XMM-Newton view of the N 206 superbubble in the Large Magellanic Cloud

We perform an analysis of the X-ray superbubble in the N 206 HII region in the Large Magellanic Cloud using current generation facilities to gain a better understanding of the physical processes at work in the superbubble and to improve our knowledge of superbubble evolution. We used XMM-Newton observations of the N 206 region to produce images and extract spectra of the superbubble diffuse emission. Morphological comparisons with Halpha images from the Magellanic Cloud Emission Line Survey were performed, and spectral analysis of the diffuse X-ray emission was carried out. We derived the physical properties of the hot gas in the superbubble based on the results of the spectral analysis. We also determined the total energy stored in the superbubble and compared this to the expected energy input from the stellar population to assess the superbubble growth rate discrepancy for N 206. We find that the brightest region of diffuse X-ray emission is confined by a Halpha shell, consistent with the superbubble model. In addition, faint emission extending beyond the Halpha shell was found, which we attribute to a blowout region. The spectral analysis of both emission regions points to a hot shocked gas as the likely origin of the emission. We determine the total energy stored in the bubble and the expected energy input by the stellar population. However, due to limited data on the stellar population, the input energy is poorly constrained and, consequently, no definitive indication of a growth rate discrepancy is seen. Using the high-sensitivity X-ray data from XMM-Newton and optical data from the Magellanic Cloud Emission Line Survey has allowed us to better understand the physical properties of the N 206 superbubble and address some key questions of superbubble evolution.Comment: 12 pages, 7 figures. Accepted for publication in A&

The Complex Interstellar Na I Absorption toward h and Chi Persei

Recent high spatial and spectral resolution investigations of the diffuse interstellar medium (ISM) have found significant evidence for small-scale variations in the interstellar gas on scales less than or equal to 1 pc. To better understand the nature of small-scale variations in the ISM, we have used the KPNO WIYN Hydra multi-object spectrograph, which has a mapping advantage over the single-axis, single-scale limitations of studies using high proper motion stars and binary stars, to obtain moderate resolution (~12 km/s) interstellar Na I D absorption spectra of 172 stars toward the double open cluster h and Chi Persei. All of the sightlines toward the 150 stars with spectra that reveal absorption from the Perseus spiral arm show different interstellar Na I D absorption profiles in the Perseus arm gas. Additionally, we have utilized the KPNO Coude Feed spectrograph to obtain high-resolution (~3 km/s) interstellar Na I D absorption spectra of 24 of the brighter stars toward h and Chi Per. These spectra reveal an even greater complexity in the interstellar Na I D absorption in the Perseus arm gas and show individual components changing in number, velocity, and strength from sightline to sightline. If each of these individual velocity components represents an isolated cloud, then it would appear that the ISM of the Perseus arm gas consists of many small clouds. Although the absorption profiles vary even on the smallest scales probed by these high-resolution data (~30";~0.35pc), our analysis reveals that some interstellar Na I D absorption components from sightline to sightline are related, implying that the ISM toward h and Chi Per is probably comprised of sheets of gas in which we detect variations due to differences in the local physical conditions of the gas.Comment: 27 pages text; 8 figure

X-rays from Superbubbles in the Large Magellanic Cloud IV: The Blowout Structure of N44

We have used optical echelle spectra along with ROSAT and ASCA X-ray spectra to test the hypothesis that the southern portion of the N44 X-ray bright region is the result of a blowout structure. Three pieces of evidence now support this conclusion. First, the filamentary optical morphology corresponding with the location of the X-ray bright South Bar suggests the blowout description (Chu et al 1993). Second, optical echelle spectra show evidence of high velocity (~90 km/sec) gas in the region of the blowout. Third, X-ray spectral fits show a lower temperature for the South Bar than the main superbubble region of Shell 1. Such a blowout can affect the evolution of the superbubble and explain some of the discrepancy discussed by Oey & Massey (1995) between the observed shell diameter and the diameter predicted on the basis of the stellar content and Weaver et al.'s (1977) pressure-driven bubble model.Comment: 15 pages, LaTeX + psfig, 1 tex file, 2 sty files, 7 PS files, also available at: http://www.astro.washington.edu/gene/papers/papers.htm

A Morphological Diagnostic for Dynamical Evolution of Wolf-Rayet Bubbles

We have observed H-alpha and [OIII] emission from eight of the most well defined Wolf-Rayet ring nebulae in the Galaxy. We find that in many cases the outermost edge of the [OIII] emission leads the H-alpha emission. We suggest that these offsets, when present, are due to the shock from the Wolf-Rayet bubble expanding into the circumstellar envelope. Thus, the details of the WR bubble morphology at H-alpha and [OIII] can then be used to better understand the physical condition and evolutionary stage of the nebulae around Wolf-Rayet stars, as well as place constraints on the nature of the stellar progenitor and its mass loss history.Comment: 11 pages, LaTex, 8 figures, accepted for publication in AJ, November 200

