the central 2deg * 2deg region of our Galaxy. The polarization of point sources has been measured in the J, H, and Ks bands using the near-infrared polarimetric camera SIRPOL on the 1.4 m telescope IRSF. Comparing the Stokes parameters between high extinction stars and relatively low extinction ones, we obtain polarization originating from magnetically aligned dust grains in the central few-hundred pc of our Galaxy. We find that near the Galactic plane, the magnetic field is almost parallel to the Galactic plane (i.e., toroidal configuration) but at high Galactic latitudes (| b | > 0.4deg), the field is nearly perpendicular to the plane (i.e., poloidal configuration). This is the first detection of a smooth transition of the large-scale magnetic field configuration in this region., 12 pages, 3 figures, Accepted for publication in ApJ Letters. For higher resolution, see http://www.kusastro.kyoto-u.ac.jp/~shogo/papers/10NishiyamaMagFGC2.pdf

We have carried out near-IR imaging polarimetry toward RCW 106 with the JHKs-simultaneous imaging polarimeter SIRPOL mounted on the IRSF 1.4m telescope at SAAO, in March and May, 2017 and January, 2018. We have observed 29 fields and covered mostly the southern part of the giant molecular cloud complex associated with the ${{\rm{H}}_\mathbb{I}}$ region RCW 106, which is located at a distance of 3.5 kpc (Moises etal 2011) and is elongated approximately in the north-south direction with a size of ∼70×15 pc. Our preliminary analysis indicates that the magnetic field seems to globally run along the complex elongation, unlike many other elongated clouds that are often reported to have their global elongations perpendicular to the magnetic fields. The RCW 106 complex consists of many small filaments or clumps. Some of such filaments seem to parallel to the magnetic fields, but some others perpendicular. Around the central part of the ${{\rm{H}}_\mathbb{I}}$ region RCW 106, the magnetic field appears to be influenced by the expansion of this ${{\rm{H}}_\mathbb{I}}$ region. Here, we present our preliminary results by comparing with the archival molecular line and far- to mid-IR data.

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 = 156) in 13 CO and C18 O 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 18 O clumps. The masses of the clouds and clumps range from 460 to 36 000 and from 55 to 740 , 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.

We have determined interstellar extinction law toward the Galactic center (GC) at the wavelength from 1.2 to 8.0 micron, using point sources detected in the IRSF/SIRIUS near-infrared survey and those in the 2MASS and Spitzer/IRAC/GLIMPSE II catalogs. The central region |l| < 3deg and |b| < 1deg has been surveyed in the J, H and Ks bands with the IRSF telescope and the SIRIUS camera whose filters are similar to the Mauna Kea Observatories (MKO) near-infrared photometric system. Combined with the GLIMPSE II point source catalog, we made Ks versus (Ks - lambda) color-magnitude diagrams where lambda = 3.6, 4.5, 5.8, and 8.0 micron. The Ks magnitudes of bulge red clump stars and the (Ks - lambda) colors of red giant branches are used as a tracer of the reddening vector in the color-magnitude diagrams. From these magnitudes and colors, we have obtained the ratios of total to selective extinction A(Ks)/E(Ks-lambda) for the four IRAC bands. Combined with A(lambda)/A(Ks) for the J and H bands derived by Nishiyama et al., we obtain A(J):A(H):A(Ks):A([3.6]):A([4.5]):A([5.8]):A([8.0])=3.02:1.73:1:0.50:0.39:0.36:0.43 for the line of sight toward the GC. This confirms the flattening of the extinction curve at lambda > 3 micron from a simple extrapolation of the power-law extinction at shorter wavelengths, in accordance with recent studies. The extinction law in the 2MASS JHKs bands has also been calculated, and a good agreement with that in the MKO system is found. In nearby molecular clouds and diffuse interstellar medium, the lack of reliable measurements of the total to selective extinction ratios hampers unambiguous determination of the extinction law; however, observational results toward these lines of sight cannot be reconciled with a single extinction law., 27 pages, 10 figures, Accepted for publication in ApJ

