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Gamma rays consist of high-energy photons that selectively interact with nuclei, induce and mediate nuclear reactions and elementary particle interactions, and exceed x-rays in penetrating power and thus are indispensable for analysis and modification of dense or compressed object interior. Yet, the available gamma sources lack power and brightness which, if available, would revolutionize science and technology. The predicted laser-driven Gamma Flash (GF) would be the highest-power and the brightest terrestrial gamma source with a 30-40% laser-to-gamma energy conversion. It is based on inverse Compton scattering in a laser-solid interaction at irradiance typically above $10^{23}W/cm^2$. GF is one of the motivating goals for the most advanced laser facilities. However, till now GF remains overshadowed by simultaneously generated low-brightness Bremsstrahlung. Here we experimentally differentiate these two mechanisms and demonstrate a GF dominant regime producing several times the number of Bremsstrahlung photons. We found steep GF yield growth with the laser power and irradiance. Simulations revealed a Terawatt GF with nanometre source and an attosecond pulse train with a record brightness of $~10^{22}photons/mm^2mrad^2s0.1\%BW$ at up to tens of MeV. The small source size and high brightness paves the way towards spatially coherent gamma rays. At high photon energies, our regime is comparable in brightness to astrophysical Gamma Ray Bursts. We anticipate that the gamma ray source based on our findings will facilitate a breakthrough in research on future inertial fusion energy by enabling high-spatial-resolution time-resolved radiography of fuel mixing instabilities in extremely compressed targets. Such a new compact bright ultrafast gamma source could facilitate significant advances in time-resolved nuclear physics, homeland security, and nuclear waste management and non-proliferation., Comment: 36 pages, 11 figures

Pressure-sensitive paint (PSP) was applied to the surface of a NACA0012 airfoil to investigate pressure fluctuations associated with trailing edge (TE) noise under low-velocity flow conditions. The primary focus is to assess the feasibility of employing laser pulses exposed at the airfoil surface to mitigate TE noise. However, the weak pressure fluctuations accompanying TE noise pose a challenge, as they are overshadowed by image sensor noise in high-speed cameras capturing PSP emission changes. A novel time-resolved phase-locking technique was introduced to address this issue, utilizing the signal from a semiconductor pressure transducer at the trailing edge as a phase-lock trigger source. By repetitively conducting phase-locked measurements (1150 times), time series ensemble-averaged data based on PSP emission images were obtained, enabling the capture of these subtle pressure fluctuations. Quantitatively, fluctuations with a dominant frequency of 679 Hz and an amplitude of 50 Pa are resolved within an accuracy of about 15 Pa, achieved at a recording rate of 19.2 kHz. Both the suppression and subsequent redevelopment of the pressure field with the TE noise offer valuable insights into the dynamics of TE noise and open avenues for targeted noise reduction strategies in aerodynamic applications.

High-order harmonics are ubiquitous in nature and present in electromagnetic, acoustic, and gravitational waves. They are generated by periodic nonlinear processes or periodic high-frequency pulses. However, this periodicity is often inexact, such as that in chirped (frequency-swept) optical waveforms or interactions with nonstationary matter -- for instance, reflection from accelerating mirrors. Spectra observed in such cases contain complicated sets of harmonic-like fringes. We encountered such fringes in our experiment on coherent extreme ultraviolet generation via BISER, and could not interpret them using currently available knowledge. Here, we present a comprehensive theory based on interference of harmonics with different orders fully explaining the formation of these fringes, which we call alloharmonics. Like atomic spectra, the complex alloharmonic spectra depend on several integer numbers and bear a unique imprint of the emission process, which the theory can decipher, avoiding confusion or misinterpretation. We also demonstrate the alloharmonics in simulations of gravitational waves emitted by binary black hole mergers. Further, we predict the presence of alloharmonics in the radio spectra of pulsars and in optical frequency combs, and propose their use for measurement of extremely small accelerations necessary for testing gravity theories. The alloharmonics phenomenon generalizes classical harmonics and is critical in research fields such as laser mode locking, frequency comb generation, attosecond pulse generation, pulsar studies, and future gravitational wave spectroscopy., Comment: 29 pages, 9 figures, 3 tables

