arXiv subjects

R. A. Gruendl

Publications and source records attributed to R. A. Gruendl.

At least 469 records · Page 26Linked to original sources

arXiv2004

XMM-Newton Detection of Hot Gas in the Eskimo Nebula: Shocked Stellar Wind or Collimated Outflows?

The Eskimo Nebula (NGC 2392) is a double-shell planetary nebula (PN) known for the exceptionally large expansion velocity of its inner shell, ~90 km/s, and the existence of a fast bipolar outflow with a line-of-sight expansion velocity approaching 200 km/s. We have obtained XMM-Newton observations of the Eskimo and detected diffuse X-ray emission within its inner shell. The X-ray spectra suggest thin plasma emission with a temperature of ~2x10^6 K and an X-ray luminosity of L_X = (2.6+/-1.0)x10^31 (d/1150 pc)^2 ergs/s, where d is the distance in parsecs. The diffuse X-ray emission shows noticeably different spatial distributions between the 0.2-0.65 keV and 0.65-2.0 keV bands. High-resolution X-ray images of the Eskimo are needed to determine whether its diffuse X-ray emission originates from shocked fast wind or bipolar outflows.

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arXiv2004

FUSE Observations of Nebular O VI Emission from NGC 6543

NGC 6543 is one of the few planetary nebulae (PNe) whose X-ray emission has been shown to be extended and originate from hot interior gas. Using FUSE observations we have now detected nebular O VI emission from NGC 6543. Its central star, with an effective temperature of ~50,000 K, is too cool to photoionize O V, so the O VI ions must have been produced by thermal collisions at the interface between the hot interior gas and the cool nebular shell. We modeled the O VI emission incorporating thermal conduction, but find that simplistic assumptions for the AGB and fast wind mass loss rates overproduce X-ray emission and O VI emission. We have therefore adopted the pressure of the interior hot gas for the interface layer and find that expected O VI emission to be comparable to the observations.

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arXiv2004

A 4.8- and 8.6-GHz Survey of the Large Magellanic Cloud: I The Images

Detailed 4.8- and 8.6-GHz radio images of the entire Large Magellanic Cloud with half-power beamwidths of 33" at 4.8 GHz and 20" at 8.6 GHz have been obtained using the Australia Telescope Compact Array. A total of 7085 mosaic positions were used to cover an area of 6 degrees on a side. Full polarimetric observations were made. These images have sufficient spatial resolution (~8 and 5 pc, respectively) and sensitivity (3-sigma of 1 mJy/beam) to identify most of the individual SNRs and H II regions and also, in combination with available data from the Parkes 64-m telescope, the structure of the smooth emission in that galaxy. In addition, limited data using the sixth antenna at 4.5 to 6-km baselines are available to distinguish bright point sources (<3 and 2 arcsec, respectively) and to help estimate sizes of individual sources smaller than the resolution of the full survey. The resultant database will be valuable for statistical studies and comparisons with x-ray, optical and infrared surveys of the LMC with similar resolution.

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arXiv2004

An Optical and X-ray Examination of Two Radio Supernova Remnant Candidates in 30 Doradus

The giant HII region 30 Doradus is known for its violent internal motions and bright diffuse X-ray emission, suggesting the existence of supernova remnants (SNRs), but no nonthermal radio emission has been detected. Recently, Lazendic et al. compared the H-alpha/H-beta and radio/H-alpha ratios and suggested two small radio sources to be nonthermal and thus SNR candidates; however, no optical or X-ray counterparts were detected. We have used high-resolution optical images and high-dispersion spectra to examine the morphological, spectral, and kinematic properties of these two SNR candidates, and still find no optical evidence supporting their identification as SNRs. We have also determined the X-ray luminosities of these SNR candidates, and find them 1-3 orders of magnitude lower than those commonly seen in young SNRs. High extinction can obscure optical and X-ray signatures of an SNR, but would prohibit the use of a high radio/H-alpha ratio to identify nonthermal radio emission. We suggest that the SNR candidate MCRX J053831.8-690620 is associated with a young star forming region; while the radio emission originates from the obscured star forming region, the observed optical emission is dominated by the foreground. We suggest that the SNR candidate MCRX J053838.8-690730 is associated with a dust/molecular cloud, which obscures some optical emission but not the radio emission.

