27 research outputs found
Nonthermal Radio Emission from Planetary Nebulae
In a recent analysis of the radio emission from the planetary nebula A30,
Dgani, Evans & White (1998) claim that the emission, located in the inner
region, is probably dominated by nonthermal emission.
We propose a model to explain this.
We assume that the fast wind, blown by the central star of A30 carries a very
weak magnetic field. The interaction of this wind with a cluster of dense
condensations traps the magnetic field lines for a long time and stretches
them, leading to a strong magnetic field.
If relativistic particles are formed as the fast wind is shocked, then the
enhanced magnetic field will result in nonthermal radio emission.
The typical nonthermal radio flux at 1 GHz can be up to several milli-Jansky.
In order to detect the nonthermal emission, the emitting region should be
spatially resolved from the main optical nebula.
We list other planetary nebulae which may possess nonthermal radio emission.Comment: 11 page
X-ray emission from Planetary Nebulae. I. Spherically symmetric numerical simulations
(abridged) The interaction of a fast wind with a spherical Asymptotic Giant
Branch (AGB) wind is thought to be the basic mechanism for shaping
Pre-Planetary Nebulae (PPN) and later Planetary Nebulae (PN). Due to the large
speed of the fast wind, one expects extended X-ray emission from these objects,
but X-ray emission has only been detected in a small fraction of PNs and only
in one PPN. Using numerical simulations we investigate the constraints that can
be set on the physical properties of the fast wind (speed, mass-flux, opening
angle) in order to produce the observed X-ray emission properties of PPNs and
PNs. We combine numerical hydrodynamical simulations including radiative
cooling using the code FLASH with calculations of the X-ray properties of the
resulting expanding hot bubble using the atomic database ATOMDB. In this first
study, we compute X-ray fluxes and spectra using one-dimensional models.
Comparing our results with analytical solutions, we find some agreements and
many disagreements. In particular, we test the effect of different time
histories of the fast wind on the X-ray emission and find that it is determined
by the final stage of the time history during which the fast wind velocity has
its largest value. The disagreements which are both qualitative and
quantitative in nature argue for the necessity of using numerical simulations
for understanding the X-ray properties of PNs.Comment: 17 pages, accepted for publication in ApJ (July 27, 2006), uses
emulateap
On the Luminosities and Temperatures of Extended X-ray Emission from Planetary Nebulae
We examine mechanisms that may explain the luminosities and relatively low
temperatures of extended X-ray emission in planetary nebulae. By building a
simple flow structure for the wind from the central star during the proto, and
early, planetary nebulae phase, we estimate the temperature of the X-ray
emitting gas and its total X-ray luminosity. We conclude that in order to
account for the X-ray temperature and luminosity, both the evolution of the
wind from the central star and the adiabatic cooling of the post-shocked wind's
material must be considered. The X-ray emitting gas results mainly from shocked
wind segments that were expelled during the early planetary nebulae phase, when
the wind speed was moderate. Alternatively, the X-ray emitting gas may result
from a collimated fast wind blown by a companion to the central star. Heat
conduction and mixing between hot and cool regions are likely to occur in some
cases and may determine the detailed X-ray morphology of a nebula, but are not
required to explain the basic properties of the X-ray emitting gas.Comment: ApJ, submitted; 16 page
Suzaku Reveals Helium-burning Products in the X-ray Emitting Planetary Nebula BD+303639
BD+303639, the brightest planetary nebula at X-ray energies, was observed
with Suzaku, an X-ray observatory launched on 2005 July 10. Using the X-ray
Imaging Spectrometer, the K-lines from C VI, O VII, and O VIII were resolved
for the first time, and C/O, N/O, and Ne/O abundance ratios determined. The C/O
and Ne/O abundance ratios exceed the solar value by a factor of at least 30 and
5, respectively. These results indicate that the X-rays are emitted mainly by
helium shell-burning products.Comment: 12 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
The Angular Expansion and Distance of the Planetary Nebula BD+30 3639
The WFPC2 camera aboard the HST was used to obtain images of the planetary
nebula BD+30 3639 at two epochs separated by 5.663 years. The expansion of the
nebula in the H-alpha and [N II] bands has been measured using several methods.
