arXiv subjects

Saurabh Sharma

Publications and source records attributed to Saurabh Sharma.

At least 109 records · Page 6Linked to original sources

arXiv2019

Polarimetric and photometric investigation of a dark globule LDN 1225: distance, extinction law, and magnetic fields

We present the results based on the optical $R$-band polarization observations of 280 stars distributed towards the dark globule LDN\,1225. {\it Gaia} data release 2 parallaxes along with the polarization data of $\sim$200 stars have been used to (a) constrain the distance of LDN\,1225 as 830$\pm$83~pc, (b) determine the contribution of interstellar polarization (ISP), and (c) characterize the dust properties and delineate the magnetic field (B-field) morphology of LDN\,1225. We find that B-fields are more organized and exhibit a small dispersion of 12$\degr$. Using the $^{12}$CO molecular line data from the Purple Mountain Observatory (PMO), along with the column density, dispersion in B-fields, we estimate B-field strength to be $\sim$56\,$\pm$\,10\,$μ$G, magnetic to turbulence pressure to be $\sim$3\,$\pm$\,2, and the mass-to-magnetic flux ratio (in units of critical value) to be~$<$\,1. These results indicate the dominant role of B-fields in comparison to turbulence and gravity in rendering the cloud support. B-fields are aligned parallel to the low-density parts (traced by $^{12}$CO map) of the cloud, in contrast they are neither parallel nor perpendicular to the high-density core structures (traced by $^{13}$CO and C$^{18}$O maps). LDN\,1225 hosts two 70\,$μ$m sources which seem to be of low-mass Class 0 sources. The total-to-selective extinction derived using optical and near-infrared photometric data is found to be anomalous ($R_{V}$~$=$~3.4), suggesting dust grain growth in LDN\,1225. Polarization efficiency of dust grains follows a power-law index of $-$0.7 inferring that optical polarimetry traces B-fields in the outer parts of the cloud.

astro-ph.GA↗

arXiv2019

Understanding formation of young, distributed low-mass stars and clusters in the W4 cloud complex

It is well known that most of the stars form in rich clusters. However, recent $Spitzer$ observations have shown that a significant number of stars also form in distributed mode, origin of which is not well understood. In this work, we aim to investigate clustered and distributed mode of star formation in the W4 complex. To do so, we identified and characterized the young stellar population associated with the region using homogeneous infra-red data-sets obtained from 2MASS, GLIMPSE, MIPS and WISE surveys. We make stellar surface density and minimum spanning tree maps to identify young clusters, and use {\it Spitzer} images to identify irradiated structures, such as elephant trunk-like structures (ETLSs) and pillars in the region. The surface density distribution of the young stellar objects (YSOs) reveals three new clusterings and $\sim$ 50\% distributed protostars in the H{\sc ii} region. The clusters are of low-mass nature but significantly younger than the central cluster IC~1805. We identified $\sim$ 38 ETLSs in the region, a majority of which consist of one or a few stars at their tips. We find these stars are low-mass ($<$ 2~M$_\odot$) YSOs, located at the outskirts ($>$ 17 pc) of the cluster IC~1805 and are part of scattered distributed population. We argued that the star formation in the ETLSs of W4 is going on possibly due to triggering effect of expanding W4 bubble. Although high-resolution photometric and spectroscopic data would be required to confirm the scenario, nonetheless, we discuss the implications of this scenario for our understanding of distributed low-mass star formation in cloud complexes as opposed to other mechanisms such as turbulent fragmentation and dynamical ejection.

astro-ph.SR↗

arXiv2017

Star formation in the Sh 2-53 region influenced by accreting molecular filaments

We present a multi-wavelength analysis of a $\sim$30$' \times$30$'$ area around the Sh 2-53 region (hereafter S53 complex), which is associated with at least three H II regions, two mid-infrared bubbles (N21 and N22), and infrared dark clouds. The $^{13}$CO line data trace the molecular content of the S53 complex in a velocity range of 36--60 km s$^{-1}$, and show the presence of at least three molecular components within the selected area along this direction. Using the observed radio continuum flux of the H II regions, the derived spectral types of the ionizing sources agree well with the previously reported results. The S53 complex harbors clusters of young stellar objects (YSOs) that are identified using the photometric 2--24 $μ$m magnitudes. It also hosts several massive condensations (3000-30000 $M_\odot$) which are traced in the {\em Herschel} column density map. The complex is found at the junction of at least five molecular filaments, and the flow of gas toward the junction is evident in the velocity space of the $^{13}$CO data. Together, the S53 complex is embedded in a very similar "hub-filament" system to those reported in Myers, and the active star formation is evident towards the central "hub" inferred by the presence of the clustering of YSOs.

