40 research outputs found
Detection of the Central Star of the Planetary Nebula NGC 6302
NGC 6302 is one of the highest ionization planetary nebulae known and shows
emission from species with ionization potential >300eV. The temperature of the
central star must be >200,000K to photoionize the nebula, and has been
suggested to be up to ~ 400,000K. On account of the dense dust and molecular
disc, the central star has not convincingly been directly imaged until now. NGC
6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as
part of the Servicing Mission 4 Early Release Observations. The central star is
directly detected for the first time, and is situated at the nebula centre on
the foreground side of the tilted equatorial disc. The magnitudes of the
central star have been reliably measured in two filters(F469N and F673N).
Assuming a hot black body, the reddening has been measured from the
(4688-6766\AA) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main
sequence binary companion can be excluded. The position of the star on the HR
diagram suggests a fairly massive PN central star of about 0.64,M_sun close to
the white dwarf cooling track. A fit to the evolutionary tracks for
(T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained;
however the luminosity and temperature remain uncertain. The model tracks
predict that the star is rapidly evolving, and fading at a rate of almost 1 %
per year. Future observations could test this prediction.Comment: 13 pages, 5 figures, submitted to ApJ Letters on 25.09.2009 accepted
on 19.10.200
Identifying Young Stellar Objects in the Outer Galaxy: l = 224 deg Region in Canis Major
We study a very young star-forming region in the outer Galaxy that is the
most concentrated source of outflows in the Spitzer Space Telescope GLIMPSE360
survey. This region, dubbed CMa-l224, is located in the Canis Major OB1
association. CMa-l224 is relatively faint in the mid-infrared, but it shines
brightly at the far-infrared wavelengths as revealed by the Herschel Space
Observatory data from the Hi-GAL survey. Using the 3.6 and 4.5 m data from
the Spitzer/GLIMPSE360 survey, combined with the JHK 2MASS and the 70-500
m Herschel/Hi-GAL data, we develop a young stellar object (YSO) selection
criteria based on color-color cuts and fitting of the YSO candidates' spectral
energy distributions with YSO 2D radiative transfer models. We identify 293 YSO
candidates and estimate physical parameters for 210 sources well-fit with YSO
models. We select an additional 47 sources with GLIMPSE360-only photometry as
`possible YSO candidates'. The vast majority of these sources are associated
with high H column density regions and are good targets for follow-up
studies. The distribution of YSO candidates at different evolutionary stages
with respect to Herschel filaments supports the idea that stars are formed in
the filaments and become more dispersed with time. Both the supernova-induced
and spontaneous star formation scenarios are plausible in the environmental
context of CMa-l224. However, our results indicate that a spontaneous
gravitational collapse of filaments is a more likely scenario. The methods
developed for CMa-l224 can be used for larger regions in the Galactic plane
where the same set of photometry is available.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series; 54 pages including appendice
Validating post-AGB candidates in the LMC and SMC using SALT spectra
We selected a sample of post-AGB candidates in the Magellanic Clouds on the
basis of their near- and mid-infrared colour characteristics. Fifteen of the
most optically bright post-AGB candidates were observed with the South African
Large Telescope in order to determine their stellar parameters and thus to
validate or discriminate their nature as post-AGB objects in the Magellanic
Clouds. The spectral types of absorption-line objects were estimated according
to the MK classification, and effective temperatures were obtained by means of
stellar atmosphere modelling. Emission-line objects were classified on the
basis of the fluxes of the emission lines and the presence of the continuum.
Out of 15 observed objects, only 4 appear to be genuine post-AGB stars (27\%).