Detailed expansion maps for both emission lines were constructed from nearly
200 almost independent features. There is good agreement between the
(independent) H-alpha and [N II] proper motions. The central velocity split is
measured from the STIS echelle spectra of the C II] 2326A multiplet to be
+/-36.3 km/s at a position angle of 99 degrees. The angular displacement along
this slit position was measured to be 4.25 mas/yr at 2".47 from the center. We
constructed a tilted ellipsoidal shell model by fitting the radio brightness
variation of the 5 and 15 GHz VLA observations, and making use of the
ground-based echelle spectra from Bryce & Mellema (1999), to estimate distance.
Our model has an axial ratio of 1.56, is inclined to the line of sight by 9.7
degrees, and exhibits an expansion in the plane of the sky which is 2/3 that in
the radial direction, leading to a distance of 1.2 kpc. Not all the kinematic
data fits this simple model, so the distance must still be regarded as
uncertain. Based on the recent model atmosphere of Crowther et al. (2002), a
distance of 1.2 kpc implies a stellar luminosity of 4250 L_sun. The kinematic
age of the nebula varies somewhat from region to region; a good average value
is 800 years, while the expansion along the position of the echelle slit gives
about 600 years.Comment: 21 pages, 9 figures, accepted by AJ, May 200
Discovery of Extended X-ray Emission from the Planetary Nebula NGC 7027 by the Chandra X-ray Observatory
We report the discovery of X-ray emission from NGC 7027, a prototypical
object for the study of the formation and evolution of planetary nebulae (PNs).
Observations with the Advanced CCD Imaging Spectrometer (ACIS) aboard the
Chandra X-ray Observatory show that the X-ray emission from NGC 7027 is
extended and is bipolar in morphology. The ACIS spectrum displays strong
emission from highly ionized Ne and weaker emission features which we attribute
to O, Mg, and Si. Model fits to this spectrum suggest a characteristic
temperature T_x ~ 3x10^6 K and an intrinsic (unabsorbed) X-ray luminosity of
L_x ~ 1.3x10^32 ergs/s. The intranebular absorption of X-ray emission is highly
nonuniform, but the modeling indicates an average column density N_H ~ 6x10^21
cm^-2, consistent with previous measurements of relatively large visual
extinction within the nebula. We suggest that the X-ray emission from NGC 7027
is or was generated by a hitherto undetected fast wind from the central star of
NGC 7027, or from a companion to this star. Chandra's detection of extended,
high-temperature X-ray emission from BD +30 3639, NGC 6543, and now NGC 7027
suggests that such emission is a common feature of young planetary nebulae.Comment: 13 pages, 4 figures (fig 1 is color); accepted by The Astrophysical
Journal (Letters
What are the hot R Coronae Borealis stars?
We investigate the evolutionary status of four stars: V348 Sgr, DY Cen, and MV Sgr in the Galaxy and HV 2671 in the LMC. These stars have in common random deep declines in visual brightness, which are characteristic of R Coronae Borealis (RCB) stars. RCB stars are typically cool hydrogen-deficient supergiants. The four stars studied in this paper are hotter (Teff = 15–20 kK) than the majority of RCB stars (Teff = 5000–7000 K). Although these are commonly grouped together as the hot RCB stars they do not necessarily share a common evolutionary history. We present new observational data and an extensive collection of archival and previously published data that is reassessed to ensure internal consistency. We find temporal variations of various properties on different timescales that will eventually help us to uncover the evolutionary history of these objects. DY Cen and MV Sgr have typical RCB helium abundances, which exclude any currently known post–asymptotic giant branch (post-AGB) evolutionary models. Moreover, their carbon and nitrogen abundances present us with further problems for their interpretation. V348 Sgr and HV 2671 are in general agreement with a born-again post-AGB evolution, and their abundances are similar to Wolf-Rayet central stars of planetary nebulae (PNs). The three Galactic stars in the sample have circumstellar nebulae, which produce forbidden line radiation (for HV 2671 we have no information). V348 Sgr and DY Cen have low-density, low-expansion velocity nebulae (resolved in the case of V348 Sgr), while MV Sgr has a higher density, higher expansion velocity nebula. All three stars, on the other hand, have split emission lines, which indicate the presence of an equatorial bulge but not of a Keplerian disk. In addition, the historical light curves for the three Galactic hot RCB stars show evidence