55 research outputs found
Infrared Spectroscopy of U Equulei's Warm Circumstellar Gas
Medium and high resolution spectroscopy of U Equulei from 1 to 4 microns
during 1997-2003 has revealed information about its unusual circumstellar
envelope, observed previously at optical and radio wavelengths. Strong
absorption bands of H2O and of CO dominate the 1-4um spectrum. The gas has a
mean temperature of 600 K and 12C/13C =< 10. The CO 2-0 line profiles and
velocities imply no net ejection or infall and indicate either rapid radial gas
motions being seen along a narrow continuum beam, or absorption by orbiting gas
that is nearly coincident with a highly extended continuum source. The gas
could be located in a disk-like structure. The observed high column densities
of warm CO and H2 normally would be associated with sufficient dust to
completely obscure the star at optical wavelengths. The observations thus
indicate either a highly abnormal gas-to-dust ratio, consistent with the
earlier optical observation of abundant refractory metal oxides in the
circumstellar gas, or peculiar geometry and/or illumination.Comment: 21 pages incl. 8 postscript figures and 1 table; typos correcte
Remarkable changes in the near-infrared spectrum of the nova-like variable V4332 Sgr
We report on recent near-IR observations of V4332 Sgr - the nova-like
variable that erupted in 1994. Its rapid, post-outburst evolution to a cool M
type giant/supergiant, soon after its outburst, had showed that it was an
unusual object differing from other eruptive variables like classical/symbiotic
novae or born-again AGB stars. The present study of V4332 Sgr was motivated by
the keen interest in the recent eruption of V838 Mon - which along with V4332
Sgr - is believed to belong to a new class of objects (we propose they may be
called "quasi-novae"). Our observations show new developments in the evolution
of V4332 Sgr. The most striking feature is the detection of several molecular
bands of AlO - a rarely seen molecule in astronomical spectra - in the JHK
spectra. Many of these bands are being detected for the first time. The only
other detection of some of these AlO bands are in V838 Mon, thereby showing
further spectral similarities between the two objects. JHK photometry shows the
development of a new dust shell around V4332 Sgr with a temperature of ~ 900K.
This dust shell does not appear to be associated with ejecta of the 1994
outburst but is due to a second mass-loss episode which is not expected in a
classical nova outburst. The cold molecular environment, suggested by the AlO
emission, is also not expected in novae ejecta. We model the AlO bands and also
discuss the possible formation mechanism of the AlO.Comment: To appear in Ap.J(L), 3 figure
Optical studies of V4332 Sagittarii - detection of unusually strong KI and NaI lines in emission
We present optical observations of the enigmatic nova-like variable V4332
Sgr. The importance of this object should not be understated since it is
considered to be the possible prototype of a new class of eruptive variables.
These objects have been the subject of considerable studies at present
primarily because of the spectacular eruption of V838 Mon - another member of
this class - recently in 2002. The cause of the outburst in such objects is not
well understood. Our recent work has shown striking changes in the near-IR
spectrum of V4332 Sgr since its 1994 outburst. The optical spectrum presented
here confirms that V4332 Sgr is indeed an unusual and extremely interesting
object. This spectrum, the first to be taken after a hiatus of nearly 10 years
after the outburst, shows several lines in emission but is dominated by
exceptionally strong emission in the resonance doublet of KI at 7665 and
7699{\AA} and to a slightly lesser strength in the unresolved NaI doublet at
5890 and 5896{\AA}. The KI lines are shown to be optically thick and
considerably broadened. We investigate the site of origin of the KI and NaI
emission. We discuss whether V4332 Sgr is related to L or T type dwarf based on
BVRI photometry.Comment: 10 pages, 2 figures, 2 tables. Appeared in Ap.J(L), 2004, 604,
L57-L6
Radio Frequency Interference Mitigation for Detection of Extended Sources with an Interferometer
Radio frequency interference (RFI) is a significant problem for current and
future radio telescopes. We describe here a method for post-correlation
cancellation of RFI for the special case of an extended source observed with an
interferometer that spatially resolves the astronomical signal. In this
circumstance, the astronomical signal is detected through the auto-correlations
of each antenna but is not present in the cross-correlations between antennas.
We assume that the RFI is detected in both auto- and cross-correlations, which
is true for many cases. The large number of cross-correlations can provide a
very high interference to noise ratio reference signal which can be adaptively
subtracted from the autocorrelation signals. The residual signal is free of
interference to significant levels. We discuss the application of this
technique for detection of the spin-flip transition of interstellar deuterium
with the Allen Telescope Array. The technique may also be of use for epoch of
reionization experiments and with multi-beam feeds on single dish telescopes.Comment: 7 pages, 3 figures, Radio Science, accepte
Further detections of OH masers in carbon stars with silicate features
A sample of J-type carbon stars was searched for OH maser emission. The new
detection of three OH lines towards two silicate carbon stars is reported. In
V778 Cyg, previously known as the main-lines (1665 and 1667 MHz) maser source,
the satellite 1612 MHz emission was discovered while in NSV 2814 the main OH
lines were detected. The presence of OH maser lines confirms the former
suggestion that oxygen-rich material is located in the vicinity (
cm) of silicate carbon stars.Comment: LaTeX2e, 4 pages with 2 figure
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
The Temperature Scale of Metal-Rich M Giants Based on TiO Bands: Population Synthesis in the Near Infrared
We have computed a grid of high resolution synthetic spectra for cool stars
(2500<Teff<6000 K) in the wavelength range 6000 -- 10200A, by employing an
updated line list of atomic and molecular lines, together with state-of-the-art
model atmospheres.
