B. C. Bhatt

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.

Discovery of Main-Belt Comet P/2006 VW139 by Pan-STARRS1

The main-belt asteroid (300163) 2006 VW139 (later designated P/2006 VW139) was discovered to exhibit comet-like activity by the Pan-STARRS1 (PS1) survey telescope using automated point-spread-function analyses performed by PS1’s Moving Object Processing System. Deep follow-up observations show both a short (∼10 �� ) antisolar dust tail and a longer (∼60 �� ) dust trail aligned with the object’s orbit plane, similar to the morphology observed for another main-belt comet (MBC), P/2010 R2 (La Sagra), and other well-established comets, implying the action of a long-lived, sublimation-driven emission event. Photometry showing the brightness of the near-nucleus coma remaining constant over ∼30 days provides further evidence for this object’s cometary nature, suggesting it is in fact an MBC, and not a disrupted asteroid. A spectroscopic search for CN emission was unsuccessful, though we find an upper limit CN production rate of QCN 100 Myr, while a search for a potential asteroid family around the object reveals a cluster of 24 asteroids within a cutoff distance of 68 m s −1 .A t 70 ms −1 , this cluster merges with the Themis family, suggesting that it could be similar to the Beagle family to which another MBC, 133P/Elst-Pizarro, belongs.

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.

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.

THE RAPIDLY FLARING AFTERGLOW OF THE VERY BRIGHT AND ENERGETIC GRB 070125

We report on multiwavelength observations, ranging from X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O I, Si II, and C IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of similar to 7.0 degrees, and implying an intrinsic GRB energy in the 1-10,000 keV band of around E-gamma = (6.3-6.9) x 10(51) ergs (based on the fluences measured by the gamma-ray detectors of the IPN). GRB 070125 is among the brightest afterglows observed to date. The SED implies a host extinction of A(V) < 0. 9 mag. Two rebrightening episodes are observed, one with excellent time coverage, showing an increase in flux of 56% in similar to 8000s. The evolution of the afterglow light curve is achromatic at all times. Late-time observations of the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host galaxy samples. Evidence for strong Mg II absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight.

Be phenomenon in open clusters: Results from a survey of emission-line stars in young open clusters

Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and evolutionary state. 207 open star clusters were observed using the slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20 per cent clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0-10 and 20-30 Myr age bin (∼40 per cent each). Most of the emission stars in our survey belong to Classical Be class (∼92 per cent) while a few are Herbig Be stars (∼6 per cent) and Herbig Ae stars (∼2 per cent). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and 1624 (all 4 Myr old) while NGC 6756 (125-150 Myr) is the oldest cluster to have Classical Be stars. The Classical Be stars are located all along the main sequence (MS) in the optical colour-magnitude diagrams (CMDs) of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. The distribution of Classical Be stars as a function of spectral type shows peaks at B1-B2 and B6-B7 spectral types. The Be star fraction [N(Be)/N(B+Be)] is found to be less than 10 per cent for most of the clusters and NGC 2345 is found to have the largest fraction (∼26 per cent). Our results indicate there could be two mechanisms responsible for the Classical Be phenomenon. Some are born Classical Be stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to Classical Be stars, within the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20-30 Myr age bin.

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.

A multiwavelength census of stellar contents in the young cluster NGC 1624

We present a comprehensive multiwavelength analysis of the young cluster NGC 1624 associated with the H II region Sh2-212 using optical UBVRI photometry, optical spectroscopy and GMRT radio continuum mapping along with the near-infrared (NIR) JHK archival data. From optical observations of the massive stars, reddening E(B - V) and distance to the cluster are estimated to be 0.76-1.00 mag and 6.0 ± 0.8 kpc, respectively. The present analysis yields a spectral class of O6.5V for the main ionizing source of the region, and the maximum post-main-sequence age of the cluster is estimated as ∼4 Myr. Detailed physical properties of the young stellar objects (YSOs) in the region are analysed using a combination of optical/NIR colour-colour and colour-magnitude diagrams. The distribution of YSOs in the (J - H)/ (H - K) NIR colour-colour diagram shows that a majority of them have A V ≤ 4 mag. However, a few YSOs show A V values higher than 4 mag. Based on the NIR excess characteristics, we identified 120 probable candidate YSOs in this region, which yield a disc frequency of ∼20 per cent. However, this should be considered as a lower limit. These YSOs are found to have an age spread of ∼5 Myr with a median age of ∼2―3 Myr and a mass range of ∼0.1― 3.0 M ⊙ . A significant number of YSOs are located close to the cluster centre and we detect an enhanced density of reddened YSOs located/projected close to the molecular clumps detected by Deharveng et al. at the periphery of NGC 1624. This indicates that the YSOs located within the cluster core are relatively older in comparison to those located/projected near the clumps. From the radio continuum flux, the spectral class of the ionizing source of the ultracompact H II (UCH II) region at the periphery of Sh2-212 is estimated to be ∼B0.5V. From the optical data, the slope of the mass function (MF) Γ, in the mass range 1.2 ≤ M/M ⊙ < 27, can be represented by a single power law with a slope ―1.18 ± 0.10, whereas the NIR data in the mass range 0.65 ≤ M/M ⊙ < 27 yield Γ = ―1.31 ± 0.15. Thus the MF agrees fairly with the Salpeter value. The slope of the K-band luminosity function (KLF) for the cluster is found to be 0.30 ± 0.06, which is in agreement with the values obtained for other young clusters.

A MULTIWAVELENGTH STUDY OF STAR FORMATION IN THE VICINITY OF GALACTIC H II REGION Sh 2-100

Original article can be found at: http://iopscience.iop.org/0004-637X/ Copyright American Astronomical Society [Full text of this article is not available in the UHRA]

NGC 7419: a young open cluster with a number of very young intermediate mass pre‐MS stars

We present a photometric and spectroscopic study of the young open cluster NGC 7419, which is known to host a large number of classical Be stars for reasons not well understood. Based on CCD photometric observations of 327 stars in UBV passbands, we estimated the cluster parameters as, reddening (E(B V)) = 1.65±0.15 mag and distance = 2900 ± 400 pc. The turn-off age of the cluster was estimated as 25± 5 Myr using isochrone fits. UBV data of the stars were combined with the JHK data from 2MASS and were used to create the near infrared (NIR) (J H) vs (H K) colourcolour diagram. A large fraction of stars (42%) was found to have NIR excess and their location in the diagram was used to identify them as intermediate mass pre-MS stars. The isochrone fits to pre-MS stars in the optical colour-magnitude diagram showed that the turn-on age of the cluster is 0.3 –3 Myr. This indicates that there has been a recent episode of star formation in the vicinity of the cluster.