We present the results from our time-series imaging data taken with the 1.3m Devasthal fast optical telescope and 0.81m Tenagara telescope in $V$, $R_{c}$, $I_{c}$ bands covering an area of $\sim18^\prime.4\times 18^\prime.4$ towards the star-forming region Sh 2-190. This photometric data helped us to explore the nature of the variability of pre-main sequence (PMS) stars. We have identified 85 PMS variables, i.e., 37 Class II and 48 Class III sources. Forty-five of the PMS variables are showing periodicity in their light curves. We show that the stars with thicker discs and envelopes rotate slower and exhibit larger photometric variations compared to their disc-less counterparts. This result suggests that rotation of the PMS stars is regulated by the presence of circumstellar discs. We also found that the period of the stars show a decreasing trend with increasing mass in the range of $\sim$0.5-2.5 M$_\odot$. Our result indicates that most of the variability in Class II sources is ascribed to the presence of thick disc, while the presence of cool spots on the stellar surface causes the brightness variation in Class III sources. X-ray activities in the PMS stars were found to be at the saturation level reported for the main sequence (MS) stars. The younger counterparts of the PMS variables are showing less X-ray activity hinting towards a less significant role of a stellar disc in X-ray generation., Comment: Accepted for publication in ApJ

We present multi-epoch deep ($\sim$20 mag) $I_{c}$~band photometric monitoring of the Sh 2-170 star-forming region to understand the variability properties of pre-main-sequence (PMS) stars. We report identification of 47 periodic and 24 non-periodic variable stars with periods and amplitudes ranging from $\sim$4 hrs to 18 days and from $\sim$0.1 to 2.0 mag, respectively. We have further classified 49 variables as PMS stars (17 Class\,{\sc ii} and 32 Class\,{\sc iii}) and 17 as main-sequence (MS)/field star variables. A larger fraction of MS/field variables (88\%) show periodic variability as compared to the PMS variables (59\%). The ages and masses of the PMS variable stars are found to be comparable with those of T-Tauri stars. Their variability amplitudes show an increasing trend with the near-IR/mid-IR excess. The period distribution of the PMS variables shows two peaks, one near $\sim$1.5 days and the other near $\sim$4.5 days. It is found that the younger stars with thicker discs and envelopes seem to rotate slower than their older counterparts. These properties of the PMS variables support the disc-locking mechanism. Both the period and amplitude of PMS stars show decrease with increasing mass probably due to the effective dispersal of circumstellar discs in massive stars. Our results favour the notion that cool spots on weak line T-Tauri stars are responsible for most of their variations, while hot spots on classical T-Tauri stars resulting from variable mass accretion from an inner disc contribute to their larger amplitudes and irregular behaviours., Comment: Accepted for publication in MNRAS, 22 pages, 23 figures, 5 tables

New observations in the $VI$ bands along with archival data from the 2MASS and $WISE$ surveys have been used to generate a catalog of young stellar objects (YSOs) covering an area of about $6^\circ\times6^\circ$ in the Auriga region centered at $l\sim173^\circ$ and $b\sim1^\circ.5$. The nature of the identified YSOs and their spatial distribution are used to study the star formation in the region. The distribution of YSOs along with that of the ionized and molecular gas reveals two ring-like structures stretching over an area of a few degrees each in extent. We name these structures as Auriga Bubbles 1 and 2. The center of the Bubbles appears to be above the Galactic mid-plane. The majority of Class\,{\sc i} YSOs are associated with the Bubbles, whereas the relatively older population, i.e., Class\,{\sc ii} objects are rather randomly distributed. Using the minimum spanning tree analysis, we found 26 probable sub-clusters having 5 or more members. The sub-clusters are between $\sim$0.5 pc - $\sim$ 3 pc in size and are somewhat elongated. The star formation efficiency in most of the sub-cluster region varies between 5$\%$ - 20$\%$ indicating that the sub-clusters could be bound regions. The radii of these sub-clusters also support it., Comment: Accepted for publication in MNRAS, 30 Pages, 10 Tables and 18 figures

