Katsuo Ogura

A catalog of bright-rimmed clouds with IRAS point sources : candidates for star formation by radiation-driven implosion. I, The northern hemisphere

Forty-four bright-rimmed clouds associated with IRAS sources have been selected from the Palomar Sky Survey prints. They are good candidates for the sites of star formation induced by radiation-driven implosions. Nine of the bright-rimmed clouds are known to be associated with molecular outflows and two (including one with an outflow) with HH objects. Most of their sizes are ≃ 1 pc, similar to those of Bok globules. The luminosities of the associated IRAS sources are relatively large compared to those of the IRAS sources associated with dark globules or dense cores in quark cloud complexes. IRAS luminosity to cloud mass ratios are significantly greater than those in dark globules or in dense cores of dark cloud complexes

Young Star Clusters in Bright-rimmed Clouds: Small-Scale Sequential Star Formation?

A JHK imaging survey has been made for 44 bright-rimmed clouds associated with IRAS point sources detected by Sugitani et al. We have found small clusters of near-infrared sources having young stellar object (YSO) colors in some of these objects; most of the cluster members are considered to be older than the IRAS point sources and to be pre-main-sequence stars such as T Tauri stars. In at least six bright-rimmed clouds, the clusters are elongated toward the bright-rim tip or the exciting star(s) of the bright rim with the IRAS sources situated near the other end. There is a tendency for bluer (i.e., older) stars to be located closer to the exciting star(s) and for redder (i.e., younger) stars to be closer to the IRAS sources. This asymmetric distribution of the cluster members strongly suggests small-scale sequential star formation or propagation of star formation from the side of the exciting star(s) to the IRAS position in a few times 105 yr, as a result of the advance of the shock caused by the UV radiation from the exciting star(s).

NGC 7654: An interesting cluster to study star formation history ?

CCD UBV IC photometry in a wide eld around the open cluster NGC 7654 has been carried out for 17 860 stars, down to V 20. The reddening across the cluster region is found to be variable with E(B V )min = 0:46 to E(B V )max =0 :80. The cluster is situated at a distance of 1380 70 pc. The colour magnitude diagrams show a large age spread in the ages. Star formation was biased towards relatively higher masses during the early phase of star formation whereas most of the low mass stars of the cluster were formed during the later phase. The star formation seems to have been a gradual process that proceeded sequentially in mass and terminated with the formation of most massive stars. The present data do not support a uniform mass function (MF) for dierent regions in the cluster. Although for the whole cluster region, the MF in the observed mass range 0:8 M 4:5, can be represented by a single power law with a slope = 1:400:07, however it indicates various features when examined carefully. In three subregions of the cluster the slope , for the mass range 1:5 <M < 4:0, comes out to be 1:07 0:08 and 1:28 0:20 for the inner and intermediate regions and it becomes steeper in the outermost region ( = 2:78 0:21). For stars having masses <1.5 M the MF, in the inner and intermediate regions, can be represented by a power law having a steeper slope, whereas in the outer region a turnover can be seen in the MF at M 1:5 M. The age of NGC 7654 is found to be comparable to its two-body relaxation time-scale; therefore this may also be a reason for the observed mass segregation in the cluster.

Stellar contents and star formation in the young open cluster Stock 8

We present UBVIc CCD photometry of the young open cluster Stock 8 with the aim of studying its basic properties such as the amount of interstellar extinction, distance, age, stellar contents and initial mass function (IMF). We also studied the star formation scenario in this region. From optical data, the radius of the cluster is found to be ∼6 arcmin (∼3.6 pc) and the reddening within the cluster region varies from E(B − V) = 0.40 to 0.60 mag. The cluster is located at a distance of 2.05 ± 0.10 kpc. Using Hα slitless spectroscopy and Two Micron All Sky Survey (2MASS) near-infrared (NIR) data we identified Hα emission and NIR-excess young stellar objects (YSOs), respectively. From their locations in the colour‐magnitude diagrams, majority of them seem to have ages between 1 and 5 Myr. The spread in their ages indicate a possible noncoeval star formation in the cluster. Massive stars in the cluster region reveal an average age of 2 Myr. In the cluster region (r 6 arcmin) the slope of the mass function (MF), � , in the mass range ∼1.0 M/M� < 13.4 can be represented by a power law having a slope of −1.38 ± 0.12, which agrees well with Salpeter value (−1.35). In the mass range 0.3 M/M� < 1.0, the MF is also found to follow a power law with a shallower slope of � =− 0.58 ± 0.23 indicating a break in the slope of the IMF at ∼ 1M � . The slope of the K-band luminosity function for the cluster (r 6 arcmin) is found to be 0.31 ± 0.02, which is smaller than the average value (∼0.4) obtained for embedded star clusters. A significant number of YSOs are distributed along a Nebulous Stream towards the east side of the cluster. A small cluster is embedded in the Nebulous Stream. The YSOs lying in the Nebulous Stream and in the embedded cluster are found to be younger than the stars in the cluster Stock 8. The radio continuum, MSX, IRAS mid- and far-infrared maps and the ratio of [S II]/Hα intensities indicate that the eastern region of Stock 8 is ionization bounded whereas the western region is density bounded. The morphology seems to indicate that the ionization/shock front caused by the ionizing sources located in the Stock 8 region and westwards of Stock 8 has not reached the Nebulous Stream. It appears that star formation activity in the Nebulous Stream and embedded cluster may be independent from that of Stock 8.

