666 research outputs found
Can El Nino-Southern Oscillation (ENSO) events modulate intraseasonal oscillations of Indian summer monsoon?
Prediction of interannual variability (IAV) of Indian summer monsoon (ISM) rainfall is limited by "internal" dynamics, and the monsoon intraseasonal oscillations (MISOs) seems to be at the heart of producing internal IAV of the ISM. If one could find an identifiable way through which these MISOs are modulated by slowly varying "external" forcing, such as El Nino-Southern Oscillation (ENSO), the uncertainty in the prediction of IAV could be reduced, leading to improvement of seasonal prediction. Such efforts, so far, have been inconclusive. In this study, the modulation of MISOs by ENSO is assessed by using a nonlinear pattern recognition technique known as the Self-Organizing Map (SOM). The SOM technique is efficient in handling the nonlinearity/event-to-event variability of the MISOs and capable of identifying various shades of MISO from large-scale dynamical/thermodynamical indices, without providing information on rainfall. It is shown that particular MISO phases are preferred during ENSO years, that is, the canonical break phase is preferred more in the El Nino years and the typical active phase is preferred during La Nina years. Interestingly, if the SOM clustering is done by removing the ENSO effect on seasonal mean, the preference for the break node remains relatively unchanged; whereas, the preference reduces/vanishes for the active node. The results indicate that the El Nino-break relationship is almost independent of the ENSO-monsoon relationship on seasonal scale whereas the La Nina-active association seems to be interwoven with the seasonal relationship
Analysis of MAGSAT data of the Indian region
Progress in the development of software for reading MAGSAT data tapes and for the reduction of anomaly data, and in the preparation of data for magnetic anomaly maps is reported
Evidence for Asphericity in the Type IIn Supernova 1998S
We present optical spectropolarimetry obtained at the Keck-II 10-m telescope
on 1998 March 7 UT along with total flux spectra spanning the first 494 days
after discovery (1998 March 2 UT) of the peculiar type IIn supernova (SN)
1998S. The SN is found to exhibit a high degree of linear polarization,
implying significant asphericity for its continuum-scattering environment.
Prior to removal of the interstellar polarization, the polarization spectrum is
characterized by a flat continuum (at p ~ 2%) with distinct changes in
polarization associated with both the broad (FWZI >= 20,000 km/s) and narrow
(unresolved, FWHM < 300 km/s) line emission seen in the total flux spectrum.
When analyzed in terms of a polarized continuum with unpolarized broad-line
recombination emission, an intrinsic continuum polarization of p ~ 3% results
(the highest yet found for a SN), suggesting a global asphericity of >= 45%
from the oblate, electron-scattering dominated models of Hoflich (1991). The
smooth, blue continuum evident at early times is shown to be inconsistent with
a reddened, single-temperature blackbody, instead having a color temperature
that increases with decreasing wavelength. Broad emission-line profiles with
distinct blue and red peaks are seen in the total flux spectra at later times,
perhaps suggesting a disk-like or ring-like morphology for the dense (n_e ~
10^7 cm^{-3}) circumstellar medium. Implications of the circumstellar
scattering environment for the spectropolarimetry are discussed, as are the
effects of uncertain removal of interstellar polarization.Comment: 25 pages + 2 tables + 14 figures, Submitted to The Astrophysical
Journa
Mira's wind explored in scattering infrared CO lines
We have observed the intermediate regions of the circumstellar envelope of
Mira (o Ceti) in photospheric light scattered by three vibration-rotation
transitions of the fundamental band of CO, from low-excited rotational levels
of the ground vibrational state, at an angular distance of beta = 2"-7" away
from the star. The data were obtained with the Phoenix spectrometer mounted on
the 4 m Mayall telescope at Kitt Peak. The spatial resolution is approximately
0.5" and seeing limited. Our observations provide absolute fluxes, leading to
an independent new estimate of the mass-loss rate of approximately 3e-7
Msun/yr, as derived from a simple analytic wind model. We find that the
scattered intensity from the wind of Mira for 2" < beta < 7" decreases as
beta^-3, which suggests a time constant mass-loss rate, when averaged over 100
years, over the past 1200 years.Comment: accepted for publication in the Astrophysical Journa
The Chandra X-ray Survey of Planetary Nebulae (ChanPlaNS): Probing Binarity, Magnetic Fields, and Wind Collisions
We present an overview of the initial results from the Chandra Planetary
Nebula Survey (ChanPlaNS), the first systematic (volume-limited) Chandra X-ray
Observatory survey of planetary nebulae (PNe) in the solar neighborhood. The
first phase of ChanPlaNS targeted 21 mostly high-excitation PNe within ~1.5 kpc
of Earth, yielding 4 detections of diffuse X-ray emission and 9 detections of
X-ray-luminous point sources at the central stars (CSPNe) of these objects.