Berkeley 59 is a nearby ($\sim$1 kpc) young cluster associated with the Sh2-171 H{\sc ii} region. We present deep optical observations of the central $\sim$2.5$\times$2.5 pc$^2$ area of the cluster, obtained with the 3.58-m Telescopio Nazionale Galileo. The $V$/($V$-$I$) color-magnitude diagram manifests a clear pre-main-sequence (PMS) population down to $\sim$ 0.2 M$_\odot$. Using the near-infrared and optical colors of the low-mass PMS members we derive a global extinction of A$_V$= 4 mag and a mean age of $\sim$ 1.8 Myr, respectively, for the cluster. We constructed the initial mass function and found that its global slopes in the mass ranges of 0.2 - 28 M$_\odot$ and 0.2 - 1.5 M$_\odot$ are -1.33 and -1.23, respectively, in good agreement with the Salpeter value in the solar neighborhood. We looked for the radial variation of the mass function and found that the slope is flatter in the inner region than in the outer region, indicating mass segregation. The dynamical status of the cluster suggests that the mass segregation is likely primordial. The age distribution of the PMS sources reveals that the younger sources appear to concentrate close to the inner region compared to the outer region of the cluster, a phenomenon possibly linked to the time evolution of star-forming clouds is discussed. Within the observed area, we derive a total mass of $\sim$ 10$^3$ M$_\odot$ for the cluster. Comparing the properties of Berkeley 59 with other young clusters, we suggest it resembles more to the Trapezium cluster., Comment: 14 pages, 10 figures, accepted for publication in the Astronomical Journal

W4 is a giant H{\sc ii} region ionized by the OB stars of the cluster IC~1805. The H{\sc ii} region/cluster complex has been a subject of numerous investigations as it is an excellent laboratory for studying the feedback effect of massive stars on the surrounding region. However, the low-mass stellar content of the cluster IC~1805 remains poorly studied till now. With the aim to unravel the low-mass stellar population of the cluster, we present the results of a multiwavelength study based on deep optical data obtained with the Canada-France-Hawaii Telescope, infrared data from 2MASS, $Spitzer$ Space Telescope and X-ray data from $Chandra$ Space Telescope. The present optical dataset is complete enough to detect stars down to 0.2~M$_\odot$, which is the deepest optical observations so far for the cluster. We identified 384 candidate young stellar objects (YSOs; 101 Class I/II and 283 Class III) within the cluster using various colour-colour and colour-magnitude diagrams. We inferred the mean age of the identified YSOs to be $\sim$ 2.5 Myr and mass in the range 0.3 - 2.5 M$_\odot$. The mass function of our YSO sample has a power law index of -1.23 $\pm$ 0.23, close to the Salpeter value (-1.35), and consistent with those of other star-forming complexes. We explored the disk evolution of the cluster members and found that the diskless sources are relatively older compared to the disk bearing YSO candidates. We examined the effect of high-mass stars on the circumstellar disks and found that within uncertainties, the influence of massive stars on the disk fraction seems to be insignificant. We also studied the spatial correlation of the YSOs with the distribution of gas and dust of the complex to conclude that IC 1805 would have formed in a large filamentary cloud., Comment: Accepted for publication in MNRAS; 34 pages, 10 figures

Deep optical photometric data on the NGC 7538 region were collected and combined with archival data sets from $Chandra$, 2MASS and {\it Spitzer} surveys in order to generate a new catalog of young stellar objects (YSOs) including those not showing IR excess emission. This new catalog is complete down to 0.8 M$_\odot$. The nature of the YSOs associated with the NGC 7538 region and their spatial distribution are used to study the star formation process and the resultant mass function (MF) in the region. Out of the 419 YSOs, $\sim$91\% have ages between 0.1 to 2.5 Myr and $\sim$86\% have masses between 0.5 to 3.5 M$_\odot$, as derived by spectral energy distribution fitting analysis. Around 24\%, 62\% and 2\% of these YSOs are classified to be the Class I, Class II and Class III sources, respectively. The X-ray activity in the Class I, Class II and Class III objects is not significantly different from each other. This result implies that the enhanced X-ray surface flux due to the increase in the rotation rate may be compensated by the decrease in the stellar surface area during the pre-main sequence evolution. Our analysis shows that the O3V type high mass star `IRS 6' might have triggered the formation of young low mass stars up to a radial distance of 3 pc. The MF shows a turn-off at around 1.5 M$_\odot$ and the value of its slope `$\Gamma$' in the mass range $1.5 <$M/M$_\odot < 6$ comes out to be $-1.76\pm0.24$, which is steeper than the Salpeter value., Comment: 25 pages, 10 tables, 26 figures, accepted in MNRAS main journal for publication

