A Very Large Telescope/Naco Study of Star Formation in the Massive Embedded Cluster RCW 38 (original) (raw)
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Spectral Classification of the Brightest Objects in the Galactic Star-forming Region W40
The Astronomical Journal, 2012
We present high signal-to-noise, moderate resolution (R ≈ 2000) near-infrared spectra, as well as 10 μm imaging, for the brightest members of the central stellar cluster in the W40 H ii region, obtained using the SpeX and MIRSI instruments at NASA's Infrared Telescope Facility. Using these observations combined with archival Spitzer Space Telescope data, we have determined the spectral classifications, extinction, distances, and spectral energy distributions (SEDs) for the brightest members of the cluster. Of the eight objects observed, we identify four main-sequence (MS) OB stars (one late-O, three early-B), two Herbig Ae/Be stars, and two low-mass young stellar objects (Class II). Strong He i absorption at 1.083 μm in the MS star spectra strongly suggests that at least some of these sources are in fact close binaries. Two out of the four MS stars also show significant infrared excesses typical of circumstellar disks. Extinctions and distances were determined for each MS star by fitting model stellar atmospheres to the SEDs. We estimate a distance to the cluster of between 455 and 535 pc, which agrees well with earlier (but far less precise) distance estimates. We conclude that the late-O star we identify is the dominant source of Lyman continuum luminosity needed to power the W40 H ii region and is the likely source of the stellar wind that has blown a large (≈4 pc) pinched-waist bubble observed in wide-field mid-IR images. We also suggest that 3.6 cm radio emission observed from some of the sources in the cluster is likely not due to emission from ultracompact H ii regions, as suggested in other work, due to size constraints based on our derived distance to the cluster. Finally, we also present a discussion of the curious source IRS 3A, which has a very strong mid-IR excess (despite its B3 MS classification) and appears to be embedded in a dusty envelope roughly 2700 AU in size.
The Astronomical Journal, 2006
We report on results of a 96.7 ks Chandra observation of one of the youngest, most embedded, and most massive young stellar clusters studied in X-rays: RCW 38. We detect 460 sources in the field, of which 360 are confirmed to be associated with the RCW 38 cluster. The cluster members range in luminosity from 10 30 to 10 33.5 ergs s À1 . Over 10% of the cluster members with over 100 counts exhibit flares, while about 15% of the cluster members with over 30 counts are variable. Of the sources identified as cluster members, 160 have near-infrared (NIR) counterparts either in the Two Micron All Sky Survey database or detected via Very Large Telescope observations. Of these, about 20% appear to have optically thick disks. An additional 353 members are identified through NIR observations, of which at least 50% possess optically thick disks. We fit over 100 X-ray sources as absorbed Raymond-Smith-type plasmas and find that the column to the cluster members varies from 10 21.5 to 10 23 cm À2 . We compare the gas to dust absorption signatures in these stars and find N H ¼ A V 2 ; 10 21 ð Þcm À2 . We find that the cluster contains 31 candidate OB stars and is centered about 10 00 (0.1 pc) west of the primary source of the ionization, the O5 star IRS 2. The cluster has a peak central density of about 400 X-ray sources pc À2 . We estimate that the total cluster membership exceeds 2000 stars.
2006
We report on the results of an approximately 90 ks Chandra observation of a complex region that hosts multiple sites of recent and active star formation in ARA OB1a. The field is centered on the embedded cluster RCW 108-IR and includes a large portion of the open cluster NGC 6193. We detect over 420 X-ray sources in the field and combined these data with deep near-IR, Spitzer/IRAC and Midcourse Space Experiment (MSX) mid-IR data. We find about 360 of the X-ray sources have near-IR counterparts. We divide the region into five parts based on the X-ray point source characteristics and extended 8 µm emission. The most clearly defined regions are the central region, identified by embedded sources with high luminosities in the both the near-IR and X-ray as well as high X-ray temperatures (∼3 keV), and the eastern region, identified by low extinction and ∼1 keV X-ray temperatures. Other regions, identified by their directional relationship to RCW 108-IR, are less uniformrepresenting combinations of the first two regions, independent star formation epochs, or both. The cluster members range in X-ray luminosity from 10 29 to 10 33 erg s −1 . Over 18% of the cluster members with over 100 counts exhibit flares. All sources with over 350 counts are variable. Overall about 10% (16% in RCW 108-IR) appear to have optically thick disks as derived from their position in the (J − H ), (H − K) diagram. The disk fraction becomes much higher when IRAC data are employed. The largest fraction of X-ray sources is best described as possessing some disk material via a more detailed extinction fitting. We fit the bulk of the X-ray spectra as absorbed Raymond-Smith-type plasmas, and find that the column to the RCW 108-IR members varies from 10 21 to 10 23 cm −2 . We find that the field contains 41 candidate O or B stars, and estimate that the total number of pre-main-sequence stars in the field is about 1600 ± 200. Approximately 800 are confined to the 3 (∼1.1 pc) central region.