Combining these results with those obtained from Chandra archival data for all
(14) other PNe within ~1.5 kpc that have been observed to date, we find an
overall X-ray detection rate of ~70%. Roughly 50% of the PNe observed by
Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing
shocks formed by energetic wind collisions is detected in ~30%; five objects
display both diffuse and point-like emission components. The presence of X-ray
sources appears correlated with PN density structure, in that molecule-poor,
elliptical nebulae are more likely to display X-ray emission (either point-like
or diffuse) than molecule-rich, bipolar or Ring-like nebulae. All but one of
the X-ray point sources detected at CSPNe display X-ray spectra that are harder
than expected from hot (~100 kK) central star photospheres, possibly indicating
a high frequency of binary companions to CSPNe. Other potential explanations
include self-shocking winds or PN mass fallback. Most PNe detected as diffuse
X-ray sources are elliptical nebulae that display a nested shell/halo structure
and bright ansae; the diffuse X-ray emission regions are confined within inner,
sharp-rimmed shells. All sample PNe that display diffuse X-ray emission have
inner shell dynamical ages <~5x10^3 yr, placing firm constraints on the
timescale for strong shocks due to wind interactions in PNe.Comment: 41 pages, 6 figures; submitted to the Astronomical Journa
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
Infrared Spectroscopic Study of a Selection of AGB and Post-AGB Stars
We present here near-infrared spectroscopy in the H and K bands of a
selection of nearly 80 stars that belong to various AGB types, namely S type, M
type and SR type. This sample also includes 16 Post-AGB (PAGB) stars. From
these spectra, we seek correlations between the equivalent widths of some
important spectral signatures and the infrared colors that are indicative of
mass loss. Repeated spectroscopic observations were made on some PAGB stars to
look for spectral variations. We also analyse archival SPITZER mid-infrared
spectra on a few PAGB stars to identify spectral features due to PAH molecules
providing confirmation of the advanced stage of their evolution. Further, we
model the SEDs of the stars (compiled from archival data) and compare
circumstellar dust parameters and mass loss rates in different types.
Our near-infrared spectra show that in the case of M and S type stars, the
equivalent widths of the CO(3-0) band are moderately correlated with infrared
colors, suggesting a possible relationship with mass loss processes. A few PAGB
stars revealed short term variability in their spectra, indicating episodic
mass loss: the cooler stars showed in CO first overtone bands and the hotter
ones showed in HI Brackett lines. Our spectra on IRAS 19399+2312 suggest that
it is a transition object. From the SPITZER spectra, there seems to be a
dependence between the spectral type of the PAGB stars and the strength of the
PAH features. Modelling of SEDs showed among the M and PAGB stars that the
higher the mass loss rates, the higher the [K-12] colour in our sample.Comment: 14 pages; accepted in MNRAS, 200
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&
A Spitzer IRS Spectral Atlas of Luminous 8 micron Sources in the Large Magellanic Cloud
We present an atlas of Spitzer Space Telescope Infrared Spectrograph (IRS)
spectra of highly luminous, compact mid-infrared sources in the Large
Magellanic Cloud. Sources were selected on the basis of infrared colors and 8
micron (MSX) fluxes indicative of highly evolved, intermediate- to high-mass
stars with current or recent mass loss at large rates. We determine the
chemistry of the circumstellar envelope from the mid-IR continuum and spectral
features and classify the spectral types of the stars. In the sample of 60
sources, we find 21 Red Supergiants (RSGs), 16 C-rich Asymptotic Giant Branch
(AGB) stars, 11 HII regions, 4 likely O-rich AGB stars, 4 Galactic O-rich AGB
stars, 2 OH/IR stars, and 2 B[e] supergiants with peculiar IR spectra. We find
that the overwhelming majority of the sample AGB stars (with typical IR
luminosities ~1.0E4 L_sun) have C-rich envelopes, while the O-rich objects are
predominantly luminous RSGs with L_IR ~ 1.0E5 L_sun. We determine mean
bolometric corrections to the stellar K-band flux densities and find that for
carbon stars, the bolometric corrections depend on the infrared color, whereas
for RSGs, the bolometric correction is independent of IR color. Our results
reveal that objects previously classified as PNe on the basis of IR colors are
in fact compact HII regions with very red IRS spectra that include strong
atomic recombination lines and PAH emission features. We demonstrate that the
IRS spectral classes in our sample separate clearly in infrared color-color
diagrams that use combinations of 2MASS data and synthetic IRAC/MIPS fluxes
derived from the IRS spectra. On this basis, we suggest diagnostics to identify
and classify, with high confidence levels, IR-luminous evolved stars and HII
regions in nearby galaxies using Spitzer and near-infrared photometry.Comment: 46 pages, 9 figures; accepted for publication in AJ; abstract
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