In various media the elementary components can emit traveling waves such as electromagnetic, gravitational or acoustic types. If these elementary emitters are synchronized, the resulting emission is coherent. Moreover, the faster the emitters approach an observer, the more intense and directional their apparent emission is, with associated frequency increase. Multi-stream flows ubiquitously occur in media (such as with shock waves and jets in astrophysical and laboratory plasmas) and produce fast moving density singularities, where high concentration and synchronism can bring constructive interference. However, a singularity emitting such characteristic coherent radiation has not been demonstrated yet. We show this general phenomenon in laser-driven relativistic plasma, which is an ideal medium for realizing these effects in the laboratory under controllable conditions. Our experiments and simulations reveal bright coherent soft x-ray radiation from nanoscale electron density singularities in multi-stream plasma. They constitute a new compact x-ray source of ultrashort duration, demanded in numerous applications. In general, singularities can be bright sources of other types of traveling waves. Thus our findings open new opportunities in different fields of science. For example, gravitational wave generation, as proposed in ultrahigh-energy accelerators, can be significantly enhanced by intentionally induced density singularities in the particle bunches. Further, we anticipate that multi-stream flows in cosmic media can produce intense bursts of coherent electromagnetic and/or gravitational waves, especially at longer wavelengths which facilitate constructive interference. We can then expect to observe more directional short wavelength bursts from cosmic emitters approaching at relativistic speeds. Thus, we present a new framework for interpreting a broad range of experimental results.

We present a multiwavelength investigation of the young stellar population and star formation activities around the \hii region Sharpless 311. Using our deep near-infrared observations and archival {\it Spitzer}-IRAC observations, we have detected a total of 125 young stellar objects (YSOs) in an area of $\sim$86 arcmin$^2$. The YSOs sample include 8 Class I and 117 Class II candidate YSOs. The mass completeness of the identified YSOs sample is estimated to be 1.0 \msun. The ages and masses of the majority of the candidate YSOs are estimated to be in the range of $\sim$0.1$-$5 Myr and $\sim$0.3$-$6 \msun, respectively. The 8 \mum image of S311 displays an approximately spherical cavity around the ionizing source which is possibly created due to the expansion of the \hii region. The spatial distribution of the candidate YSOs reveals that a significant number of them are distributed systematically along the 8 $\mu$m emission with a majority clustered around the eastern border of the \hii region. Four clumps/compact \hii regions are detected in the radio continuum observations at 1280 MHz, which might have been formed during the expansion of the \hii region. The estimated dynamical age of the region, main-sequence lifetime of the ionizing source, the spatial distribution and ages of the candidate YSOs indicate triggered star formation in the complex., Comment: 21 pages, 5 tables, 15 figures, Monthly Notices of the Royal Astronomical Society (2016)

We carried out deep and wide-field near- and mid-infrared observations for a sample of 8 bright-rimmed clouds (BRCs). Supplemented with the $Spitzer$ archival data, we have identified and classified 44 to 433 young stellar objects (YSOs) associated with these BRCs. The Class I sources are generally located towards the places with higher extinction and are relatively closer to each other than the Class II sources, confirming that the young protostars are usually found in regions having denser molecular material. On the other hand the comparatively older population, Class II objects, are more randomly found throughout the regions, which can be due to their dynamical evolution. Using the minimal sampling tree analyses, we have extracted 13 stellar cores of 8 or more members, which contains 60\% of the total YSOs. The typical core is $\sim$0.6 pc in radii and somewhat elongated (aspect ratio of 1.45), of relatively low stellar density (surface density 60 pc$^{-2}$), consisting of a small (35) number of YSOs of relatively young sources (66\% Class I ), and partially embedded (median $A_K$ =1.1 mag). But the cores show a wide range in their mass distribution ($\sim$20 to 2400 M$_\odot$) with a median value of around 130 M$_\odot$. We have found the star formation efficiencies in the cores to be between 3\% and 30\% with an average of $\sim$14\%, which agree with the efficiencies needed to link the core mass function to the initial mass function. We also found a linear relation between the density of the clouds and the number of YSOs. The peaked nearest neighbor spacing distributions of the YSOs and the ratio of Jeans lengths to the YSOs separations indicates a significant degree of non-thermally driven fragmentation in these BRCs., Comment: 43 pages, 25 figures, 6 tables, accepted for publishing in the Astronomical Journal. arXiv admin note: text overlap with arXiv:0906.0201, arXiv:1403.3082 by other authors

Dependence of the energy of ions accelerated during interaction of the laser pulse obliquelly incident on the thin foil target on the laser polarization is studied experimentally and theoretically. We found that the ion energy being maximal for the p-polarization gradually decreases when the pulse becomes s-polarized. The experimentally found dependences of the ion energy are explained by invoking the anomalous electron heating which results in high electrostatic potential formation at the target surface. Anomalous heating of electrons beyond the energy of quiver motion in the laser field is described within the framework of theoretical model of driven oscillator with a step-like nonlinearity. We have demonstrated that the electron anomalous heating can be realized in two regimes: nonlinear resonance and stochastic heating, depending on the extent of stochasticity. We have found the accelerated ion energy scaling determined by the laser intensity, pulse duration, polarization angle and incident angle.

