48 research outputs found
IC 4406: a radio-infrared view
IC 4406 is a large (about 100'' x 30'') southern bipolar planetary nebula,
composed of two elongated lobes extending from a bright central region, where
there is evidence for the presence of a large torus of gas and dust. We show
new observations of this source performed with IRAC (Spitzer Space Telescope)
and the Australia Telescope Compact Array. The radio maps show that the flux
from the ionized gas is concentrated in the bright central region and
originates in a clumpy structure previously observed in H_alpha, while in the
infrared images filaments and clumps can be seen in the extended nebular
envelope, the central region showing toroidal emission. Modeling of the
infrared emission leads to the conclusion that several dust components are
present in the nebula.Comment: 22 pages, 7 figures, accepted for publication in The Astrophysical
Journal; v.2 has changes in both figures and content; preprint forma
Filaments as Possible Signatures of Magnetic Field Structure in Planetary Nebulae
We draw attention to the extreme filamentary structures seen in
high-resolution optical images of certain planetary nebulae. We determine the
physical properties of the filaments in the nebulae IC 418, NGC 3132, and NGC
6537, and based on their large length-to-width ratios, longitudinal coherence,
and morphology, we suggest that they may be signatures of the underlying
magnetic field. The fields needed for the coherence of the filaments are
probably consistent with those measured in the precursor circumstellar
envelopes. The filaments suggest that magnetic fields in planetary nebulae may
have a localized and thread-like geometry.Comment: 26 pages with 7 figures. To be published in PASP. For full resolution
images see http://physics.nyu.edu/~pjh
Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple HII regions in Leo A and GR 8
obtained from long slit optical spectroscopy of these two nearby low luminosity
dwarf irregular galaxies. As expected from their luminosities, and in agreement
with previous observations, the derived oxygen abundances are extremely low in
both galaxies. High signal-to-noise ratio observations of a planetary nebula in
Leo A yield 12 + log(O/H) = 7.30 +/- 0.05; "semi-empirical" calculations of the
oxygen abundance in four HII regions in Leo A indicate 12 + log(O/H) = 7.38 +/-
0.10. These results confirm that Leo A has one of the lowest ISM metal
abundances of known nearby galaxies. Based on results from two HII regions with
high signal-to-noise measurements of the weak [O III] 4363 line, the mean
oxygen abundance of GR 8 is 12 + log(O/H) = 7.65 +/- 0.06; using "empirical"
and "semi-empirical" methods, similar abundances are derived for 6 other GR 8
HII regions. Similar to previous results in other low metallicity galaxies, the
mean log(N/O) = -1.53 +/- 0.09 for Leo A and -1.51 +/- 0.07 for GR 8. There is
no evidence of significant variations in either O/H or N/O in the HII regions.
The metallicity-luminosity relation for nearby (D < 5 Mpc) dwarf irregular
galaxies with measured oxygen abundances has a mean correlation of 12 +
log(O/H) = 5.67 - 0.151 M_B with a dispersion in oxygen about the relationship
of 0.21. These observations confirm that gas-rich low luminosity galaxies have
extremely low elemental abundances in the ionized gas-phase of their
interstellar media. Although Leo A has one of the lowest metal abundances of
known nearby galaxies, detection of tracers of an older stellar population
indicate that it is not a newly formed galaxy as has been proposed for some
other similarly low metallicity star forming galaxies.Comment: 36 pages, 7 figures. Accepted to Ap
Are He and N Abundances in Type I PNe as High as Empirically Derived?
Type I planetary nebulae (PNe) are defined as those with high He and N
abundances (Peimbert & Torres-Peimbert 1983). These objects present in general
bipolar geometries and have high stellar temperatures (Corradi & Schwarz 1995,
Torres-Peimbert & Peimbert 1997). In this paper we analyse the empirical
methods for abundance determination in order to check if the He and N
overabundances in Type I PNe are a consequence of a geometrical effect, due to
the bipolarity, or the ionization stratification, due to the stellar
temperature. For this, we obtain simulated spherically symmetrical as well as
bipolar nebulae, using a 3D photoionization code. From the projected emission
line intensities for: a) the whole nebula; b) for a slit crossing the nebula;
as well as c) for different positions in the nebula, we applied the formulae
used in the literature to obtain empirical abundances. These empirical
abundances are then compared with the adopted ones. We show that empirical
abundances depend on the particular line of sight covered by the observation
and can simulate an overabundance and/or the presence of abundance gradients of
He and N in planetary nebulae with high stellar temperature. The geometrical
effects are also discussed. Systematic errors in abundance determinations by
empirical methods are higher for the N/H ratio than for N/O. Thus, it seems
better to use the N/O value when discussing N rich objects.Comment: 19 pages, 5 figures, aastex package, to be published at Ap.