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arXiv2004

Supernova Remnants in the Magellanic Clouds. IV. X-Ray Emission from the Largest SNR in the LMC

We present the first X-ray detection of SNR 0450-70.9 the largest known supernova remnant (SNR) in the Large Magellanic Cloud. To study the physical conditions of this SNR, we have obtained XMM-Newton X-ray observations, optical images and high-dispersion spectra, and radio continuum maps. Optical images of SNR 0450-70.9 show a large, irregular elliptical shell with bright filaments along the eastern and western rims and within the shell interior. The interior filaments have higher [S II]/Halpha ratios and form an apparent inner shell morphology. The X-ray emission region is smaller than the full extent of the optical shell, with the brightest X-ray emission found within the small interior shell and on the western rim of the large shell. The expansion velocity of the small shell is ~220 km/s, while the large shell is ~120 km/s. The radio image shows central brightening and a fairly flat radio spectral index over the SNR. However, no point X-ray or radio source corresponding to a pulsar is detected and the X-ray emission is predominantly thermal. Therefore, these phenomena can be most reasonably explained in terms of the advanced age of the large SNR. Using hydrodynamic models combined with a nonequilibrium ionization model for thermal X-ray emission, we derived a lower limit on the SNR age of about 45,000 yr, well into the later stages of SNR evolution. Despite this, the temperature and density derived from spectral fits to the X-ray emission indicate that the remnant is still overpressured, and thus that the development is largely driven by hot gas in the SNR interior.

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arXiv2004

Hot Interstellar Gas and Stellar Energy Feedback in the Antennae Galaxies

We have analyzed Chandra archival observations of the Antennae galaxies to study the distribution and physical properties of its hot interstellar gas. Eleven distinct diffuse X-ray emission regions are selected according to their underlying interstellar structures and star formation activity. The X-ray spectra of these regions are used to determine their thermal energy contents and cooling timescales. Young star clusters in these regions are also identified and their photometric measurements are compared to evolutionary stellar population synthesis models to assess their masses and ages. The cluster properties are then used to determine the stellar wind and supernova energies injected into the ISM. Comparisons between the thermal energy in the hot ISM and the expected stellar energy input show that young star clusters are sufficient to power the X-ray-emitting gas in some, but not all, active star formation regions. Super-star clusters, with masses >= 1x10^5 M_sol, heat the ISM, but the yield of hot interstellar gas is not directly proportional to the cluster mass. Finally, there exist diffuse X-ray emission regions which do not show active star formation or massive young star clusters. These regions may be powered by field stars or low-mass clusters formed within the last ~100 Myr.

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arXiv2003

X-ray Observations of Planetary Nebulae

Planetary nebulae (PNe) are an exciting addition to the zoo of X-ray sources. Recent Chandra and XMM-Newton observations have detected diffuse X-ray emission from shocked fast winds in PN interiors as well as bow-shocks of fast collimated outflows impinging on the nebular envelope. Point X-ray sources associated with PN central stars are also detected, with the soft X-ray (<0.5 keV) emission originating from the photospheres of stars hotter than ~100,000 K, and the hard X-ray (>>0.5 keV) emission from instability shocks in the fast stellar wind itself or from a low-mass companion's coronal activity. X-ray observations of PNe offer a unique opportunity to directly examine the dynamic effects of fast stellar winds and collimated outflows, and help us understand the formation and evolution of PNe.

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arXiv2003

Hot Gas in Planetary Nebulae

Diffuse X-ray emission has been detected in a small number of planetary nebulae (PNe), indicating the existence of shocked fast stellar winds and providing support for the interacting-stellar-winds formation scenario of PNe. However, the observed X-ray luminosities are much lower than expected, similar to the situation seen in bubbles or superbubbles blown by massive stars. Ad hoc assumptions have been made to reconcile the discrepancy between observations and theoretical expectations. We have initiated FUSE programs to observe OVI absorption and emission from PNe, and our preliminary results indicate that OVI emission provides an effective diagnostic for hot gas in PN interiors.

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arXiv2003

Bubbles and Superbubbles

An isolated massive star can blow a bubble, while a group of massive stars can blow superbubbles. In this paper, we examine three intriguing questions regarding bubbles and superbubbles: (1) why don't we see interstellar bubbles around every O star? (2) how hot are the bubble interiors? and (3) what is going on at the hot/cold gas interface in a bubble?

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arXiv2003

An Intriguing X-ray Arc Surrounding the X-ray Source RX J053335-6854.9 toward the Large Magellanic Cloud

ROSAT observations of the Large Magellanic Cloud (LMC) have revealed a large diffuse X-ray arc around the point source RX J053335-6854.9. The relative locations of the diffuse and point sources suggest that they might originate from a common supernova explosion. We have analyzed the physical properties of the diffuse X-ray emission and determined that it is most likely a supernova remnant in a low-density medium in the LMC. We have also analyzed the X-ray and optical observations of RX J053335-6854.9 and concluded that it is a foreground dMe star in the solar neighborhood. Therefore, despite their positional coincidence, these two X-ray sources are physically unrelated.

