S. Vig

A NEAR-INFRARED STUDY OF THE NGC 7538 STAR-FORMING REGION

We present subarcsecond (FWHM ~ 0farcs7), near-infrared (NIR) JHKs-band images and a high-sensitivity radio continuum image at 1280 MHz, using SIRIUS on the University of Hawaii 88 inch (2.2 m) telescope and the Giant Metrewave Radio Telescope (GMRT). The NIR survey covers an area of ~24 arcmin^2 with 10 σ limiting magnitudes of ~19.5, 18.4, and 17.3 in the J, H, and Ks bands, respectively. Our NIR images are deeper than any JHK surveys to date for the larger area of the NGC 7538 star-forming region. We construct JHK color-color and J - H/J and H - K/K color-magnitude diagrams to identify young stellar objects (YSOs) and to estimate their masses. Based on these color-color and color-magnitude 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 pre-main-sequence stars with intrinsic color excesses. Most of the YSOs in NGC 7538 are arranged from the northwest toward the southeast regions, forming a sequence in age: a diffuse H II region (northwest and oldest, where most of the Class II and Class I sources are detected), a compact IR core (center), and regions with an extensive IR reflection nebula and a cluster of red young stars (southeast and south). We find that the slope of the Ks-band luminosity function of NGC 7538 is lower than the typical values reported for young embedded clusters, although equally low values have also been reported in the W3 Main star-forming region. From the slope of the Ks-band luminosity function and the analysis by Megeath and coworkers, we infer that the embedded stellar population is composed of YSOs with an age of ~1 Myr. Based on the comparison of models of pre-main-sequence stars with the observed color-magnitude diagram, we find that the stellar population in NGC 7538 is primarily composed of low-mass pre-main-sequence stars similar to those observed in the W3 Main star-forming region. The radio continuum image from the GMRT observations at 1280 MHz shows an arc-shaped structure due to the interaction between the H II region and the adjacent molecular cloud. The ionization front at the interface between the H II region and the molecular cloud is clearly delineated by comparing the radio continuum, molecular line, and NIR images.

A necklace of dense cores in the high-mass star forming region G35.20-0.74 N: ALMA observations

Context. The formation process of high-mass stars (with masses >8 M_⊙) is still poorly understood, and represents a challenge from both the theoretical and observational points of view. The advent of the Atacama Large Millimeter Array (ALMA) is expected to provide observational evidence to better constrain the theoretical scenarios. Aims. The present study aims at characterizing the high-mass star forming region G35.20−0.74 N, which is found associated with at least one massive outflow and contains multiple dense cores, one of them recently found associated with a Keplerian rotating disk. Methods. We used the radio-interferometer ALMA to observe the G35.20−0.74 N region in the submillimeter continuum and line emission at 350 GHz. The observed frequency range covers tracers of dense gas (e.g., H^(13)CO^+, C^(17)O), molecular outflows (e.g., SiO), and hot cores (e.g., CH_3CN, CH_3OH). These observations were complemented with infrared and centimeter data. Results. The ALMA 870 μm continuum emission map reveals an elongated dust structure (~0.15 pc long and ~0.013 pc wide; full width at half maximum) perpendicular to the large-scale molecular outflow detected in the region, and fragmented into a number of cores with masses ~1–10 M_⊙ and sizes ~1600 AU (spatial resolution ~960 AU). The cores appear regularly spaced with a separation of ~0.023 pc. The emission of dense gas tracers such as H^(13)CO^+ or C^(17)O is extended and coincident with the dust elongated structure. The three strongest dust cores show emission of complex organic molecules characteristic of hot cores, with temperatures around 200 K, and relative abundances 0.2–2 × 10^(-8) for CH_3CN and 0.6–5 × 10^(-6) for CH_3OH. The two cores with highest mass (cores A and B) show coherent velocity fields, with gradients almost aligned with the dust elongated structure. Those velocity gradients are consistent with Keplerian disks rotating about central masses of 4–18 M_⊙. Perpendicular to the velocity gradients we have identified a large-scale precessing jet/outflow associated with core B, and hints of an east-west jet/outflow associated with core A. Conclusions. The elongated dust structure in G35.20−0.74 N is fragmented into a number of dense cores that may form high-mass stars. Based on the velocity field of the dense gas, the orientation of the magnetic field, and the regularly spaced fragmentation, we interpret this elongated structure as the densest part of a 1D filament fragmenting and forming high-mass stars.