Temperature of the central stars of planetary nebulae and the effect of the nebular optical depth
The effect of the nebula optical depth on the determination of the
temperature (T*) of the central stars in planetary nebulae is discussed. Based
on photoionization models for planetary nebulae with different optical depths,
we show, quantitatively, that the details of the distribution of the H and He
II Zanstra temperatures are mainly explained by an optical depth effect; in
particular, that the discrepancy is larger for low stellar temperatures. The
results also show that for high stellar temperatures the He II Zanstra
temperature underestimates the stellar temperature, even for high optical
depths. The stellar temperature, as well as the optical depth, can be obtained
from a Zanstra temperature ratio (ZR) plot ZR = Tz(He II)/Tz(H) versus Tz(He
II). The effects of departures from a blackbody spectrum, as well as of the He
abundance in the nebulae, are also discussed. For nebulae of very low optical
depth and/or high stellar temperature the distribution ZR versus Tz(He II) only
provides lower limits for T*. In order to obtain better values for the optical
depth and T*, we propose the use of the line intensity ratio He II/He I versus
Tz(He II) diagram.Comment: 24 pages, 4 figures; to appear in ApJ, Nov 200
On the Origin of Planetary Nebula K648 in Globular Cluster M15
We examine two scenarios for formation of the planetary nebula K648: a prompt
scenario where the planetary nebula is ejected and formed immediately after a
helium shell flash and a delayed scenario where a third dredge up occurs and
the envelope is ejected during the following interpulse phase. We present
models of both scenarios and find that each can produce K648-like systems. We
suggest that the prompt scenario is more favorable but cannot rule out the
delayed scenario.Comment: 19 pages, 2 figures, latex, 2 files containing postscript figures.
Paper to appear in Astrophysical Journal, 483, 837 (1997
A Hubble Space Telescope Survey for Resolved Companions of Planetary-Nebula Nuclei
We report results of an HST "snapshot" survey aimed at finding resolved
binary companions of the central stars of Galactic planetary nebulae (PNe).
Using WF/PC and WFPC2, we searched the fields of 113 PNe for stars whose close
proximity to the central star suggests a physical association. We find 10
binary nuclei that are very likely to be physically associated, and another six
that are possible binary associations. By correcting for interstellar
extinction and placing the central stars' companions on the main sequence, we
derive distances to the objects, and thereby significantly increase the number
of PNe with reliable distances.
Comparison of our derived distances with those obtained from various
statistical methods shows that all of the latter have systematically
overestimated the distances, by factors ranging up to a factor of two or more.
We show that this error is most likely due to the fact that the properties of
our PNe with binary nuclei are systematically different from those of PNe used
heretofore to calibrate statistical methods. Specifically, our PNe tend to have
lower surface brightnesses at the same physical radius than the traditional
calibration objects. This difference may arise from a selection effect: the PNe
in our survey are typically nearby, old nebulae, whereas most of the objects
that calibrate statistical techniques are low-latitude,
high-surface-brightness, and more distant nebulae. As a result, the statistical
methods that seem to work well with samples of distant PNe, e.g., those in the
Galactic bulge or external galaxies, may not be applicable to the more diverse
population of local PNe.Comment: 37 text pages, 17 table pages, 9 figures. Accepted by Astronomical
Journal for June 1999 issu
3He in Planetary Nebulae: A Challenge to Stellar Evolution Models
The discrepancy between the observed abundances of 3He in the ISM and those
predicted by stellar and galactic chemical evolution remains largely
unexplained. In this paper, we attempt to shed some light on this unsolved
problem by presenting a quantitative comparison of the 3He abundances recently
measured in six planetary nebulae (PNe) with the corresponding predictions of
stellar evolution theory. The determination of the mass of the PNe progenitors
allows us to dismiss, to a good degree of confidence, the hypothesis that the
abundance of 3He in the envelope of all low-mass stars is strongly reduced with
respect to the standard theoretical values by some mixing mechanism acting in
the latest phases of stellar evolution. The abundance versus mass correlation,
allowance made for the limitation of the sample, is in fact found to be fully
consistent with the classical prediction of stellar evolution. We examine the
implications of this result on the galactic evolution of 3He with the help of a
series of models with standard and non-standard nucleosynthesis prescriptions.
The results are found to be consistent with the observed galactic abundances
only if the vast majority of low-mass stars follows non-standard prescriptions.
This implies that either the sample of PNe nebulae under exam is highly biased,
or the solution to the 3He problem lies elsewhere.Comment: 13 pages, LaTeX, 4 Figures. To be published in The Astrophysical
Journa
Investigating potential planetary nebula/cluster pairs
Fundamental parameters characterizing the end-state of intermediate-mass
stars may be constrained by discovering planetary nebulae (PNe) in open
clusters (OCs). Cluster membership may be exploited to establish the distance,
luminosity, age, and physical size for PNe, and the intrinsic luminosity and
mass of its central star. Four potential PN-OC associations were investigated,
to assess the cluster membership for the PNe. Radial velocities were measured
from intermediate-resolution optical spectra, complemented with previous
estimates in the literature. When the radial velocity study supported the PN/OC
association, we analyzed if other parameters (e.g., age, distance, reddening,
central star brightness) were consistent with this conclusion. Our measurements
imply that the PNe VBe3 and HeFa1 are not members of the OCs NGC5999 and
NGC6067, respectively, and likely belong to the background bulge population.
Conversely, consistent radial velocities indicate that NGC2452/NGC2453 could be
associated, but our results are not conclusive and additional observations are
warranted. Finally, we demonstrate that all the available information point to
He2-86 being a young, highly internally obscured PN member of NGC4463. New
near-infrared photometry acquired via the Vista Variables in the Via Lactea ESO
public survey was used in tandem with existing UBV photometry to measure the
distance, reddening, and age of NGC4463, finding d=1.55+-0.10 kpc,
E(B-V)=0.41+-0.02, and tau=65+-10 Myr, respectively. The same values should be
adopted for the PN if the proposed cluster membership will be confirmed.Comment: Accepted for publication in A&