191 research outputs found
X-ray Emission from Planetary Nebulae and their Central Stars: a Status Report
In the era of Chandra and XMM-Newton, the detection (or nondetection) of
diffuse and/or point-like X-ray sources within planetary nebulae (PNe) yields
important, unique insight into PN shaping processes. Diffuse X-ray sources,
whether due to ``hot bubbles'' or to collimated outflows or jets, allow us to
probe the energetic shocks within PN wind interaction regions. Meanwhile, X-ray
point sources provide potential diagnostics of magnetic fields, accretion
disks, and/or binary companions at PN cores. Here, I highlight recent X-ray
observational results and trends that have the potential to shed new light on
the origin and evolution of the structure of PNe.Comment: 8 pages, 3 figures; to appear in proceedings of "Asymmetrical
Planetary Nebulae IV" (ed. R. Corradi et al.
The Egg Nebula (AFGL 2688): Deepening Enigma
Recent observations of the Egg Nebula (AFGL 2688), obtained at
ever-increasing spatial and spectral resolution, have revealed a perplexing
array of phenomena. Many of these phenomena present challenges to our
understanding of this object as an emerging, bipolar planetary nebula. Here, we
consider two particularly intriguing aspects of the Egg: the peculiar structure
and kinematics of its equatorial regions, and the nature of an apparent widely
separated companion to the central star. In the first case, we use recently
acquired Hubble Space Telescope images to demonstrate that the H2 emission
distributed east and west of the central star is spatially coincident with a
dusty, equatorial disk or torus. The H2 is thus constrained to lie near the
equatorial plane, casting doubt on pure radial outflow models for the
equatorial kinematics. In the second case, we show that the apparent companion
(``Peak A'') may be an accreting white dwarf that has undergone one or more
thermonuclear bursts.Comment: 4 pages, 1 figure; to appear in "Asymmetrical Planetary Nebulae III"
editors M. Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series
Constraining the X-ray Luminosities of Asymptotic Giant Branch Stars: TX Cam and T Cas
To probe the magnetic activity levels of asymptotic giant branch (AGB) stars,
we used XMM-Newton to search for X-ray emission from two well-studied objects,
TX Cam and T Cas. The former star displays polarized maser emission indicating
magnetic field strengths of B ~ 5 G; the latter is one of the nearest known AGB
stars. Neither star was detected by XMM-Newton. We use the upper limits on EPIC
(CCD detector) count rates to constrain the X-ray luminosities of these stars,
and derive L_X < 10^{31} erg s^{-1} (<10^{30} erg s^{-1}) for an assumed X-ray
emission temperature T_X = 3x10^6 K (10^7 K). These limits represent <~ 10% (<~
1%) of the X-ray luminosity expected under models in which AGB magnetic fields
are global and potentially play an important role in collimating and/or
launching AGB winds. We suggest, instead, that the B field strengths inferred
from maser observations are representative of localized, magnetic clouds.Comment: 12 pages, 1 figure; submitted to the Astrophysical Journa
Serendipitous XMM-Newton Detection of X-ray Emission from the Bipolar Planetary Nebula Hb 5
We report the serendipitous detection by the XMM-Newton X-ray Observatory of
an X-ray source at the position of the Type I (He- and N-rich) bipolar
planetary nebula Hb 5. The Hb 5 X-ray source appears marginally resolved. While
the small number of total counts (~170) and significant off-axis angle of the
X-ray source (~7.8') precludes a definitive spatial analysis, the morphology of
the X-ray emission appears to trace the brightest features seen in optical
images of Hb 5. The X-ray spectrum is indicative of a thermal plasma at a
temperature between 2.4 and 3.7 MK and appears to display strong Neon emission.
The inferred X-ray luminosity is L_X = 1.5 x 10^32 ergs/s. These results
suggest that the detected X-ray emission is dominated by shock-heated gas in
the bipolar nebula, although we cannot rule out the presence of a point-like
component at the position of the central star. The implications for and
correspondence with current models of shock-heated gas in planetary nebulae is
discussed.Comment: 11 pages, 2 figures, Accepted by The Astrophysical Journa
Pinpointing the Position of the Post-AGB Star at the Core of RAFGL 2688 using Polarimetric Imaging with NICMOS
We have used infrared polarimetric imaging with NICMOS to determine precisely
the position of the star that illuminates (and presumably generated) the
bipolar, pre-planetary reflection nebula RAFGL 2688 (the Egg Nebula). The
polarimetric data pinpoint the illuminating star, which is not detected
directly at wavelengths less than or equal to 2 microns, at a position well
within the dark lane that bisects the nebula, 0.55" (about 550 AU) southwest of
the infrared peak which was previously detected at the southern tip of the
northern polar lobe. The inferred position of the central star corresponds to
the geometric center of the tips of the four principle lobes of near-infrared
H2 emission; identifying the central star at this position also reveals the
strong point symmetric structure of the nebula, as seen both in the intensity
and polarization structure of the polar lobes. The polarimetric and imaging
data indicate that the infrared peak directly detected in the NICMOS images is
a self-luminous source and, therefore, is most likely a distant binary
companion to the illuminating star. Although present theory predicts that
bipolar structure in pre-planetary and planetary nebulae is a consequence of
binary star evolution, the separation between the components of the RAFGL 2688
binary system, as deduced from these observations, is much too large for the
presence of the infrared companion to have influenced the structure of the
RAFGL 2688 nebula.Comment: 15 pages, 6 figures, to appear in The Astrophysical Journa
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