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 UV 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., Comment: 30 pages, 7 Figures, 6 Tables, accepted for publication in Monthly Notices of the Royal Astronomical Society

The stellar content in and around Sh2-311 region have been studied using the deep optical observations as well as near-infrared (NIR) data from 2MASS. The region contains three clusters, viz. NGC 2467, Haffner 18 and Haffner 19. We have made an attempt to distinguish the stellar content of these individual regions as well as to re-determine their fundamental parameters such as distance, reddening, age, onto the basis of a new and more extended optical and infrared photometric data set. NGC 2467 and Haffner 19 are found to be located in the Perseus arm at the distances of 5.0 $\pm$ 0.4 kpc and 5.7 $\pm$ 0.4 kpc, respectively, whereas Haffner 18 is located at the distance of 11.2 $\pm$ 1.0 kpc. The clusters NGC 2467 and Haffner 19 might have formed from the same molecular cloud, whereas the cluster Haffner 18 is located in the outer galactic arm, i.e. the Norma-Cygnus arm. We identify 8 class II young stellar objects (YSOs) using the NIR $(J - H)/(H - K)$ two colour diagram. We have estimated the age and mass of the YSOs identified in the present work and those by Snider et al. (2009) using the $V/(V - I)$ colour-magnitude diagram. The estimated ages and mass range of the majority of the YSOs are $\lesssim$1 Myr and $\sim$0.4 - 3.5 \msun, respectively, indicating that these sources could be T-Tauri stars or their siblings. Spatial distribution of the YSOs shows that some of the YSOs are distributed around the H II region Sh2-311, suggesting a triggered star formation at its periphery., Comment: 19 pages, 13 figures, 9 table; Accepted for publication in New Astronomy

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. This complex is composed of five embedded clusters associated with the sub-regions A, C, E, NGC 2175s and Teu 136. Using 2MASS-NIR and Spitzer-IRAC, MIPS photometry we identified 577 young stellar objects (YSOs), of which, 163 are Class I, 400 are Class II and 14 are transition disk YSOs. Spatial distribution of the candidate YSOs shows that they are mostly clustered around the sub-regions 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 sub-regions 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 semi-circular 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 HII region as one of the major processes 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 CHII regions A and B., Comment: 19 pages, 13 figures, Accepted for publication in MNRAS

We present the analyses of the stellar contents associated with the HII region Sh2-252 using UBVRI photometry, slit and slitless spectroscopy along with the NIR data from 2MASS for an area ~1 degree x 1 degree. We studied the sub-regions of Sh2-252 which includes four compact-HII (CHII) regions, namely A, B, C and E and two clusters NGC 2175s and Teutsch 136 (Teu 136). Of the fifteen spectroscopically observed bright stars, eight have been identified as massive members of spectral class earlier than B3. From the spectro-photometric analyses, we derived the average distance of the region as 2.4+/-0.2 kpc and the reddening of the massive members is found to vary between 0.35 to 2.1 mag. 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 CHII regions. 61 H_alpha emission sources are identified using slitless spectroscopy. The distribution of the H_alpha emission sources and candidate YSOs with IR excess on the V/(V-I) CMD shows that a majority of them have approximate ages between 0.1 - 5 Myr and masses in the range of 0.3 - 2.5 M_sun. The CMDs of the candidate YSOs in the individual regions also show an age spread of 0.1 - 5 Myr for each of them. We calculated the 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 < 5 Myr. We also estimated the mass functions (MFs) of the PMS sample of the individual regions in the mass range of 0.3 - 2.5 M_sun. In general, the slopes of the MFs of all the sub-regions are found comparable to the Salpeter value., Comment: published in MNRAS