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arXiv2003

Hard X-ray Emission Associated with White Dwarfs II

We have previously conducted a search for X-ray sources coincident with white dwarfs using the white dwarf catalog compiled by McCook & Sion (1999} and the ROSAT sources in the WGACAT (Paper I). To include the white dwarfs discovered since 1999 and to include the X-ray sources detected in ROSAT Position Sensitive Proportional Counter (PSPC) observations made with a boron filter, we have carried out another search using an updated list of white dwarfs and the final catalogs of the ROSAT PSPC observations with and without a boron filter. Forty-seven new X-ray sources convincingly coincident with white dwarfs are found and reported in this paper. Among these, only 5 show hard X-ray emission: three possess confirmed or suggested late-type companions, one is apparently single, and the other is likely a misclassified BL Lac object. The apparently single white dwarf with hard X-ray emission, KPD 0005+5106, was discussed extensively in Paper I. Photospheric origin for the hard X-ray emission from hot DO and DQZO white dwarfs remains a tantalizing possibility, but high-quality near IR spectroscopic observations and monitoring of the H-alpha emission line are needed to rule out the existence of a faint dMe companion.

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arXiv2003

Hot Gas in the Circumstellar Bubble S308

S308 is a circumstellar bubble blown by the WN4 star HD50896. It is one of the only two single-star bubbles that show detectable diffuse X-ray emission. We have obtained XMM-Newton EPIC observations of the northwest quadrant of S308. The diffuse X-ray emission shows a limb-brightened morphology, with a clear gap extending from the outer edge of the diffuse X-ray emission to the outer rim of the nebular shell. The X-ray spectrum of the diffuse emission is very soft, and is well fitted by an optically thin plasma model for a N-enriched plasma at temperatures of ~1.1x10^6 K. A hotter gas component may exist but its temperature is not well constrained as it contributes less than 6% of the observed X-ray flux. The total X-ray luminosity of S308, extrapolated from the bright northwest quadrant, is <=(1.2+-0.5)x10^{34} ergs/s. We have used the observed bubble dynamics and the physical parameters of the hot interior gas of S308 in conjunction with the circumstellar bubble model of Garcia-Segura & Mac Low (1995) to demonstrate that the X-ray-emitting gas must be dominated by mixed-in nebular material.

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arXiv2003

Diffuse X-Ray Emission from the Quiescent Superbubble M17, the Omega Nebula

The emission nebula M17 contains a young ~1 Myr-old open cluster; the winds from the OB stars of this cluster have blown a superbubble around the cluster. ROSAT observations of M17 detected diffuse X-ray emission peaking at the cluster and filling the superbubble interior. The young age of the cluster suggests that no supernovae have yet occurred in M17; therefore, it provides a rare opportunity to study hot gas energized solely by shocked stellar winds in a quiescent superbubble. We have analyzed the diffuse X-ray emission from M17, and compared the observed X-ray luminosity of ~2.5*10^33 ergs/s and the hot gas temperature of ~8.5*10^6 K and mass of ~1 M_Sun to model predictions. We find that bubble models with heat conduction overpredict the X-ray luminosity by two orders of magnitude; the strong magnetic fields in M17, as measured from HI Zeeman observations, have most likely inhibited heat conduction and associated mass evaporation. Bubble models without heat conduction can explain the X-ray properties of M17, but only if cold nebular gas can be dynamically mixed into the hot bubble interior and the stellar winds are clumpy with mass-loss rates reduced by a factor of >=3. Future models of the M17 superbubble must take into account the large-scale density gradient, small-scale clumpiness, and strong magnetic field in the ambient interstellar medium.

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arXiv2003

Hard X-ray Emission Associated with White Dwarfs

We have used the WGACAT to search for hard X-ray sources associated with white dwarfs (WDs) from the catalog of McCook & Sion (1999). We find 17 X-ray sources coincident with WDs showing significant hard X-ray emission at energies >0.5 keV. Twelve of these WDs are in known binary systems, in two of which the accretion of the close companion's material onto the white dwarf produces the hard X-ray emission, and in the other ten of which the late-type companions' coronal activity emits hard X-rays. One WD is projected near an AGN which is responsible for the hard X-ray emission. The remaining four WDs and two additional white dwarfs with hard X-ray emission appear single. The lack of near-IR excess from the apparently single WDs suggests that either X-ray observations are more effective than near-IR photometry in diagnosing faint companions or a different emission mechanism is needed. It is intriguing that 50% of the six apparently single WDs with hard X-ray emission are among the hottest WDs. We have compared X-ray properties of 11 hot WDs with different spectral types, and conclude that stellar pulsation and fast stellar winds are not likely the origin of the hard X-ray emission, but a leakage of the high-energy Wien tail of emission from deep in the stellar atmosphere remains a tantalizing source of hard X-ray emission from hot DO and DQZO WDs. (This abstract is an abridged version.)