We present a polarimetric map of a 20'x20' area toward the Galactic center. The polarization of point sources has been measured in the J, H, and Ks bands using the near-infrared polarimetric camera SIRPOL on the 1.4 m telescope IRSF. One percent or better accuracy of polarization degree is achieved for sources with J, 14 pages, 11 figures, Accepted for publication in ApJ, For higher resolution, see http://www.kusastro.kyoto-u.ac.jp/~shogo/papers/08NishiyamaMagFGC.pdf

We present a method of mapping dust column density in dark clouds by using near-infrared scattered light. Our observations of the Lupus 3 dark cloud indicate that there is a well defined relation between (1) the H-Ks color of an individual star behind the cloud, i.e., dust column density, and (2) the surface brightness of scattered light toward the star in each of the J, H, and Ks bands. In the relation, the surface brightnesses increase at low H-Ks colors, then saturate and decrease with increasing H-Ks. Using a simple one-dimensional radiation transfer model, we derive empirical equations which plausibly represent the observed relationship between the surface brightness and the dust column density. By using the empirical equations, we estimate dust column density of the cloud for any directions toward which even no background stars are seen. We obtain a dust column density map with a pixel scale of 2.3 x 2.3 arcsec^2 and a large dynamic range up to Av = 50 mag. Compared to the previous studies by Juvela et al., this study is the first to use color excess of the background stars for calibration of the empirical relationship and to apply the empirical relationship beyond the point where surface brightness starts to decrease with increasing column density.

We present the results of a deep near-infrared (NIR) imaging survey searching for very low mass young stellar objects (YSOs) in the embedded cluster associated with the Perseus molecular cloud. Our observations cover an area of ~5'× 5' in the NGC 1333-S region at J-, H-, and Ks-bands. The 10σ limiting magnitudes exceed 18 mag in all three bands. Based on NIR color-color diagrams, embedded YSO candidates were identified using NIR excesses. The derived frequency of these YSO candidates with NIR excess emission among all the detected sources is 58+10 –13%. The higher frequency of YSOs with NIR excesses implies that NGC 1333 is an active and young star-forming region. Approximately half of the YSO candidates exhibit extremely low luminosity, indicating very low mass. Combining the reddening-corrected luminosity with theoretical evolutionary models, the low-luminosity YSO candidates are considered to be young substellar-mass objects. Furthermore, some sources could have planetary masses. In addition, we carried out CO molecular-line observations of the same cloud region. The results suggested that the YSO candidates are likely formed on the side of the parent molecular cloud and star-formation activity could be high. The K-band luminosity function of all the detected sources revealed a significant population of extremely low-luminosity sources in NGC 1333. The reddening-corrected J-band luminosity function of the YSO candidates does not clearly decline to the completeness limit and seems to be bimodal. We also argue that the fraction of substellar objects is larger than those in other young clusters and the mass function of the YSO candidates appears to be increasing toward the substellar-mass regime, similar to that of dust clumps. It implies that substellar-mass distributions may depend on the initial conditions of the molecular cloud.

We have made near-infrared (JHKs) imaging polarimetry toward 24 bright-rimmed clouds in the southern hemisphere in order to reveal their magnetic field structures. The obtained polarization vector maps show that the magnetic field directions inside the bright rim are different from its ambient magnetic field direction, implying that magnetic field structures just inside the ionized front of the clouds are due to the gas compression by UV radiation from nearby massive star. Our investigation into the relation between the magnetic field configuration and the shape of the cloud suggests that the magnetic field configuration affects the evolution of the cloud shape.