As a by-product, by fitting TiO bandheads in spectra of well-known M giants,
we have derived the electronic oscillator strengths of the TiO gamma prime,
delta, epsilon and phi systems. The derived oscillator strenghts for the gamma
prime, epsilon and phi systems differ from the laboratory and ab initio values
found in the literature, but are consistent with the model atmospheres and line
lists employed, resulting in a good match to the observed spectra of M giants
of known parameters.
The behavior of TiO bands as a function of the stellar parameters Teff, log g
and [Fe/H] is presented and the use of TiO spectral indices in stellar
population studies is discussed.Comment: ApJ accepted, 27 pages + 11 figures, AASLatex v4.
Dust temperature and density profiles of AGB and post-AGB stars from mid-infrared observations
First mid-infrared images of a sample of AGB and post-AGB carbon stars (V
Hya, IRC +10216, CIT 6 and Roberts 22) obtained at La Silla Observatory (ESO,
Chile) are reported. CIT 6 presents a cometary-like feature clearly seen in the
9.7m image, Roberts 22 shows an envelope slightly elongated in the
north-east direction while images of V Hya and IRC+10216 are roughly
spherically symmetric. Using inversion technique, the dust emissivity was
derived from the observed intensity profiles, allowing a determination of the
grain temperature and density distributions inside the envelope for these
stars. Dust masses and mass-loss rates were estimated for V Hya and IRC +10216.
Our results are comparable to those obtained in previous studies if dust grains
have dimensions in the range 0.01 - 0.2 m. Color maps suggest the
presence of temperature inhomogeneities in the central regions of the dust
envelopes. In the case of V Hya, an eccentric hot point, which direction
coincides with the jet previously seen in [SII] emission, suggest that we are
observing a material ejected in a previous mass-loss event. Bipolar lobes are
clearly seen in the color maps of Roberts 22 and IRC +10216.Comment: accepted for publication in Astronomy and Astrophysic
Comparison of RFI Mitigation Strategies for Dispersed Pulse Detection
Impulsive radio-frequency signals from astronomical sources are dispersed by
the frequency dependent index of refraction of the interstellar media and so
appear as chirped signals when they reach earth. Searches for dispersed
impulses have been limited by false detections due to radio frequency
interference (RFI) and, in some cases, artifacts of the instrumentation. Many
authors have discussed techniques to excise or mitigate RFI in searches for
fast transients, but comparisons between different approaches are lacking. This
work develops RFI mitigation techniques for use in searches for dispersed
pulses, employing data recorded in a "Fly's Eye" mode of the Allen Telescope
Array as a test case. We gauge the performance of several RFI mitigation
techniques by adding dispersed signals to data containing RFI and comparing
false alarm rates at the observed signal-to-noise ratios of the added signals.
We find that Huber filtering is most effective at removing broadband
interferers, while frequency centering is most effective at removing narrow
frequency interferers. Neither of these methods is effective over a broad range
of interferers. A method that combines Huber filtering and adaptive
interference cancellation provides the lowest number of false positives over
the interferers considered here. The methods developed here have application to
other searches for dispersed pulses in incoherent spectra, especially those
involving multiple beam systems.Comment: Accepted for publication in Ap
Evidence for a companion to BM Gem, a silicate carbon star
Balmer and Paschen continuum emission as well as Balmer series lines of P
Cygni-type profile from H_gamma through H_23 are revealed in the violet spectra
of BM Gem, a carbon star associated with an oxygen-rich circumstellar shell
(`silicate carbon star') observed with the high dispersion spectrograph (HDS)
on the Subaru telescope. The blue-shifted absorption in the Balmer lines
indicates the presence of an outflow, the line of sight velocity of which is at
least 400 km s^-1, which is the highest outflow velocity observed to date in a
carbon star. We argue that the observed unusual features in BM Gem are strong
evidence for the presence of a companion, which should form an accretion disk
that gives rise to both an ionized gas region and a high velocity, variable
outflow. The estimated luminosity of ~0.2 (0.03-0.6) L_sun for the ionized gas
can be maintained by a mass accretion rate to a dwarf companion of ~10^-8 M_sun
yr^-1, while ~10^-10 M_sun yr^-1 is sufficient for accretion to a white dwarf
companion. These accretion rates are feasible for some detached binary
configurations on the basis of the Bond-Hoyle type accretion process. We
concluded that the carbon star BM Gem is in a detached binary system with a
companion of low mass and low luminosity. However, we are unable to determine
whether this companion object is a dwarf or a white dwarf. The upper limits for
binary separation are 210 AU and 930 AU for a dwarf and a white dwarf,
respectively. We also note that the observed features of BM Gem mimic those of
Mira (omi Cet), which may suggest actual similarities in their binary
configurations and circumstellar structures.Comment: 11 pages, 2 figures, 1 table, accepted for publication in Ap
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