D. K. Ojha

Triggered star formation and evolution of T-Tauri stars in and around bright-rimmed clouds

The aim of this paper is to quantitatively testify the ‘small-scale sequential star formation’ hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al., we have carried out BVI c photometry of four more BRC aggregates along with deeper re-observations of two previously observed BRCs. Again, quantitative age gradients are found in almost all the BRCs studied in the present work. Archival Spitzer/Infrared Array Camera data also support this result. The global distribution of near-infrared excess stars in each H ii region studied here clearly shows evidence that a series of radiation-driven implosion processes proceeded in the past from near the central O star(s) towards the peripheries of the H ii region. We found that in general weak-line T-Tauri stars (WTTSs) are somewhat older than classical T-Tauri stars (CTTSs). Also the fraction of CTTSs among the T-Tauri stars (TTSs) associated with the BRCs is found to decrease with age. These facts are in accordance with the recent conclusion by Bertout, Siess & Cabrit that CTTSs evolve into WTTSs. It seems that in general the equivalent width of Hα emission in TTSs associated with the BRCs decreases with age. The mass function (MF) of the aggregates associated with the BRCs of the morphological type ‘A’ seems to follow that found in young open clusters, whereas ‘B/C’-type BRCs show significantly steeper MF.

Stellar contents and star formation in the young star cluster Be 59

We present UBV I c CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be ∼10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between E(B - V) ≃ 1.4 to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as 3.7 ± 0.3. The distance of the cluster is found to be 1.00 ± 0.05 kpc. Using near-infrared (NIR) colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between < 1 and ∼2 Myr, whereas the mean age of the massive stars in the cluster region is found to be ∼2 Myr. There is evidence for second-generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, Γ, in the mass range 2.5 < M/M ⊙ ≤ 28 is found to be -1.01 ± 0.11 which is shallower than the Salpeter value (-1.35), whereas in the mass range 1.5 < M/M ⊙ ≤ 2.5 the slope is almost flat. The slope of the K-band luminosity function is estimated as 0.27 ± 0.02, which is smaller than the average value (∼0.4) reported for young embedded clusters. Approximately 32 per cent of Ha emission stars of Be 59 exhibit NIR excess indicating that inner discs of the T Tauri star (TTS) population have not dissipated. The Midcourse Space Experiment (MSX) and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust.

Star formation in bright-rimmed clouds and clusters associated with the W5 E H ii region

The aim of this paper is to present the results of photometric investigations of the central cluster of the W5 E H II region as well as a follow-up study of the triggered star formation in and around bright-rimmed clouds (BRCs). We have carried out wide-field UBVI C and deep VI C photometry of the W5 E H II region. A distance of ∼2.1 kpc and a mean age of ∼1.3 Myr have been obtained for the central cluster. The young stellar objects (YSOs) associated with the region are identified on the basis of near-infrared and mid-infrared observations. We confirmed our earlier results that the average age of the YSOs lying on/inside the rim is younger than those lying outside the rim. The global distribution of the YSOs shows an aligned distribution from the ionizing source to the BRCs. These facts indicate that a series of radiation-driven implosion processes proceeded from near the central ionizing source towards the periphery of the W5 E H II region. We found that, in general, the age distributions of the Class II and Class III sources are the same. This result is apparently in contradiction to the conclusion by Bertout, Siess & Cabrit and Chauhan et al. that classical T Tauri stars evolve to weak-line T Tauri stars. The initial mass function of the central cluster region in the mass range 0.4 < M/M ⊙ ≤ 30 can be represented by Γ = ―1.29 ± 0.04. The cumulative mass functions indicate that in the mass range 0.2 ≤ M/M ⊙ ≤ 0.8, the cluster region and BRC NW have more low-mass YSOs compared to BRCs 13 and 14.

The molecular complex associated with the Galactic H II region Sh2-90: a possible site of triggered star formation