We present a multiwavelength study of the NGC 281 complex which contains the young cluster IC 1590 at the center, using deep wide-field optical UBVI_c photometry, slitless spectroscopy along with archival data sets in the near-infrared (NIR) and X-ray. The extent of IC 1590 is estimated to be ~6.5 pc. The cluster region shows a relatively small amount of differential reddening. The majority of the identified young stellar objects (YSOs) are low mass PMS stars having age <1-2 Myr and mass 0.5-3.5 M_\odot. The slope (\Gamma) of the mass function for IC 1590, in the mass range 2 < M/M_\odot \le 54, is found to be -1.11+-0.15. The slope of the K-band luminosity function (0.37+-0.07) is similar to the average value (~0.4) reported for young clusters. The distribution of gas and dust obtained from the IRAS, CO and radio maps indicates clumpy structures around the central cluster. The radial distribution of the young stellar objects, their ages, \Delta(H-K) NIR-excess, and the fraction of classical T Tauri stars suggest triggered star formation at the periphery of the cluster region. However, deeper optical, NIR and MIR observations are needed to have a conclusive view of star formation scenario in the region. The properties of the Class 0/I and Class II sources detected by using the Spitzer mid-infrared observations indicate that a majority of the Class II sources are X-ray emitting stars, whereas X-ray emission is absent from the Class 0/I sources. The spatial distribution of Class 0/I and Class II sources reveals the presence of three sub-clusters in the NGC 281 West region., Comment: 29 pages, 21 figures and 11 tables, Accepted for the publication in PASJ

We demonstrate a new high-order harmonic generation mechanism reaching the `water window' spectral region in experiments with multi-terawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving uJ/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.

The aim of this paper is to present the results of photometric investigations of the central cluster of the W5 E 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{\sc ii} region. A distance of $\sim$2.1 kpc and a mean age of $\sim$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 are younger than those lying outside the rim. The global distribution of the YSOs shows an aligned distribution from the ionising source to the BRCs. These facts indicate that a series of radiation driven implosion processes proceeded from near the central ionising source towards the periphery of the W5 E H{\sc 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 with the conclusion by Bertout, Siess & Cabrit (2007) and Chauhan et al. (2009) 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 $0.4 \le M/M_\odot \le 30$ can be represented by $\Gamma = -1.29 \pm 0.03$. The cumulative mass functions indicate that in the mass range $0.2 \le M/M_\odot \le 0.8$, the cluster region and BRC NW have more low mass YSOs in comparison to BRCs 13 and 14., Comment: 43 pages, 17 figures, accepted for publication in MNRAS

We present a comprehensive multi-wavelength 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. 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. Present analysis yields a spectral class of O6.5V for the main ionizing source of the region. The distribution of YSOs in (J-H)/ (H-K) NIR colour-colour diagram shows that a majority of them have A_V $\le$ 4 mag. Based on the NIR excess characteristics, we identified 120 probable candidate YSOs in this region which yield a disk frequency of ~ 20%. 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 $M_\odot$. 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 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, spectral class of the ionizing source of the ultra-compact H II region at the periphery of Sh2-212 is estimated to be ~ B0.5V. From optical data, slope of the mass function (MF) $\Gamma$, in the mass range $1.2 \le M/M_{\odot}<27$ can be represented by a single power law with a slope -1.18 +/- 0.10, whereas the NIR data in the mass range $0.65 \le M/M_{\odot}<27$ yields $\Gamma$ = -1.31 +/- 0.15. 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., Comment: Accepted for publication in MNRAS

The aim of this paper is to quantitatively testify the "{\it small-scale sequential star formation}" hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al. (2007, Paper I), we have carried out $BVI_{c}$ photometry of four more BRC aggregates along with deeper re-observations of 2 previously observed BRCs. Again quantitative age gradients are found in almost all the BRCs studied in the present work. Archival Spitzer/IRAC data also support this result. The global distribution of NIR excess stars in each HII region studied here clearly shows evidence that a series of radiation driven implosion (RDI) processes proceeded in the past from near the central O star(s) towards the peripheries of the HII 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 et al. (2007) that CTTSs evolve into WTTSs. It seems that in general the EW of H$\alpha$ 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., Comment: Accepted for publication in MNRAS, 29 pages, 18 figures

We demonstrate generation of 10-20 MeV/u ions with a compact 4 TW laser using a gas target mixed with submicron clusters, corresponding to tenfold increase in the ion energies compared to previous experiments with solid targets. It is inferred that the high energy ions are generated due to formation of a strong dipole vortex structure. The demonstrated method has a potential to construct compact and high repetition rate ion sources for hadron therapy and other applications., Comment: 7 pages, 4 figures