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arXiv2002

The Wind of the B[e] Supergiant Hen S22 Viewed through a Reflection Nebula in DEM L 106

Narrow-band HST WFPC2 images reveal a bow-shock-like halo around the HII region N30B toward the B[e] supergiant Hen S22 located within the larger DEM L 106 nebula in the Large Magellanic Cloud. High-dispersion spectra of N30B show a narrow H-alpha emission component from the ionized gas; the velocity variations indicate a gas flow of -5 to -10 km/s in the vicinity of the HII regions, which is resultant from interactions with Hen S22's stellar wind and responsible for the bow-shock morphology. Spectra of N30B's halo show broad H-alpha profiles extending over >1000 km/s, similar to that of Hen S22, indicating that the halo is a reflection nebula of Hen S22. Broad-band morphologies of N30B's halo are also consistent with the reflection nebula interpretation. We use dust-scattering properties and the observed brightnesses of the reflection nebula and Hen S22 to constrain the reflection geometry. The reflected stellar H-alpha emission and absorption vary across the reflection nebula as a result of viewing S22's anisotropic wind across varying angles. This reflection nebula, together with the edge-on orientation of Hen S22's disk, provides an invaluable opportunity to study the disk and polar winds of a B[e] supergiant.

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arXiv2001

Nature of widely separated ultraluminous infrared galaxies

In the complete sample of ultraluminous infrared galaxies (ULIRGs) compiled by Kim (1995) about 5% consists of widely separated galaxies which are presumably in the early phase of interaction. This fact is contrary to the conventional view that ULIRGs are in the final stages of the merger of two gas-rich disk galaxies. We have undertaken high resolution CO(J=1-0) observations for the ultraluminous infrared galaxies that have nuclear separations larger than 20 kpc. We have detected the CO emission in 5 out of 6 systems, but only in one component of the ULIRG pairs. 4 of them have LINER spectral type and 1 galaxy has Seyfert II spectral type. In K'-band images these components are also brighter than the other components which have either HII-region spectra or no detectable emission lines. Using the standard conversion factor, the molecular gas content is estimated to be a few times 10 $^{10}$ M$_\odot$, similar to that of the other ultraluminous galaxies. The result indicates that the galaxy containing the molecular gas is also the source of most, if not all, of the huge far-infrared luminosity of the system. The optical and K'-band imaging observations and optical spectra suggest multiple merger scenario for 1 system. If the remaining systems are in an early stage of a binary tidal interaction, the commonly accepted interpretation of the ULIRG phenomenon as the final merger stage of two disk galaxies may need to be re-examined.

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arXiv2001

Chandra Reveals the X-ray Glint in the Cat's Eye

We have obtained Chandra ACIS-S observations of NGC 6543, the Cat's Eye Nebula. The X-ray emission from NGC 6543 is clearly resolved into a point source at the central star and diffuse emission confined within the central elliptical shell and two extensions along the major axis. Spectral analysis of the diffuse component shows that the abundances of the X-ray-emitting gas are similar to those of the fast (1,750 km/s) stellar wind but not those of the nebula. Furthermore, the temperature of this gas is ~1.7 million K, which is 100 times lower than the expected post-shock temperature of the fast stellar wind. The combination of low temperature and wind abundances is puzzling. The thermal pressure of this hot gas is about twice the pressure in the cool nebular shell; thus, the hot gas plays an essential role in the ongoing evolution of the nebula.

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arXiv2000

A Search for Jovian Planets around Hot White Dwarfs

Current searches for extrasolar planets have concentrated on observing the reflex Doppler shift of solar-type stars. Little is known, however, about planetary systems around non-solar-type stars. We suggest a new method to extend planetary searches to hot white dwarfs. Near a hot white dwarf, the atmosphere of a Jovian planet will be photoionized and emit hydrogen recombination lines, which may be detected by high- dispersion spectroscopic observations. Multi-epoch monitoring can be used to distinguish between non-LTE stellar emission and planetary emission, and to establish the orbital parameters of the detected planets. In the future, high-precision astrometric measurements of the hot white dwarf will allow the masses of the detected planets to be determined. Searches for Jovian planets around hot white dwarfs will provide invaluable new insight on the development of planetary systems around stars more massive than the Sun and on how stellar evolution affects these systems. We present high-dispersion spectroscopic observations of the white dwarf Feige 34 to demonstrate the complexity and feasibility of the search method.

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