A systematic investigation on the evolution of a prolate cloud at an Hii boundary is conducted using Smoothed Particle Hydrodynamics (SPH) in order to understand the mechanism for a variety of irregular morphological structures found at the boundaries of various Hii regions. The prolate molecular clouds in this investigation are set with their semi-major axes at inclinations between 0 and 90 degrees to a plane parallel ionizing radiation flux. A set of 4 parameters, the number density n, the ratio of major to minor axis gamma, the inclination angle phi and the incident flux F_EUV, are used to define the initial state of the simulated clouds. The dependence of the evolution of a prolate cloud under Radiation Driven Implosion (RDI) on each of the four parameters is investigated. It is found that: i) in addition to the well studied standard type A, B or C Bright Rimmed Clouds (BRCs), many other types such as asymmetrical BRCs, filamentary structures and irregular horse-head structures could also be developed at Hii boundaries with only simple initial conditions; ii) the final morphological structures are very sensitive to the 4 initial parameters, especially to the initial density and the inclination; iii) The previously defined ionizing radiation penetration depth can still be used as a good indicator of the final morphology. Based on the simulation results, the efficiency of the RDI triggered star formation from clouds of different initial conditions is also estimated. Finally a unified mechanism for the various morphological structures found in many different Hii boundaries is suggested., Comment: 16 pages, 19 figures, 4 tables, submitted to MNRAS

Using the Wide Field Grism Spectrograph 2 (WFGS2), we have carried out slit-less spectroscopy, g'r'i' photometry, and slit spectroscopy on the L1014 dense core. We detected three Halpha emission line stars. We interpret one as weak-line T Tauri star (WTTS) and the others as classical T Tauri stars (CTTS). Since their g'-i' colors and/or classified spectral types are consistent with those of T Tauri stars and two of them show less extinction than the cloud, these three stars are likely to be T Tauri stars associated with L1014. Adopting an age range for T Tauri stars, 1-10 Myr, the color-magnitude diagram suggests a distance of ~400-900 pc, rather than the previously assumed distance, 200 pc. This could strongly affect on the mass estimate of L1014-IRS, which is thought to be either a very young protostar or proto-brown dwarf., Comment: 5 pages, 5 figures, to be published in Vol.58, No.5, October 25, 2006

We present new candidates for protostars and embedded clusters detected in our deep near-infrared (J, H, KS) imaging survey of cloud C of the Vela Molecular Ridge (Vela C). We selected protostar candidates on the basis of positional coincidences with Midcourse Space Experiment (MSX) and/or IRAS sources and the slopes of their spectral energy distributions from 2 to 25 ?m. For embedded clusters, we selected them on the basis of positional coincidence with MSX and/or IRAS sources and enhancement of stellar number densities. We identified 31 sources as protostar candidates and 5 clusters as embedded clusters; 28 protostar candidates and 3 embedded clusters were newly identified. Most of them are associated with the C18O clumps distributed throughout Vela C. We calculated the bolometric luminosities of the protostar candidates by integrating the observed fluxes from near- to far-infrared, and then estimated the mass range to be from 0.9 to 4.0 M?, assuming their ages at the birth line. There is a correlation between the number of members and the KS magnitude of the brightest star in the embedded clusters.

We have carried out a deep near-infrared survey of a bright-rimmed molecular cloud, BRC 14 (IC 1848A). The 10 sigma limiting magnitude of the survey is 17.7 mag at the K-band. Seventy-four sources are classified as young stellar object (YSO) candidates based on the near-infrared color-color diagram. The faintest YSO candidates may have masses of an order of tenths of the solar mass, assuming the age of 1 Myr. We examined three values as indicators of star formation; fraction of the YSO candidates, extinctions of all sources, and near-infrared excesses of the YSO candidates. All indicators increase from outside of the rim to the center of the molecular cloud, which suggests that the formation of the low-mass stars in the BRC 14 region proceeds from outside to the center of the cloud., Comment: 12 pages, 5 figures, 1 table, PASJ accepted

