892 research outputs found
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.Comment: 5 pages, 3 figures; to appear in "Asymmetrical Planetary Nebulae III"
editors M. Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series
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.Comment: 4 pages, 2 figures, accepted in Astronomy and Astrophysics Letter
Luminous IR Galaxies in a Merger Sequence: BIMA CO Imaging
In order to trace observationally the conditions in the interstellar medium
(ISM) that lead to starbursts, we have used the newly expanded
Berkeley-Illinois-Maryland Association (BIMA) millimeter-wave array to map the
molecular ISM in a sample of LIGs chosen to represent different phases of the
interacting/merging process. CO images of 6 galaxy mergers at the
early/intermediate stage are shown here and preliminary results are summarized.Comment: 4 pages including a postscript figure; talk presentation at IAU Symp.
186 "Galaxy Interactions at Low and High Redshift", J. Barnes & D. Sanders
(eds.) in pres
X-ray Emission from the Wolf-Rayet Bubble NGC 6888. I. Chandra ACIS-S Observations
We analyze Chandra observations of the Wolf-Rayet (WR) bubble NGC 6888. This
WR bubble presents similar spectral and morphological X-ray characteristics to
those of S 308, the only other WR bubble also showing X-ray emission. The
observed spectrum is soft, peaking at the N VII line emission at 0.5 keV with
additional line emission at 0.7 - 0.9 keV and a weak tail of harder emission up
to ~1.5 keV. This spectrum can be described by a two-temperature optically thin
plasma emission model (T_{1}~1.4x10^{6} K, T_{2}~7.4x10^{6} K). We confirm the
results of previous X-ray observations that no noticeable temperature
variations are detected in the nebula. The X-ray-emitting plasma is distributed
in three apparent morphological components: two caps along the tips of the
major axis and an extra contribution toward the northwest blowout not reported
in previous analysis of the X-ray emission toward this WR nebula. Using the
plasma model fits of the Chandra ACIS spectra for the physical properties of
the hot gas and the ROSAT PSPC image to account for the incomplete coverage of
Chandra observations, we estimate a luminosity of L_X = (7.7\pm0.1)x10^{33}
erg/s for NGC 6888 at a distance of 1.26 kpc. The average rms electron density
of the X-ray-emitting gas is >= 0.4 cm^{-3} for a total mass >= 1.2 Msun.Comment: 8 pages, 5 figures, 1 table; accepted for publication in Astronomical
Journa
Variable dust formation by the colliding-wind Wolf-Rayet system HD 36402 in the Large Magellanic Cloud
Infrared photometry of the probable triple WC4(+O?)+O8I: Wolf-Rayet system HD
36402 (= BAT99-38) in the Large Magellanic Cloud (LMC) shows emission
characteristic of heated dust. The dust emission is variable on a time-scale of
years, with a period near 4.7 yr, possibly associated with orbital motion of
the O8 supergiant and the inner P ~ 3.03-d WC4+O binary. The phase of maximum
dust emission is close to that of the X-ray minimum, consistent with both
processes being tied to colliding wind effects in an elliptical binary orbit.
It is evident that Wolf-Rayet dust formation occurs also in metal-poor
environments.Comment: 8 pages, 4 figures, to be published in MNRA
Hot gas in the Wolf-Rayet nebula NGC3199
The Wolf-Rayet (WR) nebula NGC3199 has been suggested to be a bow shock
around its central star WR18, presumably a runaway star, because optical images
of the nebula show a dominating arc of emission south-west of the star. We
present the XMM-Newton detection of extended X-ray emission from NGC3199,
unveiling the powerful effect of the fast wind from WR18. The X-ray emission is
brighter in the region south-east of the star and analysis of the spectral
properties of the X-ray emission reveals abundance variations: i) regions close
to the optical arc present nitrogen-rich gas enhanced by the stellar wind from
WR18 and ii) gas at the eastern region exhibits abundances close to those
reported for nebular abundances derived from optical studies, signature of an
efficient mixing of the nebular material with the stellar wind. The dominant
plasma temperature and electron density are estimated to be
10 K and =0.3 cm with an X-ray
luminosity in the 0.3-3.0 keV energy range of
=2.610 erg s. Combined with information
derived from Herschel and the recent Gaia first data release, we conclude that
WR18 is not a runaway star and the formation, chemical variations, and shape of
NGC3199 depend on the initial configuration of the interstellar medium.Comment: 12 pages, 6 figures, 1 table; Accepted for publication in Ap
Diffuse X-ray Emission within Wolf-Rayet Nebulae
We discuss our most recent findings on the diffuse X-ray emission from
Wolf-Rayet (WR) nebulae. The best-quality X-ray observations of these objects
are those performed by XMM-Newton and Chandra towards S308, NGC2359, and
NGC6888. Even though these three WR nebulae might have different formation
scenarios, they all share similar characteristics: i) the main plasma
temperatures of the X-ray-emitting gas is found to be =[1-2]10
K, ii) the diffuse X-ray emission is confined inside the [O III] shell, and
iii) their X-ray luminosities and electron densities in the 0.3-2.0~keV energy
range are 10-10~erg~s and
0.1-1~cm, respectively. These properties and the
nebular-like abundances of the hot gas suggest mixing and/or thermal conduction
is taking an important role reducing the temperature of the hot bubble.Comment: 3 pages, 1 figure; International Workshop on Wolf-Rayet Star
X-ray emission from the Wolf-Rayet bubble NGC 6888. II. XMM-Newton EPIC observations
We present deep XMM-Newton EPIC observations of the Wolf-Rayet (WR) bubble
NGC6888 around the star WR136. The complete X-ray mapping of the nebula
confirms the distribution of the hot gas in three maxima spatially associated
with the caps and northwest blowout hinted at by previous Chandra observations.