astro-ph.GA↗

arXiv2017

Star-disk interactions in multi-band photometric monitoring of the classical T Tauri star GI Tau

The variability of young stellar objects is mostly driven by star-disk interactions. In long-term photometric monitoring of the accreting T Tauri star GI Tau, we detect extinction events with typical depths of $ΔV \sim 2.5$ mag that last for days-to-months and often appear to occur stochastically. In 2014 - 2015, extinctions that repeated with a quasi-period of 21 days over several months is the first empirical evidence of slow warps predicted from MHD simulations to form at a few stellar radii away from the central star. The reddening is consistent with $R_V=3.85\pm0.5$ and, along with an absence of diffuse interstellar bands, indicates that some dust processing has occurred in the disk. The 2015 -- 2016 multi-band lightcurve includes variations in spot coverage, extinction, and accretion, each of which results in different traces in color-magnitude diagrams. This lightcurve is initially dominated by a month-long extinction event and return to the unocculted brightness. The subsequent light-curve then features spot modulation with a 7.03 day period, punctuated by brief, randomly-spaced extinction events. The accretion rate measured from $U$-band photometry ranges from $1.3\times10^{-8}$ to $1.1\times10^{-10}$ M$_\odot$ yr$^{-1}$ (excluding the highest and lowest 5% of high- and low- accretion rate outliers), with an average of $4.7 \times 10^{-9}$ M$_\odot$ yr$^{-1}$. A total of 50% of the mass is accreted during bursts of $>12.8\times10^{-9}$ M$_\odot$ yr${^{-1}}$, which indicates limitations on analyses of disk evolution using single-epoch accretion rates.

astro-ph.SR↗

arXiv2017

The stellar contents and star formation in the NGC 7538 region

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 `$Γ$' 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.

astro-ph.SR↗

arXiv2016

A multiwavelength investigation of the HII region S311: Young stellar population and star formation

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 $μ$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.

astro-ph.GA↗

arXiv2016

Structural studies of eight bright rimmed clouds in the southern hemisphere

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.

astro-ph.SR↗

arXiv2014

Pre-processing of Domain Ontology Graph Generation System in Punjabi

This paper describes pre-processing phase of ontology graph generation system from Punjabi text documents of different domains. This research paper focuses on pre-processing of Punjabi text documents. Pre-processing is structured representation of the input text. Pre-processing of ontology graph generation includes allowing input restrictions to the text, removal of special symbols and punctuation marks, removal of duplicate terms, removal of stop words, extract terms by matching input terms with dictionary and gazetteer lists terms.

cs.CL↗

arXiv2014

New galactic star clusters discovered in the VVV survey. Candidates projected on the inner disk and bulge

VISTA Variables in the Vía Láctea (VVV) is one of six ESO Public Surveys using the 4 meter Visible and Infrared Survey Telescope for Astronomy (VISTA). The VVV survey covers the Milky Way bulge and an adjacent section of the disk, and one of the principal objectives is to search for new star clusters within previously unreachable obscured parts of the Galaxy. The primary motivation behind this work is to discover and analyze obscured star clusters in the direction of the inner Galactic disk and bulge. Regions of the inner disk and bulge covered by the VVV survey were visually inspected using composite JHKs color images to select new cluster candidates on the basis of apparent overdensities. DR1, DR2, CASU, and PSF photometry of 10x10 arcmin fields centered on each candidate cluster were used to construct color-magnitude and color-color diagrams. Follow-up spectroscopy of the brightest members of several cluster candidates was obtained in order to clarify their nature. We report the discovery of 58 new infrared cluster candidates. Fundamental parameters such as age, distance, and metallicity were determined for 20 of the most populous clusters.

astro-ph.GA↗

arXiv2014

VISTA Variables in the Vía Láctea (VVV): Halfway Status and Results

The VISTA Variables in the Vía Láctea (VVV) survey is one of six public ESO surveys, and is now in its 4th year of observing. Although far from being complete, the VVV survey has already delivered many results, some directly connected to the intended science goals (detection of variables stars, microlensing events, new star clusters), others concerning more exotic objects, e.g. novae. Now, at the end of the fourth observing period, and comprising roughly 50% of the proposed observations, the actual status of the survey, as well some of the results based on the VVV data, are presented.