We present the result of a deep near-infrared survey of the newly identified X-ray luminous cluster of galaxies CIZA J1324.7-5736 in the Great Attractor (GA) region. In a 35 x 35 arcmin 2 region, 111 galaxy candidates with r Ks 20 > arcsec are identified. Comparison of the extinction-corrected K s -band luminosity function of CIZA J 1324.7-5736 with those of nearby clusters indicates that the richness class of CIZA J1324.7-5736 is almost the same as, or richer than, the Pavo. Centaurus and Hydra clusters but poorer than the Coma, Perseus and Norma clusters. CIZA J1324.7-5736 is possibly the second richest cluster in the GA region following the Norma cluster. The position of CIZA J1324.7-5736 [in the (l, b, v) space] is close to the Centaurus-Crux cluster and the agglomeration of galaxies detected by the Parkes Hi survey. CIZA J1324.7-5736, together with the Centaurus-Crux cluster and the Hi galaxy agglomeration, is most likely to be one of the richest local concentrations in the GA overdensity of galaxies.

We determine and compare the color excess ratios EJ-H/E for the ρ Ophiuchi and Chamaeleon dark clouds with the J-, H-, and Ks-band simultaneous camera, SIRIUS, on the IRSF 1.4 m telescope at SAAO. Determining accurate ratios is indispensable for discussions of the extinction law. However, previous surveys suffered from uncertainties in transforming one photometric system to another when comparing data on different systems. To overcome the problem, we observe both clouds in exactly the same photometric system on the same telescope, greatly simplifying the comparison of the extinction laws between two clouds. From our surveys covering about 1.56 deg2 (ρ Oph) and 2.77 deg2 (Chamaeleon), which are complete for J ~ 19.0, H ~ 18.3, and KS ~ 17.0 mag at a 10 σ limiting magnitude, we did not find a significant difference in the extinction law EJ-H/E for the two clouds, in contrast to previous work. From calculations of a model star with the effects of filter and atmosphere transmissions, the differences of the color excess ratios EJ-H/EH-K for two clouds described in the past literature can be attributed to the different photometric systems so far employed. We also note a change of the color excess ratio that occurs with increasing optical depth.

Near-infrared observations have been carried out to survey young stellar objects in the second-largest H II region in the Large Magellanic Cloud, N11. A total area of about 700 arcmin2 is covered in the J, H, and KS bands. We selected a total of 559 OB and 127 Herbig Ae/Be star candidates out of the detected sources based on their near-infrared colors and magnitudes. The existence of these young stellar objects indicates that star formation activity is underway in the whole N11 region. Many Herbig Ae/Be star candidates are distributed around the periphery of the OB association LH 9. Spatial correlations of the OB and Herbig Ae/Be star candidates with the objects observed at other wavelengths (optical, radio continuum, Hα, CO, and X-ray) suggest that the birth of the young stellar populations in peripheral molecular clouds was triggered originally by LH 9. It is likely that the trigger for this star formation was an expanding supershell blown by the OB association. In N11 a new generation of stars would have been formed in the clouds developed from swept-up interstellar medium.

We present a study of the newly discovered McNeil's nebula in Orion using the JHKs-band simultaneous observations with the near-infrared (NIR) camera SIRIUS on the IRSF 1.4m telescope. The cometary infrared nebula is clearly seen extending toward north and south from the NIR source (V1647 Orionis) that illuminates McNeil's nebula. The compact nebula has an apparent diameter of about 70 arcsec. The nebula is blue (bright in J) and has a cavity structure with two rims extending toward north-east and north-west. The north-east rim is brighter and sharp, while the north-west rim is diffuse. The north-east rim can be traced out to ~ 40 arcsec from the location of the NIR source. In contrast, no cavity structure is seen toward the south, although diffuse nebula is extended out to ~ 20 arcsec. New NIR photometric data show a significant variation in the magnitudes (> 0.15 mag) of the source of McNeil's nebula within a period of one week, that is possibly under the phase of eruptive variables like FUors or EXors., 13 pages, 5 figures in JPEG format. Accepted for the publication in PASJ Letters