Using homogeneous CCD photometric data from the 105-cm Kiso Schmidt telescope covering a 50' x 50' field, we study the mass functions (MFs) of nine open clusters. The ages and Galactocentric distances of the target clusters vary from 16 - 2000 Myr and 9-10.8 kpc, respectively. The values of MF slopes vary from -1.1 to -2.1. The classical value derived by Salpeter (1955) for the slope of the IMF is \Gamma = -1.35. The MFs in the outer regions of the clusters are found to be steeper than in the inner regions, indicating the presence of mass segregation in the clusters.The MF slopes (in the outer region as well as the whole cluster) undergo an exponential decay with the evolutionary parameter \tau (= age/ relaxation time). It seems that the evaporation of low-mass members from outer regions of the clusters is not significant at larger Galactocentric distances. It is concluded that the initial mass function (IMF) in the anticentre direction of the Galaxy might have been steeper than the IMF in the opposite direction. A comparison of the observed CMDs of the clusters with synthetic CMDs gives a photometric binary content of ~40%., Comment: Accepted for the publication in AJ, 21 Pages, 11 Figures, 9 Tables

We present $UBVI_c$ CCD photometry of the young open cluster Stock 8 with the aim to study the basic properties and star formation scenario in this region. The radius of the cluster is found to be $\sim 6^{\prime}$ ($\sim 3.6$ 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 $2.05 \pm 0.10$ kpc. Using H$\alpha$ slitless spectroscopy and 2MASS NIR data we identified H$\alpha$ emission and NIR excess young stellar objects (YSOs), respectively. The colour-magnitude diagrams of these YSOs reveal that majority of these objects have ages between 1 to 5 Myr indicating a non-coeval star formation in the cluster. Massive stars in the cluster region reveal an average age of $\le$ 2 Myr. In the cluster region ($r \le 6^\prime$) the slope of the mass function (MF), $\Gamma$, in the mass range $\sim 1.0 \le M/M_\odot < 13.4$ can be represented by a power law having a slope of $-1.38\pm0.12$, which agrees well with Salpeter value (-1.35). In the mass range $0.3 \le M/M_\odot < 1.0$, the MF is also found to follow a power law with a shallower slope of $\Gamma = -0.58\pm 0.23$ indicating a break in the slope of the IMF at $\sim 1 M_\odot$. The slope of the $K$-band luminosity function for the cluster ($r \le 6^\prime$) is found to be $0.31\pm0.02$, which is smaller than the average value ($\sim$ 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. It appears that star formation activity in the Nebulous Stream and embedded cluster may be independent from that of Stock 8., Comment: 48 pages, 29 figures, accepted for publication in MNRAS

We present $UBVI_C$ 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 $\sim$ 10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between $E(B-V) \simeq$ 1.4 to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as $3.7\pm0.3$. The distance of the cluster is found to be $1.00\pm0.05$ kpc. Using near-infrared 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 $<1$ Myr to $\sim$ 2 Myr, whereas the mean age of the massive stars in the cluster region is found to be $\sim$ 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, $\Gamma$, in the mass range $2.5 < M/M_\odot \le 28$ is found to be $-1.01\pm0.11$ which is shallower than the Salpeter value (-1.35), whereas in the mass range $1.5 < M/M_\odot \le 2.5$ the slope is almost flat. The slope of the K-band luminosity function is estimated as $0.27\pm0.02$, which is smaller than the average value ($\sim$0.4) reported for young embedded clusters. Approximately 32% of H$\alpha$ emission stars of Be 59 exhibit NIR excess indicating that inner disks of the T-Tauri star (TTS) population have not dissipated. The 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., Comment: Accepted for the publication in MNRAS, 39 pages, 26 Figures, 8 Tables

Since the advent of chirped pulse amplification1 the peak power of lasers has grown dramatically and opened the new branch of high field science, delivering the focused irradiance, electric fields of which drive electrons into the relativistic regime. In a plasma wake wave generated by such a laser, modulations of the electron density naturally and robustly take the shape of paraboloidal dense shells, separated by evacuated regions, moving almost at the speed of light. When we inject another counter-propagating laser pulse, it is partially reflected from the shells, acting as relativistic flying (semi-transparent) mirrors, producing an extremely time-compressed frequency-multiplied pulse which may be focused tightly to the diffraction limit. This is as if the counterstreaming laser pulse bounces off a relativistically swung tennis racket, turning the ball of the laser photons into another ball of coherent X-ray photons but with a form extremely relativistically compressed to attosecond and zeptosecond levels. Here we report the first demonstration of the frequency multiplication detected from the reflection of a weak laser pulse in the region of the wake wave generated by the driver pulse in helium plasma. This leads to the possibility of very strong pulse compression and extreme coherent light intensification. This Relativistic Tennis with photon beams is demonstrated leading to the possibility toward reaching enormous electromagnetic field intensification and finally approaching the Schwinger field, toward which the vacuum nonlinearly warps and eventually breaks, producing electron-positron pairs., Comment: 17 pages, 5 figures

