Aims. The formation and properties of star clusters at the edge of H II regions are poorly known, partly due to limitations in angular resolution and sensitivity, which become particularly critical ...when dealing with extragalactic clusters. In this paper we study the stellar content and star-formation processes in the young N159W region in the Large Magellanic Cloud. Methods. We investigate the star-forming sites in N159W at unprecedented spatial resolution using JHKs-band images obtained with the GeMS/GSAOI instrument on the Gemini South telescope. The typical angular resolution of the images is ~100 mas, with a limiting magnitude of H ~ 22 mag (90% completeness). Photometry from our images is used to identify candidate young stellar objects (YSOs) in N159W. We also determine the H-band luminosity function of the star cluster at the centre of the H II region and use this to estimate its initial mass function (IMF). Results. We estimate an age of 2 ± 1 Myr for the central cluster, with its IMF described by a power-law with an index of Γ = −1.05 ± 0.2, and with a total estimated mass of ~1300 M⊙. We also identify 104 candidate YSOs, which are concentrated in clumps and subclusters of stars, principally at the edges of the H II region. These clusters display signs of recent and active star-formation such as ultra-compact H II regions, and molecular outflows. This suggests that the YSOs are typically younger than the central cluster, pointing to sequential star-formation in N159W, which has probably been influenced by interactions with the expanding H II bubble.
We present the analysis of the stellar content of NGC 2282, a young cluster in the Monoceros constellation, using deep optical BVI and INT Photometric Hα Survey (IPHAS) photometry along with infrared ...(IR) data from UKIRT Infrared Deep Sky Survey and Spitzer-IRAC. Based on the stellar surface density analysis using nearest neighbourhood method, the radius of the cluster is estimated as ∼3.15 arcmin. From optical spectroscopic analysis of eight bright sources, we have classified three early B-type members in the cluster, which includes, HD 289120, a previously known B2V-type star, a Herbig Ae/Be star (B0.5 Ve) and a B5 V star. From spectrophotometric analyses, the distance to the cluster has been estimated as ∼1.65 kpc. The K-band extinction map is estimated using nearest neighbourhood technique, and the mean extinction within the cluster area is found to be A
V
∼ 3.9 mag. Using IR colour–colour criteria and Hα-emission properties, we have identified a total of 152 candidate young stellar objects (YSOs) in the region, of which, 75 are classified as Class II, nine are Class I YSOs. Our YSO catalogue also includes 50 Hα-emission line sources, identified using slitless spectroscopy and IPHAS photometry data. Based on the optical and near-IR colour–magnitude diagram analyses, the cluster age has been estimated to be in the range of 2–5 Myr, which is in agreement with the estimated age from disc fraction (∼58 per cent). Masses of these YSOs are found to be ∼0.1–2.0 M⊙. Spatial distribution of the candidate YSOs shows spherical morphology, more or less similar to the surface density map.
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.
ABSTRACT We report the results of our observations of the S255IR area with the Submillimeter Array (SMA) at 1.3 mm in the very extended configuration and at 0.8 mm in the compact configuration as ...well as with the IRAM 30 m at 0.8 mm. The best achieved angular resolution is about 0.4 arcsec. The dust continuum emission and several tens of molecular spectral lines are observed. The majority of the lines is detected only toward the S255IR-SMA1 clump, which represents a rotating structure (probably a disk) around the young massive star. The achieved angular resolution is still insufficient to make any conclusions about the Keplerian or non-Keplerian character of the rotation. The temperature of the molecular gas reaches 130-180 K. The size of the clump is about 500 AU. The clump is strongly fragmented as follows from the low beam-filling factor. The mass of the hot gas is significantly lower than the mass of the central star. A strong DCN emission near the center of the hot core most probably indicates a presence of a relatively cold ( 80 K) and rather massive clump there. High-velocity emission is observed in the CO line as well as in lines of high-density tracers HCN, HCO+, CS and other molecules. The outflow morphology obtained from a combination of the SMA and IRAM 30 m data is significantly different from that derived from the SMA data alone. The CO emission detected with the SMA traces only one boundary of the outflow. The outflow is most probably driven by jet bow shocks created by episodic ejections from the center. We detected a dense high velocity clump associated apparently with one of the bow shocks. The outflow strongly affects the chemical composition of the surrounding medium.
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..., 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 x 10... cm..., respectively. The column density maps reveal an A...>...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 x 0.8 pc, and a mass of 1023 M... (ProQuest: ... denotes formulae/symbols omitted.)