The Young Massive Stellar Cluster Associated with RCW 121
The Astronomical Journal, 2006
We report NIR broad and narrow band photometric observations in the direction of the IRAS17149-3916 source that reveal the presence of a young cluster of massive stars embedded in an HII region coincident with RCW121. These observations, together with published radio data, MSX and Spitzer images were used to determine some of the physical parameters of the region. We found 96 cluster member candidates in an area of about 1.5 × 2.0 square arcmin, 30% of them showing excess emission in the NIR. IRS 1, the strongest source in the cluster with an estimated spectral type of O5V-O6V ZAMS based on the color-magnitude diagram, is probably the main ionizing source of the HII region detected at radio wavelengths. Using the integrated Brγ and the 5 GHz flux densities, we derived a mean visual extinction A V = 5.49± 2.06 1.32 magnitudes. From the observed size of the Brγ extended emission, we calculated the emission measure E = 4.5 × 10 24 cm −5 and the electron density n e = 2.6 × 10 3 cm −3 , characteristic of compact HII regions.
IRAS 18317−0757: A Cluster of Embedded Massive Stars and Protostars
The Astrophysical Journal, 2004
We present high resolution, multiwavelength continuum and molecular line images of the massive star-forming region IRAS 18317−0757. The global infrared through millimeter spectral energy distribution can be approximated by a two temperature model (25K and 63K) with a total luminosity of approximately log(L/L ⊙ ) = 5.2. Previous submillimeter imaging resolved this region into a cluster of five dust cores, one of which is associated with the ultracompact H II region G23.955+0.150, and another with a water maser. In our new 2.7mm continuum image obtained with BIMA, only the UCH II region is detected, with total flux and morphology in good agreement with the free-free emission in the VLA centimeterwave maps. For the other four objects, the non-detections at 2.7mm and in the MSX mid-infrared bands are consistent with cool dust emission with a temperature of 13-40K and a luminosity of 1000-40000 L ⊙ . By combining single-dish and interferometric data, we have identified over two dozen virialized C 18 O cores in this region which contain ≈ 40% of the total molecular gas mass present. While the overall extent of the C 18 O and dust emission is similar, their emission peaks do not correlate well in detail. At least 11 of the 123 infrared stars identified by 2MASS in this region are likely to be associated with the star-forming cluster. Two of these objects (both associated with UCH II) were previously identified as O stars via infrared spectroscopy. Most of the rest of the reddened stars have no obvious correlation with the C 18 O cores or the dust continuum sources. In summary, our observations indicate that considerable fragmentation of the molecular cloud has taken place during the time required for the UCH II region to form and for the O stars to become detectable at infrared wavelengths. Additional star formation appears to be ongoing on the periphery of the central region where up to four B-type (proto)stars have formed amongst a substantial number of C 18 O cores.
Astron J, 2004
We describe the results of two near-infrared (K-band) imaging surveys and a three-color (JHK) survey of the vicinity of NGC 7538. The limiting magnitudes are K~=16.5 and 17.5 mag for the K-band surveys and K~=15 mag for the JHK survey. We identify more than 2000 and 9000 near-infrared (NIR) sources on the images of the two K-band surveys and 786 NIR sources in the JHK survey. From color-color diagrams, we derive a reddening law for background stars and identify 238 stars with NIR excesses. Contour maps indicate a high-density peak coincident with a concentration of stars with NIR excesses. We identify this peak as a young embedded cluster and confirm this result with the K-band luminosity function (KLF), color histograms, and color-magnitude diagrams. The center of the cluster is at R.A.=23h13m39s34, decl.=61deg29'18.9". The cluster radius is ~3', ~2.5 pc for an adopted distance d~=2.8 kpc. For d=2.8 kpc and reddening EJ-K=0.55 mag, the slope of the logarithmic KLF of the cluster, s~0.32+/-0.03, agrees well with previous results for L1630 (s=0.34) and M17 (s=0.26) found by C. Lada, E. Lada, and coworkers. Observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.
The Astronomical Journal, 2004
We describe the results of two near infrared (K-band) imaging surveys and a three color (JHK) survey of the vicinity of NGC 7538. The limiting magnitudes are K ≃ 16.5 and K ≃ 17.5 mag for the K-band surveys and K ≃ 15 mag for the JHK survey. We identify more than 2000 and 9000 near-infrared (NIR) sources on the images of the two 1 Observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.