We present colour-magnitude diagrams of open clusters, located in the range $112^\circ < l < 252^\circ$, manifesting stellar populations in the background of clusters. Some of the populations are found to be located beyond the Perseus arm and may be the part of Norma-Cygnus (outer) arm. The outer arm seems to be continued from $l\sim120^\circ$ to $l\sim235^\circ$. The background populations follow the downward warp of the Galactic plane around $l\sim240^\circ$., Comment: Accepted for the publication in MNRAS, 9 pages, 9 figures, 2 tables

In this paper we study the evolution of core and corona of nine open clusters using the projected radial density profiles derived from homogeneous CCD photometric data obtained through the 105-cm Kiso Schmidt telescope. The age and galactocentric distance of the target clusters varies from 16 Myr to 2000 Myr and 9 kpcto 10.8 kpc respectively. Barring Be 62, which is young open cluster, other clusters show a uniform reddening across the cluster region. The reddening in Be 62varies from $E(B-V)_{min}$= 0.70 mag to $E(B-V)_{max}$= 1.00 mag. The corona of six of the clusters in the present sample is found to be elongated, however on the basis of the present sample it is not possible to establish any correlation between the age and shape of the core. The elongated core in the case of young cluster Be 62 may reflect the initial conditions in the parental molecular cloud. The other results of the present study are (i) Core radius `$r_c$' and corona size $`r_{cn}$'/cluster radius $`r_{cl}$' are linearly correlated. (ii) The $r_c/r_{cn}/r_{cl}$ are linearly correlated with the number of stars in that region. (iii) In the age range 10-1000 Myr, the core and corona shrink with age. (iv) We find that in the galactocentric distance range 9 - 10 kpc, the core and corona/cluster extent of the clusters increase with the galactocentric distance., Comment: 53 pages, 14 figures, accepted for the publication in AJ, contact saurabh@aries.ernet.in for high resolution figures

The regimes of quasi-mono-energetic electron beam generation were experimentally studied in the sub-relativistic intensity laser plasma interaction. The observed electron acceleration regime is unfolded with two-dimensional-particle-in-cell simulations of laser-wakefield generation in the self-modulation regime., Comment: 10 pages, 5 figures

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., Comment: 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 a deep JHKs-band imaging survey of the W3 Main star forming region, using the near-infrared camera, SIRIUS, mounted on the University of Hawaii 2.2m telescope. The near-infrared survey covers an area of ~ 24 sq. arcmin with 10 sigma limiting magnitudes of ~ 19.0, 18.1, and 17.3 in J, H, and Ks-band, respectively. We construct JHK color-color and J/J-H and K/H-K color-magnitude diagrams to identify young stellar objects and estimate their masses. Based on these color-color and color-magnitude diagrams, a rich population of YSOs is identified which is associated with the W3 Main region. A large number of previously unreported red sources (H-K > 2) have also been detected around W3 Main. We argue that these red stars are most probably pre-main sequence stars with intrinsic color excesses. We find that the slope of the Ks-band luminosity function of W3 Main is lower than the typical values reported for the young embedded clusters. The derived slope of the KLF is the same as that found by Megeath et al. (1996), from which analysis by Megeath et al. indicates that the W3 Main region has an age in the range of 0.3--1 Myr. Based on the comparison between models of pre-main sequence stars with the observed color-magnitude diagram we find that the stellar population in W3 Main is primarily composed of low mass pre-main sequence stars. We also report the detection of isolated young stars with large infrared excesses which are most probably in their earliest evolutionary phases., Comment: 30 pages, 11 figures in JPEG format. Accepted for the publication in ApJ. The preprint is also available at http://www.tifr.res.in/~ojha/W3Main.html

In this paper we present a detailed study of the intra-cluster reddening material in the young open clusters located around $l \sim 130^o$ using colour-excess diagrams and two-colour diagrams. The study supports the universality of the extinction curves for $\lambda \geq \lambda_J$, whereas for shorter wavelengths the curve depends upon the value of the $R_{cluster}$ (total-to-selective absorption in the cluster region). The value of $R_{cluster}$ in the case of NGC 654, NGC 869 and NGC 884 is found to be normal, whereas the value of $R_{cluster}$ in the cluster regions NGC 1502 and IC 1805 indicates an anomalous reddening law in these regions. 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., Comment: 11 pages, 13 figures, accepted in A&A