astro-ph.IM↗

arXiv2014

Deep optical survey of the stellar content of Sh2-311 region

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.

astro-ph.SR↗

arXiv2014

Investigation of the stellar content in the western part of the Carina nebula

We obtained deep $UBVRI$ H$α$ photometric data of the field situated to the west of the main Carina nebula and centered on WR~22. Medium-resolution optical spectroscopy of a subsample of X-ray selected objects along with archival data sets from $Chandra$, $XMM-Newton$ and 2MASS surveys were used for the present study. Different sets of color-color and color-magnitude diagrams are used to determine reddening for the region and to identify young stellar objects (YSOs) and estimate their age and mass. Our spectroscopic results indicate that the majority of the X-ray sources are late spectral type stars. The region shows a large amount of differential reddening with minimum and maximum values of $E(B-V)$ as 0.25 and 1.1 mag, respectively. Our analysis reveals that the total-to-selective absorption ratio $R \rm_{V}$ is $\sim$3.7 $\pm$ 0.1, suggesting an abnormal grain size in the observed region. We identified 467 YSOs and studied their characteristics. The ages and masses of the 241 optically identified YSOs range from $\sim$0.1 to 10 Myr and $\sim$0.3 to 4.8M$_\odot$, respectively. However, the majority of them are younger than 1 Myr and have masses below 2 M$_\odot$. The high mass star WR 22 does not seem to have contributed to the formation of YSOs in the CrW region. The initial mass function slope, $Γ$, in this region is found to be -1.13 $\pm$ 0.20 in the mass range of 0.5 < M/M$_\odot$ < 4.8. The $K$-band luminosity function slope ($α$) is also estimated as 0.31 $\pm$ 0.01. We also performed minimum spanning tree analysis of the YSOs in this region, which reveals that there are at least ten YSO cores associated with the molecular cloud, and that leads to an average core radius of 0.43 pc and a median branch length of 0.28 pc.

astro-ph.SR↗

arXiv2014

Main-sequence variable stars in young open cluster NGC 1893

In this paper we present time series photometry of 104 variable stars in the cluster region NGC 1893. The association of the present variable candidates to the cluster NGC 1893 has been determined by using $(U-B)/(B-V)$ and $(J-H)/(H-K)$ two colour diagrams, and $V/(V-I)$ colour magnitude diagram. Forty five stars are found to be main-sequence variables and these could be B-type variable stars associated with the cluster. We classified these objects as $β$ Cep, slowly pulsating B stars and new class variables as discussed by Mowlavi et al. (2013). These variable candidates show $\sim$0.005 to $\sim$0.02 mag brightness variations with periods of $<$ 1.0 d. Seventeen new class variables are located in the $H-R$ diagram between the slowly pulsating B stars and $δ$ Scuti variables. Pulsation could be one of the causes for periodic brightness variations in these stars. The X-ray emission of present main-sequence variables associated with the cluster lies in the saturated region of X-ray luminosity versus period diagram and follows the general trend by Pizzolato et al. (2003).

astro-ph.SR↗

arXiv2013

Photometric study of five open star clusters

$UBVRI$ photometry of the five open clusters Czernik 4, Berkeley 7, NGC 2236, NGC 7226 and King 12 has been carried out using ARIES 104 cm telescope, Nainital. Fundamental cluster parameters such as foreground reddening $E(B-V)$, distance, and age have been derived by means of the observed two colour and colour-magnitude diagrams, coupled to comparisons with theoretical models. $E(B-V)$ values range from 0.55 to 0.74 mag, while ages derived for these clusters range from $\sim$10 to $\sim$500 Myr. We have also studied the spatial structure, mass function and mass segregation effects. The present study shows that evaporation of low mass stars from the halo of the clusters increases as they evolve.