Interstellar Bubbles in Two Young H II Regions

peer reviewedMassive stars are expected to produce wind-blown bubbles in the interstellar medium; however, ring nebulae, suggesting the existence of bubbles, are rarely seen around main-sequence O stars. To search for wind-blown bubbles around main-sequence O stars, we have obtained high-resolution Hubble Space Telescope WFPC2 images and high-dispersion echelle spectra of two pristine H II regions, N11B and N180B, in the Large Magellanic Cloud. These H II regions are ionized by OB associations that still contain O3 stars, suggesting that the H II regions are young and have not hosted any supernova explosions. Our observations show that wind-blown bubbles in these H II regions can be detected kinematically, but not morphologically, because their expansion velocities are comparable to or only slightly higher than the isothermal sound velocity in the H II regions. Bubbles are detected around concentrations of massive stars, individual O stars, and even an evolved red supergiant (a fossil bubble). Comparisons between the observed bubble dynamics and model predictions show a large discrepancy (1-2 orders of magnitude) between the stellar wind luminosity derived from bubble observations and models and that derived from observations of stellar winds. The number and distribution of bubbles in N11B differ from those in N180B, which can be explained by the difference in the richness of stellar content between these two H II regions. Most of the bubbles observed in N11B and N180B show a blister structure, indicating that the stars were formed on the surfaces of dense clouds. Numerous small dust clouds, similar to Bok globules or elephant trunks, are detected in these H II regions, and at least one of them hosts on-going star formation

Dust Destruction in Type Ia Supernova Remnants in the Large Magellanic Cloud

We present first results from an extensive survey of Magellanic Clouds supernova remnants (SNRs) with the Spitzer Space Telescope. We describe IRAC and MIPS imaging observations at 3.6, 4.5, 5.8, 8, 24, and 70 microns of four Balmer-dominated Type Ia SNRs in the Large Magellanic Cloud (LMC): DEM L71 (0505-67.9), 0509--67.5, 0519--69.0, and 0548-70.4. None was detected in the four short-wavelength IRAC bands, but all four were clearly imaged at 24 microns, and two at 70 microns. A comparison of these images to Chandra broadband X-ray images shows a clear association with the blast wave, and not with internal X-ray emission associated with ejecta. Our observations are well described by 1-D shock models of collisionally heated dust emission, including grain size distributions appropriate for the LMC, grain heating by collisions with both ions and electrons, and sputtering of small grains. Model parameters are constrained by X-ray, optical, and far-ultraviolet observations. Our models can reproduce observed 70/24 micron flux ratios only by including sputtering, destroying most grains smaller than 0.03-0.04 microns in radius. We infer total dust masses swept up by the SNR blast waves, before sputtering, of order 0.01 solar masses, several times less than those implied by a dust/gas mass ratio of 0.3 percent as often assumed for the LMC. Substantial dust destruction has implications for gas-phase abundances.Comment: 11 pages, 1 figure, submitted to the Astrophysical Journal Letter

The Hourglass Nebulae of Sher 25 and SN 1987 A: Two of a Kind?

We have performed a detailed study of the morphology and kinematics of the hourglass-shaped nebula around the blue supergiant Sher 25 in the galactic giant HII region NGC 3603. Near-infrared high resolution adaptive optics images in the Br gamma line and HST/NICMOS images in the HeI 1.08mu line were compared with iso-velocity maps in the H alpha and [NII] lines. The adaptive optics observations clearly resolved the width of the ring (0.9", i.e., 0.027 pc), yielding delta R / R = 1:8. We show that the H alpha and [NII] lines trace the entire silhouette of the hourglass. The bipolar lobes of the hourglass expand at 70 km/s, whereas the ring around the waist of the hourglass expands at 30 km/s. Both the ring and the bipolar lobes have about the same dynamical age, indicating a common origin and a major outburst and mass-loss event 6630 yr ago. The ionized mass within the hourglass is between 0.3 Mo and 0.6 Mo - quite comparable to the total mass suggested for the expanding (pre-supernova) shell around SN 1987 A. The hourglass structure around Sher 25 is similar to that of SN 1987 A in spatial extent, mass, and velocities. The major differences between these two nebulae might arise from environmental effects. Both internal and external ionization sources are available for Sher 25's nebula. Furthermore, Sher 25 and its hourglass-shaped nebula appear to be moving to the south-west with respect to the ambient interstellar medium, and ram pressure has apparently deformed the hourglass. We conclude that the circumstellar nebulae around SN 1987 A and Sher 25 are very similar and define a new class of nebulae around blue supergiants in their final evolutionary stage.Comment: 9 pages, Latex, also available at ftp://ftp.astro.uiuc.edu/pub/brandner/sher25_apjl or at http://www.astro.uiuc.edu/~brandner/pub.html , accepted for publication in ApJ Letter

Sub-percent Photometry: Faint DA White Dwarf Spectophotometric Standards for Astrophysical Observatories

We have established a network of 19 faint (16.5 mag $< V <$19 mag) northern and equatorial DA white dwarfs as spectrophotometric standards for present and future wide-field observatories. Our analysis infers SED models for the stars that are tied to the three CALSPEC primary standards. Our SED models are consistent with panchromatic Hubble Space Telescope ($HST$) photometry to better than 1%. The excellent agreement between observations and models validates the use of non-local-thermodynamic-equilibrium (NLTE) DA white dwarf atmospheres extinguished by interstellar dust as accurate spectrophotometric references. Our standards are accessible from both hemispheres and suitable for ground and space-based observatories covering the ultraviolet to the near infrared. The high-precision of these faint sources make our network of standards ideally suited for any experiment that has very stringent requirements on flux calibration, such as studies of dark energy using the Large Synoptic Survey Telescope (LSST) and the Wide-Field Infrared Survey Telescope ($WFIRST$).Comment: 46 pages, 23 figures, 8 tables, accepted for publication in ApJ