We present the result of a near-infrared (J H Ks) survey along the Galactic plane, -10.5deg < l < +10.5deg and b=+1.0deg, with the IRSF 1.4m telescope and the SIRIUS camera. Ks vs. H-Ks color-magnitude diagrams reveal a well-defined population of red clump (RC) stars whose apparent magnitude peak changes continuously along the Galactic plane, from Ks=13.4 at l=-10deg to Ks=12.2 at l=+10deg after dereddening. This variation can be explained by the bar-like structure found in previous studies, but we find an additional inner structure at |l| < 4deg, where the longitude - apparent magnitude relation is distinct from the outer bar, and the apparent magnitude peak changes by only 0.1 mag over the central 8deg. The exact nature of this inner structure is as yet uncertain., 8 pages, 4 figures. accepted by ApJL

著者人数: 14名, Accepted: 2004-10-20, 資料番号: SA1003336000

We present sub-arcsecond (FWHM ~ 0".7), NIR JHKs-band images and a high sensitivity radio continuum image at 1280 MHz, using SIRIUS on UH 88-inch telescope and GMRT. The NIR survey covers an area of ~ 24 arcmin^2 with 10-sigma limiting mags of ~ 19.5, 18.4, and 17.3 in J, H, and Ks-band, respectively. Our NIR images are deeper than any JHK surveys to date for the larger area of NGC 7538 star forming region. We construct JHK CC and J-H/J and H-K/K CM diagrams to identify YSOs and to estimate their masses. Based on these CC and CM diagrams, we identified a rich population of YSOs (Class I and Class II), associated with the NGC 7538 region. A large number of red sources (H-K > 2) have also been detected around NGC 7538. We argue that these red stars are most probably PMS stars with intrinsic color excesses. Most of YSOs in NGC 7538 are arranged from the N-W toward S-E regions, forming a sequence in age: the diffuse H II region (N-W, oldest: where most of the Class II and Class I sources are detected); the compact IR core (center); and the regions with the extensive IR reflection nebula and a cluster of red young stars (S-E and S). We find that the slope of the KLF of NGC 7538 is lower than the typical values reported for the young embedded clusters, although equally low values have also been reported in the W3 Main star forming region. From the slope of the KLF and the analysis by Megeath et al. (1996), we infer that the embedded stellar population is comprised of YSOs with an age of ~ 1 Myr. Based on the comparison between models of PMS stars with the observed CM diagram we find that the stellar population in NGC 7538 is primarily composed of low mass PMS stars similar to those observed in the W3 Main star forming region., 36 pages, 13 figures in JPEG format. Accepted for the publication in ApJ. Report is also available at : http://www.tifr.res.in/~ojha/NGC7538.html

We present the results of a deep near-infrared survey of a 36 x 36 arcmin 2 region centred on the giant elliptical radio galaxy PKS 1343-601, suggested to be the core of an unknown rich cluster located at the low Galactic latitude of b = 1°73 in the Great Attractor (GA) region. 19 obvious galaxies and 38 galaxy candidates have been detected; only three of them were previously identified as a galaxy. The total Galactic extinction A K towards our survey area is estimated to be 0.6-0.8 mag from the J - K colour of foreground giants. This is systematically lower by about 0.4 mag than A K taken from the IRAS/DIRBE extinction map. The number density of galaxies brighter than an extinction-corrected Ks band magnitude of 13 is 42 galaxies deg -2 , five times as high as the overall average in the GA region. However, the number of galaxies within the central 270-kpc radius is less than that of the Norma, Pavo and Centaurus clusters in the GA region. We found no evidence that a rich cluster is associated with PKS 1343-601.