Aims. We investigate the star formation activity in the molecular complex associated with the Galactic H II region Sh2-90. Methods. We obtain the distribution of the ionized and cold neutral gas using radio-continuum and Herschel observations. We use near-infrared and Spitzer data to investigate the stellar content of the complex. We discuss the evolutionary status of embedded massive young stellar objects (MYSOs) using their spectral energy distribution. Results. The Sh2-90 region presents a bubble morphology in the mid-infrared. Radio observations suggest it is an evolved H II region with an electron density similar to 144 cm(-3), emission measure similar to 6.7 x 10(4) cm(-6) pc and an ionized mass similar to 55 M-circle dot. From Herschel and CO (J = 3-2) observations we found that the H II region is part of an elongated extended molecular cloud (H-2 column density \textgreater= 3 x 10(21) cm(-2) and dust temperature 18-27 K) of total mass = 1 x 10(4) M-circle dot. We identify the ionizing cluster of Sh2-90, the main exciting star being an O8-O9 V star. Five cold dust clumps, four mid-IR blobs around B stars, and a compact H II region are found at the edge of the bubble. The velocity information derived from CO data cubes suggest that most of them are associated with the Sh2-90 region. One hundred and twenty-nine low mass (\textless= 3 M-circle dot) YSOs have been identified, and they are found to be distributed mostly in the regions of high column density. Four candidate Class 0/I MYSOs have been found. We suggest that multi-generation star formation is present in the complex. From evidence of interaction, time scales involved, and evolutionary status of stellar/protostellar sources, we argue that the star formation at the edges of Sh2-90 might have been triggered. However, several young sources in this complex are probably formed by some other processes.

The IMF of stellar clusters: effects of accretion and feedback

We have developed a model which describes the co-evolution of the mass function of dense gravitationally bound cores and of the stellar mass function in a protocluster clump. In the model, dense cores are injected, at a uniform rate, at different locations in the clump and evolve under the effect of gas accretion. Gas accretion on to the cores follows a time-dependent accretion rate that describes accretion in a turbulent medium. Once the accretion time-scales of cores of a given age, of a given mass and located at a given distance from the centre of the protocluster clumps exceed their contraction time-scales, they are turned into stars. The stellar initial mass function (IMF) is thus built up from successive generations of cores that undergo this accretion-collapse process. We also include the effect of feedback by the newly formed massive stars through their stellar winds. A fraction of the wind’s energy is assumed to counter gravity and disperse the gas from the protocluster and as a consequence quench further star formation. The latter effect sets the final IMF of the cluster. We apply our model to a clump that is expected to resemble the progenitor clump of the Orion Nebula Cluster (ONC). The ONC is the only known cluster for which a well-determined IMF exists for masses ranging from the sub-stellar regime to very massive stars. Our model is able to reproduce both the shape and normalization of the ONC’s IMF and the mass function of dense submillimetre cores in Orion. The complex features of the ONC’s present-day IMF, namely a shallow slope in the mass range of ∼[0.3–2.5] M� , a steeper slope in the mass range of ∼[2.5–12] M� and a nearly flat tail at the high-mass end, are reproduced. The model predicts a ‘rapid’ star formation process with an age spread for the stars of 2.3 × 10 5 yr which is consistent with the fact that 80 per cent of the ONC’s stars have ages of 0.3 Myr. The model also predicts a primordial mass segregation with the most massive stars being born in the region between two and four times the core radius of the cluster. In parallel, the model also reproduces, at the time the IMF is set and star formation quenched, the mass distribution of dense cores in the Orion star-forming complex. We study the effects of varying some of the model parameters on the resulting IMF and we show that the IMF of stellar clusters is expected to show significant variations, provided variations in the clumps’ and cores’ physical properties exist.

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.

Near-IR spectra of ISOGAL sources in the inner Galactic Bulge

We present near-IR spectra (HK-band) of a sample of 107 sources with mid-IR excesses at 7 and 15m detected during the ISOGAL survey. Making use of the DENIS interstellar extinction map from Schultheis et al. (1999) we derive luminosities and find that the Mbol vs. 12 CO and Mbol vs: H2O diagrams are powerful tools for identifying supergiants, AGB stars, giants and young stellar objects. The majority of our sample are AGB stars (80%) while we find four good supergiant candidates, nine young stellar objects and 12 RGB candidates. We have used the most recent K0 (15) relation by Jeong et al. (2002) based on recent theoretical modeling of dust formation of AGB stars to determine mass-loss rates. The mass-loss rates of the supergiants are comparable with those in the solar neighbourhood while the long-period variables cover a mass-loss range from 5< log ˙ M< 7. The red giant candidates lie at the lower end of the mass-loss rate range between 6:5< log ˙ M< 9. We used the equivalent width of the CO bandhead at 2.3m, the NaI doublet and the CaI triplet to estimate metallicities using the relation by Ram´ irez et al. (2000b). The metallicity distribution of the ISOGAL objects shows a mean (Fe=H) 0:25 dex with a dispersion of0:40 dex which is in agreement with the values of Ram´ irez et al. (2000b) for Galactic Bulge fields between b= 4 o and b= 1:3 o . A comparison with the solar neighbourhood sample of Lancon & Wood (2000) shows that our sample is0.5 dex more metal-rich on average.

