Protostars and Planets V 2005 8229.pdf

CHANDRA OBSERVATION OF MASSIVE STAR-FORMING COMPLEX NGC6357: THE H II REGION G353.2+0.9 AND THE MASSIVE OPEN CLUSTER PISMIS 24. Junfeng Wang, Leisa Townsley, Eric Feigelson, Gordon Garmire, 525 Davey Lab, Department of Astronomy & Astrophysics, Pennsylvania State University, University Park PA 16802, USA ([email protected]).

50 −1 X-ray emission has been detected from stars of various bright H II region. Based on the 10 UV photons s required masses and stages, ranging from T Tauri stars to Herbig Ae/Be by the observed Hβ flux, they demonstrated that most of the stars, from protostars to supernova remnant. The Chandra ionizing power comes from the hot O stars in Pismis 24. X-ray Observatory (CXO) offers high spatial resolution and Despite the intensive optical/infrared/radio studies on NGC sensitivity to hard photons, which is critical to resolve the 6357, X-ray observations in this field are extremely rare. An stellar population and to penetrate through the molecular gas unpublished ROSAT PSPC observation detected a few strong and intervening absorbing columns to study in detail the early X-ray sources. No ASCA observations were made and no phases of star formation. XMM-Newton observations are planned. NGC 6357, a large H II region complex and Galactic ring We present the first high-spatial-resolution X-ray image of nebula, is located in the Sagittarius spiral arm at a distance NGC 6357, obtained in a 38-ksec observation using Advanced of 2.5 kpc and spanning 400 × 600 in the southern sky. In CCD Imaging Spectrometer (ACIS) on board Chandra. Our its immediate neighborhood is NGC 6334, a prominent star- observation aims to separate point sources from truly diffuse forming complex which is possibly associated with NGC 6357. emission and to probe the young stellar populations embedded NGC 6357 contains three major radio peaks: G353.2+0.9, in the denser cloud. Given the exposure time and typical 1 33 −1 G353.2+0.7, and G353.1+0.6. The H II region G353.2+0.9 spectra (e.g., θ C Ori [8]), Chandra can detect 10 erg s O 5 is the youngest (≤ 10 yrs) and also the brightest in visual, stars through heavy absorption (AV ∼ 50 mag), therefore we infrared, and radio bands. A number of embedded sources can examine the open questions mentioned above from an X- were discovered via infrared observations [1]. In a recent Very ray perspective. With Chandra, the OB association is resolved Large Array (VLA) survey of six H II regions, Healy et al.[2] at the arcsecond level, over 900 point sources are detected, and reported non-detection of water maser emission in NGC 6357 more than 500 of them have optical/IR counterparts. X-ray to a 3-σ detection limit of 0.1 Jy. The contrast between the emission was detected from ∼ 20 O-type and early B-type numerous masers found in NGC 6334 and non-detection in stars, including the W-R/O binary, and from hundreds of pre- NGC 6357 led to the conclusion that the formation of massive main-sequence stars. Figure 1 shows the smoothed X-ray stars seems to have stopped in NGC 6357. image. We will present X-ray luminosities, light curves, and The OB cluster Pismis 24 is located ∼ 10 south of the spectral fits obtained for bright sources with over 100 counts. G353.2+0.9 ionization front. Early optical study [3] of Pismis Support for this work was provided to Gordon Garmire, 24 revealed ∼ 20 O-type and early B-type stars, including the ACIS Principal Investigator, by the National Aeronautics the Wolf-Rayet (W-R)/O star binary HD 157504 (=WR93; and Space Administration (NASA) through NASA Contract WC7+O7-9). Two of the cluster members, namely Pis 24- NAS8-38252 and Chandra Contract SV4-74018 issued by the 1=HDE 319718 and Pis 24-17, were recently classified as Chandra X-ray Observatory Center, which is operated by the spectral type O3, the brightest and bluest stars known [4]. Smithsonian Astrophysical Observatory for and on behalf of Only 5 other O3 stars are found in the Milky Way. Felli et NASA under contract NAS8-03060. J.W. acknowledges travel al.[5] suggested that Pismis 24 may be unrelated to the H II support from Zaccheus Daniel Foundation for Astronomy and region, rather a coincidence due to a projection effect; the H II the PPV travel grant. region is ionized by embedded massive stars instead of the References:[1]Persi, P., Ferrari-Toniolo, M., Roth, M., & O3-O7 stars in Pismis 24 and signicant amount of molecular Tapia, M. 1986, A&A, 170, 97 [2]Healy, K. R., Hester, J. J., gas should still be present between Pismis 24 and the southern & Claussen, M. J. 2004, ApJ, 610, 835 [3]Neckel, T. 1978, edge of H II region. However, Massi et al.[6] have shown that A&A, 69, 51 [4]Walborn, N. R., et al. 2002, AJ, 123, 2754 most molecular gas can be found behind the H II region or [5]Felli, M., Persi, P., Roth, M., Tapia, M., Ferrari-Toniolo, northern part further away from the edge directly exposed to M., & Cervelli, A. 1990, A&A, 232, 477 [6]Massi, F., Brand, Pismis 24, which implies that G353.2+0.9 is a face-on blister J., & Felli, M. 1997, A&A, 320, 972 [7]Bohigas, J., Tapia, M., type H II region. Bohigas et al.[7] obtained optical spectrum Roth, M., & Ruiz, M. T. 2004, AJ, 127, 2826 [8]Feigelson, and narrowband imaging (e.g., Hα, Hβ, [S II]6724A)˚ of this E. D., et al. 2005, AJ, COUP special issue, astro-ph/0506503 Protostars and Planets V 2005 8229.pdf

Figure 1: Our 38-ksec Chandra/ACIS X-ray observation of the cluster Pismis 24 and the H II region G353.2+0.9 in NGC 6357. This dataset releaved over 900 X-ray point sources. In this smoothed image, red represents the soft-band photons (0.5–2.0 keV) and blue represents the hard-band photons (2.0– 8.0 keV).