In the SMC, 1 out of 4 is post-AGB, and in the LMC, 3 out 11 are post-AGB
objects. Thus, we can conclude that the selected region in the colour-colour
diagram, while selecting the genuine post-AGB objects, overlaps severely with
other types of objects, in particular young stellar objects and planetary
nebulae. Additional classification criteria are required to distinguish between
post-AGB stars and other types of objects. In particular, photometry at far-IR
wavelengths would greatly assist in distinguishing young stellar objects from
evolved ones. On the other hand, we showed that the low-resolution optical
spectra appear to be sufficient to determine whether the candidates are
post-AGB objects.Comment: 19 pages, 19 figures, A&A in pres
HST Snapshot Survey of Post-AGB Objects
The results from a Hubble Space Telescope (HST) snapshot survey of post-AGB
objects are shown. The aim of the survey is to complement existing HST images
of PPN and to connect various types of nebulosities with physical and chemical
properties of their central stars. Nebulosities are detected in 15 of 33
sources. Images and photometric and geometric measurements are presented. For
sources with nebulosities we see a morphological bifurcation into two groups,
DUPLEX and SOLE, as previous studies have found. We find further support to the
previous results suggesting that this dichotomy is caused by a difference in
optical thickness of the dust shell. The remaining 18 sources are classified as
stellar post-AGB objects, because our observations indicate a lack of
nebulosity. We show that some stellar sources may in fact be DUPLEX or SOLE
based on their infrared colors. The cause of the differences among the groups
are investigated. We discuss some evidence suggesting that high progenitor-mass
AGB stars tend to become DUPLEX post-AGB objects. Intermediate progenitor-mass
AGB stars tend to be SOLE post-AGB objects. Most of the stellar sources
probably have low mass progenitors and do not seem to develop nebulosities
during the post-AGB phase and therefore do not become planetary nebulae.Comment: 21 pages, 11 figure
Survey of Planetary Nebulae at 30 GHz with OCRA-p
We report the results of a survey of 442 planetary nebulae at 30 GHz. The
purpose of the survey is to develop a list of planetary nebulae as calibration
sources which could be used for high frequency calibration in future. For 41
PNe with sufficient data, we test the emission mechanisms in order to evaluate
whether or not spinning dust plays an important role in their spectra at 30
GHz.
The 30-GHz data were obtained with a twin-beam differencing radiometer,
OCRA-p, which is in operation on the Torun 32-m telescope. Sources were scanned
both in right ascension and declination. We estimated flux densities at 30 GHz
using a free-free emission model and compared it with our data.
The primary result is a catalogue containing the flux densities of 93
planetary nebulae at 30 GHz. Sources with sufficient data were compared with a
spectral model of free-free emission. The model shows that free-free emission
can generally explain the observed flux densities at 30 GHz thus no other
emission mechanism is needed to account for the high frequency spectra.Comment: 10 pages, 7 Postscript figures, to be published in A&
New 6cm and 11cm observations of the supernova remnant CTA 1
(Abridged) We conducted new 6cm and 11cm observations of CTA 1 using the
Urumqi 25-m and Effelsberg 100-m telescopes. Data at other wavelengths were
included to investigate the spectrum and polarisation properties. We obtained
new total intensity and polarisation maps at 6cm and 11cm with angular
resolutions of 9.5 arcmin and 4.4 arcmin, respectively. We derived a spectral
index of alpha=-0.63+/-0.05 based on the integrated flux densities at 408 MHz,
1420 MHz, 2639 MHz, and 4800 MHz. The spectral index map calculated from data
at the four frequencies shows a clear steepening of the spectrum from the
strong shell emission towards the north-western breakout region with weak
diffuse emission. The decrease of the spectral index is up to about 0.3. The RM
map derived from polarisation data at 6cm and 11cm shows a sharp transition
between positive RMs in the north-eastern and negative RMs in the south-western
part of the SNR. We note a corresponding RM pattern of extragalactic sources
and propose the existence of a large-diameter Faraday screen in front of CTA 1,
which covers the north-eastern part of the SNR. The RM of the Faraday screen is
estimated to be about +45 rad/m2. A RM structure function of CTA 1 indicates a
very regular magnetic field within the Faraday screen, which is larger than
about 2.7 microG in case of 500 pc distance.Comment: 9 pages, 12 figures, minor changes, accepted for publication in A&
Variability in Proto-Planetary Nebulae: I. Light Curve Studies of 12 Carbon-Rich Objects
We have carried out long-term (14 years) V and R photometric monitoring of 12
carbon-rich proto-planetary nebulae. The light and color curves display
variability in all of them. The light curves are complex and suggest multiple
periods, changing periods, and/or changing amplitudes, which are attributed to
pulsation. A dominant period has been determined for each and found to be in
the range of ~150 d for the coolest (G8) to 35-40 d for the warmest (F3). A
clear, linear inverse relationship has been found in the sample between the
pulsation period and the effective temperature and also an inverse linear
relationship between the amplitude of light variation and the effective
temperature. These are consistent with the expectation for a pulsating post-AGB
star evolving toward higher temperature at constant luminosity. The published