Chasing discs around O-type (proto)stars: Evidence from ALMA observations

This work is supported by a H2020 Marie Sklodowska-Curie Action (GESTATE 661249) funded by the European Research Commission. Reproduced with permission from Astronomy & Astrophysics.

Post-outburst phase of McNeil's nebula (V1647 Orionis)

We present a detailed study of the post-outburst phase of McNeils nebula (V1647 Orionis) using optical B, V, R, I and near-infrared (NIR) J, H, K photometric and low-resolution optical spectroscopic observations. The observations were carried out with the Himalaya Faint Object Spectrograph Camera (HFOSC), NIR camera (NIRCAM), the Tata Institute of Fundamental Research (TIFR) Near-Infrared Camera (TIRCAM) and NICMOS cameras on the 2-m Himalayan Chandra Telescope (HCT) and 1.2-m Physical Research Laboratory (PRL) telescopes during the period 2004 February-2005 December. The optical and NIR observations show a general decline in the brightness of the exciting source of McNeils nebula (V1647 Ori). Our recent optical images show that V1647 Ori has faded by more than 3 mag since February 2004. McNeils nebula has also faded considerably. The optical/NIR photometric data also show a significant variation in the magnitudes (ΔV = 0.78, ΔR = 0.44, ΔI = 0.21, ΔJ = 0.24 and ΔH = 0.20 mag) of V1647 Ori within a period of one month, which is possibly undergoing a phase similar to eruptive variables, like EXors or FUors. The optical spectra show a few features such as strong Ha emission with blue-shifted absorption and the Ca II IR triplet (8498, 8542 and 8662 A) in emission. As compared to the period just after outburst, there is a decrease in the depth and extent of the blue-shifted absorption component, indicating a weakening in the powerful stellar wind. The presence of the Ca II IR triplet in emission confirms that V1647 Ori is a pre-main-sequence star. The long-term, post-outburst photometric observations of V1647 Ori suggest an EXor rather than an FUor event. An optical/IR comparison of the region surrounding McNeils nebula shows that the optical nebula is more widely and predominantly extended to the north, whereas the IR nebula is relatively confined (diameter ∼60 arcsec), but definitely extended, to the south, too. The large colour gradient from north to south and the sudden absence of an optical nebula to the south are suggestive of a large-scale disc-like structure (or envelope) surrounding the central source that hides the southern nebula.

Warp signatures of the Galactic disk as seen in mid infrared from Midcourse Space Experiment

The gross features in the distribution of stars as well as warm IT? 100 K) interstellar dust in the Galactic disk have been investigated using the recent mid infrared survey by Midcourse Space Experiment (MSX) at 8, 12, 14 and 21 μm bands. An attempt has been made to determine the location of the Galactic mid-plane at various longitudes, using two approaches: (i) fitting exponential functions to the latitude profiles and (ii) statistical indicators. The former method is successful for the inner Galaxy (-90° < l < 90°), and quantifies characteristic angular scales, y, along latitude. These ys have been translated to linear scale heights (z h ) and radial length scales (R l ) using geometric description of the Galactic disk. The distribution of warm dust in the Galactic disk is found to be characterised by R 1 < 6 kpc and 60? z h ? 100 pc, consistent with other studies. The location of the Galactic mid-plane as a function of longitude (in all 4 MSX bands), for stars as well as warm dust, has been searched for signatures of warp-like feature in their distribution, by fitting sinusoid with phase and amplitude as parameters. In every case, the warp signature has been detected. Carrying out an identical analysis of the DIRBE/COBE data (with lower angular resolution) in all its ten bands covering the entire infrared spectrum (1.25-240 μm), also leads to detection of warp signatures with very similar phase as found from the MSX data. Our results have been compared with those from other studies.