We study the evolution of elliptical galaxies at z >~ 1 on the basis of a sample of 247 Extremely Red Objects (EROs) with R-Ks >= 3.35 (AB) and Ks ~ 1.5 found by several authors, we conclude that the majority of ellipticals seen at present have not established either a red color or a smooth 1/4-law profile before z ~ 1.5$. (abridged), Comment: 46 pages, 15 PostScript figures. Submitted to ApJ

The star-forming region S269 is studied with deep JHK(s) and H(2)v = 1-0 S(1) images and K-band spectroscopy. The JHK(s) images reveal a rich embedded cluster at the center of the field; most stars in the cluster are located in two major congregations. The colors of the two congregations and of the associated nebulosities are quite different. The color-color diagrams and color-magnitude diagrams of the two subgroups show that their intrinsic colors are not significantly different, suggesting that they could be formed at a similar age. Between the subgroups there are two near-infrared sources, IRS 2e and 2w. Their K-band spectra rise toward longer wavelengths, indicative of a dense dusty envelope surrounding them. Strong H(2) and Brgamma lines are detected on these two objects. A poorly collimated bipolar H2 jet has been detected around IRS 2e and 2w. The morphology of the infrared jet and K-band spectra of the two infrared sources suggest that the jet is likely driven by IRS 2e or 2w, or both. Several lines of evidence suggest that IRS 2e and 2w are massive young stellar objects probably younger than most of the stars in the region. Given the facts above, we speculate that lower mass star-forming activities in a stellar cluster could play an important role in the formation of a massive star.

Survey observations were carried out in the MBM 12 (sometimes referred to as Lynds 1457) dark cloud in search of Hα emission stars by using a slitless grism spectrograph on the 2.6-m reflector of Byurakan Astrophysical Observatory, Armenia. Four new Hα stars were detected by the survey and identified as candidate T Tauri stars by follow-up low-dispersion spectroscopy and/or near-infrared photometry.

We carried out deep imaging observations of the Lupus 3 dark cloud in near-infrared J, H, and Ks bands. An area of ~8' × 8' was observed, which corresponds to a projected area of ~0.4 × 0.4 pc at the distance of the cloud, ~150 pc. Lupus 3 showed itself as a near-infrared nebula that has a surface brightness higher than the adjacent sky at all the three wavelengths. In a JHKs color composite image (blue, green, and red are assigned to J, H, and Ks, respectively), three dark red cores are surrounded by a blue halo. The surface brightness was measured with 5 σ limiting magnitudes of J = 21.6, H = 21.3, and Ks = 20.6 mag arcsec-2. The appearance of the nebula depends on the wavelength. In the J band, dark cores are surrounded by a brighter halo, while in the Ks band, the dark cores of the J band are bright except for the central part of two of the cores. The appearance in the H band is intermediate between those of the J and Ks bands, having dark cores surrounded by local maxima of the surface brightness and decreased surface brightness farther out. The surface brightness is J = 20.6, H = 19.8, and Ks = 19.4 mag arcsec-2 at the maximum in each band. Photometry of the point sources was done with 10 σ limiting magnitudes of J = 20.1, H = 18.8, and Ks = 17.7. We constructed an extinction map of the background stars, using the H-K color of 1974 sources and the standard reddening law of Rieke & Lebofsky. The maximum value for the extinction is AV = 47 mag. There are three local maxima of the extinction with AV 30 mag, which we consider to be dense cores. Their positions agree with the cores identified with the surface brightness appearance. The surface brightness and its relationship with the extinction are understood in terms of scattering of starlight by dust. The values of the maximum surface brightness can be explained by scattering of starlight by dust in the cloud if we adopt a model of grain size distribution by Weingartner & Draine.

We have carried out a millimeter interferometric continuum survey toward 7 YSOs in the MBM 12 cloud. Thermal emissions associated with 2 YSOs were detected above the 3-$\sigma$ level at 2.1 mm, and one also showed a 1.3 mm thermal emission. Another object was marginally detected at 2.1 mm. Spectral energy distributions of the YSOs are well fitted by a simple power-law disk model. Masses of the circumstellar disks are estimated to be an order of $0.05 M_{\sun}$. The circumstellar disks in the MBM 12 cloud have properties in common with the disks in nearby star-forming regions, in terms of disk parameters such as a disk mass, as well as an infrared excess., Comment: 9 pages, 3 figures, accepted by ApJ Letters

We are currently conducting three kinds of IR surveys of star forming regions (SFRs) in order to seek for very low-mass young stellar populations. First is a deep JHKs-bands (simultaneous) survey with the SIRIUS camera on the IRSF 1.4m or the UH 2.2m telescopes. Second is a very deep JHKs survey with the CISCO IR camera on the Subaru 8.2m telescope. Third is a high resolution companion search around nearby YSOs with the CIAO adaptive optics coronagraph IR camera on the Subaru. In this contribution, we describe our SIRIUS camera and present preliminary results of the ongoing surveys with this new instrument.