We present a multiwavelength study of the W40 star-forming region using infrared (IR) observations in the UKIRT JHK bands, Spitzer Infrared Array Camera bands, and Herschel PACS bands, 2.12 mu m H ...sub(2) narrowband imaging, and radio continuum observations from GMRT (610 and 1280 MHz), in a field of view (FoV) of ~34' x 40'. Archival Spitzer observations in conjunction with near-IR observations are used to identify 1162 Class II/III and 40 Class I sources in the FoV. The nearest-neighbor stellar surface density analysis shows that the majority of these young stellar objects (YSOs) constitute the embedded cluster centered on the high-mass source IRS 1A South. Radio continuum analysis shows that this region has a blister morphology, with the radio peak coinciding with a protostellar source.
ABSTRACT The presence of extended Main-Sequence Turn-Off (eMSTO) in the open clusters has been attributed to various factors, such as spread in rotation rates, binary stars, and dust-like extinction ...from stellar excretion discs. We present a comprehensive analysis of the eMSTO in the open cluster NGC 2355. Using spectra from the Gaia–European Southern Observatory (ESO) archives, we find that the stars in the red part of the eMSTO have a higher mean v sin i value of 135.3 $\pm$ 4.6 km s$^{-1}$ compared to the stars in the blue part that have an average v sin i equal to 81.3 $\pm$ 5.6 km s$^{-1}$. This suggests that the eMSTO in NGC 2355 is possibly caused by the spread in rotation rates of stars. We do not find any substantial evidence of the dust-like extinction from the eMSTO stars using ultraviolet data from the Swift survey. The estimated synchronization time for low-mass ratio close binaries in the blue part of the eMSTO suggests that they would be mostly slow-rotating if present. However, the stars in the blue part of the eMSTO are preferentially located in the outer region of the cluster indicating that they may lack low-mass ratio close binaries. The spread in rotation rates of eMSTO stars in NGC 2355 is most likely caused by the star-disc interaction mechanism. The stars in the lower main sequence beyond the eMSTO region of NGC 2355 are slow rotating (mean v sin i = 26.5 $\pm$ 1.3 km s$^{-1}$) possibly due to the magnetic braking of their rotations.
Multicomponent Co20Cu20Fe20Ni20Ti20 eutectic high entropy alloy (HEA), processed by vacuum arc melting cum suction casting technique was studied. The microstructure of this eutectic HEA consists bcc ...(β) and fcc (α2) solid solution dendritic phases and eutectics (fcc (α1) plus Ti2(Ni, Co)-type Laves phase). Serrations in the flow behaviour are attributed to multi substitutional solutes in the multi-phase microstructure. Based on the detailed analysis of mechanical data together with deformed microstructural characterization, the optimum thermo-mechanical processing conditions for hot working are identified as T=930–990°C (1203–1263K) and strain rate range of 10−3s−1–10−1s−1.
The present study explores the high temperature deformation behavior of FeCoNiCrAlTi dual phase high entropy alloy (henceforth referred to as DP-HEA) in the temperature range of 900–1100 °C ...(1173–1373 K) and the strain rate varying from 0.001 to 0.1 s
−1
. The as-cast sample has been characterized using scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry, which reveals the presence of two phases, disordered γ and ordered γ′. A constitutive relationship between the process parameters (stress temperature, strain, and strain rate) has been drawn using the Arrhenius-type equation to recognize the high temperature deformation behavior of the DP-HEA. The optimum thermomechanical processing window of the DP-HEA has been determined by constructing multiple contour maps based on different parameters such as efficiency, strain rate sensitivity, etc. And the optimum processing domain has been found to lie approximately in the temperature range of 1260–1300 K and SR = 10
–2.3
–10
–2
s
−1
, 1325–1373 K and SR = 10
–1.4
–10
–1
s
−1
& 1173–1193 K and SR = 10
–1.3
–10
–1.55
s
−1
. Finally, the stable and unstable regimes in the processing maps are correlated with the microstructure of hot deformed samples.
Graphical Abstract
Our goal is to identify bipolar H II regions and to understand their morphology, their evolution, and the role they play in the formation of new generations of stars. Methods. We use the ...Spitzer-GLIMPSE, -MIPSGAL, and Herschel-Hi-GAL surveys to identify bipolar Hui regions, looking for (ionized) lobes extending perpendicular to dense filamentary structures. We search for their exciting star(s) and estimate their distances using near-IR data from the 2MASS or UKIDSS surveys. Dense molecular clumps are detected using Herschel-SPIRE data, and we estimate their temperature, column density, mass, and density. MALT9O observations allow us to ascertain their association with the central HI region. The morphology suggests that the formation of a second generation of massive stars has been triggered by the central bipolar HIT region. It occurs in the compressed material of the parental cloud.
PDF