Near-infrared study of southern massive star formation regions
Astronomy and Astrophysics, 2007
Context. We present the results of a near-infrared survey of the young stellar cluster associated with the IRAS 16571-4029 source. Aims. The main purpose of this survey is to study the cluster members and find the ionizing sources of the associated HII region. Methods. The stellar population was studied by using color-color and color-magnitude diagrams, as well as by analysing the spectral energy distributions in the near-and mid-infrared wavelengths. The extended emission was studied by the construction of contour diagrams, which were compared with near-and mid-infrared images. We computed the corresponding number of Lyman continuum photons (using the integrated Brγ flux density) and compared it with that obtained from the 5 GHz flux density to derive a mean visual extinction. Results. NIR observations in the direction of RCW116B reveal the presence of a young cluster of massive stars coincident with the IRAS 16571-4029 source. These observations, together with published radio data, MSX, and Spitzer images were used to determine some of the physical parameters of the region. We found 102 cluster member candidates in an area of about 3 × 3 square arcmin, the majority of them showing excess emission in the NIR. We found that IRAS 16571-4029 is formed by multiple infrared sources, all but one are associated with small groups of stars. This suggests that the fragmentation of massive molecular clouds generates the massive sub-clusters. We derived a mean visual extinction of A V = 12.8± 4.7 3.2. This result is independent of the assumed distance and agrees with the mean visual extinction A V = 14.4, as obtained by previous spectroscopic observations of two NIR sources in the direction of the IRAS 16571-4029 source. We also compare the results obtained in this study with those obtained in previous papers in this series finding a very good correlation between the number of cluster members N s and the cluster radius r c. The cluster radius varies from 0.2−0.3 pc (IRAS 15411-5352 and IRAS 16132-5039) until about 1 pc (IRAS 15408-5356). The youngest clusters are those associated with the RCW95 complex (IRAS 15408-5356 and IRAS 15411-5353) with ages in the range 1.5−2 × 10 6 years, while the sources associated with the RCW106 (IRAS 16132-5039, IRAS 16177-5018) and RCW116B (IRAS 16571-4029) complexes have ages in the range 2.5−3 × 10 6 years. The oldest of them is the cluster associated with the RCW121 region (IRAS 17149-4029), which has an estimated age of 4.2 × 10 6 years.
A young double stellar cluster in an HII region, emerging from its parent molecular cloud
Astronomy and Astrophysics, 2008
Aims. We investigate the star formation ocurring in the region towards IRAS 07141-0920 contained in the HII region Sh2-294 (S294). We report the discovery ⋆⋆ of a new young double stellar cluster, and we describe its properties. Methods. High-resolution optical U BVRI and Hα images obtained with ALFOSC mounted on the Nordic Optical Telescope (NOT), near-infrared JHK S images obtained with NOTCam at the Nordic Optical Telescope, and VLT/ISAAC images obtained through the H 2 (2.12 µm) filter were used to make photometric and morphological studies of the point sources and the nebula seen towards Sh2-294.
Monthly Notices of the Royal Astronomical Society, 2013
We present new deep near-infrared broad-and narrow-band imaging and low-resolution spectroscopy of the star formation region RCW 121 (IRAS 17149−3916) which we analyse in combination with Herschel (70, 160, 250, 350 and 500 µm) and Spitzer (3.6, 4.5, 5.8 and 8 µm) images. The near-infrared photometry reveals the presence of a stellar cluster of approximate size of 92 arcsec which is composed of at least 264 members, approximately 25 per cent of these showing excess emission at λ > 2.0 µm, indicative of circumstellar discs. Isochrones corresponding to ages 0.5-1.0 Myr and A V = 7.8 fit well the position of a large fraction of likely cluster members in the K s versus H − K s diagram. We find three massive star-forming cores located in the boundaries of an expanding H II region ionized by a central O-type star. From their far-infrared spectral energy distributions (SEDs) we derive masses and temperatures of the dense cores. When these young stellar objects (YSOs) have warm emission components, the 1.2-500 µm SEDs are fitted with Robitaille et al.'s star-disc-envelope model to obtain their physical parameters. The masses of the three YSOs are between 8 and 10 M. The youngest site (core I) is undetected at λ < 100 µm and is at the earliest evolutionary stage that can currently be detected. The other two cores (II and III) contain YSOs of similar masses and have near-infrared counterparts, which imply a more advanced evolutionary stage. The YSO at core II has been found to have associated a jet, with strong H 2 line emission, co-existing with an H 2 O maser source. RCW 121 is another example of multiple star formation being triggered by the expansion of a single H II region.