Interstellar extinction in the open clusters towards galactic longitude around 130

In this paper we present a detailed study of the intra-cluster reddening material in the young open clusters located around l ∼ 130° using colour-excess diagrams and two-colour diagrams. The study supports the universality of the extinction curves for A ≥ λ J , whereas for shorter wavelengths the curve depends upon the value of the R c l u s t e r (total-to-selective absorption in the cluster region). The value of R c l u s t e r in the case of NGC 654, NGC 869 and NGC 884 is found to be normal, whereas the value of R c l u s t e r in the cluster regions NGC 1502 and IC 1805 indicates an anomalous reddening law in these regions. In the case of NGC 1502 the value of R c l u s t e r is found to be lower (2.57 ′ 0.27) whereas in the case of IC 1805 it is higher (3.56 ′ 0.29) than the normal value of 3.1. Although the intra-cluster material indicates a higher value of R c l u s t e r in the NGC 663 region, the error in the estimation of R c l u s t e r is too large to conclude anything. It is also found that the extinction process in the U band in the case of NGC 663 seems to be less efficient, whereas in the case of NGC 869 the process is more efficient.

Young stellar population and ongoing star formation in the H ii complex Sh2-252

In this paper, an extensive survey of the star-forming complex Sh2-252 has been undertaken with an aim to explore its hidden young stellar population as well as to understand the structure and star formation history for the first time. This complex is composed of five prominent embedded clusters associated with the subregions A, C, E, NGC 2175s and Teu 136. We used Two Micron All Sky Survey-near-infrared and Spitzer-Infrared Array Camera, Multiband Imaging Photometer for Spitzer photometry to identify and classify the young stellar objects (YSOs) by their infrared (IR) excess emission. Using the IR colour–colour criteria, we identified 577 YSOs, of which, 163 are Class I, 400 are Class II and 14 are transition disc YSOs, suggesting a moderately rich number of YSOs in this complex. Spatial distribution of the candidate YSOs shows that they are mostly clustered around the subregions in the western half of the complex, suggesting enhanced star formation activity towards its west. Using the spectral energy distribution and optical colour–magnitude diagram-based age analyses, we derived probable evolutionary status of the subregions of Sh2-252. Our analysis shows that the region A is the youngest (∼0.5 Myr), the regions B, C and E are of similar evolutionary stage (∼1–2 Myr) and the clusters NGC 2175s and Teu 136 are slightly evolved (∼2–3 Myr). Morphology of the region in the 1.1 mm map shows a semicircular shaped molecular shell composed of several clumps and YSOs bordering the western ionization front of Sh2-252. Our analyses suggest that next generation star formation is currently under way along this border and that possibly fragmentation of the matter collected during the expansion of the H ii region as one of the major processes is responsible for such stars. We observed the densest concentration of YSOs (mostly Class I, ∼0.5 Myr) at the western outskirts of the complex, within a molecular clump associated with water and methanol masers and we suggest that it is indeed a site of cluster formation at a very early evolutionary stage, sandwiched between the two relatively evolved CH ii regions A and B.

Interferometric Observations of the T Tauri Stars in the MBM 12 Cloud

We have carried out a millimeter interferometric continuum survey toward seven young stellar objects (YSOs) in the MBM 12 cloud. Thermal emissions associated with two YSOs were detected above the 3 σ 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 of the order of 0.05 M☉. 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 disk mass, as well as an infrared excess.

Optical and near‐infrared survey of the stellar contents associated with the star‐forming complex Sh2‐252

We present the analyses of the stellar contents associated with the extended Hii region Sh2-252 using deep optical UBVRI photometry, slit and slitless spectroscopy along with the near-infrared (NIR) data from Two-Micron All-Sky Survey (2MASS) for an area similar to 1 x 1 deg2. We have studied the sub-regions of Sh2-252, which includes four compact-H?ii (CH ii ) regions, namely A, B, C and E, and two clusters, NGC 2175s and Teutsch 136 (Teu 136). Of the 15 spectroscopically observed bright stars, eight have been identified as massive members of spectral class earlier than B3. From the spectrophotometric analyses, we derived the average distance of the region as 2.4 +/- 0.2 kpc, and the reddening E(B - V) of the massive members is found to vary between 0.35 and 2.1mag. We found that NGC 2175s and Teu 136, located towards the eastern edge of the complex, are the sub-clusters of Sh2-252. The stellar surface density distribution in K band shows clustering associated with the regions A, C, E, NGC 2175s and Teu 136. We have also identified the candidate ionizing sources of the CH?ii regions. 61 H alpha emission sources are identified using slitless spectroscopy. The distribution of the Ha emission sources and candidate young stellar objects (YSOs) with IR excess on the V/(V - I) colourmagnitude diagram (CMD) shows that a majority of them have approximate ages between 0.1 and 5Myr and masses in the range of 0.32.5 M circle dot. The optical CMDs of the candidate pre-main-sequence (PMS) sources in the individual regions also show an age spread of 0.15?Myr for each of them. We calculated the K-band luminosity functions (KLFs) for the sub-regions A, C, E, NGC 2175s and Teu 136. Within errors, the KLFs for all the sub-regions are found to be similar and comparable to that of young clusters of age \textless5?Myr. We also estimated the mass function of the PMS sample of the individual regions in the mass range of 0.32.5?M circle dot In general, the slopes of the MFs of all the sub-regions are found comparable to the Salpeter value.

Stars at the Tip of Peculiar Elephant Trunk-Like Clouds in IC 1848E: A Possible Third Mechanism of Triggered Star Formation

The HII region IC 1848 harbors a lot of intricate elephant trunk-like structures that look morphologically different from usual bright-rimmed clouds (BRCs). Of particular interest is a concentration of thin and long elephant trunk-like structures in the southeastern part of IC 1848E. Some of them have an apparently associated star (or two stars) at their very tip. We conducted

Age Sequence in Small Clusters Associated with Bright-Rimmed Clouds

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