A multiwavelength investigation of G24.78+0.08 A2 using observations from VLA and VLT-VISIR

Context. G24.78+0.08 is a massive star-forming region where three large massive disk-like rotating structures (toroids), namely Al, A2 and C, have been found around massive (proto)stars. In particular, while G24 Al has been extensively investigated through multiwavelength surveys, G24 A2 has been hitherto less studied. Aims. We carry out a detailed investigation of G24 A2 and speculate the scenario of the environment in the vicinity of the central exciting object(s). Methods. We have carried out mid-infrared imaging observations of the G24.78+0.08 region at 11.9 and 18.7 μm using the VISIR located on the VLT. In addition, the radio continuum VLA observations at 1.3 cm as well as the NH 3 (2,2) line emission have been investigated. Results. Compact mid-infrared emission is observed only from one source in the G24.78+0.08 region and this is likely to be associated with G24 A2. The radio continuum measurements show a faint compact source which is resolved out at higher angular resolutions (60 mas); the diameter D of the radio emitting region is estimated to be 1000 AU < D < 2000 AU. The spectral index (∼1) points towards the possibility of this emission being from an ionised jet or an H II region with a density gradient. The NH 3 (2,2) emission from the main component is optically thick and appears self-absorbed at blue-shifted velocities. The velocity distributions of the NH 3 lines can be explained by a motion which is a combination of rotation and expansion. Based on these observations, we speculate that the expansion is due to the outflow (seen in CO) with A2 being the likely origin of this outflow. The comparison of various properties of G24 Al and G24 A2 like rotation, mass of cores, infrared emission, excitation temperature, suggests that G24 A2 and G24 Al possibly harbour similar young stellar objects, but the toroid associated with G24 A2 is more inclined than that of G24 Al.

Star formation activity in the southern Galactic H ii region G351.63−1.25

The southern Galactic high-mass star-forming region, G351.63−1.25, is an H ii region–molecular cloud complex with a luminosity of ∼2.0 × 10 5 L ⊙ , located at a distance of 2.4 kpc from the Sun. In this paper, we focus on the investigation of the associated H ii region, embedded cluster and the interstellar medium in the vicinity of G351.63−1.25. We address the identification of exciting source(s) as well as the census of the stellar populations, in an attempt to unfold star formation activity in this region. The ionized gas distribution has been mapped using the Giant Metrewave Radio Telescope, India, at three frequencies: 1280, 610 and 325 MHz. The H ii region shows an elongated morphology and the 1280 MHz map comprises six resolved high-density regions encompassed by diffuse emission spanning 1.4 × 1.0 pc 2 . Based on the measurements of flux densities at multiple radio frequencies, the brightest ultracompact core has electron temperature T e ∼7647 ±153 K and emission measure, EM∼2.0 ± 0.8×10 7 cm −6 pc. The zero-age main-sequence spectral type of the brightest radio core is O7.5. We have carried out near-infrared observations in the JHK s bands using the SIRIUS camera on the 1.4 m Infrared Survey Facility telescope. The near-infrared images reveal the presence of a cluster embedded in nebulous fan-shaped emission. The log-normal slope of the K -band luminosity function of the embedded cluster is found to be ∼0.27 ± 0.03, and the fraction of the near-infrared excess stars is estimated to be 43 per cent. These indicate that the age of the cluster is consistent with ∼1 Myr. Other available data of this region show that the warm (mid-infrared) and cold (millimetre) dust emission peak at different locations indicating progressive stages of star formation process. The champagne flow model from a flat, thin molecular cloud is used to explain the morphology of radio emission with respect to the millimetre cloud and infrared brightness.