for a significant fading in their maximum-light brightnesses of ~1 mag over the last 70 yr. From this we deduce that their effective temperatures increased by a few thousand degrees. If V348 Sgr is a born-again star, as we presume, this means that the star is returning from the born-again AGB phase to the phase of a central star of PN. Spectroscopically, no dramatic change is observed over the last 50 years for V348 Sgr and MV Sgr. However, there is some evidence that the winds of V348 Sgr and DY Cen have increased in strength in the last decade. HV 2671, located in the LMC, has not been analyzed in detail but at 5 Å… resolution is almost identical to V348 Sgr. Through the bolometric correction derived for V348 Sgr and the known distance, we can estimate the absolute ν magnitude of HV 2671 (Mν = -3.0 mag) and its bolometric luminosity (~6000 L⊙)
A Spectroscopic Study of a Large Sample of Wolf-Rayet Galaxies
We analyze long-slit spectral observations of 39 Wolf-Rayet (WR) galaxies
with heavy element mass fraction ranging over 2 orders of magnitude, from
Zsun/50 to 2Zsun. Nearly all galaxies in our sample show broad WR emission in
the blue region of the spectrum (the blue bump) consisting of an unresolved
blend of N III 4640, C III 4650, C IV 4658 and He II 4686 emission lines. Broad
C IV 5808 emission (the red bump) is detected in 30 galaxies. Additionally,
weaker WR emission lines are identified, most often the N III 4512 and Si III
4565 lines, which have very rarely or never been seen and discussed before in
WR galaxies. These emission features are characteristic of WN7-WN8 and WN9-WN11
stars respectively.
We derive the numbers of early WC (WCE) and late WN (WNL) stars from the
luminosities of the red and blue bumps, and the number of O stars from the
luminosity of the Hbeta emission line. Additionally, we propose a new technique
for deriving the numbers of WNL stars from the N III 4512 and Si III 4565
emission lines. This technique is potentially more precise than the blue bump
method because it does not suffer from contamination of WCE and early WN (WNE)
stars and nebular gaseous emission.
The N(WR)/N(O+WR) ratio decreases with decreasing metallicity, in agreement
with predictions of evolutionary synthesis models. The N(WC)/N(WN) ratios and
the equivalent widths of the blue bump EW(4650) and of the red bump EW(5808)
derived from observations are also in satisfactory agreement with theoretical
predictions.Comment: 49 pages, 9 figures, to appear in Astrophys.
Chandra X-ray Observatory Detection of Extended X-ray Emission from the Planetary Nebula BD+303639
We report the detection of well resolved, extended X-ray emission from the
young planetary nebula BD+303639 using the Advanced CCD Imaging Spectrometer
(ACIS) aboard the Chandra X-ray Observatory. The X-ray emission from BD+303639
appears to lie within, but is concentrated to one side of, the interior of the
shell of ionized gas seen in high-resolution optical and IR images. The
relatively low X-ray temperature (Tx ~ 3x10^6 K) and asymmetric morphology of
the X-ray emission suggests that conduction fronts are present and/or mixing of
shock-heated and photoionized gas has occurred and, furthermore, hints at the
presence of magnetic fields. The ACIS spectrum suggests that the X-ray emitting
region is enriched in the products of helium burning. Our detection of extended
X-ray emission from BD+303639 demonstrates the power and utility of Chandra
imaging as applied to the study of planetary nebulae.Comment: 9 pages, 3 figures; to be published in the Astrophysical Journal
(Letters
Central Stars of Planetary Nebulae in the Large Magellanic Cloud: A Far-UV Spectroscopic Analysis
We observed seven central stars of planetary nebulae (CSPN) in the Large
Magellanic Cloud (LMC) with the Far Ultraviolet Spectroscopic Explorer (FUSE),
and performed a model-based analysis of these spectra in conjunction with
Hubble Space Telescope (HST) spectra in the UV and optical range to determine
the stellar and nebular parameters. Most of the objects show wind features, and
they have effective temperatures ranging from 38 to 60 kK with mass-loss rates
of ~= 5x10^-8 Msun/yr. Five of the objects have typical LMC abundances. One
object (SMP LMC 61) is a [WC4] star, and we fit its spectra with He/C/O-rich
abundances typical of the [WC] class, and find its atmosphere to be
iron-deficient. Most objects have very hot (T ~> 2000 K) molecular hydrogen in
their nebulae, which may indicate a shocked environment. One of these (SMP LMC
62) also displays OVI 1032-38 nebular emission lines, rarely observed in PN.Comment: 53 pages, 15 figures (11 color). Accepted for publication in Ap