The global X-ray emission is well described by a two-temperature optically thin
plasma model =1.410 K, =8.210 K) with a
luminosity of =7.810 erg s in the
0.3--1.5 keV energy range. The rms electron density of the X-ray-emitting gas
is estimated to be =0.4 cm. The high-quality observations
presented here reveal spectral variations within different regions in NGC6888,
which allowed us for the first time to detect temperature and/or nitrogen
abundance inhomogeneities in the hot gas inside a WR nebula. One possible
explanation for such spectral variations is that the mixing of material from
the outer nebula into the hot bubble is less efficient around the caps than in
other nebular regions.Comment: 10 pages, 4 figures, 2 tables; Accepted to MNRA
Diffuse X-ray Emission from Planetary Nebulae with Nebular O VI
The presence of O VI ions can be indicative of plasma temperatures of a few
times 10^5 K that is expected in heat conduction layers between the hot shocked
stellar wind gas at several 10^6 K and the cooler (~10,000 K) nebular gas of
planetary nebulae (PNe). We have used FUSE observations of PNe to search for
nebular O VI emission or absorption as a diagnostic of conduction layer to
ensure the presence of hot interior gas. Three PNe showing nebular O VI, namely
IC 418, NGC 2392, and NGC 6826, have been selected for Chandra observations and
diffuse X-ray emission is indeed detected in each of these PNe. Among the
three, NGC 2392 has peculiarly high diffuse X-ray luminosity and plasma
temperature compared with those expected from its stellar wind's mechanical
luminosity and terminal velocity. The limited effects of heat conduction on the
plasma temperature of a hot bubble at the low terminal velocity of the stellar
wind of NGC 2392 may partially account for its high plasma temperature, but the
high X-ray luminosity needs to be powered by processes other than the observed
stellar wind, probably caused by the presence of an unseen binary companion of
the CSPN of NGC 2392. We have compiled relevant information on the X-ray,
stellar, and nebular properties of PNe with a bubble morphology and found that
the expectations of bubble models including heat conduction compare favorably
with the present X-ray observations of hot bubbles around H-rich CSPNe, but
have notable discrepancies for those around H-poor [WR] CSPNe. We note that PNe
with more massive central stars can produce hotter plasma and higher X-ray
surface brightness inside central hot bubbles.Comment: 12 pages, 6 figures, accepted by Ap
Physical Nature of the [S II]-Bright Shell Nebulae N70 and N185
N70 and N185 are two large (100 pc in diameter) shell nebulae in the
Large Magellanic Cloud (LMC). Their high [\ion{S}{2}]/H ratios rival
those of supernova remnants (SNRs), but they are not confirmed as SNRs. To
study their physical nature, we have obtained \emph{XMM-Newton} X-ray
observations and high-dispersion long-slit echelle spectroscopic observations
of these two nebulae. The X-ray spectra of both nebulae can be well interpreted
with an optically thin thermal (0.2 keV) plasma with the average LMC
abundance in a collisional ionization equilibrium. N70 encompasses the OB
association LH114. Although N70 has a modest expansion velocity and essentially
thermal radio emission, its diffuse X-ray luminosity (
erg s) is higher than that from a quiescent superbubble with N70's
density, size, and expansion velocity; thus, N70 is most likely a superbubble
that is recently energized by an interior SNR. N185 does not contain any known
OB association, and its X-ray luminosity is an order of magnitude lower than
expected if it is a quiescent superbubble. N185 has nonthermal radio emission
and has high-velocity material expanding at nearly 200 km s, similar to
many known SNRs in the LMC. Its X-ray luminosity ( erg
s) is also consistent with that of an evolved SNR. We therefore suggest
that N185 is energized by a recent supernova.Comment: 11 pages, 9 figures, Published in Ap
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