astro-ph.SR↗

arXiv2012

Optical photometric and polarimetric investigation of NGC 1931

We present optical photometric and polarimetric observations of stars towards NGC 1931 with the aim to derive the cluster parameters such as distance, reddening, age and luminosity/mass function as well as to understand the dust properties and star formation in the region. The distance to the cluster is found to be 2.3$\pm$0.3 kpc and the reddening E(B-V) in the region is found to be variable. The stellar density contours reveal two clustering in the region. The observations suggest differing reddening law within the cluster region. Polarization efficiency of the dust grains towards the direction of the cluster is found to be less than that for the general diffuse interstellar medium (ISM). The slope of the mass function (-0.98$\pm$0.22) in the southern region in the mass range 0.8 \textless $M/M_{\sun}$ $\textless$ 9.8 is found to be shallower in comparison to that in the northern region (-1.26$\pm$0.23), which is comparable to the Salpeter value (-1.35). The K-band luminosity function (KLF) of the region is found to be comparable to the average value of slope (~0.4) for young clusters obtained by Lada & Lada (2003), however, the slope of the KLF is steeper in the northern region as compared to the southern region. The region is probably ionized by two B2 main-sequence type stars. The mean age of the young stellar objects (YSOs) is found to be 2$\pm$1 Myr which suggests that the identified YSOs could be younger than the ionizing sources of the region. The morphology of the region, the distribution of the YSOs as well as ages of the YSOs and ionizing sources indicate a triggered star formation in the region.

astro-ph.SR↗

arXiv2012

Multiwavelength Study of NGC 281 Region

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 (Γ) 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, Δ(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.

astro-ph.GA↗

arXiv2011

The population of Young Stellar Clusters throughout the disk of M33

The properties of young stellar clusters (YSCs) in M33, identified from the center out to about twice the size of the bright star-forming disk,are investigated. We find 915 discrete MIR sources as far as the extent of the warped HI disk, i.e. 16 kpc from the galaxy center. Their surface density has a steep radial decline beyond 4.5 kpc, and flattens out beyond the optical radius at 8.5 kpc. We are able to identify YSCs out to 12 kpc. At large galactocentric radii, the paucity of very luminous clusters and the relevance of hot dust emission become evident from the analysis of the bolometric and MIR luminosity functions. The YSC mass and size are correlated with a log-log slope of 2.09, similar to that measured for giant molecular clouds in M33 and the Milky Way, which represent the protocluster environment. Most of the YSCs in our sample have low extinction and ages between 3 and 10 Myr. In the inner regions of M33 the clusters span a wide range of mass (10^2<M<3 10^5 msun) and luminosity 10^38<L{bol}<3 10^{41}erg/s, while at galactocentric radii larger than 4 kpc we find a deficiency of massive clusters. Beyond 7 kpc, where the Halpha surface brightness drops significantly, the dominant YSC population has M<10^3 msun and a slightly older age (10 Myrs). This implies the occurrence of star formation events about 10 Myr ago as far as 10-12 kpc from the center of M33. The cluster L{FUV}--L{Halpha} relation is non-linear for L{FUV}<10^{39}erg/s, in agreement with randomly sampled models of the IMF which, furthermore, shows no appreciable variation throughout the M33 disk.

astro-ph.CO↗

arXiv2011

H2 flows in the Corona Australis cloud and their driving sources

We uncover the H2 flows in the Corona Australis molecular cloud and in particular identify the flows from the Coronet cluster. Near-infrared H2 v=1--0 S(1), 2.12micron-line, narrow-band imaging survey of the R CrA cloud core was carried out. We identify the best candidate-driving source for each outflow by comparing the flow properties, available proper motions, and the known/estimated properties of the driving sources. We also adopted the thumbrule of outflow power as proportional to source luminosity and inversely proportional to the source age to reach a consensus. Results: Continuum-subtracted, narrow-band images reveal several new Molecular Hydrogen emission-line Objects (MHOs). Together with previously known MHOs and Herbig-Haro objects we catalog at least 14 individual flow components of which 11 appear to be driven by the RCrA aggregate members. The flows originating in the Coronet cluster have lengths of ~0.1-0.2 pc. Eight out of nine submillimeter cores mapped in the Coronet cluster region display embedded stars driving an outflow component. Roughly 80% of the youngest objects in the Coronet are associated with outflows. The MHO flows to the west of the Coronet display lobes moving to the west and vice-versa, resulting in nondetections of the counter lobe in our deep imaging. We speculate that these counterflows may be experiencing a stunting effect in penetrating the dense central core. Conclusions:Although this work has reduced the ambiguities for many flows in the Coronet region, one of the brightest H2 feature (MHO2014) and a few fainter features in the region remain unassociated with a clear driving source. The flows from Coronet, therefore, continue to be interesting targets for future studies.

astro-ph.SR↗