Star formation towards the southern cometary H ii region IRAS 17256−3631

IRAS 17256-3631 is a southern Galactic massive star forming region located at a distance of 2 kpc. In this paper, we present a multiwavelength investigation of the embedded cluster, the HII region, as well as the parent cloud. Radio images at 325, 610 and 1372 MHz were obtained using GMRT, India while the near-infrared imaging and spectroscopy were carried out using UKIRT and Mt. Abu Infrared Telescope, India. The near-infrared K-band image reveals the presence of a partially embedded infrared cluster. The spectral features of the brightest star in the cluster, IRS-1, spectroscopically agrees with a late O or early B star and could be the driving source of this region. Filamentary H_2 emission detected towards the outer envelope indicates presence of highly excited gas. The parent cloud is investigated at far-infrared to millimeter wavelengths and eighteen dust clumps have been identified. The spectral energy distributions (SEDs) of these clumps have been fitted as modified blackbodies and the best-fit peak temperatures are found to range from 14-33 K, while the column densities vary from 0.7-8.5x10^22 cm^-2. The radio maps show a cometary morphology for the distribution of ionized gas that is density bounded towards the north-west and ionization bounded towards the south-east. This morphology is better explained with the champagne flow model as compared to the bow shock model. Using observations at near, mid and far-infrared, submillimeter and radio wavelengths, we examine the evolutionary stages of various clumps.

STAR-FORMING ACTIVITY IN THE H ii REGIONS ASSOCIATED WITH THE IRAS 17160–3707 COMPLEX

We present a multiwavelength investigation of star formation activity toward the southern H II regions associated with IRAS 17160–3707, located at a distance of 6.2 kpc with a bolometric luminosity of 8.3 × 10 5 L ⊙ . The ionized gas distribution and dust clumps in the parental molecular cloud are examined in detail using measurements at infrared, submillimeter and radio wavelengths. The radio continuum images at 1280 and 610 MHz obtained using the Giant Metrewave Radio Telescope reveal the presence of multiple compact sources as well as nebulous emission. At submillimeter wavelengths, we identify seven dust clumps and estimate their physical properties such as temperature: 24–30 K, mass: 300–4800 M ⊙ and luminosity: 9–317 × 102 L ⊙ using modified blackbody fits to the spectral energy distributions (SEDs) between 70 and 870 μm. We find 24 young stellar objects (YSOs) in the mid-infrared, with a few of them coincident with the compact radio sources. The SEDs of the YSOs have been fitted by the Robitaille models and the results indicate that those having radio compact sources as counterparts host massive objects in early evolutionary stages with best fit age ≤0.2 Myr. We compare the relative evolutionary stages of clumps using various signposts such as masers, ionized gas, presence of YSOs and infrared nebulosity, and find six massive star-forming clumps and one quiescent clump. Of the former, five are in a relatively advanced stage and one in an earlier stage.

IRAS 18511+0146: a proto Herbig Ae/Be cluster?

Context. The evolution of a young protocluster depends on the relative spatial distribution and dynamics of both stars and gas. Aims. We study the distribution and properties of the gas and stars surrounding the luminous (10^4 L_☉) protocluster IRAS 18511+0146. Methods. IRAS 18511+0146 and the cluster associated with it has been investigated using the sub-millimetre (JCMT-SCUBA), infrared (Spitzer-MIPSGAL, Spitzer-GLIMPSE, Palomar) and radio (VLA) continuum data. Cluster simulations have been carried out in order to understand the properties of clusters as well as to compare with the observations. Results. The central most obscured part of the protocluster coincident with the compact sub-millimetre source found with SCUBA is responsible for at least 2/3 of the total luminosity. A number of cluster members have been identified which are bright in mid infrared and show rising (near to mid infrared) spectral energy distributions suggesting that these are very young stellar sources. In the mid infrared 8.0 μm image, a number of filamentary structures and clumps are detected in the vicinity of IRAS 18511+0146. Conclusions. Based on the luminosity and cluster size as well as on the evolutionary stages of the cluster members, IRAS 18511+0146 is likely to be protocluster with the most massive object being a precursor to a Herbig type star.