We conducted a deep JHK(s)-band imaging survey of the M17 region, using a near-infrared camera, the Simultaneous 3-color InfraRed Imager for Unbiased Survey ( SIRIUS), mounted on the InfraRed Survey Facility (IRSF) 1.4 m telescope at the South African Astronomical Observatory. This survey covers an area of similar to200 arcmin(2) with 10 sigma limiting magnitudes of J similar to 18.7, H similar to 18.2, and K-s similar to 17.5. The near-infrared (NIR) images reveal an unprecedented view of the region. The NIR nebulae are highly structured, with two nebular bars corresponding to, but a little larger than, the H II region defined by Felli, Massi, & Churchwell, constructing a conical shape. Fine structures are found all over the nebular area. The central region contains a congregation of intermediate- to high-mass stars. From the slope of the K-s-band luminosity function and the frequency of young stellar objects (YSOs) we infer that the central cluster has an age less than 3 Myr. The central OB cluster provides tremendous energy that heats and ionizes its surrounding materials, triggering the star formation of second-generation in the nebular bars. The second generation stars are so numerous that could they affect the star formation efficiency in the whole region. To the southwest of the central cluster and the nebular bars, where a giant molecular cloud core is located, a large number of red stars are detected. We argue that these red stars are most probably associated YSOs with intrinsic color excesses, not normal field stars reddened by the molecular cloud in front of them. Being located beyond the photodissociation region, the star-forming process in the molecular region could be independent of the impact of the central cluster.

Bright-rimmed clouds (BRCs) found in H II regions are probably sites of triggered star formation due to compression by ionization/shock fronts. Many BRCs harbor IRAS point sources of low dust temperature. They also frequently contain a small cluster of near-IR stars that is elongated along the axis of the BRC. Here we present the results of our Hα grism spectroscopy and narrowband imaging observations of BRCs in search of candidate pre–main-sequence stars of the T Tauri, Herbig Ae/Be and related types, and Herbig-Haro (HH) objects. We have detected a large number (460) of Hα emission stars down to a limiting magnitude of about R = 20 in and around all but two of the 28 BRCs observed. The present study has, for the first time, reached down nearly to the faintest classical T Tauri star population in OB associations. A total of 12 new HH objects have been found. Most are of small apparent size, emphasizing the need for deep searches at high spatial resolution, but HH 588 associated with BRC 37 represents a huge HH complex composed of two-staged bow shocks on both sides of a tiny central knot. These stars and HH objects are concentrated near the tip of bright rims (i.e., in the head of the BRCs and just outside the rims) and often make loose aggregates similar to those of near-IR stars, thus supporting our hypothesis of "small-scale sequential star formation." The presence of such a large number of Hα emission stars in the immediate vicinities of BRCs implies that second-generation formation of low-mass stars is relatively extensive and further supports the notion of cohabitation of high- and low-mass populations in OB associations.