spectral energy distributions and mid-infrared images show these objects to
have cool (200 K), detached dust shells and published models imply that
intensive mass loss ended a few thousand years ago. The detection of periods as
long as 150 d in these requires a revision in the published post-AGB evolution
models that couple the pulsation period to the mass loss rate and that assume
that intensive mass loss ended when the pulsation period had decreased to 100
d. This revision will have the effect of extending the time scale for the early
phases of post-AGB evolution. It appears that real time evolution in the
pulsation periods of individual objects may be detectable on the time scale of
two decades
The Herschel Planetary Nebula Survey (HerPlaNS) I. Data Overview and Analysis Demonstration with NGC 6781
This is the first of a series of investigations into far-IR characteristics
of 11 planetary nebulae (PNs) under the Herschel Space Observatory Open Time 1
program, Herschel Planetary Nebula Survey (HerPlaNS). Using the HerPlaNS data
set, we look into the PN energetics and variations of the physical conditions
within the target nebulae. In the present work, we provide an overview of the
survey, data acquisition and processing, and resulting data products. We
perform (1) PACS/SPIRE broadband imaging to determine the spatial distribution
of the cold dust component in the target PNs and (2) PACS/SPIRE
spectral-energy-distribution (SED) and line spectroscopy to determine the
spatial distribution of the gas component in the target PNs. For the case of
NGC 6781, the broadband maps confirm the nearly pole-on barrel structure of the
amorphous carbon-richdust shell and the surrounding halo having temperatures of
26-40 K. The PACS/SPIRE multi-position spectra show spatial variations of
far-IR lines that reflect the physical stratification of the nebula. We
demonstrate that spatially-resolved far-IR line diagnostics yield the (T_e,
n_e) profiles, from which distributions of ionized, atomic, and molecular gases
can be determined. Direct comparison of the dust and gas column mass maps
constrained by the HerPlaNS data allows to construct an empirical gas-to-dust
mass ratio map, which shows a range of ratios with the median of 195+-110. The
present analysis yields estimates of the total mass of the shell to be 0.86
M_sun, consisting of 0.54 M_sun of ionized gas, 0.12 M_sun of atomic gas, 0.2
M_sun of molecular gas, and 4 x 10^-3 M_sun of dust grains. These estimates
also suggest that the central star of about 1.5 M_sun initial mass is
terminating its PN evolution onto the white dwarf cooling track.Comment: 27 pages, 16 figures, accepted for publication in A&
An evolutionary catalogue of Galactic post-AGB and related objects
With the ongoing AKARI infrared sky survey, of much greater sensitivity than
IRAS, a wealth of post-AGB objects may be discovered. It is thus time to
organize our present knowledge of known post-AGB stars in th galaxy with a view
to using it to search for new post-AGB objects among AKARI sources. We searched
the literature available on the NASA Astrophysics Data System up to 1 October
2006, and defined criteria for classifying sources into three categories: "very
likely", "possible" and "disqualified" post-AGB objects. The category of "very
likely" post-AGB objects is made up of several classes. We have created an
evolutionary, on-line catalogue of Galactic post-AGB objects, to be referred to
as "The Torun catalogue of Galactic post-AGB and related objects". The present
version of the catalogue contains 326 "very likely", 107 "possible" and 64
"disqualified" objects. For the very likely post-AGB objects, the catalogue
gives the available optical and infrared photometry, infrared spectroscopy and
spectral types, and links to finding charts and bibliography.Comment: 9 pages, 6 figures, accepted for publication in A&
The Planetary Nebula Luminosity Function at the Dawn of Gaia
The [O III] 5007 Planetary Nebula Luminosity Function (PNLF) is an excellent
extragalactic standard candle. In theory, the PNLF method should not work at
all, since the luminosities of the brightest planetary nebulae (PNe) should be
highly sensitive to the age of their host stellar population. Yet the method
appears robust, as it consistently produces < 10% distances to galaxies of all
Hubble types, from the earliest ellipticals to the latest-type spirals and
irregulars. It is therefore uniquely suited for cross-checking the results of
other techniques and finding small offsets between the Population I and
Population II distance ladders. We review the calibration of the method and
show that the zero points provided by Cepheids and the Tip of the Red Giant
Branch are in excellent agreement. We then compare the results of the PNLF with
those from Surface Brightness Fluctuation measurements, and show that, although
both techniques agree in a relative sense, the latter method yields distances
that are ~15% larger than those from the PNLF. We trace this discrepancy back
to the calibration galaxies and argue that, due to a small systematic error
associated with internal reddening, the true distance scale likely falls
between the extremes of the two methods. We also demonstrate how PNLF
measurements in the early-type galaxies that have hosted Type Ia supernovae can
help calibrate the SN Ia maximum magnitude-rate of decline relation. Finally,
we discuss how the results from space missions such as Kepler and Gaia can help
our understanding of the PNLF phenomenon and improve our knowledge of the
physics of local planetary nebulae.Comment: 12 pages, invited review at the conference "The Fundamental Cosmic
Distance Scale: State of the Art and Gaia Perspective", to appear in
Astrophysics and Space Scienc