A multiwavelength polarimetric study towards the open cluster NGC 1893

We present multiwavelength linear polarimetric observations for 44 stars of the NGC 1893 young open cluster region along with V-band polarimetric observations of stars of four other open clusters located between l ∼ 160° and 175°. We found evidence for the presence of two dust layers located at a distance of ∼170 and ∼360 pc. The dust layers produce a polarization P v ∼ 2.2 per cent. It is evident from the clusters studied in this work that, in the Galactic longitude range from l ∼ 160° to 175° and within the Galactic plane (|b| < 2°), the polarization angles remain almost constant, with a mean of ∼163° and a dispersion of 6°. The small dispersion in polarization angle could be due to the presence of a uniform dust layer beyond 1 kpc. Present observations reveal that in the case of NGC 1893, the foreground two dust layers, in addition to the intracluster medium, seem to be responsible for the polarization effects. It is also found that towards the direction of NGC 1893, the dust layer that exists between 2 and 3 kpc has a negligible contribution towards the total observed polarization. The weighted mean for percentage of polarization (P max ) and the wavelength at maximum polarization (λ max ) are found to be 2.59 ± 0.02 per cent and 0.55 ± 0.01 μm, respectively. The estimated mean value of λmax indicates that the average size of the dust grains within the cluster is similar to that in the general interstellar medium. The spatial variation of the polarization is found to decrease towards the outer region of the cluster. In this work, we support the notion, as has already been shown in previous studies, that polarimetry, in combination with the (U - B)-(B - V) colour-colour diagram, is a useful tool for identifying non-members in a cluster.

Study of morphology and stellar content of the Galactic H ii region IRAS 16148−5011

An investigation of the IRAS 16148−5011 region – a cluster at a distance of 3.6 kpc – is presented here, carried out using multiwavelength data in near-infrared (NIR) from the 1.4 m Infrared Survey Facility telescope, mid-infrared (MIR) from the archival Spitzer GLIMPSE (Galactic Legacy Infrared Midplane Survey Extraordinaire) survey, far-infrared (FIR) from the Herschel archive, and low-frequency radio continuum observations at 1280 and 843 MHz from the Giant Metrewave Radio Telescope and Molonglo Survey archive, respectively. A combination of NIR and MIR data is used to identify 7 Class I and 133 Class II sources in the region. Spectral energy distribution (SED) analysis of selected sources reveals a 9.6 M ⊙ high-mass source embedded in nebulosity. However, Lyman continuum luminosity calculation using radio emission – which shows a compact H ii region – indicates the spectral type of the ionizing source to be earlier than B0-O9.5. Free–free emission SED modelling yields the electron density as 138 cm −3 , and thus the mass of the ionized hydrogen as ∼16.4 M ⊙ . Thermal dust emission modelling, using the FIR data from Herschel and performing modified blackbody fits, helped us construct the temperature and column density maps of the region, which show peak values of 30 K and 3.3 × 10 22 cm −2 , respectively. The column density maps reveal an A V  > 20 mag extinction associated with the nebular emission, and weak filamentary structures connecting dense clumps. The clump associated with this IRAS object is found to have dimensions of ∼ 1.1 pc × 0.8 pc, and a mass of 1023 M ⊙ .

Young stellar population of bright-rimmed clouds BRC 5, BRC 7 and BRC 39

Bright-rimmed clouds (BRCs), illuminated and shaped by nearby OB stars, are potential sites of recent/ongoing star formation. Here we present an optical and infrared photometric study of three BRCs: BRC 5, BRC 7 and BRC 39 to obtain a census of the young stellar population, thereby inferring the star formation scenario, in these regions. In each BRC, the Class I sources are found to be located mostly near the bright rim or inside the cloud, whereas the Class II sources are preferentially outside, with younger sources closer to the rim. This provides strong support to sequential star formation triggered by radiation-driven implosion due to the ultraviolet radiation. Moreover, each BRC contains a small group of young stars being revealed at its head, as the next-generation stars. In particular, the young stars at the heads of BRC 5 and BRC 7 are found to be intermediate-/high-mass stars, which, under proper conditions, may themselves trigger further star birth, thereby propagating star formation out to long distances.