We observed heads of two molecular pillars in the Eagle Nebula using the Nobeyama Millimeter Array with a spatial resolution of 3'', 5'', and 3'' in the 13CO (J = 1-0) line, C18O (J = 1-0) line, and 2.7 mm continuum, respectively. We found six 13CO subclumps and four C18O cores. The 13CO clouds are elongated so as to have a head-tail structure, with the heads orientated toward the O star exciting M16. The elongation is likely to be due to radiation or wind from the O star. The cloud surface appears to be compressed, as indicated by strong 13CO emission at the cloud rim. The shapes of the 13CO clouds are quite similar to those of the dark cloud observed in the near-infrared. Three out of the four C18O cores are located within one of the 13CO clouds. One of the three cores, located near the tip of the 13CO cloud, is associated with a 2.7 mm continuum emission peak and is most likely a protostar. It is not associated with a known near-infrared source. The other two cores are located farther from the O star and are most likely starless cores. Thus, these C18O cores are aligned in order of age, with the more evolved objects closer to the O star. This linear sequence suggests the propagation of star formation activity, i.e., sequential star formation, driven by the O star. A similar sequence of a young stellar object and a C18O core was found in the other head of the molecular pillar.

We present combined J, H, and Ks images of M16 that were obtained with SIRIUS (Simultaneous three-color InfraRed Imager for Unbiased Surveys) on the University of Hawaii 2.2 m telescope. In contrast to optical images, the infrared images show that the Eagle Nebula's "pillars" have head-tail structures, with the heads faced toward the central part of the star cluster NGC 6611 and the tails trailed toward the opposite side. We have found a number of previously unreported young stellar object (YSO) candidates associated with these head-tail pillars. In particular, the youngest YSOs are located at the tips of the pillars or globules, facing toward the central part of NGC 6611. Some are associated with near-infrared jets or cometary reflection nebulae, with indications of a circumstellar disk. This YSO distribution and the cloud structures suggest that the most recent star formation is taking place following the interaction with UV light from the OB stars in NGC 6611.

We have carried out near-infrared (IR) observations to examine star formation toward the bright-rimmed cloud SFO 12, of which the main exciting star is O7V star in W5-W. We found a small young stellar object (YSO) cluster of six members embedded in the head of SFO 12 facing its exciting star, aligned along the UV radiation incident direction from the exciting star. We carried out high-resolution near-IR observations with the Subaru adaptive optics (AO) system and revealed that three of the cluster members appear to have circumstellar envelopes, one shows an arm-like structure in its envelope. Our near-IR and L?-band photometry and Spitzer IRAC data suggest that formation of two members at the tip side occurred in advance of other members toward the central part, under our adopted assumptions. Our near-IR data and previous studies imply that more YSOs are distributed in the region just outside the cloud head on the side of the main exciting star, but there is little sign of star formation toward the opposite side. We infer that star formation has been sequentially occurring from the exciting star side to the central part. We examined archival data of far-infrared and CO (J = 3-2) which reveals that, unlike in the optical image, SFO 12 has a head-tail structure which is along the UV incident direction. This suggests that SFO 12 is affected by strong UV from the main exciting star. We discuss the formation of this head-tail structure and star formation there by comparing with a radiation-driven implosion (RDI) model.

We have made 2 mm continuum observations of 15 bright-rimmed clouds (BRCs) associated with IRAS point sources and S140 with the Nobeyama Bolometer Array (NOBA) mounted on the 45 m telescope of Nobeyama Radio Observatory. Continuum emission was detected in five BRCs and in S140. These five BRCs are known to be associated with near-infrared YSO clusters, mostly on the side facing toward the exciting star(s) and, therefore, are likely the sites of small-scale sequential star formation. The detected emission peaks of these five sources correspond to the IRAS positions within the positional errors, and the IRAS sources are considered to be protostars (Class 0/I-like sources) that were formed most recently in the BRCs. Four of them are clearly extended more than the telescope beam, suggesting the presence of circumstellar structures. The circumstellar masses derived from the 2 mm continuum are ~5–90 M⊙, which are more massive than those of the nearby Class 0/I objects (1 M⊙). This fact and the relatively large bolometric luminosities of these objects suggest that the mass of the cluster or star(s) most recently formed in these BRCs could be higher than those of the previously formed stars found in the near-infrared cluster. The comparisons with previous observations of Bok globules unassociated with bright rims and other objects are discussed. Most noteworthy is that the ratios of the bolometric luminosity to the circumstellar mass are significantly higher for these BRCs than for Bok globules.